Molecular physiology of osmoregulation in eels and other teleosts: the role of transporter isoforms and gene duplication

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Na+/Cl- cotransporter 2 is not fish-specific and is widely found in amphibians, non-avian reptiles, and select mammals

Solute carrier 12 (Slc12) is a family of electroneutral cation-coupled chloride (Cl-) cotransporters. Na+/K+/2Cl- (Nkcc) and Na+/Cl- cotransporters (Ncc) belong to the Nkcc/Ncc subfamily. Human and mouse possess one gene for the Na+/Cl- cotransporter (ncc gene: slc12a3), whereas teleost fishes possess multiple ncc genes, slc12a3 (ncc1) and slc12a10 (ncc2), in addition to their species-specific paralogs. Amphibians and squamates have two ncc genes: slc12a3 (ncc1) and ncc3. However, the evolutionary relationship between slc12a10 and ncc3 remains unresolved, and the presence of slc12a10 (ncc2) in mammals has not been clarified. Synteny and phylogenetic analyses of vertebrate genome databases showed that ncc3 is the ortholog of slc12a10, and slc12a10 is present in most ray-finned fishes, coelacanths, amphibians, reptiles, and a few mammals (e.g., platypus and horse) but pseudogenized or deleted in birds, most mammals, and some ray-finned fishes (pufferfishes). This shows that slc12a10 is widely present among bony vertebrates and pseudogenized or deleted independently in multiple lineages. Notably, as compared with some fish that show varied slc12a10 tissue expression profile, spotted gar, African clawed frog, red-eared slider turtle, and horse express slc12a10 in the ovaries or premature gonads. In horse tissues, an unexpectedly large number of splicing variants for Slc12a10 have been cloned, many of which encode truncated forms of Slc12a10, suggesting that the functional constraints of horse slc12a10 are weakened, which may be in the process of becoming a pseudogene. Our results elaborate on the evolution of Nkcc/Ncc subfamily of Slc12 in vertebrates.NEW & NOTEWORTHY slc12a10 is not a fish-specific gene and is present in a few mammals (e.g., platypus and horse), non-avian reptiles, amphibians, but was pseudogenized or deleted in most mammals (e.g., human, mouse, cat, cow, and rhinoceros), birds, and some ray-finned fishes (pufferfishes).

Early and abrupt salinity reduction impacts European eel larval culture

Reducing water salinity towards iso-osmotic conditions is a common practice applied in euryhaline fish farming to limit osmoregulation costs and enhance growth. In this respect, the present study investigated the timing of salinity reduction in an abrupt manner during European eel (Anguilla anguilla) larval culture by examining associated impacts on morphological and molecular levels. Larvae from 3 different parental combinations (families) were reared at constant 36 psu for 6 days (control) or subjected to a direct reduction to 18 psu on 1, 2, or 3 days post-hatch. Overall, salinity reduction enhanced growth and survival, resulting from more efficient energy resource utilization. In the control group, expression of growth-related igf2 remained constant, demonstrating a steady growth progression, while igf1 expression increased over time only for the salinity reduced treatments, potentially qualifying as a useful biomarker for growth performance. Even though each parental combination seems to have a different capacity to cope with salinity alterations, as observed by family-driven water-transport-related aquaporin (aqp1, aqp3) gene expression, it could be inferred that the abrupt salinity change is generally not stressful, based on non-upregulated heat shock proteins (hsp70, hsp90). However, the applied salinity reduction (irrespective of timing) induced the development of pericardial edema. As such, we conclude that despite the positive effect of salinity reduction on early growth and survival, the long-term benefit for eel larval culture lies in establishing a protocol for salinity reduction, at a precise developmental time point, without causing pericardial malformations.

Molecular characterization and structural dynamics of Aquaporin1 from walking catfish in lipid bilayers

Aquaporin's (AQPs) are the major superfamily of small integral membrane proteins that facilitates transportation of water, urea, ammonia, glycerol and ions across biological cell membranes. Despite of recent advancements made in understanding the biology of Aquaporin's, only few isoforms of aquaporin 1 (AQP1) some of the teleost fish species have been characterized at molecular scale. In this study, we made an attempt to elucidate the molecular mechanism of water transportation in AQP1 from walking catfish (Clarias batrachus), a model species capable of breathing in air and inhabits in challenging environments. Using state-of-the-art computational modelling and all-atoms molecular dynamics simulation, we explored the structural dynamics of full-length aquaporin 1 from walking catfish (CbAQP1) in lipid mimetic bilayers. Unlike AQP1 of human and bovine, structural ensembles of CbAQP1 from MD revealed discrete positioning of pore lining residues at the intracellular end. Snapshots from MD simulation displayed differential dynamics of aromatic/arginine (ar/R) filter and extracellular loop C bridging transmembrane (TM) helix H3 and H4. Distinct conformation of large extracellular loops, loop bridging TM2 domain and HB helix along with positioning of selectivity filter lining residues controls the permeability of water across the bilayer. Moreover, the identified unique and conserved lipid binding sites with 100% lipid occupancy signifies lipid mediated structural dynamics of CbAQP1. All-together, this is the first ever report on structural-dynamics of aquaporin 1 in walking catfish which will be useful to understand the molecular basis of transportation of water and other small molecules under varying degree of hyperosmotic environment.

Regulation of salt-stressed sunflower (Helianthus annuus) seedling's water status by the coordinated action of Na+/K+ accumulation, nitric oxide, and aquaporin expression

Among abiotic stresses, salt stress is a major threat to crop production all over the world. Present work demonstrates the profuse accumulation of Na+ in 2-day-old, dark-grown sunflower (Helianthus annuus L.) seedlings roots in response to salt stress (NaCl). The pattern of K+ accumulation in response to salt stress is similar to that of Na+ but on relatively lower scale. Application of nitric oxide (NO) donor (DETA) scales down Na+ accumulation in salt-stressed seedlings. The impact of NO donor on K+ accumulation is, however, different in control and salt-stressed seedling roots. In control seedlings, it enhances K+ accumulation, whereas, it gets reduced in salt-stressed seedlings. Specialised channels called 'aquaporins' (AQPs) play a major role maintaining the water status and transport across plant parts under salt-stress. Thus, accumulation of plasma-membrane intrinsic proteins (PIPs) and tonoplast-intrinsic proteins (TIPs), localised on plasma-membrane and vacuolar-membrane, respectively was undertaken in 2-day-old, dark-grown seedling roots. Salt stress increased the abundance of these isoforms, whereas, NO application resulted in decreased accumulation of PIP2 and TIP1. PIP1 and TIP2 isoforms remained undetectable. Present work thus, puts forward a correlation between AQP expression and ions (Na+ and K+) homeostasis in response to salt stress and NO.

Ion Transporters and Osmoregulation in the Kidney of Teleost Fishes as a Function of Salinity

Euryhaline teleosts exhibit major changes in renal function as they move between freshwater (FW) and seawater (SW) environments, thus tolerating large fluctuations in salinity. In FW, the kidney excretes large volumes of water through high glomerular filtration rates (GFR) and low tubular reabsorption rates, while actively reabsorbing most ions at high rates. The excreted product has a high urine flow rate (UFR) with a dilute composition. In SW, GFR is greatly reduced, and the tubules reabsorb as much water as possible, while actively secreting divalent ions. The excreted product has a low UFR, and is almost isosmotic to the blood plasma, with Mg²⁺, SO₄^2–, and Cl^– as the major ionic components. Early studies at the organismal level have described these basic patterns, while in the last two decades, studies of regulation at the cell and molecular level have been implemented, though only in a few euryhaline groups (salmonids, eels, tilapias, and fugus). There have been few studies combining the two approaches. The aim of the review is to integrate known aspects of renal physiology (reabsorption and secretion) with more recent advances in molecular water and solute physiology (gene and protein function of transporters). The renal transporters addressed include the subunits of the Na⁺, K⁺- ATPase (NKA) enzyme, monovalent ion transporters for Na⁺, Cl^–, and K⁺ (NKCC1, NKCC2, CLC-K, NCC, ROMK2), water transport pathways [aquaporins (AQP), claudins (CLDN)], and divalent ion transporters for SO₄^2–, Mg²⁺, and Ca²⁺ (SLC26A6, SLC26A1, SLC13A1, SLC41A1, CNNM2, CNNM3, NCX1, NCX2, PMCA). For each transport category, we address the current understanding at the molecular level, try to synthesize it with classical knowledge of overall renal function, and highlight knowledge gaps. Future research on the kidney of euryhaline fishes should focus on integrating changes in kidney reabsorption and secretion of ions with changes in transporter function at the cellular and molecular level (gene and protein verification) in different regions of the nephrons. An increased focus on the kidney individually and its functional integration with the other osmoregulatory organs (gills, skin and intestine) in maintaining overall homeostasis will have applied relevance for aquaculture.

