Mice with mitochondrial complex I deficiency develop a fatal encephalomyopathy

Mitochondrial complex-1 as a therapeutic target for cardiac diseases

Mitochondrial dysfunction is critical for the development and progression of cardiovascular diseases (CVDs). Complex-1 (CI) is an essential component of the mitochondrial electron transport chain that participates in oxidative phosphorylation and energy production. CI is the largest multisubunit complex (~ 1 Mda) and comprises 45 protein subunits encoded by seven mt-DNA genes and 38 nuclear genes. These subunits function as the enzyme nicotinamide adenine dinucleotide  hydrogen (NADH): ubiquinone oxidoreductase. CI dysregulation has been implicated in various CVDs, including heart failure, ischemic heart disease, pressure overload, hypertrophy, and cardiomyopathy. Several studies demonstrated that impaired CI function contributes to increased oxidative stress, altered calcium homeostasis, and mitochondrial DNA damage in cardiac cells, leading to cardiomyocyte dysfunction and apoptosis. CI dysfunction has been associated with endothelial dysfunction, inflammation, and vascular remodeling, critical processes in developing atherosclerosis and hypertension. Although CI is crucial in physiological and pathological conditions, no potential therapeutics targeting CI are available to treat CVDs. We believe that a lack of understanding of CI's precise mechanisms and contributions to CVDs limits the development of therapeutic strategies. In this review, we comprehensively analyze the role of CI in cardiovascular health and disease to shed light on its potential therapeutic target role in CVDs.

Interferon-gamma contributes to disease progression in the Ndufs4(-/-) model of Leigh syndrome

AIM

Leigh syndrome (LS), the most common paediatric presentation of genetic mitochondrial dysfunction, is a multi-system disorder characterised by severe neurologic and metabolic abnormalities. Symmetric, bilateral, progressive necrotizing lesions in the brainstem are defining features of the disease. Patients are often symptom free in early life but typically develop symptoms by about 2 years of age. The mechanisms underlying disease onset and progression in LS remain obscure. Recent studies have shown that the immune system causally drives disease in the Ndufs4(-/-) mouse model of LS: treatment of Ndufs4(-/-) mice with the macrophage-depleting Csf1r inhibitor pexidartinib prevents disease. While the precise mechanisms leading to immune activation and immune factors involved in disease progression have not yet been determined, interferon-gamma (IFNγ) and interferon gamma-induced protein 10 (IP10) were found to be significantly elevated in Ndufs4(-/-) brainstem, implicating these factors in disease. Here, we aimed to explore the role of IFNγ and IP10 in LS.

METHODS

To establish the role of IFNγ and IP10 in LS, we generated IFNγ and IP10 deficient Ndufs4(-/-)/Ifng(-/-) and Ndufs4(-/-)/IP10(-/-) double knockout animals, as well as IFNγ and IP10 heterozygous, Ndufs4(-/-)/Ifng(+/-) and Ndufs4(-/-)/IP10(+/-), animals. We monitored disease onset and progression to define the impact of heterozygous or homozygous loss of IFNγ and IP10 in LS.

RESULTS

Loss of IP10 does not significantly impact the onset or progression of disease in the Ndufs4(-/-) model. IFNγ loss significantly extends survival and delays disease progression in a gene dosage-dependent manner, though the benefits are modest compared to Csf1r inhibition.

CONCLUSIONS

IFNγ contributes to disease onset and progression in LS. Our findings suggest that IFNγ targeting therapies may provide some benefits in genetic mitochondrial disease, but targeting IFNγ alone would likely yield only modest benefits in LS.

The Blood-Brain Barrier Is Unaffected in the Ndufs4-/- Mouse Model of Leigh Syndrome

Mitochondrial dysfunction plays a major role in physiological aging and in many pathological conditions. Yet, no study has explored the consequence of primary mitochondrial deficiency on the blood-brain barrier (BBB) structure and function. Addressing this question has major implications for pharmacological and genetic strategies aimed at ameliorating the neurological symptoms that are often predominant in patients suffering from these conditions. In this study, we examined the permeability of the BBB in the Ndufs4-/- mouse model of Leigh syndrome (LS). Our results indicated that the structural and functional integrity of the BBB was preserved in this severe model of mitochondrial disease. Our findings suggests that pharmacological or gene therapy strategies targeting the central nervous system in this mouse model and possibly other models of mitochondrial dysfunction require the use of specific tools to bypass the BBB. In addition, they raise the need for testing the integrity of the BBB in complementary in vivo models.

Potassium Leak Channels and Mitochondrial Complex I Interact in Glutamatergic Interneurons of the Mouse Spinal Cord

BACKGROUND

Volatile anesthetics induce hyperpolarizing potassium currents in spinal cord neurons that may contribute to their mechanism of action. They are induced at lower concentrations of isoflurane in noncholinergic neurons from mice carrying a loss-of-function mutation of the Ndufs4 gene, required for mitochondrial complex I function. The yeast NADH dehydrogenase enzyme, NDi1, can restore mitochondrial function in the absence of normal complex I activity, and gain-of-function Ndi1 transgenic mice are resistant to volatile anesthetics. The authors tested whether NDi1 would reduce the hyperpolarization caused by isoflurane in neurons from Ndufs4 and wild-type mice. Since volatile anesthetic behavioral hypersensitivity in Ndufs4 is transduced uniquely by glutamatergic neurons, it was also tested whether these currents were also unique to glutamatergic neurons in the Ndufs4 spinal cord.

METHODS

Spinal cord neurons from wild-type, NDi1, and Ndufs4 mice were patch clamped to characterize isoflurane sensitive currents. Neuron types were marked using fluorescent markers for cholinergic, glutamatergic, and γ-aminobutyric acid-mediated (GABAergic) neurons. Norfluoxetine was used to identify potassium channel type. Neuron type-specific Ndufs4 knockout animals were generated using type-specific Cre-recombinase with floxed Ndufs4.

RESULTS

Resting membrane potentials (RMPs) of neurons from NDi1;Ndufs4, unlike those from Ndufs4, were not hyperpolarized by 0.6% isoflurane (Ndufs4, ΔRMP -8.2 mV [-10 to -6.6]; P = 1.3e-07; Ndi1;Ndufs4, ΔRMP -2.1 mV [-7.6 to +1.4]; P = 1). Neurons from NDi1 animals in a wild-type background were not hyperpolarized by 1.8% isoflurane (wild-type, ΔRMP, -5.2 mV [-7.3 to -3.2]; P = 0.00057; Ndi1, ΔRMP, 0.6 mV [-1.7 to 3.2]; P = 0.68). In spinal cord slices from global Ndufs4 animals, holding currents (HC) were induced by 0.6% isoflurane in both GABAergic (ΔHC, 81.3 pA [61.7 to 101.4]; P = 2.6e-05) and glutamatergic (ΔHC, 101.2 pA [63.0 to 146.2]; P = 0.0076) neurons. In neuron type-specific Ndufs4 knockouts, HCs were increased in cholinergic (ΔHC, 119.5 pA [82.3 to 156.7]; P = 0.00019) and trended toward increase in glutamatergic (ΔHC, 85.5 pA [49 to 126.9]; P = 0.064) neurons but not in GABAergic neurons.

CONCLUSIONS

Bypassing complex I by overexpression of NDi1 eliminates increases in potassium currents induced by isoflurane in the spinal cord. The isoflurane-induced potassium currents in glutamatergic neurons represent a potential downstream mechanism of complex I inhibition in determining minimum alveolar concentration.

EDITOR’S PERSPECTIVE

Isolevuglandins Promote Mitochondrial Dysfunction and Electrophysiologic Abnormalities in Atrial Cardiomyocytes

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia, yet the cellular and molecular mechanisms underlying the AF substrate remain unclear. Isolevuglandins (IsoLGs) are highly reactive lipid dicarbonyl products that mediate oxidative stress-related injury. In murine hypertension, the lipid dicarbonyl scavenger 2-hydroxybenzylamine (2-HOBA) reduced IsoLGs and AF susceptibility. We hypothesized that IsoLGs mediate detrimental pathophysiologic effects in atrial cardiomyocytes that promote the AF substrate. Using Seahorse XFp extracellular flux analysis and a luminescence assay, IsoLG exposure suppressed intracellular ATP production in atrial HL-1 cardiomyocytes. IsoLGs caused mitochondrial dysfunction, with reduced mitochondrial membrane potential, increased mitochondrial reactive oxygen species (ROS) with protein carbonylation, and mitochondrial DNA damage. Moreover, they generated cytosolic preamyloid oligomers previously shown to cause similar detrimental effects in atrial cells. In mouse atrial and HL-1 cells, patch clamp experiments demonstrated that IsoLGs rapidly altered action potentials (AP), implying a direct effect independent of oligomer formation by reducing the maximum Phase 0 upstroke slope and shortening AP duration due to ionic current modifications. IsoLG-mediated mitochondrial and electrophysiologic abnormalities were blunted or totally prevented by 2-HOBA. These findings identify IsoLGs as novel mediators of oxidative stress-dependent atrial pathophysiology and support the investigation of dicarbonyl scavengers as a novel therapeutic approach to prevent AF.

