Voltage-dependent Na+ channels are responsible for the rapid membrane depolarization that characterizes the initial phase of an action potential in nerve, heart and muscle. In skeletal muscle, failure of Na+ channel function will lead to paralysis in spite of normal nerve, neuromuscular junction, and contractile protein function. In hereditary periodic paralysis, paroxysmal weakness is a manifestation of a defect in the muscle Na+ channel which can episodically render the myocyte electrically inexcitable. During the past two years, we and others have elucidated the molecular basis of these genetic diseases by demonstrating mutations in the human muscle Na+ channel alpha-subunit gene (SCN4A). We now know that at least nine distinct point mutations in the SCN4A gene may cause hyperkalemic periodic paralysis and paramyotonia congenita, but not hypokalemic paralysis. This research proposal outlines studies aimed at examining the dysfunction conferred by these various hereditary defects on the human muscle Na+ channel. The approach will involve a fusion of recombinant DNA technology and cellular electrophysiology. Site-directed mutagenesis will be used to create mutant Na+ channel alpha-subunit, hSkM1, will be used for these studies. Electrophysiological studies using heterologous expression of hSkM1 mutants in transfected mammalian cells will be used to define abnormal phenotypes. We will also examine the role of the Na+ channel beta1-subunit in expression of disease-producing mutant channel phenotypes. These studies will involve, in part, engineering the periodic paralysis mutations in the human cardiac Na+ channel (hH1), a channel which functions normally without the beta1-subunit. If Na+ channel alpha-subunit defects are not dependent on beta1, then mutations in hH1 patterned after those in periodic paralysis should have functional consequences. We also hypothesize that defects in the beta1-subunit gene cause hypokalemic periodic paralysis and will test this by genetic linkage analysis. This work should contribute greatly to our understanding of hereditary disorders of muscle membrane excitability, and to the structure and function of voltage-dependent Na+ channels.
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