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Regulation of actin dynamics by phosphoinositides during epithelial closure

Pickering, Karen

[Thesis]. Manchester, UK: The University of Manchester; 2013.

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Abstract

Epithelia act as protective barriers and it is therefore essential that wounded epithelia are rapidly repaired to maintain barrier function. Cells surrounding epithelial wounds become motile following wounding, which involves generating dynamic actin structures that drive closure of the wound. These actin structures include filopodia which are important in the final stage of epithelial closure in which the opposing epithelial edges are joined together. The molecular mechanisms that trigger wound edge cells to become motile are not well understood. Using Drosophila wound healing and the morphogenetic process dorsal closure as models, we find that phosphatidylinositol 3,4,5-triphosphate (PIP3) regulates epithelial closure by promoting the formation of filopodia at epithelial edges. PIP3 accumulates at epithelial edges and genetically depleting PIP3 results in reduced filopodia and defects in epithelial closure. We demonstrate that the GTPase Rac and guanine nucleotide exchange factor Myoblast City function downstream of PIP3 to promote filopodia formation. We also demonstrated that the scaffolding protein Par3/Bazooka and the lipid phosphatase PTEN are responsible for restricting the localisation of PIP3 and consequently the downstream signals to the epithelial leading edge, so acting to determine the location of filopodia formation. This project reveals a novel mechanism by which actin protrusions, required for epithelial closure, are formed in response to epithelial damage. Additionally, we have identified an additional role for PIP3 in regulating the extrusion of cells from epithelial sheets in the Drosophila embryo. This finding implicates PIP3 in the regulation of tissue homoeostasis, and could contribute to our understanding of tumour initiation as unregulated tissue growth can result in the formation of tumours.