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

Structure and function of ionocytes in the freshwater fish gill

Freshwater fishes lose ions to the external medium owing to the steep concentration gradients between the body fluids and the water. To maintain homeostasis, they use ionocytes to actively extract Na(+), Cl(-), and Ca(2+) from the dilute external medium and excrete acidic (H(+)) or basic (HCO(3)(-)) equivalents by specialized cells termed ionocytes that are responsible for transport of ions. Freshwater fishes have evolved diverse approaches to solving these similar ionic and acid-base problems. In the few well-studied species, there are clearly different patterns in the physiology and morphology for ionocytes in the gill. In this review, we describe the varying nomenclature of ionocytes that have been used in the past 80 years to allow direct comparison of ionocytes and their common functions in different species. We focus on the recent advancement in our understanding of the molecular mechanisms of ion and acid-base regulation as represented by ionocyte subtypes found in rainbow trout, killifish, tilapia and zebrafish gill.

Seven things fish know about ammonia and we don't

In this review we pose the following seven questions related to ammonia and fish that represent gaps in our knowledge. 1. How is ammonia excretion linked to sodium uptake in freshwater fish? 2. How much does branchial ammonia excretion in seawater teleosts depend on Rhesus (Rh) glycoprotein-mediated NH(3) diffusion? 3. How do fish maintain ammonia excretion rates if branchial surface area is reduced or compromised? 4. Why does high environmental ammonia change the transepithelial potential across the gills? 5. Does high environmental ammonia increase gill surface area in ammonia tolerant fish but decrease gill surface area in ammonia intolerant fish? 6. How does ammonia contribute to ventilatory control? 7. What do Rh proteins do when they are not transporting ammonia? Mini reviews on each topic, which are able to present only partial answers to each question at present, are followed by further questions and/or suggestions for research approaches targeted to uncover answers.

New insights into the many functions of carbonic anhydrase in fish gills

Carbonic anhydrase (CA) is a zinc metalloenzyme that catalyzes the reversible reactions of carbon dioxide and water: CO(2) + H(2)O ↔ H(+) + HCO(3)(-). It has long been recognized that CA is abundant in the fish gill, with attention focused on the role of CA in catalyzing the hydration of CO(2) to provide H(+) and HCO(3)(-) for the branchial ion transport processes that underlie systemic ionic and acid-base regulation. Recent work has explored the diversity of CA isoforms in the fish gill. By linking these isoforms to different cell types in the gill, and by exploiting the diversity of fish species available for study, this work is increasing our understanding of the many roles that CA plays in the fish gill. In particular, recent work has revealed that fish utilize more than one model of CO(2) excretion, that to understand the role of CA and the gill in ionic regulation and acid-base balance means characterizing the transporter and CA complement of individual cell types, and that CA plays roles in branchial sensory mechanisms. The goal of this brief review is to summarize these new developments, while at the same time highlighting key areas in which further research is needed.

Mechanisms and regulation of Na(+) uptake by freshwater fish

Mechanisms of ion uptake by freshwater (FW) fish have received considerable attention over the past 80 years. Through an assortment of techniques incorporating whole animal physiology, electrophysiology and molecular biological approaches, three models have been proposed to account for Na(+) uptake. (1) Direct exchange of Na(+) and H(+) via one or more types of Na(+)/H(+) exchanger (slc9), (2) uptake of Na(+) through epithelial Na(+) channels energized by an electrical gradient created by H(+)-ATPase and (3) Na(+)/Cl(-) co-transport (slc12). While each mechanism is supported at least in part by theoretical or experimental data, there are several outstanding questions that have not yet been fully resolved. Furthermore, there are few details concerning how these Na(+) uptake mechanisms are fine tuned in response to the fluctuating FW environments. In this review, we summarize the current understanding of these three Na(+) uptake mechanisms and discuss their regulation by endocrine (cortisol and prolactin) and neurohumoral (catecholamines) factors.

Acid secretion by mitochondrion-rich cells of medaka (Oryzias latipes) acclimated to acidic freshwater

In the present study, medaka embryos were exposed to acidified freshwater (pH 5) to investigate the mechanism of acid secretion by mitochondrion-rich (MR) cells in embryonic skin. With double or triple in situ hybridization/immunocytochemistry, the Na+/H+ exchanger 3 (NHE3) and H+-ATPase were localized in two distinct subtypes of MR cells. NHE3 was expressed in apical membranes of a major proportion of MR cells, whereas H+-ATPase was expressed in basolateral membranes of a much smaller proportion of MR cells. Gill mRNA levels of NHE3 and H+-ATPase and the two subtypes of MR cells in yolk sac skin were increased by acid acclimation; however, the mRNA level of NHE3 was remarkably higher than that of H+-ATPase. A scanning ion-selective electrode technique was used to measure H+, Na+, and NH4+ transport by individual MR cells in larval skin. Results showed that Na+ uptake and NH4+ excretion by MR cells increased after acid acclimation. These findings suggested that the NHE3/Rh glycoprotein-mediated Na+ uptake/NH4+ excretion mechanism plays a critical role in acidic equivalent (H+/NH4+) excretion by MR cells of the freshwater medaka.

Rhcg1 and NHE3b are involved in ammonium-dependent sodium uptake by zebrafish larvae acclimated to low-sodium water

To investigate whether Na+ uptake by zebrafish is dependent on NH4+ excretion, a scanning ion-selective electrode technique was applied to measure Na+ and NH4+ gradients at the yolk-sac surface of zebrafish larvae. Low-Na+ acclimation induced an inward Na+ gradient (uptake), and a combination of low Na+ and high NH4+ induced a larger inward Na+ gradient. When measuring the ionic gradients, raising the external NH4+ level (5 mM) blocked NH4+ excretion and Na+ uptake; in contrast, raising the external Na+ level (10 mM) simultaneously enhanced Na+ uptake and NH4+ excretion. The addition of MOPS buffer (5 mM), which is known to block NH4+ excretion, also suppressed Na+ uptake. These results showed that Na+ uptake and NH4+ excretion by larval skin are associated when ambient Na+ level is low. Knockdown of Rhcg1 translation with morpholino-oligonucleotides decreased both NH4+ excretion and Na+ uptake by the skin and Na+ content of whole larvae. Knockdown of nhe3b translation or inhibitor (5-ethylisopropyl amiloride) treatment also decreased both the NH4+ excretion and Na+ uptake. This study provides loss-of-function evidence for the involvement of Rhcg1 and NHE3b in the ammonium-dependent Na+ uptake mechanism in zebrafish larvae subjected to low-Na+ water.

Ammonia excretion in the freshwater planarian Schmidtea mediterranea

In aquatic invertebrates metabolic nitrogenous waste is excreted predominately as ammonia. Very little is known, however, of the underlying mechanisms of ammonia excretion, particularly in freshwater species. Our results indicate that in the non-parasitic freshwater planarian Schmidtea mediterranea ammonia excretion depends on an acidification of the apical unstirred layer of the body surface and consequent ammonia trapping. Buffering of the environment to a pH of 7 or higher decreased excretion rate. Inhibitor experiments suggested further that the excretion mechanism involves the participation of the V-type H+-ATPase and carbonic anhydrase and possibly also the Na+/K+-ATPase and Na+/H+ exchangers (NHEs). Alkalinization (pH 8.5, 2 days) of the environment led to a 1.9-fold increase in body ammonia levels and to a down-regulation of V-ATPase (subunit A) and Rh-protein mRNA. Further, a two day exposure to non-lethal ammonia concentrations (1 mmol L-1) caused a doubling of body ammonia levels and led to an increase in Rh-protein and Na+/K+-ATPase (α-subunit) mRNA expression levels. In-situ hybridization studies indicated a strong mRNA expression of the Rh-protein in the epidermal epithelium. The ammonia excretion mechanism proposed for S. mediterranea reveals striking similarities to the current model suggested to function in gills of freshwater fish.

