Functional characterization of water transport and cellular localization of three aquaporin paralogs in the salmonid intestine

Population genomics analysis to identify ion and water transporter genes involved in the adaptation of Tibetan naked carps to brackish water

Understanding how evolutionary processes shape the genetic variations and influence the response of species to environmental alterations is critical for biodiversity conservation and molecular breeding. Gymnocypris przewalskii przewalskii is the only known cyprinid fish that dwells in the brackish water of Lake Qinghai on the Qinghai-Tibetan Plateau. To reveal the genetic basis of its adaptation to high salinity and alkalinity, whole-genome sequencing was performed in G. p. przewalskii and its freshwater relatives Gymnocypris eckloni and Gymnocypris przewalskii ganzihonensis. Compared with freshwater species, lower genetic diversity and higher linkage disequilibrium were observed in G. p. przewalskii. Selective sweep analysis identified 424 core-selective genes enriched in transport activities. Transfection analysis showed that genetic changes in the positively selected gene aquaporin 3 (AQP3) improved cell viability after salt treatment, suggesting its involvement in brackish water adaptation. Our analysis indicates that ion and water transporter genes experienced intensive selection, which might have contributed to the maintenance of high osmolality and ion content in G. p. przewalskii. The current study identified key molecules involved in the adaptation of fish to brackish water, providing valuable genomic resources for the molecular breeding of salt-tolerant fish.

The role of environmental salinity on Na+-dependent intestinal amino acid uptake in rainbow trout (Oncorhynchus mykiss)

Na⁺/K⁺-ATPases (NKA) in the basolateral membrane of the intestinal enterocytes create a Na⁺-gradient that drives both ion-coupled fluid uptake and nutrient transport. Being dependent on the same gradient as well as on the environmental salinity, these processes have the potential to affect each other. In salmonids, L-lysine absorption has been shown to be higher in freshwater (FW) than in seawater (SW) acclimated fish. Using electrophysiology (Ussing chamber technique), the aim was to explore if the decrease in L-lysine transport was due to allocation of the Na⁺-gradient towards ion-driven fluid uptake in SW, at the cost of amino acid transport. Intestinal NKA activity was higher in SW compared to FW fish. Exposure to ouabain, an inhibitor of NKA, decreased L-lysine transport. However, exposure to bumetanide and hydrochlorothiazide, inhibitors of Na⁺, K⁺, 2Cl^--co-transporter (NKCC) and Na⁺, Cl^--co-transporter (NCC) respectively, did not affect the rate of intestinal L-lysine transport. In conclusion, L-lysine transport is Na⁺-dependent in rainbow trout and the NKA activity and thus the available Na⁺-gradient increases after SW acclimation. This increased Na⁺-gradient is most likely directed towards osmoregulation, as amino acid transport is not compromised in SW acclimated fish.

Bile components affect the functions and transcriptome of the rainbow trout intestinal epithelial cell line RTgutGC

The concomitant increase in cultivation of fish and decrease in supply of marine ingredients, have greatly increased the demand for new nutrient sources. This also regards so-called functional ingredients which may benefit health and welfare of the fish. In vitro cell line-based intestinal epithelial barrier models may serve as tools for narrowing down the broad range of ingredient options, to identify the most promising candidates before in vivo feeding trials are run. In vivo, differentiation of the various epithelial cells in the fish intestine, from the multipotent stem cells, takes place in the presence of a variety of substances from dietary and endogenous origin. Among these, bile salts have recently received attention as regulators of epithelial function in health and disease but have not, until now, been included in the medium when culturing fish gut epithelial cells in vitro. As bile salts are present at high levels in the chyme of the fish intestine, in particular in salmon and rainbow trout, mostly as taurocholate (>90%), their role for effects of diet ingredients on the in vitro gut cell model should be understood. With this study, we wanted to investigate whether inclusion of bile from rainbow trout or pure taurocholate in the culture media would modulate functions of the RTgutGC epithelial cells. Here, we demonstrated that the rainbow trout intestinal epithelial cell line RTgutGC responded significantly to the presence of bile components. Treatment with rainbow trout bile taken from the gall bladder (RTbile) or pure taurocholate (TC) at taurocholate concentrations of ≤0.5 mg/mL retained normal cell morphology, cell viability as in cell oxidation-reduction metabolic activity and membrane integrity, and barrier features, while high concentrations of bile salts (≥1 mg/mL) were cytotoxic to the cells. After long-term (4 days) bile treatment, transcriptome responses showed how bile salts play important roles in intestinal epithelial cell metabolism. qPCR data demonstrated that barrier function genes, brush border enzyme genes and immune genes were significantly affected. Although similar trends were seen, treatment with bile salt as a component of rainbow trout bile or pure taurocholate, induced somewhat different effects. In conclusion, this study clearly indicates that bile salts should be included in the cell medium when running in vitro studies of gut cell functions, not at least immune functions, preferably at the level of ∼0.5 mg/mL supplemented as pure taurocholate to ensure reproducibility.

