The zebrafish: a new model organism for integrative physiology

Oxidative stress response and gene expression with acute copper exposure in zebrafish (Danio rerio)

In fish, environmental pollution is one factor that induces oxidative stress, and this can disturb the natural antioxidant defense system. Oxidative stress has been well characterized in vitro, yet the in vivo effects of metal-induced oxidative stress have not been extensively studied. In two experiments we examined the impacts of copper (Cu) on gene expression, oxidative damage, and cell oxidative capacity in liver and gill of zebrafish. In the first experiment, soft water-acclimated zebrafish were exposed to 8 and 15 mug/l Cu for 48 h. This exposure resulted in significant increases in gene expression of cytochrome c oxidase subunit 17 (COX-17) and catalase, associated with both increased Cu load and protein carbonyl concentrations in the gill and liver after 48 h. In addition, we examined the potential protective effects of increased waterborne Ca(2+) (3.3 mM) and Na(+) (10 mM) on acute Cu toxicity. While both treatments were effective at reducing liver and/or gill Cu loads and attenuating oxidative damage at 48 h, 10 mM Na(+) was more protective than 3.3 mM Ca(2+). There were variable changes in the maximal activities of COX and citrate synthase (CS), indicating possible alterations in cell oxidative capacity. Moreover, Cu affected COX-to-CS ratios in both gill and liver, suggesting that Cu alters normal mitochondrial biogenic processes, possibly though metallochaperones like COX-17. Overall, this study provides important steps in determining the transcriptional and physiological endpoints of acute Cu toxicity in a model tropical species.

A positive regulatory loop between foxi3a and foxi3b is essential for specification and differentiation of zebrafish epidermal ionocytes

BACKGROUND

Epidermal ionocytes play essential roles in the transepithelial transportation of ions, water, and acid-base balance in fish embryos before their branchial counterparts are fully functional. However, the mechanism controlling epidermal ionocyte specification and differentiation remains unknown.

METHODOLOGY/PRINCIPAL FINDINGS

In zebrafish, we demonstrated that Delta-Notch-mediated lateral inhibition plays a vital role in singling out epidermal ionocyte progenitors from epidermal stem cells. The entire epidermal ionocyte domain of genetic mutants and morphants, which failed to transmit the DeltaC-Notch1a/Notch3 signal from sending cells (epidermal ionocytes) to receiving cells (epidermal stem cells), differentiates into epidermal ionocytes. The low Notch activity in epidermal ionocyte progenitors is permissive for activating winged helix/forkhead box transcription factors of foxi3a and foxi3b. Through gain- and loss-of-function assays, we show that the foxi3a-foxi3b regulatory loop functions as a master regulator to mediate a dual role of specifying epidermal ionocyte progenitors as well as of subsequently promoting differentiation of Na(+),K(+)-ATPase-rich cells and H(+)-ATPase-rich cells in a concentration-dependent manner.

CONCLUSIONS/SIGNIFICANCE

This study provides a framework to show the molecular mechanism controlling epidermal ionocyte specification and differentiation in a low vertebrate for the first time. We propose that the positive regulatory loop between foxi3a and foxi3b not only drives early ionocyte differentiation but also prevents the complete blockage of ionocyte differentiation when the master regulator of foxi3 function is unilaterally compromised.

Gill membrane remodeling with soft-water acclimation in zebrafish (Danio rerio)

Little is known regarding the ionoregulatory abilities of zebrafish exposed to soft water despite the popularity of this model organism for physiology and aquatic toxicology. We examined genomic and nongenomic changes to gills of zebrafish as they were progressively acclimated from moderately hard freshwater to typical soft water over 7 days and held in soft water for another 7 days. Gills were sampled daily and mRNA expression levels of gill Na(+)-K(+)-ATPase (NKA) alpha1a subunit, epithelium calcium channel (ECaC), carbonic anhydrase-1 and 2 (CA-1, CA-2), Na(+)/H(+) exchanger (NHE-2), V-type proton (H(+))-ATPase, and copper transport protein (CTR-1) were quantified by real-time PCR. Changes in enzyme activities of gill NKA were determined and protein levels of NKA and ECaC were quantified by Western blotting. Levels of mRNA for ECaC increased fourfold after day 6, with an associated increase in ECaC protein levels after 1 wk in soft water. CA-1 and CA-2 exhibited a 1.5- and 6-fold increase in gene expression on days 6 and 5, respectively. Likewise, there was a fivefold increase in NHE-2 expression after day 6. Surprisingly, CTR-1 mRNA showed a large transient increase (over threefold) on day 6, while H(+)-ATPase mRNA did not change. These data demonstrate a high degree of phenotypic plasticity in zebrafish gills exposed to an ion-poor environment. This not only enhances our understanding of ionoregulatory processes in fish but also highlights the need for proper experimental design for studies involving preacclimation to soft water (e.g., metal toxicity).

Pro-apoptotic effect of fly ash leachates in hepatocytes of freshwater fish (Channa punctata Bloch)

The pro-apoptotic effect of fly ash leachates (FAL) was studied in the hepatocytes of an Indian freshwater fish, Channa punctata Bloch. Hepatocytes were exposed to different concentrations of '7-day' FAL for 24 and 48h and various parameters of apoptosis were studied using standardized procedures. FAL-induced apoptosis in hepatocytes was indicated by cytological examination, DNA fragmentation and DNA laddering. The induction in cytochrome-c release, caspases 3, 7, 10 and 9 activities and lactate dehydrogenase level provide mechanistic platform for FAL-induced apoptosis. Cytological examination showed an unambiguous apoptotic effect of ash leachates in fish hepatocytes. Exposed hepatocytes also showed increased production of H(2)O(2), superoxide ions and an increase in lipid peroxidation (LPO). The present study suggests a possible role of reactive oxygen species (ROS) in FAL-induced apoptosis in hepatocytes. Lactate dehydrogenase, LPO and apoptosis as biomarkers of cytotoxicity have recently been used for assessment of ecotoxicological impact of environmental chemicals. Our findings show that these biomarkers may also be used for evaluation of ecotoxicological impact of complex chemical mixture such as fly ash and its leachates.

