Fragile X Mental Retardation-1 Knockout Zebrafish Shows Precocious Development in Social Behavior

Loss of FMRP affects ovarian development and behaviour through multiple pathways in a zebrafish model of fragile X syndrome

Fragile X syndrome (FXS) is an inherited neurodevelopmental disorder and the leading genetic cause of autism spectrum disorders. FXS is caused by loss of function mutations in Fragile X mental retardation protein (FMRP), an RNA binding protein that is known to regulate translation of its target mRNAs, predominantly in the brain and gonads. The molecular mechanisms connecting FMRP function to neurodevelopmental phenotypes are well understood. However, neither the full extent of reproductive phenotypes, nor the underlying molecular mechanisms have been as yet determined. Here, we developed new fmr1 knockout zebrafish lines and show that they mimic key aspects of FXS neuronal phenotypes across both larval and adult stages. Results from the fmr1 knockout females also showed that altered gene expression in the brain, via the neuroendocrine pathway contribute to distinct abnormal phenotypes during ovarian development and oocyte maturation. We identified at least three mechanisms underpinning these defects, including altered neuroendocrine signaling in sexually mature females resulting in accelerated ovarian development, altered expression of germ cell and meiosis promoting genes at various stages during oocyte maturation, and finally a strong mitochondrial impairment in late stage oocytes from knockout females. Our findings have implications beyond FXS in the study of reproductive function and female infertility. Dissection of the translation control pathways during ovarian development using models like the knockout lines reported here may reveal novel approaches and targets for fertility treatments.

fmr1 Mutation Alters the Early Development of Sensory Coding and Hunting and Social Behaviors in Larval Zebrafish

Autism spectrum disorders (ASDs) are developmental in origin; however, little is known about how they affect the early development of behavior and sensory coding. The most common inherited form of autism is Fragile X syndrome (FXS), caused by a mutation in FMR1 Mutation of fmr1 in zebrafish causes anxiety-like behavior, hyperactivity, and hypersensitivity in auditory and visual processing. Here, we show that zebrafish fmr1-/- mutant larvae of either sex also display changes in hunting behavior, tectal coding, and social interaction. During hunting, they were less successful at catching prey and displayed altered behavioral sequences. In the tectum, representations of prey-like stimuli were more diffuse and had higher dimensionality. In a social behavioral assay, they spent more time observing a conspecific but responded more slowly to social cues. However, when given a choice of rearing environment fmr1-/- larvae preferred one with reduced visual stimulation, and rearing them in this environment reduced genotype-specific effects on tectal excitability. Together, these results shed new light on how fmr1-/- changes the early development of neural systems and behavior in a vertebrate.SIGNIFICANCE STATEMENT Autism spectrum disorders (ASDs) are caused by changes in early neural development. Animal models of ASDs offer the opportunity to study these developmental processes in greater detail than in humans. Here, we found that a zebrafish mutant for a gene which in humans causes one type of ASD showed early alterations in hunting behavior, social behavior, and how visual stimuli are represented in the brain. However, we also found that mutant fish preferred reduced visual stimulation, and rearing them in this environment reduced alterations in neural activity patterns. These results suggest interesting new directions for using zebrafish as a model to study the development of brain and behavior in ASDs, and how the impact of ASDs could potentially be reduced.

Disruption of grin2B, an ASD-associated gene, produces social deficits in zebrafish

BACKGROUND

Autism spectrum disorder (ASD), like many neurodevelopmental disorders, has complex and varied etiologies. Advances in genome sequencing have identified multiple candidate genes associated with ASD, including dozens of missense and nonsense mutations in the NMDAR subunit GluN2B, encoded by GRIN2B. NMDARs are glutamate-gated ion channels with key synaptic functions in excitatory neurotransmission. How alterations in these proteins impact neurodevelopment is poorly understood, in part because knockouts of GluN2B in rodents are lethal.

METHODS

Here, we use CRISPR-Cas9 to generate zebrafish lacking GluN2B (grin2B-/-). Using these fish, we run an array of behavioral tests and perform whole-brain larval imaging to assay developmental roles and functions of GluN2B.

