The role of cell polarity in vertebrate limb outgrowth

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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 investigator's long term objective 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 Wnt/planar cell polarity (PCP) signaling in establishing polarity of the actin cytoskeleton and mitotic spindle 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 by which 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 investigator has collected preliminary data suggesting that AER signals activate the expression of the secreted Wnt ligand, Wnt5a in the limb mesenchyme. In fish and amphibian models, Wnt5a has previously been demonstrated to signal through the Wnt/planar cell polarity (PCP) pathway. Wnt/PCP signaling plays a critical role in establishing cellular polarity which underlies directional cell movements and cell divisions during gastrulation, essential for elongating the embryonic axis. He hypothesizes that Fgf signaling from the AER establishes a gradient of Wnt5a expression in the limb mesenchyme, which then establishes polarity in these cells and in turn directs proximal to distal cell movements and/or cell divisions that result in limb outgrowth. In this application the investigator will test the hypothesis that Fgf and Wnt/PCP signaling are necessary and sufficient for polarization of the actin cytoskeleton or the mitotic spindle. Furthermore he will examine the epistatic relationship between Fgf and Wnt/PCP signaling in regulating cell polarity.