Extensive respiratory chain defects in inhibitory interneurones in patients with mitochondrial disease

Activation of Neurotoxic Astrocytes Due to Mitochondrial Dysfunction Triggered by POLG Mutation

Mitochondrial diseases are associated with neuronal death and mtDNA depletion. Astrocytes respond to injury or stimuli and damage to the central nervous system. Neurodegeneration can cause astrocytes to activate and acquire toxic functions that induce neuronal death. However, astrocyte activation and its impact on neuronal homeostasis in mitochondrial disease remain to be explored. Using patient cells carrying POLG mutations, we generated iPSCs and then differentiated these into astrocytes. POLG astrocytes exhibited mitochondrial dysfunction including loss of mitochondrial membrane potential, energy failure, loss of complex I and IV, disturbed NAD+/NADH metabolism, and mtDNA depletion. Further, POLG derived astrocytes presented an A1-like reactive phenotype with increased proliferation, invasion, upregulation of pathways involved in response to stimulus, immune system process, cell proliferation and cell killing. Under direct and indirect co-culture with neurons, POLG astrocytes manifested a toxic effect leading to the death of neurons. We demonstrate that mitochondrial dysfunction caused by POLG mutations leads not only to intrinsic defects in energy metabolism affecting both neurons and astrocytes, but also to neurotoxic damage driven by astrocytes. These findings reveal a novel role for dysfunctional astrocytes that contribute to the pathogenesis of POLG diseases.

Mitochondrial complex I deficiency stratifies idiopathic Parkinson's disease

Idiopathic Parkinson's disease (iPD) is believed to have a heterogeneous pathophysiology, but molecular disease subtypes have not been identified. Here, we show that iPD can be stratified according to the severity of neuronal respiratory complex I (CI) deficiency, and identify two emerging disease subtypes with distinct molecular and clinical profiles. The CI deficient (CI-PD) subtype accounts for approximately a fourth of all cases, and is characterized by anatomically widespread neuronal CI deficiency, a distinct cell type-specific gene expression profile, increased load of neuronal mtDNA deletions, and a predilection for non-tremor dominant motor phenotypes. In contrast, the non-CI deficient (nCI-PD) subtype exhibits no evidence of mitochondrial impairment outside the dopaminergic substantia nigra and has a predilection for a tremor dominant phenotype. These findings constitute a step towards resolving the biological heterogeneity of iPD with implications for both mechanistic understanding and treatment strategies.

Delineating mechanisms underlying parvalbumin neuron impairment in different neurological and neurodegenerative disorders: the emerging role of mitochondrial dysfunction

Given the current paucity of effective treatments in many neurological disorders, delineating pathophysiological mechanisms among the major psychiatric and neurodegenerative diseases may fuel the development of novel, potent treatments that target shared pathways. Recent evidence suggests that various pathological processes, including bioenergetic failure in mitochondria, can perturb the function of fast-spiking, parvalbumin-positive neurons (PV+). These inhibitory neurons critically influence local circuit regulation, the generation of neuronal network oscillations and complex brain functioning. Here, we survey PV+ cell vulnerability in the major neuropsychiatric, and neurodegenerative diseases and review associated cellular and molecular pathophysiological alterations purported to underlie disease aetiology.

A novel mouse model of mitochondrial disease exhibits juvenile-onset severe neurological impairment due to parvalbumin cell mitochondrial dysfunction

Mitochondrial diseases comprise a common group of neurometabolic disorders resulting from OXPHOS defects, that may manifest with neurological impairments, for which there are currently no disease-modifying therapies. Previous studies suggest inhibitory interneuron susceptibility to mitochondrial impairment, especially of parvalbumin-expressing interneurons (PV+). We have developed a mouse model of mitochondrial dysfunction specifically in PV+ cells via conditional Tfam knockout, that exhibited a juvenile-onset progressive phenotype characterised by cognitive deficits, anxiety-like behaviour, head-nodding, stargazing, ataxia, and reduced lifespan. A brain region-dependent decrease of OXPHOS complexes I and IV in PV+ neurons was detected, with Purkinje neurons being most affected. We validated these findings in a neuropathological study of patients with pathogenic mtDNA and POLG variants showing PV+ interneuron loss and deficiencies in complexes I and IV. This mouse model offers a drug screening platform to propel the discovery of therapeutics to treat severe neurological impairment due to mitochondrial dysfunction.

Elevated susceptibility to exogenous seizure triggers and impaired interneuron excitability in a mouse model of Leigh syndrome epilepsy

Mutations in the NADH dehydrogenase (ubiquinone reductase) iron‑sulfur protein 4 (NDUFS4) gene, which encodes for a key structural subunit of the OXFOS complex I (CI), lead to the most common form of mitochondrial disease in children known as Leigh syndrome (LS). As in other mitochondrial diseases, epileptic seizures constitute one of the most significant clinical features of LS. These seizures are often very difficult to treat and are a sign of poor disease prognosis. Mice with whole-body Ndufs4 KO are a well-validated model of LS; they exhibit epilepsy and several other clinical features of LS. We have previously shown that mice with Ndufs4 KO in only GABAergic interneurons (Gad2-Ndufs4-KO) reproduce the severe epilepsy phenotype observed in the global KO mice. This observation indicated that these mice represent an excellent model of LS epilepsy isolated from other clinical manifestations of the disease. To further characterize this epilepsy phenotype, we investigated seizure susceptibility to selected exogenous seizure triggers in Gad2-Ndufs4-KO mice. Then, using electrophysiology, imaging, and immunohistochemistry, we studied the cellular, physiological, and neuroanatomical consequences of Ndufs4 KO in GABAergic interneurons. Homozygous KO of Ndufs4 in GABAergic interneurons leads to a prominent susceptibility to exogenous seizure triggers, impaired interneuron excitability and interneuron loss. Finally, we found that the hippocampus and cortex participate in the generation of seizure activity in Gad2-Ndufs4-KO mice. These findings further define the LS epilepsy phenotype and provide important insights into the cellular mechanisms underlying epilepsy in LS and other mitochondrial diseases.

