Chronic metabotropic glutamate receptor 5 inhibition corrects local alterations of brain activity and improves cognitive performance in fragile X mice

Group I and group II metabotropic glutamate receptors are upregulated in the synapses of infant rats prenatally exposed to valproic acid

RATIONALE

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired social interaction and restricted/stereotyped behavior. Prenatal exposure to valproic acid (VPA) is associated with an increased risk of developing ASD in humans and autistic-like behaviors in rodents. Increasing evidence indicates that dysfunctions of glutamate receptors at synapses are associated with ASD. In the VPA rat model, an involvement of glutamate receptors in autism-like phenotypes has been suggested; however, few studies were carried out on metabotropic glutamate (mGlu) receptors.

OBJECTIVES

We examined the protein expression levels of group I (mGlu1 and mGlu5) and group II (mGlu2/3) mGlu receptors in rats prenatally exposed to VPA and evaluated the effect of mGlu receptor modulation on an early autism-like phenotype in these animals.

METHODS

We used western blotting analysis on synaptosomes obtained from forebrain of control and VPA rats at different ages (postnatal day P13, 35, 90) and carried out ultrasonic vocalization (USV) emission test in infant control and VPA rats.

RESULTS

The expression levels of all these receptors were significantly increased in infant VPA rats. No changes were detected in adolescent and adult rats. An acute treatment with the preferential mGlu2/3 antagonist, LY341495, attenuated the impairment in the USV emission in VPA rats. No effect was observed after a treatment with the mGlu5 selective antagonist, MTEP.

CONCLUSIONS

Our findings demonstrate that the expression of group I and group II mGlu receptors is upregulated at synapses of infant VPA rats and suggest that mGlu2/3 receptor modulation may have a therapeutic potential in ASD.

Effects of AFQ056 on language learning in fragile X syndrome

BACKGROUNDFXLEARN, the first-ever large multisite trial of effects of disease-targeted pharmacotherapy on learning, was designed to explore a paradigm for measuring effects of mechanism-targeted treatment in fragile X syndrome (FXS). In FXLEARN, the effects of metabotropic glutamate receptor type 5 (mGluR5) negative allosteric modulator (NAM) AFQ056 on language learning were evaluated in 3- to 6-year-old children with FXS, expected to have more learning plasticity than adults, for whom prior trials of mGluR5 NAMs have failed.METHODSAfter a 4-month single-blind placebo lead-in, participants were randomized 1:1 to AFQ056 or placebo, with 2 months of dose optimization to the maximum tolerated dose, then 6 months of treatment during which a language-learning intervention was implemented for both groups. The primary outcome was a centrally scored videotaped communication measure, the Weighted Communication Scale (WCS). Secondary outcomes were objective performance-based and parent-reported cognitive and language measures.RESULTSFXLEARN enrolled 110 participants, randomized 99, and had 91 who completed the placebo-controlled period. Although both groups made language progress and there were no safety issues, the change in WCS score during the placebo-controlled period was not significantly different between the AFQ056 and placebo-treated groups, nor were there any significant between-group differences in change in any secondary measures.CONCLUSIONDespite the large body of evidence supporting use of mGluR5 NAMs in animal models of FXS, this study suggests that this mechanism of action does not translate into benefit for the human FXS population and that better strategies are needed to determine which mechanisms will translate from preclinical models to humans in genetic neurodevelopmental disorders.TRIAL REGISTRATIONClincalTrials.gov NCT02920892.FUNDING SOURCESNeuroNEXT network NIH grants U01NS096767, U24NS107200, U24NS107209, U01NS077323, U24NS107183, U24NS107168, U24NS107128, U24NS107199, U24NS107198, U24NS107166, U10NS077368, U01NS077366, U24NS107205, U01NS077179, and U01NS077352; NIH grant P50HD103526; and Novartis IIT grant AFQ056X2201T for provision of AFQ056.

Early Administration of the Phytocannabinoid Cannabidivarin Prevents the Neurobehavioral Abnormalities Associated with the Fmr1-KO Mouse Model of Fragile X Syndrome

Phytocannabinoids, including the non-addictive cannabis component cannabidivarin (CBDV), have been reported to hold therapeutic potential in several neurodevelopmental disorders (NDDs). Nonetheless, the therapeutic value of phytocannabinoids for treating Fragile X syndrome (FXS), a major NDD, remains unexplored. Here, we characterized the neurobehavioral effects of CBDV at doses of 20 or 100 mg/kg in the Fmr1-knockout (Fmr1-KO) mouse model of FXS using two temporally different intraperitoneal regimens: subchronic 10-day delivery during adulthood (Study 1: rescue treatment) or chronic 5-week delivery at adolescence (Study 2: preventive treatment). Behavioral tests assessing FXS-like abnormalities included anxiety, locomotor, cognitive, social and sensory alterations. Expression of inflammatory and plasticity markers was investigated in the hippocampus and prefrontal cortex. When administered during adulthood (Study 1), the effects of CBDV were marginal, rescuing at the lower dose only the acoustic hyper-responsiveness of Fmr1-KO mice and at both doses their altered hippocampal expression of neurotrophins. When administered during adolescence (Study 2), CBDV at both doses prevented the cognitive, social and acoustic alterations of adult Fmr1-KO mice and modified the expression of several inflammatory brain markers in both wild-type littermates and mutants. These findings warrant the therapeutic potential of CBDV for preventing neurobehavioral alterations associated with FXS, highlighting the relevance of its early administration.

Updates on the Physiopathology of Group I Metabotropic Glutamate Receptors (mGluRI)-Dependent Long-Term Depression

Group I metabotropic glutamate receptors (mGluRI), including mGluR1 and mGluR5 subtypes, modulate essential brain functions by affecting neuronal excitability, intracellular calcium dynamics, protein synthesis, dendritic spine formation, and synaptic transmission and plasticity. Nowadays, it is well appreciated that the mGluRI-dependent long-term depression (LTD) of glutamatergic synaptic transmission (mGluRI-LTD) is a key mechanism by which mGluRI shapes connectivity in various cerebral circuitries, directing complex brain functions and behaviors, and that it is deranged in several neurological and psychiatric illnesses, including neurodevelopmental disorders, neurodegenerative diseases, and psychopathologies. Here, we will provide an updated overview of the physiopathology of mGluRI-LTD, by describing mechanisms of induction and regulation by endogenous mGluRI interactors, as well as functional physiological implications and pathological deviations.

