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

Effects of replacing fishmeal with cottonseed protein concentrate on growth performance, blood metabolites, and the intestinal health of juvenile rainbow trout (Oncorhynchus mykiss)

Cottonseed protein concentrate (CPC) is a potential non-food protein source for fishmeal replacement in fish feed. However, a high inclusion level of CPC in diets may have adverse effects on the metabolism and health of carnivorous fish. This study aimed to investigate CPC as a fishmeal alternative in the diet of rainbow trout Oncorhynchus mykiss based on growth performance, blood metabolites, and intestinal health. Five isonitrogenous (46% crude protein) and isolipidic (16% crude lipid) diets were formulated: a control diet (30% fishmeal) and four experimental diets with substitution of fishmeal by CPC at 25%, 50%, 75%, and 100%. A total of 600 fish (mean body weight 11.24g) were hand-fed the five formulated diets to apparent satiation for eight weeks. The results showed no adverse effects on growth performance when 75% dietary fishmeal was replaced by CPC. However, reduced growth and feed intake were observed in rainbow trout fed a fishmeal-free diet based on CPC (CPC100%). Changes in serum metabolites were also observed in CPC100% compared with the control group, including an increase in alanine aminotransferase (ALT), a decrease in alkaline phosphatase (ALP), alterations in free amino acids, and reductions in cholesterol metabolism. In addition, the CPC-based diet resulted in reduced intestinal trypsin, decreased villus height and width in the distal intestine, upregulated mRNA expression levels of inflammatory cytokines in the intestine, and impaired gut microbiota with reduced bacterial diversity and decreased abundance of Bacillaceae compared with the control group. The findings suggest that the optimum substitution rate of dietary fishmeal by CPC for rainbow trout should be less than 75%.

Establishment of a striped catfish skin explant model for studying the skin response in Aeromonas hydrophila infections

Teleost fish skin serves as the first line of defense against pathogens. The interaction between pathogen and host skin determines the infection outcome. However, the mechanism(s) that modulate infection remain largely unknown. A proper tissue culture model that is easier to handle but can quantitatively and qualitatively monitor infection progress may shed some lights. Here, we use striped catfish (Pangasius hypophthalmus) to establish an ex vivo skin explant tissue culture model to explore host pathogen interactions. The skin explant model resembles in vivo skin in tissue morphology, integrity, and immune functionality. Inoculation of aquatic pathogen Aeromonas hydrophila in this model induces epidermal exfoliation along with epithelial cell dissociation and inflammation. We conclude that this ex vivo skin explant model could serve as a teleost skin infection model for monitoring pathogenesis under various infection conditions. The model can also potentially be translated into a platform to study prevention and treatment of aquatic infection on the skin in aquaculture applications.

Mummichog gill and operculum exhibit functionally consistent claudin-10 paralog profiles and Claudin-10c hypersaline response

Claudin (Cldn)-10 tight junction (TJ) proteins are hypothesized to form the paracellular Na⁺ secretion pathway of hyposmoregulating mummichog (Fundulus heteroclitus) branchial epithelia. Organ-specific expression profiles showed that only branchial organs [the gill and opercular epithelium (OE)] exhibited abundant cldn-10 paralog transcripts, which typically increased following seawater (SW) to hypersaline (2SW) challenge. Post-translational properties, protein abundance, and ionocyte localization of Cldn-10c, were then examined in gill and OE. Western blot analysis revealed two Cldn-10c immunoreactive bands in the mummichog gill and OE at ∼29 kDa and ∼40 kDa. The heavier protein could be eliminated by glycosidase treatment, demonstrating the novel presence of a glycosylated Cldn-10c. Protein abundance of Cldn-10c increased in gill and OE of 2SW-exposed fish. Cldn-10c localized to the sides of gill and OE ionocyte apical crypts and partially colocalized with cystic fibrosis transmembrane conductance regulator and F-actin, consistent with TJ complex localization. Cldn-10c immunofluorescent intensity increased but localization was unaltered by 2SW conditions. In support of our hypothesis, cldn-10/Cldn-10 TJ protein dynamics in gill and OE of mummichogs and TJ localization are functionally consistent with the creation and maintenance of salinity-responsive, cation-selective pores that facilitate Na⁺ secretion in hyperosmotic environments.

