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.