Reduced Lateral Inhibition Impairs Olfactory Computations and Behaviors in a Drosophila Model of Fragile X Syndrome

Neurophysiological effects of a combined treatment of lovastatin and minocycline in patients with fragile X syndrome: Ancillary results of the LOVAMIX randomized clinical trial

Fragile X syndrome (FXS) is the primary hereditary cause of intellectual disability and autism spectrum disorder. It is characterized by exacerbated neuronal excitability, and its correction is considered an objective measure of treatment response in animal models, a marker albeit rarely used in clinical trials. Here, we used an extensive transcranial magnetic stimulation (TMS) battery to assess the neurophysiological effects of a therapy combining two disease-modifying drugs, lovastatin (40 mg) and minocycline (100 mg), administered alone for 8 weeks and in combination for 12 weeks, in 19 patients (mean age of 23.58 ± 1.51) with FXS taking part in the LOVAmix trial. The TMS battery, which included the resting motor threshold, short-interval intracortical inhibition, long-interval intracortical inhibition, corticospinal silent period, and intracortical facilitation, was completed at baseline after 8 weeks of monotherapy (visit 2 of the clinical trial) and after 12 weeks of dual therapy (visit 4 of the clinical trial). Repeated measure ANOVAs were performed between baseline and visit 2 (monotherapy) and visit 3 (dual therapy) with interactions for which monotherapy the participants received when they began the clinical trial. Results showed that dual therapy was associated with reduced cortical excitability after 20 weeks. This was reflected by a significant increase in the resting-motor threshold after dual therapy compared to baseline. There was a tendency for enhanced short-intracortical inhibition, a marker of GABAa-mediated inhibition after 8 weeks of monotherapy compared to baseline. Together, these results suggest that a combined therapy of minocycline and lovastatin might act on the core neurophysiopathology of FXS. This trial was registered at clinicaltrials.gov (NCT02680379).

Deficits in olfactory system neurogenesis in neurodevelopmental disorders

The role of neurogenesis in neurodevelopmental disorders (NDDs) merits much attention. The complex process by which stem cells produce daughter cells that in turn differentiate into neurons, migrate various distances, and form synaptic connections that are then refined by neuronal activity or experience is integral to the development of the nervous system. Given the continued postnatal neurogenesis that occurs in the mammalian olfactory system, it provides an ideal model for understanding how disruptions in distinct stages of neurogenesis contribute to the pathophysiology of various NDDs. This review summarizes and discusses what is currently known about the disruption of neurogenesis within the olfactory system as it pertains to attention-deficit/hyperactivity disorder, autism spectrum disorder, Down syndrome, Fragile X syndrome, and Rett syndrome. Studies included in this review used either human subjects, mouse models, or Drosophila models, and lay a compelling foundation for continued investigation of NDDs by utilizing the olfactory system.

Hyperexcitability in the Olfactory Bulb and Impaired Fine Odor Discrimination in the Fmr1 KO Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is the single most common monogenetic cause of autism spectrum disorders (ASDs) in humans. FXS is caused by loss of expression of the fragile X mental retardation protein (FMRP), an mRNA-binding protein encoded on the X chromosome involved in suppressing protein translation. Sensory processing deficits have been a major focus of studies of FXS in both humans and rodent models of FXS, but olfactory deficits remain poorly understood. Here, we conducted experiments in wild-type (WT) and Fmr1 knock-out (KO; Fmr1-/y ) mice (males) that lack expression of the gene encoding FMRP to assess olfactory circuit and behavioral abnormalities. In patch-clamp recordings conducted in slices of the olfactory bulb, output mitral cells (MCs) in Fmr1 KO mice displayed greatly enhanced excitation under baseline conditions, as evidenced by a much higher rate of occurrence of spontaneous network-level events known as long-lasting depolarizations (LLDs). The higher probability of spontaneous LLDs (sLLDs), which appeared to be because of a decrease in GABAergic synaptic inhibition in glomeruli leading to more feedforward excitation, caused a reduction in the reliability of stimulation-evoked responses in MCs. In addition, in a go/no-go operant discrimination paradigm, we found that Fmr1 KO mice displayed impaired discrimination of odors in difficult tasks that involved odor mixtures but not altered discrimination of monomolecular odors. We suggest that the Fmr1 KO-induced reduction in MC response reliability is one plausible mechanism for the impaired fine odor discrimination.SIGNIFICANCE STATEMENT Fragile X syndrome (FXS) in humans is associated with a range of debilitating deficits including aberrant sensory processing. One sensory system that has received comparatively little attention in studies in animal models of FXS is olfaction. Here, we report the first comprehensive physiological analysis of circuit defects in the olfactory bulb in the commonly-used Fmr1 knock-out (KO) mouse model of FXS. Our studies indicate that Fmr1 KO alters the local excitation/inhibition balance in the bulb, similar to what Fmr1 KO does in other brain circuits, but through a novel mechanism that involves enhanced feedforward excitation. Furthermore, Fmr1 KO mice display behavioral impairments in fine odor discrimination, an effect that may be explained by changes in neural response reliability.

Evidence for Prepulse Inhibition of Visually Evoked Motor Response in Drosophila melanogaster

Prepulse inhibition (PPI) is a widely investigated behavior to study the mechanisms of disorders such as anxiety, schizophrenia, and bipolar mania. PPI has been observed across various vertebrate and invertebrate species; however, it has not yet been reported in adult Drosophila melanogaster. In this study, we describe the first detection of PPI of visually evoked locomotor arousal in flies. To validate our findings, we demonstrate that PPI in Drosophila can be partially reverted by the N-methyl D-aspartate (NMDA) receptor antagonist MK-801, known for inducing sensorimotor gating deficits in rodent models. Additionally, we show that the visually evoked response can be inhibited by multiple stimuli presentation, which can also be affected by MK-801. Given the versatility of Drosophila as a model organism for genetic screening and analysis, our results suggest that high-throughput behavioral screenings of adult flies can become a valuable tool for investigating the mechanisms behind PPI.

