Abstract
Although previous studies have suggested that cumulus cells (CCs) accelerate oocyte aging by secreting soluble and heat-sensitive paracrine factors, the factors involved are not well characterized. Because Fas-mediated apoptosis represents a major pathway in induction of apoptosis in various cells, we proposed that CCs facilitate oocyte aging by releasing soluble Fas ligand (sFasL). In this study, we reported that when the aging of freshly ovulated mouse oocytes were studied in vitro, both the apoptotic rates of CCs and the amount of CCs produced sFasL increased significantly with the culture time. We found that oocytes expressed stable levels of Fas receptors up to 24 h of in vitro aging. Moreover, culture of cumulus-denuded oocytes in CCs-conditioned CZB medium (CM), in CZB supplemented with recombinant sFasL, or in CM containing sFasL neutralizing antibodies all showed that sFasL impaired the developmental potential of the oocytes whereas facilitating activation and fragmentation of aging oocytes. Furthermore, CCs from the FasL-defective gld mice did not accelerate oocyte aging due to the lack of functional FasL. In conclusion, we propose that CCs surrounding aging oocytes released sFasL in an apoptosis-related manner, and the released sFasL accelerated oocyte aging by binding to Fas receptors.
Highlights
Previous studies have suggested that cumulus cells (CCs) accelerate oocyte aging by secreting soluble and heat-sensitive paracrine factors, the factors involved are not well characterized
Because Fas-mediated apoptosis represents a major pathway in induction of apoptosis in various cells, we proposed that CCs facilitate oocyte aging by releasing soluble Fas ligand
Western blot analysis revealed similar dynamics fluctuations of Fas receptors during oocyte aging (Fig. 2F). These results suggested that CCs released soluble Fas ligand (sFasL) in an apoptotic state-related manner; the maximal release was observed at 36 h of culture, and the presence of H2O2 further increased the apoptotic rates and the sFasL secretion of CCs
Summary
Previous studies have suggested that cumulus cells (CCs) accelerate oocyte aging by secreting soluble and heat-sensitive paracrine factors, the factors involved are not well characterized. Because Fas-mediated apoptosis represents a major pathway in induction of apoptosis in various cells, we proposed that CCs facilitate oocyte aging by releasing soluble Fas ligand (sFasL). Studies on mechanisms of oocyte aging are important for both normal and assisted reproduction Oocytes that mature both in vivo and in vitro are enclosed within cumulus cells (CCs), forming the so-called cumulus-oocyte-complexes (COCs). One of our studies showed that medium conditioned with CCs promoted aging of cumulus-denuded oocyte (DOs) in vitro but the aging-promoting effect is ablated when the conditioned medium (CM) was heated to 56uC for 15 min[22] This suggests that CCs accelerate oocyte aging by secreting soluble and heat-sensitive factors. It is worthy of studying whether the Fas/FasL system plays any role in oocyte aging
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