Review: Central nervous system involvement in mitochondrial disease

Metabolic Determinants of Cerebellar Circuit Formation and Maintenance

Cells configure their metabolism in a synchronized and timely manner to meet their energy demands throughout development and adulthood. Transitions of developmental stages are coupled to metabolic shifts, such that glycolysis is highly active during cell proliferation, whereas oxidative phosphorylation prevails in postmitotic states. In the cerebellum, metabolic transitions are remarkable given its protracted developmental timelines. Such distinctive feature, along with its high neuronal density and metabolic demands, make the cerebellum highly vulnerable to metabolic insults. Despite the expansion of metabolomic approaches to uncover biological mechanisms, little is known about the role of metabolism on cerebellar development and maintenance. To illuminate the intricate connections between metabolism, physiology, and cerebellar disorders, we examined here the impact of metabolism on cerebellar growth, maturation, and adulthood through the lens of inborn errors of metabolism.

Increased expression of SLC25A18 is associated with Alzheimer's disease and is involved in Aβ42-induced mitochondrial dysfunction and apoptosis in neuronal cells

Alzheimer's disease (AD) is currently one of the most serious public health concerns in the world. However, the best approach to treat AD has yet to be discovered, implying that we must continue to work hard to find new AD target genes. In this study, we further analysed Gene Expression Omnibus (GEO) data and discovered that the expression of the Mitochondria glutamate carrier SLC25A18 is associated with AD by screening the differentially expressed genes in different regions of the brains of Alzheimer's disease patients. To verify the expression of SLC25A18 during Alzheimer's disease development, we analysed animal models (5×FAD transgenic AD animal model, chemically induced AD animal model, natural ageing animal model), and the results showed that the expression of SLC25A18 was increased in animal models of AD. Further investigation of the different regions found that SLC25A18 expression was elevated in the EC, TeA, and CA3, and expressed in neurons. Next, We found that Aβ42 treatment elevated SLC25A18 expression in Neuro 2A cells. Reducing SLC25A18 expression attenuated mitochondrial dysfunction and neuronal apoptosis caused by Aβ42. Overexpression of SLC25A18 increased ATP and intracellular superoxide anions but decreased mitochondrial membrane potential. The results indicate that SLC25A18 affects mitochondrial function and neuronal apoptosis, and is related to AD, which makes it a potential target for treating brain dysfunction.

Mechanisms and pathologies of human mitochondrial DNA replication and deletion formation

Human mitochondria possess a multi-copy circular genome, mitochondrial DNA (mtDNA), that is essential for cellular energy metabolism. The number of copies of mtDNA per cell, and their integrity, are maintained by nuclear-encoded mtDNA replication and repair machineries. Aberrant mtDNA replication and mtDNA breakage are believed to cause deletions within mtDNA. The genomic location and breakpoint sequences of these deletions show similar patterns across various inherited and acquired diseases, and are also observed during normal ageing, suggesting a common mechanism of deletion formation. However, an ongoing debate over the mechanism by which mtDNA replicates has made it difficult to develop clear and testable models for how mtDNA rearrangements arise and propagate at a molecular and cellular level. These deletions may impair energy metabolism if present in a high proportion of the mtDNA copies within the cell, and can be seen in primary mitochondrial diseases, either in sporadic cases or caused by autosomal variants in nuclear-encoded mtDNA maintenance genes. These mitochondrial diseases have diverse genetic causes and multiple modes of inheritance, and show notoriously broad clinical heterogeneity with complex tissue specificities, which further makes establishing genotype-phenotype relationships challenging. In this review, we aim to cover our current understanding of how the human mitochondrial genome is replicated, the mechanisms by which mtDNA replication and repair can lead to mtDNA instability in the form of large-scale rearrangements, how rearranged mtDNAs subsequently accumulate within cells, and the pathological consequences when this occurs.

Origins of tissue and cell-type specificity in mitochondrial DNA (mtDNA) disease

Mutations of mitochondrial (mt)DNA are a major cause of morbidity and mortality in humans, accounting for approximately two thirds of diagnosed mitochondrial disease. However, despite significant advances in technology since the discovery of the first disease-causing mtDNA mutations in 1988, the comprehensive diagnosis and treatment of mtDNA disease remains challenging. This is partly due to the highly variable clinical presentation linked to tissue-specific vulnerability that determines which organs are affected. Organ involvement can vary between different mtDNA mutations, and also between patients carrying the same disease-causing variant. The clinical features frequently overlap with other non-mitochondrial diseases, both rare and common, adding to the diagnostic challenge. Building on previous findings, recent technological advances have cast further light on the mechanisms which underpin the organ vulnerability in mtDNA diseases, but our understanding is far from complete. In this review we explore the origins, current knowledge, and future directions of research in this area.

Hydrogen sulfide supplementation as a potential treatment for primary mitochondrial diseases

Primary mitochondrial diseases (PMD) are amongst the most common inborn errors of metabolism causing fatal outcomes within the first decade of life. With marked heterogeneity in both inheritance patterns and physiological manifestations, these conditions present distinct challenges for targeted drug therapy, where effective therapeutic countermeasures remain elusive within the clinic. Hydrogen sulfide (H2S)-based therapeutics may offer a new option for patient treatment, having been proposed as a conserved mitochondrial substrate and post-translational regulator across species, displaying therapeutic effects in age-related mitochondrial dysfunction and neurodegenerative models of mitochondrial disease. H2S can stimulate mitochondrial respiration at sites downstream of common PMD-defective subunits, augmenting energy production, mitochondrial function and reducing cell death. Here, we highlight the primary signalling mechanisms of H2S in mitochondria relevant for PMD and outline key cytoprotective proteins/pathways amenable to post-translational restoration via H2S-mediated persulfidation. The mechanisms proposed here, combined with the advent of potent mitochondria-targeted sulfide delivery molecules, could provide a framework for H2S as a countermeasure for PMD disease progression.