Partial mGlu5 Negative Allosteric Modulator M-5MPEP Demonstrates Antidepressant-Like Effects on Sleep Without Affecting Cognition or Quantitative EEG

Selective negative allosteric modulators (NAMs) targeting the metabotropic glutamate receptor subtype 5 (mGlu5) demonstrate anxiolytic-like and antidepressant-like effects yet concern regarding adverse effect liability remains. Functional coupling of mGlu5 with ionotropic N-methyl-D-aspartate receptors (NMDARs) represents a potential mechanism through which full inhibition leads to adverse effects, as NMDAR inhibition can induce cognitive impairments and psychotomimetic-like effects. Recent development of "partial" mGlu5 NAMs, characterized by submaximal but saturable levels of blockade, may represent a novel development approach to broaden the therapeutic index of mGlu5 NAMs. This study compared the partial mGlu5 NAM, M-5MPEP, with the full mGlu5 NAM, VU0424238 on sleep, cognition, and brain function alone and in combination with a subthreshold dose of the NMDAR antagonist, MK-801, using a paired-associates learning (PAL) cognition task and electroencephalography (EEG) in rats. M-5MPEP and VU0424238 decreased rapid eye movement (REM) sleep and increased REM sleep latency, both putative biomarkers of antidepressant-like activity. Neither compound alone affected accuracy, but 30 mg/kg VU0424238 combined with MK-801 decreased accuracy on the PAL task. Using quantitative EEG, VU0424238, but not M-5MPEP, prolonged arousal-related elevations in high gamma power, and, in combination, VU0424238 potentiated effects of MK-801 on high gamma power. Together, these studies further support a functional interaction between mGlu5 and NMDARs that may correspond with cognitive impairments. Present data support further development of partial mGlu5 NAMs given their potentially broader therapeutic index than full mGlu5 NAMs and use of EEG as a translational biomarker to titrate doses aligning with therapeutic versus adverse effects.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

(S)-5-(2'-Fluorophenyl)-N,N-dimethyl-1,2,3,4-tetrahydronaphthalen-2-amine, a Serotonin Receptor Modulator, Possesses Anticonvulsant, Prosocial, and Anxiolytic-like Properties in an Fmr1 Knockout Mouse Model of Fragile X Syndrome and Autism Spectrum Disorder

Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by intellectual disabilities and a plethora of neuropsychiatric symptoms. FXS is the leading monogenic cause of autism spectrum disorder (ASD), which is defined clinically by repetitive and/or restrictive patterns of behavior and social communication deficits. Epilepsy and anxiety are also common in FXS and ASD. Serotonergic neurons directly innervate and modulate the activity of neurobiological circuits altered in both disorders, providing a rationale for investigating serotonin receptors (5-HTRs) as targets for FXS and ASD drug discovery. Previously we unveiled an orally active aminotetralin, (S)-5-(2'-fluorophenyl)-N,N-dimethyl-1,2,3,4-tetrahydronaphthalen-2-amine (FPT), that exhibits partial agonist activity at 5-HT_1ARs, 5-HT_2CRs, and 5-HT₇Rs and that reduces repetitive behaviors and increases social approach behavior in wild-type mice. Here we report that in an Fmr1 knockout mouse model of FXS and ASD, FPT is prophylactic for audiogenic seizures. No FPT-treated mice displayed audiogenic seizures, compared to 73% of vehicle-treated mice. FPT also exhibits anxiolytic-like effects in several assays and increases social interactions in both Fmr1 knockout and wild-type mice. Furthermore, FPT increases c-Fos expression in the basolateral amygdala, which is a preclinical effect produced by anxiolytic medications. Receptor pharmacology assays show that FPT binds competitively and possesses rapid association and dissociation kinetics at 5-HT_1ARs and 5-HT₇Rs, yet has slow association and rapid dissociation kinetics at 5-HT_2CRs. Finally, we reassessed and report FPT's affinity and function at 5-HT_1ARs, 5-HT_2CRs, and 5-HT₇Rs. Collectively, these observations provide mounting support for further development of FPT as a pharmacotherapy for common neuropsychiatric symptoms in FXS and ASD.

Enhanced Function and Overexpression of Metabotropic Glutamate Receptors 1 and 5 in the Spinal Cord of the SOD1G93A Mouse Model of Amyotrophic Lateral Sclerosis during Disease Progression

Glutamate (Glu)-mediated excitotoxicity is a major cause of amyotrophic lateral sclerosis (ALS) and our previous work highlighted that abnormal Glu release may represent a leading mechanism for excessive synaptic Glu. We demonstrated that group I metabotropic Glu receptors (mGluR1, mGluR5) produced abnormal Glu release in SOD1^G93A mouse spinal cord at a late disease stage (120 days). Here, we studied this phenomenon in pre-symptomatic (30 and 60 days) and early-symptomatic (90 days) SOD1^G93A mice. The mGluR1/5 agonist (S)-3,5-Dihydroxyphenylglycine (3,5-DHPG) concentration dependently stimulated the release of [³H]d-Aspartate ([³H]d-Asp), which was comparable in 30- and 60-day-old wild type mice and SOD1^G93A mice. At variance, [³H]d-Asp release was significantly augmented in 90-day-old SOD1^G93A mice and both mGluR1 and mGluR5 were involved. The 3,5-DHPG-induced [³H]d-Asp release was exocytotic, being of vesicular origin and mediated by intra-terminal Ca²⁺ release. mGluR1 and mGluR5 expression was increased in Glu spinal cord axon terminals of 90-day-old SOD1^G93A mice, but not in the whole axon terminal population. Interestingly, mGluR1 and mGluR5 were significantly augmented in total spinal cord tissue already at 60 days. Thus, function and expression of group I mGluRs are enhanced in the early-symptomatic SOD1^G93A mouse spinal cord, possibly participating in excessive Glu transmission and supporting their implication in ALS. Please define all abbreviations the first time they appear in the abstract, the main text, and the first figure or table caption.

SYNGAP1 mutations: Clinical, genetic, and pathophysiological features

SYNGAP1 is a gene that encodes the cytosolic protein SYNGAP1 (SYNaptic GTPase Activating Protein), an essential component of the postsynaptic density at excitatory glutamatergic neurons. SYNGAP1 plays critical roles in synaptic development, structure, function, and plasticity. Mutations in SYNGAP1 result in a neurodevelopmental disorder termed Mental retardation-type 5 (MRD5, OMIM #612621) with a phenotype consisting of intellectual disability, motor impairments, and epilepsy, attesting to the importance of this protein for normal brain development. Here we review the clinical and pathophysiological aspects of SYNGAP1 mutations with a focus on their effect on synaptogenesis, neural circuit function, and cellular plasticity. We conclude by comparing the molecular pathogenesis of SYNGAP1 mutations with those of another neurodevelopmental disorder that affects dendritic function and cellular plasticity, fragile X syndrome. Insights into the molecular similarities and differences underlying these disorders could lead to rationale therapy development.

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.

