Stem Cell Niche Positioning in the Germ Line of C. elegans

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Stem cells are essential for proper development and tissue homeostasis in multicellular organisms. Differentiating daughters of dividing stem cells form the desired tissue, while self-renewing daughter cells retain their stem cell character. The stem cell niche provides the signal that regulates the balance between stem cell self-renewal and differentiation, and establishes the spatial organization that is critical for this balance to be properly regulated. The position of the niche is key to ensuring that cells differentiate at the proper time and in the proper location. In the C. elegans germ line the niche is the Distal Tip Cell (DTC), which is at distal end of each gonad arm. We will exploit our discovery that loss of cup-2, which encodes a conserved Derlin protein, results in the DTC becoming displaced from the distal end of the gonad to a more proximal location along the gonad arm. In the most extreme cases of DTC displacement where the DTC has moved ~10-15 germ cell diameters from the distal end, the stem cell population also moves to this new location immediately adjacent to the displaced DTC. In order to understand the mechanisms involved in maintaining the DTC at its proper location, we will first determine if changes in DTC morphology contribute to the cup-2 mutant displacement phenotype. The DTC contains both short and long processes, as well as a plexus that surrounds cells immediately adjacent to the DTC. We will compare the numbers and lengths of these processes, as well as the size of the plexus, in cup-2 mutants with those found in wild-type animals. A change in DTC morphology in cup-2 mutants could suggest a role for these structures in maintaining the DTC in the distal end of the gonad. Next we will determine cup-2's role in DTC adhesion. CUP-2/Derlin proteins are known to function in Endoplasmic Reticulum Associated Protein Degradation (ERAD). Therefore, to determine if DTC displacement in a cup-2 mutant is due to a disruption in ERAD we will analyze DTC displacement when ER stress is chemically induced, or when the functions of other ERAD components are reduced or eliminated. If disrupting ERAD through chemicals or mutants does not result in DTC displacement, then this could suggest that cup-2 has an ERAD independent function that is involved in DTC adhesion. Finally, we will utilize the cup-2 mutant as a starting point to identify additional factors involved in maintaining the DTC in the distal end of the gonad. Since the displacement observed in cup-2 mutants is incompletely penetrant, it is an ideal genetic background to perform an enhancer mutant screen to identify additional factors. Performing this screen and characterizing the identified factors will be a long-term focus of my lab. Determining how niche cell positioning is controlled is essential for us to understand how stem cell self-renewal and differentiation are precisely controlled to allow for proper development and tissue homeostasis.