The bugeye mutant zebrafish exhibits visual deficits that arise with the onset of an enlarged eye phenotype

Socs3b regulates the development and function of innate immune cells in zebrafish

Introduction

Suppressor of cytokine signaling 3 (SOCS3) is a critical component of the negative feedback regulation that controls signaling by cytokines and other factors thereby ensuring that important processes such as hematopoiesis and inflammation occur at appropriate levels.

Methods

To gain further insights into SOCS3 function, the zebrafish socs3b gene was investigated through analysis of a knockout line generated using CRISPR/Cas9-mediated genome editing.

Results

Zebrafish socs3b knockout embryos displayed elevated numbers of neutrophils during primitive and definitive hematopoiesis but macrophage numbers were not altered. However, the absence of socs3b reduced neutrophil functionality but enhanced macrophage responses. Adult socs3b knockout zebrafish displayed reduced survival that correlated with an eye pathology involving extensive infiltration of neutrophils and macrophages along with immune cell dysregulation in other tissues.

Discussion

These findings identify a conserved role for Socs3b in the regulation of neutrophil production and macrophage activation.

Zebrafish Model in Ophthalmology to Study Disease Mechanism and Drug Discovery

Visual impairment and blindness are common and seriously affect people’s work and quality of life in the world. Therefore, the effective therapies for eye diseases are of high priority. Zebrafish (Danio rerio) is an alternative vertebrate model as a useful tool for the mechanism elucidation and drug discovery of various eye disorders, such as cataracts, glaucoma, diabetic retinopathy, age-related macular degeneration, photoreceptor degeneration, etc. The genetic and embryonic accessibility of zebrafish in combination with a behavioral assessment of visual function has made it a very popular model in ophthalmology. Zebrafish has also been widely used in ocular drug discovery, such as the screening of new anti-angiogenic compounds or neuroprotective drugs, and the oculotoxicity test. In this review, we summarized the applications of zebrafish as the models of eye disorders to study disease mechanism and investigate novel drug treatments.

Loss of αklotho causes reduced motor ability and short lifespan in zebrafish

The klotho gene encodes a transmembrane protein αKlotho that interacts with a fibroblast growth factor (FGF) receptor in renal tubular epithelial cells and functions as a co-receptor for FGF23, which is an osteocytes-derived hormone. This bone-to-kidney signal promotes urinary phosphate excretion. Interestingly, αKlotho knockout mice show an accelerated aging and a shortened life span. Similarly, C. elegans lacking the αklotho homologue showed a short life span. However, the physiological basis of aging-related function of αklotho remain unclear. The αklotho-deficient vertebrate animals other than mice have been awaited as an alternative model of premature aging. We here employed zebrafish in our study and revealed that αklotho mutant zebrafish appeared to be normal at 3 months postfertilization (mpf) but eventually underwent premature death by 9 mpf, while normal zebrafish is known to survive for 42 months. We also assessed the motor ability of zebrafish in a forced swimming assay and found that αklotho mutant zebrafish displayed reduced swimming performance before their survival declined. A recent study also reported a similar finding that αklotho-deficient zebrafish exhibited a short life span and reduced spontaneous movements. Taken together, these results suggest that αKlotho mutant zebrafish show premature aging and are useful to investigate aging in vertebrates.

Igf signaling couples retina growth with body growth by modulating progenitor cell division

How the body and organs balance their relative growth is of key importance for coordinating size and function. This is of particular relevance in organisms, which continue to grow over their entire life span. We addressed this issue in the neuroretina of medaka fish (Oryzias latipes), a well-studied system with which to address vertebrate organ growth. We reveal that a central growth regulator, Igf1 receptor (Igf1r), is necessary and sufficient for proliferation control in the postembryonic retinal stem cell niche: the ciliary marginal zone (CMZ). Targeted activation of Igf1r signaling in the CMZ uncouples neuroretina growth from body size control, and we demonstrate that Igf1r operates on progenitor cells, stimulating their proliferation. Activation of Igf1r signaling increases retinal size while preserving its structural integrity, revealing a modular organization in which progenitor differentiation and neurogenesis are self-organized and highly regulated. Our findings position Igf signaling as a key module for controlling retinal size and composition, with important evolutionary implications.