New insights into ion regulation of cephalopod molluscs: a role of epidermal ionocytes in acid-base regulation during embryogenesis

The constraints of an active life in a pelagic habitat led to numerous convergent morphological and physiological adaptations that enable cephalopod molluscs and teleost fishes to compete for similar resources. Here, we show for the first time that such convergent developments are also found in the ontogenetic progression of ion regulatory tissues; as in teleost fish, epidermal ionocytes scattered on skin and yolk sac of cephalopod embryos appear to be responsible for ionic and acid-base regulation before gill epithelia become functional. Ion and acid-base regulation is crucial in cephalopod embryos, as they are surrounded by a hypercapnic egg fluid with a Pco(2) between 0.2 and 0.4 kPa. Epidermal ionocytes were characterized via immunohistochemistry, in situ hybridization, and vital dye-staining techniques. We found one group of cells that is recognized by concavalin A and MitoTracker, which also expresses Na(+)/H(+) exchangers (NHE3) and Na(+)-K(+)-ATPase. Similar to findings obtained in teleosts, these NHE3-rich cells take up sodium in exchange for protons, illustrating the energetic superiority of NHE-based proton excretion in marine systems. In vivo electrophysiological techniques demonstrated that acid equivalents are secreted by the yolk and skin integument. Intriguingly, epidermal ionocytes of cephalopod embryos are ciliated as demonstrated by scanning electron microscopy, suggesting a dual function of epithelial cells in water convection and ion regulation. These findings add significant knowledge to our mechanistic understanding of hypercapnia tolerance in marine organisms, as it demonstrates that marine taxa, which were identified as powerful acid-base regulators during hypercapnic challenges, already exhibit strong acid-base regulatory abilities during embryogenesis.

Acquisition of glial cells missing 2 enhancers contributes to a diversity of ionocytes in zebrafish

Glial cells missing 2 (gcm2) encoding a GCM-motif transcription factor is expressed in the parathyroid in amniotes. In contrast, gcm2 is expressed in pharyngeal pouches (a homologous site of the parathyroid), gills, and H(+)-ATPase-rich cells (HRCs), a subset of ionocytes on the skin surface of the teleost fish zebrafish. Ionocytes are specialized cells that are involved in osmotic homeostasis in aquatic vertebrates. Here, we showed that gcm2 is essential for the development of HRCs and Na(+)-Cl(-) co-transporter-rich cells (NCCCs), another subset of ionocytes in zebrafish. We also identified gcm2 enhancer regions that control gcm2 expression in ionocytes of zebrafish. Comparisons of the gcm2 locus with its neighboring regions revealed no conserved elements between zebrafish and tetrapods. Furthermore, We observed gcm2 expression patterns in embryos of the teleost fishes Medaka (Oryzias latipes) and fugu (Fugu niphobles), the extant primitive ray-finned fishes Polypterus (Polypterus senegalus) and sturgeon (a hybrid of Huso huso × Acipenser ruhenus), and the amphibian Xenopus (Xenopus laevis). Although gcm2-expressing cells were observed on the skin surface of Medaka and fugu, they were not found in Polypterus, sturgeon, or Xenopus. Our results suggest that an acquisition of enhancers for the expression of gcm2 contributes to a diversity of ionocytes in zebrafish during evolution.

Ammonia excretion via Rhcg1 facilitates Na⁺ uptake in larval zebrafish, Danio rerio, in acidic water

The involvement of a Na(+)/H(+) exchanger (NHE) in mediating Na(+) uptake by freshwater fish is currently debated. Although supported indirectly by empirical molecular and pharmacological data, theoretically its operation should be constrained thermodynamically, owing to unfavorable chemical gradients. Recently, there has been an increasing focus on ammonia channels (Rh proteins) as potentially contributing to Na(+) uptake across the freshwater fish gill. In this study, we tested the hypothesis that Rhcg1, a specific apical isoform of Rh protein, is critically important in facilitating Na(+) uptake in zebrafish larvae via its interaction with NHE. Treating larvae (4 days postfertilization) with 5-(N-ethyl-N-isopropyl) amiloride (EIPA), an inhibitor of NHE, caused a significant reduction in Na(+) uptake in fish reared in acidic water (pH ∼ 4.0). A role for NHE in Na(+) uptake was further confirmed by translational knockdown of NHE3b, an isoform of NHE thought to be responsible for Na(+)/H(+) exchange in zebrafish larvae. Exposing the larvae reared in acidic water to 5 mM external ammonium sulfate or increasing the buffering capacity of the water with 10 mM HEPES caused concurrent reductions in ammonia excretion and Na(+) uptake. Furthermore, translational knockdown of Rhcg1 significantly reduced ammonia excretion and Na(+) uptake in larvae chronically (4 days) or acutely (24 h) exposed to acidic water. Unlike in sham-injected larvae, EIPA did not affect Na(+) uptake in fish experiencing Rhcg1 knockdown. Additionally, exposure of larvae to bafilomycin A1 (an inhibitor of H(+)-ATPase) significantly reduced Na(+) uptake in fish reared in acidic water. These observations suggest the existence of multiple mechanisms of Na(+) uptake in larval zebrafish in acidic water: one in which Na(+) uptake via NHE3b is linked to ammonia excretion via Rhcg1, and another facilitated by H(+)-ATPase.

Freshwater fish gill ion transport: August Krogh to morpholinos and microprobes

August Krogh proposed that freshwater fishes (and other freshwater animals) maintain body NaCl homoeostasis by extracting these ions from the environment via separate Na(+) /NH(4)(+) and Cl(-) /HCO(3)(-) exchangers in the gill epithelium. Subsequent data from other laboratories suggested that Na(+) uptake was more probably coupled to H(+) secretion via a vesicular proton pump (V-ATPase) electrically coupled to a Na(+) channel. However, despite uncertainty about electrochemical gradients, evidence has accrued that epithelial Na(+) /H(+) exchange indeed may be an alternative pathway for Na(+) uptake. The specific pathways for Na(+) uptake may be species and environment specific. An apical Cl(-) /HCO(3)(-) exchanger is generally accepted for most species (some species do not extract Cl(-) from freshwater), but the relative roles of anion exchanger-like (SLC4A1) vs. pendrin-like (SLC26Z4) exchangers are unknown, and also may be species specific. Most recently, data have supported the presence of an apical Na(+) + Cl(-) cotransporter (NCC-type), despite thermodynamic uncertainty. Ammonia extrusion may be via NH(3) diffusing through the paracellular junctions or NH(4) (+) substitution on both basolateral and apical ionic exchangers (Na(+) + K(+) -ATPase; Na(+) + K(+) + Cl(-) - cotransporter; and Na(+) /H(+) exchanger), but recent evidence suggests that Rhesus-glycoproteins mediate both basolateral and apical movement of ammonia.

