Benjamin Podbilewicz et al. (2006) Development & Evolution Meeting "Mechanisms of Cell-Cell Fusion"

Amir Sapir, Additya Mittal, Clari Valansi, Evgenia Leikina, Tamar Gattegno, Benjamin Podbilewicz, Gidi Shemer, Meital Suissa, Leonid Chernomordik

[Development & Evolution Meeting, 2006]

In C. elegans, the sperm and eggs and one third of all somatic cells fuse during development. Fusion proteins are essential and sufficient for intracellular membrane fusion and entry of enveloped viruses. Little is known about the membrane pathways and the identities of fusion proteins involved in fertilization and syncytia formation in bones, muscles and placenta. In contrast, we study the novel C. elegans EFF-1 proteins that are candidate developmental cell-cell fusion proteins. To study the mechanisms of cell fusion, we look for early and late intermediates of cell membrane fusion using genetic, biochemical and kinetic analyses. Using a new in vitro system and mosaic analysis in C. elegans we demonstrate that EFF-1 transmembrane type I proteins are homotypic fusogens. Thus, EFF-1-mediated cell fusion is different than heterotypic viral and intracellular membrane fusion. We explore cell fusion in developing embryos using a semi-automated system to simultaneously monitor the rates of multiple cell fusion processes in epithelia. Using live confocal and multiphoton microscopy we found that fusion in the epidermis comprise mechanistically distinct stages of initiation and completion of membrane fusion. 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. Surprisingly, different sides of the same cell differ in their fusogenicity: while the left and right membrane domains are fusion-incompetent, the anterior and posterior membrane domains fuse with autonomous kinetics in wild-type embryos. We will discuss how the homotypic nature of EFF-1-mediated cell-cell fusion may be an efficient mechanism to control the size and shape of syncytia in different organs in developing C. elegans.

Amir Sapir et al. (2007) International Worm Meeting "AFF-1, a Novel FOS-1-regulated Fusogen Mediates Anchor-Cell fusion."

Anna P. Newman, Leonid V. Chernomordik, Benjamin Podbilewicz, Amir Sapir, Jaebok Choi, Clari Valansi, Evgenia Leikina, Ori Avinoam

[International Worm Meeting, 2007]

Cell fusion is required for fundamental biological processes like reproduction, organ formation, viral pathogenesis, and tumor progression. Little is known about the proteins that mediate cell fusion and the molecular mechanisms by which these fusogens exert their activity. Fusion between most C. elegans epithelial cells is mediated by EFF-1, the first developmental fusogen identified in metazoans. Fusion between the anchor-cell (AC) and the utse syncytium that establishes a continuous uterine-vulval tube proceeds normally in eff-1 mutants. This fusion, therefore, is facilitated by an additional unidentified fusogen. Using a forward genetic screen, we isolated mutations in the aff-1 gene in which the AC differentiates normally but fails to fuse with the utse cells. AFF-1 ectopic expression results in fusion of cells that normally do not fuse indicating that AFF-1 is sufficient to fuse cells. The fusogenic activity of AFF-1 was further confirmed by its ability to fuse heterologous Sf9 insect cells. AFF-1 and EFF-1 differ in their fusogenic activity and expression patterns but share eight conserved predicted disulfide bonds in their ectodomains including a putative TGF-beta-type-I-Receptor domain. Identification of this conserved domain is the first step in deciphering the molecular mechanism of developmental cell fusion. In addition we found that FOS-1, the Fos transcription factor ortholog that controls anchor-cell invasion during nematode development, is a specific activator of aff-1-mediated anchor-cell fusion. Thus, FOS-1 links cell invasion and fusion in a novel developmental cascade.

Benjamin Podbilewicz et al. (2006) European Worm Meeting "Mechanisms and Functions of Homotypic Cell Fusion"

Leonid V. Chernomordik, Amir Sapir, Additya Mittal, Evgenia Leikina, Clari Valansi, Benjamin Podbilewicz, Tamar Gattegno, Gidi Shemer, Meital Suissa

[European Worm Meeting, 2006]

Cell fusion is found in germlines, muscles, bones, placentas and stem cells. In C. elegans, the sperm and eggs and one third of all somatic cells fuse during development. To dissect the pathway of cell fusion, we use genetic and kinetic analyses using live confocal and multiphoton microscopy to find early and late intermediates of cell membrane fusion. We explore cell fusion in developing embryos using a semi-automated system to simultaneously monitor the rates of multiple cell fusion processes in epithelia. We found that fusion in the epidermis comprise mechanistically distinct stages of initiation and completion of membrane fusion. 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. Surprisingly, different sides of the same cell differ in their fusogenicity: while the left and right membrane domains are fusion-incompetent, the anterior and posterior membrane domains fuse with autonomous kinetics in wild-type embryos. We demonstrate that EFF-1 transmembrane type I proteins are homotypic fusogens using a new in vitro heterologous system and mosaic analysis in C. elegans. Thus, EFF-1-mediated cell fusion is different than heterotypic viral and intracellular membrane fusion. We will discuss how the homotypic nature of cell-cell fusion may be another efficient mechanism to control the size and shape of syncytia.