Early life stress behavior

A striking example of epigenetic adaptive phenomenon came from the studies of rat maternal nursing behavior (licking and grooming and arched-back nursing) effects on the offspring responses to stress (Weaver et al., 2004). As adults, the offspring of high- nursing (HN) mothers appeared less fearful and show more modest hypothalamus-pituitary axis (HPA) responses to stress compared with the offspring of low-nursing (LN) mothers. Intriguingly, the biological offspring of LN mothers reared by HN females resemble the normal offspring of HN mothers and vice versa. The adult offspring of HN mothers show increased hippocampal glucocorticoid receptor gene Nr3c1 expression and enhanced glucocorticoid feedback sensitivity on hypothalamic corticotropin-releasing hormone release and HPA response to stress compared with offspring of LN mothers. This increase in Nr3c1 gene expression has been shown to be mediated by 5-hydroxytryptamine (serotonin) (5-HT) activity at 5-HT₇ receptors and subsequent activation of cAMP-dependent protein kinase activity. This effect is also accompanied by an increased hippocampal expression of nerve growth factor-inducible protein A (NGFI-A, a transcription factor also known as egr-1, krox-24, zenk, and zif-268). The noncoding exon 1 region of the hippocampal Nr3c1, exon 1₇, includes a promoter region containing a binding site for NGFI-A. Splice variants of the Nr3c1 mRNA containing the exon 1₇ sequence are found predominantly in brain, and the expression of Nr3c1 mRNAs containing the exon 1₇ sequence is increased in the offspring of HN mothers. In hippocampal DNA a CpG site within NGFI-A consensus sequence was found to be always methylated in the offspring of LN mothers and rarely methylated in the offspring of HN females. When biological offspring of HN and LN mothers was cross-fostered, the patterns of the exon 1₇ promoter methylation in offspring were associated with the rearing female, not the biological mother. Thus, variations in maternal care directly alter the methylation status of the Nr3c1 gene in offspring brain DNA. Just before birth (E20), the entire sequence of the exon 1₇ Nr3c1 promoter is unmethylated in both offspring groups. In one day after birth (P1), this region is de novo methylated to the same extent in both animal groups. The differences in the methylation status emerge between P1 and P6, the period when differences in the maternal behavior are apparent. By P6, the NGFI-A response element CpG dinucleotide is effectively demethylated in the HN group, but not in the LN group. This difference remains consistent to adulthood (P90). Thus, the group difference in DNA methylation occurs as a function of a maternal behavior over the first week of life. It was shown that there is a significantly greater histone H3K9ac association and threefold greater binding of NGFI-A protein to the hippocampal exon 1₇ Nr3c1 promoter in the offspring of HN mothers compared with offspring of LN mothers. Central infusion of the HDAC inhibitor trichostatin A enhances histone H3K9 acetylation of the exon 1₇ Nr3c1 promoter in the offspring of the LN mothers, increases NGFI-A binding to its cognate sequence, induces hypometh- ylation of CpGs in the promoter, and eliminates the maternal effect on hippocampal Nr3c1 expression and the HPA response to stress.

Similar results were obtained in a study of the estrogen receptor gene ERa expression and methylation in medial preoptic area (MPOA) of the hypothalamus in the offspring of HN and LN mothers (Champagne et al., 2006). ERa expression in the MPOA of the P6 offspring of HN mothers was found to be significantly increased compared with that of the offspring of LN mothers, whereas the methylation levels of CpG sites across the ERa promoter were significantly higher in the offspring of LN mothers. By a cross-fostering paradigm a highly significant effect of rearing female was demonstrated. Among the CpG sites differentially methylated was one contained within a consensus Stat5b binding site. A significantly greater binding of Stat5b to the ERa promoter in the adult offspring of HN compared with LN mothers was demonstrated. Since ERa expression is regulated through activation of the Janus kinase-Stat5b pathway, inhibition of Stst5b binding to its methylated binding site could be a mechanism, by which DNA methylation suppresses ERa expression. Thus, maternal care is associated with increased ERa expression, together with demethylation of its promoter and enhanced Stat5b binding.

