Foxo1 is required in mouse spermatogonial stem cells for their maintenance and the initiation of spermatogenesis
Meredith J. Goertz, … , F. Kent Hamra, Diego H. Castrillon
Meredith J. Goertz, … , F. Kent Hamra, Diego H. Castrillon
Published August 25, 2011
Citation Information: J Clin Invest. 2011;121(9):3456-3466. https://doi.org/10.1172/JCI57984.
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Research Article Reproductive biology

Foxo1 is required in mouse spermatogonial stem cells for their maintenance and the initiation of spermatogenesis

Abstract

Spermatogonial stem cells (SSCs) capable of self-renewal and differentiation are the foundation for spermatogenesis. Although several factors important for these processes have been identified, the fundamental mechanisms regulating SSC self-renewal and differentiation remain unknown. Here, we investigated a role for the Foxo transcription factors in mouse spermatogenesis and found that Foxo1 specifically marks mouse gonocytes and a subset of spermatogonia with stem cell potential. Genetic analyses showed that Foxo1 was required for both SSC homeostasis and the initiation of spermatogenesis. Combined deficiency of Foxo1, Foxo3, and Foxo4 resulted in a severe impairment of SSC self-renewal and a complete block of differentiation, indicating that Foxo3 and Foxo4, although dispensable for male fertility, contribute to SSC function. By conditional inactivation of 3-phosphoinositide–dependent protein kinase 1 (Pdk1) and phosphatase and tensin homolog (Pten) in the male germ line, we found that PI3K signaling regulates Foxo1 stability and subcellular localization, revealing that the Foxos are pivotal effectors of PI3K-Akt signaling in SSCs. We also identified a network of Foxo gene targets — most notably Ret — that rationalized the maintenance of SSCs by the Foxos. These studies demonstrate that Foxo1 expression in the spermatogenic lineage is intimately associated with the stem cell state and revealed what we believe to be novel Foxo-dependent mechanisms underlying SSC self-renewal and differentiation, with implications for common diseases, including male infertility and testicular cancer, due to abnormalities in SSC function.

Authors

Meredith J. Goertz, Zhuoru Wu, Teresa D. Gallardo, F. Kent Hamra, Diego H. Castrillon

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Figure 8

Model for essential role of Foxos in control of SSC self-maintenance and initiation of spermatogenesis.

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Model for essential role of Foxos in control of SSC self-maintenance and...
Shown is a SSC capable of self-renewal or commitment to differentiation. Foxo1 is expressed and in an active (nuclear) state in SSCs (brown), promoting transcription of Ret mRNA and high levels of protein on the cell surface. Upon binding to Gdnf, an essential SSC growth factor, Ret/Gfra1 receptor (blue) signals via diverse signaling pathways to promote SSC survival and self-renewal. Through mechanisms that are not understood but likely to be PI3K-Akt–dependent (data not shown), Foxo1 becomes inactivated and degraded. Loss of Foxo1 activity then leads to Ret downregulation and commitment to spermatogenic differentiation. This study showed that in addition to serving as a marker for SSCs, Foxo1 is functionally associated with the stem cell state through the regulation of diverse targets, including Ret. For example, after Pten ablation, Foxo1 was inactivated, and Ret expression was low. In contrast, Pdk1 ablation led to constitutive activation of Foxo1 and high Ret expression, promoting continued SSC self-renewal. These findings also imply that PI3K signaling must be carefully titrated to balance between SSC self-renewal and differentiation via the Foxos. Among outstanding remaining questions are (a) how the sequential expression of the Ret/Gfra1 and Kit receptors is coordinated and regulated directly or indirectly by Foxo1, (b) how these receptors exert differential responses (given that they signal through similar signaling intermediaries), and (c) how other well-known SSC maintenance and differentiation factors, such as Plzf and retinoic acid interact, with PI3K-Foxo. KL, Kit ligand.