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.

Case report: MELAS and T3271C mitochondrial mutation in an adult woman

Introduction

Patients with mitochondrial disorders always show neurological deficits. However, the diversity of clinical manifestations, genetic heterogeneity and threshold effect caused by maternal heredity make its diagnosis very challenging.

Case presentation

A 30-year-old female presented to our neurology department with a recurrence of symmetrical weakness proximally in the lower extremities. Seven years ago, the patient had a sudden onset of persistent weakness in bilateral proximal lower extremities, along with elevated creatinine kinase (CK) and CK-MB. Given the diagnosis of Guillain-Barre syndrome, she was treated with high-dose glucocorticoid (GC) therapy at the local hospital and recovered. After admission to our hospital, laboratory analysis revealed elevated CK and alpha-hydroxybutyrate dehydrogenase in serum. Electrocardiography showed sinus tachycardia and left high ventricular voltage. Electromyography (EMG) and evoked potential (EP) suggested peripheral neurogenic damage of the upper and lower extremities with myogenic wear. Chronic inflammatory demyelinating polyneuropathy (CIDP) was initially considered, but neurological symptoms were not significantly improved with glucocorticoid shock therapy. An elevated level of lactate was found. The short-tau inversion recovery (STIR) axial magnetic resonance image (MRI) revealed mild hyperintensities, indicating muscle edema. Meanwhile, muscle biopsies suggested pathological changes in mitochondrial disorders (MIDs) and neuronal damage. Further mitochondrial genome analysis revealed a heteroplasmic m3271 T>C mutation in the mitochondrial tRNA-Leu gene (UUR). Collectively, the patient was finally diagnosed with mitochondrial disorder and apparently improved after the corresponding treatment to regulate energy metabolism.

Conclusions

To our knowledge, it's the first report about MELAS with 3271 mutation that have only shown peripheral nerve motion impairment. Proximal weakness is also common in CIDP. In the context of this patient's experience, mitochondrial genome analysis provides an auxiliary criterion for differential diagnosis between MIDs and CIDP. In the meantime, we discussed the clinical effect of GCs on MIDs.

Refractory Hypotension in a Late-Onset Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like Episodes (MELAS) Male with m.3243 A>G Mutation: A Case Report

(1) Introduction: Symptom spectrum can be of great diversity and heterogeneity in mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) patients in clinical practice. Here, we report a case of MELAS presenting asymptomatic refractory hypotension with m.3243 A>G mutation. (2) Case representation: A 51-year-old male patient presented with a headache, vertigo, and difficulty in expression and understanding. The magnetic resonance imaging of the brain revealed an acute stroke-like lesion involving the left temporoparietal lobe. A definitive diagnosis of MELAS was given after the genetic test identified the chrM-3243 A>G mutation. The patient suffered recurrent stroke-like episodes in the 1-year follow-up. Notably, refractory hypotension was observed during hospitalizations, and no significant improvement in blood pressure was found after continuous use of vasopressor drugs and fluid infusion therapy. (3) Conclusions: We report a case of refractory hypotension which was unresponsive to fluid infusion therapy found in a patient with MELAS. Our case suggests that comprehensive management should be paid attention to during treatment. A further study on the pathological mechanism of the multisystem symptoms in MELAS would be beneficial to the treatment of patients.

Clinical and radiological description of 120 pediatric stroke-like episodes

BACKGROUND AND PURPOSE

Stroke-like episodes (SLEs) are defined as acute onset of neurological symptoms mimicking a stroke and radiological lesions non-congruent to vascular territory. We aimed to analyze the acute clinical and radiological features of SLEs to determine their pathophysiology.

METHODS

We performed a monocenter retrospective analysis of 120 SLEs in 60 children over a 20-year period. Inclusion criteria were compatible clinical symptoms and stroke-like lesions on brain magnetic resonance imaging (MRI; performed for all 120 events) with focal hyperintensity on diffusion-weighted imaging in a non-vascular territory.

RESULTS

Three groups were identified: children with mitochondrial diseases (n = 22) involving mitochondrial DNA mutations (55%) or nuclear DNA mutations (45%); those with other metabolic diseases or epilepsy disorders (n = 22); and those in whom no etiology was found despite extensive investigations (n = 16). Age at first SLE was younger in the group with metabolic or epilepsy disorders (18 months vs. 128 months; p < 0.0001) and an infectious trigger was more frequent (69% vs. 20%; p = 0.0001). Seizures occurred in 75% of episodes, revealing 50% episodes of SLEs and mainly leading to status epilepticus (90%). Of the 120 MRI scans confirming the diagnosis, 28 were performed within a short and strict 48-h period and were further analyzed to better understand the underlying mechanisms. The scans showed primary cortical hyperintensity (n = 28/28) with decreased apparent diffusion coefficient in 52% of cases. Systematic hyperperfusion was found on spin labeling sequences when available (n = 18/18).

CONCLUSION

Clinical and radiological results support the existence of a vicious circle based on two main mechanisms: energy deficit and neuronal hyperexcitability at the origin of SLE.

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.

