Medical Physiology, 3rd Edition

Molecular Physiology of Ion Channels

Classes of ion channels can be distinguished on the basis of electrophysiology, pharmacological and physiological ligands, intracellular messengers, and sequence homology

Mammalian cells express a remarkable array of ion channels. One way of making sense of this diversity is to classify channels according to their functional characteristics. Among these characteristics are electrophysiological behavior, inhibition or stimulation by various pharmacological agents, activation by extracellular agonists, and modulation by intracellular regulatory molecules. In addition, we can classify channels by structural characteristics, such as amino-acid sequence homology and the kinds of subunits of which they are composed.

Electrophysiology

The electrophysiological approach consists of analyzing ionic currents by voltage-clamp techniques and then characterizing channels on the basis of ionic selectivity, dependence of gating on membrane potential, and kinetics of opening and closing.

One of the most striking differences among channels is their selectivity for various ions. Indeed, channels are generally named according to which ion they are most permeable to—for example, Na+ channels, Ca2+ channels, K+ channels, and Cl channels.

Another major electrophysiological characteristic of channels is their voltage dependence. In electrically excitable cells (e.g., nerve, skeletal muscle, heart), a major class of channels becomes activated—and often inactivated—as a steep function of Vm. For example, the opening probability of the Na+ channel in nerve and muscle cells increases steeply as Vm becomes more positive (see Figs. 7-7B and 7-8B). Such voltage-gated channels are generally highly selective for Na+, Ca2+, or K+.

Channels are also distinguished by the kinetics of gating behavior. For example, imagine two channels, each with an open probability (Po) of 0.5. One channel might exhibit openings and closures with a duration of 10 ms each on average, whereas the other may have the same Po with openings and closures of 1 ms each on average. Complex gating patterns of some channels are characterized by bursts of many brief openings, followed by longer silent periods.

Pharmacological Ligands

Currents that are virtually indistinguishable by electrophysiological criteria can sometimes be distinguished pharmacologically. For example, subtypes of voltage-gated Na+ channels can be distinguished by their sensitivity to the peptide toxin µ-conotoxin, which is produced by Conus geographus, a member of a family of venomous marine mollusks called cone snails. This toxin strongly inhibits the Na+ channels of adult rat skeletal muscle but has little effect on the Na+ channels of neurons and cardiac myocytes. Another conotoxin (ω-conotoxin) from a different Conus species specifically inhibits voltage-gated Ca2+ channels in the spinal cord. A synthetic version of this conotoxin (ziconotide) is available for treatment of neuropathic pain in patients.

Physiological Ligands

Some channels are characterized by their unique ability to be activated by the binding of a particular molecule termed an agonist. For example, at the vertebrate neuromuscular junction, a channel called the nicotinic acetylcholine (ACh) receptor imageN6-19 opens in response to the binding of ACh released from a presynaptic nerve terminal. This ACh receptor is an example of the pentameric Cys-loop superfamily of ligand-gated channels or agonist-gated channels (see p. 213). Other agonist-gated channels are activated directly by neurotransmitters such as glutamate, serotonin (5-hydroxytryptamine [5-HT]), gamma-aminobutyric acid (GABA), and glycine.

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Structure of the Nicotinic Acetylcholine Receptor

Contributed by Ed Moczydlowski

The nicotinic acetylcholine receptors (AChRs), which are all ligand-gated ion channels, come in two major subtypes, N1 and N2. The N1 nicotinic AChRs are at the neuromuscular junction, whereas the N2AChRs are found in the autonomic nervous system (on the postsynaptic membrane of the postganglionic sympathetic and parasympathetic neurons) and in the central nervous system. Both N1 and N2 are ligand-gated ion channels activated by ACh or nicotine. However, whereas the N1 receptors at the neuromuscular junction are stimulated by decamethonium and preferentially blocked by d-tubocurarine and α-bungarotoxin, the autonomic N2 receptors are stimulated by tetramethylammonium, blocked by hexamethonium, but resistant to α-bungarotoxin. When activated, N1 and N2 receptors are both permeable to Na+ and K+, with the entry of Na+ dominating. Thus, the nicotinic stimulation leads to rapid depolarization.

The nicotinic AChRs in skeletal muscle and autonomic ganglia are heteropentamers. That is, five nonidentical protein subunits surround a central pore, in a rosette fashion. imageN6-20 Because the five subunits are not identical, the structure exhibits pseudosymmetry, rather than true symmetry. There are at least ten α subunits (α1 to α10) and four β subunits (β1 to β4). As we will see below, the basis for these differences is a difference in subunit composition.

