The cis-trans regulatory circuit mediating RTK/Ras signaling in visceral muscle founder cell specification
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Cell fate specification is a fundamental approach for a cell to acquire unique identity from a group of undifferentiated cells during metazoan development, and it requires fine transcriptional regulation of target genes that are expressed within the specified cell type. Receptor Tyrosine Kinase (RTK) signaling plays a crucial role in specifying a wide range of cell types yet use a relatively redundant and small set of signaling components, leading to the importance of understanding RTK signaling specificity at the transcriptional level. A fine example is RTK signaling in Drosophila muscle development. The RTK/Ras/MAP Kinase signaling pathway is required to specify the muscle founder cells in all major muscle types. These founder cells (FCs) then fuse with fusion-competent myoblasts in order to form multinucleate muscle fibers. Compared to the well-established transcriptional network in somatic and cardiac founder cell specification, relatively little is known at the transcriptional level in visceral founder cell specification. Although the well-established Ras effector Pnt is typically assumed to be the primary transcription factor acting in FC specification, this present work has established that FC specification in the visceral mesoderm is mediated by a Ras-dependent but Pnt-independent pathway. Analysis of an FC-specific transcriptional enhancer for the mib2 gene has identified critical transcription factor binding sites required for Ras-dependent FC specification. Mutagenesis of these sites leads to an expansion of mib2 FC enhancer activity throughout the trunk visceral mesoderm, suggesting a possible derepression model for RTK-based induction of FC fates. High-throughput yeast one-hybrid screening has yielded the ETS factor Ets21C that binds to these wild-type sites but not the mutated sites on the mib2 FC enhancer. Visceral founder cell fate expands in embryos with Ets21C knockdown. These results suggest that the founder cell fate is induced by an Ets21C-mediated Ras/MAPK signaling pathway downstream of RTK in the visceral mesoderm possibly through de-repression. Taken together, this present work provides the molecular basis of the cis-trans -regulatory network mediating Pnt-independent Ras/MAPK signaling pathway downstream of RTK activation in visceral founder cell specification, and provides insight into further understanding the mechanisms underlying Ras/MAPK signaling specificity in the context of cell fate specification.