Defined neuronal populations drive fatal phenotype in a mouse model of Leigh syndrome

Evaluating the efficacy of vatiquinone in preclinical models of mitochondrial disease

Background

Genetic mitochondrial diseases are a major challenge in modern medicine, impacting around 1:4,000 individuals. Leigh syndrome is the most common pediatric presentation of mitochondrial disease. There are currently no effective clinical treatments for mitochondrial disease. In humans, patients are often treated with antioxidants, vitamins, and strategies targeting energetics. The vitamin-E related compound vatiquinone (EPI-743, α-tocotrienol quinone) has been the subject of at least 19 clinical trials in the US since 2012, but the effects of vatiquinone on an animal model of mitochondrial disease have not yet been reported. Here, assessed the impact of vatiquinone on disease progression and in two animal models of mitochondrial disease.

Methods

The efficacy of vatiquinone in vitro was assessed using human fibroblasts treated with the general mitochondrial oxidative stress inducer paraquat, the GPX4 inhibitor RSL3, or the glutathione synthase inhibitor BSO in combination with excess iron. The therapeutic potential of vatiquinone in vivo was assessed using tamoxifen-induced mouse model for GPX4 deficiency and the Ndufs4 knockout mouse model of Leigh syndrome. In both models, animals were treated daily with vatiquinone or vehicle and relevant disease endpoints were assessed.

Results

Vatiquinone robustly prevented death in cultured cells induced by RSL3 or BSO/iron, but had no effect on paraquat induced cell death. Vatiquinone had no impact on disease onset, progression, or survival in either the tamoxifen-inducible GPX4 deficient model or the Ndufs4(-/-) mouse model, though the drug may have reduced seizure risk.

Conclusions

Vatiquinone provided no benefit to survival in two mouse models of disease, but may prevent seizures in the Ndufs4(-/-) model. Our findings are consistent with recent press statements regarding clinical trial results and have implications for drug trial design and reporting in patients with rare diseases.

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.

AAV-mediated interneuron-specific gene replacement for Dravet syndrome

Dravet syndrome (DS) is a devastating developmental epileptic encephalopathy marked by treatment-resistant seizures, developmental delay, intellectual disability, motor deficits, and a 10-20% rate of premature death. Most DS patients harbor loss-of-function mutations in one copy of SCN1A , which has been associated with inhibitory neuron dysfunction. Here we developed an interneuron-targeting AAV human SCN1A gene replacement therapy using cell class-specific enhancers. We generated a split-intein fusion form of SCN1A to circumvent AAV packaging limitations and deliver SCN1A via a dual vector approach using cell class-specific enhancers. These constructs produced full-length Na V 1.1 protein and functional sodium channels in HEK293 cells and in brain cells in vivo . After packaging these vectors into enhancer-AAVs and administering to mice, immunohistochemical analyses showed telencephalic GABAergic interneuron-specific and dose-dependent transgene biodistribution. These vectors conferred strong dose-dependent protection against postnatal mortality and seizures in two DS mouse models carrying independent loss-of-function alleles of Scn1a, at two independent research sites, supporting the robustness of this approach. No mortality or toxicity was observed in wild-type mice injected with single vectors expressing either the N-terminal or C-terminal halves of SCN1A , or the dual vector system targeting interneurons. In contrast, nonselective neuronal targeting of SCN1A conferred less rescue against mortality and presented substantial preweaning lethality. These findings demonstrate proof-of-concept that interneuron-specific AAV-mediated SCN1A gene replacement is sufficient for significant rescue in DS mouse models and suggest it could be an effective therapeutic approach for patients with DS.

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.

