After studying this chapter, you should be able to:
Name the prominent cellular organelles and state their functions in cells.
Name the building blocks of the cellular cytoskeleton and state their contributions to cell structure and function.
Name the intercellular and cellular to extracellular connections.
Define the processes of exocytosis and endocytosis, and describe the contribution of each to normal cell function.
Define proteins that contribute to membrane permeability and transport.
Recognize various forms of intercellular communication and describe ways in which chemical messengers (including second messengers) affect cellular physiology.
The cell is the fundamental working unit of all organisms. In humans, cells can be highly specialized in both structure and function; alternatively, cells from different organs can share features and function. In the previous chapter, we examined some basic principles of biophysics and the catabolism and metabolism of building blocks found in the cell. In some of those discussions, we examined how the building blocks could contribute to basic cellular physiology (eg, DNA replication, transcription, and translation). In this chapter, we will briefly review more of the fundamental aspects of cellular and molecular physiology. Additional aspects that concern specialization of cellular and molecular physiology are considered in the next chapters concerning immune function and excitable cells, and, within the sections that highlight each physiological system.
FUNCTIONAL MORPHOLOGY OF THE CELL
A basic knowledge of cell biology is essential to an understanding of the organ systems and the way they function in the body. A key tool for examining cellular constituents is the microscope. A light microscope can resolve structures as close as 0.2 μm, while an electron microscope can resolve structures as close as 0.002 μm. Although cell dimensions are quite variable, this resolution can give us a good look at the inner workings of the cell. The advent of common access to phase contrast, fluorescent, confocal, and many other microscopy techniques along with specialized probes for both static and dynamic cellular structures further expanded the examination of cell structure and function. Equally revolutionary advances in modern biophysical, biochemical, and molecular biological techniques have also greatly contributed to our knowledge of the cell.
The specialization of the cells in the various organs is considerable, and no cell can be called “typical” of all cells in the body. However, a number of structures (organelles) are common to most cells. These structures are shown in Figure 2–1. Many of them can be isolated by ultracentrifugation combined with other techniques. When cells are homogenized and the resulting suspension is centrifuged, the nuclei sediment first, followed by the mitochondria. High-speed centrifugation that generates forces of 100,000 times gravity or more causes a fraction made up of granules called the microsomes to sediment. This fraction includes organelles such as the ribosomes and peroxisomes.
FIGURE 2–1 Diagram showing a hypothetical cell in the center as seen with the light microscope. Individual organelles are expanded for closer examination. (Adapted from Bloom and Fawcett. Reproduced with permission from Junqueira LC, Carneiro J, Kelley RO: Basic Histology, 9th ed. McGraw-Hill, 1998.)
The membrane that surrounds the cell is a remarkable structure. It is made up of lipids and proteins and is semipermeable, allowing some substances to pass through it and excluding others. However, its permeability can also be varied because it contains numerous regulated ion channels and other transport proteins that can change the amounts of substances moving across it. It is generally referred to as the plasma membrane. The nucleus and other organelles in the cell are bound by similar membranous structures.
Although the chemical structures of membranes and their properties vary considerably from one location to another, they have certain common features. They are generally about 7.5 nm (75 Å) thick. The major lipids are phospholipids such as phosphatidylcholine, phosphotidylserine, and phosphatidylethanolamine. The shape of the phospholipid molecule reflects its solubility properties: the “head” end of the molecule contains the phosphate portion and is relatively soluble in water (polar, hydrophilic) and the “tail” ends are relatively insoluble (nonpolar, hydrophobic). The possession of both hydrophilic and hydrophobic properties makes the lipid an amphipathic molecule. In the membrane, the hydrophilic ends of the molecules are exposed to the aqueous environment that bathes the exterior of the cells and the aqueous cytoplasm; the hydrophobic ends meet in the water-poor interior of the membrane (Figure 2–2). In prokaryotes (ie, bacteria in which there is no nucleus), the membranes are relatively simple, but in eukaryotes (cells containing nuclei), cell membranes contain various glycosphingolipids, sphingomyelin, and cholesterol in addition to phospholipids and phosphatidylcholine.
FIGURE 2–2 Organization of the phospholipid bilayer and associated proteins in a biological membrane. The phospholipid molecules each have two fatty acid chains (wavy lines) attached to a phosphate head (open circle). Proteins are shown as irregular colored globules. Many are integral proteins, which extend into the membrane, but peripheral proteins are attached to the inside or outside (not shown) of the membrane. Specific protein attachments and cholesterol commonly found in the bilayer are omitted for clarity. (Reproduced with permission from Widmaier EP, Raff H, Strang K: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.)
Many different proteins are embedded in the membrane. They exist as separate globular units and many pass through or are embedded in one leaflet of the membrane (integral proteins), whereas others (peripheral proteins) are associated with the inside or outside of the membrane (Figure 2–2). The amount of protein varies significantly with the function of the membrane but makes up on average 50% of the mass of the membrane; that is, there is about one protein molecule per 50 of the much smaller phospholipid molecules. The proteins in the membrane carry out many functions. Some are cell adhesion molecules (CAMs) that anchor cells to their neighbors or to basal laminas. Some proteins function as pumps, actively transporting ions across the membrane. Other proteins function as carriers, transporting substances down electrochemical gradients by facilitated diffusion. Still others are ion channels, which, when activated, permit the passage of ions into or out of the cell. The role of the pumps, carriers, and ion channels in transport across the cell membrane is discussed below. Proteins in another group function as receptors that bind ligands or messenger molecules, initiating physiologic changes inside the cell. Proteins also function as enzymes, catalyzing reactions at the surfaces of the membrane. Examples from each of these groups are discussed later in this chapter.
The uncharged, hydrophobic portions of the proteins are usually located in the interior of the membrane, whereas the charged, hydrophilic portions are located on the surfaces. Peripheral proteins are attached to the surfaces of the membrane in various ways. One common way is attachment to glycosylated forms of phosphatidylinositol. Proteins held by these glycosylphosphatidylinositol (GPI) anchors (Figure 2–3) include enzymes such as alkaline phosphatase, various antigens, a number of CAMs, and three proteins that combat cell lysis by complement. Over 45 GPI-linked cell surface proteins have now been described in humans. Other proteins are lipidated, that is, they have specific lipids attached to them (Figure 2–3). Proteins may be myristoylated, palmitoylated, or prenylated (ie, attached to geranylgeranyl or farnesyl groups).
FIGURE 2–3 Protein linkages to membrane lipids. Some are linked by their amino terminals, others by their carboxyl terminals. Many are attached via glycosylated forms of phosphatidylinositol (GPI anchors). (Adapted with permission from Fuller GM, Shields D: Molecular Basis of Medical Cell Biology. McGraw-Hill, 1998.)
The protein structure—and particularly the enzyme content—of biologic membranes varies not only from cell to cell, but also within the same cell. For example, some of the enzymes embedded in cell membranes are different from those in mitochondrial membranes. In epithelial cells, the enzymes in the cell membrane on the mucosal surface differ from those in the cell membrane on the basal and lateral margins of the cells; that is, the cells are polarized.Such polarization makes directional transport across epithelia possible. The membranes are dynamic structures, and their constituents are being constantly renewed at different rates. Some proteins are anchored to the cytoskeleton, but others move laterally in the membrane.
Underlying most cells is a thin, “fuzzy” layer plus some fibrils that collectively make up the basement membrane or, more properly, the basal lamina. The basal lamina and, more generally, the extracellular matrix are made up of many proteins that hold cells together, regulate their development, and determine their growth. These include collagens, laminins, fibronectin, tenascin, and various proteoglycans.
Over a billion years ago, aerobic bacteria were engulfed by eukaryotic cells and evolved into mitochondria, providing the eukaryotic cells with the ability to form the energy-rich compound ATP by oxidative phosphorylation.Mitochondria perform other functions, including a role in the regulation of apoptosis (programmed cell death), but oxidative phosphorylation is the most crucial. Each eukaryotic cell can have hundreds to thousands of mitochondria. In mammals, they are generally depicted as sausage-shaped organelles (Figure 2–1), but their shape can be quite dynamic. Each has an outer membrane, an intermembrane space, an inner membrane, which is folded to form shelves (cristae), and a central matrix space. The enzyme complexes responsible for oxidative phosphorylation are lined up on the cristae (Figure 2–4).
FIGURE 2–4 Components involved in oxidative phosphorylation in mitochondria and their origins. As enzyme complexes I through IV convert 2-carbon metabolic fragments to CO2 and H2O, protons (H+) are pumped into the intermembrane space. The proteins diffuse back to the matrix space via complex V, ATP synthase (AS), in which ADP is converted to ATP. The enzyme complexes are made up of subunits coded by mitochondrial DNA (mDNA) and nuclear DNA (nDNA), and the figures document the contribution of each DNA to the complexes.
Consistent with their origin from aerobic bacteria, the mitochondria have their own genome. There is much less DNA in the mitochondrial genome than in the nuclear genome, and 99% of the proteins in the mitochondria are the products of nuclear genes, but mitochondrial DNA is responsible for certain key components of the pathway for oxidative phosphorylation. Specifically, human mitochondrial DNA is a double-stranded circular molecule containing approximately 16,500 base pairs (compared with over a billion in nuclear DNA). It codes for 13 protein subunits that are associated with proteins encoded by nuclear genes to form four enzyme complexes plus two ribosomal and 22 transfer RNAs that are needed for protein production by the intramitochondrial ribosomes.
The enzyme complexes responsible for oxidative phosphorylation illustrate the interactions between the products of the mitochondrial genome and the nuclear genome. For example, complex I, reduced nicotinamide adenine dinucleotide dehydrogenase (NADH), is made up of seven protein subunits coded by mitochondrial DNA and 39 subunits coded by nuclear DNA. The origin of the subunits in the other complexes is shown in Figure 2–4. Complex II, succinate dehydrogenase-ubiquinone oxidoreductase; complex III, ubiquinonecytochrome c oxidoreductase; and complex IV, cytochrome c oxidase, act with complex I, coenzyme Q, and cytochrome c to convert metabolites to CO2 and water. Complexes I, III, and IV pump protons (H+) into the intermembrane space during this electron transfer. The protons then flow down their electrochemical gradient through complex V, ATP synthase, which harnesses this energy to generate ATP.
As zygote mitochondria are derived from the ovum, their inheritance is maternal. This maternal inheritance has been used as a tool to track evolutionary descent. Mitochondria have an ineffective DNA repair system, and the mutation rate for mitochondrial DNA is over 10 times the rate for nuclear DNA. A large number of relatively rare diseases have now been traced to mutations in mitochondrial DNA. These include disorders of tissues with high metabolic rates in which energy production is defective as a result of abnormalities in the production of ATP, as well as other disorders (Clinical Box 2–1).
CLINICAL BOX 2–1
Mitochondrial diseases encompass at least 40 diverse disorders that are grouped because of their links to mitochondrial failure. These diseases can occur following inheritance or spontaneous mutations in mitochondrial or nuclear DNA that lead to altered functions of the mitochondrial proteins (or RNA). Depending on the target cell and/or tissues affected, symptoms resulting from mitochondrial diseases may include altered motor control, altered muscle output, gastrointestinal dysfunction, altered growth, diabetes, seizures, visual/hearing problems, lactic acidosis, developmental delays, susceptibility to infection or, cardiac, liver and respiratory disease. Although there is evidence for tissue-specific isoforms of mitochondrial proteins, mutations in these proteins do not fully explain the highly variable patterns or targeted organ systems observed with mitochondrial diseases.
With the diversity of disease types and the overall importance of mitochondria in energy production, it is not surprising that there is no single cure for mitochondrial diseases and focus remains on treating the symptoms when possible. For example, in some mitochondrial myopathies (ie, mitochondrial diseases associated with neuromuscular function), physical therapy may help to extend the range of movement of muscles and improve dexterity.
In the cytoplasm of the cell there are large, somewhat irregular structures surrounded by membranes. The interior of these structures, which are called lysosomes, is more acidic than the rest of the cytoplasm, and external material such as endocytosed bacteria, as well as worn-out cell components, are digested in them. The interior is kept acidic by the action of a proton pump, or H+ATPase. This integral membrane protein uses the energy of ATP to move protons from the cytosol up their electrochemical gradient and keep the lysosome relatively acidic, near pH 5.0. Lysosomes can contain over 40 types of hydrolytic enzymes, some of which are listed in Table 2–1. Not surprisingly, these enzymes are all acid hydrolases, in that they function best at the acidic pH of the lysosomal compartment. This can be a safety feature for the cell; if the lysosomes were to break open and release their contents, the enzymes would not be efficient at the near neutral cytosolic pH (7.2), and thus would be unable to digest cytosolic enzymes they may encounter. Diseases associated with lysosomal dysfunction are discussed in Clinical Box 2–2.
TABLE 2–1 Some of the enzymes found in lysosomes and the cell components that are their substrates.