Cloning and Expression of Four Aquaporin Homologs from the Chinese Black Sleeper (Bostrychus sinensis): The Effects of Salinity Acclimation

Several fish species are known to possess mechanisms that allow them to adapt to environments with different salinities. The aim of this study was to investigate the effects of salinity on the expression of aquaporins (aqp1a, aqp3a, aqp8a, and aqp9a) in the gills and intestines of Chinese black sleeper. After 30 days of acclimation, the expression of aqp1a, aqp3a, and aqp9a in the gills was significantly higher in fish transferred to 5 ppt than in those transferred to 40 ppt seawater, whereas aqp8 expression was lower. In contrast, aqp1a, aqp3a, and aqp8a expression in the intestines was higher in fish acclimated in 40 ppt than in those acclimated in 5 ppt. During abrupt salinity acclimation, the levels of aqp1a and aqp9a in the gills varied over time in fish acclimated in 5 ppt, but not in 40 ppt. The aqp3a levels in gills were higher in the 5 ppt group after 24 h than in the 40 ppt. The expression level of aqp8a in gills was higher in 40 ppt than in 5 ppt, except for that at 12 h. In the intestines, expression level of aqp1a and aqp8a were significantly upregulated from 12 to 48 h following acclimation in 40 ppt and aqp3a was higher in 40 ppt group than in 5 ppt, while aqp9a expression exhibited an opposite trend. These findings suggest that aqp1a, aqp3a, aqp8a and aqp9a may play a major osmoregulatory role in water transport in the gills and intestines during acclimation to different salinity environment.

Role of Transportome in the Gills of Chinese Mitten Crabs in Response to Salinity Change: A Meta-Analysis of RNA-Seq Datasets

Chinese mitten crab (CMC) or Eriocheir sinensis is a strong osmoregulator that can keep rigorous cellular homeostasis. CMC can flourish in freshwater, as well as seawater, habitats and represents the most important species for freshwater aquaculture. Salt stress can have direct effects on several stages (e.g., reproduction, molting, growth, etc.) of the CMC life cycle. To get a better overview of the genes involved in the gills of CMC under different salinity conditions, we conducted an RNA-Seq meta-analysis on the transcriptomes of four publicly available datasets. The meta-analysis identified 405 differentially expressed transcripts (DETs), of which 40% were classified into various transporter classes, including accessory factors and primary active transporters as the major transport classes. A network analysis of the DETs revealed that adaptation to salinity is a highly regulated mechanism in which different functional modules play essential roles. To the best of our knowledge, this study is the first to conduct a transcriptome meta-analysis of gills from crab RNA-Seq datasets under salinity. Additionally, this study is also the first to focus on the differential expression of diverse transporters and channels (transportome) in CMC. Our meta-analysis opens new avenues for a better understanding of the osmoregulation mechanism and the selection of potential transporters associated with salinity change.

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.

Physiological trade-offs, acid-base balance and ion-osmoregulatory plasticity in European sea bass (Dicentrarchus labrax) juveniles under complex scenarios of salinity variation, ocean acidification and high ammonia challenge

In this era of global climate change, ocean acidification is becoming a serious threat to the marine ecosystem. Despite this, it remains almost unknown how fish will respond to the co-occurrence of ocean acidification with other conventional environmental perturbations typically salinity fluctuation and high ammonia threat. Therefore, the present work evaluated the interactive effects of elevated pCO₂, salinity reduction and high environmental ammonia (HEA) on the ecophysiological performance of European sea bass (Dicentrarchus labrax). Fish were progressively acclimated to seawater (32 ppt), to brackish water (10 ppt) and to hyposaline water (2.5 ppt). Following acclimation to different salinities for at least two weeks, fish were exposed to CO₂-induced water acidification representing present-day (control pCO₂, 400 μatm, LoCO₂) and future (high pCO₂, 1000 μatm, HiCO₂) sea-surface CO₂ level for 3, 7 and 21 days. At the end of each exposure period, fish were challenged with HEA for 6 h (1.18 mM representing 50% of 96 h LC₅₀). Results show that, in response to the individual HiCO₂ exposure, fish within each salinity compensated for blood acidosis. Fish subjected to HiCO₂ were able to maintain ammonia excretion rate (J_amm) within control levels, suggesting that HiCO₂ exposure alone had no impact on J_amm at any of the salinities. For 32 and 10 ppt fish, up-regulated expression of Na⁺/K⁺-ATPase was evident in all exposure groups (HEA, HiCO₂ and HEA/HiCO₂ co-exposed), whereas Na⁺/K⁺/2Cl^- co-transporter was up-regulated mainly in HiCO₂ group. Plasma glucose and lactate content were augmented in all exposure conditions for all salinity regimes. During HEA and HEA/HiCO₂, J_amm was inhibited at different time points for all salinities, which resulted in a significant build-up of ammonia in plasma and muscle. Branchial expressions of Rhesus glycoproteins (Rhcg isoforms and Rhbg) were upregulated in response to HiCO₂ as well as HEA at 10 ppt, with a more moderate response in 32 ppt groups. Overall, our findings denote that the adverse effect of single exposures of ocean acidification or HEA is exacerbated when present together, and suggests that fish are more vulnerable to these environmental threats at low salinities.

Salinity reduction benefits European eel larvae: Insights at the morphological and molecular level

European eel (Anguilla anguilla) is a euryhaline species, that has adapted to cope with both, hyper- and hypo-osmotic environments. This study investigates the effect of salinity, from a morphological and molecular point of view on European eel larvae reared from 0 to 12 days post hatch (dph). Offspring reared in 36 practical salinity units (psu; control), were compared with larvae reared in six scenarios, where salinity was decreased on 0 or 3 dph and in rates of 1, 2 or 4 psu/day, towards iso-osmotic conditions. Results showed that several genes relating to osmoregulation (nkcc2α, nkcc2β, aqp1dup, aqpe), stress response (hsp70, hsp90), and thyroid metabolism (thrαA, thrαB, thrβB, dio1, dio2, dio3) were differentially expressed throughout larval development, while nkcc1α, nkcc2β, aqp3, aqp1dup, aqpe, hsp90, thrαA and dio3 showed lower expression in response to the salinity reduction. Moreover, larvae were able to keep energy metabolism related gene expression (atp6, cox1) at stable levels, irrespective of the salinity reduction. As such, when reducing salinity, an energy surplus associated to reduced osmoregulation demands and stress (lower nkcc, aqp and hsp expression), likely facilitated the observed increased survival, improved biometry and enhanced growth efficiency. Additionally, the salinity reduction decreased the amount of severe deformities such as spinal curvature and emaciation but also induced an edematous state of the larval heart, resulting in the most balanced mortality/deformity ratio when salinity was decreased on 3 dph and at 2 psu/day. However, the persistency of the pericardial edema and if or how it represents an obstacle in further larval development needs to be further clarified. In conclusion, this study clearly showed that salinity reduction regimes towards iso-osmotic conditions facilitated the European eel pre-leptocephalus development and revealed the existence of highly sensitive and regulated osmoregulation processes at such early life stage of this species.