Localization of the Neuropeptide Arginine Vasotocin and Its Receptor in the Osmoregulatory Organs of Black Porgy, Acanthopagrus schlegelii: Gills, Kidneys, and Intestines

The neurohypophysial hormone arginine vasotocin (avt) and its receptor (avtr) regulates ions in the osmoregulatory organs of euryhaline black porgy (Acanthopagrus schlegelii). The localization of avt and avtr transcripts in the osmoregulatory organs has yet to be demonstrated. Thus, in the present study, we performed an in situ hybridization analysis to determine the localization of avt and avtr in the gills, kidneys, and intestines of the black porgy. The avt and avtr transcripts were identified in the filament and lamellae region of the gills in the black porgy. However, the basal membrane of the filament contained more avt and avtr transcripts. Fluorescence double tagging analysis revealed that avt and avtr mRNAs were partially co-localized with α-Nka-ir cells in the gill filament. The proximal tubules, distal tubules, and collecting duct of the kidney all had positive hybridization signals for the avt and avtr transcripts. Unlike the α-Nka immunoreactive cells, the avt and avtr transcripts were found on the basolateral surface of the distal convoluted tubule and in the entire cells of the proximal convoluted tubules of the black porgy kidney. In the intestine, the avt and avtr transcripts were found in the basolateral membrane of the enterocytes. Collectively, this study provides a summary of evidence suggesting that the neuropeptides avt and avtr with α-Nka-ir cells may have functions in the gills, kidneys, and intestines via ionocytes.

Molecular targets of prolactin in mummichogs (Fundulus heteroclitus): Ion transporters/channels, aquaporins, and claudins

Prolactin (Prl) was identified over 60 years ago in mummichogs (Fundulus heteroclitus) as a "freshwater (FW)-adapting hormone", yet the cellular and molecular targets of Prl in this model teleost have remained unknown. Here, we conducted a phylogenetic analysis of two mummichog Prl receptors (Prlrs), designated Prlra and Prlrb, prior to describing the tissue- and salinity-dependent expression of their associated mRNAs. We then administered ovine Prl (oPrl) to mummichogs held in brackish water and characterized the expression of genes associated with FW- and seawater (SW)-type ionocytes. Within FW-type ionocytes, oPrl stimulated the expression of Na⁺/Cl^- cotransporter 2 (ncc2) and aquaporin 3 (aqp3). Alternatively, branchial Na⁺/H⁺ exchanger 2 and -3 (nhe2 and -3) expression did not respond to oPrl. Gene transcripts associated with SW-type ionocytes, including Na⁺/K⁺/2Cl^- cotransporter 1 (nkcc1), cystic fibrosis transmembrane regulator 1 (cftr1), and claudin 10f (cldn10f) were reduced by oPrl. Isolated gill filaments incubated with oPrl in vitro exhibited elevated ncc2 and prlra expression. Given the role of Aqps in supporting gastrointestinal fluid absorption, we assessed whether several intestinal aqp transcripts were responsive to oPrl and found that aqp1a and -8 levels were reduced by oPrl. Our collective data indicate that Prl promotes FW-acclimation in mummichogs by orchestrating the expression of solute transporters/channels, water channels, and tight-junction proteins across multiple osmoregulatory organs.

Intestinal osmoregulatory mechanisms differ in Mediterranean and Atlantic European sea bass: A focus on hypersalinity

European sea bass (Dicentrarchus labrax) migrate towards habitats where salinity can reach levels over 60‰, notably in Mediterranean lagoons. D. labrax are genetically subdivided in Atlantic and Mediterranean lineages and have evolved in slightly different salinities. We compared Atlantic and West-Mediterranean populations regarding their capacity to tolerate hypersalinity with a focus on the involvement of the intestine in solute-driven water reabsorption. Fish were analyzed following a two-week transfer from seawater (SW, 36‰) to either SW or hypersaline water (HW, 55‰). Differences among lineages were observed in posterior intestines of fish maintained in SW regarding NKA activities and mRNA expressions of nkaα1a, aqp8b, aqp1a and aqp1b with systematic higher levels in Mediterranean sea bass. High salinity transfer triggered similar responses in both lineages but at different magnitudes which may indicate slight different physiological strategies between lineages. High salinity transfer did not significantly affect the phenotypic traits measured in the anterior intestine. In the posterior intestine however, the size of enterocytes and NKA activity were higher in HW compared to SW. In this tissue, nka-α1a, nkcc2, aqp8ab and aqp8aa mRNA levels were higher in HW compared to SW as well as relative protein expression of AQP8ab. For aqp1a, 1b, 8aa and 8b, an opposite trend was observed. The sub-apical localization of AQP8ab in enterocytes suggests its role in transepithelial water reabsorption. Strong apical NKCC2/NCC staining indicates an increased Na⁺ and Cl^- reuptake by enterocytes which could contribute to solute-coupled water reuptake in cells where AQP8ab is expressed.

