Cortisol reduces paracellular permeability and increases occludin abundance in cultured trout gill epithelia

Voltage-gated ion channels in cultured gill epithelia of rainbow trout, Oncorhynchus mykiss, change in transcript abundance with exposure to freshwater

Salmonid fishes are well adapted to transition between salinities as part of a diadromid lifestyle, and many species are both economically and environmentally important. Ion-transporting gill epithelium helps fishes maintain ion balance during salinity transition. Recent transcriptomic surveys suggest that voltage-gated ion channels (VGICs) are present in gill epithelium of fishes. However, fish gill epithelia are architecturally complex and structurally heterogeneous (which includes layers of excitable tissues), which necessitates a model to study isolated gill epithelial cells. In the present study, we isolated gill epithelial cells, used them to reconstruct primary cultured gill epithelium model, and exposed the reconstructed epithelia to apical freshwater (FW). Using RNAseq and molecular biology we demonstrate that multiple VGICs are expressed in cultured gill epithelia of a salmonid, rainbow trout Oncorhynchus mykiss. Following apical exposure to FW, multiple subunits of voltage-gated calcium (CaV) channels, as well as KCNE2 were upregulated in mRNA abundance. Using a custom-made antibody, we demonstrated that CaV1.3 immunolocalized to the apical membrane of epithelia in intact trout gill, as well as in the cultured gill epithelium. Pharmacological inhibition of CaV1 in FW-exposed cultured epithelia led to increased transepithelial resistance. Therefore, we propose that VGICs are present in gill epithelia of fishes, and may rapidly and autonomously respond to environmental salinity changes to help the fish maintain salt and water balance, where CaV1 specifically may play a particularly important role in rapid adjustment of gill epithelia barrier properties and resistivity and potentially in responding to regulatory cell volume decrease in vitro.

Participation of membrane-initiated cortisol effects on the rapid acclimation of rainbow trout (Oncorhynchus mykiss) to increased salinity

Cortisol, a fundamental slow-acting hormone in teleosts, plays a crucial role in acclimating to changes in saline environments. Cortisol effects are associated with its interaction with intracellular glucocorticoid (GR) and mineralocorticoid (MR) receptors, which subsequently regulate gene expression through the cortisol-receptor complex. This mechanism is known as a genomic cortisol signaling and has been studied extensively. However, recent studies have begun to explore a membrane-initiated cortisol pathway that is initiated on the cellular surface, revealing its critical role in the initial metabolic adjustments during the physiological stress response. Nevertheless, the role of this novel membrane-mediated cortisol action during acclimatization to saline environments remain to be elucidated. To investigate this, an in vivo assay was performed in which juvenile rainbow trout were maintained in freshwater (FW) (0.1 ppt), intraperitoneally injected with vehicle, cortisol or cortisol-BSA (three hours of treatment), and transferred to saline water (15 ppt) for one additional hour. Samples of blood and gills were obtained from each fish in order to measure the plasma cortisol, glucose and chloride concentrations, as well as the expression levels of the gr1, gr2, mr, and key osmoregulatory genes. Membrane-initiated cortisol action increased plasma glucose and chloride levels in fish after one hour of saline transfer in comparison with the vehicle group. Furthermore, cortisol exerts a novel regulatory influence on the expression of gr2, as well as tight junction proteins claudin10e and cldn30 in the gills. In contrast, other osmoregulation-related genes, such as cftr and nkcc1, are exclusively mediated by genomic cortisol signaling. These results suggest that membrane-initiated cortisol action plays a significant role in the rapid acclimation of fish to changes in salinity environments.

Effect of salinity on the physiological response and transcriptome of spotted seabass (Lateolabrax maculatus)

Salinity fluctuations significantly impact the reproduction, growth, development, as well as physiological and metabolic activities of fish. To explore the osmoregulation mechanism of aquatic organisms acclimating to salinity stress, the physiological and transcriptomic characteristics of spotted seabass (Lateolabrax maculatus) in response to varying salinity gradients were investigated. In this study, different salinity stress exerted inhibitory effects on lipase activity, while the impact on amylase activity was not statistically significant. Notably, a moderate increase in salinity (24 psu) demonstrated the potential to enhance the efficient utilization of proteins by spotted seabass. Both Na+/K+-ATPase and malondialdehyde showed a fluctuating trend of increasing and then decreasing, peaking at 72 h. Transcriptomic analysis revealed that most differentially expressed genes were involved in energy metabolism, signal transduction, the immune response, and osmoregulation. These results will provide insights into the molecular mechanisms of salinity adaptation and contribute to sustainable development of the global aquaculture industry.

Dysregulation of Intestinal Physiology by Aflatoxicosis in the Gilthead Seabream (Sparus aurata)

Aflatoxin B1 (AFB1) is a mycotoxin often present in food. This study aimed to understand the physiological effects of AFB1 on the seabream (Sparus aurata) gastrointestinal system. In a first in vitro approach, we investigated ion transport using the short-circuit current (Isc) technique in Ussing chambers in the anterior intestine (AI). Application of apical/luminal AFB1 concentrations of 8 and 16 μM to healthy tissues was without effect on tissue transepithelial electrical resistance (TER), and apparent tissue permeability (Papp) was measured using fluorescein FITC (4 kD). However, it resulted in dose-related effects on Isc. In a second approach, seabream juveniles fed with different AFB1 concentrations (1 and 2 mg AFB1 kg⁻¹ fish feed) for 85 days showed significantly reduced gill Na⁺/K⁺-ATPase (NKA) and H⁺-ATPase (HA) activities in the posterior intestine (PI). Moreover, dietary AFB1 modified Isc in the AI and PI, significantly affecting TER in the AI. To understand this effect on TER, we analyzed the expression of nine claudins and three occludins as markers of intestinal architecture and permeability using qPCR. Around 80% of the genes presented significantly different relative mRNA expression between AI and PI and had concomitant sensitivity to dietary AFB1. Based on the results of our in vitro, in vivo, and molecular approaches, we conclude that the effects of dietary AFB1 in the gastrointestinal system are at the base of the previously reported growth impairment caused by AFB1 in fish.

Reductionist approaches to the study of ionoregulation in fishes

The mechanisms underlying ionoregulation in fishes have been studied for nearly a century, and reductionist methods have been applied at all levels of biological organization in this field of research. The complex nature of ionoregulatory systems in fishes makes them ideally suited to reductionist methods and our collective understanding has been dramatically shaped by their use. This review provides an overview of the broad suite of techniques used to elucidate ionoregulatory mechanisms in fishes, from the whole-animal level down to the gene, discussing some of the advantages and disadvantages of these methods. We provide a roadmap for understanding and appreciating the work that has formed the current models of organismal, endocrine, cellular, molecular, and genetic regulation of ion balance in fishes and highlight the contribution that reductionist techniques have made to some of the fundamental leaps forward in the field throughout its history.

Effects of copper on a reconstructed freshwater rainbow trout gill epithelium: Paracellular and intracellular aspects

The barrier properties and intracellular responses of a primary cultured trout gill epithelium (containing both mitochondria-rich and pavement cells) were examined over 24 h of copper (Cu) exposure (0, 200 and 1000 μg/L) in apical fresh water. Transepithelial resistance (TER) and mRNA abundance of tight junction proteins zonula occludens-1, occludin, cingulin, claudin-8d and -28b were examined as endpoints of barrier function and the paracellular pathway. Intracellular endpoints analyzed were Cu accumulation, Na⁺ content, carbonic anhydrase activity and mRNA abundance of carbonic anhydrase (ca-II) and Na⁺/K⁺ ATPase (nka α1a and nka α1b isoforms). After a brief initial drop in TER in the 1000 μg Cu/L treatment, Cu at both levels increased TER over the first 6 h of exposure but there were no differences among groups from 12 h onwards. After 24 h of Cu exposure, there were no differences in mRNA abundance of any of the tight junction proteins examined. Cu accumulation occurred at 1000 μg Cu/L (5.5-fold increase), but no depletion of Na⁺ content. Carbonic anhydrase activity decreased significantly (by 76%), however Cu exposure did not alter the transcript abundance of ca-II, nka α1a, or nka α1b. This study provides a first report of carbonic anhydrase sensitivity to Cu exposure in a cultured model gill epithelium. We conclude that Cu impacts the permeability of this model during the early stages of exposure and that the use of carbonic anhydrase inhibition as an endpoint of metal toxicity in this model preparation may be useful for future mechanistic investigations and environmental monitoring.

