Estrogen receptor-alpha gene expression in the cortex: sex differences during development and in adulthood

Abstract

17β-estradiol is a hormone with far-reaching organizational, activational and protective actions in both male and female brains. The organizational effects of early estrogen exposure are essential for long-lasting behavioral and cognitive functions. Estradiol mediates many of its effects through the intracellular receptors, estrogen receptor-alpha (ERα) and estrogen receptor-beta (ERβ). In the rodent cerebral cortex, estrogen receptor expression is high early in postnatal life and declines dramatically as the animal approaches puberty. This decline is accompanied by decreased expression of ERα mRNA. This change in expression is the same in both males and females in the developing isocortex and hippocampus. An understanding of the molecular mechanisms involved in the regulation of estrogen receptor alpha (ERα) gene expression is critical for understanding the developmental, as well as changes in postpubertal expression of the estrogen receptor. One mechanism of suppressing gene expression is by the epigenetic modification of the promoter regions by DNA methylation that results in gene silencing. The decrease in ERα mRNA expression during development is accompanied by an increase in promoter methylation. Another example of regulation of ERα gene expression in the adult cortex is the changes that occur following neuronal injury. Many animal studies have demonstrated that the endogenous estrogen, 17β-estradiol, is neuroprotective. Specifically, low levels of estradiol protect the cortex from neuronal death following middle cerebral artery occlusion (MCAO). In females, this protection is mediated through an ERα-dependent mechanism. ERα expression is rapidly increased following MCAO in females, but not in males. This increase is accompanied by a decrease in methylation of the promoter suggesting a return to the developmental program of gene expression within neurons. Taken together, during development and in adulthood, regulation of ERα gene expression in the cortex can occur by DNA methylation and in a sex-dependent fashion in the adult brain.

Figure 1. ERα mRNA expression during early postnatal development

RNA was extracted from brain tissue taken from female and male mice at PND 4, 10, 18 and 25. RNA was reverse-transcribed and the resulting cDNA was used for real time PCR using ERα-specific primers. (A.) ERα mRNA expression was high in both the male and female isocortex at PND 4. ERα was significantly decreased in both male and female cortex at PND 10, 18 and 25 (Prewitt, 2007). (B). ERα mRNA expression was high in both the male and female PFC at PND 4. ERα was significantly increased in the female, but not male PFC at PND 10. ERα was significantly decreased in both male and female PFC at PND 18 and 25. (C). ERα mRNA expression was high in both the male and female hippocampus at PND 4. ERα was significantly decreased in both male and female hippocampus at PND 10, 18 and 25. Data was normalized to Histone 3.1 and compared to PND 4. Samples were run in triplicate. In all three brain regions, two-way ANOVA revealed a significant overall effect of age (p<0.01). Asterisks on the graph indicate significant differences from PND 4 (p< 0.05, n=6-8 per timepoint).

Figure 2. Summary of sex differences in ERα promoter methylation in the brain

N/D not determined.