Peptide insertion in the DNA-binding domain of fish glucocorticoid receptor is encoded by an additional exon and confers particular functional properties

Human and fish differences in steroid receptors activation: A review

Steroid receptors (SRs) are transcription factors activated by steroid hormones (SHs) that belong to the nuclear receptors (NRs) superfamily. Several studies have shown that SRs are targets of endocrine disrupting chemicals (EDCs), widespread substances in the environment capable of interfering with the endogenous hormonal pathways and causing adverse health effects in living organisms and/or their progeny. Cell lines with SRs reporter gene are currently used for in vitro screening of large quantities of chemicals with suspected endocrine-disrupting activities. However, most of these cell lines express human SRs and therefore the toxicological data obtained are also extrapolated to non-mammalian species. In parallel, in vivo tests have recently been developed on fish species whose data are also extrapolated to mammalian species. As some species-specific differences in SRs activation by natural and synthetic chemicals have been recently reported, the aim of this review is to summarize those between human and fish SRs, as representatives of mammalian and non-mammalian toxicology, respectively. Overall, this literature study aims to improve inter-species extrapolation of toxicological data on EDCs and to understand which reporter gene cell lines expressing human SRs are relevant for the assessment of effects in fish and whether in vivo tests on fish can be properly used in the assessment of adverse effects on human health.

Effects of Osmotic Stress on the mRNA Expression of prl, prlr, gr, gh, and ghr in the Pituitary and Osmoregulatory Organs of Black Porgy, Acanthopagrus schlegelii

In euryhaline teleost black porgy, Acanthopagrus schlegelii, the glucocorticoid receptor (gr), growth hormone receptor (ghr), prolactin (prl)-receptor (prlr), and sodium–potassium ATPase alpha subunit (α-nka) play essential physiological roles in the osmoregulatory organs, including the gill, kidney, and intestine, during osmotic stress. The present study aimed to investigate the impact of pituitary hormones and hormone receptors in the osmoregulatory organs during the transfer from freshwater (FW) to 4 ppt and seawater (SW) and vice versa in black porgy. Quantitative real-time PCR (Q-PCR) was carried out to analyze the transcript levels during salinity and osmoregulatory stress. Increased salinity resulted in decreased transcripts of prl in the pituitary, α-nka and prlr in the gill, and α-nka and prlr in the kidney. Increased salinity caused the increased transcripts of gr in the gill and α-nka in the intestine. Decreased salinity resulted in increased pituitary prl, and increases in α-nka and prlr in the gill, and α-nka, prlr, and ghr in the kidney. Taken together, the present results highlight the involvement of prl, prlr, gh, and ghr in the osmoregulation and osmotic stress in the osmoregulatory organs (gill, intestine, and kidney). Pituitary prl, and gill and intestine prlr are consistently downregulated during the increased salinity stress and vice versa. It is suggested that prl plays a more significant role in osmoregulation than gh in the euryhaline black porgy. Furthermore, the present results highlighted that the gill gr transcript’s role was solely to balance the homeostasis in the black porgy during salinity stress.

Regulation of the kiss2 promoter in yellowtail clownfish (Amphiprion clarkii) by cortisol via GRE-dependent GR pathway

Kisspeptin plays a vital role in mediating the stress-induced reproductive regulation. Cortisol, known as a stress-related hormone, is involved in gonadal development and sexual differentiation by binding with glucocorticoid receptor (GR) to regulate the expression of kiss gene. In the present study, cortisol treatment in yellowtail clownfish (Amphiprion clarkii) showed that the expression of kiss (kiss1 and kiss2) and gr (gr1 and gr2) genes were increased significantly. We demonstrated that the yellowtail clownfish Kiss neurons co-express the glucocorticoid receptors in the telencephalon, mesencephalon, cerebellum, and hypothalamus. We further cloned the promoter of kiss2 gene in yellowtail clownfish and identified the presence of putative binding sites for glucocorticoid receptors, estrogen receptors, androgen receptors, progesterone receptors, AP1, and C/EBP. Applying transient transfection in HEK293T cells of the yellowtail clownfish kiss2 promoter, cortisol (dexamethasone) treatment was shown to enhance the promoter activities of the yellowtail clownfish kiss2 gene in the presence of GRs. Deletion analysis of kiss2 promoter indicated that cortisol-induced promoter activities were located between position -660 and -433 with GR1, and -912 and -775 with GR2, respectively. Finally, point mutation studies on the kiss2 promoter showed that cortisol-stimulated promoter activity was mediated by one GRE site located at position -573 in the presence of GR1 and by each GRE site located at position -883, -860, -851, and -843 in the presence of GR2. Results of the present study provide novel evidence that cortisol could regulate the transcription of kiss2 gene in the yellowtail clownfish via GRE-dependent GR pathway.

