Although most sodium channels share common structural elements, a number of different sodium channel isoforms are expressed in different tissues or even in the same tissue. The expression of different sodium channel isoforms is closely regulated, suggesting that each isoform has a unique function. Mammalian skeletal muscle expresses at least two sodium channel alpha subunits that are encoded by separate genes. SkM1 is the predominant form in innervated adult skeletal muscle while SkM2 is expressed in developing muscle and again after denervation of adult muscle. In cardiac muscle, however, SkM1 expression is never activated whereas SkM2 is expressed throughout life. SkM1 and SkM2 are independently regulated by activity in tissue culture and neuromuscular blockade in mature muscle. The mechanisms that control differential expression of these two genes during development and following denervation in skeletal muscle are not understood. The differences in developmental regulation between skeletal and cardiac muscle which result in the disparate sodium channel expression are also not known. The goal of this research proposal is to analyze the genomic regulatory elements that control expression of the two sodium channel isoforms in vivo during development, following denervation, and in skeletal versus cardiac muscle. I will determine which aspects of sodium channel gene regulation are shared with other muscle specific genes and which are specific to each sodium channel isoform. Candidate promoters and regulatory elements previously defined in tissue culture will be evaluated for -their role in modulating expression of a reporter gene in postnatal development and in adult muscle after denervation, using recombinant adenovirus as a vehicle for introduction of test constructs into muscle in vivo. After the genomic regulatory elements important in sodium channel expression in vivo have been identified, recombinant adenovirus will be used to introduce members of the MyoD family of transcription factors into skeletal muscle following denervation. The role of the MyoD family in directing SkM2 expression following denervation will be determined. My ultimate goal is to better understand the transcriptional regulation of muscle genes during development and following denervation. Understanding the regulation of muscle gene expression will have implications for neuromuscular diseases where denervation plays a prominent role.
