Laser-induced gene expression in specific cells of transgenic zebrafish

Alk8 is required for neural crest cell formation and development of pharyngeal arch cartilages

The type I TGFbeta family member receptor alk8 acts in bone morphogenetic protein (BMP) signaling pathways to establish dorsoventral patterning in the early zebrafish embryo. Here, we present evidence that alk8 is required for neural crest cell (NCC) formation and that alk8 signaling gradients direct the proper patterning of premigratory NCCs. We extend our previous functional studies of alk8 to demonstrate that ectopic expression of constitutively active and dominant negative Alk8, consistently results in more medially or laterally positioned premigratory NCCs, respectively. We also demonstrate that patterning defects in premigratory NCCs, induced by alk8 misexpression, correlate with subsequent defects in NCC-derived pharyngeal arch cartilages. Furthermore, an anteroposterior effect is revealed, where overexpression of Alk8 more severely affects anterior arch cartilages and decreased Alk8 activity more severely affects posterior arch cartilage formation. Ectopic expression studies of alk8 are supported by analyses of zygotic and maternal-zygotic laf/alk8 mutants and of several BMP pathway mutants. Pharyngeal mesodermal and endodermal defects in laf/alk8 mutants suggest additional roles for alk8 in patterning of these tissues. Our results provide insight into alk8-mediated BMP signaling gradients and the establishment of premigratory NCC mediolateral positioning, and extend the model for BMP patterning of the neural crest to include that of NCC-derived pharyngeal arch cartilages.

Heat shock response:hsp70 in environmental monitoring

Heat shock proteins (Hsps) are a ubiquitous feature of cells in which these proteins cope with stress-induced denaturation of other proteins. Among the different families of Hsps, the 70 kDa family (hsp70) is the most highly conserved and has been most extensively studied. Apart from their primary role in cellular defense under stress condition, a number of studies in recent years have shown the immense potential of hsp70 in pollution monitoring using even transgenic approach both in vivo and in vitro. This article reviews the recent developments in the widespread application of hsp70 in environmental risk assessment.

Chemical discovery and global gene expression analysis in zebrafish

The zebrafish (Danio rerio) provides an excellent model for studying vertebrate development and human disease because of its ex utero, optically transparent embryogenesis and amenability to in vivo manipulation. The rapid embryonic developmental cycle, large clutch sizes and ease of maintenance at large numbers also add to the appeal of this species. Considerable genomic data has recently become publicly available that is aiding the construction of zebrafish microarrays, thus permitting global gene expression analysis. The zebrafish is also suitable for chemical genomics, in part as a result of the permeability of its embryos to small molecules and consequent avoidance of external confounding maternal effects. Finally, there is increasing characterization and analysis of zebrafish models of human disease. Thus, the zebrafish offers a high-quality, high-throughput bioassay tool for determining the biological effect of small molecules as well as for dissecting biological pathways.

Cardiac neural crest contributes to cardiomyogenesis in zebrafish

In birds and mammals, cardiac neural crest is essential for heart development and contributes to conotruncal cushion formation and outflow tract septation. The zebrafish prototypical heart lacks outflow tract septation, raising the question of whether cardiac neural crest exists in zebrafish. Here, results from three distinct lineage-labeling approaches identify zebrafish cardiac neural crest cells and indicate that these cells have the ability to generate MF20-positive muscle cells in the myocardium of the major chambers during development. Fate-mapping demonstrates that cardiac neural crest cells originate both from neural tube regions analogous to those found in birds, as well as from a novel region rostral to the otic vesicle. In contrast to other vertebrates, cardiac neural crest invades the myocardium in all segments of the heart, including outflow tract, atrium, atrioventricular junction, and ventricle in zebrafish. Three distinct groups of premigratory neural crest along the rostrocaudal axis have different propensities to contribute to different segments in the heart and are correspondingly marked by unique combinations of gene expression patterns. Zebrafish will serve as a model for understanding interactions between cardiac neural crest and cardiovascular development.