Highlighted Article: Targeted activation of Igf1r signaling in the retinal stem cell niche increases retina size through expanding the progenitor but not stem cell population.

Neurodegeneration, Neuroprotection and Regeneration in the Zebrafish Retina

Neurodegenerative retinal diseases, such as glaucoma and diabetic retinopathy, involve a gradual loss of neurons in the retina as the disease progresses. Central nervous system neurons are not able to regenerate in mammals, therefore, an often sought after course of treatment for neuronal loss follows a neuroprotective or regenerative strategy. Neuroprotection is the process of preserving the structure and function of the neurons that have survived a harmful insult; while regenerative approaches aim to replace or rewire the neurons and synaptic connections that were lost, or induce regrowth of damaged axons or dendrites. In order to test the neuroprotective effectiveness or the regenerative capacity of a particular agent, a robust experimental model of retinal neuronal damage is essential. Zebrafish are being used more often in this type of study because their eye structure and development is well-conserved between zebrafish and mammals. Zebrafish are robust genetic tools and are relatively inexpensive to maintain. The large array of functional and behavioral tests available in zebrafish makes them an attractive model for neuroprotection studies. Some common insults used to model retinal disease and study neuroprotection in zebrafish include intense light, chemical toxicity and mechanical damage. This review covers the existing retinal neuroprotection and regeneration literature in the zebrafish and highlights their potential for future studies.

Photoreceptor Degeneration Accompanies Vascular Changes in a Zebrafish Model of Diabetic Retinopathy

Purpose

Diabetic retinopathy (DR) is a leading cause of vision impairment and blindness worldwide in the working-age population, and the incidence is rising. Until now it has been difficult to define initiating events and disease progression at the molecular level, as available diabetic rodent models do not present the full spectrum of neural and vascular pathologies. Zebrafish harboring a homozygous mutation in the pancreatic transcription factor pdx1 were previously shown to display a diabetic phenotype from larval stages through adulthood. In this study, pdx1 mutants were examined for retinal vascular and neuronal pathology to demonstrate suitability of these fish for modeling DR.

Methods

Vessel morphology was examined in pdx1 mutant and control fish expressing the fli1a:EGFP transgene. We further characterized vascular and retinal phenotypes in mutants and controls using immunohistochemistry, histology, and electron microscopy. Retinal function was assessed using electroretinography.

Results

Pdx1 mutants exhibit clear vascular phenotypes at 2 months of age, and disease progression, including arterial vasculopenia, capillary tortuosity, and hypersprouting, could be detected at stages extending over more than 1 year. Neural-retinal pathologies are consistent with photoreceptor dysfunction and loss, but do not progress to blindness.

Conclusions

This study highlights pdx1 mutant zebrafish as a valuable complement to rodent and other mammalian models of DR, in particular for research into the mechanistic interplay of diabetes with vascular and neuroretinal disease. They are furthermore suited for molecular studies to identify new targets for treatment of early as well as late DR.

Establishment of an adult zebrafish model of retinal neurodegeneration induced by NMDA

AIM

To establish a model of retinal neurodegeneration induced by N-Methyl-D-aspartic acid (NMDA) in adult zebrafish.

METHODS

We compared the effects of three different NMDA delivery methods on retinal neurodegeneration in adult zebrafish: immersion (I.M.), intravitreal injection (I.V.), and intraperitoneal injection (I.P.), and examined retinal pathology and degeneration by hematoxylin and eosin and TUNEL staining in the treated zebrafish. Effects of the NMDA receptor antagonist MK-801 and the natural product resveratrol on NMDA-induced retinal neurodegeneration were also assessed.

