Mitochondria-rich cell activity in the yolk-sac membrane of tilapia(Oreochromis mossambicus) larvae acclimatized to different ambient chloride levels

Acidified water promotes silver-induced toxicity in zebrafish embryos

Freshwater acidification is a global environmental challenge, yet the effects of acidic water on fish resistance to toxic Ag+ remain an unexplored area. To address this knowledge gap, zebrafish embryos were exposed to different concentrations (0 (control), 0.1, and 0.25 mg/L) of AgNO3 under pH 5 or 7 for 7 days. Notably, AgNO3 at 0.25 mg/L resulted in 100 % mortality in both pH conditions, while AgNO3 at 0.1 mg/L resulted in higher mortality at pH 5 (85 %) compared to pH 7 (20 %), indicating that acidic water enhanced Ag+ toxicity. Several parameters, including body length, inner ear (otic vesicle and otolith) and yolk sac areas, lateral line hair cell number and morphology, the number of ionocytes (H+-ATP-rich cells and Na+/K+-ATP-rich cells), and ion contents (Ag+, Na+, and Ca2+) were assessed at 96 h (day 4) to investigate individual and combined effects of Ag+ and acid on embryos. Acid alone did not significantly alter most parameters, but it decreased the yolk sac area and increased the ionocyte number. Conversely, Ag+ alone caused reductions in most parameters, including body length, the inner ear area, hair cell number, and ionocyte number. Combining acid and Ag+ resulted in greater suppression of the otolith area, hair cell number, and Na+/Ca2+ contents. In conclusion, acidification of freshwater poses a potential risk to fish embryo viability by increasing their susceptibility to silver toxicity, specifically affecting sensory function and ion regulation.

Retention of ion channel genes expression increases Japanese medaka survival during seawater reacclimation

Euryhaline teleosts exhibit varying acclimability to survive in environments that alternate between being hypotonic and hypertonic. Such ability is conferred by ion channels expressed by ionocytes, the ion-regulating cells in the gills or skin. However, switching between environments is physiologically challenging, because most channels can only perform unidirectional ion transportation. Coordination between acute responses, such as gene expression, and long-term responses, such as cell differentiation, is believed to strongly facilitate adaptability. Moreover, the pre-acclimation to half seawater salinity can improve the survivability of Japanese medaka (Oryzias latipes) during direct transfer to seawater; here, the ionocytes preserve hypertonic acclimability while performing hypotonic functions. Whether acclimability can be similarly induced in a closed species and their corresponding responses in terms of ion channel expression remain unclear. In the present study, Japanese medaka pre-acclimated in brackish water were noted to have higher survival rates while retaining higher expression of the three ion channel genes ATP1a1a.1, ATP1b1b, and SLC12a2a. This retention was maintained up to 2 weeks after the fish were transferred back into freshwater. Notably, this induced acclimability was not found in its close kin, Indian medaka (Oryzias dancena), the natural habitat of which is brackish water. In conclusion, Japanese medaka surpassed Indian medaka in seawater acclimability after experiencing exposure to brackish water, and this ability coincided with seawater-retention gene expression.

Why can Mozambique Tilapia Acclimate to Both Freshwater and Seawater? Insights From the Plasticity of Ionocyte Functions in the Euryhaline Teleost

In teleost fishes, ionocytes in the gills are important osmoregulatory sites in maintaining ionic balance. During the embryonic stages before the formation of the gills, ionocytes are located in the yolk-sac membrane and body skin. In Mozambique tilapia embryos, quintuple-color immunofluorescence staining allowed us to classify ionocytes into four types: type I, showing only basolateral Na⁺/K⁺-ATPase (NKA) staining; type II, basolateral NKA and apical Na⁺, Cl⁻ cotransporter 2; type III, basolateral NKA, basolateral Na⁺, K⁺, 2Cl⁻ cotransporter 1a (NKCC1a) and apical Na⁺/H⁺ exchanger 3; and type IV, basolateral NKA, basolateral NKCC1a and apical cystic fibrosis transmembrane conductance regulator Cl⁻ channel. The ionocyte population consisted mostly of type I, type II and type III in freshwater, while type I and IV dominated in seawater. In adult tilapia, dual observations of whole-mount immunocytochemistry and scanning electron microscopy showed morphofunctional alterations in ionocytes. After transfer from freshwater to seawater, while type-II ionocytes closed their apical openings to suspend ion absorption, type-III ionocytes with a concave surface were transformed into type IV with a pit via a transitory surface. The proposed model of functional classification of ionocytes can account not only for ion uptake in freshwater and ion secretion in seawater, but also for plasticity in ion-transporting functions of ionocytes in tilapia.

