Analysis of optokinetic response in zebrafish by computer-based eye tracking

Critical insights into the potential risks of antipsychotic drugs to fish, including through effects on behaviour

Antipsychotic drugs (APDs) are a diverse class of neuroactive pharmaceuticals increasingly detected in surface and ground waters globally. Some APDs are classified as posing a high environmental risk, due, in part, to their tendency to bioaccumulate in wildlife, including fish. Additional risk drivers for APDs relate to their behavioural effects, potentially impacting fitness outcomes. However, standard ecotoxicological tests used in environmental risk assessment (ERA) do not currently account for these mechanisms. In this review, we critically appraise the environmental risks of APDs to fish. We begin by reading-across from human and mammalian effects data to standard ecotoxicological effects endpoints in fish. We then explore the wide range of behaviours suitable for ecotoxicological assessment of APDs (and other neuroactive) pharmaceuticals, principally through laboratory studies with zebrafish, and assess the potential for using these behavioural phenotypes to predict adverse individual- and population-level outcomes in wild fish, taking into account phenotypic plasticity. Next, we illustrate the advantages and challenges of measuring and applying behavioural endpoints for fish, including within current regulatory risk assessments. In our final analysis, the implications of relying on apical endpoints for ERA of neuroactive drugs (including APDs) are assessed and recommendations provided for the development of a more refined and tailored mechanistic approach, which would enable more robust assessment of their environmental risk(s).

NQO1 protects against clioquinol toxicity

Clioquinol (CQ) was widely used as oral antibiotic before being taken off the market in many countries in 1970, after it was linked to subacute myelo-optic neuropathy (SMON) in Japan, leading to vision loss with many patients left wheelchair-bound. The common pathology of CQ-associated SMON was reproduced in animals but none of the proposed modes of toxicity explained the restriction of CQ-induced SMON to Japan. Given a re-emergence of CQ and related analogues as neuroprotectants, it is crucial to understand the underlying mechanism of CQ-induced toxicity to prevent any potential CQ-associated risks to future patients. A small molecule screen to find drugs that induce mitochondrial dysfunction in vitro identified CQ and the structurally related 8-hydroxyquinoline (8-OHQ). Their mitochondrial liability, pro-oxidative and cytotoxic activity was subsequently confirmed in some cell lines but surprisingly not in others. Subsequent studies in isogenic cell lines demonstrated that the antioxidant protein NQO1 is differentially expressed in the cell lines tested and potently protects against CQ toxicity. CQ-induced reduction of cellular ATP levels, increased lipid peroxidation and elevated cell death was also attenuated by antioxidants, implicating oxidative stress as the core mechanism of CQ-induced toxicity. These in-vitro findings were replicated in zebrafish. Visual acuity in zebrafish larvae that do not express NQO1, was reduced by CQ in a dose-dependent manner, while CQ did not affect visual function in the adult zebrafish that express NQO1. Similarly, pharmacological inhibition of NQO1 activity resulted in CQ-induced oxidative stress in the retina and severe acute systemic toxicity in the adult fish. Given the much higher prevalence of the inactivating C609T NQO1 polymorphism in the Japanese population compared to the European population, the results of this study could for the first time indicate how the geographic restriction of SMON cases to Japan could be explained. Importantly, if CQ or its derivatives are to be used safely for the treatment of neurodegenerative diseases, it seems imperative that NQO1 levels and activity of prospective patients should be ascertained.

Vision and sensorimotor defects associated with loss of Vps11 function in a zebrafish model of genetic leukoencephalopathy

Genetic Leukoencephalopathies (gLEs) are heritable white matter disorders that cause progressive neurological abnormalities. A founder mutation in the human endolysosomal trafficking protein VPS11 has been identified in Ashkenazi Jewish patients manifesting classic gLE symptoms of hypomyelination, developmental delay, motor and systemic deficits. In this study, we characterized the visual and sensorimotor function of two zebrafish vps11 mutant lines: the previously reported vps11(plt), and a new vps11(-/-) null mutant line, using behavioral analysis to track larval motor responses to visual and acoustic stimuli. We found that mutant larvae from both vps11(plt) and vps11(-/-) lines were able to visually distinguish light and dark, but showed a progressive loss of a normal sensorimotor response to visual stimuli from 5 days post fertilization (dpf) to 7dpf. Additionally, optokinetic response analysis performed at 5dpf indicated that the mutants were significantly visually impaired. Both mutant lines also displayed a progressively lower sensorimotor response to a singular acoustic stimulus from 5-7dpf. Next, we tested the habituation response of the mutant lines to series of acoustic taps. We found both mutant lines habituated faster than their siblings, and that vps11(plt) mutants habituated faster than the vps11(-/-) mutants. Together, these data suggest that loss of Vps11 function results in progressive visual and sensorimotor abnormalities in the zebrafish vps11(plt) and vps11(-/-) mutant lines. This is the first study to characterize behavioral deficits in a vertebrate model of Vps11-dependent gLE. The mutants and behavioral assays described here could be a valuable model system in which to test potential pharmacological interventions for gLE.