Salmo salar glucocorticoid receptors analyses of alternative splicing variants under stress conditions

Cortisol is the main corticosteroid in teleosts, exerting multiple functions by activating glucocorticoid receptors (GR). Most teleost species have two GR genes, gr-1 and gr-2. Some teleost also presents two splice variants for gr-1; gr-1a and gr-1b. In this study, we report for first time the presence of 2 homeologous genes for gr-1 and gr-2, located on chromosomes 4q-13q (gr-1) and 5p-9q (gr-2) of the Salmo salar genome. Furthermore, our results describe gr-1 splice variants derived from chromosome 4 and 13, sharing typical teleost GR elements such as the 9 amino acid insertion in the DNA binding domain (DBD) and variations in the length of the ligand binding domain (LBD). Three splice variants were predicted for the gr-2 homeologous gene in chromosome 5, with differences of a 5 amino acid insertion in the DBD. We also identified an uncommon truncated gr-2 gene in chromosome 9 in salmon, which lacked the DBD and LBD domains. Finally, by designing specific primers for each predicted splice variant, we validated and evaluated the expression of their transcripts in S. salar subjected to stress caused by stocking density. Differences were observed in the expression of all identified mRNAs, revealing that gr-1 and gr-2 splice variants were upregulated in head kidney and gills of post-stressed fish. In conclusion, our findings suggest that from specific salmonid genomic duplication (125 MYA), two gene copies of each GR receptor were generated in S. salar. The identified splice variants could contribute to the variability of GR receptor complex modulation expression during stressful events, leading to variations in physiological responses in fish.

Molecular characterization and expression patterns of glucocorticoid receptor (GR) genes in protandrous hermaphroditic yellowtail clownfish, Amphiprion clarkii

Sexual differentiation and ovotestis development are closely associated with cortisol levels, the principal indicator of stress, via the glucocorticoid receptor (GR) in teleosts. Thus, GR is regarded as a mediator to expound the relationship between social stress and gonad development. In the present study, two gr genes (gr1 and gr2) were cloned and analyzed from a protandrous hermaphroditic teleost, the yellowtail clownfish (Amphiprion clarkii). GR1 was found to display a conserved nine-amino-acid insert, WRARQNTDG, between two zinc finger domains. The phylogenetic tree of GR showed that yellowtail clownfish GR1 and GR2 are clustered to teleost GR1 and teleost GR2 separately, and differ from tetrapod GR. The result of real-time PCR revealed that high-level gr1 was mainly distributed in the cerebellum, hypothalamus and heart. The gr2 gene was abundant in the pituitary and liver of females and nonbreeders, while gr2 was mainly detected in the medulla oblongata and middle kidney of males. Moreover, GRs can be expressed in cultured eukaryotic cells and functionally interact with dexamethasone (exogenous glucocorticoid), thereby triggering downstream signaling pathways of different potentials. GR1 and GR2 can be activated by 10 nM dexamethasone treatment in HEK-293T cells. Notably, real-time PCR analysis among three social status groups demonstrated that gr2 expression was the highest in the hypothalamus of nonbreeders, but gr1 was no difference. We speculate that social stress would increase the expression of gr2 gene expression in the hypothalamus to inhibit sexual development. These data provide evidence of social stress involving reproductive regulation, which may help to elucidate the underlying mechanism of sex differentiation and change.

