CYTOSKELETON AND CONTRACTILE DYSFUNCTION IN HYPERTROPHY

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The initial goal of this project was to determine whether cytoskeletal alterations have any role in the contractile dysfunction of hypertrophied myocardium. Important accomplishments have been 1) the demonstration of microtubule-based contractile dysfunction in cardiocytes from the hypertrophied and failing RV and LV, 2) extension of these findings to isolated tissue and to the intact heart, 3) biophysical characterization of the etiology of microtubule-based contractile dysfunction, 4) the finding that this mechanism is tightly restricted to pressure overload hypertrophy in which wall stress is persistently increased, 5) the demonstration that microtubules are the only major extra-myofilament cytoskeletal protein so affected, 6) the finding that this phenomenon is based both on increased tubulin, and thus microtubules, and on increased stability of the microtubules once formed, 7) the finding that MAP4, the major cardiac microtubule-stabilizing protein, is markedly upregulated in pressure overload hypertrophy, and 8) the finding that transcriptional upregulation of two minor beta -tubulin isoforms during hypertrophy, which we found to mimic the developmental regulation of these genes, accounts for the increase in beta-tubulin. The first goal for the work proposed is to use transgenic mice having cardiac-restricted expression of mutant beta-tubulins that cause reduced or increased microtubule stability in order to directly test the hypothesis that enhanced microtubule network stability and thus density cause contractile dysfunction when these alterations of cardiocyte microtubules occur in hypertrophied myocardium. This first goal will focus directly on microtubule stability per se, absent the many attendant complexities of the hypertrophic cardiac environment. The second goal is to seek in surgical models of pressure overload hypertrophy, in transgenic mice having cardiac-restricted expression of MAP4, and in isolated cardiocytes with adenovirus-mediated expression of MAP4 the basis for increased microtubule network stability and density in terms of increased microtubule affinity of MAP4, the major microtubule-associated protein of the heart. Since we have found increased MAP4 expression in hypertrophy, and we have just discovered that this MAP4 is dephosphorylated, thus increasing MAP4 -microtubule affinity, this second goal will focus on the mechanisms of hypertrophic MAP4 dephosphorylation.