Chapter 4 Early Pressure Screens

Transient window of resilience during early development minimizes teratogenic effects of heat in zebrafish embryos

Background: Transient heat shock during early development is an established experimental paradigm for doubling the genome of the zebrafish zygote, which has practical applications in expedited identification of recessive mutations in genetic screens. Despite the simplicity of the strategy and the genetic tractability of zebrafish, heat shock has not been used for genome doubling since the proof‐of‐principle experiments done in the 1980s. This is because of poor survival of embryos that ensue from transient heat shocks and gross developmental abnormalities in the few survivors, which is incompatible with phenotype driven screens. Results: We show that heat shocks during early zebrafish development uncouple the second cycle of DNA and centrosome duplication. Interestingly, the developmental time of the heat shock that triggers the dissociation between DNA and centrosome duplication cycles significantly affect the potential of embryos to survive and attain normal morphology. The potential to develop normally after a heat shock alters in a developmental time span of 2 min in zebrafish embryos, a phenomenon that has not been reported in any species. Conclusions: The existence of heat resilient developmental windows and reduced heat teratogenicity during these windows could be an effective step forward in practical application of transient heat for experimental manipulation of ploidy in zebrafish. More broadly, heat resilience before zygotic genome activation suggests that metazoan embryos may possess innate protective features against heat beyond the canonical heat shock response. Developmental Dynamics 247:992–1004, 2018. © 2018 Wiley Periodicals, Inc.

Zebrafish embryos at the end of pronuclear fusion and before initiation of zygotic mitosis are resistant to teratogenic effects of heat.

The teratogenic heat resilient window exists transiently during the maternally controlled phase of development.

Heat shock during the teratogenic heat resilient window enables generation of morphologically normal zebrafish tetraploids.

Diploidization of haploids by transient heat shocks during the teratogenic heat resilient windows aids in effective generation of gynogenic diploids.

Pharyngeal morphogenesis requires fras1-itga8-dependent epithelial-mesenchymal interaction

Both Fras1 and Itga8 connect mesenchymal cells to epithelia by way of an extracellular 'Fraser protein complex' that functions in signaling and adhesion; these proteins are vital to the development of several vertebrate organs. We previously found that zebrafish fras1 mutants have craniofacial defects, specifically, shortened symplectic cartilages and cartilage fusions that spare joint elements. During a forward mutagenesis screen, we identified a new zebrafish mutation, b1161, that we show here disrupts itga8, as confirmed using CRISPR-generated itga8 alleles. fras1 and itga8 single mutants and double mutants have similar craniofacial phenotypes, a result expected if loss of either gene disrupts function of the Fraser protein complex. Unlike fras1 mutants or other Fraser-related mutants, itga8 mutants do not show blistered tail fins. Thus, the function of the Fraser complex differs in the craniofacial skeleton and the tail fin. Focusing on the face, we find that itga8 mutants consistently show defective outpocketing of a late-forming portion of the first pharyngeal pouch, and variably express skeletal defects, matching previously characterized fras1 mutant phenotypes. In itga8 and fras1 mutants, skeletal severity varies markedly between sides, indicating that both mutants have increased developmental instability. Whereas fras1 is expressed in epithelia, we show that itga8 is expressed complementarily in facial mesenchyme. Paired with the observed phenotypic similarity, this expression indicates that the genes function in epithelial-mesenchymal interactions. Similar interactions between Fras1 and Itga8 have previously been found in mouse kidney, where these genes both regulate Nephronectin (Npnt) protein abundance. We find that zebrafish facial tissues express both npnt and the Fraser gene fibrillin2b (fbn2b), but their transcript levels do not depend on fras1 or itga8 function. Using a revertible fras1 allele, we find that the critical window for fras1 function in the craniofacial skeleton is between 1.5 and 3 days post fertilization, which coincides with the onset of fras1-dependent and itga8-dependent morphogenesis. We propose a model wherein Fras1 and Itga8 interact during late pharyngeal pouch morphogenesis to sculpt pharyngeal arches through epithelial-mesenchymal interactions, thereby stabilizing the developing craniofacial skeleton.