NDUFS4 regulates cristae remodeling in diabetic kidney disease

The mitochondrial electron transport chain (ETC) is a highly adaptive process to meet metabolic demands of the cell, and its dysregulation has been associated with diverse clinical pathologies. However, the role and nature of impaired ETC in kidney diseases remains poorly understood. Here, we generate diabetic mice with podocyte-specific overexpression of Ndufs4, an accessory subunit of mitochondrial complex I, as a model investigate the role of ETC integrity in diabetic kidney disease (DKD). We find that conditional male mice with genetic overexpression of Ndufs4 exhibit significant improvements in cristae morphology, mitochondrial dynamics, and albuminuria. By coupling proximity labeling with super-resolution imaging, we also identify the role of cristae shaping protein STOML2 in linking NDUFS4 with improved cristae morphology. Together, we provide the evidence on the central role of NDUFS4 as a regulator of cristae remodeling and mitochondrial function in kidney podocytes. We propose that targeting NDUFS4 represents a promising approach to slow the progression of DKD.

The striking differences in the bioenergetics of brain and liver mitochondria are enhanced in mitochondrial disease

Mitochondrial disorders are hallmarked by the dysfunction of oxidative phosphorylation (OXPHOS) yet are highly heterogeneous at the clinical and genetic levels. Striking tissue-specific pathological manifestations are a poorly understood feature of these conditions, even if the disease-causing genes are ubiquitously expressed. To investigate the functional basis of this phenomenon, we analyzed several OXPHOS-related bioenergetic parameters, including oxygen consumption rates, electron transfer system (ETS)-related coenzyme Q (mtCoQ) redox state and production of reactive oxygen species (ROS) in mouse brain and liver mitochondria fueled by different substrates. In addition, we determined how these functional parameters are affected by ETS impairment in a tissue-specific manner using pathologically relevant mouse models lacking either Ndufs4 or Ttc19, leading to Complex I (CI) or Complex III (CIII) deficiency, respectively. Detailed OXPHOS analysis revealed striking differences between brain and liver mitochondria in the capacity of the different metabolic substrates to fuel the ETS, reduce the ETS-related mtCoQ, and to induce ROS production. In addition, ETS deficiency due to either CI or CIII dysfunction had a much greater impact on the intrinsic bioenergetic parameters of brain compared with liver mitochondria. These findings are discussed in terms of the still rather mysterious tissue-specific manifestations of mitochondrial disease.

Hypoxia and intra-complex genetic suppressors rescue complex I mutants by a shared mechanism

The electron transport chain (ETC) of mitochondria, bacteria, and archaea couples electron flow to proton pumping and is adapted to diverse oxygen environments. Remarkably, in mice, neurological disease due to ETC complex I dysfunction is rescued by hypoxia through unknown mechanisms. Here, we show that hypoxia rescue and hyperoxia sensitivity of complex I deficiency are evolutionarily conserved to C. elegans and are specific to mutants that compromise the electron-conducting matrix arm. We show that hypoxia rescue does not involve the hypoxia-inducible factor pathway or attenuation of reactive oxygen species. To discover the mechanism, we use C. elegans genetic screens to identify suppressor mutations in the complex I accessory subunit NDUFA6/nuo-3 that phenocopy hypoxia rescue. We show that NDUFA6/nuo-3(G60D) or hypoxia directly restores complex I forward activity, with downstream rescue of ETC flux and, in some cases, complex I levels. Additional screens identify residues within the ubiquinone binding pocket as being required for the rescue by NDUFA6/nuo-3(G60D) or hypoxia. This reveals oxygen-sensitive coupling between an accessory subunit and the quinone binding pocket of complex I that can restore forward activity in the same manner as hypoxia.

Interleukin-6-elicited chronic neuroinflammation may decrease survival but is not sufficient to drive disease progression in a mouse model of Leigh syndrome

BACKGROUND

Mitochondrial diseases (MDs) are genetic disorders characterized by dysfunctions in mitochondria. Clinical data suggest that additional factors, beyond genetics, contribute to the onset and progression of this group of diseases, but these influencing factors remain largely unknown. Mounting evidence indicates that immune dysregulation or distress could play a role. Clinical observations have described the co-incidence of infection and the onset of the disease as well as the worsening of symptoms following infection. These findings highlight the complex interactions between MDs and immunity and underscore the need to better understand their underlying relationships.

RESULTS

We used Ndufs4 KO mice, a well-established mouse model of Leigh syndrome (one of the most relevant MDs), to test whether chronic induction of a neuroinflammatory state in the central nervous system before the development of neurological symptoms would affect both the onset and progression of the disease in Ndufs4 KO mice. To this aim, we took advantage of the GFAP-IL6 mouse, which overexpresses interleukin-6 (IL-6) in astrocytes and produces chronic glial reactivity, by generating a mouse line with IL-6 overexpression and NDUFS4 deficiency. IL-6 overexpression aggravated the mortality of female Ndufs4 KO mice but did not alter the main motor and respiratory phenotypes measured in any sex. Interestingly, an abnormal region-dependent microglial response to IL-6 overexpression was observed in Ndufs4 KO mice compared to controls.

CONCLUSION

Overall, our data indicate that chronic neuroinflammation may worsen the disease in Ndufs4 KO female mice, but not in males, and uncovers an abnormal microglial response due to OXPHOS dysfunction, which may have implications for our understanding of the effect of OXPHOS dysfunction in microglia.

Gene therapy for mitochondrial disorders

In this review, we detail the current state of application of gene therapy to primary mitochondrial disorders (PMDs). Recombinant adeno-associated virus-based (rAAV) gene replacement approaches for nuclear gene disorders have been undertaken successfully in more than ten preclinical mouse models of PMDs which has been made possible by the development of novel rAAV technologies that achieve more efficient organ targeting. So far, however, the greatest progress has been made for Leber Hereditary Optic Neuropathy, for which phase 3 clinical trials of lenadogene nolparvovec demonstrated efficacy and good tolerability. Other methods of treating mitochondrial DNA (mtDNA) disorders have also had traction, including refinements to nucleases that degrade mtDNA molecules with pathogenic variants, including transcription activator-like effector nucleases, zinc-finger nucleases, and meganucleases (mitoARCUS). rAAV-based approaches have been used successfully to deliver these nucleases in vivo in mice. Exciting developments in CRISPR-Cas9 gene editing technology have achieved in vivo gene editing in mouse models of PMDs due to nuclear gene defects and new CRISPR-free gene editing approaches have shown great potential for therapeutic application in mtDNA disorders. We conclude the review by discussing the challenges of translating gene therapy in patients both from the point of view of achieving adequate organ transduction as well as clinical trial design.

Ndufs4 KO mice: A model to study comorbid mood disorders associated with mitochondrial dysfunction

The Ndufs4 knockout (KO) mouse is a validated and robust preclinical model of mitochondrial diseases (specifically Leigh syndrome), that displays a narrow window of relative phenotypical normality, despite its inherent mitochondrial complex I dysfunction and severe phenotype. Preclinical observations related to psychiatric comorbidities that arise in patients with mitochondrial diseases and indeed in Leigh syndrome are, however, yet to be investigated in this model. Strengthening this narrative is the fact that major depression and bipolar disorder are known to present with deficits in mitochondrial function. We therefore screened the behavioural profile of male and female Ndufs4 KO mice (relative to heterozygous; HET and wildtype; WT mice) between postnatal days 28 and 35 for locomotor, depressive- and anxiety-like alterations and linked it with selected brain biomarkers, viz. serotonin, kynurenine, and redox status in brain areas relevant to psychiatric pathologies (i.e., prefrontal cortex, hippocampus, and striatum). The Ndufs4 KO mice initially displayed depressive-like behaviour in the tail suspension test on PND31 but not on PND35 in the forced swim test. In the mirror box test, increased risk resilience was observed. Serotonin levels of KO mice, compared to HET controls, were increased on PND36, together with increased tryptophan to serotonin and kynurenine turnover. Kynurenine to kynurenic acid turnover was however decreased, while reduced versus oxidized glutathione ratio (GSH/GSSG) was increased. When considering the comorbid psychiatric traits of patients with mitochondrial disorders, this work elaborates on the neuropsychiatric profile of the Ndufs KO mouse. Secondly, despite locomotor differences, Ndufs4 KO mice present with a behavioural profile not unlike rodent models of bipolar disorder, namely variable mood states and risk-taking behaviour. The model may elucidate the bio-energetic mechanisms underlying mood disorders, especially in the presence of mitochondrial disease. Studies are however required to further validate the model's translational relevance.

