Downstream targets of methyl CpG binding protein 2 and their abnormal expression in the frontal cortex of the human Rett syndrome brain

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.

Protective effects of pyrroloquinoline quinone in brain folate deficiency

BACKGROUND

Folates (Vitamin B9) are critical for normal neurodevelopment and function, with transport mediated by three major pathways: folate receptor alpha (FRα), proton-coupled folate transporter (PCFT), and reduced folate carrier (RFC). Cerebral folate uptake primarily occurs at the blood-cerebrospinal fluid barrier (BCSFB) through concerted actions of FRα and PCFT, with impaired folate transport resulting in the neurological disorder cerebral folate deficiency (CFD). Increasing evidence suggests that disorders associated with CFD also present with neuroinflammation, oxidative stress, and mitochondrial dysfunction, however the role of brain folate deficiency in inducing these abnormalities is not well-understood. Our laboratory has identified the upregulation of RFC by nuclear respiratory factor 1 (NRF-1) at the blood-brain barrier (BBB) once indirectly activated by the natural compound pyrroloquinoline quinone (PQQ). PQQ is also of interest due to its anti-inflammatory, antioxidant, and mitochondrial biogenesis effects. In this study, we examined the effects of folate deficiency and PQQ treatment on inflammatory and oxidative stress responses, and changes in mitochondrial function.

METHODS

Primary cultures of mouse mixed glial cells exposed to folate-deficient (FD) conditions and treated with PQQ were analyzed for changes in gene expression of the folate transporters, inflammatory markers, oxidative stress markers, and mitochondrial DNA (mtDNA) content through qPCR analysis. Changes in cellular reactive oxygen species (ROS) levels were analyzed in vitro through a DCFDA assay. Wildtype (C57BL6/N) mice exposed to FD (0 mg/kg folate), or control (2 mg/kg folate) diets underwent a 10-day (20 mg/kg/day) PQQ treatment regimen and brain tissues were collected and analyzed.

RESULTS

Folate deficiency resulted in increased expression of inflammatory and oxidative stress markers in vitro and in vivo, with increased cellular ROS levels observed in mixed glial cells as well as a reduction of mitochondrial DNA (mtDNA) content observed in FD mixed glial cells. PQQ treatment was able to reverse these changes, while increasing RFC expression through activation of the PGC-1α/NRF-1 signaling pathway.

CONCLUSION

These results demonstrate the effects of brain folate deficiency, which may contribute to the neurological deficits commonly seen in disorders of CFD. PQQ may represent a novel treatment strategy for disorders associated with CFD, as it can increase folate uptake, while in parallel reversing many abnormalities that arise with brain folate deficiency.

Mitochondrial Proteome Changes in Rett Syndrome

Rett syndrome (RTT) is a genetic neurodevelopmental disorder with mutations in the X-chromosomal MECP2 (methyl-CpG-binding protein 2) gene. Most patients are young girls. For 7-18 months after birth, they hardly present any symptoms; later they develop mental problems, a lack of communication, irregular sleep and breathing, motor dysfunction, hand stereotypies, and seizures. The complex pathology involves mitochondrial structure and function. Mecp2^-/y hippocampal astrocytes show increased mitochondrial contents. Neurons and glia suffer from oxidative stress, a lack of ATP, and increased hypoxia vulnerability. This spectrum of changes demands comprehensive molecular studies of mitochondria to further define their pathogenic role in RTT. Therefore, we applied a comparative proteomic approach for the first time to study the entity of mitochondrial proteins in a mouse model of RTT. In the neocortex and hippocampus of symptomatic male mice, two-dimensional gel electrophoresis and subsequent mass-spectrometry identified various differentially expressed mitochondrial proteins, including components of respiratory chain complexes I and III and the ATP-synthase FoF1 complex. The NADH-ubiquinone oxidoreductase 75 kDa subunit, NADH dehydrogenase [ubiquinone] iron-sulfur protein 8, NADH dehydrogenase [ubiquinone] flavoprotein 2, cytochrome b-c1 complex subunit 1, and ATP synthase subunit d are upregulated either in the hippocampus alone or both the hippocampus and neocortex of Mecp2^-/y mice. Furthermore, the regulatory mitochondrial proteins mitofusin-1, HSP60, and 14-3-3 protein theta are decreased in the Mecp2^-/y neocortex. The expressional changes identified provide further details of the altered mitochondrial function and morphology in RTT. They emphasize brain-region-specific alterations of the mitochondrial proteome and support the notion of a metabolic component of this devastating disorder.

Chronic treatment with the anti-diabetic drug metformin rescues impaired brain mitochondrial activity and selectively ameliorates defective cognitive flexibility in a female mouse model of Rett syndrome

Metformin is the most common anti-diabetic drug and a promising therapy for disorders beyond diabetes, including Rett syndrome (RTT), a rare neurologic disease characterized by severe intellectual disability. A 10-day-long treatment rescued aberrant mitochondrial activity and restrained oxidative stress in a female RTT mouse model. However, this treatment regimen did not improve the phenotype of RTT mice. In the present study, we demonstrate that a 4-month-long treatment with metformin (150 mg/Kg/day, delivered in drinking bottles) provides a selective normalization of cognitive flexibility defects in RTT female mice at an advanced stage of disease, but it does not affect their impaired general health status and abnormal motor skills. The 4-month-long treatment also rescues the reduced activity of mitochondrial respiratory chain complex activities, the defective brain ATP production and levels as well as the increased production of reactive oxidizing species in the whole blood of RTT mice. A significant boost of PGC-1α-dependent pathways related to mitochondrial biogenesis and antioxidant defense occurs in the brain of RTT mice that received the metformin treatment. Further studies will have to verify whether these effects may underlie its long-lasting beneficial effects on brain energy metabolism.

Docosahexaenoic acid increased MeCP2 mediated mitochondrial respiratory complexes II and III enzyme activities in cortical astrocytes

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the methyl-CpG-binding protein 2 (MeCP2) in the neurons and glial cells of the central nervous system. Currently, therapeutics for RTT is aimed at restoring the loss-of-function by MeCP2 gene therapy, but that approach has multiple challenges. We have already reported impaired mitochondrial bioenergetics in MeCP2 deficient astrocytes. Docosahexaenoic acid (DHA), a polyunsaturated fatty acid, has been shown with health benefits, but its impact on mitochondrial functions in MeCP2 deficient astrocytes has never been paid much attention. The present study aimed to investigate the effects of DHA on mitochondrial respiratory chain regulation in MeCP2 knockdown astrocytes. We determined NADH dehydrogenase (ubiquinone) flavoprotein 2 (Ndufv2-complex-I), Ubiquinol cytochrome c reductase core protein (Uqcrc1-complex-III) genes expression, Ndufv2 protein expression, respiratory electron transport chain complex I, II, III, and IV enzyme activities, intracellular Ca⁺² , reactive oxygen species (ROS) and mitochondrial membrane potential (MMP) in DHA pre-incubated MeCP2 knock-down rat primary cortical astrocytes. Our study demonstrates that 100 µM DHA increases MeCP2 gene and protein expression. Increases brain-derived neurotrophic factor (BDNF) and Uqcrc1 gene expression, Ndufv2 protein expression, but has no effect on glial fibrillary acidic protein (GFAP) gene expression. DHA treatment also increases mitochondrial respiratory Complexes II and III activities and reduces intracellular calcium levels. Taken together, the effects of DHA seem independent of MeCP2 deficiency in astrocytes. Hence, further studies are warranted to understand the complicated mechanisms of DHA and for its therapeutic significance in MeCP2-mediated mitochondrial dysfunction and in RTT disease.