Use of Zebrafish Models to Boost Research in Rare Genetic Diseases

Rare genetic diseases are a group of pathologies with often unmet clinical needs. Even if rare by a single genetic disease (from 1/2000 to 1/more than 1,000,000), the total number of patients concerned account for approximatively 400 million peoples worldwide. Finding treatments remains challenging due to the complexity of these diseases, the small number of patients and the challenge in conducting clinical trials. Therefore, innovative preclinical research strategies are required. The zebrafish has emerged as a powerful animal model for investigating rare diseases. Zebrafish combines conserved vertebrate characteristics with high rate of breeding, limited housing requirements and low costs. More than 84% of human genes responsible for diseases present an orthologue, suggesting that the majority of genetic diseases could be modelized in zebrafish. In this review, we emphasize the unique advantages of zebrafish models over other in vivo models, particularly underlining the high throughput phenotypic capacity for therapeutic screening. We briefly introduce how the generation of zebrafish transgenic lines by gene-modulating technologies can be used to model rare genetic diseases. Then, we describe how zebrafish could be phenotyped using state-of-the-art technologies. Two prototypic examples of rare diseases illustrate how zebrafish models could play a critical role in deciphering the underlying mechanisms of rare genetic diseases and their use to identify innovative therapeutic solutions.

A duplication on chromosome 16q12 affecting the IRXB gene cluster is associated with autosomal dominant cone dystrophy with early tritanopic color vision defect

Cone dystrophies are a rare subgroup of inherited retinal dystrophies and hallmarked by color vision defects, low or decreasing visual acuity and central vision loss, nystagmus and photophobia. Applying genome-wide linkage analysis and array comparative genome hybridization, we identified a locus for autosomal dominant cone dystrophy on chromosome 16q12 in four independent multigeneration families. The locus is defined by duplications of variable size with a smallest region of overlap of 608 kb affecting the IRXB gene cluster and encompasses the genes IRX5 and IRX6. IRX5 and IRX6 belong to the Iroquois (Iro) protein family of homeodomain-containing transcription factors involved in patterning and regionalization of embryonic tissue in vertebrates, including the eye and the retina. All patients presented with a unique progressive cone dystrophy phenotype hallmarked by early tritanopic color vision defects. We propose that the disease underlies a misregulation of the IRXB gene cluster on chromosome 16q12 and demonstrate that overexpression of Irx5a and Irx6a, the two orthologous genes in zebrafish, results in visual impairment in 5-day-old zebrafish larvae.

Approaches to Test the Neurotoxicity of Environmental Contaminants in the Zebrafish Model: From Behavior to Molecular Mechanisms

The occurrence of neuroactive chemicals in the aquatic environment is on the rise and poses a potential threat to aquatic biota of currently unpredictable outcome. In particular, subtle changes caused by these chemicals to an organism's sensation or behavior are difficult to tackle with current test systems that focus on rodents or with in vitro test systems that omit whole-animal responses. In recent years, the zebrafish (Danio rerio) has become a popular model organism for toxicological studies and testing strategies, such as the standardized use of zebrafish early life stages in the Organisation for Economic Co-operation and Development's guideline 236. In terms of neurotoxicity, the zebrafish provides a powerful model to investigate changes to the nervous system from several different angles, offering the ability to tackle the mechanisms of action of chemicals in detail. The mechanistic understanding gained through the analysis of this model species provides a good basic knowledge of how neuroactive chemicals might interact with a teleost nervous system. Such information can help infer potential effects occurring to other species exposed to neuroactive chemicals in their aquatic environment and predicting potential risks of a chemical for the aquatic ecosystem. In the present article, we highlight approaches ranging from behavioral to structural, functional, and molecular analysis of the larval zebrafish nervous system, providing a holistic view of potential neurotoxic outcomes. Environ Toxicol Chem 2021;40:989-1006. © 2020 SETAC.