Summary: The role of claudin-10 tight junction proteins in paracellular salt secretion across fish branchial epithelia is indicated by organ-specific responses to hyperosmotic conditions and their association with salt secreting transcellular proteins

Dietary Phytogenics and Galactomannan Oligosaccharides in Low Fish Meal and Fish Oil-Based Diets for European Sea Bass (Dicentrarchus labrax) Juveniles: Effects on Gill Structure and Health and Implications on Oxidative Stress Status

An effective replacement for fish meal (FM) and fish oil (FO) based on plant-based raw materials in the feed of marine fish species is necessary for the sustainability of the aquaculture sector. However, the use of plant-based raw materials to replace FM and FO has been associated with several negative health effects, some of which are related to oxidative stress processes that can induce functional and morphological alterations in mucosal tissues. This study aimed to evaluate the effects of dietary oligosaccharides of plant origin (5,000 ppm; galactomannan oligosaccharides, GMOS) and a phytogenic feed additive (200 ppm; garlic oil and labiatae plant extract mixture, PHYTO) on the oxidative stress status and mucosal health of the gills of juvenile European sea bass (Dicentrarchus labrax). The experimental diets, low FM and FO diets (10%FM/6%FO) were supplemented with GMOS from plant origin and PHYTO for 63 days. GMOS and PHYTO did not significantly affect feed utilization, fish growth, and survival. GMOS and PHYTO downregulated the expression of β-act, sod, gpx, cat, and gr in the gills of the fish compared with that in fish fed the control diet. The expression of hsp70 and ocln was upregulated and downregulated, respectively, in the GMOS group compared with that in the control group, whereas the expression of zo-1 was downregulated in the PHYTO group compared with that in the GMOS group. The morphological, histopathological, immunohistochemical, and biochemical parameters of the fish gills were mostly unaffected by GMOS and PHYTO. However, the PHYTO group had lower incidence of lamellar fusion than did the control group after 63 days. Although the tissular distribution of goblet cells was unaffected by GMOS and PHYTO, goblet cell size showed a decreasing trend (−11%) in the GMOS group. GMOS and PHYTO significantly reduced the concentration of PCNA+ in the epithelium of the gills. The above findings indicated that GMOS and PHYTO in low FM/FO-based diets protected the gill epithelia of D. labrax from oxidative stress by modulating the expression of oxidative enzyme-related genes and reducing the density of PCNA+ cells in the gills of the fish.

Palmitic Acid Affects Intestinal Epithelial Barrier Integrity and Permeability In Vitro

Palmitic acid (PA), a long-chain saturated fatty acid, might activate innate immune cells. PA plays a role in chronic liver disease, diabetes and Crohn’s disease, all of which are associated with impaired intestinal permeability. We investigated the effect of PA, at physiological postprandial intestinal concentrations, on gut epithelium as compared to lipopolysaccharide (LPS) and ethanol, using an in vitro gut model, the human intestinal epithelial cell line Caco-2 grown on transwell inserts. Cytotoxicity and oxidative stress were evaluated; epithelial barrier integrity was investigated by measuring the paracellular flux of fluorescein, and through RT-qPCR and immunofluorescence of tight junction (TJ) and adherens junction (AJ) mRNAs and proteins, respectively. In PA-exposed Caco-2 monolayers, cytotoxicity and oxidative stress were not detected. A significant increase in fluorescein flux was observed in PA-treated monolayers, after 90 min and up to 360 min, whereas with LPS and ethanol, this was only observed at later time-points. Gene expression and immunofluorescence analysis showed TJ and AJ alterations only in PA-exposed monolayers. In conclusion, PA affected intestinal permeability without inducing cytotoxicity or oxidative stress. This effect seemed to be faster and stronger than those with LPS and ethanol. Thus, we hypothesized that PA, besides having an immunomodulatory effect, might play a role in inflammatory and functional intestinal disorders in which the intestinal permeability is altered.

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.

