In order to investigate the function of nonmuscle myosin II-A, we created a nonmuscle myosin heavy chain II-A (NMHC II-A) null mouse by inserting the neomycin resistance cassette into the gene at exon 3. Previously, we have shown that ablation of nonmuscle myosin heavy chain II-B in mice results in abnormalities in the development of the heart and brain with the lethality occurring between E12 and the day of birth. The other organs of the B-/- mice appear normal, suggesting that NMHC II-A could substitute for II-B in these organs. In contrast to the NMHC II-B knockout, ablation of the NMHC II-A resulted in lethality at E7.5 or earlier, suggesting a role for NMHC II-A during gastrulation. Immunocytochemistry of wild type mice at E7.5 showed the presence of NMHC II-A in all cell layers. At this stage, however, NMHC II-B was absent from the visceral endoderm. NMHCII-A null mice are lacking or have a severely underdeveloped visceral endoderm suggesting that the absence of both isoforms of NMHC II contributed to the defect.Embryonic stem (ES) cells express both NMHC II-A and II-B. NMHC II-A-/- ES cells were generated by re-electroporation of NMHC II-A+/- cells followed by selection at increased concentrations of G418. NMHC II-A-/- ES cells which only express NMHC II-B appear to proliferate at a faster rate than NMHC II-A+/- or wild type cells and do not form embryonic stem cell colonies.