The mineralocorticoid receptor contributes to barrier function of a model fish gill epithelium

Cortisol-induced epithelial tightening of a primary cultured rainbow trout gill epithelium model occurs in association with reduced paracellular permeability and increased abundance of select barrier-forming tight junction (TJ) proteins. Corticosteroid receptor (CR) pharmacological blocker studies have suggested that to produce this tightening effect, cortisol acts on the mineralocorticoid receptor (MR) as well as glucocorticoid receptors (GRs). This study considered how cortisol influences model gill epithelium permeability and TJ properties by transcriptional knockdown of the gene encoding the MR (mr-KD) using double-stranded RNA. Following mr-KD, a significant reduction in MR protein abundance was observed in the epithelium. The mr-KD epithelium demonstrated reduced transepithelial resistance (TER) and an increase in the paracellular flux of [³H]polyethylene glycol (MW 400 kDa, PEG-400). Concurrently, mRNA abundance of gr2 and 11βhsd increased, indicating a possible compensatory response to mr-KD. Transcript abundance of claudin (cldn)-6, -8d, -23a and -28b decreased while that of cldn-20a increased in mr-KD preparations. Cortisol-induced epithelial tightening was enhanced in mr-KD preparations, suggesting that alterations in CRs and TJ composition augmented model epithelium barrier function in response to lowered MR abundance. Cortisol treatment significantly increased the transcript and protein abundance of TJ proteins such as Cldn-8d and -28b. However, in mr-KD preparations, Cldn-28b protein abundance did not significantly alter in response to cortisol treatment, while Cldn-8d abundance was significantly elevated. Data suggest that mr-KD compromises normal barrier function of a primary cultured rainbow trout gill epithelium in both the presence and absence of cortisol and that Cldn-28b protein abundance may be modulated by cortisol via the MR only.

Transcriptomic analysis reveal an efficient osmoregulatory system in Siberian sturgeon Acipenser baeri in response to salinity stress

Sturgeons are euryhaline fish species that have developed specific mechanisms of osmotic and ion regulation to adapt to waters of varying salinity. For the aim to elucidate the osmoregulation strategy behind its high salinity tolerance of sturgeons, the transcriptomes of gills in Siberian sturgeon Acipenser baeri under salinity stress (30 ppt) were sequenced using deep-sequencing platform Illumina/HiSeq-2500 and differential expression genes (DEGs) were identified. A total of 167, 501, 278 clean reads were obtained and 280, 238 unigenes were composed of those clean reads with the mean length of 520nt, and the N50 of 630 bp. Unigenes Sequence alignment was implemented via KEGG, KOG, NT, NR, PFAM, Swiss-Prot, and GO databases. 62, 242 unigenes (22.21%) were annoated in at least one database. 11380 significantly differentially expressed unigenes were found, 6969 of which were up-regulated and 4411 were down-regulated by salinity stress. Amongst the top 20 KEGG pathways with the most amount of annotation sequences, some pathways such as glycerophospholipid metabolism, fatty-acid biosynthesis, glycolysis/gluconeogenesis, oxidative phosphorylation have been comprehensively proved to be relevant to osmoregulation. Despite of these, three possible osmoregulation-related signaling pathways as lipid metabolism related pathways, tight junction pathway and thyroid hormone signaling pathway have been widely analyzed in the current study. In all DEGs, some of the typical genes involved in osmoregulation, including calcium-transporting ATPase 4 (ATP2B4), Na⁺/K⁺-ATPase alpha subunit (α-NKA), potassium-transporting ATPase alpha chain 1 (ATP4A) and Ras GTPase-activating protein (RasGAP) etc were also identified. RNA-seq results were validated with quantitative real-time PCR (qPCR), the 12 selected genes showed a consistent direction in both DGE library and qPCR analysis, proving that the RNA-seq results are reliable. The present results would be helpful to elucidate the osmoregulation mechanism of aquatic animals adapting to salinity challenge.

Tricellular tight junction-associated angulins in the gill epithelium of rainbow trout

Molecular physiology of the tricellular tight junction (tTJ)-associated proteins lipolysis-stimulated lipoprotein receptor ( lsr, = angulin-1) and an immunoglobulin-like domain-containing receptor ( ildr2, ≈angulin-3) was examined in model trout gill epithelia. Transcripts encoding lsr and ildr2 are broadly expressed in trout organs. A reduction in lsr and ildr2 mRNA abundance was observed during and after confluence in flask-cultured gill cells. In contrast, as high-resistance and low-permeability characteristics developed in a model gill epithelium cultured on permeable polyethylene terephthalate membrane inserts, lsr and ildr2 transcript abundance increased. However, as epithelia entered the developmental plateau phase, lsr abundance returned to initial values, while ildr2 transcript abundance remained elevated. When mitochondrion-rich cells were introduced to model preparations, lsr mRNA abundance was unaltered and ildr2 mRNA abundance significantly increased. Transcript abundance of ildr2 was not altered in association with corticosteroid-induced tightening of the gill epithelium, while lsr mRNA abundance decreased. Transcriptional knockdown of the tTJ protein tricelluin (Tric) reduced Tric abundance, increased gill epithelium permeability, and increased lsr without significantly altering ildr2 transcript abundance. Data suggest that angulins contribute to fish gill epithelium barrier properties but that Lsr and Ildr2 seem likely to play different roles. This is because ildr2 typically exhibited increased abundance in association with decreased model permeability, while lsr abundance changed in a manner that suggested a role in Tric recruitment to the tTJ.

Claudin-8d is a cortisol-responsive barrier protein in the gill epithelium of trout

The influence of claudin (Cldn) 8 tight junction (TJ) proteins on cortisol-mediated alterations in gill epithelium permeability was examined using a primary cultured trout gill epithelium model. Genes encoding three Cldn-8 proteins (cldn-8b, -8c and -8d) have been identified in trout and all are expressed in the model gill epithelium. Cortisol treatment 'tightened' the gill epithelium, as indicated by increased transepithelial resistance (TER) and reduced paracellular [³H]polyethylene glycol (MW 400 Da; PEG-400) flux. This occurred in association with elevated cldn-8d mRNA abundance, but no alterations in cldn-8b and -8c mRNA abundance were observed. Transcriptional knockdown (KD) of cldn-8d inhibited a cortisol-induced increase in Cldn-8d abundance and reduced the 'epithelium tightening' effect of cortisol in association with increased paracellular PEG-400 flux. Under simulated in vivo conditions (i.e. apical freshwater), cldn-8d KD hindered a cortisol-mediated reduction in basolateral to apical Na⁺ and Cl^- flux (i.e. reduced the ability of cortisol to mitigate ion loss). However, cldn-8d KD did not abolish the tightening effect of cortisol on the gill epithelium. This is likely due, in part, to the effect of cortisol on genes encoding other TJ proteins, which in some cases appeared to exhibit a compensatory response. Data support the idea that Cldn-8d is a barrier protein of the gill epithelium TJ that contributes significantly to corticosteroid-mediated alterations in gill epithelium permeability.