Claudins of sea lamprey (Petromyzon marinus) - organ-specific expression and transcriptional responses to water of varying ion content

The role of lamprey epithelium tight junctions (TJs) in the regulation of salt and water balance is poorly understood. This study reported on claudin (Cldn) TJ protein transcripts of pre-metamorphic larval and post-metamorphic juvenile sea lamprey (Petromyzon marinus) and the transcriptional response of genes encoding Cldns to changed environmental ion levels. Transcripts encoding Cldn-3b, -4, -5, -10, -14, -18 and -19 were identified, and mRNA expression profiles revealed the organ-specific presence of cldn-5 and -14, broad expression of cldn-3b, -4, -10, -18 and -19 and spatial differences in the mRNA abundance of cldn-4, -3b and -14 along the ammocoete intestine. Expression profiles were qualitatively similar in ammocoetes and juvenile fishes. Transcript abundance of genes encoding Cldns in osmoregulatory organs (gill, kidney, intestine and skin) was subsequently investigated after exposure of ammocoetes to ion-poor water (IPW) and juveniles to hyperosmotic conditions [60% sea water (SW)]. IPW-acclimated ammocoetes increased mRNA abundance of nearly all cldns in the gill. Simultaneously, cldn-10 abundance increased in the skin, whereas cldn-4, -14 and -18 decreased in the kidney. Ammocoete cldn mRNA abundance in the intestine was altered in a region-specific manner. In contrast, cldn transcript abundance was mostly downregulated in osmoregulatory organs of juvenile fish acclimated to SW - cldn-3b, -10 and -19 in the gill; cldn-3b, -4, -10 and -19 in the skin; cldn-3b in the kidney; and cldn-3b and -14 in the intestine. Data support the idea that Cldn TJ proteins play an important role in the osmoregulatory physiology of pre- and post-metamorphic sea lamprey and that Cldn participation can occur across organs, in an organ-specific manner, as well as differ spatially within organs, which contributes to the regulation of salt and water balance in these fishes.

C-type natriuretic peptide regulates the molecular components of the rainbow trout gill epithelium tight junction complex

Freshwater (FW) fish experience passive paracellular loss of ions into the surrounding environment across water-exposed epithelia such as the gill. The mitigation of paracellular ion loss is thought to be regulated by proteins of the tight junction (TJ) complex and in particular, the large superfamily of claudin (cldn) TJ proteins plays an important role. Transcript and protein levels of TJ proteins in teleosts are known to be under endocrine control of several important osmoregulatory hormones and the current study was aimed at determining whether the osmoregulatory hormone, C-type natriuretic peptide (CNP), can alter paracellular permeability and TJ protein abundance in a primary cultured gill epithelium derived from rainbow trout. Natriuretic peptide receptors were detected in the cultured trout gill epithelium. It was found that (i) developing cultured gill epithelia "grown" in the presence of 10 nM CNP, and (ii) mature cultured gill epithelia exposed to 10 nM CNP for 48 h, exhibited augmented barrier properties. This occurred in association with reduced flux rates of a paracellular permeability marker (polyethylene glycol, molecular mass 400; PEG-400) and, reduced ion efflux (i.e. ion loss) when preparations were exposed to apical FW. Exposure to CNP altered mRNA abundance of cldn-3a, -5a, -6, - 8c, -20a, -25b, -28a, -32a and cgn, but differences in the transcriptional response were observed between chronic and acute CNP exposure. In contrast, chronic and acute exposure to CNP resulted in reduced cldn-10e/Cldn-10e abundance. Data suggest that CNP may play a role in regulating the molecular physiology of the TJ complex in the fish gill epithelium and contribute to the regulation of salt and water balance by influencing the paracellular permeability properties of this tissue.

Section-specific H+ flux in renal tubules of fasted and fed goldfish

A recent study demonstrated that in response to a feeding-induced metabolic acidosis, goldfish (Carassius auratus) adjust epithelial protein and/or mRNA expression in their kidney tubules for multiple transporters known to be relevant for acid-base regulation. These include Na⁺/H⁺ exchanger (NHE), V-type H⁺-ATPase (V-ATPase), cytoplasmic carbonic anhydrase, HCO₃ ^- transporters and Rhesus proteins. Consequently, renal acid output in the form of protons and NH₄ ⁺ increases. However, little is known about the mechanistic details of renal acid-base regulation in C. auratus and teleost fishes in general. The present study applied the scanning ion-selective electrode technique (SIET) to measure proton flux in proximal, distal and connecting tubules of goldfish. We detected increased H⁺ efflux into the extracellular fluid from the tubule in fed animals, resulting from paracellular back-flux of H⁺ through the tight junction. By applying inhibitors for selected acid-base regulatory epithelial transporters, we found that cytosolic carbonic anhydrase and HCO₃ ^- transporters were important in mediating H⁺ flux in all three tubule segments of fed goldfish. Contrastingly, V-ATPase seemed to play a role in H⁺ flux only in proximal and distal tubules, and NHE in proximal and connecting tubules. We developed working models for transport of acid-base relevant equivalents (H⁺, HCO₃ ^-, NH₃/NH₄ ⁺) for each tubule segment in C. auratus kidney. While the proximal tubule appears to play a major role in both H⁺ secretion and HCO₃ ^- reabsorption, the distal and connecting tubules seem to mainly serve for HCO₃ ^- reabsorption and NH₃/NH₄ ⁺ secretion.

