Male germ cells with Bag5 deficiency show reduced spermiogenesis and exchange of basic nuclear proteins

Bcl-2 associated athanogene-5 (BAG5) represents a unique BAG cochaperone family member, regulating chaperone activity. We first demonstrated significant differences in Bag5 expression by RNA seq analysis of teratozoospermia and healthy male sperm samples, but the genetic and molecular mechanisms governing this process remain elusive. We further found that BAG5 has highest expression in human and mouse testes. BAG5 expression is elevated in late stage pachytene spermatocytes and spermatids. Targeted Bag5 inactivation in mice induces massive apoptosis in male germ cells and abrogates male infertility. The ordered loading of sperm basic nuclear proteins on chromatin is altered, with lost TNPs and PRMs, resulting in severe sperm head deformity and partial 9 + 2 microtubule structure disorder. In terms of mechanism, immunoprecipitation (IP)-mass spectroscopy (MS) revealed BAG5 interacts with HSPA2, a testis-specific HSP70 family member regulating the transcription of the transition protein TNPs as well as spermatogenesis. RNA-sequencing assessment of Bag5 deficient testis confirmed Bag5 participation in transcriptional repression and revealed significant changes in Hspa2 expression. Bag5 deficiency resulted in decreased levels of HSPA2, germ cell apoptosis and subsequent inappropriate nuclear protein deposition and chromatin condensation. Decreased BAG5 expression levels in patients with non-obstructive azoospermia and oligoasthenospermia were also detected. These results uncovered an intriguing HSPA2-mediated key function of BAG5, which may constitute a potential prognostic biomarker of male infertility.