Abrogation of Stem Loop Binding Protein (Slbp) function leads to a failure of cells to transition from proliferation to differentiation, retinal coloboma and midline axon guidance deficits

Through forward genetic screening for mutations affecting visual system development, we identified prominent coloboma and cell-autonomous retinal neuron differentiation, lamination and retinal axon projection defects in eisspalte (ele) mutant zebrafish. Additional axonal deficits were present, most notably at midline axon commissures. Genetic mapping and cloning of the ele mutation showed that the affected gene is slbp, which encodes a conserved RNA stem-loop binding protein involved in replication dependent histone mRNA metabolism. Cells throughout the central nervous system remained in the cell cycle in ele mutant embryos at stages when, and locations where, post-mitotic cells have differentiated in wild-type siblings. Indeed, RNAseq analysis showed down-regulation of many genes associated with neuronal differentiation. This was coincident with changes in the levels and spatial localisation of expression of various genes implicated, for instance, in axon guidance, that likely underlie specific ele phenotypes. These results suggest that many of the cell and tissue specific phenotypes in ele mutant embryos are secondary to altered expression of modules of developmental regulatory genes that characterise, or promote transitions in, cell state and require the correct function of Slbp-dependent histone and chromatin regulatory genes.

Maintenance of Zebrafish Lines at the European Zebrafish Resource Center

We have established a European Zebrafish Resource Center (EZRC) at the KIT. This center not only maintains and distributes a large number of existing mutant and transgenic zebrafish lines but also gives zebrafish researchers access to screening services and technologies such as imaging and high-throughput sequencing, provided by the Institute of Toxicology and Genetics (ITG). The EZRC maintains and distributes the stock collection of the Nüsslein-Volhard laboratory, comprising over 2000 publicly released mutations, as frozen sperm samples. Within the framework of the ZF-HEALTH EU project, the EZRC distributes over 10,000 knockout mutations from the Sanger Institute (United Kingdom), as well as over 100 mutant and transgenic lines from other sources. In this article, we detail the measures we have taken to ensure the health of our fish, including hygiene, quarantine, and veterinary inspections.

Report of Workshop on Euthanasia for Zebrafish-A Matter of Welfare and Science

The increasing importance of zebrafish as a biomedical model organism is reflected by the steadily growing number of publications and laboratories working with this species. Regulatory recommendations for euthanasia as issued in Directive 2010/63/EU are, however, based on experience with fish species used for food production and do not take the small size and specific physiology of zebrafish into account. Consequently, the currently recommended methods of euthanasia in the Directive 2010/63/EU are either not applicable or may interfere with research goals. An international workshop was held in Karlsruhe, Germany, March 9, 2017, to discuss and propose alternative methods for euthanasia of zebrafish. The aim was to identify methods that adequately address the physiology of zebrafish and its use as a biomedical research model, follow the principles of the 3Rs (Replacement, Reduction, and Refinement) in animal experimentation and consider animal welfare during anesthesia and euthanasia. The results of the workshop are summarized here in the form of a white paper.

The Calcineurin-FoxO-MuRF1 signaling pathway regulates myofibril integrity in cardiomyocytes

Altered Ca²⁺ handling is often present in diseased hearts undergoing structural remodeling and functional deterioration. However, whether Ca²⁺ directly regulates sarcomere structure has remained elusive. Using a zebrafish ncx1 mutant, we explored the impacts of impaired Ca²⁺ homeostasis on myofibril integrity. We found that the E3 ubiquitin ligase murf1 is upregulated in ncx1-deficient hearts. Intriguingly, knocking down murf1 activity or inhibiting proteasome activity preserved myofibril integrity, revealing a MuRF1-mediated proteasome degradation mechanism that is activated in response to abnormal Ca²⁺ homeostasis. Furthermore, we detected an accumulation of the murf1 regulator FoxO in the nuclei of ncx1-deficient cardiomyocytes. Overexpression of FoxO in wild type cardiomyocytes induced murf1 expression and caused myofibril disarray, whereas inhibiting Calcineurin activity attenuated FoxO-mediated murf1 expression and protected sarcomeres from degradation in ncx1-deficient hearts. Together, our findings reveal a novel mechanism by which Ca²⁺ overload disrupts myofibril integrity by activating a Calcineurin-FoxO-MuRF1-proteosome signaling pathway.

Spermidine, but not spermine, is essential for pigment pattern formation in zebrafish

Polyamines are small poly-cations essential for all cellular life. The main polyamines present in metazoans are putrescine, spermidine and spermine. Their exact functions are still largely unclear; however, they are involved in a wide variety of processes affecting cell growth, proliferation, apoptosis and aging. Here we identify idefix, a mutation in the zebrafish gene encoding the enzyme spermidine synthase, leading to a severe reduction in spermidine levels as shown by capillary electrophoresis-mass spectrometry. We show that spermidine, but not spermine, is essential for early development, organogenesis and colour pattern formation. Whereas in other vertebrates spermidine deficiency leads to very early embryonic lethality, maternally provided spermidine synthase in zebrafish is sufficient to rescue the early developmental defects. This allows us to uncouple them from events occurring later during colour patterning. Factors involved in the cellular interactions essential for colour patterning, likely targets for spermidine, are the gap junction components Cx41.8, Cx39.4, and Kir7.1, an inwardly rectifying potassium channel, all known to be regulated by polyamines. Thus, zebrafish provide a vertebrate model to study the in vivo effects of polyamines.

Summary: We show that the polyamine spermidine, but not spermine, in addition to more general functions during early development, also specifically regulates colour pattern formation in adult zebrafish.

pigk Mutation underlies macho behavior and affects Rohon-Beard cell excitability

The study of touch-evoked behavior allows investigation of both the cells and circuits that generate a response to tactile stimulation. We investigate a touch-insensitive zebrafish mutant, macho (maco), previously shown to have reduced sodium current amplitude and lack of action potential firing in sensory neurons. In the genomes of mutant but not wild-type embryos, we identify a mutation in the pigk gene. The encoded protein, PigK, functions in attachment of glycophosphatidylinositol anchors to precursor proteins. In wild-type embryos, pigk mRNA is present at times when mutant embryos display behavioral phenotypes. Consistent with the predicted loss of function induced by the mutation, knock-down of PigK phenocopies maco touch insensitivity and leads to reduced sodium current (INa) amplitudes in sensory neurons. We further test whether the genetic defect in pigk underlies the maco phenotype by overexpressing wild-type pigk in mutant embryos. We find that ubiquitous expression of wild-type pigk rescues the touch response in maco mutants. In addition, for maco mutants, expression of wild-type pigk restricted to sensory neurons rescues sodium current amplitudes and action potential firing in sensory neurons. However, expression of wild-type pigk limited to sensory cells of mutant embryos does not allow rescue of the behavioral touch response. Our results demonstrate an essential role for pigk in generation of the touch response beyond that required for maintenance of proper INa density and action potential firing in sensory neurons.

