PROJECT SUMMARY: The long-term goal of our research is to improve our understanding of the pathologic mechanisms of Duchenne muscular dystrophy (DMD) and to leverage that knowledge to advance therapeutics. Progress in treating respiratory insufficiency and other complications of muscle deterioration has led to cardiac disease emerging as a principal cause of morbidity and mortality in DMD. Consequently, there is a critical need to identify the specific pathogenic mechanisms that promote DMD-related cardiac disease so that targeted therapeutics can be developed. The complex pathology initiates with mechanically compromised myocytes but is intertwined with secondary defects that contribute importantly to disease progression. Fibrosis is one of the earliest clinical manifestations of cardiac involvement in DMD occurring prior to ventricular dysfunction indicating the presence of activated fibroblasts early in the progression of pathology. Fibroblasts produce extracellular matrix that comprises fibrotic scar, but it is not known whether they possess other maladaptive functions that affect disease progression. Our preliminary findings now show that fibroblast growth factor (FGF) 23 is greatly increased in cardiac fibroblasts of mdx mice, a genetic mouse model of DMD. Elevated FGF23 is remarkably associated with important clinical events related to cardiovascular disease and mortality in patients with renal disease and in the general population. FGF23 is a hormone that mostly works in cooperation with its co-receptor Klotho, but can also stimulate pathological signaling in atypical cellular targets. FGF23 exerts pathologic effects on cardiac myocytes by binding to FGF receptor (FGFR) 4 independent of Klotho, which is not expressed in the heart. Our central hypothesis is that dystrophic cardiac fibroblasts exert paracrine effects on cardiac myocytes via FGF23, thereby activating FGFR4 to promote cardiac disease in DMD. Our preliminary findings showing that Klotho is cardioprotective in mdx mice strongly supports our hypothesis that FGF23 signaling contributes importantly to cardiac remodeling. The central hypothesis will be tested by pursuing two comprehensive specific aims. In Aim 1 we will test whether FGF23 is a paracrine factor produced by cardiac fibroblasts that promotes dystrophin-deficient cardiac pathology and functional defects using a conditional genetic approach combined with ex vivo cell culture experiments. In Aim 2 we will determine whether FGFR4 activation in cardiac myocytes affects dystrophin-deficient cardiac remodeling and function using complementary loss-of-function and gain-of- function genetic approaches. The proposed research is innovative because it is predicted to show that fibroblasts exert regulatory effects on cardiac myocytes and establish FGF23/FGFR4 signaling as a novel pathogenic mechanism of DMD. This contribution is significant because it is expected to provide strong mechanistic justification for modulating FGF23/FGF...