Stem cells can replenish tissue throughout an organism's life. But they must also replenish themselves, maintaining a delicate balance that produces the correct proportion of differentiated daughter cells and new stem cells.
Specialized microenvironments called niches control this balance. Our long-term goal is to identify and characterize the factors that regulate niche function in vivo and to find out why stem cells and their niches become less active as an organism ages. Mammalian tissues are unamenable to this work, so we seek to build upon previous work using the Drosophila ovary as a powerful model for stem-cell niches.
Our preliminary data suggests that niches carry out previously unrecognized functions that are likely to be important for stem-cell health and maintenance over time. Future studies include:
- Assessing changes in gene expression within the Drosophila ovarian germline stem cell niche during aging.
- Testing whether niches help to protect stem cells from microbes introduced through natural routes or via injury.
- Probing the molecular mechanisms by which a superoxide dismutase helps to prolong proper niche function late in life.
This comprehensive analysis of in vivo niche function during the course of aging will provide key insights into why stem cell activity declines with age and will reveal new molecular targets for the development of therapies to boost tissue maintenance and regeneration in aging organisms.
We have also explored how the size of the ovarian germline stem cell niche is limited. For example, we found that Lsd1 mutants produce a population of cells that resemble germline stem cells. Using a clonal analysis and cell-specific rescue experiments, we determined that Lsd1 does not act in the germline cells themselves, but rather in somatic cells outside of the normal stem cell niche. Further work showed that Lsd1 functions to keep these cells healthy and prevents them acting as ectopic niches. These results suggest that cells immediately adjacent to the niche retain the capacity to express niche-specific signals. Such plasticity would allow niches to expand and contract in response to various environment cues or injuries and potentially represents a common feature of in vivo stem cell niches across species.
This work has led us to begin to further characterize how germ cells interact with cells in their immediate vicinity, through direct physical contact and regulated signaling. Collaborating with Dr. Yukiko Yamashita at the University of Michigan, we have also discovered novel microtubule-dependent extensions (MT-nanotubes) that foster BMP signaling within germline stem cells of the Drosophila testis.