Mutations in the transcriptional regulator MeCP2 severely impact key cellular and molecular signatures of human astrocytes during maturation

MECP2 duplication syndrome: Recent advances in pathophysiology and therapeutic perspectives

MECP2 duplication syndrome (MDS) is an X-linked neurodevelopmental disorder caused by duplication or extra copies of MECP2 gene. It primarily affects males and is characterized by intellectual disability, hypotonia, epilepsy, recurrent infections, and autistic features. Methyl-CpG binding protein 2 (MeCP2) encoded by MECP2 is a crucial epigenetic regulator of brain function. Expression levels are strictly regulated during brain development and maturation, and altered levels lead to severe neurodevelopmental disorders; excessive levels are associated with MDS, while insufficient levels cause Rett syndrome. This review provides a comprehensive overview of the recent advances in the pathophysiology and therapeutic perspectives of MDS, focusing on its pathophysiology, clinical features, disease models, and therapeutic strategies. Advances in studies using animal and patient-derived induced pluripotent stem cells (iPSCs)-derived neuronal models have provided insights into the molecular and cellular abnormalities associated with MDS and have facilitated therapeutic development. Among the emerging treatments, antisense oligonucleotide (ASO) therapy has gained significant attention as a promising approach for selectively suppressing MeCP2 overexpression. Preclinical studies using MDS mouse models and iPSCs-derived neurons have demonstrated that ASO treatment can partially restore neuronal abnormalities and clinical trials are currently underway.

Interpreting the rich dialogue between astrocytes and neurons: An overview in Rett syndrome

Rett syndrome (RTT) is a severe neurodevelopmental disorder primarily affecting females, with an incidence of 1 in 10,000 live births. It is caused mainly by de novo mutations in the X-linked MECP2 gene, which encodes methyl-CpG binding protein 2 (Mecp2), a key epigenetic regulator. MECP2 mutations have profound impacts on neurons, which exhibit morphological, synaptic and functional impairments. However, more recent evidence highlights a crucial role of astrocytes in RTT pathogenesis. Indeed, RTT astrocytes exhibit structural and functional impairments, failing to support neuronal growth and function through non-cell autonomous mechanisms. Studies reveal that MECP2 deficient astrocytes secrete abnormal factors that impair neuronal growth and synaptic function. Furthermore, they show dysregulated calcium signalling, disrupted glutamate and potassium homeostasis, and increased inflammatory responses, all of which contribute to neuronal dysfunction. Understanding these neuron-astrocyte interactions may offer novel therapeutic targets for RTT. In the review we aim at presenting the current knowledge of astrocyte-neuron crosstalk in RTT, describing the different mechanisms highlighted so far through which MECP2 mutant astrocytes impair neurons. Finally, we discuss existing and prospective methodological approaches for investigating cell-to-cell communication in RTT.

Exploring the complexity of MECP2 function in Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder that is mainly caused by mutations in the methyl-DNA-binding protein MECP2. MECP2 is an important epigenetic regulator that plays a pivotal role in neuronal gene regulation, where it has been reported to function as both a repressor and an activator. Despite extensive efforts in mechanistic studies over the past two decades, a clear consensus on how MECP2 dysfunction impacts molecular mechanisms and contributes to disease progression has not been reached. Here, we review recent insights from epigenomic, transcriptomic and proteomic studies that advance our understanding of MECP2 as an interacting hub for DNA, RNA and transcription factors, orchestrating diverse processes that are crucial for neuronal function. By discussing findings from different model systems, we identify crucial epigenetic details and cofactor interactions, enriching our understanding of the multifaceted roles of MECP2 in transcriptional regulation and chromatin structure. These mechanistic insights offer potential avenues for rational therapeutic design for RTT.

The methyl-CpG-binding protein 2 (Mecp2) regulates the hypothalamic mitochondrial function and white adipose tissue lipid metabolism

OBJECTIVE

The neuroepigenetic factor Mecp2 regulates gene expression and is thought to play a crucial role in energy homeostasis. Body weight is regulated at the hypothalamic level, where mitochondrial energy metabolism is necessary for its proper functioning, allowing the hypothalamus to respond to peripheral signals to maintain energy balance and modulate energy expenditure through the sympathetic nervous system. Since the mechanism by which genetic and environmental factors contribute to regulating energy balance is unclear, this study aims to understand the contribution of gene-environment interaction to maintaining energy balance and how its disruption alters hypothalamic cellular energy production, impacting the control of systemic metabolism.

METHODS

We used a mouse model of epigenetic disruption (Mecp2-null) to evaluate the impact of Mecp2 deletion on systemic and hypothalamic metabolism using physiological and cellular approaches.