Pituitary-interrenal interaction in zebrafish interrenal organ development

To further elucidate pituitary adrenal interactions during development, we studied the organogenesis of the interrenal organ, the teleost homolog of the mammalian adrenal gland, in zebrafish. To this end we compared wild-type zebrafish interrenal development with that of mutants lacking pituitary cell types including corticotrophs. In addition, we studied the effects of ACTH receptor (Mc2r) knockdown and dexamethasone (dex) on interrenal development and pituitary feedback. Until 2 d post fertilization (2 dpf) interrenal development assessed by transcripts of key steroidogenic genes (cyp11a1, mc2r, star) is independent of proopiomelanocortin (Pomc) as demonstrated in aal/eya1and lia/fgf3 mutants. However, at 5 dpf lack of pituitary cells leads to reduced expression of steroidogenic genes at both the transcriptional and the protein level. Pituitary control of interrenal development resides in corticotrophs, because pit1 mutants lacking pituitary cells except corticotrophs have a phenotype similar to that of wild-type controls. Furthermore, development in mc2r knockdown morphants does not differ from aal/eya1 and lia/fgf3 mutants. Inhibition of steroidogenesis by mc2r knockdown induces up-regulation of pomc expression in the anterior domain of pituitary corticotrophs. Accordingly, dex suppresses pomc in the anterior domain only, leading to impaired expression of steroidogenic genes commencing at 3 dpf and interrenal hypoplasia via reduced interrenal proliferation. In contrast, negative feedback on pituitary corticotrophs by dex is evident at 2 dpf and precedes effects of Pomc on the interrenal primordium. These data demonstrate a gradual transition from early pituitary-independent interrenal organogenesis to developmental control by the anterior domain of pituitary corticotrophs acting via Mc2 receptors.

Identification of novel hydroxysteroid-sulfating cytosolic SULTs, SULT2 ST2 and SULT2 ST3, from zebrafish: Cloning, expression, characterization, and developmental expression

By searching the expressed sequence tag database, two zebrafish cDNAs encoding putative cytosolic sulfotransferases (SULTs) were identified. Sequence analysis indicated that these two zebrafish SULTs belong to the cytosolic SULT2 gene family. The recombinant form of these two novel zebrafish SULTs, designated SULT2 ST2 and SULT2 ST3, were expressed using the pGEX-2TK glutathione S-transferase (GST) gene fusion system and purified from transformed BL21 (DE3) Escherichia coli cells. Purified GST-fusion protein form of SULT2 ST2 and SULT2 ST3 exhibited strong sulfating activities toward dehydroepiandrosterone (DHEA) and corticosterone, respectively, among various endogenous compounds tested as substrates. Both enzymes displayed pH optima at approximately 6.5. Kinetic constants of the two enzymes, as well as the GST-fusion protein form of the previously identified SULT2 ST1, with DHEA and corticosterone as substrates were determined. Developmental stage-dependent expression experiments revealed distinct patterns of expression of SULT2 ST2 and SULT2 ST3, as well as the previously identified SULT2 ST1, during embryonic development and throughout the larval stage onto maturity.

Zebrafish dax1 is required for development of the interrenal organ, the adrenal cortex equivalent

Mutations in the human nuclear receptor, DAX1, cause X-linked adrenal hypoplasia congenita (AHC). We report the isolation and characterization of a DAX1 homolog, dax1, in zebrafish. The dax1 cDNA encodes a protein of 264 amino acids, including the conserved carboxy-terminal ligand binding-like motif; but the amino-terminal region lacks the unusual repeats of the DNA binding-like domain in mammals. Genomic sequence analysis indicates that the dax1 gene structure is conserved also. Whole-mount in situ hybridization revealed the onset of dax1 expression in the developing hypothalamus at approximately 26 h post fertilization (hpf). Later, at about 28 hpf, a novel expression domain for dax1 appeared in the trunk. This bilateral dax1-expressing structure was located immediately above the yolk sac, between the otic vesicle and the pronephros. Interestingly, weak and transient expression of dax1 was observed in the interrenal glands (adrenal cortical equivalents) at approximately 31 hpf. This gene was also expressed in the liver after 3 dpf in the zebrafish larvae. Disruption of dax1 function by morpholino oligonucleotides (MO) down-regulated expression of steroidogenic genes, cyp11a and star, and led to severe phenotypes similar to ff1b (SF1) MO-injected embryos. Injection of dax1 MO did not affect ff1b expression, whereas ff1b MO abolished dax1 expression in the interrenal organ. Based on these results, we propose that dax1 is the mammalian DAX1 ortholog, functions downstream of ff1b in the regulatory cascades, and is required for normal development and function of the zebrafish interrenal organ.

Proton pump-rich cell secretes acid in skin of zebrafish larvae

The mammalian kidney excretes its metabolic acid load through the proton-transporting cells, intercalated cells, in the distal nephron and collecting duct. Fish excrete acid through external organs, gill, or skin; however, the cellular function is still controversial. In this study, molecular and electrophysiological approaches were used to identify a novel cell type secreting acid in skin of zebrafish ( Danio rerio) larvae. Among keratinocytes covering the larval surface, novel proton-secreting ionocytes, proton pump (H+-ATPase)-rich cells, were identified to generate strong outward H+flux. The present work demonstrates for the first time, with a noninvasive technique, H+-secreting cells in an intact animal model, the zebrafish, showing it to be a suitable model in which to study the functions of vertebrate transporting epithelia in vivo.

