Forward and reverse genetic approaches to the analysis of eye development in zebrafish

Protocol for lens removal in embryonic fish and its application on the developmental effects of eye regression

The lens plays a central role in the development of the optic cup. In fish, regression of the eye early in development affects the development of the craniofacial skeleton, the size of the olfactory pits, the optic nerve, and the tectum. Lens removal further affects olfaction, prey capture, and aggression. The similarity of the fish eye to other vertebrates is the basis for its use as an excellent animal model of human defects. Questions regarding the effects of eye regression are specifically well-suited to be addressed by using fish from the genus Astyanax. The species has two morphs; an eyeless cave morph and an eyed, surface morph. In the cavefish, a lens initially develops in embryos, but then degenerates by apoptosis. The cavefish retina is subsequently disorganized, degenerates, and retinal growth is arrested. The same effect is observed in surface fish when the lens is removed or exchanged for a cavefish lens. While studies can greatly benefit from a control group of surface fish with regressed eyes brought through lensectomies, few studies include them because of technical difficulties and the low survivorship of embryos that undergo this procedure. Here we describe a technique with significant modification for improvement for conducting lensectomy in one-day-old Astyanax and other fish, including zebrafish. Yields of up to 30 live embryos were obtained using this technique from a single spawn, thus enabling studies that require large sample sizes.

IMI - Report on Experimental Models of Emmetropization and Myopia

The results of many studies in a variety of species have significantly advanced our understanding of the role of visual experience and the mechanisms of postnatal eye growth, and the development of myopia. This paper surveys and reviews the major contributions that experimental studies using animal models have made to our thinking about emmetropization and development of myopia. These studies established important concepts informing our knowledge of the visual regulation of eye growth and refractive development and have transformed treatment strategies for myopia. Several major findings have come from studies of experimental animal models. These include the eye's ability to detect the sign of retinal defocus and undergo compensatory growth, the local retinal control of eye growth, regulatory changes in choroidal thickness, and the identification of components in the biochemistry of eye growth leading to the characterization of signal cascades regulating eye growth and refractive state. Several of these findings provided the proofs of concepts that form the scientific basis of new and effective clinical treatments for controlling myopia progression in humans. Experimental animal models continue to provide new insights into the cellular and molecular mechanisms of eye growth control, including the identification of potential new targets for drug development and future treatments needed to stem the increasing prevalence of myopia and the vision-threatening conditions associated with this disease.

Zebrafish WNK lysine deficient protein kinase 1 (wnk1) affects angiogenesis associated with VEGF signaling

The WNK1 (WNK lysine deficient protein kinase 1) protein is a serine/threonine protein kinase with emerging roles in cancer. WNK1 causes hypertension and hyperkalemia when overexpressed and cardiovascular defects when ablated in mice. In this study, the role of Wnk1 in angiogenesis was explored using the zebrafish model. There are two zebrafish wnk1 isoforms, wnk1a and wnk1b, and both contain all the functional domains found in the human WNK1 protein. Both isoforms are expressed in the embryo at the initiation of angiogenesis and in the posterior cardinal vein (PCV), similar to fms-related tyrosine kinase 4 (flt4). Using morpholino antisense oligonucleotides against wnk1a and wnk1b, we observed that wnk1 morphants have defects in angiogenesis in the head and trunk, similar to flk1/vegfr2 morphants. Furthermore, both wnk1a and wnk1b mRNA can partially rescue the defects in vascular formation caused by flk1/vegfr2 knockdown. Mutation of the kinase domain or the Akt/PI3K phosphorylation site within wnk1 destroys this rescue capability. The rescue experiments provide evidence that wnk1 is a downstream target for Vegfr2 (vascular endothelial growth factor receptor-2) and Akt/PI3K signaling and thereby affects angiogenesis in zebrafish embryos. Furthermore, we found that knockdown of vascular endothelial growth factor receptor-2 (flk1/vegfr2) or vascular endothelial growth factor receptor-3 (flt4/vegfr3) results in a decrease in wnk1a expression, as assessed by insitu hybridization and q-RT-PCR analysis. Thus, the Vegf/Vegfr signaling pathway controls angiogenesis in zebrafish via Akt kinase-mediated phosphorylation and activation of Wnk1 as well as transcriptional regulation of wnk1 expression.

