JIM iifliifiiirtiffl

FIGURE 13.3 Topology of voltage-gated cation channels. (From Palle Christophersen.)

Gate

FIGURE 13.4 (A) Potassium channel structure with selectivity filter at the outer pore and gating mechanism at the inner pore. (B) Selectivity mechanism. The distance between the K ions and the oxygen atoms is the same in water as in the selectivity filter enabling the K ions to enter the pore at no energy cost. This is different for Na+ ions, so they are excluded from the pore. (From Alberts, B. et al., Molecular Biology of the Cell, 2002 Garland Publishing Inc. and Doyle, D.A. et al. Science 280: 69-77, 1998.)

Gate

FIGURE 13.4 (A) Potassium channel structure with selectivity filter at the outer pore and gating mechanism at the inner pore. (B) Selectivity mechanism. The distance between the K ions and the oxygen atoms is the same in water as in the selectivity filter enabling the K ions to enter the pore at no energy cost. This is different for Na+ ions, so they are excluded from the pore. (From Alberts, B. et al., Molecular Biology of the Cell, 2002 Garland Publishing Inc. and Doyle, D.A. et al. Science 280: 69-77, 1998.)

the pore is unchanged. By this means high selectivity and high permeability of the ions passing in single file is obtained. Although Na+ ions are smaller than K+ ions they will not enter the pore since it is energetically unfavorable.

13.2 PHYSIOLOGY AND PHARMACOLOGY OF VOLTAGE-GATED ION CHANNELS—POTASSIUM CHANNELS

The 2-TM Kir channel family gives rise to six subtypes, which play diverse roles in the body. Many Kir channels are open at resting membrane potential and clamp the potential at -70 and -90 mV in nerve and heart cells, respectively (e.g., Kir4 and Kir2). The Kir3 channels are gated by binding of the Py-subunit from the Gi-protein. This mechanism is important in the atria of the heart, where stimulation of the para-sympathetic vagus nerve leads to release of acetylcholine, activating the Gi protein and subsequently the Kir3 channel to hyperpolarize the pacemaker cells.

The Kir6 channels are expressed both in heart, vasculature, nerve, and in the pancreatic P-cells. This channel subtype can only be expressed in cells when it coassembles with its accessory subunit, the so-called sulfonyl urea receptors (SUR) of the ABC transporter family. The P-cell subtype is composed of 4 Kir6.2 + 4 SUR1. Like other Kir channels it is activated by binding of phosphatidylinositol-4, 5-bisphosphate (PIP2). In contrast the complex is blocked by ATP binding to the internal surface of Kir6.2 and activated by MgADP binding to the nucleotide-binding domains of SUR1. The channel complex is also denoted as the KATP channel and it is interesting for two reasons: it is a key regulatory protein in the P-cells coupling plasma glucose levels to insulin secretion, and the SUR has a well-exploited high-affinity drug-binding site.

Briefly, insulin secretion is regulated by the following mechanism: an increase in plasma glucose leads, through an increased ATP level in the P-cells, to block the KATP channel, depolarization, Ca2+-influx, and insulin secretion (Figure 13.5). If this regulation is dysfunctional as in many type-2 diabetic patients, a similar functional effect can be obtained by directly blocking the KATP channel pharmacologically. The drug-binding site on SUR1 is on the inside of TM15 (plus partly on the inside of TM14), and the bulky substitution mutation S1237Y disrupts the site. Tolbutamide

Glucose

Antidiabetics

FIGURE 13.5 Ion channels in pancreatic P-cells and insulin secretion. The K channel subtype is called KATP and it is composed of the two molecular subunits Kir6.2 and SUR1.

Glucose

Antidiabetics

FIGURE 13.5 Ion channels in pancreatic P-cells and insulin secretion. The K channel subtype is called KATP and it is composed of the two molecular subunits Kir6.2 and SUR1.

Metiglinide

Glibenclamide

Glibenclamide

Cromakalim

Cromakalim

Tolbutamide s o' no Diazoxide

Metiglinide

Retigabine

XE-991

XE-991

Dofetilide

D-Sotalol

FIGURE 13.6 Structures of the KATP channel blockers glibenclamide, tolbutamide, and metiglinide; the Katp channel openers cromakalim and diazoxide; the KV7 channel opener retigabine; the KV7 channel blocker

XE-991; the KV11 channel blockers doletilide and D-sotalol.

binds to this site only, whereas glibenclamide and metiglinide (Figure 13.6) binds to this as well as to a neighboring benzamido site. The latter low-affinity site is shared with the cardiac and vascular subunits SUR2A and SUR2B, respectively.