Structure and autonomic innervation of the swim bladder in the zebrafish (Danio rerio)

Many teleosts actively regulate buoyancy by using a gas-filled swim bladder, which is thought to be under autonomic control. Here we investigated the swim bladder in the zebrafish to determine possible mechanisms of gas-content regulation. Fluorescently labelled phalloidin revealed myocytes that appeared to form a possible sphincter at the junction of the pneumatic duct and esophagus. Myocytes also formed thick bands along the ventral surface of the anterior chamber and bilaterally along the posterior chamber. Thinner layers of myocytes were located elsewhere. Staining of peroxidase within erythrocytes revealed a putative rete and smaller blood vessels in muscle bands and elsewhere. The antibodies zn-12, a general neuronal marker, and SV2, a synaptic vesicle marker labelling presynaptic terminals, revealed widespread innervation of the swim bladder system. Widespread innervation of the swim bladder was also indicated by acetylcholinesterase histochemistry, but choline acetyltransferase-immunoreactive (-IR) somata and fibers were limited to the junction of the pneumatic duct and esophagus. In contrast, varicose tyrosine hydroxylase-IR fibers innervated muscles and blood vessels throughout the system. Neuropeptide Y-IR somata were located near the junction of the duct and esophagus and varicose fibers innervated muscles and vasculature of the posterior chamber and duct. Vasoactive intestinal polypeptide immunoreactivity was abundant throughout the anterior chamber but sparsely distributed elsewhere. Serotonin-IR fibers and varicosities were located only along blood vessels near the junction of the pneumatic duct and posterior chamber. Our results suggest that the zebrafish swim bladder is a complex and richly innervated organ and that buoyancy-regulating effectors may be controlled by multiple populations of autonomic neurons.

CYP3C1, the first member of a new cytochrome P450 subfamily found in zebrafish (Danio rerio)

We report a new cytochrome P450 (CYP) subfamily CYP3C and the cloning through PCR from zebrafish (Danio rerio) of the first member, CYP3C1. The CYP3C1 gene is on Chromosome 3 with 13 ORF exons encoding a 505 amino acid protein which has 44-54% identities with mammalian and teleost CYP3A and CYP3B forms. As evidenced by spectral analysis, the CYP3C1 protein heterologously expressed in yeast is functional. In silico analysis identified, on the same region of the chromosome, three more genes encoding CYP3C1-like proteins that formed a clade with CYP3C1 in a phylogenetic tree. Using RT-PCR, the CYP3C1 mRNA was detected in 1-6dpf embryo/larvae and in adult fish liver and seven extrahepatic tissues. Whole-mount in situ hybridization using a riboprobe demonstrated expression in the brain during 12-120 hpf. At the 120 hpf larval stage, CYP3C1 mRNA was also detected in the pharynx and gastrointestinal tract. TCDD, dexamethasone, and rifampicin, which up-regulated CYP3A65 mRNA in zebrafish larvae, did not alter the CYP3C1 transcript levels suggesting regulatory differences between CYP3A and CYP3C enzymes in this species.

High-affinity peptide transporter PEPT2 (SLC15A2) of the zebrafish Danio rerio: functional properties, genomic organization, and expression analysis

Solute carrier 15 (SLC15) membrane proteins PEPT1 (SLC15A1) and PEPT2 (SLC15A2) have been described in great detail in mammals. In contrast, information in lower vertebrates is limited. We characterized the functional properties of a novel zebrafish peptide transporter orthologous to mammalian and avian PEPT2, described its gene (pept2) structure, and determined mRNA tissue distribution. An expressed sequence tag (EST) cDNA (Integrated Molecular Analysis of Gene Expression; IMAGE) corresponding to zebrafish pept2 was completed by inserting a stretch of 75 missing nucleotides in the coding sequence to obtain a 3,238-bp functional clone. The complete open reading frame (ORF) was 2,160 bp and encoded a 719-amino acid protein. Electrophysiological analysis after cRNA injection in Xenopus laevis oocytes suggested that zebrafish PEPT2 is a high-affinity/low-capacity transporter (K(0.5) for glycyl-L-glutamine approximately 18 microM at -120 mV and pH 7.5). Zebrafish pept2 gene was 19,435 kb, thus being the shortest vertebrate pept2 fully characterized so far. Also, zebrafish pept2 exhibited 23 exons and 22 introns, whereas human and rodent pept2 genes contain 22 exons and 21 introns only. Zebrafish pept2 mRNA was mainly detected in brain, kidney, gut, and, interestingly, otic vesicle, the embryonic structure that develops into the auditory/vestibular organ, homolog to the higher vertebrate inner ear, of the adult fish. Characterization of zebrafish pept2 will contribute to the investigation of peptide transporters using a well-established genetic model and will allow the elucidation of the evolutionary and functional relationships among vertebrate peptide transporters. Moreover, it can represent a useful marker to screen mutations that affect choroid plexus and inner ear development.

Identification of a novel thyroid hormone-sulfating cytosolic sulfotransferase, SULT1 ST5, from zebrafish

By employing RT-PCR in conjunction with 3'-RACE, a full-length cDNA encoding a novel zebrafish cytosolic sulfotransferase (SULT) was cloned and sequenced. Sequence analysis revealed that this zebrafish SULT (designated SULT1 ST5) is, at the amino acid sequence level, close to 50% identical to human and dog SULT1B1 (thyroid hormone SULT). A recombinant form of zebrafish SULT1 ST5 was expressed using the pGEX-2TK bacterial expression system and purified from transformed BL21 (DE3) cells. Purified zebrafish SULT1 ST5 migrated as a 34 kDa protein and displayed substrate specificity for thyroid hormones and their metabolites among various endogenous compounds tested. The enzyme also exhibited sulfating activities toward some xenobiotic phenolic compounds. Its pH optima were 6.0 and 9.0 with 3,3',5-triiodo-l-thyronine (l-T3) as substrate and 6.0 with beta-naphthol as substrate. Kinetic constants of the enzyme with thyroid hormones and their metabolites as substrates were determined. Quantitative evaluation of the regulatory effects of divalent metal cations on the l-T3-sulfating activity of SULT1 ST5 revealed that Fe2+, Hg2+, Co2+, Zn2+, Cu2+, Cd2+ and Pb2+ exhibited dramatic inhibitory effects, whereas Mn2+ showed a significant stimulation. Developmental stage-dependent expression experiments revealed a significant level of expression of this novel zebrafish thyroid hormone-sulfating SULT at the beginning of the hatching period during embryogenesis, which gradually increased to a high level of expression throughout the larval stage into maturity.

