Abstract
Investigation of cellular responses to thermal heat stress can lead to identifying novel targets of intervention to tackle the increasing fertility decline due to climate change-associated seasonal heat stress. We aimed to study the response of granulosa cells to seasonal thermal stress. Dry beef cows (n=11) were kept together during the study and subjected to synchronization and stimulation for follicular dynamics. Ovum pick-up (OPU) was conducted on all animals in the winter and summer. Once OPU-fluid settled, cumulus-oocyte-complexes (COCs) and granulosa cells were isolated form the OPU fluid and stored in a -80 freezer after snap freezing. In addition, blood samples and rectal temperature data were collected on each OPU. Environmental data were collected daily three weeks before each OPU using the Florida Automated Weather Network. Data was analyzed using the Glimmix procedure of SAS. Data are presented as mean ± standard error of the mean. RNA was extracted from five pools of granulosa cells followed by library preparation and sequencing (NovaSeq; Illumina). As expected, environmental conditions were contrasting: average air temperature (11.5°C vs. 27.5°C), average max air temperature (16.9°C vs. 33.7°C), relative humidity (83.5% vs. 82.3%), and temperature-humidity index (53.39 vs. 79.16) for winter and summer, respectively. Average rectal temperature was higher (P = 0.03) in summer (39.2 ± 0.2°C) than winter (38.8 ± 0.2°C). Of the total differentially expressed genes, granulosa cells during summer showed up and down regulation of 971 and 1038 transcripts, respectively compared to the winter season (Fold Change ≤ -2 or ≥ 2 and FDR p-value ≤ 0.05). Those upregulated transcripts are found to be involved in cell cycle and division, regulation of cell death, steroid biosynthesis, progesterone mediated oocyte maturation and oocyte meiosis pathways. In conclusion, exposure of cows to thermal stress can induce significant alteration in transcriptome of granulosa cells which may have negative impact on follicular physiology.