The N1 receptors have different subunit compositions depending upon location and developmental stage. The subunit composition of α2βγδ is found in fetal skeletal muscle, as well as the nonjunctional regions of denervated adult skeletal muscle. The electric organ of the electric eel (Torpedo), from which the channel was first purified, has the same subunit composition. The subunit composition of α2βεδ is found at the neuromuscular junction of adult skeletal muscle. Here, the ε subunit replaces the γ subunit. In both the α2βγδ and α2βεδ pentamers, the α subunits are of the α1 subtype and the β subunits are of the β1 subtype.

In the Torpedo N1 AChRs, the α, β, γ, and δ subunits have polypeptide lengths of 437 to 501 amino acids. eFigure 6-1 shows side and top views of this AChR.

The N2 receptors in the nervous system, like those in muscle, are heteromers, probably heteropentamers. N2 receptors use α2 to α10 and β2 to β4.

Nicotinic Receptors

Receptor Type

Agonists

Antagonists

N1 nicotinic ACh

ACh (nicotine decamethonium)

d-tubocurarine
α-bungarotoxin

N2 nicotinic ACh

ACh (nicotine tetramethylammonium)

Hexamethonium

image

EFIGURE 6-1 Three-dimensional view of the Torpedo or human fetal nicotinic AChR channel. (Data from Toyoshima C, Unwin N: Ion channel of acetylcholine receptor reconstructed from images of postsynaptic membranes. Nature 336:247–250, 1988.)

References

Unwin N. Refined structure of the nicotinic acetylcholine receptor at 4Å resolution. J Mol Biol. 2005;346:968–989.

Unwin N, Fujiyoshi Y. Gating movement of acetylcholine receptor caught by plunge-freezing. J Mol Biol. 2012;422:617–634.

Intracellular Messengers

Channels can be categorized by their physiological regulation by intracellular messengers. For example, increases in [Ca2+]i stimulate some ionic currents, in particular K+ and Cl currents. Channels underlying such currents are known as Ca2+-gated K+ channels and Ca2+-gated Cl channels, respectively. Another example is seen in the plasma membrane of light-sensitive rod cells of the retina, in which a particular type of channel is directly activated by intracellular cGMP.

The four functional criteria for characterizing channels—electrophysiology, pharmacology, extracellular agonists, and intracellular regulators—are not mutually exclusive. For example, one of the major types of Ca2+-activated K+ channels is also voltage gated.

Sequence Homology

The diversity of channels implied by functional criteria ultimately requires a molecular biological approach to channel classification. Such an approach began in the 1970s and 1980s with the biochemical purification of channel proteins. Membrane biochemists originally used rich natural sources of ion channels, such as the electric organs of the Torpedo ray and Electrophorus eel, to isolate channel proteins such as the nicotinic ACh receptor (see pp. 210–212) and the voltage-gated Na+ channel, respectively. Partial amino-acid sequencing of purified channel proteins provided the information needed to prepare oligonucleotide probes for isolating the complete coding sequences of the channel. Genes encoding many different types of ion channel proteins were initially identified in this way. Sequences of ion channel genes—now available directly from the human genome—show that channels are far more diverse than first suggested by physiological studies.

On the basis of amino-acid sequences of mammalian channel proteins, we now can identify 20 distinct families of channel proteins, which are further subclassified into a larger number of gene subfamilies (Table 6-2). Substantial progress in revealing the three-dimensional structures of channels comes from two sources: x-ray crystallographic analysis of three-dimensional protein crystals and cryoelectron microscopy of membrane preparations containing densely packed, two-dimensional crystalline arrays of proteins. Molecular information gleaned from sequence and structural analyses of channel proteins has revealed a number of important themes that we discuss in the remainder of this subchapter.

Many channels are formed by a radially symmetric arrangement of subunits or domains around a central pore

The essential function of a channel is to facilitate the passive flow of ions across the hydrophobic membrane bilayer according to the electrochemical gradient. This task requires the channel protein to form an aqueous pore. The ionophore gramicidin is a small peptide that forms a unique helix dimer that spans the membrane; the hollow cylindrical region inside the helix is the channel pore. Another interesting type of channel structure is that of the porin channel proteins (see p. 109), which are present in the outer membranes of mitochondria and gram-negative bacteria. This protein forms a large pore through the center of a barrel-like structure; the 16 staves of the barrel are formed by 16 strands of the protein, each of which is in a β-sheet conformation. However, the structural motifs of a hole through a helix (gramicidin) or a hole through a 16-stranded β barrel (porin) appear to be exceptions rather than the rule.