Color as an important biological variable in zebrafish models: Implications for translational neurobehavioral research

Color is an important environmental factor that in multiple ways affects human and animal behavior and physiology. Widely used in neuroscience research, various experimental (animal) models may help improve our understanding of how different colors impact brain and behavioral processes. Complementing laboratory rodents, the zebrafish (Danio rerio) is rapidly emerging as an important novel model species to explore complex neurobehavioral processes. The growing utility of zebrafish in biomedicine makes it timely to consider the role of colors in their behavioral and physiological responses. Here, we summarize mounting evidence implicating colors as a critical variable in zebrafish models and neurobehavioral traits, with a particular relevance to CNS disease modeling, genetic and pharmacological modulation, as well as environmental enrichment and animal welfare. We also discuss the growing value of zebrafish models to study color neurobiology and color-related neurobehavioral phenomics, and outline future directions of research in this field.

Evoking and tracking zebrafish eye movement in multiple larvae with ZebEyeTrack

Reliable measurement of spontaneous and evoked eye movement is critical for behavioral vision research. Zebrafish are increasingly used as a model organism for visual neural circuits, but ready-to-use eye-tracking solutions are scarce. Here, we present a protocol for automated real-time measurement of angular horizontal eye position in up to six immobilized larval fish using a custom-built LabVIEW-based software, ZebEyeTrack. We provide its customizable source code, as well as a streamlined and compiled version, ZebEyeTrack Light. The full version of ZebEyeTrack controls all required hardware and synchronizes six essential aspects of the experiment: (i) stimulus design; (ii) visual stimulation with moving bars; (ii) eye detection and tracking, as well as general motion detection; (iv) real-time analysis; (v) eye-position-dependent closed-loop event control; and (vi) recording of external event times. This includes optional integration with external hardware such as lasers and scanning microscopes. Once installation is complete, experiments, including stimulus design, can be completed in <10 min, and recordings can last anywhere between seconds and many hours. Results include digitized angular eye positions and hardware status, which can be used to compute tuning curves, optokinetic gain, and other custom data analysis. After the experiment, or based on existing videos, optokinetic response (OKR) performance can be analyzed semi-automatically via the graphical user interface, and results can be exported. ZebEyeTrack has been used successfully for psychophysics experiments, for optogenetic stimulation, and in combination with calcium imaging.

Eyes shut homolog is important for the maintenance of photoreceptor morphology and visual function in zebrafish

Mutations in eyes shut homolog (EYS), a gene predominantly expressed in the photoreceptor cells of the retina, are among the most frequent causes of autosomal recessive (ar) retinitis pigmentosa (RP), a progressive retinal disorder. Due to the absence of EYS in several rodent species and its retina-specific expression, still little is known about the exact function of EYS and the pathogenic mechanism underlying EYS-associated RP. We characterized eys in zebrafish, by RT-PCR analysis on zebrafish eye-derived RNA, which led to the identification of a 8,715 nucleotide coding sequence that is divided over 46 exons. The transcript is predicted to encode a 2,905-aa protein that contains 39 EGF-like domains and five laminin A G-like domains, which overall shows 33% identity with human EYS. To study the function of EYS, we generated a stable eysrmc101/rmc101 mutant zebrafish model using CRISPR/Cas9 technology. The introduced lesion is predicted to result in premature termination of protein synthesis and lead to loss of Eys function. Immunohistochemistry on retinal sections revealed that Eys localizes at the region of the connecting cilium and that both rhodopsin and cone transducin are mislocalized in the absence of Eys. Electroretinogram recordings showed diminished b-wave amplitudes in eysrmc101/rmc101 zebrafish (5 dpf) compared to age- and strain-matched wild-type larvae. In addition, decreased locomotor activity in response to light stimuli was observed in eys mutant larvae. Altogether, our study shows that absence of Eys leads to a disorganized retinal architecture and causes visual dysfunction in zebrafish.