Activating Transcription Factor 4-mediated Mitochondrial Unfolded Protein Response Alleviates Hippocampal Neuronal Damage in an In Vitro Model of Epileptiform Discharges

The mitochondrial unfolded protein response (mtUPR) has been shown to restore protein homeostasis and cell function under stress, and recent studies have confirmed that the activating transcription factor 4 (ATF4) regulates mtUPR. However, the role of ATF4-mediated mtUPR in a hippocampal neuronal culture model of seizures remains unclear. Our results showed that the expression of mtUPR-related proteins (HSP60 and CLpP) increased in primary hippocampal neurons with seizures induced by a magnesium-free solution, suggesting mtUPR activation. Furthermore, ATF4 overexpression by lentiviral vector transfection enhanced the expression of HSP60 and CLpP, whereas ATF4 low expression by lentiviral vector transfection weakened the expression of HSP60 and CLpP. In addition, ATF4 overexpression increased neuronal viability and reduced seizure-induced apoptosis. ATF4 overexpression reduced reactive oxygen species (ROS) production and improved mitochondrial membrane potential damage during seizures. Moreover, ATF4 overexpression reduced the BCL2-associated X protein (Bax) expression and increased the expression of B-cell lymphoma 2 (BCL2). In contrast, ATF4 expression showed the opposite trend. In conclusion, our results showed that ATF4-mediated mtUPR may delay the cascade activation of apoptotic pathways by reducing ROS-mediated oxidative stress, thereby attenuating seizure-induced stress injury.

Potential of Capric Acid in Neurological Disorders: An Overview

To solve the restrictions of a classical ketogenic diet, a modified medium-chain triglyceride diet was introduced which required only around 60% of dietary energy. Capric acid (CA), a small molecule, is one of the main components because its metabolic profile offers itself as an alternate source of energy to the brain in the form of ketone bodies. This is possible with the combined capability of CA to cross the blood-brain barrier and achieve a concentration of 50% concentration in the brain more than any other fatty acid in plasma. Natural sources of CA include vegetable oils such as palm oil and coconut oil, mammalian milk and some seeds. Several studies have shown that CA has varied action on targets that include AMPA receptors, PPAR-γ, inflammatory/oxidative stress pathways and gut dysbiosis. Based on these lines of evidence, CA has proved to be effective in the amelioration of neurological diseases such as epilepsy, affective disorders and Alzheimer's disease. But these studies still warrant more pre-clinical and clinical studies that would further prove its efficacy. Hence, to understand the potential of CA in brain disease and associated comorbid conditions, an advance and rigorous molecular mechanistic study, apart from the reported in-vitro/in-vivo studies, is urgently required for the development of this compound through clinical setups.

Comparing Outcomes of a Discrete Choice Experiment and Case 2 Best-Worst Scaling: An Application to Neuromuscular Disease Treatment

BACKGROUND AND OBJECTIVES

Case 2 best-worst scaling (BWS-2) is an increasingly popular method to elicit patient preferences. Because BWS-2 potentially has a lower cognitive burden compared with discrete choice experiments, the aim of this study was to compare treatment preference weights and relative importance scores.

METHODS

Patients with neuromuscular diseases completed an online survey at two different moments in time, completing one method per occasion. Patients were randomly assigned to either first a discrete choice experiment or BWS-2. Attributes included: muscle strength, energy endurance, balance, cognition, chance of blurry vision, and chance of liver damage. Multinomial logit was used to calculate overall relative importance scores and latent class logit was used to estimate heterogeneous preference weights and to calculate the relative importance scores of the attributes for each latent class.

RESULTS

A total of 140 patients were included for analyses. Overall relative importance scores showed differences in attribute importance rankings between a discrete choice experiment and BWS-2. Latent class analyses indicated three latent classes for both methods, with a specific class in both the discrete choice experiment and BWS-2 in which (avoiding) liver damage was the most important attribute. Ex-post analyses showed that classes differed in sex, age, level of education, and disease status. The discrete choice experiment was easier to understand compared with BWS-2.

CONCLUSIONS

This study showed that using a discrete choice experiment and BWS-2 leads to different outcomes, both in preference weights as well as in relative importance scores, which might have been caused by the different framing of risks in BWS-2. However, a latent class analysis revealed similar latent classes between methods. Careful consideration about method selection is required, while keeping the specific decision context in mind and pilot testing the methods.

Clinical characterization of non-ketotic hyperglycemia-related seizures: A systematic review and individual participant data meta-analysis

New-onset epileptic seizures and status epilepticus (SE) are the most frequent neurological manifestations of non-ketotic hyperglycemia (NKH), an acute complication of diabetes mellitus (DM). Treatment consists of the correction of the underlying metabolic disorder, whereas anti-seizure medications (ASMs) may even worsen seizures. Evidence on NKH-related seizures is currently restricted to case reports and small case-series. We conducted a systematic review of the PubMed, Embase, and Cochrane Library databases to provide a comprehensive description of NKH-related seizures. Statistical analyses were performed to explore possible associations of glycemic and osmolarity levels with clinical variables. We selected 130 publications and 332 patients (186 males, mean age: 61.1 years). DM was newly-diagnosed in 40%. Mean glycemia and osmolarity levels at presentation were 529.7 mg/dL and 309.6 mmol/mol, respectively; 22.6% showed other neurological symptoms besides seizures. Focal motor seizures were the prominent seizure type (49.4%); non-motor focal seizures (23.2%) most commonly manifested as visual symptoms. Reflex seizures occurred in 10.5%. Brain MRI in 48.7% of cases showed focal T2 subcortical hypodensity and/or overlying cortical T2 hyperintensity with DWI restriction. ASMs were administered in 54.2% of cases, achieving seizure control in just 18.3%. Higher osmolarity levels were associated with newly-diagnosed DM (p = 0.002) and other symptoms at presentation (p < 0.001). Glycemic values were higher in patients with focal aware seizures with motor onset compared to those with focal seizures without motor onset (p = 0.0046) or focal seizures with impaired awareness (p = 0.0306). Lower glycemic values were associated with reflex seizures (p = 0.036) and ASM administration (p < 0.001). NKH-related seizures should be suspected in adults with new-onset clustering focal seizures arising from the motor or posterior cortices, even in the absence of a history of DM. Typical focal changes on brain MRI, while not pathognomonic, can drive the clinical diagnosis. Statistical associations suggest a key role of hyperglycemia in the excitability of higher-energy-demanding cortical areas.