Vestibular CCK signaling drives motion sickness-like behavior in mice

Travel can induce motion sickness (MS) in susceptible individuals. MS is an evolutionary conserved mechanism caused by mismatches between motion-related sensory information and past visual and motion memory, triggering a malaise accompanied by hypolocomotion, hypothermia, hypophagia, and nausea. Vestibular nuclei (VN) are critical for the processing of movement input from the inner ear. Motion-induced activation of VN neurons recapitulates MS-related signs. However, the genetic identity of VN neurons mediating MS-related autonomic and aversive responses remains unknown. Here, we identify a central role of cholecystokinin (CCK)-expressing VN neurons in motion-induced malaise. Moreover, we show that CCK VN inputs onto the parabrachial nucleus activate Calca-expressing neurons and are sufficient to establish avoidance to novel food, which is prevented by CCK-A receptor antagonism. These observations provide greater insight into the neurobiological regulation of MS by identifying the neural substrates of MS and providing potential targets for treatment.

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.

Epilepsy: Mitochondrial connections to the 'Sacred' disease

Over 65 million people suffer from recurrent, unprovoked seizures. The lack of validated biomarkers specific for myriad forms of epilepsy makes diagnosis challenging. Diagnosis and monitoring of childhood epilepsy add to the need for non-invasive biomarkers, especially when evaluating antiseizure medications. Although underlying mechanisms of epileptogenesis are not fully understood, evidence for mitochondrial involvement is substantial. Seizures affect 35%-60% of patients diagnosed with mitochondrial diseases. Mitochondrial dysfunction is pathophysiological in various epilepsies, including those of non-mitochondrial origin. Decreased ATP production caused by malfunctioning brain cell mitochondria leads to altered neuronal bioenergetics, metabolism and neurological complications, including seizures. Iron-dependent lipid peroxidation initiates ferroptosis, a cell death pathway that aligns with altered mitochondrial bioenergetics, metabolism and morphology found in neurodegenerative diseases (NDDs). Studies in mouse genetic models with seizure phenotypes where the function of an essential selenoprotein (GPX4) is targeted suggest roles for ferroptosis in epilepsy. GPX4 is pivotal in NDDs, where selenium protects interneurons from ferroptosis. Selenium is an essential central nervous system micronutrient and trace element. Low serum concentrations of selenium and other trace elements and minerals, including iron, are noted in diagnosing childhood epilepsy. Selenium supplements alleviate intractable seizures in children with reduced GPX activity. Copper and cuproptosis, like iron and ferroptosis, link to mitochondria and NDDs. Connecting these mechanistic pathways to selenoproteins provides new insights into treating seizures, pointing to using medicines including prodrugs of lipoic acid to treat epilepsy and to potential alternative therapeutic approaches including transcranial magnetic stimulation (transcranial), photobiomodulation and vagus nerve stimulation.

Glutamine metabolism in diseases associated with mitochondrial dysfunction

Mitochondrial dysfunction can arise from genetic defects or environmental exposures and impact a wide range of biological processes. Among these are metabolic pathways involved in glutamine catabolism, anabolism, and glutamine-glutamate cycling. In recent years, altered glutamine metabolism has been found to play important roles in the pathologic consequences of mitochondrial dysfunction. Glutamine is a pleiotropic molecule, not only providing an alternate carbon source to glucose in certain conditions, but also playing unique roles in cellular communication in neurons and astrocytes. Glutamine consumption and catabolic flux can be significantly altered in settings of genetic mitochondrial defects or exposure to mitochondrial toxins, and alterations to glutamine metabolism appears to play a particularly significant role in neurodegenerative diseases. These include primary mitochondrial diseases like Leigh syndrome (subacute necrotizing encephalopathy) and MELAS (mitochondrial myopathy with encephalopathy, lactic acidosis, and stroke-like episodes), as well as complex age-related neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Pharmacologic interventions targeting glutamine metabolizing and catabolizing pathways appear to provide some benefits in cell and animal models of these diseases, indicating glutamine metabolism may be a clinically relevant target. In this review, we discuss glutamine metabolism, mitochondrial disease, the impact of mitochondrial dysfunction on glutamine metabolic processes, glutamine in neurodegeneration, and candidate targets for therapeutic intervention.

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.

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.

