Syntrophins are adapter proteins that recruit signaling proteins to the dystrophin complex. Although the dystrophin complex is best known for its role in muscle health (dystrophin mutations cause Duchenne muscular dystrophy), it is now certain that dystrophin-related complexes are important in many different cells, including the central nervous system. In addition to its well-established role in membrane stabilization, this complex is a scaffold for membrane-associated signaling proteins. The syntrophins are key proteins in this signaling function. Five syntrophin isoforms all have a common domain structure. The syntrophin PDZ domain binds signaling proteins (nNOS and several kinases) and channels (potassium and sodium channels and aquaporin-4). Gene-targeted mice lacking a-syntrophin do not express nNOS and aquaporin-4 on the sarcolemma, and also are deficient in utrophin at the neuromuscular synapse, a-syntrophin null mice also have a brain phenotype. Because aquaporin-4 is absent from its proper location in perivascular astrocytic endfeet, a-syntrophin null mice are resistant to brain edema and the infarct volume following ischemia is substantially smaller. The goal of this application is to understand the molecular mechanisms of syntrophin-ligand regulation and to apply this knowledge to in vivo function. We will determine the mechanisms by which the interaction of syntrophins with the dystrophin family and with signaling proteins is regulated. The role of PH domain interaction with phosphatidylinositol lipids in targeting utrophin to the sarcolemma will be determined. The importance of post-translational modifications, such as phosphorylation, will be studied in muscle and astrocytes. Linker proteins that may mediate interaction between the a-syntrophin PDZ domain and aquaporin-4 will be identified. Finally, based on results from DNA chip array experiments that compared gene expression in normal and a-syntrophin null muscle, we will examine the role of a-syntrophin in the expression and localization of the epsilon subunit of the nicotinic receptor, two potassium channels and the transient receptor channel, TRPC 1. These results will expand our understanding of the regulation of signaling proteins by syntrophins and may reveal new therapeutic targets for the treatment of muscular dystrophy, epilepsy, and brain edema following stroke.
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