Developmental expression of methyl-CpG binding protein 2 is dynamically regulated in the rodent brain

Variable expression of MECP2, CDKL5, and FMR1 in the human brain: Implications for gene restorative therapies

MECP2, CDKL5, and FMR1 are three X-linked neurodevelopmental genes associated with Rett, CDKL5-, and fragile-X syndrome, respectively. These syndromes are characterized by distinct constellations of severe cognitive and neurobehavioral anomalies, reflecting the broad but unique expression patterns of each of the genes in the brain. As these disorders are not thought to be neurodegenerative and may be reversible, a major goal has been to restore expression of the functional proteins in the patient's brain. Strategies have included gene therapy, gene editing, and selective Xi-reactivation methodologies. However, tissue penetration and overall delivery to various regions of the brain remain challenging for each strategy. Thus, gaining insights into how much restoration would be required and what regions/cell types in the brain must be targeted for meaningful physiological improvement would be valuable. As a step toward addressing these questions, here we perform a meta-analysis of single-cell transcriptomics data from the human brain across multiple developmental stages, in various brain regions, and in multiple donors. We observe a substantial degree of expression variability for MECP2, CDKL5, and FMR1 not only across cell types but also between donors. The wide range of expression may help define a therapeutic window, with the low end delineating a minimum level required to restore physiological function and the high end informing toxicology margin. Finally, the inter-cellular and inter-individual variability enable identification of co-varying genes and will facilitate future identification of biomarkers.

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.

Regulation and function of alternative polyadenylation in development and differentiation

Alternative processing of nascent mRNAs is widespread in eukaryotic organisms and greatly impacts the output of gene expression. Specifically, alternative cleavage and polyadenylation (APA) is a co-transcriptional molecular process that switches the polyadenylation site (PAS) at which a nascent mRNA is cleaved, resulting in mRNA isoforms with different 3'UTR length and content. APA can potentially affect mRNA translation efficiency, localization, stability, and mRNA seeded protein-protein interactions. APA naturally occurs during development and cellular differentiation, with around 70% of human genes displaying APA in particular tissues and cell types. For example, neurons tend to express mRNAs with long 3'UTRs due to preferential processing at PASs more distal than other PASs used in other cell types. In addition, changes in APA mark a variety of pathological states, including many types of cancer, in which mRNAs are preferentially cleaved at more proximal PASs, causing expression of mRNA isoforms with short 3'UTRs. Although APA has been widely reported, both the function of APA in development and the mechanisms that regulate the choice of 3'end cut sites in normal and pathogenic conditions are still poorly understood. In this review, we summarize current understanding of how APA is regulated during development and cellular differentiation and how the resulting change in 3'UTR content affects multiple aspects of gene expression. With APA being a widespread phenomenon, the advent of cutting-edge scientific techniques and the pressing need for in-vivo studies, there has never been a better time to delve into the intricate mechanisms of alternative cleavage and polyadenylation.

Aberrant brain functional and structural developments in MECP2 duplication rats

Transgenic animal models with homologous etiology provide a promising way to pursue the neurobiological substrates of the behavioral deficits in autism spectrum disorder (ASD). Gain-of-function mutations of MECP2 cause MECP2 duplication syndrome, a severe neurological disorder with core symptoms of ASD. However, abnormal brain developments underlying the autistic-like behavioral deficits of MECP2 duplication syndrome are rarely investigated. To this end, a human MECP2 duplication (MECP2-DP) rat model was created by the bacterial artificial chromosome transgenic method. Functional and structural magnetic resonance imaging (MRI) with high-field were performed on 16 male MECP2-DP rats and 15 male wildtype rats at postnatal 28 days, 42 days, and 56 days old. Multimodal fusion analyses guided by locomotor-relevant metrics and social novelty time separately were applied to identify abnormal brain networks associated with diverse behavioral deficits induced by MECP2 duplication. Aberrant functional developments of a core network primarily composed of the dorsal medial prefrontal cortex (dmPFC) and retrosplenial cortex (RSP) were detected to associate with diverse behavioral phenotypes in MECP2-DP rats. Altered developments of gray matter volume were detected in the hippocampus and thalamus. We conclude that gain-of-function mutations of MECP2 induce aberrant functional activities in the default-mode-like network and aberrant volumetric changes in the brain, resulting in autistic-like behavioral deficits. Our results gain critical insights into the biomarker of MECP2 duplication syndrome and the neurobiological underpinnings of the behavioral deficits in ASD.

Region-Specific KCC2 Rescue by rhIGF-1 and Oxytocin in a Mouse Model of Rett Syndrome

Rett syndrome (RTT) is characterized by dysfunction in neuronal excitation/inhibition (E/I) balance, potentially impacting seizure susceptibility via deficits in K+/Cl- cotransporter 2 (KCC2) function. Mice lacking the Methyl-CpG binding protein 2 (MeCP2) recapitulate many symptoms of RTT, and recombinant human insulin-like growth factor-1 (rhIGF-1) restores KCC2 expression and E/I balance in MeCP2 KO mice. However, clinical trial outcomes of rhIGF-1 in RTT have been variable, and increasing its therapeutic efficacy is highly desirable. To this end, the neuropeptide oxytocin (OXT) is promising, as it also critically modulates KCC2 function during early postnatal development. We measured basal KCC2 expression levels in MeCP2 KO mice and identified 3 key frontal brain regions showing KCC2 alterations in young adult mice, but not in postnatal P10 animals. We hypothesized that deficits in an IGF-1/OXT signaling crosstalk modulating KCC2 may occur in RTT during postnatal development. Consistently, we detected alterations of IGF-1 receptor and OXT receptor levels in those brain areas. rhIGF-1 and OXT treatments in KO mice rescued KCC2 expression in a region-specific and complementary manner. These results suggest that region-selective combinatorial pharmacotherapeutic strategies could be most effective at normalizing E/I balance in key brain regions subtending the RTT pathophysiology.

Regulation, diversity and function of MECP2 exon and 3'UTR isoforms

The methyl-CpG-binding protein 2 (MECP2) is a critical global regulator of gene expression. Mutations in MECP2 cause neurodevelopmental disorders including Rett syndrome (RTT). MECP2 exon 2 is spliced into two alternative messenger ribonucleic acid (mRNA) isoforms encoding MECP2-E1 or MECP2-E2 protein isoforms that differ in their N-termini. MECP2-E2, isolated first, was used to define the general roles of MECP2 in methyl-deoxyribonucleic acid (DNA) binding, targeting of transcriptional regulatory complexes, and its disease-causing impact in RTT. It was later found that MECP2-E1 is the most abundant isoform in the brain and its exon 1 is also mutated in RTT. MECP2 transcripts undergo alternative polyadenylation generating mRNAs with four possible 3'untranslated region (UTR) lengths ranging from 130 to 8600 nt. Together, the exon and 3'UTR isoforms display remarkable abundance disparity across cell types and tissues during development. These findings indicate discrete means of regulation and suggest that protein isoforms perform non-overlapping roles. Multiple regulatory programs have been explored to explain these disparities. DNA methylation patterns of the MECP2 promoter and first intron impact MECP2-E1 and E2 isoform levels. Networks of microRNAs and RNA-binding proteins also post-transcriptionally regulate the stability and translation efficiency of MECP2 3'UTR isoforms. Finally, distinctions in biophysical properties in the N-termini between MECP2-E1 and E2 lead to variable protein stabilities and DNA binding dynamics. This review describes the steps taken from the discovery of MECP2, the description of its key functions, and its association with RTT, to the emergence of evidence revealing how MECP2 isoforms are differentially regulated at the transcriptional, post-transcriptional and post-translational levels.