RESULTS

We demonstrate that zebrafish GluN2B displays similar structural and functional properties to human GluN2B. Zebrafish lacking GluN2B (grin2B-/-) surprisingly survive into adulthood. Given the prevalence of social deficits in ASD, we assayed social preference in the grin2B-/- fish. Wild-type fish develop a strong social preference by 3 weeks post fertilization. In contrast, grin2B-/- fish at this age exhibit significantly reduced social preference. Notably, the lack of GluN2B does not result in a broad disruption of neurodevelopment, as grin2B-/- larvae do not show alterations in spontaneous or photic-evoked movements, are capable of prey capture, and exhibit learning. Whole-brain imaging of grin2B-/- larvae revealed reduction of an inhibitory neuron marker in the subpallium, a region linked to ASD in humans, but showed that overall brain size and E/I balance in grin2B-/- is comparable to wild type.

LIMITATIONS

Zebrafish lacking GluN2B, while useful in studying developmental roles of GluN2B, are unlikely to model nuanced functional alterations of human missense mutations that are not complete loss of function. Additionally, detailed mammalian homologies for larval zebrafish brain subdivisions at the age of whole-brain imaging are not fully resolved.

CONCLUSIONS

We demonstrate that zebrafish completely lacking the GluN2B subunit of the NMDAR, unlike rodent models, are viable into adulthood. Notably, they exhibit a highly specific deficit in social behavior. As such, this zebrafish model affords a unique opportunity to study the roles of GluN2B in ASD etiologies and establish a disease-relevant in vivo model for future studies.

Behavioral and neurochemical profile of MK-801 adult zebrafish model: Forebrain β2-adrenoceptors contribute to social withdrawal and anxiety-like behavior

Deficits in social communication and interaction are core clinical symptoms characterizing multiple neuropsychiatric conditions, including autism spectrum disorder (ASD) and schizophrenia. Interestingly, elevated anxiety levels are a common comorbid psychopathology characterizing individuals with aberrant social behavior. Despite recent progress, the underlying neurobiological mechanisms that link anxiety with social withdrawal remain poorly understood. The present study developed a zebrafish pharmacological model displaying social withdrawal behavior, following a 3-h exposure to 4 μΜ (+)-MK-801, a non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, for 7 days. Interestingly, MK-801-treated zebrafish displayed elevated anxiety levels along with higher frequency of stereotypical behaviors, rendering this zebrafish model appropriate to unravel a possible link of catecholaminergic and ASD-like phenotypes. MK-801-treated zebrafish showed increased telencephalic protein expression of metabotropic glutamate 5 receptor (mGluR5), dopamine transporter (DAT) and β₂-adrenergic receptors (β₂-ARs), supporting the presence of excitation/inhibition imbalance along with altered dopaminergic and noradrenergic activity. Interestingly, β₂-ARs expression, was differentially regulated across the Social Decision-Making (SDM) network nodes, exhibiting increased levels in ventral telencephalic area (Vv), a key-area integrating reward and social circuits but decreased expression in dorso-medial telencephalic area (Dm) and anterior tuberal nucleus (ATN). Moreover, the co-localization of β₂-ARs with elements of GABAergic and glutamatergic systems, as well as with GAP-43, a protein indicating increased brain plasticity potential, support the key-role of β₂-ARs in the MK-801 zebrafish social dysfunctions. Our results highlight the importance of the catecholaminergic neurotransmission in the manifestation of ASD-like behavior, representing a site of potential interventions for amelioration of ASD-like symptoms.

Oxytocin receptors influence the development and maintenance of social behavior in zebrafish (Danio rerio)

Zebrafish are highly social teleost fish and an excellent model to study social behavior. The neuropeptide Oxytocin is associated different social behaviors as well as disorders resulting in social impairment like autism spectrum disorder. However, how Oxytocin receptor signaling affects the development and expression kinetics of social behavior is not known. In this study we investigated the role of the two oxytocin receptors, Oxtr and Oxtrl, in the development and maintenance of social preference and shoaling behavior in 2- to 8-week-old zebrafish. Using CRISPR/Cas9 mediated oxtr and oxtrl knock-out fish, we found that the development of social preference is accelerated if one of the Oxytocin receptors is knocked-out and that the knock-out fish reach significantly higher levels of social preference. Moreover, oxtr^−/− fish showed impairments in the maintenance of social preference. Social isolation prior to testing led to impaired maintenance of social preference in both wild-type and oxtr and oxtrl knock-out fish. Knocking-out either of the Oxytocin receptors also led to increased group spacing and reduced polarization in a 20-fish shoal at 8 weeks post fertilization, but not at 4. These results show that the development and maintenance of social behavior is influenced by the Oxytocin receptors and that the effects are not just pro- or antisocial, but dependent on both the age and social context of the fish.