Primary Mitochondrial Disorders in the Neonate

Primary mitochondrial disorders (PMDs) are a heterogeneous group of disorders characterized by functional or structural abnormalities in the mitochondria that lead to a disturbance of cellular energy, reactive oxygen species, and free radical production, as well as impairment of other intracellular metabolic functions, causing single- or multiorgan dysfunction. PMDs are caused by pathogenic variants in nuclear and mitochondrial genes, resulting in distinct modes of inheritance. Onset of disease is variable and can occur in the neonatal period, with a high morbidity and mortality. In this article, we review the most common methods used for the diagnosis of PMDs, as well as their prenatal and neonatal presentations. We highlight the shift in the diagnostic approach for PMDs since the introduction of nontargeted molecular tests into clinical practice, which has significantly reduced the use of invasive studies. We discuss common PMDs that can present in the neonate, including general, nonsyndromic presentations as well as specific syndromic disorders. We also review current treatment advances, including the use of mitochondrial "cocktails" based on limited scientific evidence and theoretical reasoning, as well as the impending arrival of personalized mitochondrial-specific treatments.

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.

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.

Differential effects of mTOR inhibition and dietary ketosis in a mouse model of subacute necrotizing encephalomyelopathy

Genetic mitochondrial diseases are the most frequent cause of inherited metabolic disorders and one of the most prevalent causes of heritable neurological disease. Leigh syndrome is the most common clinical presentation of pediatric mitochondrial disease, typically appearing in the first few years of life, and involving severe multisystem pathologies. Clinical care for Leigh syndrome patients is difficult, complicated by the wide range of symptoms including characteristic progressive CNS lesion, metabolic sequelae, and epileptic seizures, which can be intractable to standard management. While no proven therapies yet exist for the underlying mitochondrial disease, a ketogenic diet has led to some reports of success in managing mitochondrial epilepsies, with ketosis reducing seizure risk and severity. The impact of ketosis on other aspects of disease progression in Leigh syndrome has not been studied, however, and a rigorous study of the impact of ketosis on seizures in mitochondrial disease is lacking. Conversely, preclinical efforts have identified the intracellular nutrient signaling regulator mTOR as a promising therapeutic target, with data suggesting the benefits are mediated by metabolic changes. mTOR inhibition alleviates epilepsies arising from defects in TSC, an mTOR regulator, but the therapeutic potential of mTOR inhibition in seizures related to primary mitochondrial dysfunction is unknown. Given that ketogenic diet is used clinically in the setting of mitochondrial disease, and mTOR inhibition is in clinical trials for intractable pediatric epilepsies of diverse causal origins, a direct experimental assessment of their effects is imperative. Here, we define the impact of dietary ketosis on survival and CNS disease in the Ndufs4(KO) mouse model of Leigh syndrome and the therapeutic potential of both dietary ketosis and mTOR inhibition on seizures in this model. These data provide timely insight into two important clinical interventions.

Leukocytes mediate disease pathogenesis in the Ndufs4(KO) mouse model of Leigh syndrome

Symmetric, progressive, necrotizing lesions in the brainstem are a defining feature of Leigh syndrome (LS). A mechanistic understanding of the pathogenesis of these lesions has been elusive. Here, we report that leukocyte proliferation is causally involved in the pathogenesis of LS. Depleting leukocytes with a colony-stimulating factor 1 receptor inhibitor disrupted disease progression, including suppression of CNS lesion formation and a substantial extension of survival. Leukocyte depletion rescued diverse symptoms, including seizures, respiratory center function, hyperlactemia, and neurologic sequelae. These data reveal a mechanistic explanation for the beneficial effects of mTOR inhibition. More importantly, these findings dramatically alter our understanding of the pathogenesis of LS, demonstrating that immune involvement is causal in disease. This work has important implications for the mechanisms of mitochondrial disease and may lead to novel therapeutic strategies.