Integrin antagonists affect growth and pathfinding of ventral motor nerves in the trunk of embryonic zebrafish

Integrins are thought to be important receptors for extracellular matrix (ECM) components on growing axons. Ventral motor axons in the trunk of embryonic zebrafish grow in a midsegmental pathway through an environment rich in ECM components. To test the role of integrins in this process, integrin antagonists (the disintegrin echistatin in native and recombinant form, as well as the Arg-Gly-Asp-Ser peptide) were injected into embryos just prior to axon outgrowth at 14-16 h postfertilization (hpf). All integrin antagonists affected growth of ventral motor nerves in a similar way and native echistatin was most effective. At 24 hpf, when only the three primary motor axons per trunk hemisegment had grown out, 80% (16 of 20) of the embryos analyzed had abnormal motor nerves after injection of native echistatin, corresponding to 19% (91 of 480) of all nerves. At 33 hpf, when secondary motor axons were present in the pathway, 100% of the embryos were affected (24 of 24), with 20% of all nerves analyzed (196 of 960) being abnormal. Phenotypes comprised abnormal branching (64% of all abnormal nerves) and truncations (36% of all abnormal nerves) of ventral motor nerves at 24 hpf and mostly branching of the nerves at 33 hpf (94% of all abnormal nerves). Caudal branches were at least twice as frequent as rostral branches. Surrounding trunk tissue and a number of other axon fascicles were apparently not affected by the injections. Thus integrin function contributes to both growth and pathfinding of axons in ventral motor nerves in the trunk of zebrafish in vivo.

Windows into development: historic, current, and future perspectives on transgenic zebrafish

The recent explosion of transgenic zebrafish lines in the literature demonstrates the value of this model system for detailed in vivo analysis of gene regulation and morphogenetic movements. The optical clarity and rapid early development of zebrafish provides the ability to follow these events as they occur in live, developing embryos. This article will review the development of transgenic technology in zebrafish as well as the current and future uses of transgenic zebrafish to explore the dynamic environment of the developing vertebrate embryo.

From cells to circuits: development of the zebrafish spinal cord

The ability of an animal to carry out its normal behavioral repertoire requires generation of an enormous diversity of neurons and glia. The relative simplicity of the spinal cord makes this an especially attractive part of the nervous system for addressing questions about the development of vertebrate neural specification and function. The last decade has witnessed an explosion in our understanding of spinal cord development and the functional interactions among spinal cord neurons and glia. Cellular, genetic, molecular, physiological and behavioral studies in zebrafish have all been important in providing insights into questions that remained unanswered by studies from other vertebrate model organisms. This is the case because many zebrafish spinal neurons can be individually identified and followed over time in living embryos and larvae. In this review, we discuss what is currently known about the cellular, genetic and molecular mechanisms involved in specifying distinct cell types in the zebrafish spinal cord and how these cells establish the functional circuitry that mediates particular behaviors. We start by describing the early signals and morphogenetic movements that form the nervous system, and in particular, the spinal cord. We then provide an overview of the cell types within the spinal cord and describe how they are specified and patterned. We begin ventrally with floor plate and proceed dorsally, through motoneurons and oligodendrocytes, interneurons, astrocytes and radial glia, spinal sensory neurons and neural crest. We next describe axon pathfinding of spinal neurons. Finally, we discuss the roles of particular spinal cord neurons in specific behaviors.

Induction of neural differentiation by electrically stimulated gene expression of NeuroD2

Regulation of cell differentiation is an important assignment for cellular engineering. One of the techniques for regulation is gene transfection into undifferentiated cells. Transient expression of NeuroD2, one of neural bHLH transcription factors, converted mouse N1E-115 neuroblastoma cells into differentiated neurons. The regulation of neural bHLH expression should be a novel strategy for cell differentiation. In this study, we tried to regulate neural differentiation by NeuroD2 gene inserted under the control of heat shock protein-70 (HSP) promoter, which can be activated by electrical stimulation. Mouse neuroblastoma cell line, N1E-115, was stably transfected with expression vector containing mouse NeuroD2 cDNA under HSP promoter. Transfected cells were cultured on the electrode surface and applied electrical stimulation. After stimulation, NeuroD2 expression was induced, and transfected cells adopt a neuronal morphology at 3 days after stimulation. These results suggest that neural differentiation can be induced by electrically stimulated gene expression of NeuroD2.