CLINICAL BOX 2–2
When a lysosomal enzyme is congenitally absent, the lysosomes become engorged with the material the enzyme normally degrades. This eventually leads to one of the lysosomal diseases (also called lysosomal storage diseases). There are over 50 such diseases currently recognized. For example, Fabry disease is caused by a deficiency in α-galactosidase; Gaucher disease is caused by a deficiency in β-galactocerebrosidase and Tay–Sachs disease, which causes mental retardation and blindness, is caused by the loss of hexosaminidase A, a lysosomal enzyme that catalyzes the biodegradation of gangliosides (fatty acid derivatives). Such individual lysosomal diseases are rare, but they are serious and can be fatal.
Since there are many different lysosomal disorders, treatments vary considerably and “cures” remain elusive for most of these diseases. Much of the care is focused on managing symptoms of each specific disorder. Enzyme replacement therapy has shown to be effective for certain lysosomal diseases, including Gaucher’s disease and Fabry’s disease. However, the long-term effectiveness and the tissue specific effects of many of the enzyme replacement treatments have not yet been established. Alternative approaches include bone marrow or stem cell transplantation. Again, these are limited in use and medical advances are necessary to fully combat this group of diseases.
Peroxisomes are 0.5 μm in diameter, are surrounded by a membrane, and contain enzymes that can either produce H2O2(oxidases) or break it down (catalases). Proteins are directed to the peroxisome by a unique signal sequence with the help of protein chaperones, peroxins. The peroxisome membrane contains a number of peroxisome-specific proteins that are concerned with transport of substances into and out of the matrix of the peroxisome. The matrix contains more than 40 enzymes, which operate in concert with enzymes outside the peroxisome to catalyze a variety of anabolic and catabolic reactions (eg, breakdown of lipids). Peroxisomes can form by budding of the endoplasmic reticulum, or by division. A number of synthetic compounds were found to cause proliferation of peroxisomes by acting on receptors in the nuclei of cells. These peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily. When activated, they bind to DNA, producing changes in the production of mRNAs. The known effects for PPARs are extensive and can affect most tissues and organs.
All cells have a cytoskeleton, a system of fibers that not only maintains the structure of the cell but also permits it to change shape and move. The cytoskeleton is made up primarily of microtubules, intermediate filaments, and microfilaments (Figure 2–5), along with proteins that anchor them and tie them together. In addition, proteins and organelles move along microtubules and microfilaments from one part of the cell to another, propelled by molecular motors.
FIGURE 2–5 Cytoskeletal elements of the cell. Artistic impressions that depict the major cytoskeletal elements are shown on the left, with basic properties of these elements on the right. (Reproduced with permission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.)
Microtubules (Figure 2–5 and Figure 2–6) are long, hollow structures with 5-nm walls surrounding a cavity 15 nm in diameter. They are made up of two globular protein subunits: α- and β-tubulin. A third subunit, γ-tubulin, is associated with the production of microtubules by the centrosomes. The α and β subunits form heterodimers, which aggregate to form long tubes made up of stacked rings, with each ring usually containing 13 subunits. The tubules interact with GTP to facilitate their formation. Although microtubule subunits can be added to either end, microtubules are polar with assembly predominating at the “+” end and disassembly predominating at the “−” end. Both processes occur simultaneously in vitro. The growth of microtubules is temperature sensitive (disassembly is favored under cold conditions) as well as under the control of a variety of cellular factors that can directly interact with microtubules in the cell.
FIGURE 2–6 Microfilaments and microtubules. Electronmicrograph (Left) of the cytoplasm of a fibroblast, displaying actin microfilaments (MF) and microtubules (MT). Fluorescent micrographs of airway epithelial cells displaying actin microfilaments stained with phalloidin (Middle) and microtubules visualized with an antibody to β-tubulin (Right). Both fluorescent micrographs are counterstained with Hoechst dye (blue) to visualize nuclei. Note the distinct differences in cytoskeletal structure. (For left; Courtesy of E Katchburian.)
Because of their constant assembly and disassembly, microtubules are a dynamic portion of the cytoskeleton. They provide the tracks along which several different molecular motors move transport vesicles, organelles such as secretory granules, and mitochondria from one part of the cell to another. They also form the spindle, which moves the chromosomes in mitosis. Cargo can be transported in either direction on microtubules.
There are several drugs available that disrupt cellular function through interaction with microtubules. Microtubule assembly is prevented by colchicine and vinblastine. The anticancer drug paclitaxel (Taxol) binds to microtubules and makes them so stable that organelles cannot move. Mitotic spindles cannot form, and the cells die.
Intermediate filaments (Figure 2–5) are 8–14 nm in diameter and are made up of various subunits. Some of these filaments connect the nuclear membrane to the cell membrane. They form a flexible scaffolding for the cell and help it resist external pressure. In their absence, cells rupture more easily, and when they are abnormal in humans, blistering of the skin is common. The proteins that make up intermediate filaments are cell-type specific, and are thus frequently used as cellular markers. For example, vimentin is a major intermediate filament in fibroblasts, whereas cytokeratin is expressed in epithelial cells.
Microfilaments (Figures 2–5 and 2–6) are long solid fibers with a 4–6 nm diameter that are made up of actin. Although actin is most often associated with muscle contraction, it is present in all types of cells. It is the most abundant protein in mammalian cells, sometimes accounting for as much as 15% of the total protein in the cell. Its structure is highly conserved; for example, 88% of the amino acid sequences in yeast and rabbit actin are identical. Actin filaments polymerize and depolymerize in vivo, and it is not uncommon to find polymerization occurring at one end of the filament while depolymerization is occurring at the other end. Filamentous (F) actin refers to intact microfilaments and globular (G) actin refers to the unpolymerized protein actin subunits. F-actin fibers attach to various parts of the cytoskeleton and can interact directly or indirectly with membrane-bound proteins. They reach to the tips of the microvilli on the epithelial cells of the intestinal mucosa. They are also abundant in the lamellipodia that cells put out when they crawl along surfaces. The actin filaments interact with integrin receptors and form focal adhesion complexes, which serve as points of traction with the surface over which the cell pulls itself. In addition, some molecular motors use microfilaments as tracks.
The molecular motors that move proteins, organelles, and other cell parts (collectively referred to as “cargo”) to all parts of the cell are 100–500 kDa ATPases. They attach to their cargo at one end of the molecule and to microtubules or actin polymers with the other end, sometimes referred to as the “head.” They convert the energy of ATP into movement along the cytoskeleton, taking their cargo with them. There are three super families of molecular motors: kinesin, dynein, and myosin. Examples of individual proteins from each superfamily are shown in Figure 2–7. It is important to note that there is extensive variation among superfamily members, allowing for the specialization of function (eg, choice of cargo, cytoskeletal filament type, and/or direction of movement).
FIGURE 2–7 Three examples of molecular motors. Conventional kinesin is shown attached to cargo, in this case a membrane-bound organelle. The way that myosin V “walks” along a microtubule is also shown. Note that the heads of the motors hydrolyze ATP and use the energy to produce motion.
The conventional form of kinesin is a doubleheaded molecule that tends to move its cargo toward the “+” ends of microtubules. One head binds to the microtubule and then bends its neck while the other head swings forward and binds, producing almost continuous movement. Some kinesins are associated with mitosis and meiosis. Other kinesins perform different functions, including, in some instances, moving cargo to the “−” end of microtubules. Dyneinshave two heads, with their neck pieces embedded in a complex of proteins. Cytoplasmic dyneins have a function like that of conventional kinesin, except they tend to move particles and membranes to the “−” end of the microtubules. The multiple forms of myosin in the body are divided into 18 classes. The heads of myosin molecules bind to actin and produce motion by bending their neck regions (myosin II) or walking along microfilaments, one head after the other (myosin V). In these ways, they perform functions as diverse as contraction of muscle and cell migration.
Near the nucleus in the cytoplasm of eukaryotic animal cells is a centrosome. The centrosome is made up of two centrioles and surrounding amorphous pericentriolar material. The centrioles are short cylinders arranged so that they are at right angles to each other. Microtubules in groups of three run longitudinally in the walls of each centriole (Figure 2–1). Nine of these triplets are spaced at regular intervals around the circumference.
The centrosomes are microtubule-organizing centers (MTOCs) that contain γ-tubulin. The microtubules grow out of this γ-tubulin in the pericentriolar material. When a cell divides, the centrosomes duplicate themselves, and the pairs move apart to the poles of the mitotic spindle, where they monitor the steps in cell division. In multinucleate cells, a centrosome is near each nucleus.
Cilia are specialized cellular projections that are used by unicellular organisms to propel themselves through liquid and by multicellular organisms to propel mucus and other substances over the surface of various epithelia. Additionally, virtually all cells in the human body contain a primary cilium that emanates from the surface. The primary cilium serves as a sensory organelle that receives both mechanical and chemical signals from other cells and the environment. Cilia are functionally indistinct from the eukaryotic flagella of sperm cells. Within the cilium there is an axoneme that comprises a unique arrangement of nine outer microtubule doublets and two inner microtubules (“9+2” arrangement). Along this cytoskeleton is axonemal dynein. Coordinated dynein–microtubule interactions within the axoneme are the basis of ciliary and sperm movement. At the base of the axoneme and just inside lies the basal body. It has nine circumferential triplet microtubules, like a centriole, and there is evidence that basal bodies and centrioles are interconvertible. A wide variety of diseases and disorders arise from dysfunctional cilia (Clinical Box 2–3).
CLINICAL BOX 2–3
Primary ciliary dyskinesia refers to a set of inherited disorders that limit ciliary structure and/or function. Disorders associated with ciliary dysfunction have long been recognized in the conducting airway. Altered ciliary function in the conducting airway can slow the mucociliary escalator and result in airway obstruction and increased infection. Dysregulation of ciliary function in sperm cells has also been well characterized to result in loss of motility and infertility. Ciliary defects in the function or structure of primary cilia have been shown to have effects on a variety of tissues/organs. As would be expected, such diseases are quite varied in their presentation, largely due to the affected tissue and include mental retardation, retinal blindness, obesity, polycystic kidney disease, liver fibrosis, ataxia, and some forms of cancer.
The severity in ciliary disorders can vary widely, and treatments targeted to individual organs also vary. Treatment of ciliary dyskinesia in the conducting airway is focused on keeping the airways clear and free of infection. Strategies include routine washing and suctioning of the sinus cavities and ear canals and liberal use of antibiotics. Other treatments that keep the airway from being obstructed (eg, bronchodilators, mucolytics, and steroids) are also commonly used.
CELL ADHESION MOLECULES
Cells are attached to the basal lamina and to each other by CAMs that are prominent parts of the intercellular connections described below. These adhesion proteins have attracted great attention in recent years because of their unique structural and signaling functions found to be important in embryonic development and formation of the nervous system and other tissues, in holding tissues together in adults, in inflammation and wound healing, and in the metastasis of tumors. Many CAMs pass through the cell membrane and are anchored to the cytoskeleton inside the cell. Some bind to like molecules on other cells (homophilic binding), whereas others bind to nonself molecules (heterophilic binding). Many bind to laminins, a family of large cross-shaped molecules with multiple receptor domains in the extracellular matrix.
Nomenclature in the CAM field is somewhat chaotic, partly because the field is growing so rapidly and partly because of the extensive use of acronyms, as in other areas of modern biology. However, the CAMs can be divided into four broad families: (1) integrins, heterodimers that bind to various receptors; (2) adhesion molecules of the IgG superfamily of immunoglobulins; (3) cadherins, Ca2+-dependent molecules that mediate cell-to-cell adhesion by homophilic reactions; and (4) selectins, which have lectin-like domains that bind carbohydrates. Specific functions of some of these molecules are addressed in later chapters.
The CAMs not only fasten cells to their neighbors, but they also transmit signals into and out of the cell. For example, cells that lose their contact with the extracellular matrix via integrins have a higher rate of apoptosis than anchored cells, and interactions between integrins and the cytoskeleton are involved in cell movement.
Intercellular junctions that form between the cells in tissues can be broadly split into two groups: junctions that fasten the cells to one another and to surrounding tissues, and junctions that permit transfer of ions and other molecules from one cell to another. The types of junctions that tie cells together and endow tissues with strength and stability include tight junctions, which are also known as the zonula occludens (Figure 2–8). The desmosome and zonula adherens also help to hold cells together, and the hemidesmosome and focal adhesions attach cells to their basal laminas. The gap junction forms a cytoplasmic “tunnel” for diffusion of small molecules (< 1000 Da) between two neighboring cells.
FIGURE 2–8 Intercellular junctions in the mucosa of the small intestine. Tight junctions (zonula occludens), adherens junctions (zonula adherens), desmosomes, gap junctions, and hemidesmosomes are all shown in relative positions in a polarized epithelial cell.
