Comparisons of calcium regulation in fish larvae

Influence of the aquatic environment and 1α,25(OH)2 vitamin D3 on calcium influx in the intestine of adult zebrafish

We investigated the effects of environment calcium challenge and 1α,25(OH)2 vitamin D3 (1,25-D3) on 45Ca2+ influx in the intestine of zebrafish (ZF). In vitro45Ca2+ influx was analyzed using intestines from fed and fasted fish. ZF were held in water containing Ca2+ (0.02, 0.7, 2.0 mM) to analyze the ex vivo45Ca2+ influx in the intestine and for histology. Intestines from fish held in water with Ca2+ were incubated ex vivo to characterize ion channels, receptors, ATPases and ion exchangers that orchestrate 45Ca2+ influx. For in vitro studies, intestines were incubated with antagonists/agonist or inhibitors to study the mechanism of 1,25-D3 on 45Ca2+ influx. Fasted ZF reached a plateau for 45Ca2+ influx at 30 min. In vivo fish at high Ca2+ stimulated ex vivo45Ca2+ influx and increased the height of intestinal villi in low calcium. In the normal calcium, 45Ca2+ influx was maintained by the reverse-mode Na+/Ca2+ (NCX) activation, Na+/K+-ATPase pump and sarco/endoplasmic reticulum calcium ATPase (SERCA) pump. However, Ca2+ hyperosmolarity is supported by L-type voltage-dependent calcium channels (L-VDCC), transient receptor potential vanilloid subfamily 1 (TRPV1) and Na+/K+-ATPase activity. The calcium challenge causes morphological alteration and changes the ion type-channels involved in the intestine to maintain hyperosmolarity. 1,25-D3 stimulates Ca2+ influx in normal osmolarity coordinated by L-VDCC activation and SERCA inhibition to keeps high intracellular calcium in intestine. Our data showed that the adult ZF regulates the calcium challenge (per se osmolarity), independently of the hormonal regulation to maintain the calcium balance through the intestine to support ionic adaptation.

Rearing in strontium-enriched water induces vaterite otoliths in the Japanese rice fish, Oryzias latipes

Sagittal otoliths, typically composed of aragonite, are frequently laid down rather as vaterite during growth in hatchery-reared fish populations. Sagittal vateritization is believed to impair individual hearing/balancing abilities, but the causal mechanism remains unclear. Here we experimentally demonstrated that rearing in Sr-rich water induces sagittal vateritization in the HdrR-II1 inbred strain of the Japanese rice fish, Oryzias latipes. Both sagittae were partly vateritized in 70% of individuals subjected to the Sr²⁺ treatment (n = 10), whereas fish reared in normal tap water showed no sagittal vateritization (n = 8). Our result is consistent with the theoretical prediction that vaterite becomes thermodynamically more stable than aragonite as the Sr²⁺ concentration in solution increases. A vateritic layer develops surrounding the original aragonitic sagitta in vateritized otoliths, some of which take on a comma-like shape. Electron probe microanalysis demonstrates that the vateritized phase is characterized by lower Sr²⁺ and higher Mg²⁺ concentrations than the aragonitic phase. It is unlikely that increased environmental Sr²⁺ is responsible for the sagittal vateritization in farmed fish. However, our findings likely help to establish an in vivo assay using O. latipes to understand the physiological process underlying the sagittal vateritization in farmed fish.

Transcriptome reveals overview of Ca2+ dose-dependent metabolism disorders in zebrafish larvae after Cd2+ exposure

Cadmium (Cd), a ubiquitous environmental hazardous heavy metal, poses a significant threat to the health of aquatic organisms, including teleosts. Although the toxic profile of Cd is well recognized, little is known regarding the overall view of toxic responses to varying aquatic environmental parameters (e.g., water hardness) at an individual level. Herein, differences in water hardness were partially mimicked by adjusting Ca²⁺ levels in E3 medium. As an in vivo model, zebrafish embryos were exposed to variable Ca²⁺ levels (NV, normal Ca²⁺; LV, low Ca²⁺; HV, high Ca²⁺) alone or combined with 30.7 µg/L Cd²⁺ (NC, LC, and HC, respectively) until 144 hr post-fertilization. The genome-wide transcriptome revealed differentially expressed genes between groups. Functional enrichment analysis found that biological processes related to metabolism, particularly lipid metabolism, were significantly disrupted in NC and LC treatments, while a remission was observed in the HC group. Biochemical assays confirmed that the decrease in Ca²⁺ enhanced synthesis, inhibited mobilization and increased the storage of lipids in Cd²⁺ treatments. This study suggests that the toxic effect of Cd on biological pathways will be influenced by Ca²⁺, which will improve the toxicological understanding and facilitate accurate assessment of Cd.

Calcium influx and spermatogenesis in the testis and liver enzyme activities in the zebrafish are rapidly modulated by the calcium content of the water

This study investigated the effects of varying environmental Ca²⁺ concentrations on the influx of Ca²⁺ to the testis, testicular morphology, and liver enzymes in the zebrafish. Adult zebrafish (Danio rerio) were held in water containing low (0.02 mM), control (0. 7 mM) or high (2 mM) Ca²⁺ concentrations for 12 h. Testes were then incubated in vitro with 0.1 μCi/mL ⁴⁵Ca²⁺ to measure Ca²⁺ influx at 30 and 60 min and qualitative and quantitative testicular histological analyses were conducted. In addition, activity of alanine aminotransferase (ALT), aspartate aminotransferase (AST) and gamma-glutamyl transpeptidase (GGT), enzymes that indicate tissue damage, were evaluated in the liver. The testes from zebrafish exposed in vivo to low (0.02 mM) and high (2 mM) Ca²⁺ content water had a higher Ca²⁺ influx than the control group after 30 min of incubation, and at 60 min (high Ca²⁺ group only). There were morphological changes in the testes from the low and high Ca²⁺ groups including spermatozoa distributed in dense agglomerates and apoptotic cells. Furthermore, zebrafish exposed to high Ca²⁺ containing water had an increased density of haploid cells (spermatids and spermatozoa). In addition, both low and high Ca²⁺ water affected liver function by increasing ALT and GGT activities. Collectively, these studies show that alterations in calcium homeostasis in the testis, stimulation of the spermatogenic wave and hepatic injury were rapid responses to changes in the concentration of Ca²⁺ in the water.