RESULTS

Our study shows that the previously reported body weight gain in mice lacking the expression of Mecp2 is preceded by a hypothalamic mitochondrial dysfunction that disrupts hypothalamic function, leading to a dysfunctional communication between the hypothalamus and adipose tissue, thus impairing lipid metabolism. Our study has revealed three crucial aspects of the contribution of this critical epigenetic factor pivotal for a proper gene-environment interaction: i) Mecp2 drives a molecular mechanism to maintain cellular energy homeostasis, which is necessary for the proper functioning of the hypothalamus. ii) Mecp2 is necessary to maintain lipid metabolism in adipose tissue. iii) Mecp2 is a molecular bridge linking hypothalamic cellular energy metabolism and adipose tissue lipid metabolism.

CONCLUSIONS

Our results show that Mecp2 regulates the hypothalamic mitochondrial function and white adipose tissue lipid metabolism and probably alters the communication between these two tissues, which is critical for corporal energy homeostasis maintenance.

Glial Cells in Spinal Muscular Atrophy: Speculations on Non-Cell-Autonomous Mechanisms and Therapeutic Implications

Spinal muscular atrophy (SMA) is a neuromuscular disorder caused by homozygous deletions or mutations in the SMN1 gene, leading to progressive motor neuron degeneration. While SMA has been classically viewed as a motor neuron-autonomous disease, increasing evidence indicates a significant role of glial cells-astrocytes, microglia, oligodendrocytes, and Schwann cells-in the disease pathophysiology. Astrocytic dysfunction contributes to motor neuron vulnerability through impaired calcium homeostasis, disrupted synaptic integrity, and neurotrophic factor deficits. Microglia, through reactive gliosis and complement-mediated synaptic stripping, exacerbate neurodegeneration and neuroinflammation. Oligodendrocytes exhibit impaired differentiation and metabolic support, while Schwann cells display abnormalities in myelination, extracellular matrix composition, and neuromuscular junction maintenance, further compromising motor function. Dysregulation of pathways such as NF-κB, Notch, and JAK/STAT, alongside the upregulation of complement proteins and microRNAs, reinforces the non-cell-autonomous nature of SMA. Despite the advances in SMN-restorative therapies, they do not fully mitigate glial dysfunction. Targeting glial pathology, including modulation of reactive astrogliosis, microglial polarization, and myelination deficits, represents a critical avenue for therapeutic intervention. This review comprehensively examines the multifaceted roles of glial cells in SMA and highlights emerging glia-targeted strategies to enhance treatment efficacy and improve patient outcomes.

Harnessing the potential of human induced pluripotent stem cells, functional assays and machine learning for neurodevelopmental disorders

Neurodevelopmental disorders (NDDs) affect 4.7% of the global population and are associated with delays in brain development and a spectrum of impairments that can lead to lifelong disability and even mortality. Identification of biomarkers for accurate diagnosis and medications for effective treatment are lacking, in part due to the historical use of preclinical model systems that do not translate well to the clinic for neurological disorders, such as rodents and heterologous cell lines. Human-induced pluripotent stem cells (hiPSCs) are a promising in vitro system for modeling NDDs, providing opportunities to understand mechanisms driving NDDs in human neurons. Functional assays, including patch clamping, multielectrode array, and imaging-based assays, are popular tools employed with hiPSC disease models for disease investigation. Recent progress in machine learning (ML) algorithms also presents unprecedented opportunities to advance the NDD research process. In this review, we compare two-dimensional and three-dimensional hiPSC formats for disease modeling, discuss the applications of functional assays, and offer insights on incorporating ML into hiPSC-based NDD research and drug screening.

Neuroglia pathology in genetic and epigenetic disorders of the central nervous system

Glial cells are increasingly recognized for their important interactions with both developing and mature neurons, in particular for maintenance of dendritic ramifications and spines, synapses, and neurotransmitter uptake. MicroRNA abnormalities are demonstrated in individual astrocytes with alterations in neurological diseases. Alexander disease is a prototype astrocytic disease because of genetically altered glial fibrillary acidic protein (GFAP) filaments. Other genetic diseases are now recognized as involving glial cells in their pathogenesis: Rett, Fragile-X, Aicardi-Goutières, and Down syndromes, as well as epigenetic effects in the mechanism of fetal alcohol spectrum disorder. Many involve glial production of cytokines and neuroinflammation. Microglia also may contribute. The heat-shock protein α-B-crystallin is expressed in the Rosenthal fibers of Alexander disease, in which the molecular structure of GFAP is altered, in astrocytes secreting neurotoxic cytokines, and focally at or near epileptic foci. Satellite glial cells adherent to neuronal soma are frequent and diagnostically nonspecific but may contribute to neuronal degeneration, especially of hypermetabolic epileptogenic neurons. Glial cells have distorted size and morphology in mTOR malformations. Failure of glial apoptosis in the fetal lamina terminalis is the likely pathogenesis of callosal agenesis and of other cerebral dysgeneses.