Challenges and opportunities to bridge translational to clinical research for personalized mitochondrial medicine

Mitochondrial disorders are a group of rare and heterogeneous genetic diseases characterized by dysfunctional mitochondria leading to deficient adenosine triphosphate synthesis and chronic energy deficit in patients. The majority of these patients exhibit a wide range of phenotypic manifestations targeting several organ systems, making their clinical diagnosis and management challenging. Bridging translational to clinical research is crucial for improving the early diagnosis and prognosis of these intractable mitochondrial disorders and for discovering novel therapeutic drug candidates and modalities. This review provides the current state of clinical testing in mitochondrial disorders, discusses the challenges and opportunities for converting basic discoveries into clinical settings, explores the most suited patient-centric approaches to harness the extraordinary heterogeneity among patients affected by the same primary mitochondrial disorder, and describes the current outlook of clinical trials.

Structural insights into respiratory complex I deficiency and assembly from the mitochondrial disease-related ndufs4-/- mouse

Respiratory complex I (NADH:ubiquinone oxidoreductase) is essential for cellular energy production and NAD+ homeostasis. Complex I mutations cause neuromuscular, mitochondrial diseases, such as Leigh Syndrome, but their molecular-level consequences remain poorly understood. Here, we use a popular complex I-linked mitochondrial disease model, the ndufs4-/- mouse, to define the structural, biochemical, and functional consequences of the absence of subunit NDUFS4. Cryo-EM analyses of the complex I from ndufs4-/- mouse hearts revealed a loose association of the NADH-dehydrogenase module, and discrete classes containing either assembly factor NDUFAF2 or subunit NDUFS6. Subunit NDUFA12, which replaces its paralogue NDUFAF2 in mature complex I, is absent from all classes, compounding the deletion of NDUFS4 and preventing maturation of an NDUFS4-free enzyme. We propose that NDUFAF2 recruits the NADH-dehydrogenase module during assembly of the complex. Taken together, the findings provide new molecular-level understanding of the ndufs4-/- mouse model and complex I-linked mitochondrial disease.

The mediating role of inflammaging between mitochondrial dysfunction and sarcopenia in aging: a review

Sarcopenia, characterized by the insidious reduction of skeletal muscle mass and strength, detrimentally affects the quality of life in elderly cohorts. Present therapeutic strategies are confined to physiotherapeutic interventions, signaling a critical need for elucidation of the etiological underpinnings to facilitate the development of innovative pharmacotherapies. Recent scientific inquiries have associated mitochondrial dysfunction and inflammation with the etiology of sarcopenia. Mitochondria are integral to numerous fundamental cellular processes within muscle tissue, including but not limited to apoptosis, autophagy, signaling via reactive oxygen species, and the maintenance of protein equilibrium. Deviations in mitochondrial dynamics, coupled with compromised oxidative capabilities, autophagic processes, and protein equilibrium, result in disturbances to muscular architecture and functionality. Mitochondrial dysfunction is particularly detrimental as it diminishes oxidative phosphorylation, escalates apoptotic activity, and hinders calcium homeostasis within muscle cells. Additionally, deleterious feedback loops of deteriorated respiration, exacerbated oxidative injury, and diminished quality control mechanisms precipitate the acceleration of muscular senescence. Notably, mitochondria exhibiting deficient energetic metabolism are pivotal in precipitating the shift from normative muscle aging to a pathogenic state. This analytical review meticulously examines the complex interplay between mitochondrial dysfunction, persistent inflammation, and the pathogenesis of sarcopenia. It underscores the imperative to alleviate inflammation and amend mitochondrial anomalies within geriatric populations as a strategy to forestall and manage sarcopenia. An initial overview provides a succinct exposition of sarcopenia and its clinical repercussions. The discourse then progresses to an examination of the direct correlation between mitochondrial dysfunction and the genesis of sarcopenia. Concomitantly, it accentuates potential synergistic effects between inflammatory responses and mitochondrial insufficiencies during the aging of skeletal muscle, thereby casting light upon emergent therapeutic objectives. In culmination, this review distills the prevailing comprehension of the mitochondrial and inflammatory pathways implicated in sarcopenia and delineates extant lacunae in knowledge to orient subsequent scientific inquiry.

Tetracyclines activate mitoribosome quality control and reduce ER stress to promote cell survival

Mitochondrial diseases are a group of disorders defined by defects in oxidative phosphorylation caused by nuclear- or mitochondrial-encoded gene mutations. A main cellular phenotype of mitochondrial disease mutations is redox imbalances and inflammatory signaling underlying pathogenic signatures of these patients. One method to rescue this cell death vulnerability is the inhibition of mitochondrial translation using tetracyclines. However, the mechanisms whereby tetracyclines promote cell survival are unknown. Here, we show that tetracyclines inhibit the mitochondrial ribosome and promote survival through suppression of endoplasmic reticulum (ER) stress. Tetracyclines increase mitochondrial levels of the mitoribosome quality control factor MALSU1 (Mitochondrial Assembly of Ribosomal Large Subunit 1) and promote its recruitment to the mitoribosome large subunit, where MALSU1 is necessary for tetracycline-induced survival and suppression of ER stress. Glucose starvation induces ER stress to activate the unfolded protein response and IRE1α-mediated cell death that is inhibited by tetracyclines. These studies establish a new interorganelle communication whereby inhibition of the mitoribosome signals to the ER to promote survival, implicating basic mechanisms of cell survival and treatment of mitochondrial diseases.

Peripheral macrophages drive CNS disease in the Ndufs4(-/-) model of Leigh syndrome

Subacute necrotizing encephalopathy, or Leigh syndrome (LS), is the most common pediatric presentation of genetic mitochondrial disease. LS is a multi-system disorder with severe neurologic, metabolic, and musculoskeletal symptoms. The presence of progressive, symmetric, and necrotizing lesions in the brainstem are a defining feature of the disease, and the major cause of morbidity and mortality, but the mechanisms underlying their pathogenesis have been elusive. Recently, we demonstrated that high-dose pexidartinib, a CSF1R inhibitor, prevents LS CNS lesions and systemic disease in the Ndufs4(-/-) mouse model of LS. While the dose-response in this study implicated peripheral immune cells, the immune populations involved have not yet been elucidated. Here, we used a targeted genetic tool, deletion of the colony-stimulating Factor 1 receptor (CSF1R) macrophage super-enhancer FIRE (Csf1rΔFIRE), to specifically deplete microglia and define the role of microglia in the pathogenesis of LS. Homozygosity for the Csf1rΔFIRE allele ablates microglia in both control and Ndufs4(-/-) animals, but onset of CNS lesions and sequalae in the Ndufs4(-/-), including mortality, are only marginally impacted by microglia depletion. The overall development of necrotizing CNS lesions is not altered, though microglia remain absent. Finally, histologic analysis of brainstem lesions provides direct evidence of a causal role for peripheral macrophages in the characteristic CNS lesions. These data demonstrate that peripheral macrophages play a key role in the pathogenesis of disease in the Ndufs4(-/-) model.

Defective function of α-ketoglutarate dehydrogenase exacerbates mitochondrial ATP deficits during complex I deficiency

The NDUFS4 knockout (KO) mouse phenotype resembles the human Complex I deficiency Leigh Syndrome. The irreversible succination of protein thiols by fumarate is increased in select regions of the NDUFS4 KO brain affected by neurodegeneration. We report that dihydrolipoyllysine-residue succinyltransferase (DLST), a component of the α-ketoglutarate dehydrogenase complex (KGDHC) of the tricarboxylic acid (TCA) cycle, is succinated in the affected regions of the NDUFS4 KO brain. Succination of DLST reduced KGDHC activity in the brainstem (BS) and olfactory bulb (OB) of KO mice. The defective production of KGDHC derived succinyl-CoA resulted in decreased mitochondrial substrate level phosphorylation (SLP), further aggravating the existing oxidative phosphorylation (OXPHOS) ATP deficit. Protein succinylation, an acylation modification that requires succinyl-CoA, was reduced in the KO mice. Modeling succination of a cysteine in the spatial vicinity of the DLST active site or introduction of succinomimetic mutations recapitulates these metabolic deficits. Our data demonstrate that the biochemical deficit extends beyond impaired Complex I assembly and OXPHOS deficiency, functionally impairing select components of the TCA cycle to drive metabolic perturbations in affected neurons.