Heavy metal toxicity in Buriganga river alters the immunology of Nile tilapia (Oreochromis niloticus L)

The objective of the current study was to evaluate the biochemical and immunological responses of tilapia, Oreochromis niloticus due to heavy metals pollution. Histomorphological alterations in the liver and kidney suggested tissue damages due to this polluted water exposure. The brain acetylcholinesterase (AChE) as an indicator of neurotoxicity was significantly (P < 0.01) decreased after 10 days exposure of fish to heavy metal contained river water, while plasma glutamate oxalacetate transaminase and plasma glutamate pyruvate transaminase were significantly increased (P < 0.01). Moreover, superoxide dismutase, catalase, glutathione peroxidase, glutathione S-transferase enzyme activities, as well as reduced glutathione and malondialdehyde levels were significantly increased in heavy metals contained river water treated fish compared to the control. Additionally, glucose level and blood serum Ca2+ concentrations were significantly (P < 0.01) decreased in fish exposed to heavy metal contained river water compared to the control. Hematological indices such as Hemoglobin, RBC, WBC, MCV etc. of polluted river water treated fish were significantly (P < 0.01) different in comparison to that of control fish. The cytokines i.e. IL-1β, IL-6, and TNF-α level were significantly (P < 0.01) increased in the fish exposed to heavy metals contained river water in comparison to that of control fish. The present findings explored the detrimental effects of heavy metal contained river water on fish at biochemical and immunological levels.

Dibutyl phthalate rapidly alters calcium homeostasis in the gills of Danio rerio

This study investigates the impacts of exposure to an environment Ca²⁺ challenge and the mechanism of action of dibutyl phthalate (DBP) on Ca²⁺ influx in the gills of Danio rerio. In vitro profile of ⁴⁵Ca²⁺ influx in gills was verified through the basal time-course. Fish were exposed to low, normal and high Ca²⁺ concentrations (0.02, 0.7 and 2 mM) for 12 h. So, gills were morphologically analysed and ex vivo⁴⁵Ca²⁺ influx at 30 and 60 min was determined. For the in vitro studies, gills were treated for 60 min with DBP (1 pM, 1 nM and 1 μM) with/without blockers/activators of ionic channels, Ca²⁺ chelator, inhibitors of ATPases, ionic exchangers and protein kinase C to study the mechanism of DBP-induced ⁴⁵Ca²⁺ influx. Exposure to high environmental Ca²⁺ augmented ⁴⁵Ca²⁺ influx when compared to fish exposed to normal and low Ca²⁺ concentrations. Additionally, histopathological changes were observed in the gills of fish maintained for 12 h in low and high Ca²⁺. In vitro exposure of gills to DBP (1 pM) disturbed Ca²⁺ homeostasis. DBP stimulated ⁴⁵Ca²⁺ influx in gills through the transitory receptor potential vanilloid 1 (TRPV1), and reverse-mode Na⁺/Ca²⁺ exchanger (NCX) activation, protein kinase C and K⁺ channels and sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA). These data suggest that in vivo short-term exposure of gills to low and high Ca²⁺ leads to ⁴⁵Ca²⁺ influx and histopathological changes. Additionally, the DBP-induced rapid ⁴⁵Ca²⁺ influx is mediated by TRPV1, NCX activation with the involvement of PKC, K⁺-channels and SERCA, thereby altering Ca²⁺ homeostasis.

Antioxidant system status in threatened native fish Barbus meridionalis from the Osor River (Iberian Peninsula): I. Characterization and II. In vitro Zn assays

The evaluation of antioxidant system capacity is important in aquatic toxicology. It was aimed to characterize the liver antioxidant enzymes (SOD, CAT, GPX, GR, and GST) and to test the in vitro Zn effect (200 and 400 ZnSO₄ μg/L) in native fish Barbus meridionalis obtained from the Osor River (NE, Spain) influenced by Zn contamination. The maximal enzyme activities were at pH 7.0-7.5 and 100 mM phosphate buffer. Barbel showed high catalytic activity (high Vmax and low Km) indicating the efficient antioxidant detoxification ability. Direct Zn effect caused an antioxidant system imbalance. Mostly upon lower Zn concentration, GPX activity decreased (95-100 %) though CAT, GR, and GST increased (36-1543 %). GSH values either stimulated (290 %) or inhibited (85-93 %) due to tissue differences. The first record of barbel antioxidant enzyme characterization and in vitro data presenting an unbalanced antioxidant pattern could be significant to evaluate the metal pollution in the Osor River for further in vivo studies.

Uptake of Sulfate from Ambient Water by Freshwater Animals

To better understand how the sulfate (SO4 2-) anion may contribute to the adverse effects associated with elevated ionic strength or salinity in freshwaters, we measured the uptake and efflux of SO4 2- in four freshwater species: the fathead minnow (Pimephales promelas, Teleostei: Cyprinidae), paper pondshell (Utterbackia imbecillis, Bivalvia: Unionidae), red swamp crayfish (Procambarus clarkii, Crustacea: Cambaridae), and two-lined mayfly (Hexagenia bilineata, Insecta: Ephemeridae). Using δ( 34 S/ 32 S) stable isotope ratios and the concentrations of S and SO4 2-, we measured the SO4 2- influx rate (J in ), net flux (J net ), and efflux rate (Jout) during a 24 h exposure period. For all four species, the means of J in for SO4 2- were positive, and J in was significantly greater than 0 at both target SO4 2- concentrations in the fish and mollusk and at the lower SO4 2- concentration in the crayfish. The means of J out and J net were much more variable than those for J in , but several species by target SO4 2- concentration combinations for J out and J net , were negative, which suggests the net excretion of SO4 2- by the animals. The results of our experiments suggest a greater regulation of SO4 2- in freshwater animals than has been previously reported.