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.

Integrative Population and Physiological Genomics Reveals Mechanisms of Adaptation in Killifish

Adaptive divergence between marine and freshwater (FW) environments is important in generating phyletic diversity within fishes, but the genetic basis of this process remains poorly understood. Genome selection scans can identify adaptive loci, but incomplete knowledge of genotype-phenotype connections makes interpreting their significance difficult. In contrast, association mapping (genome-wide association mapping [GWAS], random forest [RF] analyses) links genotype to phenotype, but offer limited insight into the evolutionary forces shaping variation. Here, we combined GWAS, RF, and selection scans to identify loci important in adaptation to FW environments. We utilized FW-native and brackish water (BW)-native populations of Atlantic killifish (Fundulus heteroclitus) as well as a naturally admixed population between the two. We measured morphology and multiple physiological traits that differ between populations and may contribute to osmotic adaptation (salinity tolerance, hypoxia tolerance, metabolic rate, body shape) and used a reduced representation approach for genome-wide genotyping. Our results show patterns of population divergence in physiological capabilities that are consistent with local adaptation. Population genomic scans between BW-native and FW-native populations identified genomic regions evolving by natural selection, whereas association mapping revealed loci that contribute to variation for each trait. There was substantial overlap in the genomic regions putatively under selection and loci associated with phenotypic traits, particularly for salinity tolerance, suggesting that these regions and genes are important for adaptive divergence between BW and FW environments. Together, these data provide insight into the mechanisms that enable diversification of fishes across osmotic boundaries.

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-10 isoform expression and cation selectivity change with salinity in salt-secreting epithelia of Fundulusheteroclitus

To provide insight into claudin (Cldn) tight junction (TJ) protein contributions to branchial salt secretion in marine teleost fishes, this study examined cldn-10 TJ protein isoforms of a euryhaline teleost (mummichog; Fundulus heteroclitus) in association with salinity change and measurements of transepithelial cation selectivity. Mummichogs were transferred from freshwater (FW) to seawater (SW, 35‰) and from SW to hypersaline SW (2SW, 60‰) in a time course with transfer control groups (FW to FW, and SW to SW). FW to SW transfer increased mRNA abundance of cldn-10d and cldn-10e twofold, whilst cldn-10c and cldn-10f transcripts were unchanged. Transfer from SW to 2SW did not alter cldn-10d, and transiently altered cldn-10e abundance, but increased cldn-10c and cldn-10f fourfold. This was coincident with an increased number of single-stranded junctions (observed by transmission electron microscopy). For both salinity transfers, (1) cldn-10e mRNA was acutely responsive (i.e. after 24 h), (2) other responsive cldn-10 isoforms increased later (3-7 days), and (3) cystic fibrosis transmembrane conductance regulator (cftr) mRNA was elevated in accordance with established changes in transcellular Cl^- movement. Changes in mRNA encoding cldn-10c and -10f appeared linked, consistent with the tandem repeat locus in the Fundulus genome, whereas mRNA for tandem cldn-10d and cldn-10e seemed independent of each other. Cation selectivity sequence measured by voltage and conductance responses to artificial SW revealed Eisenman sequence VII: Na⁺>K⁺>Rb⁺∼Cs⁺>Li⁺ Collectively, these data support the idea that Cldn-10 TJ proteins create and maintain cation-selective pore junctions in salt-secreting tissues of teleost fishes.

