PROJECT SUMMARY Chronic kidney disease (CKD) is a health epidemic that increases risk of death due to cardiovascular disease. Hyperphosphatemia and elevations in serum levels of the hormone fibroblast growth factor (FGF) 23 are a hallmark of CKD and associated with an increased risk of cardiac injury and death. We previously found that in rodents, hyperphosphatemia induced by the administration of a high-phosphate diet elevates serum FGF23 levels and causes cardiac hypertrophy in the absence of kidney damage, suggesting that elevated phosphate and/or FGF23 per se might act as circulating factors that can damage the heart. However, whether phosphate or FGF23 can directly target the heart and induce injury is unknown. FGF23 is mainly produced by bone, and we previously found that FGF23 can activate specific signaling events in cultured cardiac myocytes and induce hypertrophy. However, whether FGF23 can directly target cardiac myocytes in vivo remains unknown. Based on global FGFR4 deletion and pharmacological FGFR4 inhibition studies, we have recently identified FGFR4 as the FGF23 receptor that is required for the development of cardiac hypertrophy in animal models of CKD as well as in mice on high-phosphate diet. For the proposed study we have generated a novel mouse model for the cardiac myocyte-specific deletion of FGFR4. By inducing CKD via an adenine diet or by administration of a high- phosphate diet, we will be able to determine whether FGF23/FGFR4 activation in the heart is required for the development of cardiac injury. Furthermore, direct effects of phosphate on cardiac myocytes have not been studied to date. Our preliminary work indicates that elevations of extracellular phosphate levels in cardiac myocyte cultures induce pro-inflammatory signaling and osteogenic gene programs. Here we will determine whether these changes lead to pathologic cardiac remodeling in mice, including cardiac hypertrophy, fibrosis, inflammation and calcification. We also found that phosphate treatments of cultured cardiac myocytes induce the expression of FGF23 that under normal conditions is not found in the heart. It has been reported that in CKD the heart starts to produce FGF23. However, whether heart-derived FGF23 can cause cardiac remodeling, is unclear. To determine the consequences of cardiac FGF23 production, we will generate a new mouse model with cardiac myocyte-specific deletion of FGF23, followed by the administration of high-phosphate or adenine diets, and a detailed analysis of cardiac structure and function. Overall, our study is designed to test the hypothesis that hyperphosphatemia - by itself as well as in the context of CKD - contributes to cardiac injury by inducing FGF23 production in the heart. FGF23 is a potent pro-hypertrophic factor, and like circulating FGF23, paracrine FGF23 might directly target cardiac myocytes via FGFR4 and induce cardiac injury. We postulate that pharmacologic blockade of FGFR4 can inhibit the pathologic actions ...