Mitochondrial disorders in children: toward development of small-molecule treatment strategies

Pathways controlling neurotoxicity and proteostasis in mitochondrial complex I deficiency

Neuromuscular disorders caused by dysfunction of the mitochondrial respiratory chain are common, severe and untreatable. We recovered a number of mitochondrial genes, including electron transport chain components, in a large forward genetic screen for mutations causing age-related neurodegeneration in the context of proteostasis dysfunction. We created a model of complex I deficiency in the Drosophila retina to probe the role of protein degradation abnormalities in mitochondrial encephalomyopathies. Using our genetic model, we found that complex I deficiency regulates both the ubiquitin/proteasome and autophagy/lysosome arms of the proteostasis machinery. We further performed an in vivo kinome screen to uncover new and potentially druggable mechanisms contributing to complex I related neurodegeneration and proteostasis failure. Reduction of RIOK kinases and the innate immune signaling kinase pelle prevented neurodegeneration in complex I deficiency animals. Genetically targeting oxidative stress, but not RIOK1 or pelle knockdown, normalized proteostasis markers. Our findings outline distinct pathways controlling neurodegeneration and protein degradation in complex I deficiency and introduce an experimentally facile model in which to study these debilitating and currently treatment-refractory disorders.

Mitochondrial Stress and Mitokines: Therapeutic Perspectives for the Treatment of Metabolic Diseases

Mitochondrial stress and the dysregulated mitochondrial unfolded protein response (UPRmt) are linked to various diseases, including metabolic disorders, neurodegenerative diseases, and cancer. Mitokines, signaling molecules released by mitochondrial stress response and UPRmt, are crucial mediators of inter-organ communication and influence systemic metabolic and physiological processes. In this review, we provide a comprehensive overview of mitokines, including their regulation by exercise and lifestyle interventions and their implications for various diseases. The endocrine actions of mitokines related to mitochondrial stress and adaptations are highlighted, specifically the broad functions of fibroblast growth factor 21 and growth differentiation factor 15, as well as their specific actions in regulating inter-tissue communication and metabolic homeostasis. Finally, we discuss the potential of physiological and genetic interventions to reduce the hazards associated with dysregulated mitokine signaling and preserve an equilibrium in mitochondrial stress-induced responses. This review provides valuable insights into the mechanisms underlying mitochondrial regulation of health and disease by exploring mitokine interactions and their regulation, which will facilitate the development of targeted therapies and personalized interventions to improve health outcomes and quality of life.

Mitochondrial diseases in Hong Kong: prevalence, clinical characteristics and genetic landscape

OBJECTIVE

To determine the prevalence of mitochondrial diseases (MD) in Hong Kong (HK) and to evaluate the clinical characteristics and genetic landscape of MD patients in the region.

METHODS

This study retrospectively reviewed the phenotypic and molecular characteristics of MD patients from participating public hospitals in HK between January 1985 to October 2020. Molecularly and/or enzymatically confirmed MD cases of any age were recruited via the Clinical Analysis and Reporting System (CDARS) using relevant keywords and/or International Classification of Disease (ICD) codes under the HK Hospital Authority or through the personal recollection of treating clinicians among the investigators.

RESULTS

A total of 119 MD patients were recruited and analyzed in the study. The point prevalence of MD in HK was 1.02 in 100,000 people (95% confidence interval 0.81-1.28 in 100,000). 110 patients had molecularly proven MD and the other nine were diagnosed by OXPHOS enzymology analysis or mitochondrial DNA depletion analysis with unknown molecular basis. Pathogenic variants in the mitochondrial genome (72 patients) were more prevalent than those in the nuclear genome (38 patients) in our cohort. The most commonly involved organ system at disease onset was the neurological system, in which developmental delay, seizures or epilepsy, and stroke-like episodes were the most frequently reported presentations. The mortality rate in our cohort was 37%.

CONCLUSION

This study is a territory-wide overview of the clinical and genetic characteristics of MD patients in a Chinese population, providing the first available prevalence rate of MD in Hong Kong. The findings of this study aim to facilitate future in-depth evaluation of MD and lay the foundation to establish a local MD registry.

ENT characteristics and therapeutic results in multisystemic disorders of mitochondrial encephalomyopathy

Here we report the evaluation of the frequency of subjective and objective otolaryngologic findings and therapeutic results in 32 patients with mitochondrial encephalomyopathy (MEM) from September 2001 to June 2021. Our analysis included studying the patients’ family histories, the clinical manifestations of MEM, and the therapeutic effects of treatments. The patients’ ages ranged from 2 to 77 years, with a median age of 12.3 years. We found that MEM ENT symptoms were characterized by hearing loss, dysphagia, and facial weakness. Most cases of sensorineural hearing loss were bilateral symmetrical progressive or sudden deafness since adolescence, which were often underestimated. Associated neuromuscular symptoms required mtDNA testing. Dysphagia and facial weakness occurred preferentially in middle-aged patients, and muscle biopsies were advised. Distortion product otoacoustic emissions and auditory brainstem responsetesting were more sensitive and reliable than pure tone averages for objective monitoring of pathogenesis. Administration of the mitochondrial synthase complex benefited patients with acute episodes. If patients did not fully recover and exhibitedresidual language deficits, hearing aids or cochlear implants were recommended. Counsel was given regarding synthetical treatments for facial weakness, endoscopic circopharyngealmyotomy for dysphagia, and surgical correction of ptosis. This study demonstrates that increased awareness of these symptoms is important to address appropriate interventions and avoid complications such as ablepsia, aphasia, social isolation, malnutrition, aspiration pneumonia, and heart failure in the setting of MEM.

The role of mitochondria in the pathogenesis of Kawasaki disease

Kawasaki disease is a systemic vasculitis, especially of the coronary arteries, affecting children. Despite extensive research, much is still unknown about the principal driver behind the amplified inflammatory response. We propose mitochondria may play a critical role. Mitochondria serve as a central hub, influencing energy generation, cell proliferation, and bioenergetics. Regulation of these biological processes, however, comes at a price. Release of mitochondrial DNA into the cytoplasm acts as damage-associated molecular patterns, initiating the development of inflammation. As a source of reactive oxygen species, they facilitate activation of the NLRP3 inflammasome. Kawasaki disease involves many of these inflammatory pathways. Progressive mitochondrial dysfunction alters the activity of immune cells and may play a role in the pathogenesis of Kawasaki disease. Because they contain their own genome, mitochondria are susceptible to mutation which can propagate their dysfunction and immunostimulatory potential. Population-specific variants in mitochondrial DNA have also been linked to racial disparities in disease risk and treatment response. Our objective is to critically examine the current literature of mitochondria’s role in coordinating proinflammatory signaling pathways, focusing on potential mitochondrial dysfunction in Kawasaki disease. No association between impaired mitochondrial function and Kawasaki disease exists, but we suggest a relationship between the two. We hypothesize a framework of mitochondrial determinants that may contribute to ethnic/racial disparities in the progression of Kawasaki disease.

The reliability and validity of the perceive, recall, plan and perform assessment in children with a mitochondrial disorder

PURPOSE

To investigate the reliability and validity of the Perceive, Recall, Plan and Perform System of Task Analysis (PRPP-Assessment) by parent-provided videos of children with mitochondrial disorders.

METHODS

Videos of 13 children performing 3-7 activities each were the subject of study, resulting in 65 activities. These were scored using the PRPP-Assessment by 11 raters. Internal consistency was calculated with Cronbach's alpha. Intra-rater reliability was evaluated by Bland-Altman Plots on 44 PRPP-Assessment scores. Inter-rater reliability was calculated with ICC_Agreement on 128 PRPP-Assessment scores. Construct validity was assessed by comparing the PRPP-Assessment scores to the Canadian Occupational Performance Measure using Cohen's Kappa. PRPP-Assessments scores were evaluated with a multi-faceted Rasch Analysis.