Slc45a2 and V-ATPase are regulators of melanosomal pH homeostasis in zebrafish, providing a mechanism for human pigment evolution and disease

We present here the positional cloning of the Danio rerio albino mutant and show that the affected gene encodes Slc45a2. The human orthologous gene has previously been shown to be involved in human skin color variation, and mutations therein have been implicated in the disease OCA4. Through ultrastructural analysis of the melanosomes in albino alleles as well as the tyrosinase-deficient mutant sandy, we add new insights into the role of Slc45a2 in the production of melanin. To gain further understanding of the role of Slc45a2 and its possible interactions with other proteins involved in melanization, we further analyzed the role of the V-ATPase as a melanosomal acidifier. We show that it is possible to rescue the melanization potential of the albino melanosomes through genetic and chemical inhibition of V-ATPase, thereby increasing internal melanosome pH.

Slc45a2 and V-ATPase are regulators of melanosomal pH homeostasis in zebrafish, providing a mechanism for human pigment evolution and disease

We present here the positional cloning of the Danio rerio albino mutant and show that the affected gene encodes Slc45a2. The human orthologous gene has previously been shown to be involved in human skin color variation, and mutations therein have been implicated in the disease OCA4. Through ultrastructural analysis of the melanosomes in albino alleles as well as the tyrosinase-deficient mutant sandy, we add new insights into the role of Slc45a2 in the production of melanin. To gain further understanding of the role of Slc45a2 and its possible interactions with other proteins involved in melanization, we further analyzed the role of the V-ATPase as a melanosomal acidifier. We show that it is possible to rescue the melanization potential of the albino melanosomes through genetic and chemical inhibition of V-ATPase, thereby increasing internal melanosome pH.

EuFishBioMed (COST Action BM0804): a European network to promote the use of small fishes in biomedical research

Small fresh water fishes such as the zebrafish (Danio rerio) have become important model organisms for biomedical research. They currently represent the best vertebrate embryo models in which it is possible to derive quantitative data on gene expression, signaling events, and cell behavior in real time in the living animal. Relevant phenotypes in fish mutants are similar to those of other vertebrate models and human diseases. They can be analyzed in great detail and much faster than in mammals. In recent years, approximately 2500 genetically distinct fish lines have been generated by European research groups alone. Their potential, including their possible use by industry, is far from being exploited. To promote zebrafish research in Europe, EuFishBioMed was founded and won support by the EU COST programme ( http://www.cost.esf.org/ ). The main objective of EuFishBioMed is to establish a platform of knowledge exchange for research on small fish models with a strong focus on widening its biomedical applications and an integration of European research efforts and resources. EuFishBioMed currently lists more than 300 member laboratories in Europe, offers funding for short-term laboratory visits, organizes and co-sponsors meetings and workshops, and has successfully lobbied for the establishment of a European Zebrafish Resource Centre. To maintain this network in the future, beyond the funding period of the COST Action, we are currently establishing the European Society for Fish Models in Biology and Medicine.

Zebrafish embryos as an alternative to animal experiments--a commentary on the definition of the onset of protected life stages in animal welfare regulations

Worldwide, the zebrafish has become a popular model for biomedical research and (eco)toxicology. Particularly the use of embryos is receiving increasing attention, since they are considered as replacement method for animal experiments. Zebrafish embryos allow the analysis of multiple endpoints ranging from acute and developmental toxicity determination to complex functional genetic and physiological analysis. Particularly the more complex endpoints require the use of post-hatched eleutheroembryo stages. According to the new EU Directive 2010/63/EU on the protection of animals used for scientific purposes, the earliest life-stages of animals are not defined as protected and, therefore, do not fall into the regulatory frameworks dealing with animal experimentation. Independent feeding is considered as the stage from which free-living larvae are subject to regulations for animal experimentation. However, despite this seemingly clear definition, large variations exist in the interpretation of this criterion by national and regional authorities. Since some assays require the use of post-hatched stages up to 120 h post fertilization, the literature and available data are reviewed in order to evaluate if this stage could still be considered as non-protected according to the regulatory criterion of independent feeding. Based on our analysis and by including criteria such as yolk consumption, feeding and swimming behavior, we conclude that zebrafish larvae can indeed be regarded as independently feeding from 120 h after fertilization. Experiments with zebrafish should thus be subject to regulations for animal experiments from 120 h after fertilization onwards.

Fine-tuning of Hh signaling by the RNA-binding protein Quaking to control muscle development

The development of the different muscles within the somite is a complex process that involves the Hedgehog (Hh) signaling pathway. To specify the proper number of muscle cells and organize them spatially and temporally, the Hh signaling pathway needs to be precisely regulated at different levels, but only a few factors external to the pathway have been described. Here, we report for the first time the role of the STAR family RNA-binding protein Quaking A (QkA) in somite muscle development. We show in zebrafish that the loss of QkA function affects fast muscle fiber maturation as well as Hh-induced muscle derivative specification and/or morphogenesis. Mosaic analysis reveals that fast fiber maturation depends on the activity of QkA in the environment of fast fiber progenitors. We further show that Hh signaling requires QkA activity for muscle development. By an in silico approach, we screened the 3'UTRs of known Hh signaling component mRNAs for the Quaking response element and found the transcription factor Gli2a, a known regulator of muscle fate development. Using destabilized GFP as a reporter, we show that the gli2a mRNA 3'UTR is a functional QkA target. Consistent with this notion, the loss of QkA function rescued slow muscle fibers in yot mutant embryos, which express a dominant-negative Gli2a isoform. Thus, our results reveal a new mechanism to ensure muscle cell fate diversity by fine-tuning of the Hh signaling pathway via RNA-binding proteins.