Neurological Phenotypes in Mouse Models of Mitochondrial Disease and Relevance to Human Neuropathology

Mitochondrial diseases represent the most common inherited neurometabolic disorders, for which no effective therapy currently exists for most patients. The unmet clinical need requires a more comprehensive understanding of the disease mechanisms and the development of reliable and robust in vivo models that accurately recapitulate human disease. This review aims to summarise and discuss various mouse models harbouring transgenic impairments in genes that regulate mitochondrial function, specifically their neurological phenotype and neuropathological features. Ataxia secondary to cerebellar impairment is one of the most prevalent neurological features of mouse models of mitochondrial dysfunction, consistent with the observation that progressive cerebellar ataxia is a common neurological manifestation in patients with mitochondrial disease. The loss of Purkinje neurons is a shared neuropathological finding in human post-mortem tissues and numerous mouse models. However, none of the existing mouse models recapitulate other devastating neurological phenotypes, such as refractory focal seizures and stroke-like episodes seen in patients. Additionally, we discuss the roles of reactive astrogliosis and microglial reactivity, which may be driving the neuropathology in some of the mouse models of mitochondrial dysfunction, as well as mechanisms through which cellular death may occur, beyond apoptosis, in neurons undergoing mitochondrial bioenergy crisis.

Astrocytic pathology in Alpers' syndrome

Refractory epilepsy is the main neurological manifestation of Alpers' syndrome, a severe childhood-onset mitochondrial disease caused by bi-allelic pathogenic variants in the mitochondrial DNA (mtDNA) polymerase gamma gene (POLG). The pathophysiological mechanisms underpinning neuronal hyperexcitabilty leading to seizures in Alpers' syndrome remain unknown. However, pathological changes to reactive astrocytes are hypothesised to exacerbate neural dysfunction and seizure-associated cortical activity in POLG-related disease. Therefore, we sought to phenotypically characterise astrocytic pathology in Alpers' syndrome. We performed a detailed quantitative investigation of reactive astrocytes in post-mortem neocortical tissues from thirteen patients with Alpers' syndrome, eight neurologically normal controls and five sudden unexpected death in epilepsy (SUDEP) patients, to control for generalised epilepsy-associated astrocytic pathology. Immunohistochemistry to identify glial fibrillary acidic protein (GFAP)-reactive astrocytes revealed striking reactive astrogliosis localised to the primary visual cortex of Alpers' syndrome tissues, characterised by abnormal-appearing hypertrophic astrocytes. Phenotypic characterisation of individual GFAP-reactive astrocytes demonstrated decreased abundance of mitochondrial oxidative phosphorylation (OXPHOS) proteins and altered expression of key astrocytic proteins including Kir4.1 (subunit of the inwardly rectifying K⁺ ion channel), AQP4 (astrocytic water channel) and glutamine synthetase (enzyme that metabolises glutamate). These phenotypic astrocytic changes were typically different from the pathology observed in SUDEP tissues, suggesting alternative mechanisms of astrocytic dysfunction between these epilepsies. Crucially, our findings provide further evidence of occipital lobe involvement in Alpers' syndrome and support the involvement of reactive astrocytes in the pathogenesis of POLG-related disease.

Stroke-like episodes in adult mitochondrial disease

Stroke-like episode is a paroxysmal neurological manifestation which affects a specific group of patients with mitochondrial disease. Focal-onset seizures, encephalopathy, and visual disturbances are prominent findings associated with stroke-like episodes, with a predilection for the posterior cerebral cortex. The most common cause of stroke-like episodes is the m.3243A>G variant in MT-TL1 gene followed by recessive POLG variants. This chapter aims to review the definition of stroke-like episode and delineate the clinical phenomenology, neuroimaging and EEG findings typically seen in patients. In addition, several lines of evidence supporting neuronal hyper-excitability as the key mechanism of stroke-like episodes are discussed. The management of stroke-like episodes should focus on aggressive seizure management and treatment for concomitant complications such as intestinal pseudo-obstruction. There is no robust evidence to prove the efficacy of l-arginine for both acute and prophylactic settings. Progressive brain atrophy and dementia are the sequalae of recurrent stroke-like episode, and the underlying genotype in part predicts prognosis.

Mitochondrial encephalomyopathy

Mitochondrial dysfunction, especially perturbation of oxidative phosphorylation and adenosine triphosphate (ATP) generation, disrupts cellular homeostasis and is a surprisingly frequent cause of central and peripheral nervous system pathology. Mitochondrial disease is an umbrella term that encompasses a host of clinical syndromes and features caused by in excess of 300 different genetic defects affecting the mitochondrial and nuclear genomes. Patients with mitochondrial disease can present at any age, ranging from neonatal onset to late adult life, with variable organ involvement and neurological manifestations including neurodevelopmental delay, seizures, stroke-like episodes, movement disorders, optic neuropathy, myopathy, and neuropathy. Until relatively recently, analysis of skeletal muscle biopsy was the focus of diagnostic algorithms, but step-changes in the scope and availability of next-generation sequencing technology and multiomics analysis have revolutionized mitochondrial disease diagnosis. Currently, there is no specific therapy for most types of mitochondrial disease, although clinical trials research in the field is gathering momentum. In that context, active management of epilepsy, stroke-like episodes, dystonia, brainstem dysfunction, and Parkinsonism are all the more important in improving patient quality of life and reducing mortality.

