Differential physiological and endocrine responses of rainbow trout, Oncorhynchus mykiss, transferred from fresh water to ion-poor or salt water

Endocrine control of gill ionocyte function in euryhaline fishes

The endocrine system is an essential regulator of the osmoregulatory organs that enable euryhaline fishes to maintain hydromineral balance in a broad range of environmental salinities. Because branchial ionocytes are the primary site for the active exchange of Na+, Cl-, and Ca2+ with the external environment, their functional regulation is inextricably linked with adaptive responses to changes in salinity. Here, we review the molecular-level processes that connect osmoregulatory hormones with branchial ion transport. We focus on how factors such as prolactin, growth hormone, cortisol, and insulin-like growth-factors operate through their cognate receptors to direct the expression of specific ion transporters/channels, Na+/K+-ATPases, tight-junction proteins, and aquaporins in ion-absorptive (freshwater-type) and ion-secretory (seawater-type) ionocytes. While these connections have historically been deduced in teleost models, more recently, increased attention has been given to understanding the nature of these connections in basal lineages. We conclude our review by proposing areas for future investigation that aim to fill gaps in the collective understanding of how hormonal signaling underlies ionocyte-based processes.

Early regulation of corticosteroid receptor expression in rainbow trout (Oncorhynchus mykiss) gills is mediated by membrane-initiated cortisol signaling

Cortisol is a key stress-related hormone involved in the physiological adjustments of fish. In gills, cortisol contributes to acclimatization to changes in environmental salinity, promoting both ion uptake or salt excretion. Cortisol exerts its biological effects through its interaction with specific intracellular glucocorticoid (GR) and mineralocorticoid (MR) receptors. Additionally, the further identification of GR and MR on the surface of different tissues, together with the existence of cortisol-mediated effects observed using membrane-impermeable analogs (e.g., cortisol-BSA), supports the existence of membrane-initiated cortisol actions in fish. Nevertheless, the impact of this alternative cortisol mechanism in relevant tissues for fish salinity acclimation, such as gill, is unknown. In this work, we sought to explore the contribution of rapid membrane-initiated cortisol on GR and MR regulation in rainbow trout (Oncorhynchus mykiss) gills using in vivo and in vitro approaches. Juvenile rainbow trout intraperitoneally injected with cortisol or cortisol-BSA showed increased gr2 but no gr1 or mr mRNA levels in gills after one hour of treatment. This result was further confirmed using RT-gills-W1 cell lines stimulated with both versions of cortisol. Interestingly, after three and six hours of cortisol or cortisol-BSA treatment, there were no changes in the mRNA levels of any corticosteroid receptor in RT-gills-W1 cells. Finally, using immunofluorescence analysis, we identified GR and MR in rainbow trout gill cells localized on the cell surface. Considering the in vivo and in vitro results of this work, we suggest that membrane-initiated cortisol action contributes to the early expression of gr2 in rainbow trout gills during salinity acclimation.

Osmoregulatory actions of prolactin in the gastrointestinal tract of fishes

In fishes, prolactin (Prl) signaling underlies the homeostatic regulation of hydromineral balance by controlling essential solute and water transporting functions performed by the gill, gastrointestinal tract, kidney, urinary bladder, and integument. Comparative studies spanning over 60 years have firmly established that Prl promotes physiological activities that enable euryhaline and stenohaline teleosts to reside in freshwater environments; nonetheless, the specific molecular and cellular targets of Prl in ion- and water-transporting tissues are still being resolved. In this short review, we discuss how particular targets of Prl (e.g., ion cotransporters, tight-junction proteins, and ion pumps) confer adaptive functions to the esophagus and intestine. Additionally, in some instances, Prl promotes histological and functional transformations within esophageal and intestinal epithelia by regulating cell proliferation. Collectively, the demonstrated actions of Prl in the gastrointestinal tract of teleosts indicate that Prl operates to promote phenotypes supportive of freshwater acclimation and to inhibit phenotypes associated with seawater acclimation. We conclude our review by underscoring that future investigations are warranted to determine how growth hormone/Prl-family signaling evolved in basal fishes to support the gastrointestinal processes underlying hydromineral balance.

