Identification of intestinal bicarbonate transporters involved in formation of carbonate precipitates to stimulate water absorption in marine teleost fish

slc26a12-A novel member of the slc26 family, is located in tandem with slc26a2 in coelacanths, amphibians, reptiles, and birds

Solute carrier family 26 (Slc26) is a family of anion exchangers with 11 members in mammals (named Slc26a1-a11). Here, we identified a novel member of the slc26 family, slc26a12, located in tandem with slc26a2 in the genomes of several vertebrate lineages. BLAST and synteny analyses of various jawed vertebrate genome databases revealed that slc26a12 is present in coelacanths, amphibians, reptiles, and birds but not in cartilaginous fishes, lungfish, mammals, or ray-finned fishes. In some avian and reptilian lineages such as owls, penguins, egrets, and ducks, and most turtles examined, slc26a12 was lost or pseudogenized. Phylogenetic analysis showed that Slc26a12 formed an independent branch with the other Slc26 members and Slc26a12, Slc26a1 and Slc26a2 formed a single branch, suggesting that these three members formed a subfamily in Slc26. In jawless fish, hagfish have two genes homologous to slc26a2 and slc26a12, whereas lamprey has a single gene homologous to slc26a2. African clawed frogs express slc26a12 in larval gills, skin, and fins. These results show that slc26a12 was present at least before the separation of lobe-finned fish and tetrapods; the name slc26a12 is appropriate because the gene duplication occurred in the distant past.

Bicarbonate secretion and acid/base sensing by the intestine

The transport of bicarbonate across the enterocyte cell membrane regulates the intracellular as well as the luminal pH and is an essential part of directional fluid movement in the gut. Since the first description of "active" transport of HCO3- ions against a concentration gradient in the 1970s, the fundamental role of HCO3- transport for multiple intestinal functions has been recognized. The ion transport proteins have been identified and molecularly characterized, and knockout mouse models have given insight into their individual role in a variety of functions. This review describes the progress made in the last decade regarding novel techniques and new findings in the molecular regulation of intestinal HCO3- transport in the different segments of the gut. We discuss human diseases with defects in intestinal HCO3- secretion and potential treatment strategies to increase luminal alkalinity. In the last part of the review, the cellular and organismal mechanisms for acid/base sensing in the intestinal tract are highlighted.

The role of Na+-coupled bicarbonate transporters (NCBT) in health and disease

Cellular and organism survival depends upon the regulation of pH, which is regulated by highly specialized cell membrane transporters, the solute carriers (SLC) (For a comprehensive list of the solute carrier family members, see: https://www.bioparadigms.org/slc/ ). The SLC4 family of bicarbonate (HCO3-) transporters consists of ten members, sorted by their coupling to either sodium (NBCe1, NBCe2, NBCn1, NBCn2, NDCBE), chloride (AE1, AE2, AE3), or borate (BTR1). The ionic coupling of SLC4A9 (AE4) remains controversial. These SLC4 bicarbonate transporters may be controlled by cellular ionic gradients, cellular membrane voltage, and signaling molecules to maintain critical cellular and systemic pH (acid-base) balance. There are profound consequences when blood pH deviates even a small amount outside the normal range (7.35-7.45). Chiefly, Na+-coupled bicarbonate transporters (NCBT) control intracellular pH in nearly every living cell, maintaining the biological pH required for life. Additionally, NCBTs have important roles to regulate cell volume and maintain salt balance as well as absorption and secretion of acid-base equivalents. Due to their varied tissue expression, NCBTs have roles in pathophysiology, which become apparent in physiologic responses when their expression is reduced or genetically deleted. Variations in physiological pH are seen in a wide variety of conditions, from canonically acid-base related conditions to pathologies not necessarily associated with acid-base dysfunction such as cancer, glaucoma, or various neurological diseases. The membranous location of the SLC4 transporters as well as recent advances in discovering their structural biology makes them accessible and attractive as a druggable target in a disease context. The role of sodium-coupled bicarbonate transporters in such a large array of conditions illustrates the potential of treating a wide range of disease states by modifying function of these transporters, whether that be through inhibition or enhancement.

Role of the kidneys in acid-base regulation and ammonia excretion in freshwater and seawater fish: implications for nephrocalcinosis

Maintaining normal pH levels in the body fluids is essential for homeostasis and represents one of the most tightly regulated physiological processes among vertebrates. Fish are generally ammoniotelic and inhabit diverse aquatic environments that present many respiratory, acidifying, alkalinizing, ionic and osmotic stressors to which they are able to adapt. They have evolved flexible strategies for the regulation of acid-base equivalents (H⁺, NH₄ ⁺, OH^- and HCO₃ ^-), ammonia and phosphate to cope with these stressors. The gills are the main regulatory organ, while the kidneys play an important, often overlooked accessory role in acid-base regulation. Here we outline the kidneys role in regulation of acid-base equivalents and two of the key 'urinary buffers', ammonia and phosphate, by integrating known aspects of renal physiology with recent advances in the molecular and cellular physiology of membrane transport systems in the teleost kidneys. The renal transporters (NHE3, NBC1, AE1, SLC26A6) and enzymes (V-type H⁺ATPase, CAc, CA IV, ammoniagenic enzymes) involved in H⁺ secretion, bicarbonate reabsorption, and the net excretion of acidic and basic equivalents, ammonia, and inorganic phosphate are addressed. The role of sodium-phosphate cotransporter (Slc34a2b) and rhesus (Rh) glycoproteins (ammonia channels) in conjunction with apical V-type H⁺ ATPase and NHE3 exchangers in these processes are also explored. Nephrocalcinosis is an inflammation-like disorder due to the precipitation of calcareous material in the kidneys, and is listed as one of the most prevalent pathologies in land-based production of salmonids in recirculating aquaculture systems. The causative links underlying the pathogenesis and etiology of nephrocalcinosis in teleosts is speculative at best, but acid-base perturbation is probably a central pathophysiological cause. Relevant risk factors associated with nephrocalcinosis are hypercapnia and hyperoxia in the culture water. These raise internal CO₂ levels in the fish, triggering complex branchial and renal acid-base compensations which may promote formation of kidney stones. However, increased salt loads through the rearing water and the feed may increase the prevalence of nephrocalcinosis. An increased understanding of the kidneys role in acid-base and ion regulation and how this relates to renal diseases such as nephrocalcinosis will have applied relevance for the biologist and aquaculturist alike.

Metabolic cost of osmoregulation by the gastro-intestinal tract in marine teleost fish

Introduction: Although dozens of studies have attempted to determine the metabolic cost of osmoregulation, mainly by comparing standard metabolic rates (SMR) in fish acclimated to different salinities, consensus is still lacking. Methods: In the present study, using the Gulf toadfish, Opsanus beta, we aimed to determine the metabolic cost of esophageal and intestinal osmoregulatory processes by estimating ATP consumption from known ion transport rates and pathways and comparing these estimates with measurements on isolated tissues. Further, we performed whole animal respirometry on fish acclimated to 9, 34 and 60 ppt. Results and Discussion: Our theoretical estimates of esophageal and intestinal osmoregulatory costs were in close agreement with direct measurements on isolated tissues and suggest that osmoregulation by these tissues amounts to ∼2.5% of SMR. This value agrees well with an earlier attempt to estimate osmoregulation cost from ion transport rates and combined with published measurements of gill osmoregulatory costs suggests that whole animal costs of osmoregulation in marine teleosts is ∼7.5% of SMR. As in many previous studies, our whole animal measurements were variable between fish and did not seem suited to determine osmoregulatory costs. While the esophagus showed constant metabolic rate regardless of acclimation salinity, the intestine of fish acclimated to higher salinities showed elevated metabolic rates. The esophagus and the intestine had 2.1 and 3.2-fold higher metabolic rates than corresponding whole animal mass specific rates, respectively. The intestinal tissue displays at least four different Cl- uptake pathways of which the Na+:Cl-:2 K+ (NKCC) pathway accounts for 95% of the Cl- uptake and is the most energy efficient. The remaining pathways are via apical anion exchange and seem to primarily serve luminal alkalinization and the formation of intestinal CaCO3 which is essential for water absorption.

Role of the luminal composition on intestinal metal toxicity, bioavailability and bioreactivity: An in vitro approach based on the cell line RTgutGC

The bioavailability of metal complexes is poorly understood. To evaluate bioavailability and toxicity of neutral and charged complexes as well as free metal ions, Visual Minteq, a chemical equilibrium model, was used to design media containing different metal species. Two non-essential (silver and cadmium) and two essential (copper and zinc) metals were selected. The rainbow trout (Oncorhynchus mykiss) gut cell line (RTgutGC) was used to investigate bioavailability, bioreactivity and toxicity of the different metal species. Toxicity was measured using a multiple endpoint cytotoxicity assay, bioavailability by measuring intracellular metal concentration, and bioreactivity by quantification of mRNA level of the metal responsive genes, metallothionein (MT), glutathione reductase (GR) and zinc transporter 1 (ZnT1). Speciation calculations showed that silver and cadmium preferentially bind chloride, copper phosphate and bicarbonate, and zinc remained primarily as a free ion. Cysteine avidly complexed with all metals reducing toxicity, bioavailability and bioreactivity. Silver and copper toxicity was not affected by inorganic metal speciation, whereas cadmium and zinc toxicity was decreased by chloride complexation. Moreover, reduction of calcium concentration in the medium increased toxicity and bioavailability of cadmium and zinc. Bioavailability of silver and zinc was reduced by low chloride while cadmium bioavailability was increased by low chloride and in presence of bicarbonate. Copper bioavailability was not affected by the medium composition. Cadmium and silver were more bioreactive, independently from the medium composition, in comparison to copper and zinc (i.e., higher induction of MT and GR). Cadmium was the only metal able to induce MT in presence of cysteine. ZnT1 was induced by cadmium in low-chloride, by zinc in low-chloride low-calcium and by cadmium and copper in the bicarbonate media. Overall, this study demonstrates that metal complexation alone is not sufficient to explain metal toxicity, and that anion exchange mechanisms play a role in metal uptake and bioreactivity.