Delineating selective vulnerability of inhibitory interneurons in Alpers’ syndrome

Aims

Methods

Results

Conclusions

Forecasting stroke-like episodes and outcomes in mitochondrial disease

In this retrospective, multicentre, observational cohort study, we sought to determine the clinical, radiological, EEG, genetics and neuropathological characteristics of mitochondrial stroke-like episodes and to identify associated risk predictors. Between January 1998 and June 2018, we identified 111 patients with genetically determined mitochondrial disease who developed stroke-like episodes. Post-mortem cases of mitochondrial disease (n = 26) were identified from Newcastle Brain Tissue Resource. The primary outcome was to interrogate the clinico-radiopathological correlates and prognostic indicators of stroke-like episode in patients with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes syndrome (MELAS). The secondary objective was to develop a multivariable prediction model to forecast stroke-like episode risk. The most common genetic cause of stroke-like episodes was the m.3243A>G variant in MT-TL1 (n = 66), followed by recessive pathogenic POLG variants (n = 22), and 11 other rarer pathogenic mitochondrial DNA variants (n = 23). The age of first stroke-like episode was available for 105 patients [mean (SD) age: 31.8 (16.1)]; a total of 35 patients (32%) presented with their first stroke-like episode ≥40 years of age. The median interval (interquartile range) between first and second stroke-like episodes was 1.33 (2.86) years; 43% of patients developed recurrent stroke-like episodes within 12 months. Clinico-radiological, electrophysiological and neuropathological findings of stroke-like episodes were consistent with the hallmarks of medically refractory epilepsy. Patients with POLG-related stroke-like episodes demonstrated more fulminant disease trajectories than cases of m.3243A>G and other mitochondrial DNA pathogenic variants, in terms of the frequency of refractory status epilepticus, rapidity of progression and overall mortality. In multivariate analysis, baseline factors of body mass index, age-adjusted blood m.3243A>G heteroplasmy, sensorineural hearing loss and serum lactate were significantly associated with risk of stroke-like episodes in patients with the m.3243A>G variant. These factors informed the development of a prediction model to assess the risk of developing stroke-like episodes that demonstrated good overall discrimination (area under the curve = 0.87, 95% CI 0.82-0.93; c-statistic = 0.89). Significant radiological and pathological features of neurodegeneration were more evident in patients harbouring pathogenic mtDNA variants compared with POLG: brain atrophy on cranial MRI (90% versus 44%, P < 0.001) and reduced mean brain weight (SD) [1044 g (148) versus 1304 g (142), P = 0.005]. Our findings highlight the often idiosyncratic clinical, radiological and EEG characteristics of mitochondrial stroke-like episodes. Early recognition of seizures and aggressive instigation of treatment may help circumvent or slow neuronal loss and abate increasing disease burden. The risk-prediction model for the m.3243A>G variant can help inform more tailored genetic counselling and prognostication in routine clinical practice.

The metabolic basis of epilepsy

The brain is a highly energy-demanding organ and requires bioenergetic adaptability to balance normal activity with pathophysiological fuelling of spontaneous recurrent seizures, the hallmark feature of the epilepsies. Recurrent or prolonged seizures have long been known to permanently alter neuronal circuitry and to cause excitotoxic injury and aberrant inflammation. Furthermore, pathological changes in bioenergetics and metabolism are considered downstream consequences of epileptic seizures that begin at the synaptic level. However, as we highlight in this Review, evidence is also emerging that primary derangements in cellular or mitochondrial metabolism can result in seizure genesis and lead to spontaneous recurrent seizures. Basic and translational research indicates that the relationships between brain metabolism and epileptic seizures are complex and bidirectional, producing a vicious cycle that compounds the deleterious consequences of seizures. Metabolism-based treatments such as the high-fat, antiseizure ketogenic diet have become mainstream, and metabolic substrates and enzymes have become attractive molecular targets for seizure prevention and recovery. Moreover, given that metabolism is crucial for epigenetic as well as inflammatory changes, the idea that epileptogenesis can be both negatively and positively influenced by metabolic changes is rapidly gaining ground. Here, we review evidence that supports both pathophysiological and therapeutic roles for brain metabolism in epilepsy.

Comprehensive summary of mitochondrial DNA alterations in the postmortem human brain: A systematic review

BACKGROUND

Mitochondrial DNA (mtDNA) encodes 37 genes necessary for synthesizing 13 essential subunits of the oxidative phosphorylation system. mtDNA alterations are known to cause mitochondrial disease (MitD), a clinically heterogeneous group of disorders that often present with neuropsychiatric symptoms. Understanding the nature and frequency of mtDNA alterations in health and disease could be a cornerstone in disentangling the relationship between biochemical findings and clinical symptoms of brain disorders. This systematic review aimed to summarize the mtDNA alterations in human brain tissue reported to date that have implications for further research on the pathophysiological significance of mtDNA alterations in brain functioning.

METHODS

We searched the PubMed and Embase databases using distinct terms related to postmortem human brain and mtDNA up to June 10, 2021. Reports were eligible if they were empirical studies analysing mtDNA in postmortem human brains.

FINDINGS

A total of 158 of 637 studies fulfilled the inclusion criteria and were clustered into the following groups: MitD (48 entries), neurological diseases (NeuD, 55 entries), psychiatric diseases (PsyD, 15 entries), a miscellaneous group with controls and other clinical diseases (5 entries), ageing (20 entries), and technical issues (5 entries). Ten entries were ascribed to more than one group. Pathogenic single nucleotide variants (pSNVs), both homo- or heteroplasmic variants, have been widely reported in MitD, with heteroplasmy levels varying among brain regions; however, pSNVs are rarer in NeuD, PsyD and ageing. A lower mtDNA copy number (CN) in disease was described in most, but not all, of the identified studies. mtDNA deletions were identified in individuals in the four clinical categories and ageing. Notably, brain samples showed significantly more mtDNA deletions and at higher heteroplasmy percentages than blood samples, and several of the deletions present in the brain were not detected in the blood. Finally, mtDNA heteroplasmy, mtDNA CN and the deletion levels varied depending on the brain region studied.