Epigenetic Regulation and Neurodevelopmental Disorders: From MeCP2 to the TCF20/PHF14 Complex

BACKGROUND

Neurodevelopmental disorders (NDDs) affect approximately 15% of children and adolescents worldwide. This group of disorders is often polygenic with varying risk factors, with many associated genes converging on shared molecular pathways, including chromatin regulation and transcriptional control. Understanding how NDD-associated chromatin regulators and protein complexes orchestrate these regulatory pathways is crucial for elucidating NDD pathogenesis and developing targeted therapeutic strategies. Recently, the TCF20/PHF14 chromatin complex was identified in the mammalian brain, expanding the list of chromatin regulatory remodelers implicated in NDDs. This complex-which includes MeCP2, RAI1, TCF20, PHF14, and HMG20A-plays a vital role in epigenetic and transcriptional regulation.

METHODS

We review and summarize current research and clinical reports pertaining to the different components of the MeCP2-interacting TCF20/PHF14 complex. We examine the NDDs associated with the TCF20/PHF14 complex, explore the molecular and neuronal functions of its components, and discuss emerging therapeutic strategies targeting this complex to mitigate symptoms, with broader applicability to other NDDs.

RESULTS

Mutations in the genes encoding the components of the MeCP2-interacting TCF20/PHF14 complex have been linked to various NDDs, underscoring its critical contribution to brain development and NDD pathogenesis.

CONCLUSIONS

The MeCP2-interacting TCF20/PHF14 complex and its associated NDDs could serve as a model system to provide insight into the interplay between epigenetic regulation and NDD pathogenesis.

Molecular Mechanisms of Rett Syndrome: Emphasizing the Roles of Monoamine, Immunity, and Mitochondrial Dysfunction

Rett syndrome (RTT), which predominantly affects females, arises in most cases from mutations in the Methyl-CpG-binding Protein-2 (MECP2) gene. When MeCP2 is impaired, it disrupts the regulation of numerous genes, causing the production of dysfunctional proteins associated with various multi-systemic issues in RTT. In this review, we explore the current insights into molecular signaling related to monoamines, immune response, and mitochondrial function, and their implications for the pathophysiology of RTT. Research has shown that monoamines-such as dopamine, norepinephrine, epinephrine, serotonin, and histamine-exhibit alterations in RTT, contributing to a range of neurological symptoms. Furthermore, the immune system in RTT individuals demonstrates dysfunction through the abnormal activity of microglia, macrophages, lymphocytes, and non-immune cells, leading to the atypical release of inflammatory mediators and disruptions in the NF-κB signaling pathway. Moreover, mitochondria, essential for energy production and calcium storage, also show dysfunction in this condition. The delicate balance of producing and scavenging reactive oxygen species-termed redox balance-is disrupted in RTT. Targeting these molecular pathways presents a promising avenue for developing effective therapies.

Neural precursor cells rescue symptoms of Rett syndrome by activation of the Interferon γ pathway

The beneficial effects of Neural Precursor Cell (NPC) transplantation in several neurological disorders are well established and they are generally mediated by the secretion of immunomodulatory and neurotrophic molecules. We therefore investigated whether Rett syndrome (RTT), that represents the first cause of severe intellectual disability in girls, might benefit from NPC-based therapy. Using in vitro co-cultures, we demonstrate that, by sensing the pathological context, NPC-secreted factors induce the recovery of morphological and synaptic defects typical of Mecp2 deficient neurons. In vivo, we prove that intracerebral transplantation of NPCs in RTT mice significantly ameliorates neurological functions. To uncover the molecular mechanisms underpinning the mediated benefic effects, we analyzed the transcriptional profile of the cerebellum of transplanted animals, disclosing the possible involvement of the Interferon γ (IFNγ) pathway. Accordingly, we report the capacity of IFNγ to rescue synaptic defects, as well as motor and cognitive alterations in Mecp2 deficient models, thereby suggesting this molecular pathway as a potential therapeutic target for RTT.

Sex-specific single cell-level transcriptomic signatures of Rett syndrome disease progression

Dominant X-linked diseases are uncommon due to female X chromosome inactivation (XCI). While random XCI usually protects females against X-linked mutations, Rett syndrome (RTT) is a female neurodevelopmental disorder caused by heterozygous MECP2 mutation. After 6-18 months of typical neurodevelopment, RTT girls undergo a poorly understood regression. We performed longitudinal snRNA-seq on cerebral cortex in a construct-relevant Mecp2e1 mutant mouse model of RTT, revealing transcriptional effects of cell type, mosaicism, and sex on progressive disease phenotypes. Across cell types, we observed sex differences in the number of differentially expressed genes (DEGs) with 6x more DEGs in mutant females than males. Unlike males, female DEGs emerged prior to symptoms, were enriched for homeostatic gene pathways in distinct cell types over time and correlated with disease phenotypes and human RTT cortical cell transcriptomes. Non-cell-autonomous effects were prominent and dynamic across disease progression of Mecp2e1 mutant females, indicating that wild-type-expressing cells normalize transcriptional homeostasis. These results advance our understanding of RTT progression and treatment.

