Proton pump-rich cell secretes acid in skin of zebrafish larvae

Potassium Regulation in Medaka (Oryzias latipes) Larvae Acclimated to Fresh Water: Passive Uptake and Active Secretion by the Skin Cells

Molecular mechanisms of Na⁺, Cl⁻, and Ca²⁺ regulation in ionocytes of fish have been well investigated. However, the regulatory mechanism of K⁺ in fishes has been largely unknown. In this study, we investigated the mechanism of K⁺ regulation in medaka larvae acclimated to fresh water. Using a scanning ion-selective electrode technique (SIET) to measure the K⁺ fluxes at skin cells, significant K⁺ effluxes were found at ionocytes; in contrast, significant K⁺ influxes were found at the boundaries between keratinocytes. High K⁺ water (HK) acclimation induced the K⁺ effluxes at ionocytes and suppressed the K⁺ influxes at keratinocytes. The K⁺ effluxes of ionocytes were suppressed by VU591, bumetanide and ouabain. The K⁺ influxes of keratinocytes were suppressed by TAP. In situ hybridization analysis showed that mRNA of ROMKa was expressed by ionocytes in the skin and gills of medaka larvae. Quantitative PCR showed that mRNA levels of ROMKa and NKCC1a in gills of adult medaka were upregulated after HK acclimation. This study suggests that medaka obtain K⁺ through a paracellular pathway between keratinocytes and extrude K⁺ through ionocytes; apical ROMKa and basolateral NKCC1a are involved in the K⁺ secretion by ionocytes.

Effect of combined stress (salinity and temperature) in European sea bass Dicentrarchus labrax osmoregulatory processes

European sea bass Dicentrarchus labrax undertake seasonal migrations to estuaries and lagoons that are characterized by fluctuations in environmental conditions. Their ability to cope with these unstable habitats is undeniable, but it is still not clear how and to what extent salinity acclimation mechanisms are affected at temperatures higher than in the sea. In this study, juvenile sea bass were pre-acclimated to seawater (SW) at 18°C (temperate) or 24°C (warm) for 2weeks and then transferred to fresh water (FW) or SW at the respective temperature. Transfer to FW for two weeks resulted in decreased blood osmolalities and plasma Cl^- at both temperatures. In FW warm conditions, plasma Na⁺ was ~15% lower and Cl^- was ~32% higher than in the temperate-water group. Branchial Na⁺/K⁺-ATPase (NKA) activity measured at the acclimation temperature (V_apparent) did not change according to the conditions. Branchial Na⁺/K⁺-ATPase activity measured at 37°C (V_max) was lower in warm conditions and increased in FW compared to SW conditions whatever the considered temperature. Mitochondrion-rich cell (MRC) density increased in FW, notably due to the appearance of lamellar MRCs, but this increase was less pronounced in warm conditions where MRC's size was lower. In SW warm conditions, pavement cell apical microridges are less developed than in other conditions. Overall gill morphometrical parameters (filament thickness, lamellar length and width) differ between fish that have been pre-acclimated to different temperatures. This study shows that a thermal change affects gill plasticity affecting whole-organism ion balance two weeks after salinity transfer.

Triportheus albus Cope, 1872 in the Blackwater, Clearwater, and Whitewater of the Amazon: A Case of Phenotypic Plasticity?

The Amazon basin includes 1000s of bodies of water, that are sorted according to their color in three types: blackwater, clearwater, and whitewater, which significantly differ in terms of their physicochemical parameters. More than 3,000 species of fish live in the rivers of the Amazon, among them, the sardine, Triportheus albus, which is one of the few species that inhabit all three types of water. The purpose of our study was to analyze if the gene expression of T. albus is determined by the different types of water, that is, if the species presents phenotypic plasticity to live in blackwater, clearwater, and whitewater. Gills of T. albus were collected at well-characterized sites for each type of water. Nine cDNA libraries were constructed, three biological replicates of each condition and the RNA was sequenced (RNA-Seq) on the MiSeq^® Platform (Illumina^®). A total of 51.6 million of paired-end reads, and 285,456 transcripts were assembled. Considering the FDR ≤ 0.05 and fold change ≥ 2, 13,754 differentially expressed genes were detected in the three water types. Two mechanisms related to homeostasis were detected in T. albus that live in blackwater, when compared to the ones in clearwater and whitewater. The acidic blackwater is a challenging environment for many types of aquatic organisms. The first mechanism is related to the decrease in cellular permeability, highlighting the genes coding for claudin proteins, actn4, itgb3b, DSP, Gap junction protein, and Ca²⁺-ATPase. The second with ionic and acid-base regulation [rhcg1, slc9a6a (NHE), ATP6V0A2, Na⁺/K⁺-ATPase, slc26a4 (pedrin) and slc4a4b]. We suggest T. albus is a good species of fish for future studies involving the ionic and acid-base regulation of Amazonian species. We also concluded that, T. albus, shows well defined phenotypic plasticity for each water type in the Amazon basin.

Insights into molecular and cellular mechanisms of hormonal actions on fish ion regulation derived from the zebrafish model

Fish have sophisticated mechanisms of ionic and acid-base regulation for maintaining body fluid homeostasis. Many hormones have been proposed to control the ionic and acid-base regulation mechanisms in fishes; however, lots of the proposed actions lack convincing cellular/molecular evidence. With the advantages of available genetic databases and molecular manipulation techniques, zebrafish has become an emerging model for research into ion transport physiology and functional regulation. Different types of ionocytes were found to transport ions through various sets of ion transporters, and the molecular mechanisms of ionocyte proliferation and differentiation have also been dissected, providing a competent platform with which to precisely study the ion transport pathways and ionocytes targeted by hormones, including isotocin, prolactin, cortisol, stanniocalcin-1, calcitonin, endothelin-1, vitamin D, parathyroid hormone 1, catecholamines, the renin-angiotensin-system, estrogen-related receptor α, and calcitonin gene-related peptide, which have been demonstrated to positively or negatively regulate ion transport through specific receptors at different molecular levels (transcriptional, translational, or posttranslational) or at different developmental stages of ionocytes (proliferation or differentiation). The knowledge obtained in zebrafish not only enhances our understanding of the hormonal control of fish ion regulation, but also informs studies on other animal species, thereby providing insights into related fields.

Ion transport in the zebrafish kidney from a human disease angle: possibilities, considerations, and future perspectives

Unique experimental advantages, such as its embryonic/larval transparency, high-throughput nature, and ease of genetic modification, underpin the rapid emergence of the zebrafish ( Danio rerio) as a preeminent model in biomedical research. Particularly in the field of nephrology, the zebrafish provides a promising model for studying the physiological implications of human solute transport processes along consecutive nephron segments. However, although the zebrafish might be considered a valuable model for numerous renal ion transport diseases and functional studies of many channels and transporters, not all human renal electrolyte transport mechanisms and human diseases can be modeled in the zebrafish. With this review, we explore the ontogeny of zebrafish renal ion transport, its nephron structure and function, and thereby demonstrate the clinical translational value of this model. By critical assessment of genomic and amino acid conservation of human proteins involved in renal ion handling (channels, transporters, and claudins), kidney and nephron segment conservation, and renal electrolyte transport physiology in the zebrafish, we provide researchers and nephrologists with an indication of the possibilities and considerations of the zebrafish as a model for human renal ion transport. Combined with advanced techniques envisioned for the future, implementation of the zebrafish might expand beyond unraveling pathophysiological mechanisms that underlie distinct genetic or environmentally, i.e., pharmacological and lifestyle, induced renal transport deficits. Specifically, the ease of drug administration and the exploitation of improved genetic approaches might argue for the adoption of the zebrafish as a model for preclinical personalized medicine for distinct renal diseases and renal electrolyte transport proteins.

