Reconstitution of the voltage-sensitive sodium channel of rat brain from solubilized components

A structural atlas of druggable sites on Nav channels

Voltage-gated sodium (Nav) channels govern membrane excitability by initiating and propagating action potentials. Consistent with their physiological significance, dysfunction, or mutations in these channels are associated with various channelopathies. Nav channels are thereby major targets for various clinical and investigational drugs. In addition, a large number of natural toxins, both small molecules and peptides, can bind to Nav channels and modulate their functions. Technological breakthrough in cryo-electron microscopy (cryo-EM) has enabled the determination of high-resolution structures of eukaryotic and eventually human Nav channels, alone or in complex with auxiliary subunits, toxins, and drugs. These studies have not only advanced our comprehension of channel architecture and working mechanisms but also afforded unprecedented clarity to the molecular basis for the binding and mechanism of action (MOA) of prototypical drugs and toxins. In this review, we will provide an overview of the recent advances in structural pharmacology of Nav channels, encompassing the structural map for ligand binding on Nav channels. These findings have established a vital groundwork for future drug development.

Structural Advances in Voltage-Gated Sodium Channels

Voltage-gated sodium (Na_V) channels are responsible for the rapid rising-phase of action potentials in excitable cells. Over 1,000 mutations in Na_V channels are associated with human diseases including epilepsy, periodic paralysis, arrhythmias and pain disorders. Natural toxins and clinically-used small-molecule drugs bind to Na_V channels and modulate their functions. Recent advances from cryo-electron microscopy (cryo-EM) structures of Na_V channels reveal invaluable insights into the architecture, activation, fast inactivation, electromechanical coupling, ligand modulation and pharmacology of eukaryotic Na_V channels. These structural analyses not only demonstrate molecular mechanisms for Na_V channel structure and function, but also provide atomic level templates for rational development of potential subtype-selective therapeutics. In this review, we summarize recent structural advances of eukaryotic Na_V channels, highlighting the structural features of eukaryotic Na_V channels as well as distinct modulation mechanisms by a wide range of modulators from natural toxins to synthetic small-molecules.

Structural Pharmacology of Voltage-Gated Sodium Channels

Voltage-gated sodium (Na_V) channels initiate and propagate action potentials in excitable tissues to mediate key physiological processes including heart contraction and nervous system function. Accordingly, Na_V channels are major targets for drugs, toxins and disease-causing mutations. Recent breakthroughs in cryo-electron microscopy have led to the visualization of human Na_V1.1, Na_V1.2, Na_V1.4, Na_V1.5 and Na_V1.7 channel subtypes at high-resolution. These landmark studies have greatly advanced our structural understanding of channel architecture, ion selectivity, voltage-sensing, electromechanical coupling, fast inactivation, and the molecular basis underlying Na_V channelopathies. Na_V channel structures have also been increasingly determined in complex with toxin and small molecule modulators that target either the pore module or voltage sensor domains. These structural studies have provided new insights into the mechanisms of pharmacological action and opportunities for subtype-selective Na_V channel drug design. This review will highlight the structural pharmacology of human Na_V channels as well as the potential use of engineered and chimeric channels in future drug discovery efforts.

Reconstitution of purified GABAA receptors: ligand binding and chloride transporting properties

GABAA receptors have been solubilized from bovine brain membranes and, following purification by benzodiazepine affinity chromatography, have been reconstituted into phospholipid vesicles. Reconstituted vesicles were about 120 nm in diameter, and, on average, each vesicle contained fewer than one GABAA receptor which was reconstituted in an outside-out orientation. These preparations have been used in parallel studies of radiolabeled ligand binding and chloride flux, the latter being measured by following the fluorescence changes of a chloride-sensitive probe which was trapped within the vesicles at the time of reconstitution. The benzodiazepine [3H]flunitrazepam binds to an apparently homogeneous population of sites in these preparations (Kd of 5 nM) whereas the GABA analogue [3H]muscimol binds to both high- and low-affinity sites (KdS of 10 nM and 0.27 microM). Muscimol stimulated chloride flux with an EC50 of 0.2 microM and, at similar concentrations (EC50 = 0.16 microM), potentiated [3H]flunitrazepam binding, suggesting that occupancy of the low-affinity sites may be important for these effects. Diazepam shifted the dose-response curve for muscimol-stimulated flux to about 4-fold lower concentrations without affecting the maximum response. Diazepam did not, however, alter the equilibrium binding of [3H]muscimol. The purified receptor showed densensitization since flux responses were abolished by prior exposure to muscimol. The competitive antagonist bicuculline and the channel blocker picrotoxin completely inhibited ion flux mediated by 3 microM muscimol with EC50 values of 5.3 and 2.5 microM, respectively. These results are discussed in terms of possible mechanisms for activation, inhibition, and modulation of GABAA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of two saxitoxin-sensitive sodium channel subtypes from rat brain with distinct biochemical and functional properties

