Primary Oxidative Phosphorylation Research Articles

Primary oxidative phosphorylation defects lead to perturbations in the human B cell repertoire.

The majority of studies on oxidative phosphorylation in immune cells have been performed in mouse models, necessitating human translation. To understand the impact of oxidative phosphorylation (OXPHOS) deficiency on human immunity, we studied children with primary mitochondrial disease (MtD). scRNAseq analysis of peripheral blood mononuclear cells was performed on matched children with MtD (N = 4) and controls (N = 4). To define B cell function we performed phage display immunoprecipitation sequencing on a cohort of children with MtD (N = 19) and controls (N = 16). Via scRNAseq, we found marked reductions in select populations involved in the humoral immune response, especially antigen presenting cells, B cell and plasma populations, with sparing of T cell populations. MTRNR2L8, a marker of bioenergetic stress, was significantly elevated in populations that were most depleted. mir4485, a miRNA contained in the intron of MTRNR2L8, was co-expressed. Knockdown studies of mir4485 demonstrated its role in promoting survival by modulating apoptosis. To determine the functional consequences of our findings on humoral immunity, we studied the antiviral antibody repertoire in children with MtD and controls using phage display and immunoprecipitation sequencing. Despite similar viral exposomes, MtD displayed antiviral antibodies with less robust fold changes and limited polyclonality. Overall, we show that children with MtD display perturbations in the B cell repertoire which may impact humoral immunity and the ability to clear viral infections.

Mitochondrial complex III deficiency drives c-MYC overexpression and illicit cell cycle entry leading to senescence and segmental progeria

Accumulating evidence suggests mitochondria as key modulators of normal and premature aging, yet whether primary oxidative phosphorylation (OXPHOS) deficiency can cause progeroid disease remains unclear. Here, we show that mice with severe isolated respiratory complex III (CIII) deficiency display nuclear DNA damage, cell cycle arrest, aberrant mitoses, and cellular senescence in the affected organs such as liver and kidney, and a systemic phenotype resembling juvenile-onset progeroid syndromes. Mechanistically, CIII deficiency triggers presymptomatic cancer-like c-MYC upregulation followed by excessive anabolic metabolism and illicit cell proliferation against lack of energy and biosynthetic precursors. Transgenic alternative oxidase dampens mitochondrial integrated stress response and the c-MYC induction, suppresses the illicit proliferation, and prevents juvenile lethality despite that canonical OXPHOS-linked functions remain uncorrected. Inhibition of c-MYC with the dominant-negative Omomyc protein relieves the DNA damage in CIII-deficient hepatocytes in vivo. Our results connect primary OXPHOS deficiency to genomic instability and progeroid pathogenesis and suggest that targeting c-MYC and aberrant cell proliferation may be therapeutic in mitochondrial diseases.

Metabolomics of Ndufs4−/− skeletal muscle: Adaptive mechanisms converge at the ubiquinone-cycle

Leigh syndrome is one of the most common childhood-onset neurometabolic disorders resulting from a primary oxidative phosphorylation dysfunction and affecting mostly brain tissues. Ndufs4−/− mice have been widely used to study the neurological responses in this syndrome, however the reason why these animals do not display strong muscle involvement remains elusive. We combined biochemical strategies and multi-platform metabolomics to gain insight into the metabolism of both glycolytic (white quadriceps) and oxidative (soleus) skeletal muscles from Ndufs4−/− mice. Enzyme assays confirmed severely reduced (80%) CI activity in both Ndufs4−/− muscle types, compared to WTs. No significant alterations were evident in other respiratory chain enzyme activities; however, Ndufs4−/− solei displayed moderate decreases in citrate synthase (12%) and CIII (18%) activities. Through hypothesis-generating metabolic profiling, we provide the first evidence of adaptive responses to CI dysfunction involving non-classical pathways fueling the ubiquinone (Q) cycle. We report a respective 48 and 34 discriminatory metabolites between Ndufs4−/− and WT white quadriceps and soleus muscles, among which the most prominent alterations indicate the involvement of the glycerol-3-phosphate shuttle, electron transfer flavoprotein system, CII, and proline cycle in fueling the Q cycle. By restoring the electron flux to CIII via the Q cycle, these adaptive mechanisms could maintain adequate oxidative ATP production, despite CI deficiency. Taken together, our results shed light on the underlying pathogenic mechanisms of CI dysfunction in skeletal muscle. Upon further investigation, these pathways could provide novel targets for therapeutic intervention in CI deficiency and potentially lead to the development of new treatment strategies.