Analysis of Astroglial Secretomic Profile in the Mecp2-Deficient Male Mouse Model of Rett Syndrome

Mutations in the X-linked MECP2 gene are responsible for Rett syndrome (RTT), a severe neurological disorder. MECP2 is a transcriptional modulator that finely regulates the expression of many genes, specifically in the central nervous system. Several studies have functionally linked the loss of MECP2 in astrocytes to the appearance and progression of the RTT phenotype in a non-cell autonomous manner and mechanisms are still unknown. Here, we used primary astroglial cells from Mecp2-deficient (KO) pups to identify deregulated secreted proteins. Using a differential quantitative proteomic analysis, twenty-nine proteins have been identified and four were confirmed by Western blotting with new samples as significantly deregulated. To further verify the functional relevance of these proteins in RTT, we tested their effects on the dendritic morphology of primary cortical neurons from Mecp2 KO mice that are known to display shorter dendritic processes. Using Sholl analysis, we found that incubation with Lcn2 or Lgals3 for 48 h was able to significantly increase the dendritic arborization of Mecp2 KO neurons. To our knowledge, this study, through secretomic analysis, is the first to identify astroglial secreted proteins involved in the neuronal RTT phenotype in vitro, which could open new therapeutic avenues for the treatment of Rett syndrome.

Overshooting Subcellular Redox-Responses in Rett-Mouse Hippocampus during Neurotransmitter Stimulation

Rett syndrome (RTT) is a neurodevelopmental disorder associated with disturbed neuronal responsiveness and impaired neuronal network function. Furthermore, mitochondrial alterations and a weakened cellular redox-homeostasis are considered part of the complex pathogenesis. So far, overshooting redox-responses of MeCP2-deficient neurons were observed during oxidant-mediated stress, hypoxia and mitochondrial inhibition. To further clarify the relevance of the fragile redox-balance for the neuronal (dys)function in RTT, we addressed more physiological stimuli and quantified the subcellular redox responses to neurotransmitter-stimulation. The roGFP redox sensor was expressed in either the cytosol or the mitochondrial matrix of cultured mouse hippocampal neurons, and the responses to transient stimulation by glutamate, serotonin, dopamine and norepinephrine were characterized. Each neurotransmitter evoked more intense oxidizing responses in the cytosol of MeCP2-deficient than in wildtype neurons. In the mitochondrial matrix the neurotransmitter-evoked oxidizing changes were more moderate and more uniform among genotypes. This identifies the cytosol as an important reactive oxygen species (ROS) source and as less stably redox buffered. Fura-2 imaging and extracellular Ca²⁺ withdrawal confirmed cytosolic Ca²⁺ transients as a contributing factor of neurotransmitter-induced redox responses and their potentiation in the cytosol of MeCP2-deficient neurons. Chemical uncoupling demonstrated the involvement of mitochondria. Nevertheless, cytosolic NADPH- and xanthine oxidases interact to play the leading role in the neurotransmitter-mediated oxidizing responses. As exaggerated redox-responses were already evident in neonatal MeCP2-deficient neurons, they may contribute remarkably to the altered neuronal network performance and the disturbed neuronal signaling, which are among the hallmarks of RTT.

The MeCP2E1/E2-BDNF-miR132 Homeostasis Regulatory Network Is Region-Dependent in the Human Brain and Is Impaired in Rett Syndrome Patients

Rett Syndrome (RTT) is a rare and progressive neurodevelopmental disorder that is caused by de novo mutations in the X-linked Methyl CpG binding protein 2 (MECP2) gene and is subjected to X-chromosome inactivation. RTT is commonly associated with neurological regression, autistic features, motor control impairment, seizures, loss of speech and purposeful hand movements, mainly affecting females. Different animal and cellular model systems have tremendously contributed to our current knowledge about MeCP2 and RTT. However, the majority of these findings remain unexamined in the brain of RTT patients. Based on previous studies in rodent brains, the highly conserved neuronal microRNA “miR132” was suggested to be an inhibitor of MeCP2 expression. The neuronal miR132 itself is induced by Brain Derived Neurotrophic Factor (BDNF), a neurotransmitter modulator, which in turn is controlled by MeCP2. This makes the basis of the MECP2-BDNF-miR132 feedback regulatory loop in the brain. Here, we studied the components of this feedback regulatory network in humans, and its possible impairment in the brain of RTT patients. In this regard, we evaluated the transcript and protein levels of MECP2/MeCP2E1 and E2 isoforms, BDNF/BDNF, and miR132 (both 3p and 5p strands) by real time RT-PCR, Western blot, and ELISA in four different regions of the human RTT brains and their age-, post-mortem delay-, and sex-matched controls. The transcript level of the studied elements was significantly compromised in RTT patients, even though the change was not identical in different parts of the brain. Our data indicates that MeCP2E1/E2-BDNF protein levels did not follow their corresponding transcript trends. Correlational studies suggested that the MECP2E1/E2-BDNF-miR132 homeostasis regulation might not be similarly controlled in different parts of the human brain. Despite challenges in evaluating autopsy samples in rare diseases, our findings would help to shed some light on RTT pathobiology, and obscurities caused by limited studies on MeCP2 regulation in the human brain.

Altered trajectories of neurodevelopment and behavior in mouse models of Rett syndrome

Rett Syndrome (RTT) is a genetic disorder that is caused by mutations in the x-linked gene coding for methyl-CpG-biding-protein 2 (MECP2) and that mainly affects females. Male and female transgenic mouse models of RTT have been studied extensively, and we have learned a great deal regarding RTT neuropathology and how MeCP2 deficiency may be influencing brain function and maturation. In this manuscript we review what is known concerning structural and coinciding functional and behavioral deficits in RTT and in mouse models of MeCP2 deficiency. We also introduce our own corroborating data regarding behavioral phenotype and morphological alterations in volume of the cortex and striatum and the density of neurons, aberrations in experience-dependent plasticity within the barrel cortex and the impact of MeCP2 loss on glial structure. We conclude that regional structural changes in genetic models of RTT show great similarity to the alterations in brain structure of patients with RTT. These region-specific modifications often coincide with phenotype onset and contribute to larger issues of circuit connectivity, progression, and severity. Although the alterations seen in mouse models of RTT appear to be primarily due to cell-autonomous effects, there are also non-cell autonomous mechanisms including those caused by MeCP2-deficient glia that negatively impact healthy neuronal function. Collectively, this body of work has provided a solid foundation on which to continue to build our understanding of the role of MeCP2 on neuronal and glial structure and function, its greater impact on neural development, and potential new therapeutic avenues.

Epigenetic modifications associated with pathophysiological effects of lead exposure

Lead (Pb) exposure during different stages of development has demonstrated dose, duration, sex, and tissue-specific pathophysiological outcomes due to altered epigenetic regulation via (a) DNA methylation, (b) histone modifications, (c) miRNAs, and (d) chromatin accessibility. Pb-induced alteration of epigenetic regulation causes neurotoxic and extra-neurotoxic pathophysiological outcomes. Neurotoxic effects of Pb include dysfunction of memory and learning, behavioral disorder, attention deficit hyperactivity disorder, autism spectrum disorder, aging, Alzheimer's disease, tauopathy, and neurodegeneration. Extra-neurotoxic effects of Pb include altered body weight, metabolic disorder, cardiovascular disorders, hematopoietic disorder, and reproductive impairment. Pb exposure either early in life or at any stage of development results in undesirable pathophysiological outcomes that tends to sustain and maintain for a lifetime.