Ploidy manipulation and induction of alternate cleavage patterns through inhibition of centrosome duplication in the early zebrafish embryo

BACKGROUND

Whole genome duplication is a useful genetic tool because it allows immediate and complete genetic homozygosity in gynogenetic offspring. A whole genome duplication method in zebrafish, Heat Shock, involves a heat pulse in the period 13-15 min postfertilization (mpf) to inhibit cytokinesis of the first mitotic cycle. However, Heat Shock produces a relatively low yield of gynogenotes.

RESULTS

A heat pulse at a later time point during the first cell cycle (22 mpf, HS2) results in a high (>80%) frequency of embryos exhibiting a precise one-cell division stall during the second cell cycle, inducing whole genome duplication. Coupled with haploid production, HS2 generates viable gynogenetic diploids with yields up to 4 times higher than those achieved through standard Heat Shock. The cell cycle delay also causes blastomere cleavage pattern variations, supporting a role for cytokinesis in spindle orientation during the following cell cycle.

CONCLUSIONS

Our studies provide a new tool for whole genome duplication, induced gynogenesis, and cleavage pattern alteration in zebrafish, based on a time period before the initiation of cell division that is sensitive to temperature-mediated interference with centrosome duplication. Targeting of this period may also facilitate genetic and developmental manipulations in other organisms.

Planar polarity pathway and Nance-Horan syndrome-like 1b have essential cell-autonomous functions in neuronal migration

Components of the planar cell polarity (PCP) pathway are required for the caudal tangential migration of facial branchiomotor (FBM) neurons, but how PCP signaling regulates this migration is not understood. In a forward genetic screen, we identified a new gene, nhsl1b, required for FBM neuron migration. nhsl1b encodes a WAVE-homology domain-containing protein related to human Nance-Horan syndrome (NHS) protein and Drosophila GUK-holder (Gukh), which have been shown to interact with components of the WAVE regulatory complex that controls cytoskeletal dynamics and with the polarity protein Scribble, respectively. Nhsl1b localizes to FBM neuron membrane protrusions and interacts physically and genetically with Scrib to control FBM neuron migration. Using chimeric analysis, we show that FBM neurons have two modes of migration: one involving interactions between the neurons and their planar-polarized environment, and an alternative, collective mode involving interactions between the neurons themselves. We demonstrate that the first mode of migration requires the cell-autonomous functions of Nhsl1b and the PCP components Scrib and Vangl2 in addition to the non-autonomous functions of Scrib and Vangl2, which serve to polarize the epithelial cells in the environment of the migrating neurons. These results define a role for Nhsl1b as a neuronal effector of PCP signaling and indicate that proper FBM neuron migration is directly controlled by PCP signaling between the epithelium and the migrating neurons.

Practical approaches for implementing forward genetic strategies in zebrafish

The tropical fresh water minnow, Danio rerio, more commonly known as zebrafish, has emerged rapidly over the last decade as a powerful tool for developmental geneticists. External fertilization, high fecundity, a short generation time, and optical transparency of embryos during early development combined with the amenability to a variety of genetic manipulations constitute in the zebrafish the convergence of several unique advantages for a vertebrate model system. Traditional forward genetic screens, which employ the use of a chemical mutagen such as N-ethyl-N-nitrosourea to induce mutations in the male genome, have also proven to be highly successful in the zebrafish. This chapter provides experimental approaches to successfully induce pre-meiotic mutations in the male zebrafish germline and genetic strategies to recover and maintain such mutations in subsequent generations (Section 3.1). Though discussed specifically in the context of zebrafish research in this chapter, many of these genetic approaches may also be broadly applicable in other model systems. We also discuss experimental techniques to manipulate the ploidy of zebrafish embryos, which when used in combination with the standard mutagenesis protocol significantly expedite the identification of the induced mutations (Section 3.2). Additional stand-alone procedures are provided in Section 3.3, which are also required for the execution of the experiments discussed in its preceding sections.