Replacement of mitochondrion-rich cells during regeneration of the gills and opercular epithelium in zebrafish (Danio rerio)

Transport epithelia maintain the volume, ion concentration and acid-base balance of blood and extracellular fluids. In teleost fish, mitochondrion-rich cells (MRCs) are specialized ionocytes that perform this role. These cells are found in epithelia of the gills and buccal surface of the operculum (the bony structure covering the gills). Proliferation of MRCs in response to changes in water salinity and other environmental stressors is well documented, but the cellular mechanisms underlying MRC proliferation are poorly understood. Recently, regeneration and epithelial cell replacement in the gill filaments was demonstrated in the model vertebrate, zebrafish (Danio rerio), raising the question of whether MRCs are replaced during regrowth of transport epithelia. We chose two anatomical sites where MRCs are found-the gills and the opercular epithelium-to investigate whether MRCs were replaced following surgical resection of these structures. In live imaging experiments, we observed gradual replacement of the branchiostegal valve, an extension of the operculum, in zebrafish over a period of 21 days post-resection (dpr). In regenerating epithelia of both the operculum and gills, we detected MRCs by immunohistochemical localization of the α subunit of plasma membrane Na⁺/K⁺-ATPase. In both tissues, MRCs appeared soon after resection, and as early as 1 dpr in the gill filaments. We report regeneration of the operculum and proliferation of MRCs in regenerating tissue in adult zebrafish. These studies may contribute to our understanding of how MRC populations are regulated during the regenerative process, which may occur following exposure to environmental stressors, chemical toxicity or disease.

Reductionist approaches to the study of ionoregulation in fishes

The mechanisms underlying ionoregulation in fishes have been studied for nearly a century, and reductionist methods have been applied at all levels of biological organization in this field of research. The complex nature of ionoregulatory systems in fishes makes them ideally suited to reductionist methods and our collective understanding has been dramatically shaped by their use. This review provides an overview of the broad suite of techniques used to elucidate ionoregulatory mechanisms in fishes, from the whole-animal level down to the gene, discussing some of the advantages and disadvantages of these methods. We provide a roadmap for understanding and appreciating the work that has formed the current models of organismal, endocrine, cellular, molecular, and genetic regulation of ion balance in fishes and highlight the contribution that reductionist techniques have made to some of the fundamental leaps forward in the field throughout its history.

Exposure to copper nanoparticles impairs ion uptake, and acid and ammonia excretion by ionocytes in zebrafish embryos

The potential toxicity of copper nanoparticles (CuNPs) to early stages of fishes is not fully understood, and little is known about their effects on ionocytes and associated functions. This study used zebrafish embryos as a model to investigate the toxic effects of CuNPs on two subtypes of ionocytes. Zebrafish embryos were exposed to 0.1, 1, and 3 mg L^-1 CuNPs for 96 h. After exposure, whole-body Na⁺ and Ca²⁺ contents were significantly reduced at ≥0.1 mg L^-1, while the K⁺ content had decreased at ≥1 mg L^-1. H⁺ and NH₄⁺ excretion by the skin significantly decreased at ≥1 mg L^-1. The number of living ionocytes labeled with rhodamine-123 had significantly decreased with ≥0.1 mg L^-1 CuNPs. The ionocyte subtypes of H⁺-ATPase-rich (HR) and Na⁺/K⁺-ATPase-rich (NaR) cells were labeled by immunostaining and had decreased with ≥1 mg L^-1. Shrinkage of the apical opening of ionocytes was revealed by scanning electronic microscopy. Functional impairment was also reflected by changes in gene expressions, including ion transporters/channels and Ca²⁺-regulatory hormones. This study shows that CuNP exposure can impair two subtypes of ionocytes and their associated functions, including Na⁺/Ca²⁺ uptake and H⁺/NH₄⁺ excretion in zebrafish embryos.