Transcriptomic Analysis of Mecp2 Mutant Mice Reveals Differentially Expressed Genes and Altered Mechanisms in Both Blood and Brain

Rett syndrome is a rare neuropsychiatric disorder with a wide symptomatology including impaired communication and movement, cardio-respiratory abnormalities, and seizures. The clinical presentation is typically associated to mutations in the gene coding for the methyl-CpG-binding protein 2 (MECP2), which is a transcription factor. The gene is ubiquitously present in all the cells of the organism with a peak of expression in neurons. For this reason, most of the studies in Rett models have been performed in brain. However, some of the symptoms of Rett are linked to the peripheral expression of MECP2, suggesting that the effects of the mutations affect gene expression levels in tissues other than the brain. We used RNA sequencing in Mecp2 mutant mice and matched controls, to identify common genes and pathways differentially regulated across different tissues. We performed our study in brain and peripheral blood, and we identified differentially expressed genes (DEGs) and pathways in each tissue. Then, we compared the genes and mechanisms identified in each preparation. We found that some genes and molecular pathways that are differentially expressed in brain are also differentially expressed in blood of Mecp2 mutant mice at a symptomatic-but not presymptomatic-stage. This is the case for the gene Ube2v1, linked to ubiquitination system, and Serpin1, involved in complement and coagulation cascades. Analysis of biological functions in the brain shows the enrichment of mechanisms correlated to circadian rhythms, while in the blood are enriched the mechanisms of response to stimulus-including immune response. Some mechanisms are enriched in both preparations, such as lipid metabolism and response to stress. These results suggest that analysis of peripheral blood can reveal ubiquitous altered molecular mechanisms of Rett and have applications in diagnosis and treatments' assessments.

Epigenetic Regulations in Neuropsychiatric Disorders

Precise genetic and epigenetic spatiotemporal regulation of gene expression is critical for proper brain development, function and circuitry formation in the mammalian central nervous system. Neuronal differentiation processes are tightly regulated by epigenetic mechanisms including DNA methylation, histone modifications, chromatin remodelers and non-coding RNAs. Dysregulation of any of these pathways is detrimental to normal neuronal development and functions, which can result in devastating neuropsychiatric disorders, such as depression, schizophrenia and autism spectrum disorders. In this review, we focus on the current understanding of epigenetic regulations in brain development and functions, as well as their implications in neuropsychiatric disorders.

Lack of MeCP2 leads to region-specific increase of doublecortin in the olfactory system

The protein doublecortin is mainly expressed in migrating neuroblasts and immature neurons. The X-linked gene MECP2, associated to several neurodevelopmental disorders such as Rett syndrome, encodes the protein methyl-CpG-binding protein 2 (MeCP2), a regulatory protein that has been implicated in neuronal maturation and refinement of olfactory circuits. Here, we explored doublecortin immunoreactivity in the brain of young adult female Mecp2-heterozygous and male Mecp2-null mice and their wild-type littermates. The distribution of doublecortin-immunoreactive somata in neurogenic brain regions was consistent with previous reports in rodents, and no qualitative differences were found between genotypes or sexes. Quantitatively, we found a significant increase in doublecortin cell density in the piriform cortex of Mecp2-null males as compared to WT littermates. A similar increase was seen in a newly identified population of doublecortin cells in the olfactory tubercle. In these olfactory structures, however, the percentage of doublecortin immature neurons that also expressed NeuN was not different between genotypes. By contrast, we found no significant differences between genotypes in doublecortin immunoreactivity in the olfactory bulbs. Nonetheless, in the periglomerular layer of Mecp2-null males, we observed a specific decrease of immature neurons co-expressing doublecortin and NeuN. Overall, no differences were evident between Mecp2-heterozygous and WT females. In addition, no differences could be detected between genotypes in the density of doublecortin-immunoreactive cells in the hippocampus or striatum of either males or females. Our results suggest that MeCP2 is involved in neuronal maturation in a region-dependent manner.

Surface- and voxel-based brain morphologic study in Rett and Rett-like syndrome with MECP2 mutation

Rett syndrome (RTT) is a rare congenital disorder which in most cases (95%) is caused by methyl-CpG binding protein 2 (MECP2) mutations. RTT is characterized by regression in global development, epilepsy, autistic features, acquired microcephaly, habitual hand clapping, loss of purposeful hand skills, and autonomic dysfunctions. Although the literature has demonstrated decreased volumes of the cerebrum, cerebellum, and the caudate nucleus in RTT patients, surface-based brain morphology including cortical thickness and cortical gyrification analyses are lacking in RTT. We present quantitative surface- and voxel-based morphological measurements in young children with RTT and Rett-like syndrome (RTT-l) with MECP2 mutations. The 8 structural T1-weighted MR images were obtained from 7 female patients with MECP2 mutations (3 classic RTT, 2 variant RTT, and 2 RTT-l) (mean age 5.2 [standard deviation 3.3] years old). Our analyses demonstrated decreased total volumes of the cerebellum in RTT/RTT-l compared to gender- and age-matched controls (t (22)=-2.93, p = .008, Cohen's d = 1.27). In contrast, global cerebral cortical surface areas, global/regional cortical thicknesses, the degree of global gyrification, and global/regional gray and white matter volumes were not statistically significantly different between the two groups. Our findings, as well as literature findings, suggest that early brain abnormalities associated with RTT/RTT-l (with MECP2 mutations) can be detected as regionally decreased cerebellar volumes. Decreased cerebellar volume may be helpful for understanding the etiology of RTT/RTT-l.

Correcting deregulated Fxyd1 expression rescues deficits in neuronal arborization and potassium homeostasis in MeCP2 deficient male mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the MECP2 gene. In the absence of MeCP2, expression of FXYD domain-containing transport regulator 1 (FXYD1) is deregulated in the frontal cortex (FC) of mice and humans. Because Fxyd1 is a membrane protein that controls cell excitability by modulating Na⁺, K⁺-ATPase activity (NKA), an excess of Fxyd1 may reduce NKA activity and contribute to the neuronal phenotype of Mecp2 deficient (KO) mice. To determine if Fxyd1 can rescue these RTT deficits, we studied the male progeny of Fxyd1 null males bred to heterozygous Mecp2 female mice. Maximal NKA enzymatic activity was not altered by the loss of MeCP2, but it increased in mice lacking one Fxyd1 allele, suggesting that NKA activity is under Fxyd1 inhibitory control. Deletion of one Fxyd1 allele also prevented the increased extracellular potassium (K⁺) accumulation observed in cerebro-cortical neurons from Mecp2 KO animals in response to the NKA inhibitor ouabain, and rescued the loss of dendritic arborization observed in FC neurons of Mecp2 KO mice. These effects were gene-dose dependent, because the absence of Fxyd1 failed to rescue the MeCP2-dependent deficits, and mimicked the effect of MeCP2 deficiency in wild-type animals. These results indicate that excess of Fxyd1 in the absence of MeCP2 results in deregulation of endogenous K⁺ conductances functionally associated with NKA and leads to stunted neuronal growth.

Are dopamine receptor and transporter changes in Rett syndrome reflected in Mecp2-deficient mice?

We tested the claim that the dopaminergic dysfunction of Rett Syndrome (RTT) also occurs in Mecp2-deficient mice that serve as a model of the syndrome. We used positron emission tomography (PET) to image dopamine D₂ receptors (D₂R) and transporters (DAT) in women with RTT and in Mecp2-deficient mice, and D₁R and D₂R density was measured in postmortem human tissue by autoradiography. Results showed 1) significantly reduced D₂R density in the striatum of women with RTT compared to control subjects. 2) PET imaging of mouse striatum similarly demonstrated significant reductions in D₂R density of 7-10 week-old hemizygous (Mecp2-null) and heterozygous (HET) mice compared to wild type (WT) mice. With age, the density of D₂R declined in WT mice but not HET mice. 3) In contrast, postmortem autoradiography revealed no group differences in the density of D₁R and D₂R in the caudate and putamen of RTT versus normal control subjects. 4) In humans and in the mouse model, PET revealed only marginal group differences in DAT. The results confirm that dopaminergic dysfunction in RTT is also present in Mecp2-deficient mice and that reductions in D₂R more likely explain the impaired ambulation and progressive rigidity observed rather than alterations in DAT.

