Zebrafish: bridging the gap between development and disease

Differential stability of lead sulfide nanoparticles influences biological responses in embryonic zebrafish

As the number of nanoparticle-based products increase in the marketplace, there will be increased potential for human exposures to these engineered materials throughout the product life cycle. We currently lack sufficient data to understand or predict the inherent nanomaterial characteristics that drive nanomaterial-biological interactions and responses. In this study, we utilized the embryonic zebrafish (Danio rerio) model to investigate the importance of nanoparticle (NP) surface functionalization, in particular as it pertains to nanoparticle stability, on in vivo biological responses. This is a comparative study where two lead sulfide nanoparticles (PbS-NPs) with nearly identical core sizes, but functionalized with either sodium 3-mercaptopropanesulfonate (MT) or sodium 2,3-dimercaptopropanesulfonate (DT) ligand, were used. Developmental exposures and assessments revealed differential biological responses to these engineered nanoparticles. Exposures beginning at 6 h post fertilization (hpf) to MT-functionalized nanoparticles (PbS-MT) led to 100% mortality by 120 hpf while exposure to DT-functionalized nanoparticles (PbS-DT) produced less than a 5% incident in mortality at the same concentration. Exposure to the MT and DT ligands themselves did not produce adverse developmental effects when not coupled to the NP core. Following exposure, we confirmed that the embryos took up both PbS-MT and PbS-DT material using inductively coupled plasma-mass spectrometry (ICP-MS). The stability of the nanoparticles in the aqueous solution was also characterized. The nanoparticles decompose and precipitate upon exposure to air. Soluble lead ions were observed following nanoparticle precipitation and in greater concentration for the PbS-MT sample compared to the PbS-DT sample. These studies demonstrate that in vivo assessments can be effectively used to characterize the role of NP surface functionalization in predicting biological responses.

Zebrafish as a model for long QT syndrome: the evidence and the means of manipulating zebrafish gene expression

Congenital long QT syndrome (LQT) is a group of cardiac disorders associated with the dysfunction of cardiac ion channels. It is characterized by prolongation of the QT-interval, episodes of syncope and even sudden death. Individuals may remain asymptomatic for most of their lives while others present with severe symptoms. This heterogeneity in phenotype makes diagnosis difficult with a greater emphasis on more targeted therapy. As a means of understanding the molecular mechanisms underlying LQT syndrome, evaluating the effect of modifier genes on disease severity as well as to test new therapies, the development of model systems remains an important research tool. Mice have predominantly been the animal model of choice for cardiac arrhythmia research, but there have been varying degrees of success in recapitulating the human symptoms; the mouse cardiac action potential (AP) and surface electrocardiograms exhibit major differences from those of the human heart. Against this background, the zebrafish is an emerging vertebrate disease modelling species that offers advantages in analysing LQT syndrome, not least because its cardiac AP much more closely resembles that of the human. This article highlights the use and potential of this species in LQT syndrome modelling, and as a platform for the in vivo assessment of putative disease-causing mutations in LQT genes, and of therapeutic interventions.

Systematics of the subfamily Danioninae (Teleostei: Cypriniformes: Cyprinidae)

The members of the cyprinid subfamily Danioninae form a diverse and scientifically important group of fishes, which includes the zebrafish, Danio rerio. The diversity of this assemblage has attracted much scientific interest but its monophyly and the relationships among its members are poorly understood. The phylogenetic relationships of the Danioninae are examined herein using sequence data from mitochondrial cytochrome b, mitochondrial cytochrome c oxidase I, nuclear opsin, and nuclear recombination activating gene 1. A combined data matrix of 4117 bp for 270 taxa was compiled and analyzed. The resulting topology supports some conclusions drawn by recent studies on the group and certain portions of the traditional classification, but our results also contradict key aspects of the traditional classification. The subfamily Danioninae is not monophyletic, with putative members scattered throughout Cyprinidae. Therefore, we restrict Danioninae to the monophyletic group that includes the following genera: Amblypharyngodon, Barilius, Cabdio, Chela, Chelaethiops, Danio, Danionella, Devario (including Inlecypris), Esomus, Horadandia, Laubuca, Leptocypris, Luciosoma, Malayochela, Microdevario, Microrasbora, Nematabramis, Neobola, Opsaridium, Opsarius, Paedocypris, Pectenocypris, Raiamas, Rasbora (including Boraras and Trigonostigma), Rasboroides, Salmostoma, Securicula, and Sundadanio. This Danioninae sensu stricto is divided into three major lineages, the tribes Chedrini, Danionini, and Rasborini, where Chedrini is sister to a Danionini-Rasborini clade. Each of these tribes is monophyletic, following the restriction of Danioninae. The tribe Chedrini includes a clade of exclusively African species and contains several genera of uncertain monophyly (Opsarius, Raiamas, Salmostoma). Within the tribe Rasborini, the species-rich genus Rasbora is rendered non-monophyletic by the placement of two monophyletic genera, Boraras and Trigonostigma, hence we synonymize those two genera with Rasbora. In the tribe Danionini, the miniature genus Danionella is recovered as the sister group of Danio, with D. nigrofasciatus sister to D. rerio.

