Depolarization-activated gating pore current conducted by mutant sodium channels in potassium-sensitive normokalemic periodic paralysis

Abstract

Some inherited periodic paralyses are caused by mutations in skeletal muscle Na V1.4 sodium channels that alter channel gating and impair action potential generation. In the case of hypokalemic periodic paralysis, mutations of one of the outermost two gating charges in the S4 voltage sensor in domain II of the Na V1.4 ␣ subunit induce gating pore current, resulting in a leak of sodium or protons through the voltage sensor that causes depolarization, sodium overload, and contractile failure correlated with low serum potassium. Potassium-sensitive normokalemic periodic paralysis (NormoPP) is caused by mutations in the third gating charge in domain II of the Na V1.4 channel. Here, we report that these mutations in rat Na V1.4 (R669Q/G/W) cause gating pore current that is activated by depolarization and therefore is conducted in the activated state of the voltage sensor. In addition, we find that this gating pore current is retained in the slow-inactivated state and is deactivated only at hyperpolarized membrane potentials. Gating pore current through the mutant voltage sensor of slowinactivated NormoPP channels would cause increased sodium influx at the resting membrane potential and during trains of action potentials, depolarize muscle fibers, and lead to contractile failure and cellular pathology in NormoPP.skeletal muscle ͉ Nav 1.4 ͉ gating charge ͉ voltage sensor V oltage-gated sodium channels in skeletal muscle (Na V 1.4) generate action potentials that initiate muscle contraction in response to nerve stimulation. They are complexes of a large pore-forming ␣-subunit and a small, auxiliary ␤1-subunit (1-4). The ␣-subunits are organized in 4 repeated domains with 6 transmembrane segments (S1-S6) and a reentrant P loop between S5 and S6 (4, 5). The voltage sensitivity of sodium channels arises from the force of the transmembrane electric field exerted on arginine residues in 3-residue repeat motifs in the S4 transmembrane segments, which move outward upon depolarization and initiate a conformational change that opens the central pore (4, 6).The periodic paralyses are rare, dominantly inherited muscle disorders characterized by episodic attacks of muscle weakness (7). Hyperkalemic periodic paralysis (HyperPP) and paramyotonia congenita are caused by mutations in the ␣ subunit of skeletal muscle Na V 1.4 channels that are widely spread through the protein and usually cause a gain-of-function by impairing fast and/or slow inactivation (8). Increased sodium channel activity leads to depolarization, hyperexcitability, and either repetitive firing or depolarization block. In contrast, hypokalemic periodic paralysis (HypoPP) is caused by mutations in both the ␣-subunit of the Na V 1.4 channel and the homologous ␣1-subunit of the skeletal muscle Ca V 1.1 channel, which initiates excitationcontraction coupling (8). In HypoPP, mutations in both of these large channel proteins specifically target the outermost two gating-charge-carrying arginine residues in their S4 voltage sensors in domains II, III, or IV. The convergence...

Leaky sodium channels from voltage sensor mutations in periodic paralysis, but not paramyotonia

“…Curiously, a set of 3 missense mutations at the third arginine in domain II produce gating pore currents that are activated by depolarization and are closed at the resting potential. 21 These mutations (R657G/Q/W) cause potassium-sensitive normokalemic periodic paralysis, 22 for which the mechanistic link to a depolarization-activated gating pore current remains to be established. A variety of gain-of-function changes for the Na ϩ current (impaired inactivation or enhanced activation) have been observed for more than 20 PMC and hyperkalemic periodic paralysis mutations studied to date, 23 but conversely have never been observed in studies of Na V 1.4-HyppoPP mutants.…”

Double gaps alongShakerS4 demonstrate omega currents at three different closed states

“…19 On the other hand, mutation of the third arginine in the same domain produced an outward omega current at depolarized potentials. 20 This would indicate that S4 in domain II of Na V 1.4 moves only two steps during gating and the omega pore spans only the size of one arginine.…”

Gating-Pore Currents Demonstrate Selective and Specific Modulation of Individual Sodium Channel Voltage-Sensors by Biological Toxins

“…The gating-pore currents generated by single-charge mutations and carried by cations such as Na 1 and H 1 are #1% of the central-pore current in VGSCs, but in potassium channels, because of their 4-fold symmetry, they can be larger (∼6% of the central-pore current) (Sokolov et al, 2005). Depending on which charged residues in the S4 segments are mutated, gating-pore currents arise either in the resting or the activated configuration (Sokolov et al, 2007;Sokolov et al, 2008b) (Fig. 1C).…”