טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentOren Meital
SubjectSufficiency and Tissue Specific Functions of eff-1 in
C. elegans
DepartmentDepartment of Biology
Supervisor Professor Benjamin Podbilewicz


Abstract

Cell fusion is a universal process that involves the merger of two membranes into one, creating a multinucleated cell. C. elegans makes an ideal model for studying the mechanism of cell fusion, as in C. elegans one third of its somatic cells fuse during development. The eff-1 gene encodes novel type I membrane proteins required for epithelial cell fusion. It was previously shown that eff-1 is essential for cell fusion. In order to test whether eff-1 is also sufficient to promote fusion in C. elegans, we ectopically expressed it in epithelial cells under a heat shock promoter. Ectopic expression of EFF-1 in embryos and larvae led to fusion of these cells with the surrounding epidermis. This ectopic fusion was followed by cytoplasmic mixing and disappearance of apical junctions. Thus, eff-1 is sufficient to promote fusion of cells that normally escape fusion. By analyzing eff-1 genetic mosaic animals we found that eff-1 is required in both neighboring cells in order for them to fuse, and that expression of eff-1 in only one of those cells is not sufficient for fusion.

When exploring eff-1 effects on different tissues, we found that eff-1 affects PVD neuronal pattern. The PVD neurons in C. elegans are sensory neurons, which exhibit extensive branching pattern along the lateral sides of adult animals, creating a repetitive "Menorah"-like pattern.  It is known that precise arborization is essential for the correct wiring of neuronal circuitry. In eff-1 mutants the branching pattern of the PVD is disrupted, showing abnormal circumferential branches that differ in the number and in the organization of the candelabra from wild type. Quantification of the number and types of PVD branches showed that the initial defect of the PVD in eff-1 mutants is ectopic branching. We also found that eff-1 mutants have two-fold more menorahs than wild-type. These mutant menorahs show more retrograde branches and excess bifurcations. When expressing EFF-1 in the PVD of eff-1 mutants, we found that it partially rescues PVD patterning defects, but also leads to cell fusion of the nearby unfused hypodermal seam cells. The positional correlation between the seam cells and the PVD neurons led us to a model in which the seam cells act to regulate PVD patterning, by expressing spatial cues. These hypothetical cues may be eff-1 or other unknown guidance molecules that direct the PVD branches to their correct position and organization.