Long and winding roads: testis differentiation in zebrafish

Zebrafish sex determination, gonad differentiation and reproduction are far from being fully understood. Although the mode of sex determination is still being disputed, most experimental data point towards the lack of sex chromosomes and a multigenic sex determination system. Secondary effects from the environment and/or (xeno)hormones may influence the process, resulting in biased sex ratios. The exact time point of sex determination is unknown. Gonad differentiation involves a compulsory 'juvenile ovary' stage with subsequent transformation of the gonad into a testis in males. As the latter is a late event, there is a delay between sex determination and testis differentiation in zebrafish, in contrast to mammals. Information on the expression of several candidate genes thought to be involved in these processes has been supplemented with data from large-scale gonadal transcriptomic studies. New approaches and methodologies provide hope that answers to a number of important questions will be deciphered in the future.

Characterization of vascular mural cells during zebrafish development

Development and maturation of the nascent cardiovascular system requires the recruitment of mural cells (MCs) around the vascular tree in a process called vascular myogenesis. Understanding the origin and development of vascular MCs has been hampered by difficulties in observing these cells in vivo and performing defined genetic and experimental manipulations in available model organisms. Here, we investigate the origin of vascular MCs using molecular and genetic tools in zebrafish. We show that vascular MCs are present around the lateral dorsal aortae (LDA) and anterior mesenteric arteries (AMA) of developing animals, and that they also contribute to the outflow tract of the developing heart and ventral aorta (VA). Genetic data indicate that the vascular MCs of the LDA and AMA do not arise from blood or endothelial progenitors but from other derivatives of the lateral plate mesoderm. We further show that zebrafish vascular MCs share many of the morphological, molecular and functional characteristics of vascular smooth muscle cells and pericytes found in higher vertebrates. These data establish the zebrafish as a useful cellular and genetic model to study vascular myogenesis as well as tumor angiogenesis and other MC-associated diseases.

Recommendations for control of pathogens and infectious diseases in fish research facilities

Concerns about infectious diseases in fish used for research have risen along with the dramatic increase in the use of fish as models in biomedical research. In addition to acute diseases causing severe morbidity and mortality, underlying chronic conditions that cause low-grade or subclinical infections may confound research results. Here we present recommendations and strategies to avoid or minimize the impacts of infectious agents in fishes maintained in the research setting. There are distinct differences in strategies for control of pathogens in fish used for research compared to fishes reared as pets or in aquaculture. Also, much can be learned from strategies and protocols for control of diseases in rodents used in research, but there are differences. This is due, in part, the unique aquatic environment that is modified by the source and quality of the water provided and the design of facilities. The process of control of pathogens and infectious diseases in fish research facilities is relatively new, and will be an evolving process over time. Nevertheless, the goal of documenting, detecting, and excluding pathogens in fish is just as important as in mammalian research models.

Antipsychotic drugs inhibit nucleotide hydrolysis in zebrafish (Danio rerio) brain membranes

Haloperidol (HAL), olanzapine (OLZ), and sulpiride (SULP) are antipsychotic drugs widely used in the pharmacotherapy of psychopathological symptoms observed in schizophrenia or mood-related psychotic symptoms in affective disorders. Here, we tested the in vitro effects of different concentrations of a typical (HAL) and two atypical (OLZ and SULP) antipsychotic drugs on ectonucleotidase activities from zebrafish brain membranes. HAL inhibited ATP (28.9%) and ADP (26.5%) hydrolysis only at 250 microM. OLZ decreased ATPase activity at all concentrations tested (23.8-60.7%). SULP did not promote significant changes on ATP hydrolysis but inhibited ADP hydrolysis at 250 microM (25.6%). All drugs tested, HAL, OLZ, and SULP, did not promote any significant changes on 5'-nucleotidase activity in the brain membranes of zebrafish. These findings demonstrated that antipsychotic drugs could inhibit NTPDase activities whereas did not change 5'-nucleotidase. Such modulation can alter the adenosine levels, since the ectonucleotidase pathway is an important source of extracellular adenosine. Thus, it is possible to suggest that changes promoted by antipsychotic drugs in the bilayer membrane could alter the NTPDase activities, modulating extracellular ATP and adenosine levels.

Developmental gene regulatory networks in the zebrafish embryo

The genomic developmental program operates mainly through the regulated expression of genes encoding transcription factors and signaling pathways. Complex networks of regulatory genetic interactions control developmental cell specification and fates. Development in the zebrafish, Danio rerio, has been studied extensively and large amounts of experimental data, including information on spatial and temporal gene expression patterns, are available. A wide variety of maternal and zygotic regulatory factors and signaling pathways have been discovered in zebrafish, and these provide a useful starting point for reconstructing the gene regulatory networks (GRNs) underlying development. In this review, we describe in detail the genetic regulatory subcircuits responsible for dorsoanterior-ventroposterior patterning and endoderm formation. We describe a number of regulatory motifs, which appear to act as the functional building blocks of the GRNs. Different positive feedback loops drive the ventral and dorsal specification processes. Mutual exclusivity in dorsal-ventral polarity in zebrafish is governed by intra-cellular cross-inhibiting GRN motifs, including vent/dharma and tll1/chordin. The dorsal-ventral axis seems to be determined by competition between two maternally driven positive-feedback loops (one operating on Dharma, the other on Bmp). This is the first systematic approach aimed at developing an integrated model of the GRNs underlying zebrafish development. Comparison of GRNs' organizational motifs between different species will provide insights into developmental specification and its evolution. The online version of the zebrafish GRNs can be found at http://www.zebrafishGRNs.org.