INTERPRETATION

mtDNA alterations are well known to affect human tissues, including the brain. In general, we found that studies of MitD, NeuD, PsyD, and ageing were highly variable in terms of the type of disease or ageing process investigated, number of screened individuals, studied brain regions and technology used. In NeuD and PsyD, no particular type of mtDNA alteration could be unequivocally assigned to any specific disease or diagnostic group. However, the presence of mtDNA deletions and mtDNA CN variation imply a role for mtDNA in NeuD and PsyD. Heteroplasmy levels and threshold effects, affected brain regions, and mitotic segregation patterns of mtDNA alterations may be involved in the complex inheritance of NeuD and PsyD and in the ageing process. Therefore, more information is needed regarding the type of mtDNA alteration, the affected brain regions, the heteroplasmy levels, and their relationship with clinical phenotypes and the ageing process.

FUNDING

Hospital Universitari Institut Pere Mata; Institut d'Investigació Sanitària Pere Virgili; Instituto de Salud Carlos III, Ministerio de Ciencia e Innovación (PI18/00514).

Clinical Characteristics of Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-Like Episodes

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome, a maternally inherited mitochondrial disorder, is characterized by its genetic, biochemical and clinical complexity. The most common mutation associated with MELAS syndrome is the mtDNA A3243G mutation in the MT-TL1 gene encoding the mitochondrial tRNA-leu(UUR), which results in impaired mitochondrial translation and protein synthesis involving the mitochondrial electron transport chain complex subunits, leading to impaired mitochondrial energy production. Angiopathy, either alone or in combination with nitric oxide (NO) deficiency, further contributes to multi-organ involvement in MELAS syndrome. Management for MELAS syndrome is amostly symptomatic multidisciplinary approach. In this article, we review the clinical presentations, pathogenic mechanisms and options for management of MELAS syndrome.

A perspective on human cell models for POLG-spectrum disorders: advantages and disadvantages of CRISPR-Cas-based vs. patient-derived iPSC models

Neurogenetic diseases represent a broad group of diseases with variable genetic causes and clinical manifestations. Among these, polymerase-gamma (POLG)-spectrum disorders are relatively frequent with an estimated disease frequency of ∼1:10.000. Also, mutations in the POLG gene are by far the most important cause for mitochondriopathy. POLG-spectrum disorders usually result in progressive loss of brain function and may involve severe and deadly encephalopathy, seizures, and neuromuscular disease, as well as cardiac and hepatic failure in some cases. Onset of disease may range from birth to late adulthood, and disease duration ranges from weeks in severe cases to decades. There is no curative treatment; current animal models do not faithfully recapitulate human disease, complicating preclinical therapeutic studies. Human-based preclinical model systems must be developed to understand the human disease mechanisms and develop therapeutic approaches. In this review, we provide an overview of the current approaches to model neurogenetic disorders in a human cellular and neuronal environment with a focus on POLG-spectrum disorders. We discuss the necessity of using neuronal cells and the advantages and pitfalls of currently available cell model approaches, namely (i) CRISPR-based (i. e., genetically engineered) and induced pluripotent stem cell (iPSC) (i. e., stem cell like)-derived neuronal models and (ii) the reprogramming of patient-derived cells into iPSCs and derived neurons. Despite the fact that cell models are by definition in vitro systems incapable of recapitulating all aspects of human disease, they are still the reasonable point of start to discover disease mechanisms and develop therapeutic approaches to treat neurogenetic diseases.

Epilepsy in Mitochondrial Diseases-Current State of Knowledge on Aetiology and Treatment

Mitochondrial diseases are a heterogeneous group of diseases resulting from energy deficit and reduced adenosine triphosphate (ATP) production due to impaired oxidative phosphorylation. The manifestation of mitochondrial disease is usually multi-organ. Epilepsy is one of the most common manifestations of diseases resulting from mitochondrial dysfunction, especially in children. The onset of epilepsy is associated with poor prognosis, while its treatment is very challenging, which further adversely affects the course of these disorders. Fortunately, our knowledge of mitochondrial diseases is still growing, which gives hope for patients to improve their condition in the future. The paper presents the pathophysiology, clinical picture and treatment options for epilepsy in patients with mitochondrial disease.

Insights into Lewy body disease from rare neurometabolic disorders

Professor Kurt Jellinger is well known for his seminal work on the neuropathology of age-associated neurodegenerative disorders, particularly Lewy body diseases. However, it is less well known that he also contributed important insights into the neuropathological features of several paediatric neurometabolic diseases, including Alpers–Huttenlocher syndrome, a syndrome of mitochondrial disease caused by POLG mutations, and infantile neuroaxonal dystrophy, a phenotype resulting from PLA2G6 mutations. Despite these rare diseases occurring in early life, they share many important pathological overlaps with age-associated Lewy body disease, particularly dysregulation of α-synuclein. In this review, we describe several neurometabolic diseases linked to Lewy body disease mechanisms, and discuss the wider context to pathological overlaps between neurometabolic and Lewy body diseases. In particular, we will focus on how understanding disease mechanisms in neurometabolic disorders with dysregulated α-synuclein may generate insights into predisposing factors for α-synuclein aggregation in idiopathic Lewy body diseases.

m.3243A > G-Induced Mitochondrial Dysfunction Impairs Human Neuronal Development and Reduces Neuronal Network Activity and Synchronicity

Epilepsy, intellectual and cortical sensory deficits, and psychiatric manifestations are the most frequent manifestations of mitochondrial diseases. How mitochondrial dysfunction affects neural structure and function remains elusive, mostly because of a lack of proper in vitro neuronal model systems with mitochondrial dysfunction. Leveraging induced pluripotent stem cell technology, we differentiated excitatory cortical neurons (iNeurons) with normal (low heteroplasmy) and impaired (high heteroplasmy) mitochondrial function on an isogenic nuclear DNA background from patients with the common pathogenic m.3243A > G variant of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS). iNeurons with high heteroplasmy exhibited mitochondrial dysfunction, delayed neural maturation, reduced dendritic complexity, and fewer excitatory synapses. Micro-electrode array recordings of neuronal networks displayed reduced network activity and decreased synchronous network bursting. Impaired neuronal energy metabolism and compromised structural and functional integrity of neurons and neural networks could be the primary drivers of increased susceptibility to neuropsychiatric manifestations of mitochondrial disease.

