A DELAYED-IMPLANTATION-ASSOCIATED PROTEIN PREVENTS RESUMPTION OF DNA-SYNTHESIS BY THE TROPHECTODERM OF DELAYED-IMPLANTING MOUSE BLASTOCYSTSIN-VITRO

Objective of this study was to determine the ability of a delayed-impl antation-associated protein (MW 170,000, DIAP170K) to inhibit DNA synt hesis by mouse blastocysts. Mice were ovariectomized on day 3 of pregn ancy and treated with daily injections with I mg progesterone till day 7 to induce delayed implantation. Blastocysts were collected on day 8 with or without a single injection of 25 ng estradiol-17 beta on day 7 that activates blastocyst metabolisms (activated blastocysts and del ayed-implanting blastocysts respectively). DNA synthesis was determine d by measuring [H-3]thymidine incorportion by blastocysts. DIAP170K at 10 mu g/ml suppressed resumption of DNA synthesis by delayed-implanti ng blastocysts and suppression was maximal at 50 mu g/ml. However, DIA P170K did not affect DNA synthesis by blastocysts obtained on day 5 of pregnancy (normal blastocysts) and activated blastocysts. Resumption of DNA synthesis in the inner cell mass (ICM) and trophectoderm from d elayed-implanting blastocysts was then separately assessed. DNA synthe sis resumed in the trophectoderm of intact blastocysts during 24-hr cu lture but not in the trophectoderm cultured apart from the ICM. DIAP17 0K inhibited the resumption of DNA synthesis by the trophectoderm of i ntact delayed-implanting blastocysts but did not affect DNA synthesis by the ICM. In conclusion, DIAP170K inhibits resumption of DNA synthes is by trophectoderm of delayed-implanting blastocysts. This action of DIAP170K may play a central role in maintaining, but not achieving, do rmancy of DNA synthesis by delayed-implanting blastocysts in mice.