MeCP2 deficiency enhances glutamate release through NF-κB signaling in myeloid derived cells

Molecular Signatures of Response to Mecasermin in Children With Rett Syndrome

Rett syndrome (RTT) is a devastating neurodevelopmental disorder without effective treatments. Attempts at developing targetted therapies have been relatively unsuccessful, at least in part, because the genotypical and phenotypical variability of the disorder. Therefore, identification of biomarkers of response and patients' stratification are high priorities. Administration of Insulin-like Growth Factor 1 (IGF-1) and related compounds leads to significant reversal of RTT-like symptoms in preclinical mouse models. However, improvements in corresponding clinical trials have not been consistent. A 20-weeks phase I open label trial of mecasermin (recombinant human IGF-1) in children with RTT demonstrated significant improvements in breathing phenotypes. However, a subsequent randomised controlled phase II trial did not show significant improvements in primary outcomes although two secondary clinical endpoints showed positive changes. To identify molecular biomarkers of response and surrogate endpoints, we used RNA sequencing to measure differential gene expression in whole blood samples of participants in the abovementioned phase I mecasermin trial. When all participants (n = 9) were analysed, gene expression was unchanged during the study (baseline vs. end of treatment, T0-T3). However, when participants were subclassified in terms of breathing phenotype improvement, specifically by their plethysmography-based apnoea index, individuals with moderate-severe apnoea and breathing improvement (Responder group) displayed significantly different transcript profiles compared to the other participants in the study (Mecasermin Study Reference group, MSR). Many of the differentially expressed genes are involved in the regulation of cell cycle processes and immune responses, as well as in IGF-1 signalling and breathing regulation. While the Responder group showed limited gene expression changes in response to mecasermin, the MSR group displayed marked differences in the expression of genes associated with inflammatory processes (e.g., neutrophil activation, complement activation) throughout the trial. Our analyses revealed gene expression profiles associated with severe breathing phenotype and its improvement after mecasermin administration in RTT, and suggest that inflammatory/immune pathways and IGF-1 signalling contribute to treatment response. Overall, these data support the notion that transcript profiles have potential as biomarkers of response to IGF-1 and related compounds.

Proteomic and transcriptional changes associated with MeCP2 dysfunction reveal nodes for therapeutic intervention in Rett syndrome

Mutations in the methyl-CpG binding protein 2 (MECP2) gene cause Rett syndrome (RTT), an X-linked neurodevelopmental disorder predominantly impacting females. MECP2 is an epigenetic transcriptional regulator acting mainly to repress gene expression, though it plays multiple gene regulatory roles and has distinct molecular targets across different cell types and specific developmental stages. In this review, we summarize MECP2 loss-of-function associated transcriptome and proteome disruptions, delving deeper into the latter which have been comparatively severely understudied. These disruptions converge on multiple biochemical and cellular pathways, including those involved in synaptic function and neurodevelopment, NF-κB signaling and inflammation, and the vitamin D pathway. RTT is a complex neurological disorder characterized by myriad physiological disruptions, in both the central nervous system and peripheral systems. Thus, treating RTT will likely require a combinatorial approach, targeting multiple nodes within the interactomes of these cellular pathways. To this end, we discuss the use of dietary supplements and factors, namely, vitamin D and polyunsaturated fatty acids (PUFAs), as possible partial therapeutic agents given their demonstrated benefit in RTT and their ability to restore homeostasis to multiple disrupted cellular pathways simultaneously. Further unravelling the complex molecular alterations induced by MECP2 loss-of-function, and contextualizing them at the level of proteome homeostasis, will identify new therapeutic avenues for this complex disorder.

What can clinical immunology learn from inborn errors of epigenetic regulators?

The epigenome is at the interface between environmental factors and the genome, regulating gene transcription, DNA repair, and replication. Epigenetic modifications play a crucial role in establishing and maintaining cell identity and are especially crucial for neurology, musculoskeletal integrity, and the function of the immune system. Mutations in genes encoding for the components of the epigenetic machinery lead to the development of distinct disorders, especially involving the central nervous system and host defense. In this review, we focus on the role of epigenetic modifications for the function of the immune system. By studying the immune phenotype of patients with monogenic mutations in components of the epigenetic machinery (inborn errors of epigenetic regulators), we demonstrate the importance of DNA methylation, histone modifications, chromatin remodeling, noncoding RNAs, and mRNA processing for immunity. Moreover, we give a short overview on therapeutic strategies targeting the epigenome.