Toxicological perspective on the osmoregulation and ionoregulation physiology of major ions by freshwater animals: Teleost fish, crustacea, aquatic insects, and Mollusca

Anthropogenic sources increase freshwater salinity and produce differences in constituent ions compared with natural waters. Moreover, ions differ in physiological roles and concentrations in intracellular and extracellular fluids. Four freshwater taxa groups are compared, to investigate similarities and differences in ion transport processes and what ion transport mechanisms suggest about the toxicity of these or other ions in freshwater. Although differences exist, many ion transporters are functionally similar and may belong to evolutionarily conserved protein families. For example, the Na+ /H+ -exchanger in teleost fish differs from the H+ /2Na+ (or Ca2+ )-exchanger in crustaceans. In osmoregulation, Na+ and Cl- predominate. Stenohaline freshwater animals hyperregulate until they are no longer able to maintain hypertonic extracellular Na+ and Cl- concentrations with increasing salinity and become isotonic. Toxic effects of K+ are related to ionoregulation and volume regulation. The ionic balance between intracellular and extracellular fluids is maintained by Na+ /K+ -adenosine triphosphatase (ATPase), but details are lacking on apical K+ transporters. Elevated H+ affects the maintenance of internal Na+ by Na+ /H+ exchange; elevated HCO3- inhibits Cl- uptake. The uptake of Mg2+ occurs by the gills or intestine, but details are lacking on Mg2+ transporters. In unionid gills, SO42- is actively transported, but most epithelia are generally impermeant to SO42- . Transporters of Ca2+ maintain homeostasis of dissolved Ca2+ . More integration of physiology with toxicology is needed to fully understand freshwater ion effects. Environ Toxicol Chem 2017;36:576-600. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Microtubule-dependent changes in morphology and localization of chloride transport proteins in gill mitochondria-rich cells of the tilapia, Oreochromis mossambicus

The tilapia (Oreochromis mossambicus) is a euryhaline fish exhibiting adaptive changes in cell size, phenotype, and ionoregulatory functions upon salinity challenge. Na(+) /Cl(-) cotransporter (NCC) and Na(+) /K(+) /2Cl(-) cotransporter (NKCC) are localized in the apical and basolateral membranes of mitochondria-rich (MR) cells of the gills. These cells are responsible for chloride absorption (NCC) and secretion (NKCC), respectively, thus, the switch of gill NCC and NKCC expression is a crucial regulatory mechanism for salinity adaptation in tilapia. However, little is known about the interaction of cytoskeleton and these adaptive changes. In this study, we examined the time-course of changes in the localization of NKCC/NCC in the gills of tilapia transferred from fresh water (FW) to brackish water (20‰) and from seawater (SW; 35‰) to FW. The results showed that basolateral NKCC disappeared and NCC was expressed in the apical membrane of MR cells. To further clarify the process of these adaptive changes, colchicine, a specific inhibitor of microtubule-dependent cellular regulating processes was used. SW-acclimated tilapia were transferred to SW, FW, and FW with colchicine (colchicine-FW) for 96 h. Compared with the FW-treatment group, in the MR cells of colchicine-FW-treatment group, (1) the average size was significantly larger, (2) only wavy-convex-subtype apical surfaces were found, and (3) the basolateral (cytoplasmic) NKCC signals were still exhibited. Taken together, our results suggest that changes in size, phenotype, as well as the expression of NCC and NKCC cotransporters of MR cells in the tilapia are microtubule-dependent. J. Morphol. 277:1113-1122, 2016. © 2016 Wiley Periodicals, Inc.

Branchial Na(+):K(+):2Cl(-) cotransporter 1 and Na(+)/K(+)-ATPase α-subunit in a brackish water-type ionocyte of the euryhaline freshwater white-rimmed stingray, Himantura signifer

Himantura signifer is a freshwater stingray which inhabits rivers in Southeast Asia. It can survive in brackish water but not seawater. In brackish water, it becomes partially ureosmotic, but how it maintains its plasma hypoionic to the external medium is enigmatic because of the lack of a rectal gland. Here, we report for the first time the expression of Na(+):K(+):2Cl(-) cotransporter 1 (nkcc1) in the gills of freshwaterH. signifer, and its moderate up-regulation (~2-fold) in response to brackish water (salinity 20) acclimation. The absence of the Ste20-related proline-alanine-rich kinase and oxidation stress response kinase 1 interaction site from the N-terminus of H. signifer Nkcc1 suggested that it might not be effectively activated by stress kinases in response to salinity changes as in more euryhaline teleosts. The increased activity of Nkcc1 during salt excretion in brackish water would lead to an influx of Na(+) into ionocytes, and the maintenance of intracellular Na(+) homeostasis would need the cooperation of Na(+)/K(+)-ATPase (Nka). We demonstrated for the first time the expression of nkaα1, nkaα2 and nkaα3 in the gills of H. signifer, and the up-regulation of the mRNA expression of nkaα3 and the overall protein abundance of Nkaα in response to acclimation to brackish water. Immunofluorescence microscopy revealed the presence of a sub-type of ionocyte, co-expressing Nkcc1 and Nkaα, near the base of the secondary lamellae in the gills of H. signifer acclimated to brackish water, but this type of ionocyte was absent from the gills of fish kept in fresh water. Hence, there could be a change in the function of the gills of H. signifer from salt absorption to salt excretion during brackish water acclimation in the absence of a functioning rectal gland.