RESULTS

The thickened inner retina was seen in histology with 100 µmol/L NMDA by I.M. administration. Significant apoptosis in the retinal ganglion cell layer and retinal thickness reduction occurred in 0.5 mol/L NMDA I.P. administration group.Seizure-like behavioral changes, but no retinal histological alteration occurred in 16 mg/kg NMDA I.P. administration group. Resveratrol and MK-801 prevented NMDA-induced retinal neurodegeneration in the zebrafish.

CONCLUSION

Among the three drug administration methods, I.V. injection of NMDA is the most suitable for establishment of an acute retinal damage model in zebrafish. I.M. with NMDA is likely the best for use as a chronic retinal damage model. I.P. treatment with NMDA causes brain damage. Resveratrol and MK801 may be a clinically valuable treatment for retinal neurodegeneration.

Zebrafish Models of Retinal Disease

Visual defects affect a large proportion of humanity, have a significant negative impact on quality of life, and cause significant economic burden. The wide variety of visual disorders and the large number of gene mutations responsible require a flexible animal model system to carry out research for possible causes and cures for the blinding conditions. With eyes similar to humans in structure and function, zebrafish are an important vertebrate model organism that is being used to study genetic and environmental eye diseases, including myopia, glaucoma, retinitis pigmentosa, ciliopathies, albinism, and diabetes. This review details the use of zebrafish in modeling human ocular diseases.

Psychophysical testing in rodent models of glaucomatous optic neuropathy

Processing of visual information begins in the retina, with photoreceptors converting light stimuli into neural signals. Ultimately, signals are transmitted to the brain through signaling networks formed by interneurons, namely bipolar, horizontal and amacrine cells providing input to retinal ganglion cells (RGCs), which form the optic nerve with their axons. As part of the chronic nature of glaucomatous optic neuropathy, the increasing and irreversible damage and ultimately loss of neurons, RGCs in particular, occurs following progressive damage to the optic nerve head (ONH), eventually resulting in visual impairment and visual field loss. There are two behavioral assays that are typically used to assess visual deficits in glaucoma rodent models, the visual water task and the optokinetic drum. The visual water task can assess an animal's ability to distinguish grating patterns that are associated with an escape from water. The optokinetic drum relies on the optomotor response, a reflex turning of the head and neck in the direction of the visual stimuli, which usually consists of rotating black and white gratings. This reflex is a physiological response critical for keeping the image stable on the retina. Driven initially by the neuronal input from direction-selective RGCs, this reflex is comprised of a number of critical sensory and motor elements. In the presence of repeatable and defined stimuli, this reflex is extremely well suited to analyze subtle changes in the circuitry and performance of retinal neurons. Increasing the cycles of these alternating gratings per degree, or gradually reducing the contrast of the visual stimuli, threshold levels can be determined at which the animal is no longer tracking the stimuli, and thereby visual function of the animal can be determined non-invasively. Integrating these assays into an array of outcome measures that determine multiple aspects of visual function is a central goal in vision research and can be realized, for example, by the combination of measuring optomotor reflex function with electroretinograms (ERGs) and optical coherence tomography (OCT) of the retina. These structure-function correlations in vivo are urgently needed to identify disease mechanisms as potential new targets for drug development. Such a combination of the experimental assessment of the optokinetic reflex (OKR) or optomotor response (OMR) with other measures of retinal structure and function is especially valuable for research on GON. The chronic progression of the disease is characterized by a gradual decrease in function accompanied by a concomitant increase in structural damage to the retina, therefore the assessment of subtle changes is key to determining the success of novel intervention strategies.

