Morphological and functional alterations in the substantia nigra pars compacta of the Mecp2-null mouse

Do Neurochemicals Reflect Psychophysiological Dimensions in Behaviors? A Transdisciplinary Perspective Based on Analogy with Maslow's Needs Pyramid

All behaviors, including motivated behaviors, result from integration of information in the brain via nerve impulses, with two main means of communication: electrical gap-junctions and chemical signaling. The latter enables information transfer between brain cells through release of biochemical messengers, such as neurotransmitters. Neurochemical studies generate plentiful biochemical data, with many variables per individual, since there are many methods to quantify neurotransmitters, precursors and metabolites. The number of variables can be far higher using other concomitant techniques to monitor behavioral parameters on the same subject of study. Surprisingly, while many quantitative variables are obtained, data analysis and discussion focus on just a few or only on the neurotransmitter known to be involved in the behavior, and the other biochemical data are, at best, regarded as less important for scientific interpretation. The present article aims to provide novel transdisciplinary arguments that all neurochemical data can be regarded as items of psychophysiological dimensions, just as questionnaire items identify modified behaviors or disorders using latent classes. A first proof of concept on nonmotivated and motivated behaviors using a multivariate data-mining approach is presented.

GM1 Oligosaccharide Ameliorates Rett Syndrome Phenotypes In Vitro and In Vivo via Trk Receptor Activation

Rett syndrome (RTT) is a severe neurodevelopmental disorder primarily caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene. Despite advancements in research, no cure exists due to an incomplete understanding of the molecular effects of MeCP2 deficiency. Previous studies have identified impaired tropomyosin receptor kinase (Trk) neurotrophin (NTP) signaling and mitochondrial redox imbalances as key drivers of the pathology. Moreover, altered glycosphingolipid metabolism has been reported in RTT. GM1 ganglioside is a known regulator of the nervous system, and growing evidence indicates its importance in maintaining neuronal homeostasis via its oligosaccharide chain, coded as GM1-OS. GM1-OS directly interacts with the Trk receptors on the cell surface, triggering neurotrophic and neuroprotective pathways in neurons. In this study, we demonstrate that GM1-OS ameliorates RTT deficits in the Mecp2-null model. GM1-OS restored synaptogenesis and reduced mitochondrial oxidative stress of Mecp2-knock-out (ko) cortical neurons. When administered in vivo, GM1-OS mitigated RTT-like symptoms. Our findings indicate that GM1-OS effects were mediated by Trk receptor activation on the neuron's plasma membrane. Overall, our results highlight GM1-OS as a promising candidate for RTT treatment.

The Alpha-Synuclein Gene (SNCA) is a Genomic Target of Methyl-CpG Binding Protein 2 (MeCP2)-Implications for Parkinson's Disease and Rett Syndrome

Mounting evidence suggests a prominent role for alpha-synuclein (a-syn) in neuronal cell function. Alterations in the levels of cellular a-syn have been hypothesized to play a critical role in the development of Parkinson's disease (PD); however, mechanisms that control expression of the gene for a-syn (SNCA) in cis and trans as well as turnover of a-syn are not well understood. We analyzed whether methyl-CpG binding protein 2 (MeCP2), a protein that specifically binds methylated DNA, thus regulating transcription, binds at predicted binding sites in intron 1 of the SNCA gene and regulates a-syn protein expression. Chromatin immunoprecipitation (ChIP) and electrophoretic mobility-shift assays (EMSA) were used to confirm binding of MeCP2 to regulatory regions of SNCA. Site-specific methylation and introduction of localized mutations by CRISPR/Cas9 were used to investigate the binding properties of MeCP2 in human SK-N-SH neuroblastoma cells. The significance of MeCP2 for SNCA regulation was further investigated by overexpressing MeCP2 and mutated variants of MeCP2 in MeCP2 knockout cells. We found that methylation-dependent binding of MeCP2 at a restricted region of intron 1 of SNCA had a significant impact on the production of a-syn. A single nucleotide substitution near to CpG1 strongly increased the binding of MeCP2 to intron 1 of SNCA and decreased a-syn protein expression by 60%. In contrast, deletion of a single nucleotide closed to CpG2 led to reduced binding of MeCP2 and significantly increased a-syn levels. In accordance, knockout of MeCP2 in SK-N-SH cells resulted in a significant increase in a-syn production, demonstrating that SNCA is a genomic target for MeCP2 regulation. In addition, the expression of two mutated MeCP2 variants found in Rett syndrome (RTT) showed a loss of their ability to reduce a-syn expression. This study demonstrates that methylation of CpGs and binding of MeCP2 to intron 1 of the SNCA gene plays an important role in the control of a-syn expression. In addition, the changes in SNCA regulation found by expression of MeCP2 variants carrying mutations found in RTT patients may be of importance for the elucidation of a new molecular pathway in RTT, a rare neurological disorder caused by mutations in MECP2.

Early-life exposure to methylmercury induces reversible behavioral impairments and gene expression modifications in one isogenic lineage of mangrove rivulus fish Kryptolebias marmoratus

Methylmercury (MeHg) is a ubiquitous bioaccumulative neurotoxicant present in aquatic ecosystems. It is known to alter behaviors, sensory functions and learning abilities in fish and other vertebrates. Developmental and early-life stages exposure to MeHg can lead to brain damage with immediate consequences on larvae behavior, but may also induce long term effects in adults after a detoxification period. However, very little is known about developmental origin of behavioral impairment in adults due to early exposure to MeHg. The aim of this study is to assess whether early-life MeHg exposure induces immediate and/or delayed effects on behaviors, related genes expression and DNA methylation (one of epigenetic mechanisms). To reach this goal, newly hatched larvae of mangrove rivulus fish, Kryptolebias marmoratus, were exposed to two sub-lethal concentrations of MeHg (90 μg/L and 135 µg/L) for 7 days, and immediate and delayed effects were assessed respectively in 7 dph (days post-hatching) and 90 dph fish. This species naturally produces isogenic lineages due to its self-fertilizing reproduction system, which is unique among vertebrates. It allows to study how environment stressors can influence organism's phenotype while minimizing genetic variability. As results, both MeHg exposures are associated with a decreased foraging efficiency and thigmotaxis, and a dose-dependent reduction in larvae locomotor activity. Regarding molecular analysis in larvae whole bodies, both MeHg exposures induced significant decreased expression of DNMT3a, MAOA, MeCP2 and NIPBL, and significant increase of GSS, but none of those genes underwent methylation changes in targeted CpGs. None of significant behavioral and molecular impairments observed in 7-dph larvae were found in 90-dph adults, which highlight a distinction between immediate and delayed effects of developmental MeHg exposure. Our results suggest implications of aminergic system and its neurotransmitters, redox/methylation trade-off and possibly other epigenetic mechanisms in MeHg neurotoxicity underlying behavioral alterations in rivulus.

