Meiosis-Activating Sterol Synthesis in Rat Preovulatory Follicle: Is It Involved in Resumption of Meiosis?1

Abstract

Meiosis-activating sterol (MAS) was shown to overcome the inhibitory effect of hypoxanthine on spontaneous maturation of mouse oocytes and was suggested to mediate the stimulation of meiosis by gonadotropins. Follicular fluid (FF)-MAS is synthesized by cytochrome P450 lanosterol 14alpha-demethylase (LDM). Follicular LDM was preferentially localized in oocytes by immunohistochemistry. Using [3H]acetate or R-[5-3H]mevalonate as precursors as well as high-performance liquid chromatographic and thin-layer chromatographic separation, we have measured the concentrations of de novo-synthesized lanosterol, FF-MAS, and cholesterol in rat graafian follicles, cumulus-oocyte complexes (COCs), and denuded oocytes (DOs) treated with LH, AY-9944 (an inhibitor of Delta14-reductase, which was anticipated to increase FF-MAS levels by inhibiting its metabolism), or both after 8 h of culture. In follicles, both LH and AY-9944 increased the accumulation of FF-MAS as compared to controls. In COCs, AY-9944 caused a marked increase in FF-MAS, but we were unable to detect accumulation of FF-MAS in DOs. Neither the endogenous increases in FF-MAS accumulation nor the addition of FF-MAS to the culture medium could overcome the inhibition on resumption of meiosis by phosphodiesterase inhibitors. Compared to LH-induced resumption of meiosis in follicles, that induced by AY-9944 was much delayed. These results call into question any role of FF-MAS as an obligatory mediator of LH activity on germinal vesicle breakdown. The discrepancy between the positive staining for LDM in oocytes and our inability to detect de novo synthesized FF-MAS in DOs may relate to the sensitivity of the methodology employed and either the number of oocytes used or a deficiency in LDM synthetic activity in such oocytes. Further studies are required to confirm any of these alternatives.

Autologous mature follicular fluid: its role in in vitro maturation of human cumulus-removed oocytes

“…Therefore the existence of cumulus cells has a positive impact on the maturation and subsequent development of oocytes. This observation is confirmed by studies in other mammals (9)(10)(11)(12)(13).…”

cDNA Cloning, Genomic Structure and Expression Analysis of the Bovine Lanosterol 14.ALPHA.-Demethylase (CYP51) in Gonads

“…The proposal was supported by recent study that LH and AY-9944 (an inhibitor of Delta14-reductase) can increased the accumulation of FF-MAS in follicles and COCs after 8 h culture, but they were unable to detect the accumulation of FF-MAS in DOs. 23) In our result the expression of CYP51 reached highest level in corpus lutea (Fig. 6).…”

Differential response of cumulus cell-enclosed and denuded mouse oocytes in a meiotic induction model system

“…MAS is particularly potent in DO and has been proposed to be the putative meiosis-inducing factor secreted by CCs under coculture conditions (Byskov et al, 1997). However, results from several labs have raised doubts as to the physiological role of MAS (Tsafriri et al, 1998;Hegele-Hartung et al, 1999Downs et al, 2001;Vaknin et al, 2001;Cao et al, 2004), and its involvement in meiotic regulation in vivo remains unclear. Other factors advanced as potential paracrine modifiers of meiotic maturation include nitric oxide (Bu et al, 2003), progesterone (Yamashita et al, 2003), androgens (Gill et al, 2004), Leydig insulin-like 3 (Kawamura et al, 2004), pyruvate (Downs, 2001), and epidermal growth factor family members (Tsafriri et al, 1989;Ashkenazi et al, 2005).…”