Several lines of evidence have linked all-trans retinoic acid (atRA) with inhibition of hypertrophy and hyperplasia in cardiomyocytes, vascular smooth muscle cells, and fibroblasts. We have generated several mouse models in which myocyte intracellular retinoid signaling is obliterated, and have demonstrated that retinoids play a pivotal role in maintaining cardiac function during adulthood. However, the intracellular mechanism by which retinoids function in the mature myocardium are vastly unknown. In order to gain insight into the retinoid- mediated signaling in the myocardium, we have evaluated the role of several candidate signal transducers in the myocytes and have recently discovered that mutation of a novel orphan type I - GPCR rescues dilated cardiomyopathy and ventricular contractility upon retinoid deficiency in vivo. We hypothesize that the retinoid and 20RH pathways converge in the myocytes to regulate cardiac function; and propose the following Specific Aims: Specific Aim 1. To determine the existence of cross regulation between the retinoid receptors and the 20RH type I-GPCR; Specific Aim 2. To asses whether 20RH and retinoid-receptor pathways converge in the regulation of cardiac myocyte contractility; Specific Aim 3 will analyze the signaling downstream of 20RH. Finally, Specific Aim 4 will establish whether down-regulation of 20RH can be used as an exportable therapeutic system in diverse models of myocyte dysfunction. Completion of this proposal will establish the molecular basis for a novel link between nuclear receptors and GPCR signaling in cardiac failure and demonstrate different levels of regulation among these two pathways (cross-regulation and downstream convergence). In addition, since mutation of 20RH does not display a basal phenotype, systems that reduce the activity of 20RH may be an excellent novel therapeutic target. Beyond the scope of this proposal, is the identification of endogenous ligands and chemical agonists/antagonists for 20RH