Kidney morphology and candidate gene expression shows plasticity in sticklebacks adapted to divergent osmotic environments

Novel physiological challenges in different environments can promote the evolution of divergent phenotypes, either through plastic or genetic changes. Environmental salinity serves as a key barrier to the distribution of nearly all aquatic organisms, and species diversification is likely to be enabled by adaptation to alternative osmotic environments. The threespine stickleback (Gasterosteus aculeatus) is a euryhaline species with populations found both in marine and freshwater environments. It has evolved both highly plastic and locally adapted phenotypes due to salinity-derived selection, but the physiological and genetic basis of adaptation to salinity is not fully understood. We integrated comparative cellular morphology of the kidney, a key organ for osmoregulation, and candidate gene expression to explore the underpinnings of evolved variation in osmotic plasticity within two populations of sticklebacks from distinct salinity zones in the Baltic Sea: the high salinity Kattegat, representative of the ancestral marine habitat, and the low salinity Bay of Bothnia. A common-garden experiment revealed that kidney morphology in the ancestral high salinity population had a highly plastic response to salinity conditions, whereas this plastic response was reduced in the low salinity population. Candidate gene expression in kidney tissue revealed a similar pattern of population-specific differences, with a higher degree of plasticity in the native high salinity population. Together these results suggest that renal cellular morphology has become canalized to low salinity, and that these structural differences may have functional implications for osmoregulation.

Decreasing salinity of seawater moderates immune response and increases survival rate of giant groupers post betanodavirus infection

Giant groupers (Epinephelus lanceolatus), an important aquaculture fish in Asia, are attacked by nervous necrosis virus (NNV), belonging to betanodavirus. Environmental salinity can affect fish immunity and physiology. We examined whether decreasing salinity from 30 to 15 ppt during acclimation of groupers could affect survival with NNV infection and the associated factors. Although NNV infection decreased muscle moisture, up-regulated the gene expression of Na(+)-K(+)-2Cl(-) cotransporter isoform 2, and elevated plasma cortisol level in groupers, these factors were not related to the higher mortality of groupers reared at 30-ppt salinity (S30-groupers), compared to 15-ppt reared groupers (S15-groupers). Infected S30-groupers exhibited high leukocyte count and innate immune gene expression level. Moreover, NNV-infected dead S30-groupers showed high IL-1β gene expression level but low NNV load in the brain. The high or excess IL-1β gene expression levels in the brain of NNV-infected S30-groupers may be the factor in high mortality.

Effects of salinity on upstream-migrating, spawning sea lamprey, Petromyzon marinus

The sea lamprey, Petromyzon marinus, is an anadromous, semelparous species that is vulnerable to endangered in parts of its native range due in part to loss of spawning habitat because of man-made barriers. The ability of lampreys to return to the ocean or estuary and search out alternative spawning river systems would be limited by their osmoregulatory ability in seawater. A reduction in tolerance to salinity has been documented in migrants, although the underlying mechanisms have not been characterized. We examined the capacity for marine osmoregulation in upstream spawning migrants by characterizing the physiological effects of salinity challenge from a molecular perspective. Estuarine-captured migrants held in freshwater (FW) for ∼1 week (short-term acclimation) or 2 months (long-term acclimation) underwent an incremental salinity challenge until loss of equilibrium occurred and upper thresholds of 25 and 17.5, respectively, occurred. Regardless of salinity tolerance, all lamprey downregulated FW ion-uptake mechanisms [gill transcripts of Na(+):Cl(-) cotransporter (NCC/slc12a3) and epithelial Na(+) channel (ENaC/scnn1) and kidney Na(+)/K(+)-ATPase (NKA) protein and activity but not transcript]. At their respective salinity limits, lamprey displayed a clear osmoregulatory failure and were unable to regulate [Na(+)] and [Cl(-)] in plasma and intestinal fluid within physiological limits, becoming osmocompromised. A >90% drop in haematocrit indicated haemolysis, and higher plasma concentrations of the cytosolic enzymes alanine aminotransferase, aspartate aminotransferase and lactate dehydrogenase indicated damage to other tissues, including liver. However, >80% of short-term FW-acclimated fish were able to osmoregulate efficiently, with less haemolysis and tissue damage. This osmoregulatory ability was correlated with significant upregulation of the secretory form of Na(+):K(+):2Cl(-) cotransporter (NKCC1/slc12a2) transcript levels and the re-emergence of seawater-type ionocytes detected through immunohistochemical NKA immunoreactivity in the gill, the central ionoregulatory organ. This work sheds light on the molecular and physiological limits to the potential return to seawater for lampreys searching for alternative FW systems in which to spawn.

Seahorse Brood Pouch Transcriptome Reveals Common Genes Associated with Vertebrate Pregnancy

Viviparity (live birth) has evolved more than 150 times in vertebrates, and represents an excellent model system for studying the evolution of complex traits. There are at least 23 independent origins of viviparity in fishes, with syngnathid fishes (seahorses and pipefish) unique in exhibiting male pregnancy. Male seahorses and pipefish have evolved specialized brooding pouches that provide protection, gas exchange, osmoregulation, and limited nutrient provisioning to developing embryos. Pouch structures differ widely across the Syngnathidae, offering an ideal opportunity to study the evolution of reproductive complexity. However, the physiological and genetic changes facilitating male pregnancy are largely unknown. We used transcriptome profiling to examine pouch gene expression at successive gestational stages in a syngnathid with the most complex brood pouch morphology, the seahorse Hippocampus abdominalis. Using a unique time-calibrated RNA-seq data set including brood pouch at key stages of embryonic development, we identified transcriptional changes associated with brood pouch remodeling, nutrient and waste transport, gas exchange, osmoregulation, and immunological protection of developing embryos at conception, development and parturition. Key seahorse transcripts share homology with genes of reproductive function in pregnant mammals, reptiles, and other live-bearing fish, suggesting a common toolkit of genes regulating pregnancy in divergent evolutionary lineages.

Expression and activity of V-H+ -ATPase in gill and kidney of marbled eel Anguilla marmorata in response to salinity challenge

The full-length complementary (c)DNA of vacuolar-type-H(+) -ATPase B1 gene (vhab1) in marbled eel Anguilla marmorata with 1741 base pairs (bp) was identified. It contained a 1512 bp open reading frame encoding a polypeptide with 503 amino acids (55·9 kDa), an 83 bp 5'-untranslated region (UTR) and a 146 bp 3'-UTR. The expression levels of A. marmorata vhab1 in gill and kidney of A. marmorata were evaluated at different intervals during the exposure to various salinities (0, 10 and 25). The results indicated that the expression levels of A. marmorata vhab1 messenger (m)RNA in gill and kidney had a significant increase and reached the highest level at 1 h in brackish water (BW, salinity 10) group and 6 h in seawater (SW, salinity 25) group. Therefore, salinity did affect the relative expression level of A. marmorata vhab1 mRNA in gills, which exhibited the enhancement by c. 44 times in SW group when compared with that in fresh water. No remarkable difference in the expression of A. marmorata vhab1 mRNA was observed after 15 days of SW exposure (P > 0·05). V-H(+) -ATPase activity exhibited an increase by two- to three-fold when compared with that in gill and kidney from the control group. The consequence primarily suggested that A. marmorata vhab1 gene product in elvers from A. marmorata plays an important role in adaptation response to SW.