Pathophysiology of Stroke-Like Episodes in MELAS: A Possible Role of Neuron-Astrocyte Uncoupling in Cellular Damage during Synaptic Activation under Primary Oxidative Phosphorylation Defect (IN7-1.004)

Pathophysiology of Stroke-Like Episodes in MELAS: A Possible Role of Neuron-Astrocyte Uncoupling in Cellular Damage during Synaptic Activation under Primary Oxidative Phosphorylation Defect (IN7-1.004)

Pathophysiology of Stroke-Like Episodes in MELAS: A Possible Role of Neuron-Astrocyte Uncoupling in Cellular Damage during Synaptic Activation under Primary Oxidative Phosphorylation Defect (P01.033)

Objective: To speculate the pathophysiological mechanism of stroke-like episodes (SEs) in MELAS based on clinical data and current concept of neuron-astrocyte communications. Background The pathogenesis of SEs remains unknown. We previously reported that SEs are non-ischemic neurovascular events presumably associated with neuronal hyperexcitability (Neurology 2002, Curr Neurovasc Res 2005, Future Neurology 2010), however, the basic molecular mechanism underlying neuronal hyperexcitability remains unknown. It has been demonstrated that astrocytic processes sense synaptic activity and transmit the signals to the arterioles. Astrocytes communicate with a number of neurons or astrocytes by releasing gliotransmitters. Thus, gliotransmitters are considered to control synaptic transmission, neuronal synchrony and neuronal excitability. During synaptic activation, neurons preferentially use lactate which is continuously supplied through astrocyte-neuron lactate shuttle. Design/Methods: Retrospective review of 30 consecutive SEs in 8 patients with MELAS (A3243G (n=7), G13513A (n=1)) who were admitted to our hospital since 1990. Results: Common presenting symptoms included headache, visual symptoms, altered levels of consciousness or seizure, and psychiatric symptoms. Seizure developed during the course of SEs (69%), suggesting sustained neuronal hyperexcitability. SEs were often accompanied by focal hyperperfusion, focal periodic epileptiform discharges, spreading cortical edema and cortical laminar necrosis. Focal lactate accumulation with leptomeningeal vasodilatation was seen at early stage. Calcitonin gene-related peptide (CGRP) was elevated in CSF. Conclusions: SEs are episodic events characterized by neuronal hyperexcitability, increased anaerobic glycolysis, vasogenic edema and vasodilatation. We speculate that 1) sustained astrocyte activation following synaptic activation contributes to focal periodic epileptiform discharges and hyperperfusion, 2) trigeminovascular activation causes headache, 3) limited availability of ATP impairs sodium-dependent glutamate transporter, resulting in decreased clearance of glutamate from the synaptic cleft, 4) low extracellular adenosine level due to decreased ATP production may cause disinhibition of synaptic activity, and 5) sustained release of lactate from activated astrocytes potentially abort glycolytic pathway. Neuron-astrocyte uncoupling may contribute to cellular damage during SEs. Disclosure: Dr. Iizuka has nothing to disclose. Dr. Hamada has nothing to disclose. Dr. Sakai has nothing to disclose.