Glutaminase in microglia: A novel regulator of neuroinflammation

Neuroinflammation is the inflammatory responses that are involved in the pathogenesis of most neurological disorders. Glutaminase (GLS) is the enzyme that catalyzes the hydrolysis of glutamine to produce glutamate. Besides its well-known role in cellular metabolism and excitatory neurotransmission, GLS has recently been increasingly noticed to be up-regulated in activated microglia under pathological conditions. Furthermore, GLS overexpression induces microglial activation, extracellular vesicle secretion, and neuroinflammatory microenvironment formation, which, are compromised by GLS inhibitors in vitro and in vivo. These results indicate that GLS has more complicated implications in brain disease etiology than what are previously known. In this review, we introduce GLS isoforms, expression patterns in the body and the brain, and expression/activities regulation. Next, we discuss the metabolic and neurotransmission functions of GLS. Afterwards, we summarize recent findings of GLS-mediated microglial activation and pro-inflammatory extracellular vesicle secretion, which, in turns, induces neuroinflammation. Lastly, we provide a comprehensive discussion for the involvement of microglial GLS in the pathogenesis of various neurological disorders, indicating microglial GLS as a promising target to treat these diseases.

Vitamin D Supplementation Rescues Aberrant NF-κB Pathway Activation and Partially Ameliorates Rett Syndrome Phenotypes in Mecp2 Mutant Mice

Rett syndrome (RTT) is a severe, progressive X-linked neurodevelopmental disorder caused by mutations in the transcriptional regulator MECP2. We previously identified aberrant NF-κB pathway upregulation in brains of Mecp2-null mice and demonstrated that genetically attenuating NF-κB rescues some characteristic neuronal RTT phenotypes. These results raised the intriguing question of whether NF-κB pathway inhibitors might provide a therapeutic avenue in RTT. Here, we investigate whether the known NF-κB pathway inhibitor vitamin D ameliorates neuronal phenotypes in Mecp2-mutant mice. Vitamin D deficiency is prevalent among RTT patients, and we find that Mecp2-null mice similarly have significantly reduced 25(OH)D serum levels compared with wild-type littermates. We identify that vitamin D rescues aberrant NF-κB pathway activation and reduced neurite outgrowth of Mecp2 knock-down cortical neurons in vitro. Further, dietary supplementation with vitamin D in early symptomatic male Mecp2 hemizygous null and female Mecp2 heterozygous mice ameliorates reduced neocortical dendritic morphology and soma size phenotypes and modestly improves reduced lifespan of Mecp2-nulls. These results elucidate fundamental neurobiology of RTT and provide foundation that NF-κB pathway inhibition might be a therapeutic target for RTT.

Compromised immune/inflammatory responses in Rett syndrome

Mutations in X-linked gene methyl-CpG-binding protein 2 (MECP2), a key transcriptional regulator, account for most cases of Rett syndrome (RTT), a devastating neurodevelopmental disorder with no known cure. Despite extensive research to elucidate MeCP2 functions, the mechanisms underlying RTT pathophysiology are still unclear. In addition to a variety of neurological symptoms, RTT also includes a plethora of additional phenotypical features including altered lipid metabolism, redox imbalance, immune dysfunction and mitochondrial abnormalities that explain its multisystemic nature. Here, we provide an overview of the current knowledge on the potential role of dysregulated inflammatory and immune responses in RTT. The findings show that abnormalities of humoral and cell-mediated immunity together with chronic low-grade inflammation in multiple organs represent not only clinical manifestations of RTT but rather can contribute to its development and deteriorating course. A future research challenge could be to target therapeutically immune dysfunction as a novel means for RTT management.