Non-synaptic Cell-Autonomous Mechanisms Underlie Neuronal Hyperactivity in a Genetic Model of PIK3CA-Driven Intractable Epilepsy

Patients harboring mutations in the PI3K-AKT-MTOR pathway-encoding genes often develop a spectrum of neurodevelopmental disorders including epilepsy. A significant proportion remains unresponsive to conventional anti-seizure medications. Understanding mutation-specific pathophysiology is thus critical for molecularly targeted therapies. We previously determined that mouse models expressing a patient-related activating mutation in PIK3CA, encoding the p110α catalytic subunit of phosphoinositide-3-kinase (PI3K), are epileptic and acutely treatable by PI3K inhibition, irrespective of dysmorphology. Here we report the physiological mechanisms underlying this dysregulated neuronal excitability. In vivo, we demonstrate epileptiform events in the Pik3ca mutant hippocampus. By ex vivo analyses, we show that Pik3ca-driven hyperactivation of hippocampal pyramidal neurons is mediated by changes in multiple non-synaptic, cell-intrinsic properties. Finally, we report that acute inhibition of PI3K or AKT, but not MTOR activity, suppresses the intrinsic hyperactivity of the mutant neurons. These acute mechanisms are distinct from those causing neuronal hyperactivity in other AKT-MTOR epileptic models and define parameters to facilitate the development of new molecularly rational therapeutic interventions for intractable epilepsy.

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.

Cross-comparison of systemic and tissue-specific metabolomes in a mouse model of Leigh syndrome

INTRODUCTION

The value of metabolomics in multi-systemic mitochondrial disease research has been increasingly recognized, with the ability to investigate a variety of biofluids and tissues considered a particular advantage. Although minimally invasive biofluids are the generally favored sample type, it remains unknown whether systemic metabolomes provide a clear reflection of tissue-specific metabolic alterations.

OBJECTIVES

Here we cross-compare urine and tissue-specific metabolomes in the Ndufs4 knockout mouse model of Leigh syndrome-a complex neurometabolic MD defined by progressive focal lesions in specific brain regions-to identify and evaluate the extent of common and unique metabolic alterations on a systemic and brain regional level.

METHODS

Untargeted and semi-targeted multi-platform metabolomics were performed on urine, four brain regions, and two muscle types of Ndufs4 KO (n≥19) vs wildtype (n≥20) mice.

RESULTS

Widespread alterations were evident in alanine, aspartate, glutamate, and arginine metabolism in Ndufs4 KO mice; while brain-region specific metabolic signatures include the accumulation of branched-chain amino acids, proline, and glycolytic intermediates. Furthermore, we describe a systemic dysregulation in one-carbon metabolism and the tricarboxylic acid cycle, which was not clearly reflected in the Ndufs4 KO brain.

CONCLUSION

Our results confirm the value of urinary metabolomics when evaluating MD-associated metabolites, while cautioning against mechanistic studies relying solely on systemic biofluids.

Ndufs4 ablation decreases synaptophysin expression in hippocampus

Altered function of mitochondrial respiratory chain in brain cells is related to many neurodegenerative diseases. NADH Dehydrogenase (Ubiquinone) Fe-S protein 4 (Ndufs4) is one of the subunits of mitochondrial complex I and its mutation in human is associated with Leigh syndrome. However, the molecular biological role of Ndufs4 in neuronal function is poorly understood. In this study, upon Ndufs4 expression confirmation in NeuN-positive neurons, and GFAP-positive astrocytes in WT mouse hippocampus, we found significant decrease of mitochondrial respiration in Ndufs4-KO mouse hippocampus. Although there was no change in the number of NeuN positive neurons in Ndufs4-KO hippocampus, the expression of synaptophysin, a presynaptic protein, was significantly decreased. To investigate the detailed mechanism, we silenced Ndufs4 in Neuro-2a cells and we observed shorter neurite lengths with decreased expression of synaptophysin. Furthermore, western blot analysis for phosphorylated extracellular regulated kinase (pERK) revealed that Ndufs4 silencing decreases the activity of ERK signalling. These results suggest that Ndufs4-modulated mitochondrial activity may be involved in neuroplasticity via regulating synaptophysin expression.