Regulation of seizure-induced MeCP2 Ser421 phosphorylation in the developing brain

Neonatal seizures disrupt normal synaptic maturation and often lead to later-life epilepsy and cognitive deficits. During early life, the brain exhibits heightened synaptic plasticity, in part due to a developmental overabundance of Ca_V1.2 L-type voltage gated calcium (Ca²⁺) channels (LT-VGCCs) and Ca²⁺-permeable AMPARs (CP-AMPARs) lacking GluA2 subunits. We hypothesized that early-life seizures overactivate these channels, in turn dysregulating Ca²⁺-dependent signaling pathways including that of methyl CPG binding protein 2 (MeCP2), a transcription factor implicated in the autism spectrum disorder (ASD) Rett Syndrome. Here, we show that in vivo hypoxia-induced seizures (HS) in postnatal day (P)10 rats acutely induced phosphorylation of the neuronal-specific target of activity-dependent MeCP2 phosphorylation, S421, as well as its upstream activator CaMKII T286. We next identified mechanisms by which activity-dependent Ca²⁺ influx induced MeCP2 phosphorylation using in vitro cortical and hippocampal neuronal cultures at embryonic day (E)18 + 10 days in vitro (DIV). In contrast to the prevalent role of NMDARs in the adult brain, we found that both CP-AMPARs and LT-VGCCs mediated MeCP2 S421 and CaMKII T286 phosphorylation induced by kainic acid (KA) or high potassium chloride (KCl) stimulation. Furthermore, in vivo post-seizure treatment with the broad-spectrum AMPAR antagonist NBQX, the CP-AMPAR blocker IEM-1460, or the LT-VGCC antagonist nimodipine blocked seizure-induced MeCP2 phosphorylation. Collectively, these results demonstrate that early-life seizures dysregulate critical activity-dependent developmental signaling pathways, in part via CP-AMPAR and LT-VGCC activation, providing novel age-specific therapeutic targets for convergent pathways underlying epilepsy and ASDs.

Long INterspersed nuclear Elements (LINEs) in brain and non-brain tissues of the rat

Long INterspersed Element-1 (L1) is a transposable element that can insert copies of itself in new genomic locations causing genomic instability. In somatic cells, L1 retrotransposition activity is usually repressed but somatic L1 retrotransposition has recently been observed during neuronal differentiation. In this study, we evaluate whether L1 elements are differentially active in rat tissues during postnatal development. To this purpose, we quantified L1 in genomic DNA extracted from the olfactory bulb (OB), cerebellum (CE), cortex (CO) and heart (H). Each analysis was repeated on rats aged 7, 21 and 60 days. We found that L1 content in OB and CE tissue was significantly higher than H tissue, in rats of all three ages studied, suggesting that L1 activity could be modulated in postnatal development and neurogenesis.

DNA Methylation program in normal and alcohol-induced thinning cortex

While cerebral underdevelopment is a hallmark of fetal alcohol spectrum disorders (FASD), the mechanism(s) guiding the broad cortical neurodevelopmental deficits are not clear. DNA methylation is known to regulate early development and tissue specification through gene regulation. Here, we examined DNA methylation in the onset of alcohol-induced cortical thinning in a mouse model of FASD. C57BL/6 (B6) mice were administered a 4% alcohol (v/v) liquid diet from embryonic (E) days 7-16, and their embryos were harvested at E17, along with isocaloric liquid diet and lab chow controls. Cortical neuroanatomy, neural phenotypes, and epigenetic markers of methylation were assessed using immunohistochemistry, Western blot, and methyl-DNA assays. We report that cortical thickness, neuroepithelial proliferation, and neuronal migration and maturity were found to be deterred by alcohol at E17. Simultaneously, DNA methylation, including 5-methylcytosine (5mC) and 5-hydroxcylmethylcytosine (5hmC), which progresses as an intrinsic program guiding normal embryonic cortical development, was severely affected by in utero alcohol exposure. The intricate relationship between cortical thinning and this DNA methylation program disruption is detailed and illustrated. DNA methylation, dynamic across the multiple cortical layers during the late embryonic stage, is highly disrupted by fetal alcohol exposure; this disruption occurs in tandem with characteristic developmental abnormalities, ranging from structural to molecular. Finally, our findings point to a significant question for future exploration: whether epigenetics guides neurodevelopment or whether developmental conditions dictate epigenetic dynamics in the context of alcohol-induced cortical teratogenesis.

Cerebellar and Striatal Pathologies in Mouse Models of Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition with a strong genetic component. To date, several hundred different genetic mutations have been identified to play a role in its aetiology. The heterogeneity of genetic abnormalities combined with the different brain regions where aberrations are found makes the search for causative mechanisms a daunting task. Even within a limited number of brain regions, a myriad of different neural circuit dysfunctions may lead to ASD. Here, we review mouse models that incorporate mutations of ASD risk genes causing pathologies in the cerebellum and striatum and highlight the vulnerability of related circuit dysfunctions within these brain regions in ASD pathophysiology.

Expression of Phospho-MeCP2s in the Developing Rat Brain and Function of Postnatal MeCP2 in Cerebellar Neural Cell Development

Abnormal expression and dysfunction of methyl-CpG binding protein 2 (MeCP2) cause Rett syndrome (RTT). The diverse phosphorylation modifications modulate MeCP2 function in neural cells. Using western blot and immunohistochemistry, we examined the expression patterns of MeCP2 and three phospho-MeCP2s (pMeCP2s) in the developing rat brain. The expression of MeCP2 and phospho-S80 (pS80) MeCP2 increased while pS421 MeCP2 and pS292 MeCP2 decreased with brain maturation. In contrast to the nuclear localization of MeCP2 and pS80 MeCP2, pS421 MeCP2 and pS292 MeCP2 were mainly expressed in the cytoplasmic compartment. Apart from their distribution in neurons, they were also detected at a low level in astrocytes. Postnatally-initiated MeCP2 deficiency affected cerebellar neural cell development, as determined by the abnormal expression of GFAP, DCX, Tuj1, MAP-2, and calbindin-D28k. Together, these results demonstrate that MeCP2 and diverse pMeCP2s have distinct features of spatio-temporal expression in the rat brain, and that the precise levels of MeCP2 in the postnatal period are vital to cerebellar neural cell development.

Down-regulation of nestin in mesenchymal stem cells derived from peripheral blood through blocking bone morphogenesis pathway

Different signaling pathways are implicated in proliferation and differentiation of stem cells. Bone Morphogenesis Pathway (BMP) signaling was known to display an important function in osteogenic and adipogenic differentiation of mesenchymal stem cells (MSCs). In the present study, the authors investigated whether blocking BMP signaling was associated with down regulation of Nestin expression as neural stem cell marker in peripheral blood derived mesenchymal stem cells (PB-MSCs). At first, MSCs were isolated from peripheral blood by plastic adherent ability and flow cytometry analysis. After reaching the confluence, the cells were treated with medium containing Noggin as antagonist of BMP signaling upon 8 days. Real time PCR analysis indicated that the expression of Nestin was diminished in PB-MSCs by attenuating BMP signaling. The obtained results suggested that BMP signaling might have a regulatory function on the Nestin expression in mesenchymal stem cells.