Timing of MeCP2 Expression Determines Midbrain Dopamine Neuron Phenotype Specification

Midbrain dopamine (DA) neurons are associated with locomotor and psychiatric disorders. DA phenotype is specified in ancestral neural precursor cells (NPCs) and maintained throughout neuronal differentiation. Here we show that endogenous expression of MeCP2 coincides with DA phenotype specification in mouse mesencephalon, and premature expression of MeCP2 prevents in vitro cultured NPCs from acquiring DA phenotype through interfering NURR1 transactivation of DA phenotype genes. By contrast, ectopic MeCP2 expression does not disturb DA phenotype in the DA neurons. By analyzing the dynamic change of DNA methylation along DA neuronal differentiation at the promoter of DA phenotype gene tyrosine hydroxylase (Th), we show that Th expression is determined by TET1-mediated de-methylation of NURR1 binding sites within Th promoter. Chromatin immunoprecipitation assays demonstrate that premature MeCP2 dominates the DNA binding of the corresponding sites thereby blocking TET1 function in DA NPCs, whereas TET1-mediated de-methylation prevents excessive MeCP2 binding in DA neurons. The significance of temporal DNA methylation status is further confirmed by targeted methylation/demethylation experiments showing that targeted de-methylation in DA NPCs protects DA phenotype specification from ectopic MeCP2 expression, whereas targeted methylation disturbs phenotype maintenance in MeCP2-overexpressed DA neurons. These findings suggest the appropriate timing of MeCP2 expression as a novel determining factor for guiding NPCs into DA lineage.

The State of the Dopaminergic and Glutamatergic Systems in the Valproic Acid Mouse Model of Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is a progressive neurodevelopmental disorder mainly characterized by deficits in social communication and stereotyped behaviors and interests. Here, we aimed to investigate the state of several key players in the dopamine and glutamate neurotransmission systems in the valproic acid (VPA) animal model that was administered to E12.5 pregnant females as a single dose (450 mg/kg). We report no alterations in the number of mesencephalic dopamine neurons or in protein levels of tyrosine hydroxylase in either the striatum or the nucleus accumbens. In females prenatally exposed to VPA, levels of dopamine were slightly decreased while the ratio of DOPAC/dopamine was increased in the dorsal striatum, suggesting increased turn-over of dopamine tone. In turn, levels of D1 and D2 dopamine receptor mRNAs were increased in the nucleus accumbens of VPA mice suggesting upregulation of the corresponding receptors. We also report decreased protein levels of striatal parvalbumin and increased levels of p-mTOR in the cerebellum and the motor cortex of VPA mice. mRNA levels of mGluR1, mGluR4, and mGluR5 and the glutamate receptor subunits NR1, NR2A, and NR2B were not altered by VPA, nor were protein levels of NR1, NR2A, and NR2B and those of BDNF and TrkB. These findings are of interest as clinical trials aiming at the dopamine and glutamate systems are being considered.

Modeling dopamine dysfunction in autism spectrum disorder: From invertebrates to vertebrates

Autism Spectrum Disorder (ASD) is a highly heterogeneous neurodevelopmental disorder characterized by deficits in social communication and by patterns of restricted interests and/or repetitive behaviors. The Simons Foundation Autism Research Initiative's Human Gene and CNV Modules now list over 1000 genes implicated in ASD and over 2000 copy number variant loci reported in individuals with ASD. Given this ever-growing list of genetic changes associated with ASD, it has become evident that there is likely not a single genetic cause of this disorder nor a single neurobiological basis of this disorder. Instead, it is likely that many different neurobiological perturbations (which may represent subtypes of ASD) can result in the set of behavioral symptoms that we called ASD. One such of possible subtype of ASD may be associated with dopamine dysfunction. Precise regulation of synaptic dopamine (DA) is required for reward processing and behavioral learning, behaviors which are disrupted in ASD. Here we review evidence for DA dysfunction in ASD and in animal models of ASD. Further, we propose that these studies provide a scaffold for scientists and clinicians to consider subcategorizing the ASD diagnosis based on the genetic changes, neurobiological difference, and behavioral features identified in individuals with ASD.

Movement disorders in patients with Rett syndrome: A systematic review of evidence and associated clinical considerations

AIM

This systematic review identified and thematically appraised clinical evidence of movement disorders in patients with Rett syndrome (RTT).

METHOD

Using PRISMA criteria, six electronic databases were searched from inception to April 2021. A thematic analysis was then undertaken on the extracted data to identify potential themes.

RESULTS

Following the thematic analysis, six themes emerged: (i) clinical features of abnormal movement behaviors; (ii) mutational profile and its impact on movement disorders; (iii) symptoms and stressors that impact on movement disorders; (iv) possible underlying neurobiological mechanisms; (v) quality of life and movement disorders; and (vi) treatment of movement disorders. Current guidelines for managing movement disorders in general were then reviewed to provide possible treatment recommendations for RTT.

CONCLUSION

Our study offers an enriched data set for clinical investigations and treatment of fine and gross motor issues in RTT. A detailed understanding of genotype-phenotype relationships of movement disorders allows for more robust genetic counseling for families but can also assist healthcare professionals in terms of monitoring disease progression in RTT. The synthesis also showed that environmental enrichment would be beneficial for improving some aspects of movement disorders. The cerebellum, basal ganglia, alongside dysregulation of the cortico-basal ganglia-thalamo-cortical loop, are likely anatomical targets. A review of treatments for movement disorders also helped to provide recommendations for treating and managing movement disorders in patients with RTT.

Dopaminergic Dysregulation in Syndromic Autism Spectrum Disorders: Insights From Genetic Mouse Models

Autism spectrum disorder (ASD) is a neurodevelopmental disorder defined by altered social interaction and communication, and repetitive, restricted, inflexible behaviors. Approximately 1.5-2% of the general population meet the diagnostic criteria for ASD and several brain regions including the cortex, amygdala, cerebellum and basal ganglia have been implicated in ASD pathophysiology. The midbrain dopamine system is an important modulator of cellular and synaptic function in multiple ASD-implicated brain regions via anatomically and functionally distinct dopaminergic projections. The dopamine hypothesis of ASD postulates that dysregulation of dopaminergic projection pathways could contribute to the behavioral manifestations of ASD, including altered reward value of social stimuli, changes in sensorimotor processing, and motor stereotypies. In this review, we examine the support for the idea that cell-autonomous changes in dopaminergic function are a core component of ASD pathophysiology. We discuss the human literature supporting the involvement of altered dopamine signaling in ASD including genetic, brain imaging and pharmacologic studies. We then focus on genetic mouse models of syndromic neurodevelopmental disorders in which single gene mutations lead to increased risk for ASD. We highlight studies that have directly examined dopamine neuron number, morphology, physiology, or output in these models. Overall, we find considerable support for the idea that the dopamine system may be dysregulated in syndromic ASDs; however, there does not appear to be a consistent signature and some models show increased dopaminergic function, while others have deficient dopamine signaling. We conclude that dopamine dysregulation is common in syndromic forms of ASD but that the specific changes may be unique to each genetic disorder and may not account for the full spectrum of ASD-related manifestations.

