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MOLECULAR RESPONSES OF HEAT STRESS DURING EARLY EMBRYONIC DEVELOPMENT AND ALLEVIATION STRATEGIES

It is well-known that exposure of female farm animals to heat elevation negatively affects all biological activities required to establish pregnancy. The reduction in pregnancy rate during the hot season can approach 50% in dairy cows and 60% in dairy buffaloes. Indeed, earlier investigations done in buffalo indicated seasonal fluctuations of oocyte quality and most studies confirmed a significant reduction in morphological oocytes quality when recovered during hot season. In vivo studies have reported a clear reduction in meiotic maturation of oocytes exposed to heat stress due to decreased blood flow to reproductive tract, low progesterone level and increased glucocorticoids profile. Buffalo oocytes recovered during summer had a lower percentage of nuclear maturation in vitro (42.9%) compared to those collected in winter (85%). Heat shock at 42 °C for 6 hours during in vitro maturation of camel oocytes significantly decreased the polar body extrusion rate (20.7%) compared to control group (32.3%). At the cellular level, decreased nuclear maturation during heat stress was coupled with reduction in mitochondrial activity, protein biosynthesis and impairment of cellular microstructure system organizing nuclear progression to accomplish maturation such as spindle formation, microfilament and microtubule distribution. Heat-induced alterations in cytoplasmic as well as nuclear maturation were coupled with down-regulation of oocyte transcripts such as GDF9, POU5F1, and C-MOS and genes involved in metabolism (GLUT1) and antioxidant defense (SOD2) that regulate maturation process and preimplantation development. These cellular and molecular defects led to compromising early cleavage and blastocyst formation. Blastocyst rate was lower (42%) for camel oocytes exposed to heat shock compared with control counterparts (48%). The rate of developed buffalo blastocyst was lower during summer (13%) than winter (28.3%) which explained by high rate of chromosome abnormalities and alteration of genes regulating key molecular processes that compromise viability of embryos. Providing females farm animals with optimum management system including housing is the first strategy to reduce environmental heat load. Changing composition of animal feeds is a second possible way to alleviate heat stress. Genetic selection for heat tolerance is long term breeding goal with continuous global warming that ensure good productive as well as reproductive performance.