The experience of the mother can also be translated through an epigenetic inheritance into phenotypic variation in the offspring, resulting in the transmission of adaptive responses across generations. The effect of maternal care on the hippocampal transcrip- tome seems to be gene specific. Of total hippocampus transcriptome only 253 transcripts (0.81%) were found to be upregulated and 50 transcripts (0.16%) downregulated in the offspring of HN mothers compared with offspring of LN mothers, whereas 30,796 (99%) transcripts remained unaltered (Weaver, Meaney, & Szyf, 2006). Thus, maternal care during early life programs the expression of hundreds of genes in the adult offspring.

Familial childhood adversities are associated with altered HPA stress responses and increased risk for multiple forms of psychopathology in humans. There is evidence for decreased hippocampal glucocorticoid receptor expression at several psychopathological conditions associated with suicide, including schizophrenia and mood disorders. Suicide is also strongly associated with a history of the childhood abuse and neglect. An investigation of the glucocorticoid receptor gene NR3C1 expression in hippocampal samples obtained from suicide victims showed a significant reduction of NR3C1 mRNA in suicide victims with a history of childhood abuse relative to nonabused suicide victims or controls (McGowan et al., 2009). A significant effect on NR3C1 promoter methyla- tion was also found.

Early prenatal stress (E-PS) in mice was found to significantly increase the corticotropin-releasing hormone (Crh) gene expression in the amygdala central nucleus and decrease Nr3c1 gene expression in the hippocampal CA3 area and dentate gyrus (Mueller & Bale, 2008). In the hypothalamus of E-PS males, reduced methylation of the Crh gene promoter and increased methylation of exon 1₇-specific part of the Nr3c1 gene promoter were observed. Thus, dysregulation of Crh and stress pathway programming may be a key contributor to the stress-sensitive phenotype in E-PS male mice.

Early life stress (ELS) during the first 10 days of life in mice led to sustained hyperactivity of the HPA. ELS induces increased expression of pituitary pro-opiomelanocortin (Pomc) mRNA. Pomc expression is induced by the hypothalamic neuropeptides arginine vasopressin (Avp) and Crh, and all of them are under negative feedback control by the glucocorticoid receptor. Conspicuously, levels of Nr3c1 mRNA in the hippocampus, hypothalamic paraventricular nucleus (PVN) and pituitary were either unchanged or upregulated in ELS-treated mice, arguing against impaired corticosterone feedback as the primary cause of the observed increases in Pomc expression and glucocorticoid secretion (Mutgatroyd et al., 2009). ELS did not influence the hypothalamic Crh mRNA expression, but resulted in a significant upregulation of Avp mRNA. The changes in Avp expression persisted for at least 1 year and were restricted to the parvocellular subpopulation of neurons in the PVN that drive the pituitary-adrenal axis. ELS also produced long-lasting memory deficits in an inhibitory avoidance task. Of four Avp gene CpG islands, promoter CGI1 and exon CGI2 are sparsely methylated in control mice, whereas intergenic region (includes a composite enhancer region between tail-to-tail-orientated Avp and oxytocin genes) CGI3 has high levels of CpG methylation, and CGI4 and the adjacent oxytocin tissue-specific enhancer region have a less-dense methylated pattern with few, irregularly spaced highly methylated CpG dinucleotides. A hypomethylation of multiple CpGs throughout the downstream Avp enhancer region was found in PVN tissue of adult ELS mice. Of the 11 CpGs significantly hypomethylated in 6-week-old ELS mice, 7 in CGI3 had methylation patterns that strongly correlated with Avp mRNA levels. This means that the ELS-induced changes in the methylation status of relevant CpG residues are persistent and sustain elevated Avp expression.

Thus, adverse events in early life can leave persistent epigenetic marks on specific genes that may prime susceptibility to neuroendocrine and behavioral dysfunctions. In general, postmitotic epigenetic modifications regulate gene expression and related neuronal function, which can serve to facilitate or disfavor physiological and behavioral adaptations.