Ion regulation in fish gills: recent progress in the cellular and molecular mechanisms

Fish encounter harsh ionic/osmotic gradients on their aquatic environments, and the mechanisms through which they maintain internal homeostasis are more challenging compared with those of terrestrial vertebrates. Gills are one of the major organs conducting the internal ionic and acid-base regulation, with specialized ionocytes as the major cells carrying out active transport of ions. Exploring the iono/osmoregulatory mechanisms in fish gills, extensive literature proposed several models, with many conflicting or unsolved issues. Recent studies emerged, shedding light on these issues with new opened windows on other aspects, on account of available advanced molecular/cellular physiological approaches and animal models. Respective types of ionocytes and ion transporters, and the relevant regulators for the mechanisms of NaCl secretion, Na(+) uptake/acid secretion/NH(4)(+) excretion, Ca(2+) uptake, and Cl(-) uptake/base secretion, were identified and functionally characterized. These new ideas broadened our understanding of the molecular/cellular mechanisms behind the functional modification/regulation of fish gill ion transport during acute and long-term acclimation to environmental challenges. Moreover, a model for the systematic and local carbohydrate energy supply to gill ionocytes during these acclimation processes was also proposed. These provide powerful platforms to precisely study transport pathways and functional regulation of specific ions, transporters, and ionocytes; however, very few model species were established so far, whereas more efforts are needed in other species.

Functional and developmental expression of a zebrafish Kir1.1 (ROMK) potassium channel homologue Kcnj1

The zebrafish, Danio rerio, is emerging as an important model organism for the pathophysiological study of some human kidney diseases, but the sites of expression and physiological roles of a number of protein orthologues in the zebrafish nephron remain mostly undefined. Here we show that a zebrafish potassium channel is orthologous to the mammalian kidney potassium channel, ROMK. The cDNA (kcnj1) encodes a protein (Kcnj1) that when expressed in Xenopus laevis oocytes displayed pH- and Ba2+-sensitive K+-selective currents, but unlike the mammalian channel, was completely insensitive to the peptide inhibitor tertiapin-Q. In the pronephros, kcnj1 transcript expression was restricted to a distal region and overlapped with that of sodium–chloride cotransporter Nkcc, chloride channel ClC-Ka, and ClC-Ka/b accessory subunit Barttin, indicating the location of the diluting segment. In a subpopulation of surface cells, kcnj1 was coexpressed with the a1a.4 isoform of the Na+/K+-ATPase, identifying these cells as potential K+ secretory cells in this epithelium. At later stages of development, kcnj1 appeared in cells of the developing gill that also expressed the a1a.4 subunit.Morpholino antisense-mediated knockdown of kcnj1 was accompanied by transient tachycardia followed by bradycardia, effects consistent with alterations in extracellular K+ concentration in the embryo.Our findings indicate that Kcnj1 is expressed in cells associated with osmoregulation and acts as a K+ efflux pathway that is important in maintaining extracellular levels of K+ in the developing embryo.

Functional plasticity of mitochondrion-rich cells in the skin of euryhaline medaka larvae (Oryzias latipes) subjected to salinity changes

A noninvasive technique, the scanning ion-selective electrode technique (SIET) was applied to measure Na(+) and Cl(-) transport by the yolk-sac skin and individual mitochondrion-rich cells (MRCs) in intact medaka larvae (Oryzias latipes). In seawater (SW)-acclimated larvae, significant outward Na(+) and Cl(-) gradients were measured at the yolk-sac surface, indicating secretions of Na(+) and Cl(-) from the yolk-sac skin. With Na(+) pump immunostaining and microscopic observation, two groups of MRCs were identified on the yolk-sac skin of SW-larvae. These were single MRCs (s-MRCs), which do not have an accompanying accessory cell (AC), and multicellular complex MRCs (mc-MRCs), which usually consist of an MRC and an accompanying AC. The percentage of mc-MRC was ∼60% in 30 parts per thousand of SW, and it decreased with the decrease of external salinity. By serial SIET probing over the surface of the MRCs and adjacent keratinocytes (KCs), significant outward fluxes of Na(+) and Cl(-) were detected at the apical opening (membrane) of mc-MRCs, whereas only outward Cl(-) flux, but not Na(+) flux, was detected at s-MRCs. Treatment with 100 μM ouabain or bumetanide effectively blocked the Na(+) and Cl(-) secretion. Following freshwater (FW) to SW transfer, Na(+) and Cl(-) secretions by the yolk-sac skin were fully developed in 5 h and 2 h, respectively. In contrast, both Na(+) and Cl(-) secretions downregulated rapidly after SW to FW transfer. Sequential probing at individual MRCs found that Na(+) and Cl(-) secretions declined dramatically after SW to FW transfer and Na(+)/Cl(-) uptake was detected at the same s-MRCs and mc-MRCs after 5 h. This study provides evidence demonstrating that ACs are required for Na(+) excretion and MRCs possess a functional plasticity in changing from a Na(+)/Cl(-)-secreting cell to a Na(+)/Cl(-)-absorbing cell.

Embryonic frog epidermis: a model for the study of cell-cell interactions in the development of mucociliary disease

Specialised epithelia such as mucociliary, secretory and transporting epithelia line all major organs, including the lung, gut and kidney. Malfunction of these epithelia is associated with many human diseases. The frog embryonic epidermis possesses mucus-secreting and multiciliated cells, and has served as an excellent model system for the biogenesis of cilia. However, ionic regulation is important for the function of all specialised epithelia and it is not clear how this is achieved in the embryonic frog epidermis. Here, we show that a third cell type develops alongside ciliated and mucus-secreting cells in the tadpole skin. These cells express high levels of ion channels and transporters; therefore, we suggest that they are analogous to ionocytes found in transporting epithelia such as the mammalian kidney. We show that frog ionocytes express the transcription factor foxi1e, which is required for the development of these cells. Depletion of ionocytes by foxi1e knockdown has detrimental effects on the development of multiciliated cells, which show fewer and aberrantly beating cilia. These results reveal a newly identified role for ionocytes and suggest that the frog embryonic skin is a model system that is particularly suited to studying the interactions of different cell types in mucociliary, as well as in secretory and transporting, epithelia.

Anion exchanger 1b, but not sodium-bicarbonate cotransporter 1b, plays a role in transport functions of zebrafish H+-ATPase-rich cells

Similar to mammalian proximal tubular cells, H(+)-ATPase rich (HR) cells in zebrafish skin and gills are also responsible for Na(+) uptake and acid secretion functions. However, the basolateral transport pathways in HR cells are still unclear. In the present study, we tested the hypothesis if there are specific slc4 members involved in basolateral ion transport pathways in HR cells. Fourteen isoforms were identified in the zebrafish(z) slc4 family, and the full-length cDNAs of two novel isoforms, zslc4a1b (anion exchanger, zAE1b) and zslc4a4b (Na(+)/HCO(3)(-) cotransporter, zNBCe1b), were sequenced. mRNA signals of zslc4a1b and zslc4a4b were mainly detected in certain groups of ionocytes in zebrafish skin/gills. Further double immunocytochemistry or in situ hybridization demonstrated that zAE1b, but not zNBCe1b, was localized to basolateral membranes of HR cells. Acclimation to low-Na(+) or acidic environments stimulated the mRNA expression of zslc4a1b in zebrafish gills, and loss-of-function of zslc4a1b with specific morpholinos caused significant decreases in both the whole body Na(+) content and the skin H(+) activity in the morphants. On the basis of these results, it was concluded that zAE1b, but not zNBCe1b, is involved in the basolateral transport pathways in Na(+) uptake/acid secretion mechanisms in zebrafish HR cells.