Adaptive divergence and underlying mechanisms in response to salinity gradients between two Crassostrea oysters revealed by phenotypic and transcriptomic analyses

Comparing the responses of closely related species to environmental changes is an efficient method to explore adaptive divergence, for a better understanding of the adaptive evolution of marine species under rapidly changing climates. Oysters are keystone species thrive in intertidal and estuarine areas where frequent environmental disturbance occurs including fluctuant salinity. The evolutionary divergence of two sister species of sympatric estuarine oysters, Crassostrea hongkongensis and Crassostrea ariakensis, in response to euryhaline habitats on phenotypes and gene expression, and the relative contribution of species effect, environment effect, and their interaction to the divergence were explored. After a 2-month outplanting at high- and low-salinity locations in the same estuary, the high growth rate, percent survival, and high tolerance indicated by physiological parameters suggested that the fitness of C. ariakensis was higher under high-salinity conditions and that of C. hongkongensis was higher under low-salinity conditions. Moreover, a transcriptomic analysis showed the two species exhibited differentiated transcriptional expression in high- and low-salinity habitats, largely caused by the species effect. Several of the important pathways enriched in divergent genes between species were also salinity-responsive pathways. Specifically, the pyruvate and taurine metabolism pathway and several solute carriers may contribute to the hyperosmotic adaptation of C. ariakensis, and some solute carriers may contribute to the hypoosmotic adaptation of C. hongkongensis. Our findings provide insights into the phenotypic and molecular mechanisms underlying salinity adaptation in marine mollusks, which will facilitate the assessment of the adaptive capacity of marine species in the context of climate change and will also provide practical information for marine resource conservation and aquaculture.

Current opinion on the regulation of small intestinal magnesium absorption

Magnesium (Mg²⁺) has an important role in numerous biological functions, and Mg²⁺ deficiency is associated with several diseases. Therefore, adequate intestinal absorption of Mg²⁺ is vital for health. The small intestine was previously thought to absorb digested Mg²⁺ exclusively through an unregulated paracellular mechanism, which is responsible for approximately 90% of total Mg²⁺ absorption. Recent studies, however, have revealed that the duodenum, jejunum, and ileum absorb Mg²⁺ through both transcellular and paracellular routes. Several regulatory factors of small intestinal Mg²⁺ uptake also have been explored, e.g., parathyroid hormone, fibroblast growth factor-23, apical acidity, proton pump inhibitor, and pH-sensing channel and receptors. The mechanistic factors underlying proton pump inhibitor suppression of small intestinal Mg²⁺, such as magnesiotropic protein dysfunction, higher mucosal bicarbonate secretion, Paneth cell dysfunction, and intestinal inflammation, are currently being explored. The potential role of small intestinal microbiomes in Mg²⁺ absorption has also been proposed. In this article, we reviewed the current knowledge on the mechanisms and regulatory factors of small intestinal Mg²⁺ absorption.

Seawater fish use an electrogenic boric acid transporter, Slc4a11A, for boric acid excretion by the kidney

Boric acid is a vital micronutrient in animals; however, excess amounts are toxic to them. Little is known about whole-body boric acid homeostasis in animals. Seawater (SW) contains 0.4 mM boric acid, and since marine fish drink SW, their urinary system was used here as a model of the boric acid excretion system. We determined that the bladder urine of a euryhaline pufferfish (river pufferfish, Takifugu obscurus) acclimated to fresh water and SW contained 0.020 and 19 mM of boric acid, respectively (a 950-fold difference), indicating the presence of a powerful excretory renal system for boric acid. Slc4a11 is a potential animal homolog of the plant boron transporter BOR1; however, mammalian Slc4a11 mediates H+ (OH-) conductance but does not transport boric acid. We found that renal expression of the pufferfish paralog of Slc4a11, Slc4a11A, was markedly induced after transfer from fresh water to SW, and Slc4a11A was localized to the apical membrane of kidney tubules. When pufferfish Slc4a11A was expressed in Xenopus oocytes, exposure to media containing boric acid and a voltage clamp elicited whole-cell outward currents, a marked increase in pHi, and increased boron content. In addition, the activity of Slc4a11A was independent of extracellular Na+. These results indicate that pufferfish Slc4a11A is an electrogenic boric acid transporter that functions as a B(OH)4- uniporter, B(OH)3-OH- cotransporter, or B(OH)3/H+ exchanger. These observations suggest that Slc4a11A is involved in the kidney tubular secretion of boric acid in SW fish, probably induced by the negative membrane potential and low pH of urine.

Na+/HCO3- cotransporter 1 (nbce1) isoform gene expression during smoltification and seawater acclimation of Atlantic salmon

The life history of Atlantic salmon (Salmo salar) includes an initial freshwater phase (parr) that precedes a springtime migration to marine environments as smolts. The development of osmoregulatory systems that will ultimately support the survival of juveniles upon entry into marine habitats is a key aspect of smoltification. While the acquisition of seawater tolerance in all euryhaline species demands the concerted activity of specific ion pumps, transporters, and channels, the contributions of Na⁺/HCO₃^- cotransporter 1 (Nbce1) to salinity acclimation remain unresolved. Here, we investigated the branchial and intestinal expression of three Na⁺/HCO₃^- cotransporter 1 isoforms, denoted nbce1.1, -1.2a, and -1.2b. Given the proposed role of Nbce1 in supporting the absorption of environmental Na⁺ by ionocytes, we first hypothesized that expression of a branchial nbce1 transcript (nbce1.2a) would be attenuated in salmon undergoing smoltification and following seawater exposure. In two separate years, we observed spring increases in branchial Na⁺/K⁺-ATPase activity, Na⁺/K⁺/2Cl^- cotransporter 1, and cystic fibrosis transmembrane regulator 1 expression characteristic of smoltification, whereas there were no attendant changes in nbce1.2a expression. Nonetheless, branchial nbce1.2a levels were reduced in parr and smolts within 2 days of seawater exposure. In the intestine, gene transcript abundance for nbce1.1 increased from spring to summer in the anterior intestine, but not in the posterior intestine or pyloric caeca, and nbce1.1 and -1.2b expression in the intestine showed season-dependent transcriptional regulation by seawater exposure. Collectively, our data indicate that tissue-specific modulation of all three nbce1 isoforms underlies adaptive responses to seawater.

Ion Pathways in Biomineralization: Perspectives on Uptake, Transport, and Deposition of Calcium, Carbonate, and Phosphate

Minerals are formed by organisms in all of the kingdoms of life. Mineral formation pathways all involve uptake of ions from the environment, transport of ions by cells, sometimes temporary storage, and ultimately deposition in or outside of the cells. Even though the details of how all this is achieved vary enormously, all pathways need to respect both the chemical limitations of ion manipulation, as well as the many "housekeeping" roles of ions in cell functioning. Here we provide a chemical perspective on the biological pathways of biomineralization. Our approach is to compare and contrast the ion pathways involving calcium, phosphate, and carbonate in three very different organisms: the enormously abundant unicellular marine coccolithophores, the well investigated sea urchin larval model for single crystal formation, and the complex pathways used by vertebrates to form their bones. The comparison highlights both common and unique processes. Significantly, phosphate is involved in regulating calcium carbonate deposition and carbonate is involved in regulating calcium phosphate deposition. One often overlooked commonality is that, from uptake to deposition, the solutions involved are usually supersaturated. This therefore requires not only avoiding mineral deposition where it is not needed but also exploiting this saturated state to produce unstable mineral precursors that can be conveniently stored, redissolved, and manipulated into diverse shapes and upon deposition transformed into more ordered and hence often functional final deposits.

Sulfate homeostasis in Atlantic salmon is associated with differential regulation of salmonid-specific paralogs in gill and kidney

Sulfate ( SO 4 2 - ) regulation is challenging for euryhaline species as they deal with large fluctuations of SO 4 2 - during migratory transitions between freshwater (FW) and seawater (SW), while maintaining a stable plasma SO 4 2 - concentration. Here, we investigated the regulation and potential role of sulfate transporters in Atlantic salmon during the preparative switch from SO 4 2 - uptake to secretion. A preparatory increase in kidney and gill sodium/potassium ATPase (Nka) enzyme activity during smolt development indicate preparative osmoregulatory changes. In contrast to gill Nka activity a transient decrease in kidney Nka after direct SW exposure was observed and may be a result of reduced glomerular filtration rates and tubular flow through the kidney. In silico analyses revealed that Atlantic salmon genome comprises a single slc13a1 gene and additional salmonid-specific duplications of slc26a1 and slc26a6a, leading to new paralogs, namely the slc26a1a and -b, and slc26a6a1 and -a2. A kidney-specific increase in slc26a6a1 and slc26a1a during smoltification and SW transfer, suggests an important role of these sulfate transporters in the regulatory shift from absorption to secretion in the kidney. Plasma SO 4 2 - in FW smolts was 0.70 mM, followed by a transient increase to 1.14 ± 0.33 mM 2 days post-SW transfer, further decreasing to 0.69 ± 0.041 mM after 1 month in SW. Our findings support the vital role of the kidney in SO 4 2 - excretion through the upregulated slc26a6a1, the most likely secretory transport candidate in fish, which together with the slc26a1a transporter likely removes excess SO 4 2 - , and ultimately enable the regulation of normal plasma SO 4 2 - levels in SW.