AHR-dependent misregulation of Wnt signaling disrupts tissue regeneration

The origins of molecular toxicology can be traced to understanding the interactions between halogenated aromatic hydrocarbons and the aryl hydrocarbon receptor (AHR). The physiological consequences of activation of the aryl hydrocarbon receptor are diverse, and we are just beginning to understand the importance of the AHR signal transduction pathway in homeostasis and disease. The many downstream targets that mediate these biological responses remain undefined. Studies have exploited the power of the zebrafish model to elucidate the mechanisms by which AHR activation disrupts biological signaling. Recent genomic analysis performed in a zebrafish tissue regeneration model revealed functional cross talk between AHR and the well-established Wnt/beta-catenin signal transduction pathway. This review focuses on the development of the zebrafish model of AHR biology and the application of in vivo toxicogenomics to unravel molecular mechanisms.

Analysis of ethanol developmental toxicity in zebrafish

It is largely accepted that vertebrates are more susceptible to chemical insult during the early life stage. It is implied that if a chemical such as ethanol is developmentally toxic, it must interfere with, or modulate, critical signaling pathways. The probable molecular explanation for increased embryonic susceptibility is that collectively there is no other period of an animal's lifespan when the full repertoire of molecular signaling is active. Understanding the mechanism by which ethanol exposure disrupts vertebrate embryonic development is enormously challenging; it requires a thorough understanding of the normal molecular program to understand how transient ethanol exposure disrupts signaling and results in detrimental long-lasting effects. During the past several years, investigators have recognized the advantages of the zebrafish model to discover the signaling events that choreograph embryonic development. External development coupled with the numerous molecular and genetic methods make this model a valuable tool to unravel the mechanisms by which ethanol disrupts embryonic development. In this chapter we describe procedures used to evaluate and define the morphological, cellular and molecular responses to ethanol in zebrafish.

Transcriptomic analyses reveal novel genes with sexually dimorphic expression in the zebrafish gonad and brain

BACKGROUND

Our knowledge on zebrafish reproduction is very limited. We generated a gonad-derived cDNA microarray from zebrafish and used it to analyze large-scale gene expression profiles in adult gonads and other organs.

METHODOLOGY/PRINCIPAL FINDINGS

We have identified 116638 gonad-derived zebrafish expressed sequence tags (ESTs), 21% of which were isolated in our lab. Following in silico normalization, we constructed a gonad-derived microarray comprising 6370 unique, full-length cDNAs from differentiating and adult gonads. Labeled targets from adult gonad, brain, kidney and 'rest-of-body' from both sexes were hybridized onto the microarray. Our analyses revealed 1366, 881 and 656 differentially expressed transcripts (34.7% novel) that showed highest expression in ovary, testis and both gonads respectively. Hierarchical clustering showed correlation of the two gonadal transcriptomes and their similarities to those of the brains. In addition, we have identified 276 genes showing sexually dimorphic expression both between the brains and between the gonads. By in situ hybridization, we showed that the gonadal transcripts with the strongest array signal intensities were germline-expressed. We found that five members of the GTP-binding septin gene family, from which only one member (septin 4) has previously been implicated in reproduction in mice, were all strongly expressed in the gonads.

CONCLUSIONS/SIGNIFICANCE

We have generated a gonad-derived zebrafish cDNA microarray and demonstrated its usefulness in identifying genes with sexually dimorphic co-expression in both the gonads and the brains. We have also provided the first evidence of large-scale differential gene expression between female and male brains of a teleost. Our microarray would be useful for studying gonad development, differentiation and function not only in zebrafish but also in related teleosts via cross-species hybridizations. Since several genes have been shown to play similar roles in gonadogenesis in zebrafish and other vertebrates, our array may even provide information on genetic disorders affecting gonadal phenotypes and fertility in mammals.

Development of a marine fish model for studying in vivo molecular responses in ecotoxicology

A protocol for fixation and processing of whole adult marine medaka (Oryzias melastigma) was developed in parallel with in situ hybridization (ISH) and immunohistochemistry (IHC) for molecular analysis of in vivo gene and protein responses in fish. Over 200 serial sagittal sections (5microm) can be produced from a single adult medaka to facilitate simultaneous localization and quantification of gene-specific mRNAs and proteins in different tissues and subcellular compartments of a single fish. Stereological analysis (as measured by volume density, V(v)) was used to quantify ISH and IHC signals on tissue sections. Using the telomerase reverse transcriptase (omTERT) gene, omTERT and proliferating cell nuclear antigen (PCNA) proteins as examples, we demonstrated that it is possible to localize, quantify and correlate their tissue expression profiles in a whole fish system. Using chronic hypoxia (1.8+/-0.2 mgO(2)L(-1) for 3 months) as an environmental stressor, we were able to identify significant alterations in levels of omTERT mRNA, omTERT protein, PCNA (cell proliferation marker) and TUNEL (apoptosis) in livers of hypoxic O. melastigma (p<0.05). Overall, the results suggest that O. melastigma can serve as a model marine fish for assessing multiple in vivo molecular responses to stresses in the marine environment.

