Characterizing the postnatal hypothalamic-pituitary-adrenal axis response of in utero heat stressed pigs at 10 and 15 weeks of age

Review: Phenotypic and molecular evidence of inter- and trans-generational effects of heat stress in livestock mammals and humans

Internal and external factors can change an individual's phenotype. A significant external threat to humans and livestock is environmental heat load, a combination of high ambient temperatures and humidity. A heat stress response occurs when an endothermal animal is exposed to a heat load that challenges its' thermoregulation capacity. With the ongoing climate change trends, the incidence of chronically elevated temperatures causing heat stress is expected to rise, posing an even greater risk to the health and survival of all species. Heat stress is generally related to adverse effects on food intake, health, and performance in mammal livestock species and humans. Evidence from epidemiological and experimental studies of humans and livestock demonstrated that exposing pregnant females to heat stress affects the phenotype of the newborn in various ways. For instance, in utero heat stress is related to lower BW at birth and changes in metabolic and immune functions in the newborn. In cows, the effects of heat stress on the performance of the offspring last for three or four generations, suggesting intergenerational effects. The molecular mechanism orchestrating these effects of heat stress may be epigenetic regulation, as various epigenetic mechanisms control genome reprogramming. Epigenetic modifications are attached to DNA and histone proteins and can influence how specific genes are expressed, resulting in phenotypic changes. Epigenetic modifications can be triggered in response to environmental heat stress without altering the DNA sequence. Heat stress insults during critical periods of organ development (i.e., fetal exposure) can trigger epigenetic modifications that impact health and productivity across generations. Thus, epigenetic changes caused by extreme temperatures can be passed down to the offspring if the mother is exposed to the insult during pregnancy. Understanding the phenotypic and molecular consequences of maternal heat stress, including the carry-over lingering effects on the resulting progeny, is necessary to develop effective mitigation strategies and gain translational knowledge about the fundamental processes leading to intergenerational and transgenerational inheritance. This review examines the phenotypic and molecular evidence of how maternal exposure to extreme heat can affect future generations in several species, including humans, swine, sheep, goats, and cattle. The current knowledge of the molecular mechanisms involved in intergenerational and transgenerational epigenetic inheritance will also be presented and discussed.

Programming effects of intrauterine hyperthermia on adrenal gland development

Maternal heat stress during late pregnancy can lead to intrauterine hyperthermia and affect fetal hypothalamic-pituitary-adrenal axis development and function. Herein, we investigated the effects of chronic environmental heat stress exposure of Holstein cows in the last 2 mo of gestation on their offspring's adrenal gland histomorphology and transcriptome. Cows in their last 54 ± 5 d of gestation were either heat stressed (housed under the shade of a freestall barn) or provided heat stress abatement via active cooling (via water soakers and fans) during a subtropical summer (temperature-humidity index >68). Respiration rate (RR) and skin temperature (ST) were elevated in heat-stressed dams relative to the cows with access to heat abatement (23 breaths/min and 2°C higher for RR and ST, respectively). Heifers born to heat-stressed cows experienced heat stress in utero (HS), whereas heifers born to actively cooled cows did not (CL). The adrenal gland was harvested from 6 heifers per group that were euthanized at birth (d 0; n = 12) or 1 wk after weaning (d 63; n = 12). Circulating cortisol was measured from blood samples collected weekly throughout the preweaning period. At d 63, heifers that experienced HS while developing in utero had heavier adrenal glands, with a greater total tissue surface area and thickness of the zona glomerulosa (ZG), fasciculata (ZF), and reticularis (ZR), compared with CL heifers. In addition, the adrenal gland of HS heifers had fewer cells in the ZG, more and larger cells in the ZF, and larger cells in the ZR, relative to CL heifers. Although no changes in circulating cortisol were observed through the preweaning period, the transcriptomic profile of the adrenal tissue was altered by fetal exposure to hyperthermia. Both at birth and on d 63, approximately 30 pathways were differentially expressed in the adrenal glands of HS heifers relative to CL. These pathways were associated with immune function, inflammation, prolactin signaling, cell function, and calcium transport. Upstream regulators significantly activated or inhibited in the adrenal glands of heifers exposed to intrauterine hyperthermia were identified. Maternal exposure to heat stress during late gestation caused an enlargement of their offspring's adrenal glands by inducing ZG and ZF cell hypertrophy, and caused gene expression changes. These phenotypic, histological, and molecular changes in the adrenal gland might lead to alterations in stress, immune, and metabolic responses later in life.