The cardiovascular side effects of the SUR-blockers are minimal whereas SUR-activators, such as cromakalim and diazoxide, which have been attempted primarily for the treatment of arterial hypertension had to be abandoned since they cause orthostatic hypotension and reflex tachycardia.

The Kv channels fall into 12 subfamilies, which are all gated by changes in the membrane potential, but they exhibit different kinetics. Kv channels can be composed of four different subunits from the same subfamily giving numerous possibilities for variations. Several Kv channel subfamilies are interesting drug targets. Retigabine is an activator of the Kv7.2/3 heteromultimeric channel being developed for the treatment of epilepsy, and XE-991 is a memory enhancing compound blocking the same channel (Figure 13.6).

Class III antiarrhythmics block Kv channels in the heart (Kv1, Kv4, and Kv11 subtypes) leading to a prolonged cardiac AP and termination of so-called reentry arrhythmia. Dofetilide, D-sotalol and other antiarrhythmics are selective for the Kv11 channels (hERG channels). These drugs show antiarrhythmic effects in some patients whereas they are proarrhythmic in others. The reason for the latter is that although the prolongation of the AP may terminate some arrhythmias, then blocking an important cardiac K+ conductance being responsible for repolarizing the AP may destabilize the heart against triggered impulses (after depolarizations).

The Kv11 channel has a high-affinity binding site in the pore, which interacts with drugs of very different classes including antihistamines, antipsychotics, antidepressants, antibiotics, and many more. Proarrhythmia caused by drug binding to this site and channel block has been a major reason for withdrawal of drugs from the market and discontinued drug development projects, so the Kv11 channel has become a major cardiac safety pharmacology issue. The Ca-activated K+ channels, K,^, are divided into three families depending on their single-channel conductance. They are gated by Ca2+ binding either directly to the channel or indirectly to a constitutively bound calmodulin. The channels are generally involved in attenuating the activity of a given cell by hyperpolarizing this, when the internal Ca2+ concentration rises.

TRP belong structurally to this group having six TM, a voltage sensor in S4, and a pore loop between S5 and S6. Despite the structural similarity to Kv channels the 28 different TRP subtypes can be either selective to Na+/K+, Mg2+, or Ca2+, and functionally they may associate with G-protein-coupled receptors, tyrosine kinases, or phospholipase C. A large number of TRP channel subunits have been cloned and based on their amino acid homology they can be divided into the TRPV-, TRPC-, and TRPM-families.

Due to their relatively recent discovery, only few TRP-subtype selective ligands have been identified. But there is no doubt that the one TRP channel that has attracted the most attention as a potential drug target is the TRPV1-channel. This ion channel is activated by heat but also by capsaicin, a constituent of chili pepper and TRPV1 is indeed responsible for the "hot" sensation induced by ingesting chili. TRPV1 has also been found to be upregulated in various animal models of chronic pain and selective antagonists of TRPV1 reduce pain sensation in these models. Selective antagonists of TRPV1 are currently undergoing clinical trials in patients suffering from different types of chronic pain.

13.3 VOLTAGE-GATED CALCIUM CHANNELS 13.3.1 Structure and Molecular Biology

The discovery of voltage-gated calcium channels (Ca,) was originally made in the 1950s, through an investigation of crab leg muscle contraction. These experiments revealed that both membrane depolarization and muscle contraction depend on extracellular calcium ions, inferring that the muscle cells posses some membrane molecules enabling calcium to selectively permeate. By use of electro-physiological techniques, it was later found that a variety of functionally distinct Cavs exist and that these ion channels are also expressed in nerve cells.

Functionally, Cavs are closed at the resting membrane potential (i.e., -50 to -80 mV), but are activated by depolarization. Two distinct classes of Cav-mediated currents can be distinguished by this feature: high-voltage-activated calcium currents, requiring membrane potentials of ca. -20 to +10 mV to activate and low-voltage activated currents, which activate at much more negative membrane potentials, typically -50 to -40 mV. Following activation, Cavs inactivate in the presence of sustained membrane depolarization, although the speed of inactivation can vary from ~50 ms to several seconds. Therefore, different types of Cavs can be distinguished on the basis of biophysical, i.e., activation and inactivation characteristics, and on pharmacological properties. Voltage-activated calcium currents, measured in native tissues, have traditionally been classified as L-, N-, P/Q-, or R-type or T-type currents (see Table 13.2).