Mitochondrial DNA Copy Number in Rett Syndrome Caused by Methyl-CpG-Binding Protein-2 Variants

OBJECTIVE

To determine changes in mitochondrial DNA (mtDNA) copy number in peripheral blood in Rett syndrome caused by methyl-CpG-binding protein-2 (MECP2) variants and explore the mechanism of mitochondrial dysfunction in Rett syndrome.

STUDY DESIGN

Female patients who were diagnosed with Rett syndrome and had an MECP2 variant (n = 142) were recruited in this study, along with the same number of age- and sex-matched healthy controls. MtDNA copy number was quantified by real-time quantitative polymerase chain reaction with TaqMan probes. The differences in mtDNA copy number between the Rett syndrome group and the control group were analyzed using the independent-samples t test. Linear regression, biserial correlation analysis, and one-way ANOVA were applied for the correlations between mtDNA copy number and age, clinical severity, variant types, functional domains, and hot-spot variants.

RESULTS

MtDNA copy number was found to be significantly increased in the patients with Rett syndrome with MECP2 gene variants compared with the control subjects. Age, clinical severity, variant types, functional domains, and hot-spot variants were not related to mtDNA copy number in patients with Rett syndrome.

CONCLUSIONS

MtDNA copy number is increased significantly in patients with Rett syndrome, suggesting that changes in mitochondrial function in Rett syndrome trigger a compensatory increase in mtDNA copy number and providing new possibilities for treating Rett syndrome, such as mitochondria-targeted therapies.

The neuropathology of autism: A systematic review of post-mortem studies of autism and related disorders

Post-mortem studies allow for the direct investigation of brain tissue in those with autism and related disorders. Several review articles have focused on aspects of post-mortem abnormalities but none has brought together the entire post-mortem literature. Here, we systematically review the evidence from post-mortem studies of autism, and of related disorders that present with autistic features. The literature consists of a small body of studies with small sample sizes, but several remarkably consistent findings are evident. Cortical layering is largely undisturbed, but there are consistent reductions in minicolumn numbers and aberrant myelination. Transcriptomics repeatedly implicate abberant synaptic, metabolic, proliferation, apoptosis and immune pathways. Sufficient replicated evidence is available to implicate non-coding RNA, aberrant epigenetic profiles, GABAergic, glutamatergic and glial dysfunction in autism pathogenesis. Overall, the cerebellum and frontal cortex are most consistently implicated, sometimes revealing distinct region-specific alterations. The literature on related disorders such as Rett syndrome, Fragile X and copy number variations (CNVs) predisposing to autism is particularly small and inconclusive. Larger studies, matched for gender, developmental stage, co-morbidities and drug treatment are required.

Transcriptomic and Epigenomic Landscape in Rett Syndrome

Rett syndrome (RTT) is an extremely invalidating, cureless, developmental disorder, and it is considered one of the leading causes of intellectual disability in female individuals. The vast majority of RTT cases are caused by de novo mutations in the X-linked Methyl-CpG binding protein 2 (MECP2) gene, which encodes a multifunctional reader of methylated DNA. MeCP2 is a master epigenetic modulator of gene expression, with a role in the organization of global chromatin architecture. Based on its interaction with multiple molecular partners and the diverse epigenetic scenario, MeCP2 triggers several downstream mechanisms, also influencing the epigenetic context, and thus leading to transcriptional activation or repression. In this frame, it is conceivable that defects in such a multifaceted factor as MeCP2 lead to large-scale alterations of the epigenome, ranging from an unbalanced deposition of epigenetic modifications to a transcriptional alteration of both protein-coding and non-coding genes, with critical consequences on multiple downstream biological processes. In this review, we provide an overview of the current knowledge concerning the transcriptomic and epigenomic alterations found in RTT patients and animal models.

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

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

Mitochondria May Mediate Prenatal Environmental Influences in Autism Spectrum Disorder

We propose that the mitochondrion, an essential cellular organelle, mediates the long-term prenatal environmental effects of disease in autism spectrum disorder (ASD). Many prenatal environmental factors which increase the risk of developing ASD influence mitochondria physiology, including toxicant exposures, immune activation, and nutritional factors. Unique types of mitochondrial dysfunction have been associated with ASD and recent studies have linked prenatal environmental exposures to long-term changes in mitochondrial physiology in children with ASD. A better understanding of the role of the mitochondria in the etiology of ASD can lead to targeted therapeutics and strategies to potentially prevent the development of ASD.

Seizures and Epilepsy in Autism Spectrum Disorder

Epilepsy and autism frequently co-occur. Epilepsy confers an increased risk of autism and autism confers an increased risk of epilepsy. Specific epilepsy syndromes, intellectual disability, and female gender present a particular risk of autism in individuals with epilepsy. Epilepsy and autism are likely to share common etiologies, which predispose individuals to either or both conditions. Genetic factors, metabolic disorders, mitochondrial disorders, and immune dysfunction all can be implicated.

Intensified mitochondrial hydrogen peroxide release occurs in all brain regions, affects male as well as female Rett mice, and constitutes a life-long burden

The neurodevelopmental disorder Rett syndrome (RTT) affects mostly females. Upon an apparently normal initial development, cognitive impairment, irregular breathing, motor dysfunction, and epilepsy occur. The complex pathogenesis includes, among others, mitochondrial impairment, redox imbalance, and oxidative damage. As these arise already in neonatal Rett mice, they were proposed contributors of disease progression. Several mitochondrial studies in RTT used either full brains or selected brain regions only. Here, we mapped mitochondria-related ROS generation brain wide. Using sophisticated multi-sample spectrofluorimetry, H₂O₂ release by isolated mitochondria was quantified in a coupled reaction of Amplex UltraRed and horseradish peroxidase. All brain regions and the entire lifespan were characterized in male and female mice. In WT mice, mitochondrial H₂O₂ release was usually highest in cortex and lowest in hippocampus. Maximum rates occurred at postnatal day (PD) 10 and they slightly declined with further maturation. Already at PD 10, male and female Rett mice showed exaggerated mitochondrial H₂O₂ releases in first brain regions and persistent brain-wide increases from PD 50 on. Interestingly, female Rett mice were more intensely affected than male Rett mice, with their brainstem, midbrain and hippocampus being most severely struck. In conclusion, we used a reliable multi-sample cuvette-based assay on mitochondrial ROS release to perform brain-wide analyzes along the entire lifespan. Mitochondrial H₂O₂ release in Rett mice is intensified in all brain regions, affects hemizygous males and heterozygous females, and involves all maturational stages. Therefore, intensified mitochondrial H₂O₂ release seriously needs to be considered throughout RTT pathogenesis and may constitute a potential therapeutic target.

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

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

Drug Studies on Rett Syndrome: From Bench to Bedside

Drug studies on Rett syndrome (RTT) have drastically increased over the past few decades. This review aims to provide master data on bench-to-bedside drug studies involving RTT. A comprehensive literature review was performed by searching in PUBMED, MEDLINE and Google Scholar, international, national and regional clinical trial registries and pharmaceutical companies using the keywords "Rett syndrome treatment and/or drug or compound or molecule". Seventy drugs were investigated in non-clinical (N = 65 animal/cell line-based studies; N = 5 iPSC-based study) and clinical trials (N = 34) for ameliorating the symptoms of RTT. Though there is good progress in both clinical and non-clinical studies, none of these drugs entered phase III/IV for being launched as a therapeutic agent for RTT.

