HYPOXANTHINE REGULATION OF OOCYTE MATURATION IN THE MOUSE - INSIGHTS USING HYPOXANTHINE PHOSPHORIBOSYLTRANSFERASE-DEFICIENT ANIMALS

In this study the effects of hypoxanthine (HX) on meiotic maturation w ere compared using oocytes from mice possessing a hypoxanthine phospho ribosyltransferase null mutation (HPRT-) and from the corresponding HP RT-competent background strain (HPRT+). Oocyte-cumulus cell complexes and cumulus cell-enclosed oocytes (oocytes cultured while enclosed by cumulus cells) from HPRT+, but not HPRT-, mice took up HX and containe d significant levels of HPRT activity. In addition, FSH increased, and HX suppressed, the de novo synthesis of purines in HPRT+ complexes, w hereas de novo synthesis was elevated in HPRT- complexes and was unaff ected by FSH or HX. After 3 h of HX treatment, lower frequencies of ge rminal vesicle breakdown (GVB) were observed in cumulus cell-enclosed than in denuded HPRT+ oocytes; however, identical frequencies of matur ation were observed in denuded and cumulus cell-enclosed HPRT- oocytes . This demonstrates a direct inhibitory action of HX on the oocyte tha t does not depend on salvage, plus an additional action of the cumulus cells that requires HPRT activity. Nevertheless, cumulus cells from H PRT mice are capable of exerting an additional inhibitory action of di butyryl cAMP (dbcAMP) on the oocyte. A kinetics analysis of FSH action on HX-arrested cumulus cell-enclosed HPRT+ and HPRT- oocytes revealed , first, that the inhibitory effect of the cumulus cells is transient and, second, that HPRT activity is not required for FSH induction of G VB in HX-arrested oocytes. When dbcAMP- or HX-arrested oocytes were tr eated with FSH, GVB was blocked to the same extent in HPRT- oocytes wi th the purine de novo synthesis inhibitor, azaserine, but this drug wa s less effective in HX-treated HPRT+ oocytes. These results confirm th e importance of the de novo pathway in hormone-induced maturation and also support a role for purine salvage as an alternative source of nuc leotide in this process.