Pharmacological inhibition of the CB1 cannabinoid receptor restores abnormal brain mitochondrial CB1 receptor expression and rescues bioenergetic and cognitive defects in a female mouse model of Rett syndrome

BACKGROUND

Defective mitochondria and aberrant brain mitochondrial bioenergetics are consistent features in syndromic intellectual disability disorders, such as Rett syndrome (RTT), a rare neurologic disorder that severely affects mainly females carrying mutations in the X-linked MECP2 gene. A pool of CB1 cannabinoid receptors (CB1R), the primary receptor subtype of the endocannabinoid system in the brain, is located on brain mitochondrial membranes (mtCB1R), where it can locally regulate energy production, synaptic transmission and memory abilities through the inhibition of the intra-mitochondrial protein kinase A (mtPKA). In the present study, we asked whether an overactive mtCB1R-mtPKA signaling might underlie the brain mitochondrial alterations in RTT and whether its modulation by systemic administration of the CB1R inverse agonist rimonabant might improve bioenergetics and cognitive defects in mice modeling RTT.

METHODS

Rimonabant (0.3 mg/kg/day, intraperitoneal injections) was administered daily to symptomatic female mice carrying a truncating mutation of the Mecp2 gene and its effects on brain mitochondria functionality, systemic oxidative status, and memory function were assessed.

RESULTS

mtCB1R is overexpressed in the RTT mouse brain. Subchronic treatment with rimonabant normalizes mtCB1R expression in RTT mouse brains, boosts mtPKA signaling, and restores the defective brain mitochondrial bioenergetics, abnormal peripheral redox homeostasis, and impaired cognitive abilities in RTT mice.

LIMITATIONS

The lack of selectivity of the rimonabant treatment towards mtCB1R does not allow us to exclude that the beneficial effects exerted by the treatment in the RTT mouse model may be ascribed more broadly to the modulation of CB1R activity and distribution among intracellular compartments, rather than to a selective effect on mtCB1R-mediated signaling. The low sample size of few experiments is a further limitation that has been addressed replicating the main findings under different experimental conditions.

CONCLUSIONS

The present data identify mtCB1R overexpression as a novel molecular alteration in the RTT mouse brain that may underlie defective brain mitochondrial bioenergetics and cognitive dysfunction.

Ex vivo disease modelling of Rett syndrome: the transcriptomic and metabolomic implications of direct neuronal conversion

BACKGROUND

Rett syndrome (RTT) is a rare neurodevelopmental disorder that primarily affects females and is characterized by a period of normal development followed by severe cognitive, motor, and communication impairment. The syndrome is predominantly caused by mutations in the MECP2. This study aimed to use comprehensive multi-omic analysis to identify the molecular and metabolic alterations associated with Rett syndrome.

METHODS AND RESULTS

Transcriptomic and metabolomic profiling was performed using neuron-like cells derived from the fibroblasts of 3 Rett syndrome patients with different MECP2 mutations (R168X, P152R, and R133C) and 1 healthy control. Differential gene expression, alternative splicing events, and metabolite changes were analyzed to identify the key pathways and processes affected in patients with Rett syndrome. Transcriptomic analysis showed there was significant down-regulation of genes associated with the extracellular matrix (ECM) and cytoskeletal components, which was particularly notable in patient P3 (R133C mutation), who had non-random X chromosome inactivation. Additionally, significant changes in microtubule-related gene expression and alternative splicing events were observed, especially in patient P2 (P152R mutation). Metabolomic profiling showed that there were alterations in metabolic pathways, particularly up-regulation of ketone body synthesis and degradation pathways, in addition to an increase in free fatty acid levels. Integrated analysis highlighted the interplay between structural gene down-regulation and metabolic shifts, underscoring the adaptive responses to cellular stress in Rett neurons.

CONCLUSION

The present findings provide valuable insights into the molecular and metabolic landscape of Rett syndrome, emphasizing the importance of combining omic data to enlighten the molecular pathophysiology of this syndrome.