Proteomic profiling reveals mitochondrial alterations in Rett syndrome

Rett syndrome (RTT) is a pervasive neurodevelopmental disorder associated with mutation in MECP2 gene. Despite a well-defined genetic cause, there is a growing consensus that a metabolic component could play a pivotal role in RTT pathophysiology. Indeed, perturbed redox homeostasis and inflammation, i.e. oxinflammation, with mitochondria dysfunction as the central hub between the two phenomena, appear as possible key contributing factors to RTT pathogenesis and its clinical features. While these RTT-related changes have been widely documented by transcriptomic profiling, proteomics studies supporting these evidences are still limited. Here, using primary dermal fibroblasts from control and patients, we perform a large-scale proteomic analysis that, together with data mining approaches, allow us to carry out the first comprehensive characterization of RTT cellular proteome, showing mainly changes in expression of proteins involved in the mitochondrial network. These findings parallel with an altered expression of key mediators of mitochondrial dynamics and mitophagy associated with abnormal mitochondrial morphology. In conclusion, our proteomic analysis confirms the pathological relevance of mitochondrial dysfunction in RTT pathogenesis and progression.

Expansion of the phenotypic spectrum of de novo missense variants in kinesin family member 1A (KIF1A)

Defects in the motor domain of kinesin family member 1A (KIF1A), a neuron-specific ATP-dependent anterograde axonal transporter of synaptic cargo, are well-recognized to cause a spectrum of neurological conditions, commonly known as KIF1A-associated neurological disorders (KAND). Here, we report one mutation-negative female with classic Rett syndrome (RTT) harboring a de novo heterozygous novel variant [NP_001230937.1:p.(Asp248Glu)] in the highly conserved motor domain of KIF1A. In addition, three individuals with severe neurodevelopmental disorder along with clinical features overlapping with KAND are also reported carrying de novo heterozygous novel [NP_001230937.1:p.(Cys92Arg) and p.(Pro305Leu)] or previously reported [NP_001230937.1:p.(Thr99Met)] variants in KIF1A. In silico tools predicted these variants to be likely pathogenic, and 3D molecular modeling predicted defective ATP hydrolysis and/or microtubule binding. Using the neurite tip accumulation assay, we demonstrated that all novel KIF1A variants significantly reduced the ability of the motor domain of KIF1A to accumulate along the neurite lengths of differentiated SH-SY5Y cells. In vitro microtubule gliding assays showed significantly reduced velocities for the variant p.(Asp248Glu) and reduced microtubule binding for the p.(Cys92Arg) and p.(Pro305Leu) variants, suggesting a decreased ability of KIF1A to move along microtubules. Thus, this study further expanded the phenotypic characteristics of KAND individuals with pathogenic variants in the KIF1A motor domain to include clinical features commonly seen in RTT individuals.

Seizures and Epilepsy in Autism Spectrum Disorder

Epilepsy and autism frequently co-occur. Epilepsy confers an increased risk of autism and autism confers an increased risk of epilepsy. Specific epilepsy syndromes, intellectual disability, and female gender present a particular risk of autism in individuals with epilepsy. Epilepsy and autism are likely to share common etiologies, which predispose individuals to either or both conditions. Genetic factors, metabolic disorders, mitochondrial disorders, and immune dysfunction all can be implicated.

Transcriptome data of temporal and cingulate cortex in the Rett syndrome brain

Rett syndrome is an X-linked neurodevelopmental disorder caused by mutation in the methyl-CpG-binding protein 2 gene (MECP2) in the majority of cases. We describe an RNA sequencing dataset of postmortem brain tissue samples from four females clinically diagnosed with Rett syndrome and four age-matched female donors. The dataset contains 16 transcriptomes, including two brain regions, temporal and cingulate cortex, for each individual. We compared our dataset with published transcriptomic analyses of postmortem brain tissue from Rett syndrome and found consistent gene expression alterations among regions of the cerebral cortex. Our data provide a valuable resource to explore the biology of the human brain in Rett syndrome.

Nuclear respiratory factor 1 (NRF-1) upregulates the expression and function of reduced folate carrier (RFC) at the blood-brain barrier

Folates are important for neurodevelopment and cognitive function. Folate transport across biological membranes is mediated by three major pathways: folate receptor alpha (FRα), proton-coupled folate transporter (PCFT), and reduced folate carrier (RFC). Brain folate transport primarily occurs at the choroid plexus through FRα and PCFT; inactivation of these transport systems results in suboptimal folate levels in the cerebrospinal fluid (CSF) causing childhood neurological disorders. Our group has reported that upregulation of RFC at the blood-brain barrier (BBB) through interactions with specific transcription factors, that is, vitamin D receptor (VDR) could increase brain folate delivery. This study investigates the role of nuclear respiratory factor 1 (NRF-1) in the regulation of RFC at the BBB. Activation of NRF-1/PGC-1α signaling through treatment with its specific ligand, pyrroloquinoline quinone (PQQ), significantly induced RFC expression and transport activity in hCMEC/D3 cells. In contrast, transfection with NRF-1 or PGC-1α targeting siRNA downregulated RFC functional expression in the same cell system. Applying chromatin immunoprecipitation (ChIP) assay, we further demonstrated that PQQ treatment increased NRF-1 binding to putative NRF-1 binding sites within the SLC19A1 promoter, which encodes for RFC. Additionally, in vivo treatment of wild type mice with PQQ-induced RFC expression in isolated mouse brain capillaries. Together, these findings demonstrate that NRF-1/PGC-1α activation by PQQ upregulates RFC functional expression at the BBB and could potentially enhance brain folate uptake.

Tau overexpression impairs neuronal endocytosis by decreasing the GTPase dynamin 1 through the miR-132/MeCP2 pathway

Tauopathies are a class of neurodegenerative diseases that are characterized by pathological aggregation of tau protein, which is accompanied by synaptic disorders. However, the role of tau in endocytosis, a fundamental process in synaptic transmission, remains elusive. Here, we report that forced expression of human tau (hTau) in mouse cortical neurons impairs endocytosis by decreasing the level of the GTPase dynamin 1 via disruption of the miR‐132‐MeCP2 pathway; this process can also be detected in the brains of Alzheimer's patients and hTau mice. Our results provide evidence for a novel role of tau in the regulation of presynaptic function.

Neuronal Redox-Imbalance in Rett Syndrome Affects Mitochondria as Well as Cytosol, and Is Accompanied by Intensified Mitochondrial O 2 Consumption and ROS Release

Rett syndrome (RTT), an X chromosome-linked neurodevelopmental disorder affecting almost exclusively females, is associated with various mitochondrial alterations. Mitochondria are swollen, show altered respiratory rates, and their inner membrane is leaking protons. To advance the understanding of these disturbances and clarify their link to redox impairment and oxidative stress, we assessed mitochondrial respiration in defined brain regions and cardiac tissue of male wildtype (WT) and MeCP2-deficient (Mecp2^-/y) mice. Also, we quantified for the first time neuronal redox-balance with subcellular resolution in cytosol and mitochondrial matrix. Quantitative roGFP1 redox imaging revealed more oxidized conditions in the cytosol of Mecp2^-/y hippocampal neurons than in WT neurons. Furthermore, cytosol and mitochondria of Mecp2^-/y neurons showed exaggerated redox-responses to hypoxia and cell-endogenous reactive oxygen species (ROS) formation. Biochemical analyzes exclude disease-related increases in mitochondrial mass in Mecp2^-/y hippocampus and cortex. Protein levels of complex I core constituents were slightly lower in Mecp2^-/y hippocampus and cortex than in WT; those of complex V were lower in Mecp2^-/y cortex. Respiratory supercomplex-formation did not differ among genotypes. Yet, supplied with the complex II substrate succinate, mitochondria of Mecp2^-/y cortex and hippocampus consumed more O₂ than WT. Furthermore, mitochondria from Mecp2^-/y hippocampus and cortex mediated an enhanced oxidative burden. In conclusion, we further advanced the molecular understanding of mitochondrial dysfunction in RTT. Intensified mitochondrial O₂ consumption, increased mitochondrial ROS generation and disturbed redox balance in mitochondria and cytosol may represent a causal chain, which provokes dysregulated proteins, oxidative tissue damage, and contributes to neuronal network dysfunction in RTT.