Dietary threonine deficiency depressed the disease resistance, immune and physical barriers in the gills of juvenile grass carp (Ctenopharyngodon idella) under infection of Flavobacterium columnare

This study was conducted to investigate the effects of dietary threonine on the disease resistance, gill immune and physical barriers function of juvenile grass carp (Ctenopharyngodon idella). A total of 1080 juveniles were fed six iso-nitrogenous diets containing graded levels of threonine (3.99-21.66 g kg^-1 diet) for 8 weeks, and then challenged with Flavobacterium columnare. Results showed that threonine deficiency (3.99 g kg^-1 diet): (1) increased the gill rot morbidity after exposure to F. columnare; (2) attenuated the gill immune barrier function by decreasing antimicrobial substances production, up-regulating the mRNA levels of pro-inflammatory cytokines (except IL-12p40), and down-regulating the anti-inflammatory cytokines partly due to the modulation of NF-κB and TOR signaling. (3) disrupt the gill tight junction complexes by down-regulating TJs (claudin-3, -b, -c, 12, occludin, ZO-1 and ZO-2) and up-regulating TJs (claudin-7a, -7b) as well as related signaling molecule myosin light chain kinase mRNA levels (P < 0.05). (4) exacerbated the gill apoptosis by up-regulating cysteinyl aspartic acid-protease-3, 8, 9, c-Jun N-terminal kinases and mediating apoptosis related factors mRNA levels (P < 0.05); (5) exacerbated oxidative injury with increased reactive oxygen species, malondialdehyde and protein carbonyl contents (P < 0.05), decreased the antioxidant related enzymes activities and corresponding mRNA levels (except glutathione peroxidase-1b and glutathione-S-transferase-omega 2) as well as glutathione contents (P < 0.05) partly ascribe to the abridgement of NF-E2-related factor 2 signaling [Nrf2/Keap1a (not Keap1b)] in fish gill. Overall, threonine deficiency depressed the disease resistance, and impaired immune and physical barriers in fish gill. Finally, based on the gill rot morbidity and biochemical indices (immune indices LA activity and antioxidant indices MDA content), threonine requirements for juvenile grass carp (9.53-53.43 g) were estimated to be 15.32 g kg^-1 diet (4.73 g 100 g^-1 protein), 15.52 g kg^-1 diet (4.79 g 100 g^-1 protein), 15.46 g kg^-1 diet (4.77 g 100 g^-1 protein), respectively.

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.

Thiamin deficiency induces impaired fish gill immune responses, tight junction protein expression and antioxidant capacity: Roles of the NF-κB, TOR, p38 MAPK and Nrf2 signaling molecules

In this study, we investigate the effects of dietary thiamin deficiency on immune responses, tight junctions, antioxidant capacity and related signaling molecules in the gills of young grass carp (Ctenopharyngodon idella). Fish were fed diets that contained 0.12-2.04 mg thiamin kg(-1) for 8 weeks. We found that dietary thiamin deficiency resulted in reduced complement 3 content, lysozyme and acid phosphatase activities, mRNA levels of hepcidin, liver-expressed antimicrobial peptides 2, transforming growth factor (TGF)-β1, interleukin (IL)-10, inhibitor protein-κBα (IκBα), ribosomal S6 protein kinase 1 and target of rapamycin (TOR) and increased expression of interferon-γ2, tumor necrosis factor-α, TGF-β2, IL-1β, IL-8, IκB kinases (IKKβ and IKKγ) and nuclear factor-κB p65 (NF-κB p65). Our findings showed that thiamin deficiency reduced the immune status of fish gills. Furthermore, thiamin deficiency resulted in reduced mRNA transcript levels of claudin b, claudin 3, claudin 12, zonula occludens 1 (ZO-1) and occludin and increased mRNA transcript levels of claudin 15a, myosin light-chain kinase (MLCK) and p38 mitogen-activated protein kinase (p38 MAPK) in fish gill tissues. These data suggested that thiamin deficiency disrupted tight junction-mediated fish gill barrier function. Additionally, reactive oxygen species, malondialdehyde and protein carbonyl levels and both the activities and expression levels of Cu/Zn superoxide dismutase, catalase, glutathione peroxidase, glutathione-S-transferases and glutathione reductase, as well as NF-E2-related factor 2 gene expression in fish gills, were lower in fish fed a thiamin-deficient diet. By contrast, thiamin deficiency increased levels of Kelch-like-ECH-associated protein 1a (Keap1a) and Keap1b mRNA transcript expression in fish gills. Taken together, our findings indicated that thiamin deficiency impaired fish gill health by effects on the expression of genes encoding cytokines, tight junction proteins, antioxidant enzymes, NF-κB p65, MLCK and Nrf2.

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.

