Quantitative analysis of male germline stem cell differentiation reveals a role for the p53-mTORC1 pathway in spermatogonial maintenance.

p53 protects cells from DNA damage by inducing cell-cycle arrest upon encountering genomic stress. Among other pathways, p53 elicits such an effect by inhibiting mammalian target of rapamycin complex 1 (mTORC1), the master regulator of cell proliferation and growth. Although recent studies have indicated roles for both p53 and mTORC1 in stem cell maintenance, it remains unclear whether the p53-mTORC1 pathway is conserved to mediate this process under normal physiological conditions. Spermatogenesis is a classic stem cell-dependent process in which undifferentiated spermatogonia undergo self-renewal and differentiation to maintain the lifelong production of spermatozoa. To better understand this process, we have developed a novel flow cytometry (FACS)-based approach that isolates spermatogonia at consecutive differentiation stages. By using this as a tool, we show that genetic loss of p53 augments mTORC1 activity during early spermatogonial differentiation. Functionally, loss of p53 drives spermatogonia out of the undifferentiated state and causes a consistent expansion of early differentiating spermatogonia until the stage of preleptotene (premeiotic) spermatocyte. The frequency of early meiotic spermatocytes is, however, dramatically decreased. Thus, these data suggest that p53-mTORC1 pathway plays a critical role in maintaining the homeostasis of early spermatogonial differentiation. Moreover, our FACS approach could be a valuable tool in understanding spermatogonial differentiation.