Smek1-Dependent Transcriptional Repression of Usp10 Is Essential for the Progression of Murine Spermatogenesis

Meiotic progression is critically dependent on precise regulatory networks to ensure genomic stability. SMEK1 is recognized as a regulatory subunit of the protein phosphatase 4 complex. However, its potential phosphatase-independent functions in mammalian meiosis remain largely unexplored. Given the association of genetic variants near the SMEK1 locus with human infertility, we sought to define its specific role and mechanism in murine spermatogenesis. We generated a germ cell-specific Smek1 knockout mice model by crossing Smek1^f/f mice with Stra8^GFP-Cre mice. The phenotypic consequences were mainly assessed by histological analysis, chromosome spreading, and immunofluorescence staining. The molecular mechanisms were predominantly investigated using chromatin-immunoprecipitation, luciferase reporter assays, and co-immunoprecipitation analysis both in vivo and in vitro studies. Germ cell-specific ablation of Smek1 resulted in complete sterility due to a total arrest of spermatogenesis. The deficiency of Smek1 caused severe defects in prophase I, including an increased proportion of diplotene-stage spermatocytes, impaired synaptonemal complex dynamics, incomplete DSB repair, and a reduction in crossover. Notably, a subset of spermatocytes survived the initial checkpoint monitoring but arrested at metaphase I with a disrupted spindle structure and hyperactivation of the spindle assembly checkpoint. Mechanistically, we identified that SMEK1 functions independently of PP4 as a transcriptional repressor. It binds to the promoter of the deubiquitinating enzyme gene USP10. In the absence of SMEK1, increased USP10 protein stabilized BUBR1 (the core spindle assembly checkpoint component) and therefore delayed further progression beyond the metaphase I stage. Our study discovered a novel role for SMEK1 as a transcriptional regulator essential for meiotic progression. This SMEK1-USP10-BUBR1 pathway provides a fundamental mechanistic insight into the causes of male infertility and identifies a potential therapeutic target for human azoospermia and infertility.