Induction of hyperglycaemia in zebrafish (Danio rerio) leads to morphological changes in the retina

Diabetes affects over 16 million Americans yearly, resulting in hyperglycaemia and microvascular complications, including retinopathy, neuropathy and nephropathy. Animal models have been developed to examine the immunological aspects of type 1 diabetes and the pathogenic mechanisms associated with diabetic retinopathy, but the methods of diabetes induction raise concerns regarding these models. Zebrafish (Danio rerio) have been used extensively to study developmental processes and mutant zebrafish strains have been used to examine vision disease present in humans. In this paper, we have induced hyperglycaemia in zebrafish by alternately immersing the fish in glucose solution or water. Eyes from untreated fish or fish exposed to alternating glucose/water solutions for 28 days were dissected, sectioned and stained to visualise cell bodies in the retina. In untreated fish retinas, the inner plexiform layer (IPL) and inner nuclear layer (INL) were approximately the same thickness, whereas in fish repeatedly exposed to glucose solutions the IPL was approximately 55% the thickness of the INL. Both the IPL and INL were significantly reduced in retinas of treated fish, compared to untreated fish, similar to that seen in other animal models of diabetes and in diabetic patients. These results suggest that zebrafish may be used as an animal model in which to study diabetic retinopathy.

Kit-like immunoreactivity in the zebrafish gastrointestinal tract reveals putative ICC

Gastrointestinal (GI) motility results from the coordinated actions of enteric neurons, interstitial cells of Cajal (ICC), and smooth muscle cells. The GI tract of the zebrafish has a cellular anatomy that is essentially similar to humans. Although enteric nerves and smooth muscle cells have been described, it is unknown if ICC are present in the zebrafish. Immunohistochemistry and PCR were used determine expression for the zebrafish Kit orthologue in the zebrafish gastrointestinal tract. Cells displaying Kit-like immunoreactivity were identified in the muscular layers of the adult zebrafish gastrointestinal tract. Two layers of Kit-positive cells were identified, one with multipolar cells located between the longitudinal and circular smooth muscle layers and one with simple bipolar cells located deep in the circular muscle layer. Primers specifically designed to amplify mRNA coding for two zebrafish kit genes, kita and kitb, and two kit ligands, kitla and kitlb, amplified the expected transcript from total RNA isolated from zebrafish GI tissues. The Sparse mutant, a kita null mutant, showed reduced contraction frequency and increased size of the GI tract indicating a functional role for kita. These data establish the presence of a cellular network with Kit-like immunoreactivity in the myenteric plexus region of the zebrafish GI tract, adjacent to enteric neurons. Expression of kita and kitb, and the ligands kitla and kitlb, were verified in the adult GI tract. The anatomical arrangement of the Kit-positive cells strongly suggests that they are ICC.

In vivo evaluation of carbon fullerene toxicity using embryonic zebrafish

There is a pressing need to develop rapid whole animal-based testing assays to assess the potential toxicity of engineered nanomaterials. To meet this challenge, the embryonic zebrafish model was employed to determine the toxicity of fullerenes. Embryonic zebrafish were exposed to graded concentrations of fullerenes [C(60), C(70), and C(60)(OH)(24)] during early embryogenesis and the resulting morphological and cellular responses were defined. Exposure to 200 μg/L C(60) and C(70) induced a significant increased in malformations, pericardial edema, and mortality; while the response to C(60)(OH)(24) exposure was less pronounced at concentrations an order of magnitude higher. Exposure to C(60) induced both necrotic and apoptotic cellular death throughout the embryo. While C(60)(OH)(24) induced an increase in embryonic cellular death, it did not induce apoptosis. Our findings concur with results obtained in other models indicating that C(60)(OH)(24) is significantly less toxic than C(60). These studies also suggest that that the embryonic zebrafish model is well-suited for the rapid assessment of nanomaterial toxicity.

Vascular anatomy of the developing medaka, Oryzias latipes: a complementary fish model for cardiovascular research on vertebrates

The zebrafish has become a very useful vertebrate model for cardiovascular research, but detailed morphogenetic studies have revealed that it differs from mammals in certain aspects of the primary circulatory system, in particular, the early vitelline circulation. We searched for another teleost species that might serve as a complementary model for the formation of these early primary vessels. Here (and online at http://www.shigen.nig.ac.jp/medaka/atlas/), we present a detailed characterization of the vascular anatomy of the developing medaka embryo from the stage 24 (1 day 20 hr) through stage 30 (3 days 10 hr). Three-dimensional images using confocal microangiography show that the medaka, Oryzias latipes, follows the common embryonic circulatory pattern consisting of ventral aorta, aortic arches, dorsal aorta, transverse vessels, vitelline capillary plexus, and marginal veins. The medaka, thus, may serve as a valuable model system for genetic analysis of the primary vasculature of vertebrates.

Recapitulation of human betaB1-crystallin promoter activity in transgenic zebrafish

Development of the eye is morphologically similar among vertebrates, indicating that the underlying mechanism regulating the process may have been highly conserved during evolution. Herein we analyzed the promoter of the human betaB1-crytallin gene in zebrafish by transgenic experiments. To delineate the evolutionarily conserved regulatory elements, we performed serial deletion assays in the promoter region. The results demonstrated that the -90/+61-bp upstream proximal promoter region is sufficient to confer lens-tissue specificity to the human betaB1-crystallin gene in transgenic zebrafish. Through phylogenetic sequence comparisons and an electrophoretic mobility shift assay (EMSA), a highly conserved cis-element of a six-base pair sequence TG(A/C)TGA, the consensus sequence for the Maf protein binding site, within the proximal promoter region was revealed. Further, a site-mutational assay showed that this element is crucial for promoter activity. These data suggest that the fundamental transcriptional regulatory mechanism of the betaB1-crystallin gene has been well conserved between humans and zebrafish, and plausibly among all vertebrates, during evolution.