The evolution, structure and function of the ray finned fish (Actinopterygii) glucocorticoid receptors

Basal ray-finned fish (Actinopterygii) possess a single glucocorticoid receptor (GR) and when compared to the lobe-finned vertebrate (Sarcopterygii) GR possess nine additional amino acids between the zinc-finger of the DNA binding domain. A whole genome duplication event which occurred between 320 and 350MYA in the teleost lineage following the split from the basal ray-finned fish resulted in 2 GRs: one GR group, GR1, has retained the 9 amino acids insert whereas the other group, GR2, has not. The exception to this is the zebrafish, that have lost one of the GRs, but they do possess 2 GRs with a splice variant that lacks the C-terminal portion of the GR to form GRβ which acts as a dominant-repressor of the wildtype GR. Another splice variant sees the basal ray-finned GR and teleost GR1 without the 9 amino acids insert. The molecular basis for GRs retention is beginning to be unravelled. In Pantadon buchholzi, rainbow trout, carp, marine and Japanese medaka GR2 is more sensitive to glucocorticoids (GC), thus potentially playing a more significant role in regulating gene expression at basal circulatory GC concentrations. However, this division in GC sensitivity is not seen in other species. The few studies to evaluate the significance of the 9 amino acid insert have shown that it affect maximal transactivational activity the extent to which is dependent on the number of glucocorticoid response elements (GREs) present in the reporter plasmid. The retention of these GRs would suggest there was an evolutionary advantage, which saw the development of a complex regulatory process to mediate the actions of the glucocorticoids.

Social regulation of cortisol receptor gene expression

In many social species, individuals influence the reproductive capacity of conspecifics. In a well-studied African cichlid fish species, Astatotilapia burtoni, males are either dominant (D) and reproductively competent or non-dominant (ND) and reproductively suppressed as evidenced by reduced gonadotropin releasing hormone (GnRH1) release, regressed gonads, lower levels of androgens and elevated levels of cortisol. Here, we asked whether androgen and cortisol levels might regulate this reproductive suppression. Astatotilapia burtoni has four glucocorticoid receptors (GR1a, GR1b, GR2 and MR), encoded by three genes, and two androgen receptors (ARα and ARβ), encoded by two genes. We previously showed that ARα and ARβ are expressed in GnRH1 neurons in the preoptic area (POA), which regulates reproduction, and that the mRNA levels of these receptors are regulated by social status. Here, we show that GR1, GR2 and MR mRNAs are also expressed in GnRH1 neurons in the POA, revealing potential mechanisms for both androgens and cortisol to influence reproductive capacity. We measured AR, MR and GR mRNA expression levels in a microdissected region of the POA containing GnRH1 neurons, comparing D and ND males. Using quantitative PCR (qPCR), we found D males had higher mRNA levels of ARα, MR, total GR1a and GR2 in the POA compared with ND males. In contrast, ND males had significantly higher levels of GR1b mRNA, a receptor subtype with a reduced transcriptional response to cortisol. Through this novel regulation of receptor type, neurons in the POA of an ND male will be less affected by the higher levels of cortisol typical of low status, suggesting GR receptor type change as a potential adaptive mechanism to mediate high cortisol levels during social suppression.

Differential expression patterns and localization of glucocorticoid and mineralocorticoid receptor transcripts in the osmoregulatory organs of tilapia during salinity stress

The glucocorticoid receptor (GR) plays an essential role during seawater (SW) acclimation. However, the regulation of GR isoforms 1 and 2 (GR1 and GR2) and the mineralocorticoid receptor (MR) during SW acclimation is poorly understood. To address this, we localized and examined the GR1, GR2 and MR transcripts in the tilapia gill, kidney and intestine. Our results indicated that the GR1, GR2 and MR levels were increased in the kidney and intestine on day 1 in seawater (SW) fish, which is in agreement with the recognized osmoregulatory role of the corticosteroid receptors. The SW transfer increased the GR2 and MR transcripts in the gill on day 1 and 4, respectively. Surprisingly, no significant difference was obtained for the GR1 mRNA level. Analysis of the plasma parameters in freshwater (FW) and SW tilapia showed that the plasma cortisol levels were significantly increased at day 1 in the SW fish compared to the FW fish. This is the first study that focused on the spatial distribution of GR1, GR2 and MR in the osmoregulatory organs of freshwater (FW)- and SW-acclimated tilapia by in situ hybridization. Consistent with the Q-PCR results, the expression levels of the GR1, GR2 and MR transcripts were increased or decreased in the SW-acclimated tilapia's gill, kidney and intestine compared to the FW fish. We observed that GR1, GR2 and MR were localized in the branchial epithelial cells and chloride cells of the gill, proximal tubules of the kidney and columnar cells of the intestine. Together, these results indicate that the mobilization of corticosteroid receptors is dependent on the target tissue, salinity and exposure time.