A role for sodium-chloride cotransporters in the rapid regulation of ion uptake following acute environmental acidosis: new insights from the zebrafish model

The effects of acute exposure to acidic water on Na⁺ and Cl^- homeostasis, and the mechanisms underlying their compensatory regulation, were investigated in the larval zebrafish Danio rerio Exposure to acidic water (pH 4.0; control pH 7.6) for 2 h significantly reduced Na⁺ uptake and whole body Na⁺ content. Nevertheless, the capacity for Na⁺ uptake was substantially increased in fish preexposed to acidic water but measured in control water. Based on the accumulation of the Na⁺-selective dye, Sodium Green, two ionocyte subtypes exhibited intracellular Na⁺ enrichment after preexposure to acidic water: H⁺-ATPase rich (HR) cells, which coexpress the Na⁺/H⁺ exchanger isoform 3b (NHE3b), and a non-HR cell population. In fish experiencing Na⁺-Cl^- cotransporter (NCC) knockdown, we observed no Sodium Green accumulation in the latter cell type, suggesting the non-HR cells were NCC cells. Elimination of NHE3b-expressing HR cells did not prevent the increased Na⁺ uptake following acid exposure. On the other hand, the increased Na⁺ uptake was abolished when the acidic water was enriched with Na⁺ and Cl^-, but not with Na⁺ only, indicating that the elevated Na⁺ uptake after acid exposure was associated with the compensatory regulation of Cl^- Further examinations demonstrated that acute acid exposure also reduced whole body Cl^- levels and increased the capacity for Cl^- uptake. Moreover, knockdown of NCC prevented the increased uptake of both Na⁺ and Cl^- after exposure to acidic water. Together, the results of the present study revealed a novel role of NCC in the compensatory regulation of Na⁺ and Cl^- uptake following acute acidosis.

Differential Lectin Binding Patterns Identify Distinct Heart Regions in Giant Danio ( Devario aequipinnatus) and Zebrafish ( Danio rerio) Hearts

Lectins are carbohydrate-binding proteins commonly used as biochemical and histochemical tools to study glycoconjugate (glycoproteins, glycolipids) expression patterns in cells, tissues, including mammalian hearts. However, lectins have received little attention in zebrafish ( Danio rerio) and giant danio ( Devario aequipinnatus) heart studies. Here, we sought to determine the binding patterns of six commonly used lectins-wheat germ agglutinin (WGA), Ulex europaeus agglutinin, Bandeiraea simplicifolia lectin (BS lectin), concanavalin A (Con A), Ricinus communis agglutinin I (RCA I), and Lycopersicon esculentum agglutinin (tomato lectin)-in these hearts. Con A showed broad staining in the myocardium. WGA stained cardiac myocyte borders, with binding markedly stronger in the compact heart and bulbus. BS lectin, which stained giant danio coronaries, was used to measure vascular reconstruction during regeneration. However, BS lectin reacted poorly in zebrafish. RCA I stained the compact heart of both fish. Tomato lectin stained the giant danio, and while low reactivity was seen in the zebrafish ventricle, staining was observed in their transitional cardiac myocytes. In addition, we observed unique staining patterns in the developing zebrafish heart. Lectins' ability to reveal differential glycoconjugate expression in giant danio and zebrafish hearts suggests they can serve as simple but important tools in studies of developing, adult, and regenerating fish hearts.

Tumor suppression in basal keratinocytes via dual non-cell-autonomous functions of a Na,K-ATPase beta subunit

The molecular pathways underlying tumor suppression are incompletely understood. Here, we identify cooperative non-cell-autonomous functions of a single gene that together provide a novel mechanism of tumor suppression in basal keratinocytes of zebrafish embryos. A loss-of-function mutation in atp1b1a, encoding the beta subunit of a Na,K-ATPase pump, causes edema and epidermal malignancy. Strikingly, basal cell carcinogenesis only occurs when Atp1b1a function is compromised in both the overlying periderm (resulting in compromised epithelial polarity and adhesiveness) and in kidney and heart (resulting in hypotonic stress). Blockade of the ensuing PI3K-AKT-mTORC1-NFκB-MMP9 pathway activation in basal cells, as well as systemic isotonicity, prevents malignant transformation. Our results identify hypotonic stress as a (previously unrecognized) contributor to tumor development and establish a novel paradigm of tumor suppression.

Oestrogen-related receptor α is required for transepithelial H+ secretion in zebrafish

Oestrogen-related receptor α (ERRα) is an orphan nuclear receptor which is important for adaptive metabolic responses under conditions of increased energy demand, such as cold, exercise and fasting. Importantly, metabolism under these conditions is usually accompanied by elevated production of organic acids, which may threaten the body acid-base status. Although ERRα is known to help regulate ion transport by the renal epithelia, its role in the transport of acid-base equivalents remains unknown. Here, we tested the hypothesis that ERRα is involved in acid-base regulation mechanisms by using zebrafish as the model to examine the effects of ERRα on transepithelial H(+) secretion. ERRα is abundantly expressed in H(+)-pump-rich cells (HR cells), a group of ionocytes responsible for H(+) secretion in the skin of developing embryos, and its expression is stimulated by acidic (pH 4) environments. Knockdown of ERRα impairs both basal and low pH-induced H(+) secretion in the yolk-sac skin, which is accompanied by decreased expression of H(+)-secreting-related transporters. The effect of ERRα on H(+) secretion is achieved through regulating both the total number of HR cells and the function of individual HR cells. These results demonstrate, for the first time, that ERRα is required for transepithelial H(+) secretion for systemic acid-base homeostasis.

Different mechanisms of Na+ uptake and ammonia excretion by the gill and yolk sac epithelium of early life stage rainbow trout

In rainbow trout, the dominant site of Na+ uptake (JNain) and ammonia excretion (Jamm) shifts from the skin to the gills over development. Post-hatch (PH; 7 days post-hatch) larvae utilize the yolk sac skin for physiological exchange, whereas by complete yolk sac absorption (CYA; 30 days post-hatch), the gill is the dominant site. At the gills, JNain and Jamm occur via loose Na+/NH4+ exchange, but this exchange has not been examined in the skin of larval trout. Based on previous work, we hypothesized that, contrary to the gill model, JNain by the yolk sac skin of PH trout occurs independently of Jamm. Following a 12-h exposure to high environmental ammonia (HEA; 0.5 mmol l−1 NH4HCO3; [Na+]=600 µmol l−1; pH=8), Jamm by the gills of CYA trout and the yolk sac skin of PH larvae, which were isolated using divided chambers, increased significantly. However, this was coupled to an increase in JNain across the gills only, supporting our hypothesis. Moreover, gene expression of proteins involved in JNain (Na+/H+-exchanger-2 (NHE2) and H+-ATPase) increased in response to HEA only in the CYA gills. We further identified expression of the apical Rhesus (Rh) proteins Rhcg2 in putative pavement cells and Rhcg1 (co-localized with apical NHE2 and NHE3b and Na+/K+-ATPase) in putative peanut lectin agglutinin-positive (PNA+) ionocytes in gill sections. Similar Na+/K+-ATPase-positive cells expressing Rhcg1 and NHE3b, but not NHE2, were identified in the yolk sac epithelium. Overall, our findings suggest that the mechanisms of JNain and Jamm by the dominant exchange epithelium at two distinct stages of early development are fundamentally different.

Recent advances in understanding trans-epithelial acid-base regulation and excretion mechanisms in cephalopods

Cephalopods have evolved complex sensory systems and an active lifestyle to compete with fish for similar resources in the marine environment. Their highly active lifestyle and their extensive protein metabolism has led to substantial acid-base regulatory abilities enabling these organisms to cope with CO2 induced acid-base disturbances. In convergence to teleost, cephalopods possess an ontogeny-dependent shift in ion-regulatory epithelia with epidermal ionocytes being the major site of embryonic acid-base regulation and ammonia excretion, while gill epithelia take these functions in adults. Although the basic morphology and excretory function of gill epithelia in cephalopods were outlined almost half a century ago, modern immunohistological and molecular techniques are bringing new insights to the mechanistic basis of acid-base regulation and excretion of nitrogenous waste products (e.g. NH3/NH4 (+)) across ion regulatory epithelia of cephalopods. Using cephalopods as an invertebrate model, recent findings reveal partly conserved mechanisms but also novel aspects of acid-base regulation and nitrogen excretion in these exclusively marine animals. Comparative studies using a range of marine invertebrates will create a novel and exciting research direction addressing the evolution of pH regulatory and excretory systems.

