Cyclical expression of GDNF is required for spermatogonial stem cell homeostasis.
In the murine testis self-renewal of spermatogonial stem cells (SSCs) requires Glial cell line-derived neurotrophic factor (GDNF) secreted from neighboring somatic cells. However, it not clear how GDNF promotes self-renewal in vivo or what downstream signaling pathways are required for SSC maintenance. We found that GDNF is normally expressed cyclically during spermatogenesis. Stage-specific ectopic expression of GDNF caused the accumulation of a GFRA1+ LIN28- Asingle population, which has enhanced SSC activity compared to wild-type, suggesting that GDNF normally limits self-renewal to specific stages. Despite the increase in SSC cell number, EdU labeling during steady-stage spermatogenesis, and during recovery following busulfan-mediated spermatogonial depletion, indicated that GDNF promotes self-renewal by blocking differentiation and not by promoting proliferation. Increased GDNF signaling lead to increased phosphorylation of AKT3 in undifferentiated spermatogonia, but not AKT1 or AKT2, and was independent of RPS6 phosphorylation, suggesting that AKT3 functions in SSC self-renewal or progenitor cell expansion. Development 2018 Feb 13 [Epub ahead of print]