Two different 3H-saxitoxin-binding proteins, with distinct biochemical and functional properties, were isolated from rat brain using a combination of anion exchange and lectin affinity chromatography as well as high resolution size exclusion and anion exchange HPLC. The alpha subunits of the binding proteins had different apparent molecular weights on SDS-PAGE (Type A: 235,000; Type B: 260,000). When reconstituted into planar lipid bilayers, the two saxitoxin-binding proteins formed sodium channels with different apparent single-channel conductances in the presence of batrachotoxin (Type A: 22 pS; Type B: 12 pS) and veratridine (Type A: 9 pS; Type B: 5 pS). The subtypes were further distinguished by scorpion (Leiurus quinquestriatus) venom which had different effects on single-channel conductance and gating of veratridine-activated Type A and Type B channels. Scorpion venom caused a 19% increase in single-channel conductance of Type A channels and a 35-mV hyperpolarizing shift in activation. Scorpion venom doubled the single-channel conductance of Type B channels and shifted activation by at least 85 mV.

Molecular aspects of human brain sodium channel

The sodium channel content of human brain was measured by tritiated tetrodotoxin specific binding. After solubilization, the sodium channel was submitted to chromatography on diethylaminoethyl(cellulose) Sephadex, hydroxylapatite and wheat germ agglutinin sepharose. An increase of tritiated tetrodotoxin binding specific activity was subsequently observed. Eluted sodium channels from wheat germ agglutinin sepharose were overlaid on a sucrose gradient. Electrophoretical analysis of the material obtained after the sedimentation step revealed two co-purified peptides, alpha (Mr = 275,000 mol. wt) and beta (Mr = 30,000-36,000 mol. wt.). Alpha showed an exceptionally high free electrophoretic mobility, which is a common feature for all sodium channels previously described. However, the high denaturation rate of the solubilized tetrodotoxin receptor site 1 did not allow tetrodotoxin receptor quantification by the tritiated toxin binding in sucrose fractions. Sodium channel effective reconstitution in liposomes was demonstrated: (1) 22Na+ influx in proteoliposomes was sensitive to sodium channel-specific neurotoxins: (2) reconstituted proteins showed a cation selectivity similar to that previously described for animal sodium channels. The sodium channel preparation obtained after four chromatographic steps shows two peptides on the electrophoretic analysis. Reconstituted sodium channels displayed some physiological properties found in intact conducting membranes.

Functional reconstitution of the bovine brain GABAA receptor from solubilized components

The GABAA/benzodiazepine receptor has been solubilized from membrane preparations of bovine cerebral cortex and has been reconstituted, in a functionally active form, into phospholipid vesicles. In preliminary experiments, the receptor was labeled with the photoactive benzodiazepine [3H]flunitrazepam prior to solubilization. A peptide of apparent molecular weight 53,500 was specifically labeled by this method, and this was used as a marker for the receptor during the reconstitution procedures. The labeled protein was solubilized with approximately 40% efficiency by 1% beta-octyl glucoside. Reconstitution was achieved by mixing the solubilized proteins with a 4:1 mixture of soybean asolectin and bovine brain phospholipids, followed by chromatography on Sephadex G-50-80 to remove detergent. The incorporation of the GABAA receptor into membrane vesicles has been verified by sucrose gradient centrifugation in which the [3H]-flunitrazepam-labeled peptide comigrated with [14C]phosphatidylcholine used as a lipid marker. Vesicles prepared without labeled markers retained the ability to bind both [3H]flunitrazepam and the GABA analogue [3H]muscimol. Furthermore, the binding parameters were very similar to those measured using native membrane preparations. A novel fluorescence technique has been used to measure chloride transport mediated by the GABAA receptor in reconstituted vesicles. Chloride influx was rapidly stimulated in the presence of micromolar concentrations of muscimol and was blocked by preincubation of the membranes with muscimol (desensitization). Flux was also blocked by pretreatment with the competitive GABAA receptor blocker bicuculline or with the noncompetitive GABAA receptor antagonist picrotoxin.

