Restoration of Mecp2 expression in GABAergic neurons is sufficient to rescue multiple disease features in a mouse model of Rett syndrome

Metabolic characterization of neurogenetic disorders involving glutamatergic neurotransmission

The study of inborn errors of neurotransmission has been mostly focused on monoamine disorders, GABAergic and glycinergic defects. The study of the glutamatergic synapse using the same approach than classic neurotransmitter disorders is challenging due to the lack of biomarkers in the CSF. A metabolomic approach can provide both insight into their molecular basis and outline novel therapeutic alternatives. We have performed a semi-targeted metabolomic analysis on CSF samples from 25 patients with neurogenetic disorders with an important expression in the glutamatergic synapse and 5 controls. Samples from patients diagnosed with MCP2, CDKL5-, GRINpathies and STXBP1-related encephalopathies were included. We have performed univariate (UVA) and multivariate statistical analysis (MVA), using Wilcoxon rank-sum test, principal component analysis (PCA), and OPLS-DA. By using the results of both analyses, we have identified the metabolites that were significantly altered and that were important in clustering the respective groups. On these, we performed pathway- and network-based analyses to define which metabolic pathways were possibly altered in each pathology. We have observed alterations in the tryptophan and branched-chain amino acid metabolism pathways, which interestingly converge on LAT1 transporter-dependency to cross the blood-brain barrier (BBB). Analysis of the expression of LAT1 transporter in brain samples from a mouse model of Rett syndrome (MECP2) revealed a decrease in the transporter expression, that was already noticeable at pre-symptomatic stages. The study of the glutamatergic synapse from this perspective advances the understanding of their pathophysiology, shining light on an understudied feature as is their metabolic signature.

Potentiation of the M 1 muscarinic acetylcholine receptor normalizes neuronal activation patterns and improves apnea severity in Mecp2+/- mice

Rett syndrome (RTT) is a neurodevelopmental disorder that is caused by loss-of-function mutations in the methyl-CpG binding protein 2 ( MeCP2 ) gene. RTT patients experience a myriad of debilitating symptoms, which include respiratory phenotypes that are often associated with lethality. Our previous work established that expression of the M 1 muscarinic acetylcholine receptor (mAchR) is decreased in RTT autopsy samples, and that potentiation of the M 1 receptor improves apneas in a mouse model of RTT; however, the population of neurons driving this rescue is unclear. Loss of Mecp2 correlates with excessive neuronal activity in cardiorespiratory nuclei. Since M 1 is found on cholinergic interneurons, we hypothesized that M 1 -potentiating compounds decrease apnea frequency by tempering brainstem hyperactivity. To test this, Mecp2 +/- and Mecp2 +/+ mice were screened for apneas before and after administration of the M 1 positive allosteric modulator (PAM) VU0453595 (VU595). Brains from the same mice were then imaged for c-Fos, ChAT, and Syto16 using whole-brain light-sheet microscopy to establish genotype and drug-dependent activation patterns that could be correlated with VU595's efficacy on apneas. The vehicle-treated Mecp2 +/- brain exhibited broad hyperactivity when coupled with the phenotypic prescreen, which was significantly decreased by administration of VU595, particularly in regions known to modulate the activity of respiratory nuclei (i.e. hippocampus and striatum). Further, the extent of apnea rescue in each mouse showed a significant positive correlation with c-Fos expression in non-cholinergic neurons in the striatum, thalamus, dentate gyrus, and within the cholinergic neurons of the brainstem. These results indicate that Mecp2 +/- mice are prone to hyperactivity in brain regions that regulate respiration, which can be normalized through M 1 potentiation.

Physical and functional convergence of the autism risk genes Scn2a and Ank2 in neocortical pyramidal cell dendrites

Dysfunction in sodium channels and their ankyrin scaffolding partners have both been implicated in neurodevelopmental disorders, including autism spectrum disorder (ASD). In particular, the genes SCN2A, which encodes the sodium channel NaV1.2, and ANK2, which encodes ankyrin-B, have strong ASD association. Recent studies indicate that ASD-associated haploinsufficiency in Scn2a impairs dendritic excitability and synaptic function in neocortical pyramidal cells, but how NaV1.2 is anchored within dendritic regions is unknown. Here, we show that ankyrin-B is essential for scaffolding NaV1.2 to the dendritic membrane of mouse neocortical neurons and that haploinsufficiency of Ank2 phenocopies intrinsic dendritic excitability and synaptic deficits observed in Scn2a+/- conditions. These results establish a direct, convergent link between two major ASD risk genes and reinforce an emerging framework suggesting that neocortical pyramidal cell dendritic dysfunction can contribute to neurodevelopmental disorder pathophysiology.

RIPK1 activation in Mecp2-deficient microglia promotes inflammation and glutamate release in RTT

Rett syndrome (RTT) is a devastating neurodevelopmental disorder primarily caused by mutations in the methyl-CpG binding protein 2 (Mecp2) gene. Here, we found that inhibition of Receptor-Interacting Serine/Threonine-Protein Kinase 1 (RIPK1) kinase ameliorated progression of motor dysfunction after onset and prolonged the survival of Mecp2-null mice. Microglia were activated early in myeloid Mecp2-deficient mice, which was inhibited upon inactivation of RIPK1 kinase. RIPK1 inhibition in Mecp2-deficient microglia reduced oxidative stress, cytokines production and induction of SLC7A11, SLC38A1, and GLS, which mediate the release of glutamate. Mecp2-deficient microglia release high levels of glutamate to impair glutamate-mediated excitatory neurotransmission and promote increased levels of GluA1 and GluA2/3 proteins in vivo, which was reduced upon RIPK1 inhibition. Thus, activation of RIPK1 kinase in Mecp2-deficient microglia may be involved both in the onset and progression of RTT.

Mitochondrial modulation with leriglitazone as a potential treatment for Rett syndrome

BACKGROUND

Rett syndrome is a neuropediatric disease occurring due to mutations in MECP2 and characterized by a regression in the neuronal development following a normal postnatal growth, which results in the loss of acquired capabilities such as speech or purposeful usage of hands. While altered neurotransmission and brain development are the center of its pathophysiology, alterations in mitochondrial performance have been previously outlined, shaping it as an attractive target for the disease treatment.

METHODS

We have thoroughly described mitochondrial performance in two Rett models, patients' primary fibroblasts and female Mecp2tm1.1Bird-/+ mice brain, discriminating between different brain areas. The characterization was made according to their bioenergetics function, oxidative stress, network dynamics or ultrastructure. Building on that, we have studied the effect of leriglitazone, a PPARγ agonist, in the modulation of mitochondrial performance. For that, we treated Rett female mice with 75 mg/kg/day leriglitazone from weaning until sacrifice at 7 months, studying both the mitochondrial performance changes and their consequences on the mice phenotype. Finally, we studied its effect on neuroinflammation based on the presence of reactive glia by immunohistochemistry and through a cytokine panel.

RESULTS

We have described mitochondrial alterations in Rett fibroblasts regarding both shape and bioenergetic functions, as they displayed less interconnected and shorter mitochondria and reduced ATP production along with increased oxidative stress. The bioenergetic alterations were recalled in Rett mice models, being especially significant in cerebellum, already detectable in pre-symptomatic stages. Treatment with leriglitazone recovered the bioenergetic alterations both in Rett fibroblasts and female mice and exerted an anti-inflammatory effect in the latest, resulting in the amelioration of the mice phenotype both in general condition and exploratory activity.

CONCLUSIONS

Our studies confirm the mitochondrial dysfunction in Rett syndrome, setting the differences through brain areas and disease stages. Its modulation through leriglitazone is a potential treatment for this disorder, along with other diseases with mitochondrial involvement. This work constitutes the preclinical necessary evidence to lead to a clinical trial.

