Mecp2 regulates tnfa during zebrafish embryonic development and acute inflammation

Zebrafish in understanding molecular pathophysiology, disease modeling, and developing effective treatments for Rett syndrome

Rett syndrome (RTT) is a rare but dreadful X-linked genetic disease that mainly affects young girls. It is a neurological disease that affects nerve cell development and function, resulting in severe motor and intellectual disabilities. To date, no cure is available for treating this disease. In 90% of the cases, RTT is caused by a mutation in methyl-CpG-binding protein 2 (MECP2), a transcription factor involved in the repression and activation of transcription. MECP2 is known to regulate several target genes and is involved in different physiological functions. Mouse models exhibit a broad range of phenotypes in recapitulating human RTT symptoms; however, understanding the disease mechanisms remains incomplete, and many potential RTT treatments developed in mouse models have not shown translational effectiveness in human trials. Recent data hint that the zebrafish model emulates similar disrupted neurological functions following mutation of the mecp2 gene. This suggests that zebrafish can be used to understand the onset and progression of RTT pathophysiology and develop a possible cure. In this review, we elaborate on the molecular basis of RTT pathophysiology in humans and model organisms, including rodents and zebrafish, focusing on the zebrafish model to understand the molecular pathophysiology and the development of therapeutic strategies for RTT. Finally, we propose a rational treatment strategy, including antisense oligonucleotides, small interfering RNA technology and induced pluripotent stem cell-derived cell therapy.

High species homology potentiates quantitative inflammation profiling in zebrafish using immunofluorescence

Due to substantial homology between the human and zebrafish genome and a high level of conservation of the innate immune system across species, zebrafish larvae have become an invaluable research tool for studying inflammation and modelling inflammatory disease. However, further microscopy techniques need to be developed for better profiling of inflammation and in particular, integrated cytokine responses to different stimuli - approaches are currently largely limited to assessment of changes in cytokine gene transcription and in vivo visualisation using transgenics, which is limited in terms of the number of cytokines that may be assessed at once. In this study, after confirming substantial homology of human vs zebrafish cytokine amino acid sequences, immunofluorescence staining using antibodies directed at human cytokines was performed. Inflammatory cytokine signalling responses to experimental tailfin transection was assessed over 24 h (1 hpi (hours post injury), 2 hpi, 4 hpi, 24 hpi) in zebrafish larvae, with experimental end point at 120 h post fertilization (hpf). When immunofluorescence results were compared to responses observed in rodent and human literature, it is clear that the cytokines follow a similar response, albeit with a condensed total time course. Notably, tumor necrosis factor-α and monocyte chemoattractant protein-1 increased and remained elevated over the 24-h period. In contrast, interleukin-1β and interleukin-6 peaked at 4 hpi and 2 hpi respectively but had both returned to baseline levels by 24 hpi. Macrophage migration inhibitory factor was lowest at 1 hpi, potentially encouraging macrophage movement into the site of injury, followed by a sharp increase. This protocol provides valuable insight into inflammation over a time course and more so, provides an affordable and accessible method to comprehensively assess inflammation in zebrafish disease models.

Fluoxetine and Curcumin Prevent the Alterations in Locomotor and Exploratory Activities and Social Interaction Elicited by Immunoinflammatory Activation in Zebrafish: Involvement of BDNF and Proinflammatory Cytokines

The increase in proinflammatory cytokine expression causes behavioral changes consistent with sickness behavior, and this led to the suggestion that depression might be a psychoneuroimmunological phenomenon. Here, we evaluated the effects of the pretreatment with fluoxetine (10 mg/kg, i.p.) and curcumin (0.5 mg/kg, i.p.) on the immune response elicited by the inoculation of an Aeromonas hydrophila bacterin in zebrafish. Non-pretreated but A. hydrophila-inoculated and sham-inoculated groups of fish served as controls. The social preference, locomotor, exploratory activities, and cerebral expression of il1b, il6, tnfa, and bdnf mRNA were compared among the groups. Behavioral changes characteristic of sickness behavior and a significant increase in the expression of il1b and il6 cytokines were found in fish from the immunostimulated group. The behavioral alterations caused by the inflammatory process were different between males and females, which was coincident with the increased expression of cerebral BDNF. Fluoxetine and curcumin prevented the sickness behavior induced by A. hydrophila and the increased expression of proinflammatory cytokines. Our results point to the potential of zebrafish as a translational model in studies related to neuroinflammation and demonstrate for the first time the effects of fluoxetine and curcumin on zebrafish sickness behavior.