Hyperexcitability in the olfactory bulb and impaired fine odor discrimination in the Fmr1 KO mouse model of fragile X syndrome

Fragile X syndrome (FXS) is the single most common monogenetic cause of autism spectrum disorders in humans. FXS is caused by loss of expression of the Fragile X mental retardation protein (FMRP), an mRNA-binding protein encoded on the X chromosome involved in suppressing protein translation. Sensory processing deficits have been a major focus of studies of FXS in both humans and rodent models of FXS, but olfactory deficits remain poorly understood. Here we conducted experiments in wild-type and Fmr1 KO ( Fmr1 -/y ) mice (males) that lack expression of the gene encoding FMRP to assess olfactory circuit and behavioral abnormalities. In patch-clamp recordings conducted in slices of the olfactory bulb, output mitral cells (MCs) in Fmr1 KO mice displayed greatly enhanced excitation, as evidenced by a much higher rate of occurrence of spontaneous network-level events known as long-lasting depolarizations (LLDs). The higher probability of LLDs did not appear to reflect changes in inhibitory connections onto MCs but rather enhanced spontaneous excitation of external tufted cells (eTCs) that provide feedforward excitation onto MCs within glomeruli. In addition, in a go/no-go operant discrimination paradigm, we found that Fmr1 KO mice displayed impaired discrimination of odors in difficult tasks that involved odor mixtures but not altered discrimination of monomolecular odors. We suggest that the higher excitability of MCs in Fmr1 KO mice may impair fine odor discrimination by broadening odor tuning curves of MCs and/or altering synchronized oscillations through changes in transient inhibition.

Significance Statement

Fragile X syndrome (FXS) in humans is associated with a range of debilitating deficits including aberrant sensory processing. One sensory system that has received comparatively little attention in studies in animal models of FXS is olfaction. Here, we report the first comprehensive physiological analysis of circuit defects in the olfactory bulb in the commonly-used Fmr1 knockout (KO) mouse model of FXS. Our studies indicate that Fmr1 KO alters the local excitation/inhibition balance in the bulb - similar to what Fmr1 KO does in other brain circuits - but through a novel mechanism that involves enhanced feedforward excitatory drive. Furthermore, Fmr1 KO mice display behavioral impairments in fine odor discrimination, an effect that may be explained by enhanced neural excitability.

A simple high-throughput method for automated detection of Drosophila melanogaster light-dependent behaviours

BACKGROUND

Like most living organisms, the fruit fly Drosophila melanogaster exhibits strong and diverse behavioural reactions to light. Drosophila is a diurnal animal that displays both short- and long-term responses to light, important for, instance, in avoidance and light wavelength preference, regulation of eclosion, courtship, and activity, and provides an important model organism for understanding the regulation of circadian rhythms both at molecular and circuit levels. However, the assessment and comparison of light-based behaviours is still a challenge, mainly due to the lack of a standardised platform to measure behaviour and different protocols created across studies. Here, we describe the Drosophila Interactive System for Controlled Optical manipulations (DISCO), a low-cost, automated, high-throughput device that records the flies' activity using infrared beams while performing LED light manipulations.

RESULTS

To demonstrate the effectiveness of this tool and validate its potential as a standard platform, we developed a number of distinct assays, including measuring the locomotor response of flies exposed to sudden darkness (lights-off) stimuli. Both white-eyed and red-eyed wild-type flies exhibit increased activity after the application of stimuli, while no changes can be observed in Fmr1 null allele flies, a model of fragile X syndrome. Next, to demonstrate the use of DISCO in long-term protocols, we monitored the circadian rhythm of the flies for 48 h while performing an alcohol preference test. We show that increased alcohol consumption happens intermittently throughout the day, especially in the dark phases. Finally, we developed a feedback-loop algorithm to implement a place preference test based on the flies' innate aversion to blue light and preference for green light. We show that both white-eyed and red-eyed wild-type flies were able to learn to avoid the blue-illuminated zones.

CONCLUSIONS

Our results demonstrate the versatility of DISCO for a range of protocols, indicating that this platform can be used in a variety of ways to study light-dependent behaviours in flies.

Neocortical localization and thalamocortical modulation of neuronal hyperexcitability contribute to Fragile X Syndrome

Fragile X Syndrome (FXS) is a monogenetic form of intellectual disability and autism in which well-established knockout (KO) animal models point to neuronal hyperexcitability and abnormal gamma-frequency physiology as a basis for key disorder features. Translating these findings into patients may identify tractable treatment targets. Using source modeling of resting-state electroencephalography data, we report findings in FXS, including 1) increases in localized gamma activity, 2) pervasive changes of theta/alpha activity, indicative of disrupted thalamocortical modulation coupled with elevated gamma power, 3) stepwise moderation of low and high-frequency abnormalities based on female sex, and 4) relationship of this physiology to intellectual disability and neuropsychiatric symptoms. Our observations extend findings in Fmr1-/- KO mice to patients with FXS and raise a key role for disrupted thalamocortical modulation in local hyperexcitability. This systems-level mechanism has received limited preclinical attention but has implications for understanding fundamental disease mechanisms.