β-sitosterol reduces anxiety and synergizes with established anxiolytic drugs in mice

Anxiety and stress-related conditions represent a significant health burden in modern society. Unfortunately, most anxiolytic drugs are prone to side effects, limiting their long-term usage. Here, we employ a bioinformatics screen to identify drugs for repurposing as anxiolytics. Comparison of drug-induced gene-expression profiles with the hippocampal transcriptome of an importin α5 mutant mouse model with reduced anxiety identifies the hypocholesterolemic agent β-sitosterol as a promising candidate. β-sitosterol activity is validated by both intraperitoneal and oral application in mice, revealing it as the only clear anxiolytic from five closely related phytosterols. β-sitosterol injection reduces the effects of restraint stress, contextual fear memory, and c-Fos activation in the prefrontal cortex and dentate gyrus. Moreover, synergistic anxiolysis is observed when combining sub-efficacious doses of β-sitosterol with the SSRI fluoxetine. These preclinical findings support further development of β-sitosterol, either as a standalone anxiolytic or in combination with low-dose SSRIs.

Neural Mechanisms Underlying Repetitive Behaviors in Rodent Models of Autism Spectrum Disorders

Autism spectrum disorder (ASD) is comprised of several conditions characterized by alterations in social interaction, communication, and repetitive behaviors. Genetic and environmental factors contribute to the heterogeneous development of ASD behaviors. Several rodent models display ASD-like phenotypes, including repetitive behaviors. In this review article, we discuss the potential neural mechanisms involved in repetitive behaviors in rodent models of ASD and related neuropsychiatric disorders. We review signaling pathways, neural circuits, and anatomical alterations in rodent models that display robust stereotypic behaviors. Understanding the mechanisms and circuit alterations underlying repetitive behaviors in rodent models of ASD will inform translational research and provide useful insight into therapeutic strategies for the treatment of repetitive behaviors in ASD and other neuropsychiatric disorders.

Methyl-CpG Binding Protein 2 in Alzheimer Dementia

Despite decades of research on Alzheimer disease, understanding the complexity of the genetic and molecular interactions involved in its pathogenesis remains far from our grasp. Methyl-CpG Binding Protein 2 (MeCP2) is an important epigenetic regulator enriched in the brain, and recent findings have implicated MeCP2 as a crucial player in Alzheimer disease. Here, we provide comprehensive insights into the pathophysiological roles of MeCP2 in Alzheimer disease. In particular, we focus on how the alteration of MeCP2 expression can impact Alzheimer disease through risk genes, amyloid-β and tau pathology, cell death and neurodegeneration, and cellular senescence. We suggest that Alzheimer disease can be adversely affected by upregulated MeCP2-dependent repression of risk genes (MEF2C, ADAM10, and PM20D1), increased tau accumulation, and neurodegeneration through neuronal cell death (excitotoxicity and apoptosis). In addition, we propose that the progression of Alzheimer disease could be caused by reduced MeCP2-mediated enhancement of astrocytic and microglial senescence and consequent glial SASP (senescence-associated secretory phenotype)-dependent neuroinflammation. We surmise that any imbalance in MeCP2 function would accelerate or cause Alzheimer disease pathogenesis, implying that MeCP2 may be a potential drug target for the treatment and prevention of Alzheimer disease.

Dysfunction of the corticostriatal pathway in autism spectrum disorders

The corticostriatal pathway that carries sensory, motor, and limbic information to the striatum plays a critical role in motor control, action selection, and reward. Dysfunction of this pathway is associated with many neurological and psychiatric disorders. Corticostriatal synapses have unique features in their cortical origins and striatal targets. In this review, we first describe axonal growth and synaptogenesis in the corticostriatal pathway during development, and then summarize the current understanding of the molecular bases of synaptic transmission and plasticity at mature corticostriatal synapses. Genes associated with autism spectrum disorder (ASD) have been implicated in axonal growth abnormalities, imbalance of the synaptic excitation/inhibition ratio, and altered long-term synaptic plasticity in the corticostriatal pathway. Here, we review a number of ASD-associated high-confidence genes, including FMR1, KMT2A, GRIN2B, SCN2A, NLGN1, NLGN3, MET, CNTNAP2, FOXP2, TSHZ3, SHANK3, PTEN, CHD8, MECP2, DYRK1A, RELN, FOXP1, SYNGAP1, and NRXN, and discuss their relevance to proper corticostriatal function.

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.

Brain-enriched microRNAs circulating in plasma as novel biomarkers for Rett syndrome

Rett syndrome (RTT) is a severe neurodevelopmental disorder caused by mutations in the X-linked gene MECP2 (methyl-CpG-binding protein 2). Minimally invasive and accurate biomarkers of disease progression and treatment response could facilitate screening of therapeutic compounds in animal models, enrollment of better-defined participants into clinical trials, and treatment monitoring. In this study, we used a targeted approach based on analysis of brain-enriched microRNAs (miRNAs) circulating in plasma to identify miRNA biomarkers of RTT using Mecp2-mutant mice as a model system and human plasma samples. An “miRNA pair” approach, i.e. the ratio between two miRNAs, was used for data normalization. Specific miRNA pairs and their combinations (classifiers) analyzed in plasma differentiated wild-type from Mecp2 male and female mice with >90% accuracy. Individual miRNA pairs were more effective in distinguishing male (homozygous) animals than female (heterozygous) animals, suggesting that disease severity correlated with the levels of the miRNA biomarkers. In the human study, 30 RTT patients were compared with age-matched controls. The results of this study showed that miRNA classifiers were able to differentiate RTT patients from controls with 85–100% sensitivity. In addition, a comparison of various age groups demonstrated that the dynamics in levels of miRNAs appear to be associated with disease development (involvement of liver, muscle and lipid metabolism in the pathology). Importantly, certain miRNA biomarker pairs were common to both the animal models and human subjects, indicating the similarity between the underlying pathological processes. The data generated in this feasibility study suggest that circulating miRNAs have the potential to be developed as markers of RTT progression and treatment response. Larger clinical studies are needed to further evaluate the findings presented here.

