The impact of MeCP2 loss- or gain-of-function on synaptic plasticity

Human microglial cells as a therapeutic target in a neurodevelopmental disease model

Although microglia are macrophages of the central nervous system, their involvement is not limited to immune functions. The roles of microglia during development in humans remain poorly understood due to limited access to fetal tissue. To understand how microglia can impact human neurodevelopment, the methyl-CpG binding protein 2 (MECP2) gene was knocked out in human microglia-like cells (MGLs). Disruption of the MECP2 in MGLs led to transcriptional and functional perturbations, including impaired phagocytosis. The co-culture of healthy MGLs with MECP2-knockout (KO) neurons rescued synaptogenesis defects, suggesting a microglial role in synapse formation. A targeted drug screening identified ADH-503, a CD11b agonist, restored phagocytosis and synapse formation in spheroid-MGL co-cultures, significantly improved disease progression, and increased survival in MeCP2-null mice. These results unveil a MECP2-specific regulation of human microglial phagocytosis and identify a novel therapeutic treatment for MECP2-related conditions.

Epigenome editing technologies for discovery and medicine

Epigenome editing has rapidly evolved in recent years, with diverse applications that include elucidating gene regulation mechanisms, annotating coding and noncoding genome functions and programming cell state and lineage specification. Importantly, given the ubiquitous role of epigenetics in complex phenotypes, epigenome editing has unique potential to impact a broad spectrum of diseases. By leveraging powerful DNA-targeting technologies, such as CRISPR, epigenome editing exploits the heritable and reversible mechanisms of epigenetics to alter gene expression without introducing DNA breaks, inducing DNA damage or relying on DNA repair pathways.

Imbalance between hippocampal projection cell and parvalbumin interneuron architecture increases epileptic susceptibility in mouse model of methyl CpG binding protein 2 duplication syndrome

OBJECTIVE

Methyl CpG-binding protein 2 (MECP2) duplication syndrome is a rare X-linked genomic disorder affecting predominantly males, which is usually manifested as epilepsy and autism spectrum disorder (ASD) comorbidity. The transgenic line MeCP2Tg1 was used for mimicking MECP2 duplication syndrome and showed autism-epilepsy co-occurrence. Previous works suggested that the excitatory/inhibitory (E/I) imbalance is a potential common mechanism for both epilepsy and ASD. The projection neurons and parvalbumin (PV) interneurons account for the majority of E/I balance in the hippocampus. Therefore, we explored how structural changes of projection and PV+ neurons occur in the hippocampus of MeCP2Tg1 mice and whether these morphological changes contribute to epilepsy susceptibility.

METHODS

We used the interneuron Designer receptors exclusively activated by designer drugs mouse model to inhibit inhibitory neurons in the hippocampus to verify the epilepsy susceptibility of MeCP2Tg1 (FVB, an inbred strain named as sensitivity to Friend leukemia virus) mice. Electroencephalograms were recorded for the definition of seizure. We performed retro-orbital injection of virus in MeCP2Tg1 (FVB):CaMKIIα-Cre (C57BL/6) mice or MeCP2Tg1:PV-Cre (C57BL/6) mice and their littermate controls to specifically label projection and PV+ neurons for structural analysis.

RESULTS

Epilepsy susceptibility was increased in MeCP2Tg1 mice. There was a reduced number of PV neurons and reduced dendritic complexity in the hippocampus of MeCP2Tg1 mice. The dendritic complexity in MeCP2Tg1 mice was increased compared to wild-type mice, and total dendritic spine density in dentate gyrus of MeCP2Tg1 mice was also increased. Total dendritic spine density was increased in CA1 of MeCP2Tg1 mice.

SIGNIFICANCE

Overexpression of MeCP2 may disrupt crucial signaling pathways, resulting in decreased dendritic complexity of PV interneurons and increased dendritic spine density of projection neurons. This reciprocal modulation of excitatory and inhibitory neuronal structures associated with MeCP2 implies its significance as a potential target in the development of epilepsy and offers a novel perspective on the co-occurrence of autism and epilepsy.

Mecp2 Deficiency in Peripheral Sensory Neuron Improves Cognitive Function by Enhancing Hippocampal Dendritic Spine Densities in Mice

Methyl-CpG-binding protein 2 (Mecp2) is an epigenetic modulator and numerous studies have explored its impact on the central nervous system manifestations. However, little attention has been given to its potential contributions to the peripheral nervous system (PNS). To investigate the regulation of Mecp2 in the PNS on specific central regions, we generated Mecp2fl/flAdvillincre mice with the sensory-neuron-specific deletion of the Mecp2 gene and found the mutant mice had a heightened sensitivity to temperature, which, however, did not affect the sense of motion, social behaviors, and anxiety-like behavior. Notably, in comparison to Mecp2fl/fl mice, Mecp2fl/flAdvillincre mice exhibited improved learning and memory abilities. The levels of hippocampal synaptophysin and PSD95 proteins were higher in Mecp2fl/flAdvillincre mice than in Mecp2fl/fl mice. Golgi staining revealed a significant increase in total spine density, and dendritic arborization in the hippocampal pyramidal neurons of Mecp2fl/flAdvillincre mice compared to Mecp2fl/fl mice. In addition, the activation of the BDNF-TrkB-CREB1 pathway was observed in the hippocampus and spinal cord of Mecp2fl/flAdvillincre mice. Intriguingly, the hippocampal BDNF/CREB1 signaling pathway in mutant mice was initiated within 5 days after birth. Our findings suggest a potential therapeutic strategy targeting the BDNF-TrkB-CREB1 signaling pathway and peripheral somasensory neurons to treat learning and cognitive deficits associated with Mecp2 disorders.

Tactile sensory processing deficits in genetic mouse models of autism spectrum disorder

Altered sensory processing is a common feature in autism spectrum disorder (ASD), as recognized in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). Although altered responses to tactile stimuli are observed in over 60% of individuals with ASD, the neurobiological basis of this phenomenon is poorly understood. ASD has a strong genetic component and genetic mouse models can provide valuable insights into the mechanisms underlying tactile abnormalities in ASD. This review critically addresses recent findings regarding tactile processing deficits found in mouse models of ASD, with a focus on behavioral, anatomical, and functional alterations. Particular attention was given to cellular and circuit-level functional alterations, both in the peripheral and central nervous systems, with the objective of highlighting possible convergence mechanisms across models. By elucidating the impact of mutations in ASD candidate genes on somatosensory circuits and correlating them with behavioral phenotypes, this review significantly advances our understanding of tactile deficits in ASD. Such insights not only broaden our comprehension but also pave the way for future therapeutic interventions.