Roles of mitochondria in neuronal development

Mitochondria are ubiquitous and multi-functional organelles involved in diverse metabolic processes, namely energy production and biomolecule synthesis. The intracellular mitochondrial morphology and distribution change dynamically, which reflect the metabolic state of a given cell type. A dramatic change of the mitochondrial dynamics has been observed in early development that led to further investigations on the relationship between mitochondria and the process of development. A significant developmental process to focus on, in this review, is a differentiation of neural progenitor cells into neurons. Information on how mitochondria- regulated cellular energetics is linked to neuronal development will be discussed, followed by functions of mitochondria and associated diseases in neuronal development. Lastly, the potential use of mitochondrial features in analyzing various neurodevelopmental diseases will be addressed. [BMB Reports 2018; 51(11): 549-556].

Clusterin in Alzheimer's Disease: Mechanisms, Genetics, and Lessons From Other Pathologies

Clusterin (CLU) or APOJ is a multifunctional glycoprotein that has been implicated in several physiological and pathological states, including Alzheimer's disease (AD). With a prominent extracellular chaperone function, additional roles have been discussed for clusterin, including lipid transport and immune modulation, and it is involved in pathways common to several diseases such as cell death and survival, oxidative stress, and proteotoxic stress. Although clusterin is normally a secreted protein, it has also been found intracellularly under certain stress conditions. Multiple hypotheses have been proposed regarding the origin of intracellular clusterin, including specific biogenic processes leading to alternative transcripts and protein isoforms, but these lines of research are incomplete and contradictory. Current consensus is that intracellular clusterin is most likely to have exited the secretory pathway at some point or to have re-entered the cell after secretion. Clusterin's relationship with amyloid beta (Aβ) has been of great interest to the AD field, including clusterin's apparent role in altering Aβ aggregation and/or clearance. Additionally, clusterin has been more recently identified as a mediator of Aβ toxicity, as evidenced by the neuroprotective effect of CLU knockdown and knockout in rodent and human iPSC-derived neurons. CLU is also the third most significant genetic risk factor for late onset AD and several variants have been identified in CLU. Although the exact contribution of these variants to altered AD risk is unclear, some have been linked to altered CLU expression at both mRNA and protein levels, altered cognitive and memory function, and altered brain structure. The apparent complexity of clusterin's biogenesis, the lack of clarity over the origin of the intracellular clusterin species, and the number of pathophysiological functions attributed to clusterin have all contributed to the challenge of understanding the role of clusterin in AD pathophysiology. Here, we highlight clusterin's relevance to AD by discussing the evidence linking clusterin to AD, as well as drawing parallels on how the role of clusterin in other diseases and pathways may help us understand its biological function(s) in association with AD.

Mitochondrial Dysfunction Is Inducible in Lymphoblastoid Cell Lines From Children With Autism and May Involve the TORC1 Pathway

We previously developed a lymphoblastoid cell line (LCL) model of mitochondrial dysfunction in autism spectrum disorder (ASD); some individuals with ASD showed mitochondrial dysfunction (AD-A) while other individuals (AD-N) demonstrated mitochondrial respiration similar to controls (CNT). To test the hypothesis that mitochondrial dysfunction could be a consequence of environmental exposures through chronic elevations in reactive oxygen species (ROS), we exposed LCLs to prolonged ROS. We also examined expression of metabolic regulatory genes and the modulating effect of the mechanistic target of rapamycin (mTOR) pathway. Prolonged ROS exposure induced or worsened mitochondrial dysfunction in all LCL groups. Expression of genes associated with ROS protection was elevated in both AD-N and AD-A LCLs, but mitochondrial fission/fusion and mitoplasticity gene expression was only increased in AD-N LCLs. Partial least squares discriminant analysis showed that mTOR, UCP2 (uncoupling protein 2), SIRT1 (sirtuin 1), and MFN2 (mitofusin-2) gene expression differentiated LCL groups. Low-dose rapamycin (0.1 nM) normalized respiration with the magnitude of this normalization greater for AD-A LCLs, suggesting that the mammalian target of rapamycin complex 1 (mTORC1) pathway may have a different dynamic range for regulating mitochondrial activity in individuals with ASD with and without mitochondrial dysfunction, potentially related to S6K1 (S6 kinase beta-1) regulation. Understanding pathways that underlie mitochondrial dysfunction in ASD may lead to novel treatments.

Mitochondrial Electron Transport Chain Complex Dysfunction in MeCP2 Knock-Down Astrocytes: Protective Effects of Quercetin Hydrate

Astrocytes play the central role in CNS metabolism to support neuronal functions. Mehyl-CpG-binding protein 2 (MeCP2) is the global transcription factor with differential expression in neuronal and non-neuronal cells. MeCP2 mutation and downstream detrimental effects have been reported in astrocytes also in MeCP2-associated neurodevelopmental disorder-Rett syndrome. Several studies have shown mitochondrial impairment linked to ROS production and reduced ATP synthesis in Rett patients and models, but consequences of MeCP2 deficiency on mitochondrial electron transport chain complexes in astrocytes and effect of known antioxidant quercetin aglycone has not yet been reported. The present study aimed to investigate effect of quercetin on mitochondrial functioning in MeCP2-deficient astrocytes. Our data show onefold upregulated Uqcrc1 and Ndufv2 gene expression, subtle change in protein expression, and significantly reduced mitochondrial respiratory chain complex-II and complex-III enzyme activities in MeCP2 knock-down astrocytes. Intracellular calcium robustly increased and mitochondrial membrane potential decreased, while no change in ROS was observed in MeCP2 knock-down astrocytes. Quercetin increased MeCP2 and normalized Uqcrc1 and Ndufv2 gene expression but did not modulate MeCP2 and Ndufv2 proteins expression. Interestingly, quercetin upregulated significantly the mitochondrial respiratory complex-II, complex-III, and complex-IV activities in dose-dependent manner. It also restored intracellular calcium level and mitochondrial membrane potential. In vitro observations suggest the beneficial effect of quercetin in mitochondrial functioning in MeCP2-deficient condition. There are no reports focusing on role of quercetin in mitochondrial function in MeCP2-deficient astrocytes, and these observations serve as preliminary data to evaluate quercetin's effects in vivo.

Roles of mitochondria in neuronal development

Mitochondria are ubiquitous and multi-functional organelles involved in diverse metabolic processes, namely energy production and biomolecule synthesis. The intracellular mitochondrial morphology and distribution change dynamically, which reflect the metabolic state of a given cell type. A dramatic change of the mitochondrial dynamics has been observed in early development that led to further investigations on the relationship between mitochondria and the process of development. A significant developmental process to focus on, in this review, is a differentiation of neural progenitor cells into neurons. Information on how mitochondria- regulated cellular energetics is linked to neuronal development will be discussed, followed by functions of mitochondria and associated diseases in neuronal development. Lastly, the potential use of mitochondrial features in analyzing various neurodevelopmental diseases will be addressed. [BMB Reports 2018; 51(11): 549-556].