Claudin multigene family in channel catfish and their expression profiles in response to bacterial infection and hypoxia as revealed by meta-analysis of RNA-Seq datasets

Claudins are one of the major groups of transmembrane proteins that play crucial roles in tight junctions. In addition to their function in the regulation of paracellular permeability, claudins are also involved in a number of biological processes related to pathogen infection, embryonic development, organ development and hypoxia response. Despite its importance, analyses of claudin genes in channel catfish have not been systematically performed. In this study, a total of 52 claudin genes were identified and characterized in channel catfish. Phylogenetic analyses were conducted to determine their identities and identify a number of lineage-specific claudin gene duplications in channel catfish. Expression profiles of catfish claudin genes in response to enteric septicemia of catfish (ESC) disease and hypoxia stress were determined by analyzing existing RNA-Seq datasets. Claudin genes were significantly down-regulated in the intestine at 3h post-infection, indicating that pathogens may disrupt the mucosal barrier by suppressing the expression of claudin genes. A total of six claudin genes were significantly regulated in the gill after hypoxia stress. Among them, the expressions of cldn-11b and cldn-10d were dramatically altered when comparing hypoxia tolerant fish with intolerant fish, though their specific roles involved in response to hypoxia stress remained unknown.

Transepithelial resistance and claudin expression in trout RTgill-W1 cell line: effects of osmoregulatory hormones

In the present study, we examined the trout gill cell line RTgill-W1 as a possible tool for in vitro investigation of epithelial gill function in fish. After seeding in transwells, transepithelial resistance (TER) increased until reaching a plateau after 1-2 days (20-80Ω⋅cm(2)), which was then maintained for more than 6 days. Tetrabromocinnamic acid, a known stimulator of TER via casein kinase II inhibition, elevated TER in the cell line to 125% of control values after 2 and 6h. Treatment with ethylenediaminetetraacetic acid induced a decrease in TER to <15% of pre-treatment level. Cortisol elevated TER after 12-72 h in a concentration-dependent manner, and this increase was antagonized by growth hormone (Gh). The effects of three osmoregulatory hormones, Gh, prolactin, and cortisol, on the mRNA expression of three tight junction proteins were examined: claudin-10e (Cldn-10e), Cldn-30, and zonula occludens-1 (Zo-1). The expression of cldn-10e was stimulated by all three hormones but with the strongest effect of Gh (50-fold). cldn-30 expression was stimulated especially by cortisol (20-fold) and also by Gh (4-fold). Finally, zo-1 was unresponsive to hormone treatment. Western blot analysis detected Cldn-10e and Cldn-30 immunoreactive proteins of expected molecular weight in samples from rainbow trout gills but not from RTgill-W1 cultures, possibly due to low expression levels. Collectively, these results show that the RTgill-W1 cell layers have tight junctions between cells, are sensitive to hormone treatments, and may provide a useful model for in vitro study of some in vivo gill phenomena.

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.

MgcRacGAP interacts with cingulin and paracingulin to regulate Rac1 activation and development of the tight junction barrier during epithelial junction assembly

The Rac1 inhibitor MgcRacGAP regulates Rac1 activation and TJ barrier development during junction assembly in epithelial cells. CGN and CGNL1 recruit MgcRacGAP to the TJ and interact with MgcRacGAP.

The regulation of Rho-family GTPases is crucial to direct the formation of cell–cell junctions and tissue barriers. Cingulin (CGN) and paracingulin (CGNL1) control RhoA activation in epithelial cells by interacting with RhoA guanidine exchange factors. CGNL1 depletion also inhibits Rac1 activation during junction assembly. Here we show that, unexpectedly, Madin–Darby canine kidney epithelial cells depleted of both CGN and CGNL1 (double-KD cells) display normal Rac1 activation and tight junction (TJ) formation, despite decreased junctional recruitment of the Rac1 activator Tiam1. The expression of the Rac1 inhibitor MgcRacGAP is decreased in double-KD cells, and the barrier development and Rac1 activation phenotypes are rescued by exogenous expression of MgcRacGAP. MgcRacGAP colocalizes with CGN and CGNL1 at TJs and forms a complex and interacts directly in vitro with CGN and CGNL1. Depletion of either CGN or CGNL1 in epithelial cells results in decreased junctional localization of MgcRacGAP but not of ECT2, a centralspindlin-interacting Rho GEF. These results provide new insight into coordination of Rho-family GTPase activities at junctions, since apical accumulation of CGN and CGNL1 at TJs during junction maturation provides a mechanism to spatially restrict down-regulation of Rac1 activation through the recruitment of MgcRacGAP.