Estrogen Receptor Null Mice - Endocrine Reviews
Estrogen Receptor Null Mice - Endocrine Reviews
Estrogen Receptor Null Mice - Endocrine Reviews
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
378 COUSE AND KORACH Vol. 20, No. 3<br />
age-matched superovulated wild-type and heterozygous females.<br />
Therefore, intraovarian ER� action does not appear to<br />
be essential to ovulation when stimulated with exogenous<br />
gonadotropins.<br />
4. �ERKO ovary. As discussed above, the �ERKO has provided<br />
a number of indications to suggest that intraovarian<br />
ER� action is not critical to ovarian function. Furthermore,<br />
several presumed functions of estradiol in ovarian granulosa<br />
cells appear to be preserved in the �ERKO, such as the<br />
attenuation of apoptosis and the induction of LH receptors.<br />
Based on the reported localization of ER� mRNA (49, 50, 63)<br />
and protein (69, 103, 104) to the granulosa cells of growing<br />
follicles, as well as the maintenance of a normal expression<br />
pattern for ER� in the �ERKO ovary (93, 142), it is likely that<br />
the newly discovered ER is the predominant mediator of<br />
estrogen action in the follicle. In the rat ovary, ER� is easily<br />
detectable by immunohistochemistry and exhibits an almost<br />
ubiquitous expression pattern within the granulosa cells of<br />
follicles ranging from the primary up to the ovulatory or<br />
Graafian stage (69, 103, 104). Byers et al. (63) demonstrated<br />
that ER� mRNA levels remain relatively constant during the<br />
follicular stage of the ovarian cycle but are rapidly decreased<br />
after the gonadotropin surge-induced luteinization of the<br />
granulosa cells. Interestingly, in 1975, Richards (216) reported<br />
a similar profile for high-affinity [ 3 H]-E 2 binding in<br />
the granulosa cells of the rat ovary, showing increased binding<br />
levels as follicles matured followed by marked decrease<br />
after luteinization. Because the affinities of the two ERs for<br />
estradiol do not significantly differ, it was not possible at that<br />
time to realize that the receptor being detected in the ovary<br />
was distinctly different (i.e., ER�) from that which was being<br />
concurrently described in the uterus (i.e., ER�).<br />
As stated previously, female mice homozygous for the<br />
targeted disruption of the ER� gene possess no gross aberrant<br />
phenotypes as neonates or during adulthood (47). However,<br />
during a continuous mating study of 8 weeks in which<br />
sexually mature wild-type or �ERKO females were housed<br />
with a known fertile wild-type male, a significant deficit in<br />
fertility in the �ERKO became obvious. As shown in Table<br />
3, �ERKO females produced substantially fewer litters as<br />
well as significantly less numbers of pups per litter when<br />
compared with their wild-type littermates (47). Whereas the<br />
average litter size among wild-type females was 8.8 (�2.5)<br />
pups per litter, this number was reduced to 3.1 (�1.8) in<br />
�ERKO females (Table 3) (47). Furthermore, two of the tested<br />
�ERKO females yielded no litters, despite the observation of<br />
seminal plugs on multiple occasions, suggesting that abnormal<br />
sexual behavior was not a cause for the infertility. Gross<br />
TABLE 3. Fertility and superovulation data in the �ERKO female mice<br />
Genotype<br />
analysis of the uteri from the �ERKO females used in this<br />
study demonstrated no indication of embryo resorption during<br />
gestation. Therefore, the observed subfertility in the<br />
�ERKO females did not appear to be due to uterine dysfunction<br />
that results in premature termination of pregnancy.<br />
However, a possible defect in embryo implantation due to a<br />
lack of ER� could not be assessed in these studies.<br />
The nature of the subfertility in the �ERKO female described<br />
above strongly suggested an ovarian phenotype. Gross analysis<br />
of ovaries from �ERKO females indicated no distinct differences<br />
in size or morphology when compared with those of<br />
wild-type females. As shown in Fig. 5, histological analysis of<br />
ovaries from sexually mature �ERKO females illustrated the<br />
presence of a relatively normal interstitial compartment and<br />
follicles at various stages of the follicular cycle, ranging from<br />
primordial and primary to those with a clearly defined antrum,<br />
all possessing the expected thecal shell. Therefore, as demonstrated<br />
for ER� by the �ERKO, ER� does not appear to be<br />
essential for the establishment of germ cell number or ovarian<br />
development. Several of the speculated intraovarian roles of<br />
estrogen were discussed previously and include a proposed<br />
critical role in proliferation of the granulosa cells in the maturing<br />
follicle (217–220, 231). However, the multiple large follicles<br />
present in the �ERKO ovaries indicate no marked differences<br />
in granulosa cell number. Furthermore, serum estradiol levels<br />
in adult �ERKO females do not appear to differ from agematched<br />
wild-type females, at 24.2 (�3.3) and 30.5 (�2.8) pg/<br />
ml, respectively. Biological evidence of estradiol synthesis in the<br />
�ERKO ovary is also illustrated by the apparently normal<br />
uterus and vagina, which exhibit the proper cyclic changes in<br />
morphology of a sexually mature wild-type mouse. Nonetheless,<br />
there were indications of an increased number of early<br />
atretic follicles and a sparse presence of corpora lutea in the<br />
�ERKO ovary when compared with the wild-type (47), suggesting<br />
that the observed subfertility in the �ERKO may be due<br />
to a reduction in completed folliculogenesis.<br />
A phenotype of compromised follicular maturation that<br />
more often terminates in atresia rather than ovulation, as<br />
described in the �ERKO female, is similar to that reported in<br />
a number of other mutant mice. As previously discussed,<br />
arrested folliculogenesis is described in knockout mice for<br />
other genes known to be expressed in granulosa cells, including<br />
the gene for FSH-receptor (242), P450 arom (ArKO)<br />
(257), IGF-I (245), and connexin-37 (247). It is therefore possible<br />
that a loss of ER� action has also resulted in alterations<br />
in the expression and/or function of one or more of these<br />
gene products. However, the normal serum estradiol levels<br />
and the appearance of estrogenic effects in the reproductive<br />
Continuous mating results Superovulation results<br />
n Litters per<br />
female (SEM)<br />
Pups per<br />
litter (SEM)<br />
n Oocytes per<br />
female (SEM)<br />
Wild-type 6 2.8 � 0.4 8.8 � 2.5 10 33.7 � 4.8 9–57<br />
Heterozygous nd nd nd 11 52.5 � 5.7 a<br />
20–77<br />
�ERKO 11 1.7 � 1.0 a<br />
3.1 � 1.8 b<br />
11 6.0 � 1.5 a<br />
nd, Not determined.<br />
0–13<br />
a<br />
P � 0.05, Student’s two tailed t-test vs. wild-type<br />
b<br />
P � 0.001, Student’s two tailed t-test vs. wild-type<br />
Range