ELEVATED-TEMPERATURE INCREASES HEAT-SHOCK PROTEIN-70 SYNTHESIS IN BOVINE 2-CELL EMBRYOS AND COMPROMISES FUNCTION OF MATURING OOCYTES

Exposure of heifers to heat stress during oocyte maturation leads to e mbryos with reduced development. The objectives of the present study w ere threefold: 1) to evaluate effects of heat shock on oocyte function as assessed by cleavage and development, and associated effects on me mbrane integrity and protein synthesis, 2) to determine whether respon ses of oocytes to heat shock are modified by the presence of cumulus c ells, and 3) to determine whether oocytes and 2-cell embryos are capab le of synthesizing heat shock proteins in response to heat shock. Expo sure of cumulus oocyte complexes (COCs) to 41 degrees C did not alter the number of embryos that cleaved but reduced the number that develop ed to the blastocyst stage. In contrast exposure to 42 degrees C reduc ed both cleavage and development rates. Maturation of COCs for 12 or 2 4 h at 41 degrees C or 42 degrees C did not affect membrane integrity as determined by fluorescein diacetate/ethidium bromide staining. Howe ver, exposure to 41 degrees C or 42 degrees C reduced protein synthesi s by oocytes when occurring during the first or last 12 h of maturatio n. Removal of cumulus cells prior to maturation reduced protein synthe sis by oocytes; furthermore, exposure to 42 degrees C caused a greater percentage of reduction in protein synthesis in denuded oocytes than in those with intact cumulus. Analysis of [S-35]methionine and [S-35]c ysteine-labeled proteins by two-dimensional SDS-PACE and fluorography showed that nonmatured oocytes at 39 degrees C produced heat shock pro tein 68 (HSP68) and two other putative heat shock proteins of 71 and 7 0 kDa (P71 and P70, respectively); exposure to 42 degrees C did not in crease synthesis of any of these proteins. The same was true for matur ed oocytes, except that P71 was absent at 39 degrees C and 42 degrees C. Two-cell embryos synthesized P70, P71, and slight amounts of HSP68 at 39 degrees C; HSP68 synthesis was greatly increased at 42 degrees C . Results indicate that oocyte thermolability may be due to alteration s in protein synthesis and absence of heat inducibility of heat shock proteins. Further, heat shock of bovine embryos induces alterations in protein synthesis and possibly gene expression as early as the 2-cell stage of development. Results also suggest a possible thermoprotectiv e role for cumulus cells during oocyte maturation.