Mitochondrial dysfunction and increased reactive oxygen species production in MECP2 mutant astrocytes and their impact on neurons

Studies on MECP2 function and its implications in Rett Syndrome (RTT) have traditionally centered on neurons. Here, using human embryonic stem cell (hESC) lines, we modeled MECP2 loss-of-function to explore its effects on astrocyte (AST) development and dysfunction in the brain. Ultrastructural analysis of RTT hESC-derived cerebral organoids revealed significantly smaller mitochondria compared to controls (CTRs), particularly pronounced in glia versus neurons. Employing a multiomics approach, we observed increased gene expression and accessibility of a subset of nuclear-encoded mitochondrial genes upon mutation of MECP2 in ASTs compared to neurons. Analysis of hESC-derived ASTs showed reduced mitochondrial respiration and altered key proteins in the tricarboxylic acid cycle and electron transport chain in RTT versus CTRs. Additionally, RTT ASTs exhibited increased cytosolic amino acids under basal conditions, which were depleted upon increased energy demands. Notably, mitochondria isolated from RTT ASTs exhibited increased reactive oxygen species and influenced neuronal activity when transferred to cortical neurons. These findings underscore MECP2 mutation's differential impact on mitochondrial and metabolic pathways in ASTs versus neurons, suggesting that dysfunctional AST mitochondria may contribute to RTT pathophysiology by affecting neuronal health.

Mitochondrial regulation of adult hippocampal neurogenesis: Insights into neurological function and neurodevelopmental disorders

Mitochondria are essential regulators of cellular energy metabolism and play a crucial role in the maintenance and function of neuronal cells. Studies in the last decade have highlighted the importance of mitochondrial dynamics and bioenergetics in adult neurogenesis, a process that significantly influences cognitive function and brain plasticity. In this review, we examine the mechanisms by which mitochondria regulate adult neurogenesis, focusing on the impact of mitochondrial function on the behavior of neural stem/progenitor cells and the maturation and plasticity of newborn neurons in the adult mouse hippocampus. In addition, we explore the link between mitochondrial dysfunction, adult hippocampal neurogenesis and genes associated with cognitive deficits in neurodevelopmental disorders. In particular, we provide insights into how alterations in the transcriptional regulator NR2F1 affect mitochondrial dynamics and may contribute to the pathophysiology of the emerging neurodevelopmental disorder Bosch-Boonstra-Schaaf optic atrophy syndrome (BBSOAS). Understanding how genes involved in embryonic and adult neurogenesis affect mitochondrial function in neurological diseases might open new directions for therapeutic interventions aimed at boosting mitochondrial function during postnatal life.

Early differential impact of MeCP2 mutations on functional networks in Rett syndrome patient-derived human cerebral organoids

Human cerebral organoids derived from induced pluripotent stem cells can recapture early developmental processes and reveal changes involving neurodevelopmental disorders. Mutations in the X-linked methyl-CpG binding protein 2 (MECP2) gene are associated with Rett syndrome, and disease severity varies depending on the location and type of mutation. Here, we focused on neuronal activity in Rett syndrome patient-derived organoids, analyzing two types of MeCP2 mutations - a missense mutation (R306C) and a truncating mutation (V247X) - using calcium imaging with three-photon microscopy. Compared to isogenic controls, we found abnormal neuronal activity in Rett organoids and altered network function based on graph theoretic analyses, with V247X mutations impacting functional responses and connectivity more severely than R306C mutations. These changes paralleled EEG data obtained from patients with comparable mutations. Labeling DLX promoter-driven inhibitory neurons demonstrated differences in activity and functional connectivity of inhibitory and excitatory neurons in the two types of mutation. Transcriptomic analyses revealed HDAC2-associated impairment in R306C organoids and decreased GABAA receptor expression in excitatory neurons in V247X organoids. These findings demonstrate mutation-specific mechanisms of vulnerability in Rett syndrome and suggest targeted strategies for their treatment.

Biochemical and molecular determinants of the subclinical inflammatory mechanisms in Rett syndrome

To date, Rett syndrome (RTT), a genetic disorder mainly caused by mutations in the X-linked MECP2 gene, is increasingly considered a broad-spectrum pathology, instead of just a neurodevelopmental disease, due to the multitude of peripheral co-morbidities and the compromised metabolic pathways, affecting the patients. The altered molecular processes include an impaired mitochondrial function, a perturbed redox homeostasis, a chronic subclinical inflammation and an improper cholesterol metabolism. The persistent subclinical inflammatory condition was first defined ten years ago, as a previously unrecognized feature of RTT, playing a role in the pathology progress and modulation of phenotypical severity. In light of this, the present work aims at reviewing the current knowledge on the chronic inflammatory status and the altered immune/inflammatory functions in RTT, as well as investigating the emerging mechanisms underlying this condition with a special focus on the latest findings about inflammasome system, autoimmunity responses and intestinal micro- and mycobiota. On these bases, although further research is needed, future therapeutic strategies able to re-establish an adequate immune/inflammatory response could represent potential approaches for RTT patients.