Cortisol Regulates Acid Secretion of H(+)-ATPase-rich Ionocytes in Zebrafish (Danio rerio) Embryos

Systemic acid-base regulation is vital for physiological processes in vertebrates. Freshwater (FW) fish live in an inconstant environment, and thus frequently face ambient acid stress. FW fish have to efficiently modulate their acid secretion processes for body fluid acid-base homeostasis during ambient acid challenge; hormonal control plays an important role in such physiological regulation. The hormone cortisol was previously proposed to be associated with acid base regulation in FW fish; however, the underlying mechanism has not been fully described. In the present study, mRNA expression of acid-secreting related transporters and cyp11b (encoding an enzyme involved in cortisol synthesis) in zebrafish embryos was stimulated by treatment with acidic FW (AFW, pH 4.0) for 3 d. Exogenous cortisol treatment (20 mg/L, 3 d) resulted in upregulated expression of transporters related to acid secretion and increased acid secretion function at the organism level in zebrafish embryos. Moreover, cortisol treatment also significantly increased the acid secretion capacity of H(+)-ATPase-rich cells (HRCs) at the cellular level. In loss-of-function experiments, microinjection of glucocorticoid receptor (GR) morpholino (MO) suppressed the expression of acid-secreting related transporters, and decreased acid secretion function at both the organism and cellular levels; on the other hand, mineralocorticoid receptor (MR) MO did not induce any effects. Such evidence supports the hypothesized role of cortisol in fish acid-base regulation, and provides new insights into the roles of cortisol; cortisol-GR signaling stimulates zebrafish acid secretion function through transcriptional/translational regulation of the transporters and upregulation of acid secretion capacity in each acid-secreting ionocyte.

Aquaporin 1 Is Involved in Acid Secretion by Ionocytes of Zebrafish Embryos through Facilitating CO2 Transport

Mammalian aquaporin 1 (AQP1) is well known to function as a membrane channel for H₂O and CO₂ transport. Zebrafish AQP1a.1 (the homologue of mammalian AQP1) was recently identified in ionocytes of embryos; however its role in ionocytes is still unclear. In this study, we hypothesized that zebrafish AQP1a.1 is involved in the acid secretion by ionocytes through facilitating H₂O and CO₂ diffusion. A real-time PCR showed that mRNA levels of AQP1a.1 in embryos were induced by exposure to 1% CO₂ hypercapnia for 3 days. In situ hybridization and immunohistochemistry showed that the AQP1a.1 transcript was highly expressed by acid-secreting ionocytes, i.e., H⁺-ATPase-rich (HR) cells. A scanning ion-selective electrode technique (SIET) was applied to analyze CO₂-induced H⁺ secretion by individual ionocytes in embryos. H⁺ secretion by HR cells remarkably increased after a transient loading of CO₂ (1% for 10 min). AQP1a.1 knockdown with morpholino oligonucleotides decreased the H⁺ secretion of HR cells by about half and limited the CO₂ stimulated increase. In addition, exposure to an AQP inhibitor (PCMB) for 10 min also suppressed CO₂-induced H⁺ secretion. Results from this study support our hypothesis and provide in vivo evidence of the physiological role of AQP1 in CO₂ transport.

Mechanism of ammonia excretion in the freshwater leech Nephelopsis obscura: characterization of a primitive Rh protein and effects of high environmental ammonia

Remarkably little is known about nitrogenous excretion in freshwater invertebrates. In the current study, the nitrogen excretion mechanism in the carnivorous ribbon leech, Nephelopsis obscura, was investigated. Excretion experiments showed that the ribbon leech is ammonotelic, excreting 166.0 ± 8.6 nmol·grams fresh weight (gFW)(-1)·h(-1) ammonia and 14.7 ± 1.9 nmol·gFW(-1)·h(-1) urea. Exposure to high and low pH hampered and enhanced, respectively, ammonia excretion rates, indicating an acid-linked ammonia trapping mechanism across the skin epithelia. Accordingly, compared with body tissues, the skin exhibited elevated mRNA expression levels of a newly identified Rhesus protein and at least in tendency the Na(+)/K(+)-ATPase. Pharmacological experiments and enzyme assays suggested an ammonia excretion mechanism that involves the V-ATPase, Na(+)/K(+)-ATPase, and carbonic anhydrase, but not necessarily a functional microtubule system. Most importantly, functional expression studies of the identified Rh protein cloned from leech skin tissue revealed an ammonia transport capability of this protein when expressed in yeast. The leech Rh-ammonia transporter (NoRhp) is a member of the primitive Rh protein family, which is a sister group to the common ancestor of vertebrate ammonia-transporting Rh proteins. Exposure to high environmental ammonia (HEA) caused a new adjustment of body ammonia, accompanied with a decrease in NoRhp and Na(+)/K(+)-ATPase mRNA levels, but unaltered ammonia excretion rates. To our knowledge, this is only the second comprehensive study regarding the ammonia excretion mechanisms in a freshwater invertebrate, but our results show that basic processes of ammonia excretion appear to also be comparable to those found in freshwater fish, suggesting an early evolution of ionoregulatory mechanisms in freshwater organisms.

Osmoregulation in zebrafish: ion transport mechanisms and functional regulation

Fish, like mammals, have to maintain their body fluid ionic and osmotic homeostasis through sophisticated iono-/osmoregulation mechanisms, which are conducted mainly by ionocytes of the gill (the skin in embryonic stages), instead of the renal tubular cells in mammals. Given the advantages in terms of genetic database availability and manipulation, zebrafish is an emerging model for research into regulatory and integrative physiology. At least five types of ionocytes, HR, NaR, NCC, SLC26, and KS cells, have been identified to carry out Na(+) uptake/H(+) secretion/NH4 (+) excretion, Ca(2+) uptake, Na(+)/Cl(-) uptake, K(+) secretion, and Cl(-) uptake/HCO3 (-) secretion, respectively, through distinct sets of transporters. Several hormones, namely isotocin, prolactin, cortisol, stanniocalcin-1, calcitonin, endothelin-1, vitamin D, parathyorid hormone 1, catecholamines, and the renin-angiotensin-system, have been demonstrated to positively or negatively regulate ion transport through specific receptors at different ionocytes stages, at either the transcriptional/translational or posttranslational level. The knowledge obtained using zebrafish answered many long-term contentious or unknown issues in the field of fish iono-/osmoregulation. The homology of ion transport pathways and hormone systems also means that the zebrafish model informs studies on mammals or other animal species, thereby providing insights into related fields.

Induction of Phosphoenolpyruvate Carboxykinase (PEPCK) during Acute Acidosis and Its Role in Acid Secretion by V-ATPase-Expressing Ionocytes

Vacuolar-Type H⁺-ATPase (V-ATPase) takes the central role in pumping H⁺ through cell membranes of diverse organisms, which is essential for surviving acid-base fluctuating lifestyles or environments. In mammals, although glucose is believed to be an important energy source to drive V-ATPase, and phosphoenolpyruvate carboxykinase (PEPCK), a key enzyme for gluconeogenesis, is known to be activated in response to acidosis, the link between acid secretion and PEPCK activation remains unclear. In the present study, we used zebrafish larva as an in vivo model to show the role of acid-inducible PEPCK activity in glucose production to support higher rate of H⁺ secretion via V-ATPase, by utilizing gene knockdown, glucose supplementation, and non-invasive scanning ion-selective electrode technique (SIET). Zebrafish larvae increased V-ATPase-mediated acid secretion and transiently expression of Pck1, a zebrafish homolog of PEPCK, in response to acid stress. When pck1 gene was knocked down by specific morpholino, the H⁺ secretion via V-ATPase decreased, but this effect was rescued by supplementation of glucose into the yolk. By assessing changes in amino acid content and gene expression of respective enzymes, glutamine and glutamate appeared to be the major source for replenishment of Krebs cycle intermediates, which are subtracted by Pck1 activity. Unexpectedly, pck1 knockdown did not affect glutamine/glutamate catalysis, which implies that Pck1 does not necessarily drive this process. The present study provides the first in vivo evidence that acid-induced PEPCK provides glucose for acid-base homeostasis at an individual level, which is supported by rapid pumping of H⁺ via V-ATPase at the cellular level.