Brain-wide visual habituation networks in wild type and fmr1 zebrafish

Habituation is a form of learning during which animals stop responding to repetitive stimuli, and deficits in habituation are characteristic of several psychiatric disorders. Due to technical challenges, the brain-wide networks mediating habituation are poorly understood. Here we report brain-wide calcium imaging during larval zebrafish habituation to repeated visual looming stimuli. We show that different functional categories of loom-sensitive neurons are located in characteristic locations throughout the brain, and that both the functional properties of their networks and the resulting behavior can be modulated by stimulus saliency and timing. Using graph theory, we identify a visual circuit that habituates minimally, a moderately habituating midbrain population proposed to mediate the sensorimotor transformation, and downstream circuit elements responsible for higher order representations and the delivery of behavior. Zebrafish larvae carrying a mutation in the fmr1 gene have a systematic shift toward sustained premotor activity in this network, and show slower behavioral habituation.

Fragile X Syndrome: From Molecular Aspect to Clinical Treatment

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by the full mutation as well as highly localized methylation of the fragile X mental retardation 1 (FMR1) gene on the long arm of the X chromosome. Children with FXS are commonly co-diagnosed with Autism Spectrum Disorder, attention and learning problems, anxiety, aggressive behavior and sleep disorder, and early interventions have improved many behavior symptoms associated with FXS. In this review, we performed a literature search of original and review articles data of clinical trials and book chapters using MEDLINE (1990-2021) and ClinicalTrials.gov. While we have reviewed the biological importance of the fragile X mental retardation protein (FMRP), the FXS phenotype, and current diagnosis techniques, the emphasis of this review is on clinical interventions. Early non-pharmacological interventions in combination with pharmacotherapy and targeted treatments aiming to reverse dysregulated brain pathways are the mainstream of treatment in FXS. Overall, early diagnosis and interventions are fundamental to achieve optimal clinical outcomes in FXS.

Zebrafish Modeling of Autism Spectrum Disorders, Current Status and Future Prospective

Autism spectrum disorder (ASD) refers to a complicated range of childhood neurodevelopmental disorders which can occur via genetic or non-genetic factors. Clinically, ASD is associated with problems in relationships, social interactions, and behaviors that pose many challenges for children with ASD and their families. Due to the complexity, heterogeneity, and association of symptoms with some neuropsychiatric disorders such as ADHD, anxiety, and sleep disorders, clinical trials have not yielded reliable results and there still remain challenges in drug discovery and development pipeline for ASD patients. One of the main steps in promoting lead compounds to the suitable drug for commercialization is preclinical animal testing, in which the efficacy and toxicity of candidate drugs are examined in vivo. In recent years, zebrafish have been able to attract the attention of many researchers in the field of neurological disorders such as ASD due to their outstanding features. The presence of orthologous genes for ASD modeling, the anatomical similarities of parts of the brain, and similar neurotransmitter systems between zebrafish and humans are some of the main reasons why scientists draw attention to zebrafish as a prominent animal model in preclinical studies to discover highly effective treatment approaches for the ASD through genetic and non-genetic modeling methods.

Zebrafish Models of Autosomal Dominant Ataxias

Hereditary dominant ataxias are a heterogeneous group of neurodegenerative conditions causing cerebellar dysfunction and characterized by progressive motor incoordination. Despite many efforts put into the study of these diseases, there are no effective treatments yet. Zebrafish models are widely used to characterize neuronal disorders due to its conserved vertebrate genetics that easily support genetic edition and their optic transparency that allows observing the intact CNS and its connections. In addition, its small size and external fertilization help to develop high throughput assays of candidate drugs. Here, we discuss the contributions of zebrafish models to the study of dominant ataxias defining phenotypes, genetic function, behavior and possible treatments. In addition, we review the zebrafish models created for X-linked repeat expansion diseases X-fragile/fragile-X tremor ataxia. Most of the models reviewed here presented neuronal damage and locomotor deficits. However, there is a generalized lack of zebrafish adult heterozygous models and there are no knock-in zebrafish models available for these diseases. The models created for dominant ataxias helped to elucidate gene function and mechanisms that cause neuronal damage. In the future, the application of new genetic edition techniques would help to develop more accurate zebrafish models of dominant ataxias.