Mecp2 deletion results in profound alterations of developmental and adult functional connectivity

As a regressive neurodevelopmental disorder with a well-established genetic cause, Rett syndrome and its Mecp2 loss-of-function mouse model provide an excellent opportunity to define potentially translatable functional signatures of disease progression, as well as offer insight into the role of Mecp2 in functional circuit development. Thus, we applied widefield optical fluorescence imaging to assess mesoscale calcium functional connectivity (FC) in the Mecp2 cortex both at postnatal day (P)35 in development and during the disease-related decline. We found that FC between numerous cortical regions was disrupted in Mecp2 mutant males both in juvenile development and early adulthood. Female Mecp2 mice displayed an increase in homotopic contralateral FC in the motor cortex at P35 but not in adulthood, where instead more posterior parietal regions were implicated. An increase in the amplitude of connection strength, both with more positive correlations and more negative anticorrelations, was observed across the male cortex in numerous functional regions. Widespread rescue of MeCP2 protein in GABAergic neurons rescued none of these functional deficits, nor, surprisingly, the expected male lifespan. Altogether, the female results identify early signs of disease progression, while the results in males indicate MeCP2 protein is required for typical FC in the brain.

Genetic and Protein Network Underlying the Convergence of Rett-Syndrome-like (RTT-L) Phenotype in Neurodevelopmental Disorders

Mutations of the X-linked gene encoding methyl-CpG-binding protein 2 (MECP2) cause classical forms of Rett syndrome (RTT) in girls. A subset of patients who are recognized to have an overlapping neurological phenotype with RTT but are lacking a mutation in a gene that causes classical or atypical RTT can be described as having a 'Rett-syndrome-like phenotype (RTT-L). Here, we report eight patients from our cohort diagnosed as having RTT-L who carry mutations in genes unrelated to RTT. We annotated the list of genes associated with RTT-L from our patient cohort, considered them in the light of peer-reviewed articles on the genetics of RTT-L, and constructed an integrated protein-protein interaction network (PPIN) consisting of 2871 interactions connecting 2192 neighboring proteins among RTT- and RTT-L-associated genes. Functional enrichment analysis of RTT and RTT-L genes identified a number of intuitive biological processes. We also identified transcription factors (TFs) whose binding sites are common across the set of RTT and RTT-L genes and appear as important regulatory motifs for them. Investigation of the most significant over-represented pathway analysis suggests that HDAC1 and CHD4 likely play a central role in the interactome between RTT and RTT-L genes.

Inhibitory Synaptic Influences on Developmental Motor Disorders

During development, GABA and glycine play major trophic and synaptic roles in the establishment of the neuromotor system. In this review, we summarise the formation, function and maturation of GABAergic and glycinergic synapses within neuromotor circuits during development. We take special care to discuss the differences in limb and respiratory neuromotor control. We then investigate the influences that GABAergic and glycinergic neurotransmission has on two major developmental neuromotor disorders: Rett syndrome and spastic cerebral palsy. We present these two syndromes in order to contrast the approaches to disease mechanism and therapy. While both conditions have motor dysfunctions at their core, one condition Rett syndrome, despite having myriad symptoms, has scientists focused on the breathing abnormalities and their alleviation-to great clinical advances. By contrast, cerebral palsy remains a scientific quagmire or poor definitions, no widely adopted model and a lack of therapeutic focus. We conclude that the sheer abundance of diversity of inhibitory neurotransmitter targets should provide hope for intractable conditions, particularly those that exhibit broad spectra of dysfunction-such as spastic cerebral palsy and Rett syndrome.

GENE TARGET: A framework for evaluating Mendelian neurodevelopmental disorders for gene therapy

Interest in gene-based therapies for neurodevelopmental disorders is increasing exponentially, driven by the rise in recognition of underlying genetic etiology, progress in genomic technology, and recent proof of concept in several disorders. The current prioritization of one genetic disorder over another for development of therapies is driven by competing interests of pharmaceutical companies, advocacy groups, and academic scientists. Although these are all valid perspectives, a consolidated framework will facilitate more efficient and rational gene therapy development. Here we outline features of Mendelian neurodevelopmental disorders that warrant consideration when determining suitability for gene therapy. These features fit into four broad domains: genetics, preclinical validation, clinical considerations, and ethics. We propose a simple mnemonic, GENE TARGET, to remember these features and illustrate how they could be scored using a preliminary scoring rubric. In this suggested rubric, for a given disorder, scores for each feature may be added up to a composite GENE TARGET suitability (GTS) score. In addition to proposing a systematic method to evaluate and compare disorders, our framework helps identify gaps in the translational pipeline for a given disorder, which can inform prioritization of future research efforts.

Reversibility and therapeutic development for neurodevelopmental disorders, insights from genetic animal models

Neurodevelopmental Disorders (NDDs) encompass a broad spectrum of conditions resulting from atypical brain development. Over the past decades, we have had the fortune to witness enormous progress in diagnosis, etiology discovery, modeling, and mechanistic understanding of NDDs from both fundamental and clinical research. Here, we review recent neurobiological advances from experimental models of NDDs. We introduce several examples and highlight breakthroughs in reversal studies of phenotypes using genetically engineered models of NDDs. The in-depth understanding of brain pathophysiology underlying NDDs and evaluations of reversibility in animal models paves the foundation for discovering novel treatment options. We discuss how the expanding property of cutting-edge technologies, such as gene editing and AAV-mediated gene delivery, are leveraged in animal models for the therapeutic development of NDDs. We envision opportunities and challenges toward faithful modeling and fruitful clinical translation.

MeCP2 deficiency impairs motor cortical circuit flexibility associated with motor learning

Loss of function mutations in the X-linked gene encoding methyl-CpG binding protein 2 (MECP2) cause Rett syndrome (RTT), a postnatal neurological disorder. The loss of motor function is an important clinical feature of RTT that manifests early during the course of the disease. RTT mouse models with mutations in the murine orthologous Mecp2 gene replicate many human phenotypes, including progressive motor impairments. However, relatively little is known about the changes in circuit function during the progression of motor deficit in this model. As the motor cortex is the key node in the motor system for the control of voluntary movement, we measured firing activity in populations of motor cortical neurons during locomotion on a motorized wheel-treadmill. Different populations of neurons intermingled in the motor cortex signal different aspects of the locomotor state of the animal. The proportion of running selective neurons whose activity positively correlates with locomotion speed gradually decreases with weekly training in wild-type mice, but not in Mecp2-null mice. The fraction of rest-selective neurons whose activity negatively correlates with locomotion speed does not change with training in wild-type mice, but is higher and increases with the progression of locomotion deficit in mutant mice. The synchronization of population activity that occurs in WT mice with training did not occur in Mecp2-null mice, a phenotype most clear during locomotion and observable across all functional cell types. Our results could represent circuit-level biomarkers for motor regression in Rett syndrome.

Selective Xi reactivation and alternative methods to restore MECP2 function in Rett syndrome

The human X-chromosome harbors only 4% of our genome but carries over 20% of genes associated with intellectual disability. Given that they inherit only one X-chromosome, males are more frequently affected by X-linked neurodevelopmental genetic disorders than females. However, despite inheriting two X-chromosomes, females can also be affected because X-chromosome inactivation enables only one of two X-chromosomes to be expressed per cell. For Rett syndrome and similar X-linked disorders affecting females, disease-specific treatments have remained elusive. However, a cure may be found within their own cells because every sick cell carries a healthy copy of the affected gene on the inactive X (Xi). Therefore, selective Xi reactivation may be a viable approach that would address the root cause of various X-linked disorders. Here, we discuss Rett syndrome and compare current approaches in the pharmaceutical pipeline to restore MECP2 function. We then focus on Xi reactivation and review available methods, lessons learned, and future directions.

Dysregulated cortical synaptic plasticity under methyl-CpG binding protein 2 deficiency and its implication in motor impairments

Caused by the mutation of methyl-CpG binding protein 2 (MeCP2), Rett syndrome leads to a battery of severe neural dysfunctions including the regression of motor coordination and motor learning. Current understanding has revealed the motor cortex as the critical region mediating voluntary movement. In this review article, we will summarize major findings from human patients and animal models regarding the cortical synaptic plasticity under the regulation of MeCP2. We will also discuss how mutation of MeCP2 leads to the disruption of cortical circuitry homeostasis to cause motor deficits. Lastly, potential values of physical exercise and neuromodulation approaches to recover neural plasticity and motor function will be evaluated. All of this evidence may help to accelerate timely diagnosis and effective interventions for Rett syndrome patients.