iPSC-derived healthy human astrocytes selectively load miRNAs targeting neuronal genes into extracellular vesicles

Astrocytes are in constant communication with neurons during the establishment and maturation of functional networks in the developing brain. Astrocytes release extracellular vesicles (EVs) containing microRNA (miRNA) cargo that regulates transcript stability in recipient cells. Astrocyte released factors are thought to be involved in neurodevelopmental disorders. Healthy astrocytes partially rescue Rett Syndrome (RTT) neuron function. EVs isolated from stem cell progeny also correct aspects of RTT. EVs cross the blood-brain barrier (BBB) and their cargo is found in peripheral blood which may allow non-invasive detection of EV cargo as biomarkers produced by healthy astrocytes. Here we characterize miRNA cargo and sequence motifs in healthy human astrocyte derived EVs (ADEVs). First, human induced Pluripotent Stem Cells (iPSC) were differentiated into Neural Progenitor Cells (NPCs) and subsequently into astrocytes using a rapid differentiation protocol. iPSC derived astrocytes expressed specific markers, displayed intracellular calcium transients and secreted ADEVs. miRNAs were identified by RNA-Seq on astrocytes and ADEVs and target gene pathway analysis detected brain and immune related terms. The miRNA profile was consistent with astrocyte identity, and included approximately 80 miRNAs found in astrocytes that were relatively depleted in ADEVs suggestive of passive loading. About 120 miRNAs were relatively enriched in ADEVs and motif analysis discovered binding sites for RNA binding proteins FUS, SRSF7 and CELF5. miR-483-5p was the most significantly enriched in ADEVs. This miRNA regulates MECP2 expression in neurons and has been found differentially expressed in blood samples from RTT patients. Our results identify potential miRNA biomarkers selectively sorted into ADEVs and implicate RNA binding protein sequence dependent mechanisms for miRNA cargo loading.

Functional neurogenomics in autism spectrum disorders: A decade of progress

Advances in autism spectrum disorder (ASD) genetics have identified many genetic causes, reflecting remarkable progress while at the same time identifying challenges such as heterogeneity and pleiotropy, which complicate attempts to connect genetic risk to mechanisms. High-throughput functional genomic approaches have yielded progress by defining a molecular pathology in the brain of individuals with ASD and in identifying convergent biological pathways through which risk genes are predicted to act. These studies indicate that ASD genetic risk converges in early brain development, primarily during the period of cortical neurogenesis. Over development, genetic perturbations in turn lead to broad neuronal signaling dysregulation, most prominent in glutamatergic cortical-cortical projecting neurons and somatostatin positive interneurons, which is accompanied by glial dyshomeostasis throughout the cerebral cortex. Connecting these developmental perturbations to disrupted neuronal and glial function in the postnatal brain is an important direction in current research. Coupling functional genomic approaches with advances in induced pluripotent stem cell-derived neural organoid development provides a promising avenue for connecting brain pathology to developmental mechanisms.

Sex-specific single cell-level transcriptomic signatures of Rett syndrome disease progression

Dominant X-linked diseases are uncommon due to female X chromosome inactivation (XCI). While random XCI usually protects females against X-linked mutations, Rett syndrome (RTT) is a female neurodevelopmental disorder caused by heterozygous MECP2 mutation. After 6-18 months of typical neurodevelopment, RTT girls undergo poorly understood regression. We performed longitudinal snRNA-seq on cerebral cortex in a construct-relevant Mecp2e1 mutant mouse model of RTT, revealing transcriptional effects of cell type, mosaicism, and sex on progressive disease phenotypes. Across cell types, we observed sex differences in the number of differentially expressed genes (DEGs) with 6x more DEGs in mutant females than males. Unlike males, female DEGs emerged prior to symptoms, were enriched for homeostatic gene pathways in distinct cell types over time, and correlated with disease phenotypes and human RTT cortical cell transcriptomes. Non-cell-autonomous effects were prominent and dynamic across disease progression of Mecp2e1 mutant females, indicating wild-type-expressing cells normalizing transcriptional homeostasis. These results improve understanding of RTT progression and treatment.