Characterization of transgenic zebrafish lines that express GFP in the retina, pineal gland, olfactory bulb, hatching gland, and optic tectum

Transgenic animals are powerful tools to study gene function invivo. Here we characterize several transgenic zebrafish lines that express green fluorescent protein (GFP) under the control of the LCR(RH2)-RH2-1 or LCR(RH2)-RH2-2 green opsin regulatory elements. Using confocal immunomicroscopy, stereo-fluorescence microscopy, and Western blotting, we show that the Tg(LCR(RH2)-RH2-1:GFP)(pt112) and Tg(LCR(RH2)-RH2-2:GFP)(pt115) transgenic zebrafish lines express GFP in the pineal gland and certain types of photoreceptors. In addition, some of these lines also express GFP in the hatching gland, optic tectum, or olfactory bulb. Some of the expression patterns differ significantly from previously published similar transgenic fish lines, making them useful tools for studying the development of the corresponding tissues and organs. In addition, the variations of GFP expression among different lines corroborate the notion that transgenic expression is often subjected to position effect, thus emphasizing the need for careful verification of expression patterns when transgenic animal models are utilized for research.

Translational vision research models program

ENU mutagenesis is an efficient method to identify new animal models of ocular disease. The new alleles described herein will be a useful resource to further examine the role of the affected molecules and the effects of their disruption within the retina.

Mouse model resources for vision research

The need for mouse models, with their well-developed genetics and similarity to human physiology and anatomy, is clear and their central role in furthering our understanding of human disease is readily apparent in the literature. Mice carrying mutations that alter developmental pathways or cellular function provide model systems for analyzing defects in comparable human disorders and for testing therapeutic strategies. Mutant mice also provide reproducible, experimental systems for elucidating pathways of normal development and function. Two programs, the Eye Mutant Resource and the Translational Vision Research Models, focused on providing such models to the vision research community are described herein. Over 100 mutant lines from the Eye Mutant Resource and 60 mutant lines from the Translational Vision Research Models have been developed. The ocular diseases of the mutant lines include a wide range of phenotypes, including cataracts, retinal dysplasia and degeneration, and abnormal blood vessel formation. The mutations in disease genes have been mapped and in some cases identified by direct sequencing. Here, we report 3 novel alleles of Crx^tvrm65, Rp1^tvrm64, and Rpe65^tvrm148 as successful examples of the TVRM program, that closely resemble previously reported knockout models.

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.

Zebrafish ale oko, an essential determinant of sensory neuron survival and the polarity of retinal radial glia, encodes the p50 subunit of dynactin

Although microtubule-dependent motors are known to play many essential functions in eukaryotic cells, their role in the context of the developing vertebrate embryo is less well understood. Here we show that the zebrafish ale oko (ako) locus encodes the p50 component of the dynactin complex. Loss of ako function results in a degeneration of photoreceptors and mechanosensory hair cells. Additionally, mutant Müller cells lose apical processes and their perikarya translocate rapidly towards the vitreal surface of the retina. This is accompanied by the accumulation of the apical determinants Nok and Has/aPKC in their cell bodies. ako is required cell-autonomously for the maintenance of the apical process but not for cell body positioning in Müller glia. At later stages, the retinotectal projection also degenerates in ako mutants. These results indicate that the p50 component of the dynactin complex is essential for the survival of sensory neurons and the maintenance of ganglion cell axons, and functions as a major determinant of apicobasal polarity in retinal radial glia.

Lens transplantation in zebrafish and its application in the analysis of eye mutants

The lens plays an important role in the development of the optic cup. Using the zebrafish as a model organism, questions regarding lens development can be addressed. The zebrafish is useful for genetic studies due to several advantageous characteristics, including small size, high fecundity, short lifecycle, and ease of care. Lens development occurs rapidly in zebrafish. By 72 hpf, the zebrafish lens is functionally mature. Abundant genetic and molecular resources are available to support research in zebrafish. In addition, the similarity of the zebrafish eye to those of other vertebrates provides basis for its use as an excellent animal model of human defects. Several zebrafish mutants exhibit lens abnormalities, including high levels of cell death, which in some cases leads to a complete degeneration of lens tissues. To determine whether lens abnormalities are due to intrinsic causes or to defective interactions with the surrounding tissues, transplantation of a mutant lens into a wild-type eye is performed. Using fire-polished metal needles, mutant or wild-type lenses are carefully dissected from the donor animal, and transferred into the host. To distinguish wild-type and mutant tissues, a transgenic line is used as the donor. This line expresses membrane-bound GFP in all tissues, including the lens. This transplantation technique is an essential tool in the studies of zebrafish lens mutants.