Tissue and salinity specific Na+/Cl- cotransporter (NCC) orthologues involved in the adaptive osmoregulation of sea lamprey (Petromyzon marinus)

Two orthologues of the gene encoding the Na⁺-Cl⁻ cotransporter (NCC), termed ncca and nccb, were found in the sea lamprey genome. No gene encoding the Na⁺-K⁺-2Cl⁻ cotransporter 2 (nkcc2) was identified. In a phylogenetic comparison among other vertebrate NCC and NKCC sequences, the sea lamprey NCCs occupied basal positions within the NCC clades. In freshwater, ncca mRNA was found only in the gill and nccb only in the intestine, whereas both were found in the kidney. Intestinal nccb mRNA levels increased during late metamorphosis coincident with salinity tolerance. Acclimation to seawater increased nccb mRNA levels in the intestine and kidney. Electrophysiological analysis of intestinal tissue ex vivo showed this tissue was anion absorptive. After seawater acclimation, the proximal intestine became less anion absorptive, whereas the distal intestine remained unchanged. Luminal application of indapamide (an NCC inhibitor) resulted in 73% and 30% inhibition of short-circuit current (I_sc) in the proximal and distal intestine, respectively. Luminal application of bumetanide (an NKCC inhibitor) did not affect intestinal I_sc. Indapamide also inhibited intestinal water absorption. Our results indicate that NCCb is likely the key ion cotransport protein for ion uptake by the lamprey intestine that facilitates water absorption in seawater. As such, the preparatory increases in intestinal nccb mRNA levels during metamorphosis of sea lamprey are likely critical to development of whole animal salinity tolerance.

Osmoregulatory actions of prolactin in the gastrointestinal tract of fishes

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

Gut immune functions and health in Atlantic salmon (Salmo salar) from late freshwater stage until one year in seawater and effects of functional ingredients: A case study from a commercial sized research site in the Arctic region

The present study was conducted to strengthen the knowledge on gut immune functions and health in Atlantic salmon under large scale, commercial conditions in the Arctic region of Norway. Two groups of fish were monitored, one fed a series of diets without functional ingredients (Ref) and the other diets with functional ingredients (Test). The nutritional composition of the two diet series varied in parallel according to the nutrient requirements of the fish during the observation time. The content of functional ingredients in the Test diets, i.e. nucleotides, yeast cell walls, a prebiotic and essential fatty acids, varied in accordance with a strategy developed by the feed company. The fish were observed at four sampling time points, the first (FW) in May 2016 two weeks before seawater transfer, the other three throughout the following seawater period until the fish reached a size of about 2 kg, i.e. in June, four weeks after seawater transfer (SW1); in November (SW2), and in April the following year (SW3). Gut health was assessed based on histopathological indicators of lipid malabsorption and gut inflammation, expression of gut immune, barrier and other health related genes, plasma biomarkers, somatic indices of intestinal sections, as well as biomarkers of digestive functions. Seawater transfer of the fish (SW1 compared to FW) caused a marked lowering of expression of genes related to immune and barrier functions in the distal intestine, i.e. cytokines (il1β, il10, tgfβ, ifnγ), T-cell markers (cd3γδ), myd88 and tight junction proteins (zo-1, claudin-15, claudin-25b), indicating suppressed immune and barrier functions. At SW2 and SW3, most of the immune biomarkers showed values similar to those observed at FW. The development of plasma cholesterol and triglyceride levels showed similar picture, with markedly lower levels after seawater transfer. Lipid malabsorption was observed in particular in fish from SW1 and SW2, as indicated by hyper-vacuolation of the pyloric caeca enterocytes with concurrently increased expression levels of plin2. Regarding effects of functional ingredients, significantly lower condition factor and plasma triglyceride level were observed for Test-fed fish at SW2, indicating a metabolic cost of use of a mixture of nucleotides, yeast cell walls and essential fatty acids. No clear effects of functional ingredients on expression of gut immune genes and other health indexes were observed through the observation period. The great, temporary lowering of expression of gut immune and barrier genes at SW1 is suggested to be an important factor underlying the increased vulnerability of the fish at this time point. Our findings regarding supplementation with functional ingredients raise questions whether some of these ingredients overall are beneficial or might come with a metabolic cost. Our results highlight the need for a better understanding of the cause and consequences of the suppression of gut immune functions of farmed Atlantic salmon just after seawater transfer, and the use of functional ingredients under commercial conditions.

Drinking and Water Handling in the Medaka Intestine: A Possible Role of Claudin-15 in Paracellular Absorption?

When euryhaline fish move between fresh water (FW) and seawater (SW), the intestine undergoes functional changes to handle imbibed SW. In Japanese medaka, the potential transcellular aquaporin-mediated conduits for water are paradoxically downregulated during SW acclimation, suggesting paracellular transport to be of principal importance in hyperosmotic conditions. In mammals, intestinal claudin-15 (CLDN15) forms paracellular channels for small cations and water, which may participate in water transport. Since two cldn15 paralogs, cldn15a and cldn15b, have previously been identified in medaka, we examined the salinity effects on their mRNA expression and immunolocalization in the intestine. In addition, we analyzed the drinking rate and intestinal water handling by adding non-absorbable radiotracers, 51-Cr-EDTA or 99-Tc-DTPA, to the water. The drinking rate was >2-fold higher in SW than FW-acclimated fish, and radiotracer experiments showed anterior accumulation in FW and posterior buildup in SW intestines. Salinity had no effect on expression of cldn15a, while cldn15b was approximately 100-fold higher in FW than SW. Despite differences in transcript dynamics, Cldn15a and Cldn15b proteins were both similarly localized in the apical tight junctions of enterocytes, co-localizing with occludin and with no apparent difference in localization and abundance between FW and SW. The stability of the Cldn15 protein suggests a physiological role in water transport in the medaka intestine.