Hypo-osmotic stress-induced physiological and ion-osmoregulatory responses in European sea bass (Dicentrarchus labrax) are modulated differentially by nutritional status

We investigated the impact of nutritional status on the physiological, metabolic and ion-osmoregulatory performance of European sea bass (Dicentrarchus labrax) when acclimated to seawater (32 ppt), brackish water (20 and 10 ppt) and hyposaline water (2.5 ppt) for 2 weeks. Following acclimation to different salinities, fish were either fed or fasted (unfed for 14 days). Plasma osmolality, [Na(+)], [Cl(-)] and muscle water content were severely altered in fasted fish acclimated to 10 and 2.5 ppt in comparison to normal seawater-acclimated fish, suggesting ion regulation and acid-base balance disturbances. In contrast to feed-deprived fish, fed fish were able to avoid osmotic perturbation more effectively. This was accompanied by an increase in Na(+)/K(+)-ATPase expression and activity, transitory activation of H(+)-ATPase (only at 2.5 ppt) and down-regulation of Na(+)/K(+)/2Cl(-) gene expression. Ammonia excretion rate was inhibited to a larger extent in fasted fish acclimated to low salinities while fed fish were able to excrete efficiently. Consequently, the build-up of ammonia in the plasma of fed fish was relatively lower. Energy stores, especially glycogen and lipid, dropped in the fasted fish at low salinities and progression towards the anaerobic metabolic pathway became evident by an increase in plasma lactate level. Overall, the results indicate no osmotic stress in both feeding treatments within the salinity range of 32 to 20 ppt. However, at lower salinities (10-2.5 ppt) feed deprivation tends to reduce physiological, metabolic, ion-osmo-regulatory and molecular compensatory mechanisms and thus limits the fish's abilities to adapt to a hypo-osmotic environment.

Ontogeny and osmoregulatory function of the urinary system in the Persian sturgeon, Acipenser persicus (Borodin, 1897)

The structure of the kidney and the localization of Na(+), K(+)-ATPase (NKA) immunopositive cells were examined throughout the postembryonic development of the Persian sturgeon, Acipenser persicus, from newly hatched prelarvae (10mm) to 20 days post hatch (20 DPH) larvae (31mm). Investigations were conducted through histology and immunohistochemistry by using the light and immunofluorescence microscopy. The pronephros was observed in newly hatched prelarvae. The cells lining the distal pronephric tubules and their collecting ducts showed laterally expressed NKA immunofluorescence that later extended throughout the whole cytoplasm. Mesonephrogenous placodes and pre-glomeruli were distinguished at 2 DPH along the collecting ducts posteriorly. Their tubules were formed and present in kidney mesenchyma, differentiated into neck, proximal, distal and collecting segments at 7 DPH when NKA immunopositive cells were observed. Their distal and collecting tubules showed an increasing immunofluorescence throughout their cytoplasm while the glomeruli remained unstained. From D 9 to D 17, the epithelial layer of pronephric collecting duct changed along the mesonephros to form ureters. Ureters, possessing isolated strong NKA immunopositive cells, appeared as two sac-like structures hanging under the trunk kidney. Since NKA immunopositive cells were not observed on the tegument or along the digestive tract of newly hatched prelarva, and also the gills are not formed yet, the pronephros is the only osmoregulatory organ until 4 DPH. At the larval stage, the pronephros and mesonephros are functional osmoregulatory organs and actively reabsorb necessary ions from the filtrate.

Functional and population genomic divergence within and between two species of killifish adapted to different osmotic niches

Adaptation to salinity affects species distributions, promotes speciation, and guides many evolutionary patterns in fishes. To uncover the basis of a complex trait like osmoregulation, genome-level analyses are sensible. We combine population genomic scans with genome expression profiling to discover candidate genes and pathways associated with divergence between osmotic environments. We compared transcriptome sequence divergence between multiple freshwater and saltwater populations of the rainwater killifish, Lucania parva. We also compared sequence divergence between L. parva and its sister species, Lucania goodei, a freshwater specialist. We found highly differentiated single nucleotide polymorphisms (SNPs) between freshwater and saltwater L. parva populations in cell junction and ion transport genes, including V-type H(+) ATPase. Between species, we found divergence in reproduction and osmotic stress genes. Genes that were differentially expressed between species during osmotic acclimation included genes involved in ion transport and cell volume regulation. Gene sets that were divergent in coding sequence and divergent in expression did not overlap, although they did converge in function. Like many studies using genomic scans, our approach may miss some loci that contribute to adaptation but have complicated patterns of allelic variation. Our study suggests that gene expression and coding sequence may evolve independently as populations adapt to a complex physiological challenge.

Adaptation to different salinities exposes functional specialization in the intestine of the sea bream (Sparus aurata L.)

The processing of intestinal fluid, in addition to a high drinking rate, is essential for osmoregulation in marine fish. This study analyzed the long-term response of the sea bream (Sparus aurata L.) to relevant changes of external salinity (12, 35 and 55 p.p.t.), focusing on the anterior intestine and in the less-often studied rectum. Intestinal water absorption, epithelial HCO(3)(-) secretion and gene expression of the main molecular mechanisms (SLC26a6, SLC26a3, SLC4a4, atp6v1b, CFTR, NKCC1 and NKCC2) involved in Cl(-) and HCO(3)(-) movements were examined. The anion transporters SLC26a6 and SLC26a3 are expressed severalfold higher in the anterior intestine, while the expression of Atp6v1b (V-type H(+)-ATPase β-subunit) is severalfold higher in the rectum. Prolonged exposure to altered external salinity was without effect on water absorption but was associated with concomitant changes in intestinal fluid content, epithelial HCO(3)(-) secretion and salinity-dependent expression of SLC26a6, SLC26a3 and SLC4a4 in the anterior intestine. However, the most striking response to external salinity was obtained in the rectum, where a 4- to 5-fold increase in water absorption was paralleled by a 2- to 3-fold increase in HCO(3)(-) secretion in response to a salinity of 55 p.p.t. In addition, the rectum of high salinity-acclimated fish shows a sustained (and enhanced) secretory current (I(sc)), identified in vitro in Ussing chambers and confirmed by the higher expression of CFTR and NKCC1 and by immunohistochemical protein localization. Taken together, the present results suggest a functional anterior-posterior specialization with regard to intestinal fluid processing and subsequently to salinity adaptation of the sea bream. The rectum becomes more active at higher salinities and functions as the final controller of intestinal function in osmoregulation.

Cloning and expression of three aquaporin homologues from the European eel (Anguilla anguilla): effects of seawater acclimation and cortisol treatment on renal expression

BACKGROUND INFORMATION

The European eel (Anguilla anguilla) is able to osmoregulate over a wide range of environmental salinities from FW (freshwater) to hyperconcentrated SW (seawater). Successful acclimation is associated with strict regulation of ion and water transport pathways within key osmoregulatory epithelia to enable animals to survive the dehydrating or oedematous conditions. These observations suggested that homologues of the AQP (aquaporin) water channel family were expressed in the eel and that these proteins may contribute to the water transport and osmoregulation in all euryhaline teleosts.

RESULTS

Complementary DNAs encoding a homologue of the mammalian aquaglyceroporins (termed AQPe) and two homologues of mammalian aquaporin-1 [termed AQP1 and AQP1dup (aquaporin-1 duplicate)] were isolated from the European eel. Northern-blot analysis revealed (i) two AQP1 transcripts exhibiting a wide tissue distribution, (ii) a single AQP1dup mRNA transcript found in the kidney and the oesophagus, and (iii) a single AQPe mRNA detectable mainly in the kidney and the intestine. The relative expression of isoforms within the kidney was AQP1dup>AQPe>AQP1. SW acclimation significantly reduced the abundance of AQP1, AQP1dup and AQPe transcripts in the kidney of yellow eels by approx. 72, 66 and 34% respectively, whereas the expression levels in silver eels were independent of salinity and equivalent to those observed in yellow SW-acclimated fish. AQP1 protein expression was primarily located within the vascular endothelium in yellow eels and the epithelial apical brush border in some renal tubules in silver eels. Infusion of cortisol into FW eels had no effect on AQPe mRNA expression, but induced significant decreases in AQP1 and AQP1dup mRNA levels in the kidney of yellow eels. Cortisol infusion had no effect on the expression of any isoform in the silver eels.

CONCLUSIONS

These results suggest that SW-acclimation or cortisol infusion induces a down-regulation of renal AQP expression in yellow eels. However, the lower levels of aquaporin expression found within the silver eel kidney were not further reduced by salinity transfer or steroid infusion. These differences in mRNA expression were accompanied by changes in the cellular distribution of the AQP1 protein between vascular endothelial and tubular epithelial cells.