Collagen XIXa1 is crucial for motor axon navigation at intermediate targets

During development, motor axons navigate from the spinal cord to their muscle targets in the periphery using stereotyped pathways. These pathways are broken down into shorter segments by intermediate targets where axon growth cones are believed to coordinate guidance cues. In zebrafish stumpy mutants, embryonic development proceeds normally; however, as trunk motor axons stall at their intermediate targets, suggesting that Stumpy is needed specifically for motor axon growth cones to proceed past intermediate targets. Fine mapping and positional cloning revealed that stumpy was the zebrafish homolog of the atypical FACIT collagen collagenXIXa1 (colXIX). colXIX expression was observed in a temporal and spatial pattern, consistent with a role in motor axon guidance at intermediate targets. Knocking down zebrafish ColXIX phenocopied the stumpy phenotype and this morpholino phenotype could be rescued by adding back either mouse or zebrafish colXIX RNA. The stumpy phenotype was also partially rescued in mutants by first knocking down zebrafish ColXIX and adding back colXIX RNA, suggesting that the mutation is acting as a dominant negative. Together, these results demonstrate a novel function for a FACIT collagen in guiding vertebrate motor axons through intermediate targets.

The lta4h locus modulates susceptibility to mycobacterial infection in zebrafish and humans

Exposure to Mycobacterium tuberculosis produces varied early outcomes, ranging from resistance to infection to progressive disease. Here we report results from a forward genetic screen in zebrafish larvae that identify multiple mutant classes with distinct patterns of innate susceptibility to Mycobacterium marinum. A hypersusceptible mutant maps to the lta4h locus encoding leukotriene A(4) hydrolase, which catalyzes the final step in the synthesis of leukotriene B(4) (LTB(4)), a potent chemoattractant and proinflammatory eicosanoid. lta4h mutations confer hypersusceptibility independent of LTB(4) reduction, by redirecting eicosanoid substrates to anti-inflammatory lipoxins. The resultant anti-inflammatory state permits increased mycobacterial proliferation by limiting production of tumor necrosis factor. In humans, we find that protection from both tuberculosis and multibacillary leprosy is associated with heterozygosity for LTA4H polymorphisms that have previously been correlated with differential LTB(4) production. Our results suggest conserved roles for balanced eicosanoid production in vertebrate resistance to mycobacterial infection.

Analysis of the retina in the zebrafish model

The zebrafish is one of the leading models for the analysis of the vertebrate visual system. A wide assortment of molecular, genetic, and cell biological approaches is available to study zebrafish visual system development and function. As new techniques become available, genetic analysis and imaging continue to be the strengths of the zebrafish model. In particular, recent developments in the use of transposons and zinc finger nucleases to produce new generations of mutant strains enhance both forward and reverse genetic analysis. Similarly, the imaging of developmental and physiological processes benefits from a wide assortment of fluorescent proteins and the ways to express them in the embryo. The zebrafish is also highly attractive for high-throughput screening of small molecules, a promising strategy to search for compounds with therapeutic potential. Here we discuss experimental approaches used in the zebrafish model to study morphogenetic transformations, cell fate decisions, and the differentiation of fine morphological features that ultimately lead to the formation of the functional vertebrate visual system.

Zebrafish mutants with disrupted early T-cell and thymus development identified in early pressure screen

Generation of mature T lymphocytes requires an intact hematopoietic stem cell compartment and functional thymic epithelium. We used the zebrafish (Danio rerio) to isolate mutations that affect the earliest steps in T lymphopoiesis and thymic organogenesis. Here we describe the results of a genetic screen in which gynogenetic diploid offspring from heterozygous females were analyzed by whole-mount in situ hybridization for the expression of rag-1. To assess immediately if a global defect in hematopoiesis resulted in the mutant phenotype, alpha-embryonic globin expression was simultaneously assayed for multilineage defects. In this report, we present the results obtained with this strategy and show representative mutant phenotypes affecting early steps in T-cell development and/or thymic epithelial cell development. We discuss the advantage of this strategy and the general usefulness of the zebrafish as a model system for vertebrate lymphopoiesis and thymic organogenesis.

