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

Zebrafish as an animal model to study ion homeostasis

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

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

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

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Bicarbonate secreted from the pancreas contributed to the formation of Ca precipitates in Japanese eel, Anguilla japonica

Marine teleosts produce Ca precipitates in the intestine as a product of osmoregulation. Ca precipitates are formed by a chemical reaction of Mg(2+) and Ca(2+) derived from ingested seawater with bicarbonate (HCO(3)(-)). It has been reported that HCO(3)(-) originates from the intestine; however, the pancreas is predicted to be another organ that may supply HCO(3)(-) to the intestinal tract. In the present study, the pancreas was surgically removed from Japanese eel to confirm its contribution to Ca precipitate formation. Pancreatectomized eel produced significantly less Ca precipitates than control eel in seawater, indicating that HCO(3)(-) from the pancreas contributes substantially to the formation of Ca precipitates. To further examine the molecular mechanisms of HCO(3)(-) secretion, we cloned cDNAs encoding HCO(3)(-) transporters and identified those transporters that elevated their mRNA expression in the intestine and pancreas following seawater transfer. In the intestine, mRNA expression of Slc26a6A was increased shortly after seawater transfer, whereas Slc26a1 mRNA expression increased gradually following seawater transfer. In the pancreas, Slc26a3 mRNA expression was high during the early stage of seawater acclimation, whereas Slc26a1 expression increased gradually after transfer to seawater. In the intestine and pancreas, therefore, both transient and progressively increasing types of HCO(3)(-) transporters are likely to be involved in HCO(3)(-) secretion into the intestinal lumen in a coordinated manner.

Loss of Slc4a1b chloride/bicarbonate exchanger function protects mechanosensory hair cells from aminoglycoside damage in the zebrafish mutant persephone

Mechanosensory hair cell death is a leading cause of hearing and balance disorders in the human population. Hair cells are remarkably sensitive to environmental insults such as excessive noise and exposure to some otherwise therapeutic drugs. However, individual responses to damaging agents can vary, in part due to genetic differences. We previously carried out a forward genetic screen using the zebrafish lateral line system to identify mutations that alter the response of larval hair cells to the antibiotic neomycin, one of a class of aminoglycoside compounds that cause hair cell death in humans. The persephone mutation confers resistance to aminoglycosides. 5 dpf homozygous persephone mutants are indistinguishable from wild-type siblings, but differ in their retention of lateral line hair cells upon exposure to neomycin. The mutation in persephone maps to the chloride/bicarbonate exchanger slc4a1b and introduces a single Ser-to-Phe substitution in zSlc4a1b. This mutation prevents delivery of the exchanger to the cell surface and abolishes the ability of the protein to import chloride across the plasma membrane. Loss of function of zSlc4a1b reduces hair cell death caused by exposure to the aminoglycosides neomycin, kanamycin, and gentamicin, and the chemotherapeutic drug cisplatin. Pharmacological block of anion transport with the disulfonic stilbene derivatives DIDS and SITS, or exposure to exogenous bicarbonate, also protects hair cells against damage. Both persephone mutant and DIDS-treated wild-type larvae show reduced uptake of labeled aminoglycosides. persephone mutants also show reduced FM1-43 uptake, indicating a potential impact on mechanotransduction-coupled activity in the mutant. We propose that tight regulation of the ionic environment of sensory hair cells, mediated by zSlc4a1b activity, is critical for their sensitivity to aminoglycoside antibiotics.

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.

New insights into gill ionocyte and ion transporter function in euryhaline and diadromous fish

Teleost fishes are able to acclimatize to seawater by secreting excess NaCl by means of specialized "ionocytes" in the gill epithelium. Antibodies against Na(+)/K(+)-ATPase (NKA) have been used since 1996 as a marker for identifying branchial ionocytes. Immunohistochemistry of NKA by itself and in combination with Na(+)/K(+)/2Cl(-) cotransporter and CFTR Cl(-) channel provided convincing evidence that ionocytes are functional during seawater acclimation, and also revealed morphological variations in ionocytes among teleost species. Recent development of antibodies to freshwater- and seawater-specific isoforms of the NKA alpha-subunit has allowed functional distinction of ion absorptive and secretory ionocytes in Atlantic salmon. Cutaneous ionocytes of tilapia embryos serve as a model for branchial ionocytes, allowing identification of 4 types: two involved in ion uptake, one responsible for salt secretion and one with unknown function. Combining molecular genetics, advanced imaging techniques and immunohistochemistry will rapidly advance our understanding of both the unity and diversity of ionocyte function and regulation in fish osmoregulation.

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

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

Euryhaline pufferfish NBCe1 differs from nonmarine species NBCe1 physiology

Marine fish drink seawater and eliminate excess salt by active salt transport across gill and gut epithelia. Euryhaline pufferfish (Takifugu obscurus, mefugu) forms a CaCO(3) precipitate on the luminal gut surface after transitioning to seawater. NBCe1 (Slc4a4) at the basolateral membrane of intestinal epithelial cell plays a major role in transepithelial intestinal HCO(3)(-) secretion and is critical for mefugu acclimation to seawater. We assayed fugu-NBCe1 (fNBCe1) activity in the Xenopus oocyte expression system. Similar to NBCe1 found in other species, fNBCe1 is an electrogenic Na(+)/HCO(3)(-) cotransporter and sensitive to the stilbene inhibitor DIDS. However, our experiments revealed several unique and distinguishable fNBCe1 transport characteristics not found in mammalian or other teleost NBCe1-orthologs: electrogenic Li(+)/nHCO(3)(-) cotransport; HCO(3)(-) independent, DIDS-insensitive transport; and increased basal intracellular Na(+) accumulation. fNBCe1 is a voltage-dependent Na(+)/nHCO(3)(-) cotransporter that rectifies, independently from the extracellular Na(+) or HCO(3)(-) concentration, around -60 mV. Na(+) removal (0Na(+) prepulse) is necessary to produce the true HCO(3)(-)-elicited current. HCO(3)(-) addition results in huge outward currents with quick current decay. Kinetic analysis of HCO(3)(-) currents reveals that fNBCe1 has a much higher transport capacity (higher maximum current) and lower affinity (higher K(m)) than human kidney NBCe1 (hkNBCe1) does in the physiological range (membrane potential = -80 mV; [HCO(3)(-)] = 10 mM). In this state, fNBCe1 is in favor of operating as transepithelial HCO(3)(-) secretion, opposite of hkNBCe1, from blood to the luminal side. Thus, fugu-NBCe1 represents the first ortholog-based tool to study amino acid substitutions in NBCe1 and how those change ion and voltage dependence.

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.