Osmoregulatory plasticity during hypersaline acclimation in red drum, Sciaenops ocellatus

Prolonged drought and freshwater diversion are making periods of hypersalinity more common in coastal ecosystems. This is especially true in the Laguna Madre system along the Texas coast where salinities can exceed 60 g/kg. As such, the ability to tolerate hypersalinity is critical to the success of endemic species, such as the commercially important red drum (Sciaenops ocellatus). This study evaluated acclimation of red drum to hypersalinity (60 g/kg) using a direct transfer protocol. Hypersalinity exposure resulted in significant impacts on plasma osmolality and muscle water in the first 24 h, but returned to control values coincident with a significant increase in intestinal water volume. Hypersalinity acclimation resulted in significant branchial and intestinal plasticity. The gill showed significant elevated nka α1a, nkcc1 and vha (B subunit) mRNA abundance, as well as NKA enzyme activity. The posterior intestine showed a stronger plasticity signal than the anterior intestine, which included a 12-fold increase in nkcc2 mRNA abundance and significant increases in NKA and VHA enzyme activity. These changes were corroborated by a significant threefold increase in bumetanide-sensitive absorptive short circuit current. These data suggest that the dynamic regulation of NKCC2-mediated intestinal water absorption is an important compliment to HCO₃^--mediated water absorption during hypersalinity exposure and acclimation.

Osmoregulatory actions of prolactin in the gastrointestinal tract of fishes

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

Slc4 Gene Family in Spotted Sea Bass (Lateolabrax maculatus): Structure, Evolution, and Expression Profiling in Response to Alkalinity Stress and Salinity Changes

The solute carrier 4 (SLC4) family is a class of cell membranes transporters involved in base transport that plays crucial roles in diverse physiological processes. In our study, 15 slc4 genes were identified and annotated in spotted sea bass, including five members of Cl⁻/HCO₃⁻ exchangers, eight genes coding Na⁺-dependent HCO₃⁻ transporters, and two copies of Na⁺-coupled borate transporters. The gene sequence and structure, chromosomal and syntenic arrangement, phylogenetic and evolution profiles were analyzed. Results showed that the slc4 gene in teleosts obviously expanded compared with higher vertebrates, arising from teleost-specific whole genome duplication event. Most gene sites of slc4 in spotted sea bass were under strong purifying selection during evolution, while positive selection sites were only detected in slc4a1b, slc4a8, and slc4a10b. Additionally, qRT-PCR results showed that different slc4 genes exhibited distinct branchial expression patterns after alkalinity and salinity stresses, of which the strongly responsive members may play essential roles during these physiological processes. Our study provides the systemic overview of the slc4 gene family in spotted sea bass and enables a better understanding for the evolution of this family and further deciphering the biological roles in maintaining ion and acid–base homeostasis in teleosts.

A marine teleost, Opsanus beta, compensates acidosis in hypersaline water by H+ excretion or reduced HCO3- excretion rather than HCO3- uptake

Increases in ambient salinity demand parallel increases in intestinal base secretion for maintenance of osmoregulatory status, which is likely the cause of a transient acidosis following transfer of euryhaline fish from freshwater to seawater. It was predicted that transfer of the marine Gulf toadfish (Opsanus beta) from seawater (35 ppt) to hypersaline (60 ppt) seawater (HSW) would lead to a transient acidosis that would be compensated by increases in branchial acid excretion to offset the acid-base disturbance. Toadfish exposed to HSW showed a significant decrease in blood pH and [HCO₃^-] but no increase in pCO₂, followed by a full recovery after 48-96 h. A similar metabolic acidosis and recovery was found when fish were exposed to 60-ppt HCO₃^--free seawater (HEPES-buffered), which may suggest that compensation for intestinal base loss during hypersaline treatment is from gill H⁺ excretion rather than gill HCO₃^- uptake. However, we cannot rule out that reduced branchial HCO₃^- excretion contributed to an increase in net acid excretion. Since colchicine prevents full compensation, translocation of H⁺ and/or HCO₃^- transporters between cytosolic compartments and plasma membrane fractions might be involved in compensating for the hypersalinity-induced acidosis. Translocation of transporters rather than de novo synthesis may represent a faster and less energetically demanding response to rapidly fluctuating and high salinities encountered by toadfish in their natural environment.

Construction of a High-Density Genetic Map and Identification of Quantitative Trait Loci for Nitrite Tolerance in the Pacific White Shrimp (Litopenaeus vannamei)

Nitrite is a major environmental toxin in aquaculture systems that disrupts multiple physiological functions in aquatic animals. Although nitrite tolerance in shrimp is closely related to successful industrial production, few genetic studies of this trait are available. In this study, we constructed a high-density genetic map of Litopenaeus vannamei with 17,242 single nucleotide polymorphism markers spanning 6,828.06 centimorgans (cM), with an average distance of 0.4 cM between adjacent markers on 44 linkage groups (LGs). Using this genetic map, we identified two markers associated with nitrite tolerance. We then sequenced the transcriptomes of the most nitrite-tolerant and nitrite-sensitive individuals from each of four genetically distinct L. vannamei families (LV-I–4). We found 2,002, 1,983, 1,954, and 1,867 differentially expressed genes in families LV-1, LV-2, LV-3, and LV-4, respectively. By integrating QTL and transcriptomics analyses, we identified a candidate gene associated with nitrite tolerance. This gene was annotated as solute carrier family 26 member 6 (SLC26A6). RNA interference (RNAi) analysis demonstrated that SLC26A6 was critical for nitrite tolerance in L. vannamei. The present study increases our understanding of the molecular mechanisms underlying nitrite tolerance in shrimp and provides a basis for molecular-marker-assisted shrimp breeding.

Immunohistochemical characterization and change in location of branchial ionocytes after transfer from freshwater to seawater in the euryhaline obscure puffer, Takifugu obscurus

The obscure puffer Takifugu obscurus is a euryhaline fish species suitable for studying the molecular mechanism of osmoregulation. The distributional changes of branchial ionocytes were detected following the transfer from freshwater (FW) to seawater (SW) based on two main ion transporters, Na⁺/K⁺-ATPase (NKA) and Na⁺/K⁺/ 2Cl^- cotransporter 1 (NKCC1). The mRNA and protein expression levels of NKA and NKCC1 in the gills all increased rapidly in the first four days after transfer to SW. Double immunofluorescence staining showed that NKCC1 and NKA were colocalized in the branchial ionocytes and the immunoreaction of NKCC1 was stronger after transfer. Moreover, following transfer to SW, the number of lamellar ionocytes in the gills is reduced and the number of filament ionocytes is increased significantly. Taken together, these findings indicated that SW transfer of obscure puffer promotes the changes of distribution, function and size of branchial ionocytes.

The teleost fish intestine is a major oxalate-secreting epithelium

Oxalate is a common constituent of kidney stones, but the mechanism of its transport across epithelia is not well understood. With prior research on the role of the intestine focused on mammals, the present study considered oxalate handling by teleost fish. Given the osmotic challenge of seawater (SW), marine teleosts have limited scope for urinary oxalate excretion relative to freshwater (FW) taxa. The marine teleost intestine was hypothesized as the principal route for oxalate elimination, thus demanding epithelial secretion. To test this, intestinal 14C-oxalate flux was compared between FW- and SW-acclimated sailfin molly (Poecilia latipinna). In SW, oxalate was secreted at remarkable rates (367.90±22.95 pmol cm-2 h-1), which were similar following FW transfer (387.59±27.82 pmol cm-2 h-1), implying no regulation by salinity. Nevertheless, this ability to secrete oxalate at rates 15-19 times higher than the mammalian small intestine supports this proposal of the teleost gut as a major, previously unrecognized excretory pathway.

Increased intestinal carbonate precipitate abundance in the sea bream (Sparus aurata L.) in response to ocean acidification

Marine fish contribute to the carbon cycle by producing mineralized intestinal precipitates generated as by-products of their osmoregulation. Here we aimed at characterizing the control of epithelial bicarbonate secretion and intestinal precipitate presence in the gilthead sea bream in response to predicted near future increases of environmental CO₂. Our results demonstrate that hypercapnia (950 and 1800 μatm CO₂) elicits higher intestine epithelial HCO₃^- secretion ex vivo and a subsequent parallel increase of intestinal precipitate presence in vivo when compared to present values (440 μatm CO₂). Intestinal gene expression analysis in response to environmental hypercapnia revealed the up-regulation of transporters involved in the intestinal bicarbonate secretion cascade such as the basolateral sodium bicarbonate co-transporter slc4a4, and the apical anion transporters slc26a3 and slc26a6 of sea bream. In addition, other genes involved in intestinal ion uptake linked to water absorption such as the apical nkcc2 and aquaporin 1b expression, indicating that hypercapnia influences different levels of intestinal physiology. Taken together the current results are consistent with an intestinal physiological response leading to higher bicarbonate secretion in the intestine of the sea bream paralleled by increased luminal carbonate precipitate abundance and the main related transporters in response to ocean acidification.