Transcriptome level analysis in Rett syndrome using human samples from different tissues

The mechanisms of neuro-genetic disorders have been mostly investigated in the brain, however, for some pathologies, transcriptomic analysis in multiple tissues represent an opportunity and a challenge to understand the consequences of the genetic mutation. This is the case for Rett Syndrome (RTT): a neurodevelopmental disorder predominantly affecting females that is characterised by a loss of purposeful movements and language accompanied by gait abnormalities and hand stereotypies. Although the genetic aetiology is largely associated to Methyl CpG binding protein 2 (MECP2) mutations, linking the pathophysiology of RTT and its clinical symptoms to direct molecular mechanisms has been difficult.One approach used to study the consequences of MECP2 dysfunction in patients, is to perform transcriptomic analysis in tissues derived from RTT patients or Induced Pluripotent Stem cells. The growing affordability and efficiency of this approach has led to a far greater understanding of the complexities of RTT syndrome but is also raised questions about previously held convictions such as the regulatory role of MECP2, the effects of different molecular mechanisms in different tissues and role of X Chromosome Inactivation in RTT.In this review we consider the results of a number of different transcriptomic analyses in different patients-derived preparations to unveil specific trends in differential gene expression across the studies. Although the analyses present limitations- such as the limited sample size- overlaps exist across these studies, and they report dysregulations in three main categories: dendritic connectivity and synapse maturation, mitochondrial dysfunction, and glial cell activity.These observations have a direct application to the disorder and give insights on the altered mechanisms in RTT, with implications on potential diagnostic criteria and treatments.

iPSC-derived neurons profiling reveals GABAergic circuit disruption and acetylated α-tubulin defect which improves after iHDAC6 treatment in Rett syndrome

Mutations in MECP2 gene have been identified in more than 95% of patients with classic Rett syndrome, one of the most common neurodevelopmental disorders in females. Taking advantage of the breakthrough technology of genetic reprogramming, we investigated transcriptome changes in neurons differentiated from induced Pluripotent Stem Cells (iPSCs) derived from patients with different mutations. Profiling by RNA-seq in terminally differentiated neurons revealed a prominent GABAergic circuit disruption along with a perturbation of cytoskeleton dynamics. In particular, in mutated neurons we identified a significant decrease of acetylated α-tubulin which can be reverted by treatment with selective inhibitors of HDAC6, the main α-tubulin deacetylase. These findings contribute to shed light on Rett pathogenic mechanisms and provide hints for the treatment of Rett-associated epileptic behavior as well as for the definition of new therapeutic strategies for Rett syndrome.

MeCP2 Deficiency Leads to Loss of Glial Kir4.1

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder usually caused by mutations in methyl-CpG-binding protein 2 (MeCP2). RTT is typified by apparently normal development until 6-18 mo of age, when motor and communicative skills regress and hand stereotypies, autonomic symptoms, and seizures present. Restoration of MeCP2 function selectively to astrocytes reversed several deficits in a murine model of RTT, but the mechanism of this rescue is unknown. Astrocytes carry out many essential functions required for normal brain functioning, including extracellular K⁺ buffering. Kir4.1, an inwardly rectifying K⁺ channel, is largely responsible for the channel-mediated K⁺ regulation by astrocytes. Loss-of-function mutations in Kir4.1 in human patients result in a severe neurodevelopmental disorder termed EAST or SESAME syndrome. Here, we evaluated astrocytic Kir4.1 expression in a murine model of Rett syndrome. We demonstrate by chromatin immunoprecipitation analysis that Kir4.1 is a direct molecular target of MeCP2. Astrocytes from Mecp2-deficient mice express significantly less Kir4.1 mRNA and protein, which translates into a >50% deficiency in Ba²⁺-sensitive Kir4.1-mediated currents, and impaired extracellular potassium dynamics. By examining astrocytes in isolation, we demonstrate that loss of Kir4.1 is cell autonomous. Assessment through postnatal development revealed that Kir4.1 expression in Mecp2-deficient animals never reaches adult, wild-type levels, consistent with a neurodevelopmental disorder. These are the first data implicating a direct MeCP2 molecular target in astrocytes and provide novel mechanistic insight explaining a potential mechanism by which astrocytic dysfunction may contribute to RTT.

Mining for mitochondrial mechanisms: Linking known syndromes to mitochondrial function

Mitochondrial disorders (MDs) are caused by defects in 1 or multiple complexes of the oxidative phosphorylation (OXPHOS) machinery. MDs are associated with a broad range of clinical signs and symptoms, and have considerable clinical overlap with other neuromuscular syndromes. This overlap might be due to involvement of mitochondrial pathways in some of these non-mitochondrial syndromes. Here, we give an overview of around 25 non-mitochondrial syndromes, diagnosed in patients who were initially suspected to have a MD on the basis of clinical and biochemical parameters. In addition, we highlight the mitochondrial connections of 6 of these non-mitochondrial syndromes (eg, Rett syndrome and Dravet syndrome) diagnosed in multiple patients. Further research to unravel the interplay between these genes and mitochondria may help to increase knowledge on these syndromes. Additionally, it may open new avenues for research on pathways interacting with mitochondrial function in order to find new targets for therapeutics to treat MDs. The data presented in this review underline the importance of careful assessment of clinical, genetic, and biochemical data in all patients suspected of a neuromuscular syndrome, and highlights the importance of the role of clinical geneticists, physicians, and clinical biochemists in recognizing the possible mitochondrial connection of non-mitochondrial syndromes.

RNA sequencing and proteomics approaches reveal novel deficits in the cortex of Mecp2-deficient mice, a model for Rett syndrome

BACKGROUND

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused by mutations in the transcriptional regulator MeCP2. Much of our understanding of MeCP2 function is derived from transcriptomic studies with the general assumption that alterations in the transcriptome correlate with proteomic changes. Advances in mass spectrometry-based proteomics have facilitated recent interest in the examination of global protein expression to better understand the biology between transcriptional and translational regulation.

METHODS

We therefore performed the first comprehensive transcriptome-proteome comparison in a RTT mouse model to elucidate RTT pathophysiology, identify potential therapeutic targets, and further our understanding of MeCP2 function. The whole cortex of wild-type and symptomatic RTT male littermates (n = 4 per genotype) were analyzed using RNA-sequencing and data-independent acquisition liquid chromatography tandem mass spectrometry. Ingenuity® Pathway Analysis was used to identify significantly affected pathways in the transcriptomic and proteomic data sets.

RESULTS

Our results indicate these two "omics" data sets supplement one another. In addition to confirming previous works regarding mRNA expression in Mecp2-deficient animals, the current study identified hundreds of novel protein targets. Several selected protein targets were validated by Western blot analysis. These data indicate RNA metabolism, proteostasis, monoamine metabolism, and cholesterol synthesis are disrupted in the RTT proteome. Hits common to both data sets indicate disrupted cellular metabolism, calcium signaling, protein stability, DNA binding, and cytoskeletal cell structure. Finally, in addition to confirming disrupted pathways and identifying novel hits in neuronal structure and synaptic transmission, our data indicate aberrant myelination, inflammation, and vascular disruption. Intriguingly, there is no evidence of reactive gliosis, but instead, gene, protein, and pathway analysis suggest astrocytic maturation and morphological deficits.

CONCLUSIONS

This comparative omics analysis supports previous works indicating widespread CNS dysfunction and may serve as a valuable resource for those interested in cellular dysfunction in RTT.

Autism, Mitochondria and Polybrominated Diphenyl Ether Exposure

BACKGROUND

Autism spectrum disorders (ASD) are a growing concern with more than 1 in every 68 children affected in the United States by age 8. Limited scientific advances have been made regarding the etiology of autism, with general agreement that both genetic and environmental factors contribute to this disorder.

OBJECTIVE

To explore the link between exposure to PBDE, mitochondrial dysfunction and autism risk.

RESULTS

Perinatal exposures to PBDEs may contribute to the etiology or morbidity of ASD including mitochondrial dysfunction based on (i) their increased environmental abundance and human exposures, (ii) their activity towards implicated in neuronal development and synaptic plasticity including mitochondria, and (iii) their bioaccumulation in mitochondria.

