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Novel molecular mechanisms regulating Shh expression and limb patterning

Normal patterning of the developing limb requires tight restriction of Sonic hedgehog (Shh) mRNA to the posterior margin of the limb. While several positive and negative regulatory factors have been shown to control Shh expression, these factors do not adequately explain the exquisite domain of Shh. Using several approaches, we provide novel insight into the molecular mechanisms which regulate Shh expression. First, we identify the dorsal‐ventral ectoderm border proximal to the AER as a new signaling center in limb development that serves to regulate the restricted expression of Tbx2 and Shh. In separate experiments, we have analyzed the function of ETV4 and ETV5, two members of the ETS family of transcriptional factors, which we show are expressed in the distal mesenchyme of developing limb bud in a FGF dependent manner. Surprisingly, repressing ETV4/5 transcriptional activity in the limb mesenchyme resulted in polydactyly and ectopic induction of Shh expression in the anterior distal limb bud, indicating ETV4/5 normally function to suppress Shh expression in the anterior mesenchyme. Further, we demonstrate that ETV4/5 positively contribute to FGF‐controlled limb growth. Taken together, our work expands the current understanding of molecular mechanisms regulating Shh expression and pattern formation in the limb.