The vertebrate Aqp14 water channel is a neuropeptide-regulated polytransporter

Water channels (aquaporins) were originally discovered in mammals with fourteen subfamilies now identified (AQP0-13). Here we show that a functional Aqp14 subfamily phylogenetically related to AQP4-type channels exists in all vertebrate lineages except hagfishes and eutherian mammals. In contrast to the water-selective classical aquaporins, which have four aromatic-arginine constriction residues, Aqp14 proteins present five non-aromatic constriction residues and facilitate the permeation of water, urea, ammonia, H2O2 and glycerol. Immunocytochemical assays suggest that Aqp14 channels play important osmoregulatory roles in piscine seawater adaptation. Our data indicate that Aqp14 intracellular trafficking is tightly regulated by the vasotocinergic/isotocinergic neuropeptide and receptor systems, whereby protein kinase C and A transduction pathways phosphorylate highly conserved C-terminal residues to control channel plasma membrane insertion. The neuropeptide regulation of Aqp14 channels thus predates the vasotocin/vasopressin regulation of AQP2-5-6 orthologs observed in tetrapods. These findings demonstrate that vertebrate Aqp14 channels represent an ancient subfamily of neuropeptide-regulated polytransporters.

Osmoregulatory role of the intestine in the sea lamprey (Petromyzon marinus)

Lampreys are the most basal vertebrates with an osmoregulatory strategy. Previous research has established that the salinity tolerance of sea lamprey increases dramatically during metamorphosis, but underlying changes in the gut have not been examined. In the present work, we examined changes in intestinal function during metamorphosis and seawater exposure of sea lamprey (Petromyzon marinus). Fully metamorphosed juvenile sea lamprey had 100% survival after direct exposure to 35 parts per thousand seawater (SW) and only slight elevations in plasma chloride (Cl^-) levels. Drinking rates of sea lamprey juveniles in seawater were 26-fold higher than juveniles in freshwater (FW). Na⁺-K⁺-ATPase (NKA) activity in the anterior and posterior intestine increased 12- and 3-fold, respectively, during metamorphosis, whereas esophageal NKA activity was lower than in the intestine and did not change with development. Acclimation to SW significantly enhanced NKA activity in the posterior intestine but did not significantly change NKA activity in the anterior intestine, which remained higher than that in the posterior intestine. Intestinal Cl^- and water uptake, which were observed in ex vivo preparations of anterior and posterior intestine under both symmetric and asymmetric conditions, were higher in juveniles than in larvae and were similar in magnitude of those of teleost fish. Inhibition of NKA by ouabain in ex vivo preparations inhibited intestinal water absorption by 64%. Our results indicate drinking and intestinal ion and water absorption are important to osmoregulation in SW and that preparatory increases in intestinal NKA activity are important to the development of salinity tolerance that occurs during sea lamprey metamorphosis.

Auto-Adhesion Potential of Extraocular Aqp0 during Teleost Development

AQP0 water channels are the most abundant proteins expressed in the mammalian lens fiber membranes where they are essential for lens development and transparency. Unlike other aquaporin paralogs, mammalian AQP0 has a low intrinsic water permeability, but can form cell-to-cell junctions between the lens fibers. It is not known whether the adhesive properties of AQP0 is a derived feature found only in mammals, or exists as a conserved ancestral trait in non-mammalian vertebrates. Here we show that a tetraploid teleost, the Atlantic salmon, expresses four Aqp0 paralogs in the developing lens, but also expresses significant levels of aqp0 mRNAs and proteins in the epithelia of the pronephros, presumptive enterocytes, gill filament and epidermis. Quantitative PCR reveals that aqp0 mRNA titres increase by three orders of magnitude between the onset of somitogenesis and pigmentation of the eye. Using in situ hybridization and specific antisera, we show that at least two of the channels (Aqp0a1, -0b1 and/or -0b2) are localized in the extraocular basolateral and apical membranes, while Aqp0a2 is lens-specific. Heterologous expression of the Aqp0 paralogs in adhesion-deficient mouse fibolast L-cells reveals that, as for human AQP0, each intact salmon channel retains cell-to-cell adhesive properties. The strongest Aqp0 interactions are auto-adhesion, suggesting that homo-octamers likely form the intercellular junctions of the developing lens and epithelial tissues. The present data are thus the first to show the adhesion potential of Aqp0 channels in a non-mammalian vertebrate, and further uncover a novel extraocular role of the channels during vertebrate development.