Association of the Psoriatic Microenvironment With Treatment Response

Importance

The ability to predict the efficacy of systemic psoriasis therapy based on immune profiles in skin biopsies could reduce the use of inappropriate treatment and its associated costs and adverse events. It could considerably decrease drug development trial costs as well.

Objective

To develop a bioinformatic gene signature score derived from skin mRNA to predict psoriasis treatment outcomes for a variety of therapies.

Design, Setting, and Participants

In this decision analytical model using 1145 skin samples from different cohorts of 12 retrospective psoriasis studies, samples were analyzed using the CIBERSORT algorithm to define the immune landscape of psoriasis lesions and controls. Random forest classification and principal component analysis algorithms were used to estimate psoriatic microenvironment (PME) signature genes and construct a PME score. Overall, 85 and 421 psoriasis lesions from 1 and 4 independent cohorts were used as discovery and validation studies, respectively. Among them, 157, 71, 89, and 90 psoriasis lesions were treated with etanercept, tofacitinib, adalimumab, and methotrexate, respectively.

Main Outcomes and Measures

Number of weeks after treatment initiation when responders and nonresponders could be predicted.

Results

Overall, 22 immune cell subtypes formed infiltration patterns that differentiated psoriasis lesions from healthy skin. In psoriasis lesions, the expression of 33 PME signature genes defined 2 immune phenotypes and in aggregate could be simplified to a numerical PME score. A high PME score, characterized by keratinocyte differentiation, correlated with a better treatment response (Psoriasis Area and Severity Index [PASI] reduction, 75.8%; 95% CI, 69.4% to 82.2%; P = .03), whereas a low PME score exhibited an immune activation signature and was associated with a worse response (PASI reduction, 53.5%; 95% CI, 45.3% to 61.7%; P = .03). The PME score at week 4 after treatment initiation correlated with future responder vs nonresponder to treatment status 8 to 12 weeks earlier than PASI reduction for etanercept, methotrexate plus adalimumab, and tofacitinib.

Conclusions and Relevance

The PME score is a biometric score that may predict clinical efficacy of systemic psoriasis therapy in advance of clinical responses. As an application of personalized medicine, it may reduce the exposure of patients with psoriasis to ineffective and expensive therapies.

Aberrant BCAA and glutamate metabolism linked to regional neurodegeneration in a mouse model of Leigh syndrome

The dysfunction of respiratory chain complex I (CI) is the most common form of mitochondrial disease that most often presents as Leigh syndrome (LS) in children - a severe neurometabolic disorder defined by progressive focal lesions in specific brain regions. The mechanisms underlying this region-specific vulnerability to CI deficiency, however, remain elusive. Here, we examined brain regional respiratory chain enzyme activities and metabolic profiles in a mouse model of LS with global CI deficiency to gain insight into regional vulnerability to neurodegeneration. One lesion-resistant and three lesion-prone brain regions were investigated in Ndufs4 knockout (KO) mice at the late stage of LS. Enzyme assays confirmed significantly decreased (60-80%) CI activity in all investigated KO brain regions, with the lesion-resistant region displaying the highest residual CI activity (38% of wild type). A higher residual CI activity, and a less perturbed NADH/NAD⁺ ratio, correlate with less severe metabolic perturbations in KO brain regions. Moreover, less perturbed BCAA oxidation and increased glutamate oxidation seem to distinguish lesion-resistant from -prone KO brain regions, thereby identifying key areas of metabolism to target in future therapeutic intervention studies.