Reciprocal crosslink among MeCP2/BDNF /CREB signaling pinpointed in autism spectrum disorder

Autism spectrum disorder, or individual disability (ID), is a condition characterized by complications in social interaction, restricted repetitive behavior, and difficulties in social communication. Neuquinon (NQ) possess a powerful therapeutic potential in various neurodegenerative disease. Nevertheless, contributing to NQ's low water solubility and bioavailability, its medicinal use has been constrained. Liposomes were supposed to be prospective drug-delivering agents for NQ, crossing the blood-brain barrier (BBB), and reaching the target organs. The current investigation aims to track the signaling pathways that govern NQ and liposomal neuquinon (LNQ) action in autistic models generated by ethyl formic acid. The neurotransmitters gamma amino-butyric acid (GABA), acetylcholine (ACh), and acetylcholinesterase (AChE) in addition to, the gene expressions of brain-derived neurotrophic factor (BDNF), cAMP response element-binding protein (CREB), and methyl-CpG-binding protein 2 (MeCP2) and the DNA damage COMET analysis at different time intervals of the study, were assessed. EFA in a dose of 500 mg/kg BW was used to induce autism in rats, and then NQ and LNQ were administered in 10 mg/kg and 2 mg/kg BW, respectively. The results revealed that NQ and LNQ significantly down-regulated BDNF, GABA, and AChE; on the other hand, they up-regulated MeCP2, CREB gene expressions, and ACh action. NQ and LNQ displayed improvement in DNA damage in almost all brain regions after EFA alterations; even better results were noticed post-LNQ therapy. Therefore, it may be concluded that neuquinon and liposomal-loaded neuquinon have a therapeutic index versus EFA-induced autism in a rat model.

Mecp2 fine-tunes quiescence exit by targeting nuclear receptors

Quiescence (G0) maintenance and exit are crucial for tissue homeostasis and regeneration in mammals. Here, we show that methyl-CpG binding protein 2 (Mecp2) expression is cell cycle-dependent and negatively regulates quiescence exit in cultured cells and in an injury-induced liver regeneration mouse model. Specifically, acute reduction of Mecp2 is required for efficient quiescence exit as deletion of Mecp2 accelerates, while overexpression of Mecp2 delays quiescence exit, and forced expression of Mecp2 after Mecp2 conditional knockout rescues cell cycle reentry. The E3 ligase Nedd4 mediates the ubiquitination and degradation of Mecp2, and thus facilitates quiescence exit. A genome-wide study uncovered the dual role of Mecp2 in preventing quiescence exit by transcriptionally activating metabolic genes while repressing proliferation-associated genes. Particularly disruption of two nuclear receptors, Rara or Nr1h3, accelerates quiescence exit, mimicking the Mecp2 depletion phenotype. Our studies unravel a previously unrecognized role for Mecp2 as an essential regulator of quiescence exit and tissue regeneration.

POMC-specific knockdown of MeCP2 leads to adverse phenotypes in mice chronically exposed to high fat diet

Methyl-CpG binding protein 2 (MeCP2) is an epigenetic factor associated with the neurodevelopmental disorders Rett Syndrome and MECP2 duplication syndrome. Previous studies have demonstrated that knocking out MeCP2 globally in the central nervous system leads to an obese phenotype and hyperphagia, however it is not clear if the hyperphagia is the result of an increased preference for food reward or due to an increase in motivation to obtain food reward. We show that mice deficient in MeCP2 specifically in pro-opiomelanocortin (POMC) neurons have an increased preference for high fat diet as measured by conditioned place preference but do not have a greater motivation to obtain food reward using a progressive ratio task, relative to wildtype littermate controls. We also demonstrate that POMC-Cre MeCP2 knockout (KO) mice have increased body weight after long-term high fat diet exposure as well as elevated plasma leptin and corticosterone levels compared to wildtype mice. Taken together, these results are the first to show that POMC-specific loss-of-function Mecp2 mutations leads to dissociable effects on the rewarding/motivational properties of food as well as changes to hormones associated with body weight homeostasis and stress.

Long-term cortical plasticity following sensory deprivation is reduced in male Rett model mice

PURPOSE/AIM

Rett (RTT) syndrome, a neurodevelopmental disorder, results from loss-of-function mutations in methyl-CpG-binding protein 2. We studied activity-dependent plasticity induced by sensory deprivation via whisker trimming in early symptomatic male mutant mice to assess neural rewiring capability.

METHODS

One whisker was trimmed for 0-14 days and intrinsic optical imaging of the transient reduction of brain blood oxygenation resulting from neural activation by 1 second of wiggling of the whisker stump was compared to that of an untrimmed control whisker.

RESULTS

Cortical evoked responses to wiggling a non-trimmed whisker were constant for 14 days, reduced for a trimmed whisker by 49.0 ± 4.3% in wild type (n = 14) but by only 22.7 ± 4.6% in mutant (n = 18, p = 0.001).

CONCLUSION

As the reduction in neural activation following sensory deprivation in whisker barrel cortex is known to be dependent upon evoked and basal neural activity, impairment of cortical re-wiring following whisker trimming provides a paradigm suitable to explore mechanisms underlying deficiencies in the establishment and maintenance of synapses in RTT, which can be potentially targeted by therapeutics.

Selective Deletion of Methyl CpG Binding Protein 2 from Parvalbumin Interneurons in the Auditory Cortex Delays the Onset of Maternal Retrieval in Mice

Mutations in MECP2 cause the neurodevelopmental disorder Rett syndrome. MECP2 codes for methyl CpG binding protein 2 (MECP2), a transcriptional regulator that activates genetic programs for experience-dependent plasticity. Many neural and behavioral symptoms of Rett syndrome may result from dysregulated timing and thresholds for plasticity. As a model of adult plasticity, we examine changes to auditory cortex inhibitory circuits in female mice when they are first exposed to pups; this plasticity facilitates behavioral responses to pups emitting distress calls. Brainwide deletion of Mecp2 alters expression of markers associated with GABAergic parvalbumin interneurons (PVins) and impairs the emergence of pup retrieval. We hypothesized that loss of Mecp2 in PVins disproportionately contributes to the phenotype. Here, we find that deletion of Mecp2 from PVins delayed the onset of maternal retrieval behavior and recapitulated the major molecular and neurophysiological features of brainwide deletion of Mecp2 We observed that when PVin-selective mutants were exposed to pups, auditory cortical expression of PVin markers increased relative to that in wild-type littermates. PVin-specific mutants also failed to show the inhibitory auditory cortex plasticity seen in wild-type mice on exposure to pups and their vocalizations. Finally, using an intersectional viral genetic strategy, we demonstrate that postdevelopmental loss of Mecp2 in PVins of the auditory cortex is sufficient to delay onset of maternal retrieval. Our results support a model in which PVins play a central role in adult cortical plasticity and may be particularly impaired by loss of Mecp2 SIGNIFICANCE STATEMENT Rett syndrome is a neurodevelopmental disorder that includes deficits in both communication and the ability to update brain connections and activity during learning (plasticity). This condition is caused by mutations in the gene MECP2 We use a maternal behavioral test in mice requiring both vocal perception and neural plasticity to probe the role of Mecp2 in social and sensory learning. Mecp2 is normally active in all brain cells, but here we remove it from a specific population (parvalbumin neurons). We find that this is sufficient to delay learned behavioral responses to pups and recreates many deficits seen in whole-brain Mecp2 deletion. Our findings suggest that parvalbumin neurons specifically are central to the consequences of loss of Mecp2 activity and yield clues as to possible mechanisms by which Rett syndrome impairs brain function.