Zebrafish grainyhead-like1 is a common marker of different non-keratinocyte epidermal cell lineages, which segregate from each other in a Foxi3-dependent manner

Grainyhead/CP2 transcription factor family members are widely conserved among the animal kingdom and have been implicated in different developmental processes. Thus far, nothing has been known about their roles in zebrafish. Here we identify seven zebrafish grainyhead-like (grhl) / cp2 genes, with focus on grhl1, which is expressed in the periderm and in epidermal ionocyte progenitors, but downregulated when ionocytes differentiate. In addition, expression was detected in other "non-keratinocyte" cell types of the epidermis, such as pvalb8-expressing cells, which according to our lineage tracing experiments are derived from the same pool of progenitor cells like keratinocytes and ionocytes. Antisense morpholino oligonucleotide-based loss-of-function analysis revealed that grhl1 is dispensable for the development and function of all investigated epidermal cell types, but required as a negative regulator of its own transcription during ionocyte differentiation. Knockdown of the transcription factor Foxi3a, which is expressed in a subset of the grhl1 population, caused a loss of ionocytes and a corresponding increase in the number of pvalb8-expressing cells, while leaving the number of grhl1-positive cells unaltered. We propose that grhl1 is a novel common marker of all or most "non-keratinocyte" epidermal progenitors, and that the sub-functionalisation of these cells is regulated by differential positive and negative effects of Foxi3 factors.

Identification of zebrafish Fxyd11a protein that is highly expressed in ion-transporting epithelium of the gill and skin and its possible role in ion homeostasis

FXYD proteins, small single-transmembrane proteins, have been proposed to be auxiliary regulatory subunits of Na⁺–K⁺-ATPase and have recently been implied in ion osmoregulation of teleost fish. In freshwater (FW) fish, numerous ions are actively taken up through mitochondrion-rich cells (MRCs) of the gill and skin epithelia, using the Na⁺ electrochemical gradient generated by Na⁺–K⁺-ATPase. In the present study, to understand the molecular mechanism for the regulation of Na⁺–K⁺-ATPase in MRCs of FW fish, we sought to identify FXYD proteins expressed in MRCs of zebrafish. Reverse-transcriptase PCR studies of adult zebrafish tissues revealed that, out of eight fxyd genes found in zebrafish database, only zebrafish fxyd11 (zfxyd11) mRNA exhibited a gill-specific expression. Double immunofluorescence staining showed that zFxyd11 is abundantly expressed in MRCs rich in Na⁺–K⁺-ATPase (NaK-MRCs) but not in those rich in vacuolar-type H⁺-transporting ATPase. An in situ proximity ligation assay demonstrated its close association with Na⁺–K⁺-ATPase in NaK-MRCs. The zfxyd11 mRNA expression was detectable at 1 day postfertilization, and its expression levels in the whole larvae and adult gills were regulated in response to changes in environmental ionic concentrations. Furthermore, knockdown of zFxyd11 resulted in a significant increase in the number of Na⁺–K⁺-ATPase–positive cells in the larval skin. These results suggest that zFxyd11 may regulate the transport ability of NaK-MRCs by modulating Na⁺–K⁺-ATPase activity, and may be involved in the regulation of body fluid and electrolyte homeostasis.

Rhesus glycoprotein and urea transporter genes in rainbow trout embryos are upregulated in response to alkaline water (pH 9.7) but not elevated water ammonia

Recent studies have shown that genes for the putative ammonia transporter, Rhesus glycoproteins (Rh) and the facilitated urea transporter (UT) are expressed before hatching in rainbow trout (Oncorhychus mykiss Walbaum) embryos. We tested the hypothesis that Rh and UT gene expressions are regulated in response to environmental conditions that inhibit ammonia excretion during early life stages. Eyed-up embryos (22 days post-fertilization (dpf)) were exposed to control (pH 8.3), high ammonia (1.70 mmol l(-1) NH4HCO3) and high pH (pH 9.7) conditions for 48h. With exposure to high water ammonia, ammonia excretion rates were reversed, tissue ammonia concentration was elevated by 9-fold, but there were no significant changes in mRNA expression relative to control embryos. In contrast, exposure to high water pH had a smaller impact on ammonia excretion rates and tissue ammonia concentrations, whereas mRNA levels for the Rhesus glycoprotein Rhcg2 and urea transporter (UT) were elevated by 3.5- and 5.6-fold, respectively. As well, mRNAs of the genes for H+ATPase and Na+/H+ exchanger (NHE2), associated with NH3 excretion, were also upregulated by 7.2- and 13-fold, respectively, in embryos exposed to alkaline water relative to controls. These results indicate that the Rhcg2, UT and associated transport genes are regulated in rainbow trout embryos, but in contrast to adults, there is no effect of high external ammonia at this stage of development.

Ammonium-dependent sodium uptake in mitochondrion-rich cells of medaka (Oryzias latipes) larvae

In this study, a scanning ion-selective electrode technique (SIET) was applied to measure H+, Na+, and NH4+ gradients and apparent fluxes at specific cells on the skin of medaka larvae. Na+ uptake and NH3/NH4+ excretion were detected at most mitochondrion-rich cells (MRCs). H+ probing at MRCs revealed two group of MRCs, i.e., acid-secreting and base-secreting MRCs. Treatment with EIPA (100 μM) blocked 35% of the NH3/NH4+ secretion and 54% of the Na+ uptake, suggesting that the Na+/H+ exchanger (NHE) is involved in Na+ and NH3/NH4+ transport. Low-Na+ water (<0.001 mM) or high-NH4+ (5 mM) acclimation simultaneously increased Na+ uptake and NH3/NH4+ excretion but decreased or even reversed the H+ gradient at the skin and MRCs. The correlation between NH4+ production and H+ consumption at the skin surface suggests that MRCs excrete nonionic NH3 (base) by an acid-trapping mechanism. Raising the external NH4+ significantly blocked NH3/NH4+ excretion and Na+ uptake. In contrast, raising the acidity of the water (pH 7 to pH 6) enhanced NH3/NH4+ excretion and Na+ uptake by MRCs. In situ hybridization and real-time PCR showed that the mRNAs of the Na+/H+ exchanger ( slc9a3) and Rhesus glycoproteins ( Rhcg1 and Rhbg) were colocalized in MRCs of medaka, and their expressions were induced by low-Na+ acclimation. This study suggests a novel Na+/NH4+ exchange pathway in apical membranes of MRCs, in which a coupled NHE and Rh glycoprotein is involved and the Rh glycoprotein may drive the NHE by generating H+ gradients across apical membranes of MRCs.

Ion uptake and acid secretion in zebrafish (Danio rerio)

Transepithelial transport is one of the major processes involved in the mechanism of homeostasis of body fluids in vertebrates including fish. The current models of ion regulation in fish gill ionocytes have been proposed mainly based on studies in traditional model species like salmon, trout, tilapia, eel and killifish, but the mechanisms are still being debated due to the lack of convincing molecular physiological evidence. Taking advantage of plentiful genetic databases for zebrafish, we studied the molecular/cellular mechanisms of ion regulation in fish skin/gills. In our recently proposed model, there are at least three subtypes of ionocytes in zebrafish skin/gills: Na(+)-K(+)-ATPase-rich (NaR), Na(+)-Cl(-) cotransporter (NCC) and H(+)-ATPase-rich (HR) cells. Specific isoforms of transporters and enzymes have been identified as being expressed by these ionocytes: zECaC, zPMCA2 and zNCX1b by NaR cells; zNCC gill form by NCC cells; and zH(+)-ATPase, zNHE3b, zCA2-like a and zCA15a by HR cells. Serial molecular physiological experiments demonstrated the distinct roles of these ionocytes in the transport of various ions: HR, NaR and NCC cells are respectively responsible for acid secretion/Na(+) uptake, Ca(2+) uptake and Cl(-) uptake. The expression, regulation and function of transporters in HR and NaR cells are much better understood than those in NCC cells. The basolateral transport pathways in HR and NCC cells are still unclear, and the driving forces for the operations of apical NHE and NCC are another unresolved issue. Studies on zebrafish skin/gill ionocytes are providing new insights into fish ion-regulatory mechanisms, but the zebrafish model cannot simply be applied to other species because of species differences and a lack of sufficient molecular physiological evidence in other species.