Claudin-31 contributes to corticosteroid-induced alterations in the barrier properties of the gill epithelium

The contribution of Claudin-31 (Cldn-31) to corticosteroid-induced tightening of the trout gill epithelium was examined using a primary cultured model preparation. Cldn-31 is a ∼23 kDa protein that localizes to the periphery of gill epithelial cells and diffusely in select gill cells that are Na⁺-K⁺-ATPase-immunoreactive. Transcriptional knockdown (KD) of cldn-31 reduced Cldn-31 abundance and increased epithelium permeability. Under simulated in vivo conditions (apical freshwater), cldn-31 KD increased net ion flux rates (≡ efflux). Cortisol treatment increased Cldn-31 abundance and decreased epithelium permeability. This tightening effect was diminished, but not eliminated, by cldn-31 KD, most likely due to other cortisol-sensitive TJ proteins that were transcriptionally unperturbed or enhanced in cortisol-treated cldn-31 KD preparations. However, cldn-31 KD abolished a cortisol-induced increase in Cldn-8d abundance, which may contribute to compromised cldn-31 KD epithelium permeability. Data suggest an important barrier function for Cldn-31 and an integral role for Cldn-31 in corticosteroid-induced gill epithelium tightening.

A role for tight junction-associated MARVEL proteins in larval sea lamprey (Petromyzon marinus) osmoregulation

This study reports on tight junction-associated MARVEL proteins of larval sea lamprey (Petromyzon marinus) and their potential role in ammocoete osmoregulation. Two Occludin isoforms (designated Ocln and Ocln-a) and a tricellulin (Tric) were identified. Transcripts encoding ocln, ocln-a, and tric were broadly expressed in larval lamprey, with greatest abundance of ocln in gut, liver and kidney, ocln-a in the gill and skin, and tric in the kidney. Ocln and Ocln-a resolved as ∼63 kDa and ∼35 kDa MW proteins respectively while Tric resolved as a ∼50 kDa protein. Ocln immunolocalized to the gill vasculature and in gill mucous cells while Ocln-a localized to the gill pouch and gill epithelium. Both Ocln and Ocln-a localized in the nephron, the epidermis and the luminal side of the gut. In branchial tissue, Tric exhibited punctate localization, consistent with its presence at regions of tricellular contact. Following ion-poor water (IPW) acclimation of ammocoetes, serum [Na+] and [Cl−] reduced, but not [Ca++], and carcass moisture content increased. In association, Ocln abundance increased in skin and kidney, but reduced in gill of IPW-acclimated ammocoetes while Ocln-a abundance reduced in the kidney only. Tric abundance increased in the gill. Region-specific alterations in ocln, ocln-a and tric mRNA abundance was also observed in the gut. Data support a role for Ocln, Ocln-a and Tric in the osmoregulatory strategies of a basal vertebrate.

Claudin tight junction proteins in rainbow trout (Oncorhynchus mykiss) skin: Spatial response to elevated cortisol levels

This study examined regional distribution and corticosteroid-induced alterations of claudin (cldn) transcript abundance in teleost fish skin. Regional comparison of mRNA encoding 20 Cldns indicated that 12 exhibit differences in abundance along the dorsoventral axis of skin. However, relative abundance of cldns (i.e. most to least abundant) remained similar in different skin regions. Several cldns appear to be present in the epidermis and dermal vasculature whereas others are present only in the epidermis. Increased circulating cortisol levels significantly altered mRNA abundance of 10 cldns in a region specific manner, as well as corticosteroid receptors and 11β-hydroxysteroid dehydrogenase (type 2). Epidermis and epidermal mucous cell morphometrics also altered in response to cortisol, exhibiting changes that appear to enhance skin barrier properties. Taken together, data provide a first look at spatial variation in the molecular physiology of the teleost fish integument TJ complex and region-specific sensitivity to an endocrine factor.

Dietary vitamin C deficiency depresses the growth, head kidney and spleen immunity and structural integrity by regulating NF-κB, TOR, Nrf2, apoptosis and MLCK signaling in young grass carp (Ctenopharyngodon idella)

This study investigated the effects of dietary vitamin C on the growth, and head kidney, spleen and skin immunity, structural integrity and related signaling molecules mRNA expression levels of young grass carp (Ctenopharyngodon idella). A total of 540 grass carp (264.37 ± 0.66 g) were fed six diets with graded levels of vitamin C (2.9, 44.2, 89.1, 133.8, 179.4 and 224.5 mg/kg diet) for 10 weeks. Subsequently, a challenge test was conducted by injection of Aeromonas hydrophila and the survival rate recorded for 14 days. The results indicated that compared with optimal vitamin C supplementation, vitamin C deficiency (2.9 mg/kg diet) decreased lysozyme (LA) and acid phosphatase (ACP) activities, and complement 3 and complement 4 (C4) contents (P < 0.05), down-regulated the mRNA levels of antimicrobial peptides [liver expressed antimicrobial peptide (LEAP) 2A, LEAP-2B, hepcidin, β-defensin] and anti-inflammatory cytokines-related factors, interleukin (IL) 4/13A, IL-4/13B (only in head kidney), IL-10, IL-11, transforming growth factor (TGF) β1, TGF-β2, inhibitor of κBα and eIF4E-binding protein 1 (P < 0.05), and up-regulated pro-inflammatory cytokines-related factors, tumor necrosis factor α, interferon γ2, IL-1β, IL-6, IL-8, IL-12 P35 (only in spleen), IL-12 P40, IL-15, IL-17D, nuclear factor κB p65, IκB kinases (IKKα, IKKβ, IKKγ), target of rapamycin and ribosomal protein S6 kinase 1 mRNA levels (P < 0.05) in the head kidney and spleen under injection fish of A. hydrophila, suggesting that vitamin C deficiency could decrease fish head kidney and spleen immunity and cause inflammation. Meanwhile, compared with optimal vitamin C supplementation, vitamin C deficiency decreased the activities and mRNA levels of copper/zinc superoxide dismutase, manganese superoxide dismutase (MnSOD), catalase, glutathione peroxidase, glutathione S-transferases and glutathione reductase (P < 0.05), and down-regulated zonula occludens (ZO) 1, ZO-2, Claudin-b, -c, -3c, -7a, -7b, B-cell lymphoma-2, inhibitor of apoptosis protein, NF-E2-related factor 2 mRNA levels (P < 0.05), increased reactive oxygen species (ROS), malondialdehyde (MDA) and protein carbonyl contents (P < 0.05), and up-regulated Claudin-12, 15a, -15b, Fas ligand, mitogen-activated protein kinase kinase 6, p38 mitogen-activated protein kinase, B-cell lymphoma protein 2 associated X protein, apoptotic protease activating factor-1, caspase-3, -7, -8, -9, Kelch-like ECH-associating protein (Keap) 1a and Keap 1b mRNA levels (P < 0.05) in the head kidney and spleen under injection fish of A. hydrophila, suggesting that vitamin C deficiency could decrease fish head kidney and spleen structural integrity through depression of antioxidative ability, induction of apoptosis and disruption of tight junctional complexes. In addition, except the activities of ACP and MnSOD, and mRNA expression levels of TGF-β1, Occludin and MnSOD, the effect of vitamin C on fish head kidney, spleen and skin immunity and structural integrity other indicators model are similar under infection of A. hydrophila. Finally, the vitamin C requirement for the growth performance (PWG) of young grass carp was estimated to be 92.8 mg/kg diet. Meanwhile, the vitamin C requirement for against skin lesion morbidity of young grass carp was estimated to be 122.9 mg/kg diet. In addition, based on the biochemical indices [immune indices (LA activity in the head kidney and C4 content in the spleen) and antioxidant indices (MDA content in the head kidney and ROS content in the spleen)] the vitamin C requirements for young grass carp were estimated to be 131.2, 137.5, 135.8 and 129.8 mg/kg diet, respectively.