Chemical Genetics: Drug Screens in Zebrafish

High throughput chemical genetic screens for compounds with specific biological activity in a whole organism are feasible using zebrafish embryos. At least two medium to large scale drug screens have been carried out to date, leading to the identification of compounds that disturb zebrafish development. Chemical genetics using zebrafish embryos may become an important step in the discovery of drugs and their targets.

In vitro and other alternative approaches to developmental neurotoxicity testing (DNT)

To address the growing need for scientifically valid and humane alternatives to developmental neurotoxicity testing (DNT), we propose that basic research scientists in developmental neurobiology be brought together with mechanistic toxicologists and policy analysts to develop the science and policy for DNT alternatives that are based on evolutionarily conserved mechanisms of neurodevelopment. In this article we briefly review in vitro and other alternative models and present our rationale for proposing that resources be focused on adapting alternative simple organism systems for DNT. We recognize that alternatives to DNT will not completely replace a DNT paradigm that involves in vivo testing in mammals. However, we believe that alternatives will be of great value in prioritizing chemicals and in identifying mechanisms of developmental neurotoxicity, which in turn will be useful in refining and reducing in vivo mammalian tests for exposures most likely to be hazardous to the developing human nervous system.

Functional genomics tools for the analysis of zebrafish pigment

Genetic model organisms are increasingly valuable in the post-genomics era to provide a basis for comparative analysis of the human genome. For higher order processes of vertebrate pigment cell biology and development, the mouse has historically been the model of choice. A complementary organism, the zebrafish (Danio rerio), shares many of the signaling and biological processes of vertebrates, e.g. neural crest development. The zebrafish has a number of characteristics that make it an especially valuable model for the study of pigment cell biology and disease. Large-scale genetic screens have identified a collection of pigmentation mutants that have already made valuable contributions to pigment research. An increasing repertoire of genomic resources such as an expressed sequence tag-based Gene Index (The Institute for Genomic Research) and improving methods of mutagenesis, transgenesis, and gene targeting make zebrafish a particularly attractive model. Morpholino phosphorodiamidate oligonucleotide (MO) 'knockdown' of pigment gene expression provides a non-conventional antisense tool for the analysis of genes involved in pigment cell biology and disease. In addition, an ongoing, reverse-genetic, MO-based screen for the rapid identification of gene function promises to be a valuable complement to other high-throughput microarray and proteomic approaches for understanding pigment cell biology. Novel reagents for zebrafish transgenesis, such as the Sleeping Beauty transposon system, continue to improve the capacity for genetic analysis in this system and ensure that the zebrafish will be a valuable genetic model for understanding a variety of biological processes and human diseases for years to come.

Zebrafish as a Model Vertebrate for Investigating Chemical Toxicity

Zebrafish (Danio rerio) has been a prominent model vertebrate in a variety of biological disciplines. Substantial information gathered from developmental and genetic research, together with near-completion of the zebrafish genome project, has placed zebrafish in an attractive position for use as a toxicological model. Although still in its infancy, there is a clear potential for zebrafish to provide valuable new insights into chemical toxicity, drug discovery, and human disease using recent advances in forward and reverse genetic techniques coupled with large-scale, high-throughput screening. Here we present an overview of the rapidly increasing use of zebrafish in toxicology. Advantages of the zebrafish both in identifying endpoints of toxicity and in elucidating mechanisms of toxicity are highlighted.

Resolving mechanisms of toxicity while pursuing ecotoxicological relevance?

In this age of modern biology, aquatic toxicological research has pursued mechanisms of action of toxicants. This has provided potential tools for ecotoxicologic investigations. However, problems of biocomplexity and issues at higher levels of biological organization remain a challenge. In the 1980s and 1990s and continuing to a lesser extent today, organisms residing in highly contaminated field sites or exposed in the laboratory to calibrated concentrations of individual compounds were carefully analyzed for their responses to priority pollutants. Correlation of biochemical and structural analyses in cultured cells and tissues, as well as the in vivo exposures led to the production and application of biomarkers of exposure and effect and to our awareness of genotoxicity and its chronic manifestations, such as neoplasms, in wild fishes. To gain acceptance of these findings in the greater environmental toxicology community, "validation of the model" versus other, better-established often rodent models, was necessary and became a major focus. Resultant biomarkers were applied to heavily contaminated and reference field sites as part of effects assessment and with investigations following large-scale disasters such as oil spills or industrial accidents. Over the past 15 years, in the laboratory, small aquarium fish models such as medaka (Oryzias latipes), zebrafish (Danio rerio), platyfish (Xiphophorus species), fathead minnow (Pimephales promelas), and sheepshead minnow (Cyprinodon variegatus) were increasingly used establishing mechanisms of toxicants. Today, the same organisms provide reliable information at higher levels of biological organization relevant to ecotoxicology. We review studies resolving mechanisms of toxicity and discuss ways to address biocomplexity, mixtures of contaminants, and the need to relate individual level responses to populations and communities.

