JNK Signaling Regulates E-cadherin Junctions In Germline Cysts And Determines Primordial Follicle Formation In Mice

In mice, primordial germ cells (PGCs), called oogonia in females, reach the urogenital ridges at about 10.5 days post coitum (dpc). Subsequently, the oogonia initiate rapid mitotic division and form germline cysts accompanied by incomplete cytokinesis and an intercellular bridge connection. After oocytes progress through the leptotene, zygotene, pachytene, diplotene and arrest at the dictyate stage of meiotic prophase I, the germline cysts undergo programmed breakdown between 17.5 dpc and 4 days post partum (dpp). Lacking of germline stem cells, all the follicles available come from primordial follicle pool in female mammals. Therefore, proper cyst breakdown and folliculogenesis are essential for fertility, however, the mechanisms regulating this process remain a mystery.Considering the functional roles of JNK signaling in collective cells communication and migration, we hypothesize its participation in regulating cyst breakdown, which depends on the organelles transportation via balbiani body.Firstly, we show that JNK. was primarily expressed in oocyte cytoplasm and the levels of p-JNK gradually increased from 17.5 dpc to 4 dpp, which implies that JNK signaling is closely correlated with cyst breakdown and primordial follicle formation.Inhibition of JNK signaling with its specific inhibitor (SP600125) or knock-down technology resulted in significantly suppressed cyst breakdown and primordial follicle formation. In addition, our results show that E-cadherin was intensely expressed in germline cysts and its decline was necessary for oocyte release from the cyst. However, the inhibition of JNK signaling leads to aberrantly enhanced localization of E-cadherin at oocyte-oocyte contact sites. Moreover, JNK inhibition markedly reduced the expression of MDM2, which is in charge of E-cadherin degradation when germline cyst breakdown.The downtrend expression pattern of E-cadherin during perinatal ovary development implied that germline cyst breakdown is an EMT-like process. The expression of wnt4, another EMT-related gene, was upregulated after SP600125 treatment. Additionally, similar to SP600125 treatment, WNT4 overexpression delayed cyst breakdown; and is accompanied by abnormal E-cadherin expression patterns.In conclusion, our results suggest that JNK signaling, which is inversely correlated with WNT4, plays an important role in perinatal germline cyst breakdown and primordial follicle formation via regulating E-cadherin junctions between oocytes in mouse ovaries. Our study provides a new perspective for studying the mechanism of primordial follicle formation and has broad physiological and clinical implications for increasing our understanding of ovarian pathology.