Experimental Models to Study Autism Spectrum Disorders: hiPSCs, Rodents and Zebrafish

Autism Spectrum Disorders (ASD) affect around 1.5% of the global population, which manifest alterations in communication and socialization, as well as repetitive behaviors or restricted interests. ASD is a complex disorder with known environmental and genetic contributors; however, ASD etiology is far from being clear. In the past decades, many efforts have been put into developing new models to study ASD, both in vitro and in vivo. These models have a lot of potential to help to validate some of the previously associated risk factors to the development of the disorder, and to test new potential therapies that help to alleviate ASD symptoms. The present review is focused on the recent advances towards the generation of models for the study of ASD, which would be a useful tool to decipher the bases of the disorder, as well as to conduct drug screenings that hopefully lead to the identification of useful compounds to help patients deal with the symptoms of ASD.

Using Zebrafish to Model Autism Spectrum Disorder: A Comparison of ASD Risk Genes Between Zebrafish and Their Mammalian Counterparts

Autism spectrum disorders (ASDs) are a highly variable and complex set of neurological disorders that alter neurodevelopment and cognitive function, which usually presents with social and learning impairments accompanied with other comorbid symptoms like hypersensitivity or hyposensitivity, or repetitive behaviors. Autism can be caused by genetic and/or environmental factors and unraveling the etiology of ASD has proven challenging, especially given that different genetic mutations can cause both similar and different phenotypes that all fall within the autism spectrum. Furthermore, the list of ASD risk genes is ever increasing making it difficult to synthesize a common theme. The use of rodent models to enhance ASD research is invaluable and is beginning to unravel the underlying molecular mechanisms of this disease. Recently, zebrafish have been recognized as a useful model of neurodevelopmental disorders with regards to genetics, pharmacology and behavior and one of the main foundations supporting autism research (SFARI) recently identified 12 ASD risk genes with validated zebrafish mutant models. Here, we describe what is known about those 12 ASD risk genes in human, mice and zebrafish to better facilitate this research. We also describe several non-genetic models including pharmacological and gnotobiotic models that are used in zebrafish to study ASD.

Understanding neurobehavioral genetics of zebrafish

Due to its fully sequenced genome, high genetic homology to humans, external fertilization, fast development, transparency of embryos, low cost and active reproduction, the zebrafish (Danio rerio) has become a novel promising model organism in biomedicine. Zebrafish are a useful tool in genetic and neuroscience research, including linking various genetic mutations to brain mechanisms using forward and reverse genetics. These approaches have produced novel models of rare genetic CNS disorders and common brain illnesses, such as addiction, aggression, anxiety and depression. Genetically modified zebrafish also foster neuroanatomical studies, manipulating neural circuits and linking them to different behaviors. Here, we discuss recent advances in neurogenetics of zebrafish, and evaluate their unique strengths, inherent limitations and the rapidly growing potential for elucidating the conserved roles of genes in neuropsychiatric disorders.

A DNAzyme based knockdown model for Fragile-X syndrome in zebrafish reveals a critical window for therapeutic intervention

INTRODUCTION

FXS is the leading cause of intellectual disabilities in males and a major monogenic cause of ASD (Autism spectrum disorders). It occurs due to the loss of FMRP, whose role in early development is not well understood. In this study, we have used a novel DNAzyme based approach to create a larval model of FXS in zebrafish with specific focus on the early developmental window.

METHODS

Fmr1specific DNAzymes were electroporated into embryos to create the knockdown. Changes in RNA and protein levels of FMRP and relevant biomarkers were measured in the 0-7dpf window. Behavioral tests to measure anxiety, cognitive impairments and irritability in the larvae were conducted at the 7dpf stage. Drug treatment was carried out at various time points in the 0-7dpf window to identify the critical window for pharmacological intervention.

RESULTS

The DNAzyme based knockdown approach led to a significant knockdown of FMRP in the zebrafish embryos, accompanied by increased anxiety, irritability and cognitive impairments at 7dpf, thus creating a robust larval model of FXS. Treatment with the Mavoglurant was able to rescue the behavioral phenotypes in the FXS larvae, and found to be more efficacious in the 0-3dpf window.

DISCUSSION

The results from this study have revealed that a) a DNAzyme based knockdown approach can be used to create robust larval zebrafish model of disease, in a high-throughput manner and b) optimal window for therapeutic intervention for FXS as well as other pediatric diseases with a monogenic cause can be identified using such a model.

