WASP-Arp2/3-dependent actin polymerization influences fusogen localization during cell-cell fusion in Caenorhabditis elegans embryos

Cell-cell fusion is essential for development and physiology. Actin polymerization was implicated in the Caenorhabditis elegans fusogen EFF-1 engagement in a reconstituted Drosophila cell culture system, and the actin-binding protein spectraplakin links EFF-1 to the actin cytoskeleton and promotes cell-cell fusions in C. elegans larvae. However, it remains unclear whether and how fusogens and the actin cytoskeleton are coordinated in C. elegans embryos. Here, we used live imaging analysis of GFP knock-in and RNAi embryos to study the embryonic cell-cell fusions in C. elegans. Our results show that the inhibition of WASP-Arp2/3-dependent actin polymerization delays cell-cell fusions. EFF-1 is primarily distributed in intracellular vesicles in embryonic fusing cells, and we find that the perturbation of actin polymerization reduces the number of EFF-1-postive vesicles. Thus, the actin cytoskeleton differently promotes cell-cell fusion by regulating fusogen localization to the fusing plasma membrane in larvae or to intracellular vesicles in embryos.