Tight junctions characteristically surround the apical margins of the cells in epithelia such as the intestinal mucosa, the walls of the renal tubules, and the choroid plexus. They are also important to endothelial barrier function. They are made up of ridges—half from one cell and half from the other—which adhere so strongly at cell junctions that they almost obliterate the space between the cells. There are three main families of transmembrane proteins that contribute to tight junctions: occludin, junctional adhesion molecules (JAMs), and claudins; and several more proteins that interact from the cytosolic side. Tight junctions permit the passage of some ions and solute in between adjacent cells (paracellular pathway) and the degree of this “leakiness” varies, depending in part on the protein makeup of the tight junction. Extracellular fluxes of ions and solute across epithelia at these junctions are a significant part of overall ion and solute flux. In addition, tight junctions prevent the movement of proteins in the plane of the membrane, helping to maintain the different distribution of transporters and channels in the apical and basolateral cell membranes that make transport across epithelia possible.
In epithelial cells, each zonula adherens is usually a continuous structure on the basal side of the zonula occludens, and it is a major site of attachment for intracellular microfilaments. It contains cadherins.
Desmosomes are patches characterized by apposed thickenings of the membranes of two adjacent cells. Attached to the thickened area in each cell are intermediate filaments, some running parallel to the membrane and others radiating away from it. Between the two membrane thickenings the intercellular space contains filamentous material that includes cadherins and the extracellular portions of several other transmembrane proteins.
Hemidesmosomes look like half-desmosomes that attach cells to the underlying basal lamina and are connected intracellularly to intermediate filaments. However, they contain integrins rather than cadherins. Focal adhesions also attach cells to their basal laminas. As noted previously, they are labile structures associated with actin filaments inside the cell, and they play an important role in cell movement.
At gap junctions, the intercellular space narrows from 25 to 3 nm, and units called connexons in the membrane of each cell are lined up with one another to form the dodecameric gap junction (Figure 2–9). Each connexon is made up of six protein subunits called connexins. They surround a channel that, when lined up with the channel in the corresponding connexon in the adjacent cell, permits substances to pass between the cells without entering the ECF. The diameter of the channel is normally about 2 nm, which permits the passage of ions, sugars, amino acids, and other solutes with molecular weights up to about 1000. Gap junctions thus permit the rapid propagation of electrical activity from cell to cell, as well as the exchange of various chemical messengers. However, the gap junction channels are not simply passive, nonspecific conduits. At least 20 different genes code for connexins in humans, and mutations in these genes can lead to diseases that are highly selective in terms of the tissues involved and the type of communication between cells produced (Clinical Box 2–4). Experiments in which particular connexins are deleted by gene manipulation or replaced with different connexins confirm that the particular connexin subunits that make up connexons determine their permeability and selectivity. It should be noted that connexons can also provide a conduit for regulated passage of small molecules between the cytoplasm and the ECF. Such movement can allow additional signaling pathways between and among cells in a tissue.
FIGURE 2–9 Gap junction connecting the cytoplasm of two cells. A) A gap junction plaque, or collection of individual gap junctions, is shown to form multiple pores between cells that allow for the transfer of small molecules. Inset is electronmicrograph from rat liver (N. Gilula). B) Topographical depiction of individual connexon and corresponding six connexin proteins that traverse the membrane. Note that each connexin traverses the membrane four times. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM (editors): Principles of Neural Science, 4th ed. McGraw-Hill, 2000.)
CLINICAL BOX 2–4
Connexins in Disease
In recent years, there has been an explosion of information related to the in vivo functions of connexins, growing out of work on connexin knock-outs in mice and the analysis of mutations in human connexins. The mouse knock-outs demonstrated that connexin deletions lead to electrophysiological defects in the heart and predisposition to sudden cardiac death, female sterility, abnormal bone development, abnormal growth in the liver, cataracts, hearing loss, and a host of other abnormalities. Information from these and other studies has allowed for the identification of several connexin mutations now known to be responsible for almost 20 different human diseases. These diseases include several skin disorders such as Clouston Syndrome (a connexin 30 (Cx30) defect) and erythrokeratoderma variabilis (Cx30.3 and Cx31); inherited deafness (Cx26, Cx30, and Cx31); predisposition to myoclonic epilepsy (Cx36), predisposition to arteriosclerosis (Cx37); cataract (Cx46 and Cx50); idiopathic atrial fibrillation (Cx40); and X-linked Charcot-Marie-Tooth disease (Cx32). It is interesting to note that each of these target tissues for disease contain other connexins that do not fully compensate for loss of the crucial connexins in disease development. Understanding how loss of individual connexins alters cell physiology to contribute to these and other human diseases is an area of intense research.
NUCLEUS & RELATED STRUCTURES
A nucleus is present in all eukaryotic cells that divide. The nucleus is made up in large part of the chromosomes, the structures in the nucleus that carry a complete blueprint for all the heritable species and individual characteristics of the animal. Except in germ cells, the chromosomes occur in pairs, one originally from each parent. Each chromosome is made up of a giant molecule of DNA. The DNA strand is about 2 m long, but it can fit in the nucleus because at intervals it is wrapped around a core of histone proteins to form a nucleosome. There are about 25 million nucleosomes in each nucleus. Thus, the structure of the chromosomes has been likened to a string of beads. The beads are the nucleosomes, and the linker DNA between them is the string. The whole complex of DNA and proteins is called chromatin. During cell division, the coiling around histones is loosened, probably by acetylation of the histones, and pairs of chromosomes become visible, but between cell divisions only clumps of chromatin can be discerned in the nucleus. The ultimate units of heredity are the genes on the chromosomes). As discussed in Chapter 1, each gene is a portion of the DNA molecule.
The nucleus of most cells contains a nucleolus (Figure 2–1), a patchwork of granules rich in RNA. In some cells, the nucleus contains several of these structures. Nucleoli are most prominent and numerous in growing cells. They are the site of synthesis of ribosomes, the structures in the cytoplasm in which proteins are synthesized.
The interior of the nucleus has a skeleton of fine filaments that are attached to the nuclear membrane, or envelope (Figure 2–1), which surrounds the nucleus. This membrane is a double membrane, and spaces between the two folds are called perinuclear cisterns. The membrane is permeable only to small molecules. However, it contains nuclear pore complexes. Each complex has eightfold symmetry and is made up of about 100 proteins organized to form a tunnel through which transport of proteins and mRNA occurs. There are many transport pathways; many proteins that participate in these pathways, including importins and exportins have been isolated and characterized. Much current research is focused on transport into and out of the nucleus, and a more detailed understanding of these processes should emerge in the near future.
The endoplasmic reticulum is a complex series of tubules in the cytoplasm of the cell (Figure 2–1; Figure 2–10; and Figure 2–11). The inner limb of its membrane is continuous with a segment of the nuclear membrane, so in effect this part of the nuclear membrane is a cistern of the endoplasmic reticulum. The tubule walls are made up of membrane. In rough, or granular, endoplasmic reticulum, ribosomes are attached to the cytoplasmic side of the membrane, whereas in smooth, or agranular, endoplasmic reticulum, ribosomes are absent. Free ribosomes are also found in the cytoplasm. The granular endoplasmic reticulum is concerned with protein synthesis and the initial folding of polypeptide chains with the formation of disulfide bonds. The agranular endoplasmic reticulum is the site of steroid synthesis in steroid-secreting cells and the site of detoxification processes in other cells. A modified endoplasmic reticulum, the sarcoplasmic reticulum, plays an important role in skeletal and cardiac muscle. In particular, the endoplasmic or sarcoplasmic reticulum can sequester Ca2+ ions and allow for their release as signaling molecules in the cytosol.
FIGURE 2–10 Rough endoplasmic reticulum and protein translation. Messenger RNA and ribosomes meet up in the cytosol for translation. Proteins that have appropriate signal peptides begin translation, and then associate with the endoplasmic reticulum (ER) to complete translation. The association of ribosomes is what gives the ER its “rough” appearance. (Reproduced with permission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.)
FIGURE 2–11 Cellular structures involved in protein processing. See text for details.
The ribosomes in eukaryotes measure approximately 22 × 32 nm. Each is made up of a large and a small subunit called, on the basis of their rates of sedimentation in the ultracentrifuge, the 60S and 40S subunits. The ribosomes are complex structures, containing many different proteins and at least three ribosomal RNAs. They are the sites of protein synthesis. The ribosomes that become attached to the endoplasmic reticulum synthesize all transmembrane proteins, most secreted proteins, and most proteins that are stored in the Golgi apparatus, lysosomes, and endosomes. These proteins typically have a hydrophobic signal peptide at one end (Figure 2–10). The polypeptide chains that form these proteins are extruded into the endoplasmic reticulum. The free ribosomes synthesize cytoplasmic proteins such as hemoglobin and the proteins found in peroxisomes and mitochondria.
GOLGI APPARATUS & VESICULAR TRAFFIC
The Golgi apparatus is a collection of membrane-enclosed sacs (cisternae) that are stacked like dinner plates (Figure 2–1). One or more Golgi apparati are present in all eukaryotic cells, usually near the nucleus. Much of the organization of the Golgi is directed at proper glycosylation of proteins and lipids. There are more than 200 enzymes that function to add, remove, or modify sugars from proteins and lipids in the Golgi apparatus.
The Golgi apparatus is a polarized structure, with cis and trans sides (Figures 2–1; 2–10; 2–11). Membranous vesicles containing newly synthesized proteins bud off from the granular endoplasmic reticulum and fuse with the cistern on the cis side of the apparatus. The proteins are then passed via other vesicles to the middle cisterns and finally to the cistern on the trans side, from which vesicles branch off into the cytoplasm. From the trans Golgi, vesicles shuttle to the lysosomes and to the cell exterior via constitutive and nonconstitutive pathways, both involving exocytosis. Conversely, vesicles are pinched off from the cell membrane by endocytosis and pass to endosomes. From there, they are recycled.
Vesicular traffic in the Golgi, and between other membranous compartments in the cell, is regulated by a combination of common mechanisms along with special mechanisms that determine where inside the cell they will go. One prominent feature is the involvement of a series of regulatory proteins controlled by GTP or GDP binding (small G proteins) associated with vesicle assembly and delivery. A second prominent feature is the presence of proteins called SNAREs (for soluble N-ethylmaleimide-sensitive factor attachment receptor). The v- (for vesicle) SNAREs on vesicle membranes interact in a lock-and-key fashion with t- (for target) SNAREs. Individual vesicles also contain structural protein or lipids in their membrane that help to target them for specific membrane compartments (eg, Golgi sacs, cell membranes).
The processes involved in protein synthesis, folding, and migration to the various parts of the cell are so complex that it is remarkable that more errors and abnormalities do not occur. The fact that these processes work as well as they do is because of mechanisms at each level that are responsible for “quality control.” Damaged DNA is detected and repaired or bypassed. The various RNAs are also checked during the translation process. Finally, when the protein chains are in the endoplasmic reticulum and Golgi apparatus, defective structures are detected and the abnormal proteins are degraded in lysosomes and proteasomes. The net result is a remarkable accuracy in the production of the proteins needed for normal body function.
In addition to dividing and growing under genetic control, cells can die and be absorbed under genetic control. This process is called programmed cell death, or apoptosis (Gr. apo “away” + ptosis “fall”). It can be called “cell suicide” in the sense that the cell’s own genes play an active role in its demise. It should be distinguished from necrosis (“cell murder”), in which healthy cells are destroyed by external processes such as inflammation.
Apoptosis is a very common process during development and in adulthood. In the central nervous system (CNS), large numbers of neurons are produced and then die during the remodeling that occurs during development and synapse formation. In the immune system, apoptosis gets rid of inappropriate clones of immunocytes and is responsible for the lytic effects of glucocorticoids on lymphocytes. Apoptosis is also an important factor in processes such as removal of the webs between the fingers in fetal life and regression of duct systems in the course of sexual development in the fetus. In adults, it participates in the cyclic breakdown of the endometrium that leads to menstruation. In epithelia, cells that lose their connections to the basal lamina and neighboring cells undergo apoptosis. This is responsible for the death of the enterocytes sloughed off the tips of intestinal villi. Abnormal apoptosis probably occurs in autoimmune diseases, neurodegenerative diseases, and cancer. It is interesting that apoptosis occurs in invertebrates, including nematodes and insects. However, its molecular mechanism is much more complex than that in vertebrates.
One final common pathway bringing about apoptosis is activation of caspases, a group of cysteine proteases. Many of these have been characterized to date in mammals; 11 have been found in humans. They exist in cells as inactive proenzymes until activated by the cellular machinery. The net result is DNA fragmentation, cytoplasmic and chromatin condensation, and eventually membrane bleb formation, with cell breakup and removal of the debris by phagocytes (see Clinical Box 2–5).