Rescue of Fmr1KO phenotypes with mGluR5 inhibitors: MRZ-8456 versus AFQ-056

Metabotropic glutamate receptor 5 (mGluR₅) is a drug target for central nervous system disorders such as fragile X syndrome that involve excessive glutamate-induced excitation. We tested the efficacy of a novel negative allosteric modulator of mGluR₅ developed by Merz Pharmaceuticals, MRZ-8456, in comparison to MPEP and AFQ-056 (Novartis, a.k.a. mavoglurant) in both in vivo and in vitro assays in a mouse model of fragile X syndrome, Fmr1^KO mice. The in vivo assays included susceptibility to audiogenic-induced seizures and pharmacokinetic measurements of drug availability. The in vitro assays included dose response assessments of biomarker expression and dendritic spine length and density in cultured primary neurons. Both MRZ-8456 and AFQ-056 attenuated wild running and audiogenic-induced seizures in Fmr1^KO mice with similar pharmacokinetic profiles. Both drugs significantly reduced dendritic expression of amyloid-beta protein precursor (APP) and rescued the ratio of mature to immature dendritic spines. These findings demonstrate that MRZ-8456, a drug being developed for the treatment of motor complications of L-DOPA in Parkinson's disease and which completed a phase I clinical trial, is effective in attenuating both well-established (seizures and dendritic spine maturity) and exploratory biomarker (APP expression) phenotypes in a mouse model of fragile X syndrome.

mGluR5 Modulation of Behavioral and Epileptic Phenotypes in a Mouse Model of Tuberous Sclerosis Complex

Drugs targeting metabotropic glutamate receptor 5 (mGluR5) have therapeutic potential in autism spectrum disorders (ASD), including tuberous sclerosis complex (TSC). The question whether inhibition or potentiation of mGluR5 could be beneficial depends, among other factors, on the specific indication. To facilitate the development of mGluR5 treatment strategies, we tested the therapeutic utility of mGluR5 negative and positive allosteric modulators (an mGluR5 NAM and PAM) for TSC, using a mutant mouse model with neuronal loss of Tsc2 that demonstrates disease-related phenotypes, including behavioral symptoms of ASD and epilepsy. This model uniquely enables the in vivo characterization and rescue of the electrographic seizures associated with TSC. We demonstrate that inhibition of mGluR5 corrects hyperactivity, seizures, and elevated de novo synaptic protein synthesis. Conversely, positive allosteric modulation of mGluR5 results in the exacerbation of hyperactivity and epileptic phenotypes. The data suggest a meaningful therapeutic potential for mGluR5 NAMs in TSC, which warrants clinical exploration and the continued development of mGluR5 therapies.

Drug development for neurodevelopmental disorders: lessons learned from fragile X syndrome

Neurodevelopmental disorders such as fragile X syndrome (FXS) result in lifelong cognitive and behavioural deficits and represent a major public health burden. FXS is the most frequent monogenic form of intellectual disability and autism, and the underlying pathophysiology linked to its causal gene, FMR1, has been the focus of intense research. Key alterations in synaptic function thought to underlie this neurodevelopmental disorder have been characterized and rescued in animal models of FXS using genetic and pharmacological approaches. These robust preclinical findings have led to the implementation of the most comprehensive drug development programme undertaken thus far for a genetically defined neurodevelopmental disorder, including phase IIb trials of metabotropic glutamate receptor 5 (mGluR5) antagonists and a phase III trial of a GABA_B receptor agonist. However, none of the trials has been able to unambiguously demonstrate efficacy, and they have also highlighted the extent of the knowledge gaps in drug development for FXS and other neurodevelopmental disorders. In this Review, we examine potential issues in the previous studies and future directions for preclinical and clinical trials. FXS is at the forefront of efforts to develop drugs for neurodevelopmental disorders, and lessons learned in the process will also be important for such disorders.

Inhibition of group-I metabotropic glutamate receptors protects against prion toxicity

Prion infections cause inexorable, progressive neurological dysfunction and neurodegeneration. Expression of the cellular prion protein PrPC is required for toxicity, suggesting the existence of deleterious PrPC-dependent signaling cascades. Because group-I metabotropic glutamate receptors (mGluR1 and mGluR5) can form complexes with the cellular prion protein (PrPC), we investigated the impact of mGluR1 and mGluR5 inhibition on prion toxicity ex vivo and in vivo. We found that pharmacological inhibition of mGluR1 and mGluR5 antagonized dose-dependently the neurotoxicity triggered by prion infection and by prion-mimetic anti-PrPC antibodies in organotypic brain slices. Prion-mimetic antibodies increased mGluR5 clustering around dendritic spines, mimicking the toxicity of Aβ oligomers. Oral treatment with the mGluR5 inhibitor, MPEP, delayed the onset of motor deficits and moderately prolonged survival of prion-infected mice. Although group-I mGluR inhibition was not curative, these results suggest that it may alleviate the neurological dysfunctions induced by prion diseases.

Dysregulated NMDA-Receptor Signaling Inhibits Long-Term Depression in a Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is a neurodevelopmental disease. It is one of the leading monogenic causes of intellectual disability among boys with most also displaying autism spectrum disorder traits. Here we investigated the role of NMDA receptors on mGluR-dependent long-term depression (mGluR-LTD), a key biomarker in the disease, at four different developmental stages. First, we applied the mGluR agonist 3,5-dihydroxyphenylglycine in the absence or presence of the NMDAR blocker, APV, hereby unmasking the NMDAR component in this process. As expected, in the presence of APV, we found more LTD in the mouse KO than in WT. This, however, was only observed in the p30-60 age group. At all other age groups tested, mGluR-LTD was almost identical between KO and WT. Interestingly, at p60, in the absence of APV, no or very little LTD was found in KO that was completely restored by application of APV. This suggests that the underlying cause of the enhanced mGluR-LTD in KO (at p30) is caused by dysregulated NMDAR signaling. To investigate this further, we next used NMDAR-subunit-specific antagonists. Inhibition of GluN2B, but not GluN2A, blocked mGluR-LTD only in WT. This was in contrast in the KO where blocking GluN2B rescued mGluR-LTD, suggesting GluN2B-containing NMDARs in the KO are hyperactive. Thus, these findings suggest strong involvement of GluN2B-containing-NMDARs in the pathophysiology of FXS and highlight a potential path for treatment for the disease.

SIGNIFICANCE STATEMENT

There is currently no cure for fragile X, although medications targeting specific FXS symptoms do exist. The FXS animal model, the Fmr1 knock-out mouse, has demonstrated an increased mGluR5-mediated long-term depression (LTD) leading to several clinical trials of mGluR5 inhibitors/modulators, yet all have failed. In addition, surprisingly little information exists about the possible role of other ion channels/receptors, including NMDA receptors (NMDAR), in mGluR-LTD. Here we focus on NMDARs and their regulation of mGluR-mediated LTD at different developmental stages using several different NMDAR blockers/antagonists. Our findings suggest dysregulated NMDARs in the pathophysiology of FXS leading to altered mGluR-mediated LTD. Together, these data will help to develop new drug candidates that could lead to reversal of the FXS phenotype.

