Estrogen-Binding Sites and Their Functional Capacity in Estrogen Receptor Double Knockout Mouse Brain

Early studies found estrogen-binding sites in the ER knockout (ERαKO) mouse brain, suggesting a splice variant of ERα or another ER. The discovery of ERβ suggested that binding was due to ERβ, although questions about an ERγ remained. To test this hypothesis, ERβKO mice were generated and crossed with ERαKO mice, and ERα/βKO animals were used for in vivo binding studies with [125I]estrogen. The results revealed nuclear binding sites in the ERα/βKO hypothalamus and amygdala. As the binding resembled the distribution of ERα, we evaluated the presence of ERα splicing variants. A nonphysiological splice variant of ERα was identified in ERα/βKO brain and uterus, but was absent in wild-type mice. ERα immunoreactivity was also detected in regions of ERα/βKO brain where residual binding was seen. To ascertain the functionality of the variant, the regulation of PR was assessed in brain. The results revealed that E2 significantly increased PR expression, an indication that the variant can regulate gene transcription. These data demonstrate the presence and functionality of an ERα variant in ERα/βKO brain and suggest that the residual binding and regulation of PR in ERα/βKO brain can be accounted for by the variant.