During heart development, cells from the proepicardial organ spread over the naked heart tube to form the epicardium. From here, epicardium-derived cells (EPDCs) migrate into the myocardium. EPDCs proved to be indispensable for the formation of the ventricular compact zone and myocardial maturation, by largely unknown mechanisms. In this study we investigated in vitro how EPDCs affect cardiomyocyte proliferation, cellular alignment and contraction, as well as the expression and cellular distribution of proteins involved in myocardial maturation. Embryonic quail EPDCs induced proliferation of neonatal mouse cardiomyocytes. This required cell-cell interactions, as proliferation was not observed in transwell cocultures. Western blot analysis showed elevated levels of electrical and mechanical junctions (connexin43, N-cadherin), sarcomeric proteins (Troponin-I, alpha-actinin), extracellular matrix (collagen I and periostin) in cocultures of EPDCs and cardiomyocytes. Immunohistochemistry indicated more membrane-bound expression of Cx43, N-cadherin, the mechanotransduction molecule focal adhesion kinase, and higher expression of the sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a). Newly developed software for analysis of directionality in immunofluorescent stainings showed a quantitatively determined enhanced cellular alignment of cardiomyocytes. This was functionally related to increased contraction. The in vitro effects of EPDCs on cardiomyocytes were confirmed in three reciprocal in vivo models for EPDC-depletion (chicken and mice) in which downregulation of myocardial N-cadherin, Cx43, and FAK were observed. In conclusion, direct interaction of EPDCs with cardiomyocytes induced proliferation, correct mechanical and electrical coupling of cardiomyocytes, ECM-deposition and concurrent establishment of cellular array. These findings implicate that EPDCs are ideal candidates as adjuvant cells for cardiomyocyte integration during cardiac (stem) cell therapy.
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