Tales of regeneration in zebrafish

Complex tissue regeneration involves exquisitely coordinated proliferation and patterning of adult cells after severe injury or amputation. Certain lower vertebrates such as urodele amphibians and teleost fish have a greater capacity for regeneration than mammals. However, little is known about molecular mechanisms of regeneration, and cellular mechanisms are incompletely defined. To address this deficiency, we and others have focused on the zebrafish model system. Several helpful tools and reagents are available for use with zebrafish, including the potential for genetic approaches to regeneration. Recent studies have shed light on the remarkable ability of zebrafish to regenerate fins.

Eye regeneration at the molecular age

Eye tissues such as the lens and the retina possess remarkable regenerative abilities. In amphibians, a complete lens can be regenerated after lentectomy. The process is a classic example of transdifferentiation of one cell type to another. Likewise, retina can be regenerated, but the strategy used to replace the damaged retina differs, depending on the animal system and the age of the animal. Retina can be regenerated by transdifferentiation or by the use of stem cells. In this review, we present a synthesis on the regenerative capacity of eye tissues in different animals with emphasis on the strategy and the molecules involved. In addition, we stress the place of this field at the molecular age and the importance of the recent technologic advances.

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.

Making the connection: retinal axon guidance in the zebrafish

Genetic screens in zebrafish have identified a large number of mutations that affect neural connectivity in the developing visual system. These mutants define genes essential for accurate retinal axon guidance in the eye and brain and the characterization of these mutants is helping to define the cellular and molecular mechanisms that guide axons in the vertebrate embryo. The combination of zebrafish genetic and embryological approaches promises to greatly increase our understanding of how multiple guidance mechanisms establish the complex neural interconnectivity of the vertebrate brain.

Analysis of gene function in the zebrafish retina

Mutagenesis screens in zebrafish have uncovered several hundred mutant alleles affecting the development of the retina and established the zebrafish as one of the leading models of vertebrate eye development. In addition to forward genetic mutagenesis approaches, gene function in the zebrafish embryo is being studied using several reverse genetic techniques. Some of these rely on the overexpression of a gene product, others take advantage of antisense oligonucleotides to block function of selected loci. Here we describe these methods in the context of the developing eye.

Zebrafish as a model for hearing and deafness

The zebrafish is an especially attractive model for the study of the development and function of the vertebrate inner ear. It combines rapid and accessible embryogenesis with a host of genetic and genomic tools for systematic gene discovery and analysis. A large collection of mutations affecting development and function of the ear and a related sensory system, the lateral line, have been isolated; several of these have now been cloned, and at least five provide models for human deafness disorders. Disruption of multiple genes, using both forward and reverse genetic approaches, has established key players--both signaling molecules and autonomous factors--responsible for induction and specification of the otic placode. Vestibular and auditory defects have been detected in adult animals, making the zebrafish a useful system in which to tackle the genetic causes of late onset deafness and vestibular disease.

Rapid in vivo labeling of identified zebrafish neurons

We report a simple and rapid method to label individual neurons in live zebrafish embryos and to examine their gene expression profiles. Injection of plasmid DNA encoding an alpha-tubulin promotor driving GFP expression results in mosaic embryos containing a limited number of GFP-positive neurons. Labeled neurons express GFP in their soma and axon, providing the opportunity to analyze pathfinding behaviors of identified neurons in vivo. Moreover, the presence of only a small subset of GFP tagged neurons permits the rapid anatomical identification of these neurons based on soma position and axonal trajectory. Analysis of injected embryos reveals that most, if not all, spinal cord cell types and many other neuronal cell types elsewhere in the nervous system can be GFP tagged. Finally, by combining GFP labeling of individual neurons with fluorescent in situ hybridization, we demonstrate the potential of this method to elucidate gene expression patterns at single cell resolution.