TTC3-Mediated Protein Quality Control, A Potential Mechanism for Cognitive Impairment

The tetrapeptide repeat domain 3 (TTC3) gene falls within Down's syndrome (DS) critical region. Cognitive impairment is a common phenotype of DS and Alzheimer’s disease (AD), and overexpression of TTC3 can accelerate cognitive decline, but the specific mechanism is unknown. The TTC3-mediated protein quality control (PQC) mechanism, similar to the PQC system, is divided into three parts: it acts as a cochaperone to assist proteins in folding correctly; it acts as an E3 ubiquitin ligase (E3s) involved in protein degradation processes through the ubiquitin–proteasome system (UPS); and it may also eventually cause autophagy by affecting mitochondrial function. Thus, this article reviews the research progress on the structure, function, and metabolism of TTC3, including the recent research progress on TTC3 in DS and AD; the role of TTC3 in cognitive impairment through PQC in combination with the abovementioned attributes of TTC3; and the potential targets of TTC3 in the treatment of such diseases.

Diagnosis- and Cell Type-Specific Mitochondrial Functional Pathway Signatures in Schizophrenia and Bipolar Disorder

OBJECTIVE

The shared risk factors and clinical features in schizophrenia and bipolar disorder may be linked via mitochondrial dysfunction. However, the severity of mitochondrial dysfunction, and/or the specific mitochondrial functional pathways affected, may differ between diagnoses, especially at the level of individual cell types.

METHODS

Transcriptomic profiling data for a gene set indexing mitochondrial functional pathways were obtained for dorsolateral prefrontal cortex (DLPFC) gray matter and layer 3 and layer 5 pyramidal neurons of subjects with schizophrenia or bipolar disorder. Analyses were conducted using a dual strategy: identification of differentially expressed genes (DEGs) and their functional pathway enrichment, and application of weighted gene coexpression network analysis. These analyses were repeated in monkeys chronically exposed to antipsychotic drugs to determine their effect on mitochondrial-related gene expression.

RESULTS

In DLPFC gray matter, 41% of mitochondrial-related genes were differentially expressed in the schizophrenia group, whereas 8% were differentially expressed in the bipolar group. In the schizophrenia group, 83% of DEGs showed lower expression, and these were significantly enriched for three functional pathways, each indexing energy production. DEGs in the bipolar disorder group were not enriched for functional pathways. This disease-related pattern of findings was also identified in pyramidal neurons. None of the gene expression alterations disrupted coexpression modules, and DEGs were not attributable to antipsychotic medications.

CONCLUSIONS

Schizophrenia and bipolar disorder do not appear to share similar mitochondrial alterations in the DLPFC. The selective and coordinated down-regulation of energy production genes in schizophrenia is consistent with the effects of chronic reductions in pyramidal neuron firing, and enhancement of this activity may serve as a therapeutic target.

Alternating Hemiplegia of Childhood: gastrointestinal manifestations and correlation with neurological impairments

Background

Alternating Hemiplegia of Childhood (AHC) is caused by mutations of the ATP1A3 gene which is expressed in brain areas that include structures controling autonomic, gastrointestinal, gut motility and GABAergic functions. We aimed to investigate, in a cohort of 44 consecutive AHC patients, two hypotheses: 1) AHC patients frequently manifest gastrointestinal, particularly motility, problems. 2) These problems are often severe and their severity correlates with neurological impairments.

Results

41/44 (93%) exhibited gastrointestinal symptoms requiring medical attention. For these 41 patients, symptoms included constipation (66%), swallowing problems (63%), vomiting (63%), anorexia (46%), diarrhea (44%), nausea (37%), and abdominal pain (22%). Symptoms indicative of dysmotility occurred in 33 (80%). The most common diagnoses were oropharyngeal dysphagia (63%) and gastroesophageal reflux (63%). 16 (39%) required gastrostomy and two fundoplication. Severity of gastrointestinal symptoms correlated with non-paroxysmal neurological disability index, Gross Motor Function Classification System scores, and with the presence/absence of non-gastrointestinal autonomic dysfunction (p = 0.031, 0.043, Spearman correlations and 0.0166 Cramer’s V, respectively) but not with the paroxysmal disability index (p = 0.408).

Conclusions

Most AHC patients have gastrointestinal problems. These are usually severe, most commonly are indicative of dysmotility, often require surgical therapies, and their severity correlates with that of non-paroxysmal CNS manifestations. Our findings should help in management-anticipatory guidance of AHC patients. Furthermore, they are consistent with current understandings of the pathophysiology of AHC and of gastrointestinal dysmotility, both of which involve autonomic and GABAergic dysfunction.

Complex I reductions in the nucleus basalis of Meynert in Lewy body dementia: the role of Lewy bodies

Neurons of the nucleus basalis of Meynert (nbM) are vulnerable to Lewy body formation and neuronal loss, which is thought to underlie cognitive dysfunction in Lewy body dementia (LBD). There is continued debate about whether Lewy bodies exert a neurodegenerative effect by affecting mitochondria, or whether they represent a protective mechanism. Therefore, the present study sought to determine whether the nbM is subject to mitochondrial dysfunctional in LBD and the association of Lewy body formation with such changes. Post-mortem nbM tissue was stained for Complex I or IV and quantitated relative to porin with immunofluorescence using confocal microscopy of individual cells from LBD (303 neurons, 8 cases), control (362 neurons, 8 cases) and asymptomatic incidental LBD (iLBD) cases (99 neurons, 2 cases). Additionally, α-synuclein, tau and amyloid-β pathology were analysed using quantitative immunohistochemistry, and respiratory chain markers were compared in cells with Lewy bodies (N = 134) and unaffected cells (N = 272). The expression of Complex I normalised to mitochondrial mass was significantly lower in LBD compared to control and iLBD cases and this was unrelated to local neuropathological burdens but trended toward a relationship with neuronal loss. Furthermore, Complex I expression was higher in cells with Lewy bodies compared to adjacent cells without α-synuclein aggregates. These findings suggest that Complex I deficits in the nbM occur in symptomatic LBD cases and may relate to neuronal loss, but that contrary to the view that Lewy body formation underlies neuronal dysfunction and damage in LBD, Lewy bodies are associated with higher Complex I expression than neurons without Lewy bodies. One could speculate that Lewy bodies may provide a mechanism to encapsulate damaged mitochondria and/or α-synuclein oligomers, thus protecting neurons from their cytotoxic effects.

