Takifugu obscurus is a euryhaline fugu species very close to Takifugu rubripes and suitable for studying osmoregulation

Identification of renal transporters involved in sulfate excretion in marine teleost fish

Sulfate (SO(4)(2-)) is the second most abundant anion in seawater (SW), and excretion of excess SO(4)(2-) from ingested SW is essential for marine fish to survive. Marine teleosts excrete SO(4)(2-) via the urine produced in the kidney. The SO(4)(2-) transporter that secretes and concentrates SO(4)(2-) in the urine has not previously been identified. Here, we have identified and characterized candidates for the long-sought transporters. Using sequences from the fugu database, we have cloned cDNA fragments of all transporters belonging to the Slc13 and Slc26 families from mefugu (Takifugu obscurus). We compared Slc13 and Slc26 mRNA expression in the kidney between freshwater (FW) and SW mefugu. Among 14 clones examined, the expression of a Slc26a6 paralog (mfSlc26a6A) was the most upregulated (30-fold) in the kidney of SW mefugu. Electrophysiological analyses of Xenopus oocytes expressing mfSlc26a6A, mfSlc26a6B, and mouse Slc26a6 (mSlc26a6) demonstrated that all transporters mediate electrogenic Cl(-)/SO(4)(2-), Cl(-)/oxalate(2-), and Cl(-)/nHCO(3)(-) exchanges and electroneutral Cl(-)/formate(-) exchange. Two-electrode voltage-clamp experiments demonstrated that the SO(4)(2-)-elicited currents of mfSlc26a6A is quite large (approximately 35 microA at +60 mV) and 50- to 200-fold higher than those of mfSlc26a6B and mSlc26a6. Conversely, the currents elicited by oxalate and HCO(3)(-) are almost identical among mfSlc26a6A, mfSlc26a6B, and mSlc26a6. Kinetic analysis revealed that mfSlc26a6A has the highest SO(4)(2-) affinity as well as capacity. Immunohistochemical analyses demonstrated that mfSlc26a6A localizes to the apical (brush-border) region of the proximal tubules. Together, these findings suggest that mfSlc26a6A is the most likely candidate for the major apical SO(4)(2-) transporter that mediates SO(4)(2-) secretion in the kidney of marine teleosts.

A microarray-based transcriptomic time-course of hyper- and hypo-osmotic stress signaling events in the euryhaline fish Gillichthys mirabilis: osmosensors to effectors

Cells respond to changes in osmolality with compensatory adaptations that re-establish ion homeostasis and repair disturbed aspects of cell structure and function. These physiological processes are highly complex, and require the coordinated activities of osmosensing, signal transducing and effector molecules. Although the critical role of effector proteins such as Na+, K+-ATPases and Na+/K+/Cl(-) co-transporters during osmotic stress are well established, comparatively little information is available regarding the identity or expression of the osmosensing and signal transduction genes that may govern their activities. To better resolve this issue, a cDNA microarray consisting of 9207 cDNA clones was used to monitor gene expression changes in the gill of the euryhaline fish Gillichthys mirabilis exposed to hyper- and hypo-osmotic stress. We successfully annotated 168 transcripts differentially expressed during the first 12 h of osmotic stress exposure. Functional classifications of genes encoding these transcripts reveal that a variety of biological processes are affected. However, genes participating in cell signaling events were the dominant class of genes differentially expressed during both hyper- and hypo-osmotic stress. Many of these genes have had no previously reported role in osmotic stress adaptation. Subsequent analyses used the novel expression patterns generated in this study to place genes within the context of osmotic stress sensing, signaling and effector events. Our data indicate multiple major signaling pathways work in concert to modify diverse effectors, and that these molecules operate within a framework of regulatory proteins.

Explosive speciation of Takifugu: another use of fugu as a model system for evolutionary biology

Although the fugu Takifugu rubripes has attracted attention as a model organism for genomic studies because of its compact genome, it is not generally appreciated that there are approximately 25 closely related species with limited distributions in the waters of East Asia. We performed molecular phylogenetic analyses and constructed a time tree using whole mitochondrial genome sequences from 15 Takifugu species together with 10 outgroups to examine patterns of diversification. The resultant time tree showed that the modern Takifugu species underwent explosive speciation during the Pliocene 1.8-5.3 Ma, which is comparable with that of the Malawi cichlids and tropheine cichlids in Lake Tanganyika. Considering their limited distributions and remarkable variations in coloration, morphology, and behavior, the results of the present study strongly suggest that Takifugu species are strong candidates as a model system for evolutionary studies of speciation mechanisms in marine environments where few such organisms are available.