Paradoxical Hyperexcitability in Disorders of Neurodevelopment

Autism Spectrum Disorder (ASD), Rett syndrome (RTT) and Angelman Syndrome (AS) are neurodevelopmental disorders (NDDs) that share several clinical characteristics, including displays of repetitive movements, developmental delays, language deficits, intellectual disability, and increased susceptibility to epilepsy. While several reviews address the biological basis of non-seizure-related ASD phenotypes, here, I highlight some shared biological mechanisms that may contribute to increased seizure susceptibility. I focus on genetic studies identifying the anatomical origin of the seizure phenotype in loss-of-function, monogenic, mouse models of these NDDs, combined with insights gained from complementary studies quantifying levels of synaptic excitation and inhibition. Epilepsy is characterized by a sudden, abnormal increase in synchronous activity within neuronal networks, that is posited to arise from excess excitation, largely driven by reduced synaptic inhibition. Primarily for this reason, elevated network excitability is proposed to underlie the causal basis for the ASD, RTT, and AS phenotypes. Although, mouse models of these disorders replicate aspects of the human condition, i.e., hyperexcitability discharges or seizures on cortical electroencephalograms, measures at the synaptic level often reveal deficits in excitatory synaptic transmission, rather than too much excitation. Resolving this apparent paradox has direct implications regarding expected outcomes of manipulating GABAergic tone. In particular, in NDDs associated with seizures, cortical circuits can display reduced, rather than normal or increased levels of synaptic excitation, and therefore suggested treatments aimed at increasing inhibition could further promote hypoactivity instead of normality. In this review, I highlight shared mechanisms across animal models for ASD, RTT, and AS with reduced synaptic excitation that nevertheless promote hyperexcitability in cortical circuits.

A Critical Period for Development of Cerebellar-Mediated Autism-Relevant Social Behavior

The cerebellum has been increasingly implicated in autism spectrum disorder (ASD) with many ASD-linked genes impacting both cerebellar function and development. However, the precise timing and critical periods of when abnormal cerebellar neurodevelopment contributes to ASD-relevant behaviors remains poorly understood. In this study, we identify a critical period for the development of ASD-relevant behaviors in a cerebellar male mouse model of tuberous sclerosis complex (TSC), by using the mechanistic target of rapamycin (mTOR) inhibitor, rapamycin, to pharmacologically inhibit dysregulated downstream signaling. We find independent critical periods during which abnormal ASD-relevant behaviors develop for the two core ASD diagnostic criteria, social impairments and behavioral flexibility, and delineate an anatomic, physiological, and behavioral framework. These findings not only further our understanding of the genetic mechanisms underlying the timing of ASD-relevant behaviors but also have the capacity to inform potential therapies to optimize treatment interventions.SIGNIFICANCE STATEMENT No targeted treatments currently exist for autism spectrum disorder (ASD). This complex developmental disorder has established links to genetic and circuit aberrations, yet the precise timing and coordination of these underlying mechanisms that contribute to the spectrum of physiological and behavioral abnormalities remains unclear. Cerebellar pathology is consistently seen in ASD individuals; therefore, we sought to identify the specific windows for cerebellar involvement in the development of ASD-relevant behaviors. Using pharmacologic treatment paradigms, we outline distinct critical periods of developmental vulnerability for ASD-relevant social and inflexible behaviors. From this study, we posit a refined window of time during which ASD symptoms develop that will inform therapeutic timing.

Excitation and Inhibition Imbalance in Rett Syndrome

A loss of the excitation/inhibition (E/I) balance in the neural circuit has emerged as a common neuropathological feature in many neurodevelopmental disorders. Rett syndrome (RTT), a prevalent neurodevelopmental disorder that affects 1:10,000-15,000 women globally, is caused by loss-of-function mutations in the Methyl-CpG-binding Protein-2 (Mecp2) gene. E/I imbalance is recognized as the leading cellular and synaptic hallmark that is fundamental to diverse RTT neurological symptoms, including stereotypic hand movements, impaired motor coordination, breathing irregularities, seizures, and learning/memory dysfunctions. E/I balance in RTT is not homogeneously altered but demonstrates brain region and cell type specificity instead. In this review, I elaborate on the current understanding of the loss of E/I balance in a range of brain areas at molecular and cellular levels. I further describe how the underlying cellular mechanisms contribute to the disturbance of the proper E/I ratio. Last, I discuss current pharmacologic innervations for RTT and their role in modifying the E/I balance.

GABAergic System Dysfunction in Autism Spectrum Disorders

Autism spectrum disorder (ASD) refers to a series of neurodevelopmental diseases characterized by two hallmark symptoms, social communication deficits and repetitive behaviors. Gamma-aminobutyric acid (GABA) is one of the most important inhibitory neurotransmitters in the central nervous system (CNS). GABAergic inhibitory neurotransmission is critical for the regulation of brain rhythm and spontaneous neuronal activities during neurodevelopment. Genetic evidence has identified some variations of genes associated with the GABA system, indicating an abnormal excitatory/inhibitory (E/I) neurotransmission ratio implicated in the pathogenesis of ASD. However, the specific molecular mechanism by which GABA and GABAergic synaptic transmission affect ASD remains unclear. Transgenic technology enables translating genetic variations into rodent models to further investigate the structural and functional synaptic dysregulation related to ASD. In this review, we summarized evidence from human neuroimaging, postmortem, and genetic and pharmacological studies, and put emphasis on the GABAergic synaptic dysregulation and consequent E/I imbalance. We attempt to illuminate the pathophysiological role of structural and functional synaptic dysregulation in ASD and provide insights for future investigation.

Vitamin D modulates cortical transcriptome and behavioral phenotypes in an Mecp2 heterozygous Rett syndrome mouse model

Rett syndrome (RTT) is an X-linked neurological disorder caused by mutations in the transcriptional regulator MECP2. Mecp2 loss-of-function leads to the disruption of many cellular pathways, including aberrant activation of the NF-κB pathway. Genetically attenuating the NF-κB pathway in Mecp2-null mice ameliorates hallmark phenotypes of RTT, including reduced dendritic complexity, raising the question of whether NF-κB pathway inhibitors could provide a therapeutic avenue for RTT. Vitamin D is a known inhibitor of NF-κB signaling; further, vitamin D deficiency is prevalent in RTT patients and male Mecp2-null mice. We previously demonstrated that vitamin D rescues the aberrant NF-κB activity and reduced neurite outgrowth of Mecp2-knockdown cortical neurons in vitro, and that dietary vitamin D supplementation rescues decreased dendritic complexity and soma size of neocortical projection neurons in both male hemizygous Mecp2-null and female heterozygous mice in vivo. Here, we have identified over 200 genes whose dysregulated expression in the Mecp2+/- cortex is modulated by dietary vitamin D. Genes normalized with vitamin D supplementation are involved in dendritic complexity, synapses, and neuronal projections, suggesting that the rescue of their expression could underpin the rescue of neuronal morphology. Further, there is a disruption in the homeostasis of the vitamin D synthesis pathway in Mecp2+/- mice, and motor and anxiety-like behavioral phenotypes in Mecp2+/- mice correlate with circulating vitamin D levels. Thus, our data indicate that vitamin D modulates RTT pathology and its supplementation could provide a simple and cost-effective partial therapeutic for RTT.

JNK signaling provides a novel therapeutic target for Rett syndrome

Background

Rett syndrome (RTT) is a monogenic X-linked neurodevelopmental disorder characterized by loss-of-function mutations in the MECP2 gene, which lead to structural and functional changes in synapse communication, and impairments of neural activity at the basis of cognitive deficits that progress from an early age. While the restoration of MECP2 in animal models has been shown to rescue some RTT symptoms, gene therapy intervention presents potential side effects, and with gene- and RNA-editing approaches still far from clinical application, strategies focusing on signaling pathways downstream of MeCP2 may provide alternatives for the development of more effective therapies in vivo. Here, we investigate the role of the c-Jun N-terminal kinase (JNK) stress pathway in the pathogenesis of RTT using different animal and cell models and evaluate JNK inhibition as a potential therapeutic approach.