Expression and genomic organization of the zebrafish chaperonin gene complex

Chaperonin 10 and chaperonin 60 monomers exist within the multimeric mitochondrial chaperonin folding complex with a stoichiometry of 2:1. This complex is located in the mitochondrial matrix, where it aids in the folding and acquisition of the tertiary structure of proteins. We have previously isolated the cpn10 cDNA in zebrafish (Danio rerio), and demonstrated that it is ubiquitously expressed during embryonic development and transcriptionally upregulated after exposure to heat shock. In the present study, we have isolated a cDNA encoding chaperonin 60 (cpn60) from zebrafish, and have shown that it is similarly expressed uniformly and ubiquitously throughout early embryonic development of zebrafish. Upregulation of cpn60 expression was also observed after exposure of zebrafish embryos to a heat shock of 1 h at 37 degrees C compared with control embryos raised at 27 degrees C. The induction of the cpn60 heat shock response was greatest after 1 h of heat shock, whereas significant decreases of cpn60 mRNA were observed within 2 h following a return to 27 degrees C. We subsequently isolated genomic DNA sequences for both of these genes, and show that they are also arranged in a head-to-head organization and share a common bidirectional promoter that contains a single heat shock element (HSE). Our database analysis shows that this head-to-head organization is also found in human (Homo sapiens), rat (Rattus norvegicus), pufferfish (Fugu rubripes), and Caenorhabditis elegans, but not in Drosophila or yeast (Saccharomyces cerevisiae). The data suggest that the genomic organization of the cpn gene complex has been conserved across the vertebrates.

Development of the locomotor network in zebrafish

The zebrafish is a leading model for studies of vertebrate development and genetics. Its embryonic motor behaviors are easy to assess (e.g. for mutagenic screens), the embryos develop rapidly (hatching as larvae at 2 days) and are transparent, permitting calcium imaging and patch clamp recording in vivo. We review primarily the recent advances in understanding the cellular basis for the development of motor activities in the developing zebrafish. The motor activities are generated largely in the spinal cord and hindbrain. In the embryo these segmented structures possess a relatively small number of repeating sets of identifiable neurons. Many types of neurons as well as the two types of muscle cells have been classified based on their morphologies. Some of the molecular signals for cellular differentiation have been identified recently and mutations affecting cell development have been isolated. Embryonic motor behaviors appear in sequence and consist of an early period of transient spontaneous coiling contractions, followed by the emergence of twitching responses to touch, and later by the ability to swim. Coiling contractions are generated by an electrically coupled network of a subset of spinal neurons whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming. Swimming becomes sustained in larvae once the neuromodulatory serotonergic system develops. These results indicate many similarities between developing zebrafish and other vertebrates in the properties of the synaptic drive underlying locomotion. Therefore, the zebrafish is a useful preparation for gaining new insights into the development of the neural control of vertebrate locomotion. As the types of neurons, transmitters, receptors and channels used in the locomotor network are being defined, this opens the possibility of combining cellular neurophysiology with forward and reverse molecular genetics to understand the principles of locomotor network assembly and function.

Vascular cell biology in vivo: a new piscine paradigm?

Understanding how blood vessels form has become increasingly important in recent years yet remains difficult to study. The architecture and context of blood vessels are difficult to reproduce in vitro, and most developing blood vessels in vivo are relatively inaccessible to observation and experimental manipulation. Zebrafish, however, provide several advantages. They have small, accessible, transparent embryos and larvae, facilitating high-resolution imaging in vivo. In addition, genetic and experimental tools and methods are available for functional manipulation of the entire organism, vascular tissues or even single vascular- or non-vascular cells. Together, these features make the fish amenable to 'in vivo vascular cell biology'.