Discovery and validation of candidate smoltification gene expression biomarkers across multiple species and ecotypes of Pacific salmonids

Early marine survival of juvenile salmon is intimately associated with their physiological condition during smoltification and ocean entry. Smoltification (parr-smolt transformation) is a developmental process that allows salmon to acquire seawater tolerance in preparation for marine living. Traditionally, this developmental process has been monitored using gill Na⁺/K⁺-ATPase (NKA) activity or plasma hormones, but gill gene expression offers the possibility of another method. Here, we describe the discovery of candidate genes from gill tissue for staging smoltification using comparisons of microarray studies with particular focus on the commonalities between anadromous Rainbow trout and Sockeye salmon datasets, as well as a literature comparison encompassing more species. A subset of 37 candidate genes mainly from the microarray analyses was used for TaqMan quantitative PCR assay design and their expression patterns were validated using gill samples from four groups, representing three species and two ecotypes: Coho salmon, Sockeye salmon, stream-type Chinook salmon and ocean-type Chinook salmon. The best smoltification biomarkers, as measured by consistent changes across these four groups, were genes involved in ion regulation, oxygen transport and immunity. Smoltification gene expression patterns (using the top 10 biomarkers) were confirmed by significant correlations with NKA activity and were associated with changes in body brightness, caudal fin darkness and caudal peduncle length. We incorporate gene expression patterns of pre-smolt, smolt and de-smolt trials from acute seawater transfers from a companion study to develop a preliminary seawater tolerance classification model for ocean-type Chinook salmon. This work demonstrates the potential of gene expression biomarkers to stage smoltification and classify juveniles as pre-smolt, smolt or de-smolt.

Na+/K+-ATPase is involved in the regulation of food intake in rainbow trout but apparently not through brain glucosensing mechanisms

To assess the hypothesis that Na⁺/K⁺-ATPase (NKA) is involved in the central regulation of food intake in fish, we observed in a first experiment with rainbow trout (Oncorhynchus mykiss) that intracerebroventricular (ICV) treatment with ouabain decreased food intake. We hypothesized that this effect relates to modulation of glucosensing mechanisms in brain areas (hypothalamus, hindbrain, and telencephalon) involved in food intake control. Therefore, we evaluated in a second experiment, the effect of ICV administration of ouabain, in the absence or in the presence of glucose, on NKA activity, mRNA abundance of different NKA subunits, parameters related to glucosensing, transcription factors, and appetite-related neuropeptides in brain areas involved in the control of food intake. NKA activity and mRNA abundance of nkaα1a and nkaα1c in brain were inhibited by ouabain treatment and partially by glucose. The anorectic effect of ouabain is opposed to the orexigenic effect reported in mammals. The difference might relate to the activity of glucosensing as well as downstream mechanisms involved in food intake regulation. Ouabain inhibited glucosensing mechanisms, which were activated by glucose in hypothalamus and telencephalon. Transcription factors and neuropeptides displayed responses comparable to those elicited by glucose when ouabain was administered alone, but not when glucose and ouabain were administered simultaneously. Ouabain might therefore affect other processes, besides glucosensing mechanisms, generating changes in membrane potential and/or intracellular pathways finally modulating transcription factors and neuropeptide mRNA abundance leading to modified food intake.

Salmonid gene expression biomarkers indicative of physiological responses to changes in salinity and temperature, but not dissolved oxygen

An organism's ability to respond effectively to environmental change is critical to its survival. Yet, life stage and overall condition can dictate tolerance thresholds to heightened environmental stressors, such that stress may not be equally felt across individuals and at all times. Also, the transcriptional responses induced by environmental changes can reflect both generalized responses as well as others that are highly specific to the type of change being experienced. Thus, if transcriptional biomarkers specific to a stressor, even under multi-stressor conditions, can be identified, the biomarkers could then be applied in natural environments to determine when and where an individual experiences such a stressor. Here, we experimentally challenged juvenile Chinook salmon (Oncorhynchus tshawytscha) to validate candidate gill gene expression biomarkers. A sophisticated experimental design manipulated salinity (freshwater, brackish water and seawater), temperature (10, 14 and 18°C) and dissolved oxygen (normoxia and hypoxia) in all 18 possible combinations for 6 days using separate trials for three smolt statuses (pre-smolt, smolt and de-smolt). In addition, changes in juvenile behaviour, plasma variables, gill Na+/K+-ATPase activity, body size, body morphology and skin pigmentation supplemented the gene expression responses. We identified biomarkers specific to salinity and temperature that transcended the multiple stressors, smolt status and mortality (live, dead and moribund). Similar biomarkers for dissolved oxygen were not identified. This work demonstrates the unique power of gene expression biomarkers to identify a specific stressor even under multi-stressor conditions, and we discuss our next steps for hypoxia biomarkers using an RNA-seq study.