Results

We discovered that the c-Jun N-terminal kinase (JNK) stress pathway is activated in Mecp2-knockout, Mecp2-heterozygous mice, and in human MECP2-mutated iPSC neurons. The specific JNK inhibitor, D-JNKI1, promotes recovery of body weight and locomotor impairments in two mouse models of RTT and rescues their dendritic spine alterations. Mecp2-knockout presents intermittent crises of apnea/hypopnea, one of the most invalidating RTT pathological symptoms, and D-JNKI1 powerfully reduces this breathing dysfunction. Importantly, we discovered that also neurons derived from hiPSC-MECP2 mut show JNK activation, high-phosphorylated c-Jun levels, and cell death, which is not observed in the isogenic control wt allele hiPSCs. Treatment with D-JNKI1 inhibits neuronal death induced by MECP2 mutation in hiPSCs mut neurons.

Conclusions

As a summary, we found altered JNK signaling in models of RTT and suggest that D-JNKI1 treatment prevents clinical symptoms, with coherent results at the cellular, molecular, and functional levels. This is the first proof of concept that JNK plays a key role in RTT and its specific inhibition offers a new and potential therapeutic tool to tackle RTT.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01190-2.

Defining the nature of human pluripotent stem cell-derived interneurons via single-cell analysis

The specification of inhibitory neurons has been described for the mouse and human brain, and many studies have shown that pluripotent stem cells (PSCs) can be used to create interneurons in vitro. It is unclear whether in vitro methods to produce human interneurons generate all the subtypes found in brain, and how similar in vitro and in vivo interneurons are. We applied single-nuclei and single-cell transcriptomics to model interneuron development from human cortex and interneurons derived from PSCs. We provide a direct comparison of various in vitro interneuron derivation methods to determine the homogeneity achieved. We find that PSC-derived interneurons capture stages of development prior to mid-gestation, and represent a minority of potential subtypes found in brain. Comparison with those found in fetal or adult brain highlighted decreased expression of synapse-related genes. These analyses highlight the potential to tailor the method of generation to drive formation of particular subtypes.

Fluoxetine increases brain MeCP2 immuno-positive cells in a female Mecp2 heterozygous mouse model of Rett syndrome through endogenous serotonin

Motor skill deficit is a common and invalidating symptom of Rett syndrome (RTT), a rare disease almost exclusively affecting girls during the first/second year of life. Loss-of-function mutations of the methyl-CpG-binding protein2 (MECP2; Mecp2 in rodents) gene is the cause in most patients. We recently found that fluoxetine, a selective serotonin (5-HT) reuptake inhibitor and antidepressant drug, fully rescued motor coordination deficits in Mecp2 heterozygous (Mecp2 HET) mice acting through brain 5-HT. Here, we asked whether fluoxetine could increase MeCP2 expression in the brain of Mecp2 HET mice, under the same schedule of treatment improving motor coordination. Fluoxetine increased the number of MeCP2 immuno-positive (MeCP2⁺) cells in the prefrontal cortex, M1 and M2 motor cortices, and in dorsal, ventral and lateral striatum. Fluoxetine had no effect in the CA3 region of the hippocampus or in any of the brain regions of WT mice. Inhibition of 5-HT synthesis abolished the fluoxetine-induced rise of MeCP2⁺ cells. These findings suggest that boosting 5-HT transmission is sufficient to enhance the expression of MeCP2 in several brain regions of Mecp2 HET mice. Fluoxetine-induced rise of MeCP2 could potentially rescue motor coordination and other deficits of RTT.

Reviewing Evidence for the Relationship of EEG Abnormalities and RTT Phenotype Paralleled by Insights from Animal Studies

Rett syndrome (RTT) is a rare neurodevelopmental disorder that is usually caused by mutations of the MECP2 gene. Patients with RTT suffer from severe deficits in motor, perceptual and cognitive domains. Electroencephalogram (EEG) has provided useful information to clinicians and scientists, from the very first descriptions of RTT, and yet no reliable neurophysiological biomarkers related to the pathophysiology of the disorder or symptom severity have been identified to date. To identify consistently observed and potentially informative EEG characteristics of RTT pathophysiology, and ascertain areas most worthy of further systematic investigation, here we review the literature for EEG abnormalities reported in patients with RTT and in its disease models. While pointing to some promising potential EEG biomarkers of RTT, our review identify areas of need to realize the potential of EEG including (1) quantitative investigation of promising clinical-EEG observations in RTT, e.g., shift of mu rhythm frequency and EEG during sleep; (2) closer alignment of approaches between patients with RTT and its animal models to strengthen the translational significance of the work (e.g., EEG measurements and behavioral states); (3) establishment of large-scale consortium research, to provide adequate Ns to investigate age and genotype effects.

The role of GABAergic signalling in neurodevelopmental disorders

GABAergic inhibition shapes the connectivity, activity and plasticity of the brain. A series of exciting new discoveries provides compelling evidence that disruptions in a number of key facets of GABAergic inhibition have critical roles in the aetiology of neurodevelopmental disorders (NDDs). These facets include the generation, migration and survival of GABAergic neurons, the formation of GABAergic synapses and circuit connectivity, and the dynamic regulation of the efficacy of GABAergic signalling through neuronal chloride transporters. In this Review, we discuss recent work that elucidates the functions and dysfunctions of GABAergic signalling in health and disease, that uncovers the contribution of GABAergic neural circuit dysfunction to NDD aetiology and that leverages such mechanistic insights to advance precision medicine for the treatment of NDDs.

Deleting Mecp2 from the cerebellum rather than its neuronal subtypes causes a delay in motor learning in mice

Rett syndrome is a devastating childhood neurological disorder caused by mutations in MECP2. Of the many symptoms, motor deterioration is a significant problem for patients. In mice, deleting Mecp2 from the cortex or basal ganglia causes motor dysfunction, hypoactivity, and tremor, which are abnormalities observed in patients. Little is known about the function of Mecp2 in the cerebellum, a brain region critical for motor function. Here we show that deleting Mecp2 from the cerebellum, but not from its neuronal subtypes, causes a delay in motor learning that is overcome by additional training. We observed irregular firing rates of Purkinje cells and altered heterochromatin architecture within the cerebellum of knockout mice. These findings demonstrate that the motor deficits present in Rett syndrome arise, in part, from cerebellar dysfunction. For Rett syndrome and other neurodevelopmental disorders, our results highlight the importance of understanding which brain regions contribute to disease phenotypes.

Postnatal Arx transcriptional activity regulates functional properties of PV interneurons

The transcription factor Aristaless-related X-linked gene (Arx) is a monogenic factor in early onset epileptic encephalopathies (EOEEs) and a fundamental regulator of early stages of brain development. However, Arx expression persists in mature GABAergic neurons with an unknown role. To address this issue, we generated a conditional knockout (CKO) mouse in which postnatal Arx was ablated in parvalbumin interneurons (PVIs). Electroencephalogram (EEG) recordings in CKO mice revealed an increase in theta oscillations and the occurrence of occasional seizures. Behavioral analysis uncovered an increase in anxiety. Genome-wide sequencing of fluorescence activated cell sorted (FACS) PVIs revealed that Arx impinged on network excitability via genes primarily associated with synaptic and extracellular matrix pathways. Whole-cell recordings revealed prominent hypoexcitability of various intrinsic and synaptic properties. These results revealed important roles for postnatal Arx expression in PVIs in the control of neural circuits and that dysfunction in those roles alone can cause EOEE-like network abnormalities.