TABLE 13.2

Cav Channel Terminology and Properties

Channel Subtype

Former names

Activation threshold Blocker

L-type

High voltage Dihydropyridines Phenylalkylamines Benzothiazepines

Ca,2

Ca,3

Ca,2.1 = P/Q type Cav2.2 = N type Cav2.3 = R type High voltage

Ca^.2.1 blockers: ffl-conotoxin MVIIC, ffl-agatoxin IVA Ca^.2.2: blockers: ffl-conotoxin GVIA, ffl-conotoxin MVIIA Cav2.3 blocker: SNX-482

T-type

Low voltage

Mibefradil R-(-)-efonidipine Kurtoxin

FIGURE 13.7 Overview of the membrane topology of voltage-gated ion channel a-subunits. (A) The voltage-sensing S4 TM segments (green) contain several positively charged amino acid residues and the segments that constitute the ion channel pore (shown in red) are the S5, S6, and pore loop segments. (B) Membrane topology of auxiliary subunits of Nav, Cav, and K ion channels. (From Catterall, W.A. et al., Toxicon, 49, 124, 2007. With permission from Elsevier.)

FIGURE 13.7 Overview of the membrane topology of voltage-gated ion channel a-subunits. (A) The voltage-sensing S4 TM segments (green) contain several positively charged amino acid residues and the segments that constitute the ion channel pore (shown in red) are the S5, S6, and pore loop segments. (B) Membrane topology of auxiliary subunits of Nav, Cav, and K ion channels. (From Catterall, W.A. et al., Toxicon, 49, 124, 2007. With permission from Elsevier.)

The major component of the Ca is the large a1-subunit, consisting of ~2000 amino acid residues. This subunit has 24 TM segments, arranged in four linked homologous domains (I-IV), each comprising six TM a-helices (S1-S6), including the positively charged voltage-sensing S4 segments, and the S5-S6 pore loops, with the pore loops and S6 segments believed to line the channel lumen; the structure of the a1- and other Ca subunits is schematically shown in Figure 13.7A.

Ca„s are several 1000-fold selective for Ca2+ ions over Na+ and K+ and this amazing selectivity is created by a ring of four negatively charged glutamic acid residues projecting into the ion channel pore, one such residue being contributed by each of the four pore loops. Ten different arsubunit types have been cloned and based on their amino acid homology, these have been divided into three distinct families (Cav1, Cav2, and Cav3) that display 30%-50% amino acid identity with each other. Within each family there are three to four members (Cav1.1-Cav1.4, Cav2.1-Cav2.3, and Cav3.1-Cav3.3) that each show a much higher degree of sequence identity (~80%) with each other. The Cav1.1-subunit is only expressed in skeletal and cardiac muscle, and the Cav1.4-subunit is exclusively expressed in retina. The other a1-subunits are widely expressed in many tissues, in particular the peripheral and central nervous system (CNS) as well as many types of endocrine cells.

When expressed alone, the arsubunit can form a functional ion channel. But native Cavs are mul-tisubunit complexes in which the arsubunit interacts with a P-, an a^- and sometimes a y-subunit (Figure 13.7B). The role of these subunits is to promote incorporation of Ca into the cell membrane and to modulate the functional properties of Ca„s.

13.3.2 Physiological Roles of Voltage-Gated Calcium Channels

Ca2+ is an important second messenger molecule in eukaryotic cells where it initiates muscle contraction, neurotransmitter release, and activates many types of protein kinases. Many homeostatic mechanisms operate to keep intracellular [Ca2+] < 100 nM under resting conditions. Outside the cell, [Ca2+] is 1-2 mM, creating a 10,000-fold concentration gradient. The Ca2+-equilibrium potential is > +100 mV so Ca2+ always flows into a cell, when Ca„s are activated by depolarization. While the primary function of voltage-gated Na+ and K+ channels is to produce depolarization/repolarization of the cell membrane, voltage-gated Ca2+ channels should be thought of as "gatekeepers" of calcium entry into excitable cells.

In muscle tissue, the binding of Ca2+ to the protein troponin C allows myosin-mediated sliding of actin-filaments, leading to shortening of muscle fibers. In skeletal muscle, the calcium necessary for this process actually comes from the sarcoplasmic reticulum and is released from this into the cytoplasm via ryanodine receptors. In this particular context, the Ca functions as a voltage-sensor for the process—a direct interaction between the Ca^.1 arsubunit and the ryanodine receptors then activates the Ca2+ release.

