Nonmuscle myosin IIB regulates epicardial integrity and epicardium-derived mesenchymal cell maturation

Nonmuscle myosin IIB (NMIIB; heavy chain encoded by MYH10) is essential for cardiac myocyte cytokinesis. The role of NMIIB in other cardiac cells is not known. Here, we show that NMIIB is required in epicardial formation and functions to support myocardial proliferation and coronary vessel development. Ablation of NMIIB in epicardial cells results in disruption of epicardial integrity with a loss of E-cadherin at cell-cell junctions and a focal detachment of epicardial cells from the myocardium. NMIIB-knockout and blebbistatin-treated epicardial explants demonstrate impaired mesenchymal cell maturation during epicardial epithelial-mesenchymal transition. This is manifested by an impaired invasion of collagen gels by the epicardium-derived mesenchymal cells and the reorganization of the cytoskeletal structure. Although there is a marked decrease in the expression of mesenchymal genes, there is no change in Snail (also known as Snai1) or E-cadherin expression. Studies from epicardium-specific NMIIB-knockout mice confirm the importance of NMIIB for epicardial integrity and epicardial functions in promoting cardiac myocyte proliferation and coronary vessel formation during heart development. Our findings provide a novel mechanism linking epicardial formation and epicardial function to the activity of the cytoplasmic motor protein NMIIB. Total RNA is prepared from 8 explants each with or without treatmnet with 20mM blebbistatin.

非肌肌球蛋白IIB（Nonmuscle myosin IIB，NMIIB）的重链由MYH10基因编码，其在心肌细胞胞质分裂过程中发挥不可或缺的作用。目前学界对NMIIB在其他心脏细胞中的功能尚不清楚。本研究证实，NMIIB参与心外膜形成，并可支持心肌细胞增殖与冠状动脉血管发育。在心外膜细胞中敲除NMIIB，会破坏心外膜完整性，导致细胞间连接处的E-钙粘蛋白（E-cadherin）缺失，并引发心外膜细胞局灶性脱离心肌层。经NMIIB基因敲除或20mM布莱比司他汀（blebbistatin）处理的心外膜外植体实验显示，心外膜上皮-间质转化过程中的间充质细胞成熟过程受损。该损伤表现为心外膜来源的间充质细胞侵袭胶原凝胶的能力受损，以及细胞骨架结构重排异常。尽管间充质相关基因的表达显著下调，但Snail转录因子（亦称为Snai1）与E-钙粘蛋白的表达并未发生明显改变。心外膜特异性NMIIB基因敲除小鼠的研究结果证实，在心脏发育过程中，NMIIB对于维持心外膜完整性，以及心外膜促进心肌细胞增殖、冠状动脉血管生成的功能至关重要。本研究结果揭示了一种全新的机制，将心外膜形成与心外膜功能和胞质动力蛋白NMIIB的活性关联起来。实验材料为各8例经20mM布莱比司他汀（blebbistatin）处理或未处理的心外膜外植体，从中提取总RNA。