Rare Diseases of Neurodevelopment: Maintain the Mystery or Use a Dazzling Tool for Investigation? The Case of Rett Syndrome

The investigation on neurotransmission function during normal and pathologic development is a pivotal component needed to understand the basic mechanisms underlying neurodevelopmental pathologies. To study these diseases, many animal models have been generated which allowed to face the limited availability of human tissues and, as a consequence, most of the electrophysiology has been performed on these models of diseases. On the other hand, the technique of membrane microtransplantation in Xenopus oocytes allows the study of human functional neurotransmitter receptors thanks to the use of tissues from autopsies or surgeries, even in quantities that would not permit other kinds of functional studies. In this short article, we intend to underline how this technique is well-fit for the study of rare diseases by characterizing the electrophysiological properties of GABA_A and AMPA receptors in Rett syndrome. For our purposes, we used both tissues from Rett syndrome patients and Mecp2-null mice, a well validated murine model of the same disease, in order to strengthen the solidity of our results through the comparison of the two. Our findings retrace previous results and, in the light of this, further argue in favor of Prof. Miledi's technique of membrane microtransplantation that proves itself a very useful tool of investigation in the field of neurophysiology. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

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

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

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.

MicroRNAs regulate synaptic plasticity underlying drug addiction

Chronic use of drugs of abuse results in neurochemical, morphological and behavioral plasticity that underlies the emergence of compulsive drug seeking and vulnerability to relapse during periods of attempted abstinence. Identifying and reversing addiction-relevant plasticity is seen as a potential point of pharmacotherapeutic intervention in drug-addicted individuals. Despite considerable advances in our understanding of the actions of drugs of abuse in the brain, this information has thus far yielded few novel treatment options addicted individuals. MicroRNAs are small noncoding RNAs that can each regulate the translation of hundreds to thousands of messenger RNAs. The highly pleiotropic nature of miRNAs has focused attention on their contribution to addiction-relevant structural and functional plasticity in the brain and their potential utility as targets for medications development. In this review, we discuss the roles of miRNAs in synaptic plasticity underlying the development of addiction and then briefly discuss the possibility of using circulating miRNA as biomarkers for addiction.

A codon-optimized Mecp2 transgene corrects breathing deficits and improves survival in a mouse model of Rett syndrome

Rett syndrome (RTT) is a severe X-linked neurodevelopmental disorder that is primarily caused by mutations in the methyl CpG binding protein 2 gene (MECP2). RTT is the second most prevalent cause of intellectual disability in girls and there is currently no cure for the disease. The finding that the deficits caused by the loss of Mecp2 are reversible in the mouse has bolstered interest in gene therapy as a cure for RTT. In order to assess the feasibility of gene therapy in a RTT mouse model, and in keeping with translational goals, we investigated the efficacy of a self-complementary AAV9 vector expressing a codon-optimized version of Mecp2 (AAV9-MCO) delivered via a systemic approach in early symptomatic Mecp2-deficient (KO) mice. Our results show that AAV9-MCO administered at a dose of 2×10¹¹ viral genome (vg)/mouse was able to significantly increase survival and weight gain, and delay the occurrence of behavioral deficits. Apneas, which are one of the core RTT breathing deficits, were significantly decreased to WT levels in Mecp2 KO mice after AAV9-MCO administration. Semi-quantitative analysis showed that AAV9-MCO administration in Mecp2 KO mice resulted in 10 to 20% Mecp2 immunopositive cells compared to WT animals, with the highest Mecp2 expression found in midbrain regions known to regulate cardio-respiratory functions. In addition, we also found a cell autonomous increase in tyrosine hydroxylase levels in the A1C1 and A2C2 catecholaminergic Mecp2+ neurons in treated Mecp2 KO mice, which may partly explain the beneficial effect of AAV9-MCO administration on apneas occurrence.

Dopaminergic Neurons Exhibit an Age-Dependent Decline in Electrophysiological Parameters in the MitoPark Mouse Model of Parkinson's Disease

Dopaminergic neurons of the substantia nigra (SN) play a vital role in everyday tasks, such as reward-related behavior and voluntary movement, and excessive loss of these neurons is a primary hallmark of Parkinson's disease (PD). Mitochondrial dysfunction has long been implicated in PD and many animal models induce parkinsonian features by disrupting mitochondrial function. MitoPark mice are a recently developed genetic model of PD that lacks the gene for mitochondrial transcription factor A specifically in dopaminergic neurons. This model mimics many distinct characteristics of PD including progressive and selective loss of SN dopamine neurons, motor deficits that are improved byl-DOPA, and development of inclusion bodies. Here, we used brain slice electrophysiology to construct a timeline of functional decline in SN dopaminergic neurons from MitoPark mice. Dopaminergic neurons from MitoPark mice exhibited decreased cell capacitance and increased input resistance that became more severe with age. Pacemaker firing regularity was disrupted in MitoPark mice and ion channel conductances associated with firing were decreased. Additionally, dopaminergic neurons from MitoPark mice showed a progressive decrease of endogenous dopamine levels, decreased dopamine release, and smaller D2 dopamine receptor-mediated outward currents. Interestingly, expression of ion channel subunits associated with impulse activity (Cav1.2, Cav1.3, HCN1, Nav1.2, and NavB3) was upregulated in older MitoPark mice. The results describe alterations in intrinsic and synaptic properties of dopaminergic neurons in MitoPark mice occurring at ages both before and concurrent with motor impairment. These findings may help inform future investigations into treatment targets for prodromal PD.

SIGNIFICANCE STATEMENT

Parkinson's disease (PD) is the second most diagnosed neurodegenerative disorder, and the classic motor symptoms of the disease are attributed to selective loss of dopaminergic neurons of the substantia nigra. The MitoPark mouse is a genetic model of PD that mimics many of the key characteristics of the disease and enables the study of progressive neurodegeneration in parkinsonism. Here we have identified functional deficits in the ion channel physiology of dopaminergic neurons from MitoPark mice that both precede and are concurrent with the time course of behavioral symptomatology. Because PD is a progressive disease with a long asymptomatic phase, identification of early functional adaptations could lay the groundwork to test therapeutic interventions that halt or reverse disease progression.