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

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

Phylogenomic and functional analyses of salmon lice aquaporins uncover the molecular diversity of the superfamily in Arthropoda

BACKGROUND

An emerging field in biomedical research is focusing on the roles of aquaporin water channels in parasites that cause debilitating or lethal diseases to their vertebrate hosts. The primary vectorial agents are hematophagous arthropods, including mosquitoes, flies, ticks and lice, however very little is known concerning the functional diversity of aquaporins in non-insect members of the Arthropoda. Here we conducted phylogenomic and functional analyses of aquaporins in the salmon louse, a marine ectoparasitic copepod that feeds on the skin and body fluids of salmonids, and used the primary structures of the isolated channels to uncover the genomic repertoires in Arthropoda.

RESULTS

Genomic screening identified 7 aquaporin paralogs in the louse in contrast to 42 in its host the Atlantic salmon. Phylogenetic inference of the louse nucleotides and proteins in relation to orthologs identified in Chelicerata, Myriapoda, Crustacea and Hexapoda revealed that the arthropod aquaporin superfamily can be classified into three major grades (1) classical aquaporins including Big brain (Bib) and Prip-like (PripL) channels (2) aquaglyceroporins (Glp) and (3) unorthodox aquaporins (Aqp12-like). In Hexapoda, two additional subfamilies exist as Drip and a recently classified entomoglyceroporin (Eglp) group. Cloning and remapping the louse cDNAs to the genomic DNA revealed that they are encoded by 1-7 exons, with two of the Glps being expressed as N-terminal splice variants (Glp1_v1, -1_v2, -3_v1, -3_v2). Heterologous expression of the cRNAs in amphibian oocytes demonstrated that PripL transports water and urea, while Bib does not. Glp1_v1, -2, -3_v1 and -3_v2 each transport water, glycerol and urea, while Glp1_v2 and the Aqp12-like channels were retained intracellularly. Transcript abundance analyses revealed expression of each louse paralog at all developmental stages, except for glp1_v1, which is specific to preadult and adult males.

CONCLUSIONS

Our data suggest that the aquaporin repertoires of extant arthropods have expanded independently in the different lineages, but can be phylogenetically classified into three major grades as opposed to four present in deuterostome animals. While the aquaporin repertoire of Atlantic salmon represents a 6-fold redundancy compared to the louse, the functional assays reveal that the permeation properties of the different crustacean grades of aquaporin are largely conserved to the vertebrate counterparts.

Water transport and functional dynamics of aquaporins in osmoregulatory organs of fishes

Aquaporins play distinct roles for water transport in fishes as they do in mammals-both at the cellular, organ, and organismal levels. However, with over 32,000 known species of fishes inhabiting almost every aquatic environment, from tidal pools, small mountain streams, to the oceans and extreme salty desert lakes, the challenge to obtain consensus as well as specific knowledge about aquaporin physiology in these vertebrate clades is overwhelming. Because the integumental surfaces of these animals are in intimate contact with the surrounding milieu, passive water loss and uptake represent two of the major osmoregulatory challenges that need compensation. However, neither obligatory nor regulatory water transport nor their mechanisms have been elucidated to the same degree as, for example, ion transport in fishes. Currently fewer than 60 papers address fish aquaporins. Most of these papers identify "what is present" and describe tissue expression patterns in various teleosts. The agnathans, chondrichthyans, and functionality of fish aquaporins generally have received little attention. This review emphasizes the functional physiology of aquaporins in fishes, focusing on transepithelial water transport in osmoregulatory organs in euryhaline species - primarily teleosts, but covering other taxonomic groups as well. Most current knowledge comes from teleosts, and there is a strong need for related information on older fish clades. Our survey aims to stimulate new, original research in this area and to bring together new collaborations across disciplines.

Sexual maturation and changes in water and salt transport components in the kidney and intestine of three-spined stickleback (Gasterosteus aculeatus L.)

Mature three-spined stickleback males use spiggin threads secreted from their kidney to glue together nest material. This requires strongly hypertrophied renal proximal tubular cells, which compromises renal osmoregulatory function during the breeding period. Experimental evidence suggests that the intestine takes over hypotonic fluid secretion at that stage but the mechanism is unexplored. To unravel the molecular mechanism we analyzed and compared transcript levels of several membrane proteins involved in water and salt transport in intestinal and renal tissues, in non-mature males (NM), mature males (MM), and mature females (MF). Aquaporin paralogs aqp1a, -3a, -8aa, -8ab, -10a, and -10b, two Na(+),K(+)-ATPase alpha-1 subunit isoforms (nka547, nka976), Na(+),K(+),2Cl(-)-, and Na(+),Cl(-)-cotransporters (nkcc1a, nkcc2, ncc), the cystic fibrosis transmembrane conductance regulator (cftr) and two claudin isoforms (cldn2, cldn15a) were expressed in the intestine and kidney in all groups. There were no differences in aqp and cldn expression between intestines of NM and MM; nkcc2 was lower and nka levels tended to be higher in intestines of MM than in NM. In the kidney, aqp1 and aqp8ab levels were lower in MM than in NM, whereas aqp3a, nkcc1a, cldn15a, and spiggin were markedly elevated. This was accompanied by marked hypertrophy of kidney tubules in MM. The data support an altered kidney function in terms of water handling in mature males, whereas there was no support for modified trans-epithelial water permeability or salt-secretory activity in the intestine of mature males. Salt-absorptive activity in the intestine may, however, be down-regulated during male maturation.