CLINICAL BOX 2–5
Fundamental research on molecular aspects of genetics, regulation of gene expression, and protein synthesis has been paying off in clinical medicine at a rapidly accelerating rate.
One early dividend was an understanding of the mechanisms by which antibiotics exert their effects. Almost all act by inhibiting protein synthesis at one or another of the steps described previously. Antiviral drugs act in a similar way; for example, acyclovir and ganciclovir act by inhibiting DNA polymerase. Some of these drugs have this effect primarily in bacteria, but others inhibit protein synthesis in the cells of other animals, including mammals. This fact makes antibiotics of great value for research as well as for treatment of infections.
Single genetic abnormalities that cause over 600 human diseases have now been identified. Many of the diseases are rare, but others are more common and some cause conditions that are severe and eventually fatal. Examples include the defectively regulated Cl- channel in cystic fibrosis and the unstable trinucleotide repeats in various parts of the genome that cause Huntington’s disease, the fragile X syndrome, and several other neurologic diseases. Abnormalities in mitochondrial DNA can also cause human diseases such as Leber’s hereditary optic neuropathy and some forms of cardiomyopathy. Not surprisingly, genetic aspects of cancer are probably receiving the greatest current attention. Some cancers are caused by oncogenes, genes that are carried in the genomes of cancer cells and are responsible for producing their malignant properties. These genes are derived by somatic mutation from closely related proto-oncogenes, which are normal genes that control growth. Over 100 oncogenes have been described. Another group of genes produce proteins that suppress tumors, and more than 10 of these tumor suppressor geneshave been described. The most studied of these is the p53 gene on human chromosome 17. The p53 protein produced by this gene triggers apoptosis. It is also a nuclear transcription factor that appears to increase production of a 21-kDa protein that blocks two cell cycle enzymes, slowing the cycle and permitting repair of mutations and other defects in DNA. The p53 gene is mutated in up to 50% of human cancers, with the production of p53 proteins that fail to slow the cell cycle and permit other mutations in DNA to persist. The accumulated mutations eventually cause cancer.
TRANSPORT ACROSS CELL MEMBRANES
There are several mechanisms of transport across cellular membranes. Primary pathways include exocytosis, endocytosis, movement through ion channels, and primary and secondary active transport. Each of these are discussed below.
Vesicles containing material for export are targeted to the cell membrane (Figure 2–11), where they bond in a similar manner to that discussed in vesicular traffic between Golgi stacks, via the v-SNARE/t-SNARE arrangement. The area of fusion then breaks down, leaving the contents of the vesicle outside and the cell membrane intact. This is the Ca2+-dependent process of exocytosis (Figure 2–12).
FIGURE 2–12 Exocytosis and endocytosis. Note that in exocytosis the cytoplasmic sides of two membranes fuse, whereas in endocytosis two noncytoplasmic sides fuse.
Note that secretion from the cell occurs via two pathways (Figure 2–11). In the nonconstitutive pathway, proteins from the Golgi apparatus initially enter secretory granules, where processing of prohormones to the mature hormones occurs before exocytosis. The other pathway, the constitutive pathway, involves the prompt transport of proteins to the cell membrane in vesicles, with little or no processing or storage. The nonconstitutive pathway is sometimes called the regulated pathway, but this term is misleading because the output of proteins by the constitutive pathway is also regulated.
Endocytosis is the reverse of exocytosis. There are various types of endocytosis named for the size of particles being ingested as well as the regulatory requirements for the particular process. These include phagocytosis, pinocytosis, clathrin-mediated endocytosis, caveolae-dependent uptake, and nonclathrin/noncaveolae endocytosis.
Phagocytosis (“cell eating”) is the process by which bacteria, dead tissue, or other bits of microscopic material are engulfed by cells such as the polymorphonuclear leukocytes of the blood. The material makes contact with the cell membrane, which then invaginates. The invagination is pinched off, leaving the engulfed material in the membrane-enclosed vacuole and the cell membrane intact. Pinocytosis (“cell drinking”) is a similar process with the vesicles much smaller in size and the substances ingested are in solution. The small size membrane that is ingested with each event should not be misconstrued; cells undergoing active pinocytosis (eg, macrophages) can ingest the equivalent of their entire cell membrane in just 1 h.
Clathrin-mediated endocytosis occurs at membrane indentations where the protein clathrin accumulates. Clathrin molecules have the shape of triskelions, with three “legs” radiating from a central hub (Figure 2–13). As endocytosis progresses, the clathrin molecules form a geometric array that surrounds the endocytotic vesicle. At the neck of the vesicle, the GTP binding protein dynamin is involved, either directly or indirectly, in pinching off the vesicle. Once the complete vesicle is formed, the clathrin falls off and the three-legged proteins recycle to form another vesicle. The vesicle fuses with and dumps its contents into an early endosome (Figure 2–11). From the early endosome, a new vesicle can bud off and return to the cell membrane. Alternatively, the early endosome can become a late endosome and fuse with a lysosome (Figure 2–11) in which the contents are digested by the lysosomal proteases. Clathrin-mediated endocytosis is responsible for the internalization of many receptors and the ligands bound to them—including, for example, nerve growth factor (NGF) and low-density lipoproteins. It also plays a major role in synaptic function.
FIGURE 2–13 Clathrin molecule on the surface of an endocytotic vesicle. Note the characteristic triskelion shape and the fact that with other clathrin molecules it forms a net supporting the vesicle.
It is apparent that exocytosis adds to the total amount of membrane surrounding the cell, and if membrane were not removed elsewhere at an equivalent rate, the cell would enlarge. However, removal of cell membrane occurs by endocytosis, and such exocytosis–endocytosis coupling maintains the surface area of the cell at its normal size.
RAFTS & CAVEOLAE
Some areas of the cell membrane are especially rich in cholesterol and sphingolipids and have been called rafts. These rafts are probably the precursors of flask-shaped membrane depressions called caveolae (little caves) when their walls become infiltrated with a protein called caveolin that resembles clathrin. There is considerable debate about the functions of rafts and caveolae, with evidence that they are involved in cholesterol regulation and transcytosis. It is clear, however, that cholesterol can interact directly with caveolin, effectively limiting the protein’s ability to move around in the membrane. Internalization via caveolae involves binding of cargo to caveolin and regulation by dynamin. Caveolae are prominent in endothelial cells, where they help in the uptake of nutrients from the blood.
COATS & VESICLE TRANSPORT
It now appears that all vesicles involved in transport have protein coats. In humans, over 50 coat complex subunits have been identified. Vesicles that transport proteins from the trans Golgi to lysosomes have assembly protein 1 (AP-1) clathrin coats, and endocytotic vesicles that transport to endosomes have AP-2 clathrin coats. Vesicles that transport between the endoplasmic reticulum and the Golgi have coat proteins I and II (COPI and COPII). Certain amino acid sequences or attached groups on the transported proteins target the proteins for particular locations. For example, the amino acid sequence Asn–Pro–any amino acid–Tyr targets transport from the cell surface to the endosomes, and mannose-6-phosphate groups target transfer from the Golgi to mannose-6-phosphate receptors (MPR) on the lysosomes.
Various small G proteins of the Rab family are especially important in vesicular traffic. They appear to guide and facilitate orderly attachments of these vesicles. To illustrate the complexity of directing vesicular traffic, humans have 60 Rab proteins and 35 SNARE proteins.
MEMBRANE PERMEABILITY & MEMBRANE TRANSPORT PROTEINS
An important technique that has permitted major advances in our knowledge about transport proteins is patch clamping. A micropipette is placed on the membrane of a cell and forms a tight seal to the membrane. The patch of membrane under the pipette tip usually contains only a few transport proteins, allowing for their detailed biophysical study (Figure 2–14). The cell can be left intact (cell-attached patch clamp). Alternatively, the patch can be pulled loose from the cell, forming an inside-out patch. A third alternative is to suck out the patch with the micropipette still attached to the rest of the cell membrane, providing direct access to the interior of the cell (whole cell recording).
FIGURE 2–14 Patch clamp to investigate transport. In a patch clamp experiment, a small pipette is carefully maneuvered to seal off a portion of a cell membrane. The pipette has an electrode bathed in an appropriate solution that allows for recording of electrical changes through any pore in the membrane (shown below). The illustrated setup is termed an “inside-out patch” because of the orientation of the membrane with reference to the electrode. Other configurations include cell attached, whole cell, and outside-out patches. (Modified from Ackerman MJ, Clapham DE: Ion channels: Basic science and clinical disease. N Engl J Med 1997;336:1575.)
Small, nonpolar molecules (including O2 and N2) and small uncharged polar molecules such as CO2 diffuse across the lipid membranes of cells. However, the membranes have very limited permeability to other substances. Instead, they cross the membranes by endocytosis and exocytosis and by passage through highly specific transport proteins, transmembrane proteins that form channels for ions or transport substances such as glucose, urea, and amino acids. The limited permeability applies even to water, with simple diffusion being supplemented throughout the body with various water channels (aquaporins). For reference, the sizes of ions and other biologically important substances are summarized in Table 2–2.
TABLE 2–2 Size of hydrated ions and other substances of biologic interest.
Some transport proteins are simple aqueous ion channels, though many of these have special features that make them selective for a given substance such as Ca2+ or, in the case of aquaporins, for water. These membrane-spanning proteins (or collections of proteins) have tightly regulated pores that can be gated opened or closed in response to local changes (Figure 2–15). Some are gated by alterations in membrane potential (voltage-gated), whereas others are opened or closed in response to a ligand (ligand-gated). The ligand is often external (eg, a neurotransmitter or a hormone). However, it can also be internal; intracellular Ca2+, cAMP, lipids, or one of the G proteins produced in cells can bind directly to channels and activate them. Some channels are also opened by mechanical stretch, and these mechanosensitive channels play an important role in cell movement.
FIGURE 2–15 Regulation of gating in ion channels. Several types of gating are shown for ion channels. A) Ligand-gated channels open in response to ligand binding. B) Protein phosphorylation or dephosphorylation regulate opening and closing of some ion channels. C) Changes in membrane potential alter channel openings. D) Mechanical stretch of the membrane results in channel opening. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM (editors): Principles of Neural Science, 4th ed. McGraw-Hill, 2000.)
Other transport proteins are carriers that bind ions and other molecules and then change their configuration, moving the bound molecule from one side of the cell membrane to the other. Molecules move from areas of high concentration to areas of low concentration (down their chemical gradient), and cations move to negatively charged areas whereas anions move to positively charged areas (down their electrical gradient). When carrier proteins move substances in the direction of their chemical or electrical gradients, no energy input is required and the process is called facilitated diffusion. A typical example is glucose transport by the glucose transporter, which moves glucose down its concentration gradient from the ECF to the cytoplasm of the cell. Other carriers transport substances against their electrical and chemical gradients. This form of transport requires energy and is called active transport. In animal cells, the energy is provided almost exclusively by hydrolysis of ATP. Not surprisingly, therefore, many carrier molecules are ATPases, enzymes that catalyze the hydrolysis of ATP. One of these ATPases is sodium–potassium adenosine triphosphatase (Na, K ATPase), which is also known as the Na, K pump. There are also H, K ATPases in the gastric mucosa and the renal tubules. Ca2+ ATPase pumps Ca2+ out of cells. Proton ATPases acidify many intracellular organelles, including parts of the Golgi complex and lysosomes.
Some of the transport proteins are called uniports because they transport only one substance. Others are called symports because transport requires the binding of more than one substance to the transport protein and the substances are transported across the membrane together. An example is the symport in the intestinal mucosa that is responsible for the cotransport of Na+ and glucose from the intestinal lumen into mucosal cells. Other transporters are called antiports because they exchange one substance for another.
There are ion channels specific for K+, Na+, Ca2+, and Cl−, as well as channels that are nonselective for cations or anions. Each type of channel exists in multiple forms with diverse properties. Most are made up of identical or very similar subunits. Figure 2–16 shows the multiunit structure of various channels in diagrammatic cross-section.
FIGURE 2–16 Different ways in which ion channels form pores. Many K+ channels are tetramers A), with each protein subunit forming part of the channel. In ligand-gated cation and anion channels B) such as the acetylcholine receptor, five identical or very similar subunits form the channel. Cl- channels from the ClC family are dimers C), with an intracellular pore in each subunit. Aquaporin water channels (D) are tetramers with an intracellular channel in each subunit. (Reproduced with permission from Jentsch TJ: Chloride channels are different. Nature 2002;415:276.)