Is the duration of the smolt window related to migration distance in coho salmon Oncorhynchus kisutch?

Physiological changes during the parr-smolt transformation were investigated in short distance (Chilliwack River) and long-distance (Salmon River) migrating coho salmon Oncorhynchus kisutch populations in British Columbia, Canada. Biochemical and molecular indicators were used to monitor smolt development for fish reared at 10 °C throughout the spring. Fish grew well and developed the physical appearance of competent smolts. Both populations exhibited increases in gill Na⁺ -K⁺ -ATPase activity (NKA; an important indicator of seawater tolerance) at the same date and the duration of the increase in enzyme activity did not differ between populations. Gill messenger (m)RNA copies for two isoforms of the NKA α subunit, α1a and α1b, showed significant changes and the pattern was similar between populations. Growth hormone receptor and prolactin receptor mRNA from the gill showed modest changes associated with smolting in the spring for both populations, suggesting that these may not be useful indicators of smolt development in hatchery-reared O. kisutch. Consequently, the duration of the smolt window was not based on the region of origin in the present study.

Temporal and spatial differences in smolting among Oncorhynchus nerka populations throughout fresh and seawater migration

Physiological changes that occur in the spring are preparatory for salmonid smolts to successfully enter seawater, but variation is likely to exist within species with a wide geographic distribution. Whether differences in development of seawater tolerance exist among populations that differ in distance to the ocean, temporally during the spring, or as fish migrate downstream is not known. Juvenile sockeye salmon Oncorhynchus nerka from four regions in the Fraser River catchment, British Columbia, were intercepted to assess physiological differences among populations and at different times during migration to characterize the parr-smolt transformation. Pre-migratory fish had low levels of gill Na⁺ -K⁺ -ATPase (NKA) activity. High gill NKA activities were observed at the start of migration for some populations, but smolts leaving the lake did not consistently have higher gill NKA activity than non-migratory juvenile O. nerka sampled in their natal lakes. Gill NKA activity was highly variable at the start of migration with no relationship with distance from the ocean. Gill NKA activity changes with migration were also highly variable, but consistently smolts in the ocean had the highest enzyme activities. Internal and external factors may influence this variation, but the dynamic nature of smolting was not based on the region of origin, timing during migration or on the year of migration.

Annotation of the Nuclear Receptors in an Estuarine Fish species, Fundulus heteroclitus

The nuclear receptors (NRs) are ligand-dependent transcription factors that respond to various internal as well as external cues such as nutrients, pheromones, and steroid hormones that play crucial roles in regulation and maintenance of homeostasis and orchestrating the physiological and stress responses of an organism. We annotated the Fundulus heteroclitus (mummichog; Atlantic killifish) nuclear receptors. Mummichog are a non-migratory, estuarine fish with a limited home range often used in environmental research as a field model for studying ecological and evolutionary responses to variable environmental conditions such as salinity, oxygen, temperature, pH, and toxic compounds because of their hardiness. F. heteroclitus have at least 74 NRs spanning all seven gene subfamilies. F. heteroclitus is unique in that no RXRα member was found within the genome. Interestingly, some of the NRs are highly conserved between species, while others show a higher degree of divergence such as PXR, SF1, and ARα. Fundulus like other fish species show expansion of the RAR (NR1B), Rev-erb (NR1D), ROR (NR1F), COUPTF (NR2F), ERR (NR3B), RXR (NR2B), and to a lesser extent the NGF (NR4A), and NR3C steroid receptors (GR/AR). Of particular interest is the co-expansion of opposing NRs, Reverb-ROR, and RAR/RXR-COUPTF.

