TAF4b in the Regulation of Spermatogenic Maintenance.

Abstract

The mammalian testis is composed of multiple somatic and germ cell populations that collectively orchestrate the production of mature sperm throughout much of adult life. The normal ability to maintain spermatogenesis is largely attributed to a small population of spermatogonial stem cells (SSCs) that are found within the basement membrane of the seminiferous tubules. However, the cellular and molecular events garnering the maintenance of spermatogenesis and a precise understanding of the SSC niche remain to be elucidated. To this end, we have set out to characterize the novel functions and mechanisms of TAF4b (formerly TAFII105) in the maintenance of the SSC niche in the mouse testis. TAF4b is a gonadal-enriched component of the ubiquitous TFIID complex composed of the TATA-box binding protein (TBP) and multiple TBP-associated factors (TAFs). Analysis of mouse tissue mRNA and protein levels indicated that TAF4b is enriched in the testis and ovary and accordingly, mice lacking TAF4b are infertile. Female mice lacking TAF4b are completely infertile due multiple defects in ovarian follicle development (see poster by Lovasco et al.). In contrast, TAF4b-deficient males are able to establish spermatogenesis temporarily, but rapidly fail to maintain spermatogenesis through early adulthood. We hypothesize that TAF4b-containing TFIID complexes in the testis regulate programs of gene expression required for SSC self- renewal and spermatogenic maintenance in the postnatal mammalian testis. We have begun to characterize the normal expression patterns and functions of TAF4b in the postnatal mouse testis. Immunostaining of wild-type frozen testis sections and touch-preps reveals nuclear staining of TAF4b in spermatocytes and perinuclear TAF4b staining in spermatids. The perinuclear staining of TAF4b in round spermatids is consistent with the presence of TAF4b in the chromatoid body. Thus, TAF4b exhibits a complex and dynamic pattern of germ cell localization in the mouse testis. To assay the functional requirement for TAF4b in SSC renewal, we have used BrdU-incorporation in the TAF4b-null testis and observe a reduction of BrdU-positive germ cell populations in the TAF4b-null testis. Consistent with the loss of labeled germ cells, the TAF4b-deficient testes undergo severe testicular atrophy that worsens over time. The absence of germ cells within the TAF4b-null testis suggests that a potential SSC self-renewal defect may underlie the infertility observed in these mice. Currently, we are employing markers of SSC identity and chromatoid body components to determine the cellular and molecular events accomplished by TAF4b as it regulates SSC self-renewal. This research was supported by a New Scholar Award in Aging (RF) from the Ellison Medical Foundation.