CONCLUSION

In this review, we propose that PBDE, and possibly other environmental exposures, during child development can induce or compound mitochondrial dysfunction, which in conjunction with a dysregulated antioxidant response, increase a child's susceptibility of autism.

4-hydroxynonenal protein adducts: Key mediator in Rett syndrome oxinflammation

In the last 15 years a strong correlation between oxidative stress (OxS) and Rett syndrome (RTT), a rare neurodevelopmental disorder known to be caused in 95% of the cases, by a mutation in the methyl-CpG-binding protein 2 (MECP2) gene, has been well documented. Here, we revised, summarized and discussed the current knowledge on the role of lipid peroxidation byproducts, with special emphasis on 4-hydroxynonenal (4HNE), in RTT pathophysiology. The posttranslational modifications of proteins via 4HNE, known as 4HNE protein adducts (4NHE-PAs), causing detrimental effects on protein functions, appear to contribute to the clinical severity of the syndrome, since their levels increase significantly during the subsequent 4 clinical stages, reaching the maximum degree at stage 4, represented by a late motor deterioration. In addition, 4HNE-PA are only partially removed due to the compromised functionality of the proteasome activity, contributing therefore to the cellular damage in RTT. All this will lead to a characteristic subclinical inflammation, defined "OxInflammation", derived by a positive feedback loop between OxS byproducts and inflammatory mediators that in a long run further aggravates the clinical features of RTT patients. Therefore, in a pathology completely orphan of any therapy, aiming 4HNE as a therapeutic target could represent a coadjuvant treatment with some beneficial impact in these patients.‬‬‬.

Increased Mitochondrial Mass and Cytosolic Redox Imbalance in Hippocampal Astrocytes of a Mouse Model of Rett Syndrome: Subcellular Changes Revealed by Ratiometric Imaging of JC-1 and roGFP1 Fluorescence

Rett syndrome (RTT) is a neurodevelopmental disorder with mutations in the MECP2 gene. Mostly girls are affected, and an apparently normal development is followed by cognitive impairment, motor dysfunction, epilepsy, and irregular breathing. Various indications suggest mitochondrial dysfunction. In Rett mice, brain ATP levels are reduced, mitochondria are leaking protons, and respiratory complexes are dysregulated. Furthermore, we found in MeCP2-deficient mouse (Mecp2^−/y) hippocampus an intensified mitochondrial metabolism and ROS generation. We now used emission ratiometric 2-photon imaging to assess mitochondrial morphology, mass, and membrane potential (ΔΨm) in Mecp2^−/y hippocampal astrocytes. Cultured astrocytes were labeled with the ΔΨm marker JC-1, and semiautomated analyses yielded the number of mitochondria per cell, their morphology, and ΔΨm. Mecp2^−/y astrocytes contained more mitochondria than wild-type (WT) cells and were more oxidized. Mitochondrial size, ΔΨm, and vulnerability to pharmacological challenge did not differ. The antioxidant Trolox opposed the oxidative burden and decreased the mitochondrial mass, thereby dampening the differences among WT and Mecp2^−/y astrocytes; mitochondrial size and ΔΨm were not markedly affected. In conclusion, mitochondrial alterations and redox imbalance in RTT also involve astrocytes. Mitochondria are more numerous in Mecp2^−/y than in WT astrocytes. As this genotypic difference is abolished by Trolox, it seems linked to the oxidative stress in RTT.

Folate metabolism abnormalities in autism: potential biomarkers

Autism spectrum disorder (ASD) has been linked to abnormalities in folate metabolism. Polymorphisms in folate genes may act in complex polygenic ways to increase the risk of developing ASD. Autoantibodies that block folate transport into the brain have been associated with ASD and children with ASD and these autoantibodies respond to high doses of a reduced form of folate known as folinic acid (leucovorin calcium). Some of the same abnormalities are also found in mothers of children with ASD and supplementing folate during preconception and gestational periods reduces the risk to the offspring from developing ASD. These data suggest that folate pathway abnormalities may be a major metabolic disturbance underlying ASD that can be leveraged as biomarkers to improve symptoms and prevent ASD.

Mitochondrial Dysfunction in the Pathogenesis of Rett Syndrome: Implications for Mitochondria-Targeted Therapies

First described over 50 years ago, Rett syndrome (RTT) is a neurodevelopmental disorder caused primarily by mutations of the X-linked MECP2 gene. RTT affects predominantly females, and has a prevalence of roughly 1 in every 10,000 female births. Prior to the discovery that mutations of MECP2 are the leading cause of RTT, there were suggestions that RTT could be a mitochondrial disease. In fact, several reports documented altered mitochondrial structure, and deficiencies in mitochondrial enzyme activity in different cells or tissues derived from RTT patients. With the identification of MECP2 as the causal gene, interest largely shifted toward defining the normal function of MeCP2 in the brain, and how its absence affects the neurodevelopment and neurophysiology. Recently, though, interest in studying mitochondrial function in RTT has been reignited, at least in part due to observations suggesting systemic oxidative stress does play a contributing role in RTT pathogenesis. Here we review data relating to mitochondrial alterations at the structural and functional levels in RTT patients and model systems, and present a hypothesis for how the absence of MeCP2 could lead to altered mitochondrial function and elevated levels of cellular oxidative stress. Finally, we discuss the prospects for treating RTT using interventions that target specific aspects of mitochondrial dysfunction and/or oxidative stress.

Constitutional Epi/Genetic Conditions: Genetic, Epigenetic, and Environmental Factors

There are more than 4,000 phenotypes for which the molecular basis is at least partly known. Though defects in primary DNA structure constitute a major cause of these disorders, epigenetic disruption is emerging as an important alternative mechanism in the etiology of a broad range of congenital and developmental conditions. These include epigenetic defects caused by either localized (in cis) genetic alterations or more distant (in trans) genetic events but can also include environmental effects. Emerging evidence suggests interplay between genetic and environmental factors in the epigenetic etiology of several constitutional "epi/genetic" conditions. This review summarizes our broadening understanding of how epigenetics contributes to pediatric disease by exploring different classes of epigenomic disorders. It further challenges the simplistic dogma of "DNA encodes RNA encodes protein" to best understand the spectrum of factors that can influence genetic traits in a pediatric population.

Alterations in the carnitine cycle in a mouse model of Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disease that leads to intellectual deficit, motor disability, epilepsy and increased risk of sudden death. Although in up to 95% of cases this disease is caused by de novo loss-of-function mutations in the X-linked methyl-CpG binding protein 2 gene, it is a multisystem disease associated also with mitochondrial metabolic imbalance. In addition, the presence of long QT intervals (LQT) on the patients’ electrocardiograms has been associated with the development of ventricular tachyarrhythmias and sudden death. In the attempt to shed light on the mechanism underlying heart failure in RTT, we investigated the contribution of the carnitine cycle to the onset of mitochondrial dysfunction in the cardiac tissues of two subgroups of RTT mice, namely Mecp2^+/− NQTc and Mecp2^+/− LQTc mice, that have a normal and an LQT interval, respectively. We found that carnitine palmitoyltransferase 1 A/B and carnitine acylcarnitine translocase were significantly upregulated at mRNA and protein level in the heart of Mecp2^+/− mice. Moreover, the carnitine system was imbalanced in Mecp2^+/− LQTc mice due to decreased carnitine acylcarnitine transferase expression. By causing accumulation of intramitochondrial acylcarnitines, this imbalance exacerbated incomplete fatty acid oxidation, which, in turn, could contribute to mitochondrial overload and sudden death.