Depressive behaviour in children diagnosed with a mitochondrial disorder

A higher incidence of depression has been described in adults with primary oxidative phosphorylation disease. We evaluated the psychological characteristics of eighteen non-retarded pediatric patients diagnosed with a disorder of the oxidative phosphorylation. We found significantly higher rate of withdrawn, depressive behaviour compared to population norm scores, to children with other types of inborn errors of metabolism and also in comparison to patients with Sotos syndrome. The occurrence of depressive behaviour showed no correlation with the degree of mitochondrial dysfunction. These findings support the hypothesis that mood disorders could be associated to abnormal cerebral energy metabolism.

Major depression in adolescent children consecutively diagnosed with mitochondrial disorder

A higher incidence of major depression has been described in adults with a primary oxidative phosphorylation disease. Intriguingly however, not all patients carrying the same mutation develop symptoms of major depression, pointing out the significance of the interplay of genetic and non-genetic factors in the etiology. In a series of paediatric patients evaluated for mitochondrial dysfunction, out of 35 children with a biochemically and genetically confirmed mitochondrial disorder, we identified five cases presenting with major depression prior to the diagnosis. The patients were diagnosed respectively with mutations in MTTK, MTND1, POLG1, PDHA1 and the common 4977 bp mtDNA deletion. Besides cerebral lactic acidemia protein and glucose concentrations, immunoglobins, anti-gangliosides and neurotransmitters were normal. No significant difference could be confirmed in the disease progression or the quality of life, compared to the other, genetically confirmed mitochondrial patients. In three out of our five patients a significant stress life event was confirmed. We propose the abnormal central nervous system energy metabolism as the underlying cause of the mood disorder in our paediatric patients. Exploring the genetic etiology in children with mitochondrial dysfunction and depression is essential both for safe medication and adequate counselling.

Mitochondrial disease criteria

Based on a previous prospective clinical and biochemical study, a consensus mitochondrial disease scoring system was established to facilitate the diagnosis in patients with a suspected mitochondrial disorder. To evaluate the specificity of the diagnostic system, we applied the mitochondrial disease score in 61 children with a multisystem disease and a suspected oxidative phosphorylation disorder who underwent a muscle biopsy and were consecutively diagnosed with a genetic mutation. We evaluated data of 44 children diagnosed with a disorder in oxidative phosphorylation, carrying a mutation in the mitochondrial or nuclear DNA. We compared them with 17 children who, based on the clinical and metabolic features, also had a muscle biopsy but were finally diagnosed with a nonmitochondrial multisystem disorder by further genetic analysis. All children with a genetically established diagnosis of a primary oxidative phosphorylation disorder had a mitochondrial disease score above 6 (probable mitochondrial disorder), and 73% of the children had a score above 8 (definite mitochondrial disorder) at evaluation of the muscle biopsy. In the nonmitochondrial multisystem disorder group, the score was significantly lower, and no patients reached a score comparable with a definite respiratory chain disorder. The mitochondrial disease criteria system has a high specificity to distinguish between mitochondrial and other multisystem disorders. The method could also be applied in children with a suspected mitochondrial disorder, prior to performing a muscle biopsy.

Dietary intervention and oxidative phosphorylation capacity

Secondary deterioration of mitochondrial function has been reported in patients with anorexia and cancer-related malnutrition. Inadequate nutrition, failure to thrive and feeding problems are also common symptoms in children with primary oxidative phosphorylation defects. As a standard intervention protocol we advise an age-appropriate diet and energy intake in our patients diagnosed with a mitochondrial dysfunction. By comparing the results of the first and the second samples from a group of children who underwent repeated muscle biopsies, we observed biochemical improvement in the mitochondrial function in 7 out of 10 patients following dietary advice and intervention. We suggest evaluating the nutritional state by interpretation of the skeletal muscle biochemistry in patients with a suspected oxidative phosphorylation defect. Since an insufficient dietary intake could play a role in secondary mitochondrial dysfunction, nutritional intervention should be performed prior to the biopsy. On the other hand, our data suggest that optimizing the nutritional and energy intake might also improve the utilization of the residual mitochondrial energy-generating capacity in patients with primary oxidative phosphorylation defects.