Ca„s are also very important in cardiac and smooth muscles, where direct Ca2+-influx through the Ca itself provides the Ca2+ necessary for muscular contraction. In cardiac muscle, Ca^.2 or Ca,1.3 is responsible for the plateau-phase of the cardiac action potential, which is important for cardiac muscle contraction and for regulation of the heart rate, so dihydropyridines are used for treatment of hypertension and cardiac arrhythmia. Cav3.1- and Cav3.2-subunits are found in the sino-atrial nodes where they play important roles for cardiac pacemaking.

The release of neurotransmitters from synaptic nerve terminals is triggered by influx of Ca2+ ions via Ca,2.1- (P/Q-type) or Ca,2.2- (N-type) subunits, which are expressed in all nerve terminals. When neuronal action potentials travel down the axon and reach the nerve terminal, they provide the depolarization necessary for activation of Cavs leading to Ca2+-influx. The Cav2.1 and Ca,2.2 subunits bind directly to proteins of the protein-machinery involved in membrane fusion of neurotransmitter-containing vesicles.

A similar role of Ca,s is found in various endocrine cells such as the pancreatic P-cells in which ATP-mediated closing of KATP-channels leads to cellular depolarization, activation of Cav1.3 channels, and release of insulin-containing vesicles (Figure 13.5).

13.3.3 Pharmacology of Voltage-Gated Calcium Channels

There are two types of inhibition of Cav function, namely, blockade of the ion channel pore and allosteric modulation of ion channel function. An example of pore blockade is cadmium (Cd2+), which produces nonselective inhibition of all type of Cavs. The mechanism behind this effect is that Cd2+ binds to the ring of four glutamates in the selectivity filter of the pore with much higher affinity than Ca2+ itself and thus blocks the pore. Most of the peptide toxins, which block Cav-subtypes with high specificity, also act by producing pore block. Allosteric modulation, on the other hand, is exemplified by the dihydropyridines, which selectively affect members of the Cav1-family. The binding site for these compounds is located away from the pore and their mechanism of action relies on modification of the gating characteristics of the channel.

13.3.3.1 Cav1-Family (L-Type Currents)

The best characterized group of Cav modulators is the so-called organic calcium blockers or calcium antagonists, comprising phenylalkylamines (e.g., verapamil), benzothiazepines (e.g., diltiazem), and the dihydropyridines (e.g., nifedipine; Figure 13.8). Several dihydropyridines are widely used clinically for the treatment of cardiovascular disorders such as hypertension, angina pectoris, and cardiac arrhythmia.

The organic calcium blockers bind with high affinity and selectivity to arsubunits of the Ca^-family, and act as allosteric modulators. This is highlighted by the fact that among the

Verapamil (Cav1)

Nifedipine (Cav1)

Nifedipine (Cav1)

Verapamil (Cav1)

Bay K8644 (Cav1)

Bay K8644 (Cav1)

Mibefradil (Cav3)

Diltiazem (Cav1)

Efonidipine (Cav3)

CKGKGAKCSRLMYDCCTGSCRSGKC I_I

w-Conotoxin MVIIA (ziconotide/Prialt®) (Cav2.1)

Mibefradil (Cav3)

FIGURE 13.8 Chemical structure of drugs acting as blockers of Cav1 (L-type) and Cav3 (T-type) channels and the amino acid sequence of the highly specific peptide blocker of Ca,2.2 (N-type) channels, ra-conotoxin MVIIA.

FIGURE 13.9 Overview of the binding sites of toxins and drugs acting at (A) Cav and (B) Nav channels. ([A] From Catterall, W.A. et al., Pharmacol. Rev., 57, 385, 2005. With permission from Elsevier; (B) From Catterall W.A. et al., Toxicon, 49, 124, 2007. With permission from Elsevier.)

dihydropyridine-type compounds, positive modulators of Ca.,1 have also been identified, e.g., the compound Bay K 8644 (Figure 13.8).

Amino acid residues important for the binding of these compounds have been identified through mutagenesis studies and are located in the S5 and S6 segments of domains III and IV of the a1-subunit (Figure 13.9).