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.

Increases in apoptosis, caspase activity and expression of p53 and bax, and the transition between two types of mitochondrion-rich cells, in the gills of the climbing perch, Anabas testudineus, during a progressive acclimation from freshwater to seawater

This study aimed to test the hypothesis that branchial osmoregulatory acclimation involved increased apoptosis and replacement of mitochdonrion-rich cells (MRCs) in the climbing perch, Anabas testudineus, during a progressive acclimation from freshwater to seawater. A significant increase in branchial caspase-3/-7 activity was observed on day 4 (salinity 20), and an extensive TUNEL-positive apoptosis was detected on day 5 (salinity 25), indicating salinity-induced apoptosis had occurred. This was further supported by an up-regulation of branchial mRNA expression of p53, a key regulator of cell cycle arrest and apoptosis, between day 2 (salinity 10) and day 6 (seawater), and an increase in branchial p53 protein abundance on day 6. Seawater acclimation apparently activated both the extrinsic and intrinsic pathways, as reflected by significant increases in branchial caspase-8 and caspase-9 activities. The involvement of the intrinsic pathway was confirmed by the significant increase in branchial mRNA expression of bax between day 4 (salinity 20) and day 6 (seawater). Western blotting results revealed the presence of a freshwater Na(+)/K(+)-ATPase (Nka) α-isoform, Nka α1a, and a seawater isoform, Nka α1b, the protein abundance of which decreased and increased, respectively, during seawater acclimation. Immunofluorescence microscopy revealed the presence of two types of MRCs distinctly different in sizes, and confirmed that the reduction in Nka α1a expression, and the prominent increases in expression of Nka α1b, Na(+):K(+):2Cl(-) cotransporter 1, and cystic fibrosis transmembrane conductance regulator Cl(-) channel coincided with the salinity-induced apoptotic event. Since modulation of existing MRCs alone could not have led to extensive salinity-induced apoptosis, it is probable that some, if not all, freshwater-type MRCs could have been removed through increased apoptosis and subsequently replaced by seawater-type MRCs in the gills of A. testudineus during seawater acclimation.

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.

Mechanism of osmoregulatory adaptation in tilapia

The shortage of freshwater resource in many countries leads to a shift to develop aquaculture in brackish water and sea water. Tilapias are euryhaline that can thrive from freshwater to full sea water. They and their hybrids are the best candidate species for cultivation in brackish habitats. Thus, understanding their osmoregulatory mechanisms will help to breed or genetically engineer salt tolerant species. In this paper, we review recent progress in understanding the mechanisms of osmoregulatory adaptations in tilapia.

Ontogeny of salinity tolerance and evidence for seawater-entry preparation in juvenile green sturgeon, Acipenser medirostris

We measured the ontogeny of salinity tolerance and the preparatory hypo-osmoregulatory physiological changes for seawater entry in green sturgeon (Acipenser medirostris), an anadromous species occurring along the Pacific Coast of North America. Salinity tolerance was measured every 2 weeks starting in 40-day post-hatch (dph) juveniles and was repeated until 100% survival at 34‰ was achieved. Fish were subjected to step increases in salinity (5‰ 12 h(-1)) that culminated in a 72-h exposure to a target salinity, and treatment groups (0, 15, 20, 25, 30, 34‰; and abrupt exposure to 34‰) were adjusted as fish developed. After 100% survival was achieved (134 dph), a second experiment tested two sizes of fish for 28-day seawater (33‰) tolerance, and gill and gastrointestinal tract tissues were sampled. Their salinity tolerance increased and plasma osmolality decreased with increasing size and age, and electron microscopy revealed three types of mitochondria-rich cells: one in fresh water and two in seawater. In addition, fish held on a natural photoperiod in fresh water at 19°C showed peaks in cortisol, thyroid hormones and gill and pyloric ceca Na(+), K(+)-ATPase activities at body sizes associated with seawater tolerance. Therefore, salinity tolerance in green sturgeon increases during ontogeny (e.g., as these juveniles may move down estuaries to the ocean) with increases in body size. Also, physiological and morphological changes associated with seawater readiness increased in freshwater-reared juveniles and peaked at their seawater-tolerant ages and body sizes. Their seawater-ready body size also matched that described for swimming performance decreases, presumably associated with downstream movements. Therefore, juvenile green sturgeon develop structures and physiological changes appropriate for seawater entry while growing in fresh water, indicating that hypo-osmoregulatory changes may proceed by multiple routes in sturgeons.