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

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

Structural rather than catalytic role for mitochondrial respiratory chain supercomplexes

Mammalian mitochondrial respiratory chain (MRC) complexes are able to associate into quaternary structures named supercomplexes (SCs), which normally coexist with non-bound individual complexes. The functional significance of SCs has not been fully clarified and the debate has been centered on whether or not they confer catalytic advantages compared with the non-bound individual complexes. Mitochondrial respiratory chain organization does not seem to be conserved in all organisms. In fact, and differently from mammalian species, mitochondria from Drosophila melanogaster tissues are characterized by low amounts of SCs, despite the high metabolic demands and MRC activity shown by these mitochondria. Here, we show that attenuating the biogenesis of individual respiratory chain complexes was accompanied by increased formation of stable SCs, which are missing in Drosophila melanogaster in physiological conditions. This phenomenon was not accompanied by an increase in mitochondrial respiratory activity. Therefore, we conclude that SC formation is necessary to stabilize the complexes in suboptimal biogenesis conditions, but not for the enhancement of respiratory chain catalysis.

Animal Models of Mitochondrial Diseases Associated with Nuclear Gene Mutations

Mitochondrial diseases (MDs) associated with nuclear gene mutations are part of a large group of inherited diseases caused by the suppression of energy metabolism. These diseases are of particular interest, because nuclear genes encode not only most of the structural proteins of the oxidative phosphorylation system (OXPHOS), but also all the proteins involved in the OXPHOS protein import from the cytoplasm and their assembly in mitochondria. Defects in any of these proteins can lead to functional impairment of the respiratory chain, including dysfunction of complex I that plays a central role in cellular respiration and oxidative phosphorylation, which is the most common cause of mitopathologies. Mitochondrial diseases are characterized by an early age of onset and a progressive course and affect primarily energy-consuming tissues and organs. The treatment of MDs should be initiated as soon as possible, but the diagnosis of mitopathologies is extremely difficult because of their heterogeneity and overlapping clinical features. The molecular pathogenesis of mitochondrial diseases is investigated using animal models: i.e. animals carrying mutations causing MD symptoms in humans. The use of mutant animal models opens new opportunities in the study of genes encoding mitochondrial proteins, as well as the molecular mechanisms of mitopathology development, which is necessary for improving diagnosis and developing approaches to drug therapy. In this review, we present the most recent information on mitochondrial diseases associated with nuclear gene mutations and animal models developed to investigate them.

Expert Panel Curation of 113 Primary Mitochondrial Disease Genes for the Leigh Syndrome Spectrum

OBJECTIVE

Primary mitochondrial diseases (PMDs) are heterogeneous disorders caused by inherited mitochondrial dysfunction. Classically defined neuropathologically as subacute necrotizing encephalomyelopathy, Leigh syndrome spectrum (LSS) is the most frequent manifestation of PMD in children, but may also present in adults. A major challenge for accurate diagnosis of LSS in the genomic medicine era is establishing gene-disease relationships (GDRs) for this syndrome with >100 monogenic causes across both nuclear and mitochondrial genomes.

METHODS

The Clinical Genome Resource (ClinGen) Mitochondrial Disease Gene Curation Expert Panel (GCEP), comprising 40 international PMD experts, met monthly for 4 years to review GDRs for LSS. The GCEP standardized gene curation for LSS by refining the phenotypic definition, modifying the ClinGen Gene-Disease Clinical Validity Curation Framework to improve interpretation for LSS, and establishing a scoring rubric for LSS.

RESULTS

The GDR with LSS across the nuclear and mitochondrial genomes was classified as definitive for 31 of 114 GDRs curated (27%), moderate for 38 (33%), limited for 43 (38%), and disputed for 2 (2%). Ninety genes were associated with autosomal recessive inheritance, 16 were maternally inherited, 5 were autosomal dominant, and 3 were X-linked.

INTERPRETATION

GDRs for LSS were established for genes across both nuclear and mitochondrial genomes. Establishing these GDRs will allow accurate variant interpretation, expedite genetic diagnosis of LSS, and facilitate precision medicine, multisystem organ surveillance, recurrence risk counseling, reproductive choice, natural history studies, and determination of eligibility for interventional clinical trials. ANN NEUROL 2023;94:696-712.

Development of a novel in vitro model to study the modulatory role of the respiratory complex I in macrophage effector functions

Increasing evidence demonstrate that the electron transfer chain plays a critical role in controlling the effector functions of macrophages. In this work, we have generated a Ndufs4-/- murine macrophage cell lines. The Ndufs4 gene, which encodes a supernumerary subunit of complex I, is a mutational hotspot in Leigh syndrome patients. Ndufs4-/- macrophages showed decreased complex I activity, altered complex I assembly, and lower levels of maximal respiration and ATP production. These mitochondrial respiration alterations were associated with a shift towards a pro-inflammatory cytokine profile after lipopolysaccharide challenge and improved ability to phagocytose Gram-negative bacteria.

Mitochondria and Brain Disease: A Comprehensive Review of Pathological Mechanisms and Therapeutic Opportunities

Mitochondria play a vital role in maintaining cellular energy homeostasis, regulating apoptosis, and controlling redox signaling. Dysfunction of mitochondria has been implicated in the pathogenesis of various brain diseases, including neurodegenerative disorders, stroke, and psychiatric illnesses. This review paper provides a comprehensive overview of the intricate relationship between mitochondria and brain disease, focusing on the underlying pathological mechanisms and exploring potential therapeutic opportunities. The review covers key topics such as mitochondrial DNA mutations, impaired oxidative phosphorylation, mitochondrial dynamics, calcium dysregulation, and reactive oxygen species generation in the context of brain disease. Additionally, it discusses emerging strategies targeting mitochondrial dysfunction, including mitochondrial protective agents, metabolic modulators, and gene therapy approaches. By critically analysing the existing literature and recent advancements, this review aims to enhance our understanding of the multifaceted role of mitochondria in brain disease and shed light on novel therapeutic interventions.

TREK-1 and TREK-2 Knockout Mice Are Not Resistant to Halothane or Isoflurane

BACKGROUND

A variety of molecular targets for volatile anesthetics have been suggested, including the anesthetic-sensitive potassium leak channel, TREK-1. Knockout of TREK-1 is reported to render mice resistant to volatile anesthetics, making TREK-1 channels compelling targets for anesthetic action. Spinal cord slices from mice, either wild type or an anesthetic- hypersensitive mutant, Ndufs4, display an isoflurane-induced outward potassium leak that correlates with their minimum alveolar concentrations and is blocked by norfluoxetine. The hypothesis was that TREK-1 channels conveyed this current and contribute to the anesthetic hypersensitivity of Ndufs4. The results led to evaluation of a second TREK channel, TREK-2, in control of anesthetic sensitivity.

METHODS

The anesthetic sensitivities of mice carrying knockout alleles of Trek-1 and Trek-2, the double knockout Trek-1;Trek-2, and Ndufs4;Trek-1 were measured. Neurons from spinal cord slices from each mutant were patch clamped to characterize isoflurane-sensitive currents. Norfluoxetine was used to identify TREK-dependent currents.

RESULTS

The mean values for minimum alveolar concentrations (± SD) between wild type and two Trek-1 knockout alleles in mice (P values, Trek-1 compared to wild type) were compared. For wild type, minimum alveolar concentration of halothane was 1.30% (0.10), and minimum alveolar concentration of isoflurane was 1.40% (0.11); for Trek-1tm1Lex, minimum alveolar concentration of halothane was 1.27% (0.11; P = 0.387), and minimum alveolar concentration of isoflurane was 1.38% (0.09; P = 0.268); and for Trek-1tm1Lzd, minimum alveolar concentration of halothane was 1.27% (0.11; P = 0.482), and minimum alveolar concentration of isoflurane was 1.41% (0.12; P = 0.188). Neither allele was resistant for loss of righting reflex. The EC50 values of Ndufs4;Trek-1tm1Lex did not differ from Ndufs4 (for Ndufs4, EC50 of halothane, 0.65% [0.05]; EC50 of isoflurane, 0.63% [0.05]; and for Ndufs4;Trek-1tm1Lex, EC50 of halothane, 0.58% [0.07; P = 0.004]; and EC50 of isoflurane, 0.61% [0.06; P = 0.442]). Loss of TREK-2 did not alter anesthetic sensitivity in a wild-type or Trek-1 genetic background. Loss of TREK-1, TREK-2, or both did not alter the isoflurane-induced currents in wild-type cells but did cause them to be norfluoxetine insensitive.