Extracellular vesicles improve GABAergic transmission in Huntington's disease iPSC-derived neurons

Background: Extracellular vesicles (EVs) carry bioactive molecules associated with various biological processes, including miRNAs. In both Huntington's disease (HD) models and human samples, altered expression of miRNAs involved in synapse regulation was reported. Recently, the use of EV cargo to reverse phenotypic alterations in disease models with synaptopathy as the end result of the pathophysiological cascade has become an interesting possibility. Methods: Here, we assessed the contribution of EVs to GABAergic synaptic alterations using a human HD model and studied the miRNA content of isolated EVs. Results: After differentiating human induced pluripotent stem cells into electrophysiologically active striatal-like GABAergic neurons, we found that HD-derived neurons displayed reduced density of inhibitory synapse markers and GABA receptor-mediated ionotropic signaling. Treatment with EVs secreted by control (CTR) fibroblasts reversed the deficits in GABAergic synaptic transmission and increased the density of inhibitory synapses in HD-derived neuron cultures, while EVs from HD-derived fibroblasts had the opposite effects on CTR-derived neurons. Moreover, analysis of miRNAs from purified EVs identified a set of differentially expressed miRNAs between manifest HD, premanifest, and CTR lines with predicted synaptic targets. Conclusion: The EV-mediated reversal of the abnormal GABAergic phenotype in HD-derived neurons reinforces the potential role of EV-miRNAs on synapse regulation.

Maternal Diabetes Deregulates the Expression of Mecp2 via miR-26b-5p in Mouse Embryonic Neural Stem Cells

Maternal diabetes has been associated with a greater risk of neurodevelopmental disorders in offspring. It has been established that hyperglycemia alters the expression of genes and microRNAs (miRNAs) regulating the fate of neural stem cells (NSCs) during brain development. In this study, the expression of methyl-CpG-binding protein-2 (Mecp2), a global chromatin organizer and a crucial regulator of synaptic proteins, was analyzed in NSCs obtained from the forebrain of embryos of diabetic mice. Mecp2 was significantly downregulated in NSCs derived from embryos of diabetic mice when compared to controls. miRNA target prediction revealed that the miR-26 family could regulate the expression of Mecp2, and further validation confirmed that Mecp2 is a target of miR-26b-5p. Knockdown of Mecp2 or overexpression of miR-26b-5p altered the expression of tau protein and other synaptic proteins, suggesting that miR-26b-5p alters neurite outgrowth and synaptogenesis via Mecp2. This study revealed that maternal diabetes upregulates the expression of miR-26b-5p in NSCs, resulting in downregulation of its target, Mecp2, which in turn perturbs neurite outgrowth and expression of synaptic proteins. Overall, hyperglycemia dysregulates synaptogenesis that may manifest as neurodevelopmental disorders in offspring from diabetic pregnancy.

Selective deletion of Methyl CpG binding protein 2 from parvalbumin interneurons in the auditory cortex delays the onset of maternal retrieval in mice

Mutations in MECP2 cause the neurodevelopmental disorder Rett syndrome. MECP2 codes for methyl CpG binding protein 2 (MECP2), a transcriptional regulator that activates genetic programs for experience-dependent plasticity. Many neural and behavioral symptoms of Rett syndrome may result from dysregulated timing and threshold for plasticity. As a model of adult plasticity, we examine changes to auditory cortex inhibitory circuits in female mice when they are first exposed to pups; this plasticity facilitates behavioral responses to pups emitting distress calls. Brain-wide deletion of Mecp2 alters expression of markers associated with GABAergic parvalbumin interneurons (PVin) and impairs the emergence of pup retrieval. We hypothesized that loss of Mecp2 in PVin disproportionately contributes to the phenotype. Here we find that deletion of Mecp2 from PVin delayed the onset of maternal retrieval behavior and recapitulated the major molecular and neurophysiological features of brain-wide deletion of Mecp2 . We observed that when PVin-selective mutants were exposed to pups, auditory cortical expression of PVin markers increased relative to that in wild type littermates. PVin-specific mutants also failed to show the inhibitory auditory cortex plasticity seen in wild type mice upon exposure to pups and their vocalizations. Finally, using an intersectional viral genetic strategy, we demonstrate that post-developmental loss of Mecp2 in PVin of the auditory cortex is sufficient to delay onset of maternal retrieval. Our results support a model in which PVin play a central role in adult cortical plasticity and may be particularly impaired by loss of Mecp2 .

SIGNIFICANCE STATEMENT

Rett syndrome is a neurodevelopmental disorder that includes deficits in both communication and the ability to update brain connections and activity during learning ('plasticity'). This condition is caused by mutations in the gene MECP2 . We use a maternal behavioral test in mice requiring both vocal perception and neural plasticity to probe Mecp2' s role in social and sensory learning. Mecp2 is normally active in all brain cells, but here we remove it from a specific population ('parvalbumin neurons'). We find that this is sufficient to delay learned behavioral responses to pups and recreates many deficits seen in whole brain Mecp2 deletion. Our findings suggest that parvalbumin neurons specifically are central to the consequences of loss of Mecp2 activity and yield clues as to possible mechanisms by which Rett syndrome impairs brain function.

Multiplex epigenome editing of MECP2 to rescue Rett syndrome neurons

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused by loss-of-function heterozygous mutations of methyl CpG-binding protein 2 (MECP2) on the X chromosome in young females. Reactivation of the silent wild-type MECP2 allele from the inactive X chromosome (Xi) represents a promising therapeutic opportunity for female patients with RTT. Here, we applied a multiplex epigenome editing approach to reactivate MECP2 from Xi in RTT human embryonic stem cells (hESCs) and derived neurons. Demethylation of the MECP2 promoter by dCas9-Tet1 with target single-guide RNA reactivated MECP2 from Xi in RTT hESCs without detectable off-target effects at the transcriptional level. Neurons derived from methylation-edited RTT hESCs maintained MECP2 reactivation and reversed the smaller soma size and electrophysiological abnormalities, two hallmarks of RTT. In RTT neurons, insulation of the methylation-edited MECP2 locus by dCpf1-CTCF (a catalytically dead Cpf1 fused with CCCTC-binding factor) with target CRISPR RNA enhanced MECP2 reactivation and rescued RTT-related neuronal defects, providing a proof-of-concept study for epigenome editing to treat RTT and potentially other dominant X-linked diseases.

Epigenetics in fetal alcohol spectrum disorder

During pregnancy, alcohol abuse and its detrimental effects on developing offspring are major public health, economic and social challenges. The prominent characteristic attributes of alcohol (ethanol) abuse during pregnancy in humans are neurobehavioral impairments in offspring due to damage to the central nervous system (CNS), causing structural and behavioral impairments that are together named fetal alcohol spectrum disorder (FASD). Development-specific alcohol exposure paradigms were established to recapitulate the human FASD phenotypes and establish the underlying mechanisms. These animal studies have offered some critical molecular and cellular underpinnings likely to account for the neurobehavioral impairments associated with prenatal ethanol exposure. Although the pathogenesis of FASD remains unclear, emerging literature proposes that the various genomic and epigenetic components that cause the imbalance in gene expression can significantly contribute to the development of this disease. These studies acknowledged numerous immediate and enduring epigenetic modifications, such as methylation of DNA, post-translational modifications (PTMs) of histone proteins, and regulatory networks related to RNA, using many molecular approaches. Methylated DNA profiles, PTMs of histone proteins, and RNA-regulated expression of genes are essential for synaptic and cognitive behavior. Thus, offering a solution to many neuronal and behavioral impairments reported in FASD. In the current chapter, we review the recent advances in different epigenetic modifications that cause the pathogenesis of FASD. The information discussed can help better explain the pathogenesis of FASD and thereby might provide a basis for finding novel therapeutic targets and innovative treatment strategies.

