ReviewEstrogen receptor-alpha gene expression in the cortex: Sex differences during development and in adulthood
Research Highlights
►Estrogen receptor alpha is developmentally regulated in the cortex. ►Estrogen receptor down-regulation occurs by DNA methylation. ►Estrogen receptor alpha is re-expressed following injury in a a sex-specific manner.
Introduction
Estrogens have long been known to play a crucial role in coordinating many neuroendocrine events that control sexual development, sexual behaviour and reproduction. 17β-estradiol is the primary biologically active form of estrogen. In rodents, estradiol is critical for sexual differentiation of the brain (see review by (McCarthy, 2008)). For example, estradiol organizes neural circuits and regulates apoptosis of neurons leading to long-term differences in the male and female brain (Anderson et al., 1986, Rhees et al., 1990, Toran-Allerand, 1976). In addition to its role in development, estradiol modulates numerous facets of brain function in the adult brain (for review see (McEwen and Alves, 1999) and (Simpkins and Singh, 2008)). Estradiol prevents neuronal cell death in a variety of brain injury models, modulates learning and memory and promotes the formation of synapses (Li et al., 2004, Sherwin, 1994, Simpkins et al., 1994, Wise et al., 2001, Woolley & McEwen, 1993).
The physiological effects resulting from estradiol actions in target tissues are mediated primarily by two intracellular receptors, ERα and ERβ (Green et al., 1986, Koike et al., 1987, Kuiper et al., 1996, Mosselman et al., 1996, White et al., 1987). Both ERα and ERβ have been observed in neurons and glia in the brain (Chaban et al., 2004, Donahue et al., 2000), and both are expressed throughout the brain with distinct patterns in different brain regions and with differing levels of expression during development (Gonzalez et al., 2007, Osterlund et al., 2000a, Osterlund et al., 2000b, Prewitt, 2007, Shughrue et al., 1997).
The ERα gene is highly conserved between human and rodents, containing similar gene and protein structures. ERα is preceded by multiple promoters that generate several mRNA splice variants (Hirata et al., 2001, Kos et al., 2001, Monje et al., 2007). Alternative splicing occurs at the first exon, which is then spliced to a common splice acceptor site upstream of the translational initiation codon in Exon 1. This alternative promoter splicing results in mRNA splice variants that differ only in their 5’ untranslated region encoding the same protein. These different mRNAs may be important in regulating stability or processing of the mRNA (Kos et al., 2001). Differences in the 5'UTRs may also provide for areas of different epigenetic modifications that can then modulate changes in gene expression. The regulatory binding sites of transcription factors that control gene expression are also likely located in the regions surrounding these promoters. To date, relatively little is known about the molecular composition of the regulatory elements that control ERα gene expression.
The regulation of gene expression by epigenetic modification is an emerging mechanism for controlling neuronal gene expression. Epigenetic modification of chromatin involves changes to DNA bases and the associated proteins (for review see (Wolffe and Matzke, 1999) and (Klose and Bird, 2006)) in the absence of changes in the DNA sequence. Epigenetic modifications include histone acetylation, histone methylation and DNA methylation (Bird & Wolffe, 1999, Cooper & Krawczak, 1989). The first step in DNA methylation results in the enzymatic transfer of a methyl group to the 5’-position of the pyrimidine ring of a cytosine residue followed by a guanine (CpG dinucleotides). The modification of the cytosines in CpG residues are carried out initially by the enzyme DNA methyltransferase 3A (DNMT3A) and maintained by DNMT1 (Klose and Bird, 2006). CpG residues are often found upstream or downstream of the transcriptional start site.
The methylated CpGs are stabilized by methyl-CpG-binding proteins. The family of methyl-CpG-binding proteins contains several members including methyl binding domain (MBD) proteins 1, 2, 3, 4 and MeCP2 (Nan et al., 1998, Ng et al., 2000, Ng et al., 1999). These proteins bind to the methylated CpG residues potentially disrupting transcription. These proteins can also associate with co-repressor protein complexes of promoters that include histone deacetylases (HDAC) (Ballestar & Wolffe, 2001, Wade, 2001). Together, these complexes suppress transcription of genes with methylated promoter DNA.