The pH sensitivity of Aqp0 channels in tetraploid and diploid teleosts

Water homeostasis and the structural integrity of the vertebrate lens is partially mediated by AQP0 channels. Emerging evidence indicates that external pH may be involved in channel gating. Here we show that a tetraploid teleost, the Atlantic salmon, retains 4 aqp0 genes (aqp0a1, -0a2, -0b1, and -0b2), which are highly, but not exclusively, expressed in the lens. Functional characterization reveals that, although each paralog permeates water efficiently, the permeability is respectively shifted to the neutral, alkaline, or acidic pH in Aqp0a1, -0a2, and -0b1, whereas that of Aqp0b2 is not regulated by external pH. Mutagenesis studies demonstrate that Ser(38), His(39), and His(40) residues in the extracellular transmembrane domain of α-helix 2 facing the water pore are critical for the pH modulation of water transport. To validate these findings, we show that both zebrafish Aqp0a and -0b are functional water channels with respective pH sensitivities toward alkaline or acid pH ranges and that an N-terminal allelic variant (Ser(19)) of Aqp0b exists that abolishes water transport in Xenopus laevis oocytes. The data suggest that the alkaline pH sensitivity is a conserved trait in teleost Aqp0 a-type channels, whereas mammalian AQP0 and some teleost Aqp0 b-type channels display an acidic pH permeation preference.

Tubular localization and expressional dynamics of aquaporins in the kidney of seawater-challenged Atlantic salmon

Most vertebrate nephrons possess an inherited ability to secrete fluid in normal or pathophysiological states. We hypothesized that renal aquaporin expression and localization are functionally regulated in response to seawater and during smoltification in Atlantic salmon and thus reflect a shift in renal function from filtration towards secretion. We localized aquaporins (Aqp) in Atlantic salmon renal tubular segments by immunohistochemistry and monitored their expressional dynamics using RT-PCR and immunoblotting. Three aquaporins: Aqpa1aa, Aqp1ab and Aqp8b and two aquaglyceroporins Aqp3a and Aqp10b were localized in the kidney of salmon. The staining for all aquaporins was most abundant in the proximal kidney tubules and there was no clear effect of salinity or developmental stage on localization pattern. Aqp1aa and Aqp3a were abundant apically but extended throughout the epithelial cells. Aqp10b was expressed apically and along the lateral membrane. Aqp8b was mainly basolateral and Aqp1ab was located in sub-apical intracellular compartments. mRNAs of aqp8b and aqp10b were higher in FW smolts compared to FW parr, whereas the opposite was true for aqp1aa. Aqp mRNA levels changed in response to both SW and sham transfer. Protein levels, however, were stable for most paralogs. In conclusion, aquaporins are abundant in salmon proximal renal tubules and may participate in water secretion and thus urine modification as suggested for other vertebrates. Further studies should seek to couple functional measurements of single nephrons to expression and localization of Aqps in the salmonid kidney.

Development of intestinal ion-transporting mechanisms during smoltification and seawater acclimation in Atlantic salmon Salmo salar

This study investigated the expression of ion transporters involved in intestinal fluid absorption and presents evidence for developmental changes in abundance and tissue distribution of these transporters during smoltification and seawater (SW) acclimation of Atlantic salmon Salmo salar. Emphasis was placed on Na(+) , K(+) -ATPase (NKA) and Na(+) , K(+) , Cl(-) co-transporter (NKCC) isoforms, at both transcriptional and protein levels, together with transcription of chloride channel genes. The nka α1c was the dominant isoform at the transcript level in both proximal and distal intestines; also, it was the most abundant isoform expressed in the basolateral membrane of enterocytes in the proximal intestine. This isoform was also abundantly expressed in the distal intestine in the lower part of the mucosal folds. The protein expression of intestinal Nkaα1c increased during smoltification. Immunostaining was localized to the basal membrane of the enterocytes in freshwater (FW) fish, and re-distributed to a lateral position after SW entry. Two other Nka isoforms, α1a and α1b, were expressed in the intestine but were not regulated to the same extent during smoltification and subsequent SW transfer. Their localization in the intestinal wall indicates a house-keeping function in excitatory tissues. The absorptive form of the NKCC-like isoform (sub-apically located NKCC2 and/or Na(+) , Cl(-) co-transporter) increased during smoltification and further after SW transfer. The cellular distribution changed from a diffuse expression in the sub-apical regions during smoltification to clustering of the transporters closer to the apical membrane after entry to SW. Furthermore, transcript abundance indicates that the mechanisms necessary for exit of chloride ions across the basolateral membrane and into the lateral intercellular space are present in the form of one or more of three different chloride channels: cystic fibrosis transmembrane conductance regulator I and II and chloride channel 3.

