Defining the mechanisms whereby MEK1/2 and ERK1/2 signaling control cardiomyocyte directional growth

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Project Summary/Abstract Pathological growth of the heart can be separated into two categories: hypertrophic cardiomyopathy, whereby the ventricles undergo thickening and cardiomyocytes increase in width, and dilated cardiomyopathy, in which the ventricular walls become thinner and cardiomyocytes lengthen. New avenues of research to broaden our understanding of cardiac growth mechanisms are critical to treating the many patients worldwide suffering from these cardiovascular diseases. The experiments outlined in this proposal seek to increase the knowledge of such mechanisms by determining how cardiomyocytes grow preferentially in length or width. MEK1/2 and ERK1/2 have long been known to regulate cell growth and proliferation in many cell types. However, adult cardiomyocytes have very low proliferative capacities; therefore, the MEK1/2-ERK1/2 signaling pathway may be specialized in these cells to control growth. Indeed the sponsor's lab has previously found that these kinases regulate the switch between cardiomyocyte growth in different directions. When signaling through this pathway is increased, cardiomyocytes increase in width, adding sarcomeres in parallel. However, the inhibition of MEK1/2-ERK1/2 causes significant lengthening of cardiomyocytes and addition of sarcomeres in series. Canonical MEK1/2-ERK1/2 activation causes translocation of most ERK1/2 proteins to the nucleus to elicit cell growth; however, lengthening growth still occurs in cardiomyocytes when ERK1/2 do not translocate to the nucleus and remain in the cytoplasm. Therefore there are likely specific actions of MEK1/2 and ERK1/2 in the cytoplasm that regulate directional growth of cardiomyocytes, namely interactions with or regulation of proteins in the cytoplasm. This proposal seeks to understand the molecular underpinnings of these kinases' actions on directional growth by first determining the spatiotemporal changes in MEK1/2 and ERK1/2 during hypertrophic stimuli. The roles that these proteins' subcellular localizations play in directing cardiomyocyte lengthening or widening will also be assessed. Proteins which directly bind MEK1/2 and ERK1/2 will be identified through yeast two-hybrid and in vivo proteomics assays. These studies are underway and have already led to the identification of a novel interaction between ERK2 and the Z-disk protein nebulette. Finally a phosphoproteomic screen was conducted to determine proteins that may not necessarily bind MEK1/2 and ERK1/2 but may be downstream effectors. The results showed an array of targets, many of which are cytoskeletal or adaptor proteins, which displayed changes in phosphorylation upon the induction or inhibition of MEK1/2-ERK1/2 signaling. That so many cytoskeletal proteins were affected by changes in this signaling pathway supports the hypothesis that these kinases have specific actions in the cytoplasm to modulate directional growth. Overall, these studies will greatly expand our knowledge of the mechanisms that control cardiomyocyte growth in different directions, and thereby may provide us with new therapeutic targets to manipulate in conditions of pathological cardiac growth.