Genetic Control of Fusion Pore Expansion in the Epidermis ofCaenorhabditis elegans

Abstract

Developmental cell fusion is found in germlines, muscles, bones, placentae, and stem cells. In Caenorhabditis elegans 300 somatic cells fuse during development. Although there is extensive information on the early intermediates of viralinduced and intracellular membrane fusion, little is known about late stages in membrane fusion. To dissect the pathway of cell fusion in C. elegans embryos, we use genetic and kinetic analyses using live-confocal and electron microscopy. We simultaneously monitor the rates of multiple cell fusions in developing embryos and find kinetically distinct stages of initiation and completion of membrane fusion in the epidermis. The stages of cell fusion are differentially blocked or retarded in eff-1 and idf-1 mutants. We generate kinetic cell fusion maps for embryos grown at different temperatures. Different sides of the same cell differ in their fusogenicity: the left and right membrane domains are fusion-incompetent, whereas the anterior and posterior membrane domains fuse with autonomous kinetics in embryos. All but one cell pair can initiate the formation of the largest syncytium. The first cell fusion does not trigger a wave of orderly fusions in either direction. Ultrastructural studies show that epidermal syncytiogenesis require eff-1 activities to initiate and expand membrane merger. INTRODUCTIONCell fusion is a ubiquitous and highly controlled process in eukaryotes. Developmental cell fusion is vital for mating and fertilization in yeast and humans, respectively. Cell fusion is required for the formation and maintenance of the muscular-skeletal system in vertebrates, in muscle fibers in Drosophila, and in nearly one third of all cells in Caenorhabditis elegans (Podbilewicz and White, 1994;Heiman and Walter, 2000;Wassarman et al., 2001;Abmayr et al., 2003;Shemer and Podbilewicz, 2003;Chen and Olson, 2005). In C. elegans, diverse cell fusions essential for organ formation and cell fate determination have recently become paradigms for developmental cell fusion (Podbilewicz and White, 1994;Mohler et al., 1998;Nguyen et al., 1999;Sharma-Kishore et al., 1999;Alper and Kenyon, 2002;Mohler et al., 2002;Shemer and Podbilewicz, 2002, 2003;Witze and Rothman, 2002). Cell fusion functions to sculpt organs and to accomplish defined body shapes (Sharma-Kishore et al., 1999;Witze and Rothman, 2002;Shemer and Podbilewicz, 2003). eff-1 (epithelial fusion failure) was identified as a gene encoding type I membrane proteins required for diverse epithelial cell fusion reactions. Mutations in eff-1 result in failure of epithelial cell fusion and developmental defects in organs where cell fusion normally occurs (Mohler et al., 2002;Shemer, 2002;Shemer and Podbilewicz, 2002). The activity of eff-1 is strongly regulated by homeobox containing genes (Shemer and Podbilewicz, 2002, 2003;Cassata et al., 2005). Other transcription and signaling factors have been shown to control the cell fusion process in C. elegans (Nilsson et al., 1998;Ch'ng and Kenyon, 1999;Shemer et al., 2000;Alper and Kenyon, 2001;Chen ...

Myoblast fusion: lessons from flies and mice

“…Thus, it remains to be determined whether fusion is accomplished by a single 25-50 m micropore that expands too rapidly to be observed, or whether multiple micropores converge to form this large opening. Perhaps, as seen in mutations of other fusion systems (Gattegno et al, 2007), analysis of Drosophila mutants that block complete fusion will allow this issue to be addressed.…”

Live Cell Imaging of In Vitro Human Trophoblast Syncytialization1

“…1E). To determine whether the multinucleated cells would allow single cells to fuse or would prevent new single cell-cell fusions from occurring, we performed numerical assessment of the proportion of cells that initially formed a 2-cell fusion in the context of the total fusion indices at 3 time points (12,24, and 48 h) (Fig. 1F).…”

“…Using live-cell imaging combined with other strategies, many interesting structures or molecules related to cell-cell fusion have been elucidated. For example, the fusion pore in the C. elegans epidermis [24] and the invasive podosome-like structure in myogenesis of the Drosophila embryo [25] have been described, and the F-family fusogens in C. elegans, epithelial fusion failure 1 (EFF-1) [26][27][28], and anchor cell fusion failure 1 (AFF-1) [29] have been discovered.…”

AFF-1 fusogen can rejuvenate the regenerative potential of adult dendritic trees via self-fusion

“…EFF-1 is the first bona fide eukaryotic developmental cell-cell fusion protein. It is expressed in different cell types including neurons, and mediates fusion between cells by a homotypic mechanism (Gattegno et al, 2007;Mohler et al, 2002;Podbilewicz et al, 2006;Shemer et al, 2004). Thus, EFF-1 has to be expressed in both cellular membranes for them to merge and the high-resolution crystal structure of the EFF-1 ectodomain suggests that EFF-1 acts by a mechanism that is distinct from viral-induced fusion.…”