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.

Functional aging and gradual senescence in zebrafish

Zebrafish (Danio rerio) has been recognized as a powerful model for genetic studies in developmental biology. Recently, the zebrafish system also has given insights into several human diseases such as neurodegenerative, hematopoietic, and cardiovascular disease, and cancer. Because aging processes affect these and various other human disorders, it is important to compare zebrafish and mammalian senescence. However, the aging process of zebrafish remains largely unexplored, and little is known about functional aging and senescence in zebrafish. In our initial studies to assess aging phenotypes in zebrafish, we have identified several potential aging biomarkers in an ongoing search for suitable ones on zebrafish aging. In aging zebrafish, we detected senescence-associated beta-galactosidase activity in skin and oxidized protein accumulation in muscle. On the other hand, we did not observe lipofuscin granules (aging pigments), which accumulate in postmitotic cells, in muscle of zebrafish with advancing age. Consistently, there were continuously proliferating myocytes that incorporated BrdU in muscle tissues of the aged fish. Moreover, we demonstrated that zebrafish have constitutively abundant telomerase activity in adult somatic tissues implicating unlimited replicative ability of cells throughout their lives. Although some stress-associated markers are upregulated and minor histological changes are observed during the aging process of zebrafish, our studies together with other evidence of remarkable reproductive and regenerative abilities suggest that zebrafish show very gradual senescence. By using those biological and biochemical aging markers already characterized in normal zebrafish, transgenic fish analyses and genetic mutant fish screens can be readily performed. These efforts will help to elucidate the role and molecular mechanisms of common or different pathways of aging among vertebrates from fish to humans and also will contribute to the discovery of potential drugs applicable to age-associated diseases in the future.

Annual Fish as a Genetic Model for Aging

Advancement in the genetics of aging and identification of longevity genes has been largely due to the model organisms such as Caenorhabditis elegans and Drosophila melanogaster. However, knowledge gained from these invertebrates will not be able to identify vertebrate-specific longevity genes. The mouse has a relatively long life span of about 3 years, which limits its utility for screening of longevity genes. Fish have been used in aging studies. However, systematic comparison of survivorship curves for fish is lacking. In this study, we compared the survivorship curves of zebrafish and 2 different annual fish, namely, Cynolebias nigripinnis and Nothobranchius rachovii. These studies established that Nothobranchius rachovii has the shortest life span (8.5 months, at which time 10% of population remains). We also established that it is possible to breed Nothobranchius rachovii under laboratory conditions, and showed that their embryos can be stored for several months and hatched at any time by adding water. In addition, we have isolated 31 cDNA markers out of 71 attempted amplifications based on corresponding homologous genomic sequences in zebrafish and Fugu available from public databases, suggesting that approximately 40% of the genes from Nothobranchius rachovii could be easily isolated. Thus, the ability to be bred under laboratory conditions and the availability of cDNA markers for mapping, along with the major advantage of a relatively short life span, make Nothobranchius rachovii an attractive vertebrate genetic model for aging over other available vertebrate models.

Genetics in zebrafish, mice, and humans to dissect congenital heart disease: insights in the role of VEGF

Heart development and the establishment of a functional circulatory circuit are complex biological processes in which subtle perturbations may result in catastrophic consequences of cardiovascular birth defects. Studies in model organisms, most notably the mouse and the zebrafish, have identified genes that also cause these life-threatening defects when mutated in humans. Gradually, a framework for the genetic pathway controlling these events is now beginning to emerge. However, the puzzling phenotypic variability of the cardiovascular disease phenotype in humans and the recent identification of phenotypic modifiers using model organisms indicates that other genetic loci might interact to modify the disease phenotype. To illustrate this, we review the role of vascular endothelial growth factor (VEGF) during vascular and cardiac development and stress how zebrafish and mouse genetic studies have helped us to understand the role this growth factor has in human disease, in particular in the Di-George syndrome.

ATP and ADP hydrolysis in brain membranes of zebrafish (Danio rerio)

Nucleotides, e.g. ATP and ADP, are important signaling molecules, which elicit several biological responses. The degradation of nucleotides is catalyzed by a family of enzymes called NTPDases (nucleoside triphosphate diphosphohydrolases). The present study reports the enzymatic properties of a NTPDase (CD39, apyrase, ATP diphosphohydrolase) in brain membranes of zebrafish (Danio rerio). This enzyme was cation-dependent, with a maximal rate for ATP and ADP hydrolysis in a pH range of 7.5-8.0 in the presence of Ca(2+) (5 mM). The enzyme displayed a maximal activity for ATP and ADP hydrolysis at 37 degrees C. It was able to hydrolyze purine and pyrimidine nucleosides 5'-di and triphosphates, being insensitive to classical ATPase inhibitors, such as ouabain (1 mM), N-ethylmaleimide (0.1 mM), orthovanadate (0.1 mM) and sodium azide (0.1 mM). A significant inhibition of ATP and ADP hydrolysis (68% and 34%, respectively) was observed in the presence of 20 mM sodium azide, used as a possible inhibitor of ATP diphosphohydrolase. Levamisole (1 mM) and tetramisole (1 mM), specific inhibitors of alkaline phosphatase and P1, P(5)-di (adenosine 5'-) pentaphosphate, an inhibitor of adenylate kinase did not alter the enzyme activity. The presence of a NTPDase in brain membranes of zebrafish may be important for the modulation of nucleotide and nucleoside levels, controlling their actions on specific purinoceptors in central nervous system of this specie.