For the majority of eukaryotic channels, the aqueous pores are located at the center of an oligomeric rosette-like arrangement of homologous subunits in the plane of the membrane (Fig. 6-17). Channels can have three, four, five, or six of these subunits, each of which is a polypeptide that weaves through the membrane several times. In some cases (e.g., voltage-gated Na+ and Ca2+ channels), the channel is not a true homo-oligomer or hetero-oligomer but rather a pseudo-oligomer in which a single polypeptide contains repetitive, homologous domains. In these channels, the rosette-like arrangement of repetitive domains—rather than distinct subunits—surrounds a central pore. imageN6-20

image

FIGURE 6-17 Structure of ion channels. Most ion channels consist of three to six subunits that are arranged like a rosette in the plane of the membrane. The channel can be made up of (1) identical, distinct subunits (homo-oligomer); (2) distinct subunits that are homologous but not identical (hetero-oligomer); or (3) repetitive subunit-like domains within a single polypeptide (pseudo-oligomer). In any case, these subunits surround the central pore of the ion channel. Note that each subunit is itself made up of several transmembrane segments.

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Rosette Arrangement of Channel Subunits

Contributed by Ed Moczydlowski

The radial arrangement of subunits or domains around a central pore appears to be a common theme of channel structure. Figure 6-17 in the text illustrates that various membrane protein channels can be classified according to whether they are formed from three, four, five, or six separate subunits or from a number of subunit-like domains within a single polypeptide. An example of a channel composed of nonidentical subunits is the nicotinic ACh receptor channel. An example of a channel composed of identical subunits is the voltage-gated K+ channel. Thus, such K+ channels have a homotetrameric, symmetric subunit arrangement, whereas the gap junction has a homohexameric structure. Finally, the voltage-sensitive Na+ and Ca2+ channels are examples of channels formed by four internally homologous, nonidentical subunit-like domains within a single large ~250-kDa polypeptide α subunit. These latter channels are formed by a pseudosymmetrical arrangement of four homologous domains, rather than distinct subunits. We discuss the voltage-sensitive cation channels in more detail beginning on page 182. Thus, the major families of channel proteins found in membranes have apparently solved the problem of how to get an ion across a membrane by forming a channel at the central interface of protein subunits or domains.

Gap junction channels are made up of two connexons, each of which has six identical subunits called connexins

Gap junctions are protein channels that connect two cells via a large, unselective pore—having a diameter of ~1.4 nm at the narrowest constriction—that allows solutes as large as 1 kDa (e.g., Ca2+, glucose, cyclic nucleotides, inositol 1,4,5-trisphosphate [IP3], ADP, ATP) to pass between cells (Fig. 6-18A). These channels have been found in virtually all mammalian cells with only a few exceptions, such as adult skeletal muscle and erythrocytes. For example, gap junctions interconnect hepatocytes of the liver, cardiac muscle fibers of the heart and smooth muscle of the gut, β cells of the pancreas, and epithelial cells in the cornea of the eye, to name just a few. Gap junctions provide pathways for chemical communication and electrical coupling between cells. The gap junction comprises two apposed hexameric structures called connexons, one contributed by each cell. Two connexons contact each other to form an end-to-end protein channel with a total length of 15.5 nm, bridging a gap of ~4 nm between the two cell membranes. Each connexon has six identical subunits—connexins (Cx)—that surround a central pore in so-called radial hexameric symmetry (see Fig. 6-18B). Each Cx is an integral membrane protein with four transmembrane helices (TM1 to TM4) and a molecular mass ranging from 26 to 62 kDa for different connexin subtypes.

image

FIGURE 6-18 Gap junction channels. A, Connexins, connexons, and gap junction channels in apposing membranes. B, Crystal structure of human Cx26, showing two connexons consisting of six connexins (shown in six colors), each made up of four transmembrane segments. The aqueous pore—lined primarily by the TM1 transmembrane helix—has an inner diameter ranging from 4.0 nm at the wide funnel-like cytoplasmic entrance to 1.4 nm near the middle of the membrane. C, The left panel shows the preparation of the two cells, each of which is voltage clamped by means of a patch pipette in the whole-cell configuration (see Fig. 6-14). Because cell 1 is clamped to −40 mV and cell 2 is clamped to −80 mV, current flows through the gap junctions from cell 1 to cell 2. The right panel shows that the current recorded by the electrode in cell 1 is the mirror image of the current recorded in cell 2. The fluctuating current transitions represent the openings and closings of individual gap junction channels. D, The short N-terminal helix (NTH, shown in red) of each of the six connexins forms a circular girdle that lines the funnel of the cytoplasmic opening of the channel. The NTH contains an aspartate residue (D) as well as a tryptophan residue (W) that interacts with a methionine (M) on a transmembrane segment. The left side of the panel shows an open configuration of the gap junction channel when the transjunctional potential (Vj) is zero, that is, when the upper and lower cells are at the same potential. When the upper cell is more positive than the lower (Vj > 0), the six NTHs in the upper connexon move inward and assemble into a plug that blocks the pore. When the lower cell is more positive than the upper (Vj < 0), the six NTHs in the lower connexon move inward and assemble into a plug. (B and D adapted with permission from Maeda S, Nakagawa S, Suga M, et al: Structure of the connexin 26 gap junction channel at 3.5 Å resolution. Nature 458:597–602, 2009. C, Data from Veenstra RD, DeHaan RL: Measurement of single channel currents from cardiac gap junctions. Science 233:972–974, 1986.)