RESULTS

Internal consistency was high (0.92). Intra-rater reliability was sufficient to be good (92-96% within the 95%-Limits of the Agreement). The ICC_Agreement for stage 1 Mastery Score showed acceptable inter-rater reliability (0.646). Stage 2 of the PRPP-Assessment showed low ICCs due to a lack of variability within the sample. Four out of six hypotheses on validity were accepted. Rasch's analysis demonstrated sound goodness-of-fit, and supported the validity of the PRPP-Assessment.

CONCLUSION

The PRPP-Assessment by parent-provided videos in this heterogenic group showed sufficient to good psychometric properties. In practice, careful task selection and formulating criterion is recommended.Implications for RehabilitationPRPP-Assessment by parent-provided videos is reliable and valid in the complex, heterogenous group of children with mitochondrial disordersThe PRPP-Assessment is suitable for children with mitochondrial disorders as it showed to contain familiar, functional and meaningful tasks and activities that fit with their level of functioningProfessionals should be aware that parents might not be used to the criterium-based frame of reference of the PRPP-AssessmentWhen applying the PRPP-Assessment, it is recommended to be careful in task selection and formulating the criterion.

Application of Neural Network in the Stability of Biped Robot and Embedded Control of Walking Mode

The biped robot adopts the human movement mode. Compared with other movement modes, the gait has good flexibility and adaptability. It is very important in the research of robotics, so it has become a hot spot of robotics research. This article aims to study the application of neural networks in the stability of biped robots and the embedded control of walking mode. A method of establishing precise mathematical modeling and stability analysis is proposed. Based on this model, the motion characteristics of the biped robot’s walking mode and the local stability of joints are studied, and the motion mode of passive walking under the control of the neural network is deeply analyzed, using a neural network to control the stability of biped robot motion and adopting the research method of the embedded control system in walking mode. Essentially, the output value of the physical network is used to judge whether the robot is in a stable position so as to perform appropriate actions and control the robot’s stability of walking. The experimental results show that the biped robot can detect movement and overcome obstacles through related networks and embedded control systems. Through the control of the embedded system, the errors of each joint of the biped robot on flat ground, stairs, and obstacles are greatly reduced. The most obvious reduction of the deviation is that the ankle joint decreases from 2.5 to 0.07 when rotating, and the knee joint angle deviation is reduced from 3.8 to 2, which greatly improves the stability of the biped robot’s walking mode.

SGPL1 stimulates VPS39 recruitment to the mitochondria in MICU1 deficient cells

Objective

Mitochondrial “retrograde” signaling may stimulate organelle biogenesis as a compensatory adaptation to aberrant activity of the oxidative phosphorylation (OXPHOS) system. To maintain energy-consuming processes in OXPHOS deficient cells, alternative metabolic pathways are functionally coupled to the degradation, recycling and redistribution of biomolecules across distinct intracellular compartments. While transcriptional regulation of mitochondrial network expansion has been the focus of many studies, the molecular mechanisms promoting mitochondrial maintenance in energy-deprived cells remain poorly investigated.

Methods

We performed transcriptomics, quantitative proteomics and lifespan assays to identify pathways that are mechanistically linked to mitochondrial network expansion and homeostasis in Caenorhabditis elegans lacking the mitochondrial calcium uptake protein 1 (MICU-1/MICU1). To support our findings, we carried out biochemical and image analyses in mammalian cells and mouse-derived tissues.

Results

We report that micu-1(null) mutations impair the OXPHOS system and promote C. elegans longevity through a transcriptional program that is independent of the mitochondrial calcium uniporter MCU-1/MCU and the essential MCU regulator EMRE-1/EMRE. We identify sphingosine phosphate lyase SPL-1/SGPL1 and the ATFS-1-target HOPS complex subunit VPS-39/VPS39 as critical lifespan modulators of micu-1(null) mutant animals. Cross-species investigation indicates that SGPL1 upregulation stimulates VPS39 recruitment to the mitochondria, thereby enhancing mitochondria-lysosome contacts. Consistently, VPS39 downregulation compromises mitochondrial network maintenance and basal autophagic flux in MICU1 deficient cells. In mouse-derived muscles, we show that VPS39 recruitment to the mitochondria may represent a common signature associated with altered OXPHOS system.

Conclusions

Our findings reveal a previously unrecognized SGPL1/VPS39 axis that stimulates intracellular organelle interactions and sustains autophagy and mitochondrial homeostasis in OXPHOS deficient cells.

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micu-1(null) nematodes are long-lived mitochondrial mutants.

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MICU-1/MICU1 deficiency stimulates VPS-39/VPS39 and SPL-1/SGPL1 upregulation.

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VPS-39 sustains mitochondrial network expansion in micu-1(null) nematodes.

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VPS39 and SGPL1 expression influences mitochondria-lysosome contact sites in MICU1 deficient cells.

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VPS39/SGPL1 signaling may be a common signature of mitochondrial deficient cells.

Ndufs4 knockout mouse models of Leigh syndrome: pathophysiology and intervention

Mitochondria are small cellular constituents that generate cellular energy (ATP) by oxidative phosphorylation (OXPHOS). Dysfunction of these organelles is linked to a heterogeneous group of multisystemic disorders, including diabetes, cancer, ageing-related pathologies and rare mitochondrial diseases. With respect to the latter, mutations in subunit-encoding genes and assembly factors of the first OXPHOS complex (complex I) induce isolated complex I deficiency and Leigh syndrome. This syndrome is an early-onset, often fatal, encephalopathy with a variable clinical presentation and poor prognosis due to the lack of effective intervention strategies. Mutations in the nuclear DNA-encoded NDUFS4 gene, encoding the NADH:ubiquinone oxidoreductase subunit S4 (NDUFS4) of complex I, induce 'mitochondrial complex I deficiency, nuclear type 1' (MC1DN1) and Leigh syndrome in paediatric patients. A variety of (tissue-specific) Ndufs4 knockout mouse models were developed to study the Leigh syndrome pathomechanism and intervention testing. Here, we review and discuss the role of complex I and NDUFS4 mutations in human mitochondrial disease, and review how the analysis of Ndufs4 knockout mouse models has generated new insights into the MC1ND1/Leigh syndrome pathomechanism and its therapeutic targeting.

Ndufs4 knockout mouse models of Leigh syndrome: pathophysiology and intervention

Mitochondria are small cellular constituents that generate cellular energy (ATP) by oxidative phosphorylation (OXPHOS). Dysfunction of these organelles is linked to a heterogeneous group of multisystemic disorders, including diabetes, cancer, ageing-related pathologies and rare mitochondrial diseases. With respect to the latter, mutations in subunit-encoding genes and assembly factors of the first OXPHOS complex (complex I) induce isolated complex I deficiency and Leigh syndrome. This syndrome is an early-onset, often fatal, encephalopathy with a variable clinical presentation and poor prognosis due to the lack of effective intervention strategies. Mutations in the nuclear DNA-encoded NDUFS4 gene, encoding the NADH:ubiquinone oxidoreductase subunit S4 (NDUFS4) of complex I, induce ‘mitochondrial complex I deficiency, nuclear type 1’ (MC1DN1) and Leigh syndrome in paediatric patients. A variety of (tissue-specific) Ndufs4 knockout mouse models were developed to study the Leigh syndrome pathomechanism and intervention testing.