NeuroD regulates proliferation of photoreceptor progenitors in the retina of the zebrafish

neuroD is a member of the family of proneural genes, which function to regulate the cell cycle, cell fate determination and cellular differentiation. In the retinas of larval and adult teleosts, neuroD is expressed in two populations of post-mitotic cells, a subset of amacrine cells and nascent cone photoreceptors, and proliferating cells in the lineages that give rise exclusively to rod and cone photoreceptors. Based on previous studies of NeuroD function in vitro and the cellular pattern of neuroD expression in the zebrafish retina, we hypothesized that within the mitotic photoreceptor lineages NeuroD selectively regulates aspects of the cell cycle. To test this hypothesis, gain and loss-of-function approaches were employed, relying on the inducible expression of a NeuroD(EGFP) fusion protein and morpholino oligonucleotides to inhibit protein translation, respectively. Conditional expression of neuroD causes cells to withdraw from the cell cycle, upregulate the expression of the cell cycle inhibitors, p27 and p57, and downregulate the cell cycle progression factors, Cyclin B1, Cyclin D1, and Cyclin E2. In the absence of NeuroD, cells specific for the rod and cone photoreceptor lineage fail to exit the cell cycle, and the number of cells expressing Cyclin D1 is increased. When expression is ectopically induced in multipotent progenitors, neuroD promotes the genesis of rod photoreceptors and inhibits the genesis of Müller glia. These data show that in the teleost retina NeuroD plays a fundamental role in photoreceptor genesis by regulating mechanisms that promote rod and cone progenitors to withdraw from the cell cycle. This is the first in vivo demonstration in the retina of cell cycle regulation by NeuroD.

Formation of stromal collagen fibrils and proteoglycans in the developing zebrafish cornea

PURPOSE

Collagen fibrils and proteoglycans are the main components of the corneal extracellular matrix and corneal transparency depends crucially on their proper arrangement. In the present study, we investigated the formation of collagen fibrils and proteoglycans in the developing cornea of the zebrafish, a model organism used to study vertebrate embryonic development and genetic disease.

METHODS

We employed thin-section electron microscopy to investigate the ultrastructure of the zebrafish cornea at different developmental stages.

RESULTS

The layering of the zebrafish cornea into an epithelium, a Bowman's layer, stroma and endothelium was observed starting at 72 hr post-fertilization. At this stage, the stroma contained orthogonally arranged collagen fibrils and small proteoglycans. The density of proteoglycans increased gradually throughout subsequent development of the cornea. In the stroma of 2-week-old larvae, the collagen fibrils were organized into thin lamellae and were separated by very large, randomly distributed proteoglycans. At 4 weeks, a regular arrangement of proteoglycans in relation to the collagen fibrils was observed for the first time and the lamellae were also thickened.

CONCLUSION

The present study, for the first time, provides ultrastructural details of collagen fibril and proteoglycan development in the zebrafish cornea. Furthermore, it directly correlates the collagen fibril and proteoglycan composition of the zebrafish cornea with that of the human cornea. The similarities between the two species suggest that the zebrafish could serve as a model for investigating the genetics of human corneal development and diseases.

Genetic linkage map of the loach Misgurnus anguillicaudatus (Teleostei: Cobitidae)

In the present study, the first genetic linkage map of the loach Misgurnus anguillicaudatus was constructed with 164 microsatellite markers and a color locus, and it included 155 newly developed markers. A total of 159 microsatellite markers and a color locus were mapped in 27 linkage groups (LGs). The female map covered 784.5 cM with 153 microsatellite markers and a color locus, whereas the male map covered 662.2 cM with 119 microsatellite markers. The centromeric position in each LG was estimated by marker-centromere mapping based on half-tetrad analysis. In 4 LGs (LG2, LG3, LG4, and LG5), the centromere was estimated at the intermediate region. In LG1, LG11, and LG12, the centromere was estimated to shift from the sub-intermediate region to the end (telomeric). The number of these LGs (7) was identical to the collective number of bi-arm metacentric (5) and sub-metacentric chromosome (2) of the haploid chromosome set (n = 5) of the loach. In the other LGs, the position of the centromere was estimated at the end or outside. These results indicate satisfactory compliance between the linkage map and the chromosome set. Our map would cover approximately almost the entire loach genome because most markers were successfully mapped.