Heat shock protein genes and their functional significance in fish

Despite decades of intensive investigation, important questions remain regarding the functional, ecological, and evolutionary roles of heat shock proteins. In this paper, we discuss the utility of fish as a model system to address these questions, and review the relevant studies of heat shock protein genes and the regulation of their expression in fish. Although molecular studies of the heat shock proteins in fish are still in their early descriptive phase, data are rapidly being collected. More is known about the biotic and abiotic factors regulating heat shock proteins. We briefly review these studies and focus on the role of heat shock proteins in development, their regulation by the endocrine system, and their importance in fish in nature. Functional genomics approaches will provide the tools necessary to gain a comprehensive understanding of the significance of heat shock proteins in the cellular stress response, in the physiological processes at higher levels of organization, and in the whole animal in its natural environment.

Constructing the hindbrain: insights from the zebrafish

The hindbrain is responsible for controlling essential functions such as respiration and heart beat that we literally do not think about most of the time. In addition, cranial nerves projecting from the hindbrain control muscles in the jaw, eye, and face, and receive sensory input from these same areas. In all vertebrates that have been studied, the hindbrain passes through a segmented phase shortly after the neural tube has formed, with a series of seven bulges--the rhombomeres--forming along the anterior-posterior extent of the neural tube. Our current understanding of vertebrate hindbrain development comes from integrating data from several model systems. Work on the chick has helped us to understand the cell biology of the rhombomeres, whereas the power of mouse molecular genetics has allowed investigation of the molecular mechanisms underlying their development. This review focuses on the special insights that the zebrafish system has provided to our understanding of hindbrain development. As we will discuss, work in the zebrafish has elucidated inductive events that specify the presumptive hindbrain domain and has identified genes required for hindbrain segmentation and the specification of segment identities.

A quantitative analysis of the kinetics of Gal4 activator and effector gene expression in the zebrafish

Using a temperature-inducible hsp70:Gal4 activator and UAS:myc-notch1a-intra as effector, we determined quantitatively the kinetics of expression of both transgenes and analysed the effects of varying their expressivity on several phenotypic traits in the developing zebrafish. hsp70:Gal4 is transcribed within 15 min after temperature-mediated induction, but Gal4 RNA decays rapidly. The Gal4 protein was found to be quite stable, as functional Gal4, which was detectable 1.5 h after heat shock (HS), persisted for at least 13 h. myc-notch1a-intra RNA is expressed approximately 1.5 h after HS, but unlike the Gal4 RNA, it was found to be very stable; it continues to accumulate during the succeeding 17 h after HS. Fully penetrant phenotypic effects are obtained after a relatively long activator induction with a 30-min HS.

The heat-inducible zebrafish hsp70 gene is expressed during normal lens development under non-stress conditions

In the present study, we show that the stress-inducible hsp70 gene in zebrafish is strongly and specifically expressed during normal lens formation from 28 to 38 hours post-fertilization, and is subsequently downregulated by 2 days of age. Only weak constitutive hsp70 mRNA signal was sporadically observed in other embryonic tissues. Similarly, transgenic fish carrying a 1.5 kb fragment of the hsp70 promoter linked to eGFP exhibited fluorescence only in the lens. In contrast, both the endogenous hsp70 gene and the transgene were strongly expressed throughout the embryo following heat shock at the same developmental stages.

Green fluorescent protein expression in germ-line transmitted transgenic zebrafish under a stratified epithelial promoter from keratin8

A zebrafish cDNA encoding a novel keratin protein was characterized and named keratin8, or krt8. krt8 expression was initiated at 4.5 hr postfertilization, immediately after the time of zygotic genome activation. The expression is limited to a single layer of envelope cells on the surface of embryos and, in later stages, it also appears in the innermost epithelial layer of the anterior- and posteriormost portions of the digestive tract. In adult, its expression was limited to the surface layer of stratified epithelial tissues, including skin epidermis and epithelia of mouth, pharynx, esophagus, and rectum but not in the gastral and intestinal epithelia. By using a 2.2-kb promoter from krt8, several stable green fluorescent protein (gfp) transgenic zebrafish lines were established. All of these transgenic lines displayed GFP expression in tissues mentioned above except for the rectum; therefore, the pattern of transgenic GFP expression is essentially identical to that of the endogenous krt8 mRNAs. krt8-GFP fusion protein was also expressed in zebrafish embryos under a ubiquitous promoter, and the fusion protein was capable of assembling into intermediate filaments only in the epithelia that normally expressed krt8 mRNAs, indicating the specificity of keratin assembly in vivo.