Aberrant CHCHD2-associated mitochondriopathy in Kii ALS/PDC astrocytes

Amyotrophic Lateral Sclerosis/Parkinsonism-Dementia Complex (ALS/PDC), a rare and complex neurological disorder, is predominantly observed in the Western Pacific islands, including regions of Japan, Guam, and Papua. This enigmatic condition continues to capture medical attention due to affected patients displaying symptoms that parallel those seen in either classical amyotrophic lateral sclerosis (ALS) or Parkinson's disease (PD). Distinctly, postmortem examinations of the brains of affected individuals have shown the presence of α-synuclein aggregates and TDP-43, which are hallmarks of PD and classical ALS, respectively. These observations are further complicated by the detection of phosphorylated tau, accentuating the multifaceted proteinopathic nature of ALS/PDC. The etiological foundations of this disease remain undetermined, and genetic investigations have yet to provide conclusive answers. However, emerging evidence has implicated the contribution of astrocytes, pivotal cells for maintaining brain health, to neurodegenerative onset, and likely to play a significant role in the pathogenesis of ALS/PDC. Leveraging advanced induced pluripotent stem cell technology, our team cultivated multiple astrocyte lines to further investigate the Japanese variant of ALS/PDC (Kii ALS/PDC). CHCHD2 emerged as a significantly dysregulated gene when disease astrocytes were compared to healthy controls. Our analyses also revealed imbalances in the activation of specific pathways: those associated with astrocytic cilium dysfunction, known to be involved in neurodegeneration, and those related to major neurological disorders, including classical ALS and PD. Further in-depth examinations revealed abnormalities in the mitochondrial morphology and metabolic processes of the affected astrocytes. A particularly striking observation was the reduced expression of CHCHD2 in the spinal cord, motor cortex, and oculomotor nuclei of patients with Kii ALS/PDC. In summary, our findings suggest a potential reduction in the support Kii ALS/PDC astrocytes provide to neurons, emphasizing the need to explore the role of CHCHD2 in maintaining mitochondrial health and its implications for the disease.

Mecp2 knock-out astrocytes affect synaptogenesis by interleukin 6 dependent mechanisms

Synaptic abnormalities are a hallmark of several neurological diseases, and clarification of the underlying mechanisms represents a crucial step toward the development of therapeutic strategies. Rett syndrome (RTT) is a rare neurodevelopmental disorder, mainly affecting females, caused by mutations in the X-linked methyl-CpG-binding protein 2 (MECP2) gene, leading to a deep derangement of synaptic connectivity. Although initial studies supported the exclusive involvement of neurons, recent data have highlighted the pivotal contribution of astrocytes in RTT pathogenesis through non-cell autonomous mechanisms. Since astrocytes regulate synapse formation and functionality by releasing multiple molecules, we investigated the influence of soluble factors secreted by Mecp2 knock-out (KO) astrocytes on synapses. We found that Mecp2 deficiency in astrocytes negatively affects their ability to support synaptogenesis by releasing synaptotoxic molecules. Notably, neuronal inputs from a dysfunctional astrocyte-neuron crosstalk lead KO astrocytes to aberrantly express IL-6, and blocking IL-6 activity prevents synaptic alterations.

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.

Potential for New Therapeutic Approaches by Targeting Lactate and pH Mediated Epigenetic Dysregulation in Major Mental Diseases

Multiple lines of evidence have shown that lactate-mediated pH alterations in the brains of patients with neuropsychiatric diseases such as schizophrenia (SCZ), Alzheimer's disease (AD) and autism may be attributed to mitochondrial dysfunction and changes in energy metabolism. While neuronal activity is associated with reduction in brain pH, astrocytes are responsible for rebalancing the pH to maintain the equilibrium. As lactate level is the main determinant of brain pH, neuronal activities are impacted by pH changes due to the binding of protons (H+) to various types of proteins, altering their structure and function in the neuronal and non-neuronal cells of the brain. Lactate and pH could affect diverse types of epigenetic modifications, including histone lactylation, which is linked to histone acetylation and DNA methylation. In this review, we discuss the importance of pH homeostasis in normal brain function, the role of lactate as an essential epigenetic regulatory molecule and its contributions to brain pH abnormalities in neuropsychiatric diseases, and shed light on lactate-based and pH-modulating therapies in neuropsychiatric diseases by targeting epigenetic modifications. In conclusion, we attempt to highlight the potentials and challenges of translating lactate-pH-modulating therapies to clinics for the treatment of neuropsychiatric diseases.

Multilevel evidence of MECP2-associated mitochondrial dysfunction and its therapeutic implications

We present a male patient carrying a pathogenic MECP2 p. Arg179Trp variant with predominant negative psychiatric features and multilevel evidence of mitochondrial dysfunction who responded to the cariprazine treatment. He had delayed speech development and later experienced severe social anxiety, learning disabilities, cognitive slowing, and predominant negative psychiatric symptoms associated with rigidity. Clinical examinations showed multisystemic involvement. Together with elevated ergometric lactate levels, the clinical picture suggested mitochondrial disease, which was also supported by muscle histopathology. Exploratory transcriptome analysis also revealed the involvement of metabolic and oxidative phosphorylation pathways. Whole-exome sequencing identified a pathogenic MECP2 variant, which can explain both the dopamine imbalance and mitochondrial dysfunction in this patient. Mitochondrial dysfunction was previously suggested in classical Rett syndrome, and we detected related phenotype evidence on multiple consistent levels for the first time in a MECP2 variant carrier male. This study further supports the importance of the MECP2 gene in the mitochondrial pathways, which can open the gate for more personalized therapeutic interventions. Good cariprazine response highlights the role of dopamine dysfunction in the complex psychiatric symptoms of Rett syndrome. This can help identify the optimal treatment strategy from a transdiagnostic perspective instead of a classical diagnostic category.