A given connexon hexamer in a particular cell membrane may be formed from a single connexin (homomeric) or a mixture of different connexin proteins (heteromeric). The apposition of two identical connexon hexamers forms a homotypic channel; the apposition of dissimilar connexon hexamers forms a heterotypic channel. Such structural variation in the assembly of connexons provides for greater diversity of function and regulation.

The gating properties of gap junctions can be studied by measuring electrical currents through gap junctions using two patch electrodes simultaneously placed in a pair of coupled cells (see Fig. 6-18C). When the two cells are clamped at different values of Vm, so that current flows from one cell to the other via the gap junctions, the current measured in either cell fluctuates as a result of the opening and closing of individual gap junction channels. Because the amount of current that enters one cell is the same as the amount of current that leaves the other cell, the current fluctuations in the two cells are mirror images of one another. Studies of this type show that increases in [Ca2+]i or decreases in intracellular pH generally favor the closing of gap junction channels. Phosphorylation can also regulate gap junction channels.

The gating of many gap junction channels responds to the difference in transjunctional voltage (Vj) of the two coupled cells, a process known as transjunctional voltage gating. Gap junction channels formed by Cx26 connexin close when the Vm on the cytoplasmic side is positive (see Fig. 6-18D).

Human Cx26 connexin is present in many organs such as the liver, brain, skin, and inner ear. Point mutations throughout the sequence of human Cx26 are the major cause of nonsyndromic sensorineural deafness, the genetic basis for more than half of all cases of congenital deafness (Box 6-1).

Box 6-1

Genetic Defects in Connexin Genes

Many human genetic diseases have been identified in which the primary defect has been mapped to mutations of ion channel proteins. For example, mutations in the 21 connexin genes in the human genome cause diverse hereditary diseases resulting in craniofacial and bone deformities (Cx43), deafness (Cx26, Cx30, Cx31), myelin-related disease (Cx32), skin disorders (Cx26, Cx30), and congenital cataracts (Cx46, Cx50). Charcot-Marie-Tooth disease is a rare form of hereditary neuropathy that involves the progressive degeneration of peripheral nerves. Patients with this inherited disease have been found to have various mutations in the human gene GJB1, which encodes Cx32 imageN6-24 and is located on the X chromosome. Cx32 appears to be involved in forming gap junctions between the folds of Schwann cell membranes. These Schwann cells wrap around the axons of peripheral nerves and form a layer of insulating material called myelin, which is critical for the conduction of nerve impulses. Mutations in Cx32 appear to hinder diffusion across the concentric layers of myelin, which results in disruption of myelin and hence axonal degeneration. Mutations in Cx26, Cx30, Cx30.3, and Cx31 all cause sensorineural deafness, which may result from poor secretion of K+ into the endolymph (see Fig. 15-21). Mutations in Cx46 and Cx56—expressed in lens cells of the eye—cause a variety of congenital cataract disorders. imageN6-25 Many other human diseases—call channelopathies—involve either a genetic defect of a particular channel protein or an autoimmune response directed against a channel protein (see Table 6-2).

N6-24

Mutations in Cx32 That Cause Charcot-Marie-Tooth Disease

Contributed by Ed Moczydlowski

The protein-folding diagram of Cx32 in eFigure 6-2 indicates the locations of six point mutations (indicated in red), as well as a frameshift mutation, that have been observed in certain patients with this disease. (Mutations in other genes besides Cx32 can lead to Charcot-Marie-Tooth disease.)

image

EFIGURE 6-2 Membrane folding of connexin-32, one of the gap junction proteins. M1-M4, membrane-spanning segments 1 through 4. (Data from Bergoffen J, Scherer SS, Wang S, et al: Connexin mutations in X-linked Charcot-Marie-Tooth disease. Science 262:2039–2042, 1993.)