Modelling Autism Spectrum Disorder (ASD) and Attention-Deficit/Hyperactivity Disorder (ADHD) Using Mice and Zebrafish

Autism spectrum disorders (ASD) and attention-deficit/hyperactivity disorder (ADHD) are two debilitating neurodevelopmental disorders. The former is associated with social impairments whereas the latter is associated with inattentiveness, hyperactivity, and impulsivity. There is recent evidence that both disorders are somehow related and that genes may play a large role in these disorders. Despite mounting human and animal research, the neurological pathways underlying ASD and ADHD are still not well understood. Scientists investigate neurodevelopmental disorders by using animal models that have high similarities in genetics and behaviours with humans. Mice have been utilized in neuroscience research as an excellent animal model for a long time; however, the zebrafish has attracted much attention recently, with an increasingly large number of studies using this model. In this review, we first discuss ASD and ADHD aetiology from a general point of view to their characteristics and treatments. We also compare mice and zebrafish for their similarities and discuss their advantages and limitations in neuroscience. Finally, we summarize the most recent and existing research on zebrafish and mouse models of ASD and ADHD. We believe that this review will serve as a unique document providing interesting information to date about these models, thus facilitating research on ASD and ADHD.

Evaluation of Resveratrol and Piceatannol Anticonvulsant Potential in Adult Zebrafish (Danio rerio)

Epilepsy is a common neurological disorder which affects 50 million people worldwide. Patients with epilepsy may present cognitive deficits and psychological impairment. Currently, 30% of patients fail to respond to any available antiseizure drug, and a significant number of patients do not well tolerate the offered treatments. Then, it is necessary to find out alternatives for controlling epileptic seizures. Studies have shown that despite its neuroprotective effects, resveratrol shows poor anticonvulsant properties. Resveratrol analog, piceatannol, possesses higher biological activity than resveratrol and could be an alternative to control seizure. Thus, the present study investigated the effects of resveratrol and piceatannol in pentylenetetrazole-induced seizures in adult zebrafish (Danio rerio). Only the experimental positive control (diazepam) showed anticonvulsant effect in this study. In addition, no behavioral changes were observed 24 h after seizure occurrence. Finally, the expression of genes related to neuronal activity (c-fos), neurogenesis (p70S6Ka and p70S6Kb), inflammatory response (interleukin 1β), and cell apoptosis (caspase-3) did not change by pentylenetetrazole-induced seizures. Therefore, we failed to observe any anticonvulsant and neuroprotective potential of resveratrol and piceatannol in adult zebrafish. However, resveratrol and piceatannol benefits in epilepsy are not discharged, and more studies are necessary.

Epigenetics and Neuroinflammation Associated With Neurodevelopmental Disorders: A Microglial Perspective

Neuroinflammation is a cause of neurodevelopmental disorders such as autism spectrum disorders, fetal alcohol syndrome, and cerebral palsy. Converging lines of evidence from basic and clinical sciences suggest that dysregulation of the epigenetic landscape, including DNA methylation and miRNA expression, is associated with neuroinflammation. Genetic and environmental factors can affect the interaction between epigenetics and neuroinflammation, which may cause neurodevelopmental disorders. In this minireview, we focus on neuroinflammation that might be mediated by epigenetic dysregulation in microglia, and compare studies using mammals and zebrafish.

Zebrafish larvae as experimental model to expedite the search for new biomarkers and treatments for neonatal sepsis

Neonatal sepsis is a major cause of death and disability in newborns. Commonly used biomarkers for diagnosis and evaluation of treatment response lack sufficient sensitivity or specificity. Additionally, new targets to treat the dysregulated immune response are needed, as are methods to effectively screen drugs for these targets. Available research methods have hitherto not yielded the breakthroughs required to significantly improve disease outcomes, we therefore describe the potential of zebrafish (Danio rerio) larvae as preclinical model for neonatal sepsis. In biomedical research, zebrafish larvae combine the complexity of a whole organism with the convenience and high-throughput potential of in vitro methods. This paper illustrates that zebrafish exhibit an immune system that is remarkably similar to humans, both in terms of types of immune cells and signaling pathways. Moreover, the developmental state of the larval immune system is highly similar to human neonates. We provide examples of zebrafish larvae being used to study infections with pathogens commonly causing neonatal sepsis and discuss known limitations. We believe this species could expedite research into immune regulation during neonatal sepsis and may hold keys for the discovery of new biomarkers and novel treatment targets as well as for screening of targeted drug therapies.