Aquaporin expression in the Japanese medaka (Oryzias latipes) in freshwater and seawater: challenging the paradigm of intestinal water transport?

We investigated the salinity-dependent expression dynamics of seven aquaporin paralogs (aqp1a, aqp3a, aqp7, aqp8ab, aqp10a, aqp10b and aqp11a) in several tissues of euryhaline Japanese medaka (Oryzias latipes). All paralogs except aqp7 and aqp10a had a broad tissue distribution, and several were affected by salinity in both osmoregulatory and non-osmoregulatory tissues. In the intestine, aqp1a, aqp7, aqp8ab and aqp10a decreased upon seawater (SW) acclimation in both long-term acclimated fish and during 1-3 days of the transition period. In the gill, aqp3a was lower and aqp10a higher in SW than in freshwater (FW). In the kidney no aqps were affected by salinity. In the skin, aqp1a and aqp3a were lower in SW than in FW. In the liver, aqp8ab and aqp10a were lower in SW than in FW. Furthermore, six Na(+),K(+)-ATPase α-subunit isoform transcripts were analysed in the intestine but none showed a consistent response to salinity, suggesting that water transport is not regulated at this level. In contrast, mRNA of the Na(+),K(+),2Cl(-)-cotransporter type-2 strongly increased in the intestine in SW compared with FW fish. Using custom-made antibodies, Aqp1a, Aqp8ab and Aqp10a were localized in the apical region of enterocytes of FW fish. Apical staining intensity strongly decreased, vanished or moved to subapical regions, when fish were acclimated to SW, supporting the lower mRNA expression in SW. Western blots confirmed the decrease in Aqp1a and Aqp10a in SW. The strong decrease in aquaporin expression in the intestine of SW fish is surprising, and challenges the paradigm for transepithelial intestinal water absorption in SW fishes.

Isolation and mRNA expression analysis of aquaporin isoforms in marine medaka Oryzias dancena, a euryhaline teleost

We have identified six putative aquaporin (AQP) genes from marine medaka Oryzias dancena (named odAQPs 1, 3, 8, 10, 11 and 12). The marine medaka AQP cDNAs encode polypeptides of 259-298 amino acids, respectively. Topology predictions showed six transmembrane domains, five connecting loops, and cytoplasmic N- and C-terminal domains, all of which is conserved among AQP molecules. Although asparagine-proline-alanine (NPA) motifs are highly conserved in most odAQP isoforms, several AQPs revealed variant types of motifs such as asparagine-proline-proline (NPP), asparagine-proline-valine (NPV) or/and asparagine-proline-serine (NPS) motifs. The phylogenic analysis showed that marine medaka AQPs had closet relationship with Japanese ricefish (medaka; Oryzias latipes) counterparts. Reverse transcription (RT)-PCR analyses showed that marine medaka AQP transcripts would be expressed in not only osmoregulatory tissues but also nonosmoregulatory tissues, and also that the expression levels of certain AQP isoforms in nonosmoregulatory tissues were readily comparable or even higher than those in typically known osmoregulatory organs. Although the overall tissue distribution patterns of AQPs were not significantly different between 0- and 30-ppt acclimated fish, the expression levels under different salinities were largely variable among isoforms and tissues. This is the first report to investigate tissue expression profiles of teleostean AQPs 11 and 12 during the long-term acclimation to freshwater and salted water.

The role of aquaporin and tight junction proteins in the regulation of water movement in larval zebrafish (Danio rerio)

Teleost fish living in freshwater are challenged by passive water influx; however the molecular mechanisms regulating water influx in fish are not well understood. The potential involvement of aquaporins (AQP) and epithelial tight junction proteins in the regulation of transcellular and paracellular water movement was investigated in larval zebrafish (Danio rerio). We observed that the half-time for saturation of water influx (K_u) was 4.3±0.9 min, and reached equilibrium at approximately 30 min. These findings suggest a high turnover rate of water between the fish and the environment. Water influx was reduced by the putative AQP inhibitor phloretin (100 or 500 μM). Immunohistochemistry and confocal microscopy revealed that AQP1a1 protein was expressed in cells on the yolk sac epithelium. A substantial number of these AQP1a1-positive cells were identified as ionocytes, either H⁺-ATPase-rich cells or Na⁺/K⁺-ATPase-rich cells. AQP1a1 appeared to be expressed predominantly on the basolateral membranes of ionocytes, suggesting its potential involvement in regulating ionocyte volume and/or water flux into the circulation. Additionally, translational gene knockdown of AQP1a1 protein reduced water influx by approximately 30%, further indicating a role for AQP1a1 in facilitating transcellular water uptake. On the other hand, incubation with the Ca²⁺-chelator EDTA or knockdown of the epithelial tight junction protein claudin-b significantly increased water influx. These findings indicate that the epithelial tight junctions normally act to restrict paracellular water influx. Together, the results of the present study provide direct in vivo evidence that water movement can occur through transcellular routes (via AQP); the paracellular routes may become significant when the paracellular permeability is increased.