ANAVEX®2-73 (blarcamesine), a Sigma-1 receptor agonist, ameliorates neurologic impairments in a mouse model of Rett syndrome

Rett syndrome (RTT) is a severe neurodevelopmental disorder that is associated in most cases with mutations in the transcriptional regulator MECP2. At present, there are no effective treatments for the disorder. Despite recent advances in RTT genetics and neurobiology, most drug development programs have focused on compounds targeting the IGF-1 pathway and no pivotal trial has been completed as yet. Thus, testing novel drugs that can ameliorate RTT's clinical manifestations is a high priority. ANAVEX2-73 (blarcamesine) is a Sigma-1 receptor agonist and muscarinic receptor modulator with a strong safety record and preliminary evidence of efficacy in patients with Alzheimer's disease. Its role in calcium homeostasis and mitochondrial function, cellular functions that underlie pathological processes and compensatory mechanisms in RTT, makes blarcamesine an intriguing drug candidate for this disorder. Mice deficient in MeCP2 have a range of physiological and neurological abnormalities that mimic the human syndrome. We tested blarcamesine in female heterozygous mice carrying one null allele of Mecp2 (HET) using a two-tier approach, with age-appropriate tests. Administration of the drug to younger and older adult mice resulted in improvement in multiple motor, sensory, and autonomic phenotypes of relevance to RTT. The latter included motor coordination and balance, acoustic and visual responses, hindlimb clasping, and apnea in expiration. In line with previous animal and human studies, blarcamesine also showed a good safety profile in this mouse model of RTT. Clinical studies in RTT with blarcamesine are ongoing.

MicroRNA-223 protects neurons from degeneration in experimental autoimmune encephalomyelitis

Dysregulation of miRNAs has been observed in many neurodegenerative diseases, including multiple sclerosis. Morquette et al. show that overexpression of miR-223-3p prevents accumulation of axonal damage in a rodent model of multiple sclerosis, in part through regulation of glutamate receptor signalling. Manipulation of miRNA levels may have therapeutic potential.

Efficient and Precise CRISPR/Cas9-Mediated MECP2 Modifications in Human-Induced Pluripotent Stem Cells

Patients with Rett syndrome (RTT) have severe mental and physical disabilities. The majority of RTT patients carry a heterozygous mutation in methyl-CpG binding protein 2 (MECP2), an X-linked gene encoding an epigenetic factor crucial for normal nerve cell function. No curative therapy for RTT syndrome exists, and cellular mechanisms are incompletely understood. Here, we developed a CRISPR/Cas9-mediated system that targets and corrects the disease relevant regions of the MECP2 exon 4 coding sequence. We achieved homologous recombination (HR) efficiencies of 20% to 30% in human cell lines and iPSCs. Furthermore, we successfully introduced a MECP2^R270X mutation into the MECP2 gene in human induced pluripotent stem cells (iPSCs). Consequently, using CRISPR/Cas9, we were able to repair such mutations with high efficiency in human mutant iPSCs. In summary, we provide a new strategy for MECP2 gene targeting that can be potentially translated into gene therapy or for iPSCs-based disease modeling of RTT syndrome.

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.

The Pathophysiological Role of Microglia in Dynamic Surveillance, Phagocytosis and Structural Remodeling of the Developing CNS

In vertebrates, during an early wave of hematopoiesis in the yolk sac between embryonic day E7.0 and E9.0, cells of mesodermal leaflet addressed to macrophage lineage enter in developing central nervous system (CNS) and originate the developing native microglial cells. Depending on the species, microglial cells represent 5–20% of glial cells resident in adult brain. Here, we briefly discuss some canonical functions of the microglia, i.e., cytokine secretion and functional transition from M1 to M2 phenotype. In addition, we review studies on the non-canonical functions of microglia such as regulation of phagocytosis, synaptic pruning, and sculpting postnatal neural circuits. In this latter context the contribution of microglia to some neurodevelopmental disorders is now well established. Nasu-Hakola (NHD) disease is considered a primary microgliopathy with alterations of the DNAX activation protein 12 (DAP12)-Triggering receptor expressed on myeloid cells 2 (TREM-2) signaling and removal of macromolecules and apoptotic cells followed by secondary microglia activation. In Rett syndrome Mecp2^-/- microglia shows a substantial impairment of phagocytic ability, although the role of microglia is not yet clear. In a mouse model of Tourette syndrome (TS), microglia abnormalities have also been described, and deficient microglia-mediated neuroprotection is obvious. Here we review the role of microglial cells in neurodevelopmental disorders without inflammation and on the complex role of microglia in developing CNS.

