The role of Fgf and Wnt5a signaling in directed outgrowth of the limb mesenchyme

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DESCRIPTION (provided by applicant): Limb anomalies represent some of the most common birth defects occurring in approximately 0.7 to 1 per 1,000 human births. Therefore, understanding the mechanisms responsible for the proper outgrowth and patterning of limbs are of great interest to the medical community. The long-term objective of this application is to understand the cellular and molecular mechanisms that drive outgrowth and morphogenesis of the limb. This application specifically examines the role of fibroblast growth factors (FGFs) and Wnt5a in regulating directed migration and oriented cell division of limb mesenchyme cells as mechanisms that underlie limb outgrowth. For some time it has been known that a small ridge of ectodermal tissue that exists along the distal margin of the vertebrate embryonic limb bud is required for outgrowth of the adjacent limb mesenchyme. More recently, it has been demonstrated that members of the secreted fibroblast growth factor (FGF) family are expressed in this tissue (known as the apical ectodermal ridge or AER) and are both necessary and sufficient for outgrowth of the limb mesenchyme. While the mechanisms whereby FGFs mediate outgrowth are unknown, recent data suggest that they do so by acting as a chemoattractant to the limb mesenchyme rather than as a mitogenic or cell survival factor. How does FGF signaling mediate this chemoattraction? The investigators have collected preliminary data suggesting that AER signals activate the expression of the secreted Wnt ligand, Wnt5a, in the limb mesenchyme. In vertebrate and invertebrate models, Wnt5a has previously been demonstrated to signal through the Wnt/planar cell polarity (PCP) and/or Ror signaling pathways, both of which have in turn been shown to regulate directional cell movements and cell divisions in vivo and in vitro. The investigators hypothesize that Fgf signaling from the AER establishes a gradient of Wnt5a expression in the limb mesenchyme which then establishes cellular polarity in these cells and directs proximal to distal cell movements and/or cell divisions that result in limb outgrowth. In this application, they will test the hypotheses that Fgf and Wnt5a/Ror signaling in the distal limb are both necessary and sufficient for directed migration and oriented cell divisions of limb mesenchyme cells. PUBLIC HEALTH RELEVANCE: Limb anomalies are some of the most common birth defects in humans. The research in this application is designed to understand the mechanisms underlying normal limb development. It is anticipated that the basic mechanisms elucidated here will have important implications for future research and potential therapies.