Organic calcium blockers bind with a much higher affinity to the inactivated conformations of the Cavs, relative to the closed conformation, thereby trapping the receptors in the inactivated state. Therefore, inhibition of Cavs by these compounds has been termed "use-dependent:" the rate and extent of Cav inhibition will increase with channel activation frequency. Use-dependence is generally considered to be an attractive quality of ion channel inhibitors, since only the highly active channels—presumably the ones responsible for a given disorder—will be inhibited, while less frequently activated channels are spared, thereby reducing the risk of side effects.

13.3.3.2 Cav2-Family (N-, P/Q-, and R-Type Current)

Within this family, the Cav2.2-subunit (N-type current) has attracted the most attention as potential drug target. The most efficient inhibitors of N-type currents are peptide toxins isolated from the venom of fish-eating marine snails that use these toxins to paralyze their prey. The category includes the 25-30 amino acid residue peptides ra-conotoxin GVIA and ra-conotoxin MVIIA (Figure 13.8), which bind to Cav2.2 with very high affinity and selectivity. Binding of ra-conotoxin GVIA mainly occurs to residues located in the pore loop region of domain III, suggesting that this toxin acts as a pore blocker of the Cav2.2-subunit.

The reason for the pharmacological interest in Cav2.2 is that these channels are responsible for neurotransmitter release in neural pathways relaying pain signals to the brain. Although ra-conotoxins are poorly suited for use as drugs because of their lack of biomembrane permeability, ra-conotoxin MVIIA (Prialt®) was recently approved for use in humans. Since the drug has to be given through an intrathecal catheter to circumvent the blood-brain barrier, the clinical use of ra-conotoxin MVIIA is limited to severe pain in patients suffering from terminal cancer or AIDS. A selective, nonpeptide Cav2.2 blockers that can be administered orally has so far not been identified, despite significant efforts, and finding the "dihydropyridines of Cav2.2 calcium channels" therefore still remains an open challenge!

Cav2.1 channels (P/Q-type current) are generally involved in neurotransmitter release in most synapses throughout the brain. Cav2.1 can be selectively blocked by peptide toxins from either Conus snails (ra-conotoxin MVIIC) or from spider venom (ra-agatoxin IVA) (Table 13.2). From a drug discovery point of view, however, these Cavs are not of great interest, since their widespread role in neurotransmitter release predicts severe toxicity as a consequence of channel inhibition.

The function(s) and pharmacology of Cav2.3 channels (R-type current) are not well understood. A peptide toxin, SNX-482, isolated from tarantula venom, has been found to act as a selective blocker of Cav2.3-channels.

13.3.3.3 Cav3-Family (T-Type Current)

Certain small-molecule compounds appear to act as moderately selective blockers of Cav3. The vasodilating compound mibefradil (Figure 13.8), which has been used widely for treatment of hypertension and angina pectoris, inhibits Cav3.1-Cav3.3 channels in a use-dependent way with ~10-fold selectivity over Cav1.2 channels. Moreover, certain novel dihydropyridine compounds (e.g., R-(-)-efonidipine, Figure 13.8) inhibit Cav3 channels up to ~100-fold more potently compared to Cav1 channels. It is not yet known exactly how these compounds interact with Cav3, but this family of ion channels seems to have a great potential as drug targets for treatment of cardiovascular disease. Certain classical antiepileptic compounds, such as ethosuximide, phenytoin, and zonisamide exert their antiepileptic action at least partly via inhibition of Cav3 channels. Substances such as nickel ions (Ni2+), n-octanol, and the diuretic amiloride display moderate selectivity for Cav3 channels over the other Ca channel types. Kurtoxin is a scorpion venom toxin, which produces potent and selective blockade of Cavs containing Cav3.1- and Cav3.2- but not Cav3.3-subunits.

13.3.3.4 Auxiliary Subunits

The drugs gabapentin and the more recently developed pregabalin are used clinically for the treatment of epilepsy and neuropathic pain. Their mechanism of action was not understood before the discovery that gabapentin binds with extremely high affinity to the a25-subunit of Cavs. Functionally, gabapentin and pregabalin decrease the amplitude of calcium currents partially without producing the complete blockade seen with Cav inhibitors targeting the arsubunit. Both Cav2.1 and Ca„2.2 are involved in mediating the effects of gabapentin/pregabalin. Both drugs are nontoxic, which may be related to their partial blocking effect.