Metallothionein 1 Overexpression Does Not Protect Against Mitochondrial Disease Pathology in Ndufs4 Knockout Mice

Mitochondrial diseases (MD), such as Leigh syndrome (LS), present with severe neurological and muscular phenotypes in patients, but have no known cure and limited treatment options. Based on their neuroprotective effects against other neurodegenerative diseases in vivo and their positive impact as an antioxidant against complex I deficiency in vitro, we investigated the potential protective effect of metallothioneins (MTs) in an Ndufs4 knockout mouse model (with a very similar phenotype to LS) crossed with an Mt1 overexpressing mouse model (TgMt1). Despite subtle reductions in the expression of neuroinflammatory markers GFAP and IBA1 in the vestibular nucleus and hippocampus, we found no improvement in survival, growth, locomotor activity, balance, or motor coordination in the Mt1 overexpressing Ndufs4-/- mice. Furthermore, at a cellular level, no differences were detected in the metabolomics profile or gene expression of selected one-carbon metabolism and oxidative stress genes, performed in the brain and quadriceps, nor in the ROS levels of macrophages derived from these mice. Considering these outcomes, we conclude that MT1, in general, does not protect against the impaired motor activity or improve survival in these complex I-deficient mice. The unexpected absence of increased oxidative stress and metabolic redox imbalance in this MD model may explain these observations. However, tissue-specific observations such as the mildly reduced inflammation in the hippocampus and vestibular nucleus, as well as differential MT1 expression in these tissues, may yet reveal a tissue- or cell-specific role for MTs in these mice.

Progressive optic atrophy in a retinal ganglion cell-specific mouse model of complex I deficiency

Optic atrophy resulting from retinal ganglion cell (RGC) degeneration is a prominent ocular manifestation of mitochondrial dysfunction. Although transgenic mice lacking the mitochondrial complex I accessory subunit NDUFS4 develop early-onset optic atrophy, severe systemic mitochondrial dysfunction leads to very early death and makes this mouse line impractical for studying the pathobiology of mitochondrial optic neuropathies. Theoretically, RGC-specific inactivation of ndufs4 would allow characterization of RGC degeneration over a longer time course, provided that RGC death from mitochondrial dysfunction is a cell-autonomous process. We demonstrate that the vesicular glutamate transporter VGLUT2 may be exploited to drive robust Cre recombinase expression in RGCs without any expression observed in directly neighboring retinal cell types. Deletion of ndufs4 in RGCs resulted in reduced expression of NDUFS4 protein within the optic nerves of Vglut2-Cre;ndufs4^loxP/loxP mice. RGC degeneration in Vglut2-Cre;ndufs4^loxP/loxP retinas commenced around postnatal day 45 (P45) and progressed to loss of two-thirds of RGCs by P90, confirming that intrinsic complex I dysfunction is sufficient to induce RGC death. The rapidly-developing optic atrophy makes the Vglut2-Cre;ndufs4^loxP/loxP mouse line a promising preclinical model for testing therapies for currently untreatable mitochondrial optic neuropathies such as Leber Hereditary Optic Neuropathy.

Measuring Breathing Patterns in Mice Using Whole-body Plethysmography

Respiratory dysfunction is among the main cause of severe and fatal pathologies worldwide. The use of effective experimental models and methodologies for the study of the pulmonary pathophysiology is necessary to prevent, control and cure these diseases. Plethysmography, a technique for the assessment of lung function, has been widely applied in mice for the characterization of respiratory physiology. However, classical plethysmography methods present technical limitations such as the use of anesthesia and animal immobilization. Whole-body plethysmography (WBP) avoids these issues providing a non-invasive approach for the assessment of the respiratory function in conscious animals. WBP relies on the recording of pressure changes that are produced by the spontaneous breathing activity of an animal placed inside an airtight chamber. During normal respiration, pressure variation is directly proportional to the respiratory pattern of the animal allowing the measurement of the respiratory rate and tidal volume. These parameters are commonly used to evaluate pulmonary function in different physiological and disease models. In contrast to classical plethysmography methods, WBP technique allows reproducible serial measurements as it avoids animal restraint or the use of anesthesia. These key features rend WBP a suitable approach for longitudinal studies allowing the assessment of progressive respiratory alterations in physiological and pathological conditions. This protocol describes the procedures for the measurement of the breathing patterns in mice using the WBP method, the data analysis and results interpretation.