Investigation on the Anticonvulsant Potential of Luteolin and Micronized Luteolin in Adult Zebrafish (Danio rerio)

Epilepsy affects around 50 million people worldwide, and an important number of patients (30%) fail to respond to any available antiepileptic drug. Previous studies have shown that luteolin presents a promising potential as an anticonvulsant. On the other hand, different studies showed that luteolin does not promote anticonvulsant effects. Therefore, there is a lack of consensus about the use of luteolin for seizure control. Luteolin low bioavailability could be a limiting factor to obtain better results. Attractively, micronization technology has been applied to improve flavonoids bioavailability. Thus, the present study aimed to investigate the effects of luteolin on its raw form and micronized luteolin in a PTZ-induced seizure model in adult zebrafish (Danio rerio). Our results demonstrate that luteolin and micronized luteolin did not block PTZ-induced seizures in adult zebrafish. Also, luteolin and micronized luteolin did not provoke behavioral changes. Finally, our results show that 24 h after seizure occurrence, no changes were detected for p70S6Kb, interleukin 1β, and caspase-3 transcript levels. Altogether, we failed to observe an anticonvulsant potential of luteolin in adult zebrafish, even in its micronized form. However, we recommend new studies to investigate luteolin benefits in epilepsy.

The Gut-Brain-Microbiome Axis and Its Link to Autism: Emerging Insights and the Potential of Zebrafish Models

Research involving autism spectrum disorder (ASD) most frequently focuses on its key diagnostic criteria: restricted interests and repetitive behaviors, altered sensory perception, and communication impairments. These core criteria, however, are often accompanied by numerous comorbidities, many of which result in severe negative impacts on quality of life, including seizures, epilepsy, sleep disturbance, hypotonia, and GI distress. While ASD is a clinically heterogeneous disorder, gastrointestinal (GI) distress is among the most prevalent co-occurring symptom complex, manifesting in upward of 70% of all individuals with ASD. Consistent with this high prevalence, over a dozen family foundations that represent genetically distinct, molecularly defined forms of ASD have identified GI symptoms as an understudied area with significant negative impacts on quality of life for both individuals and their caregivers. Moreover, GI symptoms are also correlated with more pronounced irritability, social withdrawal, stereotypy, hyperactivity, and sleep disturbances, suggesting that they may exacerbate the defining behavioral symptoms of ASD. Despite these facts (and to the detriment of the community), GI distress remains largely unaddressed by ASD research and is frequently regarded as a symptomatic outcome rather than a potential contributory factor to the behavioral symptoms. Allowing for examination of both ASD's impact on the central nervous system (CNS) as well as its impact on the GI tract and the associated microbiome, the zebrafish has recently emerged as a powerful tool to study ASD. This is in no small part due to the advantages zebrafish present as a model system: their precocious development, their small transparent larval form, and their parallels with humans in genetics and physiology. While ASD research centered on the CNS has leveraged these advantages, there has been a critical lack of GI-centric ASD research in zebrafish models, making a holistic view of the gut-brain-microbiome axis incomplete. Similarly, high-throughput ASD drug screens have recently been developed but primarily focus on CNS and behavioral impacts while potential GI impacts have not been investigated. In this review, we aim to explore the great promise of the zebrafish model for elucidating the roles of the gut-brain-microbiome axis in ASD.