Exposure to GABAA Receptor Antagonist Picrotoxin in Pregnant Mice Causes Autism-Like Behaviors and Aberrant Gene Expression in Offspring

Autism spectrum disorder (ASD) is a neurodevelopmental disorder that is characterized by impairments in social interaction and restricted/repetitive behaviors. The neurotransmitter γ-aminobutyric acid (GABA) through GABAA receptor signaling in the immature brain plays a key role in the development of neuronal circuits. Excitatory/inhibitory imbalance in the mature brain has been investigated as a pathophysiological mechanism of ASD. However, whether and how disturbances of GABA signaling in embryos that are caused by GABAA receptor inhibitors cause ASD-like pathophysiology are poorly understood. The present study examined whether exposure to the GABAA receptor antagonist picrotoxin causes ASD-like pathophysiology in offspring by conducting behavioral tests from the juvenile period to adulthood and performing gene expression analyses in mature mouse brains. Here, we found that male mice that were prenatally exposed to picrotoxin exhibited a reduction of active interaction time in the social interaction test in both adolescence and adulthood. The gene expression analyses showed that picrotoxin-exposed male mice exhibited a significant increase in the gene expression of odorant receptors. Weighted gene co-expression network analysis showed a strong correlation between social interaction and enrichment of the "odorant binding" pathway gene module. Our findings suggest that exposure to a GABAA receptor inhibitor during the embryonic period induces ASD-like behavior, and impairments in odorant function may contribute to social deficits in offspring.

Hyperexcitability and Homeostasis in Fragile X Syndrome

Fragile X Syndrome (FXS) is a leading inherited cause of autism and intellectual disability, resulting from a mutation in the FMR1 gene and subsequent loss of its protein product FMRP. Despite this simple genetic origin, FXS is a phenotypically complex disorder with a range of physical and neurocognitive disruptions. While numerous molecular and cellular pathways are affected by FMRP loss, there is growing evidence that circuit hyperexcitability may be a common convergence point that can account for many of the wide-ranging phenotypes seen in FXS. The mechanisms for hyperexcitability in FXS include alterations to excitatory synaptic function and connectivity, reduced inhibitory neuron activity, as well as changes to ion channel expression and conductance. However, understanding the impact of FMR1 mutation on circuit function is complicated by the inherent plasticity in neural circuits, which display an array of homeostatic mechanisms to maintain activity near set levels. FMRP is also an important regulator of activity-dependent plasticity in the brain, meaning that dysregulated plasticity can be both a cause and consequence of hyperexcitable networks in FXS. This makes it difficult to separate the direct effects of FMR1 mutation from the myriad and pleiotropic compensatory changes associated with it, both of which are likely to contribute to FXS pathophysiology. Here we will: (1) review evidence for hyperexcitability and homeostatic plasticity phenotypes in FXS models, focusing on similarities/differences across brain regions, cell-types, and developmental time points; (2) examine how excitability and plasticity disruptions interact with each other to ultimately contribute to circuit dysfunction in FXS; and (3) discuss how these synaptic and circuit deficits contribute to disease-relevant behavioral phenotypes like epilepsy and sensory hypersensitivity. Through this discussion of where the current field stands, we aim to introduce perspectives moving forward in FXS research.

Experience-dependent plasticity modulates ongoing activity in the antennal lobe and enhances odor representations

Ongoing neural activity has been observed across several brain regions and is thought to reflect the internal state of the brain. Yet, it is important to understand how ongoing neural activity interacts with sensory experience and shapes sensory representations. Here, we show that the projection neurons of the fruit fly antennal lobe exhibit spatiotemporally organized ongoing activity. After repeated exposure to odors, we observe a gradual and cumulative decrease in the amplitude and number of calcium events occurring in the absence of odor stimulation, as well as a reorganization of correlations between olfactory glomeruli. Accompanying these plastic changes, we find that repeated odor experience decreases trial-to-trial variability and enhances the specificity of odor representations. Our results reveal an odor-experience-dependent modulation of ongoing and sensory-evoked activity at peripheral levels of the fruit fly olfactory system.

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The fruit fly antennal lobe exhibits spatiotemporally organized ongoing activity

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Repeated odor experience decreases the amplitude and number of ongoing calcium events

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Odor experience enhances the robustness and the specificity of odor representations

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Representations of different odors become more dissimilar upon repeated exposure

Interneuron Dysfunction and Inhibitory Deficits in Autism and Fragile X Syndrome

The alteration of excitatory–inhibitory (E–I) balance has been implicated in various neurological and psychiatric diseases, including autism spectrum disorder (ASD). Fragile X syndrome (FXS) is a single-gene disorder that is the most common known cause of ASD. Understanding the molecular and physiological features of FXS is thought to enhance our knowledge of the pathophysiology of ASD. Accumulated evidence implicates deficits in the inhibitory circuits in FXS that tips E–I balance toward excitation. Deficits in interneurons, the main source of an inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), have been reported in FXS, including a reduced number of cells, reduction in intrinsic cellular excitability, or weaker synaptic connectivity. Manipulating the interneuron activity ameliorated the symptoms in the FXS mouse model, which makes it reasonable to conceptualize FXS as an interneuronopathy. While it is still poorly understood how the developmental profiles of the inhibitory circuit go awry in FXS, recent works have uncovered several developmental alterations in the functional properties of interneurons. Correcting disrupted E–I balance by potentiating the inhibitory circuit by targeting interneurons may have a therapeutic potential in FXS. I will review the recent evidence about the inhibitory alterations and interneuron dysfunction in ASD and FXS and will discuss the future directions of this field.

