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

Effects of ambient cadmium with calcium on mRNA expressions of calcium uptake related transporters in zebrafish (Danio rerio) larvae

The mRNA expression levels of Ca²⁺ transporter genes including the epithelial calcium channel (ECaC), sodium/calcium exchanger 1b (NCX1b), and plasma membrane calcium ATPase 2 (PMCA2) were measured in zebrafish larvae after exposure to 0.08 μM Cd²⁺ in either water mixed with 0.2 mM Ca²⁺ (lCa) or 2 mM Ca²⁺ (hCa). The ECaC and NCX1b expression decreased at the 48 and 72 h mark, respectively; however, PMCA2 transcripts decreased at 96 h after exposure to Cd²⁺ in lCa environment. On the other hand, the ECaC transcripts were not affected; however, the PMCA2 transcripts were increased at 72 h, while the NCX1b transcripts significantly decreased at 48 and 96 h after exposure to Cd²⁺ in a hCa environment. The Ca²⁺ contents of larvae significantly decreased after Cd²⁺ exposure in a lCa environment; however, the Ca²⁺ contents were evidently higher after exposure to Cd²⁺ in a hCa environment, except for 48th h mark. In addition, ECaC morphants showed lower Ca²⁺ contents of whole-body, and there were higher levels of mortality after exposure to the same condition compared to the wild-type groups. In contrast, injection of ECaC cRNA resulted in an increase in Ca²⁺ content and the rate of Ca²⁺ influx in zebrafish embryos. Summary, the results showed that the Ca²⁺ transporters of zebrafish larvae were impacted after exposures of 0.08 μM Cd. However, in the exposure condition, the ECaC and PMCA2 transcripts could be restored to control levels after the fish were treated in an environment with hCa.

Macrophage-stimulating protein and calcium homeostasis in zebrafish

To systematically identify novel gene functions essential for osteogenesis and skeletal mineralization, we performed a forward genetic mutagenesis screen in zebrafish and isolated a mutant that showed delayed skeletal mineralization. Analysis of the mutant phenotype in an osterix:nuclear-GFP transgenic background demonstrated that mutants contain osterix-expressing osteoblasts comparable to wild-type embryos. Positional cloning revealed a premature stop mutation in the macrophage-stimulating protein (msp) gene, predicted to result in a biologically inactive protein. Analysis of the embryonic expression pattern for the receptor for Msp, Ron, shows specific expression in the corpuscles of Stannius, a teleost-specific organ that produces stanniocalcin, a pivotal hormone in fish calcium homeostasis. Knockdown of Ron resulted in identical phenotypes as observed in msp mutants. Msp mutant embryos could be rescued by excess calcium. Consistent with a role for Msp/Ron in calcium homeostasis, calcium-regulating factors, such as pth1, pth2, stc1l, and trpv5/6 were significantly affected in msp mutant larvae. While Msp and Ron have previously been shown to play a critical role in a wide variety of biological processes, we introduce here the Msp/Ron signaling axis as a previously unappreciated player in calcium homeostasis and embryonic skeletal mineralization.

Evolution and roles of stanniocalcin

In fish, stanniocalcin-1 (STC1) is a key endocrine factor that acts on gill, intestine and kidney to regulate serum calcium and phosphate homeostasis. The recent identification and study of mammalian STCs (STC1 and STC2) revealed that the hormones are made in virtually all tissues and they act primarily as paracrine/autocrine factors to regulate various biological functions. Based on their ubiquitous expression patterns and generally undetectable levels in blood serum, it is unlikely that the mammalian STCs play important roles in serum Ca(2+)/P(i) homeostasis. However current evidences still support the local action of STCs in Ca(2+) and P(i) transport, probably via their action on Ca(2+)-channels and Na(+)/P(i) co-transporter. At present, information about the sequence, expression and distribution of the STC receptor(s) is lacking. However, recent emerging evidence hints the involvement of STC1 and STC2 in the sub-cellular functions of mitochondria and endoplasmic reticulum respectively, particularly responding to oxidative stress and unfolded protein response. With increasing evidence that demonstrates the local actions of STCs, the focus of the research has been moved to cellular inflammation and carcinogenesis. This review integrates the information available on STCs in fish and mammals, focusing mainly on their embryonic origin, tissue distribution, their potential regulatory mechanisms and the modes of action, and their physiological and pathophysiological functions, particularly in cancer biology.