Knockdown of the survival motor neuron (Smn) protein in zebrafish causes defects in motor axon outgrowth and pathfinding

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by a loss of alpha motoneurons in the spinal cord. SMA is caused by low levels of the ubiquitously expressed survival motor neuron (Smn) protein. As it is unclear how low levels of Smn specifically affect motoneurons, we have modeled SMA in zebrafish, a vertebrate model organism with well-characterized motoneuron development. Using antisense morpholinos to reduce Smn levels throughout the entire embryo, we found motor axon-specific pathfinding defects. Reduction of Smn in individual motoneurons revealed that smn is acting cell autonomously. These results show for the first time, in vivo, that Smn functions in motor axon development and suggest that these early developmental defects may lead to subsequent motoneuron loss.

Fish can be first--advances in fish transgenesis for commercial applications

Over the past 15 years researchers have generated stable lines of several species of transgenic fish important for aquaculture. 'All-fish' growth hormone (GH) gene constructs and antifreeze protein (AFP) genes have been successfully introduced into the fish genome resulting in a significant acceleration of growth rate and an increase in cold and freeze tolerance. However, neither gene modification is completely understood; there are still questions to be resolved. Expression rates are still low, producing variable growth enhancement rates and less than desired levels of freeze resistance. Transgene strategies are also being developed to provide improved pathogen resistance and modified metabolism for better utilization of the diet. Additional challenges are to tailor the genetically modified fish strains to prevent release of the modified genes into the environment.

The zebrafish as a vertebrate model of functional aging and very gradual senescence

The zebrafish (Danio rerio) has been developed as a powerful model for genetic studies in developmental biology, which also gives insights into several diseases of adult humans such as cardiovascular disease and cancer. Because aging processes affect these and many other human diseases, it is important to compare zebrafish and other mammalian aging. However, the aging process of zebrafish remains largely unexplored, and little is known about its functional aging and senescence. In a survey of aging in zebrafish, we detected senescence-associated beta-galactosidase activity in skin and oxidized protein accumulation in muscle. However, we did not observe lipofuscin granules ('aging pigments'), which commonly accumulate in postmitotic cells of other vertebrates. This absence of lipofuscins may be consistent with the existence of continuously proliferating myocytes that incorporated BrdU in muscle tissues of aged zebrafish. Moreover, we demonstrated that zebrafish have constitutively abundant telomerase activity in somatic tissues from embryos to aged adults. Although some stress-associated markers are upregulated and minor histological changes are observed during the aging process of zebrafish, our studies together with other evidence of remarkable reproductive and regenerative abilities suggest that zebrafish show very gradual or sub-negligible senescence in vivo.

Application of comparative genomics in fish endocrinology

This review discusses the ways in which comparative genomics can contribute to the study of fish endocrinology. First, the phylogenetic position of fish and an overview of their specific endocrine systems are presented. The emphasis will be on teleosts because they are the most abundant fishes and because most data are available for this group. Second, the complexity of fish genomics is reviewed. With the vast array of genome sizes and ploidy levels, assignment of gene orthology is more difficult in fish, but this is an absolute prerequisite in functional analysis and it is important to be aware of such genome plasticity when cloning genes. The ease with which a gene is cloned at the genomic level is directly related to genome size and complexity, a factor that is not known in the majority of fish species. Finally, the methodology is presented along with specific examples of parathyroid hormone-related protein (PTHrP) (a previously unidentified hormone in fish), calcium-sensing receptor, and calcitonin (with a duplication of this particular ligand in Fugu rubripes). Preliminary data also suggest that there are further duplicated genes in the calcium regulatory system. Comparative genomics has provided a valuable approach for isolating and characterizing a range of fish genes involved in calcium regulation. However, for understanding the physiology and endocrine regulation of this system, particularly with regard to gene duplication, an alternative approach is required in which conventional endocrinology techniques will play a significant role.

Sarcoglycans of the zebrafish: orthology and localization to the sarcolemma and myosepta of muscle

The zebrafish is an established model of vertebrate development and is also receiving increasing attention in terms of human disease modelling. In order to provide experimental support to realize this modelling potential, we report here the identification of apparent orthologues of many critical members of the dystrophin-associated glycoprotein complex (DGC) that have been implicated in a diverse range of neuromuscular disorders. In addition, immunohistochemical studies show the localization of the DGC to the sarcolemma of adult zebrafish muscle and in particular the myosepta. Together, these data suggest that the DGC in adult zebrafish may play a highly conserved functional role in muscle architecture that, when disrupted, could offer insight into human neuromuscular disease processes.

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.