Systemic proteome phenotypes reveal defective metabolic flexibility in Mecp2 mutants

Genes mutated in monogenic neurodevelopmental disorders are broadly expressed. This observation supports the concept that monogenic neurodevelopmental disorders are systemic diseases that profoundly impact neurodevelopment. We tested the systemic disease model focusing on Rett syndrome, which is caused by mutations in MECP2. Transcriptomes and proteomes of organs and brain regions from Mecp2-null mice as well as diverse MECP2-null male and female human cells were assessed. Widespread changes in the steady-state transcriptome and proteome were identified in brain regions and organs of presymptomatic Mecp2-null male mice as well as mutant human cell lines. The extent of these transcriptome and proteome modifications was similar in cortex, liver, kidney, and skeletal muscle and more pronounced than in the hippocampus and striatum. In particular, Mecp2- and MECP2-sensitive proteomes were enriched in synaptic and metabolic annotated gene products, the latter encompassing lipid metabolism and mitochondrial pathways. MECP2 mutations altered pyruvate-dependent mitochondrial respiration while maintaining the capacity to use glutamine as a mitochondrial carbon source. We conclude that mutations in Mecp2/MECP2 perturb lipid and mitochondrial metabolism systemically limiting cellular flexibility to utilize mitochondrial fuels.

Ox-inflammasome involvement in neuroinflammation

Neuroinflammation plays a crucial role in the onset and the progression of several neuropathologies, from neurodegenerative disorders to migraine, from Rett syndrome to post-COVID 19 neurological manifestations. Inflammasomes are cytosolic multiprotein complexes of the innate immune system that fuel inflammation. They have been under study for the last twenty years and more recently their involvement in neuro-related conditions has been of great interest as possible therapeutic target. The role of oxidative stress in inflammasome activation has been described, however the exact way of action of specific endogenous and exogenous oxidants needs to be better clarified. In this review, we provide the current knowledge on the involvement of inflammasome in the main neuropathologies, emphasizing the importance to further clarify the role of oxidative stress in its activation including the role of mitochondria in inflammasome-induced neuroinflammation.

Involvement of brain metabolism in neurodevelopmental disorders

Neurodevelopmental disorders (NDDs) affect a significant portion of the global population and have a substantial social and economic impact worldwide. Most NDDs manifest in early childhood and are characterized by deficits in cognition, communication, social interaction and motor control. Due to a limited understanding of the etiology of NDDs, current treatment options primarily focus on symptom management rather than on curative solutions. Moreover, research on NDDs is problematic due to its reliance on a neurocentric approach. However, recent studies are broadening the scope of research on NDDs, to include dysregulations within a diverse network of brain cell types, including vascular and glial cells. This review aims to summarize studies from the past few decades on potential new contributions to the etiology of NDDs, with a special focus on metabolic signatures of various brain cells. In particular, we aim to convey how the metabolic functions are intimately linked to the onset and/or progression of common NDDs such as autism spectrum disorders, fragile X syndrome, Rett syndrome and Down syndrome.

Systemic Proteome Phenotypes Reveal Defective Metabolic Flexibility in Mecp2 Mutants

Genes mutated in monogenic neurodevelopmental disorders are broadly expressed. This observation supports the concept that monogenic neurodevelopmental disorders are systemic diseases that profoundly impact neurodevelopment. We tested the systemic disease model focusing on Rett syndrome, which is caused by mutations in MECP2. Transcriptomes and proteomes of organs and brain regions from Mecp2-null mice as well as diverse MECP2-null male and female human cells were assessed. Widespread changes in the steady-state transcriptome and proteome were identified in brain regions and organs of presymptomatic Mecp2-null male mice as well as mutant human cell lines. The extent of these transcriptome and proteome modifications was similar in cortex, liver, kidney, and skeletal muscle and more pronounced than in the hippocampus and striatum. In particular, Mecp2- and MECP2-sensitive proteomes were enriched in synaptic and metabolic annotated gene products, the latter encompassing lipid metabolism and mitochondrial pathways. MECP2 mutations altered pyruvate-dependent mitochondrial respiration while maintaining the capacity to use glutamine as a mitochondrial carbon source. We conclude that mutations in Mecp2/MECP2 perturb lipid and mitochondrial metabolism systemically limiting cellular flexibility to utilize mitochondrial fuels.