Distinct freshwater and seawater isoforms of Na+/K+-ATPase in gill chloride cells of Atlantic salmon

Gill Na(+)/K(+)-ATPase (NKA) in teleost fishes is involved in ion regulation in both freshwater and seawater. We have developed and validated rabbit polyclonal antibodies specific to the NKA alpha1a and alpha1b protein isoforms of Atlantic salmon (Salmo salar Linnaeus), and used western blots and immunohistochemistry to characterize their size, abundance and localization. The relative molecular mass of NKA alpha1a is slightly less than that for NKA beta1b. The abundance of gill NKA alpha1a was high in freshwater and became nearly undetectable after seawater acclimation. NKA alpha1b was present in small amounts in freshwater and increased 13-fold after seawater acclimation. Both NKA isoforms were detected only in chloride cells. NKA alpha1a was located in both filamental and lamellar chloride cells in freshwater, whereas in seawater it was present only as a faint background in filamental chloride cells. In freshwater, NKA alpha1b was found in a small number of filamental chloride cells, and after seawater acclimation it was found in all chloride cells on the filament and lamellae. Double simultaneous immunofluorescence indicated that NKA alpha1a and alpha1b are located in different chloride cells in freshwater. In many chloride cells in seawater, NKA alpha1b was present in greater amounts in the subapical region than elsewhere in the cell. The combined patterns in abundance and immunolocalization of these two isoforms can explain the salinity-related changes in total NKA and chloride cell abundance. The results indicate that there is a freshwater and a seawater isoform of NKA alpha-subunit in the gills of Atlantic salmon and that they are present in distinct chloride cells.

Hox cluster duplication in the basal teleost Hiodon alosoides (Osteoglossomorpha)

Large-scale—even genome-wide—duplications have repeatedly been invoked as an explanation for major radiations. Teleosts, the most species-rich vertebrate clade, underwent a “fish-specific genome duplication” (FSGD) that is shared by most ray-finned fish lineages. We investigate here the Hox complement of the goldeye (Hiodon alosoides), a representative of Osteoglossomorpha, the most basal teleostean clade. An extensive PCR survey reveals that goldeye has at least eight Hox clusters, indicating a duplicated genome compared to basal actinopterygians. The possession of duplicated Hox clusters is uncoupled to species richness. The Hox system of the goldeye is substantially different from that of other teleost lineages, having retained several duplicates of Hox genes for which crown teleosts have lost at least one copy. A detailed analysis of the PCR fragments as well as full length sequences of two HoxA13 paralogs, and HoxA10 and HoxC4 genes places the duplication event close in time to the divergence of Osteoglossomorpha and crown teleosts. The data are consistent with—but do not conclusively prove—that Osteoglossomorpha shares the FSGD.

Rectal water absorption in seawater-adapted Japanese eel Anguilla japonica

Marine teleosts drink large amounts of seawater to compensate for continuous osmotic water loss. We investigated a possible significant role of the rectum in water absorption in seawater-adapted eel. In rectal sacs filled with balanced salt solution (BSS) and incubated in isotonic BSS, water absorption was greater in seawater-adapted eel than in freshwater eel. Since rectal fluid osmolality was slightly lower than plasma osmolality in seawater-adapted eel, effects of rectal fluid osmolality on water absorption were examined in rectal sacs filled with artificial rectal fluid with different osmolality. Rectal water absorption was greater at lower rectal fluid osmolality, suggesting that an osmotic gradient between the blood and rectal fluid drives the water movement. Ouabain, a specific inhibitor of Na+/K(+)-ATPase, inhibited water absorption in rectal sacs, indicating that an osmotic gradient favorable to rectal water absorption was created by ion uptake driven by Na+/K(+)-ATPase. Expression levels of aquaporin 1 (AQP1), a water-selective channel, were significantly higher in the rectum than in the anterior and posterior intestines. Immunoreaction for Na+/K(+)-ATPase was detected in the mucosal epithelial cells in the rectum with more intense staining in the basal half than in the apical half, whereas AQP1 was located in the apical membrane of Na+/K(+)-ATPase-immunoreactive epithelial cells. The rectum is spatially separated from the posterior intestine by a valve structure and from the anus by a sphincter. Such structures allow the rectum to swell as intestinal fluid flows into it, and a concomitant increase in hydrostatic pressure may provide an additional force for rectal water absorption. Our findings indicate that the rectum contributes greatly to high efficiency of intestinal water absorption by simultaneous absorption of ions and water.

Aquaporin molecular characterization in the sea-bass (Dicentrarchus labrax): the effect of salinity on AQP1 and AQP3 expression

Euryhaline fish possess the ability to compensate for environmental salinity changes through hydro-mineral regulation. A number of proteins have been studied in order to understand water and ion exchanges, known as fish osmoregulation. Sea-bass (Dicentrarchus labrax) cDNA sequences encoding a homologue of mammalian aquaporin (termed AQP1) and a homologue of mammalian aquaglyceroporin (termed AQP3) have been isolated and sequenced. The aquaporin amino acid sequences share respectively more than 60% and 65% identity with other known aquaporins. We have shown that salinity influences aquaporin expression levels in the gill, kidney and digestive tract, the main osmoregulatory organs. AQP1 may have a major osmoregulatory role in water transport in kidney and gut in SW-acclimated fish, whereas AQP3 could be implicated in gill water transport in FW-acclimated fish.

The Na+/K+/2Cl- cotransporter in the sea bass Dicentrarchus labrax during ontogeny: involvement in osmoregulation

This study combines a cellular and molecular analysis of the Na(+)/K(+)/2Cl(-) cotransporter (NKCC) to determine the osmoregulatory role of this protein in different tissues during the ontogeny of the sea bass. We have characterized the complete sequence of the NKCC1 isoform isolated from the sea bass gills and have identified, by immunofluorescence, NKCC1, and other isoforms, within the epithelium of the major osmoregulatory organs. Different (absorptive and secretory) functions have been attributed to this protein according to the tissue and salinity. The effects of short- (1-4 days), medium- (7-21 days) and long (6 months)-term freshwater (FW) adaptations were investigated, in comparison with seawater (SW)-maintained sea bass. In adult sea bass after long-term adaptation to FW and SW, the gills had the highest expression of NKCC mRNA compared with the median/posterior kidney and to the posterior intestine. Expression of NKCC mRNA in the kidney was 95% (SW) and 63% (FW) lower, and in the intestine 98% (SW) and 77% (FW) lower. Compared to SW-maintained sea bass, long-term FW adaptation induced a significant 5.6-fold decrease in the branchial NKCC gene expression whereas the intestinal and renal expressions did not vary significantly. The cells of the intestine and collecting ducts as well as a part of the epithelium lining the urinary bladder expressed NKCC apically. Within the gill chloride cells, NKCC was found basolaterally in SW-acclimated fish; some apically stained cells were detected after 7 days of FW exposure and their relative number increased progressively following FW acclimation. The appearance of FW-type chloride cells induces a functional shift of the gills from a secretory to an absorptive epithelium, which was only completed after long-term exposure to FW. Short- and medium-term exposure to FW induced a progressive decrease in total NKCC content and an increase in functionally different branchial chloride cells. During development, the cotransporter was already expressed in tegumentary ionocytes and along the digestive tract of late embryos. NKCC was recorded in the branchial chamber and along the renal collecting ducts in prelarvae and also in the dorsal part of the urinary bladder in larvae. The expression of NKCC along the osmoregulatory epithelial cells and the presence of Na(+)/K(+)-ATPase within these cells contribute to the increase of the osmoregulatory capacity during sea bass ontogeny.