Most K+ channels are tetramers, with each of the four subunits forming part of the pore through which K+ ions pass. Structural analysis of a bacterial voltage-gated K+ channel indicates that each of the four subunits have a paddle-like extension containing four charges. When the channel is closed, these extensions are near the negatively charged interior of the cell. When the membrane potential is reduced, the paddles containing the charges bend through the membrane to its exterior surface, causing the channel to open. The bacterial K+ channel is very similar to the voltage-gated K+ channels in a wide variety of species, including mammals. In the acetylcholine ion channel and other ligand-gated cation or anion channels, five subunits make up the pore. Members of the ClC family of Cl- channels are dimers, but they have two pores, one in each subunit. Finally, aquaporins are tetramers with a water pore in each of the subunits. Recently, a number of ion channels with intrinsic enzyme activity have been cloned. More than 30 different voltage-gated or cyclic nucleotide-gated Na+ and Ca2+ channels of this type have been described. Representative Na+, Ca2+, and K+ channels are shown in extended diagrammatic form in Figure 2–17.
FIGURE 2–17 Diagrammatic representation of the pore-forming subunits of three ion channels. The α subunit of the Na+ and Ca2+ channels traverse the membrane 24 times in four repeats of six membrane-spanning units. Each repeat has a “P” loop between membrane spans 5 and 6 that does not traverse the membrane. These P loops are thought to form the pore. Note that span 4 of each repeat is colored in red, representing its net “+” charge. The K+channel has only a single repeat of the six spanning regions and P loop. Four K+ subunits are assembled for a functional K+ channel. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM (editors): Principles of Neural Science, 4th ed. McGraw-Hill, 2000.)
Another family of Na+ channels with a different structure has been found in the apical membranes of epithelial cells in the kidneys, colon, lungs, and brain. The epithelial sodium channels (ENaCs) are made up of three subunits encoded by three different genes. Each of the subunits probably spans the membrane twice, and the amino terminal and carboxyl terminal are located inside the cell. The α subunit transports Na+, whereas the β and γ subunits do not. However, the addition of the β and γ subunits increases Na+ transport through the α subunit. ENaCs are inhibited by the diuretic amiloride, which binds to the α subunit, and they used to be called amiloride inhibitable Na+channels.The ENaCs in the kidney play an important role in the regulation of ECF volume by aldosterone. ENaC knockout mice are born alive but promptly die because they cannot move Na+, and hence water, out of their lungs.
Humans have several types of Cl- channels. The ClC dimeric channels are found in plants, bacteria, and animals, and there are nine different ClC genes in humans. Other Cl- channels have the same pentameric form as the acetylcholine receptor; examples include the γ-aminobutyric acid A (GABAA) and glycine receptors in the CNS. The cystic fibrosis transmembrane conductance regulator (CFTR) that is mutated in cystic fibrosis is also a Cl-channel. Ion channel mutations cause a variety of channelopathies—diseases that mostly affect muscle and brain tissue and produce episodic paralyses or convulsions, but are also observed in nonexcitable tissues (Clinical Box 2–6).
CLINICAL BOX 2–6
Channelopathies include a wide range of diseases that can affect both excitable (eg, neurons and muscle) and nonexcitable cells. Using molecular genetic tools, many of the pathological defects in channelopathies have been traced to mutations in single ion channels. Examples of channelopathies in excitable cells include periodic paralysis (eg, Kir2.1, a K+ channel subunit, or Nav2.1, a Na+ channel subunit), myasthenia (eg, nicotinic Acetyl Choline Receptor, a ligand gated nonspecific cation channel), myotonia (eg, Kir1.1, a K+ channel subunit), malignant hypothermia (Ryanodine Receptor, a Ca2+ channel), long QT syndrome (both Na+ and K+ channel subunit examples) and several other disorders. Examples of channelopathies in nonexcitable cells include the underlying cause for Cystic fibrosis (CFTR, a Cl- channel) and a form of Bartter’s syndrome (Kir1.1, a K+ channel subunit). Importantly, advances in treatment of these disorders can come from the understanding of the basic defect and tailoring drugs that act to alter the mutated properties of the affected channel.
Na, K ATPase
As noted previously, Na, K ATPase catalyzes the hydrolysis of ATP to adenosine diphosphate (ADP) and uses the energy to extrude three Na+ from the cell and take two K+ into the cell for each molecule of ATP hydrolyzed. It is an electrogenic pump in that it moves three positive charges out of the cell for each two that it moves in, and it is therefore said to have a coupling ratio of 3:2. It is found in all parts of the body. Its activity is inhibited by ouabain and related digitalis glycosides used in the treatment of heart failure. It is a heterodimer made up of an α subunit with a molecular weight of approximately 100,000 and a β subunit with a molecular weight of approximately 55,000. Both extend through the cell membrane (Figure 2–18). Separation of the subunits eliminates activity. The β subunit is a glycoprotein, whereas Na+ and K+ transport occur through the α subunit. The β subunit has a single membrane-spanning domain and three extracellular glycosylation sites, all of which appear to have attached carbohydrate residues. These residues account for one third of its molecular weight. The α subunit probably spans the cell membrane 10 times, with the amino and carboxyl terminals both located intracellularly. This subunit has intracellular Na+- and ATP-binding sites and a phosphorylation site; it also has extracellular binding sites for K+ and ouabain. The endogenous ligand of the ouabain-binding site is unsettled. When Na+ binds to the α subunit, ATP also binds and is converted to ADP, with a phosphate being transferred to Asp 376, the phosphorylation site. This causes a change in the configuration of the protein, extruding Na+ into the ECF. K+ then binds extracellularly, dephosphorylating the α subunit, which returns to its previous conformation, releasing K+ into the cytoplasm.
FIGURE 2–18 Na, K ATPase. The intracellular portion of the α subunit has a Na+-binding site (1), a phosphorylation site (4), and an ATP-binding site (5). The extracellular portion has a K+-binding site (2) and an ouabain-binding site (3). (From Horisberger J-D et al: Structure–function relationship of Na, K ATPase. Annu Rev Physiol 1991;53:565. Reproduced with permission from the Annual Review of Physiology, vol. 53. Copyright 1991 by Annual Reviews.)
The α and β subunits are heterogeneous, with α1, α2, and α3 subunits and β1, β2, and β3 subunits described so far. The α1 isoform is found in the membranes of most cells, whereas α2 is present in muscle, heart, adipose tissue, and brain, and α3 is present in heart and brain. The β1 subunit is widely distributed but is absent in certain astrocytes, vestibular cells of the inner ear, and glycolytic fast-twitch muscles. The fast-twitch muscles contain only β2 subunits. The different α and β subunit structures of Na, K ATPase in various tissues probably represent specialization for specific tissue functions.
REGULATION OF Na, K ATPase
The amount of Na+ normally found in cells is not enough to saturate the pump, so if the Na+ increases, more is pumped out. Pump activity is affected by second messenger molecules (eg, cAMP and diacylglycerol [DAG]). The magnitude and direction of the altered pump effects vary with the experimental conditions. Thyroid hormones increase pump activity by a genomic action to increase the formation of Na, K ATPase molecules. Aldosterone also increases the number of pumps, although this effect is probably secondary. Dopamine in the kidney inhibits the pump by phosphorylating it, causing a natriuresis. Insulin increases pump activity, probably by a variety of different mechanisms.
SECONDARY ACTIVE TRANSPORT
In many situations, the active transport of Na+ is coupled to the transport of other substances (secondary active transport). For example, the luminal membranes of mucosal cells in the small intestine contain a symport that transports glucose into the cell only if Na+ binds to the protein and is transported into the cell at the same time. From the cells, the glucose enters the blood. The electrochemical gradient for Na+ is maintained by the active transport of Na+ out of the mucosal cell into ECF. Other examples are shown in Figure 2–19. In the heart, Na, K ATPase indirectly affects Ca2+ transport. An antiport in the membranes of cardiac muscle cells normally exchanges intracellular Ca2+ for extracellular Na+.
FIGURE 2–19 Composite diagram of main secondary effects of active transport of Na+ and K+. Na,K ATPase converts the chemical energy of ATP hydrolysis into maintenance of an inward gradient for Na+ and an outward gradient for K+. The energy of the gradients is used for countertransport, cotransport, and maintenance of the membrane potential. Some examples of cotransport and countertransport that use these gradients are shown. (Reproduced with permission from Skou JC: The Na–K pump. News Physiol Sci 1992;7:95.)
Active transport of Na+ and K+ is one of the major energy-using processes in the body. On the average, it accounts for about 24% of the energy utilized by cells, and in neurons it accounts for 70%. Thus, it accounts for a large part of the basal metabolism. A major payoff for this energy use is the establishment of the electrochemical gradient in cells.
TRANSPORT ACROSS EPITHELIA
In the gastrointestinal tract, the pulmonary airways, the renal tubules, and other structures lined with polarized epithelial cells, substances enter one side of a cell and exit another, producing movement of the substance from one side of the epithelium to the other. For transepithelial transport to occur, the cells need to be bound by tight junctions and, obviously, have different ion channels and transport proteins in different parts of their membranes. Most of the instances of secondary active transport cited in the preceding paragraph involve transepithelial movement of ions and other molecules.
SPECIALIZED TRANSPORT ACROSS THE CAPILLARY WALL
The capillary wall separating plasma from interstitial fluid is different from the cell membranes separating interstitial fluid from intracellular fluid because the pressure difference across it makes filtration a significant factor in producing movement of water and solute. By definition, filtration is the process by which fluid is forced through a membrane or other barrier because of a difference in pressure on the two sides.
The structure of the capillary wall varies from one vascular bed to another. However, near skeletal muscle and many other organs, water and relatively small solutes are the only substances that cross the wall with ease. The apertures in the junctions between the endothelial cells are too small to permit plasma proteins and other colloids to pass through in significant quantities. The colloids have a high molecular weight but are present in large amounts. Small amounts cross the capillary wall by vesicular transport, but their effect is slight. Therefore, the capillary wall behaves like a membrane impermeable to colloids, and these exert an osmotic pressure of about 25 mm Hg. The colloid osmotic pressure due to the plasma colloids is called the oncotic pressure. Filtration across the capillary membrane as a result of the hydrostatic pressure head in the vascular system is opposed by the oncotic pressure. The way the balance between the hydrostatic and oncotic pressures controls exchanges across the capillary wall is considered in detail in Chapter 31.
Vesicles are present in the cytoplasm of endothelial cells, and tagged protein molecules injected into the bloodstream have been found in the vesicles and in the interstitium. This indicates that small amounts of protein are transported out of capillaries across endothelial cells by endocytosis on the capillary side followed by exocytosis on the interstitial side of the cells. The transport mechanism makes use of coated vesicles that appear to be coated with caveolin and is called transcytosis, vesicular transport, or cytopempsis.
Cells communicate with one another via chemical messengers. Within a given tissue, some messengers move from cell to cell via gap junctions without entering the ECF. In addition, cells are affected by chemical messengers secreted into the ECF, or by direct cell–cell contacts. Chemical messengers typically bind to protein receptors on the surface of the cell or, in some instances, in the cytoplasm or the nucleus, triggering sequences of intracellular changes that produce their physiologic effects. Three general types of intercellular communication are mediated by messengers in the ECF: (1) neural communication, in which neurotransmitters are released at synaptic junctions from nerve cells and act across a narrow synaptic cleft on a postsynaptic cell; (2) endocrine communication, in which hormones and growth factors reach cells via the circulating blood or the lymph; and (3) paracrine communication, in which the products of cells diffuse in the ECF to affect neighboring cells that may be some distance away (Figure 2–20). In addition, cells secrete chemical messengers that in some situations bind to receptors on the same cell, that is, the cell that secreted the messenger (autocrine communication). The chemical messengers include amines, amino acids, steroids, polypeptides, and in some instances, lipids, purine nucleotides, and pyrimidine nucleotides. It is worth noting that in various parts of the body, the same chemical messenger can function as a neurotransmitter, a paracrine mediator, a hormone secreted by neurons into the blood (neural hormone), and a hormone secreted by gland cells into the blood.
FIGURE 2–20 Intercellular communication by chemical mediators. A, autocrine; P, paracrine.
An additional form of intercellular communication is called juxtacrine communication. Some cells express multiple repeats of growth factors such as transforming growth factor alpha (TGFα) extracellularly on transmembrane proteins that provide an anchor to the cell. Other cells have TGFα receptors. Consequently, TGFα anchored to a cell can bind to a TGFα receptor on another cell, linking the two. This could be important in producing local foci of growth in tissues.
RECEPTORS FOR CHEMICAL MESSENGERS
The recognition of chemical messengers by cells typically begins by interaction with a receptor at that cell. There have been over 20 families of receptors for chemical messengers characterized. These proteins are not static components of the cell, but their numbers increase and decrease in response to various stimuli, and their properties change with changes in physiological conditions. When a hormone or neurotransmitter is present in excess, the number of active receptors generally decreases (down-regulation), whereas in the presence of a deficiency of the chemical messenger, there is an increase in the number of active receptors (up-regulation). In its actions on the adrenal cortex, angiotensin II is an exception; it increases rather than decreases the number of its receptors in the adrenal. In the case of receptors in the membrane, receptor-mediated endocytosis is responsible for down-regulation in some instances; ligands bind to their receptors, and the ligand–receptor complexes move laterally in the membrane to coated pits, where they are taken into the cell by endocytosis (internalization). This decreases the number of receptors in the membrane. Some receptors are recycled after internalization, whereas others are replaced by de novo synthesis in the cell. Another type of down-regulation is desensitization, in which receptors are chemically modified in ways that make them less responsive.
