ONTOGENY OF TEMPERATURE-REGULATED HEAT-SHOCK-PROTEIN-70 SYNTHESIS IN PREIMPLANTATION BOVINE EMBRYOS

Development of the preimplantation embryo is very susceptible to disru ption by heat shock. As embryos proceed through development, they acqu ire resistance to heat shock, perhaps because they become transcriptio nally active and can respond to environmental changes by undergoing tr anscriptionally-regulated cellular adaptation. Objectives were to dete rmine the ontogeny of heat shock protein 70 (HSP70) synthesis in preim plantation bovine embryos and to ascertain whether heat-induced increa ses in HSP70 in embryos are caused by environmental alterations in gen e expression. Exposure of bovine embryos to heat shock induced synthes is of a 68 kDa form of HSP70 called HSP68 as early as the two-cell sta ge of development. Induction of HSP68 was alpha-amanitin independent a t the two-cell stage but was blocked by alpha-amanitin as early as the early four-cell stage. There fore, heat-induced synthesis of HSP68 is regulated at the level of transcription at a time before the major ro und of embryonic genome activation is considered to occur. Two other c onstitutive HSP70 molecules were identified called heat shock cognates (HSC) 71 and 70; both proteins were synthesized during all stages of development from the two-cell to hatched blastocyst stages. However, h eat-induced synthesis of HSC71 and HSC70 was not evident until the exp anded blastocyst stage. in conclusion, environmental signals can activ ate gene expression before the major round of embryonic genome activat ion occurs in bovine embryos. Moreover, differences in thermal sensiti vity of early embryos to heat shock is not caused by an inability to s ynthesize HSP70, suggesting that other mechanisms are involved in deve lopmental acquisition of thermotolerance. (C) 1997 Wiiey-Liss, Inc.