A High-Throughput Assay for Congenital and Age-Related Eye Diseases in Zebrafish

Behavioral Effects of the Mixture and the Single Compounds Carbendazim, Fipronil, and Sulfentrazone on Zebrafish (Danio rerio) Larvae

Pesticides are often detected in freshwater, but their impact on the aquatic environment is commonly studied based on single compounds, underestimating the potential additive effects of these mixtures. Even at low concentrations, pesticides can negatively affect organisms, altering important behaviors that can have repercussions at the population level. This study used a multi-behavioral approach to evaluate the effects of zebrafish larvae exposure to carbendazim (C), fipronil (F), and sulfentrazone (S), individually and mixed. Five behavioral tests, thigmotaxis, touch sensitivity, optomotor response, bouncing ball test, and larval exploratory behavior, were performed to assess potential effects on anxiety, fear, and spatial and social interaction. Significant changes were observed in the performance of larvae exposed to all compounds and their mixtures. Among the single pesticides, exposure to S produced the most behavioral alterations, followed by F and C, respectively. A synergistic effect between the compounds was observed in the C + F group, which showed more behavioral effects than the groups exposed to pesticides individually. The use of behavioral tests to evaluate pesticide mixtures is important to standardize methods and associate behavioral changes with ecologically relevant events, thus creating a more realistic scenario for investigating the potential environmental impacts of these compounds.

The USH3A causative gene clarin1 functions in Müller glia to maintain retinal photoreceptors

Mutations in CLRN1 cause Usher syndrome type IIIA (USH3A), an autosomal recessive disorder characterized by hearing and vision loss, and often accompanied by vestibular balance issues. The identity of the cell types responsible for the pathology and mechanisms leading to vision loss in USH3A remains elusive. To address this, we employed CRISPR/Cas9 technology to delete a large region in the coding and untranslated (UTR) region of zebrafish clrn1. Retina of clrn1 mutant larvae exhibited sensitivity to cell stress, along with age-dependent loss of function and degeneration in the photoreceptor layer. Investigation revealed disorganization in the outer retina in clrn1 mutants, including actin-based structures of the Müller glia and photoreceptor cells. To assess cell-specific contributions to USH3A pathology, we specifically re-expressed clrn1 in either Müller glia or photoreceptor cells. Müller glia re-expression of clrn1 prevented the elevated cell death observed in larval clrn1 mutant zebrafish exposed to high-intensity light. Notably, the degree of phenotypic rescue correlated with the level of Clrn1 re-expression. Surprisingly, high levels of Clrn1 expression enhanced cell death in both wild-type and clrn1 mutant animals. However, rod- or cone-specific Clrn1 re-expression did not rescue the extent of cell death. Taken together, our findings underscore three crucial insights. First, clrn1 mutant zebrafish exhibit key pathological features of USH3A; second, Clrn1 within Müller glia plays a pivotal role in photoreceptor maintenance, with its expression requiring controlled regulation; third, the reliance of photoreceptors on Müller glia suggests a structural support mechanism, possibly through direct interactions between Müller glia and photoreceptors mediated in part by Clrn1 protein.

Novel use of FDA-approved drugs identified by cluster analysis of behavioral profiles

Repurposing FDA-approved drugs is an efficient and cost-effective approach in the development of therapeutics for a broad range of diseases. However, prediction of function can be challenging, especially in the brain. We screened a small-molecule library with FDA-approved drugs for effects on behavior. The studies were carried out using zebrafish larvae, imaged in a 384-well format. We found that various drugs affect activity, habituation, startle responses, excitability, and optomotor responses. The changes in behavior were organized in behavioral profiles, which were examined by hierarchical cluster analysis. One of the identified clusters includes the calcineurin inhibitors cyclosporine (CsA) and tacrolimus (FK506), which are immunosuppressants and potential therapeutics in the prevention of Alzheimer's disease. The calcineurin inhibitors form a functional cluster with seemingly unrelated drugs, including bromocriptine, tetrabenazine, rosiglitazone, nebivolol, sorafenib, cabozantinib, tamoxifen, meclizine, and salmeterol. We propose that drugs with 'CsA-type' behavioral profiles are promising candidates for the prevention and treatment of Alzheimer's disease.