Tissue distribution, effects of salinity acclimation, and ontogeny of aquaporin 3 in the marine teleost, silver sea bream (Sparus sarba)

The purpose of the present study was to ascertain the tissue-specific expression of the water channel protein, aquaporin 3 (AQP3), during salinity acclimation and larval development of silver sea bream (Sparus sarba). A cDNA fragment encoding aquaporin 3 (aqp3) from silver sea bream gill was cloned and from the deduced amino acid sequence a polyclonal antibody was prepared. AQP3 was found to be present in gill, kidney, liver, brain, heart, and spleen but not in whole blood. The abundance of AQP3 was significantly highest in gills of hypoosmotic (6 ppt) and isoosmotic (12 ppt) acclimated sea bream when compared to seawater (33 ppt) and hypersaline (50 ppt)- acclimated sea bream. Spleen tissue also displayed significantly high levels of AQP3 protein in hypoosmotic and isoosmotic salinities whereas the AQP3 abundance in brain, liver, heart, and kidney remained unchanged across the range of salinities tested. The ontogenetic profile of AQP3 was also investigated from developing sea bream larvae and AQP3 was first detected at 14 days posthatch (dph) and increased steadily up to 28-46 dph. In conclusion, this study has demonstrated that AQP3 expression is modulated in gill and spleen tissue of salinity acclimated sea bream and that it can be detected relatively early during larval development.

The role of aquaporin 3 in teleost fish

The aquaporin isoform, AQP3 has now been identified in a number of different teleost fish species, with additional DNA sequence information on AQP3 genes in further fish species available in genome databases. In zebrafish (Danio rerio), the AQP3 gene is present as two duplicate isoforms resulting from a teleostean fish genome-wide duplication. A further splicoform/isoform has also been identified in rainbow trout (Oncorhynchus mykiss). The identification of these AQP3 isoforms in other fish species is consequently explored. The role of AQP3 in physiological/osmoregulatory processes, in various teleost organs is then described. In teleost gill, AQP3 is expressed in 'chloride' cells, and in some species, in other epithelial cell types, where it may have a number of different functions including the prevention of dehydration. In eel esophagus, immunohistochemistry shows that AQP3 is expressed in surface epithelial cells in the anterior esophagus, but in mucus cells within the epithelium of the posterior esophagus. In eel intestine, AQP3 is found in macrophage-like cells and probably plays no part in osmoregulatory processes. In the rectum, as in the posterior esophagus AQP3 is expressed in mucus cells. In eel kidney, AQP3 is expressed in a subset of renal tubules, and localizes to the apical pole of tubule cells. There is no apparent change in the location or protein abundance of renal AQP3 following the acclimation of eels from freshwater to seawater.

Digestive tract ontogeny of Dicentrarchus labrax: implication in osmoregulation

The ontogeny of the digestive tract (DT) and of Na(+)/K(+)-ATPase localization was investigated during the early postembryonic development (from yolk sac larva to juvenile) of the euryhaline teleost Dicentrarchus labrax reared at two salinities: seawater and diluted seawater. Histology, electron microscopy and immunocytochemistry were used to determine the presence and differentiation of ion transporting cells. At hatching, the DT is an undifferentiated straight tube over the yolk sac. At the mouth opening (day 5), it comprises six segments: buccopharynx, esophagus, stomach, anterior intestine, posterior intestine and rectum, well differentiated at the juvenile stage (day 72). The enterocytes displayed ultrastructural features similar to those of mitochondria-rich cells known to be involved in active ion transport. At hatching, ion transporting cells lining the intestine and the rectum exhibited a Na(+)/K(+)-ATPase activity which increased mainly after the larva/juvenile (20 mm) metamorphic transition. The immunofluorescence intensity was dependent upon the stage of development of the gut as well as on the histological configuration of the analyzed segment. The appearance and distribution of enteric ionocytes and the implication of the DT in osmoregulation are discussed.

Endocrine regulation of ion transport in the avian lower intestine

The lower intestine (colon and coprodeum) of the domestic fowl maintains a very active, transporting epithelium, with a microvillus brush border, columnar epithelial cells, and a variety of transport systems. The colon of normal or high salt-acclimated hens expresses sodium-linked glucose and amino acid cotransporters, while the coprodeum is relatively inactive. Following acclimation to low salt diets, however, both colon and coprodeum shift to a pattern of high expression of electrogenic sodium channels, and the colonic cotransporter activity is simultaneously downregulated. These changes in the transport patterns seem to be regulated, at least in part, by aldosterone. Our recent work with this tissue has focused on whether aldosterone alone can account for the low salt pattern of transport. Other work has looked at the changes in morphology and in proportions of cell types that occur during chronic acclimation to high or low salt diets, and on a cAMP-activated chloride secretion pathway. Recent findings suggesting effects of other hormones on lower intestinal transport are also presented.

Cell proliferation and apoptosis in the anterior intestine of an amphibious, euryhaline mudskipper (Periophthalmus modestus)

In order to replace the diffusive loss of water to the surrounding environment, seawater (SW)-acclimated euryhaline fishes have gastrointestinal tracts with higher ion/water flux in concert with greater permeability, and contrast that to freshwater (FW)-acclimated fish. To understand the cellular basis for these differences, we examined cell proliferation and apoptosis in the anterior intestine of mudskipper transferred from one-third SW to FW or to SW for 1 and 7 days, and those kept out of water for 1 day. The intestinal apoptosis (indicated by DNA laddering) increased during seawater acclimation. TUNEL staining detected numerous apoptotic cells over the epithelium of SW-acclimated fish. Cell proliferation ([3H]thymidine incorporation) in the FW fish was greater than those in SW 7 days after transfer. Labeling with a Proliferating cell nuclear antigen (PCNA) antibody indicated that proliferating cells were greater in number and randomly distributed in the epithelium of FW fish, whereas in SW fish they were almost entirely in the troughs of the intestinal folds. There were no changes in cell turnover in fish kept out of water. During acclimation to different salinities, modification of the cell turnover and abundance may play an important role in regulating the permeability (and transport capacity) of the gastrointestinal tract of fish.

Duplication events and the evolution of segmental identity

Duplication of genes, genomes, or morphological structures (or some combination of these) has long been thought to facilitate evolutionary change. Here we focus on studies of the teleost fishes to consider the conceptual similarities in the evolutionary potential of these three different kinds of duplication events. We review recent data that have confirmed the occurrence of a whole-genome duplication event in the ray-finned fish lineage, and discuss whether this event may have fuelled the radiation of teleost fishes. We then consider the fates of individual duplicated genes, from both a theoretical and an experimental viewpoint, focusing on our studies of teleost Hox genes and their functions in patterning the segmented hindbrain. Finally, we consider the duplication of morphological structures, once again drawing on our experimental studies of the hindbrain, which have revealed that experimentally induced duplicated neurons can produce functionally redundant neural circuits. We posit that the availability of duplicated material, independent of its nature, can lead to functional redundancy, which in turn enables evolutionary change.

Ontogeny of osmoregulation in postembryonic fish: a review

Salinity and its variations are among the key factors that affect survival, metabolism and distribution during the fish development. The successful establishment of a fish species in a given habitat depends on the ability of each developmental stage to cope with salinity through osmoregulation. It is well established that adult teleosts maintain their blood osmolality close to 300 mosM kg(-1) due to ion and water regulation effected at several sites: tegument, gut, branchial chambers, urinary organs. But fewer data are available in developing fish. We propose a review on the ontogeny of osmoregulation based on studies conducted in different species. Most teleost prelarvae are able to osmoregulate at hatch, and their ability increases in later stages. Before the occurrence of gills, the prelarval tegument where a high density of ionocytes (displaying high contents of Na+/K+-ATPase) is located appears temporarily as the main osmoregulatory site. Gills develop gradually during the prelarval stage along with the numerous ionocytes they support. The tegument and gill Na+/K+-ATPase activity varies ontogenetically. During the larval phase, the osmoregulatory function shifts from the skin to the gills, which become the main osmoregulatory site. The drinking rate normalized to body weight tends to decrease throughout development. The kidney and urinary bladder develop progressively during ontogeny and the capacity to produce hypotonic urine at low salinity increases accordingly. The development of the osmoregulatory functions is hormonally controlled. These events are inter-related and are correlated with changes in salinity tolerance, which often increases markedly at the metamorphic transition from larva to juvenile. In summary, the ability of ontogenetical stages of fish to tolerate salinity through osmoregulation relies on integumental ionocytes, then digestive tract development and drinking rate, developing branchial chambers and urinary organs. The physiological changes leading to variations in salinity tolerance are one of the main basis of the ontogenetical migrations or movements between habitats of different salinity regimes.