CONCLUSIONS

Loss of TREK channels did not alter anesthetic sensitivity in mice, nor did it eliminate isoflurane-induced transmembrane currents. However, the isoflurane-induced currents are norfluoxetine-resistant in Trek mutants, indicating that other channels may function in this role when TREK channels are deleted.

EDITOR’S PERSPECTIVE

Yeast NDI1 reconfigures neuronal metabolism and prevents the unfolded protein response in mitochondrial complex I deficiency

Mutations in subunits of the mitochondrial NADH dehydrogenase cause mitochondrial complex I deficiency, a group of severe neurological diseases that can result in death in infancy. The pathogenesis of complex I deficiency remain poorly understood, and as a result there are currently no available treatments. To better understand the underlying mechanisms, we modelled complex I deficiency in Drosophila using knockdown of the mitochondrial complex I subunit ND-75 (NDUFS1) specifically in neurons. Neuronal complex I deficiency causes locomotor defects, seizures and reduced lifespan. At the cellular level, complex I deficiency does not affect ATP levels but leads to mitochondrial morphology defects, reduced endoplasmic reticulum-mitochondria contacts and activation of the endoplasmic reticulum unfolded protein response (UPR) in neurons. Multi-omic analysis shows that complex I deficiency dramatically perturbs mitochondrial metabolism in the brain. We find that expression of the yeast non-proton translocating NADH dehydrogenase NDI1, which reinstates mitochondrial NADH oxidation but not ATP production, restores levels of several key metabolites in the brain in complex I deficiency. Remarkably, NDI1 expression also reinstates endoplasmic reticulum-mitochondria contacts, prevents UPR activation and rescues the behavioural and lifespan phenotypes caused by complex I deficiency. Together, these data show that metabolic disruption due to loss of neuronal NADH dehydrogenase activity cause UPR activation and drive pathogenesis in complex I deficiency.

NDUFS4 Regulates Cristae Remodeling in Diabetic Kidney Disease

The mitochondrial electron transport chain (ETC) is a highly adaptive process to meet metabolic demands of the cell, and its dysregulation has been associated with diverse clinical pathologies. However, the role and nature of impaired ETC in kidney diseases remains poorly understood. Here, we generated diabetic mice with podocyte-specific overexpression of Ndufs4, an accessory subunit of mitochondrial complex I, as a model to investigate the role of ETC integrity in diabetic kidney disease (DKD). We find that these conditional mice exhibit significant improvements in cristae morphology, mitochondrial dynamics, and albuminuria. By coupling proximity labeling with super-resolution imaging, we also identify the role of cristae shaping proteins in linking NDUFS4 with improved cristae morphology. Taken together, we discover the central role of NDUFS4 as a powerful regulator of cristae remodeling, respiratory supercomplexes assembly, and mitochondrial ultrastructure in vitro and in vivo. We propose that targeting NDUFS4 represents a promising approach to slow the progression of DKD.

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.

Acarbose suppresses symptoms of mitochondrial disease in a mouse model of Leigh syndrome

Mitochondrial diseases represent a spectrum of disorders caused by impaired mitochondrial function, ranging in severity from mortality during infancy to progressive adult-onset disease. Mitochondrial dysfunction is also recognized as a molecular hallmark of the biological ageing process. Rapamycin, a drug that increases lifespan and health during normative ageing, also increases survival and reduces neurological symptoms in a mouse model of the severe mitochondrial disease Leigh syndrome. The Ndufs4 knockout (Ndufs4^-/-) mouse lacks the complex I subunit NDUFS4 and shows rapid onset and progression of neurodegeneration mimicking patients with Leigh syndrome. Here we show that another drug that extends lifespan and delays normative ageing in mice, acarbose, also suppresses symptoms of disease and improves survival of Ndufs4^-/- mice. Unlike rapamycin, acarbose rescues disease phenotypes independently of inhibition of the mechanistic target of rapamycin. Furthermore, rapamycin and acarbose have additive effects in delaying neurological symptoms and increasing maximum lifespan in Ndufs4^-/- mice. We find that acarbose remodels the intestinal microbiome and alters the production of short-chain fatty acids. Supplementation with tributyrin, a source of butyric acid, recapitulates some effects of acarbose on lifespan and disease progression, while depletion of the endogenous microbiome in Ndufs4^-/- mice appears to fully recapitulate the effects of acarbose on healthspan and lifespan in these animals. To our knowledge, this study provides the first evidence that alteration of the gut microbiome plays a significant role in severe mitochondrial disease and provides further support for the model that biological ageing and severe mitochondrial disorders share underlying common mechanisms.

A ketogenic diet alters mTOR activity, systemic metabolism and potentially prevents collagen degradation associated with chronic alcohol consumption in mice

INTRODUCTION

A ketogenic diet (KD), which is a high fat, low carbohydrate diet has been shown to inhibit the mammalian target of rapamycin (mTOR) pathway and alter the redox state. Inhibition of the mTOR complex has been associated with the attenuation and alleviation of various metabolic and- inflammatory diseases such as neurodegeneration, diabetes, and metabolic syndrome. Various metabolic pathways and signalling mechanisms have been explored to assess the therapeutic potential of mTOR inhibition. However, chronic alcohol consumption has also been reported to alter mTOR activity, the cellular redox- and inflammatory state. Thus, a relevant question that remains is what effect chronic alcohol consumption would have on mTOR activity and overall metabolism during a KD-based intervention.

OBJECTIVES

The aim of this study was to evaluate the effect of alcohol and a KD on the phosphorylation of the mTORC1 target p70S6K, systemic metabolism as well as the redox- and inflammatory state in a mouse model.

METHODS

Mice were fed either a control diet with/without alcohol or a KD with/without alcohol for three weeks. After the dietary intervention, samples were collected and subjected towards western blot analysis, multi-platform metabolomics analysis and flow cytometry.

RESULTS

Mice fed a KD exhibited significant mTOR inhibition and reduction in growth rate. Alcohol consumption alone did not markedly alter mTOR activity or growth rate but moderately increased mTOR inhibition in mice fed a KD. In addition, metabolic profiling showed alteration of several metabolic pathways as well as the redox state following consumption of a KD and alcohol. A KD was also observed to potentially prevent bone loss and collagen degradation associated with chronic alcohol consumption, as indicated by hydroxyproline metabolism.

CONCLUSION

This study sheds light on the influence that a KD alongside alcohol intake can exert on not just mTOR, but also their effect on metabolic reprogramming and the redox state.

Tetracycline-dependent inhibition of mitoribosome protein elongation in mitochondrial disease mutant cells suppresses IRE1α to promote cell survival

Mitochondrial diseases are a group of disorders defined by defects in oxidative phosphorylation caused by nuclear- or mitochondrial-encoded gene mutations. A main cellular phenotype of mitochondrial disease mutations are redox imbalances and inflammatory signaling underlying pathogenic signatures of these patients. Depending on the type of mitochondrial mutation, certain mechanisms can efficiently rescue cell death vulnerability. One method is the inhibition of mitochondrial translation elongation using tetracyclines, potent suppressors of cell death in mitochondrial disease mutant cells. However, the mechanisms whereby tetracyclines promote cell survival are unknown. Here, we show that in mitochondrial mutant disease cells, tetracycline-mediated inhibition of mitoribosome elongation promotes survival through suppression of the ER stress IRE1α protein. Tetracyclines increased levels of the splitting factor MALSU1 (Mitochondrial Assembly of Ribosomal Large Subunit 1) at the mitochondria with recruitment to the mitochondrial ribosome (mitoribosome) large subunit. MALSU1, but not other quality control factors, was required for tetracycline-induced cell survival in mitochondrial disease mutant cells during glucose starvation. In these cells, nutrient stress induced cell death through IRE1α activation associated with a strong protein loading in the ER lumen. Notably, tetracyclines rescued cell death through suppression of IRE1α oligomerization and activity. Consistent with MALSU1 requirement, MALSU1 deficient mitochondrial mutant cells were sensitive to glucose-deprivation and exhibited increased ER stress and activation of IRE1α that was not reversed by tetracyclines. These studies show that inhibition of mitoribosome elongation signals to the ER to promote survival, establishing a new interorganelle communication between the mitoribosome and ER with implications in basic mechanisms of cell survival and treatment of mitochondrial diseases.