Dynamics of Gene Expression Responses for Ion Transport Proteins and Aquaporins in the Gill of a Euryhaline Pupfish during Freshwater and High-Salinity Acclimation

Pupfishes (genus Cyprinodon) evolved some of the broadest salinity tolerances of teleost fishes, with some taxa surviving in conditions from freshwater to nearly 160 ppt. In this study, we examined transcriptional dynamics of ion transporters and aquaporins in the gill of the desert Amargosa pupfish (Cyprinodon nevadensis amargosae) during rapid salinity change. Pupfish acclimated to 7.5 ppt were exposed to freshwater (0.3 ppt), seawater (35 ppt), or hypersaline (55 ppt) conditions over 4 h and sampled at these salinities over 14 d. Plasma osmolality and Cl^- concentration became elevated 8 h after the start of exposure to 35 or 55 ppt but returned to baseline levels after 14 d. Osmolality recovery was paralleled by increased gill Na⁺/K⁺-ATPase activity and higher relative levels of messenger RNAs (mRNAs) encoding cystic fibrosis transmembrane conductance regulator (cftr) and Na⁺/K⁺/2Cl^- cotransporter-1 (nkcc1). Transcripts encoding one Na⁺-HCO₃^- cotransporter-1 isoform (nbce1.1) also increased in the gills at higher salinities, while a second isoform (nbce1.2) increased expression in freshwater. Pupfish in freshwater also had lower osmolality and elevated gill mRNAs for Na⁺/H⁺ exchanger isoform-2a (nhe2a) and V-type H⁺-ATPase within 8 h, followed by increases in Na⁺/H⁺ exchanger-3 (nhe3), carbonic anhydrase 2 (ca2), and aquaporin-3 (aqp3) within 1 d. Gill mRNAs for Na⁺/Cl^- cotransporter-2 (ncc2) also were elevated 14 d after exposure to 0.3 ppt. These results offer insights into how coordinated transcriptional responses for ion transporters in the gill facilitate reestablishment of osmotic homeostasis after changes in environmental salinity and provide evidence that the teleost gill expresses two Na⁺-HCO₃^- cotransporter-1 isoforms with different roles in freshwater and seawater acclimation.

Molecular mechanisms for intestinal HCO3− secretion and its regulation by guanylin in seawater-acclimated eels

The intestine of marine teleosts secretes HCO3− into the lumen and precipitates Ca2+ and Mg2+ in the imbibed seawater as carbonates to decrease luminal fluid osmolality and facilitate water absorption. However, hormonal regulation of HCO3−secretion is largely unknown. Here, mucosally-added guanylin (GN) increased HCO3− secretion, measured by pH-stat, across isolated seawater-acclimated eel intestine bathed in saline at pH 7.4 (5% CO2). The effect of GN on HCO3− secretion was slower than that on the short-circuit current, and the time-course of the GN effect was similar to that of bumetanide. Mucosal bumetanide and serosal 4,4’-dinitrostilbene-2,2’-disulfonic acid (DNDS) inhibited the GN effect, suggesting an involvement of apical Na+-K+-2Cl− cotransporter (NKCC2) and basolateral Cl−/HCO3− exchanger (AE)/Na+-HCO3− cotransporter (NBC) in the GN effect. As mucosal DNDS failed to inhibit the GN effect, apical DNDS-sensitive AE may not be involved. To identify molecular species of transporters involved in the GN effect, we performed RNA-seq analyses followed by quantitative real-time PCR after transfer of eels to seawater. Among the genes upregulated after seawater transfer, AE genes, draa, b, and pat1a, c, on the apical membrane, and NBC genes, nbce1a, n1, n2a, and a AE gene, sat-1, on the basolateral membrane were candidates involved in HCO3− secretion. Judging from the slow effect of GN, we suggest that GN inhibits NKCC2b on the apical membrane and decreases cytosolic Cl− and Na+, which then activates apical DNDS-insensitive DRAs and basolateral DNDS-sensitive NBCs to enhance transcellular HCO3− flux across the intestinal epithelia of seawater-acclimated eels.

Effect of dendritic organ ligation on striped eel catfish Plotosus lineatus osmoregulation

Unique amongst the teleost, Plotosidae catfish possess a dendritic organ (DO) as a purported salt secreting organ, whereas other marine teleosts rely on their gill ionocytes for active NaCl excretion. To address the role of the DO in ionregulation, ligation experiments were conducted in brackish water (BW) 3‰ and seawater (SW) 34‰ acclimated Plotosus lineatus and compared to sham operated fish. Ligation in SW resulted in an osmoregulatory impairment in blood (elevated ions and hematocrit) and muscle (dehydration). However, SW ligation did not elicit compensatory changes in gill or kidney Na⁺/K⁺-ATPase (NKA) activity and/or protein expression while a decrease in anterior intestine and increased in posterior intestine were observed but this was not reflected at the protein level. Following ligation in SW, protein levels of carbonic anhydrase (CA) and V-ATPase B subunit (VHAB) were higher in kidney but either lower (CA) or unchanged (VHAB) in other tissues. Taken together, the osmotic disturbance in ligated SW fish indicates the central role of the DO in salt secretion and the absence of a compensatory response from the gill.

The Amphibious Mudskipper: A Unique Model Bridging the Gap of Central Actions of Osmoregulatory Hormones Between Terrestrial and Aquatic Vertebrates

Body fluid regulation, or osmoregulation, continues to be a major topic in comparative physiology, and teleost fishes have been the subject of intensive research. Great progress has been made in understanding the osmoregulatory mechanisms including drinking behavior in teleosts and mammals. Mudskipper gobies can bridge the gap from aquatic to terrestrial habitats by their amphibious behavior, but the studies are yet emerging. In this review, we introduce this unique teleost as a model to study osmoregulatory behaviors, particularly amphibious behaviors regulated by the central action of hormones. Regarding drinking behavior of mammals, a thirst sensation is aroused by angiotensin II (Ang II) through direct actions on the forebrain circumventricular structures, which predominantly motivates them to search for water and take it into the mouth for drinking. By contrast, aquatic teleosts can drink water that is constantly present in their mouth only by reflex swallowing, and Ang II induces swallowing by acting on the hindbrain circumventricular organ without inducing thirst. In mudskippers, however, through the loss of buccal water by swallowing, which appears to induce buccal drying on land, Ang II motivates these fishes to move to water for drinking. Thus, mudskippers revealed a unique thirst regulation by sensory detection in the buccal cavity. In addition, the neurohypophysial hormones, isotocin (IT) and vasotocin (VT), promote migration to water via IT receptors in mudskippers. VT is also dipsogenic and the neurons in the forebrain may mediate their thirst. VT regulates social behaviors as well as osmoregulation. The VT-induced migration appears to be a submissive response of subordinate mudskippers to escape from competitive and dehydrating land. Together with implications of VT in aggression, mudskippers may bridge the multiple functions of neurohypophysial hormones. Interestingly, cortisol, an important hormone for seawater adaptation and stress response in teleosts, also stimulates the migration toward water, mediated possibly via the mineralocorticoid receptor. The corticosteroid system that is responsive to external stressors can accelerate emergence of migration to alternative habitats. In this review, we suggest this unique teleost as an important model to deepen insights into the behavioral roles of these hormones in relation to osmoregulation.

Section-specific expression of acid-base and ammonia transporters in the kidney tubules of the goldfish Carassius auratus and their responses to feeding

In teleost fishes, renal contributions to acid-base and ammonia regulation are often neglected compared with the gills. In goldfish, increased renal acid excretion in response to feeding was indicated by increased urine ammonia and inorganic phosphate concentrations and decreased urine pH. By microdissecting the kidney tubules and performing quantitative real-time PCR and/or immunohistochemistry, we profiled the section-specific expression of glutamate dehydrogenase (GDH), glutamine synthetase (GS), Na⁺/H⁺-exchanger 3 (NHE3), carbonic anhydrase II (CAIIa), V-H⁺-ATPase subunit 1b, Cl^-/ HCO3- -exchanger 1 (AE1), Na⁺/ HCO3- -cotransporter 1 (NBC1), Na⁺/K⁺-ATPase subunit 1α, and Rhesus-proteins Rhbg, Rhcg1a, and Rhcg1b. Here, we show for the first time that 1) the proximal tubule appears to be the major site for ammoniagenesis, 2) epithelial transporters are differentially expressed along the renal tubule, and 3) a potential feeding-related "acidic tide" results in the differential regulation of epithelial transporters, resembling the mammalian renal response to a metabolic acidosis. Specifically, GDH and NHE3 mRNAs were upregulated and GS downregulated in the proximal tubule upon feeding, suggesting this section as a major site for ammoniagenesis and acid secretion. The distal tubule may play a major role in renal ammonia secretion, with feeding-induced upregulation of mRNA and protein for apical NHE3, cytoplasmic CAIIa, universal Rhcg1a and apical Rhcg1b, and downregulation of basolateral Rhbg and AE1. Changes in mRNA expression of the Wolffian ducts and bladder suggest supporting roles in fine-tuning urine composition. The present study verifies an important renal contribution to acid-base balance and emphasizes that studies looking at the whole kidney may overlook key section-specific responses.

