An atypical CaV1.1 mutation reveals a common mechanism for hypokalemic periodic paralysis.

Hypokalemic periodic paralysis (HypoKPP) is a rare, dominantly inherited disorder of muscle that is characterized by recurrent and prolonged episodes of severe weakness. These episodes are associated with low serum K (often <3.0 mM, normally 3.5–5.5 mM), often triggered by events that lower extracellular [K] such as carbohydrate-rich meals, and result in depolarization-induced loss of excitability. In 1994, genetic linkage and positional cloning established a HypoKPP locus in CAC NA1S, which encodes the pore-forming α subunit of the skeletal muscle L-type Ca channel CaV1.1 (Fontaine et al., 1994). The first missense mutations were identified soon after (Jurkat-Rott et al., 1994; Ptáček et al., 1994). Despite the tantalizing observation that the first three mutations were all missense substitutions of arginines in S4 segments, the identification of a functional defect with a plausible mechanistic connection to depolarization-induced loss of excitability and weakness in low K did not emerge until some 13 yr later. Two major challenges impeded functional studies of HypoKPP mutant channels. First, expression of CaV1.1 at the plasma membrane is poor in model systems not derived from muscle. Second, the critical channel defect—an anomalous inward current—is of relatively small amplitude and difficult to distinguish from a nonspecific leakage current. In the current issue of the Journal of General Physiology, Fuster et al. use in vivo electroporation into mouse muscle as a system in which to achieve high expression of human CaV1.1. They express CaV1.1 channels carrying an atypical HypoKPP mutation—V876E in S3 of domain III—and demonstrate an anomalous inward current at hyperpolarized potentials. This finding is an important advance in the field because it reveals that the one HypoKPP mutation that departs from the canonical pattern of R/X missense mutations in S4 segments still produces an anomalous inward current. Moreover, unlike all other CaV1.1 HypoKPP mutations studied to date, V876E does not alter voltage-dependent gating of the conventional Ca current, strongly supporting the notion that the anomalous inward current is the critical defect in HypoKPP. The experimental insight on the source of the anomalous inward current in HypoKPP originated from the remarkable observation that missense mutations of SCN4A, encoding the α subunit of the skeletal muscle sodium channel NaV1.4, are also a cause of HypoKPP. The clinical HypoKPP phenotype is essentially identical, regardless of whether the underlying mutation is in CaV1.1 or NaV1.4. This genetic heterogeneity was initially puzzling, especially because the functional roles for NaV1.4 and CaV1.1 in excitation-contraction coupling are very different. Moreover, the final common pathway for weakness in an episode of HypoKPP is a failure to maintain the resting potential (Vrest) in low K, and yet studies in human HypoKPP fibers showed this paradoxical depolarization is not prevented by the channel blockers tetrodotoxin or nitrendipine (Ruff, 1999; Jurkat-Rott et al., 2000). The solution to the mystery of the depolarizing current in HypoKPP was gleaned from recent advances in understanding the conformational changes that occur in the voltage sensor domain of KV channels. First, histidine-scanning mutagenesis in Shaker S4, as a tool to discern gating-dependent changes in aqueous accessibility of specific S4 residues to protons, also created a proton conduction pathway (Starace and Bezanilla, 2001). Other R/X substitutions even supported currents carried by Na or K (Tombola et al., 2005). These were initially called omega currents, to distinguish this conduction mechanism from ion flux through the conventional pore (α current). In subsequent publications, the preferred term became the “gating pore current” to convey the view that the S4 helix undergoes a voltage-dependent translocation through a crevasse in the channel—the gating pore— and furthermore that mutations at this critical interface may permit ion flux as an anomalous gating pore current. Second, a consistent pattern emerged as more HypoKPP mutations were identified (Fig. 1). All 10 HypoKPP missense mutations in NaV1.4 are in the outer arginine residues of S4 (R1 or R2) in domains I, II, or III; and 8 of 9 HypoKPP mutations in CaV1.1 are in R1