Neuron-Specific FMRP Roles in Experience-Dependent Remodeling of Olfactory Brain Innervation during an Early-Life Critical Period

Critical periods are developmental windows during which neural circuits effectively adapt to the new sensory environment. Animal models of fragile X syndrome (FXS), a common monogenic autism spectrum disorder (ASD), exhibit profound impairments of sensory experience-driven critical periods. However, it is not known whether the causative fragile X mental retardation protein (FMRP) acts uniformly across neurons, or instead manifests neuron-specific functions. Here, we use the genetically-tractable Drosophila brain antennal lobe (AL) olfactory circuit of both sexes to investigate neuron-specific FMRP roles in the odorant experience-dependent remodeling of the olfactory sensory neuron (OSN) innervation during an early-life critical period. We find targeted OSN class-specific FMRP RNAi impairs innervation remodeling within AL synaptic glomeruli, whereas global dfmr1 null mutants display relatively normal odorant-driven refinement. We find both OSN cell autonomous and cell non-autonomous FMRP functions mediate odorant experience-dependent remodeling, with AL circuit FMRP imbalance causing defects in overall glomerulus innervation refinement. We find OSN class-specific FMRP levels bidirectionally regulate critical period remodeling, with odorant experience selectively controlling OSN synaptic terminals in AL glomeruli. We find OSN class-specific FMRP loss impairs critical period remodeling by disrupting responses to lateral modulation from other odorant-responsive OSNs mediating overall AL gain control. We find that silencing glutamatergic AL interneurons reduces OSN remodeling, while conversely, interfering with the OSN class-specific GABA_A signaling enhances remodeling. These findings reveal control of OSN synaptic remodeling by FMRP with neuron-specific circuit functions, and indicate how neural circuitry can compensate for global FMRP loss to reinstate normal critical period brain circuit remodeling.SIGNIFICANCE STATEMENT Fragile X syndrome (FXS), the leading monogenic cause of intellectual disability and autism spectrum disorder (ASD), manifests severe neurodevelopmental delays. Likewise, FXS disease models display disrupted neurodevelopmental critical periods. In the well-mapped Drosophila olfactory circuit model, perturbing the causative fragile X mental retardation protein (FMRP) within a single olfactory sensory neuron (OSN) class impairs odorant-dependent remodeling during an early-life critical period. Importantly, this impairment requires activation of other OSNs, and the olfactory circuit can compensate when FMRP is removed from all OSNs. Understanding the neuron-specific FMRP requirements within a developing neural circuit, as well as the FMRP loss compensation mechanisms, should help us engineer FXS treatments. This work suggests FXS treatments could use homeostatic mechanisms to alleviate circuit-level deficits.

Critical periods shaping the social brain: A perspective from Drosophila

Many sensory processing regions of the central brain undergo critical periods of experience-dependent plasticity. During this time ethologically relevant information shapes circuit structure and function. The mechanisms that control critical period timing and duration are poorly understood, and this is of special importance for those later periods of development, which often give rise to complex cognitive functions such as social behavior. Here, we review recent findings in Drosophila, an organism that has some unique experimental advantages, and introduce novel views for manipulating plasticity in the post-embryonic brain. Critical periods in larval and young adult flies resemble classic vertebrate models with distinct onset and termination, display clear connections with complex behaviors, and provide opportunities to control the time course of plasticity. These findings may extend our knowledge about mechanisms underlying extension and reopening of critical periods, a concept that has great relevance to many human neurodevelopmental disorders.

Genetic background mutations drive neural circuit hyperconnectivity in a fragile X syndrome model

BACKGROUND

Neural circuits are initially assembled during development when neurons synapse with potential partners and later refined as appropriate connections stabilize into mature synapses while inappropriate contacts are eliminated. Disruptions to this synaptogenic process impair connectivity optimization and can cause neurodevelopmental disorders. Intellectual disability (ID) and autism spectrum disorder (ASD) are often characterized by synaptic overgrowth, with the maintenance of immature or inappropriate synapses. Such synaptogenic defects can occur through mutation of a single gene, such as fragile X mental retardation protein (FMRP) loss causing the neurodevelopmental disorder fragile X syndrome (FXS). FXS represents the leading heritable cause of ID and ASD, but many other genes that play roles in ID and ASD have yet to be identified.

RESULTS

In a Drosophila FXS disease model, one dfmr150M null mutant stock exhibits previously unreported axonal overgrowths at developmental and mature stages in the giant fiber (GF) escape circuit. These excess axon projections contain both chemical and electrical synapse markers, indicating mixed synaptic connections. Extensive analyses show these supernumerary synapses connect known GF circuit neurons, rather than new, inappropriate partners, indicating hyperconnectivity within the circuit. Despite the striking similarities to well-characterized FXS synaptic defects, this new GF circuit hyperconnectivity phenotype is driven by genetic background mutations in this dfmr150M stock. Similar GF circuit synaptic overgrowth is not observed in independent dfmr1 null alleles. Bulked segregant analysis (BSA) was combined with whole genome sequencing (WGS) to identify the quantitative trait loci (QTL) linked to neural circuit hyperconnectivity. The results reveal 8 QTL associated with inappropriate synapse formation and maintenance in the dfmr150M mutant background.

CONCLUSIONS

Synaptogenesis is a complex, precisely orchestrated neurodevelopmental process with a large cohort of gene products coordinating the connectivity, synaptic strength, and excitatory/inhibitory balance between neuronal partners. This work identifies a number of genetic regions that contain mutations disrupting proper synaptogenesis within a particularly well-mapped neural circuit. These QTL regions contain potential new genes involved in synapse formation and refinement. Given the similarity of the synaptic overgrowth phenotype to known ID and ASD inherited conditions, identifying these genes should increase our understanding of these devastating neurodevelopmental disease states.

Neurobiology of sensory processing in autism spectrum disorder

Altered sensory processing and perception has been one of the characteristics of autism spectrum disorder (ASD). In this chapter, we review the neural underpinnings of sensory abnormalities of ASD by examining the literature on clinical, behavioral and neurobiological evidence that underlies the main patterns of sensory integration function and dysfunction. Furthermore, neural differences in anatomy, function and connectivity of different regions underlying sensory processing are also discussed. We conclude that sensory integration intervention is built on the premise of neuroplasticity to improve function and behavior for individuals with ASD.