Regulation of Bicarbonate Secretion in Marine Fish Intestine by the Calcium-Sensing Receptor

In marine fish, high epithelial intestinal HCO₃− secretion generates luminal carbonate precipitates of divalent cations that play a key role in water and ion homeostasis. The present study was designed to expose the putative role for calcium and the calcium-sensing receptor (CaSR) in the regulation of HCO₃− secretion in the intestine of the sea bream (Sparus aurata L.). Effects on the expression of the CaSR in the intestine were evaluated by qPCR and an increase was observed in the anterior intestine in fed fish compared with unfed fish and with different regions of intestine. CaSR expression reflected intestinal fluid calcium concentration. In addition, anterior intestine tissue was mounted in Ussing chambers to test the putative regulation of HCO₃− secretion in vitro using the anterior intestine. HCO₃− secretion was sensitive to varying calcium levels in luminal saline and to calcimimetic compounds known to activate/block the CaSR i.e., R 568 and NPS-2143. Subsequent experiments were performed in intestinal sacs to measure water absorption and the sensitivity of water absorption to varying luminal levels of calcium and calcimimetics were exposed as well. It appears, that CaSR mediates HCO₃− secretion and water absorption in marine fish as shown by responsiveness to calcium levels and calcimimetic compounds.

Loss of stomach, loss of appetite? Sequencing of the ballan wrasse (Labrus bergylta) genome and intestinal transcriptomic profiling illuminate the evolution of loss of stomach function in fish

BACKGROUND

The ballan wrasse (Labrus bergylta) belongs to a large teleost family containing more than 600 species showing several unique evolutionary traits such as lack of stomach and hermaphroditism. Agastric fish are found throughout the teleost phylogeny, in quite diverse and unrelated lineages, indicating stomach loss has occurred independently multiple times in the course of evolution. By assembling the ballan wrasse genome and transcriptome we aimed to determine the genetic basis for its digestive system function and appetite regulation. Among other, this knowledge will aid the formulation of aquaculture diets that meet the nutritional needs of agastric species.

RESULTS

Long and short read sequencing technologies were combined to generate a ballan wrasse genome of 805 Mbp. Analysis of the genome and transcriptome assemblies confirmed the absence of genes that code for proteins involved in gastric function. The gene coding for the appetite stimulating protein ghrelin was also absent in wrasse. Gene synteny mapping identified several appetite-controlling genes and their paralogs previously undescribed in fish. Transcriptome profiling along the length of the intestine found a declining expression gradient from the anterior to the posterior, and a distinct expression profile in the hind gut.

CONCLUSIONS

We showed gene loss has occurred for all known genes related to stomach function in the ballan wrasse, while the remaining functions of the digestive tract appear intact. The results also show appetite control in ballan wrasse has undergone substantial changes. The loss of ghrelin suggests that other genes, such as motilin, may play a ghrelin like role. The wrasse genome offers novel insight in to the evolutionary traits of this large family. As the stomach plays a major role in protein digestion, the lack of genes related to stomach digestion in wrasse suggests it requires formulated diets with higher levels of readily digestible protein than those for gastric species.

Carbon dioxide induced plasticity of branchial acid-base pathways in an estuarine teleost

Anthropogenic CO₂ is expected to drive ocean pCO₂ above 1,000 μatm by 2100 – inducing respiratory acidosis in fish that must be corrected through branchial ion transport. This study examined the time course and plasticity of branchial metabolic compensation in response to varying levels of CO₂ in an estuarine fish, the red drum, which regularly encounters elevated CO₂ and may therefore have intrinsic resilience. Under control conditions fish exhibited net base excretion; however, CO₂ exposure resulted in a dose dependent increase in acid excretion during the initial 2 h. This returned to baseline levels during the second 2 h interval for exposures up to 5,000 μatm, but remained elevated for exposures above 15,000 μatm. Plasticity was assessed via gene expression in three CO₂ treatments: environmentally realistic 1,000 and 6,000 μatm exposures, and a proof-of-principle 30,000 μatm exposure. Few differences were observed at 1,000 or 6,000 μatm; however, 30,000 μatm stimulated widespread up-regulation. Translocation of V-type ATPase after 1 h of exposure to 30,000 μatm was also assessed; however, no evidence of translocation was found. These results indicate that red drum can quickly compensate to environmentally relevant acid-base disturbances using baseline cellular machinery, yet are capable of plasticity in response to extreme acid-base challenges.

High rates of intestinal bicarbonate secretion in seawater tilapia (Oreochromis mossambicus)

Osmoregulation in fish is a complex process that requires the orchestrated cooperation of many tissues. In fish facing hyperosmotic environments, the intestinal absorption of some monovalent ions and the secretion of bicarbonate are key processes to favor water absorption. In the present study, we showed that bicarbonate levels in the intestinal fluid are several fold higher in seawater than in freshwater acclimated tilapia (Oreochromis mossambicus). In addition, we analyzed gene expression of the main molecular mechanisms involved in HCO₃^- movements i.e. slc26a6, slc26a3, slc4a4 and v-type H-ATPase sub C in the intestine of tilapia acclimated to both seawater and freshwater. Our results show an anterior/posterior functional regionalization of the intestine in tilapia in terms of expression patterns, which is affected by environmental salinity mostly in the anterior and mid intestine. Analysis of bicarbonate secretion using pH-Stat in tissues mounted in Ussing chambers reveals high rates of bicarbonate secretion in tilapia acclimated to seawater from anterior intestine to rectum ranging between ~900 and ~1700nmolHCO₃^-cm^-2h^-1. However, a relationship between the expression of slc26a6, slc26a3, slc4a4 and the rate of bicarbonate secretion seems to be compromised in the rectum. In this region, the low expression of the bicarbonate transporters could not explain the high bicarbonate secretion rates here described. However, we postulate that the elevated v-type H-ATPase mRNA expression in the rectum could be involved in this process.

Molecular mechanisms underlying active desalination and low water permeability in the esophagus of eels acclimated to seawater

Marine teleosts can absorb imbibed seawater (SW) to maintain water balance, with esophageal desalination playing an essential role. NaCl absorption from luminal SW was enhanced 10-fold in the esophagus of SW-acclimated eels, and removal of Na⁺ or Cl^- from luminal SW abolished the facilitated absorption, indicating coupled transport. Mucosal/serosal application of various blockers for Na⁺/Cl^- transporters profoundly decreased the absorption. Among the transporter genes expressed in eel esophagus detected by RNA-seq, dimethyl amiloride-sensitive Na⁺/H⁺ exchanger (NHE3) and 4,4'-diisothiocyano-2,2'-disulfonic acid-sensitive Cl^-/[Formula: see text] exchanger (AE) coupled by the scaffolding protein on the apical membrane of epithelial cells, and ouabain-sensitive Na⁺-K⁺-ATPases (NKA1α1c and NKA3α) and diphenylamine-2-carboxylic acid-sensitive Cl^- channel (CLCN2) on the basolateral membrane, may be responsible for enhanced transcellular NaCl transport because of their profound upregulation after SW acclimation. Upregulated carbonic anhydrase 2a (CA2a) supplies H⁺ and [Formula: see text] for activation of the coupled NHE and AE. Apical hydrochlorothiazide-sensitive Na⁺-Cl^- cotransporters and basolateral Na⁺-[Formula: see text] cotransporter (NBCe1) and AE1 are other possible candidates. Concerning the low water permeability that is typically seen in marine teleost esophagus, downregulated aquaporin genes (aqp1a and aqp3) and upregulated claudin gene (cldn15a) are candidates for transcellular/paracellular route. In situ hybridization showed that these upregulated transporters and tight-junction protein genes were expressed in the absorptive columnar epithelial cells of eel esophagus. These results allow us to provide a full picture of the molecular mechanism of active desalination and low water permeability that are characteristic to marine teleost esophagus and gain deeper insights into the role of gastrointestinal tracts in SW acclimation.

Di- and tripeptide transport in vertebrates: the contribution of teleost fish models

Solute Carrier 15 (SLC15) family, alias H⁺-coupled oligopeptide cotransporter family, is a group of membrane transporters known for their role in the cellular uptake of di- and tripeptides (di/tripeptides) and peptide-like molecules. Of its members, SLC15A1 (PEPT1) chiefly mediates intestinal absorption of luminal di/tripeptides from dietary protein digestion, while SLC15A2 (PEPT2) mainly allows renal tubular reabsorption of di/tripeptides from ultrafiltration, SLC15A3 (PHT2) and SLC15A4 (PHT1) possibly interact with di/tripeptides and histidine in certain immune cells, and SLC15A5 has unknown function. Our understanding of this family in vertebrates has steadily increased, also due to the surge of genomic-to-functional information from 'non-conventional' animal models, livestock, poultry, and aquaculture fish species. Here, we review the literature on the SLC15 transporters in teleost fish with emphasis on SLC15A1 (PEPT1), one of the solute carriers better studied amongst teleost fish because of its relevance in animal nutrition. We report on the operativity of the transporter, the molecular diversity, and multiplicity of structural-functional solutions of the teleost fish orthologs with respect to higher vertebrates, its relevance at the intersection of the alimentary and osmoregulative functions of the gut, its response under various physiological states and dietary solicitations, and its possible involvement in examples of total body plasticity, such as growth and compensatory growth. By a comparative approach, we also review the few studies in teleost fish on SLC15A2 (PEPT2), SLC15A4 (PHT1), and SLC15A3 (PHT2). By representing the contribution of teleost fish to the knowledge of the physiology of di/tripeptide transport and transporters, we aim to fill the gap between higher and lower vertebrates.

