Purine control of mouse oocyte maturation: Evidence that nonmetabolized hypoxanthine maintains meiotic arrest

Abstract

Hypoxanthine is present in preparations of follicular fluid and has been shown to suppress the spontaneous meiotic maturation of mammalian oocytes in vitro. The present experiments examined the possible role of hypoxanthine metabolism in mediating this meiotic arrest. Four putative inhibitors of the enzyme, hypoxanthine phosphoribosyltransferase (HPRT), which metabolizes hypoxanthine to inosine monophosphate, were tested on lysates of oocyte-cumulus cell complexes. At a concentration of 1 mM, 6-mercapto-9-(tetrahydro-2-furyl)-purine (MPTF) and 6-mercaptopurine (6-MP) suppressed enzymatic activity by 86% and 98%, respectively, while 6-azauridine and 2,6-bis-(hydroxyamino)-9-beta-D-ribofuranosyl-purine had no effect. MPTF and 6-MP increased the inhibitory effect of hypoxanthine on germinal vesicle breakdown, but the other agents did not. The 2 active agents had similar effects on salvage activity and hypoxanthine-maintained meiotic arrest in denuded oocytes. Also, oocytes from XO mice were more sensitive to the meiosis-arresting action of hypoxanthine than oocytes from XX littermates, which have twice the HPRT activity. The actions of the HPRT inhibitors were not due to their conversion to nucleotides via HPRT and negative feedback on purine de novo synthesis, because azaserine and 6-methylmercaptopurine riboside, which are more potent inhibitors of de novo synthesis, had a stimulatory, rather than inhibitory, effect on hypoxanthine-arrested oocytes. Furthermore, several lines of evidence indicate that metabolism of hypoxanthine to xanthine and uric acid by xanthine oxidase does not mediate the inhibitory action of this purine base on meiotic maturation. The data therefore suggest that nonmetabolized hypoxanthine is responsible for the meiotic arrest observed, most likely through suppression of cAMP degradation.

Epiregulin can effectively mature isolated cumulus–oocyte complexes, but fails as a substitute for the hCG/epidermal growth factor stimulus on cultured follicles

“…Considering the fact that CM maintained oocyte meiotic arrest more efficiently (P!0.001) than FM suggests the possibility that meiotic arrest could be sustained by a secreted inhibitory factor of follicular origin , Downs 1993) causing resistance to EREG or EGF stimulation.…”

Inhibition of T lymphocyte activation in mice heterozygous for loss of the IMPDH II gene

“…Of the blastocysts analyzed, only 6% demonstrated targeting of both IMPDH II alleles, strongly suggesting that loss of the IMPDH type II enzyme does not permit the further development of fertilized ova. Previous studies have suggested a critical role for purines, especially for guanosine and hypoxanthine, in the maintenance of oocytes in meiotic arrest (29). Inhibitors of IMPDH such as MPA induced premature meiotic maturation and resulted in a significant loss of implantation capacity and viability after implantation (30).…”

Aging-related premature luteinization of granulosa cells is avoided by early oocyte retrieval

“…Transcription in oocytes depends on the presence of attached CCs (De La Fuente & Eppig 2001). In addition, the maintenance of oocyte arrest before recruitment also relies on the contribution of autocrine and paracrine factors synthesized in GCs, including cAMP/cGMP (Webb et al 2002, Wigglesworth et al 2013, Shuhaibar et al 2015, purine (Downs 1993), kit ligand (Ye et al 2009), and NPR2 , Tsuji et al 2012, Wigglesworth et al 2013. Oocytes from antral follicles resume and complete meiosis spontaneously after removal of surrounding CCs (Buccione et al 1990, Mehlmann 2005, suggesting that CCs control oocyte nuclear maturation.…”