Interhemispheric connections between olfactory bulbs improve odor detection

Interhemispheric connections enable interaction and integration of sensory information in bilaterian nervous systems and are thought to optimize sensory computations. However, the cellular and spatial organization of interhemispheric networks and the computational properties they mediate in vertebrates are still poorly understood. Thus, it remains unclear to what extent the connectivity between left and right brain hemispheres participates in sensory processing. Here, we show that the zebrafish olfactory bulbs (OBs) receive direct interhemispheric projections from their contralateral counterparts in addition to top-down inputs from the contralateral zebrafish homolog of olfactory cortex. The direct interhemispheric projections between the OBs reach peripheral layers of the contralateral OB and retain a precise topographic organization, which directly connects similarly tuned olfactory glomeruli across hemispheres. In contrast, interhemispheric top-down inputs consist of diffuse projections that broadly innervate the inhibitory granule cell layer. Jointly, these interhemispheric connections elicit a balance of topographically organized excitation and nontopographic inhibition on the contralateral OB and modulate odor responses. We show that the interhemispheric connections in the olfactory system enable the modulation of odor response and contribute to a small but significant improvement in the detection of a reproductive pheromone when presented together with complex olfactory cues by potentiating the response of the pheromone selective neurons. Taken together, our data show a previously unknown function for an interhemispheric connection between chemosensory maps of the olfactory system.

Interhemispheric connections enable interaction and integration of sensory information in bilaterian nervous systems and are thought to optimize sensory computations. This study shows that interhemispheric olfactory connections in the zebrafish brain improve the detection of a reproductive pheromone within a noisy odor background.

GABAergic abnormalities in the fragile X syndrome

Many pathways have been involved in pathophysiology of the fragile X syndrome, one of the more frequent genetic causes of intellectual disability and autism. This review highlights the recent insights in the role the abnormalities in the GABAergic system play in the disorder. Since the initial observations showed that the expression of specific subunits of the GABA(A) receptor were underexpressed in the fragile X knockout mouse model more than a decade ago, evidence has accumulated that the expression of approximately half of the GABAergic system is compromised in multiple species, including in fragile X patients. Functional consequences of the GABAergic deficiencies could be measured using whole-cell voltage clamp recordings. Pharmalogical treatment with agonist of the receptor was been able to restore several behavioral deficits in the fragile X mouse model, including seizures, marble burying and, in part, prepulse inhibition. Trials in patients with the same agonist have demonstrated encouraging post-hoc results in the most severely affected patients, although no effect could be demonstrated in the patient group as a whole. In conclusion, there can be little doubt that the GABAergic system is compromised in the fragile X syndrome and that these abnormalities contribute to the clinical abnormalities observed. However, at the moment the difference in treatment effectiveness of agonist of the receptor in animal models as opposed to in patients remains unexplained.

Hyperexcitability and impaired intracortical inhibition in patients with fragile-X syndrome

Fragile-X syndrome (FXS) is characterized by neurological and psychiatric problems symptomatic of cortical hyperexcitability. Recent animal studies identified deficient γ-aminobutyricacid (GABA) inhibition as a key mechanism for hyperexcitability in FXS, but the GABA system remains largely unexplored in humans with the disorder. The primary objective of this study was to assess GABA-mediated inhibition and its relationship with hyperexcitability in patients with FXS. Transcranial magnetic stimulation (TMS) was used to assess cortical and corticospinal inhibitory and excitatory mechanisms in 18 patients with a molecular diagnosis of FXS and 18 healthy controls. GABA-mediated inhibition was measured with short-interval intracortical inhibition (GABA_A), long-interval intracortical inhibition (GABA_B), and the corticospinal silent period (GABA_A+B). Net intracortical facilitation involving glutamate was assessed with intracortical facilitation, and corticospinal excitability was measured with the resting motor threshold. Results showed that FXS patients had significantly reduced short-interval intracortical inhibition, increased long-interval intracortical inhibition, and increased intracortical facilitation compared to healthy controls. In the FXS group, reduced short-interval intracortical inhibition was associated with heightened intracortical facilitation. Taken together, these results suggest that reduced GABA_A inhibition is a plausible mechanism underlying cortical hyperexcitability in patients with FXS. These findings closely match those observed in animal models, supporting the translational validity of these markers for clinical research.

Auditory EEG Biomarkers in Fragile X Syndrome: Clinical Relevance

Sensory hypersensitivities are common and distressing features of Fragile X Syndrome (FXS). While there are many drug interventions that reduce behavioral deficits in Fmr1 mice and efforts to translate these preclinical breakthroughs into clinical trials for FXS, evidence-based clinical interventions are almost non-existent potentially due to lack of valid neural biomarkers. Local circuit function in sensory networks is dependent on the dynamic balance of activity in inhibitory/excitatory synapses. Studies are needed to examine the association of electrophysiological alterations in neural systems with sensory and other clinical features of FXS to establish their clinical relevance. Adolescents and adults with FXS (n = 38, Mean age = 25.5, std = 10.1; 13 females) and age matched typically developing controls (n = 40, Mean age = 27.7, std = 12.1; 17 females) completed auditory chirp and auditory habituation tasks while undergoing dense array electroencephalography (EEG). Amplitude, latency, and percent change (habituation) in N1 and P2 event-related potential (ERP) components were characterized for the habituation task; time-frequency calculations using Morlet wavelets characterized phase-locking and single trial power for the habituation and chirp tasks. FXS patients showed increased amplitude but some evidence for reduced habituation of the N1 ERP, and reduced phase-locking in the low and high gamma frequency range and increased low gamma power to the chirp stimulus. FXS showed increased theta power in both tasks. While the habituation finding was weaker than previously found, the remaining findings replicate our previous work in a new sample of patients with FXS. Females showed less deficit in the chirp task but not the habituation task. Abnormal increases in gamma power were related to more severe behavioral and psychiatric features as well as reductions in neurocognitive abilities. Replicating electrophysiological deficits in a new group of patients using different EEG equipment at a new data collection site with differing levels of environmental noise that were robust to data processing techniques utilizing multiple researchers, indicates a potential for scalability to multi-site clinical trials. Given the robust replicability, relevance to clinical measures, and preclinical evidence for sensitivity of these EEG measures to pharmacological intervention, the observed abnormalities may provide novel translational markers of target engagement and potentially outcome measures in large-scale studies evaluating new treatments targeting neural hyperexcitability in FXS.