C2orf71a/pcare1 is important for photoreceptor outer segment morphogenesis and visual function in zebrafish

Mutations in C2orf71 are causative for autosomal recessive retinitis pigmentosa and occasionally cone-rod dystrophy. We have recently discovered that the protein encoded by this gene is important for modulation of the ciliary membrane through the recruitment of an actin assembly module, and have therefore renamed the gene to PCARE (photoreceptor cilium actin regulator). Here, we report on the identification of two copies of the c2orf71/pcare gene in zebrafish, pcare1 and pcare2. To study the role of the gene most similar to human PCARE, pcare1, we have generated a stable pcare1 mutant zebrafish model (designated pcare1^{rmc100/rmc100}) in which the coding sequence was disrupted using CRISPR/Cas9 technology. Retinas of both embryonic (5 dpf) and adult (6 mpf) pcare1^{rmc100/rmc100} zebrafish display a clear disorganization of photoreceptor outer segments, resembling the phenotype observed in Pcare^−/− mice. Optokinetic response and visual motor response measurements indicated visual impairment in pcare1^{rmc100/rmc100} zebrafish larvae at 5 dpf. In addition, electroretinogram measurements showed decreased b-wave amplitudes in pcare1^{rmc100/rmc100} zebrafish as compared to age- and strain-matched wild-type larvae, indicating a defect in the transretinal current. Altogether, our data show that lack of pcare1 causes a retinal phenotype in zebrafish and indicate that the function of the PCARE gene is conserved across species.

Genetic approaches to retinal research in zebrafish

The zebrafish (Danio rerio) possesses a vertebrate-type retina that is extraordinarily conserved in evolution. This well-organized and anatomically easily accessible part of the central nervous system has been widely investigated in zebrafish, promoting general understanding of retinal development, morphology, function and associated diseases. Over the recent years, genome and protein engineering as well as imaging techniques have experienced revolutionary advances and innovations, creating new possibilities and methods to study zebrafish development and function. In this review, we focus on some of these emerging technologies and how they may impact retinal research in the future. We place an emphasis on genetic techniques, such as transgenic approaches and the revolutionizing new possibilities in genome editing.

miR-181a/b control the assembly of visual circuitry by regulating retinal axon specification and growth

Connectivity and function of neuronal circuitry require the correct specification and growth of axons and dendrites. Here, we identify the microRNAs miR‐181a and miR‐181b as key regulators of retinal axon specification and growth. Loss of miR‐181a/b in medaka fish (Oryzias latipes) failed to consolidate amacrine cell processes into axons and delayed the growth of retinal ganglion cell (RGC) axons. These alterations were accompanied by defects in visual connectivity and function. We demonstrated that miR‐181a/b exert these actions through negative modulation of MAPK/ERK signaling that in turn leads to RhoA reduction and proper neuritogenesis in both amacrine cells and RGCs via local cytoskeletal rearrangement. Our results identify a new pathway for axon specification and growth unraveling a crucial role of miR‐181a/b in the proper establishment of visual system connectivity and function. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 75: 1252–1267, 2015

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.

Brain on the stage - spotlight on nervous system development in zebrafish: EMBO practical course, KIT, Sept. 2013

During the EMBO course ‘Imaging of Neural Development in Zebrafish’, held on September 9–15^th 2013, researchers from different backgrounds shared their latest results, ideas and practical expertise on zebrafish as a model to address open questions regarding nervous system development.

OMR-arena: automated measurement and stimulation system to determine mouse visual thresholds based on optomotor responses

Measurement of the optomotor response is a common way to determine thresholds of the visual system in animals. Particularly in mice, it is frequently used to characterize the visual performance of different genetically modified strains or to test the effect of various drugs on visual performance. Several methods have been developed to facilitate the presentation of stimuli using computer screens or projectors. Common methods are either based on the measurement of eye movement during optokinetic reflex behavior or rely on the measurement of head and/or body-movements during optomotor responses. Eye-movements can easily and objectively be quantified, but their measurement requires invasive fixation of the animals. Head movements can be observed in freely moving animals, but until now depended on the judgment of a human observer who reported the counted tracking movements of the animal during an experiment. In this study we present a novel measurement and stimulation system based on open source building plans and software. This system presents appropriate 360 stimuli while simultaneously video-tracking the animal's head-movements without fixation. The on-line determined head gaze is used to adjust the stimulus to the head position, as well as to automatically calculate visual acuity. Exemplary, we show that automatically measured visual response curves of mice match the results obtained by a human observer very well. The spatial acuity thresholds yielded by the automatic analysis are also consistent with the human observer approach and with published results. Hence, OMR-arena provides an affordable, convenient and objective way to measure mouse visual performance.

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.