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.

Mitochondrial DNA: Consensuses and Controversies

In the course of its short history, mitochondrial DNA (mtDNA) has made a long journey from obscurity to the forefront of research on major biological processes. mtDNA alterations have been found in all major disease groups, and their significance remains the subject of intense research. Despite remarkable progress, our understanding of the major aspects of mtDNA biology, such as its replication, damage, repair, transcription, maintenance, etc., is frustratingly limited. The path to better understanding mtDNA and its role in cells, however, remains torturous and not without errors, which sometimes leave a long trail of controversy behind them. This review aims to provide a brief summary of our current knowledge of mtDNA and highlight some of the controversies that require attention from the mitochondrial research community.

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.

L-arginine in Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like Episodes: A Systematic Review

Background and Objectives

Stroke management in the context of primary mitochondrial disease is clinically challenging, and the best treatment options for patients with stroke-like episodes remain uncertain. We sought to perform a systematic review of the safety and efficacy of l-arginine use in the acute and prophylactic management of stroke-like episodes in patients with mitochondrial disease.

Methods

The systematic review was registered in PROSPERO (CRD42020181230). We searched 6 databases from inception to January 15, 2021: MEDLINE, Embase, Scopus, Web of Science, CINAHL, and ClinicalTrials.gov. Original articles and registered trials available, in English, reporting l-arginine use in the acute or prophylactic management of stroke-like episodes in patients with genetically confirmed mitochondrial disease were eligible for inclusion. Data on safety and treatment response were extracted and summarized by multiple observers. Risk of bias was assessed by the methodologic quality of case reports, case series, and a risk-of-bias checklist for nonrandomized studies. Quality of evidence was synthesized with the Oxford Centre for Evidence-Based Medicine Levels of Evidence and Grade of Recommendations. The predetermined main outcome measures were clinical response to l-arginine treatment, adverse events, withdrawals, and deaths (on treatment and/or during follow-up), as defined by the author.

Results

Thirty-seven articles met inclusion criteria (0 randomized controlled trials; 3 open-label; 1 retrospective cohort; 33 case reports/case series) (N = 91 patients; 86% m.3243A>G). In the case reports, 54% of patients reported a positive clinical response to acute l-arginine, of which 40% were concomitantly treated with antiepileptic drugs. Improved headache at 24 hours was the greatest reported benefit in response to IV l-arginine in the open-label trials (31 of 39, 79%). In 15 of 48 patients (31%) who positively responded to prophylactic l-arginine, antiepileptic drugs were either used (7 of 15) or unreported (8 of 15). Moderate adverse events were reported in the follow-up of both IV and oral l-arginine treatment, and 11 patients (12%) died during follow-up or while on prophylactic treatment.

Discussion

The available evidence is of poor methodologic quality and classified as Level 5. IV and oral l-arginine confers no demonstrable clinical benefit in either the acute or prophylactic treatment of mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes, with more robust controlled trials required to assess its efficacy and safety profile.

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.

The war against Alzheimer, the mitochondrion strikes back!

Alzheimer's disease (AD) is a leading neurodegenerative pathology associated with aging worldwide. It is estimated that AD prevalence will increase from 5.8 million people today to 13.8 million by 2050 in the United States alone. AD effects in the brain are well known; however, there is still a lack of knowledge about the cellular mechanisms behind the origin of AD. It is known that AD induces cellular stress affecting the energy metabolism in brain cells. During the pathophysiological advancement of AD, damaged mitochondria enter a vicious cycle, producing reactive oxygen species (ROS), harming mitochondrial DNA and proteins, leading to more ROS and cellular death. Additionally, mitochondria are interconnected with the plaques formed by amyloid-β in AD and have underlying roles in the progression of the disease and severity. For years, the biomedical field struggled to develop new therapeutic options for AD without a significant advancement. However, mitochondria are striking back existing outside cells in a new mechanism of intercellular communication. Extracellular mitochondria are exchanged from healthy to damaged cells to rescue those with a perturbed metabolism in a process that could be applied as a new therapeutic option to repair those brain cells affected by AD. In this review we highlight key aspects of mitochondria's role in CNS' physiology and neurodegenerative disorders, focusing on AD. We also suggest how mitochondria strikes back as a therapeutic target and as a potential agent to be transplanted to repair neurons affected by AD.

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).

Spinal Cord Involvement in Adult Mitochondrial Diseases: A Cohort Study

The central nervous system is metabolically very demanding and consequently vulnerable to defects of the mitochondrial respiratory chain. While the clinical manifestations and the corresponding radiological findings of the brain involvement in mitochondrial diseases (e.g., stroke-like episodes, signal changes of the basal ganglia, cerebral and cerebellar atrophy) are well known, at present there are few data on the spinal-cord abnormalities in these pathologies, in particular in adult subjects. In this study, we present a cross-sectional cohort study on the prevalence and characterization of spinal-cord involvement in adult patients with genetically defined mitochondrial diseases.

Mitochondrial Strokes: Diagnostic Challenges and Chameleons

Mitochondrial stroke-like episodes (SLEs) are a hallmark of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS). They should be suspected in anyone with an acute/subacute onset of focal neurological symptoms at any age and are usually driven by seizures. Suggestive features of an underlying mitochondrial pathology include evolving MRI lesions, often originating within the posterior brain regions, the presence of multisystemic involvement, including diabetes, deafness, or cardiomyopathy, and a positive family history. The diagnosis of MELAS has important implications for those affected and their relatives, given it enables early initiation of appropriate treatment and genetic counselling. However, the diagnosis is frequently challenging, particularly during the acute phase of an event. We describe four cases of mitochondrial strokes to highlight the considerable overlap that exists with other neurological disorders, including viral and autoimmune encephalitis, ischemic stroke, and central nervous system (CNS) vasculitis, and discuss the clinical, laboratory, and imaging features that can help distinguish MELAS from these differential diagnoses.