The synergistic toxicity of Cd(II) and Cu(II) to zebrafish (Danio rerio): Effect of water hardness

We have evaluated the interactive toxicity of Cu(II) and Cd(II) in water with different hardness levels using adult zebrafish (Danio rerio). Zebrafish were exposed to Cd(II) (0.2-22 μM) or Cu(II) (0.1-8 μM) in single or binary exposures in very soft, moderately hard or very hard water. The whole body burdens of Cd(II) and Cu(II) reflect the net effect of biouptake and elimination, mortality was the indicator of toxicity, and whole body major ion content was measured to assess ion regulatory functions. Cu(II) was found to be more toxic than Cd(II) for zebrafish, and Cu(II) and Cd(II) exhibited a significant synergistic effect. The toxicity of metal ions increased upon decreasing the ionic strength of the exposure medium, probably due to elevated competition between metal ions with other cations in hard water and increased activity of Ca²⁺ pathways in soft water treatments. Whole body metal accumulation and the accumulation rate of both Cu and Cd increased as the metal ion concentration in the exposure medium increased. Nevertheless, neither parameter explained the observed synergistic effect on mortality. Finally, we observed a significant loss of whole body Na⁺ in fish which died during the metal exposure compared to surviving fish, irrespective of exposure conditions. Such an effect was not observed for other major cations (K⁺, Ca²⁺ and Mg²⁺). This observation suggests that, under the applied exposure conditions, survival was correlated to the capacity of the organism to maintain Na⁺ homeostasis.

The influence of Ca2+ concentration on voltage-dependent L-type calcium channels' expression in the marbled eel (Anguilla marmorata)

The catadromous species, eels, invariably exposed to variable Ca²⁺ concentrations circumstance i.e., lagoon or ocean. They need to maintain Ca²⁺ homeostasis by exchanging Ca²⁺ under different culture conditions. To understand the effects of environmental Ca²⁺ to fish, three types of genes coding for voltage-dependent L-type calcium channels (cacnb1, 2, 3) were cloned by screening an A. marmorata cDNA library. Tissue distribution analysis of Western blot showed that Cacnb1, 2, 3 had a significantly high expression in gill; while mRNA results showed the expressions of cacnb1 and cacnb3 were predominated in skin tissue but only cacnb2 was expressed in intestine. Serum osmolality and Ca²⁺ concentrations of A.marmorata were increased in a high calcium environment while reduced in a low calcium environment within 7 days; however, they were not significantly different among Ca²⁺ treatments after the eels were acclimated for 7 days. We also examined the influence of ambient Ca²⁺ levels on cacnbs expression of eels. With the increasing of exposure time, mRNA and protein expressions of cacnb1 were up-regulated in high level of Ca²⁺ (10 mM) and down-regulated in deficient Ca²⁺ (0 mM) compared to the control Ca²⁺ (2 mM). However, the opposite results were observed in cacnb2 and cacnb3. Notably, the cacnb2 expression was not significant different among Ca²⁺ treatments on day 7. Our study provided the insightful evidence that cacnbs play important roles in maintaining Ca²⁺ homeostasis of fish.

Investigating the Role of Ionoregulatory Processes in the Species- and Life-Stage-Specific Differences in Sensitivity of Rainbow Trout and White Sturgeon to Cadmium

There are huge variations in life-stage- and species-specific sensitivities among the fishes to the exposure with metals; however, the physiological mechanisms underlying these differences are not well understood to date. This study revealed significant life-stage-specific (larval, swim-up, and juvenile) and species-specific differences between two evolutionary distant species of fishes, rainbow trout ( Oncorhynchus mykiss) and white sturgeon ( Acipenser transmontanus), following acute exposures to Cd. Although the 96 h LC50 of Cd was similar in both species at the larval stage, trout demonstrated an increased sensitivity to Cd at later life stages as compared to sturgeon. Moreover, exposure to Cd disrupted calcium (Ca) uptake and whole body Ca levels in trout by a greater degree relative to that in sturgeon regardless of life stage. Finally, white sturgeon demonstrated a lower affinity for Cd uptake relative to the more sensitive rainbow trout. This infers a differential nature of the interaction between Cd and Ca transport pathways in the two species and partially explains the differences in Cd sensitivity between rainbow trout and white sturgeon described previously. Overall, our results suggest that species- and life-stage-specific differences in sensitivity to waterborne Cd in fish are likely a function of the interplay between Cd uptake and Cd-induced disruption of Ca homeostasis.

Comparison of Calcium Balancing Strategies During Hypothermic Acclimation of Tilapia (Oreochromis mossambicus) and Goldfish (Carassius auratus)

The body temperatures of teleost species fluctuate following changes in the aquatic environment. As such, decreased water temperature lowers the rates of biochemical reactions and affects many physiological processes, including active transport-dependent ion absorption. Previous studies have focused on the impacts of low temperature on the plasma ion concentrations or membrane transporters in fishes. However, very few in vivo or organism-level studies have been performed to more thoroughly elucidate the process of acclimation to low temperatures. In the present study, we compared the strategies for cold acclimation between stenothermic tilapia and eurythermic goldfish. Whole-body calcium content was more prominently diminished in tilapia than in goldfish after long-term cold exposure. This difference can be attributed to alterations in the transportation parameters for Ca²⁺ influx, i.e., maximum velocity (V_max) and binding affinity (1/K_m). There was also a significant difference in the regulation of Ca²⁺ efflux between the two fishes. Transcript levels for Ca²⁺ related transporters, including the Na⁺/Ca²⁺ exchanger and epithelial Ca²⁺ channel, were similarly regulated in both fishes. However, upregulation of plasma membrane Ca²⁺ATPase expression was more pronounced in goldfish than in tilapia. In addition, enhanced Na⁺/K⁺-ATPase abundance, which provides the major driving force for ion absorption, was only detected in tilapia, while upregulated Na⁺/K⁺-ATPase activity was only detected in goldfish. Based on the results of the present study, we have found that goldfish and tilapia differentially regulate gill epithelial plasma membrane Ca²⁺-ATPase (PMCA) expression and Na⁺/K⁺-ATPase activity in response to cold environments. These regulatory differences are potentially linked to more effective regulation of Ca²⁺ influx kinetics and better maintenance of whole body calcium content in goldfish than in tilapia.

