Independent roles of Drosophila Moesin in imaginal disc morphogenesis and hedgehog signalling.

The three ERM proteins (Ezrin, Radixin and Moesin) form a conserved family required in many developmental processes involving regulation of the cytoskeleton. In general, the molecular function of ERM proteins is to link specific membrane proteins to the actin cytoskeleton. In Drosophila, loss of moesin (moe) activity causes incorrect localisation of maternal determinants during oogenesis, failures in rhabdomere differentiation in the eye and alterations of epithelial integrity in the wing imaginal disc. Some aspects of Drosophila Moe are related to the activity of the small GTPase RhoA, because the reduction of RhoA activity corrects many phenotypes of moe mutant embryos and imaginal discs. We have analysed the phenotype of moesin loss-of-function alleles in the wing disc and adult wing, and studied the effects of reduced Moesin activity on signalling mediated by the Notch, Decapentaplegic, Wingless and Hedgehog pathways. We found that reductions in Moesin levels in the wing disc cause the formation of wing-tissue vesicles and large thickenings of the vein L3, corresponding to breakdowns of epithelial continuity in the wing base and modifications of Hedgehog signalling in the wing blade, respectively. We did not observe any effect on signalling pathways other than Hedgehog, indicating that the moe defects in epithelial integrity have not generalised effects on cell signalling. The effects of moe mutants on Hedgehog signalling depend on the correct gene-dose of rhoA, suggesting that the requirements for Moesin in disc morphogenesis and Hh signalling in the wing disc are mediated by its regulation of RhoA activity. The mechanism linking Moesin activity with RhoA function and Hedgehog signalling remains to be elucidated.