MeCP2 SUMOylation rescues Mecp2-mutant-induced behavioural deficits in a mouse model of Rett syndrome

The methyl-CpG-binding protein 2 (MeCP2) gene, MECP2, is an X-linked gene encoding the MeCP2 protein, and mutations of MECP2 cause Rett syndrome (RTT). However, the molecular mechanism of MECP2-mutation-caused RTT is less known. Here we find that MeCP2 could be SUMO-modified by the E3 ligase PIAS1 at Lys-412. MeCP2 phosphorylation (at Ser-421 and Thr-308) facilitates MeCP2 SUMOylation, and MeCP2 SUMOylation is induced by NMDA, IGF-1 and CRF in the rat brain. MeCP2 SUMOylation releases CREB from the repressor complex and enhances Bdnf mRNA expression. Several MECP2 mutations identified in RTT patients show decreased MeCP2 SUMOylation. Re-expression of wild-type MeCP2 or SUMO-modified MeCP2 in Mecp2-null neurons rescues the deficits of social interaction, fear memory and LTP observed in Mecp2 conditional knockout (cKO) mice. These results together reveal an important role of MeCP2 SUMOylation in social interaction, memory and synaptic plasticity, and that abnormal MeCP2 SUMOylation is implicated in RTT.

Post-translational modifications of methyl-CpG-binding protein 2 (MeCP2) are important for its function and dysfunction in Rett syndrome. Here, Tai et al. show a functional interaction between MeCP2 SUMOylation and phosphorylation in rodent behavior and synaptic plasticity.

MECP2 disorders: from the clinic to mice and back

Two severe, progressive neurological disorders characterized by intellectual disability, autism, and developmental regression, Rett syndrome and MECP2 duplication syndrome, result from loss and gain of function, respectively, of the same critical gene, methyl-CpG-binding protein 2 (MECP2). Neurons acutely require the appropriate dose of MECP2 to function properly but do not die in its absence or overexpression. Instead, neuronal dysfunction can be reversed in a Rett syndrome mouse model if MeCP2 function is restored. Thus, MECP2 disorders provide a unique window into the delicate balance of neuronal health, the power of mouse models, and the importance of chromatin regulation in mature neurons. In this Review, we will discuss the clinical profiles of MECP2 disorders, the knowledge acquired from mouse models of the syndromes, and how that knowledge is informing current and future clinical studies.

Identification of neurodegenerative factors using translatome-regulatory network analysis

For degenerative disorders of the CNS, the main obstacle to therapeutic advancement has been the challenge of identifying the key molecular mechanisms underlying neuronal loss. We developed a combinatorial approach including translational profiling and brain regulatory network analysis to search for key determinants of neuronal survival or death. Following the generation of transgenic mice for cell type-specific profiling of midbrain dopaminergic neurons, we established and compared translatome libraries reflecting the molecular signature of these cells at baseline or under degenerative stress. Analysis of these libraries by interrogating a context-specific brain regulatory network led to the identification of a repertoire of intrinsic upstream regulators that drive the dopaminergic stress response. The altered activity of these regulators was not associated with changes in their expression levels. This strategy can be generalized for the identification of molecular determinants involved in the degeneration of other classes of neurons.

MeCP2 in the rostral striatum maintains local dopamine content critical for psychomotor control

Methyl-CpG binding protein 2 (MeCP2) is a chromatin regulator highly expressed in mature neurons. Mutations of MECP2 gene cause >90% cases of Rett syndrome, a neurodevelopmental disorder featured by striking psychomotor dysfunction. In Mecp2-null mice, the motor deficits are associated with reduction of dopamine content in the striatum, the input nucleus of basal ganglia mostly composed of GABAergic neurons. Here we investigated the causal role of MeCP2 in modulation of striatal dopamine content and psychomotor function. We found that mice with selective removal of MeCP2 in forebrain GABAergic neurons, predominantly in the striatum, phenocopied Mecp2-null mice in dopamine deregulation and motor dysfunction. Selective expression of MeCP2 in the striatum preserved dopamine content and psychomotor function in both males and females. Notably, the dopamine deregulation was primarily confined to the rostral striatum, and focal deletion or reactivation of MeCP2 expression in the rostral striatum through adeno-associated virus effectively disrupted or restored dopamine content and locomotor activity, respectively. Together, these findings demonstrate that striatal MeCP2 maintains local dopamine content in a non-cell autonomous manner in the rostral striatum and that is critical for psychomotor control.

Improvement of the Rett syndrome phenotype in a MeCP2 mouse model upon treatment with levodopa and a dopa-decarboxylase inhibitor

Rett Syndrome is a neurodevelopmental autism spectrum disorder caused by mutations in the gene coding for methyl CpG-binding protein (MeCP2). The disease is characterized by abnormal motor, respiratory, cognitive impairment, and autistic-like behaviors. No effective treatment of the disorder is available. Mecp2 knockout mice have a range of physiological and neurological abnormalities that resemble the human syndrome and can be used as a model to interrogate new therapies. Herein, we show that the combined administration of Levodopa and a Dopa-decarboxylase inhibitor in RTT mouse models is well tolerated, diminishes RTT-associated symptoms, and increases life span. The amelioration of RTT symptomatology is particularly significant in those features controlled by the dopaminergic pathway in the nigrostratium, such as mobility, tremor, and breathing. Most important, the improvement of the RTT phenotype upon use of the combined treatment is reflected at the cellular level by the development of neuronal dendritic growth. However, much work is required to extend the duration of the benefit of the described preclinical treatment.

Is X-linked methyl-CpG binding protein 2 a new target for the treatment of Parkinson's disease

X-linked methyl-CpG binding protein 2 mutations can induce symptoms similar to those of Parkinson's disease and dopamine metabolism disorders, but the specific role of X-linked methyl-CpG binding protein 2 in the pathogenesis of Parkinson's disease remains unknown. In the present study, we used 6-hydroxydopamine-induced human neuroblastoma cell (SH-SY5Y cells) injury as a cell model of Parkinson's disease. The 6-hydroxydopamine (50 μmol/L) treatment decreased protein levels for both X-linked methyl-CpG binding protein 2 and tyrosine hydroxylase in these cells, and led to cell death. However, overexpression of X-linked methyl-CpG binding protein 2 was able to ameliorate the effects of 6-hydroxydopamine, it reduced 6-hydroxydopamine-induced apoptosis, and increased the levels of tyrosine hydroxylase in SH-SY5Y cells. These findings suggesting that X-linked methyl-CpG binding protein 2 may be a potential therapeutic target for the treatment of Parkinson's disease.

