Voltage-sensor mutations in channelopathies of skeletal muscle

Abstract

Mutations of voltage-gated ion channels cause several channelopathies of skeletal muscle, which present clinically with myotonia, periodic paralysis, or a combination of both. Expression studies have revealed both loss-of-function and gain-of-function defects for the currents passed by mutant channels. In many cases, these functional changes could be mechanistically linked to the defects of fibre excitability underlying myotonia or periodic paralysis. One remaining enigma was the basis for depolarization-induced weakness in hypokalaemic periodic paralysis (HypoPP) arising from mutations in either sodium or calcium channels. Curiously, 14 of 15 HypoPP mutations are at arginines in S4 voltage sensors, and recent observations show that these substitutions support an alternative pathway for ion conduction, the gating pore, that may be the source of the aberrant depolarization during an attack of paralysis.

Figure 1. Alignment of S4 sequences for Na_V1.4, Ca_V1.1 and Shaker K⁺ channels

Positively charged residues are delineated with a shaded background (blue) for the R0 to R7 position. Mutations of these basic residues may produce a gating pore current activated by hyperpolarization (hyperpol), by depolarization (depol), or give rise to a proton transporter (xporter). The gating pore waist lies between R2 and R3, and voltage-dependent translocation of substituted R residues on S4 through this region regulates the flow of gating pore current. HypoPP mutations (red) are clustered at arginines in the R1 or R2 position of Na_V1.4 and Ca_V1.1. Mutations of Na_V1.4 at IV-R1 (green) are associated with PMC, not HypoPP, and do not produce gating pore current. A mixed variant of periodic paralysis is associated with mutations at Na_V1.4 II-R3 (yellow) which causes a depolarization-activated gating pore.

Figure 2. Hyperpolarization-activated gating pore current in the R1H HypoPP mutation of Na_V1.4

A, currents elicited by 300 ms step depolarizations between −140 mV and +30 mV from a holding potential of −100 mV in oocytes expressing wild-type (WT) or R1H channels. Saturating concentration of TTX has been added to suppress I_Na. B, steady-state I–V relation, after subtraction of linear background leak: WT, open circles; R1H, filled circles. Modified from Struyk & Cannon (2007) with permission.

Figure 3. The gating pore current increases the susceptibility to paradoxical depolarization in low extracellular [K⁺]

The steady-state I–V relation for mammalian skeletal muscle was simulated by the combination of an inward rectifier K⁺ current, I_Kir, a delayed rectifier K⁺ current, I_KDR, and a leakage current with a reversal potential of 0 mV, I_Leak, as in Struyk & Cannon (2008). A, in 4 mm[K⁺]_o the resting potential of −91.3 mV is determined primarily from the balance of an inward I_Leak and outward I_Kir. Addition of a gating pore current, I_gp, to simulate HypoPP (dashed red line), shifts the I–V relation downward (dashed black line) but results in only a small depolarization of V_rest to −87.3 mV. B, reduction of [K⁺]_o to 2.5 mm shifts E_K and I_Kir to more negative potentials, with a predicted hyperpolarization of V_rest to −101.6 mV in WT fibres. For HypoPP fibres, however, V_rest depolarizes to −65.3 mV (arrow) because the combination of inward currents (I_Leak+I_gp) exceeds the outward current from I_Kir. Under these conditions, which would result in paralysis from inactivation of sodium channels, V_rest is now set by the balance of I_KDR and the inward currents. Because K_DR channels are active only for >−55 mV, V_rest is strongly dependent on K_DR gating, which results in an apparent decoupling of V_rest from E_K. C, phase plot of V_rest as a function of [K⁺]_o for simulated WT and HypoPP fibres. The inward gating pore current in HypoPP fibres causes only a small depolarization of V_rest from −91.3 mV to −87.3 mV in 4 mm[K⁺]_o (arrow). More importantly, the catastrophic depolarization of V_rest shifts leftward from 1.5 mm[K⁺]_o for WT to 3 mm for HypoPP. The Nernst potential for K⁺, E_K, is shown for comparison (blue line).