Molecular characterization of a fish-specific toll-like receptor 22 (TLR22) gene from common carp (Cyprinus carpio L.): Evolutionary relationship and induced expression upon immune stimulants

In the host innate immune system, various pattern recognition receptors (PRRs) recognize conserved pathogens-associated molecular patterns (PAMPs), and represent an efficient first line of defense against invading pathogens. TLR22 is one of the fish-specific Toll-like receptors (TLRs), identified in a variety of fish species. In this study, we report the cloning and identification of a TLR22 cDNA from the gills of common carp (Cyprinus carpio L.). The full-length CcTLR22 cDNA was 3301 bp long, including a 32 bp 5'-untranslated region (UTR), an open reading frame (ORF) of 2838 bp and a 432 bp 3'-UTR.The CcTLR22 protein was found to comprise a signal peptide, 16 LRR domains, a LRRCT domain in the extracellular region and a TIR domain in the cytoplasmic region, which fits with the characteristic TLR domain architecture. The genomic organization of CcTLR22 was identified, which was encoded by an uninterrupted exon. Sequence alignment and phylogenetic analysis showed that all known teleost TLR22 members were clustered into an independent clade of the TLR22 family, and showed high amino acid identities with other fish TLRs. Real-time PCR assay showed that CcTLR22 mRNA was expressed in almost all tissues examined, while the levels obviously varied among different tissues. When challenged with poly(I:C) (a viral model) or A. hydrophila bacteria, the expression level of CcTLR22 was up-regulated in a variety of common carp tissues. These results indicate that CcTLR22 plays a significant role in systemic as well as mucosal defence after viral or bacterial stimulation or infection.

Evaluation of potential candidate genes involved in salinity tolerance in striped catfish (Pangasianodon hypophthalmus) using an RNA-Seq approach

Increasing salinity levels in freshwater and coastal environments caused by sea level rise linked to climate change is now recognized to be a major factor that can impact fish growth negatively, especially for freshwater teleost species. Striped catfish (Pangasianodon hypophthalmus) is an important freshwater teleost that is now widely farmed across the Mekong River Delta in Vietnam. Understanding the basis for tolerance and adaptation to raised environmental salinity conditions can assist the regional culture industry to mitigate predicted impacts of climate change across this region. Attempt of next generation sequencing using the ion proton platform results in more than 174 million raw reads from three tissue libraries (gill, kidney and intestine). Reads were filtered and de novo assembled using a variety of assemblers and then clustered together to generate a combined reference transcriptome. Downstream analysis resulted in a final reference transcriptome that contained 60,585 transcripts with an N50 of 683 bp. This resource was further annotated using a variety of bioinformatics databases, followed by differential gene expression analysis that resulted in 3062 transcripts that were differentially expressed in catfish samples raised under two experimental conditions (0 and 15 ppt). A number of transcripts with a potential role in salinity tolerance were then classified into six different functional gene categories based on their gene ontology assignments. These included; energy metabolism, ion transportation, detoxification, signal transduction, structural organization and detoxification. Finally, we combined the data on functional salinity tolerance genes into a hypothetical schematic model that attempted to describe potential relationships and interactions among target genes to explain the molecular pathways that control adaptive salinity responses in P. hypophthalmus. Our results indicate that P. hypophthalmus exhibit predictable plastic regulatory responses to elevated salinity by means of characteristic gene expression patterns, providing numerous candidate genes for future investigations.

Prolactin and teleost ionocytes: new insights into cellular and molecular targets of prolactin in vertebrate epithelia

The peptide hormone prolactin is a functionally versatile hormone produced by the vertebrate pituitary. Comparative studies over the last six decades have revealed that a conserved function for prolactin across vertebrates is the regulation of ion and water transport in a variety of tissues including those responsible for whole-organism ion homeostasis. In teleost fishes, prolactin was identified as the "freshwater-adapting hormone", promoting ion-conserving and water-secreting processes by acting on the gill, kidney, gut and urinary bladder. In mammals, prolactin is known to regulate renal, intestinal, mammary and amniotic epithelia, with dysfunction linked to hypogonadism, infertility, and metabolic disorders. Until recently, our understanding of the cellular mechanisms of prolactin action in fishes has been hampered by a paucity of molecular tools to define and study ionocytes, specialized cells that control active ion transport across branchial and epidermal epithelia. Here we review work in teleost models indicating that prolactin regulates ion balance through action on ion transporters, tight-junction proteins, and water channels in ionocytes, and discuss recent advances in our understanding of ionocyte function in the genetically and embryonically accessible zebrafish (Danio rerio). Given the high degree of evolutionary conservation in endocrine and osmoregulatory systems, these studies in teleost models are contributing novel mechanistic insight into how prolactin participates in the development, function, and dysfunction of osmoregulatory systems across the vertebrate lineage.

