Fizzy-related 1 (FZR1) is an activator of the Anaphase promoting complex/cyclosome (APC/C) and an important regulator of the mitotic cell division cycle. Using a germ-cell-specific conditional knockout model we examined its role in entry into meiosis and early meiotic events in both sexes. Loss of APC/C FZR1 activity in the male germline led to both a mitotic and a meiotic testicular defect resulting in infertility due to the absence of mature spermatozoa. Spermatogonia in the prepubertal testes of such mice had abnormal proliferation and delayed entry into meiosis. Although early recombination events were initiated, male germ cells failed to progress beyond zygotene and underwent apoptosis. Loss of APC/C FZR1 activity was associated with raised cyclin B1 levels, suggesting that CDK1 may trigger apoptosis. By contrast, female FZR1Δ mice were subfertile, with premature onset of ovarian failure by 5 months of age. Germ cell loss occurred embryonically in the ovary, around the time of the zygotenepachytene transition, similar to that observed in males. In addition, the transition of primordial follicles into the growing follicle pool in the neonatal ovary was abnormal, such that the primordial follicles were prematurely depleted. We conclude that APC/C FZR1 is an essential regulator of spermatogonial proliferation and early meiotic prophase I in both male and female germ cells and is therefore important in establishing the reproductive health of adult male and female mammals. KEY WORDS: FZR1, Mouse, Meiosis, Spermatogenesis, Oogenesis, APC/C, CDH1 INTRODUCTIONDespite temporal differences in their meiotic programs, both sexes and indeed all eukaryotes, have a requirement for a prolonged prophase I to allow for homologous chromosome organization. In yeast, this is achieved through targeted proteolysis of M-phase promoting proteins by the Anaphase promoting complex/cyclosome (APC/C) complex, together with a meiosis specific coactivator protein, AMA1 (Cooper et al., 2000; Okaz et al., 2012). The mammalian meiotic program is punctuated by more complex 'stops' and 'starts' than those of lower eukaryotes, and the regulatory mechanisms behind these events -such as whether the meiotic role of the APC/C is conserved, has remained largely unexplored. In mammals, germ cell development encompasses a highly coordinated series of cell cycle changes. In the female, meiosis is initiated embryonically such that by birth oocytes are arrested at dictyate stage of prophase I within a primordial follicle (Adams and McLaren, 2002). Meiosis I is re-initiated at the time of ovulation but arrested once again at metaphase II until fertilization. By contrast, after a period of mitotic proliferation, after sex determination, embryonic male germ cells (gonocytes) enter quiescence. Postnatally they migrate to the basement membrane, re-enter the cell cycle and develop into undifferentiated spermatogonial cells. Some gonocytes establish the self-renewing stem cell population whereas others become differentiated spermatogonia that comple...

Characterizing the mechanisms underlying follicle development in the ovary is crucial to understanding female fertility and is an area of increasing research interest. The RNA binding protein Musashi is essential for post-transcriptional regulation of oocyte maturation in Xenopus and is expressed during ovarian development in Drosophila. In mammals Musashi is important for spermatogenesis and male fertility, but its role in the ovary has yet to be characterized. In this study we determined the expression of mammalian Musashi proteins MSI1 and MSI2 during mouse folliculogenesis, and through the use of a MSI2-specific knockout mouse model we identified that MSI2 is essential for normal follicle development. Time-course characterization of MSI1 and MSI2 revealed distinct differences in steady-state mRNA levels and protein expression/localization at important developmental time-points during folliculogenesis. Using a gene-trap mouse model that inactivates Msi2, we observed a significant decrease in ovarian mass, and change in follicle-stage composition due to developmental blocking of antral stage follicles and pre-antral follicle loss through atresia. We also confirmed that hormonally stimulated Msi2-deficient mice produce significantly fewer MII oocytes (60.9% less than controls, p < 0.05). Furthermore, the majority of these oocytes are of poor viability (62.2% non-viable/apoptotic, p < 0.05), which causes a reduction in female fertility evidenced by decreased litter size in Msi2-deficient animals (33.1% reduction to controls, p < 0.05). Our findings indicate that MSI1 and MSI2 display distinct expression profiles during mammalian folliculogenesis and that MSI2 is required for pre-antral follicle development.