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 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.

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.

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.

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.

Differential responses in gills of euryhaline tilapia, Oreochromis mossambicus, to various hyperosmotic shocks

Euryhaline tilapia (Oreochromis mossambicus) survived in brackish water (BW; 20 per thousand) but died in seawater (SW; 35 per thousand) within 6 h when transferred directly from fresh water (FW). The purpose of this study was to clarify responses in gills of FW tilapia to various hyperosmotic shocks induced by BW or SW. In FW-acclimated tilapia, scanning electron micrographs of gills revealed three subtypes of MR cell apical surfaces: wavy-convex (subtype I), shallow-basin (subtype II), and deep-hole (subtype III). Density of apical surfaces of mitochondrion-rich (MR) cell in gills of the BW-transfer tilapia decreased significantly within 3 h post-transfer due to disappearance of subtype I cells, but increased from 48 h post-transfer because of increasing density of subtype III cells. SW-transfer individuals, however, showed decreased density of MR cell openings after 1 h post-transfer because subtype I MR cell disappeared. On the other hand, relative branchial Na+/K+-ATPase (NKA) alpha1-subunit mRNA levels, protein abundance, and NKA activity of the BW-transfer group increased significantly at 6, 12, and 12 h post-transfer, respectively. In the SW-transfer group, relative mRNA and protein abundance of gill NKA alpha1-subunit did not change while NKA activity declined before dying in 5 h. Upon SW transfer, dramatic increases (nearly 2-fold) of plasma osmolality, [Na+], and [Cl(-)] were found prior to death. For the BW-transfer group, plasma osmolality was eventually controlled by 96 h post-transfer by enhancement of NKA expression and subtype III MR cell. The success or failure of NKA activation from gene to functional protein as well as the development of specific SW subtype in gills were crucial for the survival of euryhaline tilapia to various hyperosmotic shocks.

Bone development in the jaw of Nile tilapia Oreochromis niloticus (Pisces: Cichlidae)

East African cichlids have evolved feeding apparatus morphologies adapted to their diverse feeding behaviors. The evolution of the oral jaw morphologies is accomplished by the diversity of bone formation during development. To further understand this evolutionary process, we examined the skeletal elements of the jaw and their temporal and sequential emergence, categorized by developmental stages, using the Nile tilapia Oreochromis niloticus as a model cichlid. We found that chondrogenesis started in Stage 17. The deposition of osteoid for the dermal bones commenced in Stage 18. The uptake of calcium dramatically shifted from the surface of larvae to the gills in Stage 20. The bone mineralization of the skeleton began in Stage 25. These data provide important information regarding the sequential events of craniofacial development in East African cichlids and lay the groundwork for studying the molecular mechanisms underlying adaptation of jaw structure to feeding behavior.

Juvenile tilapia (Oreochromis mossambicus) strive to maintain physiological functions after waterborne copper exposure

Juvenile tilapia were acutely exposed to 0.2 and 2 mg/L Cu(2+) for up to 144 h. The Na(+)-K(+)-ATPase (NKA)-specific activity in the gills of tilapia exposed to 0.2 mg/L Cu(2+) significantly decreased over 48-72 h and was restored to the control level after 96 h, but was again depressed during 120-144 h. The whole-body Cl(-) levels significantly decreased after 48 h, but recovered shortly afterwards and continued to do so until 144 h with 0.2 mg/L Cu exposure. During 48-72 h, the numbers of the wavy-convex type of mitochondria-rich (MR) cells appeared to significantly increase and the cortisol content also significantly increased. Changes in MR cell morphology might be necessary in order to enhance Cl(-) uptake, and this might be related to changes in cortisol levels. Whole-body Na(+) concentrations had significantly decreased by 72 h, but recovered during 96-144 h. Whole-body Cu(2+) concentrations also significantly increased compared to the initial concentration during 72-144 h of Cu exposure. All measured parameters (NKA activity, Na(+) concentration, and MR cell numbers) significantly decreased in fish exposed to 2 mg/L Cu, and no recovery was observed. These data demonstrate that juvenile tilapia strived to maintain physiological functions after exposure to sub-lethal concentrations of Cu.

