Project Summary Fatty fibrosis, the replacement of healthy muscle tissue with fat and fibrotic scar tissue, is a prominent feature of chronic muscle diseases such as Duchenne muscular dystrophy (DMD), sarcopenia, the age-related loss of skeletal muscle and strength, in addition to obesity, and diabetes. This project will identify and characterize the cellular and molecular mechanisms by which intramuscular fat forms, and the functional consequences of intramuscular fat on muscle health. Recent evidence points to ciliary Hedgehog signaling as a potent ant- adipogenic signal during muscle regeneration and in mdx mice, a mouse model of DMD. In addition, Hedgehog promotes muscle repair and prevents the decline in myofiber size in mdx mice. By what mechanism(s) Hedgehog acts to balance fat formation and muscle regeneration is unknown. This proposal will use innovative and powerful mouse models to identify the role of Hedgehog signaling during muscle regeneration in the following aims: 1) Identify and characterize the responsible Hedgehog ligand; 2) determine which cell types respond to Hedgehog signaling; and 3) demonstrate if intramuscular fat directly affects muscle health. Taken together, this proposal will provide insights into the origin and function of intramuscular fat and will aid in the search for novel therapeutic interventions into human diseases affected by fatty fibrosis.