טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentShemer Gideon
SubjectCell Fusion and Organogenesis in Caenorhabditis
elegans
DepartmentDepartment of Biology
Supervisor Professor Benjamin Podbilewicz


Abstract

Cell fusion is a key developmental process that takes part in organogenesis and development. Studies of membrane fusion between enveloped viruses and their hosts identified fusogens- viral membrane glycoproteins that are essential for fusion. However, to date, no one had found fusogens that participate in cell-cell fusion.

We performed a 2-step clonal screen in the nematode Caenorhabditis elegans to look for vulval defective mutants and among them- for mutants with cell fusion defects. Using this approach, we isolated eff-1(hy21), a mutant in which cell fusion is completely blocked throughout development. eff-1 encodes a novel type I membrane protein that is expressed in cells committed to fusion.

            By analyzing eff-1(hy21) we found that cell fusion in C. elegans is essential for body elongation, locomotion, fertility, egg-laying and proper morphology of many structures and organs.

We have used the vulva of C. elegans as a model system to study how cell fusion and other cellular events contribute to the formation of a mature organ. By analyzing Ras mutants, in which ectopic function of let-60/ras leads to the formation of real and pseudo vulvae, we discovered the establishment of several primordial centers that act like “organizers”. We also found that although in wild-type the vulva is formed from two symmetrical halves, each half acts autonomously and is able to form complete pseudo vulval rings.

The Hox gene lin-39 plays a major role in regulation of cell fusion during vulva organogenesis. Using epistasis analysis and studying eff-1p::GFP expression in wild-type and lin-39 mutants, we found that lin-39 inhibits fusion by repressing the expression of eff-1 in specific vulva precursor cells (VPCs) resulting in the loss of the fusogenic activity of these cells. We also discovered that lin-39 activity is essential for the proliferation of the VPCs but not for ring formation during vulva organogenesis.