Differential expression of branchial Na+/K(+)-ATPase of two medaka species, Oryzias latipes and Oryzias dancena, with different salinity tolerances acclimated to fresh water, brackish water and seawater

Previous studies on non-diadromous euryhaline teleosts introduced a hypothesis that the lowest level of gill Na(+)/K(+)-ATPase (NKA) activity occurs in the environments with salinity close to the primary natural habitats of the studied species. To provide more evidence of the hypothesis, two medaka species, Oryzias latipes and O. dancena, whose primary natural habitats are fresh water (FW) and brackish water (BW) environments, respectively, were compared from levels of mRNA to cells in this study. The plasma osmolalities of O. latipes and O. dancena were lowest in the FW individuals. The muscle water contents of O. latipes decreased with elevated external salinities, but were constant among FW-, BW-, and seawater (SW)-acclimated O. dancena. Expression of NKA, the primary driving force of ion transporters in gill ionocytes, revealed different patterns in the two Oryzias species. The highest NKA alpha-subunit mRNA abundances were found in the gills of the SW O. latipes and the FW O. dancena, respectively. The pattern of NKA activity and alpha-subunit protein abundance in the gills of O. latipes revealed that the FW group was the lowest, while the pattern in O. dancena revealed that the BW group was the lowest. Immunohistochemical staining showed similar profiles of NKA immunoreactive (NKIR) cell activities (NKIR cell numberxcell size) in the gills of these two species among FW, BW, and SW groups. Taken together, O. latipes exhibited better hyposmoregulatory ability, while O. dancena exhibited better hyperosmoregulatory ability. Our results corresponding to the hypothesis indicated that the lowest branchial NKA activities of these two medaka species were found in the environments with salinities similar to their natural habitats.

Cloning of a river pufferfish (Takifugu obscurus) metallothionein cDNA and study of its induction profile in cadmium-exposed fish

We report here the full-length cDNA sequence of metallothionein (MT) gene from an anadromous river pufferfish, Takifugu obscurus (order: Tetradotiformes; family: Tetradontidae). Phylogenetic relationship analysis revealed that the identified MT has high sequence similarity with many Perciformes fish species. The tissue distribution and concentration- and time-dependent expression of MT mRNA were studied in fish exposed to cadmium. Liver showed the highest level of MT gene expression followed by other tissues (brain, gill and kidney) in response to cadmium exposure. Muscle showed a weak expression response of MT gene. Time-course study revealed highest early phase (at 6h) expression in the brain and late persistence of induction in the intestine. MT mRNA expression showed a concentration-dependent expression in all the tissues. However, induction in brain and liver occurred at much lower concentrations as compared to other tissues. Our results demonstrate that MT in T. obscurus is induced by cadmium exposure which indicates that it plays a functionally conserved function of metal detoxification.

Molecular cloning and beta-naphthoflavone-induced expression of a cytochrome P450 1A (CYP1A) gene from an anadromous river pufferfish, Takifugu obscurus

In recent years, there has been a decline in the wild populations of river pufferfish, Takifugu obscurus. Besides overexploitation for commercial purposes, environmental pollution is believed to have contributed to its decline. However, almost no information exists about genes involved in metabolism of xenobiotics by this species. Nevertheless, there is interest in fugu fishes, since they possess the smallest genome among vertebrates. We cloned and characterized the full-length cDNA sequence of a cytochrome P450 1A (CYP1A) gene from T. obscurus. Phylogenic relationship of T. obscurus CYP1A was also compared to other fish species. The tissue distribution and time-dependant induction of CYP1A mRNA were studied by real-time PCR in T. obscurus exposed to an aryl hydrocarbon receptor (Ahr) agonist, beta-naphthoflavone (BNF). The greatest basal expression in livers of control as well as BNF-treated individuals. However, brain, gill, gonad, intestine, and kidney also expressed CYP1A. Muscles expressed the least CYP1A. The results of the time-course study revealed induction in brain and gills after 6h and at 12h in most tissues. Except for gills, all other organs retained induced expression of CYP1A mRNA up to 96h.