N6-25

Genetic and Autoimmune Ion Channel Defects

Contributed by Ed Moczydlowski

Channel

Disease

Etiology

Voltage-Gated K+ Channels

KvLQT1 (old terminology) cardiac K+ channel, also known as KCNQ1

A form of long QT syndrome

Mutation of KCNQ gene on chromosome 11. See Box 7-3.

Cardiac K+ channel (HERG)

A form of long QT syndrome

Mutation. See Box 7-3.

Voltage-Gated Na+ Channels

Skeletal muscle Na+ channel (Nav1.4)

A form of hyperkalemic periodic paralysis (HYPP)

Mutation of SCN4A gene located on human chromosome 17. See Box 7-1.

Skeletal muscle Na+ channel (Nav1.4)

Paramyotonia congenita (PC)

Mutation of SCN4A gene located on human chromosome 17. See Box 7-1.

Cardiac muscle Na+ channel (Nav1.5)

A form of long QT syndrome

Mutation of SCN5A gene located on human chromosome 17. See Box 7-1.

Voltage-Gated Ca2+ Channels

α1S subunit (old terminology) of skeletal muscle L-type Ca2+ channel, also known as Cav1.1

A form of muscular dysgenesis

Mutation of CACNA1S gene on chromosome 1.

Presynaptic (i.e., on motor neuron) Ca2+ channels at neuromuscular junction

Lambert-Eaton syndrome

Autoimmune; most often seen in patients with certain types of cancer, such as small-cell lung carcinoma. See Box 8-1.

α1S subunit (old terminology) of skeletal muscle L-type Ca2+ channel, also known as Cav 1.1

A form of hypokalemic periodic paralysis

Mutation of CACNA1S gene on chromosome 1.

α1A subunit (old terminology) of P/Q-type Ca2+ channel, also known as Cav2.1

Familial hemiplegic migraine

Mutation of CACNA1A gene on chromosome 19. See Box 7-2.

α1A subunit (old terminology) of P/Q-type Ca2+ channel, also known as Cav2.1

Episodic ataxia type 2

Mutation of CACNA1A gene on chromosome 19; ataxia originating from the cerebellum. See Box 7-2.

Ligand-Gated Channels

N1 nicotinic ACh receptor (AChR)

Myasthenia gravis

Autoimmune disease attacking the junctional nicotinic AChR (α2βεδ pentamer). See Box 8-1.

Other Channels

Cx32 (connexin making up gap junction)

Charcot-Marie-Tooth disease

Mutation.

CFTR

Cystic fibrosis

Mutation. See p. 120 of the text for a discussion of the channel, and Box 43-1 for a discussion of the disease.

β or γ subunit of ENaC epithelial Na+ channel

Liddle syndrome

Gain-of-function mutation due to defective endocytosis of ENaC channels on the apical membrane. See pp. 758–759 of the text for a discussion of the channel. For a discussion of the syndrome, see imageN23-14.

An evolutionary tree called a dendrogram illustrates the relatedness of ion channels

Comparisons of amino-acid sequences of channels and of the nucleotide sequences of genes that encode them provide insight into the molecular evolution of these proteins. The current human genome database contains at least 263 different genes encoding channel proteins. Like other proteins, specific isoforms of channels are differentially expressed in different parts of cells in various tissues and at certain stages of development. In particular, many different kinds of channels are expressed in the brain. In the central nervous system, the great diversity of ion channels provides a means of specifically and precisely regulating the complex electrical activity of 100 billion brain neurons that are connected in numerous functional pathways.

As an example of the diversity and species interrelatedness of a channel family, consider the connexins. Figure 6-19A compares 14 sequences of homologous proteins that are members of the connexin family. Like many other proteins, connexins are encoded by a family of related genes that evolved by gene duplication and divergence. In the connexin family, various subtypes are named according to their protein molecular masses. Thus, rat Cx32 refers to a rat connexin with a protein molecular mass of ~32 kDa. The various connexins differ primarily in the length of the intracellular C-terminal domain.

image

FIGURE 6-19 Family tree of hypothetical evolutionary relationships among connexin sequences of gap junction channels. A, Dendrogram based on amino-acid sequence differences among 14 connexins in various species. The summed length of the horizontal line segments connecting two connexins is a measure of the degree of difference between the two connexins. B, Dendrogram based strictly on human sequences. (A, Data from Dermietzel R, Spray DC: Gap junctions in the brain: Where, what type, how many and why? Trends Neurosci 16:186–192, 1993; B, data from White TW: Nonredundant gap junction functions. News Physiol Sci 18:95–99, 2003.)