Influenza A induces dysfunctional immunity and death in MeCP2-overexpressing mice

Loss of function or overexpression of methyl-CpG-binding protein 2 (MeCP2) results in the severe neurodevelopmental disorders Rett syndrome and MeCP2 duplication syndrome, respectively. MeCP2 plays a critical role in neuronal function and the function of cells throughout the body. It has been previously demonstrated that MeCP2 regulates T cell function and macrophage response to multiple stimuli, and that immune-mediated rescue imparts significant benefit in Mecp2-null mice. Unlike Rett syndrome, MeCP2 duplication syndrome results in chronic, severe respiratory infections, which represent a significant cause of patient morbidity and mortality. Here, we demonstrate that MeCP2Tg3 mice, which overexpress MeCP2 at levels 3- to 5-fold higher than normal, are hypersensitive to influenza A/PR/8/34 infection. Prior to death, MeCP2Tg3 mice experienced a host of complications during infection, including neutrophilia, increased cytokine production, excessive corticosterone levels, defective adaptive immunity, and vascular pathology characterized by impaired perfusion and pulmonary hemorrhage. Importantly, we found that radioresistant cells are essential to infection-related death after bone marrow transplantation. In all, these results demonstrate that influenza A infection in MeCP2Tg3 mice results in pathology affecting both immune and nonhematopoietic cells, suggesting that failure to effectively respond and clear viral respiratory infection has a complex, multicompartment etiology in the context of MeCP2 overexpression.

Neurobiologically-based treatments in Rett syndrome: opportunities and challenges

Introduction: Rett syndrome (RTT) is an X-linked neurodevelopmental disorder that primarily affects females, typically resulting in a period of developmental regression in early childhood followed by stabilization and severe chronic cognitive, behavioral, and physical disability. No known treatment exists beyond symptomatic management, and while insights into the genetic cause, pathophysiology, neurobiology, and natural history of RTT have been gained, many challenges remain. Areas covered: Based on a comprehensive survey of the primary literature on RTT, this article describes and comments upon the general and unique features of the disorder, genetic and neurobiological bases of drug development, and the history of clinical trials in RTT, with an emphasis on drug trial design, outcome measures, and implementation. Expert opinion: Neurobiologically based drug trials are the ultimate goal in RTT, and due to the complexity and global nature of the disorder, drugs targeting both general mechanisms (e.g., growth factors) and specific systems (e.g., glutamate modulators) could be effective. Trial design should optimize data on safety and efficacy, but selection of outcome measures with adequate measurement properties, as well as innovative strategies, such as those enhancing synaptic plasticity and use of biomarkers, are essential for progress in RTT and other neurodevelopmental disorders.

Unexpected cellular players in Rett syndrome pathology

Rett syndrome is a devastating neurodevelopmental disorder, primarily caused by mutations of methyl CpG-binding protein 2 (MeCP2). Although the genetic cause of disease was identified over a decade ago, a significant gap still remains in both our clinical and scientific understanding of its pathogenesis. Neurons are known to be primary players in pathology, with their dysfunction being the key in Rett syndrome. While studies in mice have demonstrated a clear causative - and potential therapeutic - role for neurons in Rett syndrome, recent work has suggested that other tissues also contribute significantly to progression of the disease. Indeed, Rett syndrome is known to present with several common peripheral pathologies, such as osteopenia, scoliosis, gastrointestinal problems including nutritional defects, and general growth deficit. Mouse models assessing the potential role of non-neuronal cell types have confirmed both roles in disease and potential therapeutic targets. A new picture is emerging in which neurons both initiate and drive pathology, while dysfunction of other cell types and peripheral tissues exacerbate disease, possibly amplifying further neurologic problems, and ultimately result in a positive feedback loop of progressively worsening symptoms. Here, we review what is known about neuronal and non-neuronal cell types, and discuss how this new, integrative understanding of the disease may allow for additional clinical and scientific pathways for treating and understanding Rett syndrome.

Reduction of aberrant NF-κB signalling ameliorates Rett syndrome phenotypes in Mecp2-null mice

Mutations in the transcriptional regulator Mecp2 cause the severe X-linked neurodevelopmental disorder Rett syndrome (RTT). In this study, we investigate genes that function downstream of MeCP2 in cerebral cortex circuitry, and identify upregulation of Irak1, a central component of the NF-κB pathway. We show that overexpression of Irak1 mimics the reduced dendritic complexity of Mecp2-null cortical callosal projection neurons (CPN), and that NF-κB signalling is upregulated in the cortex with Mecp2 loss-of-function. Strikingly, we find that genetically reducing NF-κB signalling in Mecp2-null mice not only ameliorates CPN dendritic complexity but also substantially extends their normally shortened lifespan, indicating broader roles for NF-κB signalling in RTT pathogenesis. These results provide new insight into both the fundamental neurobiology of RTT, and potential therapeutic strategies via NF-κB pathway modulation.