Revisiting the flip side: Long-term depression of synaptic efficacy in the hippocampus

Synaptic plasticity is widely regarded as a putative biological substrate for learning and memory processes. While both decreases and increases in synaptic strength are seen as playing a role in learning and memory, long-term depression (LTD) of synaptic efficacy has received far less attention than its counterpart long-term potentiation (LTP). Never-the-less, LTD at synapses can play an important role in increasing computational flexibility in neural networks. In addition, like learning and memory processes, the magnitude of LTD can be modulated by factors that include stress and sex hormones, neurotrophic support, learning environments, and age. Examining how these factors modulate hippocampal LTD can provide the means to better elucidate the molecular underpinnings of learning and memory processes. This is in turn will enhance our appreciation of how both increases and decreases in synaptic plasticity can play a role in different neurodevelopmental and neurodegenerative conditions.

Excitation/Inhibition Imbalance in Animal Models of Autism Spectrum Disorders

Imbalances between excitation and inhibition in synaptic transmission and neural circuits have been implicated in autism spectrum disorders. Excitation and inhibition imbalances are frequently observed in animal models of autism spectrum disorders, and their correction normalizes key autistic-like phenotypes in these animals. These results suggest that excitation and inhibition imbalances may contribute to the development and maintenance of autism spectrum disorders and represent an important therapeutic target.

Delineating the Common Biological Pathways Perturbed by ASD's Genetic Etiology: Lessons from Network-Based Studies

In recent decades it has become clear that Autism Spectrum Disorder (ASD) possesses a diverse and heterogeneous genetic etiology. Aberrations in hundreds of genes have been associated with ASD so far, which include both rare and common variations. While one may expect that these genes converge on specific common molecular pathways, which drive the development of the core ASD characteristics, the task of elucidating these common molecular pathways has been proven to be challenging. Several studies have combined genetic analysis with bioinformatical techniques to uncover molecular mechanisms that are specifically targeted by autism-associated genetic aberrations. Recently, several analysis have suggested that particular signaling mechanisms, including the Wnt and Ca²⁺/Calmodulin-signaling pathways are often targeted by autism-associated mutations. In this review, we discuss several studies that determine specific molecular pathways affected by autism-associated mutations, and then discuss more in-depth into the biological roles of a few of these pathways, and how they may be involved in the development of ASD. Considering that these pathways may be targeted by specific pharmacological intervention, they may prove to be important therapeutic targets for the treatment of ASD.

Cellular distribution of the fragile X mental retardation protein in the mouse brain

The fragile X mental retardation protein (FMRP) plays an important role in normal brain development. Absence of FMRP results in abnormal neuronal morphologies in a selected manner throughout the brain, leading to intellectual deficits and sensory dysfunction in the fragile X syndrome (FXS). Despite FMRP importance for proper brain function, its overall expression pattern in the mammalian brain at the resolution of individual neuronal cell groups is not known. In this study we used FMR1 knockout and isogenic wildtype mice to systematically map the distribution of FMRP expression in the entire mouse brain. Using immunocytochemistry and cellular quantification analyses, we identified a large number of prominent cell groups expressing high levels of FMRP at the subcortical levels, in particular sensory and motor neurons in the brainstem and thalamus. In contrast, many cell groups in the midbrain and hypothalamus exhibit low FMRP levels. More important, we describe differential patterns of FMRP distribution in both cortical and subcortical brain regions. Almost all major brain areas contain high and low levels of FMRP cell groups adjacent to each other or between layers of the same cortical areas. These differential patterns indicate that FMRP expression appears to be specific to individual neuronal cell groups instead of being associated with all neurons in distinct brain regions, as previously considered. Taken together, these findings support the notion of FMRP differential neuronal regulation and strongly implicate the contribution of fundamental sensory and motor processing at subcortical levels to FXS pathology. J. Comp. Neurol. 525:818-849, 2017. © 2016 Wiley Periodicals, Inc.

Genetic and Pharmacological Reversibility of Phenotypes in Mouse Models of Autism Spectrum Disorder

As autism spectrum disorder (ASD) is largely regarded as a neurodevelopmental condition, long-time consensus was that its hallmark features are irreversible. However, several studies from recent years using defined mouse models of ASD have provided clear evidence that in mice neurobiological and behavioural alterations can be ameliorated or even reversed by genetic restoration or pharmacological treatment either before or after symptom onset. Here, we review findings on genetic and pharmacological reversibility of phenotypes in mouse models of ASD. Our review should give a comprehensive overview on both aspects and encourage future studies to better understand the underlying molecular mechanisms that might be translatable from animals to humans.

APP Causes Hyperexcitability in Fragile X Mice

Amyloid-beta protein precursor (APP) and metabolite levels are altered in fragile X syndrome (FXS) patients and in the mouse model of the disorder, Fmr1KO mice. Normalization of APP levels in Fmr1KO mice (Fmr1KO /APPHET mice) rescues many disease phenotypes. Thus, APP is a potential biomarker as well as therapeutic target for FXS. Hyperexcitability is a key phenotype of FXS. Herein, we determine the effects of APP levels on hyperexcitability in Fmr1KO brain slices. Fmr1KO /APPHET slices exhibit complete rescue of UP states in a neocortical hyperexcitability model and reduced duration of ictal discharges in a CA3 hippocampal model. These data demonstrate that APP plays a pivotal role in maintaining an appropriate balance of excitation and inhibition (E/I) in neural circuits. A model is proposed whereby APP acts as a rheostat in a molecular circuit that modulates hyperexcitability through mGluR5 and FMRP. Both over- and under-expression of APP in the context of the Fmr1KO increases seizure propensity suggesting that an APP rheostat maintains appropriate E/I levels but is overloaded by mGluR5-mediated excitation in the absence of FMRP. These findings are discussed in relation to novel treatment approaches to restore APP homeostasis in FXS.

A Subset of Autism-Associated Genes Regulate the Structural Stability of Neurons

Autism spectrum disorder (ASD) comprises a range of neurological conditions that affect individuals’ ability to communicate and interact with others. People with ASD often exhibit marked qualitative difficulties in social interaction, communication, and behavior. Alterations in neurite arborization and dendritic spine morphology, including size, shape, and number, are hallmarks of almost all neurological conditions, including ASD. As experimental evidence emerges in recent years, it becomes clear that although there is broad heterogeneity of identified autism risk genes, many of them converge into similar cellular pathways, including those regulating neurite outgrowth, synapse formation and spine stability, and synaptic plasticity. These mechanisms together regulate the structural stability of neurons and are vulnerable targets in ASD. In this review, we discuss the current understanding of those autism risk genes that affect the structural connectivity of neurons. We sub-categorize them into (1) cytoskeletal regulators, e.g., motors and small RhoGTPase regulators; (2) adhesion molecules, e.g., cadherins, NCAM, and neurexin superfamily; (3) cell surface receptors, e.g., glutamatergic receptors and receptor tyrosine kinases; (4) signaling molecules, e.g., protein kinases and phosphatases; and (5) synaptic proteins, e.g., vesicle and scaffolding proteins. Although the roles of some of these genes in maintaining neuronal structural stability are well studied, how mutations contribute to the autism phenotype is still largely unknown. Investigating whether and how the neuronal structure and function are affected when these genes are mutated will provide insights toward developing effective interventions aimed at improving the lives of people with autism and their families.

