Year 5

Stem cells are the building blocks during development of organisms as varied as plants and humans. In addition, adult or “tissue” stem cells provide for the maintenance and regeneration of tissues, such as blood and skin throughout the lifetime of an individual. The ability of stem cells to contribute to these processes depends on their unique ability to divide and generate both new stem cells (self-renewal) as well as specialized cell types (differentiation). In some tissues, cells that have already begun to specialize can revert or “de-differentiate” and assume stem cell properties, including the ability to self-renew. De-differentiation of specialized cells could provide a “reservoir” of cells that could act to replace stem cells lost due to wounding or aging. This proposal seeks to uncover the mechanisms that are utilized to regulate the process of de-differentiation and to compare these to the mechanisms that endow stem cells with the ability to self-renew using the fruit fly Drosophila melanogaster as well as pluripotent human cells.

In the most recent funding period, we have found that the multiple sex combs (msx) gene in fruit flies encodes a protein that is important for balancing the number of proteins that control DNA packaging. Our research has shown that when a subset of these proteins is present in excess, it causes a low level DNA damage response. In return, it appears that another group of proteins that regulate self-renewal and differentiation are recruited away from their ‘normal’ job and used for DNA repair. As many chemotherapeutic agents and anti-cancer strategies involved using DNA damage to kill proliferating cells, our data suggest that not only will stem cell be susceptible to killing, but the normal differentiation programs might also be affected. Because the function of this gene is conserved in human cells, we speculate that understanding the function of this gene will provide insight

In addition, we have continued to characterize the role of human Igf-II mRNA binding protein 1 (hIMP1) in pluripotent human cells and during early neural differentiation. Our primary progress in the current funding period was to identify genome-wide RNA targets for hIMP1, hIMP2, and LIN28 (a protein that associates with IMP proteins). Lastly, we have published a manuscript that shows that specialized stem cell microenvironments (also known as ‘niches’) are able to sustain similar numbers of stem cells, despite damage. This compensatory behavior is important because it suggests that the niche is an important component in sustaining stem cells after tissue damage or during wound repair. We will now use this observation as the basis for a screen to identify additional genes and pathways that regulate maintenance and regeneration of stem cell niches in more complex mammalian tissues.