Loss of MeCP2 Function Across Several Neuronal Populations Impairs Breathing Response to Acute Hypoxia

Rett Syndrome (RTT) is a neurodevelopmental disorder caused by loss of function of the transcriptional regulator Methyl-CpG-Binding Protein 2 (MeCP2). In addition to the characteristic loss of hand function and spoken language after the first year of life, people with RTT also have a variety of physiological and autonomic abnormalities including disrupted breathing rhythms characterized by bouts of hyperventilation and an increased frequency of apnea. These breathing abnormalities, that likely involve alterations in both the circuitry underlying respiratory pace making and those underlying breathing response to environmental stimuli, may underlie the sudden unexpected death seen in a significant fraction of people with RTT. In fact, mice lacking MeCP2 function exhibit abnormal breathing rate response to acute hypoxia and maintain a persistently elevated breathing rate rather than showing typical hypoxic ventilatory decline that can be observed among their wild-type littermates. Using genetic and pharmacological tools to better understand the course of this abnormal hypoxic breathing rate response and the neurons driving it, we learned that the abnormal hypoxic breathing response is acquired as the animals mature, and that MeCP2 function is required within excitatory, inhibitory, and modulatory populations for a normal hypoxic breathing rate response. Furthermore, mice lacking MeCP2 exhibit decreased hypoxia-induced neuronal activity within the nucleus tractus solitarius of the dorsal medulla. Overall, these data provide insight into the neurons driving the circuit dysfunction that leads to breathing abnormalities upon loss of MeCP2. The discovery that combined dysfunction across multiple neuronal populations contributes to breathing dysfunction may provide insight into sudden unexpected death in RTT.

Vascular amyloid accumulation alters the gabaergic synapse and induces hyperactivity in a model of cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is typified by the cerebrovascular deposition of amyloid. The mechanisms underlying the contribution of CAA to neurodegeneration are not currently understood. Although CAA is highly associated with the accumulation of β‐amyloid (Aβ), other amyloids are known to associate with the vasculature. Alzheimer's disease (AD) is characterized by parenchymal Aβ deposition and intracellular accumulation of tau as neurofibrillary tangles (NFTs), affecting synapses directly, leading to behavioral and physical impairment. CAA increases with age and is present in 70%–97% of individuals with AD. Studies have overwhelmingly focused on the connection between parenchymal amyloid accumulation and synaptotoxicity; thus, the contribution of vascular amyloid is mostly understudied. Here, synaptic alterations induced by vascular amyloid accumulation and their behavioral consequences were characterized using a mouse model of Familial Danish dementia (FDD), a neurodegenerative disease characterized by the accumulation of Danish amyloid (ADan) in the vasculature. The mouse model (Tg‐FDD) displays a hyperactive phenotype that potentially arises from impairment in the GABAergic synapses, as determined by electrophysiological analysis. We demonstrated that the disruption of GABAergic synapse organization causes this impairment and provided evidence that GABAergic synapses are impaired in patients with CAA pathology. Understanding the mechanism that CAA contributes to synaptic dysfunction in AD‐related dementias is of critical importance for developing future therapeutic interventions.

Deficits in skilled motor and auditory learning in a rat model of Rett syndrome

BACKGROUND

Rett syndrome is an X-linked neurodevelopmental disorder caused by a mutation in the gene MECP2. Individuals with Rett syndrome display developmental regression at an early age, and develop a range of motor, auditory, cognitive, and social impairments. Several studies have successfully modeled some aspects of dysfunction and Rett syndrome-like phenotypes in transgenic mouse and rat models bearing mutations in the MECP2 gene. Here, we sought to extend these findings and characterize skilled learning, a more complex behavior known to be altered in Rett syndrome.

METHODS

We evaluated the acquisition and performance of auditory and motor function on two complex tasks in heterozygous female Mecp2 rats. Animals were trained to perform a speech discrimination task or a skilled forelimb reaching task.

RESULTS

Our results reveal that Mecp2 rats display slower acquisition and reduced performance on an auditory discrimination task than wild-type (WT) littermates. Similarly, Mecp2 rats exhibit impaired learning rates and worse performance on a skilled forelimb motor task compared to WT.

CONCLUSIONS

Together, these findings illustrate novel deficits in skilled learning consistent with clinical manifestation of Rett syndrome and provide a framework for development of therapeutic strategies to improve these complex behaviors.

Sleep disordered breathing and daytime hypoventilation in a male with MECP2 mutation

Rett syndrome (RTT, MIM * 312750) is an X-linked neurodevelopmental disorder caused by pathogenic variants at the Xq28 region involving the gene methyl-CpG-binding protein 2 (MECP2, MIM * 300005). The spectrum of MECP2-related phenotypes is wide and it ranges from asymptomatic female carriers to severe neonatal-onset encephalopathy in males. Abnormal breathing represents one of the leading features, but today little is known about polysomnographic features in RTT females; no data are available about males. We report the case of a male of Moroccan origins with a MECP2 pathogenic variant and a history of encephalopathy and severe breathing disturbances in the absence of dysmorphic features. For the first time we describe in detail the polysomnographic characteristics of a MECP2-mutated male and we show the relevance of severe central apneas, which may represent a new clinical clue to suggest the diagnosis. Moreover, we want to highlight the importance to maintain a high index of suspicion for MECP2-related disorders in the presence of severe hypotonia, apneic crises, and respiratory insufficiency in males to permit an earlier diagnosis and the consequent definition of recurrence risk of the family and to avoid other useless and invasive exams.

Cell-Type-Specific Gene Inactivation and In Situ Restoration via Recombinase-Based Flipping of Targeted Genomic Region

Conditional gene inactivation and restoration are powerful tools for studying gene functions in the nervous system and for modeling neuropsychiatric diseases. The combination of the two is necessary to interrogate specific cell types within defined developmental stages. However, very few methods and animal models have been developed for such purpose. Here we present a versatile method for conditional gene inactivation and in situ restoration through reversibly inverting a critical part of its endogenous genomic sequence by Cre- and Flp-mediated recombinations. Using this method, we generated a mouse model to manipulate Mecp2, an X-linked dosage-sensitive gene whose mutations cause Rett syndrome. Combined with multiple Cre- and Flp-expressing drivers and viral tools, we achieved efficient and reliable Mecp2 inactivation and restoration in the germline and several neuronal cell types, and demonstrated phenotypic reversal and prevention on cellular and behavioral levels in male mice. This study not only provides valuable tools and critical insights for Mecp2 and Rett syndrome, but also offers a generally applicable strategy to decipher other neurologic disorders.SIGNIFICANCE STATEMENT Studying neurodevelopment and modeling neurologic disorders rely on genetic tools, such as conditional gene regulation. We developed a new method to combine conditional gene inactivation and restoration on a single allele without disturbing endogenous expression pattern or dosage. We applied it to manipulate Mecp2, a gene residing on X chromosome whose malfunction leads to neurologic disease, including Rett syndrome. Our results demonstrated the efficiency, specificity, and versatility of this new method, provided valuable tools and critical insights for Mecp2 function and Rett syndrome research, and offered a generally applicable strategy to investigate other genes and genetic disorders.

Haploinsufficiency of Cyfip2 Causes Lithium-Responsive Prefrontal Dysfunction

OBJECTIVE

Genetic variants of the cytoplasmic FMR1-interacting protein 2 (CYFIP2) encoding an actin-regulatory protein are associated with brain disorders, including intellectual disability and epilepsy. However, specific in vivo neuronal defects and potential treatments for CYFIP2-associated brain disorders remain largely unknown. Here, we characterized Cyfip2 heterozygous (Cyfip2^+/- ) mice to understand their neurobehavioral phenotypes and the underlying pathological mechanisms. Furthermore, we examined a potential treatment for such phenotypes of the Cyfip2^+/- mice and specified a neuronal function mediating its efficacy.

METHODS

We performed behavioral analyses of Cyfip2^+/- mice. We combined molecular, ultrastructural, and in vitro and in vivo electrophysiological analyses of Cyfip2^+/- prefrontal neurons. We also selectively reduced CYFIP2 in the prefrontal cortex (PFC) of mice with virus injections.

RESULTS

Adult Cyfip2^+/- mice exhibited lithium-responsive abnormal behaviors. We found increased filamentous actin, enlarged dendritic spines, and enhanced excitatory synaptic transmission and excitability in the adult Cyfip2^+/- PFC that was restricted to layer 5 (L5) neurons. Consistently, adult Cyfip2^+/- mice showed increased seizure susceptibility and auditory steady-state responses from the cortical electroencephalographic recordings. Among the identified prefrontal defects, lithium selectively normalized the hyperexcitability of Cyfip2^+/- L5 neurons. RNA sequencing revealed reduced expression of potassium channel genes in the adult Cyfip2^+/- PFC. Virus-mediated reduction of CYFIP2 in the PFC was sufficient to induce L5 hyperexcitability and lithium-responsive abnormal behavior.