A review of Rett syndrome (RTT) with induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSCs) are pluripotent stem cells generated from somatic cells by the introduction of a combination of pluripotency-associated genes such as OCT4, SOX2, along with either KLF4 and c-MYC or NANOG and LIN28 via retroviral or lentiviral vectors. Most importantly, hiPSCs are similar to human embryonic stem cells (hESCs) functionally as they are pluripotent and can potentially differentiate into any desired cell type when provided with the appropriate cues, but do not have the ethical issues surrounding hESCs. For these reasons, hiPSCs have huge potential in translational medicine such as disease modeling, drug screening, and cellular therapy. Indeed, patient-specific hiPSCs have been generated for a multitude of diseases, including many with a neurological basis, in which disease phenotypes have been recapitulated in vitro and proof-of-principle drug screening has been performed. As the techniques for generating hiPSCs are refined and these cells become a more widely used tool for understanding brain development, the insights they produce must be understood in the context of the greater complexity of the human genome and the human brain. Disease models using iPS from Rett syndrome (RTT) patient's fibroblasts have opened up a new avenue of drug discovery for therapeutic treatment of RTT. The analysis of X chromosome inactivation (XCI) upon differentiation of RTT-hiPSCs into neurons will be critical to conclusively demonstrate the isolation of pre-XCI RTT-hiPSCs in comparison to post-XCI RTT-hiPSCs. The current review projects on iPSC studies in RTT as well as XCI in hiPSC were it suggests for screening new potential therapeutic targets for RTT in future for the benefit of RTT patients. In conclusion, patient-specific drug screening might be feasible and would be particularly helpful in disorders where patients frequently have to try multiple drugs before finding a regimen that works.

Epigenetics and Human Disease

Genetic causes for human disorders are being discovered at an unprecedented pace. A growing subclass of disease-causing mutations involves changes in the epigenome or in the abundance and activity of proteins that regulate chromatin structure. This article focuses on research that has uncovered human diseases that stem from such epigenetic deregulation. Disease may be caused by direct changes in epigenetic marks, such as DNA methylation, commonly found to affect imprinted gene regulation. Also described are disease-causing genetic mutations in epigenetic modifiers that either affect chromatin in trans or have a cis effect in altering chromatin configuration.

Regulatory functions and pathological relevance of the MECP2 3'UTR in the central nervous system

Methyl-CpG-binding protein 2 (MeCP2), encoded by the gene MECP2, is a transcriptional regulator and chromatin-remodeling protein, which is ubiquitously expressed and plays an essential role in the development and maintenance of the central nervous system (CNS). Highly enriched in post-migratory neurons, MeCP2 is needed for neuronal maturation, including dendritic arborization and the development of synapses. Loss-of-function mutations in MECP2 cause Rett syndrome (RTT), a debilitating neurodevelopmental disorder characterized by a phase of normal development, followed by the progressive loss of milestones and cognitive disability. While a great deal has been discovered about the structure, function, and regulation of MeCP2 in the time since its discovery as the genetic cause of RTT, including its involvement in a number of RTT-related syndromes that have come to be known as MeCP2-spectrum disorders, much about this multifunctional protein remains enigmatic. One unequivocal fact that has become apparent is the importance of maintaining MeCP2 protein levels within a narrow range, the limits of which may depend upon the cell type and developmental time point. As such, MeCP2 is amenable to complex, multifactorial regulation. Here, we summarize the role of the MECP2 3' untranslated region (UTR) in the regulation of MeCP2 protein levels and how mutations in this region contribute to autism and other non-RTT neuropsychiatric disorders.

Cerebellar associative sensory learning defects in five mouse autism models

Sensory integration difficulties have been reported in autism, but their underlying brain-circuit mechanisms are underexplored. Using five autism-related mouse models, Shank3+/ΔC, Mecp2(R308/Y), Cntnap2-/-, L7-Tsc1 (L7/Pcp2(Cre)::Tsc1(flox/+)), and patDp(15q11-13)/+, we report specific perturbations in delay eyeblink conditioning, a form of associative sensory learning requiring cerebellar plasticity. By distinguishing perturbations in the probability and characteristics of learned responses, we found that probability was reduced in Cntnap2-/-, patDp(15q11-13)/+, and L7/Pcp2(Cre)::Tsc1(flox/+), which are associated with Purkinje-cell/deep-nuclear gene expression, along with Shank3+/ΔC. Amplitudes were smaller in L7/Pcp2(Cre)::Tsc1(flox/+) as well as Shank3+/ΔC and Mecp2(R308/Y), which are associated with granule cell pathway expression. Shank3+/ΔC and Mecp2(R308/Y) also showed aberrant response timing and reduced Purkinje-cell dendritic spine density. Overall, our observations are potentially accounted for by defects in instructed learning in the olivocerebellar loop and response representation in the granule cell pathway. Our findings indicate that defects in associative temporal binding of sensory events are widespread in autism mouse models.

Rett syndrome: a complex disorder with simple roots

Rett syndrome (RTT) is a severe neurological disorder caused by mutations in the X-linked gene MECP2 (methyl-CpG-binding protein 2). Two decades of research have fostered the view that MeCP2 is a multifunctional chromatin protein that integrates diverse aspects of neuronal biology. More recently, studies have focused on specific RTT-associated mutations within the protein. This work has yielded molecular insights into the critical functions of MeCP2 that promise to simplify our understanding of RTT pathology.

Evaluation of current pharmacological treatment options in the management of Rett syndrome: from the present to future therapeutic alternatives

Neurodevelopmental disorders are a large family of conditions of genetic or environmental origin that are characterized by deficiencies in cognitive and behavioral functions. The therapeutic management of individuals with these disorders is typically complex and is limited to the treatment of specific symptoms that characterize each disorder. The neurodevelopmental disorder Rett syndrome (RTT) is the leading cause of severe intellectual disability in females. Mutations in the gene encoding the transcriptional regulator methyl-CpG-binding protein 2 (MECP2), located on the X chromosome, have been confirmed in more than 95% of individuals meeting diagnostic criteria for classical RTT. RTT is characterized by an uneventful early infancy followed by stagnation and regression of growth, motor, language, and social skills later in development. This review will discuss the genetics, pathology, and symptoms that distinguish RTT from other neurodevelopmental disorders associated with intellectual disability. Because great progress has been made in the basic and clinical science of RTT, the goal of this review is to provide a thorough assessment of current pharmacotherapeutic options to treat the symptoms associated with this disorder. Furthermore, we will highlight recent discoveries made with novel pharmacological interventions in experimental preclinical phases, and which have reversed pathological phenotypes in mouse and cell culture models of RTT and may result in clinical trials.

MeCP2-Related Diseases and Animal Models

The role of epigenetics in human disease has become an area of increased research interest. Collaborative efforts from scientists and clinicians have led to a better understanding of the molecular mechanisms by which epigenetic regulation is involved in the pathogenesis of many human diseases. Several neurological and non-neurological disorders are associated with mutations in genes that encode for epigenetic factors. One of the most studied proteins that impacts human disease and is associated with deregulation of epigenetic processes is Methyl CpG binding protein 2 (MeCP2). MeCP2 is an epigenetic regulator that modulates gene expression by translating epigenetic DNA methylation marks into appropriate cellular responses. In order to highlight the importance of epigenetics to development and disease, we will discuss how MeCP2 emerges as a key epigenetic player in human neurodevelopmental, neurological, and non-neurological disorders. We will review our current knowledge on MeCP2-related diseases, including Rett Syndrome, Angelman Syndrome, Fetal Alcohol Spectrum Disorder, Hirschsprung disease, and Cancer. Additionally, we will briefly discuss about the existing MeCP2 animal models that have been generated for a better understanding of how MeCP2 impacts certain human diseases.

