An RNAi screen of Rab GTPase genes in C. elegans reveals that somatic cells of the reproductive system depend on rab-1 for morphogenesis but not stem cell niche maintenance

Membrane trafficking is a crucial function of all cells and is regulated at multiple levels from vesicle formation, packaging, and localization to fusion, exocytosis, and endocytosis. Rab GTPase proteins are core regulators of eukaryotic membrane trafficking, but developmental roles of specific Rab GTPases are less well characterized, potentially because of their essentiality for basic cellular function. C. elegans gonad development entails the coordination of cell growth, proliferation, and migration--processes in which membrane trafficking is known to be required. Here we take an organ-focused approach to Rab GTPase function in vivo to assess the roles of Rab genes in reproductive system development. We performed a whole-body RNAi screen of the entire Rab family in C. elegans to uncover Rabs essential for gonad development. Notable gonad defects resulted from RNAi knockdown of rab-1, the key regulator of ER-Golgi trafficking. We then examined the effects of tissue-specific RNAi knockdown of rab-1 in somatic reproductive system and germline cells. We interrogated the dual functions of the distal tip cell (DTC) as both a leader cell of gonad organogenesis and the germline stem cell niche. We find that rab-1 functions cell-autonomously and non-cell-autonomously to regulate both somatic gonad and germline development. Gonad migration, elongation, and gamete differentiation--but surprisingly not germline stem niche function--are highly sensitive to rab-1 RNAi. SUMMARYThe Rab family of GTPases regulate vesicular trafficking in cells. This study assessed the consequences for the growth of the gonad of RNAi-mediated gene knockdown of all Rab GTPase genes in C. elegans. The highly conserved primary regulator of ER-Golgi trafficking, rab-1, is essential for normal gonad and germline development. Further experiments found that rab-1 is required in the somatic gonad for gonad elongation and migration, germline proliferation, and proper gamete formation. Surprisingly, the ability of the germline stem cell niche to maintain germ cells in the proliferative stem-like state was not affected by rab-1 RNAi knockdown.