Prolactin regulates transcription of the ion uptake Na+/Cl- cotransporter (ncc) gene in zebrafish gill

Prolactin (PRL) is a well-known regulator of ion and water transport within osmoregulatory tissues across vertebrate species, yet how PRL acts on some of its target tissues remains poorly understood. Using zebrafish as a model, we show that ionocytes in the gill directly respond to systemic PRL to regulate mechanisms of ion uptake. Ion-poor conditions led to increases in the expression of PRL receptor (prlra), Na(+)/Cl(-) cotransporter (ncc; slc12a10.2), Na(+)/H(+) exchanger (nhe3b; slc9a3.2), and epithelial Ca(2+) channel (ecac; trpv6) transcripts within the gill. Intraperitoneal injection of ovine PRL (oPRL) increased ncc and prlra transcripts, but did not affect nhe3b or ecac. Consistent with direct PRL action in the gill, addition of oPRL to cultured gill filaments stimulated ncc in a concentration-dependent manner, an effect blocked by a pure human PRL receptor antagonist (Δ1-9-G129R-hPRL). These results suggest that PRL signaling through PRL receptors in the gill regulates the expression of ncc, thereby linking this pituitary hormone with an effector of Cl(-) uptake in zebrafish for the first time.

A test of maternal programming of offspring stress response to predation risk in threespine sticklebacks

Non-genetic maternal effects are widespread across taxa and challenge our traditional understanding of inheritance. Maternal experience with predators, for example, can have lifelong consequences for offspring traits, including fitness. Previous work in threespine sticklebacks showed that females exposed to simulated predation risk produced eggs with higher cortisol content and offspring with altered anti-predator behavior. However, it is unknown whether this maternal effect is mediated via the offspring glucocorticoid stress response and if it is retained over the entire lifetime of offspring. Therefore, we tested the hypothesis that maternal exposure to simulated predation risk has long-lasting effects on the cortisol response to simulated predation risk in stickleback offspring. We measured circulating concentrations of cortisol before (baseline), 15 min after, and 60 min after exposure to a simulated predation risk. We compared adult offspring of predator-exposed mothers and control mothers in two different social environments (alone or in a group). Relative to baseline, offspring plasma cortisol was highest 15 min after exposure to simulated predation risk and decreased after 60 min. Offspring of predator-exposed mothers differed in the cortisol response to simulated predation risk compared to offspring of control mothers. In general, females had higher cortisol than males, and fish in a group had lower cortisol than fish that were by themselves. The buffering effect of the social environment did not differ between maternal treatments or between males and females. Altogether the results show that while a mother's experience with simulated predation risk might affect the physiological response of her adult offspring to a predator, sex and social isolation have much larger effects on the stress response to predation risk in sticklebacks.

The role of 'mineralocorticoids' in teleost fish: relative importance of glucocorticoid signaling in the osmoregulation and 'central' actions of mineralocorticoid receptor

It has long been held that cortisol, a glucocorticoid in many vertebrates, performs glucocorticoid and mineralocorticoid actions in the teleost fish since it lacks aldosterone. However, in addition to the counterparts of tetrapod mineralocorticoid receptors (MRs), 11-deoxycorticosterone (DOC) has been recently identified as a specific endogenous ligand for the MRs in teleosts. Here, we point out the minor role of mineralocorticoid signaling (i.e., DOC-MR) in the osmoregulation compared with those of glucocorticoid signaling (i.e., cortisol-glucocorticoid receptor [GR]), and review the current findings on the physiological roles of the DOC-MR in teleosts. Cortisol promotes both freshwater and seawater adaptation via the GRs in the osmoregulatory organs such as gills and gastrointestinal tracts, but the expressions of MR mRNA are abundant in the brains especially in the key components of the stress axis and cerebellums. Together with the behavioral effects of intracerebroventricular injection with DOC, the MR is suggested to play an important role in the brain dependent behaviors. Since the abundant expression of central MRs has been reported also in higher vertebrates and the MR is thought to be ancestral to the GR, the role of MR in fish might reflect the principal and original function of corticosteroid signaling. Functional evolution of corticosteroid systems is summarized and areas in need of research like our on-going experiments with MR-knockout medaka are outlined.

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.