Genetic diseases: congenital central hypoventilation, Rett, and Prader-Willi syndromes

The present review summarizes current knowledge on three rare genetic disorders of respiratory control, congenital central hypoventilation syndrome (CCHS), Rett syndrome (RTT), and Prader-Willi syndrome (PWS). CCHS is characterized by lack of ventilatory chemosensitivity caused by PHOX2B gene abnormalities consisting mainly of alanine expansions. RTT is associated with episodes of tachypneic and irregular breathing intermixed with breathholds and apneas and is caused by mutations in the X-linked MECP2 gene encoding methyl-CpG-binding protein. PWS manifests as sleep-disordered breathing with apneas and episodes of hypoventilation and is caused by the loss of a group of paternally inherited genes on chromosome 15. CCHS is the most specific disorder of respiratory control, whereas the breathing disorders in RTT and PWS are components of a more general developmental disorder. The main clinical features of these three disorders are reviewed with special emphasis on the associated brain abnormalities. In all three syndromes, disease-causing genetic defects have been identified, allowing the development of genetically engineered mouse models. New directions for future therapies based on these models or, in some cases, on clinical experience are delineated. Studies of CCHS, RTT, and PWS extend our knowledge of the molecular and cellular aspects of respiratory rhythm generation and suggest possible pharmacological approaches to respiratory control disorders. This knowledge is relevant for the clinical management of many respiratory disorders that are far more prevalent than the rare diseases discussed here.

Synaptic plasticity and signaling in Rett syndrome

Rett syndrome (RTT) is a disorder that is caused in the majority of cases by mutations in the gene methyl-CpG-binding protein-2 (MeCP2). Children with RTT are generally characterized by normal development up to the first year and a half of age, after which they undergo a rapid regression marked by a deceleration of head growth, the onset of stereotyped hand movements, irregular breathing, and seizures. Animal models of RTT with good construct and face validity are available. Their analysis showed that homeostatic regulation of MeCP2 gene is necessary for normal CNS functioning and that multiple complex pathways involving different neuronal and glial cell types are disrupted in RTT models. However, it is increasingly clear that RTT pathogenetic mechanisms converge at synaptic level impairing synaptic transmission and plasticity. We review novel findings showing how specific synaptic mechanisms and related signaling pathways are affected in RTT models.

Autism spectrum disorder and the cerebellum

The cerebellum has been long known for its importance in motor learning and coordination. Recently, anatomical, clinical, and neuroimaging studies strongly suggest that the cerebellum supports cognitive functions, including language and executive functions, as well as affective regulation. Furthermore, the cerebellum has emerged as one of the key brain regions affected in autism. Here, we discuss our current understanding of the role of the cerebellum in autism, including evidence from genetic, molecular, clinical, behavioral, and neuroimaging studies. Cerebellar findings in autism suggest developmental differences at multiple levels of neural structure and function, indicating that the cerebellum is an important player in the complex neural underpinnings of autism spectrum disorder, with behavioral implications beyond the motor domain.

Influence of developmental lead exposure on expression of DNA methyltransferases and methyl cytosine-binding proteins in hippocampus

Developmental exposure to lead (Pb) has adverse effects on cognitive functioning and behavior that can persist into adulthood. Exposures that occur during fetal or early life periods may produce changes in brain related to physiological re-programming from an epigenetic influence such as altered DNA methylation status. Since DNA methylation is regulated by DNA methyltransferases and methyl cytosine-binding proteins, this study assessed the extent to which developmental Pb exposure might affect expression of these proteins in the hippocampus. Long Evans dams were fed chow with or without added Pb acetate (0, 150, 375, 750 ppm) prior to breeding and remained on the same diet through weaning (perinatal exposure group). Other animals were exposed to the same doses of Pb but exposure started on postnatal day 1 and continued through weaning (early postnatal exposure group). All animals were euthanized on day 55 and hippocampi were removed. Western blot analyses showed significant effects of Pb exposure on DNMT1, DNMT3a, and MeCP2 expression, with effects often seen at the lowest level of exposure and modified by sex and developmental window of Pb exposure. These data suggest potential epigenetic effects of developmental Pb exposure on DNA methylation mediated at least in part through dysregulation of methyltransferases.

A critical and cell-autonomous role for MeCP2 in synaptic scaling up

Rett syndrome (Rett) is the leading genetic cause of mental retardation in females. Most cases of Rett are caused by loss-of-function mutations in the gene coding for the transcriptional regulator methyl-CpG binding protein 2 (MeCP2), but despite much effort, it remains unclear how a loss of MeCP2 function generates the neurological deficits of Rett. Here we show that MeCP2 plays an essential and cell-autonomous role in homeostatic synaptic scaling up in response to reduced firing or reduced sensory drive in rat visual cortical pyramidal neurons. We found that acute RNAi knockdown of MeCP2 blocked synaptic scaling within targeted neocortical pyramidal neurons. Furthermore, MeCP2 knockdown decreased excitatory synapse number without affecting basal mEPSC amplitude or AMPAR accumulation at spared synapses, demonstrating that MeCP2 acts cell-autonomously to maintain both excitatory synapse number and synaptic scaling in individual neocortical neurons. Finally, we used a mouse model of Rett to show that MeCP2 loss prevents homeostatic synaptic scaling up in response to visual deprivation in vivo, demonstrating for the first time that MeCP2 loss disrupts homeostatic plasticity within the intact developing neocortex. Our results establish MeCP2 as a critical mediator of synaptic scaling and raise the possibility that some of the neurological defects of Rett arise from a disruption of homeostatic plasticity.

Acetyl-L-carnitine improves behavior and dendritic morphology in a mouse model of Rett syndrome

Rett syndrome (RTT) is a devastating neurodevelopmental disorder affecting 1 in 10,000 girls. Approximately 90% of cases are caused by spontaneous mutations in the X-linked gene encoding methyl-CpG-binding protein 2 (MeCP2). Girls with RTT suffer from severe motor, respiratory, cognitive and social abnormalities attributed to early deficits in synaptic connectivity which manifest in the adult as a myriad of physiological and anatomical abnormalities including, but not limited to, dimished dendritic complexity. Supplementation with acetyl-L-carnitine (ALC), an acetyl group donor, ameliorates motor and cognitive deficits in other disease models through a variety of mechanisms including altering patterns of histone acetylation resulting in changes in gene expression, and stimulating biosynthetic pathways such as acetylcholine. We hypothesized ALC treatment during critical periods in cortical development would promote normal synaptic maturation, and continuing treatment would improve behavioral deficits in the Mecp2(1lox) mouse model of RTT. In this study, wildtype and Mecp2(1lox) mutant mice received daily injections of ALC from birth until death (postnatal day 47). General health, motor, respiratory, and cognitive functions were assessed at several time points during symptom progression. ALC improved weight gain, grip strength, activity levels, prevented metabolic abnormalities and modestly improved cognitive function in Mecp2 null mice early in the course of treatment, but did not significantly improve motor or cognitive functions assessed later in life. ALC treatment from birth was associated with an almost complete rescue of hippocampal dendritic morphology abnormalities with no discernable side effects in the mutant mice. Therefore, ALC appears to be a promising therapeutic approach to treating early RTT symptoms and may be useful in combination with other therapies.