Developmental Accumulation of Gene Body and Transposon Non-CpG Methylation in the Zebrafish Brain

DNA methylation predominantly occurs at CG dinucleotides in vertebrate genomes; however, non-CG methylation (mCH) is also detectable in vertebrate tissues, most notably in the nervous system. In mammals it is well established that mCH is targeted to CAC trinucleotides by DNMT3A during nervous system development where it is enriched in gene bodies and associated with transcriptional repression. Nevertheless, the conservation of developmental mCH accumulation and its deposition by DNMT3A is largely unexplored and has yet to be functionally demonstrated in other vertebrates. In this study, by analyzing DNA methylomes and transcriptomes of zebrafish brains, we identified enrichment of mCH at CAC trinucleotides (mCAC) at defined transposon motifs as well as in developmentally downregulated genes associated with developmental and neural functions. We further generated and analyzed DNA methylomes and transcriptomes of developing zebrafish larvae and demonstrated that, like in mammals, mCH accumulates during post-embryonic brain development. Finally, by employing CRISPR/Cas9 technology, we unraveled a conserved role for Dnmt3a enzymes in developmental mCAC deposition. Overall, this work demonstrates the evolutionary conservation of developmental mCH dynamics and highlights the potential of zebrafish as a model to study mCH regulation and function during normal and perturbed development.

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.

Micronized Resveratrol Shows Anticonvulsant Properties in Pentylenetetrazole-Induced Seizure Model in Adult Zebrafish

Epilepsy affects 50 million people around the world, and the patients experience cognitive, psychological and social consequences. Despite the considerable quantity of antiepileptic drugs available, 30% of patients still suffer in seizure. Therefore, the advance in therapeutic alternatives is mandatory. Resveratrol has been attracting the attention of many researchers because of its pharmacological potential. However, despite its neuroprotective and anti-epileptic effects, clinical resveratrol use is impaired by its low bioavailability. Here, we applied the supercritical fluid micronization technology (SEDS) to overcome this deficit, and investigated the anticonvulsant potential of micronized resveratrol in a PTZ-induced seizure model in adult zebrafish (Danio rerio). SEDS permits obtaining significantly reduced particle size with a fine size distribution in comparison with the starting material. It can improve the pharmacotherapeutic efficacy. Our data showed that micronized resveratrol decreased the occurrence of the tonic-clonic seizure stage and slowed the development of the seizures in a similar manner of diazepam. Non-processed resveratrol was not able to protect the animals. Furthermore, diazepam decreased the locomotion and exploratory behavior. Differently from diazepam, the micronized resveratrol did not induce behavioral adverse events. In addition, our data showed that the PTZ-induced seizures increased the c-fos transcript levels following the neural excitability. However, the increase in c-fos levels was prevented by micronized resveratrol. In conclusion, our results demonstrate that the micronized resveratrol shows anticonvulsant effect, like the classical antiepileptic drug diazepam in a PTZ-induced seizure model. Excitingly, different from diazepam, micronized resveratrol did not provoke behavioral adverse events.

Compromised immune/inflammatory responses in Rett syndrome

Mutations in X-linked gene methyl-CpG-binding protein 2 (MECP2), a key transcriptional regulator, account for most cases of Rett syndrome (RTT), a devastating neurodevelopmental disorder with no known cure. Despite extensive research to elucidate MeCP2 functions, the mechanisms underlying RTT pathophysiology are still unclear. In addition to a variety of neurological symptoms, RTT also includes a plethora of additional phenotypical features including altered lipid metabolism, redox imbalance, immune dysfunction and mitochondrial abnormalities that explain its multisystemic nature. Here, we provide an overview of the current knowledge on the potential role of dysregulated inflammatory and immune responses in RTT. The findings show that abnormalities of humoral and cell-mediated immunity together with chronic low-grade inflammation in multiple organs represent not only clinical manifestations of RTT but rather can contribute to its development and deteriorating course. A future research challenge could be to target therapeutically immune dysfunction as a novel means for RTT management.