The mitochondrial epilepsies

Mitochondria are vital organelles within cells that undertake many important metabolic roles, the most significant of which is to generate energy to support organ function. Dysfunction of the mitochondrion can lead to a wide range of clinical features, predominantly affecting organs with a high metabolic demand such as the brain. One of the main neurological manifestations of mitochondrial disease is metabolic epilepsies. These epileptic seizures are more frequently of posterior quadrant and occipital lobe onset, more likely to present with non-convulsive status epilepticus which may last months and be more resistant to treatment from the onset. The onset of can be of any age. Childhood onset epilepsy is a major phenotypic feature in mitochondrial disorders such as Alpers-Huttenlocher syndrome, pyruvate dehydrogenase complex deficiencies, and Leigh syndrome. Meanwhile, adults with classical mitochondrial disease syndrome such as MELAS, MERFF or POLG-related disorders could present with either focal or generalised seizures. There are no specific curative treatments for mitochondrial epilepsy. Generally, the epileptic seizures should be managed by specialist neurologist with appropriate use of anticonvulsants. As a general rule, especially in disorders associated with mutation in POLG, sodium valproate is best avoided because hepato-toxicity can be fulminant and fatal.

Consensus-based statements for the management of mitochondrial stroke-like episodes

Background: Focal-onset seizures and encephalopathy are prominent features of a stroke-like episode, which is a severe neurological manifestation associated with subtypes of mitochondrial disease. Despite more than 30 years of research, the acute treatment of stroke-like episodes remains controversial. Methods: We used the modified Delphi process to harness the clinical expertise of a group of mitochondrial disease specialists from five European countries to produce consensus guidance for the acute management of stroke-like episodes and commonly associated complications. Results: Consensus on a new definition of mitochondrial stroke-like episodes was achieved and enabled the group to develop diagnostic criteria based on clinical features, neuroimaging and/or electroencephalogram findings. Guidelines for the management of strokelike episodes were agreed with aggressive seizure management strongly recommended at the outset of stroke-like episodes. Conclusions: Our consensus statement defines stroke-like episodes in terms of an epileptic encephalopathy and we have used this to revise both diagnostic criteria and guidelines for management. A prospective, multi-centre, randomised controlled trial is required for evaluating the efficacy of any compound on modifying the trajectory of stroke-like episodes.

The role of astrocytes in seizure generation: insights from a novel in vitro seizure model based on mitochondrial dysfunction

Approximately one-quarter of patients with mitochondrial disease experience epilepsy. Their epilepsy is often severe and resistant towards conventional antiepileptic drugs. Despite the severity of this epilepsy, there are currently no animal models available to provide a mechanistic understanding of mitochondrial epilepsy. We conducted neuropathological studies on patients with mitochondrial epilepsy and found the involvement of the astrocytic compartment. As a proof of concept, we developed a novel brain slice model of mitochondrial epilepsy by the application of an astrocytic-specific aconitase inhibitor, fluorocitrate, concomitant with mitochondrial respiratory inhibitors, rotenone and potassium cyanide. The model was robust and exhibited both face and predictive validity. We then used the model to assess the role that astrocytes play in seizure generation and demonstrated the involvement of the GABA-glutamate-glutamine cycle. Notably, glutamine appears to be an important intermediary molecule between the neuronal and astrocytic compartment in the regulation of GABAergic inhibitory tone. Finally, we found that a deficiency in glutamine synthetase is an important pathogenic process for seizure generation in both the brain slice model and the human neuropathological study. Our study describes the first model for mitochondrial epilepsy and provides a mechanistic insight into how astrocytes drive seizure generation in mitochondrial epilepsy.

Resolving complexity in mitochondrial disease: Towards precision medicine

Mitochondrial diseases, caused by mutations in either the nuclear or mitochondrial genomes (mtDNA), are the most common form of inherited neurometabolic disorders. They are remarkably heterogeneous, both in their clinical presentation and genetic etiology, presenting challenges for diagnosis, clinical management and elucidation of molecular mechanism. The multifaceted nature of these diseases, compounded by the unique characteristics of mitochondrial genetics, cement their space in the field of complex disease. In this review we examine the m.3243A>G variant, one of the most prevalent mitochondrial DNA mutations, using it as an exemplar to demonstrate the challenges presented by these complex disorders. Disease caused by m.3243A>G is one of the most phenotypically diverse of all mitochondrial diseases; we outline known causes of this heterogeneity including mtDNA heteroplasmy, mtDNA copy number and nuclear genetic factors. We consider the impact that this has in the clinic, discussing the personalized management of common manifestations attributed to this pathogenic mtDNA variant, including hearing impairment, diabetes mellitus, myopathy, cardiac disease, stroke-like episodes and gastrointestinal disturbances. Future research into this complex disorder must account for this heterogeneity, benefitting from the use of large patient cohorts to build upon current clinical expertise. Through multi-disciplinary collaboration, the complexities of this mitochondrial disease can be addressed with the variety of diagnostic, prognostic, and treatment approaches that are moulded to best fit the needs of each individual patient.