Intrafollicular paracrine communication in the zebrafish ovary: the state of the art of an emerging model for the study of vertebrate folliculogenesis

The development and function of vertebrate ovary are primarily controlled by the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), from the pituitary. However, most of the activities of FSH and LH are mediated or modulated by a variety of locally produced factors that form an intimate regulatory network within and between the follicles. As a top vertebrate model for genetic and developmental studies, the zebrafish has caught tremendous attention in the past two decades; however, its utility has quickly been extended to other areas including physiology. In the past few years, a variety of peptide growth factors have been identified and characterized in the zebrafish ovary including activin and epidermal growth factor (EGF), and lines of evidence point to the existence of an ovarian network of communication involving these factors. This article provides the state of the art of zebrafish as a model for analyzing ovarian development and its regulation.

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.

Identification of a novel estrogen-sulfating cytosolic SULT from zebrafish: molecular cloning, expression, characterization, and ontogeny study

By searching the expressed sequence tag database, a zebrafish cDNA encoding a putative cytosolic sulfotransferase (SULT) was identified. Sequence analysis indicated that this zebrafish SULT belongs to the SULT1 cytosolic SULT gene family. The recombinant form of this novel zebrafish SULT, expressed using the pGEX-2TK expression system and purified from transformed BL21 (DE3) Escherichia coli cells, displayed sulfating activities specifically for estrone and 17beta-estradiol among various endogenous compounds tested as substrates. The enzyme also exhibited sulfating activities toward some xenobiotic phenolic compounds. This new zebrafish SULT showed dual pH optima, at 6.5 and 10-10.5, with estrone or n-propyl gallate as substrate. Kinetic constants of the sulfation of estrone, 17beta-estradiol, and n-propyl gallate were determined. Developmental stage-dependent expression experiments revealed a significant level of expression of this novel zebrafish estrogen-sulfating SULT at the beginning of the hatching period during embryogenesis, which continued throughout the larval stage onto maturity.

Identification of a novel zebrafish SULT1 cytosolic sulfotransferase: Cloning, expression, characterization, and developmental expression study

By searching the zebrafish expressed sequence tag database, we had identified two partial cDNA clones encoding the 5'- and 3'-regions of a putative cytosolic sulfotransferase (SULT). Using the reverse transcription-polymerase chain reaction (RT-PCR) technique, a full-length cDNA encoding this zebrafish SULT was amplified, cloned, and sequenced. Analysis of the sequence data revealed that this novel zebrafish SULT displays 49, 46, and 45% amino acid sequence identity to human SULT1A1, mouse SULT1D1, and rat SULT1C1. This zebrafish SULT therefore appears to belong to the SULT1 cytosolic SULT gene family. Recombinant zebrafish SULT (designated SULT1 isoform 4), expressed using the pGEX-2TK prokaryotic expression vector and purified from transformed Escherichia coli cells, migrated as a 35kDa protein upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Among the endogenous compounds tested as substrates, the purified SULT1 isoform 4 displayed significant sulfating activities toward thyroid hormones, estrone, and dehydroepiandrosterone. The enzyme also showed activities toward a number of xenobiotics including some flavonoids, isoflavonoids, and other phenolic compounds, with a pH optimum at 7.0. A thermostability experiment revealed the enzyme to be relatively stable over a temperature range between 28 and 37 degrees C. Among 10 divalent metal cations tested, Fe2+, Hg2+, Co2+, Zn2+, Cu2+, and Cd2+ exhibited dramatic inhibitory effects on the activity of the enzyme. Developmental expression study using RT-PCR revealed that the zebrafish SULT1 isoform 4 showed a low level of expression in the segmentation period during the embryonic development, which gradually increased to a high level of expression throughout the larval stage onto maturity.

Cloning and characterization of full length of a novel zebrafish gene Zsrg abundantly expressed in the germline stem cells

Using the digital differential display program of the National Center for Biotechnology Information, we identified a contig of expression sequence tags (ESTs) (Accession No. BM316936), which came from zebrafish ovary and testis libraries. The full-length cDNA of this transcript was cloned and further confirmed by polymerase chain reaction and sequencing. The full-length cDNA of the novel gene is 807bp and encodes a novel protein of 187 amino acids, which shares no significant homology with any other known proteins. Characterization of genomic sequences of the gene revealed that it spans 6kb on the linkage group 3 and is composed of five exons and four introns. RT-PCR analysis showed that it was expressed in mature oocytes and one-cell stage, and persisted until 24h of development. RT-PCR also revealed that it is expressed in gonad and kidney, with the highest level of expression in the testis. The expression sites of the novel gene in adult gonad were further localized by in situ hybridization to oogonia and growing oocytes in ovary and to spermatogonia, spermatocytes but not to spermatids in testis. Based on its abundance in testis and the germline stem cell-spermatogonia and oogonia, we hypothesize that it may function as a testicular development and gametogenesis related gene that plays important roles in spermatogenesis, and named it Zsrg (zebrafish testis spermatogenesis related gene, Zsrg).

COMPARATIVE DEVELOPMENTAL PHYSIOLOGY:An Interdisciplinary Convergence

Comparative developmental physiology spans genomics to physiological ecology and evolution. Although not a new discipline, comparative developmental physiology's position at the convergence of development, physiology and evolution gives it prominent new significance. The contributions of this discipline may be particularly influential as physiologists expand beyond genomics to a true systems synthesis, integrating molecular through organ function in multiple organ systems. This review considers how developing physiological systems are directed by genes yet respond to environment and how these characteristics both constrain and enable evolution of physiological characters. Experimental approaches and methodologies of comparative developmental physiology include studying event sequences (heterochrony and heterokairy), describing the onset and progression of physiological regulation, exploiting scaling, expanding the list of animal models, using genetic engineering, and capitalizing on new miniaturized technologies for physiological investigation down to the embryonic level. A synthesis of these approaches is likely to generate a more complete understanding of how physiological systems and, indeed, whole animals develop and how populations evolve.