Glutamate Carrier Involvement in Mitochondrial Dysfunctioning in the Brain White Matter

Glutamate homeostasis is an important determinant of health of the central nervous system (CNS). Mitochondria play crucial roles in glutamate metabolism, especially in processes with a high energy demand such as action potential generation. Mitochondrial glutamate carriers (GCs) and aspartate-GCs (AGCs) regulate the transport of glutamate from the cytoplasm across the mitochondrial membrane, which is needed to control energy demand, lipid metabolism, and metabolic activity including oxidative phosphorylation and glycolysis. Dysfunction in these carriers are associated with seizures, spasticity, and/or myelin deficits, all of which are associated with inherited metabolic disorders. Since solute carrier functioning and associated processes are cell type- and context-specific, selective vulnerability to glutamate excitotoxicity and mitochondrial dysfunctioning is expected. Understanding this could offer important insights into the pathomechanisms of associated disorders. This perspective aims to explore the link between functions of both AGCs and GCs and their role in metabolic disorders, with a focus on a subclass of lysosomal storage disorders called leukodystrophies (LDs).

Mitochondrial dysfunction in GnRH neurons impaired GnRH production

The onset establishment and maintenance of gonadotropin-releasing hormone (GnRH) secretion is an important phenomenon regulating pubertal development and reproduction. GnRH neurons as well as other neurons in the hypothalamus have high-energy demands and require a constant energy supply from their mitochondria machinery to maintain active functioning. However, the involvement of mitochondrial function in GnRH neurons is still unclear. In this study, we examined the role of NADH Dehydrogenase (Ubiquinone) Fe-S protein 4 (Ndufs4), a member of the mitochondrial complex 1, on GnRH neurons using Ndufs4-KO mice and Ndufs4-KO GT1-7 cells. Ndufs4 was highly expressed in GnRH neurons in the medial preoptic area (MPOA) and NPY/AgRP and POMC neurons in the arcuate (ARC) nucleus in WT mice. Conversely, there was a significant decrease in GnRH expression in MPOA and median eminence of Ndufs4-KO mice, followed by impaired peripheral endocrine system. In Ndufs4-KO GT1-7 cells, Gnrh1 expression was significantly decreased with or without stimulation with either kisspeptin or NGF, whereas, stimulation significantly increased Gnrh1 expression in control cells. In contrast, there was no difference in cell signaling activity including ERK and CREB as well as the expression of GPR54, TrkA and p75^NTR, suggesting that Ndufs4 is involved in the transcriptional regulation system for GnRH production. These findings may be useful in understanding the mitochondrial function in GnRH neuron.

NAD+ Regeneration Rescues Lifespan, but Not Ataxia, in a Mouse Model of Brain Mitochondrial Complex I Dysfunction

Mitochondrial complex I regenerates NAD+ and proton pumps for TCA cycle function and ATP production, respectively. Mitochondrial complex I dysfunction has been implicated in many brain pathologies including Leigh syndrome and Parkinson's disease. We sought to determine whether NAD+ regeneration or proton pumping, i.e., bioenergetics, is the dominant function of mitochondrial complex I in protection from brain pathology. We generated a mouse that conditionally expresses the yeast NADH dehydrogenase (NDI1), a single enzyme that can replace the NAD+ regeneration capability of the 45-subunit mammalian mitochondrial complex I without proton pumping. NDI1 expression was sufficient to dramatically prolong lifespan without significantly improving motor function in a mouse model of Leigh syndrome driven by the loss of NDUFS4, a subunit of mitochondrial complex I. Therefore, mitochondrial complex I activity in the brain supports organismal survival through its NAD+ regeneration capacity, while optimal motor control requires the bioenergetic function of mitochondrial complex I.