Molecular biology of major components of chloride cells

Current understanding of chloride cells (CCs) is briefly reviewed with emphasis on molecular aspects of their channels, transporters and regulators. Seawater-type and freshwater-type CCs have been identified based on their shape, location and response to different ionic conditions. Among the freshwater-type CCs, subpopulations are emerging that are implicated in the uptake of Na(+), Cl(-) and Ca(2+), respectively, and can be distinguished by their shape of apical crypt and affinity for lectins. The major function of the seawater CC is transcellular secretion of Cl(-), which is accomplished by four major channels and transporters: (1). CFTR Cl(-) channel, (2). Na(+),K(+)-ATPase, (3). Na(+)/K(+)/2Cl(-) cotransporter and (4). a K(+) channel. The first three components have been cloned and characterized, but concerning the K(+) channel that is essential for the continued generation of the driving force by Na(+),K(+)-ATPase, only one candidate is identified. Although controversial, freshwater CCs seem to perform the uptake of Na(+), Cl(-) and Ca(2+) in a manner analogous to but slightly different from that seen in the absorptive epithelia of mammalian kidney and intestine since freshwater CCs face larger concentration gradients than ordinary epithelial cells. The components involved in these processes are beginning to be cloned, but their CC localization remains to be established definitively. The most important yet controversial issue is the mechanism of Na(+) uptake. Two models have been postulated: (i). the original one involves amiloride-sensitive electroneutral Na(+)/H(+) exchanger (NHE) with the driving force generated by Na(+),K(+)-ATPase and carbonic anhydrase (CA) and (ii). the current model suggests that Na(+) uptake occurs through an amiloride-sensitive epithelial sodium channel (ENaC) electrogenically coupled to H(+)-ATPase. While fish ENaC remains to be identified by molecular cloning and database mining, fish NHE has been cloned and shown to be highly expressed on the apical membrane of CCs, reviving the original model. The CC is also involved in acid-base regulation. Analysis using Osorezan dace (Tribolodon hakonensis) living in a pH 3.5 lake demonstrated marked inductions of Na(+),K(+)-ATPase, CA-II, NHE3, Na(+)/HCO(3)(-) cotransporter-1 and aquaporin-3 in the CCs on acidification, leading to a working hypothesis for the mechanism of Na(+) retention and acid-base regulation.

Permeabilities of teleost and elasmobranch gill apical membranes: evidence that lipid bilayers alone do not account for barrier function

Teleosts and elasmobranchs faced with considerable osmotic challenges living in sea water, use compensatory mechanisms to survive the loss of water (teleosts) and urea (elasmobranchs) across epithelial surfaces. We hypothesized that the gill, with a high surface area for gas exchange must have an apical membrane of exceptionally low permeability to prevent equilibration between seawater and plasma. We isolated apical membrane vesicles from the gills of Pleuronectes americanus (winter flounder) and Squalus acanthias (dogfish shark) and demonstrated approximately sixfold enrichment of the apical marker, ADPase compared to homogenate. We also isolated basolateral membranes from shark gill (enriched 2.3-fold for Na-K-ATPase) and using stopped-flow fluorometry measured membrane permeabilities to water, urea, and NH3. Apical membrane water permeabilities were similar between species and quite low (7.4 ± 0.7 × 10−4and 6.6 ± 0.8 × 10−4cm/s for shark and flounder, respectively), whereas shark basolateral membranes showed twofold higher water permeability (14 ± 2 × 10−4cm/s). Permeabilities to urea and NH3were also low in apical membranes. Because of the much lower apical to basolateral surface area we conclude that the apical membrane represents an effective barrier. However, the values we obtained were not low enough to account for low water loss (teleosts) and urea loss (elasmobranchs) measured in vivo by others. We conclude that there are other mechanisms which permit gill epithelia to serve as effective barriers. This conclusion has implications for the function of other barrier epithelia, such as the gastric mucosa, mammalian bladder, and renal thick ascending limb.

Proton pump-driven cutaneous chloride uptake in anuran amphibia

Krogh introduced the concept of active ion uptake across surface epithelia of freshwater animals, and proved independent transports of Na(+) and Cl(-) in anuran skin and fish gill. He suggested that the fluxes of Na(+) and Cl(-) involve exchanges with ions of similar charge. In the so-called Krogh model, Cl(-)/HCO(3)(-) and Na(+)/H(+) antiporters are located in the apical membrane of the osmoregulatory epithelium. More recent studies have shown that H(+) excretion in anuran skin is due to a V-ATPase in mitochondria-rich (MR) cells. The pump has been localized by immunostaining and H(+) fluxes estimated by pH-stat titration and mathematical modelling of pH-profiles in the unstirred layer on the external side of the epithelium. H(+) secretion is voltage-dependent, sensitive to carbonic-anhydrase inhibitors, and rheogenic with a charge/ion-flux ratio of unity. Cl(-) uptake from freshwater is saturating, voltage independent, and sensitive to DIDS and carbonic-anhydrase inhibitors. Depending on anuran species and probably on acid/base balance of the animal, apical exit of protons is coupled to an exchange of Cl(-) with base (HCO(3)(-)) either in the apical membrane (gamma-type of MR cell) or in the basolateral membrane (alpha-type MR cell). The gamma-cell model accounts for the rheogenic active uptake of Cl(-) observed in several anuran species. There is indirect evidence also for non-rheogenic active uptake accomplished by a beta-type MR cell with apical base secretion and basolateral proton pumping. Several studies have indicated that the transport modes of MR cells are regulated via ion- and acid/base balance of the animal, but the signalling mechanisms have not been investigated. Estimates of energy consumption by the H(+)-ATPase and the Na(+)/K(+)-ATPase indicate that the gamma-cell accomplishes uptake of NaCl in normal and diluted freshwater. Under common freshwater conditions with serosa-positive or zero V(t), the K(+) conductance of the basolateral membrane would have to maintain the inward driving force for Na(+) uptake across the apical membrane. With the K(+) equilibrium potential across the basolateral membrane estimated to -105 mV, this would apply to external Na(+) concentrations down to 40-120 micromol/l. NaCl uptake from concentrations down to 10 micromol/l, as observed by Krogh, presupposes that the H(+) pump hyperpolarizes the apical membrane, which would then have to be associated with serosa-negative V(t). In diluted freshwater, exchange of cellular HCO(3)(-) with external Cl(-) seems to be possible only if the proton pump has the additional function of keeping the external concentration of HCO(3)(-) low. Quantitative considerations also lead to the conclusion that with the above extreme demand, at physiological intracellular pH of 7.2, the influx of Cl(-) via the apical antiporter and the passive exit of Cl(-) via basolateral channels would be possible within a common range of intracellular Cl(-) concentrations.

Cl- absorption in European eel intestine and its regulation

The intestinal epithelium of the euryhaline teleost fish, Anguilla anguilla, absorbs Cl(-) transepithelially. This gives rise to a negative transepithelial potential at the basolateral side of the epithelium and to a measured short circuit current. Cl(-) absorption occurs via bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransport, localized on the luminal membrane. The cotransport operates in parallel with a luminal K(+) conductance that recycles the ion into the lumen. Cl(-) leaves the cell across the basolateral membrane by way of Cl(-) conductance and presumably via a KCl cotransport. The driving force for this process is provided by the electrochemical sodium gradient across the plasma membrane, generated and maintained by the basolateral Na(+)-K(+)-ATPase. The resulting NaCl absorption process is active and enables marine fish to take up water, thereby compensating for water that was lost passively from the body. Fresh water acclimatized eel also absorb Cl(-) actively, although in smaller quantities, utilizing the same ion transport mechanisms as marine eels. This mechanism is basically the same as the model proposed for the thick ascending limb (cTAL). Cl(-) absorption is regulated by a number of cellular factors, such as HCO(3) (-), pH, Ca(2+), cyclic nucleotides, and cytoskeletal elements. It is sensitive to osmotic stress, and therefore is a good physiological model to study ion transport mechanisms that are activated when osmotic stress induces cell volume regulation. The activation of these various ion transport pathways is dependent on cellular transduction mechanisms in which phosphorylation events (mainly by PKC and MLCK for the hypertonic response) and cytoskeletal elements, either microfilaments or microtubules, seem to play key roles.

