Challenging the role of retinoic acid in meiotic initiation

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PROJECT SUMMARY/ABSTRACT Meiosis is essential for sexual reproduction, and its specialized molecular and cellular programs have been intensely studied in lower eukaryotes and mammals. In mammals, however, the transition from mitotic spermatogonia into the meiotic program, termed meiotic initiation, has received little attention. Meiotic initiation occurs during the preleptotene phase of meiotic prophase I, as these spermatocytes replicate their DNA and prepare for meiotic recombination and segregation. Retinoic acid (RA) has been proposed to serve as the ‘meiosis inducing substance.’ In the postnatal testis, although it is clearly required for spermatogonial differentiation, our exciting preliminary data reveals RA is dispensable 8.6 days later for meiotic initiation. Thus, the role of RA in meiosis has not been properly examined, and the true meiosis- inducing factor(s) remain undefined. This proposal represents a new collaboration between the Geyer and Schindler labs, who will work together to uncover the true role(s) for RA in meiosis. In Aim 1, we will identify the specific requirement for RA in initiation and progression through meiosis to form haploid spermatids. In Aim 2, we will employ a novel transgenic mouse model with synchronized spermatogenesis for fluorescence-based isolation of millions of germ cells prior to and during meiotic initiation. Using this unique resource, we will define and compare RA-mediated changes in gene expression at the transcriptome (RNA abundance), translatome (translating RNAs), and proteome (protein abundance) levels during meiotic initiation. The outcome of this work is identification of novel regulators of this critical transition, and testis-specific proteins that are upregulated in preleptotene spermatocytes will represent putative male contraceptive targets. Indeed, preleptotene spermatocytes represent an ideal cell type for male contraceptive drug development, for two reasons: 1) they reside outside the blood-testis-barrier (BTB), which is a significant barrier to drug delivery; and 2) they are distinct from the spermatogonial stem cell (SSC) pool, and thus can be targeted without irreversibly damaging the male germline. The results from this proposal will be foundational to defining the broader mechanistic relationship between RA and meiosis that are essential for male fertility.