The zebrafish subcortical social brain as a model for studying social behavior disorders

Social behaviors are essential for the survival and reproduction of social species. Many, if not most, neuropsychiatric disorders in humans are either associated with underlying social deficits or are accompanied by social dysfunctions. Traditionally, rodent models have been used to model these behavioral impairments. However, rodent assays are often difficult to scale up and adapt to high-throughput formats, which severely limits their use for systems-level science. In recent years, an increasing number of studies have used zebrafish (Danio rerio) as a model system to study social behavior. These studies have demonstrated clear potential in overcoming some of the limitations of rodent models. In this Review, we explore the evolutionary conservation of a subcortical social brain between teleosts and mammals as the biological basis for using zebrafish to model human social behavior disorders, while summarizing relevant experimental tools and assays. We then discuss the recent advances gleaned from zebrafish social behavior assays, the applications of these assays to studying related disorders, and the opportunities and challenges that lie ahead.

Zebrafish Models of Neurodevelopmental Disorders: Limitations and Benefits of Current Tools and Techniques

For the past few years there has been an exponential increase in the use of animal models to confirm the pathogenicity of candidate disease-causing genetic variants found in patients. One such animal model is the zebrafish. Despite being a non-mammalian animal, the zebrafish model has proven its potential in recapitulating the phenotypes of many different human genetic disorders. This review will focus on recent advances in the modeling of neurodevelopmental disorders in zebrafish, covering aspects from early brain development to techniques used for modulating gene expression, as well as how to best characterize the resulting phenotypes. We also review other existing models of neurodevelopmental disorders, and the current efforts in developing and testing compounds with potential therapeutic value.

Hippocampal deficits in neurodevelopmental disorders

Neurodevelopmental disorders result from impaired development or maturation of the central nervous system. Both genetic and environmental factors can contribute to the pathogenesis of these disorders; however, the exact causes are frequently complex and unclear. Individuals with neurodevelopmental disorders may have deficits with diverse manifestations, including challenges with sensory function, motor function, learning, memory, executive function, emotion, anxiety, and social ability. Although these functions are mediated by multiple brain regions, many of them are dependent on the hippocampus. Extensive research supports important roles of the mammalian hippocampus in learning and cognition. In addition, with its high levels of activity-dependent synaptic plasticity and lifelong neurogenesis, the hippocampus is sensitive to experience and exposure and susceptible to disease and injury. In this review, we first summarize hippocampal deficits seen in several human neurodevelopmental disorders, and then discuss hippocampal impairment including hippocampus-dependent behavioral deficits found in animal models of these neurodevelopmental disorders.

Dynamic duo - FMRP and TDP-43: Regulating common targets, causing different diseases

RNA binding proteins play essential roles during development and aging, and are also involved in disease pathomechanisms. RNA sequencing and omics analyses have provided a window into systems level alterations in neurological disease, and have identified RNA processing defects among notable disease mechanisms. This review focuses on two seemingly distinct neurological disorders, the RNA binding proteins they are linked to, and their newly discovered functional relationship. When deficient, Fragile X Mental Retardation Protein (FMRP) causes developmental deficits and autistic behaviors while TAR-DNA Binding Protein (TDP-43) dysregulation causes age dependent neuronal degeneration. Recent findings that FMRP and TDP-43 associate in ribonuclear protein particles and share mRNA targets in neurons highlight the critical importance of translation regulation in synaptic plasticity and provide new perspectives on neuronal vulnerability during lifespan.

Modeling Fragile X Syndrome in Drosophila

Intellectual disability (ID) and autism are hallmarks of Fragile X Syndrome (FXS), a hereditary neurodevelopmental disorder. The gene responsible for FXS is Fragile X Mental Retardation gene 1 (FMR1) encoding the Fragile X Mental Retardation Protein (FMRP), an RNA-binding protein involved in RNA metabolism and modulating the expression level of many targets. Most cases of FXS are caused by silencing of FMR1 due to CGG expansions in the 5'-UTR of the gene. Humans also carry the FXR1 and FXR2 paralogs of FMR1 while flies have only one FMR1 gene, here called dFMR1, sharing the same level of sequence homology with all three human genes, but functionally most similar to FMR1. This enables a much easier approach for FMR1 genetic studies. Drosophila has been widely used to investigate FMR1 functions at genetic, cellular, and molecular levels since dFMR1 mutants have many phenotypes in common with the wide spectrum of FMR1 functions that underlay the disease. In this review, we present very recent Drosophila studies investigating FMRP functions at genetic, cellular, molecular, and electrophysiological levels in addition to research on pharmacological treatments in the fly model. These studies have the potential to aid the discovery of pharmacological therapies for FXS.