Evolution of the corticosteroid receptor signalling pathway in fish

Evolution of a new function by degenerative mutation in cephalochordate steroid receptors

Gene duplication is the predominant mechanism for the evolution of new genes. Major existing models of this process assume that duplicate genes are redundant; degenerative mutations in one copy can therefore accumulate close to neutrally, usually leading to loss from the genome. When gene products dimerize or interact with other molecules for their functions, however, degenerative mutations in one copy may produce repressor alleles that inhibit the function of the other and are therefore exposed to selection. Here, we describe the evolution of a duplicate repressor by simple degenerative mutations in the steroid hormone receptors (SRs), a biologically crucial vertebrate gene family. We isolated and characterized the SRs of the cephalochordate Branchiostoma floridae, which diverged from other chordates just after duplication of the ancestral SR. The B. floridae genome contains two SRs: BfER, an ortholog of the vertebrate estrogen receptors, and BfSR, an ortholog of the vertebrate receptors for androgens, progestins, and corticosteroids. BfSR is specifically activated by estrogens and recognizes estrogen response elements (EREs) in DNA; BfER does not activate transcription in response to steroid hormones but binds EREs, where it competitively represses BfSR. The two genes are partially coexpressed, particularly in ovary and testis, suggesting an ancient role in germ cell development. These results corroborate previous findings that the ancestral steroid receptor was estrogen-sensitive and indicate that, after duplication, BfSR retained the ancestral function, while BfER evolved the capacity to negatively regulate BfSR. Either of two historical mutations that occurred during BfER evolution is sufficient to generate a competitive repressor. Our findings suggest that after duplication of genes whose functions depend on specific molecular interactions, high-probability degenerative mutations can yield novel functions, which are then exposed to positive or negative selection; in either case, the probability of neofunctionalization relative to gene loss is increased compared to existing models.

The A/B domain of the teleost glucocorticoid receptors influences partial nuclear localization in the absence of hormone

The glucocorticoid (GR) and mineralocorticoid receptor (MR) of extant jawed vertebrates emerged after duplication of an ancestral corticosteroid receptor. The ancestral corticosteroid receptor resembled extant MRs in hormone selectivity, and the different ligand specificity of extant GRs is a secondary derived characteristic. An additional characteristic that distinguishes the mammalian GR from the MR is the cellular distribution pattern in the absence of hormone: the naïve GR resides in the cytoplasm, whereas the naïve MR is found in both the nucleus and cytoplasm. Our results show, by the use of green fluorescent protein-tagged fusion proteins, that the GRs [rainbow trout (rt) GR1 and rtGR2] from a lower vertebrate, the teleost fish, rainbow trout (Oncorhynchus mykiss) resemble mammalian MR rather than GR in their subcellular localization pattern. The addition of cortisol caused the remaining cytoplasmic rtGR1 and rtGR2 to migrate to the nucleus. The speed of nuclear localization was cortisol concentration dependent, with rtGR2 being more sensitive than rtGR1, mimicking the transactivational properties of the receptors in which the cortisol EC50 value is an order of magnitude lower for rtGR2. By the use of chimera constructs between the trout GRs and the rat GR C656G, we show that the E domain of the trout receptors are not involved in the nucleocytoplasmic localization of naïve trout GRs, but the A/B domain, especially if linked to the corresponding trout CD region, plays a pivotal role in the cellular distribution pattern. This is unrelated to the difference in the trout GRs transactivation sensitivity, which is determined by the receptor's E-domains.