Rapid, accurate, and non-invasive measurement of zebrafish axial length and other eye dimensions using SD-OCT allows longitudinal analysis of myopia and emmetropization

Refractive errors in vision can be caused by aberrant axial length of the eye, irregular corneal shape, or lens abnormalities. Causes of eye length overgrowth include multiple genetic loci, and visual parameters. We evaluate zebrafish as a potential animal model for studies of the genetic, cellular, and signaling basis of emmetropization and myopia. Axial length and other eye dimensions of zebrafish were measured using spectral domain-optical coherence tomography (SD-OCT). We used ocular lens and body metrics to normalize and compare eye size and relative refractive error (difference between observed retinal radial length and controls) in wild-type and lrp2 zebrafish. Zebrafish were dark-reared to assess effects of visual deprivation on eye size. Two relative measurements, ocular axial length to body length and axial length to lens diameter, were found to accurately normalize comparisons of eye sizes between different sized fish (R2=0.9548, R2=0.9921). Ray-traced focal lengths of wild-type zebrafish lenses were equal to their retinal radii, while lrp2 eyes had longer retinal radii than focal lengths. Both genetic mutation (lrp2) and environmental manipulation (dark-rearing) caused elongated eye axes. lrp2 mutants had relative refractive errors of -0.327 compared to wild-types, and dark-reared wild-type fish had relative refractive errors of -0.132 compared to light-reared siblings. Therefore, zebrafish eye anatomy (axial length, lens radius, retinal radius) can be rapidly and accurately measured by SD-OCT, facilitating longitudinal studies of regulated eye growth and emmetropization. Specifically, genes homologous to human myopia candidates may be modified, inactivated or overexpressed in zebrafish, and myopia-sensitizing conditions used to probe gene-environment interactions. Our studies provide foundation for such investigations into genetic contributions that control eye size and impact refractive errors.

Zebrafish--on the move towards ophthalmological research

Millions of people are affected by visual impairment and blindness globally, and the prevalence of vision loss is likely to increase as we are living longer. However, many ocular diseases remain poorly controlled due to lack of proper understanding of the pathogenesis and the corresponding lack of effective therapies. Consequently, there is a major need for animal models that closely mirror the human eye pathology and at the same time allow higher-throughput drug screening approaches. In this context, zebrafish as an animal model organism not only address these needs but can in many respects reflect the human situation better than the current rodent models. Over the past decade, zebrafish have become an established model to study a variety of human diseases and are more recently becoming a valuable tool for the study of human ophthalmological disorders. Many human ocular diseases such as cataract, glaucoma, diabetic retinopathy, and age-related macular degeneration have already been modelled in zebrafish. In addition, zebrafish have become an attractive model for pre-clinical drug toxicity testing and are now increasingly used by scientists worldwide for the discovery of novel treatment approaches. This review presents the advantages and uses of zebrafish for ophthalmological research.

The optokinetic response as a quantitative measure of visual acuity in zebrafish

Zebrafish are a proven model for vision research, however many of the earlier methods generally focused on larval fish or demonstrated a simple response. More recently adult visual behavior in zebrafish has become of interest, but methods to measure specific responses are new coming. To address this gap, we set out to develop a methodology to repeatedly and accurately utilize the optokinetic response (OKR) to measure visual acuity in adult zebrafish. Here we show that the adult zebrafish's visual acuity can be measured, including both binocular and monocular acuities. Because the fish is not harmed during the procedure, the visual acuity can be measured and compared over short or long periods of time. The visual acuity measurements described here can also be done quickly allowing for high throughput and for additional visual procedures if desired. This type of analysis is conducive to drug intervention studies or investigations of disease progression.

Early retinoic acid deprivation in developing zebrafish results in microphthalmia

