From maternal glucocorticoid and thyroid hormones to epigenetic regulation of offspring gene expression: An experimental study in a wild bird species

Abstract

Offspring phenotype at birth is determined by its genotype and the prenatal environment including exposure to maternal hormones. Variation in both maternal glucocorticoids and thyroid hormones can affect offspring phenotype, but the underlying molecular mechanisms, especially those contributing to long-lasting effects, remain unclear. Epigenetic changes (such as DNA methylation) have been postulated as mediators of long-lasting effects of early-life environment. In this study, we determined the effects of elevated prenatal glucocorticoid and thyroid hormones on handling stress response (breath rate) as well as DNA methylation and gene expression of glucocorticoid receptor (GR) and thyroid hormone receptor (THR) in great tits (Parus major). Eggs were injected before incubation onset with corticosterone (the main avian glucocorticoid) and/or thyroid hormones (thyroxine and triiodothyronine) to simulate variation in maternal hormone deposition. Breath rate during handling and gene expression of GR and THR were evaluated 14 days after hatching. Methylation status of GR and THR genes was analyzed from the longitudinal blood cells sampled 7 and 14 days after hatching, as well as the following autumn. Elevated prenatal corticosterone level significantly increased the breath rate during handling, indicating an enhanced metabolic stress response. Prenatal corticosterone manipulation had CpG-site-specific effects on DNA methylation at the GR putative promoter region, while it did not significantly affect GR gene expression. GR expression was negatively associated with earlier hatching date and chick size. THR methylation or expression did not exhibit any significant relationship with the hormonal treatments or the examined covariates, suggesting that TH signaling may be more robust due to its crucial role in development. This study provides some support to the hypothesis suggesting that maternal corticosterone may influence offspring metabolic stress response via epigenetic alterations, yet their possible adaptive role in optimizing offspring phenotype to the prevailing conditions, context-dependency, and the underlying molecular interplay needs further research.

Keywords: Parus major; epigenetics; hormone; maternal effects; methylation; prenatal.

FIGURE 1

Effects of prenatal corticosterone manipulation on (a): breath rate 14 days after hatching (per 60s), (b): DNA methylation (%) at the quantified 12 CpG sites of NR3C1 promoter region (black = non‐CORT, red = CORT) since a significant CORT*Site interaction was detected, and (c): glucocorticoid receptor NR3C1 relative gene expression in blood cells. Estimated marginal means and standard errors are given (a–c), with the raw data (a,b). p‐values for the effect of corticosterone from type III ANOVA (a,b) and site‐specific post‐hoc comparison with Tukey's test (c) are also shown for p < 0.10.

FIGURE 2

(a) Average methylation percentages pooled across different treatment groups at different ages for glucocorticoid receptor gene NR3C1. (b) Average methylation percentages pooled across different treatment groups at different ages for thyroid hormone receptor gene THRΒ. Black lines represent changes in methylation percentage in the overall mean (across all samples), and the grey lines represent changes in methylation percentage (averaged over all CpG sites) for each individual across ages.

FIGURE 3

(a) Correlation between breath rate (per 60s) and NR3C1 gene expression. (b) Correlation between NR3C1 relative gene expression and putative promoter mean methylation (%). Regression line and 95% confidence limits are given with the raw data.