PROJECT ABSTRACT Loss of muscle cell adhesion is emerging as a common theme in muscular dystrophies. In skeletal muscle, the dystrophin-glycoprotein complex is located at the sarcolemma and is composed of peripheral and integral membrane proteins. As a whole, this complex links the extracellular matrix to the intracellular actin cytoskeleton and provides structural stability to the sarcolemma during muscle contraction. Duchenne muscular dystrophy, the most common form of dystrophy, is caused by mutations in the dystrophin gene that result in loss of dystrophin protein and the entire dystrophin-glycoprotein complex. My research group has pioneered several key discoveries related to the function of sarcospan, an integral component of the dystrophin-glycoprotein complex. We have shown that sarcospan plays an important role in mediating protein interactions within this complex. Sarcospan affects communication between the dystrophin-glycoprotein complex and the extracellular matrix. Importantly, we demonstrate that mild sarcospan over-expression in mdx mice, which possess a mutation in the murine dystrophin gene, rescues muscular dystrophy by stabilizing expression of a complex of compensatory proteins that is functionally analogous to the dystrophin-glycoprotein complex, including a7b1 integrin. The current 4R01 proposal builds on discoveries made during the prior funding periods by interrogating specific mechanisms by which sarcospan ameliorates disease in dystrophin- deficient mdx mice. We will test the hypothesis that sarcospan enhances a7b1 integrin outside-in and inside- out signaling. We will investigate the extracellular matrix of mdx muscle that is overexpressing sarcospan to determine how sarcospan affects the composition, organization, and mechanical properties of the extracellular matrix. Lastly, we will use our decellularization protocol to isolate the extracellular matrix and test its interaction with human iPSC-derived skeletal muscle progenitors to test principles of bidirectional communication in a newly developed in vitro model system. We expect that our results will illuminate molecular pathways that counter a broad range of muscle wasting disorders due to loss of extracellular matrix contact.