The role of acid-sensing ion channels in epithelial Na+ uptake in adult zebrafish (Danio rerio)

Acid-sensing ion channels (ASICs) are epithelial Na(+) channels gated by external H(+). Recently, it has been demonstrated that ASICs play a role in Na(+) uptake in freshwater rainbow trout. Here, we investigate the potential involvement of ASICs in Na(+) transport in another freshwater fish species, the zebrafish (Danio rerio). Using molecular and histological techniques we found that asic genes and the ASIC4.2 protein are expressed in the gill of adult zebrafish. Immunohistochemistry revealed that mitochondrion-rich cells positive for ASIC4.2 do not co-localize with Na(+)/K(+)-ATPase-rich cells, but co-localize with cells expressing vacuolar-type H(+)-ATPase. Furthermore, pharmacological inhibitors of ASIC and Na(+)/H(+)-exchanger significantly reduced uptake of Na(+) in adult zebrafish exposed to low-Na(+) media, but did not cause the same response in individuals exposed to ultra-low-Na(+) water. Our results suggest that in adult zebrafish ASICs play a role in branchial Na(+) uptake in media with low Na(+) concentrations and that mechanisms used for Na(+) uptake by zebrafish may depend on the Na(+) concentration in the acclimation medium.

The zebrafish merovingian mutant reveals a role for pH regulation in hair cell toxicity and function

Control of the extracellular environment of inner ear hair cells by ionic transporters is crucial for hair cell function. In addition to inner ear hair cells, aquatic vertebrates have hair cells on the surface of their body in the lateral line system. The ionic environment of these cells also appears to be regulated, although the mechanisms of this regulation are less understood than those of the mammalian inner ear. We identified the merovingian mutant through genetic screening in zebrafish for genes involved in drug-induced hair cell death. Mutants show complete resistance to neomycin-induced hair cell death and partial resistance to cisplatin-induced hair cell death. This resistance is probably due to impaired drug uptake as a result of reduced mechanotransduction ability, suggesting that the mutants have defects in hair cell function independent of drug treatment. Through genetic mapping we found that merovingian mutants contain a mutation in the transcription factor gcm2. This gene is important for the production of ionocytes, which are cells crucial for whole body pH regulation in fish. We found that merovingian mutants showed an acidified extracellular environment in the vicinity of both inner ear and lateral line hair cells. We believe that this acidified extracellular environment is responsible for the defects seen in hair cells of merovingian mutants, and that these mutants would serve as a valuable model for further study of the role of pH in hair cell function.

Using the zebrafish lateral line to uncover novel mechanisms of action and prevention in drug-induced hair cell death

The majority of hearing loss and balance disorders are caused by the permanent loss of mechanosensory hair cells of the inner ear. Identification of genes and compounds that modulate susceptibility to hair cell death is frequently confounded by the difficulties of assaying for such complex phenomena in mammalian models. The zebrafish has emerged as a powerful animal model for genetic and chemical screening in many contexts. Several characteristics of the zebrafish, such as its small size and external location of mechanosensory hair cells within the lateral line sensory organ, uniquely position it as an ideal model organism for the study of hair cell toxicity. We have used this model to screen for genes and compounds that affect hair cell survival during ototoxin exposure and have identified agents that would not be expected to play a role in this process based on a priori knowledge of their function. The identification of such agents yields better understanding of hair cell death and holds promise to stem hearing loss and balance disorders in the human population.

Anion exchanger 1b in stereocilia is required for the functioning of mechanotransducer channels in lateral-line hair cells of zebrafish

The anion exchanger (AE) plays critical roles in physiological processes including CO2 transport and volume regulation in erythrocytes and acid-base regulation in renal tubules. Although expression of the AE in inner-ear hair cells was reported, its specific localization and function are still unclear. Using in situ hybridization, we found that the AE1b transcript is expressed in lateral-line hair cells of zebrafish larvae. An immunohistochemical analysis with a zebrafish-specific antibody localized AE1b to stereocilia of hair cells, and the expression was eliminated by morpholino knockdown of AE1b. A non-invasive, scanning ion-selective electrode technique was applied to analyze mechanotransducer (MET) channel-mediated Ca2+ influx at stereocilia of hair cells of intact fish. Ca2+ influx was effectively suppressed by AE1b morpholino knockdown and inhibitor (DIDS) treatment. Elevating external Ca2+ (0.2 to 2 mM) neutralized the inhibition of DIDS. Taken together, this study provides solid evidence to show that AE1b in stereocilia is required for the proper functioning of MET channels.

Development and application of the adverse outcome pathway framework for understanding and predicting chronic toxicity: II. A focus on growth impairment in fish

Adverse outcome pathways (AOPs) organize knowledge on the progression of toxicity through levels of biological organization. By determining the linkages between toxicity events at different levels, AOPs lay the foundation for mechanism-based alternative testing approaches to hazard assessment. Here, we focus on growth impairment in fish to illustrate the initial stages in the process of AOP development for chronic toxicity outcomes. Growth is an apical endpoint commonly assessed in chronic toxicity tests for which a replacement is desirable. Based on several criteria, we identified reduction in food intake to be a suitable key event for initiation of middle-out AOP development. To start exploring the upstream and downstream links of this key event, we developed three AOP case studies, for pyrethroids, selective serotonin reuptake inhibitors (SSRIs) and cadmium. Our analysis showed that the effect of pyrethroids and SSRIs on food intake is strongly linked to growth impairment, while cadmium causes a reduction in growth due to increased metabolic demands rather than changes in food intake. Locomotion impairment by pyrethroids is strongly linked to their effects on food intake and growth, while for SSRIs their direct influence on appetite may play a more important role. We further discuss which alternative tests could be used to inform on the predictive key events identified in the case studies. In conclusion, our work demonstrates how the AOP concept can be used in practice to assess critically the knowledge available for specific chronic toxicity cases and to identify existing knowledge gaps and potential alternative tests.

Stanniocalcin-1 controls ion regulation functions of ion-transporting epithelium other than calcium balance

Stanniocalcin-1 (STC-1) was first identified to involve in Ca(2+) homeostasis in teleosts, and was thought to act as a hypocalcemic hormone in vertebrate. Recent studies suggested that STC-1 exhibits broad effects on ion balance, not confines to Ca(2+), but the mechanism of this regulation process remains largely unknown. Here, we used zebrafish embryos as an alternative in vivo model to investigate how STC-1 regulates transepithelial ion transport function in ion-transporting epithelium. Expression of stc-1 mRNA in zebrafish embryos was increased in high-Ca(2+) environments but decreased by acidic and ion-deficient treatments while overexpression of stc-1 impaired the hypotonic acclimation by decreasing whole body Ca(2+), Na(+), and Cl(-) contents and H(+) secretion ability. Injection of STC-1 mRNA also down-regulated mRNA expressions of epithelial Ca(2+) channel, H(+)-ATPase, and Na(+)-Cl(-) cotransporter, suggesting the roles of STC-1 in regulation of ions other than Ca(2+). Knockdown of STC-1 caused an increase in ionocyte progenitors (foxi3a as the marker) and mature ionocytes (ion transporters as the markers), but did not affect epithelium stem cells (p63 as the marker) in the embryonic skin. Overexpression of STC-1 had the corresponding opposite effect on ionocyte progenitors, mature ionocytes in the embryonic skin. Taken together, STC-1 negatively regulates the number of ionocytes to reduce ionocyte functions. This process is important for body fluid ionic homeostasis, which is achieved by the regulation of ion transport functions in ionocytes. The present findings provide new insights into the broader functions of STC-1, a hypocalcemic hormone.

