Voltage-gated sodium channels at 60: structure, function and pathophysiology

Abstract

Voltage-gated sodium channels initiate action potentials in nerve, muscle and other excitable cells. The sodium current that initiates the nerve action potential was discovered by Hodgkin and Huxley using the voltage clamp technique in their landmark series of papers in The Journal of Physiology in 1952. They described sodium selectivity, voltage-dependent activation and fast inactivation, and they developed a quantitative model for action potential generation that has endured for many decades. This article gives an overview of the legacy that has evolved from their work, including development of conceptual models of sodium channel function, discovery of the sodium channel protein, analysis of its structure and function, determination of its structure at high resolution, definition of the mechanism and structural basis for drug block, and exploration of the role of the sodium channel as a target for disease mutations. Structural models for sodium selectivity and conductance, voltage-dependent activation, fast inactivation and drug block are discussed. A perspective for the future envisions new advances in understanding the structural basis for sodium channel function, the role of sodium channels in disease and the opportunity for discovery of novel therapeutics.

Figure 1. Subunit structure of voltage-gated sodium channels

A, SDS polyacrylamide gel electrophoresis patterns illustrating the α and β subunits of the brain sodium channels. Left, α and β subunits covalently labelled with ¹²⁵I-labelled scorpion toxin (Beneski & Catterall, 1980). Lane 1, specific labelling; lane 2, non-specific labelling. Right, sodium channel purified from rat brain showing the α, β1 and β2 subunits and their molecular weights (Hartshorne et al. 1982). As illustrated, the α and β2 subunits are linked by a disulfide bond. Tetrodotoxin and scorpion toxins bind to the α subunits of sodium channels as indicated and were used a molecular tags to identify and purify the sodium channel protein (Beneski & Catterall, 1980; Hartshorne et al. 1982; Hartshorne & Catterall, 1984). Inset, single channel currents conducted by a single purified sodium channel incorporated into a planar bilayer (Hartshorne et al. 1985). B, drawing of the subunit structure of the brain sodium channel based on biochemical data.

Figure 2. The primary structures of the subunits of the voltage-gated sodium channels

Cylinders represent probable α-helical segments. Bold lines represent the polypeptide chains of each subunit with length approximately proportional to the number of amino acid residues in the brain sodium channel subtypes. The extracellular domains of the β1 and β2 subunits are shown as immunoglobulin-like folds. Y, sites of probable N-linked glycosylation; P in red circles, sites of demonstrated protein phosphorylation by PKA (circles) and PKC (diamonds); green, pore-lining segments; white circles, the outer (EEEE) and inner (DEKA) rings of amino residues that form the ion selectivity filter and the tetrodotoxin binding site; yellow, S4 voltage sensors; h in blue circle, inactivation particle in the inactivation gate loop; blue circles, sites implicated in forming the inactivation gate receptor. Sites of binding of α- and β-scorpion toxins and a site of interaction between α and β1 subunits are also shown. Tetrodotoxin is a specific blocker of the pore of sodium channels (Hille, 1975b), whereas the α- and β-scorpion toxins block fast inactivation and enhance activation, respectively, and thereby generate persistent sodium current that causes depolarization block of nerve conduction (Catterall et al. 2007). Tetrodotoxin has been used as a tool to probe the pore of the sodium channel, whereas the scorpion toxins have been valuable as probes of voltage sensor function. Inset, structure of the inactivation gate in solution determined by NMR (Rohl et al. 1999).

Figure 3. Structure of NavAb

A, top view of NavAb channels coloured according to crystallographic temperature factors of the main chain (blue < 50 Ų to red > 150 Ų). B, side view of NavAb. C, structural elements in NavAb. The structural components of one subunit are highlighted (1–6, transmembrane segments S1–S6).

Figure 4. NavAb pore and selectivity filter

A, architecture of the NavAb pore. Glu177 side-chains, purple; pore volume, grey. B, the closed activation gate at the intracellular end of the pore illustrating the close interaction of Met221 residues in closing the pore. C, top view of the ion selectivity filter. Symmetry-related molecules are coloured white and yellow; P-helix residues are coloured green. Hydrogen bonds between Thr175 and Trp179 are indicated by grey dashes. Electron-densities from F_o–F_c omit maps are contoured at 4.0 σ (blue and grey) and subtle differences can be appreciated (small arrows). D, side view of the selectivity filter. Glu177 (purple) interactions with Gln172, Ser178 and the backbone of Ser180 are shown in the far subunit. F_o–F_c omit map, 4.75 σ (blue); putative cations or water molecules (red spheres, Ion_EX). Electron-density around Leu176 (grey; F_o–F_c omit map at 1.75 σ) and a putative water molecule is shown (grey sphere). Na⁺-coordination sites: Site_HFS, Site_CEN and Site_IN.

Figure 5. Membrane access to the central cavity in NavAb

A, side-view through the pore module illustrating fenestrations (portals) and hydrophobic access to central cavity. Phe203 side-chains, yellow sticks. Surface representations of NavAb residues aligning with those implicated in drug binding and block; Thr206, blue; Met209, green; Val213, orange. Membrane boundaries, grey lines. Electron-density from an F_o–F_c omit map is contoured at 2.0 σ. B, top-view sectioned below the selectivity filter, coloured as in A.

Figure 6. Model of conformational changes in the voltage sensor during gating

Transmembrane view of the ribbon representation of Rosetta models of three resting and three activated states of the VSD of NaChBac. Segments S1 through S4 coloured individually and labelled. Side chain atoms of the gating charge carrying arginines in S4 (coloured dark blue), negatively charged residues in S1, S2 and S3 segments (coloured red), polar residues in S1, S3 and S4 (coloured purple), and key hydrophobic residues in S1, S2 and S3 (coloured grey) are shown in sphere representation and labelled. The HCS is highlighted by the orange bar.