Developmental and environmental regulation of Aquaporin gene expression across Populus species: divergence or redundancy?

Aquaporins (AQPs) are membrane channels belonging to the major intrinsic proteins family and are known for their ability to facilitate water movement. While in Populus trichocarpa, AQP proteins form a large family encompassing fifty-five genes, most of the experimental work focused on a few genes or subfamilies. The current work was undertaken to develop a comprehensive picture of the whole AQP gene family in Populus species by delineating gene expression domain and distinguishing responsiveness to developmental and environmental cues. Since duplication events amplified the poplar AQP family, we addressed the question of expression redundancy between gene duplicates. On these purposes, we carried a meta-analysis of all publicly available Affymetrix experiments. Our in-silico strategy controlled for previously identified biases in cross-species transcriptomics, a necessary step for any comparative transcriptomics based on multispecies design chips. Three poplar AQPs were not supported by any expression data, even in a large collection of situations (abiotic and biotic constraints, temporal oscillations and mutants). The expression of 11 AQPs was never or poorly regulated whatever the wideness of their expression domain and their expression level. Our work highlighted that PtTIP1;4 was the most responsive gene of the AQP family. A high functional divergence between gene duplicates was detected across species and in response to tested cues, except for the root-expressed PtTIP2;3/PtTIP2;4 pair exhibiting 80% convergent responses. Our meta-analysis assessed key features of aquaporin expression which had remained hidden in single experiments, such as expression wideness, response specificity and genotype and environment interactions. By consolidating expression profiles using independent experimental series, we showed that the large expansion of AQP family in poplar was accompanied with a strong divergence of gene expression, even if some cases of functional redundancy could be suspected.

Differential expression and novel permeability properties of three aquaporin 8 paralogs from seawater-challenged Atlantic salmon smolts

Aquaporins may facilitate transepithelial water absorption in the intestine of seawater (SW) acclimated fish. Here we have characterized three full-length aqp8 paralogs from Atlantic salmon (Salmo salar). Bayesian inference revealed that each paralog is a representative of the three major classes of aqp8aa, aqp8ab and aqp8b genes found in other teleosts. The permeability properties were studied by heterologous expression in Xenopus laevis oocytes, and the expression levels examined by qPCR, immunofluorescence and immunoelectron microscopy, and immunoblotting of membrane fractions from intestines of SW challenged smolts. All three Aqp8 paralogs were permeable to water and urea, whereas Aqp8ab and -8b were, surprisingly, also permeable to glycerol. The mRNA tissue distribution of each paralog was distinct although some tissues, such as the intestine showed redundant expression of more than one paralog. Immunofluorescence microscopy localized Aqp8aa(1+2) to intracellular compartments of the liver and intestine, and Aqp8ab and Aqp8b to apical plasma membrane domains of the intestinal epithelium, with Aqp8b also in goblet cells. In a control experiment with rainbow trout, immunoelectron microscopy confirmed abundant labeling of Aqp8ab and -8b at apical plasma membranes of enterocytes in the middle intestine and also in subapical vesicular structures. During SW-challenge, Aqp8ab showed significantly increased levels of protein expression in plasma membrane enriched fractions of the intestine. These data indicate that the Atlantic salmon Aqp8 paralogs have neofunctionalized on a transcriptional as well as on a functional level, and that Aqp8ab may play a central role in the intestinal transcellular uptake of water during SW acclimation.

Intestinal fluid absorption in anadromous salmonids: importance of tight junctions and aquaporins

The anadromous salmonid life cycle includes both fresh water (FW) and seawater (SW) stages. The parr-smolt transformation (smoltification) pre-adapt the fish to SW while still in FW. The osmoregulatory organs change their mode of action from a role of preventing water inflow in FW, to absorb ions to replace water lost by osmosis in SW. During smoltification, the drinking rate increases, in the intestine the ion and fluid transport increases and is further elevated after SW entry. In SW, the intestine absorbs ions to create an inwardly directed water flow which is accomplished by increased Na⁺, K⁺-ATPase (NKA) activity in the basolateral membrane, driving ion absorption via ion channels and/or co-transporters. This review will aim at discussing the expression patterns of the ion transporting proteins involved in intestinal fluid absorption in the FW stage, during smoltification and after SW entry. Of equal importance for intestinal fluid absorption as the active absorption of ions is the permeability of the epithelium to ions and water. During the smoltification the increase in NKA activity and water uptake in SW is accompanied by decreased paracellular permeability suggesting a redirection of the fluid movement from a paracellular route in FW, to a transcellular route in SW. Increased transcellular fluid absorption could be achieved by incorporation of aquaporins (AQPs) into the enterocyte membranes and/or by a change in fatty acid profile of the enterocyte lipid bilayer. An increased incorporation of unsaturated fatty acids into the membrane phospholipids will increase water permeability by enhancing the fluidity of the membrane. A second aim of the present review is therefore to discuss the presence and regulation of expression of AQPs in the enterocyte membrane as well as to discuss the profile of fatty acids present in the membrane phospholipids during different stages of the salmonid lifecycle.