Hypokalemic periodic paralysis (HypoPP) is a dominantly inherited disorder of skeletal muscle in which recurrent attacks of weakness are caused by intermittent failure of fiber excitability. Episodes occur in association with hypokalemia (K+ <3 mM) and are often triggered by carbohydrate ingestion, exercise, or stress. The molecular defect in HypoPP is heterogeneous, with 60% of families having missense mutations in CACNA1S encoding the L-type Ca channel CaV1.1, and 20% have missense mutations in SCN4A encoding the voltage-gated Na channel NaV1.4. Despite this scientific advance, the pathogenic basis for the transient attacks of fiber depolarization and loss of excitability is not fully established, and effective clinical interventions are lacking. The proposed studies are designed to advance our understanding of the functional defects caused by CaV1.1 HypoPP mutations and to apply this information in model simulations to gain insights on disease mechanism and thereby identify experimentally testable strategies for disease modification. All 10 HypoPP mutations in NaV1.4 and 9 of 11 in CaV1.1 occur at arginine residues in S4 segments of voltage-sensor domains (VSD). For NaV1.4 HypoPP, these R/X mutations make channels leaky because of a small anomalous ?gating pore current?. By homology, a similar defect has been proposed for CaV1.1 HypoPP, but experimental confirmation has been limited. In the prior cycle of this project, we detected a CaV1.1 gating pore current in our R528H knockin mutant mouse model of HypoPP. This finding supports the notion that the anomalous gating pore current is a feature in common between NaV1.4 and CaV1.1 HypoPP mutant channels, and thereby explains how mutations of either channel can produce the same clinical phenotype. The CaV1.1-R528H mice have a robust HypoPP phenotype and are a unique resource for investigating mechanisms by which environmental triggers elicit loss of excitability and weakness. For example, we recently discovered that recovery from acidosis is a potent trigger for a transient loss of force in soleus muscle of R528H mice, which may provide the first insight on why attacks of weakness frequently occur after strenuous exercise in HypoPP patients. The availability of fully-differentiated HypoPP muscle fibers provides an excellent system to study excitation-contraction coupling, and Ca2+ release is markedly suppressed in homozygous R528H fibers. In other preliminary studies, we have achieved a 50-fold increase of CaV1.1 expression in Xenopus oocytes by co-expressing Stac3, a chaperone that promotes CaV trafficking to the membrane. This new high-expression system now makes it possible to determine whether the other CaV1.1 HypoPP mutations, especially the two atypical mutations not at R residues in S4, also support a gating pore current. The CaV1.1-R528H HypoPP mouse and the enhanced membrane expression of CaV1.1 in Xenopus oocytes will be used to address the following Specific Aims: (1) Test the hypothesis that an anomalous gating pore current is a pathomechanism in common with CaV1.1 mutations associated with HypoPP, (2) In the pH-shift model for post-exercise weakness in HypoPP, test the hypothesis that a shift in the Cl- gradient causes susceptibility to depolarization-induced loss of excitability and that maneuvers to limit Cl- accumulation reduce the severity of weakness. (3) Test whether impaired Ca2+ release is a pervasive defect in CaV1.1 R528H muscle and explore whether the defect is intrinsic to the channel mutation.
Hypokalemic periodic paralysis is a rare inherited disorder of skeletal muscle in which intermittent failure of muscle excitability causes attacks of severe weakness or paralysis lasting for hours to days. The molecular defect is well established to be mutations of either calcium channels or sodium channels, but the mechanisms by which mutant channels cause susceptibility to attacks of weakness are poorly understood and consequently rational therapeutic strategies to modify disease course are lacking. We have developed the only genetically-engineered mouse model of periodic paralysis based on a calcium channel mutation and will use this unique tool to define further the mechanisms underlying attacks of paralysis and test potential interventions to modify disease burden.