Patient Preferences in Rare Diseases: A Qualitative Study in Neuromuscular Disorders to Inform a Quantitative Preference Study

INTRODUCTION

It has become increasingly important to include patient preference information in decision-making processes for drug development. As neuromuscular disorders represent multisystem, debilitating, and progressive rare diseases with few treatment options, this study aimed to explore unmet health care needs and patient treatment preferences for two neuromuscular disorders, myotonic dystrophy type 1 (DM1) and mitochondrial myopathies (MM) to inform early stages of drug development.

METHODS

Fifteen semi-structured interviews and five focus group discussions (FGDs) were held with DM1 and MM adult patients and caregivers. Topics discussed included (1) reasons for study participation; (2) disease signs/symptoms and their impact on daily lives; (3) top desired benefits; and (4) acceptability of risks and tolerance levels for a hypothetical new treatment. Data were analyzed following a thematic 'code' approach.

RESULTS

A total of 52 participants representing a wide range of disease severities participated. 'Muscle strength' and 'energy and endurance' were the disease-related unmet needs most often mentioned. Additionally, improved 'balance', 'cognition' and 'gut function' were the top desired treatment benefits, while 'damage to the liver, kidneys or eyes' was the most concerning risk. Factors influencing their tolerance to risks related to previously having experienced the risk and differentiation between permanent and temporary risks. A few differences were elicited between patients and caregivers.

CONCLUSIONS

This qualitative study provided an open forum to elicit treatment-desired benefits and acceptable risks to be established by patients themselves. These findings can inform decisions for developing new treatments and the design of clinical trials for DM1 and MM.

[11C]PK11195-PET Brain Imaging of the Mitochondrial Translocator Protein in Mitochondrial Disease

OBJECTIVE

To explore the possibilities of radioligands against the mitochondrial outer membrane translocator protein (TSPO) as biomarkers for mitochondrial disease, we performed brain PET-MRI with [11C]PK11195 in 14 patients with genetically confirmed mitochondrial disease and 33 matched controls.

METHODS

Case-control study of brain PET-MRI with the TSPO radioligand [11C]PK11195.

RESULTS

Forty-six percent of symptomatic patients had volumes of abnormal radiotracer binding greater than the 95th percentile in controls. [11C]PK11195 binding was generally greater in gray matter and significantly decreased in white matter. This was most striking in patients with nuclear TYMP or mitochondrial m.3243A>G MT-TL1 mutations, in keeping with differences in mitochondrial density seen postmortem. Some regional binding patterns corresponded to clinical presentation and underlying mutation, even in the absence of structural changes on MRI. This was most obvious for the cerebellum, where patients with ataxia had decreased binding in the cerebellar cortex, but not necessarily volume loss. Overall, there was a positive correlation between aberrant [11C]PK11195 binding and clinical severity.

CONCLUSION

These findings endorse the use of PET imaging with TSPO radioligands as a noninvasive in vivo biomarker of mitochondrial pathology.

CLASSIFICATION OF EVIDENCE

This study provides Class III evidence that brain PET-MRI with TSPO radioligands identifies mitochondrial pathology.

An optimized method for adult zebrafish brain-tissue dissociation that allows access mitochondrial function under healthy and epileptic conditions

Mitochondria play key roles in brain metabolism. Not surprisingly, mitochondria dysfunction is a ubiquitous cause of neurodegenerative diseases. In turn, acquired forms of epilepsy etiology is specifically intriguing since mitochondria function and dysfunction remain not completely enlightened. Investigation in the field includes models of epileptic disorder using mainly rodents followed by mitochondrial function evaluation, which in general evidenced controversial data. So, we considered the efforts and limitations in this research field and we took into account that sample preparation and quality are critical for bioenergetics investigation. For these reasons the aim of the present study was to develop a thorough protocol for adult zebrafish brain-tissue dissociation to evaluate oxygen consumption flux and reach the bioenergetics profile in health and models of epileptic disorder in both, in vitro using pentylenetetrazole (PTZ) and N-methyl-D-Aspartic acid (NMDA), and in vivo after kainic acid (KA)-induced status epilepticus. In conclusion, we verify that fire-polished glass Pasteur pipette is eligible to brain-tissue dissociation and to study mitochondrial function and dysfunction in adult zebrafish. The results give evidence for large effect size in increase of coupling efficiency respiration (p/O₂) correlated to treatment with PTZ and spare respiratory capacity (SRC) in KA-induced model indicating oxidative phosphorylation (OXPHOS) variable alterations. Further investigation is needed in order to clarify the bioenergetics role as well as other mitochondrial functions in epilepsy.

A Novel Homozygous Missense Mutation in the YARS Gene: Expanding the Phenotype of YARS Multisystem Disease

Aminoacyl-tRNA synthetases (ARSs) are crucial enzymes for protein translation. Mutations in genes encoding ARSs are associated with human disease. Tyrosyl-tRNA synthetase is encoded by YARS which is ubiquitously expressed and implicated in an autosomal dominant form of Charcot-Marie-Tooth and autosomal recessive YARS-related multisystem disease.