Aplexone targets the HMG-CoA reductase pathway and differentially regulates arteriovenous angiogenesis

Arterial and venous endothelial cells exhibit distinct molecular characteristics at early developmental stages. These lineage-specific molecular programs are instructive to the development of distinct vascular architectures and physiological conditions of arteries and veins, but their roles in angiogenesis remain unexplored. Here, we show that the caudal vein plexus in zebrafish forms by endothelial cell sprouting, migration and anastomosis, providing a venous-specific angiogenesis model. Using this model, we have identified a novel compound, aplexone, which effectively suppresses venous, but not arterial, angiogenesis. Multiple lines of evidence indicate that aplexone differentially regulates arteriovenous angiogenesis by targeting the HMG-CoA reductase (HMGCR) pathway. Treatment with aplexone affects the transcription of enzymes in the HMGCR pathway and reduces cellular cholesterol levels. Injecting mevalonate, a metabolic product of HMGCR, reverses the inhibitory effect of aplexone on venous angiogenesis. In addition, aplexone treatment inhibits protein prenylation and blocking the activity of geranylgeranyl transferase induces a venous angiogenesis phenotype resembling that observed in aplexone-treated embryos. Furthermore, endothelial cells of venous origin have higher levels of proteins requiring geranylgeranylation than arterial endothelial cells and inhibiting the activity of Rac or Rho kinase effectively reduces the migration of venous, but not arterial, endothelial cells. Taken together, our findings indicate that angiogenesis is differentially regulated by the HMGCR pathway via an arteriovenous-dependent requirement for protein prenylation in zebrafish and human endothelial cells.

The coiled-coil domain containing protein CCDC40 is essential for motile cilia function and left-right axis formation

Primary ciliary dyskinesia (PCD) is a genetically heterogeneous autosomal recessive disorder characterized by recurrent infections of the respiratory tract associated with the abnormal function of motile cilia. Approximately half of individuals with PCD also have alterations in the left-right organization of their internal organ positioning, including situs inversus and situs ambiguous (Kartagener's syndrome). Here, we identify an uncharacterized coiled-coil domain containing a protein, CCDC40, essential for correct left-right patterning in mouse, zebrafish and human. In mouse and zebrafish, Ccdc40 is expressed in tissues that contain motile cilia, and mutations in Ccdc40 result in cilia with reduced ranges of motility. We further show that CCDC40 mutations in humans result in a variant of PCD characterized by misplacement of the central pair of microtubules and defective assembly of inner dynein arms and dynein regulatory complexes. CCDC40 localizes to motile cilia and the apical cytoplasm and is required for axonemal recruitment of CCDC39, disruption of which underlies a similar variant of PCD.

The zebrafish mutant bumper shows a hyperproliferation of lens epithelial cells and fibre cell degeneration leading to functional blindness

The development of the eye lens is one of the classical paradigms of induction during embryonic development in vertebrates. But while there have been numerous studies aimed at discovering the genetic networks controlling early lens development, comparatively little is known about later stages, including the differentiation of secondary lens fibre cells. The analysis of mutant zebrafish isolated in forward genetic screens is an important way to investigate the roles of genes in embryogenesis. In this study we describe the zebrafish mutant bumper (bum), which shows a transient, tumour-like hyperproliferation of the lens epithelium as well as a progressively stronger defect in secondary fibre cell differentiation, which results in a significantly reduced lens size and ectopic location of the lens within the neural retina. Interestingly, the initial hyperproliferation of the lens epithelium in bum spontaneously regresses, suggesting this mutant as a valuable model to study the molecular control of tumour progression/suppression. Behavioural analyses demonstrate that, despite a morphologically normal retina, larval and adult bum(-/-) zebrafish are functionally blind. We further show that these fish have defects in their craniofacial skeleton with normal but delayed formation of the scleral ossicles within the eye, several reduced craniofacial bones resulting in an abnormal skull shape, and asymmetric ectopic bone formation within the mandible. Genetic mapping located the mutation in bum to a 4cM interval on chromosome 7 with the closest markers located at 0.2 and 0cM, respectively.

The zebrafish dystrophic mutant softy maintains muscle fibre viability despite basement membrane rupture and muscle detachment

The skeletal muscle basement membrane fulfils several crucial functions during development and in the mature myotome and defects in its composition underlie certain forms of muscular dystrophy. A major component of this extracellular structure is the laminin polymer, which assembles into a resilient meshwork that protects the sarcolemma during contraction. Here we describe a zebrafish mutant, softy, which displays severe embryonic muscle degeneration as a result of initial basement membrane failure. The softy phenotype is caused by a mutation in the lamb2 gene, identifying laminin beta2 as an essential component of this basement membrane. Uniquely, softy homozygotes are able to recover and survive to adulthood despite the loss of myofibre adhesion. We identify the formation of ectopic, stable basement membrane attachments as a novel means by which detached fibres are able to maintain viability. This demonstration of a muscular dystrophy model possessing innate fibre viability following muscle detachment suggests basement membrane augmentation as a therapeutic strategy to inhibit myofibre loss.

Leukocyte tyrosine kinase functions in pigment cell development

A fundamental problem in developmental biology concerns how multipotent precursors choose specific fates. Neural crest cells (NCCs) are multipotent, yet the mechanisms driving specific fate choices remain incompletely understood. Sox10 is required for specification of neural cells and melanocytes from NCCs. Like sox10 mutants, zebrafish shady mutants lack iridophores; we have proposed that sox10 and shady are required for iridophore specification from NCCs. We show using diverse approaches that shady encodes zebrafish leukocyte tyrosine kinase (Ltk). Cell transplantation studies show that Ltk acts cell-autonomously within the iridophore lineage. Consistent with this, ltk is expressed in a subset of NCCs, before becoming restricted to the iridophore lineage. Marker analysis reveals a primary defect in iridophore specification in ltk mutants. We saw no evidence for a fate-shift of neural crest cells into other pigment cell fates and some NCCs were subsequently lost by apoptosis. These features are also characteristic of the neural crest cell phenotype in sox10 mutants, leading us to examine iridophores in sox10 mutants. As expected, sox10 mutants largely lacked iridophore markers at late stages. In addition, sox10 mutants unexpectedly showed more ltk-expressing cells than wild-type siblings. These cells remained in a premigratory position and expressed sox10 but not the earliest neural crest markers and may represent multipotent, but partially-restricted, progenitors. In summary, we have discovered a novel signalling pathway in NCC development and demonstrate fate specification of iridophores as the first identified role for Ltk.

Stem and other multipotent cells generate diverse cell-types, but our understanding of how they make these decisions, which is important for their therapeutic use, is incomplete. Neural crest cells are an important class of multipotent cells and generate multiple stem cell types. We have looked at how pigment cells are made from the neural crest in the zebrafish. The silver shine familiar in so many fish is due to specialised mirror-like pigment cells, called iridophores. We show that these cells are missing in zebrafish shady mutants. We identify the shady gene as encoding a cell signalling receptor, leukocyte tyrosine kinase (Ltk), that has recently been associated with human auto-immune disease. We show that in zebrafish this gene is most likely required to make iridophores from neural crest cells. Thus, we identify a novel pathway required for diversification of these multipotent cells. Our work defines the first role for Ltk in a vertebrate. It provides a mutant resource that will allow us to discover the full breadth of roles for this important gene. Furthermore, the loss of iridophores forms a simple visual screen for inhibition of LTK function and might well have implications in drug discovery.