New insights into fish ion regulation and mitochondrion-rich cells

Compared to terrestrial animals, fish have to cope with more-challenging osmotic and ionic gradients from aquatic environments with diverse salinities, ion compositions, and pH values. Gills, a unique and highly studied organ in research on fish osmoregulation and ionoregulation, provide an excellent model to study the regulatory mechanisms of ion transport. The present review introduces and discusses some recent advances in relevant issues of teleost gill ion transport and functions of gill ionocytes. Based on accumulating evidence, a conclusive model of NaCl secretion in gills of euryhaline teleosts has been established. Interpretations of results of studies on freshwater fish gill Na+/Cl- uptake mechanisms are still being debated compared with those for NaCl secretion. Current models for Na+/Cl- uptake are proposed based on studies in traditionally used model species. Many reported inconsistencies are claimed to be due to differences among species, various experimental designs, or acclimation conditions. Having the benefit of advanced techniques in molecular/cellular biology, functional genomics, and model animals, several new notions have recently been raised concerning relevant issues of Na+/Cl- uptake pathways. Several new windows have been opened particularly in terms of molecular mechanisms of ionocyte differentiation and energy metabolite transport between gill cells during environmental challenge.

Epithelial mitochondria-rich cells and associated innervation in adult and developing zebrafish

Studies of ion regulation by mitochondria-rich cells (MRCs) of transport epithelia in fish have revealed many processes by which ion homeostasis is achieved. However, the control of these mechanisms and, particularly, the extent of nervous system involvement are not completely understood. We characterized the potential innervation of MRCs in various gill and extrabranchial tissues involved in ion transport in the model vertebrate the zebrafish. Confocal and conventional microscopy of whole-mount preparations were combined with immunofluorescence techniques to label MRCs with antibodies against a subunit of the enzyme Na(+)/K(+)-ATPase and nerve fibers with a zebrafish neuronal marker, zn-12. MRCs of the gill filaments were identified by their morphology and migration out to the lamellae in response to ion-poor water acclimation. Gill MRCs were intimately associated with nerve fibers originating from outside the filaments. MRCs of the opercular epithelium resembled those of the gill and were also located adjacent to nerve fibers. Mitochondria-rich "pseudobranch cells" were identified in the pseudobranch by immunofluorescence and labeling of dissociated cells with the mitochondrial marker DASPEI. Pseudobranch MRCs resembled gill MRCs and received innervation from a dense network of nerve fibers. In larvae, MRCs were distributed across the surface of the skin. These cells were situated among a dense network of varicose nerve fibers, and some MRCs of the skin displayed extensive cytoplasmic processes. Evidence is presented suggestive of widespread association of MRCs with the nervous system in transport epithelia and the neural control of MRC-mediated ion regulation in teleost fish.

Visualization in zebrafish larvae of Na(+) uptake in mitochondria-rich cells whose differentiation is dependent on foxi3a

Uptake of Na(+) from the environment is an indispensable strategy for the survival of freshwater fish, as they easily lose Na(+) from the plasma to a diluted environment. Nevertheless, the location of and molecules involved in Na(+) uptake remain poorly understood. In this study, we utilized Sodium Green, a Na(+)-dependent fluorescent reagent, to provide direct evidence that Na(+) absorption takes place in a subset of the mitochondria-rich (MR) cells on the yolk sac surface of zebrafish larvae. Combined with immunohistochemistry, we revealed that the Na(+)-absorbing MR cells were exceptionally rich in vacuolar-type H(+)-ATPase (H(+)-ATPase) but moderately rich in Na(+)-K(+)-ATPase. We also addressed the function of foxi3a, a transcription factor that is specifically expressed in the H(+)-ATPase-rich MR cells. When foxi3a was depleted from zebrafish embryos by antisense morpholino oligonucleotide injection, differentiation of the MR cells was completely blocked and Na(+) influx was severely reduced, indicating that MR cells are the primary sites for Na(+) absorption. Additionally, foxi3a expression is initiated at the gastrula stage in the presumptive ectoderm; thus, we propose that foxi3a is a key gene in the control of MR cell differentiation. We also utilized a set of ion transport inhibitors to assess the molecules involved in the process and discuss the observations.

