Zebrafish ae2.2 encodes a second slc4a2 anion exchanger

The slc4a2b gene is required for hair cell development in zebrafish

Hair cells (HCs) function as important sensory receptors that can detect movement in their immediate environment. HCs in the inner ear can sense acoustic signals, while in aquatic vertebrates HCs can also detect movements, vibrations, and pressure gradients in the surrounding water. Many genes are responsible for the development of HCs, and developmental defects in HCs can lead to hearing loss and other sensory dysfunctions. Here, we found that the solute carrier family 4, member 2b (slc4a2b) gene, which is a member of the anion-exchange family, is expressed in the otic vesicles and lateral line neuromasts in developing zebrafish embryos. An in silico analysis showed that the slc4a2b is evolutionarily conserved, and we found that loss of function of slc4a2b resulted in a decreased number of HCs in zebrafish neuromasts due to increased HC apoptosis. Taken together, we conclude that slc4a2b plays a critical role in the development of HCs in zebrafish.

Transcriptional effects in the estuarine guppy Poecilia vivipara exposed to sanitary sewage in laboratory and in situ

The urban growth has increased sanitary sewage discharges in coastal ecosystems, negatively affecting the aquatic biota. Mangroves, one of the most human-affected coastal biomes, are areas for reproduction and nursing of several species. In order to evaluate the effects of sanitary sewage effluents in mangrove species, this study assessed the hepatic transcriptional responses of guppy fish Poecilia vivipara exposed to sanitary sewage 33% (v:v), using suppressive subtraction hybridization (SSH), high throughput sequencing of RNA (Ion-proton) and quantification of transcript levels by qPCR of some identified genes in fish kept in a sewage-contaminated environment. Genes identified are related predominantly to xenobiotic biotransformation, immune system and sexual differentiation. The qPCR results confirmed the induction of cytochrome P450 1A (CYP1A), glutathione S transferase A-like (GST A-like) methyltransferase (MET) and UDP glycosyltransferase 1A (UDPGT1A), and repression of complement component C3 (C3), doublesex and mab-3 related transcription factor 1 (DMRT1), and transferrin (TF) in the laboratory experiment. In the field exposure, the transcript levels of CYP1A, DMRT1, MET, GST A-like and UDPGT1A were higher in fishes exposed at the contaminated sites compared to the reference site. Chemical analysis in fish from the laboratory and in situ experiments, and surface sediment from the sewage-contaminated sites revealed relevant levels of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyl (PCBs) and linear alkylbenzenes (LABs). These data reinforce the use of P. vivipara as a sentinel for monitoring environmental contamination in coastal regions.

Molecular cloning and functional characterization of zebrafish Slc4a3/Ae3 anion exchanger

The zebrafish genome encodes two slc4a1 genes, one expressed in erythroid tissues and the other in the HR (H(+)-ATPase-rich) type of embryonic skin ionocytes, and two slc4a2 genes, one in proximal pronephric duct and the other in several extrarenal tissues of the embryo. We now report cDNA cloning and functional characterization of zebrafish slc4a3/ae3 gene products. The single ae3 gene on chromosome 9 generates at least two low-abundance ae3 transcripts differing only in their 5'-untranslated regions and encoding a single definitive Ae3 polypeptide of 1170 amino acids. The 7 kb upstream of the apparent initiator Met in ae3 exon 3 comprises multiple diverse, mobile repeat elements which disrupt and appear to truncate the Ae3 N-terminal amino acid sequence that would otherwise align with brain Ae3 of other species. Embryonic ae3 mRNA expression was detected by whole mount in situ hybridization only in fin buds at 24-72 hpf, but was detectable by RT-PCR across a range of embryonic and adult tissues. Epitope-tagged Ae3 polypeptide was expressed at or near the surface of Xenopus oocytes, and mediated low rates of DIDS-sensitive (36)Cl(-)/Cl(-) exchange in influx and efflux assays. As previously reported for Ae2 polypeptides, (36)Cl(-) transport by Ae3 was inhibited by both extracellular and intracellular acidic pH, and stimulated by alkaline pH. However, zebrafish Ae3 differed from Ae2 polypeptides in its insensitivity to NH4Cl and to hypertonicity. We conclude that multiple repeat elements have disrupted the 5'-end of the zebrafish ae3 gene, associated with N-terminal truncation of the protein and reduced anion transport activity.