Here, we review and discuss the role of complex I and NDUFS4 mutations in human mitochondrial disease, and review how the analysis of Ndufs4 knockout mouse models has generated new insights into the MC1ND1/Leigh syndrome pathomechanism and its therapeutic targeting.

The ketogenic diet as a therapeutic intervention strategy in mitochondrial disease

Classical mitochondrial disease (MD) represents a group of complex metabolic syndromes primarily linked to dysfunction of the mitochondrial ATP-generating oxidative phosphorylation (OXPHOS) system. To date, effective therapies for these diseases are lacking. Here we discuss the ketogenic diet (KD), being a high-fat, moderate protein, and low carbohydrate diet, as a potential intervention strategy. We concisely review the impact of the KD on bioenergetics, ROS/redox metabolism, mitochondrial dynamics and mitophagy. Next, the consequences of the KD in (models of) MD, as well as KD adverse effects, are described. It is concluded that the current experimental evidence suggests that the KD can positively impact on mitochondrial bioenergetics, mitochondrial ROS/redox metabolism and mitochondrial dynamics. However, more information is required on the bioenergetic consequences and mechanistic mode-of-action aspects of the KD at the cellular level and in MD patients.

PGC1s and Beyond: Disentangling the Complex Regulation of Mitochondrial and Cellular Metabolism

Metabolism is the central engine of living organisms as it provides energy and building blocks for many essential components of each cell, which are required for specific functions in different tissues. Mitochondria are the main site for energy production in living organisms and they also provide intermediate metabolites required for the synthesis of other biologically relevant molecules. Such cellular processes are finely tuned at different levels, including allosteric regulation, posttranslational modifications, and transcription of genes encoding key proteins in metabolic pathways. Peroxisome proliferator activated receptor γ coactivator 1 (PGC1) proteins are transcriptional coactivators involved in the regulation of many cellular processes, mostly ascribable to metabolic pathways. Here, we will discuss some aspects of the cellular processes regulated by PGC1s, bringing up some examples of their role in mitochondrial and cellular metabolism, and how metabolic regulation in mitochondria by members of the PGC1 family affects the immune system. We will analyze how PGC1 proteins are regulated at the transcriptional and posttranslational level and will also examine other regulators of mitochondrial metabolism and the related cellular functions, considering approaches to identify novel mitochondrial regulators and their role in physiology and disease. Finally, we will analyze possible therapeutical perspectives currently under assessment that are applicable to different disease states.

Comparative analysis of CI- and CIV-containing respiratory supercomplexes at single-cell resolution

Mitochondria sustain the energy demand of the cell. The composition and functional state of the mitochondrial oxidative phosphorylation system are informative indicators of organelle bioenergetic capacity. Here, we describe a highly sensitive and reproducible method for a single-cell quantification of mitochondrial CI- and CIV-containing respiratory supercomplexes (CI∗CIV-SCs) as an alternative means of assessing mitochondrial respiratory chain integrity. We apply a proximity ligation assay (PLA) and stain CI∗CIV-SCs in fixed human and mouse brains, tumorigenic cells, induced pluripotent stem cells (iPSCs) and iPSC-derived neural precursor cells (NPCs), and neurons. Spatial visualization of CI∗CIV-SCs enables the detection of mitochondrial lesions in various experimental models, including complex tissues undergoing degenerative processes. We report that comparative assessments of CI∗CIV-SCs facilitate the quantitative profiling of even subtle mitochondrial variations by overcoming the confounding effects that mixed cell populations have on other measurements. Together, our PLA-based analysis of CI∗CIV-SCs is a sensitive and complementary technique for detecting cell-type-specific mitochondrial perturbations in fixed materials.

Mitochondrial changes associated with viral infectious diseases in the paediatric population

Infectious diseases occur worldwide with great frequency in both adults and children, causing 350,000 deaths in 2017, according to the latest World Health Organization reports. Both infections and their treatments trigger mitochondrial interactions at multiple levels: (i) incorporation of damaged or mutated proteins into the complexes of the electron transport chain; (ii) impact on mitochondrial genome (depletion, deletions and point mutations) and mitochondrial dynamics (fusion and fission); (iii) membrane potential impairment; (iv) apoptotic regulation; and (v) generation of reactive oxygen species, among others. Such alterations may result in serious adverse clinical events with considerable impact on the quality of life of the children and could even cause death. Herein, we use a systematic review to explore the association between mitochondrial alterations in paediatric infections including human immunodeficiency virus, cytomegalovirus, herpes viruses, various forms of hepatitis, adenovirus, T-cell lymphotropic virus and influenza. We analyse how these paediatric viral infectious processes may cause mitochondrial deterioration in this especially vulnerable population, with consideration for the principal aspects of research and diagnosis leading to improved disease understanding, management and surveillance.

A Mitocentric View of the Main Bacterial and Parasitic Infectious Diseases in the Pediatric Population

Infectious diseases occur worldwide with great frequency in both adults and children. Both infections and their treatments trigger mitochondrial interactions at multiple levels: (i) incorporation of damaged or mutated proteins to the complexes of the electron transport chain, (ii) mitochondrial genome (depletion, deletions, and point mutations) and mitochondrial dynamics (fusion and fission), (iii) membrane potential, (iv) apoptotic regulation, (v) generation of reactive oxygen species, among others. Such alterations may result in serious adverse clinical events with great impact on children's quality of life, even resulting in death. As such, bacterial agents are frequently associated with loss of mitochondrial membrane potential and cytochrome c release, ultimately leading to mitochondrial apoptosis by activation of caspases-3 and -9. Using Rayyan QCRI software for systematic reviews, we explore the association between mitochondrial alterations and pediatric infections including (i) bacterial: M. tuberculosis, E. cloacae, P. mirabilis, E. coli, S. enterica, S. aureus, S. pneumoniae, N. meningitidis and (ii) parasitic: P. falciparum. We analyze how these pediatric infections and their treatments may lead to mitochondrial deterioration in this especially vulnerable population, with the intention of improving both the understanding of these diseases and their management in clinical practice.

Is there a special relationship between complex I activity and nigral neuronal loss in Parkinson's disease? A critical reappraisal

Parkinson’s disease (PD) is a progressive neurodegenerative disease manifesting both motor and non-motor symptoms. The motor features are generally ascribed to the selective loss of dopamine neurons within the substantia nigra pars compacta. While the precise etiology of PD remains elusive, multiple genetic and environmental elements have emerged as contributing factors. The discovery of MPTP-induced parkinsonism directed intense inquiry towards mitochondrial pathways, with a specific focus on mitochondrial complex I. Consisting of more than 40 subunits, complex I is the first enzyme of the electron transport chain that is required for mitochondrial ATP production. In this review, we present a critical analysis of studies assessing the prevalence and specificity of mitochondrial complex I deficiency in PD. In addition, we take the novel view of incorporating the features of genetically-defined bona fide complex I disorders and the prevalence of nigral involvement in such cases. Through this innovative bi-directional view, we consider both complex I changes in a disease of the substantia nigra and nigral changes in diseases of complex I. We assess the strength of association between nigral cell loss and complex I deficits, as well as the oft under-appreciated heterogeneity of complex I deficiency disorders and the variability of the PD data.