Evidence for a divergence in function between two glucocorticoid receptors from a basal teleost

Background

Duplicated glucocorticoid receptors (GR) are present in most teleost fish. The evolutionary advantage of retaining two GRs is unclear, as no subtype specific functional traits or physiological roles have been defined. To identify factors driving the retention of duplicate GRs in teleosts, the current study examined GRs in representatives of two basal ray-finned fish taxa that emerged either side of the teleost lineage whole genome duplication event (WGD) event, the acipenseriform, Acipenser ruthenus, (pre-WGD) and the osteoglossimorph, Pantodon buchholzi, (post-WGD).

Results

The study identified a single GR in A. ruthenus (ArGR) and two GRs in P. buchholzi (PbGR1 and PbGR2). Phylogenetic analyses showed that ArGR formed a distinct branch separate from the teleosts GRs. The teleost GR lineage was subdivded into two sublineages, each of which contained one of the two P. buchholzi GRs. ArGR, PbGR1 and PbGR2 all possess the unique 9 amino acid insert between the zinc-fingers of the DNA-binding domain that is present in one of the teleost GR lineages (GR1), but not the other (GR2). A splice variant of PbGR2 produces an isoform that lacked these 9 amino acids (PbGR2b). Cortisol stimulated transactivation activity of ArGR, PbGR2b and PbGR1 in vitro; with PbGR2b and PbGR1, the glucocorticoid 11-deoxycortisol was a more potent agonist than cortisol. The hormone sensitivity of PbGR2b and PbGR1 differed in the transactivation assay, with PbGR2b having lower EC50 values and greater fold induction.

Conclusions

The difference in transactivation activity sensitivity between duplicated GRs of P. buchholzi suggests potential functional differences between the paralogs emerged early in the teleost lineage. Given the pleiotropic nature of GR function in vertebrates, this finding is in accordance with the hypothesis that duplicated GRs were potentially retained through subfunctionalisation followed by gene sharing. A 9 amino acid insert in the DNA-binding domain emerged in basal ray-finned fish GRs. However, the presence of a PbGR2 splice variant that lacks this insert, as well as the loss of the exon encoding these amino acids in the genes encoding for other teleost GR2 suggests the selection of two receptors with different DNA-binding domain structures in teleosts.

Characterization of two functional glucocorticoid receptors in the marine medaka Oryzias dancena

The cDNAs that encode the glucocorticoid receptors odGR1 and odGR2 were cloned from a euryhaline teleost, the marine medaka (Oryzias dancena). The open reading frames of odGR1 and odGR2 encode 790 and 783 amino acids, respectively, and show a sequence identity of 46% with each other. When inter- and intra-species comparisons of the GR domains were made, the N-terminal AF-1 (A/B) and hinge (D) domains showed relatively low identities, whereas the DNA-binding (C) domain (DBD) and ligand-binding (E) domain showed relatively high identities. Through phylogenetic analysis, we revealed that odGR1 and odGR2 belong to the teleost GR1 and GR2 groups, respectively. Transfection of odGR1 or odGR2 expression vectors into COS-7 cells along with a reporter vector demonstrated that cortisol and dexamethasone dose-dependently induce transcriptional activity in both GRs. As described in other teleostean fish, the transactivity of odGR2 was more sensitive at far lower concentrations of ligands than the transactivity of odGR1. When treated with aldosterone, the reporter gene was activated in COS-7 cells transfected with odGR2 but not in cells transfected with odGR1. RU486 inhibited transactivation by both GRs, but odGR2 was less sensitive to the inhibitor. Interestingly, alterations in coregulators, GRIP-1 and SMILE, mediated transactivation that was more drastic for odGR2 than odGR1. A nine-amino acid insertion (WRARQNTDG) in the DBD of odGR1 had a weak but significant influence on the transactivity of odGR2 with respect to responsiveness to agonists or coregulators. Taken together, these results indicate that the two odGRs possess distinct features not only for ligand sensitivity but also for preferential coregulator recruitment.