RNA-binding proteins and post-transcriptional regulation in lens biology and cataract: Mediating spatiotemporal expression of key factors that control the cell cycle, transcription, cytoskeleton and transparency

Development of the ocular lens - a transparent tissue capable of sustaining frequent shape changes for optimal focusing power - pushes the boundaries of what cells can achieve using the molecular toolkit encoded by their genomes. The mammalian lens contains broadly two types of cells, the anteriorly located monolayer of epithelial cells which, at the equatorial region of the lens, initiate differentiation into fiber cells that contribute to the bulk of the tissue. This differentiation program involves massive upregulation of select fiber cell-expressed RNAs and their subsequent translation into high amounts of proteins, such as crystallins. But intriguingly, fiber cells achieve this while also simultaneously undergoing significant morphological changes such as elongation - involving about 1000-fold length-wise increase - and migration, which requires modulation of cytoskeletal and cell adhesion factors. Adding further to the challenges, these molecular and cellular events have to be coordinated as fiber cells progress toward loss of their nuclei and organelles, which irreversibly compromises their potential for harnessing genetically hardwired information. A long-standing question is how processes downstream of signaling and transcription, which may also participate in feedback regulation, contribute toward orchestrating these cellular differentiation events in the lens. It is now becoming clear from findings over the past decade that post-transcriptional gene expression regulatory mechanisms are critical in controlling cellular proteomes and coordinating key processes in lens development and fiber cell differentiation. Indeed, RNA-binding proteins (RBPs) such as Caprin2, Celf1, Rbm24 and Tdrd7 have now been described in mediating post-transcriptional control over key factors (e.g. Actn2, Cdkn1a (p21Cip1), Cdkn1b (p27Kip1), various crystallins, Dnase2b, Hspb1, Pax6, Prox1, Sox2) that are variously involved in cell cycle, transcription, cytoskeleton maintenance and differentiation in the lens. Furthermore, deficiencies of these RBPs have been shown to result in various eye and lens defects and/or cataract. Because fiber cell differentiation in the lens occurs throughout life, the underlying regulatory mechanisms operational in development are expected to also be recruited for the maintenance of transparency in aged lenses. Indeed, in support of this, TDRD7 and CAPRIN2 loci have been linked to age-related cataract in humans. Here, I will review the role of key RBPs in the lens and their importance in understanding the pathology of lens defects. I will discuss advances in RBP-based gene expression control, in general, and the important challenges that need to be addressed in the lens to define the mechanisms that determine the epithelial and fiber cell proteome. Finally, I will also discuss in detail several key future directions including the application of bioinformatics approaches such as iSyTE to study RBP-based post-transcriptional gene expression control in the aging lens and in the context of age-related cataract.

Development and biological characterization of a clinical gene transfer vector for the treatment of MAK-associated retinitis pigmentosa

By combining next generation whole exome sequencing and induced pluripotent stem cell (iPSC) technology we found that an Alu repeat inserted in exon 9 of the MAK gene results in a loss of normal MAK transcript and development of human autosomal recessive retinitis pigmentosa (RP). Although a relatively rare cause of disease in the general population, the MAK variant is enriched in individuals of Jewish ancestry. In this population, 1 in 55 individuals are carriers and one third of all cases of recessive RP is caused by this gene. The purpose of this study was to determine if a viral gene augmentation strategy could be used to safely restore functional MAK protein as a step toward a treatment for early stage MAK-associated RP. Patient iPSC-derived photoreceptor precursor cells were generated and transduced with viral vectors containing the MAK transcript. One week after transduction, transcript and protein could be detected via rt-PCR and western blotting respectively. Using patient-derived fibroblast cells and mak knockdown zebra fish we demonstrate that over-expression of the retinal MAK transgene restored the cells ability to regulate primary cilia length. In addition, the visual defect in mak knockdown zebrafish was mitigated via treatment with the retinal MAK transgene. There was no evidence of local or systemic toxicity at 1-month or 3-months following subretinal delivery of clinical grade vector into wild type rats. The findings reported here will help pave the way for initiation of a phase 1 clinical trial for the treatment of patients with MAK-associated RP.

The master transcription factor SOX2, mutated in anophthalmia/microphthalmia, is post-transcriptionally regulated by the conserved RNA-binding protein RBM24 in vertebrate eye development