Immunology and zebrafish: spawning new models of human disease

The zebrafish has emerged as a powerful new vertebrate model of human disease. Initially prominent in developmental biology, the zebrafish has now been adopted into varied fields of study including immunology. In this review, we describe the characteristics of the zebrafish, which make it a versatile model, including a description of its immune system with its remarkable similarities to its mammalian counterparts. We review the zebrafish disease models of innate and adaptive immunity. Models of immune system malignancies are discussed that are either based on oncogene over-expression or on our own forward-genetic screen that was designed to identify new models of immune dysregulation.

Genetic screen for mutations affecting development and function of the enteric nervous system

An intact enteric nervous system is required for normal gastrointestinal tract function. Several human conditions result from decreased innervation by enteric neurons; however, the genetic basis of enteric nervous system development and function is incompletely understood. In an effort to increase our understanding of the mechanisms underlying enteric nervous system development, we screened mutagenized zebrafish for changes in the number or distribution of enteric neurons. We also established a motility assay and rescreened mutants to learn whether enteric neuron number is correlated with gastrointestinal motility in zebrafish. We describe mutations isolated in our screen that affect enteric neurons specifically, as well as mutations that affect other neural crest derivatives or have pleiotropic effects. We show a correlation between the severity of enteric neuron loss and gastrointestinal motility defects. This screen provides biological tools that serve as the basis for future mechanistic studies.

Network of coregulated spliceosome components revealed by zebrafish mutant in recycling factor p110

The spliceosome cycle consists of assembly, catalysis, and recycling phases. Recycling of postspliceosomal U4 and U6 small nuclear ribonucleoproteins (snRNPs) requires p110/SART3, a general splicing factor. In this article, we report that the zebrafish earl grey (egy) mutation maps in the p110 gene and results in a phenotype characterized by thymus hypoplasia, other organ-specific defects, and death by 7 to 8 days postfertilization. U4/U6 snRNPs were disrupted in egy mutant embryos, demonstrating the importance of p110 for U4/U6 snRNP recycling in vivo. Surprisingly, expression profiling of the egy mutant revealed an extensive network of coordinately up-regulated components of the spliceosome cycle, providing a mechanism compensating for the recycling defect. Together, our data demonstrate that a mutation in a general splicing factor can lead to distinct defects in organ development and cause disease.

Neuromuscular synaptogenesis in wild-type and mutant zebrafish

Genetic screens for synaptogenesis mutants have been performed in many organisms, but few if any have simultaneously screened for defects in pre- and postsynaptic specializations. Here, we report the results of a small-scale genetic screen, the first in vertebrates, for defects in synaptogenesis. Using zebrafish as a model system, we identified seven mutants that affect different aspects of neuromuscular synapse formation. Many of these mutant phenotypes have not been previously reported in zebrafish and are distinct from those described in other organisms. Characterization of mutant and wild-type zebrafish, from the time that motor axons first arrive at target muscles through adulthood, has provided the new information about the cellular events that occur during neuromuscular synaptogenesis. These include insights into the formation and dispersal of prepatterned AChR clusters, the relationship between motor axon elongation and synapse size, and the development of precise appositions between presynaptic clusters of synaptic vesicles in nerve terminals and postsynaptic receptor clusters. In addition, we show that the mechanisms underlying synapse formation within the myotomal muscle itself are largely independent of those that underlie synapse formation at myotendinous junctions and that the outgrowth of secondary motor axons requires at least one cue not necessary for the outgrowth of primary motor axons, while other cues are required for both. One-third of the mutants identified in this screen did not have impaired motility, suggesting that many genes involved in neuromuscular synaptogenesis were missed in large scale motility-based screens. Identification of the underlying genetic defects in these mutants will extend our understanding of the cellular and molecular mechanisms that underlie the formation and function of neuromuscular and other synapses.