Wiring the zebrafish: axon guidance and synaptogenesis

Many zebrafish mutants have specific defects in axon guidance or synaptogenesis, particularly in the retinotectal and motor systems. Several mutants have now been characterized in detail and/or cloned. A combination of genetic studies, in vivo imaging and new techniques for misexpressing genes or blocking their function promises to reveal the molecules and principles that govern wiring of the vertebrate nervous system.

A transgenic Lef1/beta-catenin-dependent reporter is expressed in spatially restricted domains throughout zebrafish development

The Wnt/beta-catenin signaling pathway plays multiple roles during embryonic development, only a few of which have been extensively characterized. Although domains of Wnt expression have been identified throughout embryogenesis, anatomical and molecular characterization of responding cells has been mostly unexplored. We have generated a transgenic zebrafish line that expresses a destabilized green fluorescent protein (GFP) variant under the control of a beta-catenin responsive promoter. Early zygotic expression of this transgene (TOPdGFP) mirrors known domains of Wnt signaling in the embryo. Loss of Lef1 activity results in decreased reporter expression and posterior defects, while loss of Tcf3 (Headless, Hdl) activity does not alter reporter expression, even though it results in loss of forebrain structures. In addition, ectopic Wnt1 expression can activate the reporter. In older embryos, we identify a number of transgene-expressing cell populations as novel sites of beta-catenin signaling. We conclude that our TOP-dGFP reporter line faithfully illustrates domains of beta-catenin activity and enables the identification of responsive cell populations.

Spatiotemporal expression patterns of slit and robo genes in the rat brain

Diffusible chemorepellents play a major role in guiding developing axons toward their correct targets by preventing them from entering or steering them away from certain regions. Genetic studies in Drosophila revealed a repulsive guidance system that prevents inappropriate axons from crossing the central nervous system midline; this repulsive system is mediated by the secreted extracellular matrix protein Slit and its receptors Roundabout (Robo). Three distinct slit genes (slit1, slit2, and slit3) and three distinct robo genes (robo1, robo2, rig-1) have been cloned in mammals. However, to date, only Robo1 and Robo2 have been shown to be receptors for Slits. In rodents, Slits have been shown to function as chemorepellents for several classes of axons and migrating neurons. In addition, Slit can also stimulate the formation of axonal branches by some sensory axons. To identify Slit-responsive neurons and to help analyze Slit function, we have studied, by in situ hybridization, the expression pattern of slits and their receptors robo1 and robo2, in the rat central nervous system from embryonic stages to adult age. We found that their expression patterns are very dynamic: in most regions, slit and robo are expressed in a complementary pattern, and their expression is up-regulated postnatally. Our study confirms the potential role of these molecules in axonal pathfinding and neuronal migration. However, the persistence of robo and slit expression suggests that the couple slit/robo may also have an important function in the adult brain.

The art and design of genetic screens: zebrafish

Inventive genetic screens in zebrafish are revealing new genetic pathways that control vertebrate development, disease and behaviour. By exploiting the versatility of zebrafish, biological processes that had been previously obscured can be visualized and many of the responsible genes can be isolated. Coupled with gene knockdown and overexpression technologies, and small-molecule-induced phenotypes, genetic screens in zebrafish provide a powerful system by which to dissect vertebrate gene function and gene networks.