Acute renal failure in zebrafish: a novel system to study a complex disease

Acute renal failure (ARF) is characterized by a very high mortality essentially unchanged over the past 40 years. Simple vertebrate models are needed to improve our understanding of ARF and facilitate the development of novel therapies for this clinical syndrome. Here, we demonstrate that gentamicin, a commonly used nephrotoxic antibiotic, causes larval zebrafish to develop ARF characterized by histological and functional changes that mirror aminoglycoside toxicity in higher vertebrates and inability of zebrafish to maintain fluid homeostasis. We developed a novel method to quantitate renal function in larval zebrafish and demonstrate a decline in glomerular filtration rate after gentamicin exposure. The antineoplastic drug cisplatin, whose use in humans is limited by kidney toxicity, also causes typical histological changes and a decline in renal function in larval zebrafish. A specific inhibitor of Omi/HtrA2, a serine protease implicated in cisplatin-induced apoptosis, prevented renal failure and increased survival. This protective effect was confirmed in a mouse model of cisplatin-induced nephrotoxicity. Therefore, zebrafish provides a unique model system, amenable to genetic manipulation and drug screening, to explore the pathophysiology of ARF and establish novel therapies with potential use in mammals.

Characterization of Fxr1 in Danio rerio; a simple vertebrate model to study costamere development

The X-linked FMR1 gene, which is involved in the fragile X syndrome, forms a small gene family with its two autosomal homologs, FXR1 and FXR2. Mouse models for the FXR genes have been generated and proved to be valuable in elucidating the function of these genes, particularly in adult mice. Unfortunately, Fxr1 knockout mice die shortly after birth, necessitating an animal model that allows the study of the role of Fxr1p, the gene product of Fxr1, in early embryonic development. For gene function studies during early embryonic development the use of zebrafish as a model organism is highly advantageous. In this paper the suitability of the zebrafish as a model organism to study Fxr1p function during early development is explored. As a first step, we present here the initial characterization of Fxr1p in zebrafish. Fxr1p is present in all the cells from zebrafish embryos from the 2/4-cell stage onward; however, during late development a more tissue-specific distribution is found, with the highest expression in developing muscle. In adult zebrafish, Fxr1p is localized at the myoseptum and in costamere-like granules in skeletal muscle. In the testis, Fxr1p is localized in immature spermatogenic cells and in brain tissue Fxr1p displays a predominantly nuclear staining in neurons throughout the brain. Finally, the different tissue-specific isoforms of Fxr1p are characterized. Since the functional domains and the expression pattern of Fxr1p in zebrafish are comparable to those in higher vertebrates such as mouse and human, we conclude that the zebrafish is a highly suitable model for functional studies of Fxr1p.

Zebrafish tyrosylprotein sulfotransferase: molecular cloning, expression, and functional characterization

By employing the reverse transcriptase-polymerase chain reaction technique in conjunction with 3' rapid amplification of cDNA ends, a full-length cDNA encoding a zebrafish (Danio rerio) tyrosylprotein sulfotransferase (TPST) was cloned and sequenced. Sequence analysis revealed that this zebrafish TPST is, at the amino acid sequence level, 66% and 60% identical to the human and mouse TPST-1 and TPST-2, respectively. The recombinant form of the zebrafish TPST, expressed in COS-7 cells, exhibited a pH optimum at 5.75. Manganese appeared to exert a stimulatory effect on the zebrafish TPST. The activity of the enzyme determined in the presence of 20 mM MnCl2 was more than 2.5 times that determined in the absence of MnCl2. Of the other nine divalent metal cations tested at a 10 mM concentration, Co2+ also showed a considerable stimulatory effect, while Ca2+, Pb2+, and Cd2+ exerted some inhibitory effects. The other four divalent cations, Fe2+, Cu2+, Zn2+, and Hg2+, inhibited completely the sulfating activity of the zebrafish TPST. Using the wild-type and mutated P-selectin glycoprotein ligand-1 N-terminal peptides as substrates, the zebrafish TPST was shown to exhibit a high degree of substrate specificity for the tyrosine residue on the C-terminal side of the peptide. These results constitute a first study on the cloning, expression, and characterization of a zebrafish cytosolic TPST.

Characterization of a zebrafish estrogen-sulfating cytosolic sulfotransferase: inhibitory effects and mechanism of action of phytoestrogens

Cytosolic sulfotransferases (STs) are generally thought to be involved in detoxification of xenobiotics, as well as homeostasis of endogenous compounds such as thyroid/steroid hormones and catecholamine hormones/neurotransmitters. We report here the identification and characterization of a zebrafish estrogen-sulfating cytosolic ST. The zebrafish ST was bacterially expressed, purified, and examined for enzymatic activities using a variety of endogenous compounds as substrates. Results showed that the enzyme displayed much higher activities toward two endogenous estrogens, estrone (E(1)) and 17beta-estradiol (E(2)), in comparison with thyroid hormones, 3,3',5-triiodothyronine (T(3)) and thyroxine (T(4)), dopamine, dihydroxyphenylalanine (Dopa), and dehydroepiandrosterone (DHEA). The kinetic parameters, K(m), and V(max), with estrogens and thyroid hormones as substrates were determined. The calculated V(max)/K(m) for E(1), E(2), T(3), and T(4) were, respectively, 31.6, 16.7, 1.5, and 0.8 nmol min(-1) mg(-1) microM(-1), indicating clearly the estrogens being preferred physiological substrates for the enzyme. The inhibitory effects of isoflavone phytoestrogens on the sulfation of E(2) by this zebrafish ST were examined. The IC(50) determined for quercetin, genistein, and daidzein were 0.7, 2.5, and 8 microM, respectively. Kinetic analyses revealed that the mechanism underlying the inhibition by these isoflavones to be of the competitive type.