Development and adult morphology of the eye lens in the zebrafish

The zebrafish has become an important vertebrate model organism to study the development of the visual system. Mutagenesis projects have resulted in the identification of hundreds of eye mutants. Analysis of the phenotypes of these mutants relies on in depth knowledge of the embryogenesis in wild-type animals. While the morphological events leading to the formation of the retina and its connections to the central nervous system have been described in great detail, the characterization of the development of the eye lens is still incomplete. In the present study, we provide a morphological description of embryonic and larval lens development as well as adult lens morphology in the zebrafish. Our analyses show that, in contrast to other vertebrate species, the zebrafish lens delaminates from the surface ectoderm as a solid cluster of cells. Detachment of the prospective lens from the surface ectoderm is facilitated by apoptosis. Primary fibre cell elongation occurs in a circular fashion resulting in an embryonic lens nucleus with concentric shells of fibres. After formation of a monolayer of lens epithelial cells, differentiation and elongation of secondary lens fibres result in a final lens morphology similar to that of other vertebrate species. As in other vertebrates, secondary fibre cell differentiation includes the programmed degradation of nuclei, the interconnection of adjacent fibres via protrusions at the fibre cells' edges and the establishment of gap junctions between lens fibre cells. The very close spacing of the nuclei of the differentiating secondary fibres in a narrow zone close to the equatorial epithelium, however, suggests that secondary fibre cell differentiation deviates from that described for mammalian or avian lenses. In summary, while there are similarities in the development and final morphology of the zebrafish lens with mammalian and avian lenses, there are also significant differences, suggesting caution when extrapolating findings on the zebrafish to, for example, human lens development or function.

Progress towards understanding disease mechanisms in small vertebrate models of neuronal ceroid lipofuscinosis

Model systems provide an invaluable tool for investigating the molecular mechanisms underlying the NCLs, devastating neurodegenerative disorders that affect the relatively inaccessible tissues of the central nervous system. These models have enabled the assessment of behavioural, pathological, cellular, and molecular abnormalities, and also allow for development and evaluation of novel therapies. This review highlights the relative advantages of the two available small vertebrate species, the mouse and zebrafish, in modelling NCL disease, summarising how these have been useful in NCL research and their potential for the development and testing of prospective disease treatments. A panel of mouse mutants is available representing all the cloned NCL gene disorders (Cathepsin D, CLN1, CLN2, CLN3, CLN5, CLN6, CLN8). These NCL mice all have progressive neurodegenerative phenotypes that closely resemble the pathology of human NCL. The analysis of these models has highlighted several novel aspects underlying NCL pathogenesis including the selective nature of neurodegeneration, evidence for glial responses that precede neuronal loss and identification of the thalamus as an important pathological target early in disease progression. Studies in mice have also highlighted an unexpected heterogeneity underlying NCL phenotypes, and novel potential NCL-like mouse models have been described including mice with mutations in cathepsins, CLC chloride channels, and other lysosome-related genes. These new models are likely to provide significant new information on the spectrum of NCL disease. Information on NCL mice is available in the NCL Mouse Model Database (). There are homologs of most of the NCL genes in zebrafish, and NCL zebrafish models are currently in development. This model system provides additional advantages to those provided by NCL mouse models including high-throughput mutational, pharmacogenetic and therapeutic technique analyses. Mouse and zebrafish models are an important shared resource for NCL research, offering a unique possibility to dissect disease mechanisms and to develop therapeutic approaches.

Responses of Müller glia to retinal injury in adult zebrafish

In an effort to identify the cellular events that enable neuronal regeneration in the vertebrate retina, the identity and characteristics of mitotic and apoptotic cells were examined in lesioned retinas of adult zebrafish. Following lesion a complex spatiotemporal pattern of mitosis was observed, including a delayed entry of Müller glia into the cell cycle. Characteristics of these proliferative Müller glia indicated they might serve as a stem/precursor cell of regenerated retina. The results suggested a model of retinal regeneration in which lesions are filled, in part, by a localized en place cytogenesis within intact retina surrounding the lesion site.

Retinal pattern and the genetic basis of its formation in zebrafish

The vertebrate nervous system contains an immense diversity of distinct cellular components that are organized into precise spatial patterns. The importance of accurate neuronal architecture is particularly obvious in the retina, where it is necessary for the formation of visual images. The retina is structured in a distinct layered pattern that is remarkably conserved in evolution, including phyla as diverse as primates and teleost fish. Genetic analysis in zebrafish reveals mechanisms that are essential for the formation of this architecture.

