Twist protein in mouse embryogenesis

The genes in the twist family of bHLH transcription factors are essential for embryogenesis of both vertebrates and invertebrates, as demonstrated by the embryonic lethal phenotypes of the Drosophila and mouse twist-null mutants. Presented here is the embryonic distribution of murine Twist protein (MTwist). Its appearance and presence in the course of early embryogenesis was followed with a monoclonal antibody, αTwiMab-1, the first antibody generated against any of the vertebrate Twist proteins. The specificity for MTwist was demonstrated in comparative Western blot experiments, reticulocyte lysate assays, and immunohistochemistry of embryonic mouse samples. Consistent with its probable role as a transcription factor, MTwist localized to the nuclei. MTwist signal first appeared in 8- to 10-somite embryos at 8.25 dpc in the cranial neural crest cells and branchial arches, in the limb-bud mesenchyme and the somatic lateral plate, and in the sclerotome and the dermatome of the somites. The presence of MTwist protein in these tissues corroborates the reported MTwist RNA distribution and the phenotype of the MTwist-null mutants. There also emerged, however, an unexpected difference between MTwist RNA and protein expression. No protein could be detected prior to 8.25 dpc, despite the reported high levels of transcripts as early as 7.0 dpc. Also, the presomitic mesoderm, epithelial somites, and anterior mesoderm expressed abundant MTwist RNA, but no protein. The results suggest posttranscriptional downregulation of MTwist in these regions. A proposed role of MTwist in somite formation and maturation is inhibition of myogenic bHLH and MEF2 genes and thus prevention of premature and/or ectopic differentiation in the presomitic mesoderm and epithelial somites. The absence of MTwist protein from these areas indicates that its role in somitogenesis must be reevaluated.