SCO-Spondin Defects and Neuroinflammation Are Conserved Mechanisms Driving Spinal Deformity across Genetic Models of Idiopathic Scoliosis

Adolescent idiopathic scoliosis (AIS) affects 3% to 4% of children between the ages of 11 and 18 [1, 2]. This disorder, characterized by abnormal three-dimensional spinal curvatures that typically develop during periods of rapid growth, occurs in the absence of congenital vertebral malformations or neuromuscular defects [1]. Genetic heterogeneity [3] and a historical lack of appropriate animal models [4] have confounded basic understanding of AIS biology; thus, treatment options remain limited [5, 6]. Recently, genetic studies using zebrafish have linked idiopathic-like scoliosis to irregularities in motile cilia-mediated cerebrospinal fluid flow [7-9]. However, because loss of cilia motility in human primary ciliary dyskinesia patients is not fully associated with scoliosis [10, 11], other pathogenic mechanisms remain to be determined. Here, we demonstrate that zebrafish scospondin (sspo) mutants develop late-onset idiopathic-like spinal curvatures in the absence of obvious cilia motility defects. Sspo is a large secreted glycoprotein functionally associated with the subcommissural organ and Reissner's fiber [12]-ancient and enigmatic organs of the brain ventricular system reported to govern cerebrospinal fluid homeostasis [13, 14], neurogenesis [12, 15-18], and embryonic morphogenesis [19]. We demonstrate that irregular deposition of Sspo within brain ventricles is associated with idiopathic-like scoliosis across diverse genetic models. Furthermore, Sspo defects are sufficient to induce oxidative stress and neuroinflammatory responses implicated in AIS pathogenesis [9]. Through screening for chemical suppressors of sspo mutant phenotypes, we also identify potent agents capable of blocking severe juvenile spine deformity. Our work thus defines a new preclinical model of AIS and provides tools to realize novel therapeutic strategies.

Repetitive Transcranial Magnetic Stimulation Alleviates Neurological Deficits After Cerebral Ischemia Through Interaction Between RACK1 and BDNF exon IV by the Phosphorylation-Dependent Factor MeCP2

Repetitive transcranial magnetic stimulation (rTMS) is acknowledged as a form of neurostimulation, especially for functional recovery. The foundational knowledge of molecular mechanism is limited regarding its role in cerebral ischemia, for which the present study was designed. Primary neurons were treated with oxygen-glucose deprivation (OGD) and repetitive magnetic stimulation (rMS), in which brain-derived neurotrophic factor (BDNF) and transcription of BDNF exons were examined. Then, adenovirus vectors carrying siRACK1 sequence were delivered to primary neurons, followed by detection of the transcription of BDNF exons and the extent of methyl CpG binding protein 2 (MeCP2) phosphorylation. Results showed that BDNF and the transcription of BDNF exons were upregulated by rMS and OGD treatment, but decreased by extra treatment of RACK1 siRNA. Then, the mechanism investigations demonstrated that rMS increased the extent of MeCP2 phosphorylation to promote the interaction between RACK1 and BDNF exon IV. The aforementioned findings were further confirmed in vivo in middle cerebral artery occlusion (MCAO)-induced rat models, as indicated by improved neurological functions and reduced area of cerebral infarction. The study offers potential evidence for improvement of neurological deficits, highlighting the important role of rTMS for treatment of cerebral ischemia.

Spotlight on zebrafish: the next wave of translational research

Five years after the launch of the Disease Models & Mechanisms (DMM) Special Issue on zebrafish as a disease model, the field has progressed significantly. Zebrafish have been used to precisely model human genetic variants, to unpick the mechanisms of metabolic and other diseases, to study infection, inflammation and cancer, and to develop and test new therapeutic approaches. In this Editorial, we highlight recent research published in DMM that uses zebrafish to develop new experimental tools and to provide new insight into disease mechanism and therapy. The broad spectrum of subjects and approaches covered in these articles underscores the versatility of zebrafish in translational research. Further, it highlights the zebrafish community's ethos of creativity and collaboration in translating basic biological research into clinically relevant advances affecting how we understand and treat human disease.

Summary: Zebrafish are a highly versatile and relevant organism for human disease modelling. This Editorial highlights the recent zebrafish research published in DMM.

