National Centers for Translational Research in Reproduction and Infertility

Translational Regulation of Meiotic Cell Cycle Onset and Progression in the Male

Stanford University

PI:  M.T. Fuller

The specialized cell cycle of meiosis is a signature of germ cell differentiation and the key to sexual reproduction in eukaryotic organisms. In addition to pausing/arresting the meiotic cell cycle in early G2 (for oogenesis), the germ cell developmental program must specify the long G2 phase of the cell cycle termed meiotic prophase (for both oogenesis and spermatogenesis) to provide essential fime for the chromosomal and transcriptional events that prepare for the meiotic divisions and gamete differentiation. To achieve meiotic prophase, the germ cell differentiation program imposes extra layers of regulation upon the expression and activity of core cell cycle machinery components to delay cell cycle progression, with translational repression playing a central role. RNAs that encode activators of the G2/MI transition are initially kept silent by translational repression, but must be stably maintained in a competent state unfil needed. The RNAs must then be activated at the correct time in meiotic maturation for spermatocytes and oocytes to become competent to enter the meiotic divisions. Although progress has been made on understanding the last step during oocyte maturation, little is known about the underlying mechanisms during male meiosis or the early stages of oogenesis. Understanding how germ cells normally enter and progress through meiotic prophase is key to understanding germ cell development, the molecular basis of meiosis I maturation arrest infertility, and to designing effective strategies for oocyte and spermatocyte development and maturation in vitro. We are using the genetic power of the Drosophila model system to elucidate molecular mechanisms underlying each of these key requirements for regulation of the meiotic cell cycle in males. We have discovered that core cell cycle components, including cyclin B and the cell cycle G2/M activating phosphatase cdc25/twine, are regulated by both cell type specific transcriptional programs and translational repression when male germ cells enter meiotic prophase. We found that the hnRNP A family RNA-binding protein Rbp4, expressed only in primary spermatocytes, is required for translational repression of cycB, working through clsacfing sequences in a spermatocyte specific form of the 3′ UTR of the cycB RNA. We discovered that the Drosophila hnRNP R/Q homolog Racquetball (Rq) is required to preserve stability of the repressed cycB RNA through meiotic prophase. In addition, we found that Boule, the Drosophila homolog of human Deleted in Azoospermia (DAZ) and BOULE, activates translation of the cell cycle regulatory phosphatase cdc25/twine in mature spermatocytes for progression through the G2/M transition of meiosis I. Action of Boule in regulating is likely conserved, as lack of DAZ family members is correlated with meiosis I maturation arrest infertility in men. Our Specific Aims are to: 1) Investigate how the germ cell differentiation program sets up translational repression then stage specific activation of the core cell cycle component Cyclin B to regulate meiotic prophase. 2) Determine how translationally repressed cell cycle RNAs are stably maintained through meiotic prophase. 3) Investigate how the DAZ/DAZL homolog Boule regulates translation of cdc25/twine to allow entry into meiotic division.