Genes Participating in the Ensheathment of Neurons Are Affected by Postnatal Stress and Maternal Immune Activation in the Pituitary Gland

Immune challenges during gestation are associated with neurodevelopmental disorders and can interact with stress later in life. The pituitary gland participates in endocrine- and immune-related processes that influence development, growth, and reproduction and can modulate physiological and behavioral responses to challenges. The objective of this study was to investigate the effect of stressors at different time points on the molecular mechanisms of the pituitary gland and detect sex differences. RNA sequencing was used to profile the pituitary glands of female and male pigs exposed to weaning stress and virally induced maternal immune activation (MIA), relative to unchallenged groups. Significant effects (FDR-adjusted p-value < 0.05) of MIA and weaning stress were detected in 1829 and 1014 genes, respectively. Of these, 1090 genes presented significant interactions between stressors and sex. The gene ontology biological process of the ensheathment of neurons (GO:0007272), substance abuse, and immuno-related pathways, including the measles disease (ssc05162), encompasses many genes with profiles impacted by MIA and weaning stress. A gene network analysis highlighted the under-expression of myelin protein zero (Mpz) and inhibitors of DNA binding 4 (Id4) among the non-stressed males exposed to MIA, relative to the control and non-MIA males exposed to weaning stress, relative to non-stressed pigs. The detection of changes in the molecular mechanisms of the pituitary gland could advance our understanding of disruptions in the formation of the myelin sheath and the transmission of neuron-to-neuron signals in behavioral disorders associated with maternal immune activation and stress.

Dietary Soy Prevents Alcohol-Mediated Neurocognitive Dysfunction and Associated Impairments in Brain Insulin Pathway Signaling in an Adolescent Rat Model

BACKGROUND

Alcohol-related brain degeneration is linked to cognitive-motor deficits and impaired signaling through insulin/insulin-like growth factor type 1 (IGF-1)-Akt pathways that regulate cell survival, plasticity, metabolism, and homeostasis. In addition, ethanol inhibits Aspartyl-asparaginyl-β-hydroxylase (ASPH), a downstream target of insulin/IGF-1-Akt signaling and an activator of Notch networks. Previous studies have suggested that early treatment with insulin sensitizers or dietary soy could reduce or prevent the long-term adverse effects of chronic ethanol feeding.

OBJECTIVE

The goal of this study was to assess the effects of substituting soy isolate for casein to prevent or reduce ethanol's adverse effects on brain structure and function.

METHODS

Young adolescent male and female Long Evans were used in a 4-way model as follows: Control + Casein; Ethanol + Casein; Control + Soy; Ethanol + Soy; Control = 0% ethanol; Ethanol = 26% ethanol (caloric). Rats were fed isocaloric diets from 4 to 11 weeks of age. During the final experimental week, the Morris Water maze test was used to assess spatial learning (4 consecutive days), after which the brains were harvested to measure the temporal lobe expression of the total phospho-Akt pathway and downstream target proteins using multiplex bead-based enzyme-linked immunosorbent assays (ELISAs) and duplex ELISAs.

RESULTS

Ethanol inhibited spatial learning and reduced brain weight, insulin signaling through Akt, and the expression of ASPH when standard casein was provided as the protein source. The substitution of soy isolate for casein largely abrogated the adverse effects of chronic ethanol feeding. In contrast, Notch signaling protein expression was minimally altered by ethanol or soy isolate.

CONCLUSIONS

These novel findings suggest that the insulin sensitizer properties of soy isolate may prevent some of the adverse effects that chronic ethanol exposure has on neurobehavioral function and insulin-regulated metabolic pathways in adolescent brains.