Environmental calcium and variation in yolk sac size influence swimming performance in larval lake sturgeon (Acipenser fulvescens)

In many animal species, performance in the early life stages strongly affects recruitment to the adult population; however, factors that influence early life history stages are often the least understood. This is particularly relevant for lake sturgeon, Acipenser fulvescens, living in areas where environmental calcium concentrations are declining, partly due to anthropogenic activity. As calcium is important for muscle contraction and fatigue resistance, declining calcium levels could constrain swimming performance. Similarly, swimming performance could be influenced by variation in yolk sac volume, because the yolk sac is likely to affect drag forces during swimming. Testing swimming performance of larval A. fulvescens reared in four different calcium treatments spanning the range of 4-132 mg l^-1 [Ca²⁺], this study found no treatment effects on the sprint swimming speed. A novel test of volitional swimming performance, however, revealed reduced swimming performance in the low calcium environment. Specifically, volitionally swimming larvae covered a shorter distance before swimming cessation in the low calcium environment compared with the other treatments. Moreover, sprint swimming speed in larvae with a large yolk sac was significantly slower than in larvae with a small yolk sac, regardless of body length variation. Thus, elevated maternal allocation (i.e. more yolk) was associated with reduced swimming performance. Data suggest that larvae in low calcium environments or with a large yolk sac exhibit reduced swimming performance and could be more susceptible to predation or premature downstream drift. Our study reveals how environmental factors and phenotypic variation influence locomotor performance in a larval fish.

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

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

Molecular Physiology of the Hypocalcemic Action of Fibroblast Growth Factor 23 in Zebrafish (Danio rerio)

Fibroblast growth factor 23 (FGF23), a hormone required for phosphorus metabolism, was recently proposed to act on Ca2+ uptake; however, the available evidence of how FGF23 controls the body fluid Ca2+ homeostasis needs to be further clarified. The use of zebrafish as a model system revealed that FGF23 is specifically expressed in the corpuscles of Stannius (CS), an organ involved in Ca2+ homeostasis in fish, and that its expression is stimulated by ambient water with a high Ca2+ level. The overexpression of FGF23 inhibited Ca2+ uptake by downregulating the messenger RNA (mRNA) expression of epithelium calcium channel. Calcium-sensing receptor (CaSR), which senses changes in extracellular Ca2+ levels and modulates calciotropic hormones in organs controlling Ca2+ homeostasis in vertebrates, was found to be coexpressed with FGF23 in the CS. In addition, upregulated expression of FGF23 mRNA was detected in morphants of stanniocalcin 1 (stc1, another hypocalcemic factor synthesized in the CS), and knockdown of CaSR suppressed such upregulation and enhanced Ca2+ uptake. Taken together, our data indicate that FGF23 functions as a hypocalcemic hormone in zebrafish and that the CaSR/STC1-FGF23 axis is involved in body fluid Ca2+ homeostasis in vertebrates.

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.

Characterization of Biological Secretions Binding to Graphene Oxide in Water and the Specific Toxicological Mechanisms

With the widening application of graphene oxide nanosheets (GONS), their safety has attracted much attention. Secretions from aquatic organisms are ubiquitous in natural water, but the effects of secretions on the characteristics and toxicity of GONS remain largely unknown. To help fill this knowledge gap, we characterized the GONS with biological secretions (GOBS) and the associated changes in apparent toxicity. Small organic molecules, proteins, nucleotides and mucopolysaccharides from secretions in zebrafish culture water bound to GONS. Compared with GONS, GOBS showed special nanoplate topography with thicknesses of approximately 10 nm and lateral lengths ranging from 19.5 to 282 nm. GOBS with smaller lateral sizes exhibited more negative surface charges and lower aggregation state than GONS. Furthermore, GOBS triggered higher toxicity than GONS, such as death, malformation, upregulation of β-galactosidase and loss in mitochondrial membrane potential of zebrafish embryos. The well-dispersive GOBS covered embryos, inhibiting oxygen and ion exchange; these phenomena were the specific mechanisms of the adverse effects. In future work, the acquired natural coatings on nanomaterials should be paid much attention in nanotoxicology, especially for the relationships among topography, aggregation state, and toxicity.

Environmental and cortisol-mediated control of Ca(2+) uptake in tilapia (Oreochromis mossambicus)

Ca²⁺ is a vital element for many physiological processes in vertebrates, including teleosts, which live in aquatic environments and acquire Ca²⁺ from their surroundings. Ionocytes within the adult gills or larval skin are critical sites for transcellular Ca²⁺ uptake in teleosts. The ionocytes of zebrafish were found to contain transcellular Ca²⁺ transporters, epithelial Ca²⁺ channel (ECaC), plasma membrane Ca²⁺-ATPase 2 (PMCA2), and Na⁺/Ca²⁺ exchanger 1b (NCX1b), providing information about the molecular mechanism of transcellular Ca²⁺ transports mediated by ionocytes in fish. However, more evidence is required to establish whether or not a similar mechanism of transcellular Ca²⁺ transport also exists in others teleosts. In the present study, ecac, pmca2, and ncx1 were found to be expressed in the branchial ionocytes of tilapia, thereby providing further support for the mechanism of transcellular Ca²⁺ transport through ionocytes previously proposed for zebrafish. In addition, we also reveal that low Ca²⁺ water treatment of tilapia stimulates Ca²⁺ uptake and expression of ecac and cyp11b (the latter encodes a cortisol-synthesis enzyme). Treatment of tilapia with exogenous cortisol (20 mg/l) enhanced both Ca²⁺ influx and ecac expression. Therefore, increased cyp11b expression is suggested to enhance Ca²⁺ uptake capacity in tilapia exposed to low Ca²⁺ water. Furthermore, the application of cortisol receptor antagonists revealed that cortisol may regulate Ca²⁺ uptake through glucocorticoid and/or mineralocorticoid receptor (GR and/or MR) in tilapia. Taken together, the data suggest that cortisol may activate GR and/or MR to execute its hypercalcemic action by stimulating ecac expression in tilapia.