Mutations in the key transcription factor, SOX2, alone account for 20% of anophthalmia (no eye) and microphthalmia (small eye) birth defects in humans-yet its regulation is not well understood, especially on the post-transcription level. We report the unprecedented finding that the conserved RNA-binding motif protein, RBM24, positively controls Sox2 mRNA stability and is necessary for optimal SOX2 mRNA and protein levels in development, perturbation of which causes ocular defects, including microphthalmia and anophthalmia. RNA immunoprecipitation assay indicates that RBM24 protein interacts with Sox2 mRNA in mouse embryonic eye tissue. and electrophoretic mobility shift assay shows that RBM24 directly binds to the Sox2 mRNA 3'UTR, which is dependent on AU-rich elements (ARE) present in the Sox2 mRNA 3'UTR. Further, we demonstrate that Sox2 3'UTR AREs are necessary for RBM24-based elevation of Sox2 mRNA half-life. We find that this novel RBM24-Sox2 regulatory module is essential for early eye development in vertebrates. We show that Rbm24-targeted deletion using a constitutive CMV-driven Cre in mouse, and rbm24a-CRISPR/Cas9-targeted mutation or morpholino knockdown in zebrafish, results in Sox2 downregulation and causes the developmental defects anophthalmia or microphthalmia, similar to human SOX2-deficiency defects. We further show that Rbm24 deficiency leads to apoptotic defects in mouse ocular tissue and downregulation of eye development markers Lhx2, Pax6, Jag1, E-cadherin and gamma-crystallins. These data highlight the exquisite specificity that conserved RNA-binding proteins like RBM24 mediate in the post-transcriptional control of key transcription factors, namely, SOX2, associated with organogenesis and human developmental defects.

Functional Role of the RNA-Binding Protein Rbm24a and Its Target sox2 in Microphthalmia

Congenital eye defects represent a large class of disorders affecting roughly 21 million children worldwide. Microphthalmia and anophthalmia are relatively common congenital defects, with approximately 20% of human cases caused by mutations in SOX2. Recently, we identified the RNA-binding motif protein 24a (Rbm24a) which binds to and regulates sox2 in zebrafish and mice. Here we show that morpholino knockdown of rbm24a leads to microphthalmia and visual impairment. By utilizing sequential injections, we demonstrate that addition of exogenous sox2 RNA to rbm24a-deplete embryos is sufficient to suppress morphological and visual defects. This research demonstrates a critical role for understanding the post-transcriptional regulation of genes needed for development.

Zebrafish as an Animal Model for Ocular Toxicity Testing: A Review of Ocular Anatomy and Functional Assays

Xenobiotics make their way into organisms from diverse sources including diet, medication, and pollution. Our understanding of ocular toxicities from xenobiotics in humans, livestock, and wildlife is growing thanks to laboratory animal models. Anatomy and physiology are conserved among vertebrate eyes, and studies with common mammalian preclinical species (rodent, dog) can predict human ocular toxicity. However, since the eye is susceptible to toxicities that may not involve a histological correlate, and these species rely heavily on smell and hearing to navigate their world, discovering visual deficits can be challenging with traditional animal models. Alternative models capable of identifying functional impacts on vision and requiring minimal amounts of chemical are valuable assets to toxicology. Human and zebrafish eyes are anatomically and functionally similar, and it has been reported that several common human ocular toxicants cause comparable toxicity in zebrafish. Vision develops rapidly in zebrafish; the tiny larvae rely on visual cues as early as 4 days, and behavioral responses to those cues can be monitored in high-throughput fashion. This article describes the comparative anatomy of the zebrafish eye, the notable differences from the mammalian eye, and presents practical applications of this underutilized model for assessment of ocular toxicity.

Comparison of behavioral assays for assessing toxicant-induced alterations in neurological function in larval fathead minnows

Neuroactive compounds are routinely detected in surface waters at concentrations that pose potential threats to wildlife. Exposure to neurotoxicants can adversely affect exposed organism by altering ecologically-important behaviors (e.g., feeding and predator response) that are likely to have important repercussions for populations. These compounds can elicit behavioral effects at concentrations lower than those that induce overt toxicity as indicated by mortality or decreased growth. Though a wide variety of methods have been employed to assess the behavior of early life stage fish, it is unclear which assays are best suited for identifying ecologically-relevant behavioral changes following exposures to neurotoxicants. The goal of the present study was to promote the use of behavioral assays for assessing the behavioral impacts of exposure to neurotoxic compounds by comparing the performance of different behavioral assays in larval fish. To achieve this goal, the sensitivity and practicality of three behavioral assays (i.e., feeding, optomotor response, and C-start assays) were compared in larval fathead minnows exposed to a known neurotoxicant, chlorpyrifos. There were significant alterations in the performance of fathead minnow larvae in all three behavioral assays in response to a 12-d embryo-larval exposure to chlorpyrifos. However, feeding and C-start were the most practical of the selected assays, as they took less time and allowed for larger samples sizes. Further work to standardize behavioral testing methods, and to link alterations to ecologically-relevant behaviors, will help promote the use of these assays when investigating the potential environmental impacts of neurotoxic compounds.