Characterization of conserved tandem donor sites and intronic motifs required for alternative splicing in corticosteroid receptor genes

Alternative splicing events from tandem donor sites result in mRNA variants coding for additional amino acids in the DNA binding domain of both the glucocorticoid (GR) and mineralocorticoid (MR) receptors. We now show that expression of both splice variants is extensively conserved in mammalian species, providing strong evidence for their functional significance. An exception to the conservation of the MR tandem splice site (an A at position +5 of the MR+12 donor site in the mouse) was predicted to decrease U1 small nuclear RNA binding. In accord with this prediction, we were unable to detect the MR+12 variant in this species. The one exception to the conservation of the GR tandem splice site, an A at position +3 of the platypus GRgamma donor site that was predicted to enhance binding of U1 snRNA, was unexpectedly associated with decreased expression of the variant from the endogenous gene as well as a minigene. An intronic pyrimidine motif present in both GR and MR genes was found to be critical for usage of the downstream donor site, and overexpression of TIA1/TIAL1 RNA binding proteins, which are known to bind such motifs, led to a marked increase in the proportion of GRgamma and MR+12. These results provide striking evidence for conservation of a complex splicing mechanism that involves processes other than stochastic spliceosome binding and identify a mechanism that would allow regulation of variant expression.

Stress and innate immunity in carp: corticosteroid receptors and pro-inflammatory cytokines

The stress hormone cortisol is deeply involved in immune regulation in all vertebrates. Common carp (Cyprinus carpio L.) express four corticoid receptors that may modulate immune responses: three glucocorticoid receptors (GR); GR1, with two splice variants (GR1a and GR1b), GR2 and a single mineralocorticoid receptor (MR). All receptors are expressed as of 4 days post-fertilization and may thus play a critical role in development and functioning of the adult immune system. Immune tissues and cells predominantly express mRNA for GRs compared to mRNA for the MR. Three-dimensional protein structure modeling predicts, and transfection assays confirm that alternative splicing of GR1 does not influence the capacity to induce transcription of effector genes. When tested for cortisol activation, GR2 is the most sensitive corticoid receptor in carp, followed by the MR and GR1a and GR1b. Lipopolysacharide (LPS) treatment of head kidney phagocytes quickly induces GR1 expression and inhibits GR2 expression. Cortisol treatment in vivo enhances GR1a and MR mRNA expression, but only mildly, and cortisol treatment in vitro does not affect receptor expression of phagocytes. Cortisol has no direct effect on the LPS-induced receptor profile. Therefore, an immune rather than a stress stimulus regulates GR expression. Cortisol administered at stress levels to phagocytes in vitro significantly inhibits LPS-induced expression of the pro-inflammatory cytokines tumor necrosis factor alpha (TNF-alpha) and interleukin-12 (IL-12) (subunit p35) and of inducible nitric oxide synthase (iNOS) expression. A physiologically differential function for GR1 and GR2 in the immune response of fish to infection is indicated.