Synaptic Plasticity, a Prominent Contributor to the Anxiety in Fragile X Syndrome

Fragile X syndrome (FXS) is an inheritable neuropsychological disease caused by expansion of the CGG trinucleotide repeat affecting the fmr1 gene on X chromosome, resulting in silence of the fmr1 gene and failed expression of FMRP. Patients with FXS suffer from cognitive impairment, sensory integration deficits, learning disability, anxiety, autistic traits, and so forth. Specifically, the morbidity of anxiety in FXS individuals remains high from childhood to adulthood. By and large, it is common that the change of brain plasticity plays a key role in the progression of disease. But for now, most studies excessively emphasized the one-sided factor on the change of synaptic plasticity participating in the generation of anxiety during the development of FXS. Here we proposed an integrated concept to acquire better recognition about the details of this process.

Normal Performance of Fmr1 Mice on a Touchscreen Delayed Nonmatching to Position Working Memory Task

Fragile X syndrome is a neurodevelopmental disorder characterized by mild-to-severe cognitive deficits. The complete absence of Fmr1 and its protein product in the mouse model of fragile X (Fmr1 KO) provides construct validity. A major conundrum in the field is the remarkably normal performance of Fmr1 mice on cognitive tests in most reports. One explanation may be insufficiently challenging cognitive testing procedures. Here we developed a delayed nonmatching to position touchscreen task to test the hypothesis that paradigms placing demands on working memory would reveal robust and replicable cognitive deficits in the Fmr1 KO mouse. We first tested Fmr1 KO mice (Fmr1) and their wild-type (WT) littermates in a simple visual discrimination task, followed by assessment of reversal learning. We then tested Fmr1 and WT mice in a new touchscreen nonmatch to position task and subsequently challenged their working memory abilities by adding delays, representing a higher cognitive load. The performance by Fmr1 KO mice was equal to WTs on both touchscreen tasks. Last, we replicated previous reports of normal performance by Fmr1 mice on Morris water maze spatial navigation and reversal. These results indicate that, while the Fmr1 mouse model effectively recapitulates many molecular and cellular aspects of fragile X syndrome, the cognitive profile of Fmr1 mice generally does not recapitulate the primary cognitive deficits in the human syndrome, even when diverse and challenging tasks are imposed.

Convergence of Hippocampal Pathophysiology in Syngap+/- and Fmr1-/y Mice

Previous studies have hypothesized that diverse genetic causes of intellectual disability (ID) and autism spectrum disorders (ASDs) converge on common cellular pathways. Testing this hypothesis requires detailed phenotypic analyses of animal models with genetic mutations that accurately reflect those seen in the human condition (i.e., have structural validity) and which produce phenotypes that mirror ID/ASDs (i.e., have face validity). We show that SynGAP haploinsufficiency, which causes ID with co-occurring ASD in humans, mimics and occludes the synaptic pathophysiology associated with deletion of the Fmr1 gene. Syngap(+/-) and Fmr1(-/y) mice show increases in basal protein synthesis and metabotropic glutamate receptor (mGluR)-dependent long-term depression that, unlike in their wild-type controls, is independent of new protein synthesis. Basal levels of phosphorylated ERK1/2 are also elevated in Syngap(+/-) hippocampal slices. Super-resolution microscopy reveals that Syngap(+/-) and Fmr1(-/y) mice show nanoscale alterations in dendritic spine morphology that predict an increase in biochemical compartmentalization. Finally, increased basal protein synthesis is rescued by negative regulators of the mGlu subtype 5 receptor and the Ras-ERK1/2 pathway, indicating that therapeutic interventions for fragile X syndrome may benefit patients with SYNGAP1 haploinsufficiency.

SIGNIFICANCE STATEMENT

As the genetics of intellectual disability (ID) and autism spectrum disorders (ASDs) are unraveled, a key issue is whether genetically divergent forms of these disorders converge on common biochemical/cellular pathways and hence may be amenable to common therapeutic interventions. This study compares the pathophysiology associated with the loss of fragile X mental retardation protein (FMRP) and haploinsufficiency of synaptic GTPase-activating protein (SynGAP), two prevalent monogenic forms of ID. We show that Syngap(+/-) mice phenocopy Fmr1(-/y) mice in the alterations in mGluR-dependent long-term depression, basal protein synthesis, and dendritic spine morphology. Deficits in basal protein synthesis can be rescued by pharmacological interventions that reduce the mGlu5 receptor-ERK1/2 signaling pathway, which also rescues the same deficit in Fmr1(-/y) mice. Our findings support the hypothesis that phenotypes associated with genetically diverse forms of ID/ASDs result from alterations in common cellular/biochemical pathways.

Molecular medicine of fragile X syndrome: based on known molecular mechanisms

BACKGROUND

Extensive research on fragile X mental retardation gene knockout mice and mutant Drosophila models has largely expanded our knowledge on mechanism-based treatment of fragile X syndrome (FXS). In light of these findings, several clinical trials are now underway for therapeutic translation to humans.

DATA SOURCES

Electronic literature searches were conducted using the PubMed database and ClinicalTrials.gov. The search terms included "fragile X syndrome", "FXS and medication", "FXS and therapeutics" and "FXS and treatment". Based on the publications identified in this search, we reviewed the neuroanatomical abnormalities in FXS patients and the potential pathogenic mechanisms to monitor the progress of FXS research, from basic studies to clinical trials.

RESULTS

The pathological mechanisms of FXS were categorized on the basis of neuroanatomy, synaptic structure, synaptic transmission and fragile X mental retardation protein (FMRP) loss of function. The neuroanatomical abnormalities in FXS were described to motivate extensive research into the region-specific pathologies in the brain responsible for FXS behavioural manifestations. Mechanism-directed molecular medicines were classified according to their target pathological mechanisms, and the most recent progress in clinical trials was discussed.

CONCLUSIONS

Current mechanism-based studies and clinical trials have greatly contributed to the development of FXS pharmacological therapeutics. Research examining the extent to which these treatments provided a rescue effect or FMRP compensation for the developmental impairments in FXS patients may help to improve the efficacy of treatments.