13.4 VOLTAGE-GATED SODIUM CHANNELS

13.4.1 Structure and Molecular Biology of Voltage-Gated Sodium Channels

Functionally, Navs are closed at the resting membrane potential and open when the membrane becomes depolarized, activation requiring membrane potentials of -70 to -30 mV, with some variation between different Nav types. Most Navs inactivate within ~1-10 ms in the presence of sustained depolarization. In certain types of neurons, a more persistent Nav current with slow inactivation has also been identified.

At the molecular level, Navs are composed of a large (~2000 amino acid residues) a-subunit, which is structurally similar to the arsubunit of Cavs, and forms the ion conducting pore (Figure 13.7A). The high selectivity for Na+ over K+ is due to the composition of the ion selectivity filter, which consists of two rings of amino acids with each of the four homologous domains contributing one amino acid to each ring: outer ring with Glu-Glu-Asp-Asp and inner ring with Asp-Glu-Lys-Ala. The rapid inactivation of most Navs is explained by the cytoplasmic domain III-IV linker (Figure 13.9B h motiv), which functions as a "hinged lid," that simply swings in to occlude the intracellular mouth of the pore.

Nine different Nav a-subunits (Nav1.1-Nav1.9) have been cloned and all these display >50% amino acid identity with each other, so they compose one subfamily. The Na„1 family has most likely arisen from a singe ancestral gene and that their present diversity reflects gene duplication events and chromosomal rearrangements occurring late in evolution.

By analogy to the Cavs, functional Navs can be formed from expression of a-subunits alone although native Navs are protein complexes composed by a-subunits and auxiliary subunits. Only a single class of auxiliary Nav subunits (P-subunits) has been identified. P-Subunits are composed of a large extracellular part, through which it interacts with the a-subunit (Figure 13.7B) and a small C-terminal portion consisting of a single TM segment. The function of the P-subunits can be divided into: (1) modulation of the functional properties of Navs, (2) enhancement of membrane expression, and (3) mediating interactions between Navs and extracellular matrix proteins as well as various signal transduction molecules.

13.4.2 Physiological Roles of Voltage-Gated Sodium Channels

The biological importance of Navs relies on their ability to cause depolarization of cell membranes. Most of the Nav a-subunits are capable of detecting even very small increases in membrane potential and this makes the Navs activate, and subsequently inactivate, on a ms time scale. This combination of high sensitivity toward depolarization and very rapid gating kinetics makes Navs perfect for initiating and conducting action potentials.

Nav1.1, Nav1.2, Nav1.3, or Nav1.6 subunits are expressed in virtually all neurons within the CNS, in particular, at the base and along the entire length of the axon. When an excitatory synaptic signal (e.g. glutamate, released by a neighboring neuron, acting on AMPA receptors, see Chapter 15) is received, this generates a small depolarization of the neuronal membrane in the dendrites and cell body. This rather modest depolarization is sufficient for activating Navs at the initial segment of the axon, leading to the generation of an AP. Once the AP reaches the nerve terminal, this will activate Cavs, leading to release of neurotransmitter. The importance of these Navs for AP initiation and conduction is also highlighted by the fact that point mutations in the genes encoding Nav1.1, Nav1.2, and Nav1.3, which alter their functional properties, have been linked to certain forms of epilepsy.

Dorsal root ganglion (DRG) neurons are important for transmitting sensory signals, including pain, from the periphery to the CNS. Sensory stimulation leads to generation and conduction of action potentials in DRG neurons and these APs are mediated by Navs. Navs of DRG neurons contain the Nav1.7, Nav1.8, and Nav1.9 subunits, which are almost exclusively expressed in these neurons. It has also been shown that expression of these a-subunits is altered in a complex fashion in animal models of inflammatory and neuropathic pain. From a therapeutic point of view, these a-subunits are therefore of particular interest, since compounds capable of selectively blocking Nav1.7-Nav1.9 channels could have great potential as analgesics.

13.4.3 Pharmacology of Voltage-Gated Sodium Channels

A large number of natural products (peptides and alkaloids) have been found to bind Navs with high affinity. Radioligand-binding, photoaffinity-labeling, and mutagenesis techniques have been used to identify the regions of the a-subunit to which these substances bind. Six binding sites for these toxins are therefore used to provide the conceptual framework for understanding the pharmacology of Na,s (Table 13.3; Figure 13.9B). Given the high degree of homology between the Na^-subunits, very few examples of subunit-selective toxins are known. The substances mentioned in Table 13.3 thus bind to nearly all Na,1 subunits. Most toxins act as gating modifiers, and only tetrodotoxin and saxitoxin binding to site 1 are pore blockers.