The mitochondrial unfolded protein response and its diverse roles in cellular stress

Mitochondrial function is centrally involved in many cellular processes, such as energy production, metabolism of nucleotides, amino acids, and lipids, calcium buffering, and regulation of cell death. Multiple mechanisms are engaged under conditions of mitochondrial dysfunction to restore cellular and, subsequently, systemic functions. The mitochondrial unfolded protein response is a homeostatic mechanism that has attracted a lot of interest recently and has been described in several organisms, including humans. The mitochondrial unfolded protein response serves as a first-line-of-defence mechanism against stress to restore mitochondrial proteostasis and functions. Here, we discuss the canonical mechanisms via which the mitochondrial unfolded protein response is activated under stress and examine recent evidence that links the response with other processes that promote survival and the recovery of the mitochondrial network (i.e. the integrated stress response and mitophagy).

Pioglitazone and Deoxyribonucleoside Combination Treatment Increases Mitochondrial Respiratory Capacity in m.3243A>G MELAS Cybrid Cells

The lack of effective treatments for mitochondrial disease has seen the development of new approaches, including those that aim to stimulate mitochondrial biogenesis to boost ATP generation above a critical disease threshold. Here, we examine the effects of the peroxisome proliferator-activated receptor γ (PPARγ) activator pioglitazone (PioG), in combination with deoxyribonucleosides (dNs), on mitochondrial biogenesis in cybrid cells containing >90% of the m.3243A>G mutation associated with mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS). PioG + dNs combination treatment increased mtDNA copy number and mitochondrial mass in both control (CON) and m.3243A>G (MUT) cybrids, with no adverse effects on cell proliferation. PioG + dNs also increased mtDNA-encoded transcripts in CON cybrids, but had the opposite effect in MUT cybrids, reducing the already elevated transcript levels. Steady-state levels of mature oxidative phosphorylation (OXPHOS) protein complexes were increased by PioG + dNs treatment in CON cybrids, but were unchanged in MUT cybrids. However, treatment was able to significantly increase maximal mitochondrial oxygen consumption rates and cell respiratory control ratios in both CON and MUT cybrids. Overall, these findings highlight the ability of PioG + dNs to improve mitochondrial respiratory function in cybrid cells containing the m.3243A>G MELAS mutation, as well as their potential for development into novel therapies to treat mitochondrial disease.

Downregulation of respiratory complex I mediates major signalling changes triggered by TOR activation

Mitochondrial dysfunctions belong amongst the most common metabolic diseases but the signalling networks that lead to the manifestation of a disease phenotype are often not well understood. We identified the subunits of respiratory complex I, III and IV as mediators of major signalling changes during Drosophila wing disc development. Their downregulation in larval wing disc leads to robust stimulation of TOR activity, which in turn orchestrates a complex downstream signalling network. Specifically, after downregulation of the complex I subunit ND-49 (mammalian NDUFS2), TOR activates JNK to induce cell death and ROS production essential for the stimulation of compensatory apoptosis-induced proliferation within the tissue. Additionally, TOR upregulates Notch and JAK/STAT signalling and it directs glycolytic switch of the target tissue. Our results highlight the central role of TOR signalling in mediating the complex response to mitochondrial respiratory dysfunction and they provide a rationale why the disease symptoms associated with respiratory dysfunctions are often alleviated by mTOR inhibitors.

Effects of clofibrate and KH176 on life span and motor function in mitochondrial complex I-deficient mice

Mitochondrial complex I (CI), the first multiprotein enzyme complex of the OXPHOS system, executes a major role in cellular ATP generation. Consequently, dysfunction of this complex has been linked to inherited metabolic disorders, including Leigh disease (LD), an often fatal disease in early life. Development of clinical effective treatments for LD remains challenging due to the complex pathophysiological nature. Treatment with the peroxisome proliferation-activated receptor (PPAR) agonist bezafibrate improved disease phenotype in several mitochondrial disease mouse models mediated via enhanced mitochondrial biogenesis and fatty acid β-oxidation. However, the therapeutic potential of this mixed PPAR (α, δ/β, γ) agonist is severely hampered by hepatotoxicity, which is possibly caused by activation of PPARγ. Here, we aimed to investigate the effects of the PPARα-specific fibrate clofibrate in mitochondrial CI-deficient (Ndufs4^-/-) mice. Clofibrate increased lifespan and motor function of Ndufs4^-/- mice, while only marginal hepatotoxic effects were observed. Due to the complex clinical and cellular phenotype of CI-deficiency, we also aimed to investigate the therapeutic potential of clofibrate combined with the redox modulator KH176. As described previously, single treatment with KH176 was beneficial, however, combining clofibrate with KH176 did not result in an additive effect on disease phenotype in Ndufs4^-/- mice. Overall, both drugs have promising, but independent and nonadditive, properties for the pharmacological treatment of CI-deficiency-related mitochondrial diseases.

Chlamydomonas reinhardtii as a plant model system to study mitochondrial complex I dysfunction

Mitochondrial complex I, a proton-pumping NADH: ubiquinone oxidoreductase, is required for oxidative phosphorylation. However, the contribution of several human mutations to complex I deficiency is poorly understood. The unicellular alga Chlamydomonas reinhardtii was utilized to study complex I as, unlike in mammals, mutants with complete loss of the holoenzyme are viable. From a forward genetic screen for complex I-deficient insertional mutants, six mutants exhibiting complex I deficiency with assembly defects were isolated. Chlamydomonas mutants isolated from our screens, lacking the subunits NDUFV2 and NDUFB10, were used to reconstruct and analyze the effect of two human mutations in these subunit-encoding genes. The K209R substitution in NDUFV2, reported in Parkinson's disease patients, did not significantly affect the enzyme activity or assembly. The C107S substitution in the NDUFB10 subunit, reported in a case of fatal infantile cardiomyopathy, is part of a conserved C-(X)11-C motif. The cysteine substitutions, at either one or both positions, still allowed low levels of holoenzyme formation, indicating that this motif is crucial for complex I function but not strictly essential for assembly. We show that the algal mutants provide a simple and useful platform to delineate the consequences of patient mutations on complex I function.

Cell models and drug discovery for mitochondrial diseases

Mitochondrion is a semi-autonomous organelle, important for cell energy metabolism, apoptosis, the production of reactive oxygen species (ROS), and Ca²⁺ homeostasis. Mitochondrial DNA (mtDNA) mutation is one of the primary factors in mitochondrial disorders. Though much progress has been made, there remain many difficulties in constructing cell models for mitochondrial diseases. This seriously restricts studies related to targeted drug discovery and the mechanism and therapy for such diseases. Here we summarize the characteristics of patient-specific immortalized lymphoblastoid cells, fibroblastoid cells, cytoplasmic hybrid (cybrid) cell lines, and induced pluripotent stem cells (iPSCs)-derived differentiation cells in the study of mitochondrial disorders, as well as offering discussion of roles and advances of these cell models, particularly in the screening of drugs.

Advances Towards Therapeutic Approaches for mtDNA Disease

Mitochondria maintain and express their own genome, referred to as mtDNA, which is required for proper mitochondrial function. While mutations in mtDNA can cause a heterogeneous array of disease phenotypes, there is currently no cure for this collection of diseases. Here, we will cover characteristics of the mitochondrial genome important for understanding the pathology associated with mtDNA mutations, and review recent approaches that are being developed to treat and prevent mtDNA disease. First, we will discuss mitochondrial replacement therapy (MRT), where mitochondria from a healthy donor replace maternal mitochondria harbouring mutant mtDNA. In addition to ethical concerns surrounding this procedure, MRT is only applicable in cases where the mother is known or suspected to carry mtDNA mutations. Thus, there remains a need for other strategies to treat patients with mtDNA disease. To this end, we will also discuss several alternative means to reduce the amount of mutant mtDNA present in cells. Such methods, referred to as heteroplasmy shifting, have proven successful in animal models. In particular, we will focus on the approach of targeting engineered endonucleases to specifically cleave mutant mtDNA. Together, these approaches offer hope to prevent the transmission of mtDNA disease and potentially reduce the impact of mtDNA mutations.