The sodium channel of excitable and non-excitable cells

Excitation and conduction in the majority of excitable cells, as originally described in the squid axon, are initiated by a transient and highly selective increase of the membrane Na conductance, which allows this ion to move passively down its electrochemical gradient (Hodgkin & Katz, 1949; Hodgkin & Huxley, 1952). The term ‘Na channel’ was introduced to describe the mechanism involved in this conductance change (Hodgkin & Keynes, 1955).

Rapid incorporation of the solubilized dihydropyridine receptor into phospholipid vesicles

We describe the rapid incorporation of the CHAPS solubilized dihydropyridine receptor into phospholipid vesicles. A series of sucrose gradient sedimentation experiments demonstrate that the (+)-[3H]PN200-110-labeled dihydropyridine receptor is associated with lipid vesicles following detergent removal by Extracti-gel chromatography. Solubilization of the receptor results in a loss of (+)-[3H]PN200-110 binding affinity relative to that observed in native membranes; the high affinity binding of (+)-[3H]PN200-110 can be restored upon reincorporation of the receptor into phospholipid vesicles. Similarly, the incorporation of the receptor restores its stability to incubation at 37 degrees C relative to that of the detergent solubilized receptor, thereby mimicking the properties of the membrane bound form of the receptor. The dissociation rate of (+)-[3H]PN200-110 from the reconstituted receptor is shown to be allosterically regulated by verapamil and diltiazem, indicating that the binding sites for these calcium antagonists have been inserted along with the dihydropyridine receptor into phospholipid vesicles. The results presented in this report, thus demonstrate the successful reconstitution of the dihydropyridine receptor into phospholipid vesicles by a variety of criteria. The reconstitution method described here is rapid and efficient, and should now facilitate structure-function studies of this receptor and its interrelationships with other regulatory components of the voltage-sensitive calcium channel system.

The molecular basis of neuronal excitability

Neurons process and transmit information in the form of electrical signals. Their electrical excitability is due to the presence of voltage-sensitive ion channels in the neuronal plasma membrane. In recent years, the voltage-sensitive sodium channel of mammalian brain has become the first of these important neuronal components to be studied at the molecular level. This article describes the distribution of sodium channels among the functional compartments of the neuron and reviews work leading to the identification, purification, and characterization of this membrane glycoprotein.

Ethanol inhibits veratridine-stimulated sodium uptake in synaptosomes

The effects of ethanol and pentobarbital on voltage-sensitive sodium channels in whole brain (rat) synaptosomes were studied using isotopic flux measurements. Incubation of synaptosomes with ethanol or pentobarbital in vitro inhibited veratridine-stimulated 22Na+ uptake. The effect of ethanol is dose-dependent, occurs at sublethal, pharmacologically relevant concentrations and is fully reversible. These results suggest that ethanol and pentobarbital directly interfere with sodium channel function in nervous tissue. Alterations in sodium channel function may be a possible mechanism for the central nervous system (CNS) depressant action of ethanol and related compounds.

Solubilization and partial purification of putative calcium channels labelled with [3H]-nimodipine

High-affinity binding sites for the potent 1,4-dihydropyridine calcium channel blocker [3H]-nimodipine were solubilized from guinea-pig skeletal muscle microsomes with digitonin and CHAPS [3-(3-cholamidopropyl)-dimethyl-ammonio-l-propanesulfonate]. Detergent-solubilized binding sites could not be sedimented by centrifugation (50,000 X g, 4 h), passed freely through 0.2 micron nitrocellulose filters and were stable at 4 degrees C with half-lives of greater than 60 h. The solubilized 1,4-dihydropyridine binding sites were precipitable with polyethyleneglycol 6000 on Whatman GF/C filters. Saturation analysis of solubilized microsomes with [3H]-nimodipine revealed a single class of binding sites (Bmax = 0.5 to 1.7 pmol per mg of protein) with a KD of 2.2-3.6 nmol/l at 37 degrees C. Specific binding of the 1,4-dihydropyridine calcium channel label was fully reversible (k-1 = 1.5 min-1, at 37 degrees C). The solubilized drug receptors discriminated between the optical enantiomers of chiral 1,4-dihydropyridine calcium channel blockers, (-)- and (+)D-600 as well as between l-cis and d-cis-diltiazem. d-cis-Diltiazem stimulated the binding of [3H]-nimodipine (ED50:3.6 mumol/l), by increasing the Bmax and slowed the dissociation rate of the labelled 1,4-dihydropyridine calcium channel blocker. The solubilized binding sites were sensitive to pronase, alpha-chymotrypsin and phospholipases A and C indicating their protein nature as well as their lipid requirement. Chelation of endogeneous divalent cations by EDTA, EGTA or CDTA inhibits high-affinity [3H]-nimodipine binding, demonstrating that divalent cations are required for high affinity [3H]-nimodipine binding. Detergent-solubilized binding sites are adsorbed by several sepharose-immobilized lectins, including concanavalin A, wheat germ agglutinin and lentil-lectin but not by helix pomatia lectin. Preparative chromatography on concanavalin A sepharose was performed and the adsorbed [3H]-nimodipine binding sites were selectively eluted by alpha-methylmannoside; NaCl (1 mol/l) being completely ineffective as elutant. The purification factors by this method were 17-40-fold. The binding sites could be also purified (up to 10-fold) by sucrose density centrifugation. The S20, w value of the drug receptors is 12.9 s. It is concluded that the 1,4-dihydropyridine binding sites of the putative calcium channel are intimately associated with carbohydrate containing structures. Since the detergent-solubilized material shows allosteric regulation of 1,4-dihydropyridine binding, interaction with chemically different classes of calcium channel blockers, metalloprotein nature and a S20, w value which is indicative of structure large enough to span the membrane, we conclude that we have solubilized and partially purified the putative calcium channel.

