Abstract
Although previous studies indicated that cumulus cells (CCs) accelerate oocyte aging by releasing soluble factors, the factors have yet to be characterized. While demonstrating that CCs promoted oocyte aging by releasing soluble Fas ligand (sFasL), our recent study suggested that CCs might secrete other factors to mediate oocyte aging as well. This study tested whether CCs accelerate oocyte aging by secreting tumor necrosis factor (TNF)-α. The results showed that mouse CCs undergoing apoptosis released soluble TNF-α (sTNF-α) during in vitro aging. While ethanol activation rates were higher, the maturation-promoting factor (MPF) activity was lower significantly after culture of cumulus-denuded oocytes (DOs) in medium conditioned with CCs for 36 h than in medium conditioned for 24 h. Aging mouse oocytes expressed TNF-receptor 1. The CCs released equal amounts of sTNF-α and sFasL during aging in vitro, and the TNF-α-knockdown CCs secreted less sFasL than the control CCs did. Treatment of DOs in vitro with sTNF-α significantly accelerated their aging. The aging-promoting effect of sTNF-α was significantly reduced in TNF-α-knocked-down CCs and in CCs from the TNF-α-knockout mice. It is concluded that mouse CCs accelerate oocyte aging by secreting sTNF-α as well as sFasL.
Highlights
If not fertilized or activated in time after ovulation or in vitro maturation, mammalian oocytes undergo a timedependent process of aging [1,2,3]
cumulus cells (CCs) recovered from newly ovulated oocytes were cultured in CZB medium alone or with 200 μM H2O2, and at different times of the culture, rates of apoptotic cells were observed following Hoechst or annexin-V staining, and concentrations of soluble TNF-α (sTNF-α) in the conditioned medium (CM) were measured
The results suggested that CCs underwent apoptosis and continuously released sTNF-α during in vitro aging, and that H2O2 significantly enhanced apoptosis and sTNF-α secretion in CCs
Summary
If not fertilized or activated in time after ovulation or in vitro maturation, mammalian oocytes undergo a timedependent process of aging [1,2,3]. This post-ovulatory oocyte aging process has been found to affect embryo development [4,5,6] and offspring [7,8]. The mechanisms for post-ovulatory oocyte aging are largely unknown Both in vivo and in vitro matured oocytes are enclosed within cumulus cells (CCs) forming the so-called cumulus-oocyte-complexes (COCs). Apoptotic CCs accelerated porcine oocyte aging and degeneration in vitro via a paracrine manner [13]. This suggests that CCs accelerate oocyte aging by releasing soluble and heat-sensitive factors, the oocyte aging-promoting factors that CCs release have yet to be characterized
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