Glucocorticoids alter craniofacial development and increase expression and activity of matrix metalloproteinases in developing zebrafish (Danio rerio)

Teratogenic effects are observed following long-term administration of glucocorticoids, although short-term glucocorticoid therapy is still utilized to reduce fetal mortality, respiratory distress syndrome, and intraventricular hemorrhage in preterm infants. However, the mechanism of glucocorticoid-induced teratogenicity is unknown. We hypothesize that glucocorticoid-induced teratogenesis is mediated through the glucocorticoid receptor (GR) and results from altering the expression and activity of the matrix metalloproteinases (MMPs). During embryogenesis, degradation of the extracellular matrix to allow for proper cellular migration and tissue organization is a tightly regulated process requiring appropriate temporal and spatial expression and activity of the MMPs. Studies have demonstrated that MMP gene expression can be either inhibited or induced by glucocorticoids in a variety of model systems. Using the zebrafish (Danio rerio) as a model of development, the data presented here demonstrate that embryonic exposure to the glucocorticoids dexamethasone or hydrocortisone increased expression of two gelatinases, MMP-2 ( approximately 1.5-fold) and MMP-9 (7.6- to 9.0-fold), at 72 h postfertilization (hpf). Further, gelatinase activity was increased approximately threefold at 72 hpf following glucocorticoid treatment, and changes in craniofacial morphogenesis were also observed. Cotreatment of zebrafish embryos with each glucocorticoid and the GR antagonist RU486 resulted in attenuation of glucocorticoid-induced increases in MMP expression (52-84% decrease) and activity (41-94% decrease). Furthermore, the abnormal craniofacial phenotype observed following glucocorticoid exposure was less severe following RU486 cotreatment. These studies demonstrate that in the embryonic zebrafish, dexamethasone, and hydrocortisone alter expression and activity of MMP-2 and -9, and suggest that these increases may be mediated through the GR.

Evolution of the corticosteroid receptor signalling pathway in fish

The corticosteroid receptors (CR) control a vast array of physiological processes acting primarily as ligand-dependent transcription factors. The origins of the gnathostomata CRs can be traced back to an ancestral steroid receptor present in a primitive agnathan vertebrate. A genome duplication event in the early gnathostomes is believed to have produced a set of two CRs still present today in Sarcopterygii (lobe-finned fish and tetrapods), i.e. a glucocorticoid (GR) and mineralocorticoid receptor (MR), with divergent function and different ligands, cortisol and aldosterone, respectively. A further genome duplication occurred in the early evolutionary history of the teleosts, and the teleost CR system seems to have diversified, consisting now of 2 GRs and a MR. Teleosts lack aldosterone and the main corticosteroid is believed to be cortisol. However, the mineralocorticoid, 11-deoxycorticosterone (DOC), has been identified as an agonist for the rainbow trout MR, suggesting it may be the ancestral ligand for the MR. The retention of two GRs in teleosts suggests neofunctionalisation of one of the duplicated genes, but this hypothesis requires further work. In rainbow trout, transactivation and transrepression activities of the two GRs show marked differences in their sensitivity to glucocorticoids, suggesting a mechanism that may allow the two GRs to control different physiological pathways. Whether a similar mechanism is seen throughout the actinopterygii or whether this is specific to the salmonid lineage remains to be verified.

Hormone receptors in gills of smolting Atlantic salmon, Salmo salar: expression of growth hormone, prolactin, mineralocorticoid and glucocorticoid receptors and 11beta-hydroxysteroid dehydrogenase type 2

This is the first study to report concurrent dynamics in mRNA expression of growth hormone receptor (GHR), prolactin receptor (PRLR), gluco- and mineralocorticoid receptor (GR and MR) and the 11beta-hydroxysteroid dehydrogenase type-2 enzyme (11beta-HSD2) in Atlantic salmon (Salmo salar) gill during smoltification. Transcript levels were analysed by quantitative PCR in fresh water (FW) fish and after a 24-h salt water (SW) challenge. GHR transcript levels increased concurrent with gill Na(+),K(+)-ATPase activity in FW fish consistent with the SW-adaptive role of GH. SW-transfer induced an increased GHR expression levels in the early stages of smoltification but a decrease in expression at the peak of smoltification. PRLR transcript levels decreased steadily during smoltification in agreement with the recognized hyper-osmoregulatory role of PRL. Surprisingly, PRLR levels increased after SW transfer during the course of smoltification. GR mRNA levels were low early on during smoltification but increased at the peak of smoltification and remained high during de-smoltification, indicative of increased cortisol signalling at this point. Coherently, SW transfer increased GR levels to smolt levels prior to the smoltification peak. 11beta-HSD2 levels increased at the smoltification peak and MR levels increased during de-smoltification, suggesting a need for protection of MR from cortisol signalling during smoltification. This is supported by the fact that SW-transfer results in a profound up-regulation of 11beta-HSD2, whereas SW transfer down-regulates MR levels. The study concludes that GR and MR may have distinctive roles in developing hypo- and hyper-osmoregulatory mechanisms during smoltification and de-smoltification, respectively.