Characterization of stanniocalcin-1 receptors in the rainbow trout

Mammalian stanniocalcin-1 (STC-1) is one of several ligands targeted to mitochondria. High affinity STC-1 receptors are present on the mitochondrial membranes of nephron cells, myocytes, and hepatocytes, to enable ligand sequestration within the matrix. However, STC-1 receptors have not been characterized in fish. Nor is it known if mitochondrial targeting occurs in fish. The aim of the study was to address these questions. Saturation binding assays were carried out to obtain estimates of K_D and B_max. They revealed the presence of saturable, high-affinity receptors on both membranes and mitochondria of liver, muscle, and gill filament. In situ ligand binding (ISLB) was used to localize receptors at the histological level and revealed some unexpected findings. In cranium, for instance, receptors were found mainly in the cartilage matrix, as opposed to the chondrocytes. In brain, the majority of receptors were located on neuropil areas as opposed to neuronal cell bodies. In skeletal muscle, receptors were confined to periodic striations, tentatively identified as the Z lines. Receptors were even found on STC-1 producing corpuscles of Stannius cells, raising the possibility of there being an autocrine feedback loop or, perhaps, a soluble binding protein that is released with the ligand to regulate its bioavailability.

Four stanniocalcin genes in teleost fish: structure, phylogenetic analysis, tissue distribution and expression during hypercalcemic challenge

Stanniocalcin (STC), first isolated from the corpuscles of Stannius (CS) of teleost fishes and a systemic regulator of mineral metabolism, is present in all vertebrates as two isoforms, STC1 and STC2, encoded by separate genes. Here we show that the genome of Tetraodon nigroviridis, and other teleosts, possess duplicate genes for each STC isoform, designated stc1-a and -b, and stc2-a and -b. Stc1-a was cloned from CS, stc2-a from muscle and the two novel cDNAs, stc1-b and stc2-b, from brain. However, stc2-b was isolated as a conjoined (read-through) transcript with bod1 (bi-orientation defective 1, or FAM44B), and two additional alternative conjoined transcripts were also isolated. The predicted STC products shared the typical vertebrate 10 conserved cysteine residues and N-linked glycosylation motifs, in addition to specific features. Gene structure was generally conserved with four exons and three introns with the exception of stc1-a which gained an extra intron in exon three, originating one extra exon. Gene order and synteny is also maintained across vertebrates and the cpeb4 gene identified in the homologue region of the chordate Ciona was linked to vertebrate stc2 but not stc1. Immunohistochemistry in different species revealed that STC1-A was found only in CS and in a few cells in kidney. STC1-B had a restricted expression and was more prominent in the gills. STC2-A was detected in a variety of tissues, including pituitary, with most abundant immunoreaction in kidney cells and gill rakers and the CS was negative. Expression of stc1-a in CS of Tetraodon was 15-fold (p<0.05) up-regulated 2 h after transfer from 2.9 mM Ca(2+) to 10 mM Ca(2+) water and down-regulated after 12 hours to 11-fold lower than 2.9 mM Ca(2+) fish (p<0.05). With the exception of stc1-a in CS, low expression levels and high individual variation were generally found for the expression of stc transcripts in kidney and gills, with no statistically significant changes in response to the hypercalcemic shock. In conclusion, both stc1 and stc2 genes are represented by paralogues in teleosts genomes and the analysis performed suggests that only stc1-a in the CS is involved in extracellular calcium regulation. The widespread distribution of stcs in fish tissues supports pleiotropic roles.

Comparison of static immersion and intravenous injection systems for exposure of zebrafish embryos to the natural pathogen Edwardsiella tarda

Background

The zebrafish embryo is an important in vivo model to study the host innate immune response towards microbial infection. In most zebrafish infectious disease models, infection is achieved by micro-injection of bacteria into the embryo. Alternatively, Edwardsiella tarda, a natural fish pathogen, has been used to treat embryos by static immersion. In this study we used transcriptome profiling and quantitative RT-PCR to analyze the immune response induced by E. tarda immersion and injection.

Results

Mortality rates after static immersion of embryos in E. tarda suspension varied between 25-75%, while intravenous injection of bacteria resulted in 100% mortality. Quantitative RT-PCR analysis on the level of single embryos showed that expression of the proinflammatory marker genes il1b and mmp9 was induced only in some embryos that were exposed to E. tarda in the immersion system, whereas intravenous injection of E. tarda led to il1b and mmp9 induction in all embryos. In addition, microarray expression profiles of embryos subjected to immersion or injection showed little overlap. E. tarda-injected embryos displayed strong induction of inflammatory and defense genes and of regulatory genes of the immune response. E. tarda-immersed embryos showed transient induction of the cytochrome P450 gene cyp1a. This gene was also induced after immersion in Escherichia coli and Pseudomonas aeruginosa suspensions, but, in contrast, was not induced upon intravenous E. tarda injection. One of the rare common responses in the immersion and injection systems was induction of irg1l, a homolog of a murine immunoresponsive gene of unknown function.