Epigenetic signature in neural plasticity: the journey so far and journey ahead

Neural plasticity is a remarkable characteristic of the brain which allows neurons to rewire their structure in response to internal and external stimuli. Many external stimuli collectively referred to as 'epigenetic factors' strongly influence structural and functional reorganization of the brain, thereby acting as a potential driver of neural plasticity. DNA methylation and demethylation, histone acetylation, and deacetylation are some of the frontline epigenetic mechanisms behind neural plasticity. Epigenetic signature molecules (mostly proteins) play a pivotal role in epigenetic reprogramming. Though neuro-epigenetics is an incredibly important field of emerging research, the critical role of signature proteins associated with epigenetic alteration and their involvement in neural plasticity needs further attention. This study gives an integrated and systematic overview of the current state of knowledge with a clear idea of types of neural plasticity and the context-dependent role of epigenetic signature molecules and their modulation by some natural bioactive compounds.

Linking neuroanatomical abnormalities in autism spectrum disorder with gene expression of candidate ASD genes: A meta-analytic and network-oriented approach

BACKGROUND

Autism spectrum disorder (ASD) is a set of developmental conditions with widespread neuroanatomical abnormalities and a strong genetic basis. Although neuroimaging studies have indicated anatomical changes in grey matter (GM) morphometry, their associations with gene expression remain elusive.

METHODS

Here, we aim to understand how gene expression correlates with neuroanatomical atypicalities in ASD. To do so, we performed a coordinate-based meta-analysis to determine the common GM variation pattern in the autistic brain. From the Allen Human Brain Atlas, we selected eight genes from the SHANK, NRXN, NLGN family and MECP2, which have been implicated with ASD, particularly in regards to altered synaptic transmission and plasticity. The gene expression maps for each gene were built. We then assessed the correlation between the gene expression maps and the GM alteration maps. Lastly, we projected the obtained clusters of GM alteration-gene correlations on top of the canonical resting state networks, in order to provide a functional characterization of the structural evidence.

RESULTS

We found that gene expression of most genes correlated with GM alteration (both increase and decrease) in regions located in the default mode network. Decreased GM was also correlated with gene expression of some ASD genes in areas associated with the dorsal attention and cerebellar network. Lastly, single genes were found to be significantly correlated with increased GM in areas located in the somatomotor, limbic and ganglia/thalamus networks.

CONCLUSIONS

This approach allowed us to combine the well beaten path of genetic and brain imaging in a novel way, to specifically investigate the relation between gene expression and brain with structural damage, and individuate genes of potential interest for further investigation in the functional domain.

Evaluating Learning and Memory in Drosophila melanogaster to Study the Neurodevelopmental Impacts of Toxicants

Neurodevelopmental disorders are a heterogeneous group of behaviorally defined disorders with both genetic and environmental risk factors. Given that many neurodevelopmental disorders are characterized by impaired learning and/or intellectual abilities, behavioral paradigms that assess cognition in animal models have been effective tools in delineating underlying genetic variants that impact disease pathophysiology. For example, learning and memory paradigms in the common fruit fly Drosophila melanogaster have been successfully used to study risk genes and biological pathways associated with several neurodevelopmental disorders, including fragile X syndrome, autism spectrum disorder, and CHARGE syndrome. While these established Drosophila behavioral paradigms have historically been used to investigate genetic risk factors, they also have many other applications, including use in developmental neurotoxicology studies to determine environmental risk factors for neurodevelopmental disorders. There is, however, a deficit of step-by-step protocols that describe how to apply learning and memory assays in fruit flies to developmental neurotoxicology studies. Here, we describe two quantitative behavioral paradigms for Drosophila-predator-induced oviposition and courtship conditioning-that can be used to measure different forms of learning and memory in the context of a developmental neurotoxicology study. Non-associative learning and memory are measured here by examining female Drosophila oviposition behavior in response to endoparasitoid wasps, while associative learning and memory are measured in males using courtship conditioning. Our protocols outline procedures for oral toxicant exposure of developing fruit flies, culturing of endoparasitoid wasps, measuring Drosophila oviposition activity, and assessing conditioned courtship in order to identify the impact of toxicants on learning and memory in both females and males. As an example, we present the protocols using bisphenol A, a chemical utilized in the synthesis of polycarbonate plastics, to determine its impacts on learning and memory. These protocols are inexpensive and relatively simple to perform, and can be used by labs that are new to Drosophila behavioral research to evaluate how toxicant exposure influences learning and memory in male and female flies. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Preparation of toxicant-containing food and developmental exposure Basic Protocol 2: Predator-induced oviposition assay Support Protocol: Culture of Leptopilina heterotoma Basic Protocol 3: Conditioned courtship assay.

Convergent cerebrospinal fluid proteomes and metabolic ontologies in humans and animal models of Rett syndrome

MECP2 loss-of-function mutations cause Rett syndrome, a neurodevelopmental disorder resulting from a disrupted brain transcriptome. How these transcriptional defects are decoded into a disease proteome remains unknown. We studied the proteome of Rett cerebrospinal fluid (CSF) to identify consensus Rett proteome and ontologies shared across three species. Rett CSF proteomes enriched proteins annotated to HDL lipoproteins, complement, mitochondria, citrate/pyruvate metabolism, synapse compartments, and the neurosecretory protein VGF. We used shared Rett ontologies to select analytes for orthogonal quantification and functional validation. VGF and ontologically selected CSF proteins had genotypic discriminatory capacity as determined by receiver operating characteristic analysis in Mecp2 ^-/y and Mecp2 ^-/+ . Differentially expressed CSF proteins distinguished Rett from a related neurodevelopmental disorder, CDKL5 deficiency disorder. We propose that Mecp2 mutant CSF proteomes and ontologies inform putative mechanisms and biomarkers of disease. We suggest that Rett syndrome results from synapse and metabolism dysfunction.

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.

MeCP2 loss-of-function dysregulates microRNAs regionally and disrupts excitatory/inhibitory synaptic transmission balance

Rett syndrome is a leading cause of intellectual disability in females primarily caused by loss of function mutations in the transcriptional regulator MeCP2. Loss of MeCP2 leads to a host of synaptic phenotypes that are believed to underlie Rett syndrome pathophysiology. Synaptic deficits vary by brain region upon MeCP2 loss, suggesting distinct molecular alterations leading to disparate synaptic outcomes. In this study, we examined the contribution of MeCP2's newly described role in miRNA regulation to regional molecular and synaptic impairments. Two miRNAs, miR-101a and miR-203, were identified and confirmed as upregulated in MeCP2 KO mice in the hippocampus and cortex, respectively. miR-101a overexpression in hippocampal cultures led to opposing effects at excitatory and inhibitory synapses and in spontaneous and evoked neurotransmission, revealing the potential for a single miRNA to broadly regulate synapse function in the hippocampus. These results highlight the importance of regional alterations in miRNA expression and the specific impact on synaptic function with potential implications for Rett syndrome.

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.

A weakened recurrent circuit in the hippocampus of Rett syndrome mice disrupts long-term memory representations

Successful recall of a contextual memory requires reactivating ensembles of hippocampal cells that were allocated during memory formation. Altering the ratio of excitation-to-inhibition (E/I) during memory retrieval can bias cell participation in an ensemble and hinder memory recall. In the case of Rett syndrome (RTT), a neurological disorder with severe learning and memory deficits, the E/I balance is altered, but the source of this imbalance is unknown. Using in vivo imaging during an associative memory task, we show that during long-term memory retrieval, RTT CA1 cells poorly distinguish mnemonic context and form larger ensembles than wild-type mouse cells. Simultaneous multiple whole-cell recordings revealed that mutant somatostatin expressing interneurons (SOM) are poorly recruited by CA1 pyramidal cells and are less active during long-term memory retrieval in vivo. Chemogenetic manipulation revealed that reduced SOM activity underlies poor long-term memory recall. Our findings reveal a disrupted recurrent CA1 circuit contributing to RTT memory impairment.