Claudins in a primary cultured puffer fish (Tetraodon nigroviridis) gill epithelium model alter in response to acute seawater exposure

Gill epithelium permeability and qualitative/quantitative aspects of gill claudin (cldn) tight junction (TJ) protein transcriptomics were examined with a primary cultured model gill epithelium developed using euryhaline puffer fish (Tetraodon nigroviridis) gills. The model was prepared using seawater-acclimated fish gills and was cultured on permeable cell culture filter supports. The model is composed of 1-2 confluent layers of gill pavement cells (PVCs), with the outer layer exhibiting prominent apical surface microridges and TJs between adjacent cells. During development of electrophysiological characteristics, the model exhibits a sigmoidal increase in transpithelial resistance (TER) and plateaus around 30 kΩcm(2). At this point paracellular movement of [(3)H]polyethylene glycol (PEG) 4000 was low at ~1.75 cm s(-1)×10(-7). When exposed to apical seawater (SW) epithelia exhibit a marked decrease in TER while PEG flux remained unchanged for at least 6 h. In association with this, transcript encoding cldn TJ proteins cldn3c, -23b, -27a, -27c, -32a and -33b increased during the first 6 h while cldn11a decreased. This suggests that these proteins are involved in maintaining barrier properties between gill PVCs of SW fishes. Gill cldn mRNA abundance also altered 6 and 12 h following abrupt SW exposure of puffer fish, but in a manner that differed qualitatively and quantitatively from the cultured model. This most likely reflects the cellular heterogeneity of whole tissue and/or the contribution of the endocrine system in intact fish. The current study provides insight into the physiological and transcriptomic response of euryhaline fish gill cells to a hyperosmotic environment.

Effect of the liquorice root derivatives on salt and water balance in a teleost fish, rainbow trout (Oncorhynchus mykiss)

The effect of liquorice root derivatives (LRDs) glycyrrhizic acid (GL) and glycyrrhetinic acid (18βGA) on salt and water balance and end points of gill ion transport in a freshwater teleost, (rainbow trout) was examined after feeding fish diets containing GL or 18βGA (0, 5, 50 or 500 µg/g diet) for a two week period. Serum cortisol levels and gill 11β-hydroxysteroid dehydrogenase type 2 mRNA abundance decreased in fish fed GL but increased (at select doses) in fish fed 18βGA. At higher doses of GL, gill Na(+)-K(+)-ATPase and H(+)-ATPase activity increased, while cystic fibrosis transmembrane conductance regulator type II mRNA abundance significantly decreased at the lowest dose of GL. End points of gill transcellular ion transport were not significantly altered in fish fed 18βGA, except for a reduction in Na(+)-K(+)-ATPase activity at a 50 µg/g dose. In contrast, high doses of GL and 18βGA increased gill transcript abundance of the tight junction protein claudin-31 (cldn-31). Other end points of gill paracellular transport differed in fishes fed LRDs. Tricellulin mRNA abundance was increased by high dose GL and decreased by high dose 18βGA, and cldn-23a and cldn-27b mRNA abundance significantly decreased in response to GL irrespective of dose. Despite the above observations, systemic end points of salt and water balance (i.e. serum [Na(+)] and [Cl(-)] as well as muscle moisture) were unaffected by LRDs. Therefore data suggest that LRDs can alter end points of ion transport in fishes but that overall salt and water balance need not be perturbed.

Transcriptional dynamics of a conserved gene expression network associated with craniofacial divergence in Arctic charr

Background

Understanding the molecular basis of craniofacial variation can provide insights into key developmental mechanisms of adaptive changes and their role in trophic divergence and speciation. Arctic charr (Salvelinus alpinus) is a polymorphic fish species, and, in Lake Thingvallavatn in Iceland, four sympatric morphs have evolved distinct craniofacial structures. We conducted a gene expression study on candidates from a conserved gene coexpression network, focusing on the development of craniofacial elements in embryos of two contrasting Arctic charr morphotypes (benthic and limnetic).

Results

Four Arctic charr morphs were studied: one limnetic and two benthic morphs from Lake Thingvallavatn and a limnetic reference aquaculture morph. The presence of morphological differences at developmental stages before the onset of feeding was verified by morphometric analysis. Following up on our previous findings that Mmp2 and Sparc were differentially expressed between morphotypes, we identified a network of genes with conserved coexpression across diverse vertebrate species. A comparative expression study of candidates from this network in developing heads of the four Arctic charr morphs verified the coexpression relationship of these genes and revealed distinct transcriptional dynamics strongly correlated with contrasting craniofacial morphologies (benthic versus limnetic). A literature review and Gene Ontology analysis indicated that a significant proportion of the network genes play a role in extracellular matrix organization and skeletogenesis, and motif enrichment analysis of conserved noncoding regions of network candidates predicted a handful of transcription factors, including Ap1 and Ets2, as potential regulators of the gene network. The expression of Ets2 itself was also found to associate with network gene expression. Genes linked to glucocorticoid signalling were also studied, as both Mmp2 and Sparc are responsive to this pathway. Among those, several transcriptional targets and upstream regulators showed differential expression between the contrasting morphotypes. Interestingly, although selected network genes showed overlapping expression patterns in situ and no morph differences, Timp2 expression patterns differed between morphs.

Conclusion

Our comparative study of transcriptional dynamics in divergent craniofacial morphologies of Arctic charr revealed a conserved network of coexpressed genes sharing functional roles in structural morphogenesis. We also implicate transcriptional regulators of the network as targets for future functional studies.

Electronic supplementary material

The online version of this article (doi:10.1186/2041-9139-5-40) contains supplementary material, which is available to authorized users.

The physiology of fish at low pH: the zebrafish as a model system

Ionic regulation and acid–base balance are fundamental to the physiology of vertebrates including fish. Acidification of freshwater ecosystems is recognized as a global environmental problem, and the physiological responses to acid exposure in a few fish species are well characterized. However, the underlying mechanisms promoting ionic and acid–base balance for most fish species that have been investigated remain unclear. Zebrafish (Danio rerio) has emerged as a powerful model system to elucidate the molecular basis of ionic and acid–base regulation. The utility of zebrafish is related to the ease with which it can be genetically manipulated, its suitability for state-of-the-art molecular and cellular approaches, and its tolerance to diverse environmental conditions. Recent studies have identified several key regulatory mechanisms enabling acclimation of zebrafish to acidic environments, including activation of the sodium/hydrogen exchanger (NHE) and H+-ATPase for acid secretion and Na+ uptake, cortisol-mediated regulation of transcellular and paracellular Na+ movements, and ionocyte proliferation controlled by specific cell-fate transcription factors. These integrated physiological responses ultimately contribute to ionic and acid–base homeostasis in zebrafish exposed to acidic water. In the present review, we provide an overview of the general effects of acid exposure on freshwater fish, the adaptive mechanisms promoting extreme acid tolerance in fishes native to acidic environments, and the mechanisms regulating ionic and acid–base balance during acid exposure in zebrafish.

Gill cell culture systems as models for aquatic environmental monitoring

A vast number of chemicals require environmental safety assessments for market authorisation. To ensure acceptable water quality, effluents and natural waters are monitored for their potential harmful effects. Tests for market authorisation and environmental monitoring usually involve the use of large numbers of organisms and, for ethical, cost and logistic reasons, there is a drive to develop alternative methods that can predict toxicity to fish without the need to expose any animals. There is therefore a great interest in the potential to use cultured fish cells in chemical toxicity testing. This review summarises the advances made in the area and focuses in particular on a system of cultured fish gill cells grown into an epithelium that permits direct treatment with water samples.

