164. Gonadal Expression of Foxo1, but Not Foxo3, Is Conserved in Diverse Mammalian Species.

Abstract

The Foxo forkhead transcription factors are key effectors of the PI3K/Akt signaling pathway and regulate diverse physiologic processes. Two of these factors, Foxo1 and Foxo3, serve specific roles in reproduction in the mouse. Intriguingly, the functional requirement for each factor (revealed by analyses of knockout mice) mirrors the remarkably specific expression of the protein in a cell-autonomous manner. For example, Foxo3 is required for the maintenance of primordial oocytes in a quiescent state, and Foxo3 protein is highly expressed in primordial oocytes but not in other ovarian cell types. More recently, our laboratory demonstrated a specific role for Foxo1 in male fertility and the maintenance of spermatogonial stem cells in the mouse, and Foxo1 is highly expressed in spermatogonial stem cells but not in other testicular cell types. In the mouse ovary, Foxo1 is highly expressed in granulosa cells but not in oocytes or other cell types, suggesting an important functional role for Foxo1 in granulosa cells. Given that invertebrate model species such as C. elegans and D. melanogaster harbor a single ancestral Foxo homolog, these observations suggest that Foxo gene duplication conferred an evolutionary advantage by permitting the Foxos to adopt distinct but controlling roles in oogenesis and spermatogenesis. This study's objective is to determine if the remarkably specific expression patterns of Foxo1 and Foxo3 in mouse gonads (and by inference, Foxo function) are conserved in diverse mammalian species, including humans. The conservation of Foxo function in mammals would have significant implications for our understanding of reproduction and the causes of infertility in these diverse species. Western blotting was used to validate isoform-specific antibodies in mammalian species including rodents, companion animals, farm animals, non-human primates, and humans. Following validation of each antibody, immunohistochemical analysis was performed to ascertain Foxo1 and Foxo3 gonadal expression patterns. In the testis, Foxo1 expression was confined to a rare subpopulation of germ cells along the seminiferous tubule basement membrane in all species studied. In the ovary, Foxo1 expression was readily detected in the granulosa cells of developing follicles in all species. Foxo3 was highly expressed in primordial oocytes but absent by the secondary follicle stage in all rodent species. Interestingly, Foxo3 oocyte expression was not consistently conserved in non-rodent species. In some species, Foxo1 was expressed in primordial oocytes, but absent by the secondary follicle stage. In conclusion, while Foxo1 expression in spermatogonial stem cells and granulosa cells was conserved in all mammalian species evaluated, Foxo3 expression in oocytes was more variable. Our findings suggest that Foxo3 may not be uniquely required for primordial follicle maintenance in non-rodent species, and that other Foxos, particularly Foxo1, may contribute to oocyte maintenance in a functionally-redundant manner.