UPTAKE AND SALVAGE OF HYPOXANTHINE MEDIATES DEVELOPMENTAL ARREST IN PREIMPLANTATION MOUSE EMBRYOS

Preimplantation mouse embryos become arrested after first or second cl eavage when cultured in hypoxanthine-supplemented Whitten's medium. We present evidence that the hypoxanthine-induced arrest is dependent on uptake and salvage of hypoxanthine and depletion of phosphoribosylpyr ophosphate (PRPP) levels. Hypoxanthine uptake increased during the 2-c ell stage and was augmented by glucose. HPLC analysis of [C-14]hypoxan thine metabolism revealed that hypoxanthine was salvaged and converted to ATP and guanosine triphosphate (GTP), with a shift to more guanyl nucleotide production at the 3- to 4-cell stage. In embryos from mice with a null mutation for the salvage enzyme hypoxanthine-guanine phosp horibosyltransferase, hypoxanthine did not block development nor was i t taken up by the embryos. Glucose, which is required for the hypoxant hine-induced arrest, produced a 5.3-fold increase in PRPP levels at th e 2-cell stage, which was eliminated by hypoxanthine. We conclude that metabolism of hypoxanthine to nucleotides mediates its inhibitory act ion on preimplantation mouse embryos via negative feedback on PRPP syn thetase, ultimately resulting in decreased PRPP availability and arres t of other PRPP-dependent pathways. Finally, reversal of the block by EDTA and cAMP-elevating agents may be mediated by alterations in hypox anthine or glucose uptake, or by changes in the relative metabolism of hypoxanthine.