Claudin-6, -10d and -10e contribute to seawater acclimation in the euryhaline puffer fish Tetraodon nigroviridis

Expression profiles of claudin-6, -10d and -10e in the euryhaline teleost fish Tetraodon nigroviridis revealed claudin-6 in brain, eye, gill and skin tissue, while claudin-10d and -10e were found in brain, gill and skin only. In fishes, the gill and skin are important tissue barriers that interface directly with surrounding water, but these organs generally function differently in osmoregulation. Therefore, roles for gill and skin claudin-6, -10d and -10e in the osmoregulatory strategies of T. nigroviridis were investigated. In the gill epithelium, claudin-6, -10d and -10e co-localized with Na(+)-K(+)-ATPase immunoreactive (NKA-ir) ionocytes, and differences in sub-cellular localization could be observed in hypoosmotic (freshwater, FW) versus hyperosmotic (seawater, SW) environments. Claudin-10d and -10e abundance increased in the gills of fish acclimated to SW versus FW, while claudin-6 abundance decreased in the gills of fish acclimated to SW. Taken together with our knowledge of claudin-6 and -10 function in other vertebrates, data support the idea that in SW-acclimated T. nigroviridis, these claudins are abundant in gill ionocytes, where they contribute to the formation of a Na(+) shunt and 'leaky' epithelium, both of which are characteristic of salt-secreting SW fish gills. Skin claudin-10d and -10e abundance also increased in fish acclimated to SW versus those in FW, but so did claudin-6. In skin, claudin-6 was found to co-localize with NKA-ir cells, but claudin-10d and -10e did not. This study provides direct evidence that the gill epithelium contains salinity-responsive tight junction proteins that are abundant primarily in ionocytes. These same proteins also appear to play a role in the osmoregulatory physiology of the epidermis.

Tight junction protein gene expression patterns and changes in transcript abundance during development of model fish gill epithelia

In vertebrates, tight junction (TJ) proteins play an important role in epithelium formation and development, the maintenance of tissue integrity and regulation of TJ permeability. In this study, primary cultured model gill epithelia composed of pavement cells (PVCs) were used to examine TJ protein transcript abundance during the development of epithelium confluence and epithelium resistive properties. Differences in TJ protein expression patterns and transcript abundance between gill models composed of PVCs and models composed of PVCs and mitochondrion-rich cells (MRCs) were also examined. Marked alterations in TJ protein transcript abundance were observed as cells developed to confluence in flask-cultured model gill epithelia. In contrast, during the formation of tissue resistance in insert-cultured epithelia (i.e. epithelia cultured on a permeable substrate), changes in TJ protein mRNA abundance were conservative, despite paracellular marker flux decreasing by orders of magnitude. In both cases significant changes in claudin-8b, -8d, -27b, -28b and -32a transcript abundance were observed, suggesting that temporal alterations in the abundance of these genes are important end points of model gill epithelium integrity. When MRCs were present in cultured gill models, the mRNA abundance of several TJ proteins significantly altered and claudin-10c, -10d and -33b were only detected in preparations that included MRCs. These data provide insight into the role of select TJ proteins in the formation and development of gill epithelia and the maintenance of gill barrier properties. In addition, observations reveal a heterogeneous distribution of claudin TJ proteins in the gill epithelial cells of rainbow trout.

Cortisol regulates epithelial permeability and sodium losses in zebrafish exposed to acidic water

The effects of cortisol on epithelial permeability and sodium (Na(+)) handling during acid exposure were investigated in larval zebrafish (Danio rerio). The results demonstrated that the whole-body absorption of the paracellular permeability marker polyethylene glycol-4000 (PEG-4000) decreased with increasing levels of exogenous cortisol. Western blot analysis revealed that the abundance of the epithelial tight junction proteins occludin-a and claudin-b was increased after cortisol treatment. Furthermore, translational gene knockdown of claudin-b using an antisense morpholino oligonucleotide caused an increase in the permeability to PEG-4000, which was mitigated by cortisol treatment, further suggesting a role for cortisol in reducing paracellular permeability. Exposure to acidic water (pH 4.0 vs 7.6) caused an expected increase in the diffusive loss of Na(+) and a decrease in whole-body Na(+) levels. These disruptive effects of acute acid exposure on Na(+) balance were reduced by treatment of larvae with exogenous cortisol. Translational knockdown of the glucocorticoid receptor (GR) abolished the effects of cortisol on epithelial PEG permeability, suggesting that activation of GR was probably the major signaling pathway for reducing epithelial permeability. During acid exposure, the epithelial PEG permeability in the GR morphants was significantly higher than in the control fish. Additionally, GR morphants exhibited a more pronounced diffusive loss of Na(+) than the control fish during acid exposure. These findings suggest that cortisol may help to minimize the negative consequences of acid exposure on Na(+) homoeostasis via GR-mediated reductions in epithelial permeability and paracellular Na(+) loss.

A role for tricellulin in the regulation of gill epithelium permeability

The apical-most region of cell-to-cell contact in a vertebrate epithelium is the tight junction (TJ) complex. It is composed of bicellular TJs (bTJs) that bridge two adjacent epithelial cells and tricellular TJs (tTJs) that are points of contact between three adjoining epithelial cells. Tricellulin (TRIC) is a transmembrane TJ protein of vertebrates that is found in the tTJ complex. Full-length cDNA encoding rainbow trout TRIC was cloned and sequenced. In silico analysis of rainbow trout TRIC revealed a tetraspannin protein with several putative posttranslational modification sites. TRIC mRNA was broadly expressed in rainbow trout tissues and exhibited moderately greater abundance in the gill. In a primary cultured gill epithelium, TRIC localized to tTJs and TRIC protein abundance increased in association with corticosteroid-induced reductions in paracellular permeability. Sodium caprate was used to compromise cultured gill epithelium integrity by disrupting the tTJ complex. Sodium caprate treatment caused a reversible reduction in transepithelial resistance, caused an increase in paracellular permeability (as measured by [³H]PEG-4000 flux), and displaced TRIC from tTJs while leaving bTJs intact. Data from this study support the view that tTJs and the TJ protein TRIC 1) play a role in maintaining gill epithelium integrity and 2) contribute to the regulation of gill epithelium permeability.

The tight junction protein claudin-b regulates epithelial permeability and sodium handling in larval zebrafish, Danio rerio

The functional role of the tight junction protein claudin-b in larval zebrafish (Danio rerio) was investigated. We showed that claudin-b protein is expressed at epithelial cell-cell contacts on the skin. Translational gene knockdown of claudin-b protein expression caused developmental defects, including edema in the pericardial cavity and yolk sac. Claudin-b morphants exhibited an increase in epithelial permeability to the paracellular marker polyethylene glycol (PEG-4000) and fluorescein isothiocyanate-dextran (FD-4). Accumulation of FD-4 was confined mainly to the yolk sac and pericardial cavity in the claudin-b morphants, suggesting these regions became particularly leaky in the absence of claudin-b expression. Additionally, Na(+) efflux was substantially increased in the claudin-b morphants, which contributed to a significant reduction in whole-body Na(+) levels. These results indicate that claudin-b normally acts as a paracellular barrier to Na(+). Nevertheless, the elevated loss of Na(+) in the morphants was compensated by an increase in Na(+) uptake. Notably, we observed that the increased Na(+) uptake in the morphants was attenuated in the presence of the selective Na(+)/Cl(-)-cotransporter (NCC) inhibitor metolazone, or during exposure to Cl(-)-free water. These results suggested that the increased Na(+) uptake in the morphants was, at least in part, mediated by NCC. Furthermore, treatment with an H(+)-ATPase inhibitor bafilomycin A1 was found to reduce Na(+) uptake in the morphants, suggesting that H(+)-ATPase activity was essential to provide a driving force for Na(+) uptake. Overall, the results suggest that claudin-b plays an important role in regulating epithelial permeability and Na(+) handling in zebrafish.

