| Transcription factors play a key role in the regulation of many critical processes during embryonic development. Members of the homeodomain family of transcription factors have been implicated in the control of a wide variety of events during embryogenesis, including the patterning of the anterior/posterior axis and organogenesis. Here I describe the isolation and characterization of several developmentally regulated homeobox genes in the frog Xenopus laevis. The XHex gene is expressed transiently in endothelial cells of the developing vascular system immediately prior to differentiation. The over-expression of wild type XHex in the embryo results in an increased number of endothelial precursor cells, while the expression of a dominant negative version of XHex inhibits vascular development. These data suggest a key role for the XHex gene in vasculogenesis. Expression of the XNkx2-10 gene is initiated in the cells of the cardiac anlage at the early neurula stage. Transcripts are maintained in this tissue until the onset of terminal differentiation at which time XNkx2-10 is rapidly down-regulated. Like other Nkx proteins, XNkx2-10 is able to activate transcription from cardiac gene promoters in cultured cells and is rapidly down-regulated in the embryo upon exposure to retinoic acid. However, in contrast to other tinman-related proteins in Xenopus, over-expression of XNkx2-10 does not result in an increase in the size of the embryonic heart. Thus, XNkx2-10 possesses a unique set of biological properties relative to other XNkx family members. The Drosophila bagpipe homeobox gene is represented in Xenopus by at least three related genes, Xbagpipe, zampogna and koza. Like the Drosophila gene, both Xbagpipe and zampogna are expressed in the muscular layer of the midgut. In contrast, koza exhibits a more divergent pattern of expression, initially expressed broadly throughout the myotome. Later in development, koza transcripts become restricted to the slow twitch muscle cells of the lateral somite. Over-expression of koza in the early embryo results in a decreased number of cells contributing to the paraxial mesoderm, suggesting a role for koza in the regulation of cellular proliferation. |
