Estrogen receptor-related receptors in the killifish Fundulus heteroclitus: diversity, expression, and estrogen responsiveness

Abstract

The estrogen receptor-related receptors (ERRs) are a group of nuclear receptors that were originally identified on the basis of sequence similarity to the estrogen receptors. The three mammalian ERR genes have been implicated in diverse physiological processes ranging from placental development to maintenance of bone density, but the diversity, function, and regulation of ERRs in non-mammalian species are not well understood. In this study, we report the cloning of four ERR cDNAs from the Atlantic killifish, Fundulus heteroclitus, along with adult tissue expression and estrogen responsiveness. Phylogenetic analysis indicates that F. heteroclitus (Fh)ERRα is an ortholog of the single ERRα identified in mammals, pufferfish, and zebrafish. FhERRβa and FhERRβb are co-orthologs of the mammalian ERRβ. Phylogenetic placement of the fourth killifish ERR gene, tentatively identified as FhERRγb, is less clear. The four ERRs showed distinct, partially overlapping mRNA expression patterns in adult tissues. FhERRα was broadly expressed. FhERRβa was expressed at apparently low levels in eye, brain, and ovary. FhERRβb was expressed more broadly in liver, gonad, eye, brain, and kidney. FhERRγb was expressed in multiple tissues including gill, heart, kidney, and eye. Distinct expression patterns of FhERRβa and FhERRβb are consistent with subfunctionalization of the ERRβ paralogs. Induction of ERRα mRNA by exogenous estrogen exposure has been reported in some mammalian tissues. In adult male killifish, ERR expression did not significantly change following estradiol injection, but showed a trend toward a slight induction (three- to five-fold) of ERRα expression in heart. In a second, more targeted experiment, expression of ERRα in adult female killifish was downregulated 2.5-fold in the heart following estradiol injection. In summary, our results indicate that killifish contain additional ERR genes relative to mammals, including ERRβ paralogs. In addition, regulation of ERRα expression in killifish apparently differs from regulation in mammals. Together, these features may facilitate determination of both conserved and specialized ERR gene functions.