The Emerging Role of Metabotropic Glutamate Receptors in the Pathophysiology of Chronic Stress-Related Disorders

Chronic stress-related psychiatric conditions such as anxiety, depression, and alcohol abuse are an enormous public health concern. The etiology of these pathologies is complex, with psychosocial stressors being among the most frequently discussed risk factors. The brain glutamatergic neurotransmitter system has often been found involved in behaviors and pathophysiologies resulting from acute stress and fear. Despite this, relatively little is known about the role of glutamatergic system components in chronic psychosocial stress, neither in rodents nor in humans. Recently, drug discovery efforts at the metabotropic receptor subtypes of the glutamatergic system (mGlu1-8 receptors) led to the identification of pharmacological tools with emerging potential in psychiatric conditions. But again, the contribution of individual mGlu subtypes to the manifestation of physiological, molecular, and behavioral consequences of chronic psychosocial stress remains still largely unaddressed. The current review will describe animal models typically used to analyze acute and particularly chronic stress conditions, including models of psychosocial stress, and there we will discuss the emerging roles for mGlu receptor subtypes. Indeed, accumulating evidence indicates relevance and potential therapeutic usefulness of mGlu2/3 ligands and mGlu5 receptor antagonists in chronic stress-related disorders. In addition, a role for further mechanisms, e.g. mGlu7-selective compounds, is beginning to emerge. These mechanisms are important to be analyzed in chronic psychosocial stress paradigms, e.g. in the chronic subordinate colony housing (CSC) model. We summarize the early results and discuss necessary future investigations, especially for mGlu5 and mGlu7 receptor blockers, which might serve to suggest improved therapeutic strategies to treat stress-related disorders.

The utility of rodent models of autism spectrum disorders

PURPOSE OF REVIEW

This review discusses the ways that rodent models of autism spectrum disorders (ASDs) have been used to gain critical information about convergent molecular pathways, the mechanisms underlying altered microcircuit structure and function, and as a screen for potential cutting edge-treatments for ASDs.

RECENT FINDINGS

There is convergent evidence that impaired developmental pruning of connections may be a common finding among several mouse models of ASDs. Recent studies have uncovered impaired autophagy by pathological mTOR activation as a potential contributor to microcircuit dysfunction and behavior. ASD-related disinhibition and exaggerated synaptic plasticity in multiple distinct circuits in cortex and reward circuits in striatum also contribute to social dysfunction and repetitive behaviors. New exciting molecular therapeutic techniques have reversed cognitive deficits in models of ASD, indicating that mouse models could be used for preclinical translational studies of new treatments.

SUMMARY

Rodent models of ASDs coupled to new emerging technologies for genome editing, cell-specific functional and structural imaging, and neuronal activity manipulation will yield critical insights into ASD pathogenesis and fuel the emergence of new treatments.

Therapeutic Strategies in Fragile X Syndrome: From Bench to Bedside and Back

Fragile X syndrome (FXS), an inherited intellectual disability often associated with autism, is caused by the loss of expression of the fragile X mental retardation protein. Tremendous progress in basic, preclinical, and translational clinical research has elucidated a variety of molecular-, cellular-, and system-level defects in FXS. This has led to the development of several promising therapeutic strategies, some of which have been tested in larger-scale controlled clinical trials. Here, we will summarize recent advances in understanding molecular functions of fragile X mental retardation protein beyond the well-known role as an mRNA-binding protein, and will describe current developments and emerging limitations in the use of the FXS mouse model as a preclinical tool to identify therapeutic targets. We will review the results of recent clinical trials conducted in FXS that were based on some of the preclinical findings, and discuss how the observed outcomes and obstacles will inform future therapy development in FXS and other autism spectrum disorders.

Early social enrichment rescues adult behavioral and brain abnormalities in a mouse model of fragile X syndrome

Converging lines of evidence support the use of environmental stimulation to ameliorate the symptoms of a variety of neurodevelopmental disorders. Applying these interventions at very early ages is critical to achieve a marked reduction of the pathological phenotypes. Here we evaluated the impact of early social enrichment in Fmr1-KO mice, a genetic mouse model of fragile X syndrome (FXS), a major developmental disorder and the most frequent monogenic cause of autism. Enrichment was achieved by providing male KO pups and their WT littermates with enhanced social stimulation, housing them from birth until weaning with the mother and an additional nonlactating female. At adulthood they were tested for locomotor, social, and cognitive abilities; furthermore, dendritic alterations were assessed in the hippocampus and amygdala, two brain regions known to be involved in the control of the examined behaviors and affected by spine pathology in Fmr1-KOs. Enrichment rescued the behavioral FXS-like deficits displayed in adulthood by Fmr1-KO mice, that is, hyperactivity, reduced social interactions, and cognitive deficits. Early social enrichment also eliminated the abnormalities shown by adult KO mice in the morphology of hippocampal and amygdala dendritic spines, namely an enhanced density of immature vs mature types. Importantly, enrichment did not induce neurobehavioral changes in WT mice, thus supporting specific effects on FXS-like pathology. These findings show that early environmental stimulation has profound and long-term beneficial effects on the pathological FXS phenotype, thereby encouraging the use of nonpharmacological interventions for the treatment of this and perhaps other neurodevelopmental diseases.

Negative versus positive allosteric modulation of metabotropic glutamate receptors (mGluR5): indices for potential pro-cognitive drug properties based on EEG network oscillations and sleep-wake organization in rats

RATIONALE

Evidence is emerging that positive and negative modulation of the metabotropic glutamate (mGluR5) receptors has the potential for treating cognitive deficits and neuroprotection associated with psychiatric and neurodegenerative diseases, respectively. Sleep and synchronisation of disparate neuronal networks are critically involved in neuronal plasticity, and disturbance in vigilance states and cortical network connectivity contribute significantly to cognitive deficits described in schizophrenia and Alzheimer's disease. Here, we examined the circadian changes of mGluR5 density and the functional response to modulation of mGluR5 signaling.

METHODS

The current study carried out in Sprague-Dawley rats quantified the density of mGluR5 across the light-dark cycle with autoradiography. The central activity of mGluR5 negative allosteric modulators (2-methyl-6-(phenylethynyl)pyridine (MPEP) and [(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and positive allosteric modulators (S-(4-fluoro-phenyl)-{3-[3-(4-fluoro-phenyl)-[1,2,4]oxadiazol-5-yl]-piperidin-1-yl}-methanone (ADX47273) and (7S)-3-tert-butyl-7-[3-(4-fluoro-phenyl)-1,2,4-oxadiazol-5-yl]-5,6,7,8-tetrahydro[1,2,4]triazolo[4,3-a]pyridine (LSN2814617) was examined on sleep-wake architecture. The functional effect of mGluR5 modulation on cortical networks communication was described in freely moving animals.