Glucocorticoid receptor (DlGR1) is expressed in pre-larval and larval stages of the teleost fish Dicentrarchus labrax

Glucocorticoid hormone receptors (GR), members of the nuclear hormone receptor superfamily, are ligand-dependent transcription factors expressed in various tissues by binding to specific DNA sequences. Since glucocorticoids have a role in maintaining the homeostatic status in fish, we previously cloned and sequenced a GR (DlGR1) of adult Dicentrarchus labrax; we also showed mRNA expression (in situ hybridization) and tissue immunohistochemical localization of DlGR1 in several organs. This work has now been extended to the examination of the expression, tissue distribution, and cytolocalization of DlGR1 in larval developmental stages by similar methods to those used for the adult organs. The riboprobe included the DlGR1 cDNA transcriptional activation domain (1.0–1,300 nucleotide sequence) showing no significant similarity with a known second GR cDNA sequence of sea bass. The antibody was specific for an opportunely selected peptide sequence of the DlGR1 transcriptional domain. In histological sections of brain, head kidney, gills, liver, anterior intestine, and spleen cells, the riboprobe was mainly located in the cell nucleus. The antibody identified DlGR1 in the head kidney, gills, liver, and anterior intestine, mainly located in the cytosol. These results are in agreement with the receptor location in adult tissues. The greater presence of both the transcript and protein of DlGR1 in the late developmental stages suggests an increasing expression of this receptor. The cytolocalization (nuclear-cytosolic) and presumptive roles of DlGR1-containing tissues are discussed.

Paralogous vitamin D receptors in teleosts: transition of nuclear receptor function

The availability of multiple teleost (bony fish) genomes is providing unprecedented opportunities to understand the diversity and function of gene duplication events using comparative genomics. Here we describe the cloning and functional characterization of two novel vitamin D receptor (VDR) paralogs from the freshwater teleost medaka (Oryzias latipes). VDR sequences were identified through mining of the medaka genome database in which gene organization and structure was determined. Two distinct VDR genes were identified in the medaka genome and mapped to defined loci. Each VDR sequence exhibits unique intronic organization and dissimilar 5' untranslated regions, suggesting they are not isoforms of the same gene locus. Phylogenetic comparison with additional teleosts and mammalian VDR sequences illustrate that two distinct clusters are formed separating aquatic and terrestrial species. Nested within the teleost cluster are two separate clades for VDRalpha and VDRbeta. The topology of teleost VDR sequences is consistent with the notion of paralogous genes arising from a whole genome duplication event prior to teleost radiation. Functional characterization was conducted through the development of VDR expression vectors including Gal4 chimeras containing the yeast Gal4 DNA binding domain fused to the medaka VDR ligand binding domain and full-length protein. The common VDR ligand 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)(2)D(3)] resulted in significant transactivation activity with both the Gal4 and full-length constructs of medaka (m) VDRbeta. Comparatively, transactivation of mVDRalpha with 1alpha,25(OH)(2)D(3) was highly attenuated, suggesting a functional divergence between these two nuclear receptor paralogs. We additionally demonstrate through coactivator studies that mVDRalpha is still functional; however, it exhibits a different sensitivity to 1alpha,25(OH)(2)D(3), compared with VDRbeta. These results suggest that in mVDRalpha and VDRbeta have undergone a functional divergence through a process of sub- and/or neofunctionalization of VDR nuclear receptor gene pairs.

Endocrine systems in juvenile anadromous and landlocked Atlantic salmon (Salmo salar): seasonal development and seawater acclimation