Temperature Modulates the Effects of Ocean Acidification on Intestinal Ion Transport in Atlantic Cod, Gadus morhua

CO₂-driven seawater acidification has been demonstrated to enhance intestinal bicarbonate secretion rates in teleosts, leading to an increased release of CaCO₃ under simulated ocean acidification scenarios. In this study, we investigated if increasing CO₂ levels stimulate the intestinal acid–base regulatory machinery of Atlantic cod (Gadus morhua) and whether temperatures at the upper limit of thermal tolerance stimulate or counteract ion regulatory capacities. Juvenile G. morhua were acclimated for 4 weeks to three CO₂ levels (550, 1200, and 2200 μatm) covering present and near-future natural variability, at optimum (10°C) and summer maximum temperature (18°C), respectively. Immunohistochemical analyses revealed the subcellular localization of ion transporters, including Na⁺/K⁺-ATPase (NKA), Na⁺/H⁺-exchanger 3 (NHE3), Na⁺/HCO3− cotransporter (NBC1), pendrin-like Cl⁻/HCO3− exchanger (SLC26a6), V-type H⁺-ATPase subunit a (VHA), and Cl⁻ channel 3 (CLC3) in epithelial cells of the anterior intestine. At 10°C, proteins and mRNA were generally up-regulated for most transporters in the intestinal epithelium after acclimation to higher CO₂ levels. This supports recent findings demonstrating increased intestinal HCO3− secretion rates in response to CO₂ induced seawater acidification. At 18°C, mRNA expression and protein concentrations of most ion transporters remained unchanged or were even decreased, suggesting thermal compensation. This response may be energetically favorable to retain blood HCO3− levels to stabilize pH_e, but may negatively affect intestinal salt and water resorption of marine teleosts in future oceans.

Sulfate transporters involved in sulfate secretion in the kidney are localized in the renal proximal tubule II of the elephant fish (Callorhinchus milii)

Most vertebrates, including cartilaginous fishes, maintain their plasma SO4 (2-) concentration ([SO4 (2-)]) within a narrow range of 0.2-1 mM. As seawater has a [SO4 (2-)] about 40 times higher than that of the plasma, SO4 (2-) excretion is the major role of kidneys in marine teleost fishes. It has been suggested that cartilaginous fishes also excrete excess SO4 (2-) via the kidney. However, little is known about the underlying mechanisms for SO4 (2-) transport in cartilaginous fish, largely due to the extraordinarily elaborate four-loop configuration of the nephron, which consists of at least 10 morphologically distinguishable segments. In the present study, we determined cDNA sequences from the kidney of holocephalan elephant fish (Callorhinchus milii) that encoded solute carrier family 26 member 1 (Slc26a1) and member 6 (Slc26a6), which are SO4 (2-) transporters that are expressed in mammalian and teleost kidneys. Elephant fish Slc26a1 (cmSlc26a1) and cmSlc26a6 mRNAs were coexpressed in the proximal II (PII) segment of the nephron, which comprises the second loop in the sinus zone. Functional analyses using Xenopus oocytes and the results of immunohistochemistry revealed that cmSlc26a1 is a basolaterally located electroneutral SO4 (2-) transporter, while cmSlc26a6 is an apically located, electrogenic Cl(-)/SO4 (2-) exchanger. In addition, we found that both cmSlc26a1 and cmSlc26a6 were abundantly expressed in the kidney of embryos; SO4 (2-) was concentrated in a bladder-like structure of elephant fish embryos. Our results demonstrated that the PII segment of the nephron contributes to the secretion of excess SO4 (2-) by the kidney of elephant fish. Possible mechanisms for SO4 (2-) secretion in the PII segment are discussed.

The role of the rectum in osmoregulation and the potential effect of renoguanylin on SLC26a6 transport activity in the Gulf toadfish (Opsanus beta)

Teleosts living in seawater continually absorb water across the intestine to compensate for branchial water loss to the environment. The present study reveals that the Gulf toadfish (Opsanus beta) rectum plays a comparable role to the posterior intestine in ion and water absorption. However, the posterior intestine appears to rely more on SLC26a6 (a HCO3 (-)/Cl(-) antiporter) and the rectum appears to rely on NKCC2 (SLC12a1) for the purposes of solute-coupled water absorption. The present study also demonstrates that the rectum responds to renoguanylin (RGN), a member of the guanylin family of peptides that alters the normal osmoregulatory processes of the distal intestine, by inhibited water absorption. RGN decreases rectal water absorption more greatly than in the posterior intestine and leads to net Na(+) and Cl(-) secretion, and a reversal of the absorptive short-circuit current (ISC). It is hypothesized that maintaining a larger fluid volume within the distal segments of intestinal tract facilitates the removal of CaCO3 precipitates and other solids from the intestine. Indeed, the expression of the components of the Cl(-)-secretory response, apical CFTR, and basolateral NKCC1 (SLC12a2), are upregulated in the rectum of the Gulf toadfish after 96 h in 60 ppt, an exposure that increases CaCO3 precipitate formation relative to 35 ppt. Moreover, the downstream intracellular effects of RGN appear to directly inhibit ion absorption by NKCC2 and anion exchange by SLC26a6. Overall, the present findings elucidate key electrophysiological differences between the posterior intestine and rectum of Gulf toadfish and the potent regulatory role renoguanylin plays in osmoregulation.

Sulfate and thiosulfate inhibit oxalate transport via a dPrestin (Slc26a6)-dependent mechanism in an insect model of calcium oxalate nephrolithiasis

Nephrolithiasis is one of the most common urinary tract disorders, with the majority of kidney stones composed of calcium oxalate (CaOx). Given its prevalence (US occurrence 10%), it is still poorly understood, lacking progress in identifying new therapies because of its complex etiology. Drosophila melanogaster (fruitfly) is a recently developed model of CaOx nephrolithiasis. Effects of sulfate and thiosulfate on crystal formation were investigated using the Drosophila model, as well as electrophysiological effects on both Drosophila (Slc26a5/6; dPrestin) and mouse (mSlc26a6) oxalate transporters utilizing the Xenopus laevis oocyte heterologous expression system. Results indicate that both transport thiosulfate with a much higher affinity than sulfate Additionally, both compounds were effective at decreasing CaOx crystallization when added to the diet. However, these results were not observed when compounds were applied to Malpighian tubules ex vivo. Neither compound affected CaOx crystallization in dPrestin knockdown animals, indicating a role for principal cell-specific dPrestin in luminal oxalate transport. Furthermore, thiosulfate has a higher affinity for dPrestin and mSlc26a6 compared with oxalate These data indicate that thiosulfate's ability to act as a competitive inhibitor of oxalate via dPrestin, can explain the decrease in CaOx crystallization seen in the presence of thiosulfate, but not sulfate. Overall, our findings predict that thiosulfate or oxalate-mimics may be effective as therapeutic competitive inhibitors of CaOx crystallization.

The differential role of renoguanylin in osmoregulation and apical Cl−/HCO3− exchange activity in the posterior intestine of the Gulf toadfish (Opsanus beta)

The guanylin family of peptides are effective regulators of intestinal physiology in marine teleosts. In the distal intestinal segments, they inhibit or reverse fluid absorption by inhibiting the absorptive short-circuit current ( Isc). The present findings demonstrate that mRNA from guanylin and uroguanylin, as well as at least one isoform of the guanylin peptide receptor, apical guanylyl cyclase-C (GC-C), was highly expressed in the intestine and rectum of the Gulf toadfish ( Opsanus beta). In the posterior intestine, GC-C, as well as the cystic fibrosis transmembrane conductance regulator and basolateral Na+/K+/2Cl− cotransporter, which comprise a Cl−-secretory pathway, were transcriptionally upregulated in 60 parts per thousand (ppt). The present study also shows that, in intestinal tissues from Gulf toadfish held in 35 ppt, renoguanylin (RGN) expectedly causes net Cl− secretion, inhibits both the absorptive Isc and fluid absorption, and decreases HCO3− secretion. Likewise, in intestinal tissues from Gulf toadfish acclimated to 60 ppt, RGN also inhibits the absorptive Isc and fluid absorption but to an even greater extent, corresponding with the mRNA expression data. In contrast, RGN does not alter Cl− flux and, instead, elevates HCO3− secretion in the 60-ppt group, suggesting increased apical Cl−/HCO3− exchange activity by SLC26a6. Overall, these findings reinforce the hypotheses that the guanylin peptide system is important for salinity acclimatization and that the secretory response could facilitate the removal of solids, such as CaCO3 precipitates, from the intestine.