Identification of intestinal bicarbonate transporters involved in formation of carbonate precipitates to stimulate water absorption in marine teleost fish

Marine teleost fish precipitate divalent cations as carbonate deposits in the intestine to minimize the potential for excessive Ca2+ entry and to stimulate water absorption by reducing luminal osmotic pressure. This carbonate deposit formation, therefore, helps maintain osmoregulation in the seawater (SW) environment and requires controlled secretion of HCO3(-) to match the amount of Ca2+ entering the intestinal lumen. Despite its physiological importance, the process of HCO3(-) secretion has not been characterized at the molecular level. We analyzed the expression of two families of HCO3(-) transporters, Slc4 and Slc26, in fresh-water- and SW-acclimated euryhaline pufferfish, mefugu (Takifugu obscurus), and obtained the following candidate clones: NBCe1 (an Na+-HCO3(-) cotransporter) and Slc26a6A and Slc26a6B (putative Cl(-)/HCO3(-) exchangers). Heterologous expression in Xenopus oocytes showed that Slc26a6A and Slc26a6B have potent HCO3(-)-transporting activity as electrogenic Cl(-)/nHCO3(-) exchangers, whereas mefugu NBCe1 functions as an electrogenic Na+-nHCO3(-) cotransporter. Expression of NBCe1 and Slc26a6A was highly induced in the intestine in SW and expression of Slc26a6B was high in the intestine in SW and fresh water, suggesting their involvement in HCO3(-) secretion and carbonate precipitate formation. Immunohistochemistry showed staining on the apical (Slc26a6A and Slc26a6B) and basolateral (NBCe1) membranes of the intestinal epithelial cells in SW. We therefore propose a mechanism for HCO3(-) transport across the intestinal epithelial cells of marine fish that includes basolateral HCO3(-) uptake (NBCe1) and apical HCO3(-) secretion (Slc26a6A and Slc26a6B).

Structure of neurohypophysial hormone genes and changes in the levels of expression during spawning season in grass puffer (Takifugu niphobles)

Vasotocin (VT) has been shown to influence various aspects of social and sexual behaviors in a broad range of vertebrate species, but less is known about the mechanisms through which this peptide modulates behavior. Additionally, much less is known about roles of isotocin (IT) in regulation of behavior. Grass puffer, Takifugu niphobles, has unique spawning behavior; spawning occurs on beach only for several days around the spring tide and is conducted by a group of 10-60 individuals, of which one is female. As a first step toward investigating the roles of VT and IT in this species' spawning behavior, we determined the structures of the VT and IT genes from grass puffer using the genome resources of the closely related tiger puffer and green puffer. We then used these sequences to develop real-time PCR assays and examined changes in expression of the VT and IT genes over the spawning season. The structures of VT and IT genes are well conserved among three puffer species. Particularly, the sequence similarities between grass and tiger puffers were very high not only in the coding region (85-99%), but also in the non-coding regions (92-98%) that include the 5'-upstream regions. The levels of expression of VT gene increased in the brain of pre-spawning females. The levels of VT mRNA in the spawning females tended to be higher than that in the spawning males. In contrast, the levels of IT mRNA did not show such variation. The present results suggest that VT gene expression augments in the brain of females during the spawning period. The unique spawning behavior of grass puffer provides a useful model for studying the molecular mechanism of sexual behavior utilizing the genome resources of tiger puffer.

Localization of ammonia transporter Rhcg1 in mitochondrion-rich cells of yolk sac, gill, and kidney of zebrafish and its ionic strength-dependent expression