By aligning connexin sequences according to identical amino acids and computing the relative similarity of each pair of connexin sequences, it is possible to reconstruct a hypothetical phylogenetic tree of evolutionary relationships. Such a tree is called a dendrogram. The one in Figure 6-19A includes nine rat, two human, one chicken, and two frog (Xenopus) connexins. The horizontal branch lengths of the tree are approximately proportional to the sequence differences or evolutionary divergence between protein members of the family. Closely grouped clusters of sequences in the tree represent evolutionarily related groups of proteins. The connexin tree indicates that the Cx32 genes from rats and humans are very closely related, differing by only 4 amino acids of a total of 284 residues. Thus, these Cx32 proteins probably represent the same functional genes in these two species—orthologous genes. The closely related Cx43 genes from the rat and human are also likely to be orthologs.

A sequence analysis restricted to only human connexin genes reveals five families—GJA through GJE—that cluster in the dendrogram in Figure 6-19B. Related channel proteins often exhibit different patterns of tissue expression. For example, Cx32 is expressed in the liver, Schwann cells, and oligodendrocytes, whereas Cx43 is expressed in heart and many other tissues.

The functional properties of channels are generally consistent with the classification of channel subtypes based on molecular evolution. For example, ion channels that are voltage gated (see p. 189) share sequence homology of their voltage-sensing domain.

Hydrophobic domains of channel proteins can predict how these proteins weave through the membrane

From sequence information and atomic-resolution structures of many ion channels, a number of common structural principles emerge. Like other integral membrane proteins (see pp. 16–19), channel proteins generally have several segments of hydrophobic amino acids, each long enough (~20 amino acids) to span the lipid bilayer as an α helix. If the channel has N membrane-spanning segments, it also has N + 1 hydrophilic domains of variable length that connect or terminate the membrane spans. Many putative transmembrane segments predicted by hydropathy analysis (see Table 2-1) have proven to be transmembrane segments in an α-helical conformation. The intervening hydrophilic segments that link the transmembrane regions together typically fold to form extracellular and intracellular protein domains that contact the aqueous solution.

The primary sequences of channel proteins are often schematically represented by hypothetical folding diagrams, such as those shown in Figure 6-20. For example, connexin monomers have four identifiable hydrophobic transmembrane segments, known as M1, M2, M3, and M4 (see Fig. 6-20A). The crystal structure of Cx26 indicates that the N-terminal and C-terminal hydrophilic segments of connexin are located on the cytoplasmic side of the membrane. The N terminus is involved in voltage gating of the channel, and TM1 is the major pore-lining helix (see Fig. 6-18B). Mutations in Cx32 and many other connexin genes result in a wide variety of physiological abnormalities (see Box 6-1).

imageimage

FIGURE 6-20 Membrane topologic features of ion channel proteins. ATD, amino (N)–terminal domain; CaMBD, calmodulin binding domain; CNBD, cyclic nucleotide–binding domain; CTD, carboxyl (C)–terminal domain; ER, endoplasmic reticulum; NBD, nucleotide-binding domain; P, pore loop; R, regulatory domain; RCK, regulator of K+ conductance; STIM1, stromal interaction molecule 1. imageN6-26

Protein superfamilies, subfamilies, and subtypes are the structural bases of channel diversity

Table 6-2 summarizes the basic functional and structural aspects of currently recognized families of the pore-forming subunits of human ion channel proteins. The table (1) groups these channels into structurally related protein families; (2) describes their properties; (3) lists the assigned human gene symbols, number of genes, and protein names; (4) summarizes noted physiological functions; (5) lists human diseases associated with the corresponding ion channels; and (6) provides a reference to Figure 6-20 that indicates the hypothetical membrane topology. Because some of the membrane-topology diagrams in Figure 6-20 are based primarily on hydropathy analysis, they should be considered “best-guess” representations unless the three-dimensional structure has been established by structural biology. Here, we briefly summarize major aspects of the molecular physiology of human ion channel families, in the order of their presentation in Table 6-2. imageN6-22