Making waves in cancer research: new models in the zebrafish

The zebrafish (Danio rerio) has proven to be a powerful vertebrate model system for the genetic analysis of developmental pathways and is only beginning to be exploited as a model for human disease and clinical research. The attributes that have led to the emergence of the zebrafish as a preeminent embryological model, including its capacity for forward and reverse genetic analyses, provides a unique opportunity to uncover novel insights into the molecular genetics of cancer. Some of the advantages of the zebrafish animal model system include fecundity, with each female capable of laying 200-300 eggs per week, external fertilization that permits manipulation of embryos ex utero, and rapid development of optically clear embryos, which allows the direct observation of developing internal organs and tissues in vivo. The zebrafish is amenable to transgenic and both forward and reverse genetic strategies that can be used to identify or generate zebrafish models of different types of cancer and may also present significant advantages for the discovery of tumor suppressor genes that promote tumorigenesis when mutationally inactivated. Importantly, the transparency and accessibility of the zebrafish embryo allows the unprecedented direct analysis of pathologic processes in vivo, including neoplastic cell transformation and tumorigenic progression. Ultimately, high-throughput modifier screens based on zebrafish cancer models can lead to the identification of chemicals or genes involved in the suppression or prevention of the malignant phenotype. The identification of small molecules or gene products through such screens will serve as ideal entry points for novel drug development for the treatment of cancer. This review focuses on the current technology that takes advantage of the zebrafish model system to further our understanding of the genetic basis of cancer and its treatment.

Genetic screens for genes controlling motor nerve-muscle development and interactions

Motor growth cones navigate long and complex trajectories to connect with their muscle targets. Experimental studies have shown that this guidance process critically depends on extrinsic cues. In the zebrafish embryo, a subset of mesodermal cells, the adaxial cells, delineates the prospective path of pioneering motor growth cones. Genetic ablation of adaxial cells causes profound pathfinding defects, suggesting the existence of adaxial cell derived guidance factors. Intriguingly, adaxial cells are themselves migratory, and as growth cones approach they migrate away from the prospective axonal path to the lateral surface of the myotome, where they develop into slow-twitching muscle fibers. Genetic screens in embryos stained with an antibody cocktail identified mutants with specific defects in differentiation and migration of adaxial cells/slow muscle fibers, as well as mutants with specific defects in axonal pathfinding, including exit from the spinal cord and pathway selection. Together, the genes underlying these mutant phenotypes define pathways essential for nerve and muscle development and interactions between these two cell types.

A gynogenetic screen to isolate naturally occurring recessive mutations in Xenopus tropicalis

In the rapidly developing, diploid amphibian Xenopus tropicalis, genetics can be married to the already powerful tools of the amphibian system to overcome a disability that has hampered Xenopus laevis as a model organism: the difficulties inherent in conducting genetic analyses in a tetraploid organism with a longer generation time. We describe here a gynogenetic screen to uncover naturally occurring recessive mutations in wild X. tropicalis populations, a procedure that is both faster and easier than conventional genetic screens traditionally employed in model organisms to dissect early developmental pathways. During the first round of our screen, gynogenetic diploids from over 160 females comprising four different wild-caught populations were examined. Forty-two potential mutant phenotypes were isolated during this round of gynogenesis. From this group, we describe 10 lines that have genetically heritable recessive mutations. A wide range of developmental defects were obtained in this screen, encompassing effects limited to individual organs as well phenotypes characterized by more global changes in tadpole body morphology. The frequency of recessive mutations detected in our screen appears lower than that seen in other vertebrate genetic screens, but given constraints on the screening procedure used here, is likely to be consistent with rates seen in other animals, and clearly illustrates how wild-caught animals can be a productive source of developmental mutations for experimental study. The development of genetic strategies for the Xenopus system, together with new genomic resources, existing technologies for transgenesis, and other means for manipulating gene expression, as well as the power of performing embryonic manipulations, will provide an impressive set of tools for resolving complex cell and developmental phenomena in the future.