The present study compares developmental changes in plasma levels of growth hormone (GH), insulin-like growth factor I (IGF-I) and cortisol, and mRNA levels of their receptors and the prolactin receptor (PRLR) in the gill of anadromous and landlocked Atlantic salmon during the spring parr-smolt transformation (smoltification) period and following four days and one month seawater (SW) acclimation. Plasma GH and gill GH receptor (GHR) mRNA levels increased continuously during the spring smoltification period in the anadromous, but not in landlocked salmon. There were no differences in plasma IGF-I levels between strains, or any increase during smoltification. Gill IGF-I and IGF-I receptor (IGF-IR) mRNA levels increased in anadromous salmon during smoltification, with no changes observed in landlocked fish. Gill PRLR mRNA levels remained stable in both strains during spring. Plasma cortisol levels in anadromous salmon increased 5-fold in May and June, but not in landlocked salmon. Gill glucocorticoid receptor (GR) mRNA levels were elevated in both strains at the time of peak smoltification in anadromous salmon, while mineralocorticoid receptor (MR) mRNA levels remained stable. Only anadromous salmon showed an increase of gill 11beta-hydroxysteroid dehydrogenase type-2 (11beta-HSD2) mRNA levels in May. GH and gill GHR mRNA levels increased in both strains following four days of SW exposure in mid-May, whereas only the anadromous salmon displayed elevated plasma GH and GHR mRNA after one month in SW. Plasma IGF-I increased after four days in SW in both strains, decreasing in both strains after one month in SW. Gill IGF-I mRNA levels were only increased in landlocked salmon after 4days in SW. Gill IGF-IR mRNA levels in SW did not differ from FW levels in either strain. Gill PRLR mRNA did not change after four days of SW exposure, and decreased in both strains after one month in SW. Plasma cortisol levels did not change following SW exposure in either strain. Gill GR, 11beta-HSD2 and MR mRNA levels increased after four days in SW in both strains, whereas only the anadromous strain maintained elevated gill GR and 11beta-HSD2 mRNA levels after one month in SW. The results indicate that hormones and receptors of the GH and cortisol axes are present at significantly lower levels during spring development and SW acclimation in landlocked relative to anadromous salmon. These findings suggest that attenuation of GH and cortisol axes may, at least partially, result in reduced preparatory upregulation of key gill ion-secretory proteins, possibly a result of reduced selection pressure for marine adaptations in landlocked salmon.

Expression of a glucocorticoid receptor (DlGR1) in several tissues of the teleost fish Dicentrarchus labrax

Since glucocorticoids have a role in maintaining the homeostatic status in fish, in the present paper mRNA expression (in situ hybridization) and tissue immunohistochemical localization of a glucocorticoid receptor (DlGR1) in several Dicentrarchus labrax organs are reported. Riboprobe and specific antibodies were prepared by using the DlGR1 that has been previously cloned and sequenced from peritoneal cavity leukocytes. Both mRNA and receptor were identified in head kidney, spleen, gills, intestine, heart and liver tissues. The functional roles of DlGR1 localization are discussed.

Development of the corticosteroid stress axis and receptor expression in zebrafish

Using zebrafish embryos and larvae, we examined the temporal patterns of cortisol and expression of genes involved in corticosteroid synthesis and signaling. Embryonic cortisol levels decreased approximately 70% from 1.5 h postfertilization (hpf) to hatch (approximately 42 hpf) and then increased 27-fold by 146 hpf. The mRNA abundances of steroidogenic acute regulatory protein, 11beta-hydroxylase and 11beta-hydroxysteroid dehydrogenase type 2, increased severalfold after hatch and preceded the rise of cortisol levels. In contrast to other teleosts that possess two glucocorticoid receptors (GRs) and one mineralocorticoid receptor (MR), only one GR and MR were identified in zebrafish, which were cloned and sequenced. GR mRNA abundance decreased from 1.5 to 25 hpf, rebounded, and then was stable from 49 to 146 hpf. MR transcripts increased continuously from 1.5 hpf and were 52-fold higher by 97 hpf. An acute cortisol response to a stressor was not detected until 97 hpf, whereas melanocortin type 2 receptor mRNA increased between 25 and 49 hpf. Collectively, the patterns of cortisol and the expression of cortisol biosynthetic genes and melanocortin type 2 receptor suggest that the corticoid stress axis in zebrafish is fully developed only after hatch. The temporal differences in GR, MR, and 11beta-hydroxysteroid dehydrogenase type 2 gene expression lead us to propose a key role for MR signaling by maternal cortisol during embryogenesis, whereas cortisol secretion after hatch may be regulating GR expression and signaling in zebrafish.