Restoration of Spermatogenesis is Dependent on Activation of a SPRY4-ERK Checkpoint Following Germline Stem Cell Damage

Mammalian spermatogonial stem cells (SSCs) sustain male fertility through continuous self-renewal and differentiation, leading to the production of haploid spermatozoa throughout adulthood. However, SSCs are vulnerable to genotoxic drugs, and patients receiving chemotherapy face a high risk of germline instability and infertility. The molecular mechanisms and cellular pathways that choreograph SSC recovery after chemotherapeutic insult remain unknown. Previously, we identified SPRY4 as an ERK-dependent negative feedback regulator of growth factor signaling that is critical for preservation of stem cell activity in cultured mouse SSCs. Here, we demonstrate that following alkylating agent busulfan (BU)-induced injury, germline-specific Spry4 gene deletion (Spry4 ^G-KO) reduces stem cell regeneration but promotes differentiation with rapidly enhanced nuclear ERK1/2 activity in undifferentiated (A_undiff) spermatogonia (including SSCs) in adult mice. Genes essential for stem cell maintenance, including Id1 and Cxcl12, were dysregulated by loss of Spry4. Furthermore, the MEK1/2 inhibitor PD0325901, but not mTORC1 Inhibitor Rapamycin, was sufficient to promote spermatogonial proliferation in Spry4 ^G-KO testis 10 days post-BU treatment. Notably, the restoration of both spermatogonia pool and fertility was delayed in adult Spry4 ^G-KO males long-term after injury. In summary, germline-specific deletion of Spry4 results in hyper-activation of the MAPK/ERK pathway in A_undiff spermatogonia, unleashing excessive spermatogenesis after germline damage, and ultimately impairing germline regeneration in adult males. Our study indicates an essential role for SPRY4-ERK signaling as a molecular checkpoint in securing SSC recovery upon chemotherapy drug-induced germline damage, revealing how stem cells normally withstand environmental stress.