The role of UCH-L1 in mammalian spermatogonia

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Stem cells in the testis are one of the most important stem cell systems in the adult because they produce sperm that transmit genetic information across generations. Stem cells can divide in two distinct ways: they can self-renew to generate more stem cells or differentiate to a committed cell type that will proliferate and ultimately produce sperm. This process, known as stem cell fate decision, is essential for mammalian reproduction. If this delicate balance is skewed toward self-renewal, sperm cells will eventually disappear. If there is too much differentiation, however, the testis will lose all of its stem cells and consequently, sperm production will cease. In either case infertility will ensue. Given the importance of stem cell fate decision, it is perhaps surprising that the mechanisms that govern it are largely unknown. We previously identified a protein, called UCH-L1 that is specifically expressed in the nervous system and in the undifferentiated germ cells of the testis in many species. As germ cells begin to differentiate toward sperm, UCH-L1 is no longer present. Yet, while the expression of UCH-L1 is restricted to undifferentiated germ cells, its function in these cells is unknown. We have designed a series of experiments that help characterize the molecular and cellular pathways by which UCH-L1 influences germ cell fate decision. First, we will use a mouse model with a specific deletion of UCH-L1 to identify how its absence affects progression of spermatogenesis. We will then identify which proteins UCH-L1 binds to in germ cells and how it is involved in controlling germ cell differentiation. Studies in other cell types such as neurons or cancer cells show that UCH-L1 interacts with the mTORC1 pathway in the control of cell metabolism. We will investigate if this also occurs in the testis. We will also monitor differences in gene expression in the presence or absence of UCH-L1. The definitive test as to whether a cell is a true stem cell is the ability to recreate its entire cell lineage. In case of the testis, a true stem cell can colonize the testis of a recipient animal depleted of germ cells, and produce sperm. We hypothesis that the ability to reconstitute spermatogenesis is impaired in stem cells without UCH-L1 and we will use transplantation assays to test this hypothesis. Finally, we will translate our findings in the mouse model to the pig as an agriculturally important species. We will block the function of UCH-L1 in pig germ cells using a specific inhibitor or suppression of gene expression and will investigate whether the same pathways are affected as in the mouse model. We will then use transplantation assays previously developed by us to further define the role of UCH-L1 in pig germ cell differentiation. Knowing what regulates stem cell maintenance versus differentiation will allow us to develop systems to propagate testis stem cells in vitro and help design strategies for fertility regulation.