Class II non-muscle myosins are implicated in diverse biological processes such as cytokinesis, cellularization, cell shape changes and gastrulation. Two distinct non-muscle myosin heavy chain genes have been reported in all vertebrates: non-muscle myosin heavy chain-A (NMHC-A) and -B (NMHC-B). We report here the isolation of the Xenopus homolog of NMHC-A and present a comparative analysis of the developmental and spatial expression patterns of NMHC-A and the previously isolated NMHC-B to address the role of NMHCs in Xenopus development. A 7.5 kb NMHC-A mRNA is present, maternally in unfertilized eggs and throughout embryogenesis, as well as in all adult tissues examined. An additional 8.3 kb zygotic transcript for NMHC-A is also detected, but only during embryonic stages. Whole mount in situ hybridization with tailbud stage embryos shows that NMHC-A mRNA is predominantly expressed in the epidermis, whereas NMHC-B mRNA is expressed in the somites, brain, eyes and branchial arches. Interestingly, the expression of NMHC-B in developing somites is gradually restricted to the center of each somite as differentiation proceeds. DAPI nuclear staining demonstrated that NMHC-B mRNA is colocalized with the nuclei or perinuclear area. In animal cap experiments, treatment with activin A or ectopic expression of Xbra and an activated form of Xlim1 markedly up-regulates NMHC-B as well as muscle actin mRNAs and slightly down-regulates NMHC-A mRNA, consistent with NMHC-B expression in the somitic muscle and NMHC-A expression in the epidermis.
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