INTERPRETATION

These results suggest that L5-specific prefrontal dysfunction, especially hyperexcitability, underlies both the pathophysiology and the lithium-mediated amelioration of neurobehavioral phenotypes in adult Cyfip2^+/- mice, which can be implicated in CYFIP2-associated brain disorders. ANN NEUROL 2020;88:526-543.

Fluoxetine rescues rotarod motor deficits in Mecp2 heterozygous mouse model of Rett syndrome via brain serotonin

Motor skill is a specific area of disability of Rett syndrome (RTT), a rare disorder occurring almost exclusively in girls, caused by loss-of-function mutations of the X-linked methyl-CpG-binding protein2 (MECP2) gene, encoding the MECP2 protein, a member of the methyl-CpG-binding domain nuclear proteins family. Brain 5-HT, which is defective in RTT patients and Mecp2 mutant mice, regulates motor circuits and SSRIs enhance motor skill learning and plasticity. In the present study, we used heterozygous (Het) Mecp2 female and Mecp2-null male mice to investigate whether fluoxetine, a SSRI with pleiotropic effects on neuronal circuits, rescues motor coordination deficits. Repeated administration of 10 mg/kg fluoxetine fully rescued rotarod deficit in Mecp2 Het mice regardless of age, route of administration or pre-training to rotarod. The motor improvement was confirmed in the beam walking test while no effect was observed in the hanging-wire test, suggesting a preferential action of fluoxetine on motor coordination. Citalopram mimicked the effects of fluoxetine, while the inhibition of 5-HT synthesis abolished the fluoxetine-induced improvement of motor coordination. Mecp2 null mice, which responded poorly to fluoxetine in the rotarod, showed reduced 5-HT synthesis in the prefrontal cortex, hippocampus and striatum, and reduced efficacy of fluoxetine in raising extracellular 5-HT as compared to female mutants. No sex differences were observed in the ability of fluoxetine to desensitize 5-HT_1A autoreceptors upon repeated administration. These findings indicate that fluoxetine rescues motor coordination in Mecp2 Het mice through its ability to enhance brain 5-HT and suggest that drugs enhancing 5-HT neurotransmission may have beneficial effects on motor symptoms of RTT.

An Astrocytic Influence on Impaired Tonic Inhibition in Hippocampal CA1 Pyramidal Neurons in a Mouse Model of Rett Syndrome

Rett syndrome (RTT) is a severe neurodevelopmental disease caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene. Although altered interneuron development and function are clearly demonstrated in RTT mice, a particular mode of inhibition, tonic inhibition, has not been carefully examined. We report here that tonic inhibition is significantly reduced in pyramidal neurons in the CA1 region of the hippocampus in mice where Mecp2 is deleted either in all cells or specifically in astrocytes. Since no change is detected in the level of GABA receptors, such a reduction in tonic inhibition is likely a result of decreased ambient GABA level in the extracellular space. Consistent with this explanation, we observed increased expression of a GABA transporter, GABA transporter 3 (GAT3), in the hippocampus of the Mecp2 KO mice, as well as a corresponding increase of GAT3 current in hippocampal astrocytes. These phenotypes are relevant to RTT because pharmacological blockage of GAT3 can normalize tonic inhibition and intrinsic excitability in CA1 pyramidal neurons, and rescue the phenotype of increased network excitability in acute hippocampal slices from the Mecp2 KO mice. Finally, chronic administration of a GAT3 antagonist improved a composite symptom score and extended lifespan in the Mecp2 KO mice. Only male mice were used in this study. These results not only advance our understanding of RTT etiology by defining a new neuronal phenotype and revealing how it can be influenced by astrocytic alterations, but also reveal potential targets for intervention.SIGNIFICANCE STATEMENT Our study reports a novel phenotype of reduced tonic inhibition in hippocampal CA1 pyramidal neurons in the Rett syndrome mice, reveal a potential mechanism of increased GABA transporter expression/activity in the neighboring astrocytes, describe a disease-relevant consequence in hyperexcitability, and provide preliminary evidence that targeting this phenotype may slow down disease progression in Rett syndrome mice. These results help our understanding of the disease etiology and identify a new therapeutic target for treating Rett syndrome.

Intellectual and Developmental Disabilities Research Centers: A Multidisciplinary Approach to Understand the Pathogenesis of Methyl-CpG Binding Protein 2-related Disorders

Disruptions in the gene encoding methyl-CpG binding protein 2 (MECP2) underlie complex neurodevelopmental disorders including Rett Syndrome (RTT), MECP2 duplication disorder, intellectual disabilities, and autism. Significant progress has been made on the molecular and cellular basis of MECP2-related disorders providing a new framework for understanding how altered epigenetic landscape can derail the formation and refinement of neuronal circuits in early postnatal life and proper neurological function. This review will summarize selected major findings from the past years and particularly highlight the integrated and multidisciplinary work done at eight NIH-funded Intellectual and Developmental Disabilities Research Centers (IDDRC) across the US. Finally, we will outline a path forward with identification of reliable biomarkers and outcome measures, longitudinal preclinical and clinical studies, reproducibility of results across centers as a synergistic effort to decode and treat the pathogenesis of the complex MeCP2 disorders.

Developmental loss of MeCP2 from VIP interneurons impairs cortical function and behavior

Rett Syndrome is a devastating neurodevelopmental disorder resulting from mutations in the gene MECP2. Mutations of Mecp2 that are restricted to GABAergic cell types largely replicate the behavioral phenotypes associated with mouse models of Rett Syndrome, suggesting a pathophysiological role for inhibitory interneurons. Recent work has suggested that vasoactive intestinal peptide-expressing (VIP) interneurons may play a critical role in the proper development and function of cortical circuits, making them a potential key point of vulnerability in neurodevelopmental disorders. However, little is known about the role of VIP interneurons in Rett Syndrome. Here we find that loss of MeCP2 specifically from VIP interneurons replicates key neural and behavioral phenotypes observed following global Mecp2 loss of function.

Comprehensive Analysis of GABAA-A1R Developmental Alterations in Rett Syndrome: Setting the Focus for Therapeutic Targets in the Time Frame of the Disease

Rett syndrome, a serious neurodevelopmental disorder, has been associated with an altered expression of different synaptic-related proteins and aberrant glutamatergic and γ-aminobutyric acid (GABA)ergic neurotransmission. Despite its severity, it lacks a therapeutic option. Through this work we aimed to define the relationship between MeCP2 and GABAA.-A1 receptor expression, emphasizing the time dependence of such relationship. For this, we analyzed the expression of the ionotropic receptor subunit in different MeCP2 gene-dosage and developmental conditions, in cells lines, and in primary cultured neurons, as well as in different developmental stages of a Rett mouse model. Further, RNAseq and systems biology analysis was performed from post-mortem brain biopsies of Rett patients. We observed that the modulation of the MeCP2 expression in cellular models (both Neuro2a (N2A) cells and primary neuronal cultures) revealed a MeCP2 positive effect on the GABAA.-A1 receptor subunit expression, which did not occur in other proteins such as KCC2 (Potassium-chloride channel, member 5). In the Mecp2+/− mouse brain, both the KCC2 and GABA subunits expression were developmentally regulated, with a decreased expression during the pre-symptomatic stage, while the expression was variable in the adult symptomatic mice. Finally, the expression of the gamma-aminobutyric acid (GABA) receptor-related synaptic proteins from the postmortem brain biopsies of two Rett patients was evaluated, specifically revealing the GABA A1R subunit overexpression. The identification of the molecular changes along with the Rett syndrome prodromic stages strongly endorses the importance of time frame when addressing this disease, supporting the need for a neurotransmission-targeted early therapeutic intervention.

