Onset of RNA synthesis in early bovine embryos detected by reverse transcription-polymerase chain reaction following introduction of exogenous gene into their pronuclei

Induced thermotolerance in bovine two-cell embryos and the role of heat shock protein 70 in embryonic development

Induced thermotolerance is a phenomenon whereby exposure to a mild heat shock can induce heat shock proteins (HSP) and other cellular changes to make cells more resistant to a subsequent, more severe heat shock. Given that the 2-cell bovine embryo is very sensitive to heat shock, but can also produce HSP70 in response to elevated temperature, experiments were conducted to test whether 2-cell embryos could be made to undergo induced thermotolerance. Another objective was to test the role of the heat-inducible form of heat shock protein 70 (HSP70i) in development and sensitivity of bovine embryos to heat shock. To test for induced thermotolerance, 2-cell bovine embryos were first exposed to a mild heat shock (40 degrees C for 1 hr, or 41 degrees C or 42 degrees C for 80 min), allowed to recover at 38.5 degrees C and 5% (v/v) CO2 for 2 hr, and then exposed to a severe heat shock (41 degrees C for 4.5, 6, or 12 hr). Regardless of the conditions, previous exposure to mild heat shock did not reduce the deleterious effect of heat shock on development of embryos to the blastocyst stage. The role of HSP70i in embryonic development was tested in two experiments by culturing embryos with a monoclonal antibody to the inducible form of HSP70. At both 38.5 degrees C and 41 degrees C, the proportion of 2-cell embryos that developed to blastocyst was reduced (P < 0.05) by addition of anti-HSP70i to the culture medium. In contrast, sensitivity to heat shock was not generally increased by addition of antibody. In conclusion, bovine 2-cell embryos appear incapable of induced thermotolerance. Lack of capacity for induced thermotolerance could explain in part the increased sensitivity of 2-cell embryos to heat shock as compared to embryos at later stages of development. Results also implicate a role for HSP70i in normal development of bovine embryos.

Transcriptional control of development, protein synthesis, and heat-induced heat shock protein 70 synthesis in 2-cell bovine embryos

Experiments were performed to evaluate the role of transcription in early development of bovine embryos. Two transcription inhibitors-5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) and actinomycin D-were used to test whether 1) the inhibitors alter the rate of early embryonic development and protein synthesis, 2) heat shock increases the steady-state amounts of mRNA for the inducible form of heat shock protein 70 (HSP70) in embryos, and 3) this latter effect is blocked by transcription inhibitors. Addition of either DRB or actinomycin D to culture medium beginning 8 h postinsemination (hpi) reduced the proportion of oocytes that had undergone cleavage by 32-34 hpi. Both transcription inhibitors also reduced the proportion of cleaved embryos that reached the 4-cell stage by 32-34 hpi. Incorporation of (35)S-labeled amino acids into de novo synthesized protein by bovine 2-cell embryos was lower for embryos cultured with DRB. Using reverse transcription-polymerase chain reaction, HSP70 mRNA in 2- and 4-cell embryos was increased by exposure to 42 degrees C. Both inhibitors reduced amounts of HSP70 mRNA at 42 degrees C. Results indicate that bovine embryos can undergo transcription in response to heat shock as early as the 2-cell stage. Moreover, the observations that transcription inhibitors reduce rates of cleavage and early development point out the importance of transcription for development from the earliest period of embryonic life.