Abstract
Changes to the epigenome, including those to DNA methylation, have been proposed as mechanisms by which stress can induce long-term physiological changes in livestock species. Pig weaning is associated with dietary and social stress, both of which elicit an immune response and changes to the hypothalamic–pituitary–adrenal (HPA) axis. While differential methylation following stress has been assessed in model organisms, it remains poorly understood how the pig methylome is altered by stressors in production settings. We quantified changes in CpG methylation and transcript abundance in piglet peripheral blood mononuclear cells (PBMCs) following weaning and also assessed differential patterns in pigs exhibiting high and low stress response as measured by cortisol concentration and lesion scores. Blood was collected from nine gilt piglets 24 h before and after weaning, and whole-genome bisulfite sequencing (WGBS) and RNA-sequencing were performed on six and nine animals, respectively, at both time points. We identified 2,674 differentially methylated regions (DMRs) that were enriched within promoters of genes associated with lymphocyte stimulation and transcriptional regulation. Stress groups displayed unique differential methylation and expression patterns associated with activation and suppression of T cell immunity in low and high stress animals, respectively. Differential methylation was strongly associated with differential expression; specifically, upregulated genes were enriched among hypomethylated genes. We observed post-weaning hypermethylation of the glucocorticoid receptor (NR3C1) promoter and a significant decrease in NR3C1 expression (n = 9, p = 6.1 × 10–3). Our results indicate that weaning-associated stress elicits genome-wide methylation changes associated with differential gene expression, reduced T cell activation, and an altered HPA axis response.
Highlights
Livestock animals experience numerous stressors throughout their lifetime that result in shortand long-term effects on physiology and performance (Yahav and Hurwitz, 1996; Geverink et al, 1998; Molenaar et al, 2011; Li et al, 2017; Johnson et al, 2018)
Blood lymphocyte concentrations have been shown to be significantly reduced in weaned pigs (Kick et al, 2012), and additional psychosocial stress experienced during weaning has been associated with lower concentrations of T cell subpopulations (Tuchscherer et al, 2009)
We obtained 128–194M whole-genome bisulfite sequencing (WGBS) reads across samples, of which 86–89% uniquely aligned to the S. scrofa reference genome (Supplementary Table 1)
Summary
Livestock animals experience numerous stressors throughout their lifetime that result in shortand long-term effects on physiology and performance (Yahav and Hurwitz, 1996; Geverink et al, 1998; Molenaar et al, 2011; Li et al, 2017; Johnson et al, 2018). In addition to direct effects of weaning, piglets are exposed to unfamiliar individuals in nursery pens, which results in aggressive encounters and skin lesion development (Wurtz et al, 2017). One reliable indicator of stress response following pig weaning and mixing is lesion counts, as this measure has been shown to be associated with aggressive behavior, hypothalamic–pituitary– adrenal axis (HPA) activity, and risk of infection, as well as negatively associated with immunocompetence (Fernandez et al, 1994; Morrow-Tesch et al, 1994; Turner et al, 2006, 2009). Weaning stress can have long-term consequences on gut health and disease susceptibility (reviewed in Moeser et al, 2017), yet the molecular mechanisms underlying these relationships remain poorly characterized. It has been proposed that epigenetic mechanisms may link stress from environmental stimuli to lasting changes in gene expression and physiology (Tzschentke and Basta, 2002; González-Recio et al, 2015)
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