BNC1 Promotes Spermatogenesis by Regulating Transcription of Ybx2 and Papolb via Direct Binding to Their Promotor Elements

BNC1 is a transcription factor that is crucial for spermatogenesis and male fertility, although the underlying mechanism remains unclear. To study BNC1's specific role in spermatogenesis, we characterized a previously developed mouse model carrying a truncating mutation in Bnc1 (termed Bnc1^+/tr for heterozygotes and Bnc1^tr/tr for homozygotes) and found that the mutation decreased BNC1 protein levels and resulted in germ cell loss by apoptosis. Given that loss of functional Bnc1 is known to result in decreased expression of the spermatogenesis genes Ybx2 and Papolb, we aimed to explore whether and how BNC1 promotes transcription of Ybx2 and Papolb to mediate its role in spermatogenesis. We confirmed significant reduction in YBX2 and PAPOLB protein levels in testis tissue from Bnc1^+/tr and Bnc1^tr/tr males compared with wild-type mice (Bnc1^+/+). Consistently, knockdown of Bnc1 led to downregulation of Ybx2 and Papolb in CRL-2196 cells in vitro. To investigate if BNC1 directly induces Ybx2 and Papolb gene expression, chromatin immunoprecipitation using mouse testicular tissue and luciferase reporter assays in HEK293 cells were used to identify functional binding of BNC1 to the Ybx2 and Papolb promoters at defined BNC1 binding sites. Taken together, this study reveals a mechanism for BNC1's role in spermatogenesis by directly binding to BNC1 binding elements in the promoter regions of both Ybx2 and Papolb and inducing transcription of these important spermatogenesis genes.