Research development and the prospect of animal models of mitochondrial DNA-related mitochondrial diseases

Mitochondrial diseases (MDs) are genetic and clinical heterogeneous diseases caused by mitochondrial oxidative phosphorylation defects. It is not only one of the most common genetic diseases, but also the only genetic disease involving two different genomes in humans. As a result of the complicated genetic condition, the pathogenesis of MDs is not entirely elucidated at present, and there is a lack of effective treatment in the clinic. Establishing the ideal animal models is the critical preclinical platform to explore the pathogenesis of MDs and to verify new therapeutic strategies. However, the development of animal modeling of mitochondrial DNA (mtDNA)-related MDs is time-consuming due to the limitations of physiological structure and technology. A small number of animal models of mtDNA mutations have been constructed using cell hybridization and other methods. However, the diversity of mtDNA mutation sites and clinical phenotypes make establishing relevant animal models tricky. The development of gene editing technology has become a new hope for establishing animal models of mtDNA-related mitochondrial diseases.

Oxidative phosphorylation selectively orchestrates tissue macrophage homeostasis

In vitro studies have associated oxidative phosphorylation (OXPHOS) with anti-inflammatory macrophages, whereas pro-inflammatory macrophages rely on glycolysis. However, the metabolic needs of macrophages in tissues (TMFs) to fulfill their homeostatic activities are incompletely understood. Here, we identified OXPHOS as the highest discriminating process among TMFs from different organs in homeostasis by analysis of RNA-seq data in both humans and mice. Impairing OXPHOS in TMFs via Tfam deletion differentially affected TMF populations. Tfam deletion resulted in reduction of alveolar macrophages (AMs) due to impaired lipid-handling capacity, leading to increased cholesterol content and cellular stress, causing cell-cycle arrest in vivo. In obesity, Tfam depletion selectively ablated pro-inflammatory lipid-handling white adipose tissue macrophages (WAT-MFs), thus preventing insulin resistance and hepatosteatosis. Hence, OXPHOS, rather than glycolysis, distinguishes TMF populations and is critical for the maintenance of TMFs with a high lipid-handling activity, including pro-inflammatory WAT-MFs. This could provide a selective therapeutic targeting tool.

Genetic aberration analysis of mitochondrial respiratory complex I implications in the development of neurological disorders and their clinical significance

Growing neurological diseases pose difficult challenges for modern medicine to diagnose and manage them effectively. Many neurological disorders mainly occur due to genetic alteration in genes encoding mitochondrial proteins. Moreover, mitochondrial genes exhibit a higher rate of mutation due to the generation of Reactive oxygen species (ROS) during oxidative phosphorylation operating in their vicinity. Among the different complexes of Electron transport chain (ETC), NADH: Ubiquinone oxidoreductase (Mitochondrial complex I) is the most important. This multimeric enzyme, composed of 44 subunits, is encoded by both nuclear and mitochondrial genes. It often exhibits mutations resulting in development of various neurological diseases. The most prominent diseases include leigh syndrome (LS), leber hereditary optic neuropathy (LHON), mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS), myoclonic epilepsy associated with ragged-red fibers (MERRF), idiopathic Parkinson's disease (PD) and, Alzheimer's disease (AD). Preliminary data suggest that mitochondrial complex I subunit genes mutated are frequently of nuclear origin; however, most of the mtDNA gene encoding subunits are also primarily involved. In this review, we have discussed the genetic origins of neurological disorders involving mitochondrial complex I and signified recent approaches to unravel the diagnostic and therapeutic potentials and their management.

Transcriptomic analyses reveal neuronal specificity of Leigh syndrome associated genes

Leigh syndrome is a rare, inherited, complex neurometabolic disorder with genetic and clinical heterogeneity. Features present in affected patients range from classical stepwise developmental regression to ataxia, seizures, tremor, and occasionally psychiatric manifestations. Currently, more than 100 monogenic causes of Leigh syndrome have been identified, yet the pathophysiology remains unknown. Here, we sought to determine the cellular specificity within the brain of all genes currently associated with Leigh syndrome. Further, we aimed to investigate potential genetic commonalities between Leigh syndrome and other disorders with overlapping clinical features. Enrichment of our target genes within the brain was evaluated with co-expression (CoExp) network analyses constructed using existing UK Brain Expression Consortium data. To determine the cellular specificity of the Leigh associated genes, we employed expression weighted cell type enrichment (EWCE) analysis of single-cell RNA-Seq data. Finally, CoExp network modules demonstrating enrichment of Leigh syndrome associated genes were then utilised for synaptic gene ontology analysis and heritability analysis. CoExp network analyses revealed that Leigh syndrome associated genes exhibit the highest levels of expression in brain regions most affected on MRI in affected patients. EWCE revealed significant enrichment of target genes in hippocampal and somatosensory pyramidal neurons and interneurons of the brain. Analysis of CoExp modules enriched with our target genes revealed preferential association with pre-synaptic structures. Heritability studies suggested some common enrichment between Leigh syndrome and Parkinson disease and epilepsy. Our findings suggest a primary mitochondrial dysfunction as the underlying basis of Leigh syndrome, with associated genes primarily expressed in neuronal cells.

Leigh syndrome

Leigh syndrome, or subacute necrotizing encephalomyelopathy, was initially recognized as a neuropathological entity in 1951. Bilateral symmetrical lesions, typically extending from the basal ganglia and thalamus through brainstem structures to the posterior columns of the spinal cord, are characterized microscopically by capillary proliferation, gliosis, severe neuronal loss, and relative preservation of astrocytes. Leigh syndrome is a pan-ethnic disorder usually with onset in infancy or early childhood, but late-onset forms occur, including in adult life. Over the last six decades it has emerged that this complex neurodegenerative disorder encompasses more than 100 separate monogenic disorders associated with enormous clinical and biochemical heterogeneity. This chapter discusses clinical, biochemical and neuropathological aspects of the disorder, and postulated pathomechanisms. Known genetic causes, including defects of 16 mitochondrial DNA (mtDNA) genes and approaching 100 nuclear genes, are categorized into disorders of subunits and assembly factors of the five oxidative phosphorylation enzymes, disorders of pyruvate metabolism and vitamin and cofactor transport and metabolism, disorders of mtDNA maintenance, and defects of mitochondrial gene expression, protein quality control, lipid remodeling, dynamics, and toxicity. An approach to diagnosis is presented, together with known treatable causes and an overview of current supportive management options and emerging therapies on the horizon.

Iron status influences mitochondrial disease progression in Complex I-deficient mice

Mitochondrial dysfunction caused by aberrant Complex I assembly and reduced activity of the electron transport chain is pathogenic in many genetic and age-related diseases. Mice missing the Complex I subunit NADH dehydrogenase [ubiquinone] iron-sulfur protein 4 (NDUFS4) are a leading mammalian model of severe mitochondrial disease that exhibit many characteristic symptoms of Leigh Syndrome including oxidative stress, neuroinflammation, brain lesions, and premature death. NDUFS4 knockout mice have decreased expression of nearly every Complex I subunit. As Complex I normally contains at least 8 iron-sulfur clusters and more than 25 iron atoms, we asked whether a deficiency of Complex I may lead to iron perturbations, thereby accelerating disease progression. Consistent with this, iron supplementation accelerates symptoms of brain degeneration in these mice, while iron restriction delays the onset of these symptoms, reduces neuroinflammation, and increases survival. NDUFS4 knockout mice display signs of iron overload in the liver including increased expression of hepcidin and show changes in iron-responsive element-regulated proteins consistent with increased cellular iron that were prevented by iron restriction. These results suggest that perturbed iron homeostasis may contribute to pathology in Leigh Syndrome and possibly other mitochondrial disorders.

Creation of Mitochondrial Disease Models Using Mitochondrial DNA Editing

Mitochondrial diseases are a large class of human hereditary diseases, accompanied by the dysfunction of mitochondria and the disruption of cellular energy synthesis, that affect various tissues and organ systems. Mitochondrial DNA mutation-caused disorders are difficult to study because of the insufficient number of clinical cases and the challenges of creating appropriate models. There are many cellular models of mitochondrial diseases, but their application has a number of limitations. The most proper and promising models of mitochondrial diseases are animal models, which, unfortunately, are quite rare and more difficult to develop. The challenges mainly arise from the structural features of mitochondria, which complicate the genetic editing of mitochondrial DNA. This review is devoted to discussing animal models of human mitochondrial diseases and recently developed approaches used to create them. Furthermore, this review discusses mitochondrial diseases and studies of metabolic disorders caused by the mitochondrial DNA mutations underlying these diseases.

