To Be, or Not to Be: The Molecular Mechanisms that Regulate Spermatogonial Stem Cell Fate
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Spermatogonial stem cells (SSCs) are a rare population of cells in the male germ line that self-renew and differentiate into progenitor spermatogonia for the initiation of spermatogenesis. In mammalian males, this process of gametogenesis generates millions of spermatozoa every day. The production of viable gametes is vital for transmitting genetic and epigenetic information to the next generation. Dysregulation of SSCs interferes with this process and causes infertility, which is detrimental to the survival and evolutionary fitness of sexually reproductive species. In the United States, infertility is suffered by approximately 10% of all reproductive age couples, and half of these cases are due to problems in the male, factors that interfere with the production of sperm. The severity of male infertility ranges from azoospermia, low sperm production, or abnormal sperm function, to reproductive tract abnormalities that prevent the delivery of sperm. Azoospermia has been associated with genetic disorders and cancer therapies that result in dysregulation of the SSC pool. Thus, understanding the molecular mechanisms that regulate SSC maintenance has the potential to lead to improved diagnostic procedures, advances in assisted reproductive technology, and the development of gene therapy techniques to treat infertility patients. Prior to the studies in this dissertation, the field of reproductive biology had no viable means of isolating pure populations of SSCs. As a former member of the Oatley lab, I participated in projects to identify and study SSCs. In published data, we identified inhibitor of DNA binding 4 (Id4) as a putative SSC marker. Subsequently, an Id4-Gfp transgenic mouse line was generated as a tool to isolate purified populations of SSCs. Functional transplantation studies confirmed that SSC capacity is found only in the ID4-GFP+ cell population. Consequently, the ID4-GFP+/SSCs and ID4-GFP-/progenitors were isolated for transcriptome analysis that identified 11 putative genes for SSC maintenance. Undifferentiated transcription factor 1 (Utf1) was among the 11 genes suggested to regulate SSC state. This gene is particularly interesting because it has been implicated in controlling ESC pluripotency and differentiation. As a chromatin associated protein, UTF1 was shown to regulate poised genes that are expressed during ESC differentiation. Our studies demonstrated that UTF1 localizes to SSCs and a subpopulation of progenitor spermatogonia. Additionally, we were able to show that transient reduction of Utf1 decreased SSC abundance. As a result, we hypothesized that UTF1 maintains SSC state by binding to genes that are essential for spermatogenesis initiation. Taken together, the studies summaries here have provided tools to study the transcriptome and epigenome that regulates SSC self-renewal and differentiation.