20S proteasome-regulated proteostasis in ELVAs is critical for oocyte-to-embryo transition and female fertility.

Programmed degradation of maternal proteins is essential for the oocyte-to-embryo transition (OET). While pharmacological inhibition studies have established the importance of proteasomes in ovarian reserve maintenance, oocyte maturation and fertilization, the physiological impact of intrinsic proteasome insufficiency and underlying molecular mechanisms remain poorly understood. In mice, endolysosomal vesicular assemblies (ELVAs), specialized membraneless compartments composed of proteasomes, endolysosomes and autophagosomes, facilitate protein degradation during oocyte maturation and early embryogenesis. In this study, we generated mice with oocyte-specific deletion of the proteasomal core subunit Psma7, to investigate the physiological function of the 20S proteasome and its roles in ELVAs-mediated protein degradation. PSMA7-deficiency destabilized 20S proteasomes and disrupted translocation of ELVAs, leading to pronounced accumulation of ubiquitinated proteins in oocytes and zygotes. Consequently, maternal Psma7 deletion resulted in female infertility, manifested by impaired oocyte maturation and developmental arrest at one- to two-cell stage. Furthermore, we observed reduced proteasome abundance and dysfunction of ELVAs in aged oocytes, providing a mechanistic explanation for the decline in developmental competence associated with oocyte aging. Taken together, our findings elucidate the critical function of proteasome-regulated proteostasis within ELVAs in maintaining oocyte quality during OET and reproductive aging.