Oocyte secretion is regulated by somatic cells during oocyte maturation and mediates cumulus cell function

OBJECTIVE: De novo mRNA synthesis ceases during the final stages of oocyte maturation and previously synthesized mRNAs are translated accord- ing to a well-orchestrated program of recruitment to the polysomes. The aim of this study is to investigate the regulation of oocyte mRNA translation and protein secretion during maturation, using interleukin (IL)-7 as the proto- typic secreted factor, and to determine the role of this oocyte secreted factor in cumulus cell (CC) function. DESIGN: Prospective. MATERIALS AND METHODS: Studies involving in vivo/in vitro matu- ration and fertilization of oocytes from 21 day old C57BL/6 mice, and human follicular fluid before and after hCG administration. RESULTS: Microarray analysis of oocyte polysomal fraction showed that 191 of 7600 polysome-bound mRNAs encoded for secreted proteins. Specif- ically, 72 transcripts were constitutively present in the polysome fraction throughout in vivo oocyte maturation, while 80 were decreased and 39 were increased. IL-7 was selected as the prototype because its translation increased the most during oocyte maturation. In qPCR analysis, the level of polysome- bound IL-7 mRNA enhanced in MI oocyte compared to GVoocyte, and further increased with maturation to MII. Total IL-7 mRNA levels did not change dur- ing oocyte maturation. Renilla luciferase reporter under the control of IL-73’UTR was injected into cumulus enclosed oocytes (CEOs). Translation of the IL-7 reporter significantly increased as the oocytes progressed from GV to MII, and was further stimulated by amphiregulin (AREG), a somatic cell- derived EGF-like growth factor that accumulates in the follicle after LH surge. AREG-induced effect was not detected when the oocytes were denuded prior to stimulation. IL-7 protein secretion increased with oocyte maturation in CEOs and, to a less extent in denuded oocytes (DOs). AREG further enhanced IL-7 secretion in CEOs, but not in DOs. IL-7 was not detectable in spent media of CC only cultures. In humans, IL-7 levels were significantly higher in the follicular fluid containing MII oocyte (n1⁄442) compared to those containing GV oocyte (n1⁄48). IL-7 levels positively correlated with AREG concentration in the follicular fluid. After fertilization, IL-7 secretion diminished in both mouse and human embryos. In mouse, IL-7 receptor mRNA expression in CCs increased during oocyte maturation. IL-7 increased the proliferation of CCs, but did not affect CC expansion. CONCLUSIONS: Oocyte secretion during maturation is highly dynamic and regulated mRNA translation is the molecular mechanism underlying this timed secretion. The oocyte secretion is sensitive to somatic cues and modulates CC function supporting the concept that these regulated secretions are the part of cross-talk between the oocyte and surrounding CCs, which is crucial for the oocyte developmental competence. Supported by: RO1-GM097165 and T32 HD007263-28.