Differential expression of non-muscle myosin heavy chain genes during Xenopus embryogenesis

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 r eport here the isolation of the Xenopus homolog of NMHC-A and present a com parative analysis of the developmental and spatial expression patterns of N MHC-A and the previously isolated NMHC-B to address the role of NMHCs in Xe nopus development. A 7.5 kb NMHC-A mRNA is present, maternally in unfertili zed eggs and throughout embryogenesis, as well as in all adult tissues exam ined. An additional 8.3 kb zygotic transcript for NMHC-A is also detected, but only during embryonic stages. Whole mount in situ hybridization with ta ilbud stage embryos shows that NMHC-A mRNA is predominantly expressed in th e epidermis, whereas NMHC-B mRNA is expressed in the somites, brain, eyes a nd branchial arches, Interestingly, the expression of NMHC-B in developing somites is gradually restricted to the center of each somite as differentia tion proceeds. DAPI nuclear staining demonstrated that NMHC-B mRNA is coloc alized with the nuclei or perinuclear area. In animal cap experiments, trea tment 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 sligh tly down-regulates NMHC-A mRNA, consistent with NMHC-B expression in the se mitic muscle and NMHC-A expression in the epidermis. (C) 1998 Elsevier Scie nce Ireland Ltd. All rights reserved.