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

The mammalian kidney excretes its metabolic acid load through the proton-transporting cells, intercalated cells, in the distal nephron and collecting duct. Fish excrete acid through external organs, gill, or skin; however, the cellular function is still controversial. In this study, molecular and electrophysiological approaches were used to identify a novel cell type secreting acid in skin of zebrafish ( Danio rerio) larvae. Among keratinocytes covering the larval surface, novel proton-secreting ionocytes, proton pump (H+-ATPase)-rich cells, were identified to generate strong outward H+flux. The present work demonstrates for the first time, with a noninvasive technique, H+-secreting cells in an intact animal model, the zebrafish, showing it to be a suitable model in which to study the functions of vertebrate transporting epithelia in vivo.

Functional classification of mitochondrion-rich cells in euryhaline Mozambique tilapia (Oreochromis mossambicus) embryos, by means of triple immunofluorescence staining for Na+/K+-ATPase, Na+/K+/2Cl- cotransporter and CFTR anion channel

Mozambique tilapia Oreochromis mossambicus embryos were transferred from freshwater to seawater and vice versa, and short-term changes in the localization of three major ion transport proteins, Na+/K+-ATPase, Na+/K+/2Cl- cotransporter (NKCC) and cystic fibrosis transmembrane conductance regulator (CFTR) were examined within mitochondrion-rich cells (MRCs) in the embryonic yolk-sac membrane. Triple-color immunofluorescence staining allowed us to classify MRCs into four types: type I, showing only basolateral Na+/K+-ATPase staining; type II, basolateral Na+/K+-ATPase and apical NKCC; type III, basolateral Na+/K+-ATPase and basolateral NKCC; type IV, basolateral Na+/K+-ATPase, basolateral NKCC and apical CFTR. In freshwater, type-I, type-II and type-III cells were observed. Following transfer from freshwater to seawater, type-IV cells appeared at 12 h and showed a remarkable increase in number between 24 h and 48 h, whereas type-III cells disappeared. When transferred from seawater back to freshwater, type-IV cells decreased and disappeared at 48 h, type-III cells increased, and type-II cells, which were not found in seawater, appeared at 12 h and increased in number thereafter. Type-I cells existed consistently irrespective of salinity changes. These results suggest that type I is an immature MRC, type II is a freshwater-type ion absorptive cell, type III is a dormant type-IV cell and/or an ion absorptive cell (with a different mechanism from type II), and type IV is a seawater-type ion secretory cell. The intracellular localization of the three ion transport proteins in type-IV cells is completely consistent with a widely accepted model for ion secretion by MRCs. A new model for ion absorption is proposed based on type-II cells possessing apical NKCC.

Epithelial Ca(2+) channel expression and Ca(2+) uptake in developing zebrafish

The purpose of the present work was to study the possible role of the epithelial Ca(2+) channel (ECaC) in the Ca(2+) uptake mechanism in developing zebrafish (Danio rerio). With rapid amplification of cDNA ends, full-length cDNA encoding the ECaC of zebrafish (zECaC) was cloned and sequenced. The cloned zECaC was 2,578 bp in length and encoded a protein of 709 amino acids that showed up to 73% identity with previously described vertebrate ECaCs. The zECaC was found to be expressed in all tissues examined and began to be expressed in the skin covering the yolk sac of embryos at 24 h postfertilization (hpf). zECaC-expressing cells expanded to cover the skin of the entire yolk sac after embryonic development and began to occur in the gill filaments at 96 hpf, and thereafter zECaC-expressing cells rapidly increased in both gills and yolk sac skin. Corresponding to ECaC expression profile, the Ca(2+) influx and content began to increase at 36-72 hpf. Incubating zebrafish embryos in low-Ca(2+) (0.02 mM) freshwater caused upregulation of the whole body Ca(2+) influx and zECaC expression in both gills and skin. Colocalization of zECaC mRNA and the Na(+)-K(+)-ATPase alpha-subunit (a marker for mitochondria-rich cells) indicated that only a portion of the mitochondria-rich cells expressed zECaC mRNA. These results suggest that the zECaC plays a key role in Ca(2+) absorption in developing zebrafish.