MECHANISMS BY WHICH CHEMICAL MESSENGERS ACT
Receptor–ligand interaction is usually just the beginning of the cell response. This event is transduced into secondary responses within the cell that can be divided into four broad categories: (1) ion channel activation, (2) G-proteinactivation, (3) activation of enzyme activity within the cell, or (4) direct activation of transcription. Within each of these groups, responses can be quite varied. Some of the common mechanisms by which chemical messengers exert their intracellular effects are summarized in Table 2–3. Ligands such as acetylcholine bind directly to ion channels in the cell membrane, changing their conductance. Thyroid and steroid hormones, 1,25-dihydroxycholecalciferol, and retinoids enter cells and act on one or another member of a family of structurally related cytoplasmic or nuclear receptors. The activated receptor binds to DNA and increases transcription of selected mRNAs. Many other ligands in the ECF bind to receptors on the surface of cells and trigger the release of intracellular mediators such as cAMP, IP3, and DAG that initiate changes in cell function. Consequently, the extracellular ligands are called “first messengers” and the intracellular mediators are called “second messengers.” Second messengers bring about many short-term changes in cell function by altering enzyme function, triggering exocytosis, and so on, but they also can lead to the alteration of transcription of various genes. A variety of enzymatic changes, protein–protein interactions, or second messenger changes can be activated within a cell in an orderly fashion following receptor recognition of the primary messenger. The resulting cell signaling pathway provides amplification of the primary signal and distribution of the signal to appropriate targets within the cell. Extensive cell signaling pathways also provide opportunities for feedback and regulation that can fine-tune the signal for the correct physiological response by the cell.
TABLE 2–3 Common mechanisms by which chemical messengers in the ECF bring about changes in cell function.
The most predominant posttranslation modification of proteins, phosphorylation, is a common theme in cell signaling pathways. Cellular phosphorylation is under the control of two groups of proteins: kinases, enzymes that catalyze the phosphorylation of tyrosine or serine and threonine residues in proteins (or in some cases, in lipids); and phosphatases, proteins that remove phosphates from proteins (or lipids). Some of the larger receptor families are themselves kinases. Tyrosine kinase receptors initiate phosphorylation on tyrosine residues on complementary receptors following ligand binding. Serine/threonine kinase receptors initiate phosphorylation on serines or threonines in complementary receptors following ligand binding. Cytokine receptors are directly associated with a group of protein kinases that are activated following cytokine binding. Alternatively, second messenger changes can lead to phosphorylation further downstream in the signaling pathway. More than 500 protein kinases have been described. Some of the principal ones that are important in mammalian cell signaling are summarized in TABLE 2–4. In general, addition of phosphate groups changes the conformation of the proteins, altering their functions and consequently the functions of the cell. The close relationship between phosphorylation and dephosphorylation of cellular proteins allows for a temporal control of activation of cell signaling pathways. This is sometimes referred to as a “phosphate timer.” The dysregulation of the phosphate timer and subsequent cellular signaling in a cell can lead to disease (Clinical Box 2–7).
TABLE 2–4 Sample protein kinases.
CLINICAL BOX 2–7
Kinases in Cancer: Chronic Myeloid Leukemia
Kinases frequently play important roles in regulating cellular physiology outcomes, including cell growth and cell death. Dysregulation of cell proliferation or cell death is a hallmark of cancer. Although cancer can have many causes, a role for kinase dysregulation is exemplified in Chronic myeloid leukemia (CML). CML is a pluripotent hematopoietic stem cell disorder characterized by the Philadelphia (Ph) chromosome translocation. The Ph chromosome is formed following a translocation of chromosomes 9 and 22. The resultant shortened chromosome 22 (Ph chromosome). At the point of fusion a novel gene (BCR-ABL) encoding the active tyrosine kinase domain from a gene on chromosome 9 (Abelson tyrosine kinase; c-Abl) is fused to novel regulatory region of a separate gene on chromosome 22 (breakpoint cluster region; bcr). The BCR-ABL fusion gene encodes a cytoplasmic protein with constitutively active tyrosine kinase. The dysregulated kinase activity in BCR-ABL protein effectively limits white blood cell death signaling pathways while promoting cell proliferation and genetic instability. Experimental models have shown that translocation to produce the fusion BCR-ABL protein is sufficient to produce CML in animal models.
The identification of BCR-ABL as the initial transforming event in CML provided an ideal target for drug discovery. The drug imatinib (Gleevac) was developed to specifically block the tyrosine kinase activity of the BCR-ABL protein. Imatinib has proven to be an effective agent for treating chronic phase CML.
STIMULATION OF TRANSCRIPTION
The activation of transcription, and subsequent translation, is a common outcome of cellular signaling. There are three distinct pathways for primary messengers to alter transcription of cells. First, as is the case with steroid or thyroid hormones, the primary messenger is able to cross the cell membrane and bind to a nuclear receptor, which then can directly interact with DNA to alter gene expression. A second pathway to gene transcription is the activation of cytoplasmic protein kinases that can move to the nucleus to phosphorylate a latent transcription factor for activation. This pathway is a common endpoint of signals that go through the mitogen activated protein (MAP) kinasecascade. MAP kinases can be activated following a variety of receptor–ligand interactions through second messenger signaling. They comprise a series of three kinases that coordinate a stepwise phosphorylation to activate each protein in series in the cytosol. Phosphorylation of the last MAP kinase in series allows it to migrate to the nucleus where it phosphorylates a latent transcription factor. A third common pathway is the activation of a latent transcription factor in the cytosol, which then migrates to the nucleus and alters transcription. This pathway is shared by a diverse set of transcription factors that include nuclear factor kappa B (NFκB; activated following tumor necrosis family receptor binding and others), and signal transducers of activated transcription (STATs; activated following cytokine receptor binding). In all cases, the binding of the activated transcription factor to DNA increases (or in some cases, decreases) the transcription of mRNAs encoded by the gene to which it binds. The mRNAs are translated in the ribosomes, with the production of increased quantities of proteins that alter cell function.
INTRACELLULAR Ca2+ AS A SECOND MESSENGER
Ca2+ regulates a very large number of physiological processes that are as diverse as proliferation, neural signaling, learning, contraction, secretion, and fertilization, so regulation of intracellular Ca2+ is of great importance. The free Ca2+ concentration in the cytoplasm at rest is maintained at about 100 nmol/L. The Ca2+ concentration in the interstitial fluid is about 12,000 times the cytoplasmic concentration (ie, 1,200,000 nmol/L), so there is a marked inwardly directed concentration gradient as well as an inwardly directed electrical gradient. Much of the intracellular Ca2+ is stored at relatively high concentrations in the endoplasmic reticulum and other organelles (Figure 2–21), and these organelles provide a store from which Ca2+ can be mobilized via ligand-gated channels to increase the concentration of free Ca2+ in the cytoplasm. Increased cytoplasmic Ca2+ binds to and activates calcium-binding proteins. These proteins can have direct effects in cellular physiology, or can activate other proteins, commonly protein kinases, to further cell signaling pathways.
FIGURE 2–21 Ca2+ handling in mammalian cells. Ca2+ is stored in the endoplasmic reticulum and, to a lesser extent, mitochondria and can be released from them to replenish cytoplasmic Ca2+. Calcium-binding proteins (CaBP) bind cytoplasmic Ca2+ and, when activated in this fashion, bring about a variety of physiologic effects. Ca2+ enters the cells via voltage-gated (volt) and ligand-gated (lig) Ca2+ channels and store-operated calcium channels (SOCCs). It is transported out of the cell by Ca, Mg ATPases (not shown), Ca, H ATPase and a Na, Ca antiport. It is also transported into the ER by Ca ATPases.
Ca2+ can enter the cell from the extracellular fluid, down its electrochemical gradient, through many different Ca2+ channels. Some of these are ligand-gated and others are voltage-gated. Stretch-activated channels exist in some cells as well.
Many second messengers act by increasing the cytoplasmic Ca2+ concentration. The increase is produced by releasing Ca2+ from intracellular stores—primarily the endoplasmic reticulum—or by increasing the entry of Ca2+ into cells, or by both mechanisms. IP3 is the major second messenger that causes Ca2+ release from the endoplasmic reticulum through the direct activation of a ligand-gated channel, the IP3 receptor. In effect, the generation of one second messenger (IP3) can lead to the release of another second messenger (Ca2+). In many tissues, transient release of Ca2+ from internal stores into the cytoplasm triggers opening of a population of Ca2+ channels in the cell membrane (store-operated Ca2+channels; SOCCs). The resulting Ca2+ influx replenishes the total intracellular Ca2+ supply and refills the endoplasmic reticulum. Recent research has identified the physical relationships between SOCCs and regulatory interactions of proteins from the endoplasmic reticulum that gate these channels.
As with other second messenger molecules, the increase in Ca2+ within the cytosol is rapid, and is followed by a rapid decrease. Because the movement of Ca2+ outside of the cytosol (ie, across the plasma membrane or the membrane of the internal store) requires that it move up its electrochemical gradient, it requires energy. Ca2+ movement out of the cell is facilitated by the plasma membrane Ca2+ ATPase. Alternatively, it can be transported by an antiport that exchanges three Na+ for each Ca2+ driven by the energy stored in the Na+ electrochemical gradient. Ca2+ movement into the internal stores is through the action of the sarcoplasmic or endoplasmic reticulum Ca2+ATPase, also known as the SERCA pump.
Many different Ca2+-binding proteins have been described, including troponin, calmodulin, and calbindin. Troponin is the Ca2+-binding protein involved in contraction of skeletal muscle (Chapter 5). Calmodulin contains 148 amino acid residues (Figure 2–22) and has four Ca2+-binding domains. It is unique in that amino acid residue 115 is trimethylated, and it is extensively conserved, being found in plants as well as animals. When calmodulin binds Ca2+, it is capable of activating five different calmodulin-dependent kinases (CaMKs; Table 2–4), among other proteins. One of the kinases is myosin light-chain kinase, which phosphorylates myosin. This brings about contraction in smooth muscle. CaMKI and CaMKII are concerned with synaptic function, and CaMKIII is concerned with protein synthesis. Another calmodulin-activated protein is calcineurin, a phosphatase that inactivates Ca2+ channels by dephosphorylating them. It also plays a prominent role in activating T cells and is inhibited by some immunosuppressants.
FIGURE 2–22 Secondary structure of calmodulin from bovine brain. Single-letter abbreviations are used for the amino acid residues. Note the four calcium domains (purple residues) flanked on either side by stretches of amino acids that form α-helices in tertiary structure. (Reproduced with permission from Cheung WY: Calmodulin: An overview. Fed Proc 1982;41:2253.)
MECHANISMS OF DIVERSITY OF Ca2+ ACTIONS
It may seem difficult to understand how intracellular Ca2+ can have so many varied effects as a second messenger. Part of the explanation is that Ca2+ may have different effects at low and at high concentrations. The ion may be at high concentration at the site of its release from an organelle or a channel (Ca2+sparks) and at a subsequent lower concentration after it diffuses throughout the cell. Some of the changes it produces can outlast the rise in intracellular Ca2+ concentration because of the way it binds to some of the Ca2+-binding proteins. In addition, once released, intracellular Ca2+ concentrations frequently oscillate at regular intervals, and there is evidence that the frequency and, to a lesser extent, the amplitude of those oscillations codes information for effector mechanisms. Finally, increases in intracellular Ca2+ concentration can spread from cell to cell in waves, producing coordinated events such as the rhythmic beating of cilia in airway epithelial cells.
A common way to translate a signal to a biologic effect inside cells is by way of nucleotide regulatory proteins that are activated after binding GTP (G proteins). When an activating signal reaches a G protein, the protein exchanges GDP for GTP. The GTP–protein complex brings about the activating effect of the G protein. The inherent GTPase activity of the protein then converts GTP to GDP, restoring the G protein to an inactive resting state. G proteins can be divided into two principal groups involved in cell signaling: small G proteins and heterotrimeric G proteins. Other groups that have similar regulation and are also important to cell physiology include elongation factors, dynamin, and translocation GTPases.
There are six different families of small G proteins (or small GTPases) that are all highly regulated. GTPase activating proteins (GAPs) tend to inactivate small G proteins by encouraging hydrolysis of GTP to GDP in the central binding site. Guanine exchange factors (GEFs) tend to activate small G proteins by encouraging exchange of GDP for GTP in the active site. Some of the small G proteins contain lipid modifications that help to anchor them to membranes, while others are free to diffuse throughout the cytosol. Small G proteins are involved in many cellular functions. Members of the Rab family regulate the rate of vesicle traffic between the endoplasmic reticulum, the Golgi apparatus, lysosomes, endosomes, and the cell membrane. Another family of small GTP-binding proteins, the Rho/Rac family, mediates interactions between the cytoskeleton and cell membrane; and a third family, the Ras family, regulates growth by transmitting signals from the cell membrane to the nucleus.