Transgenes as screening tools to probe and manipulate the zebrafish genome

The zebrafish, originally an object of study as an inexpensive and prolific vertebrate embryological model with a plethora of genetic tricks, has over the past decade moved to large-scale chemical mutagenesis and recently came of age as a high throughput transgenic model with a sequenced genome nearing completion. Insertional mutagenesis, gene trapping and enhancer detection are all contributing to the increasing speed with which research in this biomedical model is progressing. We review here some of the recent developments in the emerging field of zebrafish developmental genomics and transgenesis.

Insertional mutagenesis in zebrafish

Insertional mutagenesis is a method for identifying genes essential for a given biological process by using the integration of DNA as the mutagen, thereby facilitating the cloning of the mutated gene. The use of retrovirus-mediated insertional mutagenesis in zebrafish has led to the mutation and rapid identification of hundreds of genes required for embryonic development and cell viability and growth, revealing the diversity of gene products required for the development of this vertebrate. Here, I will review the methodology of this approach and the results to date, as well as other potential ways to use insertional mutagenesis for genetic screens.

The use of zebrafish to understand immunity

For decades immunologists have relied heavily on the mouse model for their experimental designs. With the realization of the important role innate immunity plays in orchestrating immune responses, invertebrates such as worms and flies have been added to the repertoire. Here, we discuss the advent of the zebrafish as a powerful vertebrate model organism that promises to positively impact immunologic research.

Approaches to study neurogenesis in the zebrafish retina

Similar to other vertebrate species, the zebrafish retina is simpler than other regions of the central nervous system (CNS). Relative simplicity, rapid development, and accessibility to genetic analysis make the zebrafish retina an excellent model system for the studies of neurogenesis in the vertebrate CNS. Numerous genetic screens have led to isolation of an impressive collection of mutations affecting the retina and the retinotectal projection in zebrafish. Mutant phenotypes are being studied using a rich variety of markers: antibodies, RNA probes, retrograde and anterograde tracers, as well as transgenic lines. Particularly impressive progress has been made in the characterization of the zebrafish genome. Consequently, positional and candidate cloning of mutant genes are now fairly easy to accomplish in zebrafish. Many mutant genes have, in fact, already been cloned and their analysis has provided important insights into the gene circuitry that regulates retinal neurogenesis. Genetic screens for visual system defects will continue in the future and progressively more sophisticated screening approaches will make it possible to detect a variety of subtle mutant phenotypes in retinal development. The remarkable evolutionary conservation of the vertebrate eye provides the basis for the use of the zebrafish retina as a model of human disorders. Some of the genetic defects of the zebrafish retina indeed resemble human retinopathies. As new techniques are being introduced and improved at a rapid pace, the zebrafish will continue to be an important organism for the studies of the vertebrate visual system.

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.

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.

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.

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.

Fishing for lymphoid genes

Thymic organogenesis and T-cell lymphopoiesis are crucial interdependent processes that establish a functional vertebrate immune system. The current understanding of vertebrate thymic development during embryogenesis remains incomplete and would benefit from novel approaches. The zebrafish Danio rerio is a powerful developmental and genetic system for the dissection of early events in the ontogeny of the immune system. Forward genetic screens have uncovered genes involved in hematopoiesis, and specific screens are being designed to examine the genes that regulate T-cell development and the origin of the thymus. Studies of the zebrafish should improve our understanding of lymphoid development in vertebrates.

Harnessing the power of forward genetics--analysis of neuronal diversity and patterning in the zebrafish retina

The seven major cell classes of the vertebrate retina are organized with remarkable precision into distinct layers. The appearance of this architecture during embryogenesis raises two questions of general importance. How do individual cell classes acquire their specialized structures and functions if they all originate from a morphologically uniform cell population? What mechanisms are responsible for the formation of such a complex and exact pattern? Recent advances present an opportunity to apply the tools of forward genetic analysis to identify mutations that affect these mechanisms in zebrafish. Molecular characterization will follow, providing insight into the basis of neuronal patterning in the vertebrate CNS.