N6-22

Voltage-Gated Channels

Contributed by Ed Moczydlowski

Some families of channel proteins are so large and diverse that they are known as superfamilies. For example, the superfamily of voltage-gated channels consists of K+, Na+, and Ca2+ channels, respectively denoted Kv, Nav, and Cav. These channels have a common structural motif (see pp. 182–199). These channels play a primary role in electrical signaling in the nervous system, where they underlie the voltage-dependent depolarization (Nav, Cav) and hyperpolarization (Kv) of propagating action potentials (discussed in Chapter 7). The pore-forming complex of each of these channels consists of four subunits or domains, each of which contains six transmembrane segments denoted as S1 through S6. Voltage-gated K+ channels are believed to represent an evolutionary precursor to Nav and Cav channels because their pore-forming subunit contains only one S1 through S6 domain (see Fig. 6-20B). Voltage-gated K+ channels are homotetramers or heterotetramers of monomer subunits. The pore-forming subunits of Na+ and Ca2+ channels (see Fig. 6-20K, L) both comprise four domains (I, II, III, and IV), each of which contains the S1 through S6 structural motif that is homologous to the basic voltage-gated K+ channel subunit or monomer. Because domains I through IV of Nav and Cav channels are organized as four tandem repeats within the membrane, these domains are referred to as pseudosubunits. The molecular evolution of the four-repeat structure of Nav and Cav channels is believed to have occurred by a process involving consecutive gene duplication from a primordial gene containing S1 through S6. Members of the voltage-gated superfamily of channels are also recognized by a characteristic structure of the S4 transmembrane segment in which four to seven positively charged residues (lysine or arginine) are located at every third position. This unique S4 domain appears to function as the voltage-sensing element of voltage-gated ion channels (see p. 184).

Voltage-gated Ca2+ channels also illustrate another feature of some ion channels: they are multisubunit complexes consisting of accessory proteins in addition to the channel-forming subunits. For example, Cav channels are composed of a large pseudotetrameric α1 subunit with domains I through IV that form the pore, plus four additional structurally unrelated subunits known as α2, β, γ, and δ (see Fig. 6-20L). Like the homologous α subunit of Nav channels, the large α1 subunit of Cav channels specifies most of the basic channel functions, including ionic selectivity, voltage sensitivity, and the binding sites of various drugs. It appears that the β, γ, and δ subunits are important for modulating the activity of Ca2+ channels, but their exact functional roles are largely unknown. In some cases, accessory subunits modulate the gating activity and pharmacology of channel complexes, whereas in other cases such accessory subunits of channels may help target newly synthesized channels to their proper cellular locations.

Connexins

We discussed these channels above in the section on gap junctions, in Figures 6-186-19, and 6-20A, as well as in Box 6-1.

K+ Channels

The tetrameric K+ channels form the largest and most diverse family of ion channels. They are part of the voltage-gated–like (VGL) superfamily of channels, which includes all channels in Figure 6-20B through M. The K+-selective pore of K+ channels is formed by a highly conserved domain containing two transmembrane segments linked by a pore loop labeled P in the figure. The family includes five distinct subfamilies, all of which we will discuss beginning on p. 189: (1) Kv voltage-gated K+ channels (see pp. 193–196), (2) SKCa small- and IKCa intermediate-conductance Ca2+-activated K+ channels (see pp. 193–196), (3) BKCa large-conductance Ca2+- and voltage-activated K+ channels (see p. 196), (4) Kir inward-rectifier K+ channels (see p. 196), and (5) K2P dimeric tandem two-pore K+ channels (see p. 199). For the first two subfamilies, each of the four subunits contains six TMs denoted S1 to S6 (see Fig. 6-20B, C). BKCa channels are similar to Kv channels but have an additional S0 TM (see Fig. 6-20D). Kir channels are structurally the simplest members of the K+ channel family because each of the four subunits contains two TMs analogous to S5 and S6 in the Kv channels (see Fig. 6-20E). K2P channels are the equivalent of a tandem duplication of Kir channels so that two K2P subunits form a pseudotetrameric channel (see Fig. 6-20F).

HCN, CNG, and TRP Channels

HCN hyperpolarization-activated, cyclic nucleotide–gated cation channels (see Fig. 6-20G) play a critical role in electrical automaticity of the heart (see p. 488) and rhythmically firing neurons of the brain. CNG channels form a family of cation-selective channels that are directly activated by intracellular cGMP or cAMP. These channels play an important role in visual (see p. 368) and olfactory sensory transduction (see p. 359). The CNGs have the same basic S1 through S6 motif as K+ channels, but they contain a unique cyclic nucleotide–binding domain at the C terminus (see Fig. 6-20H). TRP transient receptor potential cation channels (see Fig. 6-20I) function in diverse sensory processes and include six subfamilies: TRPA (for ankyrin like), TRPC (for canonical), TRPM (for melastatin), MCOLN or TRPML (for mucolipin), PKD or TRPP (for polycystin 2), and TRPV (for vanilloid). TRPV1 is activated by capsaicin, the “hot” ingredient of chili peppers, whereas TRPM8 responds to menthol, the “cool”-tasting substance in eucalyptus leaves. The capsaicin receptor TRPV1 functions in pain and temperature sensation.