The role of chemical speciation, chemical fractionation and calcium disruption in manganese-induced developmental toxicity in zebrafish (Danio rerio) embryos

Manganese (Mn) is an essential nutrient that can be toxic in excess concentrations, especially during early development stages. The mechanisms of Mn toxicity is still unclear, and little information is available regarding the role of Mn speciation and fractionation in toxicology. We aimed to investigate the toxic effects of several chemical forms of Mn in embryos of Danio rerio exposed during different development stages, between 2 and 122h post fertilization. We found a stage-specific increase of lethality associated with hatching and removal of the chorion. Mn(II), ([Mn(H2O)6](2+)) appeared to be the most toxic species to embryos exposed for 48h, and Mn(II) citrate was most toxic to embryos exposed for 72 and/or 120h. Manganese toxicity was associated with calcium disruption, manganese speciation and metal fractionation, including bioaccumulation in tissue, granule fractions, organelles and denaturated proteins.

Hydrogen sulfide promotes calcium uptake in larval zebrafish

Hydrogen sulfide (H2S) can act as a signaling molecule for various ion channels and/or transporters; however, little is known about its potential involvement in Ca(2+) balance. Using developing zebrafish (Danio rerio) as an in vivo model system, the present study demonstrated that acute exposure to H2S donors increased Ca(2+) influx at 4 days postfertilization, while chronic (3-day) exposure caused a rise in whole body Ca(2+) levels. The mRNA expression of Ca(2+)-transport-related genes was unaffected by H2S exposure, suggesting that posttranscriptional modifications were responsible for the altered rates of Ca(2+) uptake. Indeed, treatment of fish with the protein kinase A inhibitor H-89 abolished the H2S-mediated stimulation of Ca(2+) influx, suggesting that H2S increased Ca(2+) influx by activating cAMP-protein kinase A pathways. Cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) are two key enzymes in the endogenous synthesis of H2S. Using an antisense morpholino knockdown approach, we demonstrated that Ca(2+) influx was reduced in CBS isoform b (CBSb)- but not in CSE-deficient fish. Interestingly, the reduction in Ca(2+) influx in CBSb-deficient fish was observed only in fish that were acclimated to low-Ca(2+) water (i.e., 25 μM Ca(2+); control: 250 μM Ca(2+)). Similarly, mRNA expression of cbsb but not cse was increased in fish acclimated to low-Ca(2+) water. Results from whole-mount immunohistochemistry further revealed that CBSb was expressed in Na(+)-K(+)-ATPase-rich cells, which are implicated in Ca(2+) uptake in zebrafish larvae. Collectively, the present study suggests a novel role for H2S in promoting Ca(2+) influx, particularly in a low-Ca(2+) environment.

Involvement of the calcium-sensing receptor in calcium homeostasis in larval zebrafish exposed to low environmental calcium

The involvement of the calcium-sensing receptor (CaSR) in Ca(2+) homeostasis was investigated in larval zebrafish, Danio rerio. The expression of CaSR mRNA was first observed at 3 h posfertilization (hpf) and increased with development until plateauing at ∼48 hpf. At 4 dpf, CaSR mRNA was increased in fish acclimated to low Ca(2+) water (25 μM vs. 250 μM in normal water). Using immunohistochemistry and confocal microscopy, we demonstrated that the CaSR is expressed in the olfactory epithelium, neuromasts, ionocytes on the yolk sac epithelium, and corpuscles of Stannius. Results of double immunohistochemistry and/or in situ hybridization indicated that the CaSR is localized to a subset of mitochondrion-rich ionocytes enriched with Na(+)/K(+)-ATPase and epithelial Ca(2+) channel (ecac). Translational knockdown of the CaSR prevented 4 dpf larvae from regulating whole body Ca(2+) levels when exposed to a low Ca(2+) environment. Further, the increases in ecac mRNA expression and Ca(2+) influx, normally associated with exposure to low-Ca(2+) water, were prevented by CaSR knockdown. These findings demonstrate that larval zebrafish lacking the CaSR lose their ability to regulate Ca(2+) when confronted with a low-Ca(2+) environment. Results from real-time PCR suggested that the mRNA expression of the hypocalcemic hormone stanniocalcin (stc-1) remained elevated in the CaSR morphants following acclimation to low-Ca(2+) water. Overall, the results suggest that the CaSR is critical for Ca(2+) homeostasis in larval zebrafish exposed to low environmental Ca(2+) levels, possibly owing to its modulation of stanniocalcin mRNA expression.

Zebrafish housing systems: a review of basic operating principles and considerations for design and functionality

The strategies for housing zebrafish used in biomedical research have evolved considerably over the past three decades. To keep pace with the rapid expansion and development of the zebrafish model system, the field has generally moved from keeping fish at the level of aquarium hobbyist to that of industrialized, recirculating aquaculture. Numerous commercial system vendors now offer increasingly sophisticated housing systems based on design principles that maximize the number of animals that can be housed in a given space footprint, and they are thus able to support large and diverse research programs. This review is designed to provide managers, lab animal veterinarians, investigators, and other parties responsible for care and use of these animals with a comprehensive overview of the basic operating and design principles of zebrafish housing systems. This information can be used to help plan the construction of new facilities and/or the upgrade and maintenance of existing operations.

Zebrafish as an animal model to study ion homeostasis

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

Action of vitamin D and the receptor, VDRa, in calcium handling in zebrafish (Danio rerio)

The purpose of the present study was to use zebrafish as a model to investigate how vitamin D and its receptors interact to control Ca²⁺ uptake function. Low-Ca²⁺ fresh water stimulated Ca²⁺ influx and expressions of epithelial calcium channel (ecac), vitamin D-25-hydroxylase (cyp2r1), vitamin D receptor a (vdra), and vdrb in zebrafish. Exogenous vitamin D increased Ca²⁺ influx and expressions of ecac and 25-hydroxyvitamin D₃-24-hydroxylase (cyp24a1), but downregulated 1α-OHase (cyp27b1) with no effects on other Ca²⁺ transporters. Morpholino oligonucleotide knockdown of VDRa, but not VDRb, was found as a consequence of calcium uptake inhibition by knockdown of ecac, and ossification of vertebrae is impaired. Taken together, vitamin D-VDRa signaling may stimulate Ca²⁺ uptake by upregulating ECaC in zebrafish, thereby clarifying the Ca²⁺-handling function of only a VDR in teleosts. Zebrafish may be useful as a model to explore the function of vitamin D-VDR signaling in Ca²⁺ homeostasis and the related physiological processes in vertebrates.