Investigator profile. An interview with Robert Geisler, Ph.D. Interview by Vicki Glaser

Robert Geisler, Ph.D., is Group Leader at the Max-Planck-Institut für Entwicklungsbiologie, in Tübingen, Germany. His research focus is on genome mapping and gene expression profiling in the zebrafish. He completed a Bachelor's degree in physiological chemistry and a doctorate in genetics at Tübingen University. Dr. Geisler pursued a postdoctoral fellowship in zebrafish genomics in the laboratory of Pascal Haffter, at MPI für Entwicklungsbiologie, where he developed methods and software for large-scale PCR-based mapping of zebrafish mutations and genes. He has been Scientific Advisor of the ZFIN database project since 2001.

The zebrafish mutant lbk/vam6 resembles human multisystemic disorders caused by aberrant trafficking of endosomal vesicles

The trafficking of intracellular vesicles is essential for a number of cellular processes and defects in this process have been implicated in a wide range of human diseases. We identify the zebrafish mutant lbk as a novel model for such disorders. lbk displays hypopigmentation of skin melanocytes and the retinal pigment epithelium (RPE), an absence of iridophore reflections, defects in internal organs (liver, intestine) as well as functional defects in vision and the innate immune system (macrophages). Positional cloning, an allele screen, rescue experiments and morpholino knock-down reveal a mutation in the zebrafish orthologue of the vam6/vps39 gene. Vam6p is part of the HOPS complex, which is essential for vesicle tethering and fusion. Affected cells in the lbk RPE, liver, intestine and macrophages display increased numbers and enlarged intracellular vesicles. Physiological and behavioural analyses reveal severe defects in visual ability in lbk mutants. The present study provides the first phenotypic description of a lack of vam6 gene function in a multicellular organism. lbk shares many of the characteristics of human diseases and suggests a novel disease gene for pathologies associated with defective vesicle transport, including the arthrogryposis-renal dysfunction-cholestasis (ARC) syndrome, the Hermansky-Pudlak syndrome, the Chediak-Higashi syndrome and the Griscelli syndrome.

Cohesin-dependent regulation of Runx genes

Runx transcription factors determine cell fate in many lineages. Maintaining balanced levels of Runx proteins is crucial, as deregulated expression leads to cancers and developmental disorders. We conducted a forward genetic screen in zebrafish for positive regulators of runx1 that yielded the cohesin subunit rad21. Zebrafish embryos lacking Rad21, or cohesin subunit Smc3, fail to express runx3 and lose hematopoietic runx1 expression in early embryonic development. Failure to develop differentiated blood cells in rad21 mutants is partially rescued by microinjection of runx1 mRNA. Significantly, monoallelic loss of rad21 caused a reduction in the transcription of runx1 and of the proneural genes ascl1a and ascl1b, indicating that downstream genes are sensitive to Rad21 dose. Changes in gene expression were observed in a reduced cohesin background in which cell division was able to proceed, indicating that cohesin might have a function in transcription that is separable from its mitotic role. Cohesin is a protein complex essential for sister chromatid cohesion and DNA repair that also appears to be essential for normal development through as yet unknown mechanisms. Our findings provide evidence for a novel role for cohesin in development, and indicate potential for monoallelic loss of cohesin subunits to alter gene expression.

Radiation hybrid maps of Medaka chromosomes LG 12, 17, and 22

The Medaka is an excellent genetic system for studies of vertebrate development and disease and environmental and evolutionary biology studies. To facilitate the mapping of markers or the cloning of affected genes in Medaka mutants identified by forward-genetic screens, we have established a panel of whole-genome radiation hybrids (RHs) and RH maps for three Medaka chromosomes. RH mapping is useful, since markers to be mapped need not be polymorphic and one can establish the order of markers that are difficult to resolve by genetic mapping owing to low genetic recombination rates. RHs were generated by fusing the irradiated donor, OLF-136 Medaka cell line, with the host B78 mouse melanoma cells. Of 290 initial RH clones, we selected 93 on the basis of high retention of fragments of the Medaka genome to establish a panel that allows genotyping in the 96-well format. RH maps for linkage groups 12, 17, and 22 were generated using 159 markers. The average retention for the three chromosomes was 19% and the average break point frequency was ∼33 kb/cR. We estimate the potential resolution of the RH panel to be ∼186 kb, which is high enough for integrating RH data with bacterial artificial chromosome clones. Thus, this first RH panel will be a useful tool for mapping mutated genes in Medaka.

The UCS factor Steif/Unc-45b interacts with the heat shock protein Hsp90a during myofibrillogenesis

Contraction of muscles is mediated by highly organized arrays of myosin motor proteins. We report here the characterization of a mutation of a UCS gene named steif/unc-45b that is required for the formation of ordered myofibrils in both the skeletal and cardiac muscles of zebrafish. We show that Steif/Unc-45b interacts with the chaperone Hsp90a in vitro. The two genes are co-expressed in the skeletal musculature and knockdown of Hsp90a leads to impaired myofibril formation in the same manner as lack of Steif/Unc-45b activity. Transcripts of both genes are up-regulated in steif mutants suggesting co-regulation of the two genes. Our data indicate a requirement of Steif/unc-45b and Hsp90a for the assembly of the contractile apparatus in the vertebrate skeletal musculature.

Acoustic cavitation, bubble dynamics and sonoluminescence

Basic facts on the dynamics of bubbles in water are presented. Measurements on the free and forced radial oscillations of single spherical bubbles and their acoustic (shock waves) and optic (luminescence) emissions are given in photographic series and diagrams. Bubble cloud patterns and their dynamics and light emission in standing acoustic fields are discussed.

Differential gene expression as a toxicant-sensitive endpoint in zebrafish embryos and larvae

The zebrafish (Danio rerio) embryo toxicity test (DarT) is under consideration as an alternative to the acute fish toxicity test. Microscopically visible developmental disorders or death are the endpoints used to report on toxicity in DarT. These endpoints are easily observed. They, however, rarely reveal mechanisms leading to a toxic effect and are relatively insensitive compared to chronic toxic effects. We hypothesized that, by using gene expression profiles as an additional endpoint, it may be possible to increase the sensitivity and predictive value of DarT. Therefore, as a proof of principle, we exposed zebrafish embryos to the reference compound 3,4-dichloroaniline (3,4-DCA) and analyzed gene expression patterns with a 14k oligonucleotide array. Important stress response genes not included in the microarray were additionally quantified by reverse transcriptase polymerase chain reaction. Six genes involved in biotransformation (cyp1a, ahr2), stress response (nfe212, maft, hmox1) and cell cycle control (fzr1) were significantly regulated. With the exception of fzr1, these genes proved to be differentially expressed in post hatch life stages as well. The identified genes point toward an aryl hydrocarbon receptor-mediated response. Differential gene expression in embryos exposed for 48 h was observed at 3,4-DCA concentrations as low as 0.78 microM, which is more than 10-fold below the concentrations that elicited visible toxic effects. Upon exposure for 5 days, differential expression was detected at concentrations as low as 0.22 microM of 3,4-DCA, which was close to the lowest observed effect concentration (0.11 microM) in the 30-day early life stage test. This study therefore indicates that gene expression analysis in DarT is able to reveal mechanistic information and may also be exploited for the development of replacement methods for chronic fish tests.