RESULTS

The density of mGluR5 in the striatal, cortical, hippocampal and thalamic structures was unchanged across the light-dark cycle. Allosteric blockade of mGluR5 consistently consolidated deep sleep, enhanced sleep efficiency and elicited prominent functional coherent network activity in slow theta and gamma oscillations. However, allosteric activation of mGluR5 increased waking, decreased deep sleep and reduced functional network connectivity following the activation of slow alpha oscillatory activity.

CONCLUSION

This functional study differentiates the pharmacology of allosteric blockade of mGluR5 from that of allosteric activation and suggests that mGluR5 blockade enhances sleep and facilitates oscillatory network connectivity, both processes being known to have relevance in cognition processes.

Targeted pharmacological treatment of autism spectrum disorders: fragile X and Rett syndromes

Autism spectrum disorders (ASDs) are genetically and clinically heterogeneous and lack effective medications to treat their core symptoms. Studies of syndromic ASDs caused by single gene mutations have provided insights into the pathophysiology of autism. Fragile X and Rett syndromes belong to the syndromic ASDs in which preclinical studies have identified rational targets for drug therapies focused on correcting underlying neural dysfunction. These preclinical discoveries are increasingly translating into exciting human clinical trials. Since there are significant molecular and neurobiological overlaps among ASDs, targeted treatments developed for fragile X and Rett syndromes may be helpful for autism of different etiologies. Here, we review the targeted pharmacological treatment of fragile X and Rett syndromes and discuss related issues in both preclinical studies and clinical trials of potential therapies for the diseases.

Antipurinergic therapy corrects the autism-like features in the Fragile X (Fmr1 knockout) mouse model

Background

This study was designed to test a new approach to drug treatment of autism spectrum disorders (ASDs) in the Fragile X (Fmr1) knockout mouse model.

Methods

We used behavioral analysis, mass spectrometry, metabolomics, electron microscopy, and western analysis to test the hypothesis that the disturbances in social behavior, novelty preference, metabolism, and synapse structure are treatable with antipurinergic therapy (APT).

Results

Weekly treatment with the purinergic antagonist suramin (20 mg/kg intraperitoneally), started at 9 weeks of age, restored normal social behavior, and improved metabolism, and brain synaptosomal structure. Abnormalities in synaptosomal glutamate, endocannabinoid, purinergic, and IP3 receptor expression, complement C1q, TDP43, and amyloid β precursor protein (APP) were corrected. Comprehensive metabolomic analysis identified 20 biochemical pathways associated with symptom improvements. Seventeen pathways were shared with human ASD, and 11 were shared with the maternal immune activation (MIA) model of ASD. These metabolic pathways were previously identified as functionally related mediators of the evolutionarily conserved cell danger response (CDR).

Conclusions

The data show that antipurinergic therapy improves the multisystem, ASD-like features of both the environmental MIA, and the genetic Fragile X models. These abnormalities appeared to be traceable to mitochondria and regulated by purinergic signaling.

Electronic supplementary material

The online version of this article (doi:10.1186/2040-2392-6-1) contains supplementary material, which is available to authorized users.

The legacy of the benzodiazepine receptor: from flumazenil to enhancing cognition in Down syndrome and social interaction in autism

The study of the psychopharmacology of benzodiazepines continues to provide new insights into diverse brain functions related to vigilance, anxiety, mood, epileptiform activity, schizophrenia, cognitive performance, and autism-related social behavior. In this endeavor, the discovery of the benzodiazepine receptor was a key event, as it supplied the primary benzodiazepine drug-target site, provided the molecular link to the allosteric modulation of GABAA receptors and, following the recognition of GABAA receptor subtypes, furnished the platform for future, more selective drug actions. This review has two parts. In a retrospective first part, it acknowledges the contributions to the field made by my collaborators over the years, initially at Hoffmann-La Roche in Basle and later, in academia, at the University and the ETH of Zurich. In the second part, the new frontier of GABA pharmacology, targeting GABAA receptor subtypes, is reviewed with special focus on nonsedative anxiolytics, antidepressants, analgesics, as well as enhancers of cognition in Down syndrome and attenuators of symptoms of autism spectrum disorders. It is encouraging that a clinical trial has been initiated with a partial inverse agonist acting on α5 GABAA receptors in an attempt to alleviate the cognitive deficits in Down syndrome.

Spaced training rescues memory and ERK1/2 signaling in fragile X syndrome model mice

Recent studies have shown that short, spaced trains of afferent stimulation produce much greater long-term potentiation (LTP) than that obtained with a single, prolonged stimulation episode. The present studies demonstrate that spaced training regimens, based on these LTP timing rules, facilitate learning in wild-type (WT) mice and can offset learning and synaptic signaling impairments in the fragile X mental retardation 1 (Fmr1) knockout (KO) model of fragile X syndrome. We determined that 5 min of continuous training supports object location memory (OLM) in WT but not Fmr1 KO mice. However, the same amount of training distributed across three short trials, spaced by one hour, produced robust long-term memory in the KOs. At least three training trials were needed to realize the benefit of spacing, and intertrial intervals shorter or longer than 60 min were ineffective. Multiple short training trials also rescued novel object recognition in Fmr1 KOs. The spacing effect was surprisingly potent: just 1 min of OLM training, distributed across three trials, supported robust memory in both genotypes. Spacing also rescued training-induced activation of synaptic ERK1/2 in dorsal hippocampus of Fmr1 KO mice. These results show that a spaced training regimen designed to maximize synaptic potentiation facilitates recognition memory in WT mice and can offset synaptic signaling and memory impairments in a model of congenital intellectual disability.

Modeling fragile X syndrome in the Fmr1 knockout mouse

Fragile X Syndrome (FXS) is a commonly inherited form of intellectual disability and one of the leading genetic causes for autism spectrum disorder. Clinical symptoms of FXS can include impaired cognition, anxiety, hyperactivity, social phobia, and repetitive behaviors. FXS is caused by a CGG repeat mutation which expands a region on the X chromosome containing the FMR1 gene. In FXS, a full mutation (> 200 repeats) leads to hypermethylation of FMR1, an epigenetic mechanism that effectively silences FMR1 gene expression and reduces levels of the FMR1 gene product, fragile X mental retardation protein (FMRP). FMRP is an RNA-binding protein that is important for the regulation of protein expression. In an effort to further understand how loss of FMR1 and FMRP contribute to FXS symptomology, several FXS animal models have been created. The most well characterized rodent model is the Fmr1 knockout (KO) mouse, which lacks FMRP protein due to a disruption in its Fmr1 gene. Here, we review the behavioral phenotyping of the Fmr1 KO mouse to date, and discuss the clinical relevance of this mouse model to the human FXS condition. While much remains to be learned about FXS, the Fmr1 KO mouse is a valuable tool for understanding the repercussions of functional loss of FMRP and assessing the efficacy of pharmacological compounds in ameliorating the molecular and behavioral phenotypes relevant to FXS.