Conclusions

Based on the differences in mortality rates between experiments and gene expression profiles of individual embryos we conclude that zebrafish embryos cannot be reproducibly infected by exposure to E. tarda in the immersion system. Induction of il1b and mmp9 was consistently observed in embryos that had been systemically infected by intravenous injection, while the early transcriptional induction of cyp1a and irg1l in the immersion system may reflect an epithelial or other tissue response towards cell membrane or other molecules that are shed or released by bacteria. Our microarray expression data provide a useful reference for future analysis of signal transduction pathways underlying the systemic innate immune response versus those underlying responses to external bacteria and secreted virulence factors and toxins.

Identification and apical membrane localization of an electrogenic Na⁺/Ca²⁺ exchanger NCX2a likely to be involved in renal Ca²⁺ excretion by seawater fish

Seawater (SW) contains ∼10 mM Ca(2+), yet marine fish must drink seawater as their major water source. Thus marine teleosts fish need to excrete Ca(2+) to maintain whole body Ca(2+) homeostasis. In the intestine, seawater Ca(2+) interreacts with epithelial-secreted HCO(3)(-) by the intestinal epithelium, and the resulting CaCO(3) precipitates, which is rectally excreted. Recently the transporters involved in intestinal HCO(3)(-) secretion were identified. Ca(2+) is also excreted by the kidney, but the protein(s) involved in renal Ca(2+) excretion have not been identified. Here we identified a candidate transporter by using SW pufferfish torafugu (Takifugu rubripes) and its closely related euryhaline species mefugu (Takifugu obscurus), which are becoming useful animal models for studying molecular mechanisms of seawater adaptation. RT-PCR analyses of Na(+)/Ca(2+) exchanger (NCX) family members in various torafugu tissues demonstrated that only NCX2a is highly expressed in the kidney. Renal expression of NCX2a was markedly elevated when mefugu were transferred from freshwater to seawater. In situ hybridization and immunohistochemical analyses indicated that NCX2a is expressed in the proximal tubule at the apical membrane. NCX2a, expressed in Xenopus oocytes, conferred [Ca(2+)](out)- and Na(+)-dependent currents. These results suggest that NCX2a mediates renal Ca(2+) secretion at the apical membrane of renal proximal tubules and has an important role in whole body Ca(2+) homeostasis of marine teleosts.

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.

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.

Stanniocalcin has deep evolutionary roots in eukaryotes

Vertebrates have a large glycoprotein hormone, stanniocalcin, which originally was shown to inhibit calcium uptake from the environment in teleost fish gills. Later, humans, other mammals, and teleost fish were shown to have two forms of stanniocalcin (STC1 and STC2) that were widely distributed in many tissues. STC1 is associated with calcium and phosphate homeostasis and STC2 with phosphate, but their receptors and signaling pathways have not been elucidated. We undertook a phylogenetic investigation of stanniocalcin beyond the vertebrates using a combination of BLAST and HMMER homology searches in protein, genomic, and expressed sequence tag databases. We identified novel STC homologs in a diverse array of multicellular and unicellular organisms. Within the eukaryotes, almost all major taxonomic groups except plants and algae have STC homologs, although some groups like echinoderms and arthropods lack STC genes. The critical structural feature for recognition of stanniocalcins was the conserved pattern of ten cysteines, even though the amino acid sequence identity was low. Signal peptides in STC sequences suggest they are secreted from the cell of synthesis. The role of glycosylation signals and additional cysteines is not yet clear, although the 11th cysteine, if present, has been shown to form homodimers in some vertebrates. We predict that large secreted stanniocalcin homologs appeared in evolution as early as single-celled eukaryotes. Stanniocalcin's tertiary structure with five disulfide bonds and its primary structure with modest amino acid conservation currently lack an established receptor-signaling system, although we suggest possible alternatives.

trpm7 regulation of in vivo cation homeostasis and kidney function involves stanniocalcin 1 and fgf23

The transient receptor potential melastatin 7 (trpm7) channel kinase is a primary regulator of magnesium homeostasis in vitro. Here we show that trpm7 is an important regulator of cation homeostasis as well as kidney function in vivo. Using zebrafish trpm7 mutants, we show that early larvae exhibit reduced levels of both total magnesium and total calcium. Accompanying these deficits, we show that trpm7 mutants express higher levels of stanniocalcin 1 (stc1), a potent regulator of calcium homeostasis. Using transgenic overexpression and morpholino oligonucleotide knockdown, we demonstrate that stc1 modulates both calcium and magnesium levels in trpm7 mutants and in the wild type and that levels of these cations are restored to normal in trpm7 mutants when stc1 activity is blocked. Consistent with defects in both calcium and phosphate homeostasis, we further show that trpm7 mutants develop kidney stones by early larval stages and exhibit increased levels of the anti-hyperphosphatemic factor, fibroblast growth factor 23 (fgf23). Finally, we demonstrate that elevated fgf23 expression contributes to kidney stone formation by morpholino knockdown of fgf23 in trpm7 mutants. Together, these analyses reveal roles for trpm7 in regulating cation homeostasis and kidney function in vivo and implicate both stc1 and fgf23 in these processes.