ETHE1 and MOCS1 deficiencies: Disruption of mitochondrial bioenergetics, dynamics, redox homeostasis and endoplasmic reticulum-mitochondria crosstalk in patient fibroblasts

Ethylmalonic encephalopathy protein 1 (ETHE1) and molybdenum cofactor (MoCo) deficiencies are hereditary disorders that affect the catabolism of sulfur-containing amino acids. ETHE1 deficiency is caused by mutations in the ETHE1 gene, while MoCo deficiency is due to mutations in one of three genes involved in MoCo biosynthesis (MOCS1, MOCS2 and GPHN). Patients with both disorders exhibit abnormalities of the mitochondrial respiratory chain, among other biochemical findings. However, the pathophysiology of the defects has not been elucidated. To characterize cellular derangements, mitochondrial bioenergetics, dynamics, endoplasmic reticulum (ER)-mitochondria communication, superoxide production and apoptosis were evaluated in fibroblasts from four patients with ETHE1 deficiency and one with MOCS1 deficiency. The effect of JP4-039, a promising mitochondrial-targeted antioxidant, was also tested on cells. Our data show that mitochondrial respiration was decreased in all patient cell lines. ATP depletion and increased mitochondrial mass was identified in the same cells, while variable alterations in mitochondrial fusion and fission were seen. High superoxide levels were found in all cells and were decreased by treatment with JP4-039, while the respiratory chain activity was increased by this antioxidant in cells in which it was impaired. The content of VDAC1 and IP3R, proteins involved in ER-mitochondria communication, was decreased, while DDIT3, a marker of ER stress, and apoptosis were increased in all cell lines. These data demonstrate that previously unrecognized broad disturbances of cellular function are involved in the pathophysiology of ETHE1 and MOCS1 deficiencies, and that reduction of mitochondrial superoxide by JP4-039 is a promising strategy for adjuvant therapy of these disorders.

Defective Mitochondrial tRNA Taurine Modification Activates Global Proteostress and Leads to Mitochondrial Disease

A subset of mitochondrial tRNAs (mt-tRNAs) contains taurine-derived modifications at 34U of the anticodon. Loss of taurine modification has been linked to the development of mitochondrial diseases, but the molecular mechanism is still unclear. Here, we showed that taurine modification is catalyzed by mitochondrial optimization 1 (Mto1) in mammals. Mto1 deficiency severely impaired mitochondrial translation and respiratory activity. Moreover, Mto1-deficient cells exhibited abnormal mitochondrial morphology owing to aberrant trafficking of nuclear DNA-encoded mitochondrial proteins, including Opa1. The mistargeted proteins were aggregated and misfolded in the cytoplasm, which induced cytotoxic unfolded protein response. Importantly, application of chemical chaperones successfully suppressed cytotoxicity by reducing protein misfolding and increasing functional mitochondrial proteins in Mto1-deficient cells and mice. Thus, our results demonstrate the essential role of taurine modification in mitochondrial translation and reveal an intrinsic protein homeostasis network between the mitochondria and cytosol, which has therapeutic potential for mitochondrial diseases.

Mitochondrial Morphofunction in Mammalian Cells

Significance: In addition to their classical role in cellular ATP production, mitochondria are of key relevance in various (patho)physiological mechanisms including second messenger signaling, neuro-transduction, immune responses and death induction. Recent Advances: Within cells, mitochondria are motile and display temporal changes in internal and external structure ("mitochondrial dynamics"). During the last decade, substantial empirical and in silico evidence was presented demonstrating that mitochondrial dynamics impacts on mitochondrial function and vice versa. Critical Issues: However, a comprehensive and quantitative understanding of the bidirectional links between mitochondrial external shape, internal structure and function ("morphofunction") is still lacking. The latter particularly hampers our understanding of the functional properties and behavior of individual mitochondrial within single living cells. Future Directions: In this review we discuss the concept of mitochondrial morphofunction in mammalian cells, primarily using experimental evidence obtained within the last decade. The topic is introduced by briefly presenting the central role of mitochondria in cell physiology and the importance of the mitochondrial electron transport chain (ETC) therein. Next, we summarize in detail how mitochondrial (ultra)structure is controlled and discuss empirical evidence regarding the equivalence of mitochondrial (ultra)structure and function. Finally, we provide a brief summary of how mitochondrial morphofunction can be quantified at the level of single cells and mitochondria, how mitochondrial ultrastructure/volume impacts on mitochondrial bioreactions and intramitochondrial protein diffusion, and how mitochondrial morphofunction can be targeted by small molecules.

Five non-mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes phenotype adult patients with m.3243A>G mutation after kidney transplantation: follow-up and review of the literature

Background

Renal involvement in patients with the m.3243A>G mutation may result in end-stage renal disease (ESRD) requiring renal replacement therapy. Although kidney transplantations have been performed in a small number of patients, short- and long-term follow-up data are lacking.

Methods

We describe five patients with the m.3243A<G mutation who received a kidney transplant, including follow-up data up to 13 years. We also summarize all cases (n = 13) of kidney transplantation in m.3243A>G carriers described in the literature.

Results

Proteinuria with or without renal failure was the first clinical presentation of renal involvement in 13 of 18 (72%) patients. Focal segmental glomerulosclerosis (FSGS) was found in 9 of 13 (69%) biopsies. Sixteen of 18 (84%) patients developed hearing loss. All patients were diagnosed with diabetes mellitus, of whom eight (44%) developed the disease after transplantation. All patients with reported follow-up data (13/18) had stable kidney function from 6 months to 13 years of follow-up after transplantation.

Conclusions

Renal involvement in carriers of the m.3243A>G mutation most commonly leads to proteinuria and FSGS and may lead to ESRD. Proper recognition of the mitochondrial origin of the renal disease in these patients is important for adequate treatment selection and suitable supportive care. This case series and review of the available literature on long-term follow-up after kidney transplantation shows it is feasible for non-mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes phenotype carriers of the m.3243A>G mutation to be considered for kidney transplantation in case of ESRD. These patients should not be excluded from transplant solely for their mitochondrial diagnosis.

Natural variability of daily physical activity measured by accelerometry in children with a mitochondrial disease

In this exploratory study we aimed to select the most valid and feasible accelerometer to measure daily physical activity at home in 10 children with mitochondrial disease. Using the experimentally-selected GENEActiv, good to excellent short- and long-term test-retest reliability of daily physical activity was found. Especially in children with more severe limitations daily physical activity seemed more stable and less susceptible to weather conditions. Moreover, small standard errors of measurement were found, indicating high precision of this measuring method. We conclude that measuring daily physical activity using accelerometry is a promising outcome measure for future studies in this unique population.

Mitochondrial Genetic Disorders: Cell Signaling and Pharmacological Therapies

Mitochondrial fatty acid oxidation (FAO) and respiratory chain (RC) defects form a large group of inherited monogenic disorders sharing many common clinical and pathophysiological features, including disruption of mitochondrial bioenergetics, but also, for example, oxidative stress and accumulation of noxious metabolites. Interestingly, several transcription factors or co-activators exert transcriptional control on both FAO and RC genes, and can be activated by small molecules, opening to possibly common therapeutic approaches for FAO and RC deficiencies. Here, we review recent data on the potential of various drugs or small molecules targeting pivotal metabolic regulators: peroxisome proliferator activated receptors (PPARs), sirtuin 1 (SIRT1), AMP-activated protein kinase (AMPK), and protein kinase A (PKA)) or interacting with reactive oxygen species (ROS) signaling, to alleviate or to correct inborn FAO or RC deficiencies in cellular or animal models. The possible molecular mechanisms involved, in particular the contribution of mitochondrial biogenesis, are discussed. Applications of these pharmacological approaches as a function of genotype/phenotype are also addressed, which clearly orient toward personalized therapy. Finally, we propose that beyond the identification of individual candidate drugs/molecules, future pharmacological approaches should consider their combination, which could produce additive or synergistic effects that may further enhance their therapeutic potential.