Asian medaka fishes offer new models for studying mechanisms of seawater adaptation

Japanese medaka (Oryzias latipes) is a freshwater (FW) teleost that is popular throughout the world for laboratory use. In this paper, we discuss the utility of Japanese medaka and related species for studying mechanisms of seawater (SW) adaptation. In addition to general advantages as an experimental animal such as their daily spawning activity, transparency of embryos, short generation time and established transgenic techniques, Japanese medaka have some adaptability to SW unlike the strictly stenohaline zebrafish (Danio rerio). Since other species in the genus Oryzias exhibit different degrees of adaptability to SW, comparative studies between Japanese medaka, where molecular-biological and genetic information is abundant, and other Oryzias species are expected to present varying approaches to solving the problems of SW adaptation. We introduce some examples of interspecies comparison for SW adaptabilities both in adult fish and in embryos. Oryzias species are good models for evolutionary, ecological and zoogeographical studies and a relationship between SW adaptability and geographic distribution has been suggested. Medaka fishes may thus deliver new insights into our understanding of how fish have expanded their distribution to a wide variety of osmotic environments.

Zebrafish pronephros: A model for understanding cystic kidney disease

The embryonic kidney of the zebrafish is the pronephros. The ease of genetic analysis and experimentation in zebrafish, coupled with the simplicity of the pronephros, make the zebrafish an ideal model system for studying kidney development and function. Several mutations have been isolated in zebrafish genetic screens that result in cyst formation in the pronephros. Cloning and characterization of these mutations will provide insight into kidney development but may also provide understanding of the molecular basis of cystic kidney diseases. In this review, we focus on the zebrafish as a model for understanding cystic kidney disease and the links between cystic kidney disease and left-right patterning.

Transgenic zebrafish with fluorescent germ cell: a useful tool to visualize germ cell proliferation and juvenile hermaphroditism in vivo

Juvenile zebrafish are hermaphroditic; undifferentiated gonads first develop into ovary-like tissues, which then either become ovaries and produce oocytes (female) or degenerate and develop into testes (male). In order to fully capture the dynamic processes of germ cells' proliferation and juvenile hermaphroditism in zebrafish, we established transgenic lines TG(beta-actin:EGFP), harboring an enhanced green fluorescent protein (EGFP) gene driven by a medaka beta-actin promoter. In TG(beta-actin:EGFP), proliferating germ cells and female gonads strongly expressed EGFP, but fluorescence was only dimly detected in male gonads. Based on the fluorescent (+) or nonfluorescent (-) appearance of germ cells seen in living animals, three distinct groups were evident among TG(beta-actin:EGFP). Transgenics in ++ group (44%) were females, had fluorescent germ cells as juveniles, and female gonads continuously fluoresced throughout sexual maturation. Transgenics in +- (23%) and -- (33%) groups were males. Fluorescent germ cells were transiently detected in +- transgenics from 14 to 34 days postfertilization (dpf), but were not detected in -- transgenics throughout their life span. Histological analyses showed that 26-dpf-old transgenics in ++, +-, and -- groups all developed ovary-like tissues: Germ cells in -- group juveniles arrested at the gonocyte stage and accumulated low quantities of EGFP, while those in ++ group juveniles highly proliferated into diplotene to perinucleolar stages and accumulated high quantities of EGFP. In +- group juveniles, degenerating oocytes, gonocytes, and spermatogonia were coexistent in transiently fluorescent gonads. Therefore, the fluorescent appearance of gonads in this study was synchronous with the differentiation of ovary-like tissues. Thus, TG(beta-actin:EGFP) can be used to visualize germ cells' proliferation and juvenile hermaphroditism in living zebrafish for the first time.

Sulfonation of environmental estrogens by zebrafish cytosolic sulfotransferases

Environmental estrogen-like chemicals are increasingly recognized as a potential hazardous factor for wildlife as well as humans. We have recently embarked on developing a zebrafish model for investigating the role of sulfonation in the metabolism and adverse functioning of environmental estrogens. Here, we report on a systematic investigation of the sulfonation of representative environmental estrogens (bisphenol A, 4-n-octylphenol, 4-n-nolylphenol, diethylstilbestrol, and 17 alpha-ethynylestradiol) by zebrafish cytosolic sulfotransferases (STs). Of the seven enzymes tested, four zebrafish STs (designated ZF ST #2, ZF ST #3, ZF ST #4, and ZF DHEA ST) exhibited differential sulfonating activities toward the five environmental estrogens tested, with ZF ST #3 being more highly active than the other three. It was further demonstrated that bisphenol A, 4-n-octylphenol, and 4-n-nonylphenol exerted concentration-dependent inhibition of the sulfonation of 17 beta-estradiol, implying a potential role of these environmental estrogens in interfering with the sulfonation, and possibly homeostasis, of endogenous estrogens. Kinetic studies revealed that the mechanism underlying the inhibition by bisphenol A or 4-n-nonylphenol to be of the competitive type.

Molecular and functional characterisation of the zebrafish (Danio rerio) PEPT1-type peptide transporter

We report the molecular and functional characterisation of a novel peptide transporter from zebrafish, orthologue to mammalian and avian PEPT1. Zebrafish PEPT1 is a low-affinity/high-capacity system. However, in contrast to higher vertebrate counterparts in which maximal transport activity is independent of extracellular pH, zebrafish PEPT1 maximal transport rates unexpectedly increase at alkaline extracellular pH. Zebrafish pept1 is highly expressed in the proximal intestine since day 4 post-fertilisation, thus preceding functional maturation of the gut, first feeding and complete yolk resorption. Zebrafish PEPT1 might help to understand the evolutionary and functional relationships among vertebrate peptide transporters. Moreover, zebrafish pept1 can be a useful marker for screening mutations that affect gut regionalisation, differentiation and morphogenesis.