Dissecting the Genetics of Autism Spectrum Disorders: A Drosophila Perspective

Autism Spectrum Disorder (ASD) is a complex group of multi-factorial developmental disorders that leads to communication and behavioral defects. Genetic alterations have been identified in around 20% of ASD patients and the use of genetic models, such as Drosophila melanogaster, has been of paramount importance in deciphering the significance of these alterations. In fact, many of the ASD associated genes, such as FMR1, Neurexin, Neuroligins and SHANK encode for proteins that have conserved functions in neurons and during synapse development, both in humans and in the fruit fly. Drosophila is a prominent model in neuroscience due to the conserved genetic networks that control neurodevelopmental processes and to the ease of manipulating its genetics. In the present review we will describe recent advances in the field of ASD with a particular focus on the characterization of genes where the use of Drosophila has been fundamental to better understand their function.

Sensory Processing Phenotypes in Fragile X Syndrome

Fragile X syndrome (FXS) is a neurodevelopmental disorder that causes intellectual disability. It is a leading known genetic cause of autism. In addition to cognitive, social, and communication deficits, humans with FXS demonstrate abnormal sensory processing including sensory hypersensitivity. Sensory hypersensitivity commonly manifests as auditory, tactile, or visual defensiveness or avoidance. Clinical, behavioral, and electrophysiological studies consistently show auditory hypersensitivity, impaired habituation to repeated sounds, and reduced auditory attention in humans with FXS. Children with FXS also exhibit significant visuospatial impairments. Studies in infants and toddlers with FXS have documented impairments in processing texture-defined motion stimuli, temporal flicker, perceiving ordinal numerical sequence, and the ability to maintain the identity of dynamic object information during occlusion. Consistent with the observations in humans with FXS, fragile X mental retardation 1 ( Fmr1) gene knockout (KO) rodent models of FXS also show seizures, abnormal visual-evoked responses, auditory hypersensitivity, and abnormal processing at multiple levels of the auditory system, including altered acoustic startle responses. Among other sensory symptoms, individuals with FXS exhibit tactile defensiveness. Fmr1 KO mice also show impaired encoding of tactile stimulation frequency and larger size of receptive fields in the somatosensory cortex. Since sensory deficits are relatively more tractable from circuit mechanisms and developmental perspectives than more complex social behaviors, the focus of this review is on clinical, functional, and structural studies that outline the auditory, visual, and somatosensory processing deficits in FXS. The similarities in sensory phenotypes between humans with FXS and animal models suggest a likely conservation of basic sensory processing circuits across species and may provide a translational platform to not just develop biomarkers but also to understand underlying mechanisms. We argue that preclinical studies in animal models of FXS can facilitate the ongoing search for new therapeutic approaches in FXS by understanding mechanisms of basic sensory processing circuits and behaviors that are conserved across species.

Neural Circuits: Reduced Inhibition in Fragile X Syndrome

The Drosophila Fragile X Syndrome model has long generated insights into this devastating neurological disease state. A recent study of olfactory neural circuitry shows that decreased lateral inhibition onto projection neurons relaying sensory input into higher brain centers causes impaired behavior.

Structural and Functional Abnormalities in the Olfactory System of Fragile X Syndrome Models

Fragile X Syndrome (FXS) is the most common inherited form of intellectual disability. It is produced by mutation of the Fmr1 gene that encodes for the Fragile Mental Retardation Protein (FMRP), an important RNA-binding protein that regulates the expression of multiple proteins located in neuronal synapses. Individuals with FXS exhibit abnormal sensory information processing frequently leading to hypersensitivity across sensory modalities and consequently a wide array of behavioral symptoms. Insects and mammals engage primarily their sense of smell to create proper representations of the external world and guide adequate decision-making processes. This feature in combination with the exquisitely organized neuronal circuits found throughout the olfactory system (OS) and the wide expression of FMRP in brain regions that process olfactory information makes it an ideal model to study sensory alterations in FXS models. In the last decade several groups have taken advantage of these features and have used the OS of fruit fly and rodents to understand neuronal alteration giving rise to sensory perception issues. In this review article, we will discuss molecular, morphological and physiological aspects of the olfactory information processing in FXS models. We will highlight the decreased inhibitory/excitatory synaptic balance and the diminished synaptic plasticity found in this system resulting in behavioral alteration of individuals in the presence of odorant stimuli.

Activity-Dependent Remodeling of Drosophila Olfactory Sensory Neuron Brain Innervation during an Early-Life Critical Period

