The Role And Mechanism Of Retinoid Acid Maintaining The Stability Of Primordial Follicle Pool In Mice

Most mammals will establish a primordial follicle pool to store germ cells in early life.Once the primordial follicle pool is formed,the majority of primodial follicles remain preserved in a dormant state to maintain the reproductive life of female mammals.Abnormal activation of primordial follicle will cause premature ovarian failure and eventually lead to female infertility.Many studies have shown that retinoic acid(RA)plays an important role in meiotic initiation and sex differentiation,but the role of RA in the primordial follicle activation is not clear.In this paper,we studied the role and molecular mechanism of RA in maintaining the stability of primordial follicle pool.The results of present study showed that the expression of retinaldehyde dehydrogenase and RA receptor was significantly decreased in ovaries as the development of neonatal mice,suggesting that the activity of RA may be negatively related to primordial follicle activation.Culture of 3 days post-parturition(dpp)ovaries with RA in vitro showed that RA could not significantly inhibit the activation of primordial follicles,but the expression of RA catabolic enzymes was significantly increased.Therefore,in this study we added Talarozole,an inhibitor of RA catabolic enzymes and RA to co-culture neonatal ovaries,the results showed that after RA+Talarozole treatment,the number of activated follicles was significantly reduced.Adding the RA receptor inhibitor AGN194301 or retinaldehyde dehydrogenase inhibitor DEAB could significantly increased the number of activated.follicles and decreased the number of primordial follicles.In vivo injection results showed that RA could significantly inhibit the activation of primordial follicles.These results indicate that RA can maintain the stability of the primordial follicle pool and prevent the excessive activation of primordial follicles.It has been reported that RA can inhibit mitogen-activated protein kinase 3/1(MAPK3/1)signaling in cardiomyocytes and keratinocytes.Therefore,the effect of MAPK3/1 signaling on primordial follicle activation and the relationship between RA and MAPK3/1 signaling pathway were studied.Culture of 3 dpp ovaries with the MAPK3/1 signaling inhibitor U0126 significantly decresed the number of activated follicles.Further studies showed that U0126 inhibits mTORC1-KITL signaling.Subsequently,we investigated whether RA prevents follicular overactivation by inhibiting MAPK3/1 signaling pathway in neonatal mouse ovaries and the mechanism of RA maintaining the stability of primordial follicular pool.The results showed that RA did not affect the activity of MAPK3/1 and the expression of KITL,which indicate that RA did not affect the stability of the primordial follicle pool by MAPK3/1 signaling pathway and KITL secretion in pre-granulosa cells.Finally,our results showed that RA could inhibit the expression of phosphorylated Akt in oocytes,resulting in a decrease of Foxo3 nuclear export,suggesting that RA could maintain the stability of the primordial follicle pool through inhibiting the PI3K-Akt signaling in oocytes.In conclusion,RA maintains the stability of primordial follicle pool in mice by inhibiting PI3K-Akt signaling pathway in oocytes.However,MAPK3/1 participate in the activation of primordial follicles through mTORCl-KITL signaling.These findings provide a new scientific evidence for elucidating the molecular mechanisms for controlling the activation of primordial follicles.