Cloning of the glucocorticoid receptor (GR) in gilthead seabream (Sparus aurata). Differential expression of GR and immune genes in gilthead seabream after an immune challenge

In order to determine the cortisol response after an immune challenge in the gilthead seabream (Sparus aurata), a cortisol receptor (GR) was cloned, sequenced and its expression determined after lipopolysaccharide (LPS) treatment. To clone the gilthead seabream GR (sbGR), consecutive PCR amplifications and screening of a pituitary cDNA library were performed. We obtained a clone of 4586 bp encoding a 784aa protein. Northern blot analysis from head kidney, heart and intestine revealed that the full length sbGR mRNA was approximately 6.5 Kb. A LPS treatment, used as an acute stress model, was employed to characterise the expression of sbGR and some selected genes involved in the immune response (IL-1beta, TNF-alpha, Mx protein, cathepsin D and PPAR-gamma). All genes were expressed in all tissues examined and responses were tissue and time dependent revealing differential gene expression profiles after LPS administration. Furthermore, analysis of plasma cortisol levels after LPS injection, showed an acute response to inflammatory stress with a significant increase two and six h after injection, recovering to basal levels 12 h post-stress in all LPS concentrations tested.

Multiple corticosteroid receptors in fish: from old ideas to new concepts

The effect of corticosteroid hormones in fish are mediated through intracellular receptors that act as ligand-binding transcription factors. Many studies have been devoted to cortisol binding using radiolabeled ligand in fish and allowed characterization of a single class of high affinity binding sites in various tissues. Molecular characterization of cortisol receptors has only been initiated recently by cloning the different receptor forms: Following a isolation of a first glucocorticoid receptor (GR), a mineralocorticoid receptor (MR) was described and the presence of various GR isoforms was recently reported. Sequence comparison and phylogenetic analysis of these sequences confirm that fish possess both GR and MR and that GR gene is duplicated. The importance of these various corticosteroid receptor forms is also illustrated by analysis of their transcriptional activity. When tested in human cell lines, these receptors showed functionally distinct actions on GR-sensitive promotors, thus suggesting a more complicated corticosteroid signaling system than initially anticipated from binding studies. These results also suggest that, whereas cortisol is certainly the physiological ligand for GR, this may not be the case for MR which showed high sensitivity for deoxycorticosterone (DOC) and aldosterone. As this last hormone is probably absent in fish, these results raise the question as to whether DOC could be a physiological ligand for MR in fish. Information on DOC effect in fish is very scarce and clarification of the differential osmoregulatory roles of cortisol and DOC in fish needs ellucidation. This will require analysis of all actors of the corticosteroid signaling system at pre-receptor, receptor, and post-receptor levels.

Quantification of a glucocorticoid receptor in sea bass (Dicentrarchus labrax, L.) reared at high stocking density

To assess the effects of environmental stress conditions on fish, the examination of a suite of biomarkers, including endocrine parameters, has been suggested. In teleosts, glucocorticoids, including cortisol and corticosterone, are known to mediate stress response. Inside the cell, they bind to a high-affinity cytosolic glucocorticoid receptor (GR), which acts as ligand-dependent transcription factor to control and regulate gene expression. Receptor number or affinity may directly influence the degree of reactivity of target cells. GR transcripts have been cloned and characterized in different fish species, but no studies, to date, are available on the quantification of GR in sea bass (Dicentrarchus labrax, L.), although this is one of the most important species in Mediterranean aquaculture. These considerations prompted our interest in cloning the GR in this species and investigating the impact of long-term exposure to crowding stress on GR expression in the liver of sea bass. Our data clearly demonstrate that a high rearing density stress affects GR mRNA, whose abundance in the liver decreased inversely with blood cortisol levels.

