Dissecting the role of the cardiac fibroblast in hypertrophy.

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Abstract The cardiac myocyte has long been the primary focus of studies attempting to elucidate the regulatory aspects underlying cardiac development and disease. However, recently the involvement of nonmyocytes has emerged as potentially just as important as myocytes in contributing to and controlling cardiac remodeling during progressive pathogenesis associated with heart failure. More specifically, the cardiac fibroblast and its ability to convert to myofibroblasts in promoting extracellular matrix (ECM) production, ventricular remodeling and the fibrotic response is now viewed as an equally critical regulator of cardiac biology. Here we will address how fibroblasts in the heart function as key determinants of disease and pathologic remodeling. We have developed important genetic tools that specifically target the fibroblast in the heart so that we can manipulate the activity of these cells. Thus, here we can now test the novel hypothesis that activated fibroblasts (myofibroblasts) are critical regulators of cardiac disease processes, not only involving fibrosis but also the ability of cardiomyocytes in the heart to properly hypertrophy. We will test the more specific hypothesis that fibroblasts regulate the density and integrity of the cardiac ECM and collagen that cardiomyocytes must sense as increased structural support in order to effectively hypertrophy in vivo. Indeed, we further hypothesize that the tension sensing mechanism within the cardiomyocyte extends outward to the ECM and its integrity or stiffness. This application has 3 specific aims: 1) To increase the structural rigor of collagen in the heart to investigate the impact on cardiomyocyte hypertrophic growth potential in vivo, 2) To genetically impair type I collagen formation in the heart to reduce ECM structural rigor, and 3) To examine the actin filamentous network as a central signaling mechanism whereby ECM integrity or stiffness impacts myocyte growth in vivo. Collectively, these specific aims will uncover how fibroblasts communicate with cardiomyocytes in the heart through the ECM and its properties. Such an understanding will lay the foundation for future studies into specific therapeutic targets in treating longstanding fibrotic heart disease states or hypertrophy in general.