The aim of this proposal is to understand the function of homeobox-containing genes in the differentiation and morphogenesis of the vertebrate endoderm. The endoderm gives rise to a major organ system with an ordered anteroposterior progression of regionalized parts: i.e. pharynx, esophagus, lung, stomach, liver, duodenum, pancreas, and intestine. Although there is good evidence for homeobox genes being involved in patterning the vertebrate mesoderm and neuroectoderm, virtually nothing is yet known regarding the genes controlling endodermal specification. We will test the hypothesis that new members of the homeobox gene family control critical early steps in the differentiation of the vertebrate endoderm. The basis for this study is the identification of a novel frog homeobox gene, XlHbox 8 (Wright et al. 1988). Immunolocalizations show that XlHbox 8 is expressed in a band of the early embryonic endoderm that gives rise to the pancreatic/duodenal region of the gut (Wright et a]. 1988). This distribution is in sharp contrast to that of all other homeobox genes so far studied, which are expressed in both neuroectoderm and mesoderm. We will characterize the XlHbox 8 gene and analyze its expression pattern in detail at the very earliest stages of endodermal development. Tissue recombination experiments will be carded out to analyze the mesoderm/endoderm interactions required for XlHbox 8 activation. In vitro cultures of explanted presumptive endoderm will be used to identify the endoderm-inducing factors produced by mesodermal cells. We will also directly test the function of XlHbox 8 in endodermal specification, using two strategies: a) loss-of-function experiments involving the microinjection of XlHbox 8-specific antibodies, and b) dominant gain-of-function experiments based on injecting synthetic MRNA encoding XlHbox 8. An endoderm-specific Xenopus CDNA library will be made, and screened for other homeobox genes that may control anteroposterior regionalization of endoderm. Products of new genes will be localized using antibodies against gene-specific fusion proteins. We will clone the mouse homolog of XlHbox 8, laying the foundation for genetic experiments based on transgenic animals and null mutations. Such approaches UGH help elucidate the biological function in embryogenesis of homeodomain-containing transcription factors expressed in specific regions of the endoderm, and how they become activated over these precise areas.
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