The inner opercular membrane of the euryhaline teleost: a useful surrogate model for comparisons of different characteristics of ionocytes between seawater- and freshwater-acclimated medaka

The inner opercular membranes of the brackish medaka, Oryzias dancena, have numerous ionocytes, similar to the gill epithelia. By histological observation, this study demonstrated that it is possible to investigate the cellular morphology and function of ionocytes in the opercular membrane. The mitochondria-rich ionocytes in the opercular membranes were traced using rhodamine 123 and a cytochrome c oxidase IV antibody in vital and fixed situations, respectively. To validate different morphologies of seawater (SW)-type and freshwater (FW)-type ionocytes of the opercular membrane of euryhaline brackish medaka, a method of dual observation including immunofluorescence staining and subsequent scanning electron microscopy was used. The apical morphologies of SW- and FW-type ionocytes were hole and flat opening, respectively. In addition, the microvilli were found on the apical surface of the FW-type ionocytes. The SW-type ionocytes exhibited basolateral Na(+), K(+), 2Cl(-) cotransporter and the apical cystic fibrosis transmembrane conductance regulator. In contrast, in the apical region of FW-type ionocytes, Na(+), Cl(-) cotransporter and villin 1-like protein were expressed. In addition, histochemical staining of AgCl precipitation counterstained with a Na(+), K(+)-ATPase α-subunit antibody on the opercular membrane illustrated the role of Cl(-) secretion in the SW-type ionocytes of the brackish medaka. A combination of different observations in this study indicated that the opercular membrane could be a useful surrogate model for histological and functional studies on the epithelial ionocytes of fish gills.

A new model for fish ion regulation: identification of ionocytes in freshwater- and seawater-acclimated medaka (Oryzias latipes)

The ion regulation mechanisms of fishes have been recently studied in zebrafish (Danio rerio), a stenohaline species. However, recent advances using this organism are not necessarily applicable to euryhaline fishes. The euryhaline species medaka (Oryzias latipes), which, like zebrafish, is genetically well categorized and amenable to molecular manipulation, was proposed as an alternative model for studying osmoregulation during acclimation to different salinities. To establish its suitability as an alternative, the present study was conducted to (1) identify different types of ionocytes in the embryonic skin and (2) analyze gene expressions of the transporters during seawater acclimation. Double/triple in situ hybridization and/or immunocytochemistry revealed that freshwater (FW) medaka contain three types of ionocyte: (1) Na(+)/H(+) exchanger 3 (NHE3) cells with apical NHE3 and basolateral Na(+)-K(+)-2Cl(-) cotransporter (NKCC), Na(+)-K(+)-ATPase (NKA) and anion exchanger (AE); (2) Na(+)-Cl(-) cotransporter (NCC) cells with apical NCC and basolateral H(+)-ATPase; and (3) epithelial Ca(2+) channel (ECaC) cells [presumed accessory (AC) cells] with apical ECaC. On the other hand, seawater (SW) medaka has a single predominant ionocyte type, which possesses apical cystic fibrosis transmembrane conductance regulator (CFTR) and NHE3 and basolateral NKCC and NKA and is accompanied by smaller AC cells that express lower levels of basolateral NKA. Reciprocal gene expressions of decreased NHE3, AE, NCC and ECaC and increased CFTR and NKCC in medaka gills during SW were revealed by quantative PCR analysis.

Endothelin-1 regulates H⁺-ATPase-dependent transepithelial H⁺ secretion in zebrafish

Endothelin-1 (EDN1) is an important regulator of H⁺ secretion in the mammalian kidney. EDN1 enhances renal tubule H⁺-ATPase activity, but the underlying mechanism remains unclear. To further elucidate the role of EDN1 in vertebrates' acid-base regulation, the present study used zebrafish as the model to examine the effects of EDN1 and its receptors on transepithelial H⁺ secretion. Expression of EDN1 and one of its receptors, EDNRAa, was stimulated in zebrafish acclimated to acidic water. A noninvasive scanning ion-selective electrode technique was used to show that edn1 overexpression enhances H⁺ secretion in embryonic skin at 3 days post fertilization. EDNRAa loss of function significantly decreased EDN1- and acid-induced H⁺ secretion. Abrogation of EDN1-enhanced H⁺ secretion by a vacuolar H⁺-ATPase inhibitor (bafilomycin A1) suggests that EDN1 exerts its action by regulating the H⁺-ATPase-mediated H⁺ secretion. EDN1 does not appear to affect H⁺ secretion through either altering the abundance of H⁺-ATPase or affecting the cell differentiation of H⁺-ATPase-rich ionocytes, because the reduction in secretion upon ednraa knockdown was not accompanied by decreased expression of H⁺-ATPase or reduced H⁺-ATPase-rich cell density. These findings provide evidence that EDN1 signaling is involved in acid-base regulation in zebrafish and enhance our understanding of EDN1 regulation of transepithelial H⁺ secretion in vertebrates.

Expression of carbonic anhydrase, cystic fibrosis transmembrane regulator (CFTR) and V-H(+)-ATPase in the lancelet Branchiostoma lanceolatum (Pallas, 1774)

Sequencing of the amphioxus genome revealed that it contains a basic set of chordate genes involved in development and cell signaling. Despite the availability of genomic data, up till now no studies have been addressed on the comprehension of the amphioxus osmoregulation. Using primers designed on Branchiostoma floridae carbonic anhydrase (CA) II, cystic fibrosis transmembrane regulator (CFTR) and V-H(+)-ATPase, a 100bp long region, containing the protein region recognized by the respective antibodies, has been amplified and sequenced in B. lanceolatum indicating the presence of hortologous V-ATPase, CFTR and carbonic anhydrase II genes in Branchiostoma lanceolatum. Immunohistochemical results showed that all three transporting proteins are expressed in almost 90% of epithelial cells of the skin in B. lanceolatum adults with a different degree of positivity in different regions of body wall and with a different localization in the cells. The comparison of results between young and adult lancelets showed that the distribution of these transporters is quite different. Indeed, in the young specimens the expression pattern of all tested molecules appears concentrated at the gut level, whereas in adult the gut loses its key role that is mostly supported by skin.

V-ATPase proton pumping activity is required for adult zebrafish appendage regeneration

The activity of ion channels and transporters generates ion-specific fluxes that encode electrical and/or chemical signals with biological significance. Even though it is long known that some of those signals are crucial for regeneration, only in recent years the corresponding molecular sources started to be identified using mainly invertebrate or larval vertebrate models. We used adult zebrafish caudal fin as a model to investigate which and how ion transporters affect regeneration in an adult vertebrate model. Through the combined use of biophysical and molecular approaches, we show that V-ATPase activity contributes to a regeneration-specific H+ ef`flux. The onset and intensity of both V-ATPase expression and H+ efflux correlate with the different regeneration rate along the proximal-distal axis. Moreover, we show that V-ATPase inhibition impairs regeneration in adult vertebrate. Notably, the activity of this H+ pump is necessary for aldh1a2 and mkp3 expression, blastema cell proliferation and fin innervation. To the best of our knowledge, this is the first report on the role of V-ATPase during adult vertebrate regeneration.

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

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

Zebrafish as a model system for mitochondrial biology and diseases

Animal models for studying human disease are essential to the continuing evolution of medicine. Rodent models are attractive for the obvious similarities in development and genetic makeup compared with humans, but have cost and technical limitations. The zebrafish (Danio rerio) represents an ideal alternative vertebrate model of human disease because of its high conservation of genetic information and physiological processes, inexpensive maintenance, and optical clarity facilitating direct observation. This review highlights recent advances in understanding genetic disease states associated with the dynamic organelle, the mitochondrion, using the zebrafish. Mitochondrial diseases that have been replicated in the zebrafish include those affecting the nervous and cardiovascular systems, as well as red blood cell function. Gene silencing techniques, including morpholino knockdown and transcription activator-like (TAL)-effector endonucleases, have been exploited to demonstrate how loss of function can induce human disease-like states in zebrafish. Moreover, modeling mitochondrial diseases has been facilitated greatly by the creation of transgenic fish with fluorescently labeled mitochondria for in vivo visualization of these structures. In addition, behavioral assays have been developed to examine changes in motor activity and sensory responses, particularly in larval stages. Zebrafish are poised to advance our understanding of the pathogenesis of human mitochondrial diseases beyond the current state of knowledge and provide a key tool in the development of novel therapeutic approaches to treat these conditions.