A Drosophila Mitochondrial Complex I Deficiency Phenotype Array

Mitochondrial diseases are a group of rare life-threatening diseases often caused by defects in the oxidative phosphorylation system. No effective treatment is available for these disorders. Therapeutic development is hampered by the high heterogeneity in genetic, biochemical, and clinical spectra of mitochondrial diseases and by limited preclinical resources to screen and identify effective treatment candidates. Alternative models of the pathology are essential to better understand mitochondrial diseases and to accelerate the development of new therapeutics. The fruit fly Drosophila melanogaster is a cost- and time-efficient model that can recapitulate a wide range of phenotypes observed in patients suffering from mitochondrial disorders. We targeted three important subunits of complex I of the mitochondrial oxidative phosphorylation system with the flexible UAS-Gal4 system and RNA interference (RNAi): NDUFS4 (ND-18), NDUFS7 (ND-20), and NDUFV1 (ND-51). Using two ubiquitous driver lines at two temperatures, we established a collection of phenotypes relevant to complex I deficiencies. Our data offer models and phenotypes with different levels of severity that can be used for future therapeutic screenings. These include qualitative phenotypes that are amenable to high-throughput drug screening and quantitative phenotypes that require more resources but are likely to have increased potential and sensitivity to show modulation by drug treatment.

Multi-omics identify xanthine as a pro-survival metabolite for nematodes with mitochondrial dysfunction

Aberrant mitochondrial function contributes to the pathogenesis of various metabolic and chronic disorders. Inhibition of insulin/IGF‐1 signaling (IIS) represents a promising avenue for the treatment of mitochondrial diseases, although many of the molecular mechanisms underlying this beneficial effect remain elusive. Using an unbiased multi‐omics approach, we report here that IIS inhibition reduces protein synthesis and favors catabolism in mitochondrial deficient Caenorhabditis elegans. We unveil that the lifespan extension does not occur through the restoration of mitochondrial respiration, but as a consequence of an ATP‐saving metabolic rewiring that is associated with an evolutionarily conserved phosphoproteome landscape. Furthermore, we identify xanthine accumulation as a prominent downstream metabolic output of IIS inhibition. We provide evidence that supplementation of FDA‐approved xanthine derivatives is sufficient to promote fitness and survival of nematodes carrying mitochondrial lesions. Together, our data describe previously unknown molecular components of a metabolic network that can extend the lifespan of short‐lived mitochondrial mutant animals.

Rescue from galactose-induced death of Leigh Syndrome patient cells by pyruvate and NAD

Cell models of mitochondrial complex I (CI) deficiency display activation of glycolysis to compensate for the loss in mitochondrial ATP production. This adaptation can mask other relevant deficiency-induced aberrations in cell physiology. Here we investigated the viability, mitochondrial morphofunction, ROS levels and ATP homeostasis of primary skin fibroblasts from Leigh Syndrome (LS) patients with isolated CI deficiency. These cell lines harbored mutations in nuclear DNA (nDNA)-encoded CI genes (NDUFS7, NDUFS8, NDUFV1) and, to prevent glycolysis upregulation, were cultured in a pyruvate-free medium in which glucose was replaced by galactose. Following optimization of the cell culture protocol, LS fibroblasts died in the galactose medium, whereas control cells did not. LS cell death was dose-dependently inhibited by pyruvate, malate, oxaloacetate, α-ketoglutarate, aspartate, and exogenous NAD⁺ (eNAD), but not by lactate, succinate, α-ketobutyrate, and uridine. Pyruvate and eNAD increased the cellular NAD⁺ content in galactose-treated LS cells to a different extent and co-incubation studies revealed that pyruvate-induced rescue was not primarily mediated by NAD⁺. Functionally, in LS cells glucose-by-galactose replacement increased mitochondrial fragmentation and mass, depolarized the mitochondrial membrane potential (Δψ), increased H₂DCFDA-oxidizing ROS levels, increased mitochondrial ATP generation, and reduced the total cellular ATP content. These aberrations were differentially rescued by pyruvate and eNAD, supporting the conclusion that these compounds rescue galactose-induced LS cell death via different mechanisms. These findings establish a cell-based strategy for intervention testing and enhance our understanding of CI deficiency pathophysiology.

Outcome measures for children with mitochondrial disease: consensus recommendations for future studies from a Delphi-based international workshop

Although there are no effective disease-modifying therapies for mitochondrial diseases, an increasing number of trials are being conducted in this rare disease group. The use of sensitive and valid endpoints is essential to test the effectiveness of potential treatments. There is no consensus on which outcome measures to use in children with mitochondrial disease. The aims of this two-day Delphi-based workshop were to (i) define the protocol for an international, multi-centre natural history study in children with mitochondrial myopathy and (ii) to select appropriate outcome measures for a validation study in children with mitochondrial encephalopathy. We suggest two sets of outcome measures for a natural history study in children with mitochondrial myopathy and for a proposed validation study in children with mitochondrial encephalopathy.

The KHENERGY Study: Safety and Efficacy of KH176 in Mitochondrial m.3243A>G Spectrum Disorders

KH176 is a potent intracellular reduction-oxidation-modulating compound developed to treat mitochondrial disease. We studied tolerability, safety, pharmacokinetics, pharmacodynamics, and efficacy of twice daily oral 100 mg KH176 for 28 days in a double-blind, randomized, placebo-controlled, two-way crossover phase IIA study in 18 adult m.3243A>G patients without cardiovascular involvement. Efficacy parameters included clinical and functional outcome measures and biomarkers. The trial was registered within ClinicalTrials.gov (NCT02909400), the European Clinical Trials Database (2016-001696-79), and ISRCTN (43372293) (The KHENERGY study). Twice daily oral 100 mg KH176 was well tolerated and appeared safe. No serious treatment-emergent adverse events were reported. No significant improvements in gait parameters or other outcome measures were obtained, except for a positive effect on alertness and mood, although a coincidence due to multiplicity cannot be ignored. The results of the study provide first data on safety and efficacy of KH176 in patients with mitochondrial disease and will be instrumental in designing future clinical trials.

Resveratrol preserves mitochondrial function in a human post-mitotic cell model

Dysfunctions caused by genetic defects in the mitochondrial DNA (mtDNA) of humans are called mitochondrial diseases; however, mtDNA mutations are also associated with aging and age-related diseases. Here, we present an original cellular model that allows gathering information on molecules that might contrast or prevent mitochondrial dysfunctions and their related diseases. This model allowed us to show that resveratrol (RSV), a phytochemical present in food, exerts protective effects at low concentrations on resting human fibroblasts carrying dysfunctional respiratory chain Complex I. Cells were maintained both in resting condition, to mimic the high energy demanding post-mitotic tissues (serum absence and gramicidin presence), and under glucose deficiency to push the synthesis of ATP via oxidative phosphorylation. Pre-incubation with RSV prolonged the viability of the fibroblasts exposed to rotenone, a well-known specific inhibitor of the respiratory chain Complex I, and decreased mitochondrial fragmentation. It significantly prevented the oxidative phosphorylation impairment indirectly caused by the rotenone-mediated Complex I inhibition, allowing for an almost complete preservation of the cellular ATP level. Indeed, RSV limited the rotenone-induced reactive oxygen species increase, allowing for the maintenance of a functional mitochondrial membrane potential. These findings indicate the potential usage of resveratrol to prevent or possibly treat many disorders, in which the bioenergetic defects and oxidative stress are the primary (mitochondrial encephalomyopathy), or the secondary (age-related diseases) causes of the pathology; and to also assist cell senescence during aging.