Members of the Rh glycoprotein family have been shown to be involved in ammonia transport in a variety of species. Here we show that zebrafish Rhcg1, a member of the Rh glycoprotein family, is highly expressed in the yolk sac, gill, and renal tubules. Molecular cloning and characterization indicate that zebrafish Rhcg1 shares 82% sequence identity with the pufferfish ortholog fRhcg1. RT-PCR, combined with in situ hybridization, revealed that Rhcg1 is first expressed in vacuolar-type H+-ATPase/mitochondrion-rich cells (vH-MRC) on the yolk sac of larvae at 3 days postfertilization (dpf) and later in vH-MRC-like cells in the gill at 4–5 dpf. Ammonia excretion from zebrafish larvae increased in parallel with the expression of Rhcg1. At larval stages, Rhcg1 mRNA was detected only on the yolk sac and gill; however, the kidney, as well as the gill, becomes a major site of Rhcg1 expression in adults. Using a zebrafish Tol2 transgenic line whose vH-MRC are labeled with green fluorescent protein (GFP) and an antibody against zebrafish Rhcg1, we demonstrate that Rhcg1 is located in the apical regions of 1) vH-MRC on the yolk sac and vH-MRC-like cells (cell population with the expression of Rhcg1 and GFP) in the gill and 2) cells in the renal distal tubule and intercalated cell-like cells in the collecting duct of the kidney. Remarkably, expression of Rhcg1 mRNA at the larval stage was changed by environmental ionic strength. These results suggest that roles of zebrafish Rhcg1 are not solely ammonia secretion to eliminate nitrogen from the gill.

Fish calcitonin receptor has novel features

Calcitonin (CT), a 32-amino acid peptide, was initially isolated from fish. Fish CT has higher affinity to mammalian CT receptor (CTR), and has activity on calcium homeostasis. Therefore, fish CT has been used as a drug for the treatment of human bone diseases. However, the physiological roles of CT in fish as well as the characteristics of the fish CTR have not been clarified. Here, we cloned and characterized CTR from mefugu (Takifugu obscurus). Full-length cDNA sequencing revealed that mfCTR (mf, mefugu) consists of N-terminal four tandem putative hormone-binding domains (HBDs). Database mining showed that the multiple HBD-containing CTR is a common feature for some other fishes. Detailed pharmacological studies revealed that mfCTR generated cAMP in response to (1) fish CT, (2) calcitonin gene-related peptide (CGRP) in combinations with receptor activity-modifying proteins (mfRAMPs) 1 and 4, and (3) amylin in combinations with mfRAMPs 1-5. Unlike mammalian CTR, mfCTR showed dual affinity sites. Corresponding EC(50) values of those are in close proximity of the in vivo concentration of CT in fish. Analyses of the deletion mutants of mfCTR demonstrated that only the nearmost HBD to the first transmembrane region is functional to the ligands. Although, fish CT has higher affinity to the human CTR, human CT did not bind to the mfCTR. This is the first report that demonstrates the structure and property of fish receptor for CT, CGRP, and amylin. Fish CTR is the first example that has multiple HBD-like sequences.

The effect of environmental salinity on the protein expression of Na+/K+-ATPase, Na+/K+/2Cl- cotransporter, cystic fibrosis transmembrane conductance regulator, anion exchanger 1, and chloride channel 3 in gills of a euryhaline teleost, Tetraodon nigroviridis

Chloride transport mechanisms in the gills of the estuarine spotted green pufferfish (Tetraodon nigroviridis) were investigated. Protein abundance of Na(+)/K(+)-ATPase (NKA) and the other four chloride transporters, i.e., Na(+)/K(+)/2Cl(-) cotransporter (NKCC), cystic fibrosis transmembrane conductance regulator (CFTR), Cl(-)/HCO(3)(-) anion exchanger 1 (AE1), and chloride channel 3 (CLC-3) in gills of the seawater- (SW; 35 per thousand) or freshwater (FW)-acclimatized fish were examined by immunoblot analysis. Appropriate negative controls were used to confirm the specificity of the antibodies to the target proteins. The relative protein abundance of NKA was higher (i.e., 2-fold) in gills of the SW group compared to the FW group. NKCC and CFTR were expressed in gills of the SW group but not in the FW group. In contrast, the levels of relative protein abundance of branchial AE1 and CLC-3 in the FW group were 23-fold and 2.7-fold higher, respectively, compared to those of the SW group. This study is first of its kind to provide direct in vivo evidence of the protein expression of CLC-3 in teleostean gills, as well as to examine the simultaneous protein expression of the Cl(-) transporters, especially AE1 and CLC-3 of FW- and SW-acclimatized teleosts. The differential protein expression of NKA, chloride transporters in gills of the FW- and SW-acclimatized T. nigroviridis observed in the present study shows their close relationship to the physiological homeostasis (stable blood osmolality), as well as explains the impressive ionoregulatory ability of this euryhaline species in response to salinity challenges.