The zebrafish udu gene encodes a novel nuclear factor and is essential for primitive erythroid cell development

Hematopoiesis is a complex process which gives rise to all blood lineages in the course of an organism's lifespan. However, the underlying molecular mechanism governing this process is not fully understood. Here we report the isolation and detailed study of a newly identified zebrafish ugly duckling (Udu) mutant allele, Udu(sq1). We show that loss-of-function mutation in the udu gene disrupts primitive erythroid cell proliferation and differentiation in a cell-autonomous manner, resulting in red blood cell (RBC) hypoplasia. Positional cloning reveals that the Udu gene encodes a novel factor that contains 2 paired amphipathic alpha-helix-like (PAH-L) repeats and a putative SANT-L (SW13, ADA2, N-Cor, and TFIIIB-like) domain. We further show that the Udu protein is predominantly localized in the nucleus and deletion of the putative SANT-L domain abolishes its function. Our study indicates that the Udu protein is very likely to function as a transcription modulator essential for the proliferation and differentiation of erythroid lineage.

A gene-based radiation hybrid map of the gilthead sea bream Sparus aurata refines and exploits conserved synteny with Tetraodon nigroviridis

Background

Comparative teleost studies are of great interest since they are important in aquaculture and in evolutionary issues. Comparing genomes of fully sequenced model fish species with those of farmed fish species through comparative mapping offers shortcuts for quantitative trait loci (QTL) detections and for studying genome evolution through the identification of regions of conserved synteny in teleosts. Here a comparative mapping study is presented by radiation hybrid (RH) mapping genes of the gilthead sea bream Sparus aurata, a non-model teleost fish of commercial and evolutionary interest, as it represents the worldwide distributed species-rich family of Sparidae.

Results

An additional 74 microsatellite markers and 428 gene-based markers appropriate for comparative mapping studies were mapped on the existing RH map of Sparus aurata. The anchoring of the RH map to the genetic linkage map resulted in 24 groups matching the karyotype of Sparus aurata. Homologous sequences to Tetraodon were identified for 301 of the gene-based markers positioned on the RH map of Sparus aurata. Comparison between Sparus aurata RH groups and Tetraodon chromosomes (karyotype of Tetraodon consists of 21 chromosomes) in this study reveals an unambiguous one-to-one relationship suggesting that three Tetraodon chromosomes correspond to six Sparus aurata radiation hybrid groups. The exploitation of this conserved synteny relationship is furthermore demonstrated by in silico mapping of gilthead sea bream expressed sequence tags (EST) that give a significant similarity hit to Tetraodon.

Conclusion

The addition of primarily gene-based markers increased substantially the density of the existing RH map and facilitated comparative analysis. The anchoring of this gene-based radiation hybrid map to the genome maps of model species broadened the pool of candidate genes that mainly control growth, disease resistance, sex determination and reversal, reproduction as well as environmental tolerance in this species, all traits of great importance for QTL mapping and marker assisted selection. Furthermore this comparative mapping approach will facilitate to give insights into chromosome evolution and into the genetic make up of the gilthead sea bream.

Large-scale mapping of mutations affecting zebrafish development

Background

Large-scale mutagenesis screens in the zebrafish employing the mutagen ENU have isolated several hundred mutant loci that represent putative developmental control genes. In order to realize the potential of such screens, systematic genetic mapping of the mutations is necessary. Here we report on a large-scale effort to map the mutations generated in mutagenesis screening at the Max Planck Institute for Developmental Biology by genome scanning with microsatellite markers.

Results

We have selected a set of microsatellite markers and developed methods and scoring criteria suitable for efficient, high-throughput genome scanning. We have used these methods to successfully obtain a rough map position for 319 mutant loci from the Tübingen I mutagenesis screen and subsequent screening of the mutant collection. For 277 of these the corresponding gene is not yet identified. Mapping was successful for 80 % of the tested loci. By comparing 21 mutation and gene positions of cloned mutations we have validated the correctness of our linkage group assignments and estimated the standard error of our map positions to be approximately 6 cM.

Conclusion

By obtaining rough map positions for over 300 zebrafish loci with developmental phenotypes, we have generated a dataset that will be useful not only for cloning of the affected genes, but also to suggest allelism of mutations with similar phenotypes that will be identified in future screens. Furthermore this work validates the usefulness of our methodology for rapid, systematic and inexpensive microsatellite mapping of zebrafish mutations.

Mutation in the delta-subunit of the nAChR suppresses the muscle defects caused by lack of Dystrophin

Normal motility of the zebrafish embryo requires a large number of gene loci, many of which have human orthologues implicated in myasthenias and other myopathies. We have identified a mutation in the zebrafish that abolishes body motility. Embryos have narrower myofibrils and lack clusters of nicotinic acetylcholine receptors (nAChRs) on the surface of the somitic muscle. We mapped the mutation to the delta-subunit of the nAChR, showing this mutant to be a new allele of the previously named sofa potato (sop). The mutant allele carries a missense mutation in the extracellular domain altering the cysteine at position 150 to an arginine. The delta-subunit is expressed in all striated muscles in embryonic and early larval stages together with the alpha1, beta1, epsilon, and gamma-subunits of nAChR. In contrast to mammals that show switching from the gamma embryonic to the adult epsilon-subunit, the two subunits are coexpressed in zebrafish embryos. We, furthermore, demonstrated that the sop/delta-nAChR mutation is a suppressor of the myopathy caused by lack of Dystrophin. The myofiber detachment phenotype of Dystroglycan-deficient embryos was not suppressed, suggesting that Dystrophin and Dystroglycan play distinct roles in muscle formation and maintenance of muscle integrity.