Negative Regulation of the IL-1 System by IL-1R2 and IL-1R8: Relevance in Pathophysiology and Disease

Interleukin-1 (IL-1) is a primary cytokine of innate immunity and inflammation. IL-1 belongs to a complex family including ligands with agonist activity, receptor antagonists, and an anti-inflammatory cytokine. The receptors for these ligands, the IL-1 Receptor (IL-1R) family, include signaling receptor complexes, decoy receptors, and negative regulators. Agonists and regulatory molecules co-evolved, suggesting the evolutionary relevance of a tight control of inflammatory responses, which ensures a balance between amplification of innate immunity and uncontrolled inflammation. IL-1 family members interact with innate immunity cells promoting innate immunity, as well as with innate and adaptive lymphoid cells, contributing to their differentiation and functional polarization and plasticity. Here we will review the properties of two key regulatory receptors of the IL-1 system, IL-1R2, the first decoy receptor identified, and IL-1R8, a pleiotropic regulator of different IL-1 family members and co-receptor for IL-37, the anti-inflammatory member of the IL-1 family. Their complex impact in pathology, ranging from infections and inflammatory responses, to cancer and neurologic disorders, as well as clinical implications and potential therapeutic exploitation will be presented.

Literature Cases Summarized Based on Their Polysomnographic Findings in Rett Syndrome

Rett syndrome (RTT) is a severe and rare neurodevelopmental disorder affecting mostly girls. In RTT, an impaired sleep pattern is a supportive criterion for the diagnosis, yet little is known regarding the sleep structure and sleep respiratory events. Aiming to delineate sleep by aggregating RTT case (series) data from published polysomnographic studies, seventy-four RTT cases were collected from eleven studies up until 6 February 2022 (PROSPERO: CRD 42020198099). We compared the polysomnographic data within RTT stratifications and to a typically developing population. MECP2 cases demonstrated shortened total sleep time (TST) with increased stage N3 and decreased REM sleep. In cases with CDKL5 mutations, TST was longer and they spent more time in stage N1 but less in stage N3 than those cases affected by MECP2 mutations and a typically developing population. Sleep-disordered breathing was confirmed by the abnormal apnea/hypopnea index of 11.92 ± 23.67/h TST in these aggregated cases. No association of sleep structure with chronological age was found. In RTT, the sleep macrostructure of MECP2 versus CDKL5 cases showed differences, particularly regarding sleep stage N3. A severe REM sleep propensity reduction was found. Aberrant sleep cycling, possibly characterized by a poor REM ‘on switch’ and preponderance in slow and high-voltage sleep, is proposed.

Multimodal Neuroimaging in Rett Syndrome With MECP2 Mutation

Rett syndrome (RTT) is a rare neurodevelopmental disorder characterized by severe cognitive, social, and physical impairments resulting from de novo mutations in the X-chromosomal methyl-CpG binding protein gene 2 (MECP2). While there is still no cure for RTT, exploring up-to date neurofunctional diagnostic markers, discovering new potential therapeutic targets, and searching for novel drug efficacy evaluation indicators are fundamental. Multiple neuroimaging studies on brain structure and function have been carried out in RTT-linked gene mutation carriers to unravel disease-specific imaging features and explore genotype-phenotype associations. Here, we reviewed the neuroimaging literature on this disorder. MRI morphologic studies have shown global atrophy of gray matter (GM) and white matter (WM) and regional variations in brain maturation. Diffusion tensor imaging (DTI) studies have demonstrated reduced fractional anisotropy (FA) in left peripheral WM areas, left major WM tracts, and cingulum bilaterally, and WM microstructural/network topology changes have been further found to be correlated with behavioral abnormalities in RTT. Cerebral blood perfusion imaging studies using single-photon emission CT (SPECT) or PET have evidenced a decreased global cerebral blood flow (CBF), particularly in prefrontal and temporoparietal areas, while magnetic resonance spectroscopy (MRS) and PET studies have contributed to unraveling metabolic alterations in patients with RTT. The results obtained from the available reports confirm that multimodal neuroimaging can provide new insights into a complex interplay between genes, neurotransmitter pathway abnormalities, disease-related behaviors, and clinical severity. However, common limitations related to the available studies include small sample sizes and hypothesis-based and region-specific approaches. We, therefore, conclude that this field is still in its early development phase and that multimodal/multisequence studies with improved post-processing technologies as well as combined PET–MRI approaches are urgently needed to further explore RTT brain alterations.

Rett Syndrome and MECP2 Duplication Syndrome: Disorders of MeCP2 Dosage

Rett syndrome (RTT) is a neurodevelopmental disorder caused predominantly by loss-of-function mutations in the gene Methyl-CpG-binding protein 2 (MECP2), which encodes the MeCP2 protein. RTT is a MECP2-related disorder, along with MECP2 duplication syndrome (MDS), caused by gain-of-function duplications of MECP2. Nearly two decades of research have advanced our knowledge of MeCP2 function in health and disease. The following review will discuss MeCP2 protein function and its dysregulation in the MECP2-related disorders RTT and MDS. This will include a discussion of the genetic underpinnings of these disorders, specifically how sporadic X-chromosome mutations arise and manifest in specific populations. We will then review current diagnostic guidelines and clinical manifestations of RTT and MDS. Next, we will delve into MeCP2 biology, describing the dual landscapes of methylated DNA and its reader MeCP2 across the neuronal genome as well as the function of MeCP2 as a transcriptional modulator. Following this, we will outline common MECP2 mutations and genotype-phenotype correlations in both diseases, with particular focus on mutations associated with relatively mild disease in RTT. We will also summarize decades of disease modeling and resulting molecular, synaptic, and behavioral phenotypes associated with RTT and MDS. Finally, we list several therapeutics in the development pipeline for RTT and MDS and available evidence of their safety and efficacy.

Rett Syndrome and Fragile X Syndrome: Different Etiology With Common Molecular Dysfunctions

Rett syndrome (RTT) and Fragile X syndrome (FXS) are two monogenetic neurodevelopmental disorders with complex clinical presentations. RTT is caused by mutations in the Methyl-CpG binding protein 2 gene (MECP2) altering the function of its protein product MeCP2. MeCP2 modulates gene expression by binding methylated CpG dinucleotides, and by interacting with transcription factors. FXS is caused by the silencing of the FMR1 gene encoding the Fragile X Mental Retardation Protein (FMRP), a RNA binding protein involved in multiple steps of RNA metabolism, and modulating the translation of thousands of proteins including a large set of synaptic proteins. Despite differences in genetic etiology, there are overlapping features in RTT and FXS, possibly due to interactions between MeCP2 and FMRP, and to the regulation of pathways resulting in dysregulation of common molecular signaling. Furthermore, basic physiological mechanisms are regulated by these proteins and might concur to the pathophysiology of both syndromes. Considering that RTT and FXS are disorders affecting brain development, and that most of the common targets of MeCP2 and FMRP are involved in brain activity, we discuss the mechanisms of synaptic function and plasticity altered in RTT and FXS, and we consider the similarities and the differences between these two disorders.

