Elevated temperature increases heat shock protein 70 synthesis in bovine two-cell embryos and compromises function of maturing oocytes.

Abstract

Exposure of heifers to heat stress during oocyte maturation leads to embryos with reduced development. The objectives of the present study were threefold: 1) to evaluate effects of heat shock on oocyte function as assessed by cleavage and development, and associated effects on membrane integrity and protein synthesis, 2) to determine whether responses of oocytes to heat shock are modified by the presence of cumulus cells, and 3) to determine whether oocytes and 2-cell embryos are capable of synthesizing heat shock proteins in response to heat shock. Exposure of cumulus oocyte complexes (COCs) to 41 degrees C did not alter the number of embryos that cleaved but reduced the number that developed to the blastocyst stage. In contrast, exposure to 42 degrees C reduced both cleavage and development rates. Maturation of COCs for 12 or 24 h at 41 degrees C or 42 degrees C did not affect membrane integrity as determined by fluorescein diacetate/ethidium reduced protein synthesis by oocytes when occurring during the first or last 12 h of maturation. Removal of cumulus cells prior to maturation reduced protein synthesis 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 [35S]methionine and [35S]cysteine-labeled proteins by two-dimensional SDS-PAGE and fluorography showed that nonmatured oocytes at 39 degrees C produced heat shock protein 68 (HSP68) and two other putative heat shock proteins of 71 and 70 kDa (P71 and P70, respectively); exposure to 42 degrees C did not increase synthesis of any of these proteins. The same was true for matured 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 alterations 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 thermoprotective role for cumulus cells during oocyte maturation.