Estrogen receptor beta in the brain: from form to function

Abstract

Estrogens have numerous effects on the brain, both in adulthood and during development. These actions of estrogen are mediated by two distinct estrogen receptor (ER) systems, ER alpha (ERalpha) and ER beta (ERbeta). In brain, ERalpha plays a critical role in regulating reproductive neuroendocrine function and behavior, however, a definitive role for ERbeta in any neurobiological function has been slow in forthcoming. Clues to the function of ERbeta in the central nervous system can be gleaned from the neuroanatomical distribution of ERbeta and the phenotypes of neurons that express ERbeta. ERbeta immunoreactivity has been found in populations of GnRH, CRH, vasopressin, oxytocin and prolactin containing neurons in the hypothalamus. Utilizing subtype-selective estrogen receptor agonists can help determine the roles for ERbeta in non-reproductive behaviors in rat models. ERbeta-selective agonists exert potent anxiolytic activity when animals were tested in a number of behavioral paradigms. Consistent with this, ERbeta-selective agonists also inhibited the ACTH and corticosterone response to stress. In contrast, ERalpha selective agonists were found to be anxiogenic and correspondingly increased the hormonal stress response. Taken together, our studies implicate ERbeta as an important modulator of some non-reproductive neurobiological systems. The molecular and neuroanatomical targets of estrogen that are mediated by ERbeta remain to be determined. A number of splice variants of ERbeta mRNA have been reported in brain tissue. Imaging of eGFP labeled chimeric receptor proteins transfected into cell lines shows that ERbeta splice variation can alter trafficking patterns and function. The originally described ERbeta (herein termed ERbeta1) is characterized by possessing a high affinity for estradiol. Similar to ERalpha, it is localized in the nucleus and is trafficked to nuclear sites termed "hyperspeckles" following ligand binding. In contrast, ERbeta2 contains an 18 amino acid insert within the ligand-binding domain and as a result can be best described as a low affinity form of ERbeta. A delta3 (delta3) variant of ERbeta has a deletion of the 3rd exon (coding for the second half of the DNA-binding domain) and as a result does not bind an estrogen response element in DNA. delta3 variants are trafficked to a unique low abundance and larger nuclear site following ligand binding. A delta4 (delta4) variant lacks exon 4 and as a result is localized to the cytoplasm. The amount of individual splice variant mRNAs varies depending upon brain region. Examination of neuropeptide promoter regulation by ERbeta splice variants demonstrates that ERbeta functions as a constitutively active transcription factor. Moreover, it appears that splice variation of ERbeta alters its ability to regulate transcription in a promoter-dependent and ligand-dependent fashion.

Figure 1

Schematic representation of ERα and ERβ protein structure and relative homology, and ERβ splice variant exon structure. Deletions are indicated by a single line, and insertions are indicated by a shaded box. DBD = DNA binding domain, LBD = ligand binding domain.

Figure 2

DPN treatment of ovariectomized female rats reduced anxiety-type behavior in the elevated plus maze. Quantification of behaviors included open arm entries (A), time spent on the open arms (B), rears (C), head dips (D), fecal boli (E), and time spent grooming (F). n = 9 animals per group, n = 8 for PPT. *, significant difference (P < 0.01) compared with control treatment. Adapted from (Lund et al., 2005).

Figure 3

GFP-ERβ splice variants differentially localize within the nucleus in transiently transfected CHO cells. Cells were transfected with constructs coding for chimeric receptor proteins containing eGFP coupled to the various receptors. Schematic diagrams at the left show the protein-encoding regions (boxes; deletions are indicated by a single line, and insertions are shaded red). Panels A, C, E show cell nuclei in cells maintained in medium containing charcoal-stripped FBS. Panels B, D, F show cell nuclei after exposure to 100 nm estradiol for 20 min. Panels G – I show a time sequence of an individual nucleus following treatment with 100 nM estradiol (G = 0 min, H = 10 min, I = 15 min). Panels J–K show a time sequence of an individual nucleus following treatment with 100 nm tamoxifen (J = 0 min, K = 2 min, L = 10 min). The foci in G–L are larger and less abundant than the hyperspeckles shown in panels B, D, F, and the appearance and disappearance of receptor from foci are rapid.