Sex differences in Mecp2-mutant Rett syndrome model mice and the impact of cellular mosaicism in phenotype development

There is currently no effective treatment for Rett syndrome (RTT), a severe X-linked progressive neurodevelopmental disorder caused by mutations in the transcriptional regulator MECP2. Because MECP2 is subjected to X-inactivation, most affected individuals are female heterozygotes who display cellular mosaicism for normal and mutant MECP2. Males who are hemizygous for mutant MECP2 are more severely affected than heterozygous females and rarely survive. Mecp2 loss-of-function is less severe in mice, however, and male hemizygous null mice not only survive until adulthood, they have been the most commonly studied model system. Although heterozygous female mice better recapitulate human RTT, they have not been as thoroughly characterized. This is likely because of the added experimental challenges that they present, including delayed and more variable phenotypic progression and cellular mosaicism due to X-inactivation. In this review, we compare phenotypes of Mecp2 heterozygous female mice and male hemizygous null mouse models. Further, we discuss the complexities that arise from the many cell-type and tissue-type specific roles of MeCP2, as well as the combination of cell-autonomous and non-cell-autonomous disruptions that result from Mecp2 loss-of-function. This is of particular importance in the context of the female heterozygous brain, composed of a mixture of MeCP2+ and MeCP2- cells, the ratio of which can alter RTT phenotypes in the case of skewed X-inactivation. The goal of this review is to provide a clearer understanding of the pathophysiological differences between the mouse models, which is an essential consideration in the design of future pre-clinical studies.

The distinct methylation landscape of maturing neurons and its role in Rett syndrome pathogenesis

Rett syndrome (RTT) is one of the most common causes of intellectual and developmental disabilities in girls, and is caused by mutations in the gene encoding methyl-CpG binding protein 2 (MECP2). Here we will review our current understanding of RTT, the landscape of pathogenic mutations and function of MeCP2, and culminate with recent advances elucidating the distinct DNA methylation landscape in the brain that may explain why disease symptoms are delayed and selective to the nervous system.

Early Postnatal Treatment with Valproate Induces Gad1 Promoter Remodeling in the Brain and Reduces Apnea Episodes in Mecp2-Null Mice

The deletion of Mecp2, the gene encoding methyl-CpG-binding protein 2, causes severe breathing defects and developmental anomalies in mammals. In Mecp2-null mice, impaired GABAergic neurotransmission is demonstrated at the early stage of life. GABAergic dysfunction in neurons in the rostral ventrolateral medulla (RVLM) is considered as a primary cause of breathing abnormality in Mecp2-null mice, but its molecular mechanism is unclear. Here, we report that mRNA expression levels of Gad1, which encodes glutamate decarboxylase 67 (GAD67), in the RVLM of Mecp2-null (Mecp2^-/y, B6.129P2(C)-Mecp2^tm1.1Bird/J) mice is closely related to the methylation status of its promoter, and valproate (VPA) can upregulate transcription from Gad1 through epigenetic mechanisms. The administration of VPA (300 mg/kg/day) together with L-carnitine (30 mg/kg/day) from day 8 to day 14 after birth increased Gad1 mRNA expression in the RVLM and reduced apnea counts in Mecp2^-/y mice on postnatal day 15. Cytosine methylation levels in the Gad1 promoter were higher in the RVLM of Mecp2^-/y mice compared to wild-type mice born to C57BL/6J females, while VPA treatment decreased the methylation levels in Mecp2^-/y mice. Chromatin immunoprecipitation assay revealed that the VPA treatment reduced the binding of methyl-CpG binding domain protein 1 (MBD1) to the Gad1 promoter in Mecp2^-/y mice. These results suggest that VPA improves breathing of Mecp2^-/y mice by reducing the Gad1 promoter methylation, which potentially leads to the enhancement of GABAergic neurotransmission in the RVLM.

Towards a better diagnosis and treatment of Rett syndrome: a model synaptic disorder

With the recent 50th anniversary of the first publication on Rett syndrome, and the almost 20 years since the first report on the link between Rett syndrome and MECP2 mutations, it is important to reflect on the tremendous advances in our understanding and their implications for the diagnosis and treatment of this neurodevelopmental disorder. Rett syndrome features an interesting challenge for biologists and clinicians, as the disorder lies at the intersection of molecular mechanisms of epigenetic regulation and neurophysiological alterations in synapses and circuits that together contribute to severe pathophysiological endophenotypes. Genetic, clinical, and neurobiological evidences support the notion that Rett syndrome is primarily a synaptic disorder, and a disease model for both intellectual disability and autism spectrum disorder. This review examines major developments in both recent neurobiological and preclinical findings of Rett syndrome, and to what extent they are beginning to impact our understanding and management of the disorder. It also discusses potential applications of knowledge on synaptic plasticity abnormalities in Rett syndrome to its diagnosis and treatment.

Rett syndrome: insights into genetic, molecular and circuit mechanisms

Rett syndrome (RTT) is a severe neurological disorder caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). Almost two decades of research into RTT have greatly advanced our understanding of the function and regulation of the multifunctional protein MeCP2. Here, we review recent advances in understanding how loss of MeCP2 impacts different stages of brain development, discuss recent findings demonstrating the molecular role of MeCP2 as a transcriptional repressor, assess primary and secondary effects of MeCP2 loss and examine how loss of MeCP2 can result in an imbalance of neuronal excitation and inhibition at the circuit level along with dysregulation of activity-dependent mechanisms. These factors present challenges to the search for mechanism-based therapeutics for RTT and suggest specific approaches that may be more effective than others.

Pharmacological enhancement of KCC2 gene expression exerts therapeutic effects on human Rett syndrome neurons and Mecp2 mutant mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the methyl CpG binding protein 2 (MECP2) gene. There are currently no approved treatments for RTT. The expression of K+/Cl- cotransporter 2 (KCC2), a neuron-specific protein, has been found to be reduced in human RTT neurons and in RTT mouse models, suggesting that KCC2 might play a role in the pathophysiology of RTT. To develop neuron-based high-throughput screening (HTS) assays to identify chemical compounds that enhance the expression of the KCC2 gene, we report the generation of a robust high-throughput drug screening platform that allows for the rapid assessment of KCC2 gene expression in genome-edited human reporter neurons. From an unbiased screen of more than 900 small-molecule chemicals, we have identified a group of compounds that enhance KCC2 expression termed KCC2 expression-enhancing compounds (KEECs). The identified KEECs include U.S. Food and Drug Administration-approved drugs that are inhibitors of the fms-like tyrosine kinase 3 (FLT3) or glycogen synthase kinase 3β (GSK3β) pathways and activators of the sirtuin 1 (SIRT1) and transient receptor potential cation channel subfamily V member 1 (TRPV1) pathways. Treatment with hit compounds increased KCC2 expression in human wild-type (WT) and isogenic MECP2 mutant RTT neurons, and rescued electrophysiological and morphological abnormalities of RTT neurons. Injection of KEEC KW-2449 or piperine in Mecp2 mutant mice ameliorated disease-associated respiratory and locomotion phenotypes. The small-molecule compounds described in our study may have therapeutic effects not only in RTT but also in other neurological disorders involving dysregulation of KCC2.

From Cannabinoids and Neurosteroids to Statins and the Ketogenic Diet: New Therapeutic Avenues in Rett Syndrome?

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused mainly by mutations in the MECP2 gene, being one of the leading causes of mental disability in females. Mutations in the MECP2 gene are responsible for 95% of the diagnosed RTT cases and the mechanisms through which these mutations relate with symptomatology are still elusive. Children with RTT present a period of apparent normal development followed by a rapid regression in speech and behavior and a progressive deterioration of motor abilities. Epilepsy is one of the most common symptoms in RTT, occurring in 60 to 80% of RTT cases, being associated with worsening of other symptoms. At this point, no cure for RTT is available and there is a pressing need for the discovery of new drug candidates to treat its severe symptoms. However, despite being a rare disease, in the last decade research in RTT has grown exponentially. New and exciting evidence has been gathered and the etiopathogenesis of this complex, severe and untreatable disease is slowly being unfolded. Advances in gene editing techniques have prompted cure-oriented research in RTT. Nonetheless, at this point, finding a cure is a distant reality, highlighting the importance of further investigating the basic pathological mechanisms of this disease. In this review, we focus our attention in some of the newest evidence on RTT clinical and preclinical research, evaluating their impact in RTT symptomatology control, and pinpointing possible directions for future research.

