Abstract 1399 Endolysosomal trafficking regulates yolk granule biogenesis in Drosophila oocytes

The endolysosomal system is essential for nearly all aspects of physiological functions of eukaryotic cells. As our understanding of membrane trafficking is mostly from in vitro studies in yeast and cultured mammalian cells, one challenge is to systematically evaluate these findings in a physiological setting. One valuable in vivo system to address this challenge is the vitellogenic oocyte in Drosophila, an organ that undergoes extensive endocytosis of Yolkless (Yl), a low-density lipoprotein receptor (LDLR) ortholog, to uptake extracellular lipoproteins into oocytes and package them into yolk granules, a type of specialized storage lysosomes used during development. Yet, there is still a lack of sufficient understanding on the molecular and cellular processes that control yolk granule biogenesis. By creating genome-tagging lines for the Yl receptor and analyzing its distribution in oocytes, we observed a close association of different endosomal structures with distinct lipid identities (phosphoinositides) and cytoskeletal dynamics. By characterizing several key Rab GTPases, we showed that Rab5 and Rab11, but surprisingly not Rab4 and Rab7, are essential for yolk granule biogenesis. We uncovered evidence for a potential role of Rab7 in actin regulation and observed a notable overlap of Rab4 and Rab7, two Rab GTPases that have long been proposed to have distinct non-overlapping spatial distribution and functions. Our results support that yolk granule biogenesis largely follows the canonical endolysosomal trafficking and maturation processes. Further, the data suggest that both the dynein motor and the RAVE/V-ATPase complexes function upstream of or in parallel with Rab7, and are involved in earlier stages of endosomal trafficking events. Together, our study provides novel insight into endolysosomal pathways and establishes vitellogenic oocytes in Drosophila as an excellent in vivo model for dissecting the highly complex membrane trafficking events in metazoans. NIH grant R01 NS110943