Expression of Ol-KIP, a cyclin-dependent kinase inhibitor, in embryonic and adult medaka (Oryzias latipes) central nervous system

From an expression screen in a fish model, the medaka, we have isolated Ol-KIP (Oryzias latipes-kinase inhibitor protein), a new member of the KIP subfamily of cyclin-dependent kinase (Cdk) inhibitors. We have analysed its expression in the developing and adult brain by in situ hybridization and by double labeling with Ol-KIP mRNA and proliferating cell nuclear antigen (PCNA) antibodies. Ol-KIP presents a complex expression pattern in several areas of the embryonic central nervous system, most often in close vicinity to proliferative neuroepithelia. We studied in great detail its expression in the optic tectum: Ol-KIP is expressed in a ring-shaped domain lying exactly between the proliferative and the postmitotic zones of this structure and is, therefore, potentially involved in cell cycle exit. In the adult CNS, Ol-KIP expression persists in numerous nuclei, both close and distant from proliferative ventricular areas. So, Ol-KIP expression is in part compatible with a sustained "stop signal" role for proliferation, but its expression in postmitotic zones suggests that KIP proteins may have late neuronal function(s), in addition to inhibiting Cdks. This first detailed study of the expression profile of a KIP gene in a nonmammalian vertebrate, thus, opens perspectives for analysing the role of these regulators in brain development and function.

Duplicate mitf genes in zebrafish: complementary expression and conservation of melanogenic potential

Mutations in the zebrafish nacre/mitfa gene, expressed in all embryonic melanogenic cells, perturb only neural crest melanocytes, suggesting redundancy of mitfa with another gene in the zebrafish retinal pigment epithelium (RPE). Here, we describe a second zebrafish mitf gene, mitfb, which may fulfill this role. The proteins encoded by the two zebrafish mitf genes appear homologous to distinct isoforms generated by alternately spliced mRNAs of the single mammalian Mitf gene, suggesting specialization of the two zebrafish genes following a duplication event. Consistent with this hypothesis, expression of mitfa and mitfb is partially overlapping. mitfb is coexpressed with mitfa in the RPE at an appropriate time to compensate for loss of mitfa function in the nacre mutant but is not expressed in neural crest melanoblasts. Additionally, mitfb is expressed in the epiphysis and olfactory bulb where mitfa is not, and where Mitf expression has not previously been reported in other species. mitfb, but not a zebrafish ortholog of the closely related gene tfe3, can rescue neural crest melanophore development in nacre/mitfa mutant embryos when expressed via the mitfa promoter. These data suggest that mitfa and mitfb together may recapitulate the expression and functions of a single ancestral Mitf gene, and that mitfb may serve additional novel functions.

Cloning and expression of three zebrafish roundabout homologs suggest roles in axon guidance and cell migration

We report the cloning and expression patterns of three novel zebrafish Roundabout homologs. The Roundabout (robo) gene encodes a transmembrane receptor that is essential for axon guidance in Drosophila and Robo family members have been implicated in cell migration. Analysis of extracellular domains and conserved cytoplasmic motifs shows that zebrafish Robo1 and Robo2 are orthologs of mammalian Robo1 and Robo2, respectively, while zebrafish Robo3 is likely to be an ortholog of mouse Rig-1. The three zebrafish robos are expressed in distinct but overlapping patterns during embryogenesis. They are highly expressed in the developing nervous system, including the olfactory system, visual system, hindbrain, cranial ganglia, spinal cord, and posterior lateral line primordium. They are also expressed in several nonneuronal tissues, including somites and fin buds. The timing and patterns of expression suggest roles for zebrafish robos in axon guidance and cell migration. Wiley-Liss, Inc.

An instructive function for Notch in promoting gliogenesis in the zebrafish retina

The Gal4-UAS technique has been used to misexpress a constitutively active Notch receptor variant (notch1a-intra) in the developing zebrafish retina. This is the first study to use this technique to misexpress genes and assess their function in neural development of the zebrafish. Expression of activated Notch1a either ubiquitously, driven by a heat-shock70 promoter, or in a spatially regulated manner, controlled by the deltaD promoter, causes a block in neuronal differentiation that affects all cell types. Developing cells take on either a glial fate or remain undifferentiated. A large number of cells eventually undergo apoptosis. These phenotypic effects of activated Notch1a are expressed cell autonomously. Cells within central regions of the retina adopt a glial fate if they express activated Notch1a in a time window that extends from 27 to 48 hours postfertilization. This period corresponds mainly to the time of origin of ganglion cells in the normal retina. Activation of notch1a at later stages results in defects in cell type specification that remain restricted to the ciliary marginal zone, whereas neuronal types are specified normally within the central region. These observations indicate that glial differentiation is initiated by Notch1a-intra expressing cells, which become postmitotic in the same time window. Our results strongly suggest that Notch1a instructs a certain cell population to enter gliogenesis, and keeps the remaining cells in an undifferentiated state. Some or all of these cells will eventually succumb to apoptosis.