Loss of O-GlcNAcylation on MeCP2 at Threonine 203 Leads to Neurodevelopmental Disorders

Mutations of the X-linked methyl-CpG-binding protein 2 (MECP2) gene in humans are responsible for most cases of Rett syndrome (RTT), an X-linked progressive neurological disorder. While genome-wide screens in clinical trials have revealed several putative RTT-associated mutations in MECP2, their causal relevance regarding the functional regulation of MeCP2 at the etiologic sites at the protein level requires more evidence. In this study, we demonstrated that MeCP2 was dynamically modified by O-linked-β-N-acetylglucosamine (O-GlcNAc) at threonine 203 (T203), an etiologic site in RTT patients. Disruption of the O-GlcNAcylation of MeCP2 specifically at T203 impaired dendrite development and spine maturation in cultured hippocampal neurons, and disrupted neuronal migration, dendritic spine morphogenesis, and caused dysfunction of synaptic transmission in the developing and juvenile mouse cerebral cortex. Mechanistically, genetic disruption of O-GlcNAcylation at T203 on MeCP2 decreased the neuronal activity-induced induction of Bdnf transcription. Our study highlights the critical role of MeCP2 T203 O-GlcNAcylation in neural development and synaptic transmission potentially via brain-derived neurotrophic factor.

A putative role for lncRNAs in epigenetic regulation of memory

The central dogma of molecular genetics is defined as encoded genetic information within DNA, transcribed into messenger RNA, which contain the instructions for protein synthesis, thus imparting cellular functionality and ultimately life. This molecular genetic theory has given birth to the field of neuroepigenetics, and it is now well established that epigenetic regulation of gene transcription is critical to the learning and memory process. In this review, we address a potential role for a relatively new player in the field of epigenetic crosstalk - long non-coding RNAs (lncRNAs). First, we briefly summarize epigenetic mechanisms in memory formation and examine what little is known about the emerging role of lncRNAs during this process. We then focus discussions on how lncRNAs interact with epigenetic mechanisms to control transcriptional programs under various conditions in the brain, and how this may be applied to regulation of gene expression necessary for memory formation. Next, we explore how epigenetic crosstalk in turn serves to regulate expression of various individual lncRNAs themselves. To highlight the importance of further exploring the role of lncRNA in epigenetic regulation of gene expression, we consider the significant relationship between lncRNA dysregulation and declining memory reserve with aging, Alzheimer's disease, and epilepsy, as well as the promise of novel therapeutic interventions. Finally, we conclude with a discussion of the critical questions that remain to be answered regarding a role for lncRNA in memory.

Primate-specific retrotransposons and the evolution of circadian networks in the human brain

The circadian rhythm of the human brain is attuned to sleep-wake cycles that entail global alterations in neuronal excitability. This periodicity involves a highly coordinated regulation of gene expression. A growing number of studies are documenting a fascinating connection between primate-specific retrotransposons (Alu elements) and key epigenetic regulatory processes in the primate brain. Collectively, these studies indicate that Alu elements embedded in the human neuronal genome mediate post-transcriptional processes that unite human-specific neuroepigenetic landscapes and circadian rhythm. Here, we review evidence linking Alu retrotransposon-mediated posttranscriptional pathways to circadian gene expression. We hypothesize that Alu retrotransposons participate in the organization of circadian brain function through multidimensional neuroepigenetic pathways. We anticipate that these pathways are closely tied to the evolution of human cognition and their perturbation contributes to the manifestation of human-specific neurological diseases. Finally, we address current challenges and accompanying opportunities in studying primate- and human-specific transposable elements.

Perinatal exposure to high fat diet alters expression of MeCP2 in the hypothalamus

Obesity is a complex disease that is the result of a number of different factors including genetic, environmental, and endocrine abnormalities. Given that monogenic forms of obesity are rare, it is important to identify other mechanisms that contribute to its etiology. Methyl-Cp-G binding protein 2 (MeCP2) is a neuroepigenetic factor that binds to methylated regions of DNA to influence transcription. Past studies demonstrate that disruption in MeCP2 function produces obesity in mice. Using a diet-induced obesity mouse model, we show that perinatal exposure to high fat diet significantly decreases MeCP2 protein expression in the hypothalamus of female mice, effects not seen when high fat diet is given to mice during adulthood. Moreover, these effects are seen specifically in a subregion of the hypothalamus known as the arcuate nucleus with females having decreased MeCP2 expression in rostral areas and males having decreased MeCP2 expression in intermediate regions of the arcuate nucleus. Interestingly, mice gain more weight when exposed to high fat diet during adulthood relative to mice exposed to high fat diet perinatally, suggesting that perhaps high fat diet exposure during adulthood may be affecting mechanisms independent of MeCP2 function. Collectively, our data demonstrate that there are developmentally sensitive periods in which MeCP2 expression is influenced by high fat diet exposure and this occurs in a sexually dimorphic manner.

Nanoscale synapse organization and dysfunction in neurodevelopmental disorders

Neurodevelopmental disorders such as those linked to intellectual disabilities or autism spectrum disorder are thought to originate in part from genetic defects in synaptic proteins. Single gene mutations linked to synapse dysfunction can broadly be separated in three categories: disorders of transcriptional regulation, disorders of synaptic signaling and disorders of synaptic scaffolding and structures. The recent developments in super-resolution imaging technologies and their application to synapses have unraveled a complex nanoscale organization of synaptic components. On the one hand, part of receptors, adhesion proteins, ion channels, scaffold elements and the pre-synaptic release machinery are partitioned in subsynaptic nanodomains, and the respective organization of these nanodomains has tremendous impact on synaptic function. For example, pre-synaptic neurotransmitter release sites are partly aligned with nanometer precision to postsynaptic receptor clusters. On the other hand, a large fraction of synaptic components is extremely dynamic and constantly exchanges between synaptic domains and extrasynaptic or intracellular compartments. It is largely the combination of the exquisitely precise nanoscale synaptic organization of synaptic components and their high dynamic that allows the rapid and profound regulation of synaptic function during synaptic plasticity processes that underlie adaptability of brain function, learning and memory. It is very tempting to speculate that genetic defects that lead to neurodevelopmental disorders and target synaptic scaffolds and structures mediate their deleterious impact on brain function through perturbing synapse nanoscale dynamic organization. We discuss here how applying super-resolution imaging methods in models of neurodevelopmental disorders could help in addressing this question.

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.

Engineered microRNA-based regulatory element permits safe high-dose miniMECP2 gene therapy in Rett mice

MECP2 gene transfer has been shown to extend the survival of Mecp2-/y knockout (KO) mice modeling Rett syndrome (RTT), an X-linked neurodevelopmental disorder. However, controlling deleterious overexpression of MeCP2 remains the critical unmet obstacle towards a safe and effective gene therapy approach for RTT. A recently developed truncated miniMECP2 gene has also been shown to be therapeutic after AAV9-mediated gene transfer in KO neonates. We show that AAV9/miniMECP2 has a similar dose-dependent toxicity profile to that of a published second-generation AAV9/MECP2 vector after treatment in adolescent mice. To overcome that toxicity, we developed a risk-driven viral genome design strategy rooted in high-throughput profiling and genome mining to rationally develop a compact, synthetic miRNA target panel (miR-Responsive Auto-Regulatory Element, "miRARE") to minimize the possibility of miniMECP2 transgene overexpression in the context of RTT gene therapy. The goal of miRARE is to have a built-in inhibitory element responsive to MeCP2 overexpression. The data provided herein show that insertion of miRARE into the miniMECP2 gene expression cassette greatly improved the safety of miniMECP2 gene transfer without compromising efficacy. Importantly, this built-in regulation system does not require any additional exogenous drug application, and no miRNAs are expressed from the transgene cassette. Although broad applications of miRARE have yet to be determined, the design of miRARE suggests a potential use in gene therapy approaches for other dose-sensitive genes.