Folic acid reduces the ethanol-induced morphological and behavioral defects in embryonic and larval zebrafish (Danio rerio) as a model for fetal alcohol spectrum disorder (FASD)

The objective of this work was to determine whether folic acid (FA) reduces the embryonic ethanol (EtOH) exposure induced behavioral and morphological defects in our zebrafish fetal alcohol spectrum disorder (FASD) model. Teratogenic effects, mortality, the excitatory light-dark locomotion (ELD), sleep (SL), thigmotaxis (TH), touch sensitivity (TS), and optomotor response (OMR) tests were evaluated in larvae (6-7 days post-fertilization) using four treatment conditions: Untreated, FA, EtOH and EtOH + FA. FA reduced morphological defects on heart, eyes and swim bladder inflation seen in EtOH exposed fish. The larvae were more active in the dark than in light conditions, and EtOH reduced the swimming activity in the ELD test. EtOH affected the sleep pattern, inducing several arousal periods and increasing inactivity in zebrafish. FA reduces these toxic effects and produced more consistent inactivity during the night, reducing the arousal periods. FA also prevented the EtOH-induced defects in thigmotaxis and optomotor response of the larvae. We conclude that in this FASD model, EtOH exposure produced several teratogenic and behavioral defects, FA reduced, but did not totally prevent, these defects. Understanding of EtOH-induced behavioral defects could help to identify new therapeutic or prevention strategies for FASD.

RNA-Binding Protein Rbm24 as a Multifaceted Post-Transcriptional Regulator of Embryonic Lineage Differentiation and Cellular Homeostasis

RNA-binding proteins control the metabolism of RNAs at all stages of their lifetime. They are critically required for the post-transcriptional regulation of gene expression in a wide variety of physiological and pathological processes. Rbm24 is a highly conserved RNA-binding protein that displays strongly regionalized expression patterns and exhibits dynamic changes in subcellular localization during early development. There is increasing evidence that it acts as a multifunctional regulator to switch cell fate determination and to maintain tissue homeostasis. Dysfunction of Rbm24 disrupts cell differentiation in nearly every tissue where it is expressed, such as skeletal and cardiac muscles, and different head sensory organs, but the molecular events that are affected may vary in a tissue-specific, or even a stage-specific manner. Recent works using different animal models have uncovered multiple post-transcriptional regulatory mechanisms by which Rbm24 functions in key developmental processes. In particular, it represents a major splicing factor in muscle cell development, and plays an essential role in cytoplasmic polyadenylation during lens fiber cell terminal differentiation. Here we review the advances in understanding the implication of Rbm24 during development and disease, by focusing on its regulatory roles in physiological and pathological conditions.

Rbm24 controls poly(A) tail length and translation efficiency of crystallin mRNAs in the lens via cytoplasmic polyadenylation

Lens transparency critically requires the abundant accumulation of crystallin proteins, and deregulation of this process causes congenital cataracts in humans. Rbm24 is an RNA-binding protein with highly conserved expression in differentiating lens fiber cells among all vertebrates. We use a zebrafish model to demonstrate that loss of Rbm24 function specifically impedes lens fiber cell differentiation, resulting in cataract formation and blindness. Molecular analyses reveal that Rbm24 interacts with cytoplasmic polyadenylation complex and binds to a large number of lens-expressed messenger RNAs to maintain their stability and protect their poly(A) tail length, thereby crucially contributing to their efficient translation into functional proteins. This work identifies an important mechanism by which Rbm24 posttranscriptionally controls lens gene expression to establish transparency and refraction power.

Lens transparency is established by abundant accumulation of crystallin proteins and loss of organelles in the fiber cells. It requires an efficient translation of lens messenger RNAs (mRNAs) to overcome the progressively reduced transcriptional activity that results from denucleation. Inappropriate regulation of this process impairs lens differentiation and causes cataract formation. However, the regulatory mechanism promoting protein synthesis from lens-expressed mRNAs remains unclear. Here we show that in zebrafish, the RNA-binding protein Rbm24 is critically required for the accumulation of crystallin proteins and terminal differentiation of lens fiber cells. In the developing lens, Rbm24 binds to a wide spectrum of lens-specific mRNAs through the RNA recognition motif and interacts with cytoplasmic polyadenylation element-binding protein (Cpeb1b) and cytoplasmic poly(A)-binding protein (Pabpc1l) through the C-terminal region. Loss of Rbm24 reduces the stability of a subset of lens mRNAs encoding heat shock proteins and shortens the poly(A) tail length of crystallin mRNAs encoding lens structural components, thereby preventing their translation into functional proteins. This severely impairs lens transparency and results in blindness. Consistent with its highly conserved expression in differentiating lens fiber cells, the findings suggest that vertebrate Rbm24 represents a key regulator of cytoplasmic polyadenylation and plays an essential role in the posttranscriptional control of lens development.