Ma Xuefei, Adelstein Robert S
From the Laboratory of Molecular Cardiology, Genetics & Developmental Biology Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD.
Circ Cardiovasc Genet. 2014 Jun;7(3):257-65. doi: 10.1161/CIRCGENETICS.113.000455. Epub 2014 May 13.
The 3 isoforms of nonmuscle myosin (NM) II (NMII-A, NMII-B, and NMII-C) play various roles during mouse embryonic development. Previous work, using knockout and hypomorphic mice, showed that Myh10 encoding myosin heavy chain II-B is critical for cardiac and brain development. Ablating or decreasing NMII-B by 80% results in cardiac (ventricular septal defect, double outlet of the right ventricle) and brain defects but not midline fusion defects. Neither NMII-A nor II-C seems to play roles in early myocardial development.
We had previously generated point mutant knock-in mice and now report novel findings as a result of expressing motor-deficient NMII-B at wild-type levels. Homozygous mice die at embryonic day 14.5 in cardiac failure, exhibiting abnormalities not seen in NMII-B null and hypomorphic mice: a failure in midline fusion resulting in a cleft palate, ectopia cordis, and a large omphalocele. Fusion of the sternum and endocardial cushions is impaired in the mutant mice associated with a failure in apoptosis of the mesenchymal cells. Failure to disassemble myocyte cell-cell adhesions during cardiac outflow tract development contributes to impaired outflow tract myocardialization and displacement of the aorta to the right ventricle.
Expression of motor-impaired NMII-B disrupts normal ventral body wall closure because of a dominant-negative effect. This is not because of the loss of NMII-B function but rather a gain-of-function resulting from prolonged cross-linking of NMII-B to actin filaments, thereby interfering with the dynamics of actomyosin cytoskeletal structure. Furthermore, impaired NMII-B motor activity inhibits outflow tract myocardialization, leading to mislocalization of the aorta.
非肌肉肌球蛋白（NM）II的三种同工型（NMII-A、NMII-B和NMII-C）在小鼠胚胎发育过程中发挥着多种作用。以往利用基因敲除和基因低表达小鼠进行的研究表明，编码肌球蛋白重链II-B的Myh10对心脏和大脑发育至关重要。去除或使NMII-B减少80%会导致心脏（室间隔缺损、右心室双出口）和大脑缺陷，但不会导致中线融合缺陷。NMII-A和II-C似乎都未在早期心肌发育中发挥作用。
我们之前构建了点突变敲入小鼠，现在报告在野生型水平表达运动缺陷型NMII-B所得到的新发现。纯合子小鼠在胚胎第14.5天死于心力衰竭，表现出在NMII-B基因敲除和基因低表达小鼠中未出现的异常：中线融合失败导致腭裂、心脏异位和巨大脐膨出。突变小鼠的胸骨和心内膜垫融合受损，这与间充质细胞凋亡失败有关。在心脏流出道发育过程中未能分解心肌细胞间的黏附会导致流出道心肌化受损以及主动脉向右心室移位。
运动受损的NMII-B的表达由于显性负效应破坏了正常的腹侧体壁闭合。这不是因为NMII-B功能丧失，而是由于NMII-B与肌动蛋白丝的长时间交联导致的功能获得，从而干扰了肌动球蛋白细胞骨架结构的动力学。此外，受损的NMII-B运动活性抑制流出道心肌化，导致主动脉定位错误。