Oxidative brain damage in Mecp2-mutant murine models of Rett syndrome

Rett syndrome (RTT) is a rare neurodevelopmental disorder affecting almost exclusively females, caused in the overwhelming majority of the cases by loss-of-function mutations in the gene encoding methyl-CpG binding protein 2 (MECP2). High circulating levels of oxidative stress (OS) markers in patients suggest the involvement of OS in the RTT pathogenesis. To investigate the occurrence of oxidative brain damage in Mecp2 mutant mouse models, several OS markers were evaluated in whole brains of Mecp2-null (pre-symptomatic, symptomatic, and rescued) and Mecp2-308 mutated (pre-symptomatic and symptomatic) mice, and compared to those of wild type littermates. Selected OS markers included non-protein-bound iron, isoprostanes (F₂-isoprostanes, F₄-neuroprostanes, F₂-dihomo-isoprostanes) and 4-hydroxy-2-nonenal protein adducts. Our findings indicate that oxidative brain damage 1) occurs in both Mecp2-null (both −/y and stop/y) and Mecp2-308 (both 308/y males and 308/+ females) mouse models of RTT; 2) precedes the onset of symptoms in both Mecp2-null and Mecp2-308 models; and 3) is rescued by Mecp2 brain specific gene reactivation. Our data provide direct evidence of the link between Mecp2 deficiency, oxidative stress and RTT pathology, as demonstrated by the rescue of the brain oxidative homeostasis following brain-specifically Mecp2-reactivated mice. The present study indicates that oxidative brain damage is a previously unrecognized hallmark feature of murine RTT, and suggests that Mecp2 is involved in the protection of the brain from oxidative stress.

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Oxidative damage is demonstrated in the brain, and more specifically in the neurons, of Mecp2 mutant mouse models.

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A direct evidence between enhanced oxidative stress and Mecp2 deficiency is provided.

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Oxidative damage precedes the behavioral abnormalities in Mecp2 mutant mice.

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Mecp2 is likely involved in the protection of the brain from oxidative stress.

GABA and glutamate pathways are spatially and developmentally affected in the brain of Mecp2-deficient mice

Proper brain functioning requires a fine-tuning between excitatory and inhibitory neurotransmission, a balance maintained through the regulation and release of glutamate and GABA. Rett syndrome (RTT) is a rare genetic disorder caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene affecting the postnatal brain development. Dysfunctions in the GABAergic and glutamatergic systems have been implicated in the neuropathology of RTT and a disruption of the balance between excitation and inhibition, together with a perturbation of the electrophysiological properties of GABA and glutamate neurons, were reported in the brain of the Mecp2-deficient mouse. However, to date, the extent and the nature of the GABA/glutamate deficit affecting the Mecp2-deficient mouse brain are unclear. In order to better characterize these deficits, we simultaneously analyzed the GABA and glutamate levels in Mecp2-deficient mice at 2 different ages (P35 and P55) and in several brain areas. We used a multilevel approach including the quantification of GABA and glutamate levels, as well as the quantification of the mRNA and protein expression levels of key genes involved in the GABAergic and glutamatergic pathways. Our results show that Mecp2-deficient mice displayed regional- and age-dependent variations in the GABA pathway and, to a lesser extent, in the glutamate pathway. The implication of the GABA pathway in the RTT neuropathology was further confirmed using an in vivo treatment with a GABA reuptake inhibitor that significantly improved the lifespan of Mecp2-deficient mice. Our results confirm that RTT mouse present a deficit in the GABAergic pathway and suggest that GABAergic modulators could be interesting therapeutic agents for this severe neurological disorder.

Dopamine mediates vagal modulation of the immune system by electroacupuncture

Previous anti-inflammatory strategies against sepsis, a leading cause of death in hospitals, had limited efficacy in clinical trials, in part because they targeted single cytokines and the experimental models failed to mimic clinical settings. Neuronal networks represent physiological mechanisms, selected by evolution to control inflammation, that can be exploited for the treatment of inflammatory and infectious disorders. Here, we report that sciatic nerve activation with electroacupuncture controls systemic inflammation and rescues mice from polymicrobial peritonitis. Electroacupuncture at the sciatic nerve controls systemic inflammation by inducing vagal activation of aromatic L-amino acid decarboxylase, leading to the production of dopamine in the adrenal medulla. Experimental models with adrenolectomized mice mimic clinical adrenal insufficiency, increase the susceptibility to sepsis and prevent the anti-inflammatory effects of electroacupuncture. Dopamine inhibits cytokine production via dopamine type 1 (D1) receptors. D1 receptor agonists suppress systemic inflammation and rescue mice with adrenal insufficiency from polymicrobial peritonitis. Our results suggest a new anti-inflammatory mechanism mediated by the sciatic and vagus nerves that modulates the production of catecholamines in the adrenal glands. From a pharmacological perspective, the effects of selective dopamine agonists mimic the anti-inflammatory effects of electroacupuncture and can provide therapeutic advantages to control inflammation in infectious and inflammatory disorders.

MeCP2-mediated alterations of striatal features accompany psychomotor deficits in a mouse model of Rett syndrome

Rett Syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the methyl-CpG-binding protein 2 (MECP2) gene. Affected individuals develop motor deficits including stereotypic hand movements, impaired motor learning and difficulties with movement. To understand the neural mechanisms of motor deficits in RTT, we characterized the molecular and cellular phenotypes in the striatum, the major input nucleus of the basal ganglia that controls psychomotor function, in mice carrying a null allele of Mecp2. These mice showed significant hypoactivity associated with impaired motor coordination and motor skill learning. We found that dopamine content was significantly reduced in the striatum of Mecp2 null mice. Reduced dopamine was accompanied by down-regulation of tyrosine hydroxylase and up-regulation of dopamine D2 receptors, particularly in the rostral striatum. We also observed that loss of MeCP2 induced compartment-specific alterations in the striatum, including reduced expression of μ-opioid receptors in the striosomes and increased number of calbindin-positive neurons in the striatal matrix. The total number of parvalbumin-positive interneurons and their dendritic arborization were also significantly increased in the striatum of Mecp2 null mice. Together, our findings support that MeCP2 regulates a unique set of genes critical for modulating motor output of the striatum, and that aberrant structure and function of the striatum due to MeCP2 deficiency may underlie the motor deficits in RTT.