Lactate metabolism is essential in early-onset mitochondrial myopathy

Myopathies secondary to mitochondrial electron transport chain (ETC) dysfunction can result in devastating disease. While the consequences of ETC defects have been extensively studied in culture, little in vivo data are available. Using a mouse model of severe, early-onset mitochondrial myopathy, we characterized the proteomic, transcriptomic, and metabolic characteristics of disease progression. Unexpectedly, ETC dysfunction in muscle results in reduced expression of glycolytic enzymes in our animal model and patient muscle biopsies. The decrease in glycolysis was mediated by loss of constitutive Hif1α signaling, down-regulation of the purine nucleotide cycle enzyme AMPD1, and activation of AMPK. In vivo isotope tracing experiments indicated that myopathic muscle relies on lactate import to supply central carbon metabolites. Inhibition of lactate import reduced steady-state levels of tricarboxylic acid cycle intermediates and compromised the life span of myopathic mice. These data indicate an unexpected mode of metabolic reprogramming in severe mitochondrial myopathy that regulates disease progression.

Continuous Hypoxia Reduces Retinal Ganglion Cell Degeneration in a Mouse Model of Mitochondrial Optic Neuropathy

Purpose

To test whether continuous hypoxia is neuroprotective to retinal ganglion cells (RGCs) in a mouse model of mitochondrial optic neuropathy.

Methods

RGC degeneration was assessed in genetically modified mice in which the floxed gene for the complex I subunit NDUFS4 is deleted from RGCs using Vlgut2-driven Cre recombinase. Beginning at postnatal day 25 (P25), Vglut2-Cre;ndufs4loxP/loxP mice and control littermates were housed under hypoxia (11% oxygen) or kept under normoxia (21% oxygen). Survival of RGC somas and axons was assessed at P60 and P90 via histological analysis of retinal flatmounts and optic nerve cross-sections, respectively. Retinal tissue was also assessed for gliosis and neuroinflammation using western blot and immunofluorescence.

Results

Consistent with our previous characterization of this model, at least one-third of RGCs had degenerated by P60 in Vglut2-Cre;ndufs4loxP/loxP mice remaining under normoxia. However, continuous hypoxia resulted in complete rescue of RGC somas and axons at this time point, with normal axonal myelination observed on electron microscopy. Though only partial, hypoxia-mediated rescue of complex I-deficient RGC somas and axons remained significant at P90. Hypoxia prevented reactive gliosis at P60, but the retinal accumulation of Iba1+ mononuclear phagocytic cells was not substantially reduced.

Conclusions

Continuous hypoxia achieved dramatic rescue of early RGC degeneration in mice with severe mitochondrial dysfunction. Although complete rescue was not durable to P90, our observations suggest that investigating the mechanisms underlying hypoxia-mediated neuroprotection of RGCs may identify useful therapeutic strategies for optic neuropathies resulting from less profound mitochondrial impairment, such as Leber hereditary optic neuropathy.

Impaired mitochondrial oxidative metabolism in skeletal progenitor cells leads to musculoskeletal disintegration

Although skeletal progenitors provide a reservoir for bone-forming osteoblasts, the major energy source for their osteogenesis remains unclear. Here, we demonstrate a requirement for mitochondrial oxidative phosphorylation in the osteogenic commitment and differentiation of skeletal progenitors. Deletion of Evolutionarily Conserved Signaling Intermediate in Toll pathways (ECSIT) in skeletal progenitors hinders bone formation and regeneration, resulting in skeletal deformity, defects in the bone marrow niche and spontaneous fractures followed by persistent nonunion. Upon skeletal fracture, Ecsit-deficient skeletal progenitors migrate to adjacent skeletal muscle causing muscle atrophy. These phenotypes are intrinsic to ECSIT function in skeletal progenitors, as little skeletal abnormalities were observed in mice lacking Ecsit in committed osteoprogenitors or mature osteoblasts. Mechanistically, Ecsit deletion in skeletal progenitors impairs mitochondrial complex assembly and mitochondrial oxidative phosphorylation and elevates glycolysis. ECSIT-associated skeletal phenotypes were reversed by in vivo reconstitution with wild-type ECSIT expression, but not a mutant displaying defective mitochondrial localization. Collectively, these findings identify mitochondrial oxidative phosphorylation as the prominent energy-driving force for osteogenesis of skeletal progenitors, governing musculoskeletal integrity.

Targeting adaptive cellular responses to mitochondrial bioenergetic deficiencies in human disease

Mitochondrial dysfunction is increasingly appreciated as a central contributor to human disease. Oxidative metabolism at the mitochondrial respiratory chain produces ATP and is intricately tied to redox homeostasis and biosynthetic pathways. Metabolic stress arising from genetic mutations in mitochondrial genes and environmental factors such as malnutrition or overnutrition is perceived by the cell and leads to adaptive and maladaptive responses that can underlie pathology. Here, we will outline cellular sensors that react to alterations in energy production, organellar redox, and metabolites stemming from mitochondrial disease (MD) mutations. MD is a heterogeneous group of disorders primarily defined by defects in mitochondrial oxidative phosphorylation from nuclear or mitochondrial-encoded gene mutations. Preclinical therapies that improve fitness of MD mouse models have been recently identified. Targeting metabolic/energetic deficiencies, maladaptive signaling processes, and hyper-oxygenation of tissues are all strategies aside from direct genetic approaches that hold therapeutic promise. A further mechanistic understanding of these curative processes as well as the identification of novel targets will significantly impact mitochondrial biology and disease research.

Dietary lipids inhibit mitochondria transfer to macrophages to divert adipocyte-derived mitochondria into the blood

Adipocytes transfer mitochondria to macrophages in white and brown adipose tissues to maintain metabolic homeostasis. In obesity, adipocyte-to-macrophage mitochondria transfer is impaired, and instead, adipocytes release mitochondria into the blood to induce a protective antioxidant response in the heart. We found that adipocyte-to-macrophage mitochondria transfer in white adipose tissue is inhibited in murine obesity elicited by a lard-based high-fat diet, but not a hydrogenated-coconut-oil-based high-fat diet, aging, or a corn-starch diet. The long-chain fatty acids enriched in lard suppress mitochondria capture by macrophages, diverting adipocyte-derived mitochondria into the blood for delivery to other organs, such as the heart. The depletion of macrophages rapidly increased the number of adipocyte-derived mitochondria in the blood. These findings suggest that dietary lipids regulate mitochondria uptake by macrophages locally in white adipose tissue to determine whether adipocyte-derived mitochondria are released into systemic circulation to support the metabolic adaptation of distant organs in response to nutrient stress.

Differential requirements for mitochondrial electron transport chain components in the adult murine liver

Mitochondrial electron transport chain (ETC) dysfunction due to mutations in the nuclear or mitochondrial genome is a common cause of metabolic disease in humans and displays striking tissue specificity depending on the affected gene. The mechanisms underlying tissue-specific phenotypes are not understood. Complex I (cI) is classically considered the entry point for electrons into the ETC, and in vitro experiments indicate that cI is required for basal respiration and maintenance of the NAD+/NADH ratio, an indicator of cellular redox status. This finding has largely not been tested in vivo. Here, we report that mitochondrial complex I is dispensable for homeostasis of the adult mouse liver; animals with hepatocyte-specific loss of cI function display no overt phenotypes or signs of liver damage, and maintain liver function, redox and oxygen status. Further analysis of cI-deficient livers did not reveal significant proteomic or metabolic changes, indicating little to no compensation is required in the setting of complex I loss. In contrast, complex IV (cIV) dysfunction in adult hepatocytes results in decreased liver function, impaired oxygen handling, steatosis, and liver damage, accompanied by significant metabolomic and proteomic perturbations. Our results support a model whereby complex I loss is tolerated in the mouse liver because hepatocytes use alternative electron donors to fuel the mitochondrial ETC.

cAMP/PKA Signaling Modulates Mitochondrial Supercomplex Organization

The oxidative phosphorylation (OXPHOS) system couples the transfer of electrons to oxygen with pumping of protons across the inner mitochondrial membrane, ensuring the ATP production. Evidence suggests that respiratory chain complexes may also assemble into supramolecular structures, called supercomplexes (SCs). The SCs appear to increase the efficiency/capacity of OXPHOS and reduce the reactive oxygen species (ROS) production, especially that which is produced by complex I. Studies suggest a mutual regulation between complex I and SCs, while SCs organization is important for complex I assembly/stability, complex I is involved in the supercomplex formation. Complex I is a pacemaker of the OXPHOS system, and it has been shown that the PKA-dependent phosphorylation of some of its subunits increases the activity of the complex, reducing the ROS production. In this work, using in ex vivo and in vitro models, we show that the activation of cAMP/PKA cascade resulted in an increase in SCs formation associated with an enhanced capacity of electron flux and ATP production rate. This is also associated with the phosphorylation of the NDUFS4 subunit of complex I. This aspect highlights the key role of complex I in cellular energy production.