Antiglycative Activity and RAGE Expression in Rett Syndrome

Rett syndrome (RTT) is a human neurodevelopmental disorder, whose pathogenesis has been linked to both oxidative stress and subclinical inflammatory status (OxInflammation). Methylglyoxal (MG), a glycolytic by-product with cytotoxic and pro-oxidant power, is the major precursor in vivo of advanced glycation end products (AGEs), which are known to exert their detrimental effect via receptor- (e.g., RAGE) or non-receptor-mediated mechanisms in several neurological diseases. On this basis, we aimed to compare fibroblasts from healthy subjects (CTR) with fibroblasts from RTT patients (N = 6 per group), by evaluating gene/protein expression patterns, and enzymatic activities of glyoxalases (GLOs), along with the levels of MG-dependent damage in both basal and MG-challenged conditions. Our results revealed that RTT is linked to an alteration of the GLOs system (specifically, increased GLO2 activity), that ensures unchanged MG-dependent damage levels. However, RTT cells underwent more pronounced cell death upon exogenous MG-treatment, as compared to CTR, and displayed lower RAGE levels than CTR, with no alterations following MG-treatment, thus suggesting that an adaptive response to dicarbonyl stress may occur. In conclusion, besides OxInflammation, RTT is associated with reshaping of the major defense systems against dicarbonyl stress, along with an altered cellular stress response towards pro-glycating insults.

Hematopoietic growth factors: the scenario in zebrafish

Humoral regulation by ligand/receptor interactions is a fundamental feature of vertebrate hematopoiesis. Zebrafish are an established vertebrate animal model of hematopoiesis, sharing with mammals conserved genetic, molecular and cell biological regulatory mechanisms. This comprehensive review considers zebrafish hematopoiesis from the perspective of the hematopoietic growth factors (HGFs), their receptors and their actions. Zebrafish possess multiple HGFs: CSF1 (M-CSF) and CSF3 (G-CSF), kit ligand (KL, SCF), erythropoietin (EPO), thrombopoietin (THPO/TPO), and the interleukins IL6, IL11, and IL34. Some ligands and/or receptor components have been duplicated by various mechanisms including the teleost whole genome duplication, adding complexity to the ligand/receptor interactions possible, but also providing examples of several different outcomes of ligand and receptor subfunctionalization or neofunctionalization. CSF2 (GM-CSF), IL3 and IL5 and their receptors are absent from zebrafish. Overall the humoral regulation of hematopoiesis in zebrafish displays considerable similarity with mammals, which can be applied in biological and disease modelling research.

Prenatal Neuropathologies in Autism Spectrum Disorder and Intellectual Disability: The Gestation of a Comprehensive Zebrafish Model

Autism spectrum disorder (ASD) and intellectual disability (ID) are neurodevelopmental disorders with overlapping diagnostic behaviors and risk factors. These include embryonic exposure to teratogens and mutations in genes that have important functions prenatally. Animal models, including rodents and zebrafish, have been essential in delineating mechanisms of neuropathology and identifying developmental critical periods, when those mechanisms are most sensitive to disruption. This review focuses on how the developmentally accessible zebrafish is contributing to our understanding of prenatal pathologies that set the stage for later ASD-ID behavioral deficits. We discuss the known factors that contribute prenatally to ASD-ID and the recent use of zebrafish to model deficits in brain morphogenesis and circuit development. We conclude by suggesting that a future challenge in zebrafish ASD-ID modeling will be to bridge prenatal anatomical and physiological pathologies to behavioral deficits later in life.

Zebrafish Models of Neurodevelopmental Disorders: Past, Present, and Future

Zebrafish are increasingly being utilized as a model system to investigate the function of the growing list of risk genes associated with neurodevelopmental disorders. This is due in large part to the unique features of zebrafish that make them an optimal system for this purpose, including rapid, external development of transparent embryos, which enable the direct visualization of the developing nervous system during early stages, large progenies, which provide considerable tractability for performing high-throughput pharmacological screens to identify small molecule suppressors of simple behavioral phenotypes, and ease of genetic manipulation, which has been greatly facilitated by the advent of CRISPR/Cas9 gene editing technologies. This review article focuses on studies that have harnessed these advantages of the zebrafish system for the functional analysis of genes that are strongly associated with the following neurodevelopmental disorders: autism spectrum disorders (ASD), epilepsy, intellectual disability (ID) and schizophrenia. We focus primarily on studies describing early morphological and behavioral phenotypes during embryonic and larval stages resulting from loss of risk gene function. We highlight insights into basic mechanisms of risk gene function gained from these studies as well as limitations of studies to date. Finally, we discuss advances in in vivo neural circuit imaging in zebrafish, which promise to transform research using the zebrafish model by illuminating novel circuit-level mechanisms with relevance to neurodevelopmental disorders.