DNA methylation map of mouse and human brain identifies target genes in Alzheimer's disease

The central nervous system has a pattern of gene expression that is closely regulated with respect to functional and anatomical regions. DNA methylation is a major regulator of transcriptional activity, and aberrations in the distribution of this epigenetic mark may be involved in many neurological disorders, such as Alzheimer’s disease. Herein, we have analysed 12 distinct mouse brain regions according to their CpG 5’-end gene methylation patterns and observed their unique epigenetic landscapes. The DNA methylomes obtained from the cerebral cortex were used to identify aberrant DNA methylation changes that occurred in two mouse models of Alzheimer’s disease. We were able to translate these findings to patients with Alzheimer’s disease, identifying DNA methylation-associated silencing of three targets genes: thromboxane A2 receptor (TBXA2R), sorbin and SH3 domain containing 3 (SORBS3) and spectrin beta 4 (SPTBN4). These hypermethylation targets indicate that the cyclic AMP response element-binding protein (CREB) activation pathway and the axon initial segment could contribute to the disease.

Insulinotropic treatments exacerbate metabolic syndrome in mice lacking MeCP2 function

Rett syndrome (RTT), an X-linked postnatal disorder, results from mutations in Methyl CpG-binding protein 2 (MECP2). Survival and breathing in Mecp2(NULL/Y) animals are improved by an N-terminal tripeptide of insulin-like growth factor I (IGF-I) treatment. We determined that Mecp2(NULL/Y) animals also have a metabolic syndrome and investigated whether IGF-I treatment might improve this phenotype. Mecp2(NULL/Y) mice were treated with a full-length IGF-I modified with the addition of polyethylene glycol (PEG-IGF-I), which improves pharmacological properties. Low-dose PEG-IGF-I treatment slightly improved lifespan and heart rate in Mecp2(NULL/Y) mice; however, high-dose PEG-IGF-I decreased lifespan. To determine whether insulinotropic off-target effects of PEG-IGF-I caused the detrimental effect, we treated Mecp2(NULL/Y) mice with insulin, which also decreased lifespan. Thus, the clinical benefit of IGF-I treatment in RTT may critically depend on the dose used, and caution should be taken when initiating clinical trials with these compounds because the beneficial therapeutic window is narrow.

Revealing the complexity of a monogenic disease: rett syndrome exome sequencing

Rett syndrome (OMIM#312750) is a monogenic disorder that may manifest as a large variety of phenotypes ranging from very severe to mild disease. Since there is a weak correlation between the mutation type in the Xq28 disease-gene MECP2/X-inactivation status and phenotypic variability, we used this disease as a model to unveil the complex nature of a monogenic disorder. Whole exome sequencing was used to analyze the functional portion of the genome of two pairs of sisters with Rett syndrome. Although each pair of sisters had the same MECP2 (OMIM*300005) mutation and balanced X-inactivation, one individual from each pair could not speak or walk, and had a profound intellectual deficit (classical Rett syndrome), while the other individual could speak and walk, and had a moderate intellectual disability (Zappella variant). In addition to the MECP2 mutation, each patient has a group of variants predicted to impair protein function. The classical Rett girls, but not their milder affected sisters, have an enrichment of variants in genes related to oxidative stress, muscle impairment and intellectual disability and/or autism. On the other hand, a subgroup of variants related to modulation of immune system, exclusive to the Zappella Rett patients are driving toward a milder phenotype. We demonstrate that genome analysis has the potential to identify genetic modifiers of Rett syndrome, providing insight into disease pathophysiology. Combinations of mutations that affect speaking, walking and intellectual capabilities may represent targets for new therapeutic approaches. Most importantly, we demonstrated that monogenic diseases may be more complex than previously thought.

Selective preservation of MeCP2 in catecholaminergic cells is sufficient to improve the behavioral phenotype of male and female Mecp2-deficient mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused primarily by mutations of the X-linked MECP2 gene. Although the loss of MeCP2 function affects many neural systems, impairments of catecholaminergic function have been hypothesized to underlie several of the cardinal behavioral deficits of RTT patients and Mecp2-deficient mice. Although recent Mecp2 reactivation studies indicate that RTT may be a reversible condition, it remains unclear whether specifically preserving Mecp2 function within a specific system will be sufficient to convey beneficial effects. Here, we test whether the selective preservation of Mecp2 within catecholaminergic cells will improve the phenotype of Mecp2-deficient mice. Our results show that this targeted preservation of Mecp2 significantly improves the lifespan, phenotypic severity and cortical epileptiform discharge activity of both male and female Mecp2-deficient mice. Further, we found that the catecholaminergic preservation of Mecp2 also improves the ambulatory rate, rearing activity, motor coordination, anxiety and nest-building performances of Mecp2-deficient mice of each gender. Interestingly, our results also revealed a gender-specific improvement, as specific cortical and hippocampal electroencephalographic abnormalities were significantly improved in male, but not female, rescue mice. Collectively, these results support the role of the catecholaminergic system in the pathogenesis of RTT and provide proof-of-principle that restoring MeCP2 function within this specific system could represent a treatment strategy for RTT.

Reward circuitry dysfunction in psychiatric and neurodevelopmental disorders and genetic syndromes: animal models and clinical findings

This review summarizes evidence of dysregulated reward circuitry function in a range of neurodevelopmental and psychiatric disorders and genetic syndromes. First, the contribution of identifying a core mechanistic process across disparate disorders to disease classification is discussed, followed by a review of the neurobiology of reward circuitry. We next consider preclinical animal models and clinical evidence of reward-pathway dysfunction in a range of disorders, including psychiatric disorders (i.e., substance-use disorders, affective disorders, eating disorders, and obsessive compulsive disorders), neurodevelopmental disorders (i.e., schizophrenia, attention-deficit/hyperactivity disorder, autism spectrum disorders, Tourette's syndrome, conduct disorder/oppositional defiant disorder), and genetic syndromes (i.e., Fragile X syndrome, Prader-Willi syndrome, Williams syndrome, Angelman syndrome, and Rett syndrome). We also provide brief overviews of effective psychopharmacologic agents that have an effect on the dopamine system in these disorders. This review concludes with methodological considerations for future research designed to more clearly probe reward-circuitry dysfunction, with the ultimate goal of improved intervention strategies.

