Dry period heat tolerance affects placental efficiency, apoptosis, and protein expression in Holstein cows

The placenta plays a key role in fetal growth and functions to transport nutrients that are vital to the fetus. Heat stress (HS) impairs placental function in cows, affecting fetal development. There is limited information on the effects of HS on the placenta of heat-tolerant and -sensitive dry cows. The objective of this study was to assess differences in placental efficiency, morphology, apoptosis, and placental proteomics in heat-tolerant and -sensitive dry cows, and to investigate the underlying mechanisms of the effect. Sixty-six dry cows were measured for respiratory rate, body surface temperature, and microenvironmental temperature-humidity index, by 56.94 ± 16.04 bpm, 36.11 ± 0.38°C, and 80.65 ± 3.31 accordingly, indicating that the cows were exposed to HS. A mixed-effects model was applied to analyze the measured indicators, and clustering was used to classify them as heat-tolerant dry cows (n = 19) and heat-sensitive dry cows (n = 47). Placentals were collected from heat-tolerant (n = 8) and -sensitive (n = 13) dry cows. The placental weight, number of placental cotyledons, and fetal birth weight were recorded. Placental efficiency (calf birth weight: placental weight ratio), morphology, apoptosis, and proteomic differences were assessed in heat-tolerant and -sensitive dry cows. Five placental cotyledon samples from each group underwent hematoxylin and eosin staining, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, and label-free proteomic analysis to assess microstructure, apoptosis, and differential protein expression, respectively. Results indicate that placental efficiency was significantly higher in heat-tolerant dry cows compared with heat-sensitive dry cows. Heat-tolerant cows exhibited a significantly lower apoptosis index (TUNEL+ cells: the total number of cells ratio) compared with heat-sensitive cows. A total of 3,691 proteins were identified, with 160 showing differential expression. The endocytosis pathway was significantly enriched in both Gene Ontology analysis and Kyoto Encyclopedia of Genes and Genomes pathway enrichment. Heat tolerance of heat-stressed dry cows improves placental efficiency, exhibits lower placental apoptosis, and maintains nutrient transport of placenta through the endocytosis pathway, which are likely due to the coordinated action of multiple pathways, contributing to better overall placental function in heat-tolerant cows.