Chloride transport in mitochondrion-rich cells of euryhaline tilapia (Oreochromis mossambicus) larvae

A noninvasive scanning ion-selective electrode technique (SIET) was applied to measure Cl- transport at individual mitochondrion-rich cells (MRCs) in the skin of euryhaline tilapia (Oreochromis mossambicus) larvae. In seawater (SW)-acclimated larvae, outward Cl- gradients (20-80 mM higher than the background) were measured at the surface, indicating a secretion of Cl- from the skin. By serial probing over the surface of MRCs and adjacent keratinocytes (KCs), a significant outward flux of Cl- was detected at the apical opening (membrane) of MRCs. Treatment with 100 microM ouabain or bumetanide inhibited the Cl- secretion by approximately 75%. In freshwater (FW)-acclimated larvae, a lower level of outward Cl- gradients (0.2-1 mM) was measured at the skin surface. Low-Cl- water (<0.005 mM) acclimation increased the apical Na+-Cl- cotransporter (NCC) immunoreactivity of MRCs in the larval skin. An inward flux of Cl- was detected when probing the exterior surface of a group of MRCs (convex-MRCs) that express the NCC. An NCC inhibitor (100 microM metolazone) reduced the flux by approximately 90%. This study provides direct and convincing evidence for Cl- transport by MRCs of SW- and FW-acclimated euryhaline tilapia and the involvement of an apical NCC in Cl- uptake of MRCs of FW-acclimated fish.

A new paradigm for ammonia excretion in aquatic animals: role of Rhesus(Rh) glycoproteins

Ammonia excretion at the gills of fish has been studied for 80 years, but the mechanism(s) involved remain controversial. The relatively recent discovery of the ammonia-transporting function of the Rhesus (Rh) proteins, a family related to the Mep/Amt family of methyl ammonia and ammonia transporters in bacteria, yeast and plants, and the occurrence of these genes and glycosylated proteins in fish gills has opened a new paradigm. We provide background on the evolution and function of the Rh proteins, and review recent studies employing molecular physiology which demonstrate their important contribution to branchial ammonia efflux. Rhag occurs in red blood cells,whereas several isoforms of both Rhbg and Rhcg occur in many tissues. In the branchial epithelium, Rhcg appears to be localized in apical membranes and Rhbg in basolateral membranes. Their gene expression is upregulated during exposure to high environmental ammonia or internal ammonia infusion, and may be sensitive to synergistic stimulation by ammonia and cortisol. Rhcg in particular appears to be coupled to H+ excretion and Na+uptake mechanisms. We propose a new model for ammonia excretion in freshwater fish and its variable linkage to Na+ uptake and acid excretion. In this model, Rhag facilitates NH3 flux out of the erythrocyte, Rhbg moves it across the basolateral membrane of the branchial ionocyte, and an apical “Na+/NH +4 exchange complex” consisting of several membrane transporters (Rhcg, V-type H+-ATPase, Na+/H+ exchanger NHE-2 and/or NHE-3, Na+ channel) working together as a metabolon provides an acid trapping mechanism for apical excretion. Intracellular carbonic anhydrase(CA-2) and basolateral Na+/HCO –3cotransporter (NBC-1) and Na+/K+-ATPase play indirect roles. These mechanisms are normally superimposed on a substantial outward movement of NH3 by simple diffusion, which is probably dependent on acid trapping in boundary layer water by H+ ions created by the catalysed or non-catalysed hydration of expired metabolic CO2. Profitable areas for future investigation of Rh proteins in fish are highlighted: their involvement in the mechanism of ammonia excretion across the gills in seawater fish, their possible importance in ammonia excretion across the skin, their potential dual role as CO2 transporters,their responses to feeding, and their roles in early life stages prior to the full development of gills.

Morphological and functional classification of ion-absorbing mitochondria-rich cells in the gills of Mozambique tilapia

To clarify ion-absorbing functions and molecular mechanisms of mitochondria-rich (MR) cells, Mozambique tilapia (Oreochromis mossambicus) were acclimated to artificial freshwaters with normal or lowered Na+ and/or Cl- concentration: (1) normal Na+/normal Cl- (control); (2) normal Na+/low Cl-; (3) low Na+/normal Cl-; and (4) low Na+/low Cl-. Scanning electron microscopy (SEM) revealed that concave and convex apical surfaces of MR cells predominantly developed in low Na+ and low Cl- waters, respectively, whereas small apical pits predominated in control conditions. Expression of Na+/H+ exchanger-3 (NHE3) mRNA in the gills was increased in low Na+ waters (low Na+/normal Cl- and low Na+/low Cl-, whereas that of Na+/Cl- cotransporter (NCC) expression was upregulated in low Cl-, but not in low Na+/low Cl-. Immunofluorescence staining showed that enlarged NHE3-immunoreactive apical regions were concave or flat in low Na+ waters, whereas NCC-immunoreactive regions were enlarged convexly in low Cl- waters. Using SEM immunocytochemistry the distribution of NHE3/NCC was compared with SEM images obtained simultaneously, it was further demonstrated that NHE3 and NCC were confined to concave and convex apical surfaces, respectively. These results indicated that small apical pits developed into concave apical surfaces to facilitate Na+ uptake through NHE3, and into convex apical surfaces to enhance Na+/Cl- uptake through NCC. Our findings integrated morphological and functional classifications of ion-absorbing MR cells in Mozambique tilapia.

Expression regulation of Na+-K+-ATPase α1-subunit subtypes in zebrafish gill ionocytes

In zebrafish ( Danio rerio), six distinct Na+-K+-ATPase (NKA) α1-subunit genes have been identified, and four of them, zatp1a1a.1, zatp1a1a.2, zatp1a1a.4, and zatp1a1a.5, are expressed in embryonic skin where different types of ionocytes appear. The present study attempted to test a hypothesis of whether these NKA α1 paralogues are specifically expressed and function in respective ionocytes. Double fluorescence in situ hybridization analysis demonstrated the specific expression of zatp1a1a.1, zatp1a1a.2, and zatp1a1a. 5 in NKA-rich (NaR) cells, Na+-Cl− cotransporter (NCC)-expressing cells, and H+-ATPase-rich (HR) cells, respectively, based on the colocalization of the three NKA α1 genes with marker genes of the respective ionocytes (epithelial Ca2+ channel in NaR cells; NCC in NCC cells; and H+-ATPase and Na+/H+ exchanger 3b in HR cells). The mRNA expression (by real-time PCR) of zatp1a1a.1, zatp1a1a.2, and zatp1a1a.5 were, respectively, upregulated by low-Ca2+, low-Cl−, and low-Na+ freshwater, which had previously been reported to stimulate uptake functions of Ca2+, Cl−, and Na+. However, zatp1a1a.4 was not colocalized with any of the three types of ionocytes, nor did its mRNA respond to the ambient ions examined. Taken together, zATP1a1a.1, zATP1a1a.2, and zATP1a1a.5 may provide driving force for Na+-coupled cotransporter activity specifically in NaR, NCC, and HR cells, respectively.