MECP2 Increases the Pro-Inflammatory Response of Microglial Cells and Phosphorylation at Serine 423 Regulates Neuronal Gene Expression upon Neuroinflammation

Methyl-CpG-binding protein 2 (MECP2) is a critical transcriptional regulator for synaptic function. Dysfunction of synapses, as well as microglia-mediated neuroinflammation, represent the earliest pathological events in Alzheimer's disease (AD). Here, expression, protein levels, and activity-related phosphorylation changes of MECP2 were analyzed in post-mortem human temporal cortex. The effects of wild type and phosphorylation-deficient MECP2 variants at serine 423 (S423) or S80 on microglial and neuronal function were assessed utilizing BV2 microglial monocultures and co-cultures with mouse cortical neurons under inflammatory stress conditions. MECP2 phosphorylation at the functionally relevant S423 site nominally decreased in the early stages of AD-related neurofibrillary pathology in the human temporal cortex. Overexpression of wild type MECP2 enhanced the pro-inflammatory response in BV2 cells upon treatment with lipopolysaccharide (LPS) and interferon-γ (IFNγ) and decreased BV2 cell phagocytic activity. The expression of the phosphorylation-deficient MECP2-S423A variant, but not S80A, further increased the pro-inflammatory response of BV2 cells. In neurons co-cultured with BV2 cells, the MECP2-S423A variant increased the expression of several genes, which are important for the maintenance and protection of neurons and synapses upon inflammatory stress. Collectively, functional analyses in different cellular models suggest that MECP2 may influence the inflammatory response in microglia independently of S423 and S80 phosphorylation, while the S423 phosphorylation might play a role in the activation of neuronal gene expression, which conveys neuroprotection under neuroinflammation-related stress.

MECP2 and the Biology of MECP2 Duplication Syndrome

MECP2 duplication syndrome (MDS), a rare X-linked genomic disorder affecting predominantly males, is caused by duplication of the chromosomal region containing the methyl CpG binding protein-2 (MECP2) gene, which encodes methyl-CpG-binding protein 2 (MECP2), a multi-functional protein required for proper brain development and maintenance of brain function during adulthood. Disease symptoms include severe motor and cognitive impairment, delayed or absent speech development, autistic features, seizures, ataxia, recurrent respiratory infections and shortened lifespan. The cellular and molecular mechanisms by which a relatively modest increase in MECP2 protein causes such severe disease symptoms are poorly understood and consequently there are no treatments available for this fatal disorder. This review summarizes what is known to date about the structure and complex regulation of MECP2 and its many functions in the developing and adult brain. Additionally, recent experimental findings on the cellular and molecular underpinnings of MDS based on cell culture and mouse models of the disorder are reviewed. The emerging picture from these studies is that MDS is a neurodegenerative disorder in which neurons die in specific parts of the central nervous system, including the cortex, hippocampus, cerebellum and spinal cord. Neuronal death likely results from astrocytic dysfunction, including a breakdown of glutamate homeostatic mechanisms. The role of elevations in the expression of glial acidic fibrillary protein (GFAP) in astrocytes and the microtubule-associated protein, Tau, in neurons to the pathogenesis of MDS is discussed. Lastly, potential therapeutic strategies to potentially treat MDS are discussed.

miR-132 downregulation alleviates behavioral impairment of rats exposed to single prolonged stress, reduces the level of apoptosis in PFC, and upregulates the expression of MeCP2 and BDNF

Post-traumatic stress disorder (PTSD) is usually accompanied by anxiety symptoms and decreased expression of brain-derived neurotrophic factor (BDNF), which played an important role in promoting neuronal proliferation and survival. Methyl CpG-binding protein 2 (MeCP2) is a positive mediator of BDNF and is regulated by miR-132-3p. In the present study, we explored the possible molecular mechanism of miR-132, focusing on the involvement of MeCP2 and BDNF in the formation of anxiety-like symptoms of PTSD. Single prolonged stress (SPS) was used to establish a model of PTSD in adult rats and the anxiety-like behavior was tested by the elevated plus-maze (EPM). The level of miR-132 in the prefrontal cortex (PFC) was increased and intraventricular injection of anti-miR-132 could significantly improve the anxiety-like behavior of rats exposed to SPS through MeCP2 and the subsequent upregulation of BDNF levels. Then tropomyosin-related kinase B (TrkB) and downstream signals, including MAP kinase ERK1/2 and phosphoinositol 3-kinase (PI3K)/Akt pathways, were activated by BDNF upregulation, and might participate in regulating dendritic complexity and the expression of postsynaptic density-95 (PSD95) and synapsin I in the PFC of SPS rats. Furthermore, we found that the apoptosis of cells in PFC induced by SPS procedure could be alleviated by miR-132 inhibition. Our results suggest that miR-132 might be involved in the formation of anxiety-like symptoms of adult rat PTSD models by targeting MeCP2, and this effect is related to BDNF/TrkB and its downstream ERK and Akt signaling pathways.

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.

Methyl-CpG-binding protein 2 mediates overlapping mechanisms across brain disorders

MECP2 and its product, Methyl-CpG binding protein 2 (MeCP2), are mostly known for their association to Rett Syndrome (RTT), a rare neurodevelopmental disorder. Additional evidence suggests that MECP2 may underlie other neuropsychiatric and neurological conditions, and perhaps modulate common presentations and pathophysiology across disorders. To clarify the mechanisms of these interactions, we develop a method that uses the binding properties of MeCP2 to identify its targets, and in particular, the genes recognized by MeCP2 and associated to several neurological and neuropsychiatric disorders. Analysing mechanisms and pathways modulated by these genes, we find that they are involved in three main processes: neuronal transmission, immuno-reactivity, and development. Also, while the nervous system is the most relevant in the pathophysiology of the disorders, additional systems may contribute to MeCP2 action through its target genes. We tested our results with transcriptome analysis on Mecp2-null models and cells derived from a patient with RTT, confirming that the genes identified by our procedure are directly modulated by MeCP2. Thus, MeCP2 may modulate similar mechanisms in different pathologies, suggesting that treatments for one condition may be effective for related disorders.

Identification of hsa-miR-1275 as a Novel Biomarker Targeting MECP2 for Human Epilepsy of Unknown Etiology

Epilepsy affects around 70 million people worldwide, with a 65% rate of unknown etiology. This rate is known as epilepsy of unknown etiology (EUE). Dysregulation of microRNAs (miRNAs) is recognized to contribute to mental disorders, including epilepsy. However, miRNA dysregulation is poorly understood in EUE. Here, we conducted miRNA expression profiling of EUE by microarray technology and identified 57 pathogenic changed miRNAs with significance. The data and bioinformatic analysis results indicated that among these miRNAs, hsa-microRNA (miR)-1275 was highly associated with neurological disorders. Subsequently, new samples of serum and cerebrospinal fluid were collected for validation of hsa-miR-1275 expression by TaqMan assays. Results show that hsa-miR-1275 in serums of EUE were increased significantly, but in cerebrospinal fluid, the miRNA was decreased. Moreover, the MECP2 gene was selected as a hsa-miR-1275 target based on target prediction tools and gene ontology analysis. Validation of in vitro tests proved that MECP2 expression was specifically inhibited by hsa-miR-1275. Additionally, overexpression of hsa-miR-1275 can elevate expression of nuclear factor κB (NF-κB) and promote cell apoptosis. Taken together, hsa-miR-1275 might represent a novel biomarker targeting MECP2 for human EUE.