Learning from small fry: the zebrafish as a genetic model organism for aquaculture fish species

In recent years, the zebrafish has become one of the most prominent vertebrate model organisms used to study the genetics underlying development, normal body function, and disease. The growing interest in zebrafish research was paralleled by an increase in tools and methods available to study zebrafish. While zebrafish research initially centered on mutagenesis screens (forward genetics), recent years saw the establishment of reverse genetic methods (morpholino knock-down, TILLING). In addition, increasingly sophisticated protocols for generating transgenic zebrafish have been developed and microarrays are now available to characterize gene expression on a near genome-wide scale. The identification of loci underlying specific traits is aided by genetic, physical, and radiation hybrid maps of the zebrafish genome and the zebrafish genome project. As genomic resources for aquacultural species are increasingly being generated, a meaningful interaction between zebrafish and aquacultural research now appears to be possible and beneficial for both sides. In particular, research on nutrition and growth, stress, and disease resistance in the zebrafish can be expected to produce results applicable to aquacultural fish, for example, by improving husbandry and formulated feeds. Forward and reverse genetics approaches in the zebrafish, together with the known conservation of synteny between the species, offer the potential to identify and verify candidate genes for quantitative trait loci (QTLs) to be used in marker-assisted breeding. Moreover, some technologies from the zebrafish field such as TILLING may be directly transferable to aquacultural research and production.

The zebrafish shocked gene encodes a glycine transporter and is essential for the function of early neural circuits in the CNS

shocked (sho) is a zebrafish mutation that causes motor deficits attributable to CNS defects during the first2dof development. Mutant embryos display reduced spontaneous coiling of the trunk, diminished escape responses when touched, and an absence of swimming. A missense mutation in the slc6a9 gene that encodes a glycine transporter (GlyT1) was identified as the cause of the sho phenotype. Antisense knock-down of GlyT1 in wild-type embryos phenocopies sho, and injection of wild-type GlyT1 mRNA into mutants rescues them. A comparison of glycine-evoked inward currents in Xenopus oocytes expressing either the wild-type or mutant protein found that the missense mutation results in a nonfunctional transporter. glyt1 and the related glyt2 mRNAs are expressed in the hindbrain and spinal cord in nonoverlapping patterns. The fact that these regions are known to be required for generation of early locomotory behaviors suggests that the regulation of extracellular glycine levels in the CNS is important for proper function of neural networks. Furthermore, physiological analysis after manipulation of glycinergic activity in wild-type and sho embryos suggests that the mutant phenotype is attributable to elevated extracellular glycine within the CNS.

The first radiation hybrid map of a perch-like fish: the gilthead seabream (Sparus aurata L)

Among Teleosts, Perciformes are the largest order of fishes and include numerous species of commercial importance. Perciformes also comprise species of primary interest for evolutionary studies and analysis of the sex determination systems and sex chromosome plasticity. Unfortunately, genomics tools and resources for Perciformes remain to be developed. Here, we report the production of a seabream whole-genome radiation hybrid (RH) panel in which quality was ascertained by the construction of a 2-Mb-resolution RH map. The map encompasses 440 markers (288 microsatellites, 82 gene-based markers, and 70 STS) suitable for linkage analysis and comparative mapping studies. Achievement of a RH panel and a whole-genome RH map should contribute to establishing seabream as a fish model among the Perciformes and should be of importance in aquaculture for marker-assisted selection, improvement of growth performance, and disease management. Development of RH maps in a cost-effective manner for other fishes with the described methodology will offer a powerful approach in aquaculture and will provide extended capabilities for comparing vertebrate genome evolution.

A mutation in the silver gene leads to defects in melanosome biogenesis and alterations in the visual system in the zebrafish mutant fading vision

Forward genetic screens have been instrumental in defining molecular components of visual function. The zebrafish mutant fading vision (fdv) has been identified in such a screen due to defects in vision accompanied by hypopigmentation in the retinal pigment epithelium (RPE) and body melanocytes. The RPE forms the outer most layer of the retina, and its function is essential for vision. In fdv mutant larvae, the outer segments of photoreceptors are strongly reduced in length or absent due to defects in RPE cells. Ultrastructural analysis of RPE cells reveals dramatic cellular changes such as an absence of microvilli and vesicular inclusions. The retinoid profile is altered as judged by biochemical analysis, arguing for a partial block in visual pigment regeneration. Surprisingly, homozygous fdv vision mutants survive to adulthood and show, despite a persistence of the hypopigmentation, a partial recovery of retinal morphology. By positional cloning and subsequent morpholino knock-down, we identified a mutation in the silver gene as the molecular defect underlying the fdv phenotype. The Silver protein is required for intralumenal fibril formation in melanosomes by amylogenic cleavage. Our data reveal an unexpected link between melanosome biogenesis and the visual system, undetectable in cell culture.

Zebrafish penner/lethal giant larvae 2 functions in hemidesmosome formation, maintenance of cellular morphology and growth regulation in the developing basal epidermis

Epithelial cells are equipped with junctional complexes that are involved in maintaining tissue architecture, providing mechanical integrity and suppressing tumour formation as well as invasiveness. A strict spatial segregation of these junctional complexes leads to the polarisation of epithelial cells. In vertebrate epithelia, basally localised hemidesmosomes mediate stable adhesion between epithelial cells and the underlying basement membrane. Although components of hemidesmosomes are relatively well known, the molecular machinery involved in governing the formation of these robust junctions, remains elusive. Here, we have identified the first component of this machinery using a forward genetic approach in zebrafish as we show that the function of penner (pen)/lethal giant larvae 2 (lgl2) is necessary for hemidesmosome formation and maintenance of the tissue integrity in the developing basal epidermis. Moreover, in pen/lgl2 mutant, basal epidermal cells hyper-proliferate and migrate to ectopic positions. Of the two vertebrate orthologues of the Drosophila tumour suppressor gene lethal giant larvae, the function of lgl2 in vertebrate development and organogenesis remained unclear so far. Here, we have unravelled an essential function of lgl2 during development of the epidermis in vertebrates.

beamter/deltaC and the role of Notch ligands in the zebrafish somite segmentation, hindbrain neurogenesis and hypochord differentiation

The Tübingen large-scale zebrafish genetic screen completed in 1996 identified a set of five genes required for orderly somite segmentation. Four of them have been molecularly identified and three were found to code for components of the Notch pathway, which are required for the coordinated oscillation of gene expression, known as the segmentation clock, in the presomitic mesoderm (PSM). Here, we show that the final member of the group, beamter (bea), codes for the Notch ligand DeltaC, and we present and characterize two new alleles, including one allele encoding for a protein truncated in the 7th EGF repeat and an allele deleting only the DSL domain which was previously shown to be necessary for ligand function. Interestingly however, when we over-express any of the mutant deltaC mRNAs, we observe antimorphic effects on both hindbrain neurogenesis and hypochord formation. Expression of bea/deltaC oscillates in the PSM, and a triple fluorescent in situ analysis of its oscillation in relation to that of other oscillating genes in the PSM reveals differences in subcellular localization of the oscillating mRNAs in individual cells in different oscillation phases. Mutations in aei/deltaD and bea/deltaC differ in the way they disrupt the oscillating expression of her1 and deltaC. Furthermore, we find that the double mutants have significantly stronger defects in hypochord formation but not in somitogenesis or hindbrain neurogenesis, indicating genetically that the two delta's may function either semi-redundantly or distinctly, depending upon context.

