Differential expression of estrogen receptor beta splice variants in rat brain: identification and characterization of a novel variant missing exon 4.

Estrogen receptor beta (ER-beta) mRNA is found in abundance in rat brain. The distribution of ER-beta mRNA in brain differs from that of ER-alpha suggesting they subserve different functions. ER-beta mRNA has been reported to be variably spliced, in contrast to ER-alpha, resulting in numerous isoforms that possess different functional properties. The present study was undertaken to determine whether the isoforms of ER-beta mRNA are differentially distributed in different brain regions. In order to assess the range of transcript forms expressed in various brain regions in the same assay, a micropunch dissection technique was combined with semiquantitative RT-PCR. The relative abundance of each ER-beta isoform (beta1>beta2>beta1delta3>beta2delta3) was similar in all ER-beta positive brain regions with the exception of the hippocampus, which contained low levels of most isoforms and a fifth ER-beta isoform, which we are calling ER-beta1delta4. Based on its sequence, ER-beta1delta4 encodes an ER-beta that is missing exon 4. Initial characterization studies of this showed that it did not bind estrogen, and that, unlike ER-beta1, it localized to the cytoplasm when expressed in cultured cells. The distribution of ER-beta1delta4 was different from that of the other isoforms in that it was expressed at high levels in the hippocampus, where the other isoforms were low, and that it was nearly undetectable in the brain regions that expressed the highest levels of the other ER-beta splice variants. These data suggest that a highly complex pattern of estrogen signaling can occur in a region specific manner in the rat brain.