Relationship between Mecp2 and NFκb signaling during neural differentiation of P19 cells

The pluripotent P19 embryo carcinoma cell line was studied to determine a signaling pathway regulating MeCP2 expression. P19 cells were induced to differentiate into neurons by RA and express β-III tubulin at one day after induction and synaptophysin by 7 days. MeCP2 was first observed after β-III tubulin expression was detected and continued to rise over the course of differentiation. Both Mecp2 e1 and e2 mRNA forms progressively increased in differentiating cells. MeCP2 expression was increased by tumor necrosis factor (TNF) in early differentiating cells, which was blocked by NFκB inhibitors. TNF did not increase MeCP2 expression in naïve cells. Moreover, TNF did not increase NFκB reporter gene activity in naïve cells even though increases were observed in early differentiating cells. The protein kinase C activator phorbol 12-myristate 13-acetate (PMA) increased MeCP2 expression in naïve P19 cells, which was also blocked by NFκB inhibitors. Interestingly, PMA increased NFκB reporter gene activity in naïve cells. Finally, PMA, but not TNF, induced IκBα degradation in naïve P19 cells. Taken together, our data indicates that MeCP2 expression is regulated in part by signaling pathways involving NFκB.

Improved survival and reduced phenotypic severity following AAV9/MECP2 gene transfer to neonatal and juvenile male Mecp2 knockout mice

Typical Rett syndrome (RTT) is a pediatric disorder caused by loss-of-function mutations in the methyl-CpG binding protein 2 (MECP2) gene. The demonstrated reversibility of RTT-like phenotypes in mice suggests that MECP2 gene replacement is a potential therapeutic option in patients. We report improvements in survival and phenotypic severity in Mecp2-null male mice after neonatal intracranial delivery of a single-stranded (ss) AAV9/chicken β-actin (CBA)-MECP2 vector. Median survival was 16.6 weeks for MECP2-treated versus 9.3 weeks for green fluorescent protein (GFP)-treated mice. ssAAV9/CBA-MECP2-treated mice also showed significant improvement in the phenotype severity score, in locomotor function, and in exploratory activity, as well as a normalization of neuronal nuclear volume in transduced cells. Wild-type (WT) mice receiving neonatal injections of the same ssAAV9/CBA-MECP2 vector did not show any significant deficits, suggesting a tolerance for modest MeCP2 overexpression. To test a MECP2 gene replacement approach in a manner more relevant for human translation, a self-complementary (sc) adeno-associated virus (AAV) vector designed to drive MeCP2 expression from a fragment of the Mecp2 promoter was injected intravenously (IV) into juvenile (4-5 weeks old) Mecp2-null mice. While the brain transduction efficiency in juvenile mice was low (~2-4% of neurons), modest improvements in survival were still observed. These results support the concept of MECP2 gene therapy for RTT.

Postnatal inactivation reveals enhanced requirement for MeCP2 at distinct age windows

Rett Syndrome is a neurological disorder caused by mutations in the X-linked MECP2 gene. Mouse models where Mecp2 is inactivated or mutated recapitulate several features of the disorder and have demonstrated a requirement for the protein to ensure brain function in adult mice. We deleted the Mecp2 gene in ~80% of brain cells at three postnatal ages to determine whether the need for MeCP2 varies with age. Inactivation at all three time points induced Rett-like phenotypes and caused premature death of the animals. We find two threshold ages beyond which the requirement for MeCP2 markedly increases in stringency. The earlier threshold (8-14 weeks), when inactivated mice develop symptoms, represents early adulthood in the mouse and coincides with the period when Mecp2-null mice exhibit terminal symptoms. Unexpectedly, we identified a later age threshold (30-45 weeks) beyond which an 80% reduction in MeCP2 is incompatible with life. This finding suggests an enhanced role for MeCP2 in the aging brain.

Does microglial dysfunction play a role in autism and Rett syndrome?

Autism spectrum disorders (ASDs) including classic autism is a group of complex developmental disabilities with core deficits of impaired social interactions, communication difficulties and repetitive behaviors. Although the neurobiology of ASDs has attracted much attention in the last two decades, the role of microglia has been ignored. Existing data are focused on their recognized role in neuroinflammation, which only covers a small part of the pathological repertoire of microglia. This review highlights recent findings on the broader roles of microglia, including their active surveillance of brain microenvironments and regulation of synaptic connectivity, maturation of brain circuitry and neurogenesis. Emerging evidence suggests that microglia respond to pre- and postnatal environmental stimuli through epigenetic interface to change gene expression, thus acting as effectors of experience-dependent synaptic plasticity. Impairments of these microglial functions could substantially contribute to several major etiological factors of autism, such as environmental toxins and cortical underconnectivity. Our recent study on Rett syndrome, a syndromic autistic disorder, provides an example that intrinsic microglial dysfunction due to genetic and epigenetic aberrations could detrimentally affect the developmental trajectory without evoking neuroinflammation. We propose that ASDs provide excellent opportunities to study the influence of microglia on neurodevelopment, and this knowledge could lead to novel therapies.

Sensitive time-windows for susceptibility in neurodevelopmental disorders

Many neurodevelopmental disorders (NDDs) are characterized by age-dependent symptom onset and regression, particularly during early postnatal periods of life. The neurobiological mechanisms preceding and underlying these developmental cognitive and behavioral impairments are, however, not clearly understood. Recent evidence using animal models for monogenic NDDs demonstrates the existence of time-regulated windows of neuronal and synaptic impairments. We propose that these developmentally-dependent impairments can be unified into a key concept: namely, time-restricted windows for impaired synaptic phenotypes exist in NDDs, akin to critical periods during normal sensory development in the brain. Existence of sensitive time-windows has significant implications for our understanding of early brain development underlying NDDs and may indicate vulnerable periods when the brain is more susceptible to current therapeutic treatments.

Reduced expression of MECP2 affects cell commitment and maintenance in neurons by triggering senescence: new perspective for Rett syndrome

MECP2 protein binds preferentially to methylated CpGs and regulates gene expression by causing changes in chromatin structure. The mechanism by which impaired MECP2 activity can induce pathological abnormalities in the nervous system of patients with Rett syndrome (RTT) is not clearly understood. To gain further insight into the role of MECP2 in human neurogenesis, we compared the neural differentiation process in mesenchymal stem cells (MSCs) obtained from a RTT patient and from healthy donors. We further analyzed neural differentiation in a human neuroblastoma cell line carrying a partially silenced MECP2 gene. Senescence and reduced expression of neural markers were observed in proliferating and differentiating MSCs from the RTT patient, which suggests that impaired activity of MECP2 protein may impair neural differentiation, as observed in RTT patients. Next, we used an inducible expression system to silence MECP2 in neuroblastoma cells before and after the induction of neural differentiation via retinoic acid treatment. This approach was used to test whether MECP2 inactivation affected the cell fate of neural progenitors and/or neuronal differentiation and maintenance. Overall, our data suggest that neural cell fate and neuronal maintenance may be perturbed by senescence triggered by impaired MECP2 activity either before or after neural differentiation.

Cell-autonomous alterations in dendritic arbor morphology and connectivity induced by overexpression of MeCP2 in Xenopus central neurons in vivo

Methyl CpG binding protein-2 (MeCP2) is an essential epigenetic regulator in human brain development. Mutations in the MeCP2 gene have been linked to Rett syndrome, a severe X-linked progressive neurodevelopmental disorder, and one of the most common causes of mental retardation in females. MeCP2 duplication and triplication have also been found to affect brain development, indicating that both loss of function and gain in MeCP2 dosage lead to similar neurological phenotypes. Here, we used the Xenopus laevis visual system as an in vivo model to examine the consequence of increased MeCP2 expression during the morphological maturation of individual central neurons in an otherwise intact brain. Single-cell overexpression of wild-type human MeCP2 was combined with time-lapse confocal microscopy imaging to study dynamic mechanisms by which MeCP2 influences tectal neuron dendritic arborization. Analysis of neurons co-expressing DsRed2 demonstrates that MeCP2 overexpression specifically interfered with dendritic elaboration, decreasing the rates of branch addition and elimination over a 48 hour observation period. Moreover, dynamic analysis of neurons co-expressing wt-hMeCP2 and PSD95-GFP revealed that even though neurons expressing wt-hMeCP2 possessed significantly fewer dendrites and simpler morphologies than control neurons at the same developmental stage, postsynaptic site density in wt-hMeCP2-expressing neurons was similar to controls and increased at a rate higher than controls. Together, our in vivo studies support an early, cell-autonomous role for MeCP2 during the morphological differentiation of neurons and indicate that perturbations in MeCP2 gene dosage result in deficits in dendritic arborization that can be compensated, at least in part, by synaptic connectivity changes.

