Integrins, heterodimeric transmembrane receptors linking extracellular matrix molecules with cytoskeletal proteins and signaling complexes, are essential in normal cardiac development, cardiomyocyte differentiation, and maintenance of sarcomere organization. Induced mutations in mice demonstrate the importance of integrins and their ligands in maintenance of cardiac muscle function. Integrins not only mediate cell adhesion, but also serve as stretch receptors and co-receptors in growth factor stimulated pathways. We hypothesize that integrins function as important modulators in the heart, sensing and responding to mechanical and endocrine stimuli to maintain homeostasis. We have used in vitro models and in vivo expression to begin to this hypotheses. In vitro studies demonstrate that integrin ligation to extracellular matrix and integrin signaling is required for cardiomyocytes to respond to hypertrophic agonists acting via G-protein coupled receptors. Complementary in vivo studies demonstrate that transgenic expression of a gain of function mutation in the integrin a5 subunit, or expressing a chimeric protein interfering with integrin function results in a severe, perinatal lethal phenotype in transgenic mice The proposed studies will utilize an integrated approach to further dissect the role of integrins in the structure and function of cardiac myocytes. The role of proximal mediators of integrin signaling in the hypertrophic response of cardiac myocytes will be evaluated with an in vitro model system of cultured myocytes. The importance of candidate integrin effectors identified in the in vitro system will be validated or refuted by overexpressing these candidate genes in transgenic animals. A refined conditional transgene expression system will determine the effect of transgene expression in the adult heart. Cardiac phenotypes resulting from transgene expression will be correlated with morphological and molecular alterations observed in myocytes in vitro. Finally, the physiological correlates of transgene expression will be examined by determining the effect of transgene expression on cardiac function under basal conditions and after hemodynamic loading. These studies will provide new insights into myocyte signaling pathways and approaches applicable to pathologic alterations in the human myocardium.
