Regulation Of Transforming Growth Factor Beta In The Maintenance And Activation Of Primordial Follicle Pool In The Mouse Ovary

Naturally, only a few follicles enter the growing follicle pool from primordial follicles each wave in mammalian ovary. However, why only a few follicles enter the growing follicle pool every time remains unknown. Recent studies using knockout mice model have showed that early follicle oocyte growth depends on signaling from the tuberous sclerosis complex (TSC), the mammalian target of rapamycin complex1(mTORCl), phosphatase and tensin homolog deleted on chromosome10(PTEN), and phosphatidylinositol3kinase (PI3K) pathways. However, whether transforming growth factor beta (TGF-β) has function on them is unknown. This study aims to identify the physiological roles of TGF-P signaling during primordial follicle activation.First of all, TGF-β1and TGF-P receptor type I (TGF-PR1) expressed in the oocytes of cysts, primordial and primary follicles in18.5days post coitus (dpc),1days post parturition (dpp) and4dpp mouse ovaries and started expressing in the cuboidal granulosa cells of primary follicles. Western blot results revealed that the protein levels of TGF-β1and TGF-βR1sharply decreased between4dpp and7dpp mouse ovaries.We found that the growth of primordial follicle oocytes in TGF-β1-treated ovaries was inhibited, while the growth of oocytes in SD208(a specific inhibitor of TGF-βR1)-treated ovaries was accelerated with an in vitro ovary culture system. Furthermore, TGF-β1dose-dependently rescued the abnormal primordial follicle activation in SD208-treated ovaries.To determine the cause of rapid oocyte growth, we found that there was dramatically less proliferation of granulosa cells and more TUNEL-positive oocytes in TGF-β1-treated ovaries and significantly more proliferation in SD208-treated ovaries as shown by5-bromo-2’-deoxyuridine (BrdU) incorporation assays and immunohistochemical staining for proliferating cell nuclear antigen (PCNA) and TUNEL assays.To investigate the molecular mechanisms underlying the accelerated enlargement of oocytes, we found that the phosphorylation levels of Akt (S473) and FOXO3a (Thr32) in SD208-treated ovaries were similar to control and TGF-β1-treated ovaries. However, the phosphorylation of S6K1at its threonine389(p-S6K1, T389) and the phosphorylation of rpS6at its serine235and236(p-rpS6, S235/6) and at its serine240and244(p-rpS6, S240/4) were elevated in SD208-treated ovaries but reduced in TGF-β1-treated ovaries compared with control ovaries. In other words, TGF-P signaling may regulate primordial follicle growth through activation of p70S6kinase1(S6K1)/ribosomal protein S6(rpS6) signaling in mouse ovaries.In conclusion, our results suggest that TGF-P signaling plays an important physiological role in the maintenance of the dormant pool of primordial follicles, which functions through activation of S6K1/rpS6signaling in mouse ovaries.