Critical periods are windows of development when the environment has a pronounced effect on brain circuitry. Models of neurodevelopmental disorders, including autism spectrum disorders, intellectual disabilities, and schizophrenia, are linked to disruption of critical period remodeling. Critical periods open with the onset of sensory experience; however, it remains unclear exactly how sensory input modifies brain circuits. Here, we examine olfactory sensory neuron (OSN) innervation of the Drosophila antennal lobe of both sexes as a genetic model of this question. We find that olfactory sensory experience during an early-use critical period drives loss of OSN innervation of antennal lobe glomeruli and subsequent axon retraction in a dose-dependent mechanism. This remodeling does not result from olfactory receptor loss or OSN degeneration, but rather from rapid synapse elimination and axon pruning in the target olfactory glomerulus. Removal of the odorant stimulus only during the critical period leads to OSN reinnervation, demonstrating that remodeling is transiently reversible. We find that this synaptic refinement requires the OSN-specific olfactory receptor and downstream activity. Conversely, blocking OSN synaptic output elevates glomeruli remodeling. We find that GABAergic neurotransmission has no detectable role, but that glutamatergic signaling via NMDA receptors is required for OSN synaptic refinement. Together, these results demonstrate that OSN inputs into the brain manifest robust, experience-dependent remodeling during an early-life critical period, which requires olfactory reception, OSN activity, and NMDA receptor signaling. This work reveals a pathway linking initial olfactory sensory experience to glutamatergic neurotransmission in the activity-dependent remodeling of brain neural circuitry in an early-use critical period.SIGNIFICANCE STATEMENT Neurodevelopmental disorders manifest symptoms at specific developmental milestones that suggest an intersection between early sensory experience and brain neural circuit remodeling. One classic example is Fragile X syndrome caused by loss of an RNA-binding translation regulator of activity-dependent synaptic refinement. As a model, Drosophila olfactory circuitry is well characterized, genetically tractable, and rapidly developing, and thus ideally suited to probe underlying mechanisms. Here, we find olfactory sensory neurons are dramatically remodeled by heightened sensory experience during an early-life critical period. We demonstrate removing the olfactory stimulus during the critical period can reverse the connectivity changes. We find that this remodeling requires neural activity and NMDA receptor-mediated glutamatergic transmission. This improved understanding may help us design treatments for neurodevelopmental disorders.

Haploinsufficiency of autism causative gene Tbr1 impairs olfactory discrimination and neuronal activation of the olfactory system in mice

Background

Autism spectrum disorders (ASD) exhibit two clusters of core symptoms, i.e., social and communication impairment, and repetitive behaviors and sensory abnormalities. Our previous study demonstrated that TBR1, a causative gene of ASD, controls axonal projection and neuronal activation of amygdala and regulates social interaction and vocal communication in a mouse model. Behavioral defects caused by Tbr1 haploinsufficiency can be ameliorated by increasing neural activity via D-cycloserine treatment, an N-methyl-D-aspartate receptor (NMDAR) coagonist. In this report, we investigate the role of TBR1 in regulating olfaction and test whether D-cycloserine can also improve olfactory defects in Tbr1 mutant mice.

Methods

We used Tbr1^+/− mice as a model to investigate the function of TBR1 in olfactory sensation and discrimination of non-social odors. We employed a behavioral assay to characterize the olfactory defects of Tbr1^+/− mice. Magnetic resonance imaging (MRI) and histological analysis were applied to characterize anatomical features. Immunostaining was performed to further analyze differences in expression of TBR1 subfamily members (namely TBR1, TBR2, and TBX21), interneuron populations, and dendritic abnormalities in olfactory bulbs. Finally, C-FOS staining was used to monitor neuronal activation of the olfactory system upon odor stimulation.

Results

Tbr1^+/− mice exhibited smaller olfactory bulbs and anterior commissures, reduced interneuron populations, and an abnormal dendritic morphology of mitral cells in the olfactory bulbs. Tbr1 haploinsufficiency specifically impaired olfactory discrimination but not olfactory sensation. Neuronal activation upon odorant stimulation was reduced in the glomerular layer of Tbr1^+/− olfactory bulbs. Furthermore, although the sizes of piriform and perirhinal cortices were not affected by Tbr1 deficiency, neuronal activation was reduced in these two cortical regions in response to odorant stimulation. These results suggest an impairment of neuronal activation in olfactory bulbs and defective connectivity from olfactory bulbs to the upper olfactory system in Tbr1^+/− mice. Systemic administration of D-cycloserine, an NMDAR co-agonist, ameliorated olfactory discrimination in Tbr1^+/− mice, suggesting that increased neuronal activity has a beneficial effect on Tbr1 deficiency.

Conclusions

Tbr1 regulates neural circuits and activity in the olfactory system to control olfaction. Tbr1^+/− mice can serve as a suitable model for revealing how an autism causative gene controls neuronal circuits, neural activity, and autism-related behaviors.

Electronic supplementary material

The online version of this article (10.1186/s13229-019-0257-5) contains supplementary material, which is available to authorized users.

Impaired GABA Neural Circuits Are Critical for Fragile X Syndrome

Fragile X syndrome (FXS) is an inheritable neuropsychological disease caused by silence of the fmr1 gene and the deficiency of Fragile X mental retardation protein (FMRP). Patients present neuronal alterations that lead to severe intellectual disability and altered sleep rhythms. However, the neural circuit mechanisms underlying FXS remain unclear. Previous studies have suggested that metabolic glutamate and gamma-aminobutyric acid (GABA) receptors/circuits are two counter-balanced factors involved in FXS pathophysiology. More and more studies demonstrated that attenuated GABAergic circuits in the absence of FMRP are critical for abnormal progression of FXS. Here, we reviewed the changes of GABA neural circuits that were attributed to intellectual-deficient FXS, from several aspects including deregulated GABA metabolism, decreased expressions of GABA receptor subunits, and impaired GABAergic neural circuits. Furthermore, the activities of GABA neural circuits are modulated by circadian rhythm of FMRP metabolism and reviewed the abnormal condition of FXS mice or patients.

Drosophila as a Model for Assessing the Function of RNA-Binding Proteins during Neurogenesis and Neurological Disease

An outstanding question in developmental neurobiology is how RNA processing events contribute to the regulation of neurogenesis. RNA processing events are increasingly recognized as playing fundamental roles in regulating multiple developmental events during neurogenesis, from the asymmetric divisions of neural stem cells, to the generation of complex and diverse neurite morphologies. Indeed, both asymmetric cell division and neurite morphogenesis are often achieved by mechanisms that generate asymmetric protein distributions, including post-transcriptional gene regulatory mechanisms such as the transport of translationally silent messenger RNAs (mRNAs) and local translation of mRNAs within neurites. Additionally, defects in RNA splicing have emerged as a common theme in many neurodegenerative disorders, highlighting the importance of RNA processing in maintaining neuronal circuitry. RNA-binding proteins (RBPs) play an integral role in splicing and post-transcriptional gene regulation, and mutations in RBPs have been linked with multiple neurological disorders including autism, dementia, amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), Fragile X syndrome (FXS), and X-linked intellectual disability disorder. Despite their widespread nature and roles in neurological disease, the molecular mechanisms and networks of regulated target RNAs have been defined for only a small number of specific RBPs. This review aims to highlight recent studies in Drosophila that have advanced our knowledge of how RBP dysfunction contributes to neurological disease.