PTHrP regulates water absorption and aquaporin expression in the intestine of the marine sea bream (Sparus aurata, L.)

Water ingestion by drinking is fundamental for ion homeostasis in marine fish. However, the fluid ingested requires processing to allow net water absorption in the intestine. The formation of luminal carbonate aggregates impacts on calcium homeostasis and requires epithelial HCO3(-) secretion to enable water absorption. In light of its endocrine importance in calcium handling and the indication of involvement in HCO3(-) secretion the present study was designed to expose the role of the parathyroid hormone-related protein (PTHrP) in HCO3(-) secretion, water absorption and the regulation of aqp1 gene expression in the anterior intestine of the sea bream. HCO3(-) secretion rapidly decreased when PTHrP(1-34) was added to anterior intestine of the sea bream mounted in Ussing chambers. The effect achieved a maximum inhibition of 60% of basal secretion rates, showing a threshold effective dose of 0.1 ng ml(-1) compatible with reported plasma values of PTHrP. When applied in combination with the adenylate cyclase inhibitor (SQ 22.536, 100 μmol l(-1)) or the phospholipase C inhibitor (U73122, 10 μmol l(-1)) the effect of PTHrP(1-34) on HCO3(-) secretion was reduced by about 50% in both cases. In parallel, bulk water absorption measured in intestinal sacs was sensitive to inhibition by PTHrP. The inhibitory action conforms to a typical dose-response curve in the range of 0.1-1000 ng ml(-1), achieves a maximal effect of 60-65% inhibition from basal rates and shows threshold significant effects at hormone levels of 0.1 ng ml(-1). The action of PTHrP in water absorption was completely abolished in the presence of the adenylate cyclase inhibitor (SQ 22.536, 100 μmol l(-1)) and was insensitive to the phospholipase C inhibitor (U73122, 10 μmol l(-1)). In vivo injections of PTHrP(1-34) or the PTH/PTHrP receptor antagonist PTHrP(7-34) evoked respectively, a significant decrease or increase of aqp1ab, but not aqp1a. Overall the present results suggest that PTHrP acts as a key regulator of carbonate aggregate formation in the intestine of marine fish via its actions on water absorption, calcium regulation and HCO3(-) secretion.

Guanylin activates Cl(-) secretion into the lumen of seawater eel intestine via apical Cl(-) channel under simulated in vivo conditions

Guanylin (GN) action on seawater eel intestine was examined under simulated in vivo conditions, where isotonic luminal fluid has low NaCl and high MgSO4 (MgSO4 Ringer). In Ussing chamber, MgSO4 Ringer induced serosa-negative potential difference (PD) even after bumetanide treatment, which is due to the higher paracellular Na(+) permeability over Cl(-), as confirmed by the replacement by MgCl2 (no Cl(-) gradient) or Na2SO4 Ringer (no Na(+) gradient). Luminal GN reversed serosa-negative PD, probably by enhancing Cl(-) secretion into the lumen, as the GN effect was blocked by apical Cl(-) channel blockers [diphenylamine-2-carboxylic acid (DPC), 5-nitro-2-(3-phenylpropylamino) benzoic acid, glibenclamide but not cystic fibrosis transmembrane regulator (CFTR)inh-172] or replacement of luminal fluid by MgCl2 Ringer. The blockers' effect was undetectable when normal Ringer was on both sides. In the sac preparation, NaCl secretion occurred into the lumen (Na(+) > Cl(-)), and GN further enhanced Cl(-) secretion (Cl(-) > Na(+)), resulting in water secretion. These GN effects were also blocked by DPC. Quantitative analyses showed that isotonic NaCl is absorbed when luminal fluid is normal Ringer, but, when luminal fluid is MgSO4 Ringer, hypertonic NaCl, almost equivalent to seawater, is secreted into the lumen after GN. These results indicate that GN stimulates the secretion of hypertonic NaCl into the lumen of seawater eel intestine, like rectal gland of marine elasmobranchs, to get rid of excess NaCl although marine teleost intestine is thought to have only absorptive-type cells with a unique Na-K-Cl cotransport system. The secreted NaCl may activate the cotransport system and further help absorb water in the final segment of seawater eel intestine.

Mechanisms of Cl(-) uptake in rainbow trout: cloning and expression of slc26a6, a prospective Cl(-)/HCO3(-) exchanger

In fresh waters, fishes continuously acquire ions to offset diffusive losses to a more dilute ambient environment and to maintain acid-base status. The objectives of the present study were to clone slc26a6, a prospective Cl(-)/HCO3(-) exchanger from rainbow trout, investigate its expression patterns in various tissues, at different developmental stages and after differential salinity exposure, and probe the mechanisms of Cl(-) uptake in rainbow trout embryos during development using a pharmacological inhibitor approach combined with (36)Cl(-) unidirectional fluxes. Results showed that the cloned gene encoded a 783 amino acid protein with conserved domains characteristic of the SLC26a family of anion exchange proteins. Phylogenetic analysis of this sequence against all subfamilies of the SLC26a family demonstrated that this translated protein shared a common ancestor with other actinopterygii and mammalian SLC26a6 isoforms and thus confirmed the identity of the cloned gene. Expression of slc26a6 was detected in all tissues and developmental stages assayed but was highest in the gill of juvenile trout. In trout embryos, Cl(-) uptake increased significantly post-hatch and was demonstrated to be mediated via an anion exchanger specific (DIDS sensitive) pathway that was also sensitive to hypercapnia. This parallels well with the predicted function of slc26a6, and the detection of the transcript in embryos and tissues of trout. In conclusion, this study is the first report of slc26a6 in rainbow trout and functional and expression analyses indicate its likely involvement in Cl(-)/HCO3(-) exchange in two life stages of rainbow trout.

Physiological impacts of elevated carbon dioxide and ocean acidification on fish

Most fish studied to date efficiently compensate for a hypercapnic acid-base disturbance; however, many recent studies examining the effects of ocean acidification on fish have documented impacts at CO2 levels predicted to occur before the end of this century. Notable impacts on neurosensory and behavioral endpoints, otolith growth, mitochondrial function, and metabolic rate demonstrate an unexpected sensitivity to current-day and near-future CO2 levels. Most explanations for these effects seem to center on increases in Pco2 and HCO3− that occur in the body during pH compensation for acid-base balance; however, few studies have measured these parameters at environmentally relevant CO2 levels or directly related them to reported negative endpoints. This compensatory response is well documented, but noted variation in dynamic regulation of acid-base transport pathways across species, exposure levels, and exposure duration suggests that multiple strategies may be utilized to cope with hypercapnia. Understanding this regulation and changes in ion gradients in extracellular and intracellular compartments during CO2 exposure could provide a basis for predicting sensitivity and explaining interspecies variation. Based on analysis of the existing literature, the present review presents a clear message that ocean acidification may cause significant effects on fish across multiple physiological systems, suggesting that pH compensation does not necessarily confer tolerance as downstream consequences and tradeoffs occur. It remains difficult to assess if acclimation responses during abrupt CO2 exposures will translate to fitness impacts over longer timescales. Nonetheless, identifying mechanisms and processes that may be subject to selective pressure could be one of many important components of assessing adaptive capacity.

Guanylin peptides regulate electrolyte and fluid transport in the Gulf toadfish (Opsanus beta) posterior intestine

The physiological effects of guanylin (GN) and uroguanylin (UGN) on fluid and electrolyte transport in the teleost fish intestine have yet to be thoroughly investigated. In the present study, the effects of GN, UGN, and renoguanylin (RGN; a GN and UGN homolog) on short-circuit current ( Isc) and the transport of Cl−, Na+, bicarbonate (HCO3−), and fluid in the Gulf toadfish ( Opsanus beta) intestine were determined using Ussing chambers, pH-stat titration, and intestinal sac experiments. GN, UGN, and RGN reversed the Isc of the posterior intestine (absorptive-to-secretory), but not of the anterior intestine. RGN decreased baseline HCO3− secretion, but increased Cl− and fluid secretion in the posterior intestine. The secretory response of the posterior intestine coincides with the presence of basolateral NKCC1 and apical cystic fibrosis transmembrane conductance regulator (CFTR), the latter of which is lacking in the anterior intestine and is not permeable to HCO3− in the posterior intestine. However, the response to RGN by the posterior intestine is counterintuitive given the known role of the marine teleost intestine as a salt- and water-absorbing organ. These data demonstrate that marine teleosts possess a tissue-specific secretory response, apparently associated with seawater adaptation, the exact role of which remains to be determined.