Haemostatic screening and identification of zebrafish mutants with coagulation pathway defects: an approach to identifying novel haemostatic genes in man

Zebrafish were used as a model to study haemostasis, a vertebrate function of paramount importance. A limitation of the zebrafish model is the difficulty in assaying small amounts of blood to detect coagulation mutants. We report the use of a rapid total coagulation activity (TCA) assay to screen for coagulation defects in individual adult zebrafish. We screened the TCA in 1000 gynogenetic half-tetrad diploids derived from 86 clutches. Each clutch was from a single F1 female offspring of males mutagenized with ethylnitrosourea (ENU). We found 30-50% defective zebrafish among six clutches, consistent with a heritable defect. The assay developed here provided a rapid screen to detect overall coagulation defects. However, because of the limited amounts of plasma, we could not detect defects in specific pathways. Therefore, a novel, ultra-sensitive kinetic method was developed to identify specific pathway defects. To test whether the kinetic assay could be used as a screening tool, 1500 Florida wild-type zebrafish pairs were analysed for naturally occurring coagulation defects. We detected 30 fish with extrinsic pathway defects, but with intact common and intrinsic pathways. We conclude that it is now possible to identify specific coagulation pathway defects in zebrafish.

Analysis of chromosomal rearrangements induced by postmeiotic mutagenesis with ethylnitrosourea in zebrafish

Mutations identified in zebrafish genetic screens allow the dissection of a wide array of problems in vertebrate biology. Most screens have examined mutations induced by treatment of spermatogonial (premeiotic) cells with the chemical mutagen N-ethyl-N-nitrosourea (ENU). Treatment of postmeiotic gametes with ENU induces specific-locus mutations at a higher rate than premeiotic regimens, suggesting that postmeiotic mutagenesis protocols could be useful in some screening strategies. Whereas there is extensive evidence that ENU induces point mutations in premeiotic cells, the range of mutations induced in postmeiotic zebrafish germ cells has been less thoroughly characterized. Here we report the identification and analysis of five mutations induced by postmeiotic ENU treatment. One mutation, snh(st1), is a translocation involving linkage group (LG) 11 and LG 14. The other four mutations, oep(st2), kny(st3), Df(LG 13)(st4), and cyc(st5), are deletions, ranging in size from less than 3 cM to greater than 20 cM. These results show that germ cell stage is an important determinant of the type of mutations induced. The induction of chromosomal rearrangements may account for the elevated frequency of specific-locus mutations observed after treatment of postmeiotic gametes with ENU.

Dissecting hematopoiesis and disease using the zebrafish

The study of blood has often defined paradigms that are relevant to the biology of other vertebrate organ systems. As examples, stem cell physiology and the structure of the membrane cytoskeleton were first described in hematopoietic cells. Much of the reason for these successes resides in the ease with which blood cells can be isolated and manipulated in vitro. The cell biology of hematopoiesis can also be illuminated by the study of human disease states such as anemia, immunodeficiency, and leukemia. The sequential development of the blood system in vertebrates is characterized by ventral mesoderm induction, hematopoietic stem cell specification, and subsequent cell lineage differentiation. Some of the key regulatory steps in this process have been uncovered by studies in mouse, chicken, and Xenopus. More recently, the genetics of the zebrafish (Danio rerio) have been employed to define novel points of regulation of the hematopoietic program. In this review, we describe the advantages of the zebrafish system for the study of blood cell development and the initial success of the system in this pursuit. The striking similarity of zebrafish mutant phenotypes and human diseases emphasizes the utility of this model system for elucidating pathophysiologic mechanisms. New screens for lineage-specific mutations are beginning, and the availability of transgenics promises a better understanding of lineage-specific gene expression. The infrastructure of the zebrafish system is growing with an NIH-directed genome initiative, providing a detailed map of the zebrafish genome and an increasing number of candidate genes for the mutations. The zebrafish is poised to contribute greatly to our understanding of normal and disease-related hematopoiesis.

Transcriptional regulation during zebrafish embryogenesis

Many zebrafish transcription factors have been isolated, and the challenge at present is to uncover the genes and pathways they regulate. The wealth of developmental mutants available for study and recent advances in zebrafish transgenic technology have allowed identification of putative transcriptional regulatory pathways, as well as characterization of promoter interactions at a molecular level.