The zebrafish mutants dre, uki, and lep encode negative regulators of the hedgehog signaling pathway

Proliferation is one of the basic processes that control embryogenesis. To identify factors involved in the regulation of proliferation, we performed a zebrafish genetic screen in which we used proliferating cell nuclear antigen (PCNA) expression as a readout. Two mutants, hu418B and hu540A, show increased PCNA expression. Morphologically both mutants resembled the dre (dreumes), uki (ukkie), and lep (leprechaun) mutant class and both are shown to be additional uki alleles. Surprisingly, although an increased size is detected of multiple structures in these mutant embryos, adults become dwarfs. We show that these mutations disrupt repressors of the Hedgehog (Hh) signaling pathway. The dre, uki, and lep loci encode Su(fu) (suppressor of fused), Hip (Hedgehog interacting protein), and Ptc2 (Patched2) proteins, respectively. This class of mutants is therefore unique compared to previously described Hh mutants from zebrafish genetic screens, which mainly show loss of Hh signaling. Furthermore, su(fu) and ptc2 mutants have not been described in vertebrate model systems before. Inhibiting Hh activity by cyclopamine rescues uki and lep mutants and confirms the overactivation of the Hh signaling pathway in these mutants. Triple uki/dre/lep mutants show neither an additive increase in PCNA expression nor enhanced embryonic phenotypes, suggesting that other negative regulators, possibly Ptc1, prevent further activation of the Hh signaling pathway. The effects of increased Hh signaling resulting from the genetic alterations in the uki, dre, and lep mutants differ from phenotypes described as a result of Hh overexpression and therefore provide additional insight into the role of Hh signaling during vertebrate development.

In a screen aimed at finding genes that control proliferation in the zebrafish embryo, three mutants were identified. Mutants showed an increase in size of several structures including the brain, the retina, and the fins. Surprisingly, although size was increased in the embryos, adults remained small. Cloning of these genes revealed that increased Hedgehog signaling was at the basis of the phenotype, because all three genes encoded known repressors of the Hedgehog signaling pathway: Ptc2, Su(Fu), and Hip.

Hedgehog is known to play a role in proliferation. For instance, ectopic Hedgehog signaling can lead to several tumors including basal cell carcinoma and medulloblastoma. However, the phenotypes were still a surprise, because earlier experiments in zebrafish embryos suggested that activation should lead to patterning rather than proliferation defects. Current models of the pathway predict that these genes act independently to inhibit the signal but curiously, redundancy amongst these genes was not found, because triple mutants looked like the single mutants.

The conclusion is that weak activation of Hedgehog signaling can already lead to stimulation of growth in the absence of patterning defects, and that the Hedgehog signal is probably kept in check by the last inhibitor: Ptc1. A mutant for the ptc1 gene has recently been created and will put the model to the test.

Gene expression profiling of gilthead sea bream during early development and detection of stress-related genes by the application of cDNA microarray technology

Large-scale gene expression studies were performed for one of the main European aquaculture species, the gilthead sea bream Sparus auratus L. For this purpose, a cDNA microarray containing 10,176 clones from a cDNA library of mixed embryonic and larval stages was constructed. In addition to its importance for aquaculture, the taxonomic position and the relatively small genome size of sea bream makes it a prospective model for evolutionary biology and comparative genomics. However, so far, no large-scale analysis of gene expression exists for this species. In the present study, gene expression was analyzed in gilthead sea bream during early development, a significant period in the determination of quantitative traits and therefore of considerable interest for aquaculture. Synexpression groups expressed primarily early and late in development were determined and were composed of both known and novel genes. Furthermore, it was possible to identify stress response genes induced by cortisol injections using the cDNA microarray generated. The creation of gene expression profiles for sea bream by microarray hybridization will accelerate identification of candidate genes involved in multifactorial traits and certain regulatory pathways and will also contribute to a better understanding of the genetic background of fish physiology, which may help to improve aquaculture practices.

Transcriptome profiling of adult zebrafish at the late stage of chronic tuberculosis due to Mycobacterium marinum infection

The Mycobacterium marinum-zebrafish infection model was used in this study for analysis of a host transcriptome response to mycobacterium infection at the organismal level. RNA isolated from adult zebrafish that showed typical signs of fish tuberculosis due to a chronic progressive infection with M. marinum was compared with RNA from healthy fish in microarray analyses. Spotted oligonucleotide sets (designed by Sigma-Compugen and MWG) and Affymetrix GeneChips were used, in total comprising 45,465 zebrafish transcript annotations. Based on a detailed comparative analysis and quantitative reverse transcriptase-PCR analysis, we present a validated reference set of 159 genes whose regulation is strongly affected by mycobacterial infection in the three types of microarrays analyzed. Furthermore, we analyzed the separate datasets of the microarrays with special emphasis on the expression profiles of immune-related genes. Upregulated genes include many known components of the inflammatory response and several genes that have previously been implicated in the response to mycobacterial infections in cell cultures of other organisms. Different marker genes of the myeloid lineage that have been characterized in zebrafish also showed increased expression. Furthermore, the zebrafish homologs of many signal transduction genes with relationship to the immune response were induced by M. marinum infection. Future functional analysis of these genes may contribute to understanding the mechanisms of mycobacterial pathogenesis. Since a large group of genes linked to immune responses did not show altered expression in the infected animals, these results suggest specific responses in mycobacterium-induced disease.

Monorail/Foxa2 regulates floorplate differentiation and specification of oligodendrocytes, serotonergic raphé neurones and cranial motoneurones

In this study, we elucidate the roles of the winged-helix transcription factor Foxa2 in ventral CNS development in zebrafish. Through cloning of monorail (mol), which we find encodes the transcription factor Foxa2, and phenotypic analysis of mol-/- embryos, we show that floorplate is induced in the absence of Foxa2 function but fails to further differentiate. In mol-/- mutants, expression of Foxa and Hh family genes is not maintained in floorplate cells and lateral expansion of the floorplate fails to occur. Our results suggest that this is due to defects both in the regulation of Hh activity in medial floorplate cells as well as cell-autonomous requirements for Foxa2 in the prospective laterally positioned floorplate cells themselves. Foxa2 is also required for induction and/or patterning of several distinct cell types in the ventral CNS. Serotonergic neurones of the raphenucleus and the trochlear motor nucleus are absent in mol-/- embryos, and oculomotor and facial motoneurones ectopically occupy ventral CNS midline positions in the midbrain and hindbrain. There is also a severe reduction of prospective oligodendrocytes in the midbrain and hindbrain. Finally, in the absence of Foxa2, at least two likely Hh pathway target genes are ectopically expressed in more dorsal regions of the midbrain and hindbrain ventricular neuroepithelium, raising the possibility that Foxa2 activity may normally be required to limit the range of action of secreted Hh proteins.