Carbonic anhydrase and acid–base regulation in fish

Carbonic anhydrase (CA) is the zinc metalloenzyme that catalyses the reversible reactions of CO2 with water. CA plays a crucial role in systemic acid–base regulation in fish by providing acid–base equivalents for exchange with the environment. Unlike air-breathing vertebrates, which frequently utilize alterations of breathing (respiratory compensation) to regulate acid–base status, acid–base balance in fish relies almost entirely upon the direct exchange of acid–base equivalents with the environment (metabolic compensation). The gill is the critical site of metabolic compensation, with the kidney playing a supporting role. At the gill, cytosolic CA catalyses the hydration of CO2 to H+ and HCO3– for export to the water. In the kidney, cytosolic and membrane-bound CA isoforms have been implicated in HCO3– reabsorption and urine acidification. In this review, the CA isoforms that have been identified to date in fish will be discussed together with their tissue localizations and roles in systemic acid–base regulation.

The effects of thermally induced gill remodeling on ionocyte distribution and branchial chloride fluxes in goldfish (Carassius auratus)

Experiments were performed to evaluate the effects of temperature-induced changes in functional gill lamellar surface area on the distribution of ionocytes and branchial chloride fluxes in goldfish (Carassius auratus). In fish acclimated to warm water (25 degrees C), the ionocytes were scattered along the lamellae and within the interlamellar regions of the filament. In cold water (7 degrees C), the ionocytes were largely absent from the lamellae and filaments but instead were mostly confined to the outer regions of an interlamellar cell mass (ILCM) that formed within the interlamellar channels. Using a ;time-differential double fluorescent staining' technique, it was determined that in fish transferred from 25 degrees to 7 degrees C, the ionocytes on the outer edge of (and within) the ILCM originated predominantly from the migration of pre-existing ionocytes and to a lesser extent from the differentiation of progenitor cells. Despite the greater functional lamellar surface area in the warm-water-acclimated fish, there was no associated statistically significant increase in passive branchial Cl(-) efflux. Because the paracellular efflux of polyethylene glycol was increased 2.5-fold at the warmer temperature, it would suggest that goldfish specifically regulate (minimize) Cl(-) loss that otherwise would accompany the increasing functional lamellar surface area. In contrast to predictions, the numbers and sizes of individual ionocytes was inversely related to functional lamellar surface area resulting in a markedly greater ionocyte surface area in fish acclimated to cold water (5219+/-438 compared with 2103+/-180 microm(2) mm(-1) of filament). Paradoxically, the activity of Na(+)/K(+)-ATPase (as measured at room temperature) also was lower in the cold-water fish (0.43+/-0.06 compared with 1.28+/-0.15 micromol mg(-1) protein h(-1)) despite the greater numbers of ionocytes. There were no statistically significant differences in the rates of Cl(-) uptake in the two groups of fish despite the differences in ionocyte abundance. It is possible that to maintain normal rates of Cl(-) uptake, a greater ionocyte surface area is required in the cold-water fish that possess an ILCM because of the unfavorable positioning of the ionocytes on and within the ILCM, a structure lacking any obvious blood supply.

Specific expression and regulation of glucose transporters in zebrafish ionocytes

Glucose, a carbohydrate metabolite, plays a major role in the energy supply for fish iono- and osmoregulation, and the way that glucose is transported in ionocytes is a critical process related to the functional operations of ionocytes. Eighteen members of glucose transporters (GLUTs, SLC2A) were cloned and identified from zebrafish. Previously, Na(+),K(+)-ATPase-rich (NaR), Na(+)-Cl(-) cotransporter-expressing (NCC), H(+)-ATPase-rich (HR), and glycogen-rich (GR) cells have been identified to be responsible for Ca(2+) uptake, Cl(-) uptake, Na(+) uptake, and the energy deposition, respectively, in zebrafish skin/gills. The purpose of the present study was to test the hypothesis of whether GLUT isoforms are specifically expressed and function in ionocytes to supply energy for ion regulatory mechanisms. On the basis of translational knockdown of foxi3a/3b (2 transcriptional factors related to the ionocytes' differentiation) and triple in situ hybridization/immunocytochemistry, 3 GLUT isoforms, zglut1a, -6, and -13.1, were specifically localized in NaR/NCC cells, GR cells, and HR cells, respectively. mRNA expression of zglut1a in embryos and adult gills were stimulated by the low Ca(2+) or low Cl(-) freshwater, which has been previously reported to upregulate the functions (monitored by epithelial Ca(2+) channel, NCC mRNA) of NaR/NCC cells, respectively while that of zglut13.1 was stimulated only by low Na(+), a situation to upregulate the function (monitored by carbonic anhydrase 15a mRNA) of HR cells. On the other hand, ambient ion compositions did not affect the zglut6 mRNA expression. Taken together, zGLUT1a, -6, and 13.1, the specific transporters in NaR/NCC cells, GR cells, and HR cells, may absorb glucose into the respective cells to fulfill different physiological demands.

miR-8 microRNAs regulate the response to osmotic stress in zebrafish embryos

MicroRNAs (miRNAs) are highly conserved small RNAs that act as translational regulators of gene expression, exerting their influence by selectively targeting mRNAs bearing complementary sequence elements. These RNAs function in diverse aspects of animal development and physiology. Because of an ability to act as rapid responders at the level of translation, miRNAs may also influence stress response. In this study, we show that the miR-8 family of miRNAs regulates osmoregulation in zebrafish embryos. Ionocytes, which are a specialized cell type scattered throughout the epidermis, are responsible for pH and ion homeostasis during early development before gill formation. The highly conserved miR-8 family is expressed in ionocytes and enables precise control of ion transport by modulating the expression of Nherf1, which is a regulator of apical trafficking of transmembrane ion transporters. Ultimately, disruption of miR-8 family member function leads to an inability to respond to osmotic stress and blocks the ability to properly traffic and/or cluster transmembrane glycoproteins at the apical surface of ionocytes.

Role of SLC12A10.2, a Na-Cl cotransporter-like protein, in a Cl uptake mechanism in zebrafish (Danio rerio)

The thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC), a member of the SLC12 family, is mainly expressed in the apical membrane of the mammalian distal convoluted tubule (DCT) cells, is responsible for cotransporting Na(+) and Cl(-) from the lumen into DCT cells and plays a major role in the mammalian renal NaCl reabsorption. The NCC has also been reported in fish, but the functional role in fish ion regulation is yet unclear. The present study used zebrafish as an in vivo model to test the hypothesis of whether the NCC plays a role in Na(+) and/or Cl(-) uptake mechanisms. Four NCCs were cloned, and only one of them, zebrafish (z) slc12a10.2 was found to predominately and specifically be expressed in gills. Double in situ hybridization/immunocytochemistry in zebrafish skin/gills demonstrated that the specific expression of zslc12a10.2 mRNA in a novel group of ionocytes differed from those of the previously-reported H(+)-ATPase-rich (HR) cells and Na(+)-K(+)-ATPase-rich (NaR) cells. Gill mRNA expression of zslc12a10.2 was induced by a low-Cl environment that stimulated fish Cl(-) influx, while a low-Na environment suppressed this expression. Incubation with metolazone, a specific inhibitor of the NCC, impaired both Na(+) and Cl(-) influx in 5-day postfertilization (dpf) zebrafish embryos. Translational knockdown of zslc12a10.2 with a specific morpholino caused significant decreases in both Cl(-) influx and Cl(-) content of 5-dpf zebrafish embryos, suggesting that the operation of zNCC-like 2 results in a net uptake of Cl(-) in zebrafish. On the contrary, zslc12a10.2 morphants showed increased Na(+) influx and content that resulted from upregulation of mRNA expressions of Na(+)-H(+) exchanger 3b and carbonic anhydrase 15a in HR cells. These results for the first time provide in vivo molecular physiological evidence for the possible role of the NCC in the Cl(-) uptake mechanism in zebrafish skin/gills.