Advances in methods for reducing mitochondrial DNA disease by replacing or manipulating the mitochondrial genome

Mitochondrial DNA (mtDNA) is a multi-copy genome whose cell copy number varies depending on tissue type. Mutations in mtDNA can cause a wide spectrum of diseases. Mutated mtDNA is often found as a subset of the total mtDNA population in a cell or tissue, a situation known as heteroplasmy. As mitochondrial dysfunction only presents after a certain level of heteroplasmy has been acquired, ways to artificially reduce or replace the mutated species have been attempted. This review addresses recent approaches and advances in this field, focusing on the prevention of pathogenic mtDNA transfer via mitochondrial donation techniques such as maternal spindle transfer and pronuclear transfer in which mutated mtDNA in the oocyte or fertilized embryo is substituted with normal copies of the mitochondrial genome. This review also discusses the molecular targeting and cleavage of pathogenic mtDNA to shift heteroplasmy using antigenomic therapy and genome engineering techniques including Zinc-finger nucleases and transcription activator-like effector nucleases. Finally, it considers CRISPR technology and the unique difficulties that mitochondrial genome editing presents.

Mitochondrial dysfunction and its role in tissue-specific cellular stress

Mitochondrial bioenergetics require the coordination of two different and independent genomes. Mutations in either genome will affect mitochondrial functionality and produce different sources of cellular stress. Depending on the kind of defect and stress, different tissues and organs will be affected, leading to diverse pathological conditions. There is no curative therapy for mitochondrial diseases, nevertheless, there are strategies described that fight the various stress forms caused by the malfunctioning organelles. Here, we will revise the main kinds of stress generated by mutations in mitochondrial genes and outline several ways of fighting this stress.

A disease-associated Aifm1 variant induces severe myopathy in knockin mice

OBJECTIVE

Mutations in the AIFM1 gene have been identified in recessive X-linked mitochondrial diseases. Functional and molecular consequences of these pathogenic AIFM1 mutations have been poorly studied in vivo.

METHODS/RESULTS

Here we provide evidence that the disease-associated apoptosis-inducing factor (AIF) deletion arginine 201 (R200 in rodents) causes pathology in knockin mice. Within a few months, posttranslational loss of the mutant AIF protein induces severe myopathy associated with a lower number of cytochrome c oxidase-positive muscle fibers. At a later stage, Aifm1 (R200 del) knockin mice manifest peripheral neuropathy, but they do not show neurodegenerative processes in the cerebellum, as observed in age-matched hypomorphic Harlequin (Hq) mutant mice. Quantitative proteomic and biochemical data highlight common molecular signatures of mitochondrial diseases, including aberrant folate-driven one-carbon metabolism and sustained Akt/mTOR signaling.

CONCLUSION

Our findings indicate metabolic defects and distinct tissue-specific vulnerability due to a disease-causing AIFM1 mutation, with many pathological hallmarks that resemble those seen in patients.

Quantification of gait in children with mitochondrial disease

Mitochondrial disorders are multisystem conditions that can potentially affect gait in many ways. The aim of this study was to select the optimal protocol to quantify the spatiotemporal parameters of gait in ambulatory children with mitochondrial disorders based on feasibility, test-retest reliability, and the difference between patients and controls. Gait at self-selected pace was quantified in ambulatory children with a genetically confirmed primary mitochondrial disease using the GAITRite electronic walkway. Three protocols were tested: pre-exercise, post-exercise (after a 3-min walking test), and recovery. In 14 ambulatory patients, we showed good to perfect reliability for velocity, cadence, step length, step time, step time variability, and step width in the recovery condition. The difference between patients and 70 individually age- and gender matched healthy controls only became apparent in the post-exercise protocol. In conclusion, measuring spatiotemporal parameters of gait using the GAITRite in ambulatory children with mitochondrial disease is feasible and reliable for most of the parameters measured. When using gait analysis in future studies in children with mitochondrial disease, we advise i) to use an exercise test prior to the gait analysis, ii) to let children practice the test before the actual data collection, and iii) not to use symmetry parameters.

Everyday Activities for Children with Mitochondrial Disorder: A Retrospective Chart Review

Background

Engagement in everyday activities is important for the health and wellbeing of children. Children with mitochondrial disorders have impaired energy production leading to limitations in activity. It is unknown which activities these children perform and if the nature of activities of low-functioning children differs from average-functioning children. Therefore, this pilot study explored the activities reported in patient records of a heterogeneous group of children with genetically confirmed mitochondrial disorders.

Methods

A retrospective qualitative directed content analysis by health care professionals reported activities (as part of their professional reasoning obligations) in hospital patient records of children with mitochondrial disorder.

Results

Seventeen patient records, presenting notes on capacities and performed activities, showed an overview of everyday activities that covered the categories: self-care, house chores, therapy, school, computing, hobby, play, sports, and mobility/transport. The activity categories of low-functioning children did not differ from average-functioning children, although descriptions of specific activities differed between groups.

Conclusion

This pilot exploration indicates that the types of activities that children with mitochondrial disorders perform are not necessarily linked to the child's impairments. However, differences in levels of independence, assistive device usage, and energy costs seem to exist. Future research should address the child's perspective on, and meaning of, activity performances.

Neuronal Dysfunction Associated with Cholesterol Deregulation

Cholesterol metabolism is crucial for cells and, in particular, its biosynthesis in the central nervous system occurs in situ, and its deregulation involves morphological changes that cause functional variations and trigger programmed cell death. The pathogenesis of rare diseases, such as Mevalonate Kinase Deficiency or Smith–Lemli–Opitz Syndrome, arises due to enzymatic defects in the cholesterol metabolic pathways, resulting in a shortage of downstream products. The most severe clinical manifestations of these diseases appear as neurological defects. Expanding the knowledge of this biological mechanism will be useful for identifying potential targets and preventing neuronal damage. Several studies have demonstrated that deregulation of the cholesterol pathway induces mitochondrial dysfunction as the result of respiratory chain damage. We set out to determine whether mitochondrial damage may be prevented by using protective mitochondria-targeted compounds, such as MitoQ, in a neuronal cell line treated with a statin to induce a biochemical block of the cholesterol pathway. Evidence from the literature suggests that mitochondria play a crucial role in the apoptotic mechanism secondary to blocking the cholesterol pathway. Our study shows that MitoQ, administered as a preventive agent, could counteract the cell damage induced by statins in the early stages, but its protective role fades over time.