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

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

Parathyroid hormone-related protein-stanniocalcin antagonism in regulation of bicarbonate secretion and calcium precipitation in a marine fish intestine

Bicarbonate secretion in the intestine (duodenum) of marine fish has been suggested to play a major role in regulation of calcium availability for uptake. However, while the end process may lead to carbonate precipitation, regulation of transport of calcium and/or bicarbonate may actually result in fine-tuning of calcium availability for transport. To test this hypothesis, sea bream ( Sparus auratus ) duodenal preparations were mounted in Ussing-type chambers and the effect of parathyroid hormone-related protein (PTHrP) and stanniocalcin 1 (STC 1) on the control of intestinal bicarbonate secretion and calcium transport was analyzed. As expected, PTHrP increased net calcium uptake, as a result of an increase of calcium uptake without changes in calcium efflux. In contrast, purified sea bream STC 1 caused a minor decrease of calcium uptake and a two- to threefold increase in calcium efflux. As a result, STC 1 was able to invert the calcium flux from net calcium uptake to net calcium loss, which is in keeping with its known actions as a hypocalcemic factor. Furthermore, both PTHrP and STC 1 regulate intestinal bicarbonate secretion. PTHrP increased calcium uptake and simultaneously reduced the single factor that induces calcium precipitation, bicarbonate secretion. In contrast, STC 1, while reversing the calcium net flux to make it secretory, promoted intestinal bicarbonate secretion, both actions directed to decrease the calcium gradient across the epithelium and promote immobilization in the form of bicarbonate in the intestinal lumen. Together our results provide robust evidence to support an antagonistic action of PTHrP and STC 1 in the fine control of movements of both calcium and bicarbonate in the intestine of seawater fish.

The involvement of SLC26 anion transporters in chloride uptake in zebrafish (Danio rerio) larvae

After demonstrating phylogenetic relatedness to orthologous mammalian genes, tools were developed to investigate the roles of three members (A3, A4 and A6c) of the SLC26 anion exchange gene family in Cl- uptake and HCO3 excretion in embryos and larvae of zebrafish (Danio rerio). Whole-mount in situ hybridization revealed the presence of SLC26 mRNA in gill primordia, mesonephros and heart (slc26a3 and a4 only) at 5-9 days postfertilization (d.p.f.). SLC26A3 protein was highly expressed in lateral line neuromasts and within the gill, was localized to a sub-population of epithelial cells, which often (but not always) coexpressed Na+/K+-ATPase. SLC26 mRNA levels increased with developmental age, peaking at 5-10 d.p.f.; the largest increases in rates of Cl- uptake (JinCl-) preceded the mRNA spike, occurring at 2-5 d.p.f. Raising zebrafish in water with a low [Cl-] caused marked increases in JinCl- at 3-10 d.p.f. and was associated with increased levels of SLC26 mRNA. Raising fish in water of high [Cl-] was without effect on JinCl- or SLC26 transcript abundance. Selective gene knockdown using morpholino antisense oligonucleotides demonstrated a significant role for SLC26A3 in Cl- uptake in larval fish raised in control water and roles for A3, A4 and A6c in fish raised in water with low [Cl-]. Prolonged (7 days) or acute (24 h) exposure of fish to elevated (2 or 5 mmol l(-1)) ambient [HCO3-] caused marked increases in Cl- uptake when determined in water of normal [HCO3-] that were accompanied by elevated levels of SLC26 mRNA. The increases in JinCl- associated with high ambient [HCO3-] were not observed in the SLC26 morphants (significant only at 5 mmol l(-1) HCO3- for A4 and 2 mmol l(-1) HCO3- for A6c). Net base excretion was markedly inhibited in the slc26a3 and a6c morphants thereby implicating these genes in Cl-/HCO3- exchange. The results suggest that under normal conditions, Cl- uptake in zebrafish larvae is mediated by SLC26A3 Cl-/HCO3- exchangers but under conditions necessitating higher rates of high affinity Cl- uptake, SlC26A4 and SLC26A6c may assume a greater role.