Prolonged membrane depolarization enhances midbrain dopamine neuron differentiation via epigenetic histone modifications

Understanding midbrain dopamine (DA) neuron differentiation is of importance, because of physiological and clinical implications of this neuronal subtype. We show that prolonged membrane depolarization induced by KCl treatment promotes DA neuron differentiation from neural precursor cells (NPCs) derived from embryonic ventral midbrain (VM). Interestingly, the depolarization-induced increase of DA neuron yields was not abolished by L-type calcium channel blockers, along with no depolarization-mediated change of intracellular calcium level in the VM-derived NPCs (VM-NPCs), suggesting that the depolarization effect is due to a calcium-independent mechanism. Experiments with labeled DA neuron progenitors indicate that membrane depolarization acts at the differentiation fate determination stage and promotes the expression of DA phenotype genes (tyrosine hydroxylase [TH] and DA transporter [DAT]). Recruitment of Nurr1, a transcription factor crucial for midbrain DA neuron development, to the promoter of TH gene was enhanced by depolarization, along with increases of histone 3 acetylation (H3Ac) and trimethylation of histone3 on lysine 4 (H3K4m3), and decreases of H3K9m3 and H3K27m3 in the consensus Nurr1 binding regions of TH promoter. Depolarization stimuli on differentiating VM-NPCs also induced dissociation of methyl CpG binding protein 2 and related repressor complex molecules (repressor element-1 silencing transcription factor corepressor and histone deacetylase 1) from the CpG sites of TH and DAT promoters. Based on these findings, we suggest that membrane depolarization promotes DA neuron differentiation by opening chromatin structures surrounding DA phenotype genes and inhibiting the binding of corepressors, thus allowing transcriptional activators such as Nurr1 to access DA neuron differentiation gene promoter regions.

Rett syndrome: genes, synapses, circuits, and therapeutics

Development of the nervous system proceeds through a set of complex checkpoints which arise from a combination of sequential gene expression and early neural activity sculpted by the environment. Genetic and environmental insults lead to neurodevelopmental disorders which encompass a large group of diseases that result from anatomical and physiological abnormalities during maturation and development of brain circuits. Rett syndrome (RTT) is a neurological disorder of genetic origin, caused by mutations in the X-linked gene methyl-CpG binding protein 2 (MeCP2). It features a range of neuropsychiatric abnormalities including motor dysfunctions and mild to severe cognitive impairment. Here, we discuss key questions and recent studies describing animal models, cell-type specific functions of methyl-CpG binding protein 2 (MeCP2), defects in neural circuit plasticity, and attempts to evaluate possible therapeutic strategies for RTT. We also discuss how genes, proteins, and overlapping signaling pathways affect the molecular etiology of apparently unrelated neuropsychiatric disorders, an understanding of which can offer novel therapeutic strategies for a range of autism spectrum disorders (ASDs).

Modification of Mecp2 dosage alters axonal transport through the Huntingtin/Hap1 pathway

Mecp2 deficiency or overexpression causes a wide spectrum of neurological diseases in humans among which Rett Syndrome is the prototype. Pathogenic mechanisms are thought to involve transcriptional deregulation of target genes such as Bdnf together with defects in the general transcriptional program of affected cells. Here we found that two master genes, Huntingtin (Htt) and huntingtin-associated protein (Hap1), involved in the control of Bdnf axonal transport, are altered in the brain of Mecp2-deficient mice. We also revealed an in vivo defect of Bdnf transport throughout the cortico striatal pathway of Mecp2-deficient animals. We found that the velocity of Bdnf-containing vesicles is reduced in vitro in the Mecp2-deficient axons and this deficit can be rescued by the re-expression of Mecp2. The defect in axonal transport is not restricted to Bdnf since transport of the amyloid precursor protein (App) that is Htt and Hap1-dependent is also altered. Finally, treating Mecp2-deficient mice with cysteamine, a molecule increasing the secretion of Bdnf vesicles, improved the lifespan and reduced motor defects, suggesting a new therapeutic strategy for Rett syndrome.

Loss of Mecp2 in substantia nigra dopamine neurons compromises the nigrostriatal pathway

Mutations in the methyl-CpG-binding protein 2 (MeCP2) result in Rett syndrome (RTT), an X-linked disorder that disrupts neurodevelopment. Girls with RTT exhibit motor deficits similar to those in Parkinson's disease, suggesting defects in the nigrostriatal pathway. This study examined age-dependent changes in dopamine neurons of the substantia nigra (SN) from wild-type, presymptomatic, and symptomatic Mecp2(+/-) mice. Mecp2(+) neurons in the SN in Mecp2(+/-) mice were indistinguishable in morphology, resting conductance, and dopamine current density from neurons in wild-type mice. However, the capacitance, total dendritic length, and resting conductance of Mecp2(-) neurons were less than those of Mecp2(+) neurons as early as 4 weeks after birth, before overt symptoms. These differences were maintained throughout life. In symptomatic Mecp2(+/-) mice, the current induced by activation of D(2) dopamine autoreceptors was significantly less in Mecp2(-) neurons than in Mecp2(+) neurons, although D(2) receptor density was unaltered in Mecp2(+/-) mice. Electrochemical measurements revealed that significantly less dopamine was released after stimulation of striatum in adult Mecp2(+/-) mice compared to wild type. The decrease in size and function of Mecp2(-) neurons observed in adult Mecp2(+/-) mice was recapitulated in dopamine neurons from symptomatic Mecp2(-/y) males. These results show that mutation in Mecp2 results in cell-autonomous defects in the SN early in life and throughout adulthood. Ultimately, dysfunction in terminal dopamine release and D(2) autoreceptor-dependent currents in dopamine neurons from symptomatic females support the idea that decreased dopamine transmission due to heterogeneous Mecp2 expression contributes to the parkinsonian features of RTT in Mecp2(+/-) mice.

Biogenic amines and their metabolites are differentially affected in the Mecp2-deficient mouse brain

Background

Rett syndrome (RTT, MIM #312750) is a severe neurological disorder caused by mutations in the X-linked methyl-CpG binding protein 2 (MECP2) gene. Female patients are affected with an incidence of 1/15000 live births and develop normally from birth to 6-18 months of age before the onset of deficits in autonomic, cognitive, motor functions (stereotypic hand movements, impaired locomotion) and autistic features. Studies on Mecp2 mouse models, and specifically null mice, revealed morphological and functional alterations of neurons. Several functions that are regulated by bioaminergic nuclei or peripheral ganglia are impaired in the absence of Mecp2.

Results

Using high performance liquid chromatography, combined with electrochemical detection (HPLC/EC) we found that Mecp2^-/y mice exhibit an alteration of DA metabolism in the ponto-bulbar region at 5 weeks followed by a more global alteration of monoamines when the disease progresses (8 weeks). Hypothalamic measurements suggest biphasic disturbances of norepinephrine and serotonin at pathology onset (5 weeks) that were found stabilized later on (8 weeks). Interestingly, the postnatal nigrostriatal dopaminergic deficit identified previously does not parallel the reduction of the other neurotransmitters investigated. Finally, dosage in cortical samples do not suggest modification in the monoaminergic content respectively at 5 and 8 weeks of age.

Conclusions

We have identified that the level of catecholamines and serotonin is differentially affected in Mecp2^-/y brain areas in a time-dependent fashion.