CDK2 kinase activity is a regulator of male germ cell fate

ABSTRACT The ability of men to remain fertile throughout their lives depends upon establishment of a spermatogonial stem cell (SSC) pool from gonocyte progenitors, and also maintaining the proper balance between SSC renewal and spermatogenic differentiation throughout life. Depletion of SSCs causes infertility with a Sertoli Cell Only Syndrome (SCOS) phenotype. We previously created a mouse strain in which an inhibitory phosphorylation site (Tyr15) of Cyclin-dependent kinase 2 ( Cdk2 ) was altered. Juvenile males homozygous for this allele ( Cdk2 Y15S ) initiate the first round of spermatogenesis, which originates from prospermatogonia, but meiocytes arrest due to chromosomal defects resembling those in Cdk2 -/- mice. Subsequent waves of spermatogonial differentiation and meiosis were largely absent, leading to an SCOS-like phenotype. Here, we demonstrate that Cdk2 Y15S/Y15S mice possess mitotically active GFRa1 + SSC-like cells, but they are impaired in their ability to differentiate. Marker analysis and single cell RNA-seq revealed defective differentiation of gonocytes into SSCs. Biochemical and genetic data demonstrated that Cdk2 Y15S is a gain-of-function allele causing deregulated kinase activity, and its phenotypic effects could be reversed by mutating the Thr160 positive regulatory site in cis . These results demonstrate that precise temporal regulation of CDK2 activity in male germ cell development and in the cell cycle is critical for long-term spermatogenic homeostasis.