Mitochondrial Proteome of Affected Glutamatergic Neurons in a Mouse Model of Leigh Syndrome

Defects in mitochondrial function lead to severe neuromuscular orphan pathologies known as mitochondrial disease. Among them, Leigh Syndrome is the most common pediatric presentation, characterized by symmetrical brain lesions, hypotonia, motor and respiratory deficits, and premature death. Mitochondrial diseases are characterized by a marked anatomical and cellular specificity. However, the molecular determinants for this susceptibility are currently unknown, hindering the efforts to find an effective treatment. Due to the complex crosstalk between mitochondria and their supporting cell, strategies to assess the underlying alterations in affected cell types in the context of mitochondrial dysfunction are critical. Here, we developed a novel virus-based tool, the AAV-mitoTag viral vector, to isolate mitochondria from genetically defined cell types. Expression of the AAV-mitoTag in the glutamatergic vestibular neurons of a mouse model of Leigh Syndrome lacking the complex I subunit Ndufs4 allowed us to assess the proteome and acetylome of a subset of susceptible neurons in a well characterized model recapitulating the human disease. Our results show a marked reduction of complex I N-module subunit abundance and an increase in the levels of the assembly factor NDUFA2. Transiently associated non-mitochondrial proteins such as PKCδ, and the complement subcomponent C1Q were also increased in Ndufs4-deficient mitochondria. Furthermore, lack of Ndufs4 induced ATP synthase complex and pyruvate dehydrogenase (PDH) subunit hyperacetylation, leading to decreased PDH activity. We provide novel insight on the pathways involved in mitochondrial disease, which could underlie potential therapeutic approaches for these pathologies.

Regional metabolic signatures in the Ndufs4(KO) mouse brain implicate defective glutamate/α-ketoglutarate metabolism in mitochondrial disease

Leigh Syndrome (LS) is a mitochondrial disorder defined by progressive focal neurodegenerative lesions in specific regions of the brain. Defects in NDUFS4, a subunit of complex I of the mitochondrial electron transport chain, cause LS in humans; the Ndufs4 knockout mouse (Ndufs4(KO)) closely resembles the human disease. Here, we probed brain region-specific molecular signatures in pre-symptomatic Ndufs4(KO) to identify factors which underlie focal neurodegeneration. Metabolomics revealed that free amino acid concentrations are broadly different by region, and glucose metabolites are increased in a manner dependent on both region and genotype. We then tested the impact of the mTOR inhibitor rapamycin, which dramatically attenuates LS in Ndufs4(KO), on region specific metabolism. Our data revealed that loss of Ndufs4 drives pathogenic changes to CNS glutamine/glutamate/α-ketoglutarate metabolism which are rescued by mTOR inhibition Finally, restriction of the Ndufs4 deletion to pre-synaptic glutamatergic neurons recapitulated the whole-body knockout. Together, our findings are consistent with mTOR inhibition alleviating disease by increasing availability of α-ketoglutarate, which is both an efficient mitochondrial complex I substrate in Ndufs4(KO) and an important metabolite related to neurotransmitter metabolism in glutamatergic neurons.

Tackling mitochondrial diversity in brain function: from animal models to human brain organoids

Mitochondria exhibit high degree of heterogeneity within various tissues, including differences in terms of morphology, quantity, or function. Mitochondria can even vary among distinct sub-compartments of the same cell. Emerging evidence suggest that the molecular diversity of mitochondria can influence the identity and functionality of a given cell type. Human pathologies affecting mitochondria typically cause tissue and cell-type-specific impairment. Mitochondrial diversity could thus play a contributing role not only in physiological cell fate specification but also during pathological disease development. In this review, we discuss the role of mitochondrial diversity in brain function during health and disease. Recent advances in induced pluripotent stem cells (iPSCs) research and the derivation of cerebral organoids could provide novel opportunities to unveil the role of mitochondrial heterogeneity for the function of the human brain. Mitochondrial diversity might be at the bases of the cell-type-specific vulnerability of mitochondrial disorders and may represent an underappreciated target of disease intervention.