Sulfation of hydroxychlorobiphenyls. Molecular cloning, expression, and functional characterization of zebrafish SULT1 sulfotransferases

As a first step toward developing a zebrafish model for investigating the role of sulfation in counteracting environmental estrogenic chemicals, we have embarked on the identification and characterization of cytosolic sulfotransferases (STs) in zebrafish. By searching the zebrafish expressed sequence tag database, we have identified two cDNA clones encoding putative cytosolic STs. These two zebrafish ST cDNAs were isolated and subjected to nucleotide sequencing. Sequence data revealed that the two zebrafish STs are highly homologous, being approximately 82% identical in their amino acid sequences. Both of them display approximately 50% amino acid sequence identity to human SULT1A1, rat SULT1A1, and mouse SULT1C1 ST. These two zebrafish STs therefore appear to belong to the SULT1 cytosolic ST gene family. Recombinant zebrafish STs (designated SULT1 STs 1 and 2), expressed using the pGEX-2TK prokaryotic expression system and purified from transformed Escherichia coli cells, migrated as approximately 35 kDa proteins on SDS/PAGE. Purified zebrafish SULT1 STs 1 and 2 displayed differential sulfating activities toward a number of endogenous compounds and xenobiotics including hydroxychlorobiphenyls. Kinetic constants of the two enzymes toward two representative hydroxychlorobiphenyls, 3-chloro-4-biphenylol and 3,3',5,5'-tetrachloro-4,4'-biphenyldiol, and 3,3',5-triiodo-l-thyronine were determined. A thermostability experiment revealed the two enzymes to be relatively stable over the range 20-43 degrees C. Among 10 different divalent metal cations tested, Co2+, Zn2+, Cd2+, and Pb2+ exhibited considerable inhibitory effects, while Hg2+ and Cu2+ rendered both enzymes virtually inactive.

cDNA cloning, expression, and functional characterization of a zebrafish SULT1 cytosolic sulfotransferase

Using the reverse transcriptase-polymerase chain reaction technique, a full-length cDNA encoding a novel zebrafish sulfotransferase was cloned and sequenced. Sequence analysis indicated that this zebrafish sulfotransferase belongs to the SULT1 cytosolic sulfotransferase gene family. The recombinant form of the zebrafish sulfotransferase, purified from Escherichia coli cells, displayed sulfating activities toward a number of endogenous compounds, in particular dopamine and thyroid hormones, in addition to xenobiotics including some flavonoids, isoflavonoids, and other phenolic compounds. The zebrafish sulfotransferase exhibited substrate dependence in pH optimum. In comparison with those determined with dopamine as substrate, the zebrafish sulfotransferase displayed much lower K(m) and higher V(max) with n-propyl gallate as substrate. A thermostability experiment revealed the enzyme to be relatively stable over a temperature range between 20 and 43 degrees C. Among 10 divalent metal cations tested, Hg(2+), Co(2+), Zn(2+), Cd(2+), Cu(2+), and Pb(2+) exhibited dramatic inhibitory effects on the activity of the zebrafish sulfotransferase.

Molecular structure and developmental expression of three muscle-type troponin T genes in zebrafish

Troponin T (Tnnt), a troponin component, interacts with tropomyosin and is crucial to the regulation of striated muscle contraction. To gain insight into the molecular evolution and developmental regulation of Tnnt gene (Tnnt) in lower vertebrates, zebrafish Tnnt1 (slow Tnnt), Tnnt2 (cardiac Tnnt), and Tnnt3b (fast Tnnt isoform b) were characterized. The polypeptides of zebrafish Tnnt1, Tnnt2, and Tnnt3b were conserved in the central tropomyosin- and C-terminal troponin I-binding domains. However, the N-terminal hypervariable regions were highly extended and rich in glutamic acid in polypeptides of Tnnt1 and Tnnt2, but not Tnnt3b. The Tnnt2 and Tnnt3b contain introns, whereas Tnnt1 is intron-free. During development, large to small, alternatively spliced variants were detected in Tnnt2, but not in Tnnt1 or Tnnt3. Whole-mount in situ hybridization showed zebrafish Tnnt1 and Tnnt2 are activated during early somitogenesis (10 hr postfertilization, hpf) and cardiogenesis (14 hpf), respectively, but Tnnt3b is not activated until middle somitogenesis (18 hpf). Tnnt2 and Tnnt3b expression was cardiac- and fast-muscle specific, but Tnnt1 was expressed in both slow and fast muscles. We propose that three, distinct, muscle-type Tnnt evolved after the divergence of fish and deuterostome invertebrates. In zebrafish, the developmental regulation of Tnnt during somitogenesis and cardiogenesis is more restricted and simpler than in tetrapods. These new findings may provide insight into the developmental regulation and molecular evolution of vertebrate Tnnt.

Parallel early development of zebrafish interrenal glands and pronephros: differential control by wt1 and ff1b

Steroids are synthesized mainly from the adrenal cortex. Adrenal deficiencies are often associated with problems related to its development, which is not fully understood. To better understand adrenocortical development, we studied zebrafish because of the ease of embryo manipulation. The adrenocortical equivalent in zebrafish is called the interrenal, because it is embedded in the kidney. We find that interrenal development parallels that of the embryonic kidney (pronephros). Primordial interrenal cells first appear as bilateral intermediate mesoderm expressing ff1b in a region ventral to the third somite. These cells then migrate toward the axial midline and fuse together. The pronephric primordia are wt1-expressing cells located next to the interrenal. They also migrate to the axial midline and fuse to become glomeruli at later developmental stages. Our gene knockdown experiments indicate that wt1 is required for its initial restricted expression in pronephric primordia, pronephric cell migration and fusion. wt1 also appears to be involved in interrenal development and ff1b expression. Similarly, ff1b is required for interrenal differentiation and activation of the differentiated gene, cyp11a1. Our results show that the zebrafish interrenal and pronephros are situated close together and go through parallel developmental processes but are governed by different signaling events.