Mechanism of development of ionocytes rich in vacuolar-type H(+)-ATPase in the skin of zebrafish larvae

Mitochondrion-rich cells (MRCs), or ionocytes, play a central role in aquatic species, maintaining body fluid ionic homeostasis by actively taking up or excreting ions. Since their first description in 1932 in eel gills, extensive morphological and physiological analyses have yielded important insights into ionocyte structure and function, but understanding the developmental pathway specifying these cells remains an ongoing challenge. We previously succeeded in identifying a key transcription factor, Foxi3a, in zebrafish larvae by database mining. In the present study, we analyzed a zebrafish mutant, quadro (quo), deficient in foxi1 gene expression and found that foxi1 is essential for development of an MRC subpopulation rich in vacuolar-type H(+)-ATPase (vH-MRC). foxi1 acts upstream of Delta-Notch signaling that determines sporadic distribution of vH-MRC and regulates foxi3a expression. Through gain- and loss-of-function assays and cell transplantation experiments, we further clarified that (1) the expression level of foxi3a is maintained by a positive feedback loop between foxi3a and its downstream gene gcm2 and (2) Foxi3a functions cell-autonomously in the specification of vH-MRC. These observations provide a better understanding of the differentiation and distribution of the vH-MRC subtype.

Cellular mechanisms of Cl- transport in trout gill mitochondrion-rich cells

We have studied Cl(-) transport mechanisms in freshwater rainbow trout gill mitochondrion-rich (MR) cells using intracellular pH (pH(i)) imaging. Scanning electron microscopy demonstrated maintenance of cellular polarity in isolated MR cells. MR cell subtypes were identified by Na(+) introduction to the bath, and Cl(-) transport mechanisms were subsequently examined. Cl(-)-free exposure resulted in an alkalinization of pH(i) in both MR cell subtypes, which was dependent on HCO(3)(-) in the bath and inhibited by 1 mM DIDS. Recovery of pH(i) from an acidified state in Na(+)-free conditions was also DIDS sensitive. These results are the first functional evidence for Cl(-)/HCO(3)(-) exchangers in fish gill MR cells. A direct switch from NaCl to Cl(-)-free conditions caused a pH(i) acidification in a subset of MR cells, which was enhanced in the absence of HCO(3)(-). The acidification was replaced by an alkalinization when Cl(-) removal was performed in the presence of NPPB (500 microM) or EIPA (500 microM). Finally, we found that the Na(+)-induced alkalinization of pH(i) found in a previous study is inhibited by EIPA. This inhibitor profile's results suggest the presence of a Cl(-)-dependent Na(+)/H(+) exchange mechanism.

Functional regulation of H+-ATPase-rich cells in zebrafish embryos acclimated to an acidic environment

It is important to maintain internal pH homeostasis in biological systems. In our previous studies, H(+)-ATPase-rich (HR) cells were found to be responsible for proton secretion in the skin of zebrafish embryos during development. In this study, zebrafish embryos were exposed to acidic and basic waters to investigate the regulation of HR cell acid secretion during pH disturbances. Our results showed that the function of HR cells on the skin of zebrafish embryos can be upregulated in pH 4 water not only by increasing the cell number but also by enlarging the acid-secreting function of single cells. We also identified an "alveolar-type" apical opening under scanning electron microscopy observations of the apical membrane of HR cells, and the density and size of the alveolar type of apical openings were also increased in pH 4 water. p63 and PCNA immunostaining results also showed that additional HR cells in pH 4 water may be differentiated not only from ionocyte precursor cells but also newly proliferating epithelial stem cells.

The transcription factor, glial cell missing 2, is involved in differentiation and functional regulation of H+-ATPase-rich cells in zebrafish (Danio rerio)

H(+)-ATPase-rich (HR) cells in zebrafish are known to be involved in acid secretion and Na(+) uptake mechanisms in zebrafish gills/skin; however, little is known about how HR cells are functionally regulated. In the present work, we studied the roles of Drosophila glial cell missing (gcm), a cell fate-related transcription factor, in the differentiation and functional regulation of zebrafish HR cells. Zebrafish gcm2 (zgcm2) was found to begin expression in zebrafish embryos at 10 h postfertilization (hpf), and to be extensively expressed in gills but only mildly so in eyes, heart, muscles, and testes. By whole mount in situ hybridization, zgcm2 mRNA signals were found in a group of cells on the zebrafish yolk sac surface initially in the tail bud stage (10 hpf); they had disappeared at 36 hpf and thereafter appeared again in the gill region from 48 hpf. Double fluorescence in situ hybridization further demonstrated specific colocalization of zgcm2 mRNA in HR cells in zebrafish embryos. Knockdown of zgcm2 with a specific morpholino oligonucleotide caused the complete disappearance of HR cells with a concomitant decrease in H(+) activity at the apical surface of HR cells, but it did not affect the occurrence of Na(+)-K(+)-ATPase-rich cells. A decrease in the H(+)-ATPase subunit A (zatp6v1a) expression and no change in zgcm2 expression in zebrafish gills were seen from 12 h to 3 days after transfer to acidic fresh water, but a compensatory stimulation in the expressions of both genes appeared 4 days post-transfer. In conclusion, functional regulation of HR cells is probably achieved by enhancing cell differentiation via zGCM2 activation.

Effects of hypothermia on gene expression in zebrafish gills: upregulation in differentiation and function of ionocytes as compensatory responses

Ectothermic vertebrates are different from mammals that are sensitive to hypothermia and have to maintain core temperature for survival. Why and how ectothermic animals survive, grow and reproduce in low temperature have been for a long time a scientifically challenging and important inquiry to biologists. We used a microarray to profile the gill transcriptome in zebrafish (Danio rerio) after exposure to low temperature. Adult zebrafish were acclimated to a low temperature of 12 degrees C for 1 day and up to 30 days, and the gill transcriptome was compared with that of control fish in 28 degrees C by oligonucleotide microarray hybridization. Results showed 11 and 22 transcripts were found to be upregulated, whereas 56 and 70 transcripts were downregulated by low-temperature treatment for 1 day and 30 days, respectively. The gill transcriptome profiles revealed that ionoregulation-related genes were highly upregulated in cold-acclimated zebrafish. This paved the way to investigate the role of ionoregulatory genes in zebrafish gills during cold acclimation. Cold acclimation caused upregulation of genes that are essential for ionocyte specification, differentiation, ionoregulation, acid-base balance and the number of cells expressing these genes increased. For instance, epithelial Ca2+ channel (EcaC; an ionoregulatory protein) mRNA increased in parallel with the level of Ca2+ influx, revealing a functional compensation after long-term acclimation to cold. Phosphohistone H3 and TUNEL staining showed that the cell turnover rate was retarded in cold-acclimated gills. Altogether, these results suggest that gills may sustain their functions by producing mature ionocytes from pre-existing undifferentiated progenitors in low-temperature environments.