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.

Auditory experience controls the maturation of song discrimination and sexual response in Drosophila

In birds and higher mammals, auditory experience during development is critical to discriminate sound patterns in adulthood. However, the neural and molecular nature of this acquired ability remains elusive. In fruit flies, acoustic perception has been thought to be innate. Here we report, surprisingly, that auditory experience of a species-specific courtship song in developing Drosophila shapes adult song perception and resultant sexual behavior. Preferences in the song-response behaviors of both males and females were tuned by social acoustic exposure during development. We examined the molecular and cellular determinants of this social acoustic learning and found that GABA signaling acting on the GABAA receptor Rdl in the pC1 neurons, the integration node for courtship stimuli, regulated auditory tuning and sexual behavior. These findings demonstrate that maturation of auditory perception in flies is unexpectedly plastic and is acquired socially, providing a model to investigate how song learning regulates mating preference in insects.

Multifarious Functions of the Fragile X Mental Retardation Protein

Fragile X syndrome (FXS), a heritable intellectual and autism spectrum disorder (ASD), results from the loss of Fragile X mental retardation protein (FMRP). This neurodevelopmental disease state exhibits neural circuit hyperconnectivity and hyperexcitability. Canonically, FMRP functions as an mRNA-binding translation suppressor, but recent findings have enormously expanded its proposed roles. Although connections between burgeoning FMRP functions remain unknown, recent advances have extended understanding of its involvement in RNA, channel, and protein binding that modulate calcium signaling, activity-dependent critical period development, and the excitation-inhibition (E/I) neural circuitry balance. In this review, we contextualize 3 years of FXS model research. Future directions extrapolated from recent advances focus on discovering links between FMRP roles to determine whether FMRP has a multitude of unrelated functions or whether combinatorial mechanisms can explain its multifaceted existence.

ESCRT-III Membrane Trafficking Misregulation Contributes To Fragile X Syndrome Synaptic Defects

The leading cause of heritable intellectual disability (ID) and autism spectrum disorders (ASD), Fragile X syndrome (FXS), is caused by loss of the mRNA-binding translational suppressor Fragile X Mental Retardation Protein (FMRP). In the Drosophila FXS disease model, we found FMRP binds shrub mRNA (human Chmp4) to repress Shrub expression, causing overexpression during the disease state early-use critical period. The FXS hallmark is synaptic overelaboration causing circuit hyperconnectivity. Testing innervation of a central brain learning/memory center, we found FMRP loss and Shrub overexpression similarly increase connectivity. The ESCRT-III core protein Shrub has a central role in endosome-to-multivesicular body membrane trafficking, with synaptic requirements resembling FMRP. Consistently, we found FMRP loss and Shrub overexpression similarly elevate endosomes and result in the arrested accumulation of enlarged intraluminal vesicles within synaptic boutons. Importantly, genetic correction of Shrub levels in the FXS model prevents synaptic membrane trafficking defects and strongly restores innervation. These results reveal a new molecular mechanism underpinning the FXS disease state.

Enhanced Excitatory Connectivity and Disturbed Sound Processing in the Auditory Brainstem of Fragile X Mice

Hypersensitivity to sounds is one of the prevalent symptoms in individuals with Fragile X syndrome (FXS). It manifests behaviorally early during development and is often used as a landmark for treatment efficacy. However, the physiological mechanisms and circuit-level alterations underlying this aberrant behavior remain poorly understood. Using the mouse model of FXS (Fmr1 KO), we demonstrate that functional maturation of auditory brainstem synapses is impaired in FXS. Fmr1 KO mice showed a greatly enhanced excitatory synaptic input strength in neurons of the lateral superior olive (LSO), a prominent auditory brainstem nucleus, which integrates ipsilateral excitation and contralateral inhibition to compute interaural level differences. Conversely, the glycinergic, inhibitory input properties remained unaffected. The enhanced excitation was the result of an increased number of cochlear nucleus fibers converging onto one LSO neuron, without changing individual synapse properties. Concomitantly, immunolabeling of excitatory ending markers revealed an increase in the immunolabeled area, supporting abnormally elevated excitatory input numbers. Intrinsic firing properties were only slightly enhanced. In line with the disturbed development of LSO circuitry, auditory processing was also affected in adult Fmr1 KO mice as shown with single-unit recordings of LSO neurons. These processing deficits manifested as an increase in firing rate, a broadening of the frequency response area, and a shift in the interaural level difference function of LSO neurons. Our results suggest that this aberrant synaptic development of auditory brainstem circuits might be a major underlying cause of the auditory processing deficits in FXS.SIGNIFICANCE STATEMENT Fragile X Syndrome (FXS) is the most common inheritable form of intellectual impairment, including autism. A core symptom of FXS is extreme sensitivity to loud sounds. This is one reason why individuals with FXS tend to avoid social interactions, contributing to their isolation. Here, a mouse model of FXS was used to investigate the auditory brainstem where basic sound information is first processed. Loss of the Fragile X mental retardation protein leads to excessive excitatory compared with inhibitory inputs in neurons extracting information about sound levels. Functionally, this elevated excitation results in increased firing rates, and abnormal coding of frequency and binaural sound localization cues. Imbalanced early-stage sound level processing could partially explain the auditory processing deficits in FXS.