Effects of hypoxia-induced gill remodelling on the innervation and distribution of ionocytes in the gill of goldfish, Carassius auratus

The presence of an interlamellar cell mass (ILCM) on the gills of goldfish acclimated to 7°C leads to preferential distribution of branchial ionocytes to the distal edges of the ILCM, where they are likely to remain in contact with the water and hence remain functional. Upon exposure to hypoxia, the ILCM retracts, and the ionocytes become localized to the lamellar surfaces and on the filament epithelium, owing to their migration and the differentiation of new ionocytes from progenitor cells. Here we demonstrate that the majority of the ionocytes receive neuronal innervation, which led us to assess the consequences of ionocyte migration and differentiation during hypoxic gill remodelling on the pattern and extent of ionocyte neuronal innervation. Normoxic 7°C goldfish (ILCM present) possessed significantly greater numbers of ionocytes/mm² (951.2 ± 94.3) than their 25°C conspecifics (ILCM absent; 363.1 ± 49.6) but a statistically lower percentage of innervated ionocytes (83.1% ± 1.0% compared with 87.8% ± 1.3%). After 1 week of exposure of goldfish to hypoxia, the pool of branchial ionocytes was composed largely of pre-existing migrating cells (555.6 ± 38.1/mm²) and to a lesser extent newly formed ionocytes (226.7 ± 15.1/mm²). The percentage of new (relative to pre-existing) ionocytes remained relatively constant (at ∼30%) after 1 or 2 weeks of normoxic recovery. After hypoxia, pre-existing ionocytes expressed a greater percentage of innervation than newly formed ionocytes in all treatment groups; however, their percentage innervation steadily decreased over 2 weeks of normoxic recovery.

Calcium deficiency-induced and TRP channel-regulated IGF1R-PI3K-Akt signaling regulates abnormal epithelial cell proliferation

Calcium deficiency causes abnormal colonic growth and increases colon cancer risk with poorly understood mechanisms. Here we elucidate a novel signaling mechanism underlying the Ca(2+) deficiency-induced epithelial proliferation using a unique animal model. The zebrafish larval yolk sac skin contains a group of Ca(2+)-transporting epithelial cells known as ionocytes. Their number and density increases dramatically when acclimated to low [Ca(2+)] environments. BrdU pulse-labeling experiments suggest that low [Ca(2+)] stimulates pre-existing ionocytes to re-enter the cell cycle. Low [Ca(2+)] treatment results in a robust and sustained activation of IGF1R-PI3K-Akt signaling in these cells exclusively. These ionocytes specifically express Igfbp5a, a high-affinity and specific binding protein for insulin-like growth factors (IGFs) and the Ca(2+)-selective channel Trpv5/6. Inhibition or knockdown of Igfbp5a, IGF1 receptor, PI3K, and Akt attenuates low [Ca(2+)]-induced ionocyte proliferation. The role of Trpv5/6 was investigated using a genetic mutant, targeted knockdown, and pharmacological inhibition. Loss-of-Trpv5/6 function or expression results in elevated pAkt levels and increased ionocyte proliferation under normal [Ca(2+)]. These increases are eliminated in the presence of an IGF1R inhibitor, suggesting that Trpv5/6 represses IGF1R-PI3K-Akt signaling under normal [Ca(2+)]. Intriguingly, blockade of Trpv5/6 activity inhibits the low [Ca(2+)]-induced activation of Akt. Mechanistic analyses reveal that the low [Ca(2+)]-induced IGF signaling is mediated through Trpv5/6-associated membrane depolarization. Low extracellular [Ca(2+)] results in a similar amplification of IGF-induced PI3K-PDK1-Akt signaling in human colon cancer cells in a TRPV6-dependent manner. These results uncover a novel and evolutionarily conserved signaling mechanism that contributes to the abnormal epithelial proliferation associated with Ca(2+) deficiency.

Origin and differentiation of ionocytes in gill epithelium of teleost fish

This paper focuses on the environmental cues that transform the gills of euryhaline teleost fish from an oxygen exchange structure into a bifunctional organ that can control both gaseous movement and water/ion transport. The cellular development that allows this structure to accomplish these tasks begins shortly after fertilization of the egg. It involves alterations of structure and function of embryonic cells [ionoblasts (IB)] that are shed from the pharyngeal anlage area of the embryo. These IB contain unique protein-receptor domains in the plasma membrane. These receptors respond specifically to the environmental cues effecting a calcium-binding protein receptor [calcium-sensing receptor (CaSR)]. The CaSR containing IB act as stem cells and are acted upon by isotocin, a heteroprotein regulator which induces them to form progenitor ionocytes (pIC). The pIC form two types of cells. The first type becomes an aquaphilic ionocyte which regulates uptake of ions and through aquaporin molecules transports water out of the cell and controls body fluids of the fish. This mechanism is essential for freshwater living. The second type becomes a halophilic ionocyte and transports ions out of the cell and controls cell shrinkage by uptake of water via aquaporin molecules. This mechanism is essential for seawater living. These differentiating events in the pIC are controlled by the cross talking of genomic mechanisms found in the precursor IB. To unravel the cross talking events it is necessary to uncover how these genetic pathways are regulated by transcriptional and translational events coming from complementary DNA. Various gene families are involved such as those found in apoptosis mechanisms, regulatory volume regulators and ionic transport systems (cystic fibrosis transmembrane conductance regulator).

Extracellular Ca2+ and Mg2+ modulate aminoglycoside blockade of mechanotransducer channel-mediated Ca2+ entry in zebrafish hair cells: an in vivo study with the SIET

Zebrafish lateral-line hair cells are an in vivo model for studying hair cell development, function, and ototoxicity. However, the molecular identification and properties of the mechanotransducer (MET) channel in hair cells are still controversial. In this study, a noninvasive electrophysiological method, the scanning ion-electrode technique (SIET), was applied for the first time to investigate properties of MET channels in intact zebrafish embryos. With the use of a Ca2+-selective microelectrode to deflect hair bundles and simultaneously record the Ca2+ flux, the inward Ca2+ flux was detected at stereocilia of hair cells in 2- to ∼4-day postfertilization embryos. Ca2+ influx was blocked by MET channel blockers (BAPTA, La3+, Gd3+, and curare). In addition, 10 μM aminoglycoside antibiotics (neomycin and gentamicin) were found to effectively block Ca2+ influx within 10 min. Elevating the external Ca2+ level (0.2–2 mM) neutralized the effects of neomycin and gentamicin. However, elevating the Mg2+ level up to 5 mM neutralized blockade by gentamicin but not by neomycin. This study demonstrated MET channel-mediated Ca2+ entry at hair cells and showed that the SIET to be a sensitive approach for functionally assaying MET channels in zebrafish.

Glucocorticoid receptor, but not mineralocorticoid receptor, mediates cortisol regulation of epidermal ionocyte development and ion transport in zebrafish (danio rerio)

Cortisol is the major endogenous glucocorticoid (GC) both in human and fish, mediated by corticosteroid receptors. Due to the absence of aldosterone production in teleost fish, cortisol is also traditionally accepted to function as mineralocorticoid (MC); but whether it acts through the glucocorticoid receptor (GR) or the mineralocorticoid receptor (MR) remains a subject of debate. Here, we used loss-of-function and rescue assays to determine whether cortisol affects zebrafish epidermal ionocyte development and function via the GR and/or the MR. GR knockdown morphants displayed a significant decrease in the major ionocytes, namely Na(+)-K(+)-ATPase-rich cells (NaRCs) and H(+)-ATPase-rich cells (HRCs), as well as other cells, including epidermal stem cells (ESCs), keratinocytes, and mucus cells; conversely, cell numbers were unaffected in MR knockdown morphants. In agreement, GR morphants, but not MR morphants, exhibited decreased NaRC-mediated Ca(2+) uptake and HRC-mediated H(+) secretion. Rescue via GR capped mRNA injection or exogenous cortisol incubation normalized the number of epidermal ionocytes in GR morphants. We also provide evidence for GR localization in epidermal cells. At the transcript level, GR mRNA is ubiquitously expressed in gill sections and present in both NaRCs and HRCs, supporting the knockdown and functional assay results in embryo. Altogether, we have provided solid molecular evidence that GR is indeed present on ionocytes, where it mediates the effects of cortisol on ionocyte development and function. Hence, cortisol-GR axis performs the roles of both GC and MC in zebrafish skin and gills.