Ammonia excretion by the skin of zebrafish (Danio rerio) larvae

The mechanism of ammonia excretion in freshwater teleosts is not well understood. In this study, scanning ion-selective electrode technique was applied to measure H+and NH4+fluxes in specific cells on the skin of zebrafish larvae. NH4+extrusion was relatively high in H+pump-rich cells, which were identified as the H+-secreting ionocyte in zebrafish. Minor NH4+extrusion was also detected in keratinocytes and other types of ionocytes in larval skin. NH4+extrusion from the skin was tightly linked to acid secretion. Increases in the external pH and buffer concentration (5 mM MOPS) diminished H+and NH4+gradients at the larval surface. Moreover, coupled decreases in NH4+and H+extrusion were found in larvae treated with an H+-pump inhibitor (bafilomycin A1) or H+-pump gene ( atp6v1a) knockdown. Knockdown of Rhcg1 with morpholino-oligonucleotides also decreased NH4+excretion. This study demonstrates ammonia excretion in epithelial cells of larval skin through an acid-trapping mechanism, and it provides direct evidence for the involvement of the H+pump and an Rh glycoprotein (Rhcg1) in ammonia excretion.

Zebrafish mutants calamity and catastrophe define critical pathways of gene-nutrient interactions in developmental copper metabolism

Nutrient availability is an important environmental variable during development that has significant effects on the metabolism, health, and viability of an organism. To understand these interactions for the nutrient copper, we used a chemical genetic screen for zebrafish mutants sensitive to developmental copper deficiency. In this screen, we isolated two mutants that define subtleties of copper metabolism. The first contains a viable hypomorphic allele of atp7a and results in a loss of pigmentation when exposed to mild nutritional copper deficiency. This mutant displays incompletely penetrant skeletal defects affected by developmental copper availability. The second carries an inactivating mutation in the vacuolar ATPase that causes punctate melanocytes and embryonic lethality. This mutant, catastrophe, is sensitive to copper deprivation revealing overlap between ion metabolic pathways. Together, the two mutants illustrate the utility of chemical genetic screens in zebrafish to elucidate the interaction of nutrient availability and genetic polymorphisms in cellular metabolism.

Copper is an essential nutrient required for multiple biologic functions. Proper uptake, transport, and excretion of copper are critical for use of this metal while reducing its inherent toxicity. While several key proteins involved in this process have been identified, there are still gaps in our understanding of copper metabolism—particularly during early development. We have used zebrafish, a genetically useful animal model system, to study genetic interactions with copper deficiency during development. We treated mutant embryonic zebrafish with a chelator that reduces the level of available copper and screened for mutants that displayed a copper deficient phenotype only in the presence of the chelator. We identified and characterized two mutants that advance our understanding of copper metabolism during the early periods of development, as well as show an interaction between copper metabolism and another fundamental pathway—that of proton transport. Our results expand our knowledge of copper metabolism and illustrate the power of this type of genetic screen in zebrafish to elucidate mechanisms of nutrient metabolism.

Theoretical considerations underlying Na(+) uptake mechanisms in freshwater fishes

Ion and acid-base regulating mechanisms have been studied at the fish gill for almost a century. Original models proposed for Na(+) and Cl(-) uptake, and their linkage with H(+) and HCO(3)(-) secretion have changed substantially with the development of more sophisticated physiological techniques. At the freshwater fish gill, two dominant mechanisms for Na(+) uptake from dilute environments have persisted in the literature. The use of an apical Na(+)/H(+) exchanger driven by a basolateral Na(+)/K(+)-ATPase versus an apical Na(+) channel electrogenically coupled to an apical H(+)-ATPase have been the source of debate for a number of years. Advances in molecular biology have greatly enhanced our understanding of the basic ion transport mechanisms at the fish gill. However, it is imperative to ensure that thermodynamic principles are followed in the development of new models for gill ion transport. This review will focus on the recent molecular advances for Na(+) uptake in freshwater fish. Emphasis will be placed on thermodynamic constraints that prevent electroneutral apical NHE function in most freshwater environments. By combining recent advances in molecular and functional physiology of fish gills with thermodynamic considerations of ion transport, our knowledge in the field should continue to grow in a logical manner.

Some insights into energy metabolism for osmoregulation in fish

A sufficient and timely energy supply is a prerequisite for the operation of iono- and osmoregulatory mechanisms in fish. Measurements of whole-fish or isolated-gill (or other organs) oxygen consumption have demonstrated regulation of the energy supply during acclimation to different osmotic environments, and such regulation is dependent on species, the situation of acclimation or acclimatization, and life habits. Carbohydrate metabolism appears to play a major role in the energy supply for iono- and osmoregulation, and the liver is the major source supplying carbohydrate metabolites to osmoregulatory organs. Compared with carbohydrates, the roles of lipids and proteins remain largely unclear. Energy metabolite translocation was recently found to occur between fish gill ionocytes and neighboring glycogen-rich (GR) cells, indicating the physiological significance of a local energy supply for gill ion regulatory mechanisms. Spatial and temporal relationships between the liver and other osmoregulatory and non-osmoregulatory organs in partitioning the energy supply for ion regulatory mechanisms during salinity challenges were also proposed. A novel glucose transporter was found to specifically be expressed and function in gill ionocytes, providing the first cue for investigating energy translocation among gill cells. Advanced molecular physiological approaches can be used to examine energy metabolism relevant to a particular cell type (e.g., gill ionocytes), and functional genomics may also provide another powerful approach to explore new metabolic pathways related to fish ion regulation.

Evidence for an apical Na-Cl cotransporter involved in ion uptake in a teleost fish

Cation-chloride cotransporters, such as the Na(+)/K(+)/2Cl(-) cotransporter (NKCC) and Na(+)/Cl(-) cotransporter (NCC), are localized to the apical or basolateral plasma membranes of epithelial cells and are involved in active ion absorption or secretion. The objectives of this study were to clone and identify ;freshwater-type' and ;seawater-type' cation-chloride cotransporters of euryhaline Mozambique tilapia (Oreochromis mossambicus) and to determine their intracellular localization patterns within mitochondria-rich cells (MRCs). From tilapia gills, we cloned four full-length cDNAs homologous to human cation-chloride cotransporters and designated them as tilapia NKCC1a, NKCC1b, NKCC2 and NCC. Out of the four candidates, the mRNA encoding NKCC1a was highly expressed in the yolk-sac membrane and gills (sites of the MRC localization) of seawater-acclimatized fish, whereas the mRNA encoding NCC was exclusively expressed in the yolk-sac membrane and gills of freshwater-acclimatized fish. We then generated antibodies specific for tilapia NKCC1a and NCC and conducted whole-mount immunofluorescence staining for NKCC1a and NCC, together with Na(+)/K(+)-ATPase, cystic fibrosis transmembrane conductance regulator (CFTR) and Na(+)/H(+) exchanger 3 (NHE3), on the yolk-sac membrane of tilapia embryos acclimatized to freshwater or seawater. The simultaneous quintuple-color immunofluorescence staining allowed us to classify MRCs clearly into four types: types I, II, III and IV. The NKCC1a immunoreactivity was localized to the basolateral membrane of seawater-specific type-IV MRCs, whereas the NCC immunoreactivity was restricted to the apical membrane of freshwater-specific type-II MRCs. Taking account of these data at the level of both mRNA and protein, we deduce that NKCC1a is the seawater-type cotransporter involved in ion secretion by type-IV MRCs and that NCC is the freshwater-type cotransporter involved in ion absorption by type-II MRCs. We propose a novel ion-uptake model by MRCs in freshwater that incorporates apically located NCC. We also reevaluate a traditional ion-uptake model incorporating NHE3; the mRNA was highly expressed in freshwater, and the immunoreactivity was found at the apical membrane of other freshwater-specific MRCs.