Vitamin A deficiency causes impaired vision and blindness in millions of children around the world. Previous studies in zebrafish have demonstrated that retinoic acid (RA), the acid form of vitamin A, plays a vital role in early eye development. The objective of this study was to describe the effects of early RA deficiency by treating zebrafish with diethylaminobenzaldehyde (DEAB), a potent inhibitor of the enzyme retinaldehyde dehydrogenase (RALDH) that converts retinal to RA. Zebrafish embryos were treated for 2 h beginning at 9 h postfertilization. Gross morphology and retinal development were examined at regular intervals for 5 days after treatment. The optokinetic reflex (OKR) test, visual background adaptation (VBA) test, and the electroretinogram (ERG) were performed to assess visual function and behavior. Early treatment of zebrafish embryos with 100 μM DEAB (9 h) resulted in reduced eye size, and this microphthalmia persisted through larval development. Retinal histology revealed that DEAB eyes had significant developmental abnormalities but had relatively normal retinal lamination by 5.5 days postfertilization. However, the fish showed neither an OKR nor a VBA response. Further, the retina did not respond to light as measured by the ERG. We conclude that early deficiency of RA during eye development causes microphthalmia as well as other visual defects, and that timing of the RA deficiency is critical to the developmental outcome.

Animal models of glaucoma

Glaucoma is a heterogeneous group of disorders that progressively lead to blindness due to loss of retinal ganglion cells and damage to the optic nerve. It is a leading cause of blindness and visual impairment worldwide. Although research in the field of glaucoma is substantial, the pathophysiologic mechanisms causing the disease are not completely understood. A wide variety of animal models have been used to study glaucoma. These include monkeys, dogs, cats, rodents, and several other species. Although these models have provided valuable information about the disease, there is still no ideal model for studying glaucoma due to its complexity. In this paper we present a summary of most of the animal models that have been developed and used for the study of the different types of glaucoma, the strengths and limitations associated with each species use, and some potential criteria to develop a suitable model.

Retinal proliferation response in the buphthalmic zebrafish, bugeye

The zebrafish retina regenerates in response to acute retinal lesions, replacing damaged neurons with new neurons. In this study we test the hypothesis that chronic stress to inner retinal neurons also triggers a retinal regeneration response in the bugeye zebrafish. Mutations in the lrp2 gene in zebrafish are associated with a progressive eye phenotype (bugeye) that models several risk factors for human glaucoma including buphthalmos (enlarged eyes), elevated intraocular pressure (IOP), and upregulation of genes related to retinal ganglion cell pathology. The retinas of adult bugeye zebrafish showed high rates of ongoing proliferation which resulted in the production of a small number of new retinal neurons, particularly photoreceptors. A marker of mechanical cell stress, Hsp27, was strongly expressed in inner retinal neurons and glia of bugeye retinas. The more enlarged eyes of individual bugeye zebrafish showed disrupted retinal lamination, and a persistent reduced density of neurons in the ganglion cell layer (GCL), although total numbers of GCL neurons were higher than in control eyes. Despite the presence of a proliferative response to damage, the adult bugeye zebrafish remained behaviorally blind. These findings suggest the existence of an unsuccessful regenerative response to a persistent pathological condition in the bugeye zebrafish.

Retinal proliferation response in the buphthalmic zebrafish, bugeye

The zebrafish retina regenerates in response to acute retinal lesions, replacing damaged neurons with new neurons. In this study we test the hypothesis that chronic stress to inner retinal neurons also triggers a retinal regeneration response in the bugeye zebrafish. Mutations in the lrp2 gene in zebrafish are associated with a progressive eye phenotype (bugeye) that models several risk factors for human glaucoma including buphthalmos (enlarged eyes), elevated intraocular pressure (IOP), and upregulation of genes related to retinal ganglion cell pathology. The retinas of adult bugeye zebrafish showed high rates of ongoing proliferation which resulted in the production of a small number of new retinal neurons, particularly photoreceptors. A marker of mechanical cell stress, Hsp27, was strongly expressed in inner retinal neurons and glia of bugeye retinas. The more enlarged eyes of individual bugeye zebrafish showed disrupted retinal lamination, and a persistent reduced density of neurons in the ganglion cell layer (GCL), although total numbers of GCL neurons were higher than in control eyes. Despite the presence of a proliferative response to damage, the adult bugeye zebrafish remained behaviorally blind. These findings suggest the existence of an unsuccessful regenerative response to a persistent pathological condition in the bugeye zebrafish.