Selective autophagic degradation of ATP citrate lyase maintains citrate homeostasis and promotes oocyte maturation

Autophagy is a cellular and energy homeostatic mechanism that contributes to maintain the number of primordial follicles, germ cell survival, and anti-ovarian aging. However, it remains unknown whether autophagy in granulosa cells affects the oocyte maturation. Here, we show a clear tendency of reduced autophagy level in human granulosa cell from women of advanced maternal age, implying a potential negative correlation between autophagy level and oocyte quality. We therefore established a co-culture system and show that either pharmacological inhibition or genetic ablation of autophagy in granulosa cells negatively affect the oocyte quality and fertilization ability. Moreover, our metabolomics analysis indicates that the adverse impact of autophagy impairment on oocyte quality is mediated by downregulated citrate levels, while exogenous supplementation of citrate can significantly restore the oocyte maturation. In molecular level, we found ATP citrate lyase (Acly), which is a crucial enzyme catalyzing the cleavage of citrate, was preferentially associated with K63-linked ubiquitin chains and recognized by the autophagy receptor protein SQSTM1/p62 for the selective autophagic degradation. In human follicles, autophagy levels in granulosa cells was downregulated with maternal aging, accompanied by decreased citrate in the follicular fluid, implying a potential correlation between citrate metabolism and oocyte quality. We also show that elevated citrate levels in porcine follicular fluid promote oocyte maturation. Collectively, our data reveal that autophagy in granulosa cells is a beneficial mechanism to maintain a certain degree of citrate by selectively targeting Acly during oocyte maturation. Overall design: The RNA-Seq experiments of the control and Atg5 KD cumulus granulosa cells.