We report on a former 34-week gestational age male who presented at 2 months of age with failure to thrive (FTT) and cholestatic hepatitis. He was subsequently diagnosed with hyperinsulinemic hypoglycemia with a negative congenital hyperinsulinism gene panel and F-DOPA positron-emission tomography (PET) scan that did not demonstrate a focal lesion. Autopsy findings were notable for overall normal pancreatic islet size and morphology. Trio whole exome sequencing identified a novel homozygous variant of uncertain significance in YARS (c.611A > C, p.Tyr204Cys) with each parent a carrier for the YARS variant. Euglycemia was maintained with diazoxide (max dose, 18 mg/kg/day), and enteral dextrose via gastrostomy tube (G-Tube). During his prolonged hospitalization, the patient developed progressive liver disease, exocrine pancreatic insufficiency, acute renal failure, recurrent infections, ichthyosis, hematologic concerns, hypotonia, and global developmental delay. Such multisystem features have been previously reported in association with pathogenic YARS mutations. Although hypoglycemia has been associated with pathogenic YARS mutations, this report provides more conclusive data that a YARS variant can cause hyperinsulinemic hypoglycemia. This case expands the allelic and clinical heterogeneity of YARS-related disease. In addition, YARS-related disease should be considered in the differential of hyperinsulinemic hypoglycemia associated with multisystem disease.

Anterior visual system imaging to investigate energy failure in multiple sclerosis

Mitochondrial failure and hypoxia are key contributors to multiple sclerosis pathophysiology. Importantly, improving mitochondrial function holds promise as a new therapeutic strategy in multiple sclerosis. Currently, studying mitochondrial changes in multiple sclerosis is hampered by a paucity of non-invasive techniques to investigate mitochondrial function of the CNS in vivo. It is against this backdrop that the anterior visual system provides new avenues for monitoring of mitochondrial changes. The retina and optic nerve are among the metabolically most active structures in the human body and are almost always affected to some degree in multiple sclerosis. Here, we provide an update on emerging technologies that have the potential to indirectly monitor changes of metabolism and mitochondrial function. We report on the promising work with optical coherence tomography, showing structural changes in outer retinal mitochondrial signal bands, and with optical coherence angiography, quantifying retinal perfusion at the microcapillary level. We show that adaptive optics scanning laser ophthalmoscopy can visualize live perfusion through microcapillaries and structural changes at the level of single photoreceptors and neurons. Advantages and limitations of these techniques are summarized with regard to future research into the pathology of the disease and as trial outcome measures.

A study protocol for quantifying patient preferences in neuromuscular disorders: a case study of the IMI PREFER Project

Objectives: Patient preference studies are increasingly used to inform decision-making during the medical product lifecycle but are rarely used to inform early stages of drug development.  The primary aim of this study is to quantify treatment preferences of patients with neuromuscular disorders, which represent serious and debilitating conditions with limited or no treatment options available. Methods: This quantitative patient preferences study was designed as an online survey, with a cross-over design.  This study will target two different diseases from the neuromuscular disorders disease group, myotonic dystrophy type 1 (DM1) and mitochondrial myopathies (MM). Despite having different physio-pathological pathways both DM1 and MM manifest in a clinically similar manner and may benefit from similar treatment options.  The sample will be stratified into three subgroups: two patient groups differentiated by age of symptom onset and one caregivers group.   Each subgroup will be randomly assigned to complete two of three different preference elicitation methods at two different time points: Q-methodology survey, discrete choice experiment, and best-worst scaling type 2, allowing cross-comparisons of the results across each study time within participants and within elicitation methods. Additional variables such as sociodemographic, clinical and health literacy will be collected to enable analysis of potential heterogeneity. Ethics and Dissemination: This study protocol has undergone ethical review and approval by the Newcastle University R&D Ethics Committee (Ref: 15169/2018). All participants will be invited to give electronic informed consent to take part in the study prior accessing the online survey. All electronic data will be anonymised prior analysis. This study is part of the Patient Preferences in Benefit-Risk Assessments during the Drug Life Cycle (IMI-PREFER) project, a public-private collaborative research project aiming to develop expert and evidence-based recommendations on how and when patient preferences can be assessed and used to inform medical product decision making.

Role of Tafazzin in Mitochondrial Function, Development and Disease

Tafazzin, an enzyme associated with the rare inherited x-linked disorder Barth Syndrome, is a nuclear encoded mitochondrial transacylase that is highly conserved across multiple species and plays an important role in mitochondrial function. Numerous studies have elucidated the mechanisms by which Tafazzin affects mitochondrial function, but its effects on development and susceptibility to adult disease are incompletely understood. The purpose of this review is to highlight previous functional studies across a variety of model organisms, introduce recent studies that show an important role in development, and also to provide an update on the role of Tafazzin in human disease. The profound effects of Tafazzin on cardiac development and adult cardiac homeostasis will be emphasized. These studies underscore the importance of mitochondrial function in cardiac development and disease, and also introduce the concept of Tafazzin as a potential therapeutic modality.

Towards Central Nervous System Involvement in Adults with Hereditary Myopathies

There is increasing evidence of central nervous system involvement in numerous neuromuscular disorders primarily considered diseases of skeletal muscle. Our knowledge on cerebral affection in myopathies is expanding continuously due to a better understanding of the genetic background and underlying pathophysiological mechanisms. Intriguingly, there is a remarkable overlap of brain pathology in muscular diseases with pathomechanisms involved in neurodegenerative or neurodevelopmental disorders. A rapid progress in advanced neuroimaging techniques results in further detailed insight into structural and functional cerebral abnormalities. The spectrum of clinical manifestations is broad and includes movement disorders, neurovascular complications, paroxysmal neurological symptoms like migraine and epileptic seizures, but also behavioural abnormalities and cognitive dysfunction. Cerebral involvement implies a high socio-economic and personal burden in adult patients sometimes exceeding the everyday challenges associated with muscle weakness. It is especially important to clarify the nature and natural history of brain affection against the background of upcoming specific treatment regimen in hereditary myopathies that should address the brain as a secondary target. This review aims to highlight the character and extent of central nervous system involvement in patients with hereditary myopathies manifesting in adulthood, however also includes some childhood-onset diseases with brain abnormalities that transfer into adult neurological care.