Isolation and fractionation of gill cells from freshwater (Lasmigona costata) and seawater (Mesodesma mactroides) bivalves for use in toxicological studies with copper

Gills cells of the freshwater mussel Lasmigona costata and the seawater clam Mesodesma mactroides were isolated (mussel: chemical dissociation; clam: mechanical dissociation) and fractionated (Percoll gradient) into Fractions I and II. Mitochondrial dyes (DASPEI: mussel; MitoTracker(®): clam) and Na(+), K(+)-ATPase activity measurement were used to distinguish between cells of Fractions I and II. For mussel and clam, 80.5 ± 1.5 and 48.3 ± 3.2 % of cells were in Fraction II, respectively. For both species, cells of Fraction II had higher fluorescence emission and higher enzyme activity than those of Fraction I, being characterized as 'cells rich in mitochondria'. Cells of Fraction II were kept in saline solutions approximating the ionic composition of hemolymph either under control conditions (no Cu addition) or exposed (3 h) to copper (Cu: 5, 9 and 20 μg Cu/L). Cell viability and Cu and Na(+) content were measured. For both species, Cu content was higher and Na(+) content was lower in cells exposed to 20 μg Cu/L. Furthermore, a strong negative correlation was observed between cell Na(+) and Cu content in the two bivalve species, indicating a possible competition between Cu and Na(+) for ion-transporting mechanisms or binding sites at gill cells of Fraction II. Considering that Cu is an ionoregulatory toxicant in aquatic invertebrates, these preliminary toxicological data support the idea of using isolated gill cells rich in mitochondria to study the mechanisms underlying the acute toxicity of waterborne Cu in freshwater and marine bivalves.

Development in a naturally acidified environment: Na+/H+-exchanger 3-based proton secretion leads to CO2 tolerance in cephalopod embryos

BACKGROUND

Regulation of pH homeostasis is a central feature of all animals to cope with acid-base disturbances caused by respiratory CO2. Although a large body of knowledge is available for vertebrate and mammalian pH regulatory systems, the mechanisms of pH regulation in marine invertebrates remain largely unexplored.

RESULTS

We used squid (Sepioteuthis lessoniana), which are known as powerful acid-base regulators to investigate the pH regulatory machinery with a special focus on proton secretion pathways during environmental hypercapnia. We cloned a Rhesus protein (slRhP), V-type H+-ATPase (slVHA) and the Na+/H+ exchanger 3 (slNHE3) from S. lessoniana, which are hypothesized to represent key players in proton secretion pathways among different animal taxa. Specifically designed antibodies for S. lessoniana demonstrated the sub-cellular localization of NKA, VHA (basolateral) and NHE3 (apical) in epidermal ionocytes of early life stages. Gene expression analyses demonstrated that slNHE3, slVHA and slRhP are up regulated in response to environmental hypercapnia (pH 7.31; 0.46 kPa pCO2) in body and yolk tissues compared to control conditions (pH 8.1; 0.045 kPa pCO2). This observation is supported by H+ selective electrode measurements, which detected increased proton gradients in CO2 treated embryos. This compensatory proton secretion is EIPA sensitive and thus confirms the central role of NHE based proton secretion in cephalopods.

CONCLUSION

The present work shows that in convergence to teleosts and mammalian pH regulatory systems, cephalopod early life stages have evolved a unique acid-base regulatory machinery located in epidermal ionocytes. Using cephalopod molluscs as an invertebrate model this work provides important insights regarding the unifying evolutionary principles of pH regulation in different animal taxa that enables them to cope with CO2 induced acid-base disturbances.

Compensatory regulation of Na+ absorption by Na+/H+ exchanger and Na+-Cl- cotransporter in zebrafish (Danio rerio)

INTRODUCTION

In mammals, internal Na+ homeostasis is maintained through Na+ reabsorption via a variety of Na+ transport proteins with mutually compensating functions, which are expressed in different segments of the nephrons. In zebrafish, Na+ homeostasis is achieved mainly through the skin/gill ionocytes, namely Na+/H+ exchanger (NHE3b)-expressing H+-ATPase rich (HR) cells and Na+-Cl- cotransporter (NCC)-expressing NCC cells, which are functionally homologous to mammalian proximal and distal convoluted tubular cells, respectively. The present study aimed to investigate whether or not the functions of HR and NCC ionocytes are differentially regulated to compensate for disruptions of internal Na+ homeostasis and if the cell differentiation of the ionocytes is involved in this regulation pathway.

RESULTS

Translational knockdown of ncc caused an increase in HR cell number and a resulting augmentation of Na+ uptake in zebrafish larvae, while NHE3b loss-of-function caused an increase in NCC cell number with a concomitant recovery of Na+ absorption. Environmental acid stress suppressed nhe3b expression in HR cells and decreased Na+ content, which was followed by up-regulation of NCC cells accompanied by recovery of Na+ content. Moreover, knockdown of ncc resulted in a significant decrease of Na+ content in acid-acclimated zebrafish.

CONCLUSIONS

These results provide evidence that HR and NCC cells exhibit functional redundancy in Na+ absorption, similar to the regulatory mechanisms in mammalian kidney, and suggest this functional redundancy is a critical strategy used by zebrafish to survive in a harsh environment that disturbs body fluid Na+ homeostasis.

Proton-facilitated ammonia excretion by ionocytes of medaka (Oryzias latipes) acclimated to seawater

The proton-facilitated ammonia excretion is critical for a fish's ability to excrete ammonia in freshwater. However, it remains unclear whether that mechanism is also critical for ammonia excretion in seawater (SW). Using a scanning ion-selective electrode technique (SIET) to measure H+ gradients, an acidic boundary layer was detected at the yolk-sac surface of SW-acclimated medaka ( Oryzias latipes) larvae. The H+ gradient detected at the surface of ionocytes was higher than that of keratinocytes in the yolk sac. Treatment with Tricine buffer or EIPA (a NHE inhibitor) reduced the H+ gradient and ammonia excretion of larvae. In situ hybridization and immunochemistry showed that slc9a2 (NHE2) and slc9a3 (NHE3) were expressed in the same SW-type ionocytes. A real-time PCR analysis showed that transfer to SW downregulated branchial mRNA expressions of slc9a3 and Rhesus glycoproteins ( rhcg1, rhcg2, and rhbg) but upregulated that of slc9a2. However, slc9a3, rhcg1, rhcg2, and rhbg expressions were induced by high ammonia in SW. This study suggests that SW-type ionocytes play a role in acid and ammonia excretion and that the Na+/H+ exchanger and Rh glycoproteins are involved in the proton-facilitated ammonia excretion mechanism.

Waste nitrogen metabolism and excretion in zebrafish embryos: effects of light, ammonia, and nicotinamide

Bony fish primarily excrete ammonia as adults however the persistence of urea cycle genes may reflect a beneficial role for urea production during embryonic stages in protecting the embryo from toxic effects of ammonia produced from a highly nitrogenous yolk. This study aimed to examine the dynamic scope for changes in rates of urea synthesis and excretion in one such species (zebrafish, Danio rerio) by manipulating the intrinsic developmental rate (by alteration of light:dark cycles), as well as by direct chemical manipulation via ammonia injection (to potentially activate urea production) and nicotinamide exposure (to potentially inhibit urea production). Continuous dark exposure delayed development in embryos as evidenced by delayed appearance of hallmark anatomical features (heartbeat, eye pigmentation, body pigmentation, lateral line, fin buds) at 30 and 48 hr post-fertilization, as well by a lower hatching rate compared to embryos reared in continuous light. Both ammonia and urea excretion were similarly effected and were generally higher in embryos continuously exposed to light. Ammonia injection resulted in significant increases (up to fourfold) of urea N excretion and no changes to ammonia excretion rates along with modest increases in yolk ammonia content during 2-6 hr post-injection. Nicotinamide (an inhibitor of urea synthesis in mammals) reduced the ammonia-induced increase in urea excretion and led to retention of ammonia in the yolk and body of the embryo. Our results indicate that there is a relatively rapid and large scope for increases in urea production/excretion rates in developing embryos. Potential mechanisms for these increases are discussed.