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.

Reverse effect of mammalian hypocalcemic cortisol in fish: cortisol stimulates Ca2+ uptake via glucocorticoid receptor-mediated vitamin D3 metabolism

Cortisol was reported to downregulate body-fluid Ca(2+) levels in mammals but was proposed to show hypercalcemic effects in teleostean fish. Fish, unlike terrestrial vertebrates, obtain Ca(2+) from the environment mainly via the gills and skin rather than by dietary means, and have to regulate the Ca(2+) uptake functions to cope with fluctuating Ca(2+) levels in aquatic environments. Cortisol was previously found to regulate Ca(2+) uptake in fish; however, the molecular mechanism behind this is largely unclear. Zebrafish were used as a model to explore this issue. Acclimation to low-Ca(2+) fresh water stimulated Ca(2+) influx and expression of epithelial calcium channel (ecac), 11β-hydroxylase and the glucocorticoid receptor (gr). Exogenous cortisol increased Ca(2+) influx and the expressions of ecac and hydroxysteroid 11-beta dehydrogenase 2 (hsd11b2), but downregulated 11β-hydroxylase and the gr with no effects on other Ca(2+) transporters or the mineralocorticoid receptor (mr). Morpholino knockdown of the GR, but not the MR, was found to impair zebrafish Ca(2+) uptake function by inhibiting the ecac expression. To further explore the regulatory mechanism of cortisol in Ca(2+) uptake, the involvement of vitamin D(3) was analyzed. Cortisol stimulated expressions of vitamin D-25hydroxylase (cyp27a1), cyp27a1 like (cyp27a1l), 1α-OHase (cyp27b1) at 3 dpf through GR, the first time to demonstrate the relationship between cortisol and vitamin D(3) in fish. In conclusion, cortisol stimulates ecac expression to enhance Ca(2+) uptake functions, and this control pathway is suggested to be mediated by the GR. Lastly, cortisol also could mediate vitamin D(3) signaling to stimulate Ca(2+) uptake in zebrafish.

Trpv5/6 is vital for epithelial calcium uptake and bone formation

Calcium is an essential ion serving a multitude of physiological roles. Aside from its role as a second messenger, it is an essential component of the vertebrate bone matrix. Efficient uptake and storage of calcium are therefore indispensable for all vertebrates. Transient receptor potential family, vanilloid type (TRPV)5 and TRPV6 channels are known players in transcellular calcium uptake, but the exact contribution of this pathway is unclear. We used forward genetic screening in zebrafish (Danio rerio) to identify genes essential in bone formation and identified a lethal zebrafish mutant (matt-und-schlapp) with severe defects in bone formation, including lack of ossification of the vertebral column and craniofacial structures. Mutant embryos show a 68% reduction in calcium content, and systemic calcium homeostasis is disturbed when compared with siblings. The phenotype can be partially rescued by increasing ambient calcium levels to 25 mM. We identified the mutation as a loss-of-function mutation in the single orthologue of TRPV5 and 6, trpv5/6. Expression in HEK293 cells showed that Trpv5/6 is a calcium-selective channel capable of inward calcium transport at physiological concentrations whereas the mutant channel is not. Taken together, this study provides both genetic and functional evidence that transcellular epithelial calcium uptake is vital to sustain life and enable bone formation.

The influence of environmental calcium concentrations on calcium flux, compensatory drinking and epithelial calcium channel expression in a freshwater cartilaginous fish

Calcium metabolism and mRNA levels of the epithelial calcium channel (ECaC) were examined in a freshwater cartilaginous fish, the lake sturgeon Acipenser fulvescens. Lake sturgeon were acclimated for ≥2 weeks to 0.1 (low), 0.4 (normal) or 3.3 (high) mmol l(-1) environmental calcium. Whole-body calcium flux was examined using (45)Ca as a radioactive marker. Net calcium flux was inward in all treatment groups; however, calcium influx was greatest in the low calcium environment and lowest in the high calcium environment, whereas efflux had the opposite relationship. A significant difference in the concentration of (45)Ca in the gastrointestinal tract (GIT) of fish in the low calcium environment led to the examination of drinking rate and calcium flux across the anterior-middle (mid) intestine. Drinking rate was not different between treatments; however, calcium influx across the mid-intestine in the low calcium treatment was significantly greater than that in both the normal and high calcium treatments. The lake sturgeon ECaC was 2831 bp in length, with a predicted protein sequence of 683 amino acids that shared a 66% identity with the closest sequenced ECaCs from the vertebrate phyla. ECaC mRNA levels were examined in the gills, kidney, pyloric caeca, mid-intestine and spiral intestine. Expression levels were highest in the gills, then the kidneys, and were orders of magnitude lower in the GIT. Contrary to existing models for calcium uptake in the teleost gill, ECaC expression was greatest in high calcium conditions and kidney ECaC expression was lowest in low calcium conditions, suggesting that cellular transport mechanisms for calcium may be distinctly different in these freshwater cartilaginous fishes.

Involvement of calcitonin and its receptor in the control of calcium-regulating genes and calcium homeostasis in zebrafish (Danio rerio)

Calcitonin (CT) is one of the hormones involved in vertebrate calcium regulation. It has been proposed to act as a hypocalcemic factor, but the regulatory pathways remain to be clarified. We investigated the CT/calcitonin gene-related peptide (CGRP) family in zebrafish and its potential involvement in calcium homeostasis. We identified the presence of four receptors: CTR, CRLR1, CRLR2, and CRLR3. From the phylogenetic analysis, together with the effect observed after CT and CGRP overexpression, we concluded that CTR appears to be a CT receptor and CRLR1 a CGRP receptor. The distribution of these two receptors shows a major presence in the central nervous system and in tissues involved in ionoregulation. Zebrafish embryos kept in high-Ca(2+)-concentration medium showed upregulation of CT and CTR expression and downregulation of the epithelial calcium channel (ECaC). Embryos injected with CT morpholino (CALC MO) incubated in high-Ca(2+) medium, showed downregulation of CTR together with upregulation on ECaC mRNA expression. In contrast, overexpression of CT cRNA induced the downregulation of ECaC mRNA synthesis, concomitant with the downregulation in the calcium content after 30 hours postfertilization. At 4 days postfertilization, CT cRNA injection induced upregulation of hypercalcemic factors, with subsequent increase in the calcium content. These results suggest that CT acts as a hypocalcemic factor in calcium regulation, probably through inhibition of ECaC synthesis.

