Testis‐Enriched F‐Box Protein FBXO39 Is Important for Spermiogenesis and Male Fertility in Mice

BACKGROUND: The SCF (Skp-Cullin-F-box) complex is a major class of E3 ubiquitin ligases. F-box proteins constitute the SCF complex and play a critical role in recognizing substrates for ubiquitination. In mice, several F-box proteins, including FBXO36 and FBXO39, are predominantly expressed in testes. OBJECTIVE: To investigate the roles of FBXO36 and FBXO39 in mouse male reproduction. MATERIALS AND METHODS: We generated Fbxo36 KO and Fbxo39 KO mouse lines using the CRISPR/Cas9 system. We evaluated male fertility, sections of reproductive organs, and sperm morphology and motility. RESULTS: No overt abnormalities were found in male fertility or sperm morphology of Fbxo36 KO mice. In contrast, Fbxo39 KO males showed subfertility, abnormal sperm morphology, and reduced sperm motility. Histological analyses of testis sections revealed that FBXO39 is critical for the morphological transformation of round spermatids into spermatozoa. DISCUSSION AND CONCLUSION: Our findings demonstrate that Fbxo39 is important for spermiogenesis in mice, whereas Fbxo36 is dispensable for male fertility. Further analyses of FBXO39 may contribute to understanding the etiology of male infertility.