Cerebral mitochondrial electron transport chain dysfunction in multiple system atrophy and Parkinson's disease

Multiple system atrophy (MSA) is a neurodegenerative disease characterised by glial cytoplasmic inclusions (GCIs), containing α-synuclein. Mutated COQ2, encoding an enzyme essential for co-enzyme Q10 (CoQ10) biosynthesis, has been associated with MSA. CoQ10 is an electron carrier in the mitochondrial electron transport chain (ETC) and antioxidant. It has been shown to be deficient in MSA brain tissue, thus implicating mitochondrial dysfunction in MSA. To investigate mitochondrial dysfunction in MSA further we examined ETC activity in MSA and control brain tissue, compared with Parkinson's disease (PD) where mitochondrial dysfunction is known to be important. Using cerebellar and occipital white matter ETC complex I, II/III and IV activities were measured spectrophotometrically, selected individual components of the ETC were assessed by immunoblotting and cellular complex IV activity was analysed by enzyme histochemistry. We show decreased complex II/III activity with increased complex I and IV activity in MSA cerebellar white matter. This corresponds with the deficit in CoQ10 previously described in MSA and reflects the high regional pathological burden of GCIs. This study highlights mitochondrial dysfunction in MSA pathogenesis, suggests an influence on selective regional vulnerability to disease and points to shared disease mechanisms in α-synucleinopathies.

Dissecting the neuronal vulnerability underpinning Alpers' syndrome: a clinical and neuropathological study

Alpers’ syndrome is an early‐onset neurodegenerative disorder often caused by biallelic pathogenic variants in the gene encoding the catalytic subunit of polymerase‐gamma (POLG) which is essential for mitochondrial DNA (mtDNA) replication. Alpers’ syndrome is characterized by intractable epilepsy, developmental regression and liver failure which typically affects children aged 6 months–3 years. Although later onset variants are now recognized, they differ in that they are primarily an epileptic encephalopathy with ataxia. The disorder is progressive, without cure and inevitably leads to death from drug‐resistant status epilepticus, often with concomitant liver failure. Since our understanding of the mechanisms contributing the neurological features in Alpers’ syndrome is rudimentary, we performed a detailed and quantitative neuropathological study on 13 patients with clinically and histologically‐defined Alpers’ syndrome with ages ranging from 2 months to 18 years. Quantitative immunofluorescence showed severe respiratory chain deficiencies involving mitochondrial respiratory chain subunits of complex I and, to a lesser extent, complex IV in inhibitory interneurons and pyramidal neurons in the occipital cortex and in Purkinje cells of the cerebellum. Diminished densities of these neuronal populations were also observed. This study represents the largest cohort of post‐mortem brains from patients with clinically defined Alpers’ syndrome where we provide quantitative evidence of extensive complex I defects affecting interneurons and Purkinje cells for the first time. We believe interneuron and Purkinje cell pathology underpins the clinical development of seizures and ataxia seen in Alpers’ syndrome. This study also further highlights the extensive involvement of GABAergic neurons in mitochondrial disease.

Anatomic & metabolic brain markers of the m.3243A>G mutation: A multi-parametric 7T MRI study

One of the most common mitochondrial DNA (mtDNA) mutations, the A to G transition at base pair 3243, has been linked to changes in the brain, in addition to commonly observed hearing problems, diabetes and myopathy. However, a detailed quantitative description of m.3243A>G patients' brains has not been provided so far. In this study, ultra-high field MRI at 7T and volume- and surface-based data analyses approaches were used to highlight morphology (i.e. atrophy)-, microstructure (i.e. myelin and iron concentration)- and metabolism (i.e. cerebral blood flow)-related differences between patients (N = 22) and healthy controls (N = 15). The use of quantitative MRI at 7T allowed us to detect subtle changes of biophysical processes in the brain with high accuracy and sensitivity, in addition to typically assessed lesions and atrophy. Furthermore, the effect of m.3243A>G mutation load in blood and urine epithelial cells on these MRI measures was assessed within the patient population and revealed that blood levels were most indicative of the brain's state and disease severity, based on MRI as well as on neuropsychological data. Morphometry MRI data showed a wide-spread reduction of cortical, subcortical and cerebellar gray matter volume, in addition to significantly enlarged ventricles. Moreover, surface-based analyses revealed brain area-specific changes in cortical thickness (e.g. of the auditory cortex), and in T₁, T₂* and cerebral blood flow as a function of mutation load, which can be linked to typically m.3243A>G-related clinical symptoms (e.g. hearing impairment). In addition, several regions linked to attentional control (e.g. middle frontal gyrus), the sensorimotor network (e.g. banks of central sulcus) and the default mode network (e.g. precuneus) were characterized by alterations in cortical thickness, T₁, T₂* and/or cerebral blood flow, which has not been described in previous MRI studies. Finally, several hypotheses, based either on vascular, metabolic or astroglial implications of the m.3243A>G mutation, are discussed that potentially explain the underlying pathobiology. To conclude, this is the first 7T and also the largest MRI study on this patient population that provides macroscopic brain correlates of the m.3243A>G mutation indicating potential MRI biomarkers of mitochondrial diseases and might guide future (longitudinal) studies to extensively track neuropathological and clinical changes.

The genetics and pathology of mitochondrial disease

Mitochondria are double-membrane-bound organelles that are present in all nucleated eukaryotic cells and are responsible for the production of cellular energy in the form of ATP. Mitochondrial function is under dual genetic control - the 16.6-kb mitochondrial genome, with only 37 genes, and the nuclear genome, which encodes the remaining ∼1300 proteins of the mitoproteome. Mitochondrial dysfunction can arise because of defects in either mitochondrial DNA or nuclear mitochondrial genes, and can present in childhood or adulthood in association with vast clinical heterogeneity, with symptoms affecting a single organ or tissue, or multisystem involvement. There is no cure for mitochondrial disease for the vast majority of mitochondrial disease patients, and a genetic diagnosis is therefore crucial for genetic counselling and recurrence risk calculation, and can impact on the clinical management of affected patients. Next-generation sequencing strategies are proving pivotal in the discovery of new disease genes and the diagnosis of clinically affected patients; mutations in >250 genes have now been shown to cause mitochondrial disease, and the biochemical, histochemical, immunocytochemical and neuropathological characterization of these patients has led to improved diagnostic testing strategies and novel diagnostic techniques. This review focuses on the current genetic landscape associated with mitochondrial disease, before focusing on advances in studying associated mitochondrial pathology in two, clinically relevant organs - skeletal muscle and brain. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Review: Central nervous system involvement in mitochondrial disease