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.

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.

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

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

Evidence that SLC26 anion transporters mediate branchial chloride uptake in adult zebrafish (Danio rerio)

Experiments were performed to test the hypothesis that three members of the SLC26 anion transporter gene family (SLC26a3, A4, and A6; hereafter termed za3, za4, and za6) mediate branchial Cl(-)/HCO(3)(-) exchange in adult zebrafish (Danio rerio). Real-time RT-PCR demonstrated that the gill expressed relatively high levels of za6 mRNA; za3 and za4 mRNA, while present, were less abundant. Also, za4 and za6 were expressed at relatively high levels in the kidney. The results of in situ hybridization or immunocytochemistry (za3 only) experiments performed on gill sections revealed that the SLC26 transporters were predominantly expressed on the filament epithelium (especially within the interlamellar regions) and to a lesser extent on the lamellar epithelium at the base of lamellae. This distribution pattern suggests that the SLC26 anion transporters are localized to mitochondrion-rich cells (ionocytes). Transferring fish to water containing low [Cl(-)] (0.02 mmol/l) resulted in significant increases in branchial SLC26 mRNA expression after 5-10 days of exposure relative to fish raised in normal water [Cl(-)] (0.4 mmol/l); transferring fish to Cl(-)-enriched water (2.0 mmol/l) was without effect on mRNA levels. Transferring fish to water containing elevated levels of NaHCO(3) (10-12.5 mmol/l) caused marked increases in branchial SLC26 mRNA expression between 3 and 10 days of transfer that was associated with a significant 40% increase in Cl(-) uptake (as measured upon return to normal water after 7 days). A decrease in whole body net acid excretion (equivalent to an increase in net base excretion) in fish previously maintained in high [NaHCO(3)] water, concurrent with increases in Cl(-) uptake and SLC26 mRNA levels, suggests a role for these anion transporters in Cl(-) uptake and acid-base regulation owing to their Cl(-)/HCO(3)(-) exchange activities.

Molecular physiology and genetics of Na+-independent SLC4 anion exchangers

Plasmalemmal Cl(-)/HCO(3)(-) exchangers are encoded by the SLC4 and SLC26 gene superfamilies, and function to regulate intracellular pH, [Cl(-)] and cell volume. The Cl(-)/HCO(3)(-) exchangers of polarized epithelial cells also contribute to transepithelial secretion and reabsorption of acid-base equivalents and Cl(-). This review focuses on Na(+)-independent electroneutral Cl(-)/HCO(3)(-) exchangers of the SLC4 family. Human SLC4A1/AE1 mutations cause the familial erythroid disorders of spherocytic anemia, stomatocytic anemia and ovalocytosis. A largely discrete set of AE1 mutations causes familial distal renal tubular acidosis. The Slc4a2/Ae2(-/-) mouse dies before weaning with achlorhydria and osteopetrosis. A hypomorphic Ae2(-/-) mouse survives to exhibit male infertility with defective spermatogenesis and a syndrome resembling primary biliary cirrhosis. A human SLC4A3/AE3 polymorphism is associated with seizure disorder, and the Ae3(-/-) mouse has increased seizure susceptibility. The transport mechanism of mammalian SLC4/AE polypeptides is that of electroneutral Cl(-)/anion exchange, but trout erythroid Ae1 also mediates Cl(-) conductance. Erythroid Ae1 may mediate the DIDS-sensitive Cl(-) conductance of mammalian erythrocytes, and, with a single missense mutation, can mediate electrogenic SO(4)(2-)/Cl(-) exchange. AE1 trafficking in polarized cells is regulated by phosphorylation and by interaction with other proteins. AE2 exhibits isoform-specific patterns of acute inhibition by acidic intracellular pH and independently by acidic extracellular pH. In contrast, AE2 is activated by hypertonicity and, in a pH-independent manner, by ammonium and by hypertonicity. A growing body of structure-function and interaction data, together with emerging information about physiological function and structure, is advancing our understanding of SLC4 anion exchangers.