Molecular biology of major components of chloride cells

Current understanding of chloride cells (CCs) is briefly reviewed with emphasis on molecular aspects of their channels, transporters and regulators. Seawater-type and freshwater-type CCs have been identified based on their shape, location and response to different ionic conditions. Among the freshwater-type CCs, subpopulations are emerging that are implicated in the uptake of Na(+), Cl(-) and Ca(2+), respectively, and can be distinguished by their shape of apical crypt and affinity for lectins. The major function of the seawater CC is transcellular secretion of Cl(-), which is accomplished by four major channels and transporters: (1). CFTR Cl(-) channel, (2). Na(+),K(+)-ATPase, (3). Na(+)/K(+)/2Cl(-) cotransporter and (4). a K(+) channel. The first three components have been cloned and characterized, but concerning the K(+) channel that is essential for the continued generation of the driving force by Na(+),K(+)-ATPase, only one candidate is identified. Although controversial, freshwater CCs seem to perform the uptake of Na(+), Cl(-) and Ca(2+) in a manner analogous to but slightly different from that seen in the absorptive epithelia of mammalian kidney and intestine since freshwater CCs face larger concentration gradients than ordinary epithelial cells. The components involved in these processes are beginning to be cloned, but their CC localization remains to be established definitively. The most important yet controversial issue is the mechanism of Na(+) uptake. Two models have been postulated: (i). the original one involves amiloride-sensitive electroneutral Na(+)/H(+) exchanger (NHE) with the driving force generated by Na(+),K(+)-ATPase and carbonic anhydrase (CA) and (ii). the current model suggests that Na(+) uptake occurs through an amiloride-sensitive epithelial sodium channel (ENaC) electrogenically coupled to H(+)-ATPase. While fish ENaC remains to be identified by molecular cloning and database mining, fish NHE has been cloned and shown to be highly expressed on the apical membrane of CCs, reviving the original model. The CC is also involved in acid-base regulation. Analysis using Osorezan dace (Tribolodon hakonensis) living in a pH 3.5 lake demonstrated marked inductions of Na(+),K(+)-ATPase, CA-II, NHE3, Na(+)/HCO(3)(-) cotransporter-1 and aquaporin-3 in the CCs on acidification, leading to a working hypothesis for the mechanism of Na(+) retention and acid-base regulation.

Multiple corticosteroid receptors in a teleost fish: distinct sequences, expression patterns, and transcriptional activities

Corticosteroid hormones, including the mineralocorticoids and the glucocorticoids, regulate diverse physiological functions in vertebrates. These hormones act through two classes of corticosteroid receptors (CR) that are ligand-dependent transcription factors: type I or mineralocorticoid receptor (MR) and type II or glucocorticoid receptor (GR). There is substantial overlap in the binding of these two receptor types to hormones and to DNA. In fish, the overlap in processes controlled by CRs may be different from that in other vertebrates, as fish are thought to synthesize only glucocorticoids, whereas they express both GR and MR. Here we describe the characterization of four CRs in a cichlid fish, Haplochromis burtoni: a previously undescribed GR (HbGR1), another GR expressed in two splice isoforms (HbGR2a and HbGR2b), and an MR (HbMR). Sequence comparison and phylogenetic analysis showed that these CRs sort naturally into GR and MR groups, and that the GR duplication we describe will probably be common to all teleosts. Quantitative PCR revealed differential patterns of CR tissue expression in organs dependent on corticosteroid action. Trans-activation assays demonstrated that the CRs were selective for corticosteroid hormones and showed that the HbMR was similar to mammalian MRs in being more sensitive to both cortisol and aldosterone than the GRs. Additionally, the two HbGR2 isoforms were expressed uniquely in different tissues and were functionally distinct in their actions on classical GR-sensitive promoters. The identification of four CR subtypes in teleosts suggests a more complicated corticosteroid signaling in fish than previously recognized.