Functional Differences between GDNF-Dependent and FGF2-Dependent Mouse Spermatogonial Stem Cell Self-Renewal

• GDNF is dispensable for spermatogonial stem cell (SSC) self-renewal
• GFRA1 is expressed in most SSCs
• In vivo depletion of FGF2 in the seminiferous tubules enriches SSCs
• Self-renewal by GDNF, but not FGF2, requires MAP2K1/2

SummarySpermatogonial stem cells (SSCs) are required for spermatogenesis. Earlier studies showed that glial cell line-derived neurotrophic factor (GDNF) was indispensable for SSC self-renewal by binding to the GFRA1/RET receptor. Mice with mutations in these molecules showed impaired spermatogenesis, which was attributed to SSC depletion. Here we show that SSCs undergo GDNF-independent self-renewal. A small number of spermatogonia formed colonies when testis fragments from a Ret mutant mouse strain were transplanted into heterologous recipients. Moreover, fibroblast growth factor 2 (FGF2) supplementation enabled in vitro SSC expansion without GDNF. Although GDNF-mediated self-renewal signaling required both AKT and MAP2K1/2, the latter was dispensable in FGF2-mediated self-renewal. FGF2-depleted testes exhibited increased levels of GDNF and were enriched for SSCs, suggesting that the balance between FGF2 and GDNF levels influences SSC self-renewal in vivo. Our results show that SSCs exhibit at least two modes of self-renewal and suggest complexity of SSC regulation in vivo.

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