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.

Loss-of-Function Mutation in Thiamine Transporter 1 in a Family With Autosomal Dominant Diabetes

Solute Carrier Family 19 Member 2 (SLC19A2) encodes thiamine transporter 1 (THTR1), which facilitates thiamine transport across the cell membrane. SLC19A2 homozygous mutations have been described as a cause of thiamine-responsive megaloblastic anemia (TRMA), an autosomal recessive syndrome characterized by megaloblastic anemia, diabetes, and sensorineural deafness. Here we describe a loss-of-function SLC19A2 mutation (c.A1063C: p.Lys355Gln) in a family with early-onset diabetes and mild TRMA traits transmitted in an autosomal dominant fashion. We show that SLC19A2-deficient β-cells are characterized by impaired thiamine uptake, which is not rescued by overexpression of the p.Lys355Gln mutant protein. We further demonstrate that SLC19A2 deficit causes impaired insulin secretion in conjunction with mitochondrial dysfunction, loss of protection against oxidative stress, and cell cycle arrest. These findings link SLC19A2 mutations to autosomal dominant diabetes and suggest a role of SLC19A2 in β-cell function and survival.

Mitochondrial Migraine: Disentangling the angiopathy paradigm in m.3243A>G patients

Migraine, characterized by recurrent attacks of predominantly unilateral throbbing headache, affects approximately 15% of the adult population and is an important cause of disability worldwide. Knowledge required for the development of new classes of antimigraine drugs might come from studying rare metabolic diseases associated with migraine. An illustrative example of a monogenetic disorder associated with migraine is the spectrum of disorders caused by the m.3243A>G mutation in the mitochondrial transfer RNA Leucine. Reported migraine prevalence figures in patients with this particular mutation vary considerably, but compared to the general population, m.3243A>G patients have a higher migraine prevalence. This burdensome symptom might sometimes even be the only clinical feature in maternal relatives carrying the m.3243A>G mutation. Although the exact sequence of events and the relative importance of factors underlying migraine in m.3243A>G MELAS spectrum disorders are still enigmatic, substantial evidence in man exist that dysfunctional mitochondria in both the vascular, the smooth muscle cells and the neuronal system and the interaction between these are at the starting point of the migraine developing pathophysiological cascade. Exclusively based on results of studies performed in patients harboring the m.3243A>G mutation, either in vivo or ex vivo, we here summarize our current understanding of mitochondrial angiopathy associated migraine in m.3243A>G patients which knowledge might lead to potential new avenues for migraine drug development.

What Can Mitochondrial DNA Analysis Tell Us About Mood Disorders?

Variants in mitochondrial DNA (mtDNA) and nuclear genes encoding mitochondrial proteins in bipolar disorder, depression, or other psychiatric disorders have been studied for decades, since mitochondrial dysfunction was first suggested in the brains of patients with these diseases. Candidate gene association studies initially resulted in findings compatible with the mitochondrial dysfunction hypothesis. Many of those studies, however, were conducted with modest sample sizes (N < 1000), which could cause false positive findings. Furthermore, the DNA samples examined in these studies, including genome-wide association studies, were generally derived from peripheral tissues. One key unanswered question is whether there is an association between mood disorders and somatic mtDNA mutations (deletions and point mutations) in brain regions that accumulate a high amount of mtDNA mutations and/or are involved in the regulation of mood. Two lines of robust evidence supporting the importance of mtDNA mutations in brain tissues for mood disorders have come from clinical observation of mitochondrial disease patients who carry primary mtDNA mutations or accumulate secondary mtDNA mutations due to nuclear mutations and an animal model study. More than half of mitochondrial disease patients have comorbid mood disorders, and mice with neuron-specific accumulation of mtDNA mutations show spontaneous depression-like episodes. In this review, we first summarize the current knowledge of mtDNA and its genetics and discuss what mtDNA analysis tells us about neuropsychiatric disorders based on an example of Parkinson's disease. We also discuss challenges and future directions beyond mtDNA analysis toward an understanding of the pathophysiology of "idiopathic" mood disorders.

Expanding the phenotype of SLC25A42-associated mitochondrial encephalomyopathy

SLC25A42 gene encodes an inner mitochondrial membrane protein that imports Coenzyme A into the mitochondrial matrix. A mutation in this gene was recently reported in a subject born to consanguineous parents who presented with mitochondrial myopathy with muscle weakness and lactic acidosis. In this report, we present 12 additional individuals with the same founder mutation who presented with variable manifestations ranging from asymptomatic lactic acidosis to a severe phenotype characterized by developmental regression and epilepsy. Our report confirms the link between SLC25A42 and mitochondrial disease in humans, and suggests that pathogenic variants in SLC25A42 should be interpreted with the understanding that the associated phenotype may be highly variable.

Mitochondrial DNA damage and reactive oxygen species in neurodegenerative disease

Mitochondria are essential organelles within the cell where most ATP is produced through oxidative phosphorylation (OXPHOS). A subset of the genes needed for this process are encoded by the mitochondrial DNA (mtDNA). One consequence of OXPHOS is the production of mitochondrial reactive oxygen species (ROS), whose role in mediating cellular damage, particularly in damaging mtDNA during ageing, has been controversial. There are subsets of neurons that appear to be more sensitive to ROS-induced damage, and mitochondrial dysfunction has been associated with several neurodegenerative disorders. In this review, we will discuss the current knowledge in the field of mtDNA and neurodegeneration, the debate about ROS as a pathological or beneficial contributor to neuronal function, bona fide mtDNA diseases, and insights from mouse models of mtDNA defects affecting the central nervous system.