Molecular cloning, expression, and functional characterization of a novel zebrafish cytosolic sulfotransferase

By searching the zebrafish expressed sequence tag (EST) database, we have identified a cDNA clone encoding a putative zebrafish cytosolic sulfotransferase (ST). This cDNA was isolated and subjected to nucleotide sequencing. Analysis of the sequence data revealed that this novel zebrafish ST displays 32-35% amino acid sequence identity to members of all major cytosolic ST gene families. Therefore, this zebrafish ST, while belonging to the cytosolic ST gene superfamily, appears to be independent from all known constituent ST gene families. Recombinant zebrafish ST, expressed using the pET23c prokaryotic expression vector and purified from transformed Escherichia coli cells, migrated as a 34-kDa protein upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Purified zebrafish ST displayed sulfating activities toward dopamine and thyroid hormones (T(3) and T(4)), with a pH optimum spanning 7-9. The enzyme also exhibited activities toward a number of xenobiotics including some flavonoids, isoflavonoids, and other phenolic compounds. A thermostability experiment revealed the enzyme to be relatively stable over a temperature range between 20 and 48 degrees C. Among 10 divalent metal cations tested, Fe(++), Hg(++), Co(++), Zn(++), Cu(++), and Cd(++) exhibited dramatic inhibitory effects on the activity of the enzyme. These results constitute a first study on the cloning, expression, and characterization of a zebrafish cytosolic ST.

The state of the art of the zebrafish model for toxicology and toxicologic pathology research--advantages and current limitations

The zebrafish (Danio rerio) is now the pre-eminent vertebrate model system for clarification of the roles of specific genes and signaling pathways in development. The zebrafish genome will be completely sequenced within the next 1-2 years. Together with the substantial historical database regarding basic developmental biology, toxicology, and gene transfer, the rich foundation of molecular genetic and genomic data makes zebrafish a powerful model system for clarifying mechanisms in toxicity. In contrast to the highly advanced knowledge base on molecular developmental genetics in zebrafish, our database regarding infectious and noninfectious diseases and pathologic lesions in zebrafish lags far behind the information available on most other domestic mammalian and avian species, particularly rodents. Currently, minimal data are available regarding spontaneous neoplasm rates or spontaneous aging lesions in any of the commonly used wild-type or mutant lines of zebrafish. Therefore, to fully utilize the potential of zebrafish as an animal model for understanding human development, disease, and toxicology we must greatly advance our knowledge on zebrafish diseases and pathology.

Voltage-gated Ca2+ channel Ca(V)1.3 subunit expressed in the hair cell epithelium of the sacculus of the trout Oncorhynchus mykiss: cloning and comparison across vertebrate classes

Full-length sequence (>6.5 kb) has been determined for the Ca(V)1.3 pore-forming subunit of the voltage-gated Ca(2+) channel from the saccular hair cells of the rainbow trout (Oncorhynchus mykiss). Primary structure was obtained from overlapping PCR and cloned fragments, amplified by primers based on teleost, avian, and mammalian sources. Trout saccular Ca(V)1.3 was localized to hair cells, as evidenced by its isolation from an epithelial layer in which the hair cell is the only intact cell type. The predicted amino acid sequence of the trout hair cell Ca(V)1.3 is approximately 70% identical to the sequences of avian and mammalian Ca(V)1.3 subunits and shows L-type characteristics. The trout hair cell Ca(V)1.3 expresses a 26-aa insert in the I-II cytoplasmic loop (exon 9a) and a 10-aa insert in the IVS2-IVS3 cytoplasmic loop (exon 30a), neither of which is appreciably represented in trout brain. The exon 9a insert also occurs in hair cell organs of chick and rat, and appears as an exon in human genomic Ca(V)1.3 sequence (but not in the Ca(V)1.3 coding sequence expressed in human brain or pancreas). The exon 30a insert, although expressed in hair cells of chick as well as trout, does not appear in comparable rat or human tissues. Further, the IIIS2 region shows a splice choice (exon 22a) that is associated with the hair cell organs of trout, chick, and rat, but is not found in human genomic sequence. The elucidation of the primary structure of the voltage-gated Ca(2+) channel Ca(V)1.3 subunit from hair cells of the teleost, representing the lowest of the vertebrate classes, suggests a generality of sensory mechanism for Ca(V)1.3 across hair cell systems. In particular, the exon 9a insert of this channel appears to be the molecular feature most consistently associated with hair cells from fish to mammal, consonant with the hypothesis that the latter region may be a signature for the hair cell.

Defining the molecular and cellular basis of toxicity using comparative models

A critical element of any experimental design is the selection of the model that will be used to test the hypothesis. As Claude Bernard proposed over 100 years ago "the solution of a physiological or pathological problem often depends solely on the appropriate choice of the animal for the experiment so as to make the result clear and searching." Likewise, the Danish physiologist August Krogh in 1929 wrote that "For a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied." This scientific principle has been validated repeatedly in the intervening years as investigators have described unique models that exploit natural differences in chemical and molecular structure, biochemical function, or physiological response between different cells, tissues, and organisms to address specific hypotheses. Despite the power of this comparative approach, investigators have generally been reluctant to utilize nonmammalian or nonclassical experimental models to address questions of human biology. The perception has been that studies in relatively simple or evolutionarily ancient organisms would provide little insight into "complex" human biology. This perception, although always somewhat misguided, is now even less tenable given the results of the genome sequencing projects, which demonstrate that the human genome is remarkably similar to that of evolutionarily ancient organisms. Thus, the various life forms on Earth share much more in common then anyone had previously envisioned. This realization provides additional rationale for the use of nonclassical experimental models and provides perhaps the strongest validation of Bernard's and Krogh's assertions. This overview emphasizes some of the special attributes of alternative animal models that may be exploited to define the molecular and cellular basis of toxicity. For each attribute, selected examples of animal models and experimental approaches are presented. It focuses on the areas of neurotoxicology, reproductive and developmental toxicology, organ systems toxicology, carcinogenesis, and functional genomics/toxicogenomics and highlights the use of fish, avian, Drosophila, Caenorhabditis elegans, and yeast models in such studies.