NAADP Receptor

NAADP (nicotinic acid adenine dinucleotide phosphate) is an intracellular signaling molecule. Two TPCN genes in the human genome encode a protein with two tandem repeats of an S1 to S6 motif (see Fig. 6-20J). These proteins function in NAADP-activated release of Ca2+ from the endoplasmic reticulum and acidic compartments.

Voltage-Gated Na+ Channels

The 10 pore-forming subunits of Nav voltage-gated Na+ channels comprise four domains (I, II, III, and IV), each of which contains the S1 to S6 structural motif (see Fig. 6-20K) that is homologous to Kv K+ channel monomers. Because domains I to IV of Nav channels are organized as four tandem repeats within the membrane, these domains are referred to as pseudosubunits. The Nav channels are associated with a unique family of auxiliary β subunits that are known to modify the gating behavior and membrane localization of the channel-forming α subunit.

Voltage-Gated Ca2+ Channels

The pore-forming subunits of Cav voltage-gated Ca2+ channels (see pp. 190–191) are analogous to those for the Nav channels (see Fig. 6-20L). Like Nav channels, Cav channels are multisubunit complexes consisting of accessory proteins in addition to the channel-forming subunits.

CatSper Channels

The unique class of CatSper tetrameric channels—with an S1 to S6 motif (see Fig. 6-20M)—is expressed in the sperm tail membrane. These channels mediate a voltage-dependent Ca2+ current that is essential for male fertility (see Fig. 56-1).

Hv Channels

The unusual Hv channel protein consists of an S1 to S4 voltage-sensing motif but lacks a conventional S5-P-S6 pore domain (see Fig. 6-20N). The Hv protein functions as an H+ channel that opens only when the cell is sufficiently depolarized to mediate H+ efflux. Hv, as a functional component of the NADPH (reduced nicotinamide adenine dinucleotide phosphate) oxidase complex, is important in the innate immune response of neutrophils; Hv is also involved in sperm activation.

Ligand-Gated Channels

The agonist-activated channels are also represented by three large and diverse gene families. The pentameric Cys-loop receptor family (see Fig. 6-20O) includes cation- or Cl -selective ion channels that are activated by binding of ACh (see pp. 212–213), serotonin, GABA, and glycine (see pp. 326–327). Glutamate-activated cation channels (see Fig. 6-20P) include two subfamilies of excitatory AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)–kainate and NMDA (N-methyl-D-aspartate) receptors (see pp. 323–324). Purinergic ligand-gated cation channels (see Fig. 6-20Q) are activated by binding of extracellular ATP and other nucleotides (see p. 327).

Other Ion Channels

ENaC amiloride-sensitive Na+ channels are prominent in Na+-transporting epithelia (see Fig. 6-20R and pp. 758–759). The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl channel (see Fig. 6-20S and p. 120) that is a member of the ABC (ATP-binding cassette) protein family. The unrelated ClC family of Cl channels are dimeric (see Fig. 6-20T). imageN6-23 Two unique genes coding for the anoctamim family of Ca2+- and voltage-activated Cl channels have been added to the channel gene collection (see Fig. 6-20U). Table 6-2 includes two types of Ca2+ release channels. ITPR (see p. 60) is present in the endoplasmic reticulum membrane and is gated by the intracellular messenger IP3 (see Fig. 6-20V). RYR (see p. 230) is located in the sarcoplasmic reticulum membrane of muscle and plays a critical role in the release of Ca2+ during muscle contraction (see Fig. 6-20W). A recently discovered family of Ca2+-selective–channel proteins known as Orai store-operated Ca2+ channels (see Fig. 6-20X) plays a role in entry of extracellular Ca2+ across the plasma membrane linked to IP3 metabolism and depletion of intracellular Ca2+ from the endoplasmic reticulum of nonexcitable cells, such as lymphocytes (see p. 247).

N6-23

Structure of ClC Channels

Contributed by Emile Boulpaep, Walter Boron

Rod MacKinnon and his group solved the x-ray structure of a ClC-type Cl channel from Escherichia coli and Salmonella (see first reference below) and have also studied the basis for the channel's Cl selectivity (see second reference).

References

Dutzler R, Campbell EB, Cadene M, et al. X-ray structure of a ClC chloride channel at 3.0 Å reveals the molecular basis of anion selectivity. Nature. 2002;415:287–294.

Dutzler R, Campbell EB, MacKinnon R. Gating the selectivity filter in ClC chloride channels. Science. 2003;300:108–112.