Developmental Gene Expression In C elegans

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

One of our main focus areas over the years is muscle development. In mice, striated muscle development requires the function of the MRFs, a set of four closely related factors (MyoD, Myf-5, MRF-4, Myogenin) that regulated muscle specification and differentiation. C. elegans has a single MRF-related factor called CeMyoD (HLH-1). One of the puzzling aspects of C. elegans myogenesis was the observation that muscles could be formed in the absence of CeMyoD, as demonstrated in mutant animals. To further define the exact role of this transcription factor, we ectopically expressed HLH-1/MyoD in the early nematode embryo and found that it was sufficient to convert all early blastomeres to a muscle-like fate. The sufficiency of HLH-1/MyoD alone to direct cells into the muscle lineage illustrated its potency and revealed a level of evolutionary conservation in function that had not previously been appreciated. Moreover, these experiments revealed a remarkable degree of plasticity of the early embryonic cells to be reprogrammed with regards to cell fate choice. Using genetic deletion alleles of three genes, we showed that muscle could form when at least one of these factors was present. However, elimination of all three gene products blocked muscle differentiation in the embryo. These results defined this trio of transcription factors as the key regulators of C. elegans muscle, explaining why muscle could form in the absence of HLH-1/MyoD. This past year, we have focused on the factors regulating anterior mesoderm specification and bodywall muscle development. Captializing on the defined cell lineage of C. elegans, we have defined a hierarchy of transcription factors involved regulating muscle development at the single cell level. This has allowed us to tease apart multiple pathways all leading to the eventual activation of HLH-1/MyoD and UNC-120/SRF, the common set of drivers for bodywall muscle fate specification and differentiation. Our results also revealed an unexpected evolutionary conservation of pathway components with anterior muscle development in vertebrates. In on-going collaboration with Dr. Eisenmann (Univ of MD, Baltimore County) we have explored gene expression regulated by Wnt signaling. The Wnt signaling pathway plays a fundamental role during metazoan development, where it regulates diverse processes, including cell fate specification, cell migration, and stem cell renewal. Activation of the beta-catenin-dependent/canonical Wnt pathway up-regulates expression of Wnt target genes to mediate a cellular response. In the nematode Caenorhabditis elegans, a canonical Wnt signaling pathway regulates several processes during larval development; however, few target genes of this pathway have been identified. To address this deficit, we used a novel approach of conditionally activated Wnt signaling during a defined stage of larval life by overexpressing an activated beta-catenin protein, then used microarray analysis to identify genes showing altered expression compared with control animals. We identified 166 differentially expressed genes, of which 104 were up-regulated. A subset of the up-regulated genes was shown to have altered expression in mutants with decreased or increased Wnt signaling; we consider these genes to be bona fide C. elegans Wnt pathway targets. In a collaboration with Dr. Liu (Cornell University) we capitalized on the genetics of C. elegans to identify a role of Bone Morphogentic Protein (BMP) in signaling developmental events in the postembryonic mesoderm. This work further revealed a role for membrane tetraspanins to modulate BMP signaling, providing a possible mechanism underlying their previously defined role in cancer development.