Frische EW et al. (2007) EMBO J "RAP-1 and the RAL-1/exocyst pathway coordinate hypodermal cell organization in Caenorhabditis elegans."

Plasterk RH, Bos JL, Pellis-van Berkel W, Tijsterman M, Frische EW, Cuppen E, Zwartkruis FJ, Van Haaften G

[EMBO J, 2007]

The small Ras-like GTPase Rap1 has been identified as a regulator of integrin activation and cadherin-mediated cell-cell contacts. Surprisingly, null mutants of RAP-1 in Caenorhabditis elegans are viable and fertile. In a synthetic lethal RNAi screen with C. elegans rap-1 mutants, the Ras-like GTPase ral-1 emerged as one of seven genes specifically required for viability. Depletion of exoc-8 and sec-5, encoding two putative RAL-1 effectors and members of the exocyst complex, also caused lethality of rap-1 mutants, but did not affect wild-type worms. The RAP-1 and the RAL-1/exocyst pathway appear to coordinate hypodermal cell movement and elongation during embryonic development. They mediate their effect in part through targeting the alpha-catenin homologue HMP-1 to the lateral membrane. Genetic interactions show that the RAP-1 and RAL-1/exocyst pathway also act in parallel during larval stages. Together these data provide in vivo evidence for the exocyst complex as a downstream RAL-1 effector in cell migration.

Kumari, P. et al. (2017) International Worm Meeting "The Exocyst complex is required for germline development and embryonic cell divisions in Caenorhabditis elegans."

Kumari, P., Mylavarapu, S.

[International Worm Meeting, 2017]

Polarized vesicular trafficking in eukaryotes is regulated by a conserved protein complex called the Exocyst complex containing eight subunits namely SEC-3, SEC-5, SEC-6, SEC-8, SEC-10, SEC-15, EXO-7 and EXO-8. The Exocyst is proposed to be a molecular-tether that facilitates fusion of Golgi-derived vesicles and recycling endosomes at distinct sites on plasma membrane. It functions in several cellular processes like polarized trafficking, migration, division and autophagy (Win and Guo, 2015). In C. elegans, the Exocyst has been shown to play role in hypodermal cell migration (Frische et. al, 2007), excretory canal luminogenesis (Armenti et al., 2014) and dendritic branching (Taylor et al., 2015). However its role in germline development and embryonic cell divisions has not been demonstrated. A combination of mutant analysis and partial depletion of Exocyst components by RNAi revealed roles for the Exocyst complex in multiple stages of development in C. elegans. Exocyst components are required for viability and fertility confirming its role in both embryonic and germline development. Exocyst depleted worms have a small germline with a poorly formed lumen. The total number of germline stem cells (GSCs) is significantly reduced and they show low rates of proliferation. The somatic stem cell niche signals GSCs to remain in mitosis via canonical Notch/Delta signaling. We found that the somatic stem cell niche plexus was shortened in the absence of Exocyst components that might reduce the amount of signaling required for GSC proliferation. Further, epistasis analysis of Exocyst with notch/glp-1 indicated that it positively regulates notch/glp-1 function in the germline. The Exocyst complex is required for proper oocyte development; however spermatogenesis appears to be unaffected. Exocyst depleted worms have very few mature oocytes in the gonad and display a delay in cellularization from the syncytial gonad. Ovulation is also severely affected in these worms. Ovulating oocytes get torn in the process of exiting the gonad and spermatheca resulting in a mass of torn cells in the uterus. The trapped oocytes in the gonadal arm exit meiotic arrest and begin endo-reduplication. In addition these oocytes show severe intracellular trafficking defects. Further, a few embryos that are formed are extremely osmo-sensitive, indicating defects in egg-shell formation. Several of the embryos failed to complete cytokinesis, resulting in multinuclear blastomeres during early embryogenesis. All these results put together indicate that the Exocyst complex is required for germline development and cell division in embryos. In the future, we will focus on elucidating the mechanistic basis of these functions by exploring the protein interactions of Exocyst components in C. elegans.