MeCP2 gates spatial learning-induced alternative splicing events in the mouse hippocampus

Long-term memory formation is supported by functional and structural changes of neuronal networks, which rely on de novo gene transcription and protein synthesis. The modulation of the neuronal transcriptome in response to learning depends on transcriptional and post-transcriptional mechanisms. DNA methylation writers and readers regulate the activity-dependent genomic program required for memory consolidation. The most abundant DNA methylation reader, the Methyl CpG binding domain protein 2 (MeCP2), has been shown to regulate alternative splicing, but whether it establishes splicing events important for memory consolidation has not been investigated. In this study, we identified the alternative splicing profile of the mouse hippocampus in basal conditions and after a spatial learning experience, and investigated the requirement of MeCP2 for these processes. We observed that spatial learning triggers a wide-range of alternative splicing events in transcripts associated with structural and functional remodeling and that virus-mediated knockdown of MeCP2 impairs learning-dependent post-transcriptional responses of mature hippocampal neurons. Furthermore, we found that MeCP2 preferentially affected the splicing modalities intron retention and exon skipping and guided the alternative splicing of distinct set of genes in baseline conditions and after learning. Lastly, comparative analysis of the MeCP2-regulated transcriptome with the alternatively spliced mRNA pool, revealed that MeCP2 disruption alters the relative abundance of alternatively spliced isoforms without affecting the overall mRNA levels. Taken together, our findings reveal that adult hippocampal MeCP2 is required to finetune alternative splicing events in basal conditions, as well as in response to spatial learning. This study provides new insight into how MeCP2 regulates brain function, particularly cognitive abilities, and sheds light onto the pathophysiological mechanisms of Rett syndrome, that is characterized by intellectual disability and caused by mutations in the Mecp2 gene.

Vagus nerve stimulation paired with tones restores auditory processing in a rat model of Rett syndrome

BACKGROUND

Rett syndrome is a rare neurological disorder associated with a mutation in the X-linked gene MECP2. This disorder mainly affects females, who typically have seemingly normal early development followed by a regression of acquired skills. The rodent Mecp2 model exhibits many of the classic neural abnormalities and behavioral deficits observed in individuals with Rett syndrome. Similar to individuals with Rett syndrome, both auditory discrimination ability and auditory cortical responses are impaired in heterozygous Mecp2 rats. The development of therapies that can enhance plasticity in auditory networks and improve auditory processing has the potential to impact the lives of individuals with Rett syndrome. Evidence suggests that precisely timed vagus nerve stimulation (VNS) paired with sound presentation can drive robust neuroplasticity in auditory networks and enhance the benefits of auditory therapy.

OBJECTIVE

The aim of this study was to investigate the ability of VNS paired with tones to restore auditory processing in Mecp2 transgenic rats.

METHODS

Seventeen female heterozygous Mecp2 rats and 8 female wild-type (WT) littermates were used in this study. The rats were exposed to multiple tone frequencies paired with VNS 300 times per day for 20 days. Auditory cortex responses were then examined following VNS-tone pairing therapy or no therapy.

RESULTS

Our results indicate that Mecp2 mutation alters auditory cortex responses to sounds compared to WT controls. VNS-tone pairing in Mecp2 rats improves the cortical response strength to both tones and speech sounds compared to untreated Mecp2 rats. Additionally, VNS-tone pairing increased the information contained in the neural response that can be used to discriminate between different consonant sounds.

CONCLUSION

These results demonstrate that VNS-sound pairing may represent a strategy to enhance auditory function in individuals with Rett syndrome.

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.

P2X7 receptor inhibition ameliorates dendritic spine pathology and social behavioral deficits in Rett syndrome mice

Dysregulated immunity has been implicated in the pathogenesis of neurodevelopmental disorders but its contribution to synaptic and behavioral deficits in Rett syndrome (RTT) remains unknown. P2X7 receptors (P2X7Rs) are unique purinergic receptors with pro-inflammatory functions. Here, we report in a MECP2-deficient mouse model of RTT that the border of the cerebral cortex exhibits increased number of inflammatory myeloid cells expressing cell-surface P2X7Rs. Total knockout of P2X7Rs in MECP2 deficient mice decreases the number of inflammatory myeloid cells, restores cortical dendritic spine dynamics, and improves the animals’ neurological function and social behavior. Furthermore, either genetic depletion of P2X7Rs in bone-marrow derived leukocytes or pharmacological block of P2X7Rs primarily outside of the central nervous system parenchyma, recapitulates the beneficial effects of total P2X7R depletion on the social behavior. Together, our results highlight the pathophysiological roles of P2X7Rs in a mouse model of RTT.

P2X7 receptors are purinergic receptors with pro-inflammatory functions. Here, the authors show that inhibition of leukocyte P2X7 receptors reduces dendritic spine pathology and social behavioral deficits in a mouse model of Rett syndrome.

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.

Maternal Experience-Dependent Cortical Plasticity in Mice Is Circuit- and Stimulus-Specific and Requires MECP2

The neurodevelopmental disorder Rett syndrome is caused by mutations in the gene Mecp2 Misexpression of the protein MECP2 is thought to contribute to neuropathology by causing dysregulation of plasticity. Female heterozygous Mecp2 mutants (Mecp2^het ) failed to acquire a learned maternal retrieval behavior when exposed to pups, an effect linked to disruption of parvalbumin-expressing inhibitory interneurons (PV) in the auditory cortex. Nevertheless, how dysregulated PV networks affect the neural activity dynamics that underlie auditory cortical plasticity during early maternal experience is unknown. Here we show that maternal experience in WT adult female mice (WT) triggers suppression of PV auditory responses. We also observe concomitant disinhibition of auditory responses in deep-layer pyramidal neurons that is selective for behaviorally relevant pup vocalizations. These neurons further exhibit sharpened tuning for pup vocalizations following maternal experience. All of these neuronal changes are abolished in Mecp2^het , suggesting that they are an essential component of maternal learning. This is further supported by our finding that genetic manipulation of GABAergic networks that restores accurate retrieval behavior in Mecp2^het also restores maternal experience-dependent plasticity of PV. Our data are consistent with a growing body of evidence that cortical networks are particularly vulnerable to mutations of Mecp2 in PV neurons. Moreover, our work links, for the first time, impaired in vivo cortical plasticity in awake Mecp2 mutant animals to a natural, ethologically relevant behavior.SIGNIFICANCE STATEMENT Rett syndrome is a genetic disorder that includes language communication problems. Nearly all Rett syndrome is caused by mutations in the gene that produces the protein MECP2, which is important for changes in brain connectivity believed to underlie learning. We previously showed that female Mecp2 mutants fail to learn a simple maternal care behavior performed in response to their pups' distress cries. This impairment appeared to critically involve inhibitory neurons in the auditory cortex called parvalbumin neurons. Here we record from these neurons before and after maternal experience, and we show that they adapt their response to pup calls during maternal learning in nonmutants, but not in mutants. This adaptation is partially restored by a manipulation that improves learning.