A call to fins! Zebrafish as a gerontological model

Among the wide variety of model organisms commonly used for studies on aging, such as worms, flies and rodents, a wide research gap exists between the invertebrate and vertebrate model systems. In developmental biology, a similar gap has been filled by the zebrafish (Danio rerio). We propose that the zebrafish is uniquely suited to serve as a bridge model for gerontology. With high fecundity and economical husbandry requirements, large populations of zebrafish may be generated quickly and cheaply, facilitating large-scale approaches including demographic studies and mutagenesis screens. A variety of mutants identified in such screens have led to modelling of human disease, including cardiac disorders and cancer. While zebrafish longevity is at least 50% longer than in commonly used mouse strains, as an ectothermic fish species, its life span may be readily modulated by caloric intake, ambient temperature and reproductive activity. These features, coupled with a growing abundance of biological resources, including an ongoing genome sequencing project, make the zebrafish a compelling model organism for studies on aging.

Mechanical properties of skeletal bone in gene-mutated stöpsel(dtl28d) and wild-type zebrafish (Danio rerio) measured by atomic force microscopy-based nanoindentation

An atomic force microscopy (AFM)-based nanoindenter was used to evaluate the mechanical properties of skeletal bones in wild-type and gene-mutated zebrafish (Danio rerio), stöpsel(dtl28d). Both skeletons were isolated from adult zebrafish and tested under a load of 5 mN. It was found that stp/stp bone has a similar nanohardness but significantly greater elastic modulus compared with that of wild-type bone. The residual indenter impressions using AFM and the fracture surfaces of both bones using scanning electron microscopy were examined and showed that the bone of zebrafish becomes more brittle after the stp mutation. This first observation of the alteration of bone mechanical behavior by gene mutation in zebrafish system is of scientific and clinical relevance to many areas of study, such as bone fracture and fragility mechanisms in human heritable disorders and bone-materials fabrication via gene engineering.

The zebrafish: a new model organism for integrative physiology

This brief review summarizes features of the zebrafish, Danio rerio, that make it a suitable model organism for studies of regulatory physiology. The review presents the argument that random mutagenesis screens are a valuable gene-finding strategy to identify genes of functional importance and that their utility, although well established for developmental issues, will extend to a variety of topics of interest to the regulatory physiologist. Particular attention is drawn to the range of functional responses amenable to mutagenesis screens in larval zebrafish. Other virtues of the organism, the range of genomic tools, the potential for innovative optical methods, and the tractability for genetic and other experimental manipulations, are also described. Finally, the review provides examples of functional studies in zebrafish, including studies in sensory neurons, cardiac rhythm disturbances, gastrointestinal function, and studies of the developing kidney, that illustrate potential applications. Because of the relative ease with which combinatorial studies can be performed, the zebrafish may eventually be particularly valuable in understanding the functional interaction between subtle gene defects that cause polygenic disorders.

Medaka--a model organism from the far East

Genome sequencing has yielded a plethora of new genes the function of which can be unravelled through comparative genomic approaches. Increasingly, developmental biologists are turning to fish as model genetic systems because they are amenable to studies of gene function. Zebrafish has already secured its place as a model vertebrate and now its Far Eastern cousin--medaka--is emerging as an important model fish, because of recent additions to the genetic toolkit available for this organism. Already, the popularity of medaka among developmental biologists has led to important insights into vertebrate development.

Comparative anatomy of the histaminergic and other aminergic systems in zebrafish (Danio rerio)

The histaminergic system and its relationships to the other aminergic transmitter systems in the brain of the zebrafish were studied by using confocal microscopy and immunohistochemistry on brain whole-mounts and sections. All monoaminergic systems displayed extensive, widespread fiber systems that innervated all major brain areas, often in a complementary manner. The ventrocaudal hypothalamus contained all monoamine neurons except noradrenaline cells. Histamine (HA), tyrosine hydroxylase (TH), and serotonin (5-HT) -containing neurons were all found around the posterior recess (PR) of the caudal hypothalamus. TH- and 5-HT-containing neurons were found in the periventricular cell layer of PR, whereas the HA-containing neurons were in the surrounding cell layer as a distinct boundary. Histaminergic neurons, which send widespread ascending and descending fibers, were all confined to the ventrocaudal hypothalamus. Histaminergic neurons were medium in size (approximately 12 microm) with varicose ascending and descending ipsilateral and contralateral fiber projections. Histamine was stored in vesicles in two types of neurons and fibers. A close relationship between HA fibers and serotonergic raphe neurons and noradrenergic locus coeruleus neurons was evident. Putative synaptic contacts were occasionally detected between HA and TH or 5-HT neurons. These results indicate that reciprocal contacts between monoaminergic systems are abundant and complex. The results also provide evidence of homologies to mammalian systems and allow identification of several previously uncharacterized systems in zebrafish mutants.

Dystrophin in adult zebrafish muscle

Mutations in the human dystrophin gene are implicated in the fatal muscle wasting disease Duchenne Muscular Dystrophy (DMD). This gene expresses a sarcolemmal-associated protein that is evolutionarily conserved, underpinning its important role in the architecture of muscle. In terms of DMD modelling, the mouse has served as a suitable vertebrate species but the pathophysiology of the disease in the mouse does not entirely mimic human DMD. We have examined the zebrafish in order to expand the repertoire of vertebrate species for muscle disease modelling, and to dissect further the functional interactions of dystrophin. We report here the identification of an apparent zebrafish orthologue of the human dystrophin gene that expresses a 400-kDa protein that is localised to the muscle membrane surface. These data suggest that the zebrafish may prove to be a beneficial vertebrate model to examine the role and functional interactions of dystrophin in disease and development.