Tissue-specific expression of two structurally different estrogen receptoralpha isoforms along the female reproductive axis of an oviparous species,the rainbow trout

In oviparous species, in addition to a full-length estrogen receptor alpha (ER alpha), another ER alpha isoform lacking the A domain and exhibiting a ligand-independent transactivation function has been consistently reported. Although both isoforms are expressed in the liver, their respective sites of expression in other potential target tissues are unknown. In contrast to the situation in Xenopus and chicken, the two isoforms of rainbow trout (O ncorhynchus mykiss) are generated from two classes of transcripts with diff erent 5 ' untranslated sequences issued from the same gene by alternative s plicing and promoter usage. The aim of this study was to take advantage of the unique organization of the rainbow trout ER alpha gene to investigate t he tissue distribution of these two messenger species along the reproductiv e axis of female trout. The S1 nuclease assay and in situ hybridization wer e used, with probes specific for each of the transcripts. Reverse transcrip tion polymerase chain reaction (RT-PCR) with primers specific for each of t he isoforms also was performed. The data indicated that the full-length ER alpha is expressed in liver, brain, pituitary, and ovary, whereas expressio n of the isoform with the truncated A domain is restricted to the liver, de monstrating a tissue-specific expression of these two ER alpha isoforms. Th e presence of a short liver-specific isoform in oviparous species suggests its role in the development and/or maintenance of the unique function of th e liver in the vitellogenesis process.