Neuroprotective Effect of Nortriptyline in Overt Hepatic Encephalopathy Through Attenuation of Mitochondrial Dysfunction

Hyperammonemia associated with overt hepatic encephalopathy (OHE) causes excitotoxic neuronal death through activation of the cytochrome C (CytC)-mediated mitochondria-dependent apoptotic pathway. We tested the therapeutic effect of nortriptyline (NT), a mitochondrial permeability transition pore (mPTP) blocker that can possibly inhibit mitochondrial CytC efflux to the cytosol on in vivo and in vitro OHE models. After ensuring the generation of OHE rats, established by bile duct ligation (BDL), they were intraperitoneally administered either 20 mg/kg NT (i.e., BDL+NT) or another vehicle (i.e., BDL+VEH) for 14 days. Compared with the control, BDL+VEH showed an increment of motor deficits, cell death, synaptic loss, apoptosis, and mitochondria with aberrant morphology in substantia nigra compacta dopaminergic (DA-ergic) neurons. However, the extent was significantly reversed in BDL+NT. Subsequently, we studied the neuroprotective mechanism of NT using PC-12 cells, a DA-ergic cell line, which exposed glutamate used as an excitotoxin. Compared with the control, the cells exposed to 15 mM glutamate (i.e., GLU) showed incremental cell death, apoptosis, and demise in mitochondrial respiration. Importantly, efflux of CytC from mitochondria to cytosol and the dissipation of mitochondrial membrane potential (△Ψm), an indicator of mPTP opening, were prominent in GLU. However, compared with the GLU, the cells cotreated with 10 μM NT (i.e., GLU+NT) showed a significant reduction in the aforementioned phenomenon. Together, we concluded that NT can be used for OHE therapeutics, mitigating the excitotoxic death of substantia nigra compacta DA-ergic neurons via mPTP-associated mitochondrial dysfunction inhibition.

mPOS is a novel mitochondrial trigger of cell death - implications for neurodegeneration

In addition to its central role in energy metabolism, the mitochondrion has many other functions essential for cell survival. When stressed, the multifunctional mitochondria are expected to engender multifaceted cell stress with complex physiological consequences. Potential extra-mitochondrial proteostatic burdens imposed by inefficient protein import have been largely overlooked. Accumulating evidence suggests that a diverse range of pathogenic mitochondrial stressors, which do not directly target the core protein import machinery, can reduce cell fitness by disrupting the proteostatic network in the cytosol. The resulting stress, named mitochondrial precursor overaccumulation stress (mPOS), is characterized by the toxic accumulation of unimported mitochondrial proteins in the cytosol. Here, we review our current understanding of how mitochondrial dysfunction can impact the cytosolic proteome and proteostatic signaling. We also discuss the intriguing possibility that the mPOS model may help untangle the cause-effect relationship between mitochondrial dysfunction and cytosolic protein aggregation, which are probably the two most prominent molecular hallmarks of neurodegenerative disease.

Inherited mitochondrial genomic instability and chemical exposures

There are approximately 1500 proteins that are needed for mitochondrial structure and function, most of which are encoded in the nuclear genome (Calvo et al., 2006). Each mitochondrion has its own genome (mtDNA), which in humans encodes 13 polypeptides, 22 tRNAs and 2 rRNAs required for oxidative phosphorylation. The mitochondrial genome of humans and most vertebrates is approximately 16.5kbp, double-stranded, circular, with few non-coding bases. Thus, maintaining mtDNA stability, that is, the ability of the cell to maintain adequate levels of mtDNA template for oxidative phosphorylation is essential and can be impacted by the level of mtDNA mutation currently within the cell or mitochondrion, but also from errors made during normal mtDNA replication, defects in mitochondrial quality control mechanisms, and exacerbated by exposures to exogenous and/or endogenous genotoxic agents. In this review, we expand on the origins and consequences of mtDNA instability, the current state of research regarding the mechanisms by which mtDNA instability can be overcome by cellular and chemical interventions, and the future of research and treatments for mtDNA instability.

Mitochondrial homeostatic disruptions are sensitive indicators of stress in neurons with defective mitochondrial DNA transactions

Neurodegeneration and mitochondrial dysfunction are closely linked across many clinical conditions. In genetic diseases that result from defects in mitochondrial DNA (mtDNA) synthesis or maintenance, neurodegeneration is a frequent and major component of the disease pathology. In sporadic neurodegenerative diseases such as Alzheimer's and Parkinson's disease, mtDNA defects have been observed clinically. Mitochondrial stress related to mtDNA dysregulation can produce neuronal dysfunction and death via impaired electron transport chain activity, which results in deficient ATP production and related increases in mitochondrial reactive oxygen species (ROS) production. However, mtDNA dysregulation in post-mitotic neurons may also produce disturbances in mitochondrial homeostasis that are known to impair neuronal function as well. In this study, we used sub-toxic doses of ethidium bromide (EtBr) to induce mtDNA-associated mitochondrial stress in primary cortical neurons and measured several aspects of mitochondrial homeostasis, mitochondrial function and cell death. We found that low-dose EtBr severely depletes mtDNA synthesis and mitochondrial mRNA levels. Furthermore, homeostatic processes are especially disrupted in toxin treated neurons while mitochondrial function is relatively preserved. Mitochondria become fragmented and motility is abolished, while respiration and mitochondrial polarization are partially maintained. Moreover at these doses, cells do not exhibit increased ROS production, clear neurite retraction or loss of viability. These results indicate that mitochondrial homeostasis is a sensitive marker of mtDNA associated stress compared to mitochondria-functional outputs or endpoints related to cellular toxicity. These homeostatic disruptions are expected to contribute to neuronal dysfunction and potentially drive neurodegenerative disease pathology.

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.