Water metabolism in the eel acclimated to sea water: from mouth to intestine

Eels seem to be a suitable model system for analysing regulatory mechanisms of drinking behavior in vertebrates, since most dipsogens and antidipsogens in mammals influence the drinking rate in the seawater eels similarly. The drinking behavior in fishes consists of swallowing alone, since they live in water and water is constantly held in the mouth for respiration. Therefore, contraction of the upper esophageal sphincter (UES) muscle limits the drinking rate in fishes. The UES of the eel was innervated by the glossopharyngeal-vagal motor complex (GVC) in the medulla oblongata (MO). The GVC neurons were immunoreactive to an antibody raised against choline acetyltransferase (ChAT), an acetylcholine (ACh) synthesizing enzyme, indicating that the eel UES muscle is controlled cholinergically by the GVC. The neuronal activity of the GVC was inhibited by adrenaline or dopamine, suggesting catecholaminergic innervation to the GVC. The AP and the commissural nucleus of Cajal (NCC) in the MO projected to the GVC and were immunoreactive to an antibody raised against tyrosine hydroxylase (TH), rate limiting enzyme to produce catecholamines from tyrosine. Therefore, it is likely that activation in the AP or the NCC may inhibit the GVC and thus relaxes the UES muscle, which allows for water to enter into the esophagus. During passing through the esophagus, the imbibed sea water (SW) was desalted to approximately 1/2 SW, which was further diluted in the stomach and arrived at the intestine as approximately 1/3 SW, almost isotonic to the plasma. Finally, from the diluted SW, the eel intestine absorbed water following the Na(+)-K(+)-2Cl(-) cotransport (NKCC2) system. The NaCl and water absorption across the intestine was regulated by various factors, especially by peptides such as atrial natriuretic peptide (ANP) and somatostatin (SS-25 II). During desalination in the esophagus, however, excess salt enters into the blood circulation, which is liable to raise the plasma osmolarity. However, the eel heart was constricted powerfully by the hyperosmolarity, suggesting that the hyperosmolarity enhances the stroke volume to the gill, where excess salt was extruded powerfully via Na(+)-K(+)-2Cl(-) cotransport (NKCC1) system.

Purification, N-terminal sequence determination and enzymatic characterization of antiquitin from the liver of grass carp

Aldehyde dehydrogenase (ALDH) is a superfamily of enzymes catalyzing the conversion of various aldehydes to the corresponding acids using the coenzymes NAD+ or NADP+. While mammalian ALDHs have been studied extensively, the non-mammalian ALDHs, notably those of teleostean origin, remain relatively unexplored. In our previous study on grass carp (Ctenopharyngodon idellus) liver ALDH, a significant amount of the ALDH activity did not adsorb on the alpha-cyanocinnamate Sepharose column which binds ALDH2. The objective of the present study was to purify the ALDH which accounts for this unadsorbed activity. Further chromatography on Affi-gel Blue agarose, followed by size exclusion on Superdex 200 successfully isolated this aldehyde-oxidizing activity. The protein was a homo-tetramer with a subunit molecular mass of 58 kDa. N-terminal sequencing of the first 21 amino acid residues, followed by blastp analysis on the NCBI database revealed the protein as antiquitin. The optimal pH for the oxidation of acetaldehyde was 9.5. At this pH, the Vmax and the Km values for acetaldehyde were 1.95 U/mg and 2.00 mM, respectively.

Channels, pumps, and exchangers in the gill and kidney of freshwater fishes: Their role in ionic and acid-base regulation

In freshwater fishes, the gill and kidney are intricately involved in ionic and acid-base regulation owing to the presence of numerous ion channels, pumps, or exchangers. This review summarizes recent developments in branchial and renal ion transport physiology and presents several models that integrate epithelial ion and acid-base movements in freshwater fishes. At the gill, three cell types are potentially involved in ionic uptake: pavement cells, mitochondria-rich (MR) PNA(+) cells, and MR PNA(-) cells. The transfer of acidic or basic equivalents between the fish and its environment is accomplished largely by the gill and is appropriately regulated to correct acid-base imbalances. The kidney, while less important than the gill in overall acid or base excretion, has an essential role in regulating systemic acid-base balance by controlling HCO(3) (-) reabsorption from the filtrate.

Expression of two isoforms of the vacuolar-type ATPase subunit B in the zebrafish Danio rerio

In the present study we tested the hypothesis that two isoforms of the regulatory subunit B of vacuolar-type ATPase (V-ATPase) are expressed in the zebrafish Danio rerio. The complete coding sequences for both isoforms, vatB1 and vatB2, were cloned and sequenced. BLASTX analysis revealed the greatest similarity to amino acid sequences of B subunits from the European eel Anguilla anguilla and rainbow trout Oncorhynchus mykiss. The isoforms were expressed in a bacterial system and the recombinant proteins verified using isoform-specific antibodies directed against vatB isoforms of the eel. The distribution of both isoforms in zebrafish tissues was investigated using reverse transcriptase-polymerase chain reaction and western blot analysis. The results revealed that at the RNA level both isoforms were expressed in all tested organs, i.e. the gills, swimbladder, heart, kidney, liver, spleen, intestine and skeletal muscle. At the protein level, however, there were tissue-specific variations in the levels of the two vatB isoforms expressed. The highest amounts of V-ATPase were detected in total protein preparations from gill, heart and liver tissue. In liver tissue, however, the western blot analysis indicated that vatB1 was not as prominent as vatB2, and immunohistochemistry revealed that antibodies directed against vatB1 yielded a very weak staining in a number of cells, while an antibody directed against vatB1 and vatB2 yielded a strong staining in virtually every cell. Similarly, neurosecretory cells of the small intestine were stained with an antibody directed against vatB1 and vatB2, but not with an antibody specific for vatB1. Therefore we conclude that the differential expression of two isoforms of the V-ATPase subunits, which may serve different functions as in several mammalian species, may also be a common phenomenon in teleost fish.

Two isoforms of the Na+/K+/2Cl- cotransporter are expressed in the European eel (Anguilla anguilla)

Two cDNA isoforms of the NKCC1 secretory cotransporter have been isolated from the European eel. The NKCC1a isoform exhibited mRNA expression in a wide range of tissues in a similar fashion to mammals, whereas NKCC1b was expressed primarily in the brain. The effect of freshwater (FW) to seawater (SW) transfer on NKCC1a expression was dependent on the developmental stage. In non-migratory yellow eels, NKCC1a mRNA expression in the gill was transiently up-regulated 4.3-fold after 2 days but also subsequently by 2.5-6-fold 3 weeks after SW transfer. Gill NKCC1a expression was localised mainly in branchial chloride cells of SW acclimated yellow eels. In contrast to yellow eels, NKCC1a mRNA abundance was not significantly different following SW acclimation in silver eel gill. NKCC1a mRNA abundance decreased in the kidney following SW acclimation and this may correlate with lower tubular ion/fluid secretion and urine flow rates in SW teleosts. Kidney NKCC1a mRNA expression in silver eels was also significantly lower than in yellow eels, suggesting some pre-acclimation of mRNA levels. NKCC1a mRNA was expressed at similar low levels in the middle intestine of FW- and SW-acclimated yellow or silver eels, suggesting the presence of an ion secretory mechanism in this gut segment.