Subcellular compartmentalization of myosin isoforms in embryonic chick heart ventricle myocytes during cytokinesis

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Abstract
Embryonic chick heart ventricle myocytes retain the ability to alternate between proliferation and functional differentiation. A cytoplasmic isoform of myosin is present in cleavage furrows of various nonmuscle cells during cytokinesis, whereas one or more of the cardiac myosin isoforms are localized in sarcomeres of beating cardiomyocytes. Antibodies were employed to reveal the subcellular localizations of cytoplasmic and cardiac myosin isoforms in embryonic chick ventricle cardiomyocytes during cytokinesis. Monoclonal anticytoplasmic myosin antibodies were prepared against myosin purified from brains of 1-day posthatched chickens and shown to react with chick brain myosin heavy chain by Western blots and/or ELISA tests. One monoclonal antibrain myosin antibody also cross-reacted with chick cardiac myosin but not with skeletal or smooth muscle myosins. Two antichick cardiac myosin monoclonal antibodies and one antichick skeletal myosin polyclonal antibody that cross-reacts with cardiac myosin were employed to identify cardiac sarcomeric myosin. Cells were isolated from day 8 embryonic chick heart ventricles, enriched for myocytes, grown in vitro for 3 days, and then examined by immunofluorescence techniques. Monoclonal antibodies against cytoplasmic myosin preferentially localized in the cleavage furrows of both cardiofibroblasts and cardiomyocytes in all stages of cytokinesis. In contrast, antibodies that recognize cardiac myosin were distributed throughout cardiomyocytes during early stages of cytokinesis, but became progressively excluded from the furrow area during middle and late stages of cytokinesis. These data suggest that in cells that contain both cytoplasmic and sarcomeric myosin isoforms, only cytoplasmic myosin isoforms are mobilized to form the contractile ring for cytokinesis.