AP-1-dependent E-Cadherin trafficking in Drosophila oogenesis. Nicolas Loyer, Roland Le Borgne. CNRS UMR 6290-IGDR, Rennes, France.

   In yeast and mammals, clathrin adaptor protein complexes AP-1 function in transport between the trans-Golgi Network (TGN) and the endosomes, and in basolateral targeting in polarized epithelial cells. These functions of AP-1 are likely to be evolutionarily conserved. Here, we have investigated the function of AP-1 during oogenesis in Drosophila. Continuously growing immature egg chambers are composed of a monolayered follicular epithelium surrounding a germline cyst of 16 cells, interconnected by cytoplasmic bridges called ring canals. As reported in adult monolayered epithelium, loss of AP-1 does not cause a loss of establishment or maintenance of follicular epithelial cell polarity. In contrast, we observed two phenotypes in the germline cyst mutant for AP-1. First, plasma membrane separating the cells progressively disappeared in late stages, giving rise to multinucleated cells. Live imaging of cultured egg chambers revealed that this phenotype was due to plasma membrane detachment from ring canals actin structures. Second, the localization of E-Cadherin, an adhesion molecule homogeneously distributed along the germline cyst cells plasma membrane and enriched around their ring canals, was affected in AP-1 mutant cysts. E-Cadherin was found enriched in enlarged Rab11-positive recycling endosomes and was specifically excluded from the plasma membrane surrounding ring canals. We hypothesize a connection between these two phenotypes and are currently testing a working model involving an AP-1 dependent E-Cadherin targeting to ring canals, where it contributes to plasma membrane anchorage to ring canals actin structures, most likely through its binding partners alpha- and beta-catenins.