KH176 Safeguards Mitochondrial Diseased Cells from Redox Stress-Induced Cell Death by Interacting with the Thioredoxin System/Peroxiredoxin Enzyme Machinery

A deficient activity of one or more of the mitochondrial oxidative phosphorylation (OXPHOS) enzyme complexes leads to devastating diseases, with high unmet medical needs. Mitochondria, and more specifically the OXPHOS system, are the main cellular production sites of Reactive Oxygen Species (ROS). Increased ROS production, ultimately leading to irreversible oxidative damage of macromolecules or to more selective and reversible redox modulation of cell signalling, is a causative hallmark of mitochondrial diseases. Here we report on the development of a new clinical-stage drug KH176 acting as a ROS-Redox modulator. Patient-derived primary skin fibroblasts were used to assess the potency of a new library of chromanyl-based compounds to reduce ROS levels and protect cells against redox-stress. The lead compound KH176 was studied in cell-based and enzymatic assays and in silico. Additionally, the metabolism, pharmacokinetics and toxicokinetics of KH176 were assessed in vivo in different animal species. We demonstrate that KH176 can effectively reduce increased cellular ROS levels and protect OXPHOS deficient primary cells against redox perturbation by targeting the Thioredoxin/Peroxiredoxin system. Due to its dual activity as antioxidant and redox modulator, KH176 offers a novel approach to the treatment of mitochondrial (-related) diseases. KH176 efficacy and safety are currently being evaluated in a Phase 2 clinical trial.

Post-Translational Oxidative Modifications of Mitochondrial Complex I (NADH: Ubiquinone Oxidoreductase): Implications for Pathogenesis and Therapeutics in Human Diseases

Mitochondrial complex I (NADH: ubiquinone oxidoreductase; CI) is central to the electron transport chain (ETC), oxidative phosphorylation, and ATP production in eukaryotes. CI is a multi-subunit complex with a complicated yet organized structure that optimally connects electron transfer with proton translocation and forms higher-order supercomplexes with other ETC complexes. Efforts to understand the molecular genetics, expression profile of subunits, and structure-function relationship of CI have increased over the years due to the direct role of the complex in human diseases. Although mutations in the nuclear and mitochondrial genes of CI and altered expression of subunits could potentially lower CI activity leading to mitochondrial dysfunction in many diseases, oxidative post-translational modifications (PTMs) have emerged as an important mechanism contributing to altered CI activity. These mainly include reversible and irreversible cysteine modifications, tyrosine nitration, carbonylation, and tryptophan oxidation that are generated following exposure to reactive oxygen species/reactive nitrogen species. Interestingly, oxidative PTMs could contribute either to CI damage, mitochondrial dysfunction, and ensuing cell death or a response mechanism with potential cytoprotective effects. This has also emerged as a promising field for structural biologists since analysis of PTMs could assist in understanding the structure-function relationship of the complex and correlate electron transfer mechanism with energy production. However, analysis of PTMs of CI and their contribution to CI function are incomplete in many physiological and pathological conditions. This review aims to highlight the role of oxidative PTMs in modulating CI activity with implications toward pathobiology of CNS diseases and novel therapeutics.

Apoptosis-Inducing Factor (AIF) in Physiology and Disease: The Tale of a Repented Natural Born Killer

Apoptosis-inducing factor (AIF) is a mitochondrial oxidoreductase that contributes to cell death programmes and participates in the assembly of the respiratory chain. Importantly, AIF deficiency leads to severe mitochondrial dysfunction, causing muscle atrophy and neurodegeneration in model organisms as well as in humans. The purpose of this review is to describe functions of AIF and AIF-interacting proteins as regulators of cell death and mitochondrial bioenergetics. We describe how AIF deficiency induces pathogenic processes that alter metabolism and ultimately compromise cellular homeostasis. We report the currently known AIFM1 mutations identified in humans and discuss the variability of AIFM1-related disorders in terms of onset, organ involvement and symptoms. Finally, we summarize how the study of AIFM1-linked pathologies may help to further expand our understanding of rare inherited forms of mitochondrial diseases.

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AIF is a mitochondrial NADH-dependent oxidoreductase.

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Nuclear translocation of AIF occurs during cell death and has been associated with human disorders.

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Under physiological settings, AIF participates to the biogenesis of the respiratory complexes.

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AIFM1 mutations have been identified in patients with impaired mitochondrial bioenergetics.

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Inherited AIFM1 mutations lead to a variety of clinical manifestations, including severe childhood-onset mitochondrial diseases.

Mitochondrial dysfunction in diabetic kidney disease

Globally, diabetes is the leading cause of chronic kidney disease and end-stage renal disease, which are major risk factors for cardiovascular disease and death. Despite this burden, the factors that precipitate the development and progression of diabetic kidney disease (DKD) remain to be fully elucidated. Mitochondrial dysfunction is associated with kidney disease in nondiabetic contexts, and increasing evidence suggests that dysfunctional renal mitochondria are pathological mediators of DKD. These complex organelles have a broad range of functions, including the generation of ATP. The kidneys are mitochondrially rich, highly metabolic organs that require vast amounts of ATP for their normal function. The delivery of metabolic substrates for ATP production, such as fatty acids and oxygen, is altered by diabetes. Changes in metabolic fuel sources in diabetes to meet ATP demands result in increased oxygen consumption, which contributes to renal hypoxia. Inherited factors including mutations in genes that impact mitochondrial function and/or substrate delivery may also be important risk factors for DKD. Hence, we postulate that the diabetic milieu and inherited factors that underlie abnormalities in mitochondrial function synergistically drive the development and progression of DKD.

WAH-1/AIF regulates mitochondrial oxidative phosphorylation in the nematode Caenorhabditis elegans

Impaired mitochondrial energy metabolism contributes to a wide range of pathologic conditions, including neurodegenerative diseases. Mitochondrial apoptosis-inducing factor (AIF) is required for the correct maintenance of mitochondrial electron transport chain. An emerging body of clinical evidence indicates that several mutations in the AIFM1 gene are causally linked to severe forms of mitochondrial disorders. Here we investigate the consequence of WAH-1/AIF deficiency in the survival of the nematode Caenorhabditis elegans. Moreover, we assess the survival of C. elegans strains expressing a disease-associated WAH-1/AIF variant. We demonstrate that wah-1 downregulation compromises the function of the oxidative phosphorylation system and reduces C. elegans lifespan. Notably, the loss of respiratory subunits induces a nuclear-encoded mitochondrial stress response independently of an evident increase of oxidative stress. Overall, our data pinpoint an evolutionarily conserved role of WAH-1/AIF in the maintenance of proper mitochondrial activity.

Evaluation of mitochondrial bioenergetics, dynamics, endoplasmic reticulum-mitochondria crosstalk, and reactive oxygen species in fibroblasts from patients with complex I deficiency

Mitochondrial complex I (CI) deficiency is the most frequent cause of oxidative phosphorylation (OXPHOS) disorders in humans. In order to benchmark the effects of CI deficiency on mitochondrial bioenergetics and dynamics, respiratory chain (RC) and endoplasmic reticulum (ER)-mitochondria communication, and superoxide production, fibroblasts from patients with mutations in the ND6, NDUFV1 or ACAD9 genes were analyzed. Fatty acid metabolism, basal and maximal respiration, mitochondrial membrane potential, and ATP levels were decreased. Changes in proteins involved in mitochondrial dynamics were detected in various combinations in each cell line, while variable changes in RC components were observed. ACAD9 deficient cells exhibited an increase in RC complex subunits and DDIT3, an ER stress marker. The level of proteins involved in ER-mitochondria communication was decreased in ND6 and ACAD9 deficient cells. |ΔΨ| and cell viability were further decreased in all cell lines. These findings suggest that disruption of mitochondrial bioenergetics and dynamics, ER-mitochondria crosstalk, and increased superoxide contribute to the pathophysiology in patients with ACAD9 deficiency. Furthermore, treatment of ACAD9 deficient cells with JP4-039, a novel mitochondria-targeted reactive oxygen species, electron and radical scavenger, decreased superoxide level and increased basal and maximal respiratory rate, identifying a potential therapeutic intervention opportunity in CI deficiency.