MeCP2 mutation results in compartment-specific reductions in dendritic branching and spine density in layer 5 motor cortical neurons of YFP-H mice

Rett Syndrome (RTT) is a neurodevelopmental disorder predominantly caused by mutations in the X-linked gene MECP2. A primary feature of the syndrome is the impaired maturation and maintenance of excitatory synapses in the central nervous system (CNS). Different RTT mouse models have shown that particular Mecp2 mutations have highly variable effects on neuronal architecture. Distinguishing MeCP2 mutant cellular phenotypes therefore demands analysis of specific mutations in well-defined neuronal subpopulations. We examined a transgenically labeled subset of cortical neurons in YFP-H mice crossed with the Mecp2(tm1.1Jae) mutant line. YFP(+) Layer 5 pyramidal neurons in the motor cortex of wildtype and hemizygous mutant male mice were examined for differences in dendrite morphology and spine density. Total basal dendritic length was decreased by 18.6% due to both shorter dendrites and reduced branching proximal to the soma. Tangential dendrite lengths in the apical tuft were reduced by up to 26.6%. Spine density was reduced by 47.4% in the apical tuft and 54.5% in secondary apical dendrites, but remained unaffected in primary apical and proximal basal dendrites. We also found that MeCP2 mutation reduced the number of YFP(+) cells in YFP-H mice by up to 72% in various cortical regions without affecting the intensity of YFP expression in individual cells. Our results support the view that the effects of MeCP2 mutation are highly context-dependent and cannot be generalized across mutation types and cell populations.

The expression of spinal methyl-CpG-binding protein 2, DNA methyltransferases and histone deacetylases is modulated in persistent pain states

Background

DNA CpG methylation is carried out by DNA methyltransferases and induces chromatin remodeling and gene silencing through a transcription repressor complex comprising the methyl-CpG-binding protein 2 (MeCP2) and a subset of histone deacetylases. Recently, we have found that MeCP2 activity had a crucial role in the pattern of gene expression seen in the superficial dorsal horn rapidly after injection of Complete Freund's Adjuvant (CFA) in the rat ankle joint. The aim of the present study was to analyse the changes in expression of MeCP2, DNA methyltransferases and a subset of histone deacetylases in the superficial dorsal horn during the maintenance phase of persistent pain states. In this process, the cell specific expression of MeCP2 was also investigated.

Results

Using immunohistochemistry, we found that neurones, oligodendrocytes and astrocytes expressed MeCP2. Microglia, oligodendrocyte precursor cells and Schwann cells never showed any positive stain for MeCP2. Quantitative analyses showed that MeCP2 expression was increased in the superficial dorsal horn 7 days following CFA injection in the ankle joint but decreased 7 days following spared nerve injury. Overall, the expression of DNA methyltransferases and a subset of histone deacetylases followed the same pattern of expression. However, there were no significant changes in the expression of the MeCP2 targets that we had previously shown are regulated in the early time points following CFA injection in the ankle joint. Finally, the expression of MeCP2 was also down regulated in damaged dorsal root ganglion neurones following spared nerve injury.

Conclusion

Our results strongly suggest that changes in chromatin compaction, regulated by the binding of MeCP2 complexes to methylated DNA, are involved in the modulation of gene expression in the superficial dorsal horn and dorsal root ganglia during the maintenance of persistent pain states.

Linking epigenetics to human disease and Rett syndrome: the emerging novel and challenging concepts in MeCP2 research

Epigenetics refer to inheritable changes beyond DNA sequence that control cell identity and morphology. Epigenetics play key roles in development and cell fate commitments and highly impact the etiology of many human diseases. A well-known link between epigenetics and human disease is the X-linked MECP2 gene, mutations in which lead to the neurological disorder, Rett Syndrome. Despite the fact that MeCP2 was discovered about 20 years ago, our current knowledge about its molecular function is not comprehensive. While MeCP2 was originally found to bind methylated DNA and interact with repressor complexes to inhibit and silence its genomic targets, recent studies have challenged this idea. Indeed, depending on its interacting protein partners and target genes, MeCP2 can act either as an activator or as a repressor. Furthermore, it is becoming evident that although Rett Syndrome is a progressive and postnatal neurological disorder, the consequences of MeCP2 deficiencies initiate much earlier and before birth. To comprehend the novel and challenging concepts in MeCP2 research and to design effective therapeutic strategies for Rett Syndrome, a targeted collaborative effort from scientists in multiple research areas to clinicians is required.

Normal mitral cell dendritic development in the setting of Mecp2 mutation

Rett syndrome (RTT) is an autism spectrum disorder caused by mutation in the gene encoding methyl CpG binding protein 2 (MECP2). Evidence to date suggests that these disorders display defects in synaptic organization and plasticity. A hallmark of the pathology in RTT has been identified as decreased dendritic arborization, which has been interpreted to represent abnormal dendritic formation and pruning during development. Our previous studies revealed that olfactory axons display defective pathfinding and targeting in the setting of Mecp2 mutation. In the present work, we use Mecp2 mutant mouse models and the olfactory system to investigate dendritic development. Here, we demonstrate that mitral cell dendritic development proceeds normally in mutant mice, resulting in typical dendritic morphology at early postnatal ages. We also failed to detect abnormalities in dendritic inputs at symptomatic stages when glomeruli from mutant mice appear smaller in area than the wild type (WT) (6 weeks postnatally). Collectively, these findings suggest that the initial defects in glomeruli impairment seen with Mecp2 mutation do not result from abnormal dendritic development. Our results using the olfactory system indicate that dendritic abnormalities are not an early feature in the abnormalities incurred by Mecp2 mutation.

Temporal and regional alterations in NMDA receptor expression in Mecp2-null mice

Our previous postmortem study of girls with Rett Syndrome (RTT), a development disorder caused by MECP2 mutations, found increases in the density of N-Methyl-D-aspartate (NMDA) receptors in the prefrontal cortex of 2-8-year-old girls, whereas girls older than 10 years had reductions in NMDA receptors compared with age-matched controls (Blue et al., Ann Neurol 1999b;45:541-545). Using [(3)H]-CGP to label NMDA-type glutamate receptors in 2- and 7-week old wild-type (WT), Mecp2-null, and Mecp2-heterozygous (HET) mice (Bird model), we found that frontal areas of the brain also exhibited a bimodal pattern in NMDA expression, with increased densities of NMDA receptors in Mecp2-null mice at 2 weeks of age but decreased densities at 7 weeks of age. Visual cortex showed a similar pattern, while other cortical regions only exhibited changes in NMDA receptor densities at 2 weeks (retrosplenial granular) or 7 weeks (somatosensory). In thalamus of null mice, NMDA receptors were increased at 2 and 7 weeks. No significant differences in density were found between HET and WT mice at both ages. Western blots for NMDAR1 expression in frontal brain showed higher levels of expression in Mecp2-null mice at 2 weeks of age but not at 1 or 7 weeks of age. Our mouse data support the notion that deficient MeCP2 function is the primary cause of the NMDA receptor changes we observed in RTT. Furthermore, the findings of regional and temporal differences in NMDA expression illustrate the importance of age and brain region in evaluating different genotypes of mice.