Contribution of Ref(2)p to regulation of Drosophila notum epithelial cell apico-basal polarity and phenotype

Cell polarity impacts on the maintenance of cell shape, cell-cell junction integrity, and protrusions formation and dynamics. Further, polarity regulates cell movement, proliferation and differentiation. Conversely when cells lose their polarity they may be susceptible to dysfunction that may underlie degenerative disorders and tumour progression. Cell polarity and polarity protein complexes are highly conserved between different organisms from unicellular to multicellular, and from invertebrates to vertebrates. The focus of this study is the apico-basal polarity that is established normally in epithelial cells. The protein p62 has been revealed to have a role in epithelial cells phenotypic alteration. It is considered as a multifunctional scaffold protein and acts as a signalling hub for different pathways, and through interactions with the polarity protein aPKC we hypothesise that it may regulate apico-basal polarity. Ref(2)p is the Drosophila homologue of p62 and using Drosophila melanogaster as a model system we investigated the effects of Ref(2)p mutation or overexpression on epithelial cell apico-basal polarity, cell shape and protrusion dynamics. Our data suggests that Ref(2)p is required to maintain normal cell size, cell-cell junction stability, and protrusion dynamics and formation. Both the PB1 and UBA domains of Ref(2)p were recognized to be essential for localizing aPKC apically. As a multifunctional protein, Ref(2)p showed a further role in cell division, chromosome segregation and tumour repression. Ref(2)p mutants, as well as aPKC mutants, showed a decrease in cell division rate and phenocopied a blebbing phenotype detected in SCAR mutant dividing cells. Mechanistically, these phenotypes are likely at least due to Ref(2)p’s interaction with aPKC and on broader scale due to changes in Ref(2)p mediated autophagy on polarity proteins. Since levels of autophagic activity, mediated by Ref(2)p, have potential effects on polarity proteins levels, which affect apico-basal polarity, cell size and actin cytoskeleton organization