ROLE OF XHOX HOMEOBOX GENES IN XENOPUS DEVELOPMENT

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A recent focus of development biology has been to isolate and study genes through to make critical decisions of cell fate and patterning in embryogenesis. Genetic studies in Drosophila indicate that genes containing the homeobox DNA-binding motif participate in such decisions.Other genes such as the vertebrate Myo-D gene, also have the compelling properties of developmental regulators. We propose to study regional specification and pattern formation in Xenopus embryos using as a focus the Xhox-1 homeobox genes and the Myo-D gene. In previous work we devised an overexpression assay to address the function of the Xhox-1A gene and demonstrated that it is likely to be involved in somite formation. Somitogenesis is a primary segmentation event which shapes the vertebrate body play. This finding is significant because it is the very first indication of a function for a vertebrate homeobox gene and it fulfills the broad expectation that homeobox genes will participate in critical developmental events. We propose to study further the role of Xhox-1A and a linked gene. Xhox-1B, in somite formation. We will use antibodies to investigate at cellular resolution the normal expression pattern of these genes. Furthermore, we will design dominant-acting mutants of the Xhox-1 genes which will be used to antagonize the action of the endogenous genes and thus perturb somitogenesis in new ways. We will also approach homeobox gene function at the biochemical level by studying the binding of Xhox-1A protein to DNA. We believe that in the long term this biochemical approach will reveal how stable developmental changes are affected. In Xenopus, most of the somite tissue becomes muscle. Nothing is known about how the patterning of muscle tissue (somitogenesis) meshes with commitment to the muscle lineage. Evidence suggests that the Myo-D gene is instructive in this commitment step. We will characterize the expression of the Xenopus Myo-D during early development and attempt to devise functional assays to test whether Myo-D gene expression can commit multipotent, embryonic ectoderm directly into the muscle lineage. These experiments are designed to investigate specific examples of cell commitment and patterning in vertebrate development. In addition, we will take a more general approach to the fundamental problem of how regional specification occurs in embryos. We will screen a pre-gastrulation cDNA library with probes for developmentally interesting genes such as other homeobox genes, believing that many of them will show restricted spatial expression and will indicate zones of cell commitment at early stages. The strength of this proposal lies in the fact that we already have a functional assay for the homeobox genes that we study and in the fact that we can relate these studies to the wealth of cellular and developmental information available for Xenopus embryos.