Cortisol regulates Na+ uptake in zebrafish, Danio rerio, larvae via the glucocorticoid receptor

Unlike other freshwater fish previously examined, zebrafish are capable of increasing their rate of Na(+) uptake during chronic exposure to acidic water (pH 4). In the present study, the potential role of cortisol in the induction of Na(+) uptake during acid-exposure was investigated. When zebrafish larvae (4 days post-fertilization) were treated with waterborne cortisol, the rate of Na(+) uptake was significantly increased; this effect was blocked by co-incubating larvae with RU-486, an antagonist selective for the glucocorticoid receptor (GR). A similar induction in Na(+) uptake, which was also blocked by RU-486, was observed when larvae were treated with dexamethasone, a selective GR agonist. Conversely, treating larvae with aldosterone, a selective agonist for the mineralocorticoid receptor (MR) had no effect on Na(+) uptake. Acid-exposure increased whole body cortisol levels and translational knockdown of GR using antisense morpholinos prevented the full induction of Na(+) uptake during exposure to acidic water, further confirming the role of cortisol and GR in Na(+) uptake stimulation. Using immunohistochemistry, GR was localized to ionocytes known to be responsible for Na(+) uptake (HR-cells). Knockdown of Rhcg1, an apical membrane ammonia channel or Na(+)/H(+) exchanger 3b (NHE3b), proteins known to play an important role in facilitating Na(+) uptake in acidic water, prevented the stimulatory effects of cortisol treatment on Na(+) uptake, suggesting that cortisol regulates Na(+) uptake by stimulating an Rhcg1-NHE3b "functional metabolon".

β-Adrenergic regulation of Na+ uptake by larval zebrafish Danio rerio in acidic and ion-poor environments

The potential role of adrenergic systems in regulating Na(+) uptake in zebrafish (Danio rerio) larvae was investigated. Treatment with isoproterenol (a generic β-adrenergic receptor agonist) stimulated Na(+) uptake, whereas treatment with phenylephrine (an α(1)-adrenergic receptor agonist) as well as clonidine (an α(2)-adrenergic receptor agonist) significantly reduced Na(+) uptake, suggesting opposing roles of α- and β-adrenergic receptors in Na(+) uptake regulation. The increase in Na(+) uptake associated with exposure to acidic water (pH = 4.0) was attenuated in the presence of the nonselective β-receptor antagonist propranolol or the β(1)-receptor blocker atenolol; the β(2)-receptor antagonist ICI-118551 was without effect. The stimulation of Na(+) uptake associated with ion-poor water (32-fold dilution of Ottawa tapwater) was unaffected by β-receptor blockade. Translational gene knockdown of β-receptors using antisense oligonucleotide morpholinos was used as a second method to assess the role of adrenergic systems in the regulation of Na(+) uptake. Whereas β(1)- or β(2B)-receptor knockdown led to significant decreases in Na(+) uptake during exposure to acidic water, only β(2A)-receptor morphants failed to increase Na(+) uptake in response to ion-poor water. In support of the pharmacology and knockdown experiments that demonstrated an involvement of β-adrenergic systems in the control of Na(+) uptake, we showed that the H(+)-ATPase-rich (HR) cell, a subtype of ionocyte known to be a site of Na(+) uptake, is innervated and appears to express β-adrenergic receptors (propranolol binding sites) at 4 days postfertilization. These data indicate an important role of adrenergic systems in regulating Na(+) uptake in developing zebrafish.

GLP-2 enhances barrier formation and attenuates TNFα-induced changes in a Caco-2 cell model of the intestinal barrier

INTRODUCTION

Tight junctions are intercellular permeability seals that regulate paracellular transport across epithelia. Tight junction function, expression and localisation of constituent proteins are significantly altered by cytokines such as TNFα. Glucagon-like peptide-2 (GLP-2) is an intestinotrophic enteroendocrine peptide. It is not known whether GLP-2 regulates the barrier or tight junctions. The aim of this study was to investigate whether GLP-2 has an effect on tight junction function or protein expression, alone or in response to TNFα exposure.

METHODS

Caco-2 cells were grown to confluence on filters in the presence or absence of GLP-2. The time course of transepithelial electrical resistance developing across the monolayer was measured; tight junction protein expression was quantified by immunoblotting. At day 20, TNFα in the presence or absence of GLP-2 was added. Changes in TEER and tight junction proteins expression were quantified. Both TNFα and GLP-2 were added on the basolateral side.

RESULTS

GLP-2 exposed Caco-2 cell monolayers showed a significant increase in transepithelial electrical resistance compared to that in untreated control cells. At the same time, expression of the tight junction proteins occludin and zona occludens-1 (ZO-1) was increased at day 17 post-seeding (1.6-fold; p=0.037 and 4.7 fold; p=0.039 respectively). Subsequent TNFα exposure induced a significant 9.3-fold (p<0.001) decrease in transepithelial electrical resistance and a corresponding reduction in the expression of ZO-1 (5.3 fold; p<0.01). However, the TNFα-induced reduction in transepithelial electrical resistance in GLP-2-exposed cells was highly attenuated to 1.8-fold (p<0.01). No change in tight junction protein expression was noted in GLP-2 exposed cells after cytokine exposure.

CONCLUSION

GLP-2 enhances formation of the epithelial barrier and its constituent proteins in Caco-2 cells, and diminishes the effects of TNFα. If these effects are replicated in vivo the GLP-2 receptor may present a therapeutic target in intestinal inflammation.

Hyperoxia disrupts pulmonary epithelial barrier in newborn rats via the deterioration of occludin and ZO-1

Background

Prolonged exposure to hyperoxia in neonates can cause hyperoxic acute lung injury (HALI), which is characterized by increased pulmonary permeability and diffuse infiltration of various inflammatory cells. Disruption of the epithelial barrier may lead to altered pulmonary permeability and maintenance of barrier properties requires intact epithelial tight junctions (TJs). However, in neonatal animals, relatively little is known about how the TJ proteins are expressed in the pulmonary epithelium, including whether expression of TJ proteins is regulated in response to hyperoxia exposure. This study determines whether changes in tight junctions play an important role in disruption of the pulmonary epithelial barrier during hyperoxic acute lung injury.

Methods

Newborn rats, randomly divided into two groups, were exposed to hyperoxia (95% oxygen) or normoxia for 1–7 days, and the severity of lung injury was assessed; location and expression of key tight junction protein occludin and ZO-1 were examined by immunofluorescence staining and immunobloting; messenger RNA in lung tissue was studied by RT-PCR; transmission electron microscopy study was performed for the detection of tight junction morphology.

Results

We found that different durations of hyperoxia exposure caused different degrees of lung injury in newborn rats. Treatment with hyperoxia for prolonged duration contributed to more serious lung injury, which was characterized by increased wet-to-dry ratio, extravascular lung water content, and bronchoalveolar lavage fluid (BALF):serum FD4 ratio. Transmission electron microscopy study demonstrated that hyperoxia destroyed the structure of tight junctions and prolonged hyperoxia exposure, enhancing the structure destruction. The results were compatible with pathohistologic findings. We found that hyperoxia markedly disrupted the membrane localization and downregulated the cytoplasm expression of the key tight junction proteins occludin and ZO-1 in the alveolar epithelium by immunofluorescence. The changes of messenger RNA and protein expression of occludin and ZO-1 in lung tissue detected by RT-PCR and immunoblotting were consistent with the degree of lung injury.

Conclusions

These data suggest that the disruption of the pulmonary epithelial barrier induced by hyperoxia is, at least in part, due to massive deterioration in the expression and localization of key TJ proteins.