Another family of G proteins, the larger heterotrimeric G proteins, couple cell surface receptors to catalytic units that catalyze the intracellular formation of second messengers or couple the receptors directly to ion channels. Despite the knowledge of the small G proteins described above, the heteromeric G proteins are frequently referred to in the shortened “G protein” form because they were the first to be identified. Heterotrimeric G proteins are made up of three subunits designated α, β, and γ (Figure 2–23). Both the α and the γ subunits have lipid modifications that anchor these proteins to the plasma membrane. The α subunit is bound to GDP. When a ligand binds to a G protein-coupled receptor (GPCR, discussed below), this GDP is exchanged for GTP and the α subunit separates from the combined β and γ subunits. The separated α subunit brings about many biologic effects. The β and γ subunits are tightly bound in the cell and together form a signaling molecule that can also activate a variety of effectors. The intrinsic GTPase activity of the α subunit then converts GTP to GDP, and this leads to re-association of the α with the βγ subunit and termination of effector activation. The GTPase activity of the α subunit can be accelerated by a family of regulators of G protein signaling (RGS).
FIGURE 2–23 Heterotrimeric G proteins. Top: Summary of overall reaction that occurs in the Gα subunit. Bottom: When the ligand (square) binds to the G protein-coupled receptor in the cell membrane, GTP replaces GDP on the α subunit. GTP-α separates from the βγ subunit and GTP-α and βγ both activate various effectors, producing physiologic effects. The intrinsic GTPase activity of GTP-α then converts GTP to GDP, and the α, β, and γ subunits reassociate.
Heterotrimeric G proteins relay signals from over 1000 GPCRs, and their effectors in the cells include ion channels and enzymes. There are 20 α, 6 β, and 12 γ genes, which allow for over 1400 α, β, and γ combinations. Not all combinations occur in the cell, but over 20 different heterotrimeric G proteins have been well documented in cell signaling. They can be divided into five families, each with a relatively characteristic set of effectors.
G PROTEIN-COUPLED RECEPTORS
All the GPCRs that have been characterized to date are proteins that span the cell membrane seven times. Because of this structure they are alternatively referred to as seven-helix receptors or serpentine receptors. A very large number have been cloned, and their functions are multiple and diverse. This is emphasized by the extensive variety of ligands that target GPCRs (Table 2–5). The structures of four GPCRs are shown in Figure 2–24. These receptors assemble into a barrel-like structure. Upon ligand binding, a conformational change activates a resting heterotrimeric G protein associated with the cytoplasmic leaf of the plasma membrane. Activation of a single receptor can result in 1, 10, or more active heterotrimeric G proteins, providing amplification as well as transduction of the first messenger. Bound receptors can be inactivated to limit the amount of cellular signaling. This frequently occurs through phosphorylation of the cytoplasmic side of the receptor. Because of their diversity and importance in cellular signaling pathways, GPCRs are prime targets for drug discovery (Clinical Box 2–8).
FIGURE 2–24 Representative structures of four G protein-coupled receptors from solved crystal structures. Each group of receptors is represented by one structure, all rendered with the same orientation and color scheme: transmembrane helices are colored light blue, intracellular regions are colored darker blue, and extracellular regions are brown. Each ligand is colored orange and rendered as sticks, bound lipids are colored yellow, and the conserved tryptophan residue is rendered as spheres and colored green. This figure highlights the observed differences seen in the extracellular and intracellular domains as well as the small differences seen in the ligand binding orientations among the four GPCRs various ligands. (Reproduced with permission from Hanson MA, Stevens RC: Discovery of new GPCR biology: one receptor structure at a time. Structure 1988 Jan 14;17(1):8–14.)
TABLE 2–5 Examples of ligands for G-protein coupled receptors.
CLINICAL BOX 2–8
Drug Development: Targeting the G-Protein Coupled Receptors (GPCRs)
GPCRs are among the most heavily investigated drug targets in the pharmaceutical industry, representing approximately 40% of all the drugs in the marketplace today. These proteins are active in just about every organ system and present a wide range of opportunities as therapeutic targets in areas including cancer, cardiac dysfunction, diabetes, central nervous system disorders, obesity, inflammation, and pain. Features of GPCRs that allow them to be drug targets are their specificity in recognizing extracellular ligands to initiate cellular response, the cell surface location of GPCRs that make them accessible to novel ligands or drugs, and their prevalence in leading to human pathology and disease.
Specific examples of successful GPCR drug targets are noted with two types of Histamine Receptors.
Histamine-1 Receptor (H1-Receptor) antagonists: allergy therapy. Allergens can trigger local mast cells or basophils to release histamine in the airway. A primary target for histamine is the H1-Receptor in several airway cell types and this can lead to transient itching, sneezing, rhinorrhea, and nasal congestion. There are a variety of drugs with improved peripheral H1 receptor selectivity that are currently used to block histamine activation of the H1-Receptor and thus limit allergen effects in the upper airway. Current H1-Receptor antagonists on the market today include loratadine, fexofenadine, cetirizine, and desloratadine. These “second” an “third” generation anti H1-Receptor drugs have improved specificity and reduced adverse side effects (eg, drowsiness and central nervous system dysfunction) associated with some of the “first” generation drugs first introduced in the late 1930’s and widely developed over the next 40 years.
Histamine-2 Receptor (H2-Receptor) antagonists: treating excess stomach acid. Excess stomach acid can result in gastroesophageal reflux disease or even peptic ulcer symptoms. The parietal cell in the stomach can be stimulated to produce acid via histamine action at the H2-Receptor. Excess stomach acid results in heartburn. Antagonists or H2-Receptor blockers, reduce acid production by preventing H2-Receptor signaling that leads to production of stomach acid. There are several drugs (eg, ranitidine, famotidine, cimetidine, and nizatidine) that specifically block the H2-receptor and thus reduce excess acid production.
INOSITOL TRISPHOSPHATE & DIACYLGLYCEROL AS SECOND MESSENGERS
The link between membrane binding of a ligand that acts via Ca2+ and the prompt increase in the cytoplasmic Ca2+ concentration is often inositol trisphosphate (inositol 1,4,5-trisphosphate; IP3). When one of these ligands binds to its receptor, activation of the receptor produces activation of phospholipase C (PLC) on the inner surface of the membrane. Ligands bound to GPCR can do this through the Gq heterotrimeric G proteins, while ligands bound to tyrosine kinase receptors can do this through other cell signaling pathways. PLC has at least eight isoforms; PLCβ is activated by heterotrimeric G proteins, while PLCγ forms are activated through tyrosine kinase receptors. PLC isoforms can catalyze the hydrolysis of the membrane lipid phosphatidylinositol 4,5-diphosphate (PIP2) to form IP3 and DAG (Figure 2–25). The IP3 diffuses to the endoplasmic reticulum, where it triggers the release of Ca2+ into the cytoplasm by binding the IP3 receptor, a ligand-gated Ca2+ channel (Figure 2–26). DAG is also a second messenger; it stays in the cell membrane, where it activates one of several isoforms of protein kinase C.
FIGURE 2–25 Metabolism of phosphatidylinositol in cell membranes. Phosphatidylinositol is successively phosphorylated to form phosphatidylinositol 4-phosphate (PIP), then phosphatidylinositol 4,5-bisphosphate (PIP2). Phospholipase Cβ and phospholipase Cγ catalyze the breakdown of PIP2 to inositol 1,4,5-trisphosphate (IP3) and diacylglycerol. Other inositol phosphates and phosphatidylinositol derivatives can also be formed. IP3 is dephosphorylated to inositol, and diacylglycerol is metabolized to cytosine diphosphate (CDP)-diacylglycerol. CDP-diacylglycerol and inositol then combine to form phosphatidylinositol, completing the cycle. (Modified from Berridge MJ: Inositol triphosphate and diacylglycerol as second messengers. Biochem J 1984;220:345.)
FIGURE 2–26 Diagrammatic representation of release of inositol triphosphate (IP3) and diacylglycerol (DAG) as second messengers. Binding of ligand to G protein-coupled receptor activates phospholipase C (PLC)β. Alternatively, activation of receptors with intracellular tyrosine kinase domains can activate PLCγ. The resulting hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP2) produces IP3, which releases Ca2+ from the endoplasmic reticulum (ER), and DAG, which activates protein kinase C (PKC). CaBP, Ca2+-binding proteins; ISF, interstitial fluid.
Another important second messenger is cyclic adenosine 3′,5′-monophosphate (cyclic AMP or cAMP; Figure 2–27). Cyclic AMP is formed from ATP by the action of the enzyme adenylyl cyclase and converted to physiologically inactive 5’AMP by the action of the enzyme phosphodiesterase. Some of the phosphodiesterase isoforms that break down cAMP are inhibited by methylxanthines such as caffeine and theophylline. Consequently, these compounds can augment hormonal and transmitter effects mediated via cAMP. Cyclic AMP activates one of the cyclic nucleotide-dependent protein kinases (protein kinase A, PKA) that, like protein kinase C, catalyzes the phosphorylation of proteins, changing their conformation and altering their activity. In addition, the active catalytic subunit of PKA moves to the nucleus and phosphorylates the cAMP-responsive element-binding protein (CREB). This transcription factor then binds to DNA and alters transcription of a number of genes.
FIGURE 2–27 Formation and metabolism of cAMP. The second messenger cAMP is made from ATP by adenylyl cyclase and broken down into AMP by phosphodiesterase.
PRODUCTION OF cAMP BY ADENLYL CYCLASE
Adenylyl cyclase is a membrane bound protein with 12 transmembrane regions. Ten isoforms of this enzyme have been described and each can have distinct regulatory properties, permitting the cAMP pathway to be customized to specific tissue needs. Notably, stimulatory heterotrimeric G proteins (Gs) activate, while inhibitory heterotrimeric G proteins (Gi) inactivate adenylyl cyclase (Figure 2–28). When the appropriate ligand binds to a stimulatory receptor, a Gs α subunit activates one of the adenylyl cyclases. Conversely, when the appropriate ligand binds to an inhibitory receptor, a Gi α sub-unit inhibits adenylyl cyclase. The receptors are specific, responding at low threshold to only one or a select group of related ligands. However, heterotrimeric G proteins mediate the stimulatory and inhibitory effects produced by many different ligands. In addition, cross-talk occurs between the phospholipase C system and the adenylyl cyclase system, as several of the isoforms of adenylyl cyclase are stimulated by calmodulin. Finally, the effects of protein kinase A and protein kinase C are very widespread and can also affect directly, or indirectly, the activity at adenylyl cyclase. The close relationship between activation of G proteins and adenylyl cyclases also allows for spatial regulation of cAMP production. All of these events, and others, allow for fine-tuning the cAMP response for a particular physiological outcome in the cell.
FIGURE 2–28 The cAMP system. Activation of adenylyl cyclase catalyzes the conversion of ATP to cAMP. Cyclic AMP activates protein kinase A, which phosphorylates proteins, producing physiologic effects. Stimulatory ligands bind to stimulatory receptors and activate adenylyl cyclase via Gs. Inhibitory ligands inhibit adenylyl cyclase via inhibitory receptors and Gi. ISF, interstitial fluid.
Two bacterial toxins have important effects on adenylyl cyclase that are mediated by G proteins. The A subunit of cholera toxin catalyzes the transfer of ADP ribose to an arginine residue in the middle of the α subunit of Gs. This inhibits its GTPase activity, producing prolonged stimulation of adenylyl cyclase. Pertussis toxin catalyzes ADP-ribosylation of a cysteine residue near the carboxyl terminal of the α subunit of Gi. This inhibits the function of Gi. In addition to the implications of these alterations in disease, both toxins are used for fundamental research on G protein function. The drug forskolin also stimulates adenylyl cyclase activity by a direct action on the enzyme.
Another cyclic nucleotide of physiologic importance is cyclic guanosine monophosphate (cyclic GMP or cGMP). Cyclic GMP is important in vision in both rod and cone cells. In addition, there are cGMP-regulated ion channels, and cGMP activates cGMP-dependent kinase, producing a number of physiologic effects.
Guanylyl cyclases are a family of enzymes that catalyze the formation of cGMP. They exist in two forms (Figure 2–29). One form has an extracellular amino terminal domain that is a receptor, a single transmembrane domain, and a cytoplasmic portion with guanylyl cyclase catalytic activity. Several such guanylyl cyclases have been characterized. Two are receptors for atrial natriuretic peptide (ANP; also known as atrial natriuretic factor), and a third binds an Escherichia coli enterotoxin and the gastrointestinal polypeptide guanylin. The other form of guanylyl cyclase is soluble, contains heme, and is not bound to the membrane. There appear to be several isoforms of the intracellular enzyme. They are activated by nitric oxide (NO) and NO-containing compounds.