Characterization of brain phosphatidylserine decarboxylase: localization in the mitochondrial inner membrane

A membranous fraction from calf brain, sedimenting at 10,000g, catalyzes the decarboxylation of exogenous phosphatidyl[14C]serine presented in an aqueous dispersion in detergent. The product formed by the enzymatic decarboxylation reaction is chemically and chromatographically identical to phosphatidyl[14C]ethanolamine. The calf brain decarboxylase activity: (1) did not require divalent cations; (2) was optimally active at neutral pH; (3) exhibited maximal activity in the presence of 0.1% Cutscum or sodium taurocholate; (4) was inhibited by hydroxylamine or p-hydroxymercuribenzoate; and (5) has an apparent Km = 2.4 mM for the phospholipid substrate. When this fraction was further separated by metrizamide density centrifugation, 90% of the phosphatidylserine decarboxylase activity was associated with the mitochondria. Resolution of the inner and outer membranes of the mitochondria revealed that greater than 99% of the decarboxylase activity was bound to the inner membrane. In contrast to this result, diacylglycerol ethanolaminephosphotransferase, another enzyme responsible for phosphatidylethanolamine biosynthesis in brain, was greatly enriched in the microsomal fraction. The highest level of phospholipid N-methyltransferase activity was also localized in the microsomal fraction. Thus, phosphatidylethanolamine formation via cytidine diphosphate ethanolamine in brain occurs at a membrane site where it should be available for the biosynthesis of phosphatidylcholine by stepwise methylation. In order for phosphatidylethanolamine formed by the decarboxylation reaction to be available for N-methylation, translocation from mitochondria to the microsomal site would be required.

Effect of proteolysis on the activity of the sodium channel in isolated lobster nerve membrane

The effect of enzymatic proteolysis on structural and functional properties of the isolated lobster nerve membrane was investigated. The membranes were treated with different amounts of either trypsin or unspecific protease. Sodium channel activity was determined by measuring the veratridine-tetrodotoxin-sensitive sodium influx in proteoliposomes prepared with nerve membrane and soybean lipids. The changes in the membrane proteins were followed by electrophoresis in polyacrylamide gradient gels. From the densitometric scan of the gels the relative area for each protein was obtained, and the ratio of enzyme-treated to control areas was evaluated. Under a similar degree of proteolysis catalyzed either by trypsin or by unspecific protease, the sensitive sodium influx is not affected by trypsin, whereas it is about 60% diminished by the unspecific protease. In this condition the zones corresponding to molecular weights of 240,000 and 166,000 daltons appear modified in the electrophoretic gels by both enzymes. The 117,000-dalton range is modified only by the unspecific protease. Increasing trypsin concentration diminishes sodium influx about 60%; and the 240,000-, 166,000-, and 117,000-dalton zones appear modified. A further increase of the protease concentration totally abolishes the sensitive sodium influx and modifies practically all of the membrane proteins. The present results indicate the rather high sensitivity of the membrane sodium channel activity to proteolytic action, and show that the membrane sites that respond to veratridine appear to be highly affected by proteolysis. In contrast, the tetrodotoxin receptor retains its binding capacity even after treatment of the membrane with protease concentrations 1,000 times higher than those affecting the sensitive sodium influx [Benzer and Raftery, 1972; Villegas et al, 1973].