Gill transcriptome response to changes in environmental calcium in the green spotted puffer fish

Background

Calcium ion is tightly regulated in body fluids and for euryhaline fish, which are exposed to rapid changes in environmental [Ca²⁺], homeostasis is especially challenging. The gill is the main organ of active calcium uptake and therefore plays a crucial role in the maintenance of calcium ion homeostasis. To study the molecular basis of the short-term responses to changing calcium availability, the whole gill transcriptome obtained by Super Serial Analysis of Gene Expression (SuperSAGE) of the euryhaline teleost green spotted puffer fish, Tetraodon nigroviridis, exposed to water with altered [Ca²⁺] was analysed.

Results

Transfer of T. nigroviridis from 10 ppt water salinity containing 2.9 mM Ca²⁺ to high (10 mM Ca²⁺ ) and low (0.01 mM Ca²⁺) calcium water of similar salinity for 2-12 h resulted in 1,339 differentially expressed SuperSAGE tags (26-bp transcript identifiers) in gills. Of these 869 tags (65%) were mapped to T. nigroviridis cDNAs or genomic DNA and 497 (57%) were assigned to known proteins. Thirteen percent of the genes matched multiple tags indicating alternative RNA transcripts. The main enriched gene ontology groups belong to Ca²⁺ signaling/homeostasis but also muscle contraction, cytoskeleton, energy production/homeostasis and tissue remodeling. K-means clustering identified co-expressed transcripts with distinct patterns in response to water [Ca²⁺] and exposure time.

Conclusions

The generated transcript expression patterns provide a framework of novel water calcium-responsive genes in the gill during the initial response after transfer to different [Ca²⁺]. This molecular response entails initial perception of alterations, activation of signaling networks and effectors and suggests active remodeling of cytoskeletal proteins during the initial acclimation process. Genes related to energy production and energy homeostasis are also up-regulated, probably reflecting the increased energetic needs of the acclimation response. This study is the first genome-wide transcriptome analysis of fish gills and is an important resource for future research on the short-term mechanisms involved in the gill acclimation responses to environmental Ca²⁺ changes and osmoregulation.

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.

Plasma membrane calcium ATPase required for semicircular canal formation and otolith growth in the zebrafish inner ear

Fish otoliths consist of &gt;90% calcium carbonate, the accretion of which depends on acellular endolymph. This study confirms the presence of plasma membrane calcium ATPase 1a isoform (Atp2b1a) in the auditory and vestibular system of a teleost fish. As shown by in situ hybridization,zebrafish atp2b1a is expressed mainly in larval otic placode and lateral-line neuromast as well as in the hair cells within the adult zebrafish inner ear chamber. Zebrafish atp2b1a knockdown by antisense morpholinos reduced the number of hair cells and produced malformation of semicircular canals and smaller otoliths. These defects coincide with unbalanced body orientation. The formation of smaller otoliths in atp2b1a morphants may stem from an impairment of calcium supply in the endolymph. However, otolith formation persists in most morphants,suggesting that other zebrafish Atp2b isoforms or paracellular pathways may also transport calcium into the endolymph. These results suggest that Atp2b1a plays an important role for normal development of the auditory and vestibular system as well as calcium transport in the inner ear of zebrafish.

Effects of stanniocalcin 1 on calcium uptake in zebrafish (Danio rerio) embryo

Stanniocalcin (STC) formerly called hypocalcin or teleocalcin, is a 50-kDa disulfide-linked homodimeric glycoprotein that was originally identified in fish and secreted from the corpuscles of Stannius (CS). One of the main functions of STC-1 is Ca(2+) uptake inhibition; however, the mechanisms remain unknown. In the present study, we provide molecular evidence to elucidate how zebrafish STC-1 regulates Ca(2+) uptake in zebrafish embryos. In a wide variety of tissues including the kidney, brain, gill, muscle, and skin, zstc-1 was expressed. Incubating zebrafish embryos in low-Ca(2+) (0.02 mM) freshwater stimulated whole body Ca(2+) influx and zebrafish epithelial Ca(2+) channel (zECaC) mRNA expression, while downregulated zstc-1 expression. A morpholino microinjection approach was used to knockdown the zSTC-1 protein, and the results showed that the Ca(2+) content, Ca(2+) influx, and zECaC mRNA expression all increased in morphants. These data suggest that zSTC-1 negatively regulates ECaC gene expression to reduce Ca(2+) uptake in zebrafish embryos.

Carbonic anhydrase 2-like a and 15a are involved in acid-base regulation and Na+ uptake in zebrafish H+-ATPase-rich cells

H(+)-ATPase-rich (HR) cells in zebrafish gills/skin were found to carry out Na+ uptake and acid-base regulation through a mechanism similar to that which occurs in mammalian proximal tubular cells. However, the roles of carbonic anhydrases (CAs) in this mechanism in zebrafish HR cells are still unclear. The present study used a functional genomic approach to identify 20 CA isoforms in zebrafish. By screening with whole mount in situ hybridization, only zca2-like a and zca15a were found to be expressed in specific groups of cells in zebrafish gills/skin, and further analyses by triple in situ hybridization and immunocytochemistry demonstrated specific colocalizations of the two zca isoforms in HR cells. Knockdown of zca2-like a caused no change in and knockdown of zca15a caused an increase in H+ activity at the apical surface of HR cells at 24 h postfertilization (hpf). Later, at 96 hpf, both the zca2-like a and zca15a morphants showed decreased H+ activity and increased Na+ uptake, with concomitant upregulation of znhe3b and downregulation of zatp6v1a (H+-ATPase A-subunit) expressions. Acclimation to both acidic and low-Na+ fresh water caused upregulation of zca15a expression but did not change the zca2-like a mRNA level in zebrafish gills. These results provide molecular physiological evidence to support the roles of these two zCA isoforms in Na+ uptake and acid-base regulation mechanisms in zebrafish HR cells.