Autistic Siblings with Novel Mutations in Two Different Genes: Insight for Genetic Workups of Autistic Siblings and Connection to Mitochondrial Dysfunction

The prevalence of autism spectrum disorder (ASD) is high, yet the etiology of this disorder is still uncertain. Advancements in genetic analysis have provided the ability to identify potential genetic changes that may contribute to ASD. Interestingly, several genetic syndromes have been linked to metabolic dysfunction, suggesting an avenue for treatment. In this case study, we report siblings with ASD who had similar initial phenotypic presentations. Whole exome sequencing (WES) revealed a novel c.795delT mutation in the WDR45 gene affecting the girl, which was consistent with her eventual progression to a Rett-like syndrome phenotype including seizures along with a stereotypical cyclic breathing pattern. Interestingly, WES identified that the brother harbored a novel heterozygous Y1546H variant in the DEP domain-containing protein 5 (DEPDC5) gene, consistent with his presentation. Both siblings underwent a metabolic workup that demonstrated different patterns of mitochondrial dysfunction. The girl demonstrated statistically significant elevations in mitochondrial activity of complex I + III in both muscle and fibroblasts and increased respiration in peripheral blood mononuclear cells (PBMCs) on Seahorse Extracellular Flux analysis. The boy demonstrates a statistically significant decrease in complex IV activity in buccal epithelium and decreased respiration in PBMCs. These cases highlight the differences in genetic abnormalities even in siblings with ASD phenotypes as well as highlights the individual role of novel mutations in the WDR45 and DEPDC5 genes. These cases demonstrate the importance of advanced genetic testing combined with metabolic evaluations in the workup of children with ASD.

Identification and Treatment of Pathophysiological Comorbidities of Autism Spectrum Disorder to Achieve Optimal Outcomes

Despite the fact that the prevalence of autism spectrum disorder (ASD) continues to rise, no effective medical treatments have become standard of care. In this paper we review some of the pathophysiological abnormalities associated with ASD and their potential associated treatments. Overall, there is evidence for some children with ASD being affected by seizure and epilepsy, neurotransmitter dysfunction, sleep disorders, metabolic abnormalities, including abnormalities in folate, cobalamin, tetrahydrobiopterin, carnitine, redox and mitochondrial metabolism, and immune and gastrointestinal disorders. Although evidence for an association between these pathophysiological abnormalities and ASD exists, the exact relationship to the etiology of ASD and its associated symptoms remains to be further defined in many cases. Despite these limitations, treatments targeting some of these pathophysiological abnormalities have been studied in some cases with high-quality studies, whereas treatments for other pathophysiological abnormalities have not been well studied in many cases. There are some areas of more promising treatments specific for ASD including neurotransmitter abnormalities, particularly imbalances in glutamate and acetylcholine, sleep onset disorder (with behavioral therapy and melatonin), and metabolic abnormalities in folate, cobalamin, tetrahydrobiopterin, carnitine, and redox pathways. There is some evidence for treatments of epilepsy and seizures, mitochondrial and immune disorders, and gastrointestinal abnormalities, particularly imbalances in the enteric microbiome, but further clinical studies are needed in these areas to better define treatments specific to children with ASD. Clearly, there are some promising areas of ASD research that could lead to novel treatments that could become standard of care in the future, but more research is needed to better define subgroups of children with ASD who are affected by specific pathophysiological abnormalities and the optimal treatments for these abnormalities.

Transcriptome analysis of human brain tissue identifies reduced expression of complement complex C1Q Genes in Rett syndrome

Background

MECP2, the gene mutated in the majority of Rett syndrome cases, is a transcriptional regulator that can activate or repress transcription. Although the transcription regulatory function of MECP2 has been known for over a decade, it remains unclear how transcriptional dysregulation leads to the neurodevelopmental disorder. Notably, little convergence was previously observed between the genes abnormally expressed in the brain of Rett syndrome mouse models and those identified in human studies.

Methods

Here we carried out a comprehensive transcriptome analysis of human brain tissue from Rett syndrome brain using both RNA-seq and microarrays.

Results

We identified over two hundred differentially expressed genes, and identified the complement C1Q complex genes (C1QA, C1QB and C1QC) as a point of convergence between gene expression changes in human and mouse Rett syndrome brain.

Conclusions

The results of our study support a role for alterations in the expression level of C1Q complex genes in RTT pathogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2746-7) contains supplementary material, which is available to authorized users.

Exploring the possible link between MeCP2 and oxidative stress in Rett syndrome

Rett syndrome (RTT, MIM 312750) is a rare and orphan progressive neurodevelopmental disorder affecting girls almost exclusively, with a frequency of 1/15,000 live births of girls. The disease is characterized by a period of 6 to 18 months of apparently normal neurodevelopment, followed by early neurological regression, with a progressive loss of acquired cognitive, social, and motor skills. RTT is known to be caused in 95% of the cases by sporadic de novo loss-of-function mutations in the X-linked methyl-CpG-binding protein 2 (MECP2) gene encoding methyl-CpG binding protein 2 (MeCP2), a nuclear protein able to regulate gene expression. Despite almost two decades of research into the functions and role of MeCP2, little is known about the mechanisms leading from MECP2 mutation to the disease. Oxidative stress (OS) is involved in the pathogenic mechanisms of several neurodevelopmental and neurodegenerative disorders, although in many cases it is not clear whether OS is a cause or a consequence of the pathology. Fairly recently, the presence of a systemic OS has been demonstrated in RTT patients with a strong correlation with the patients' clinical status. The link between MECP2 mutation and the redox imbalance found in RTT is not clear. Animal studies have suggested a possible direct correlation between Mecp2 mutation and increased OS levels. In addition, the restoration of Mecp2 function in astrocytes significantly improves the developmental outcome of Mecp2-null mice and reexpression of Mecp2 gene in the brain of null mice restored oxidative damage, suggesting that Mecp2 loss of function can be involved in oxidative brain damage. Starting from the evidence that oxidative damage in the brain of Mecp2-null mice precedes the onset of symptoms, we evaluated whether, based on the current literature, the dysfunctions described in RTT could be a consequence or, in contrast, could be caused by OS. We also analyzed whether therapies that at least partially treated some RTT symptoms can play a role in defense against OS. At this stage we can propose that OS could be one of the main causes of the dysfunctions observed in RTT. In addition, the major part of the therapies recommended to alleviate RTT symptoms have been shown to interfere with oxidative homeostasis, suggesting that MeCP2 could somehow be involved in the protection of the brain from OS.

Metabolic and mitochondrial disorders associated with epilepsy in children with autism spectrum disorder

Autism spectrum disorder (ASD) affects a significant number of individuals in the United States, with the prevalence continuing to grow. A significant proportion of individuals with ASD have comorbid medical conditions such as epilepsy. In fact, treatment-resistant epilepsy appears to have a higher prevalence in children with ASD than in children without ASD, suggesting that current antiepileptic treatments may be suboptimal in controlling seizures in many individuals with ASD. Many individuals with ASD also appear to have underlying metabolic conditions. Metabolic conditions such as mitochondrial disease and dysfunction and abnormalities in cerebral folate metabolism may affect a substantial number of children with ASD, while other metabolic conditions that have been associated with ASD such as disorders of creatine, cholesterol, pyridoxine, biotin, carnitine, γ-aminobutyric acid, purine, pyrimidine, and amino acid metabolism and urea cycle disorders have also been associated with ASD without the prevalence clearly known. Interestingly, all of these metabolic conditions have been associated with epilepsy in children with ASD. The identification and treatment of these disorders could improve the underlying metabolic derangements and potentially improve behavior and seizure frequency and/or severity in these individuals. This paper provides an overview of these metabolic disorders in the context of ASD and discusses their characteristics, diagnostic testing, and treatment with concentration on mitochondrial disorders. To this end, this paper aims to help optimize the diagnosis and treatment of children with ASD and epilepsy. This article is part of a Special Issue entitled "Autism and Epilepsy".