MeCP2: an epigenetic regulator of critical periods

Complex adult behaviors arise from the integration of sequential and often overlapping critical periods (CPs) early in life and adolescence. These processes rely on a subtle interplay between the set of genes inherited from the parents, the surrounding environment and epigenetic regulation. Methyl-CpG-binding protein 2 (MeCP2) has been shown to recognize epigenetic states and regulate gene expression by reading methylated DNA. Here, we will review the recent findings revealing the role of MeCP2 during postnatal CPs of development using mouse models of Rett (RTT) syndrome.

Rett syndrome before regression: A time window of overlooked opportunities for diagnosis and intervention

Rett syndrome (RTT) is a rare neurological disorder primarily affecting females, causing severe cognitive, social, motor and physiological impairments for which no cure currently exists. RTT clinical diagnosis is based on the peculiar progression of the disease, since patients show an apparently normal initial development with a subsequent sudden regression at around 2 years of age. Accumulating evidences are rising doubts regarding the absence of early impairments, hence questioning the concept of regression. We reviewed the published literature addressing the pre-symptomatic stage of the disease in both patients and animal models with a particular focus on behavioral, physiological and brain abnormalities. The emerging picture delineates subtle, but reliable impairments that precede the onset of overt symptoms whose bases are likely set up already during embryogenesis. Some of the outlined alterations appear transient, suggesting compensatory mechanisms to occur in the course of development. There is urgent need for more systematic developmental analyses able to detect early pathological markers to be used as diagnostic tools and precocious targets of time-specific interventions.

Nano-scale analysis of synapse morphology in an autism mouse model with 15q11-13 copy number variation using focused ion beam milling and scanning electron microscopy

Circuit-level alternations in patients of autism spectrum disorder (ASD) is under active investigation and detailed characterization of synapse morphology in ASD model mice should be informative. We utilized focused ion beam milling and scanning electron microscopy (FIB-SEM) to obtain three-dimensional images of synapses in the layer 2/3 of the somatosensory cortex from a mouse model for ASD with human 15q11-13 chromosomal duplication (15q dup mice). We found a trend of higher spine density and a higher fraction of astrocytic contact with both spine and shaft synapses in 15q dup mice. Measurement of spine synapse structure indicated that the size of the post-synaptic density (PSD), spine head volume, spine head width and spine neck width were smaller in 15q dup mice. Categorization of spine synapses into five classes suggested a trend of less frequent mushroom spines in 15q dup mice. These results suggest relative increase in excitatory synapses with immature morphology but more astrocytic contacts in 15q dup mice, which may be linked to enhanced synapse turnover seen in ASD mouse models.

Reduced computational modelling of Kölliker-Fuse contributions to breathing patterns in Rett syndrome

KEY POINTS

Reduced computational models are used to test effects of loss of inhibition to the Kölliker-Fuse nucleus (KFn). Three reduced computational models that simulate eupnoeic and vagotomized respiratory rhythms are considered. All models exhibit the emergence of respiratory perturbations associated with Rett syndrome as inhibition to the KFn is diminished. Simulations suggest that application of 5-HT_1A agonists can mitigate the respiratory pathology. The three models can be distinguished and tested based on their predictions about connections and dynamics within the respiratory circuit and about effects of perturbations on certain respiratory neuron populations.

ABSTRACT

Rett syndrome (RTT) is a developmental disorder that can lead to respiratory disturbances featuring prolonged apnoeas of variable durations. Determining the mechanisms underlying these effects at the level of respiratory neural circuits would have significant implications for treatment efforts and would also enhance our understanding of respiratory rhythm generation and control. While experimental studies have suggested possible factors contributing to the respiratory patterns of RTT, we take a novel computational approach to the investigation of RTT, which allows for direct manipulation of selected system parameters and testing of specific hypotheses. Specifically, we present three reduced computational models, developed using an established framework, all of which successfully simulate respiratory outputs across eupnoeic and vagotomized conditions. All three models show that loss of inhibition to the Kölliker-Fuse nucleus reproduces the key respiratory alterations associated with RTT and, as suggested experimentally, that effects of 5-HT_1A agonists on the respiratory neural circuit suffice to alleviate this respiratory pathology. Each of the models makes distinct predictions regarding the neuronal populations and interactions underlying these effects, suggesting natural directions for future experimental testing.

Striatal Inhibition of MeCP2 or TSC1 Produces Sociability Deficits and Repetitive Behaviors

Autism spectrum disorder (ASD) is a heterogeneous group of neurobehavioral disorders characterized by the two core domains of behavioral deficits, including sociability deficits and stereotyped repetitive behaviors. It is not clear whether the core symptoms of ASD are produced by dysfunction of the overall neural network of the brain or that of a limited brain region. Recent studies reported that excessive glutamatergic or dopaminergic inputs in the dorsal striatum induced sociability deficits and repetitive behaviors. These findings suggest that the dorsal striatum plays a crucial role in autistic-like behaviors. The present study addresses whether functional deficits of well-known ASD-related genes in the dorsal striatum also produce ASD core symptoms. This study also examines whether these behavioral changes can be modulated by rebalancing glutamate and/or dopamine receptor activity in the dorsal striatum. First, we found that the siRNA-mediated inhibition of Shank3, Nlgn3, Fmr1, Mecp2, or Tsc1 in the dorsal striatum produced mild to severe behavioral changes in sociability, cognition, and/or repetitive behaviors. The knockdown effects of Mecp2 and Tsc1 on behavioral changes were the most prominent. Next, we demonstrated that behavioral changes induced by striatal inhibition of MeCP2 and TSC1 were rescued by D-cycloserine (an NMDA agonist), fenobam (an mGluR5 antagonist), SCH23390 (a D1 antagonist), and/or ecopipam (a D1 partial antagonist), pharmacological drugs that are known to regulate ASD-like symptoms in animal models. Collectively, these results suggest that the dorsal striatum is a critical brain region that, when dysfunctional, produces the core symptoms of ASD.

Psychomotor Dysfunction in Rett Syndrome: Insights into the Neurochemical and Circuit Roots

Rett syndrome (RTT) is a monogenic neurodevelopmental disorder caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene. Patients with RTT develop symptoms after 6-18 months of age, exhibiting characteristic movement deficits, such as ambulatory difficulties and loss of hand skills, in addition to breathing abnormalities and intellectual disability. Given the striking psychomotor dysfunction, numerous studies have investigated the underlying neurochemical and circuit mechanisms from different aspects. Here, I review the evidence linking MeCP2 deficiency to alterations in neurotransmission and neural circuits that govern the psychomotor function and discuss a recently identified pathological origin underlying the psychomotor deficits in RTT.

Pharmacological reactivation of inactive X-linked Mecp2 in cerebral cortical neurons of living mice

Rett syndrome (RTT) is a genetic disorder resulting from a loss-of-function mutation in one copy of the X-linked gene methyl-CpG-binding protein 2 (MECP2). Typical RTT patients are females and, due to random X chromosome inactivation (XCI), ∼50% of cells express mutant MECP2 and the other ∼50% express wild-type MECP2. Cells expressing mutant MECP2 retain a wild-type copy of MECP2 on the inactive X chromosome (Xi), the reactivation of which represents a potential therapeutic approach for RTT. Previous studies have demonstrated reactivation of Xi-linked MECP2 in cultured cells by biological or pharmacological inhibition of factors that promote XCI (called "XCI factors" or "XCIFs"). Whether XCIF inhibitors in living animals can reactivate Xi-linked MECP2 in cerebral cortical neurons, the cell type most therapeutically relevant to RTT, remains to be determined. Here, we show that pharmacological inhibitors targeting XCIFs in the PI3K/AKT and bone morphogenetic protein signaling pathways reactivate Xi-linked MECP2 in cultured mouse fibroblasts and human induced pluripotent stem cell-derived postmitotic RTT neurons. Notably, reactivation of Xi-linked MECP2 corrects characteristic defects of human RTT neurons including reduced soma size and branch points. Most importantly, we show that intracerebroventricular injection of the XCIF inhibitors reactivates Xi-linked Mecp2 in cerebral cortical neurons of adult living mice. In support of these pharmacological results, we also demonstrate genetic reactivation of Xi-linked Mecp2 in cerebral cortical neurons of living mice bearing a homozygous XCIF deletion. Collectively, our results further establish the feasibility of pharmacological reactivation of Xi-linked MECP2 as a therapeutic approach for RTT.