The functional ontogeny of the teleost gill: Which comes first, gas or ion exchange?

For most of the last century, the need to obtain sufficient oxygen to meet the respiratory requirements of the tissues was viewed as the primary selective pressure driving gill development in teleost fish. Recently, however, it has been suggested that ionoregulatory pressures may actually be more important. This manuscript reviews the theoretical and empirical evidence dealing with the functional ontogeny of the gill in the context of the oxygen and ionoregulatory hypotheses. Gas and ion exchange are subject to similar geometric constraints in developing fish. Both initially are exclusively cutaneous but shift to the gill with tissue growth because of declining surface-to-volume ratios. Based on the appearance of mitochondria-rich cells (MRCs), ionoregulatory activity shifts to the gill in advance of gas exchange. In every species examined to date, MRCs appear on the developing gill in advance of secondary lamellae, the definitive gas exchange structure of the adult gill. Biochemical and histochemical studies indicate that these early branchial MRCs are actively involved in ion exchange. In some cases, the specific activity is many times greater than in the adult gill. In contrast, O2 microelectrode and hemoglobin ablation experiments suggest that the early gill contributes little O2 to the general systemic circulation. Any oxygen taken up appears to be consumed locally. Functional ablation experiments with zebrafish indicated that the larval gill became essential for ion balance well before it was needed for O2 uptake. Similar experiments with rainbow trout, however, found that the gill became essential in terms of gas and ion exchange at about the same time. On balance, the evidence appears to favour the ionoregulatory hypothesis but the oxygen hypothesis cannot be absolutely rejected without more information. Some of the major deficiencies in our knowledge regarding the transition from cutaneous to branchial gas and ion exchange are highlighted and potential implications of the ionoregulatory hypothesis are discussed.

Expression and water calcium dependence of calcium transporter isoforms in zebrafish gill mitochondrion-rich cells

Background

Freshwater fish absorb Ca²⁺ predominantly from ambient water, and more than 97% of Ca²⁺ uptake is achieved by active transport through gill mitochondrion-rich (MR) cells. In the current model for Ca²⁺ uptake in gill MR cells, Ca²⁺ passively enters the cytosol via the epithelium Ca²⁺ channel (ECaC), and then is extruded into the plasma through the basolateral Na⁺/Ca²⁺ exchanger (NCX) and plasma membrane Ca²⁺-ATPase (PMCA). However, no convincing molecular or cellular evidence has been available to support the role of specific PMCA and/or NCX isoforms in this model. Zebrafish (Danio rerio) is a good model for analyzing isoforms of a gene because of the plentiful genomic databases and expression sequence tag (EST) data.

Results

Using a strategy of BLAST from the zebrafish genome database (Sanger Institute), 6 isoforms of PMCAs (PMCA1a, PMCA1b, PMCA2, PMCA3a, PMCA3b, and PMCA4) and 7 isoforms of NCXs (NCX1a, NCX1b, NCX2a, NCX2b, NCX3, NCX4a, and NCX4b) were identified. In the reverse-transcriptase polymerase chain reaction (RT-PCR) analysis, 5 PMCAs and 2 NCXs were ubiquitously expressed in various tissues including gills. Triple fluorescence in situ hybridization and immunocytochemistry showed the colocalization of zecac, zpmca2, and zncx1b mRNAs in a portion of gill MR cells (using Na⁺-K⁺-ATPase as the marker), implying a subset of ionocytes specifically responsible for the transepithelial Ca²⁺ uptake in zebrafish gills. The gene expressions in gills of high- or low-Ca²⁺-acclimated zebrafish by quantitative real-time PCR analysis showed that zecac was the only gene regulated in response to environmental Ca²⁺ levels, while zpmcas and zncxs remained steady.

Conclusion

The present study provides molecular evidence for the specific isoforms of Ca²⁺ transporters, zECaC, zPMCA2, and zNCX1b, supporting the current Ca²⁺ uptake model, in which ECaC may play a role as the major regulatory target for this mechanism during environmental challenge.

Knockdown of V-ATPase subunit A (atp6v1a) impairs acid secretion and ion balance in zebrafish (Danio rerio)

In the skin of zebrafish embryo, the vacuolar H(+)-ATPase (V-ATPase, H(+) pump) distributed mainly in the apical membrane of H(+)-pump-rich cells, which pump internal acid out of the embryo and function similarly to acid-secreting intercalated cells in mammalian kidney. In addition to acid excretion, the electrogenic H(+) efflux via the H(+)-ATPases in the gill apical membrane of freshwater fish was proposed to act as a driving force for Na(+) entry through the apical Na(+) channels. However, convincing molecular physiological evidence in vivo for this model is still lacking. In this study, we used morpholino-modified antisense oligonucleotides to knockdown the gene product of H(+)-ATPase subunit A (atp6v1a) and examined the phenotype of the mutants. The H(+)-ATPase knockdown embryos revealed several abnormalities, including suppression of acid-secretion from skin, growth retardation, trunk deformation, and loss of internal Ca(2+) and Na(+). This finding reveals the critical role of H(+)-ATPase in embryonic acid -secretion and ion balance, as well.

Development of a zebrafish 4-day embryo-larval bioassay to assess toxicity of chemicals

A 4-day embryo-larval zebrafish test, from blastula stage to hatching included, was developed. The observations of embryo developmental were made at different development stages, for which morphological, physiological, and behavioral endpoints were selected and quantified for unexposed and exposed embryos. The sensitivity and the ability of these endpoints to inform about mode of action (MoA) were established in testing three model toxicants with well-known toxic effects (propranolol, malathion, cadmium). Lethal, sublethal (heart rate/edema, spontaneous movements, and hatching rate/time disturbance), and teratogenic effects were detected for all the studied compounds. This bioassay allows characterization of impairments at different biological levels: neuromuscular, physiological, morphological, and behavioral, and brings useful information about the toxic MoA of the chemicals on nontarget organisms. In this sense to answers the chemical industries and international organization (EMEA) requirements for the environmental risk assessment of new chemicals and pharmaceuticals.

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.