Mitochondrial free radical overproduction due to respiratory chain impairment in the brain of a mouse model of Rett syndrome: protective effect of CNF1

Rett syndrome (RTT) is a pervasive neurodevelopmental disorder mainly caused by mutations in the X-linked MECP2 gene associated with severe intellectual disability, movement disorders, and autistic-like behaviors. Its pathogenesis remains mostly not understood and no effective therapy is available. High circulating levels of oxidative stress markers in patients and the occurrence of oxidative brain damage in MeCP2-deficient mouse models suggest the involvement of oxidative stress in RTT pathogenesis. However, the molecular mechanism and the origin of the oxidative stress have not been elucidated. Here we demonstrate that a redox imbalance arises from aberrant mitochondrial functionality in the brain of MeCP2-308 heterozygous female mice, a condition that more closely recapitulates that of RTT patients. The marked increase in the rate of hydrogen peroxide generation in the brain of RTT mice seems mainly produced by the dysfunctional complex II of the mitochondrial respiratory chain. In addition, both membrane potential generation and mitochondrial ATP synthesis are decreased in RTT mouse brains when succinate, the complex II respiratory substrate, is used as an energy source. Respiratory chain impairment is brain area specific, owing to a decrease in either cAMP-dependent phosphorylation or protein levels of specific complex subunits. Further, we investigated whether the treatment of RTT mice with the bacterial protein CNF1, previously reported to ameliorate the neurobehavioral phenotype and brain bioenergetic markers in an RTT mouse model, exerts specific effects on brain mitochondrial function and consequently on hydrogen peroxide production. In RTT brains treated with CNF1, we observed the reactivation of respiratory chain complexes, the rescue of mitochondrial functionality, and the prevention of brain hydrogen peroxide overproduction. These results provide definitive evidence of mitochondrial reactive oxygen species overproduction in RTT mouse brain and highlight CNF1 efficacy in counteracting RTT-related mitochondrial defects.

Modulation of Rho GTPases rescues brain mitochondrial dysfunction, cognitive deficits and aberrant synaptic plasticity in female mice modeling Rett syndrome

Rho GTPases are molecules critically involved in neuronal plasticity and cognition. We have previously reported that modulation of brain Rho GTPases by the bacterial toxin CNF1 rescues the neurobehavioral phenotype in MeCP2-308 male mice, a model of Rett syndrome (RTT). RTT is a rare X-linked neurodevelopmental disorder and a genetic cause of intellectual disability, for which no effective therapy is available. Mitochondrial dysfunction has been proposed to be involved in the mechanism of the disease pathogenesis. Here we demonstrate that modulation of Rho GTPases by CNF1 rescues the reduced mitochondrial ATP production via oxidative phosphorylation in the brain of MeCP2-308 heterozygous female mice, the condition which more closely recapitulates that of RTT patients. In RTT mouse brain, CNF1 also restores the alterations in the activity of the mitochondrial respiratory chain (MRC) complexes and of ATP synthase, the molecular machinery responsible for the majority of cell energy production. Such effects were achieved through the upregulation of the protein content of those MRC complexes subunits, which were defective in RTT mouse brain. Restored mitochondrial functionality was accompanied by the rescue of deficits in cognitive function (spatial reference memory in the Barnes maze), synaptic plasticity (long-term potentiation) and Tyr1472 phosphorylation of GluN2B, which was abnormally enhanced in the hippocampus of RTT mice. Present findings bring into light previously unknown functional mitochondrial alterations in the brain of female mice modeling RTT and provide the first evidence that RTT brain mitochondrial dysfunction can be rescued by modulation of Rho GTPases.

Gastrointestinal dysfunction in autism spectrum disorder: the role of the mitochondria and the enteric microbiome

Autism spectrum disorder (ASD) affects a significant number of individuals worldwide with the prevalence continuing to grow. It is becoming clear that a large subgroup of individuals with ASD demonstrate abnormalities in mitochondrial function as well as gastrointestinal (GI) symptoms. Interestingly, GI disturbances are common in individuals with mitochondrial disorders and have been reported to be highly prevalent in individuals with co-occurring ASD and mitochondrial disease. The majority of individuals with ASD and mitochondrial disorders do not manifest a primary genetic mutation, raising the possibility that their mitochondrial disorder is acquired or, at least, results from a combination of genetic susceptibility interacting with a wide range of environmental triggers. Mitochondria are very sensitive to both endogenous and exogenous environmental stressors such as toxicants, iatrogenic medications, immune activation, and metabolic disturbances. Many of these same environmental stressors have been associated with ASD, suggesting that the mitochondria could be the biological link between environmental stressors and neurometabolic abnormalities associated with ASD. This paper reviews the possible links between GI abnormalities, mitochondria, and ASD. First, we review the link between GI symptoms and abnormalities in mitochondrial function. Second, we review the evidence supporting the notion that environmental stressors linked to ASD can also adversely affect both mitochondria and GI function. Third, we review the evidence that enteric bacteria that are overrepresented in children with ASD, particularly Clostridia spp., produce short-chain fatty acid metabolites that are potentially toxic to the mitochondria. We provide an example of this gut–brain connection by highlighting the propionic acid rodent model of ASD and the clinical evidence that supports this animal model. Lastly, we discuss the potential therapeutic approaches that could be helpful for GI symptoms in ASD and mitochondrial disorders. To this end, this review aims to help better understand the underlying pathophysiology associated with ASD that may be related to concurrent mitochondrial and GI dysfunction.

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.

Elevated 5-hydroxymethylcytosine in the Engrailed-2 (EN-2) promoter is associated with increased gene expression and decreased MeCP2 binding in autism cerebellum

Epigenetic mechanisms regulate programmed gene expression during prenatal neurogenesis and serve as a mediator between genetics and environment in postnatal life. The recent discovery of 5-hydroxymethylcytosine (5-hmC), with highest concentration in the brain, has added a new dimension to epigenetic regulation of neurogenesis and the development of complex behavior disorders. Here, we take a candidate gene approach to define the role 5-hmC in Engrailed-2 (EN-2) gene expression in the autism cerebellum. The EN-2 homeobox transcription factor, previously implicated in autism, is essential for normal cerebellar patterning and development. We previously reported EN-2 overexpression associated with promoter DNA hypermethylation in the autism cerebellum but because traditional DNA methylation methodology cannot distinguish 5-methylcytosine (5-mC) from 5-hmC, we now extend our investigation by quantifying global and gene-specific 5-mC and 5-hmC. Globally, 5-hmC was significantly increased in the autism cerebellum and accompanied by increases in the expression of de novo methyltransferases DNMT3A and DNMT3B, ten-eleven translocase genes TET1 and TET3, and in 8-oxo-deoxyguanosine (8-oxo-dG) content, a marker of oxidative DNA damage. Within the EN-2 promoter, there was a significant positive correlation between 5-hmC content and EN-2 gene expression. Based on reports of reduced MeCP2 affinity for 5-hmC, MeCP2 binding studies in the EN-2 promoter revealed a significant decrease in repressive MeCP2 binding that may contribute to the aberrant overexpression of EN-2. Because normal cerebellar development depends on perinatal EN-2 downregulation, the sustained postnatal overexpression suggests that a critical window of cerebellar development may have been missed in some individuals with autism with downstream developmental consequences. Epigenetic regulation of the programmed on-off switches in gene expression that occur at birth and during early brain development warrants further investigation.