Mitochondrial respiratory chain defects are an important cause of inherited disorders affecting approximately 1 in 5000 people in the UK population. Collectively these disorders are termed ‘mitochondrial diseases’ and they result from either mitochondrial DNA mutations or defects in nuclear DNA. Although they are frequently multisystem disorders, neurological deficits are particularly common, wide‐ranging and disabling for patients. This review details the manifold neurological impairments associated with mitochondrial disease, and describes the efforts to understand how they arise and progressively worsen in patients with mitochondrial disease. We describe advances in our understanding of disease pathogenesis through detailed neuropathological studies and how this has spurred the development of cellular and animal models of disease. We underscore the importance of continued clinical, molecular genetic, neuropathological and animal model studies to fully characterize mitochondrial diseases and understand mechanisms of neurodegeneration. These studies are instrumental for the next phase of mitochondrial research that has a particular emphasis on finding novel ways to treat mitochondrial disease to improve patient care and quality of life.

Development of passive CLARITY and immunofluorescent labelling of multiple proteins in human cerebellum: understanding mechanisms of neurodegeneration in mitochondrial disease

CLARITY enables immunofluorescent labelling and imaging of large volumes of tissue to provide a better insight into the three dimensional relationship between cellular morphology and spatial interactions between different cell types. In the current study, we optimise passive CLARITY and immunofluorescent labelling of neurons and mitochondrial proteins in mouse and human brain tissues to gain further insights into mechanisms of neurodegeneration occurring in mitochondrial disease. This is the first study to utilise human cerebellum fixed in paraformaldehyde and cryoprotected in conjunction with formalin-fixed tissues opening up further avenues for use of archived tissue. We optimised hydrogel-embedding and passive clearance of lipids from both mouse (n = 5) and human (n = 9) cerebellum as well as developing an immunofluorescent protocol that consistently labels different neuronal domains as well as blood vessels. In addition to visualising large structures, we were able to visualise mitochondrial proteins in passively cleared tissues to reveal respiratory chain deficiency associated with mitochondrial disease. We also demonstrate multiple use of tissues by stripping antibodies and re-probing the cerebellum. This technique allows interrogation of large volumes intact brain samples for better understanding of the complex pathological changes taking place in mitochondrial disease.

Mitochondrial DNA Depletion in Respiratory Chain-Deficient Parkinson Disease Neurons

Objective

To determine the extent of respiratory chain abnormalities and investigate the contribution of mtDNA to the loss of respiratory chain complexes (CI–IV) in the substantia nigra (SN) of idiopathic Parkinson disease (IPD) patients at the single‐neuron level.

Methods

Multiple‐label immunofluorescence was applied to postmortem sections of 10 IPD patients and 10 controls to quantify the abundance of CI–IV subunits (NDUFB8 or NDUFA13, SDHA, UQCRC2, and COXI) and mitochondrial transcription factors (TFAM and TFB2M) relative to mitochondrial mass (porin and GRP75) in dopaminergic neurons. To assess the involvement of mtDNA in respiratory chain deficiency in IPD, SN neurons, isolated with laser‐capture microdissection, were assayed for mtDNA deletions, copy number, and presence of transcription/replication‐associated 7S DNA employing a triplex real‐time polymerase chain reaction (PCR) assay.

Results

Whereas mitochondrial mass was unchanged in single SN neurons from IPD patients, we observed a significant reduction in the abundances of CI and II subunits. At the single‐cell level, CI and II deficiencies were correlated in patients. The CI deficiency concomitantly occurred with low abundances of the mtDNA transcription factors TFAM and TFB2M, which also initiate transcription‐primed mtDNA replication. Consistent with this, real‐time PCR analysis revealed fewer transcription/replication‐associated mtDNA molecules and an overall reduction in mtDNA copy number in patients. This effect was more pronounced in single IPD neurons with severe CI deficiency.

Interpretation

Respiratory chain dysfunction in IPD neurons not only involves CI, but also extends to CII. These deficiencies are possibly a consequence of the interplay between nDNA and mtDNA‐encoded factors mechanistically connected via TFAM. ANN NEUROL 2016;79:366–378

Investigating complex I deficiency in Purkinje cells and synapses in patients with mitochondrial disease

Aims

Cerebellar ataxia is common in patients with mitochondrial disease, and despite previous neuropathological investigations demonstrating vulnerability of the olivocerebellar pathway in patients with mitochondrial disease, the exact neurodegenerative mechanisms are still not clear. We use quantitative quadruple immunofluorescence to enable precise quantification of mitochondrial respiratory chain protein expression in Purkinje cell bodies and their synaptic terminals in the dentate nucleus.

Methods

We investigated NADH dehydrogenase [ubiquinone] 1 alpha subcomplex subunit 13 protein expression in 12 clinically and genetically defined patients with mitochondrial disease and ataxia and 10 age‐matched controls. Molecular genetic analysis was performed to determine heteroplasmy levels of mutated mitochondrial DNA in Purkinje cell bodies and inhibitory synapses.

Results

Our data reveal that complex I deficiency is present in both Purkinje cell bodies and their inhibitory synapses which surround dentate nucleus neurons. Inhibitory synapses are fewer and enlarged in patients which could represent a compensatory mechanism. Mitochondrial DNA heteroplasmy demonstrated similarly high levels of mutated mitochondrial DNA in cell bodies and synapses.

Conclusions

This is the first study to use a validated quantitative immunofluorescence technique to determine complex I expression in neurons and presynaptic terminals, evaluating the distribution of respiratory chain deficiencies and assessing the degree of morphological abnormalities affecting synapses. Respiratory chain deficiencies detected in Purkinje cell bodies and their synapses and structural synaptic changes are likely to contribute to altered cerebellar circuitry and progression of ataxia.