FIGURE 2–29 Diagrammatic representation of guanylyl cyclases, tyrosine kinases, and tyrosine phosphatases. ANP, atrial natriuretic peptide; C, cytoplasm; cyc, guanylyl cyclase domain; EGF, epidermal growth factor; ISF, interstitial fluid; M, cell membrane; PDGF, platelet-derived growth factor; PTK, tyrosine kinase domain; PTP, tyrosine phosphatase domain; ST, E. coli enterotoxin. (Modified from Koesling D, Böhme E, Schultz G: Guanylyl cyclases, a growing family of signal transducing enzymes. FASEB J 1991;5:2785.)
Growth factors have become increasingly important in many different aspects of physiology. They are polypeptides and proteins that are conveniently divided into three groups. One group is made up of agents that foster the multiplication or development of various types of cells; NGF, insulin-like growth factor I (IGF-I), activins and inhibins, and epidermal growth factor (EGF) are examples. More than 20 have been described. The cytokines are a second group. These factors are produced by macrophages and lymphocytes, as well as other cells, and are important in regulation of the immune system (see Chapter 3). Again, more than 20 have been described. The third group is made up of the colony-stimulating factors that regulate proliferation and maturation of red and white blood cells.
Receptors for EGF, platelet-derived growth factor (PDGF), and many of the other factors that foster cell multiplication and growth have a single membrane-spanning domain with an intracellular tyrosine kinase domain (Figure 2–29). When ligand binds to a tyrosine kinase receptor, it first causes a dimerization of two similar receptors. The dimerization results in partial activation of the intracellular tyrosine kinase domains and a cross-phosphorylation to fully activate each other. One of the pathways activated by phosphorylation leads, through the small G protein Ras, to MAP kinases, and eventually to the production of transcription factors in the nucleus that alter gene expression (Figure 2–30).
FIGURE 2–30 One of the direct pathways by which growth factors alter gene activity. TK, tyrosine kinase domain; Grb2, Ras activator controller; Sos, Ras activator; Ras, product of the ras gene; MAP K, mitogen-activated protein kinase; MAP KK, MAP kinase kinase; TF, transcription factors. There is a cross-talk between this pathway and the cAMP pathway, as well as a cross-talk with the IP3–DAG pathway.
Receptors for cytokines and colony-stimulating factors differ from the other growth factors in that most of them do not have tyrosine kinase domains in their cytoplasmic portions and some have little or no cytoplasmic tail. However, they initiate tyrosine kinase activity in the cytoplasm. In particular, they activate the so-called Janus tyrosine kinases (JAKs) in the cytoplasm (Figure 2–31). These in turn phosphorylate STAT proteins. The phosphorylated STATs form homo- and heterodimers and move to the nucleus, where they act as transcription factors. There are four known mammalian JAKs and seven known STATs. Interestingly, the JAK–STAT pathway can also be activated by growth hormone and is another important direct path from the cell surface to the nucleus. However, it should be emphasized that both the Ras and the JAK–STAT pathways are complex and there is cross-talk between them and other signaling pathways discussed previously.
FIGURE 2–31 Signal transduction via the JAK–STAT pathway. A) Ligand binding leads to dimerization of receptor. B) Activation and tyrosine phosphorylation of JAKs. C) JAKs phosphorylate STATs. D) STATs dimerize and move to nucleus, where they bind to response elements on DNA. (Modified from Takeda K, Kishimoto T, Akira S: STAT6: Its role in interleukin 4-mediated biological functions. J Mol Med 1997;75:317.)
Finally, note that the whole subject of second messengers and intracellular signaling has become immensely complex, with multiple pathways and interactions. It is only possible in a book such as this to list highlights and present general themes that will aid the reader in understanding the rest of physiology (see Clinical Box 2–9).
CLINICAL BOX 2–9
Receptor & G Protein Diseases
Many diseases are being traced to mutations in the genes for receptors. For example, loss-of-function receptor mutations that cause disease have been reported for the 1,25-dihydroxycholecalciferol receptor and the insulin receptor. Certain other diseases are caused by production of antibodies against receptors. Thus, antibodies against thyroid-stimulating hormone (TSH) receptors cause Graves’ disease, and antibodies against nicotinic acetylcholine receptors cause myasthenia gravis.
An example of loss of function of a receptor is the type of nephrogenic diabetes insipidus that is due to loss of the ability of mutated V2 vasopressin receptors to mediate concentration of the urine. Mutant receptors can gain as well as lose function. A gain-of-function mutation of the Ca2+ receptor causes excess inhibition of parathyroid hormone secretion and familial hypercalciuric hypocalcemia. G proteins can also undergo loss-of-function or gain-of-function mutations that cause disease (Table 2–6). In one form of pseudohypoparathyroidism, a mutated Gsα fails to respond to parathyroid hormone, producing the symptoms of hypoparathyroidism without any decline in circulating parathyroid hormone. Testotoxicosis is an interesting disease that combines gain and loss of function. In this condition, an activating mutation of Gsα causes excess testosterone secretion and prepubertal sexual maturation. However, this mutation is temperature-sensitive and is active only at the relatively low temperature of the testes (33°C). At 37°C, the normal temperature of the rest of the body, it is replaced by loss of function, with the production of hypoparathyroidism and decreased responsiveness to TSH. A different activating mutation in Gsα is associated with the rough-bordered areas of skin pigmentation and hypercortisolism of the McCune–Albright syndrome. This mutation occurs during fetal development, creating a mosaic of normal and abnormal cells. A third mutation in Gsα reduces its intrinsic GTPase activity. As a result, it is much more active than normal, and excess cAMP is produced. This causes hyperplasia and eventually neoplasia in somatotrope cells of the anterior pituitary. Forty per cent of somatotrope tumors causing acromegaly have cells containing a somatic mutation of this type.
TABLE 2–6 Examples of abnormalities caused by loss- or gain-of-function mutations of heterotrimeric G protein-coupled receptors and G proteins.
The actual environment of the cells of the body is the interstitial component of the ECF. Because normal cell function depends on the constancy of this fluid, it is not surprising that in multicellular animals, an immense number of regulatory mechanisms have evolved to maintain it. To describe “the various physiologic arrangements which serve to restore the normal state, once it has been disturbed,” W.B. Cannon coined the term homeostasis. The buffering properties of the body fluids and the renal and respiratory adjustments to the presence of excess acid or alkali are examples of homeostatic mechanisms. There are countless other examples, and a large part of physiology is concerned with regulatory mechanisms that act to maintain the constancy of the internal environment. Many of these regulatory mechanisms operate on the principle of negative feedback; deviations from a given normal set point are detected by a sensor, and signals from the sensor trigger compensatory changes that continue until the set point is again reached.
The cell and the intracellular organelles are surrounded by semipermeable membranes. Biological membranes have a lipid bilayer core that is populated by structural and functional proteins. These proteins contribute greatly to the semipermeable properties of biological membrane.
Cells contain a variety of organelles that perform specialized cell functions. The nucleus is an organelle that contains the cellular DNA and is the site of transcription. The endoplasmic reticulum and the Golgi apparatus are important in protein processing and the targeting of proteins to correct compartments within the cell. Lysosomes and peroxisomes are membrane-bound organelles that contribute to protein and lipid processing. Mitochondria are organelles that allow for oxidative phosphorylation in eukaryotic cells and also are important in specialized cellular signaling.
The cytoskeleton is a network of three types of filaments that provide structural integrity to the cell as well as a means for trafficking of organelles and other structures around the cell. Actin filaments are important in cellular contraction, migration, and signaling. Actin filaments also provide the backbone for muscle contraction. Intermediate filaments are primarily structural. Microtubules provide a dynamic structure in cells that allows for the movement of cellular components around the cell.
There are three superfamilies of molecular motor proteins in the cell that use the energy of ATP to generate force, movement, or both. Myosin is the force generator for muscle cell contraction. Cellular myosins can also interact with the cytoskeleton (primarily thin filaments) to participate in contraction as well as movement of cell contents. Kinesins and cellular dyneins are motor proteins that primarily interact with microtubules to move cargo around the cells.
Cellular adhesion molecules aid in tethering cells to each other or to the extracellular matrix as well as providing for initiation of cellular signaling. There are four main families of these proteins: integrins, immunoglobulins, cadherins, and selectins.
Cells contain distinct protein complexes that serve as cellular connections to other cells or the extracellular matrix. Tight junctions provide intercellular connections that link cells into a regulated tissue barrier and also provide a barrier to movement of proteins in the cell membrane. Gap junctions provide contacts between cells that allow for direct passage of small molecules between two cells. Desmosomes and adherens junctions are specialized structures that hold cells together. Hemidesmosomes and focal adhesions attach cells to their basal lamina.
Exocytosis and endocytosis are vesicular fusion events that allow for movement of proteins and lipids between the cell interior, the plasma membrane, and the cell exterior. Exocytosis can be constitutive or nonconstitutive; both are regulated processes that require specialized proteins for vesicular fusion. Endocytosis is the formation of vesicles at the plasma membrane to take material from the extracellular space into the cell interior.
Cells can communicate with one another via chemical messengers. Individual messengers (or ligands) typically bind to a plasma membrane receptor to initiate intracellular changes that lead to physiologic changes. Plasma membrane receptor families include ion channels, G protein-coupled receptors, or a variety of enzyme-linked receptors (eg, tyrosine kinase receptors). There are additional cytosolic receptors (eg, steroid receptors) that can bind membrane-permeant compounds. Activation of receptors leads to cellular changes that include changes in membrane potential, activation of heterotrimeric G proteins, increase in second messenger molecules, or initiation of transcription.
Second messengers are molecules that undergo a rapid concentration changes in the cell following primary messenger recognition. Common second messenger molecules include Ca2+, cyclic adenosine monophosphate (cAMP), cyclic guanine monophosphate (cGMP), inositol trisphosphate (IP3), and nitric oxide (NO).
For all questions, select the single best answer unless otherwise directed.
1. The electrogenic Na, K ATPase plays a critical role in cellular physiology by
A. using the energy in ATP to extrude 3 Na+ out of the
cell in exchange for taking two K+ into the cell.
B. using the energy in ATP to extrude 3 K+ out of the cell in exchange for taking two Na+ into the cell.
C. using the energy in moving Na+ into the cell or K+ outside the cell to make ATP.
D. using the energy in moving Na+ outside of the cell or K+ inside the cell to make ATP.
2. Cell membranes
A. contain relatively few protein molecules.
B. contain many carbohydrate molecules.
C. are freely permeable to electrolytes but not to proteins.
D. have variable protein and lipid contents depending on their location in the cell.
E. have a stable composition throughout the life of the cell.
3. Second messengers
A. are substances that interact with first messengers outside cells.
B. are substances that bind to first messengers in the cell membrane.
C. are hormones secreted by cells in response to stimulation by another hormone.
D. mediate the intracellular responses to many different hormones and neurotransmitters.
E. are not formed in the brain.
4. The Golgi complex
A. is an organelle that participates in the breakdown of proteins and lipids.
B. is an organelle that participates in posttranslational processing of proteins.
C. is an organelle that participates in energy production.
D. is an organelle that participates in transcription and translation.
E. is a subcellular compartment that stores proteins for trafficking to the nucleus.
A. includes phagocytosis and pinocytosis, but not clathrin-mediated or caveolae-dependent uptake of extracellular contents.
B. refers to the merging of an intracellular vesicle with the plasma membrane to deliver intracellular contents to the extracellular milieu.
C. refers to the invagination of the plasma membrane to uptake extracellular contents into the cell.
D. refers to vesicular trafficking between Golgi stacks.
6. G protein-coupled receptors
A. are intracellular membrane proteins that help to regulate movement within the cell.
B. are plasma membrane proteins that couple the extracellular binding of primary signaling molecules to exocytosis.
C. are plasma membrane proteins that couple the extracellular binding of primary signaling molecules to the activation of heterotrimeric G proteins.
D. are intracellular proteins that couple the binding of primary messenger molecules with transcription.
7. Gap junctions are intercellular connections that
A. primarily serve to keep cells separated and allow for transport across a tissue barrier.
B. serve as a regulated cytoplasmic bridge for sharing of small molecules between cells.
C. serve as a barrier to prevent protein movement within the cellular membrane.
D. are cellular components for constitutive exocytosis that occurs between adjacent cells.
8. F-actin is a component of the cellular cytoskeleton that
A. provides a structural component for cell movement.
B. is defined as the “functional” form of actin in the cell.
C. refers to the actin subunits that provide the molecular building blocks of the extended actin molecules found in the cell.
D. provide the molecular architecture for cell to cell communication.
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