Epigenetic modification of chromatin in neurons has been shown to play an important role in regulating gene expression during neuronal development and in learning and memory (Kiefer, 2007, Levenson & Sweatt, 2005). MeCP2 gene mutations are also the cause of some cases of Rett syndrome, a progressive neurological developmental disorder that appears during early childhood when sensory experience is driving the synaptic reorganization required for creating mature circuits in the brain (Zoghbi, 2003) (Guy et al., 2001). Furthermore, MeCP2 is present in high levels in mature neurons (Meehan et al., 1992), and studies suggest that the MeCP2 protein plays a role in forming synapses between neurons (Zhou et al., 2006). Additionally, MeCP2 is differentially expressed in the hypothalamus during a critical time in sexual differentiation of the brain (Kurian et al., 2008). Furthermore, DNMT3 expression has also been shown to be dynamically regulated in the developing brain as well as in the adult cortex (Feng et al., 2005, Siegmund et al., 2007).
The ERα gene undergoes changes in promoter methylation under normal and pathological conditions. For example, methylation of the ERα promoter has been reported to occur in the colon during aging (Issa et al., 1994) and changes in the expression of ERα have been associated with the progression of numerous types of cancerous tissues including breast and lung (Issa et al., 1994, Issa et al., 1996, Lapidus et al., 1998, O'Doherty et al., 2002, Oh et al., 2001, Ottaviano et al., 1994, Sasaki et al., 2002). Although evidence exists for multiple mechanisms of epigenetic modification of the ERα gene, DNA methylation has been the most widely described epigenetic phenomenon, and is the first epigenetic mechanism we have investigated.
Section snippets
Developmental regulation of estrogen receptor-alpha mRNA
ERα protein and mRNA levels change dramatically during postnatal brain development (Shughrue et al., 1997, Simerly et al., 1990, Toran-Allerand et al., 1992). High levels of estradiol binding in non-hypothalamic regions such as the cortex and hippocampus during the first two weeks of life have been identified by receptor autoradiography (Pfaff & Keiner, 1973, Sheridan, 1979, Shughrue et al., 1990). This expression declines as animals approach puberty. In later studies in rats and mice, ERα mRNA
Regulation of estrogen receptor-alpha mRNA following stroke
In addition to the developmental changes in expression in ERα mRNA widely described, ERα mRNA expression is also dramatically regulated following brain injury. Middle cerebral artery occlusion (MCAO) is a well-established model of focal ischemia. Studies in rats and mice have demonstrated a gender difference in neuronal cell death following MCAO. Female brains are consistently protected against cell death (Alkayed et al., 1998, Rusa et al., 1999, Simpkins et al., 1997). Pretreatment with low
Summary
Estrogens mediate many diverse and critical actions in the brain. Most of these actions require the presence of classical estrogen receptors. Thus, coordinated regulation of the estrogen receptor genes is critical for mediating these responses to estrogens in an age, gender, and brain region-specific manner (Fig. 2). Alterations in the normal regulation either during development, disease or aging could potentially interfere with estradiol action. We have begun to identify numerous physiological
Acknowledgments
This work cited from our laboratory was supported by the National Science Foundation (NSF IOS0919944), COBRE grant P20 RR15592 from the National Center for Research Resources (NCRR), and R01 HL073693 (MEW). All opinions, findings and conclusions expressed in this material are those of the authors and not those necessarily of NSF or NCRR.
References (84)
Relationships between sexual activity, plasma testosterone, and the volume of the sexually dimorphic nucleus of the preoptic area in prenatally stressed and non-stressed rats
Brain Res.
(1986)- et al.
Methylation-induced repression—belts, braces, and chromatin
Cell
(1999) Cells containing immunoreactive estrogen receptor-alpha in the human basal forebrain
Brain Res.
(2000)- et al.
Prolactin regulation of estrogen receptor expression
Trends Endocrinol. Metab.
(2003) The multiple untranslated first exons system of the human estrogen receptor beta (ER beta) gene
J. Steroid Biochem. Mol. Biol.
(2001)- et al.
Genomic DNA methylation: the mark and its mediators
Trends Biochem. Sci.
(2006) ER beta: identification and characterization of a novel human estrogen receptor
FEBS Lett.
(1996)The ontogeny of estrogen receptors in heterochronic hippocampal and neocortical transplants demonstrates an intrinsic developmental program
Brain Res. Dev. Brain Res.
(1993)Estrogen receptor mRNA alterations in the developing rat hippocampus
Brain Res. Mol. Brain Res.
(1995)Sex differences in early childhood, adolescence, and adulthood on cognitive tasks that rely on orbital prefrontal cortex
Brain Cogn.
(2004)