Integrinα5 and Delta/Notch Signaling Have Complementary Spatiotemporal Requirements during Zebrafish Somitogenesis

Somitogenesis is the process by which the segmented precursors of the skeletal muscle and vertebral column are generated during vertebrate embryogenesis. While somitogenesis appears to be a serially homologous, reiterative process, we find that there are differences between the genetic control of early/anterior and late/posterior somitogenesis. We demonstrate that point mutations can cause segmentation defects in either the anterior, middle, or posterior somites in the zebrafish. We find that mutations in zebrafish integrinalpha5 disrupt anterior somite formation, giving a phenotype complementary to the posterior defects seen in the notch pathway mutants after eight/deltaD and deadly seven/notch1a. Double mutants between the notch pathway and integrinalpha5 display somite defects along the entire body axis, with a complete loss of the mesenchymal-to-epithelial transition and Fibronectin matrix assembly in the posterior. Our data suggest that notch- and integrinalpha5-dependent cell polarization and Fibronectin matrix assembly occur concomitantly and interdependently during border morphogenesis.

Histone deacetylase 1 is required for cell cycle exit and differentiation in the zebrafish retina

Histone acetylation is an important epigenetic mechanism for the control of eukaryotic transcription. The histone deacetylase 1 (HDAC1) gene has been implicated in controlling the transcription of core cell cycle regulators, but the in vivo role of HDACs in cell cycle regulation is still poorly understood. Loss of HDAC1 activity causes underproliferation in several contexts during vertebrate development. In contrast, we show here that HDAC1 has the opposite effect in the zebrafish visual system, where loss of HDAC1 activity leads to failure of cells to exit the cell cycle in the retina and in the optic stalk. The effect of HDAC1 on cell cycle exit is cell-autonomous, and loss of HDAC1 in the retina leads to up-regulation of cyclin D and E transcripts. These results demonstrate that the in vivo role of HDAC1 in regulating cell cycle progression is region-specific, as HDAC1 promotes cell cycle exit in the retina but stimulates proliferation in other cellular contexts.

Axon Sorting in the Optic Tract Requires HSPG Synthesis by ext2 (dackel) and extl3 (boxer)

Retinal ganglion cell (RGC) axons are topographically ordered in the optic tract according to their retinal origin. In zebrafish dackel (dak) and boxer (box) mutants, some dorsal RGC axons missort in the optic tract but innervate the tectum topographically. Molecular cloning reveals that dak and box encode ext2 and extl3, glycosyltransferases implicated in heparan sulfate (HS) biosynthesis. Both genes are required for HS synthesis, as shown by biochemical and immunohistochemical analysis, and are expressed maternally and then ubiquitously, likely playing permissive roles. Missorting in box can be rescued by overexpression of extl3. dak;box double mutants show synthetic pathfinding phenotypes that phenocopy robo2 mutants, suggesting that Robo2 function requires HS in vivo; however, tract sorting does not require Robo function, since it is normal in robo2 null mutants. This genetic evidence that heparan sulfate proteoglycan function is required for optic tract sorting provides clues to begin understanding the underlying molecular mechanisms.

Mutated otopetrin 1 affects the genesis of otoliths and the localization of Starmaker in zebrafish

Otoliths in bony fishes and otoconia in mammals are composite crystals consisting of calcium carbonate and proteins. These biominerals are part of the gravity and linear acceleration detection system of the inner ear. Mutations in otopetrin 1 have been shown to result in lack of otoconia in tilted and mergulhador mutant mice. The molecular function of Otopetrin 1, a novel protein that contains ten predicted transmembrane domains, however, has remained elusive. Here we show that a mutation in the orthologous gene in zebrafish is responsible for the complete absence of otoliths in backstroke mutants. We examined the localization of Starmaker, a secreted protein that is highly abundant in otoliths in backstroke mutants. Starmaker protein accumulated within cells of the otic epithelium, indicating a possible defect in secretion. Our data suggest that Otopetrin 1 in zebrafish may be involved in the protein trafficking of components required for formation of biominerals in the ear.

iguana encodes a novel zinc-finger protein with coiled-coil domains essential for Hedgehog signal transduction in the zebrafish embryo

Signaling by lipid-modified secreted glycoproteins of the Hedgehog family play fundamental roles during pattern formation in animal development and in humans; dysfunction of Hedgehog pathway components is frequently associated with a variety of congenital abnormalities and cancer. Transcriptional regulation of Hedgehog target genes is mediated by members of the Gli zinc-finger transcription factors. The relative nuclear concentrations of Gli activator (Gli(act)) and repressor (Gli(rep)) forms, together with their nucleocytoplasmic trafficking, appear to be critical determinants for target gene expression. Whereas such stringent controls of Gli activity are critical in ensuring appropriate levels of pathway activation, the mechanisms by which these processes are regulated remain inadequately understood. Here, using genetic analysis, we show that the zebrafish iguana gene product acts downstream of the Smoothened protein to modulate Gli activity in the somites of the developing embryo. Positional cloning reveals that iguana encodes the zebrafish ortholog of Dzip1, a novel zinc-finger/coiled-coil domain protein that we show can shuttle between the cytoplasm and nucleus in a manner correlated with Hedgehog pathway activity.

Mutations in cadherin 23 affect tip links in zebrafish sensory hair cells

Hair cells have highly organized bundles of apical projections, or stereocilia, that are deflected by sound and movement. Displacement of stereocilia stretches linkages at the tips of stereocilia that are thought to gate mechanosensory channels. To identify the molecular machinery that mediates mechanotransduction in hair cells, zebrafish mutants were identified with defects in balance and hearing. In sputnik mutants, stereociliary bundles are splayed to various degrees, with individuals displaying reduced or absent mechanotransduction. Here we show that the defects in sputnik mutants are caused by mutations in cadherin 23 (cdh23). Mutations in Cdh23 also cause deafness and vestibular defects in mice and humans, and the protein is present in hair bundles. We show that zebrafish Cdh23 protein is concentrated near the tips of hair bundles, and that tip links are absent in homozygous sputnik(tc317e) larvae. Moreover, tip links are absent in larvae carrying weak alleles of cdh23 that affect mechanotransduction but not hair bundle integrity. We conclude that Cdh23 is an essential tip link component required for hair-cell mechanotransduction.