Early life stress: nature and nurture.

Recent and still nascent understanding of epigenetic marks, how they occur and are modified, has been an enormous boon to studies in the fields of endocrinology and neuroscience. Because the genetic DNA sequence does not usually change during the lifetime of the individual, it had been difficult to understand how marked and persistent changes in gene transcription rates occur in response to changes in the environment and experiences of the individual. Although not the only mechanism, hiding and exposing regulatory transcription sites on DNA through epigenetic changes in chromatin folding and unfolding allow much of the great flexibility that we observe occurring in biological processes. Methylation of cytosine residues in cytosine-phosphoguanine dinucleotides is known to result in either silencing or augmenting gene activity (see Figure 1) (1, 2). Methylated DNA can be bound by methyl-CpG-binding domain proteins. Methyl-CpG-binding domain protein then recruits additional proteins to the locus, such as histone deacetylases and other chromatin remodeling proteins that modify histones, thereby forming compact, inactive chromatin, making genes inaccessible for transcription. The link between DNA methylation and chromatin structure is very important. In particular, loss of methyl-CpGbinding protein 2 (MeCP2) has been implicated in Rett syndrome (3). In an article published in this issue of Endocrinology, Wu et al (4) show that the long-lasting effects of early life stress (ELS) inmousepups that result in increasedpituitary proopiomelanocortin mRNA, ACTH and corticosterone secretion are mediated by the reduction of methylation in a specific CpG site on the distal POMC promoter (4). Methylation of the CpG 6–8 site was shown to reduce POMC activity in vitro and in vivo and was accompanied by high levels of MeCP2, which selectively bound the CpG 6–8 methylation site. MeCP2 recruits corepressors to decrease POMC gene activity. The effect of methylation in CpG sites on transcription, in this case, is inhibitory. The authors did a very thorough job in determining the effects of ELS on pituitary POMC synthesis, although they did not determine what cellular event initiated the hypomethylation and low MeCP2 at the CpG6–8 site. However, hypomethylation of the distal POMC promoter is just one effect of ELS on gene methylation and on regulation of the hypothalamo-pituitary-adrenal axis, as the authors showed in an earlier study on the brains of the same mice that had been subjected to neonatal stress (5). In that study, they found that, through acting at a specific CpG methylation site, MeCP2 persistently down-regulated Avp expression in parvocellular cells in the paraventricular nuclei (PVN). ELS decreased MeCP2 binding to methylcytosines in the promoter region but did not alter DNA methylation. In this case, the authors show that it was likely that neuronal activity resulted in Ca -dependent phosphorylation of MeCP2 which reduced its binding to specific methylated cytosines. There was no effect on corticotropin-releasing factor mRNA (5). That is both interesting and somewhat surprising, because there are high levels of MeCP2 in parvocellular neurons in the PVN, and mice bearing a truncated MeCP2 gene (Mecp2) have elevated CRF mRNA in the PVN, amygdala, and bed nuclei of the stria terminalis (6). Earlier studies have shown that the effect of ELS on CRF expression depends on the control group that is used to compare with the group exposed to 3 hours per day of maternal separation (7–9). In untouched, unhandled litters, which was the control group in Refs. 4, 5, adults have similar CRF expression levels compared with rats exposed to ELS of 3-hour separation. However, if the reference control group is either animals experiencing normal vi-