Rett syndrome is a neurodevelopmental disorder caused by mutations in Mecp2. Here the authors show that Mecp2 loss-of-function leads to upregulation of the NF-κB pathway, and that reducing NF-κB signalling ameliorates phenotypes of Mecp2-null mice, thus offering a potential therapeutic strategy.

Dysregulation of glutamine transporter SNAT1 in Rett syndrome microglia: a mechanism for mitochondrial dysfunction and neurotoxicity

Rett syndrome (RTT) is an autism spectrum disorder caused by loss-of-function mutations in the gene encoding MeCP2, an epigenetic modulator that binds the methyl CpG dinucleotide in target genes to regulate transcription. Previously, we and others reported a role of microglia in the pathophysiology of RTT. To understand the mechanism of microglia dysfunction in RTT, we identified a MeCP2 target gene, SLC38A1, which encodes a major glutamine transporter (SNAT1), and characterized its role in microglia. We found that MeCP2 acts as a microglia-specific transcriptional repressor of SNAT1. Because glutamine is mainly metabolized in the mitochondria, where it is used as an energy substrate and a precursor for glutamate production, we hypothesize that SNAT1 overexpression in MeCP2-deficient microglia would impair the glutamine homeostasis, resulting in mitochondrial dysfunction as well as microglial neurotoxicity because of glutamate overproduction. Supporting this hypothesis, we found that MeCP2 downregulation or SNAT1 overexpression in microglia resulted in (1) glutamine-dependent decrease in microglial viability, which was corroborated by reduced microglia counts in the brains of MECP2 knock-out mice; (2) proliferation of mitochondria and enhanced mitochondrial production of reactive oxygen species; (3) increased oxygen consumption but decreased ATP production (an energy-wasting state); and (4) overproduction of glutamate that caused NMDA receptor-dependent neurotoxicity. The abnormalities could be rectified by mitochondria-targeted expression of catalase and a mitochondria-targeted peptide antioxidant, Szeto-Schiller 31. Our results reveal a novel mechanism via which MeCP2 regulates bioenergetic pathways in microglia and suggest a therapeutic potential of mitochondria-targeted antioxidants for RTT.

Domestication of the dog from the wolf was promoted by enhanced excitatory synaptic plasticity: a hypothesis

Dogs shared a much closer relationship with humans than any other domesticated animals, probably due to their unique social cognitive capabilities, which were hypothesized to be a by-product of selection for tameness toward humans. Here, we demonstrate that genes involved in glutamate metabolism, which account partially for fear response, indeed show the greatest population differentiation by whole-genome comparison of dogs and wolves. However, the changing direction of their expression supports a role in increasing excitatory synaptic plasticity in dogs rather than reducing fear response. Because synaptic plasticity are widely believed to be cellular correlates of learning and memory, this change may alter the learning and memory abilities of ancient scavenging wolves, weaken the fear reaction toward humans, and prompt the initial interspecific contact.

Microglia as a critical player in both developmental and late-life CNS pathologies

Microglia, the tissue-resident macrophages of the brain, are attracting increasing attention as key players in brain homeostasis from development through aging. Recent works have highlighted new and unexpected roles for these once-enigmatic cells in both healthy central nervous system function and in diverse pathologies long thought to be primarily the result of neuronal malfunction. In this review, we have chosen to focus on Rett syndrome, which features early neurodevelopmental pathology, and Alzheimer’s disease, a disorder associated predominantly with aging. Interestingly, receptor-mediated microglial phagocytosis has emerged as a key function in both developmental and late-life brain pathologies. In a mouse model of Rett syndrome, bone marrow transplant and CNS engraftment of microglia-like cells were associated with surprising improvements in pathology—these benefits were abrogated by block of phagocytic function. In Alzheimer’s disease, large-scale genome-wide association studies have been brought to bear as a method of identifying previously unknown susceptibility genes, which highlight microglial receptors as promising novel targets for therapeutic modulation. Multi-photon in vivo microscopy has provided a method of directly visualizing the effects of manipulation of these target genes. Here, we review the latest findings and concepts emerging from the rapidly growing body of literature exemplified for Rett syndrome and late-onset, sporadic Alzheimer’s disease.