Mavoglurant as a treatment for Parkinson's disease

INTRODUCTION

A major unresolved issue in the Parkinson's disease (PD) treatment is the development of l-DOPA-induced dyskinesias (LIDs) as a side effect of chronic L-DOPA administration. Currently, LIDs are managed in part by reducing the L-DOPA dose or by the administration of amantadine. However, this treatment is only partially effective. A potential strategy, currently under investigation, is the coadministration of metabotropic glutamate receptor 5 (mGluR5) negative allosteric modulators (NAMs) and L-DOPA; a treatment that results in the improvement of dyskinesia symptoms and that permits reductions in l-DOPA dosage frequency.

AREAS COVERED

The authors examine the role of mGluR5 in the pathophysiology of PD and the potential use of mGluR5 NAM as an adjuvant therapy together with a primary treatment with L-DOPA. Specifically, the authors look at the mavoglurant therapy and the evidence presented through preclinical and clinical trials.

EXPERT OPINION

Interaction between mGluR5 NAM and L-DOPA is an area of interest in PD research as concomitant treatment results in the improvement of LID symptoms in humans, thus enhancing the patient's quality of life. However, few months ago, Novartis decided to discontinue clinical trials of mavoglurant for the treatment of LID, due to the lack of efficacy demonstrated in trials NCT01385592 and NCT01491529, although no safety concerns were involved in this decision. Nevertheless, the potential application of mGluR5 antagonists as neuroprotective agents must be considered and further studies are warranted to better investigate their potential.

New insights into the molecular pathophysiology of fragile X syndrome and therapeutic perspectives from the animal model

Fragile X syndrome is the most common monogenetic form of intellectual disability and is a leading cause of autism. This syndrome is produced by the reduced transcription of the fragile X mental retardation (FMR1) gene, and it is characterized by a range of symptoms heterogeneously expressed in patients such as cognitive impairment, seizure susceptibility, altered pain sensitivity and anxiety. The recent advances in the understanding of the pathophysiological mechanisms involved have opened novel potential therapeutic approaches identified in preclinical rodent models as a necessary preliminary step for the subsequent evaluation in patients. Among those possible therapeutic approaches, the modulation of the metabotropic glutamate receptor signaling or the GABA receptor signaling have focused most of the attention. New findings in the animal models open other possible therapeutic approaches such as the mammalian target of rapamycin signaling pathway or the endocannabinoid system. This review summarizes the emerging data recently obtained in preclinical models of fragile X syndrome supporting these new therapeutic perspectives.

Extinction of an instrumental response: a cognitive behavioral assay in Fmr1 knockout mice

Fragile X (FX) is the most common genetic cause of intellectual disability and autism. Previous studies have shown that partial inhibition of metabotropic glutamate receptor signaling is sufficient to correct behavioral phenotypes in a mouse model of FX, including audiogenic seizures, open-field hyperactivity and social behavior. These phenotypes model well the epilepsy (15%), hyperactivity (20%) and autism (30%) that are comorbid with FX in human patients. Identifying reliable and robust mouse phenotypes to model cognitive impairments is critical considering the 90% comorbidity of FX and intellectual disability. Recent work characterized a five-choice visuospatial discrimination assay testing cognitive flexibility, in which FX model mice show impairments associated with decreases in synaptic proteins in prefrontal cortex (PFC). In this study, we sought to determine whether instrumental extinction, another process requiring PFC, is altered in FX model mice, and whether downregulation of metabotropic glutamate receptor signaling pathways is sufficient to correct both visuospatial discrimination and extinction phenotypes. We report that instrumental extinction is consistently exaggerated in FX model mice. However, neither the extinction phenotype nor the visuospatial discrimination phenotype is corrected by approaches targeting metabotropic glutamate receptor signaling. This work describes a novel behavioral extinction assay to model impaired cognition in mouse models of neurodevelopmental disorders, provides evidence that extinction is exaggerated in the FX mouse model and suggests possible limitations of metabotropic glutamate receptor-based pharmacotherapy.

Degraded speech sound processing in a rat model of fragile X syndrome

Fragile X syndrome is the most common inherited form of intellectual disability and the leading genetic cause of autism. Impaired phonological processing in fragile X syndrome interferes with the development of language skills. Although auditory cortex responses are known to be abnormal in fragile X syndrome, it is not clear how these differences impact speech sound processing. This study provides the first evidence that the cortical representation of speech sounds is impaired in Fmr1 knockout rats, despite normal speech discrimination behavior. Evoked potentials and spiking activity in response to speech sounds, noise burst trains, and tones were significantly degraded in primary auditory cortex, anterior auditory field and the ventral auditory field. Neurometric analysis of speech evoked activity using a pattern classifier confirmed that activity in these fields contains significantly less information about speech sound identity in Fmr1 knockout rats compared to control rats. Responses were normal in the posterior auditory field, which is associated with sound localization. The greatest impairment was observed in the ventral auditory field, which is related to emotional regulation. Dysfunction in the ventral auditory field may contribute to poor emotional regulation in fragile X syndrome and may help explain the observation that later auditory evoked responses are more disturbed in fragile X syndrome compared to earlier responses. Rodent models of fragile X syndrome are likely to prove useful for understanding the biological basis of fragile X syndrome and for testing candidate therapies.

Impaired activity-dependent neural circuit assembly and refinement in autism spectrum disorder genetic models

Early-use activity during circuit-specific critical periods refines brain circuitry by the coupled processes of eliminating inappropriate synapses and strengthening maintained synapses. We theorize these activity-dependent (A-D) developmental processes are specifically impaired in autism spectrum disorders (ASDs). ASD genetic models in both mouse and Drosophila have pioneered our insights into normal A-D neural circuit assembly and consolidation, and how these developmental mechanisms go awry in specific genetic conditions. The monogenic fragile X syndrome (FXS), a common cause of heritable ASD and intellectual disability, has been particularly well linked to defects in A-D critical period processes. The fragile X mental retardation protein (FMRP) is positively activity-regulated in expression and function, in turn regulates excitability and activity in a negative feedback loop, and appears to be required for the A-D remodeling of synaptic connectivity during early-use critical periods. The Drosophila FXS model has been shown to functionally conserve the roles of human FMRP in synaptogenesis, and has been centrally important in generating our current mechanistic understanding of the FXS disease state. Recent advances in Drosophila optogenetics, transgenic calcium reporters, highly-targeted transgenic drivers for individually-identified neurons, and a vastly improved connectome of the brain are now being combined to provide unparalleled opportunities to both manipulate and monitor A-D processes during critical period brain development in defined neural circuits. The field is now poised to exploit this new Drosophila transgenic toolbox for the systematic dissection of A-D mechanisms in normal versus ASD brain development, particularly utilizing the well-established Drosophila FXS disease model.