Effects of elevated circulating cortisol levels on hydromineral status and gill tight junction protein abundance in the stenohaline goldfish

A role for cortisol in the regulation of hydromineral balance and gill tight junction (TJ) protein transcript abundance in the stenohaline freshwater goldfish was investigated. Intraperitoneal cortisol implants (50, 100, 200, 400 μg cortisol/g body weight) were used to dose-dependently elevate circulating cortisol levels over a 4 day period. Elevated cortisol did not significantly alter serum osmolality, serum Na(+) or muscle water content, however serum glucose and gill Na(+)-K(+)-ATPase activity were significantly increased and serum Cl(-) levels were significantly reduced when compared to control groups. Transcript levels for glucocorticoid receptor 1 (GR1) and mineralocorticoid receptor (MR) in the gill remained unchanged by cortisol treatment, however glucocorticoid receptor 2 (GR2) mRNA abundance was significantly down-regulated. Conversely, cortisol treatment significantly increased transcript and protein abundance of the TJ protein occludin in goldfish gill tissue, as well as mRNA abundance for claudin e, 7 and 8d. Goldfish tissue expression profiles demonstrated that transcripts encoding for these claudins are particularly abundant in the gill. Overall, results suggest a 'tightened' gill epithelium in response to elevated cortisol levels in goldfish. However, negative autoregulation of gill GR2 transcript suggests a lessened capacity to respond to cortisol and thus a potentially 'dampened' corticosteroid-mediated effect in the gill. Reduced systemic Cl(-) levels also suggest that sustained cortisol elevation in goldfish may have a detrimental effect on other ionoregulatory tissues.

Permeability properties of the teleost gill epithelium under ion-poor conditions

Permeability properties of the goldfish gill epithelium were examined in vivo and in vitro following exposure to ion-poor water (IPW) conditions. In gill tissue of IPW-acclimated goldfish, transcript abundance of tight junction (TJ) proteins occludin, claudin-b, -d, -e, -h, -7, and -8d increased, whereas ZO-1 and claudin 12 mRNA decreased and claudin-c was unaltered. In association with these changes, TJ depth increased among gill pavement cells (PVCs) and gill PVCs and mitochondria-rich cells (MRCs). PVC and MRC gill cell fractions were isolated using Percoll. Transcripts encoding for occludin, claudin-b, -c, -d, -e, -h, -7, -8d, -12, and ZO-1 were present in both fractions. After IPW acclimation, occludin, claudin-b and -e, and ZO-1 mRNA abundance increased in both fractions. In contrast, claudin-8d mRNA abundance increased in PVCs only while claudin-h decreased in MRCs. Gill permeability was examined using primary cultured goldfish PVC epithelia supplemented with serum derived from IPW-acclimated goldfish. IPW serum supplementation increased transepithelial resistance, reduced [(3)H]PEG-4000 permeability, and enhanced epithelial integrity during in vitro IPW exposure. IPW serum increased mRNA abundance of occludin, claudin-8d and -e in vitro. Using small interfering RNA, we found that occludin abundance was decreased in cultured gill epithelia, resulting in an increase in [(3)H]PEG-4000 flux. As occludin increased in the gills of IPW-acclimated fish as well as cultured gill epithelia exposed to IPW serum, results suggest that occludin is a barrier-forming TJ protein in fish gill epithelia. These studies support the idea that TJ proteins play an important role in regulating gill permeability in IPW.

Glucocorticoid and mineralocorticoid receptors regulate paracellular permeability in a primary cultured gill epithelium

The role of corticosteroid receptors (CRs) in the regulation of gill permeability was examined using a primary cultured trout gill epithelium. The epithelium expressed both glucocorticoid receptors (GR1 and GR2) and a mineralocorticoid receptor (MR), and cortisol treatment significantly increased transepithelial resistance (TER) and decreased paracellular [(3)H]PEG-4000 flux. Epithelial permeability was unaffected by deoxycorticosterone or aldosterone. The GR antagonist RU486 as well as MR antagonists spironolactone and RU26752 significantly reduced, but did not completely block, the effects of cortisol. The MR antagonist eplerenone was without effect. Only RU486 + spironolactone or RU486 + RU26752 treatment completely suppressed the effects of cortisol. On its own, RU486 had cortisol-like effects which could be blocked by spironolactone, suggesting that although RU486 is a GR antagonist, in this system it may also have agonistic properties that are mediated through the MR. The GR agonist dexamethasone increased TER and reduced [(3)H]PEG-4000 flux across cultured epithelia and was unaffected by MR antagonists. Therefore, alterations in transcript abundance of select tight junction (TJ) proteins were examined in response to cortisol, dexamethasone (a GR agonist) and RU486 (as a MR agonist). Occludin and claudin-7, -8d, -12 and -31 mRNA were significantly elevated in response to cortisol, dexamethasone or RU486 treatment. Claudin-3a mRNA was significantly elevated in response to cortisol or dexamethasone only, and claudin-28b and -30 mRNA were significantly altered following cortisol or RU486 treatment only. The data support a role for the GRs and MR in regulating gill permeability and suggest that TJ proteins are responsive to cortisol through both or individual CR types.

Claudin 28b and F-actin are involved in rainbow trout gill pavement cell tight junction remodeling under osmotic stress

Permeability of rainbow trout gill pavement cells cultured on permeable supports (single seeded inserts) changes upon exposure to freshwater or treatment with cortisol. The molecular components of this change are largely unknown, but tight junctions that regulate the paracellular pathway are prime candidates in this adaptational process. Using differential display polymerase chain reaction we found a set of 17 differentially regulated genes in trout pavement cells that had been exposed to freshwater apically for 24 h. Five genes were related to the cell-cell contact. One of these genes was isolated and identified as encoding claudin 28b, an integral component of the tight junction. Immunohistochemical reactivity to claudin 28b protein was concentrated in a circumferential ring colocalized to the cortical F-actin ring. To study the contribution of this isoform to changes in transepithelial resistance and Phenol Red diffusion under apical hypo-or hyperosmotic exposure we quantified the fluorescence signal of this claudin isoform in immunohistochemical stainings together with the fluorescence of phalloidin-probed F-actin. Upon hypo-osmotic stress claudin 28b fluorescence and epithelial tightness remained stable. Under hyperosmotic stress, the presence of claudin 28b at the junction significantly decreased, and epithelial tightness was severely reduced. Cortical F-actin fluorescence increased upon hypo-osmotic stress, whereas hyperosmotic stress led to a separation of cortical F-actin rings and the number of apical crypt-like pores increased. Addition of cortisol to the basolateral medium attenuated cortical F-actin separation and pore formation during hyperosmotic stress and reduced claudin 28b in junctions except after recovery of cells from exposure to freshwater. Our results showed that short-term salinity stress response in cultured trout gill cells was dependent on a dynamic remodeling of tight junctions, which involves claudin 28b and the supporting F-actin ring.

The use of the zebrafish model in stress research

The study of the causes and mechanisms underlying psychiatric disorders requires the use of non-human models for the test of scientific hypotheses as well as for use in pre-clinical drug screening and discovery. This review argues in favor of the use of zebrafish as a novel animal model to study the impact of early (stressful) experiences on the development of differential stress phenotypes in later life. This phenomenon is evolutionary conserved among several vertebrate species and has relevance to the etiology of psychiatric disorders. Why do we need novel animal models? Although significant progress has been achieved with the use of traditional mammalian models, there are major pitfalls associated with their use that impedes progress on two major fronts: 1) uncovering of the molecular mechanisms underlying aspects of compromised (stress-exposed) brain development relevant to the etiology of psychiatric disorders, and 2) ability to develop high-throughput technology for drug discovery in the field of psychiatry. The zebrafish model helps resolve these issues. Here we present a conceptual framework for the use of zebrafish in stress research and psychiatry by addressing three specific domains of application: 1) stress research, 2) human disease mechanisms, and 3) drug discovery. We also present novel methodologies associated with the development of the zebrafish stress model and discuss how such methodologies can contribute to remove the main bottleneck in the field of drug discovery.