The cell biology of neuronal navigation

Morphogenesis of the nervous system requires the directed migration of postmitotic neurons to designated locations in the nervous system and the guidance of axon growth cones to their synaptic targets. Evidence suggests that both forms of navigation depend on common guidance molecules, surface receptors and signal transduction pathways that link receptor activation to cytoskeletal reorganization. Future challenges remain not only in identifying all the components of the signalling pathways, but also in understanding how these pathways achieve signal amplification and adaptation-two essential cellular processes for neuronal navigation.

Overexpression of a slit homologue impairs convergent extension of the mesoderm and causes cyclopia in embryonic zebrafish

Slit is expressed in the midline of the central nervous system both in vertebrates and invertebrates. In Drosophila, it is the midline repellent acting as a ligand for the Roundabout (Robo) protein, the repulsive receptor which is expressed on the growth cones of the commissural neurons. We have isolated cDNA fragments of the zebrafish slit2 and slit3 homologues and found that both genes start to be expressed by the midgastrula stage well before the axonogenesis begins in the nervous system, both in the axial mesoderm, and slit2 in the anterior margin of the neural plate and slit3 in the polster at the anterior end of the prechordal mesoderm. Later, expression of slit2 mRNA is detected mainly in midline structures such as the floor plate cells and the hypochord, and in the anterior margins of the neural plates in the zebrafish embryo, while slit3 expression is observed in the anterior margin of the prechordal plate, the floorplate cells in the hindbrain, and the motor neurons both in the hindbrain and the spinal cord. To study the role of Slit in early embryos, we overexpressed Slit2 in the whole embryos either by injection of its mRNA into one-cell stage embryos or by heat-shock treatment of the transgenic embryos which carries the slit2 gene under control of the heat-shock promoter. Overexpression of Slit2 in such ways impaired the convergent extension movement of the mesoderm and the rostral migration of the cells in the dorsal diencephalon and resulted in cyclopia. Our results shed light on a novel aspect of Slit function as a regulatory factor of mesodermal cell movement during gastrulation.

Zebrafish genetics and vertebrate heart formation

Forward-genetic analyses in Drosophila and Caenorhabditis elegans have given us unprecedented insights into many developmental mechanisms. To study the formation of organs that contain cell types and structures not present in invertebrates, a vertebrate model system amenable to forward genetics would be very useful. Recent work shows that a newly initiated genetic approach in zebrafish is already making significant contributions to understanding the development of the vertebrate heart, an organ that contains several vertebrate-specific features. These and other studies point to the utility of the zebrafish system for studying a wide range of vertebrate-specific processes.

Control of motor axon guidance in the zebrafish embryo

The zebrafish neuromuscular system has been an exemplary model for studying motor axon guidance since its detailed characterization almost two decades ago. In particular, characterization and detailed analysis has focused on the development and axogenesis of early developing primary motoneurons. During the first day of development, neuromuscular connections are limited to three primary motoneurons per spinal cord hemisegment innervating three discreet myotome territories. Observations of dye labeled primary motor axons in living embryos revealed that axogenesis is highly stereotyped with each primary motor axon extending along specific pathways and displaying particular characteristics. Exploiting the unique attributes of zebrafish, notably the ability to analyze motoneurons in living embryos and the capability to induce mutations, has allowed a comprehensive cellular, molecular and genetic approach to discerning the mechanisms that control the formation of neuromuscular connectivity. Knowledge gained from this body of work not only relates to zebrafish, but to vertebrate axon guidance in general.