The zebrafish Pard3 ortholog is required for separation of the eye fields and retinal lamination

The vertebrate retina develops from a sheet of neuroepithelial cells. Because adherens and tight junctions are critical for epithelial and neuronal differentiation in a variety of eukaryotic systems, we examined the role of Par-3, a PDZ scaffold protein that is critical in cellular membrane junction formation. We cloned the zebrafish Par-3 ortholog (pard3), which encodes two Pard3 proteins (150 and 180 kDa) that differ in their carboxyl-terminus. Immunohistochemistry revealed that Pard3 localized to the apical region of the retinal and brain neuroepithelium, partially overlapping the adherens junction-associated actin bundles. After retinal lamination, the Pard3 protein was restricted to the outer limiting membrane and the outer and inner plexiform layers in the retina. Reducing Pard3 expression with antisense morpholinos caused loss of the retinal pigmented epithelia, disruption of retinal lamination, and cell death in the ventral diencephalon, which resulted in cyclopia. Overexpressing Pard3 by injection of wild-type pard3 mRNA resulted in cyclopia and eyeless embryos. Thus, Pard3 plays a critical role in the origination and separation of zebrafish eye fields and retinal lamination.

Acridine mutagenesis of zebrafish (Danio rerio)

Mutagenesis screening, in which heritable traits are isolated following damage to the genome, is a powerful approach for investigating gene function. Among vertebrate model organisms, the zebrafish (Danio rerio) is ideally suited to mutagenesis screens. The success of large-scale screens is dependent on the way in which changes are identified. The type of damage induced is also pivotal. Single base coding region deletions and insertions are suited to abolition of gene function whilst inducing a small physical alteration to the genome. Such mutations are not commonly found following mutagenesis schemes reported to date. Here, we show that an acridine mutagen, ICR191, which in other model organisms frequently induces single base deletions and insertions, is mutagenic in zebrafish. ICR191 induces hallmark phenotypes associated with genetic damage in treated embryos. Alterations are heritable. Offspring of mutagenised fish had mutations in a marker gene and were found to produce offspring with abnormal development. Using an adaptation of a molecular mutation detection method, fluorescent arbitrary primed PCR, we identified an induced alteration directly. The estimated frequency of induced mutations was sufficiently high to make it feasible to employ this approach for mutagenesis screening.

The roles of Hedgehogs and Fibroblast Growth Factors in eye development and retinal cell rescue

Knowledge of normal eye development is crucial for the development of retinal rescue strategies. I shall focus on two signalling pathways that affect retinal development. Fibroblast growth factors function in retinal cell proliferation, retinal ganglion cell axon guidance and target recognition, craniofacial patterning and lens induction. Hedgehog proteins are required for progression of the neurogenic wave, cell proliferation, photoreceptor differentiation, retinal ganglion cell axon growth and craniofacial patterning. These signalling pathways have pleiotropic effects, can interact and have the potential to be used therapeutically. The zebrafish model organism may be well suited to studying how signalling pathways interact.

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.

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.

Theheartstringsmutation in zebrafish causes heart/fin Tbx5 deficiency syndrome

Holt-Oram syndrome is one of the autosomal dominant human ‘heart-hand’ disorders, with a combination of upper limb malformations and cardiac defects. Holt-Oram syndrome is caused by mutations in the TBX5 gene, a member of a large family of T-box transcription factors that play important roles in cell-type specification and morphogenesis. In a screen for mutations affecting zebrafish cardiac function, we isolated the recessive lethal mutant heartstrings, which lacks pectoral fins and exhibits severe cardiac dysfunction, beginning with a slow heart rate and progressing to a stretched, non-functional heart.

We mapped and cloned the heartstrings mutation and find it to encode the zebrafish ortholog of the TBX5 gene. The heartstrings mutation causes premature termination at amino acid 316. Homozygous mutant embryos never develop pectoral fin buds and do not express several markers of early fin differentiation. The total absence of any fin bud differentiation distinguishes heartstrings from most other mutations that affect zebrafish fin development, suggesting that Tbx5 functions very early in the pectoral fin induction pathway. Moderate reduction of Tbx5 by morpholino causes fin malformations, revealing an additional early requirement for Tbx5 in coordinating the axes of fin outgrowth. The heart of heartstrings mutant embryos appears to form and function normally through the early heart tube stage, manifesting only a slight bradycardia compared with wild-type siblings. However, the heart fails to loop and then progressively deteriorates, a process affecting the ventricle as well as the atrium.

Relative to mammals, fish require lower levels of Tbx5 to produce malformed appendages and display whole-heart rather than atrial-predominant cardiac defects. However, the syndromic deficiencies of tbx5 mutation are remarkably well retained between fish and mammals.