Rhcg1 and Rhbg mediate ammonia excretion by ionocytes and keratinocytes in the skin of zebrafish larvae: H+-ATPase-linked active ammonia excretion by ionocytes

In zebrafish, Rhcg1 was found in apical membranes of skin ionocytes [H⁺-ATPase-rich (HR) cells], which are similar to α-type intercalated cells in mammalian collecting ducts. However, the cellular distribution and role of Rhbg in zebrafish larvae have not been well investigated. In addition, HR cells were hypothesized to excrete ammonia against concentration gradients. In this study, we attempted to compare the roles of Rhbg and Rhcg1 in ammonia excretion by larval skin and compare the capability of skin cells to excrete ammonia against concentration gradients. Using in situ hybridization and immunohistochemistry, Rhbg was localized to both apical and basolateral membranes of skin keratinocytes. A scanning ion-selective electrode technique (SIET) was applied to measure the NH₄⁺ flux at the apical surface of keratinocytes and HR cells. Knockdown of Rhbg with morpholino oligonucleotides suppressed ammonia excretion by keratinocytes and induced compensatory ammonia excretion by HR cells. To compare the capability of cells to excrete ammonia against gradients, NH₄⁺ flux of cells was determined in larvae exposed to serial concentrations of external NH₄⁺. Results showed that HR cells excreted NH₄⁺ against higher NH₄⁺ concentration than did keratinocytes. Knockdown of the expression of either Rhcg1 or H⁺ -ATPase in HR cells suppressed the capability of HR cells.

Zebrafish as an animal model to study ion homeostasis

Zebrafish (Danio rerio) possesses several advantages as an experimental organism, including the applicability of molecular tools, ease of in vivo cellular observation and functional analysis, and rapid embryonic development, making it an emerging model for the study of integrative and regulatory physiology and, in particular, the epithelial transport associated with body fluid ionic homeostasis. Zebrafish inhabits a hypotonic freshwater environment, and as such, the gills (or the skin, during embryonic stages) assume the role of the kidney in body fluid ionic homeostasis. Four types of ionocyte expressing distinct sets of transporters have been identified in these organs: H⁺-ATPase-rich, Na⁺-K⁺-ATPase-rich, Na⁺-Cl⁻ cotransporter-expressing and K⁺-secreting cells; these ionocytes perform transepithelial H⁺ secretion/Na⁺ uptake/NH₄⁺ excretion, Ca²⁺ uptake, Na⁺/Cl⁻ uptake, and K⁺ secretion, respectively. Zebrafish ionocytes are analogous to various renal tubular cells, in terms of ion transporter expression and function. During embryonic development, ionocyte progenitors develop from epidermal stem cells and then differentiate into different types of ionocyte through a positive regulatory loop of Foxi3a/-3b and other transcription factors. Several hormones, including cortisol, vitamin D, stanniocalcin-1, calcitonin, and isotocin, were found to participate in the control pathways of ionic homeostasis by precisely studying the target ion transport pathways, ion transporters, or ionocytes of the hormonal actions. In conclusion, the zebrafish model not only enhances our understanding of body fluid ion homeostasis and hormonal control in fish but also informs studies on mammals and other animal species, thereby providing new insights into related fields.

Close Association of Carbonic Anhydrase (CA2a and CA15a), Na(+)/H(+) Exchanger (Nhe3b), and Ammonia Transporter Rhcg1 in Zebrafish Ionocytes Responsible for Na(+) Uptake

Freshwater (FW) fishes actively absorb salt from their environment to tolerate low salinities. We previously reported that vacuolar-type H⁺-ATPase/mitochondrion-rich cells (H-MRCs) on the skin epithelium of zebrafish larvae (Danio rerio) are primary sites for Na⁺ uptake. In this study, in an attempt to clarify the mechanism for the Na⁺ uptake, we performed a systematic analysis of gene expression patterns of zebrafish carbonic anhydrase (CA) isoforms and found that, of 12 CA isoforms, CA2a and CA15a are highly expressed in H-MRCs at larval stages. The ca2a and ca15a mRNA expression were salinity-dependent; they were upregulated in 0.03 mM Na⁺ water whereas ca15a but not ca2a was down-regulated in 70 mM Na⁺ water. Immunohistochemistry demonstrated cytoplasmic distribution of CA2a and apical membrane localization of CA15a. Furthermore, cell surface immunofluorescence staining revealed external surface localization of CA15a. Depletion of either CA2a or CA15a expression by Morpholino antisense oligonucleotides resulted in a significant decrease in Na⁺ accumulation in H-MRCs. An in situ proximity ligation assay demonstrated a very close association of CA2a, CA15a, Na⁺/H⁺ exchanger 3b (Nhe3b), and Rhcg1 ammonia transporter in H-MRC. Our findings suggest that CA2a, CA15a, and Rhcg1 play a key role in Na⁺uptake under FW conditions by forming a transport metabolon with Nhe3b.

Water chemistry in the microenvironment of rainbow trout Oncorhynchus mykiss embryos is affected by development, the egg capsule and crowding

The hypothesis tested was that embryonic metabolism affects the water chemistry in the boundary layer. In addition, embryo crowding would further compound the metabolic effect on the water chemistry in the boundary layer. As development progressed, the magnitude of the boundary layer gradients for O(2) and pH, but not for NH4(+), increased. The presence of the egg capsule hindered the diffusion of O(2) into and H(+) and NH4(+) out of the embryo. The magnitude of the O(2), pH and NH4(+) boundary layer gradient was significantly increased when embryos were surrounded by either sham embryos or live embryos. The majority of this crowding effect on embryo boundary layers was due to changes in water flow rather than due to metabolism directly. These results clearly show that the microenvironment adjacent to the developing rainbow trout Oncorhynchus mykiss embryo becomes more stagnant as development progresses in the presence of the egg capsule and is further intensified with embryo crowding.

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

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

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

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

Cortisol promotes differentiation of epidermal ionocytes through Foxi3 transcription factors in zebrafish (Danio rerio)

Glucocorticoid regulates epidermal cell proliferation, and is used to treat certain skin disorders. Cortisol, a glucocorticoid, is also linked to skin development in teleost fish. Cortisol increases the number of epithelial ionocytes during environmental acclimation in euryhaline fishes, but it is unclear whether this is due to increased differentiation or proliferation. To investigate, we treated zebrafish embryos with exogenous cortisol (20mg/L). The densities of the ionocytes Na(+)-K(+)-ATPase rich cells (NaRCs) and H(+)-ATPase rich cells (HRCs) were significantly increased by cortisol, and this was accompanied by an increase in the respective marker genes. Expression of the glucocorticoid receptor (GR) gene was decreased. Cortisol treatment also increased ionocytes in cultured adult zebrafish gills, and up-regulated expression of genes encoding forkhead box I3 (foxi3a and foxi3b) transcription factors, which regulate ionocyte progenitor development. GR expression was up-regulated by cortisol in vitro; as such, the observed decrease in vivo reflects a regulatory systemic-negative feedback. Notably, in situ hybridization revealed that foxi3a/b mRNA expression was increased by cortisol at 24-48h post-fertilization. Cortisol also decreased keratinocytes, but did not affect epidermal stem cells or mucus cells. We conclude that foxi3a/b transactivation by cortisol-GR favors differentiation of ionocyte progenitors, thereby facilitating proliferation of mature ionocytes.