Leber Hereditary Optic Neuropathy: Molecular Pathophysiology and Updates on Gene Therapy

Molecular pathophysiology of LHON was reviewed and the current status of gene therapy for LHON is updated.

Noninvasive Ophthalmic Imaging Measures Retinal Degeneration and Vision Deficits in Ndufs4-/- Mouse Model of Mitochondrial Complex I Deficiency

Purpose

To characterize postnatal ocular pathology in a Ndufs4^−/− mouse model of complex I deficiency using noninvasive retinal imaging and visual testing.

Methods

Ndufs4^−/− mice and wild-type (WT) littermates were analyzed at 3, 5, and 7 weeks postnatal. Retinal morphology was visualized by optical coherence tomography (OCT). OCT images were analyzed for changes in retinal thickness and reflectivity profiles. Visual function was assessed by electroretinogram (ERG) and optomotor reflex (OMR).

Results

Ndufs4^−/− animals have normal OCT morphology at weaning and develop inner plexiform layer atrophy over weeks 5 to 7. Outer retinal layers show hyporeflectivity of the external limiting membrane (ELM) and photoreceptor ellipsoid zone (EZ). Retinal function is impaired at 3 weeks, with profound deficits in b-wave, a-wave, and oscillatory potential amplitudes. The b-wave and oscillatory potential implicit times are delayed, but the a-wave implicit time is unaffected. Ndufs4^−/− animals have normal OMR at 3 weeks and present with increasing acuity and contrast OMR deficits at 5 and 7 weeks. Physiological thinning of inner retinal layers, attenuation of ELM reflectivity, and attenuation of ERG b- and a-wave amplitudes occur in WT C57BL/6 littermates between weeks 3 and 7.

Conclusions

Noninvasive ocular imaging captures early-onset retinal degeneration in Ndufs4^−/− mice and is a tractable approach for investigating retinal pathology subsequent to complex I deficiency.

Translational Relevance

Ophthalmic imaging captures clinically relevant measures of retinal disease in a fast-progressing mouse model of complex I deficiency consistent with human Leigh syndrome.

Double administration of self-complementary AAV9NDUFS4 prevents Leigh disease in Ndufs4-/- mice

Leigh disease, or subacute necrotizing encephalomyelopathy, a genetically heterogeneous condition consistently characterized by defective mitochondrial bioenergetics, is the most common oxidative-phosphorylation related disease in infancy. Both neurological signs and pathological lesions of Leigh disease are mimicked by the ablation of the mouse mitochondrial respiratory chain subunit Ndufs4^−/−, which is part of, and crucial for, normal Complex I activity and assembly, particularly in the brains of both children and mice. We previously conveyed the human NDUFS4 gene to the mouse brain using either single-stranded adeno-associated viral 9 recombinant vectors or the PHP.B adeno-associated viral vector. Both these approaches significantly prolonged the lifespan of the Ndufs4^−/− mouse model but the extension of the survival was limited to a few weeks by the former approach, whereas the latter was applicable to a limited number of mouse strains, but not to primates. Here, we exploited the recent development of new, self-complementary adeno-associated viral 9 vectors, in which the transcription rate of the recombinant gene is markedly increased compared with the single-stranded adeno-associated viral 9 and can be applied to all mammals, including humans. Either single intra-vascular or double intra-vascular and intra-cerebro-ventricular injections were performed at post-natal Day 1. The first strategy ubiquitously conveyed the human NDUFS4 gene product in Ndufs4^−/− mice, doubling the lifespan from 45 to ≈100 days after birth, when the mice developed rapidly progressive neurological failure. However, the double, contemporary intra-vascular and intra-cerebroventricular administration of self-complementary-adeno-associated viral NDUFS4 prolonged healthy lifespan up to 9 months of age. These mice were well and active at euthanization, at 6, 7, 8 and 9 months of age, to investigate the brain and other organs post-mortem. Robust expression of hNDUFS4 was detected in different cerebral areas preserving normal morphology and restoring Complex I activity and assembly. Our results warrant further investigation on the translatability of self-complementary-adeno-associated viral 9 NDUFS4-based therapy in the prodromal phase of the disease in mice and eventually humans.

Metabolic rescue ameliorates mitochondrial encephalo-cardiomyopathy in murine and human iPSC models of Leigh syndrome

BACKGROUND

Mice with deletion of complex I subunit Ndufs4 develop mitochondrial encephalomyopathy resembling Leigh syndrome (LS). The metabolic derangement and underlying mechanisms of cardio-encephalomyopathy in LS remains incompletely understood.

METHODS

We performed echocardiography, electrophysiology, confocal microscopy, metabolic and molecular/morphometric analysis of the mice lacking Ndufs4. HEK293 cells, human iPS cells-derived cardiomyocytes and neurons were used to determine the mechanistic role of mitochondrial complex I deficiency.

RESULTS

LS mice develop severe cardiac bradyarrhythmia and diastolic dysfunction. Human-induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) with Ndufs4 deletion recapitulate LS cardiomyopathy. Mechanistically, we demonstrate a direct link between complex I deficiency, decreased intracellular (nicotinamide adenine dinucleotide) NAD+ /NADH and bradyarrhythmia, mediated by hyperacetylation of the cardiac sodium channel NaV 1.5, particularly at K1479 site. Neuronal apoptosis in the cerebellar and midbrain regions in LS mice was associated with hyperacetylation of p53 and activation of microglia. Targeted metabolomics revealed increases in several amino acids and citric acid cycle intermediates, likely due to impairment of NAD+ -dependent dehydrogenases, and a substantial decrease in reduced Glutathione (GSH). Metabolic rescue by nicotinamide riboside (NR) supplementation increased intracellular NAD+ / NADH, restored metabolic derangement, reversed protein hyperacetylation through NAD+ -dependent Sirtuin deacetylase, and ameliorated cardiomyopathic phenotypes, concomitant with improvement of NaV 1.5 current and SERCA2a function measured by Ca2+ -transients. NR also attenuated neuronal apoptosis and microglial activation in the LS brain and human iPS-derived neurons with Ndufs4 deletion.

CONCLUSIONS

Our study reveals direct mechanistic explanations of the observed cardiac bradyarrhythmia, diastolic dysfunction and neuronal apoptosis in mouse and human induced pluripotent stem cells (iPSC) models of LS.

Microglial response promotes neurodegeneration in the Ndufs4 KO mouse model of Leigh syndrome

Leigh syndrome is a mitochondrial disease characterized by neurodegeneration, neuroinflammation, and early death. Mice lacking NDUFS4, a mitochondrial complex I subunit (Ndufs4 KO mice), have been established as a good animal model for studying human pathology associated with Leigh syndrome. As the disease progresses, there is an increase in neurodegeneration and neuroinflammation, thereby leading to deteriorating neurological symptoms, including motor deficits, breathing alterations, and eventually, death of the animal. However, despite the magnitude of neuroinflammation associated with brain lesions, the role of neuroinflammatory pathways and their main cellular components have not been addressed directly as relevant players in the disease pathology. Here, we investigate the role of microglial cells, the main immune cells of the CNS, in Leigh-like syndrome pathology, by pharmacologically depleting them using the colony-stimulating factor 1 receptor antagonist PLX3397. Microglial depletion extended lifespan and delayed motor symptoms in Ndufs4 KO mice, likely by preventing neuronal loss. Next, we investigated the role of the major cytokine interleukin-6 (IL-6) in the disease progression. IL-6 deficiency partially rescued breathing abnormalities and modulated gliosis but did not extend the lifespan or rescue motor decline in Ndufs4 KO mice. The present results show that microglial accumulation is pathogenic, in a process independent of IL-6, and hints toward a contributing role of neuroinflammation in the disease of Ndufs4 KO mice and potentially in patients with Leigh syndrome.