Effect of cortisol on permeability and tight junction protein transcript abundance in primary cultured gill epithelia from stenohaline goldfish and euryhaline trout

Primary cultured gill epithelia from goldfish and rainbow trout were used to investigate a role for cortisol in the regulation of paracellular permeability and tight junction (TJ) protein transcript abundance in representative stenohaline versus euryhaline freshwater (FW) fish gills. Glucocorticoid and mineralocorticoid receptors are expressed in cultured goldfish gill preparations and cortisol treatment (100, 500 and 1000 ng/mL) dose-dependently elevated transepithelial resistance (TER) and reduced paracellular [(3)H]PEG-4000 flux across cultured goldfish gill epithelia. Despite these dose-dependent 'tightening' effects of cortisol, the response of goldfish TJ protein transcripts (i.e. occludin, claudin b, c, d, e, h, 7, 8d and 12, and ZO-1) were surprisingly small, with only claudin c and h, and ZO-1 transcript levels significantly decreasing at a dose of 1000 ng/mL. Extending the duration of cortisol exposure from 24 to 48 or 96 h (at 500 ng/mL) did little to alter this phenomenon. By comparison, exposing primary cultured trout gill epithelia (i.e. a euryhaline fish gill model) to 500 ng/mL cortisol resulted in a qualitatively similar, but quantitatively stronger epithelial 'tightening' response. Furthermore, transcript abundance of orthologous trout TJ proteins (i.e. occludin, and claudin 30, 28b, 3a, 7, 8d and 12) significantly elevated as would be expected in a 'tighter' epithelium. Taken together, data suggest a conservative role for cortisol in the endocrine regulation of paracellular permeability across the goldfish gill that may relate to stenohalinity.

Strategies for maintaining Na⁺ balance in zebrafish (Danio rerio) during prolonged exposure to acidic water

The objective of the present study was to characterize the capacity of zebrafish (Danio rerio) to regulate whole body Na⁺ levels during exposure to acidic (pH 3.8-4.0) water. Exposure to acidic water significantly affected the mRNA levels of 14 claudin and two occludin isoforms, tight junction proteins thought to be involved in regulating paracellular efflux. Despite these changes, Na⁺ efflux as well as uptake of polyethylene glycol (PEG), a marker for paracellular pathway, was persistently elevated during the 2-week period of acid exposure, although there was a transient recovery between 12- and 72-h. Pre-exposing fish to acidic water for 2 weeks failed to attenuate the increase in Na⁺ efflux associated with acute exposure to acidic water of low [Ca²⁺]. However, during recovery in water of circumneutral pH following exposure to acidic water, normal rates of Na⁺ efflux were restored within 5h. The rate of Na⁺ uptake was significantly elevated between 4 and 7 days of exposure to acidic water; the increase was associated with significant increases in maximal Na⁺ uptake capacity (J(MAX)Na⁺) and affinity constant (K(M)). These results demonstrate that in acidic water, zebrafish maintain their whole body Na⁺ balance primarily by regulating Na⁺ uptake, rather than Na⁺ efflux.

Claudins in a primary cultured puffer fish (Tetraodon nigroviridis) gill epithelium

A primary cultured gill epithelium from the model organism Tetraodon nigroviridis (spotted green puffer fish) has been developed for the study of claudin tight junction (TJ) proteins and their potential role in the regulation of paracellular permeability across the gills of fishes. The cultured preparation is composed of polygonal epithelial cells that exhibit TJ protein immunoreactivity around the periphery and develop a surface morphology of concentric apical microridges. There is an absence of cells exhibiting intense Na+-K+-ATPase immunoreactivity and taken together, these characteristics indicate that the epithelium is composed of gill pavement cells only. In Tetraodon, 52 genes encoding for claudin isoforms (Tncldn) have been identified and 32 of these genes are expressed in whole gill tissue. Of these genes, 12 are responsive to alterations in environmental salinity in vivo (Tncldn3a, -3c, -6, -8d, -10d, -10e, -11a, -23b, -27a, -27c, -32a, and -33b). All claudin isoforms found in whole gill tissue can be found in cultured pavement cell gill epithelia with the exception of Tncldn6, -10d, and -10e. The cultured preparation is suitable for studying the "molecular machinery" of TJ proteins in fish gill pavement cells.

Epithelial remodeling and claudin mRNA abundance in the gill and kidney of puffer fish (Tetraodon biocellatus) acclimated to altered environmental ion levels

In water of varying ion content, the gills and kidney of fishes contribute significantly to the maintenance of salt and water balance. However, little is known about the molecular architecture of the tight junction (TJ) complex and the regulation of paracellular permeability characteristics in these tissues. In the current studies, puffer fish (Tetraodon biocellatus) were acclimated to freshwater (FW), seawater (SW) or ion-poor freshwater (IPW) conditions. Following acclimation, alterations in systemic endpoints of hydromineral status were examined in conjunction with changes in gill and kidney epithelia morphology/morphometrics, as well as claudin TJ protein mRNA abundance. T. biocellatus were able to maintain endpoints of hydromineral status within relatively tight limits across the broad range of water ion content examined. Both gill and kidney tissue exhibited substantial alterations in morphology as well as claudin TJ protein mRNA abundance. These responses were particularly pronounced when comparing fish acclimated to SW versus those acclimated to IPW. TEM observations of IPW-acclimated fish gills revealed the presence of cells that exhibited the typical characteristics of gill mitochondria-rich cells (e.g. voluminous, Na(+)-K(+)-ATPase-immunoreactive, exposed to the external environment at the apical surface), but were not mitochondria-rich. To our knowledge, this type of cell has not previously been described in hyperosmoregulating fish gills. Furthermore, modifications in the morphometrics and claudin mRNA abundance of kidney tissue support the notion that spatial alterations in claudin TJ proteins along the nephron of fishes will likely play an important role in the regulation of salt and water balance in these organisms.

Tight junction proteins in zebrafish ovarian follicles: stage specific mRNA abundance and response to 17beta-estradiol, human chorionic gonadotropin, and maturation inducing hormone

In vertebrate epithelia, the tight junction (TJ) complex plays an important role in the regulation of paracellular permeability and contributes to mechanical stability. Using zebrafish, this study examined the possibility that TJ protein 'machinery' may contribute to the complex process of ovarian follicle development in fishes and be responsive to key endocrine factors that assist in the regulation of this event. Transcript encoding for 18 zebrafish claudin (cldn) and 2 occludin (ocln) orthologs were widely distributed in zebrafish tissues. All orthologs were detected in the ovary, albeit at varying levels of abundance. Using occludin as a marker, TJs localized to the periphery of ovarian follicles, corresponding to the position of granulosa and theca cells. Of the 20 orthologs examined, mRNA encoding for cldn g, d, and ocln were most abundant in whole ovary. As follicles transitioned from pre-vitellogenic to mid-/late-stage vitellogenic status, mRNA encoding for cldn a, b, d, g, h and 19 significantly declined. Transcript encoding for other orthologs (i.e. cldn c, e, f, i, j, 2, 7, 8, 10, 11, 12, 15, ocln and ocln b) did not significantly alter during follicle development. Exogenous 17beta-estradiol had little effect on TJ machinery in pre-vitellogenic follicles, but reduced the abundance of cldn h and 10 in mid- to late-stage vitellogenic follicles. In mid-vitellogenic follicles human chorionic gonadotropin increased cldn d and g mRNA abundance, whereas maturation inducing hormone reduced cldn h and 19 mRNA abundance. The data supports a role for the endocrine regulation of TJ proteins in ovarian follicle development.