Foxo Transcription Factors Control Spermatogonial Stem Cell Self-Renewal And The Initiation Of Spermatogenesis

Spermatogonial stem cells (SSCs), as the foundation for spermatogenesis, must maintain a balance of both self-renewal and differentiation. Although several factors important for these processes have been identified, the fundamental mechanisms regulating SSC self-renewal and differentiation remain essentially unknown. The work presented here describes the discovery of a role for the Foxo forkhead transcription factors in mouse spermatogenesis and SSCs. Foxo1 was found to specifically mark mouse gonocytes, and its cytoplasmic-to-nuclear translocation delineated the gonocyte-to-SSC transition in neonatal testes. In adults, Foxo1 is specifically expressed within a subset of undifferentiated spermatogonia with stem cell potential. Genetic analyses showed that Foxo1 was required for both SSC maintenance as well as the initiation of spermatogenesis, with limited contributions from Foxo3 and Foxo4. Conditional inactivation of PI3K/Akt pathway components in the male germ line confirmed that PI3K signaling regulates Foxo1 stability and subcellular localization, revealing that the Foxos are crucial effectors of PI3K/Akt signaling in SSCs. The nuclear localization of Foxo1, indicating functional activation, was found to correlate with Gfralpha1 expression and thus stem cell potential. Subsequent gene expression analyses identified a complex network of Foxo gene targets that rationalized both the maintenance of SSCs and initiation of differentiation by the Foxos. Taken together, these findings demonstrate that the Foxos, particularly Foxo1, are essential in maintaining the spermatogonial stem cell population, and regulation of the Foxos through the PI3K/Akt signaling pathway is a critical process underlying SSC self-renewal versus differentiation.