Psychoneuroimmunology and immunopsychiatry of zebrafish

Despite the high prevalence of neural and immune disorders, their etiology and molecular mechanisms remain poorly understood. As the zebrafish (Danio rerio) is increasingly utilized as a powerful model organism in biomedical research, mounting evidence suggests these fish as a useful tool to study neural and immune mechanisms and their interplay. Here, we discuss zebrafish neuro-immune mechanisms and their pharmacological and genetic modulation, the effect of stress on cytokines, as well as relevant models of microbiota-brain interplay. As many human brain diseases are based on complex interplay between the neural and the immune system, here we discuss zebrafish models, as well as recent successes and challenges, in this rapidly expanding field. We particularly emphasize the growing utility of zebrafish models in translational immunopsychiatry research, as they improve our understanding of pathogenetic neuro-immune interactions, thereby fostering future discovery of potential therapeutic agents.

Intestinal Candida parapsilosis isolates from Rett syndrome subjects bear potential virulent traits and capacity to persist within the host

BACKGROUND

Rett syndrome (RTT) is a neurological disorder mainly caused by mutations in MeCP2 gene. It has been shown that MeCP2 impairments can lead to cytokine dysregulation due to MeCP2 regulatory role in T-helper and T-reg mediated responses, thus contributing to the pro-inflammatory status associated with RTT. Furthermore, RTT subjects suffer from an intestinal dysbiosis characterized by an abnormal expansion of the Candida population, a known factor responsible for the hyper-activation of pro-inflammatory immune responses. Therefore, we asked whether the intestinal fungal population of RTT subjects might contribute the sub-inflammatory status triggered by MeCP2 deficiency.

METHODS

We evaluated the cultivable gut mycobiota from a cohort of 50 RTT patients and 29 healthy controls characterizing the faecal fungal isolates for their virulence-related traits, antifungal resistance and immune reactivity in order to elucidate the role of fungi in RTT's intestinal dysbiosis and gastrointestinal physiology.

RESULTS

Candida parapsilosis, the most abundant yeast species in RTT subjects, showed distinct genotypic profiles if compared to healthy controls' isolates as measured by hierarchical clustering analysis from RAPD genotyping. Their phenotypical analysis revealed that RTT's isolates produced more biofilm and were significantly more resistant to azole antifungals compared to the isolates from the healthy controls. In addition, the high levels of IL-1β and IL-10 produced by peripheral blood mononuclear cells and the mixed Th1/Th17 cells population induced by RTT C. parapsilosis isolates suggest the capacity of these intestinal fungi to persist within the host, being potentially involved in chronic, pro-inflammatory responses.

CONCLUSIONS

Here we demonstrated that intestinal C. parapsilosis isolates from RTT subjects hold phenotypic traits that might favour the previously observed low-grade intestinal inflammatory status associated with RTT. Therefore, the presence of putative virulent, pro-inflammatory C. parapsilosis strains in RTT could represent an additional factor in RTT's gastrointestinal pathophysiology, whose mechanisms are not yet clearly understood.

Rett syndrome: a neurological disorder with metabolic components

Rett syndrome (RTT) is a neurological disorder caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2), a ubiquitously expressed transcriptional regulator. Despite remarkable scientific progress since its discovery, the mechanism by which MECP2 mutations cause RTT symptoms is largely unknown. Consequently, treatment options for patients are currently limited and centred on symptom relief. Thought to be an entirely neurological disorder, RTT research has focused on the role of MECP2 in the central nervous system. However, the variety of phenotypes identified in Mecp2 mutant mouse models and RTT patients implicate important roles for MeCP2 in peripheral systems. Here, we review the history of RTT, highlighting breakthroughs in the field that have led us to present day. We explore the current evidence supporting metabolic dysfunction as a component of RTT, presenting recent studies that have revealed perturbed lipid metabolism in the brain and peripheral tissues of mouse models and patients. Such findings may have an impact on the quality of life of RTT patients as both dietary and drug intervention can alter lipid metabolism. Ultimately, we conclude that a thorough knowledge of MeCP2's varied functional targets in the brain and body will be required to treat this complex syndrome.