Physiological impact of amino acids during heat stress in ruminants

Abstract

High seasonal heat and humidity decrease postruminal nutrient supply, deteriorate the ruminal and intestinal mucosa, and facilitate translocation of bioactive molecules into the bloodstream. Inflammation, oxidative stress, and misfolding of cellular proteins are physiological hallmarks of stress, including heat and humidity. These processes divert amino acid use away from productive purposes. Heat-stressed ruminants could benefit from increases in postruminal supply of arginine, cysteine, leucine, lysine, and methionine. One-carbon metabolism generates antioxidants from several indispensable and nondispensable amino acids as well as folic acid, choline, and betaine. ‘Omics’ tools are allowing discovery of physiological mechanisms that can be manipulated via the supply of specific amino acids to alleviate the impact of heat stress. The loss in value due to reductions in milk and meat production from heat stress (HS) worldwide by the end of the century have recently been estimated at $14.9 to $39.9 billion per year (Thornton et al., 2021). These losses are predicted to be most-severe in tropical regions where an increase in beef production, for example, to feed the growing population worldwide is expected by 2050 (Cooke et al., 2020). Common HS abatement practices including increasing shaded areas, increasing air velocity by use of fans, and the use of water-soaker lines to increase evaporative heat loss have been implemented on farms. Despite this, the continued increase in environmental temperature and the duration and frequency of droughts will impact not only animal numbers, but also productive efficiency worldwide and the economic return (Wankar et al., 2021). Although tackling the negative impact of global warming on productive efficiency and wellbeing requires multiple management approaches, greater knowledge of physiological mechanisms that are altered by HS will be valuable. This is particularly true in parts of the world where intensive production systems are characterized by animals of high-genetic merit that not only has resulted in marked increases in milk or beef production, but also in the amount of heat (i.e., heat increment) produced per unit of feed consumed (West, 2003; Cooke et al., 2020). Escalating global temperatures combined with global demand for livestock products has resulted in HS becoming an important ongoing challenge facing the global livestock industry. Work in model organisms clearly established that the key cellular adaptation driven by short- or long-term exposure to temperatures above the upper range (i.e., HS) is the misfolding and aggregation of misfolded proteins leading to impaired cellular function and disorders, including endoplasmic reticulum (ER) stress (Tyedmers et al., 2010). The importance of these cellular events stems from the fact that functionality of most proteins in the cell requires the formation of a proper three-dimensional structure, a process termed “folding”. Studies in the last 20 years using transcriptome, proteome, metabolome, and/or systemic biomarker analyses have confirmed that these cellular events occur in experimentally or seasonal HS cattle, i.e., animals experiencing increases of body temperature (from 1- to 2-wk in experimental challenges to several wk in seasonal challenges) in the range of 0.2 to 1.5 °C (Bernabucci et al., 2002; Kim et al., 2018; Pate et al., 2020; Koch et al., 2021, 2023; Ruiz-Gonzalez et al., 2023). Heat shock protein (HSP) HSP70 is a key “chaperone” in the protein quality control system that cells possess to facilitate the folding or refolding of misfolded proteins, hence, preventing protein aggregation. Upregulation of the spliced X-box binding protein 1 (XBP1s), a member of the cAMP-response element-binding/activating transcription factor b-ZIP family of transcription factors, is another key cellular response to HS that helps decrease protein misfolding (Loor and Elolimy, 2022). Production of HSP is controlled by heat shock factors (HSF, e.g., HSF1), which upon phosphorylation are activated and travel to the nucleus where they bind to heat shock elements and enhance transcription of HSP among other important targets (Tyedmers et al., 2010). The imbalance between reactive oxygen species (ROS) production and the available antioxidant defenses against them can induce oxidative stress, and is another key biological process that causes protein unfolding and misfolding (Tyedmers et al., 2010). Heat stress in livestock species including dairy cows and beef cattle is characterized by oxidative stress, inflammation, ER stress, and increased gut permeability (Cooke et al., 2020; Ma et al., 2021; Ruiz-Gonzalez et al., 2023) all of which can be assessed through tissue and systemic biomarkers such as inflammation-related proteins (e.g., cytokines, acute-phase proteins (APP), lipopolysaccharide binding protein). The sensitivity of these cellular systems to HS in ruminants is exemplified by the 5-fold increase in abundance of XBP1s in mammary tissue from cows experiencing subtle increases in body temperature (0.2–0.3 °C) (Pate et al., 2020, 2021). Although reductions in dry matter intake (DMI) as a result of HS are a well-known response in cattle and small ruminants (West, 2003; Cooke et al., 2020; Mishra, 2021), it is not often recognized (at least in dairy cows) that under those conditions the animal may increase the frequency of intake and water consumption as a “self-regulatory” mechanism with consequent impacts on ruminal fermentation, e.g., greater production of VFA and lower pH over extended periods of time (West, 2003). Such responses are expected to be more severe the longer that the animal experiences HS (Hou et al., 2021). Two studies recently evaluated responses of the ruminal epithelium in HS versus pair-fed (thermoneutral) lactating cows during a chronic mild [temperature-humidity index (THI) = 76] (Eslamizad et al., 2020) or moderate (THI = 83) (Guo et al., 2021) HS period using environmental chambers. The “pair-fed” approach helps determine the direct effects of HS without the confounding effect of reduced DMI. Although the mild HS did not alter genes or proteins in ruminal epithelium associated with tight-junctions (Eslamizad et al., 2020), histological analysis under moderate HS revealed greater shedding of the stratum corneum (Guo et al., 2021). This change was associated with reduced pH and greater total VFA concentrations in the rumen, with little effect on lipopolysaccharide (LPS) concentrations [an inflammatory molecule; (Plaizier et al., 2018)]. Beyond LPS, it is likely that negative impacts of HS on the ruminal epithelium would enhance the transport of other bioactive molecules as has been reported during episodes of subacute ruminal acidosis (Plaizier et al., 2018). The impacts of HS (THI = 76) on the small intestine (tissue and mesenteric lymph nodes) were reported recently in a study involving lactating cows exposed to HS for 4-d through heat chambers (Koch et al., 2019, 2021, 2023). Transcriptomics revealed a more predominant population of immune cells (mainly macrophages) in the jejunal epithelium as opposed to cells typical of intestinal epithelial lineage that were more predominant in pair-fed cows under thermoneutral conditions. Despite these differences, there was no impact of HS on the jejunal morphology, an effect that was evident in ruminal epithelium albeit when animals were exposed for a longer period (7-d) to moderate HS (THI = 83) (Guo et al., 2021). Other unique biological differences between ruminal and jejunal epithelium responses to HS pertain to molecular characteristics of the tissues. For example, whereas ruminal epithelium from cows experiencing HS exhibited a marked downregulation of genes associated with an inflammatory response, jejunal epithelium exhibited downregulation of some key tight junction proteins (tight junction protein 1, claudin 1 mRNA), upregulation of catalase, and upregulation of alkaline phosphatase (ALP). Some of these responses were also verified at the protein level, e.g., downregulation of catalase, and also highlighted upregulation in the abundance of several HSP (Koch et al., 2021). The decrease in abundance of tight-junction proteins has been associated with increased permeability and LPS transport in pigs, suggesting “gut and to increases in production and helps it to of helps bind LPS and in the gut (Koch et al., differences in the response of the ruminal and intestinal epithelium to HS be due to the unique biological function of tissue with the differences of the in these of the evident is that reductions in tissue during would a of physiological stress for the Transcriptomics have revealed unique physiological responses of the ruminal epithelium to an extended period of importance in the of is the marked upregulation of genes in associated with amino acid acid and of (Guo et al., 2021). was that among the and biological were and (Guo et al., 2021). These molecular were in the of histological and ruminal and that during HS the epithelial tissue but likely the use of such as and production to the increase in of the epithelium (Guo et al., 2021). it is likely that of responses at the ruminal and intestinal during HS are important of the in supply of to the mammary that often to lower milk protein et al., et al., 2021). associated with and and in the of and from genes using from ruminal epithelium of cows experiencing heat stress to pair-fed cows in thermoneutral conditions (Guo et al., 2021). of the using resulted in The in et al., 2020) was to the of the on the The and upregulation and of downregulation of and in dairy and beef cattle to sensitivity and is more to during events which are to the of (i.e., as by in and increased concentrations et al., et al., Kim et al., 2022). the fact that abundance of proteins associated with (e.g., and did not change in cows that HS via heat for a °C increase in body that protein not be the for increases in et al., 2023). Some of these the decrease in milk or as the supply may be away from during of HS and a in postruminal supply for production due to reduced HS can also a immune and antioxidant adaptation to conditions. 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