## 3 Estrogen Receptors
### Role of ER Beta in Bone  
Most of the knowledge obtained about ER\(\beta\) function has come from global knockout rodent models. Like ER\(\alpha\) global knockout mice, this approach has many limitations and it is important to note that ER\(\beta\) global knockouts mice show expression of spliced variants present in several tissues including bone that can lead to confounding results [55; 56; 57]. Several groups have created and characterized different ER\(\beta\) knockout mice: \(\beta\)ERKOCH, \(\beta\)ERKOKI ER\(\beta\)KOST, ER\(\beta\)KOWY, and ER\(\beta\)STL-/L-. The nomenclature refers to the institute in which the mutant was developed and differs mainly on how the deletion occurs. [PERSON] et al. generated the ER\(\beta\)KOCH (mice produced in the University of Carolina, Chapel Hill, NC) by disrupting exon 3, which encodes the first zinc finger of the DNA Binding Domain, by insertion of a neomycin-resistance gene (Neo) cassette in the reverse orientation into the Pstl site. In this animal model, no full-length mRNA was detected; however, several transcript variants were observed in which exon 3 was consistently spliced out. Mice lacking full-size ER\(\beta\) reproduced normally but had fewer and smaller litters than wild-type mice. This may be due to reduced ovarian efficiency. Older male knockout mice suffered from the bladder and prostatic epithelium hyperplasia [58]. This mutant was then transferred to the Karolinska Institute (\(\beta\)ERKOKI) in Sweden where mutant males developed prostatic intraepithelial hyperplasia. Researchers at this institution determined these mice were not only sterile, but they also had questionable behavioral conduct, leading to support the hypothesis that ER\(\beta\) is involved in the development and/or homeostasis of the brain. The female mutant mice showed a lack of ovulation [59]. In 2002, [PERSON] and colleagues reported the generation of a second ER\(\beta\) knockout mouse (ER\(\beta\)KOST) in Strasbourg, France. They inserted a Neo gene in the Spel site of exon 3 in the 5\({}^{\prime}\) to 3\({}^{\prime}\) orientation. Like the knockout previously described, authors detected the presence of several spliced variants lacking exon 3. However, they could not confirm the epithelial hyperplasia of the bladder and prostate ducts observed in the \(\beta\)ERKOCH mouse model. In this animal model, it was determined that global deletion of ER\(\beta\) did not affect male skeleton but in females, bone resorption was decreased resulting in an increase in trabecular bone volume [57]. A third ER\(\beta\)KO was generated at the Wyeth-Ayerst Research/Genetic Institute (ER\(\beta\)KOWY), which was different from those previously described. The construct used in this animal model introduces stop codons in all three reading frames of the ER\(\beta\) gene, leading to a complete knockout. In addition, they introduced a Neo cassette in the reverse orientation resulting in the truncation of exons 1 and 2 and deletion of intron 1. The authors probed for ER\(\beta\) protein expression in the brain and ovaries, tissues known to have high ER\(\beta\) expression, and confirmed deletion of the receptor in those tissues. No bone analysis was reported in this paper [60]. A fourth ER\(\beta\)-null mouse model was reported in 2007 (ER\(\beta\)STL-/L-). This is different than previously reported in that it was created using the Cre/LoxP-mediated excision. Authors claim it lacks any spliced variants downstream of exon 3; furthermore, this animal model can be used to delete ER\(\beta\) in specific tissues using the Cre/LoxP system. These animals developed none of the anomalies reported in previous animal models that appear during aging (prostatic hyperplasia, defects in several tissues including brain, lungs, heart, urinary bladder, and intestine). The authors reported that ER\(\beta\)STL-/L- mice of both sexes are sterile. In female ER\(\beta\)STL-/L- mice, infertility comes from the lack of ovulation.