Mechanisms of guanylin action on water and ion absorption at different regions of seawater eel intestine

Guanylin (GN) inhibited water absorption and short-circuit current (Isc) in seawater eel intestine. Similar inhibition was observed after bumetanide, and the effect of bumetanide was abolished by GN or vice versa, suggesting that both act on the same target, Na(+)-K(+)-2Cl(-) cotransporter (NKCC), which is a key player for the Na(+)-K(+)-Cl(-) transport system responsible for water absorption in marine teleost intestine. However, effect of GN was always greater than that of bumetanide: 10% greater in middle intestine (MI) and 40% in posterior intestine (PI) for Isc, and 25% greater in MI and 34% in PI for water absorption. After treatment with GN, Isc decreased to zero, but 20-30% water absorption still remained. The remainder may be due to the Cl(-)/HCO3 (-) exchanger and Na(+)-Cl(-) cotransporter (NCC), since inhibitors for these transporters almost nullified the remaining water absorption. Quantitative PCR analysis revealed the presence of major proteins involved in water absorption; the NKCC2β and AQP1 genes whose expression was markedly upregulated after seawater acclimation. The SLC26A6 (anion exchanger) and NCCβ genes were also expressed in small amounts. Consistent with the inhibitors' effect, expression of NKCC2β was MI > PI, and that of NCCβ was MI << PI. The present study showed that GN not only inhibits the bumetanide-sensitive Na(+)-K(+)-Cl(-) transport system governed by NKCC2β, but also regulates unknown ion transporters different from GN-insensitive SLC26A6 and NCC. A candidate is cystic fibrosis transmembrane conductance regulator Cl(-) channel, as demonstrated in mammals, but its expression is low in eel intestine, and its role may be minor, as indicated by the small effect of its inhibitors.

Identification and lateral membrane localization of cyclin M3, likely to be involved in renal Mg2+ handling in seawater fish

The kidney of marine teleosts is the major site of Mg(2+) excretion and produces urine with a high Mg(2+) concentration. However, the transporters involved in Mg(2+) excretion are poorly understood. The cyclin M (Cnnm; also known as ancient conserved domain protein) family comprises membrane proteins homologous to the bacterial Mg(2+) and Co(2+) efflux protein, CorC. To understand the molecular mechanism of Mg(2+) homeostasis in marine teleosts, we analyzed the expression of the Cnnm family genes in the seawater (SW) pufferfish, torafugu (Takifugu rubripes), and the closely related euryhaline species, mefugu (Takifugu obscurus). Database mining and phylogenetic analysis indicated that the Takifugu genome contains six members of the Cnnm family: two orthologs of Cnnm1, one of Cnnm2, one of Cnnm3, and two of Cnnm4. RT-PCR analyses indicated that Cnnm2, Cnnm3, and Cnnm4a are expressed in the kidney, whereas other members are mainly expressed in the brain. Renal expression of Cnnm3 was upregulated in SW mefugu, whereas renal expression of Cnnm2 was upregulated in freshwater (FW) mefugu. No significant difference was observed in renal expression of Cnnm4a between SW and FW mefugu. In situ hybridization and immunohistochemical analyses of the SW mefugu kidney revealed that Cnnm3 is expressed in the proximal tubule, and its product localizes to the lateral membrane. When Cnnm3 was expressed in Xenopus laevis oocytes, whole cellular Mg(2+) content and free intracellular Mg(2+) activity significantly decreased. These results suggest that Cnnm3 is involved in body fluid Mg(2+) homeostasis in marine teleosts.

The roles of acid-sensing ion channel 1a and ovarian cancer G protein-coupled receptor 1 on passive Mg2+ transport across intestinal epithelium-like Caco-2 monolayers

Intestinal passive Mg(2+) absorption, which is vital for normal Mg(2+) homeostasis, has been shown to be regulated by luminal proton. We aimed to study the regulatory role of intestinal acid sensors in paracellular passive Mg(2+) transport. Omeprazole enhanced the expressions of acid-sensing ion channel 1a (ASIC1a), ovarian cancer G protein-coupled receptor 1 (OGR1), and transient receptor potential vanilloid 4 in Caco-2 cells. It also inhibited passive Mg(2+) transport across Caco-2 monolayers. The expression and activation of OGR1 resulted in the stimulation of passive Mg(2+) transport via phospholipase C- and protein kinase C-dependent pathways. ASIC1a activation, on the other hand, enhanced apical HCO3 (-) secretion that led, at least in part, by a Ca(2+)-dependent pathway to an inhibition of paracellular Mg(2+) absorption. Our results provided supporting evidence for the roles of OGR1 and ASIC1a in the regulation of intestinal passive Mg(2+) absorption.

Claudins in teleost fishes

Teleost fishes are a large and diverse animal group that represent close to 50% of all described vertebrate species. This review consolidates what is known about the claudin (Cldn) family of tight junction (TJ) proteins in teleosts. Cldns are transmembrane proteins of the vertebrate epithelial/endothelial TJ complex that largely determine TJ permeability. Cldns achieve this by expressing barrier or pore forming properties and by exhibiting distinct tissue distribution patterns. So far, ~63 genes encoding for Cldn TJ proteins have been reported in 16 teleost species. Collectively, cldns (or Cldns) are found in a broad array of teleost fish tissues, but select genes exhibit restricted expression patterns. Evidence to date strongly supports the view that Cldns play a vital role in the embryonic development of teleost fishes and in the physiology of tissues and organ systems studied thus far.

Identification and proximal tubular localization of the Mg²⁺ transporter, Slc41a1, in a seawater fish

The second most abundant cation in seawater (SW), Mg²⁺, is present at concentrations of ~53 mM. Marine teleosts maintain plasma Mg²⁺ concentration at 1-2 mM by excreting Mg²⁺ into the urine. Urine Mg²⁺ concentrations of SW teleosts exceed 70 mM, most of which is secreted by the renal tubular epithelial cells. However, molecular mechanisms of the Mg²⁺ secretion have yet to be clarified. To identify transporters involved in Mg²⁺ secretion, we analyzed the expression of fish homologs of the Slc41 Mg²⁺ transporter family in various tissues of SW pufferfish torafugu (Takifugu rubripes) and its closely related euryhaline species mefugu (Takifugu obscurus). Takifugu genome contained five members of Slc41 genes, and only Slc41a1 was highly expressed in the kidney. Renal expression of Slc41a1 was markedly elevated when mefugu were transferred from fresh water (FW) to SW. In situ hybridization analysis and immunohistochemistry at the light and electron microscopic levels revealed that Slc41a1 is localized to vacuoles in the apical cytoplasm of the proximal tubules. These results suggest that pufferfish Slc41a1 is a Mg²⁺ transporter involved in renal tubular transepithelial Mg²⁺ secretion by mediating Mg²⁺ transport from the cytosol to the vacuolar lumen, and support the hypothesis that Mg²⁺ secretion is mediated by exocytosis of Mg²⁺-rich vacuoles to the lumen.

Expression of a novel isoform of Na(+)/H(+) exchanger 3 in the kidney and intestine of banded houndshark, Triakis scyllium

Na(+)/H(+) exchanger 3 (NHE3) provides one of the major Na(+) absorptive pathways of the intestine and kidney in mammals, and recent studies of aquatic vertebrates (teleosts and elasmobranchs) have demonstrated that NHE3 is expressed in the gill and plays important roles in ion and acid-base regulation. To understand the role of NHE3 in elasmobranch osmoregulatory organs, we analyzed renal and intestinal expressions and localizations of NHE3 in a marine elasmobranch, Japanese banded houndshark (Triakis scyllium). mRNA for Triakis NHE3 was most highly expressed in the gill, kidney, spiral intestine, and rectum. The kidney and intestine expressed a transcriptional isoform of NHE3 (NHE3k/i), which has a different amino terminus compared with that of NHE3 isolated from the gill (NHE3g), suggesting that NHE3k/i and NHE3g arise from a single gene by alternative promoter usage. Immunohistochemical analyses of the Triakis kidney demonstrated that NHE3k/i is expressed in the apical membrane of a part of the proximal and late distal tubules in the sinus zone. In the bundle zone of the kidney, NHE3k/i was expressed in the apical membrane of the early distal tubules known as the diluting segment. In the spiral intestine and rectum, NHE3k/i was localized toward the apical membrane of the epithelial cells. The transcriptional levels of NHE3k/i were increased in the kidney when Triakis was acclimated in 130% seawater, whereas those in the spiral intestine were increased in fish acclimated in diluted seawater. These results suggest that NHE3 is involved in renal Na(+) reabsorption, urine acidification, and intestinal Na(+) absorption in elasmobranchs.

Bicarbonate secreted from the pancreas contributed to the formation of Ca precipitates in Japanese eel, Anguilla japonica

Marine teleosts produce Ca precipitates in the intestine as a product of osmoregulation. Ca precipitates are formed by a chemical reaction of Mg(2+) and Ca(2+) derived from ingested seawater with bicarbonate (HCO(3)(-)). It has been reported that HCO(3)(-) originates from the intestine; however, the pancreas is predicted to be another organ that may supply HCO(3)(-) to the intestinal tract. In the present study, the pancreas was surgically removed from Japanese eel to confirm its contribution to Ca precipitate formation. Pancreatectomized eel produced significantly less Ca precipitates than control eel in seawater, indicating that HCO(3)(-) from the pancreas contributes substantially to the formation of Ca precipitates. To further examine the molecular mechanisms of HCO(3)(-) secretion, we cloned cDNAs encoding HCO(3)(-) transporters and identified those transporters that elevated their mRNA expression in the intestine and pancreas following seawater transfer. In the intestine, mRNA expression of Slc26a6A was increased shortly after seawater transfer, whereas Slc26a1 mRNA expression increased gradually following seawater transfer. In the pancreas, Slc26a3 mRNA expression was high during the early stage of seawater acclimation, whereas Slc26a1 expression increased gradually after transfer to seawater. In the intestine and pancreas, therefore, both transient and progressively increasing types of HCO(3)(-) transporters are likely to be involved in HCO(3)(-) secretion into the intestinal lumen in a coordinated manner.