Molecular and cellular adaptations of the mammary gland regulating milk constitutes and immunity of heat stressed dairy cows

The molecular and cellular mechanisms underlying the adaption of the mammary gland (MG) to increased ambient heat, and which account for changes in milk composition and immunity are not known yet. This study aimed to elucidate the expression of MG proteins and differences in milk composition and ruminal short-chain fatty acid concentrations of dairy cows experiencing 7 days of heat stress (28°C, THI = 76), pair-feeding (PF) or ad libitum feeding (CON) at thermoneutrality (16°C, THI = 60; each n=10). The feed intake of PF cows were matched to the intake of HS cows. The portion of ruminal acetate, the acetate:propionate ratio, and milk urea concentration were greater whereas milk protein yield and lactose yield were lower in HS than CON cows on day 3 and 6 of the challenge, whilst energy and fat corrected milk were lower in HS than CON and PF cows. The proteome analysis revealed induced bacterial invasion of epithelial cells, leukocyte transendothelial migration, reduction of the pyruvate and carbon metabolism, and platelet activation in the MG of HS compared to CON and/or PF cows. These results highlight adaptive metabolic and immune responses to mitigate the negative effects of ambient heat in the mammary gland.

目前尚不明确乳腺（mammary gland, MG）适应环境高温的分子与细胞机制，以及该机制如何介导乳成分与免疫功能的变化。本研究旨在阐明热中性环境（16℃，温度湿度指数Temperature-Humidity Index, THI=60，每组n=10）下，分别经受7天热应激（heat stress, HS；28℃，THI=76）、配对饲喂（pair-feeding, PF）与自由采食对照组（ad libitum feeding, CON）的奶牛的乳腺蛋白表达情况，以及乳成分与瘤胃短链脂肪酸浓度的差异。配对饲喂组奶牛的采食量与热应激组保持一致。在热应激挑战第3天和第6天时，热应激组奶牛的瘤胃乙酸占比、乙酸与丙酸比值及乳尿素浓度均显著高于对照组，而乳蛋白产量与乳糖产量则显著低于对照组；同时热应激组的能量与脂肪校正乳产量亦显著低于对照组与配对饲喂组。蛋白质组学分析显示，相较于对照组和/或配对饲喂组，热应激组奶牛乳腺组织中上皮细胞细菌侵袭、白细胞跨内皮迁移通路被激活，丙酮酸代谢与碳代谢水平下调，且血小板活化状态发生改变。本研究结果揭示了奶牛乳腺为缓解环境高温带来的负面影响所启动的适应性代谢与免疫应答机制。