In addition to these different toxins, a number of clinically used drug molecules are known to exert their pharmacological action through inhibition of Nav function. Consistent with the physiological roles of Navs, these drugs include antiepileptic compounds (carbamazepine, lam-otrigine, and phenytoin), local anesthetic and analgetic compounds (lidocaine), and drugs used to treat cardiac arrhythmia (class I antiarrhythmics including quinidine, lidocaine, mexiletine, and flecainide).

TABLE 13.3

Toxin-Binding Sites on Nav Channels

Site No. Site Location

Selectivity filter of pore Interface between the S6 segments of domains I and IV

Outer pore loop regions of domains I and IV Extracellular S3-S4 loop close to the voltage sensor Interface between the IS6 and IVS5 segments Unknown

Toxins Binding to Site

Tetrodotoxin, saxitoxin Plant alkaloid toxins: grayanotoxin, batrachotoxin, and veratridine Sea anemone peptide toxins and a-scorpion toxins Large P-scorpion peptide

Plant alkaloids ciguatoxins and brevetoxins 8-Conotoxins

Mechanism of Action

Pore block

Inhibition of inactivation and channel opening at resting potential Slow inactivation

Enhance opening at negative membrane potential Enhance activation and inhibit inactivation Slow inactivation

13.5 CHLORIDE CHANNELS

Most cells have anion channels and the primary ion permeating these is Cl-. The ClC channel family members are involved in transepithelial transport, acidification of synaptic vesicles, and endocytotic trafficking. The ClC proteins are unique and are widely expressed in intracellular organelles and not in the plasma membrane. Surprisingly, some of the ClC proteins show characteristics of ion transporters. The CFTR Cl- channel is an ABC protein involved in transepithelial transport, and dysfunction of this Cl- channel causes cystic fibrosis. The CFTR protein has been extensively used in attempts to establish a gene therapy for cystic fibrosis without success. Ca2+-activated and volume-activated Cl- currents have been characterized in native cells, but these have not been identified at a molecular level yet. The pharmacology of Cl- channels is currently quite poor.

13.6 LIGAND-GATED ION CHANNELS

Nav and Kv channels are essential for generation and conduction of neuronal action potentials and Cav channels are essential for converting action potentials into neurotransmitter release—but it is the ligand-gated ion channels which receive the chemical signals of synaptic transmission and convert them into the electrical signals that initiate action potentials. See Chapter 12 for further details about ligand-gated ion channels.

FURTHER READINGS

Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., and Walter, P. 2002. Molecular Biology of the Cell,

4th edn. Garland Publishing Inc., New York. Ashcroft, F. 2000. Ion Channels and Disease. Academic Press, London, U.K.

Catterall, W.A. 2000. From ionic currents to molecular mechanisms: The structure and function of voltage-

gated sodium currents. Neuron 26: 13-25. Catterall, W.A. et al. 2005. International Union of Pharmacology. Compendium of voltage-gated ion channels.

Pharmacol. Rev. 57: 385-540. Catterall, W.A. et al. 2007. Voltage-gated ion channels and gating modifier toxins. Toxicon 49: 124-141. Dalby-Brown, W., Hansen, H., Korsgaard, M.G., Mirza, N., and Olesen, S.-P. 2006. Kv7 channels: Function, pharmacology and channel modulators. Curr. Top. Med. Chem. 6: 999-1023. Doyle, D.A., Cabral, J.M., Pfuenzner, R.A., Kuo, A., Gulbis, J.M., Cohen, S.L., Chait, B.T., and MacKinnon, R. 1998. The structure of the potassium channel: Molecular basis of K+ conduction and selectivity. Science 280: 69-77.

Hille, B. 2001. Ionic Channels of Excitable Membranes. 3rd edn. Sinauer Associates, Sunderland, MA. Nardi, A. and Olesen, S.-P. 2008. BK channel modulators: A comprehensive overview. Curr. Med. Chem. 15: 1126-1146.

Nilius, B., Owsianik, G., Voets, T., and Peters, J.A. 2007. Transient receptor potential cation channels in disease. Physiol. Rev. 87: 165-217.

Sanguinetti, M.C. and Tristani-Firouzi, M. 2006. Herg potassium channels and cardiac arrhythmia. Nature 440: 463-469.

Was this article helpful?

0 0

Post a comment