Measuring oxidative phosphorylation in human skin fibroblasts

FADS1/2 control lipid metabolism and ferroptosis susceptibility in triple-negative breast cancer

Triple-negative breast cancer (TNBC) has limited therapeutic options, is highly metastatic and characterized by early recurrence. Lipid metabolism is generally deregulated in TNBC and might reveal vulnerabilities to be targeted or used as biomarkers with clinical value. Ferroptosis is a type of cell death caused by iron-dependent lipid peroxidation which is facilitated by the presence of polyunsaturated fatty acids (PUFA). Here we identify fatty acid desaturases 1 and 2 (FADS1/2), which are responsible for PUFA biosynthesis, to be highly expressed in a subset of TNBC with a poorer prognosis. Lipidomic analysis, coupled with functional metabolic assays, showed that FADS1/2 high-expressing TNBC are susceptible to ferroptosis-inducing agents and that targeting FADS1/2 by both genetic interference and pharmacological approach renders those tumors ferroptosis-resistant while unbalancing PUFA/MUFA ratio by the supplementation of exogenous PUFA sensitizes resistant tumors to ferroptosis induction. Last, inhibiting lipid droplet (LD) formation and turnover suppresses the buffering capacity of LD and potentiates iron-dependent cell death. These findings have been validated in vitro and in vivo in mouse- and human-derived clinically relevant models and in a retrospective cohort of TNBC patients.

Acetyl-CoA carboxylase 1 controls a lipid droplet-peroxisome axis and is a vulnerability of endocrine-resistant ER+ breast cancer

Targeting aromatase deprives ER+ breast cancers of estrogens and is an effective therapeutic approach for these tumors. However, drug resistance is an unmet clinical need. Lipidomic analysis of long-term estrogen-deprived (LTED) ER+ breast cancer cells, a model of aromatase inhibitor resistance, revealed enhanced intracellular lipid storage. Functional metabolic analysis showed that lipid droplets together with peroxisomes, which we showed to be enriched and active in the LTED cells, controlled redox homeostasis and conferred metabolic adaptability to the resistant tumors. This reprogramming was controlled by acetyl-CoA-carboxylase-1 (ACC1), whose targeting selectively impaired LTED survival. However, the addition of branched- and very long-chain fatty acids reverted ACC1 inhibition, a process that was mediated by peroxisome function and redox homeostasis. The therapeutic relevance of these findings was validated in aromatase inhibitor-treated patient-derived samples. Last, targeting ACC1 reduced tumor growth of resistant patient-derived xenografts, thus identifying a targetable hub to combat the acquisition of estrogen independence in ER+ breast cancers.

L-Isoleucine reverses hyperammonemia-induced myotube mitochondrial dysfunction and post-mitotic senescence

Perturbations in the metabolism of ammonia, a cytotoxic endogenous metabolite, occur in a number of chronic diseases, with consequent hyperammonemia. Increased skeletal muscle ammonia uptake causes metabolic, molecular, and phenotype alterations including cataplerosis of (loss of tricarboxylic acid cycle (TCA) cycle intermediate) α-ketoglutarate (αKG), mitochondrial oxidative dysfunction, and senescence-associated molecular phenotype (SAMP). L-Isoleucine (Ile) is an essential, branched-chain amino acid (BCAA) that simultaneously provides acetyl-CoA as an oxidative substrate and succinyl-CoA for anaplerosis (providing TCA cycle intermediates). Our multiomics analyses in myotubes and skeletal muscle from hyperammonemic mice and human patients with cirrhosis showed perturbations in BCAA transporters and catabolism. We, therefore, determined if Ile reverses hyperammonemia-induced impaired mitochondrial oxidative function and SAMP. Studies were performed in differentiated murine C2C12 myotubes that were early passage, late passage (senescent), or those depleted of LAT1/SLC7A5 and human induced pluripotent stem cell-derived myotubes (hiPSCM). Ile reverses hyperammonemia-induced reduction in the maximum respiratory capacity, complex I, II, and III functions in early passage murine myotubes and hiPSCM. Consistently, low ATP content and impaired global protein synthesis (high energy requiring cellular process) during hyperammonemia are reversed by Ile in murine myotubes and hiPSCM. Lower abundance of critical regulators of protein synthesis in mTORC1 signaling, and increased phosphorylation of eukaryotic initiation factor 2α are also reversed by Ile. Genetic depletion studies showed that Ile responses are independent of the amino acid transporter LAT1/SLC7A5. Our studies show that Ile reverses the hyperammonemia-induced impaired mitochondrial oxidative function, cataplerosis, and SAMP in a LAT1/SLC7A5 transporter-independent manner.

Boosting intracellular sodium selectively kills hepatocarcinoma cells and induces hepatocellular carcinoma tumor shrinkage in mice

Pharmacological treatments for advanced hepatocellular carcinoma (HCC) have a partial efficacy. Augmented Na⁺ content and water retention are observed in human cancers and offer unexplored targets for anticancer therapies. Na⁺ levels are evaluated upon treatments with the antibiotic cation ionophore Monensin by fluorimetry, ICP-MS, ²³Na-MRI, NMR relaxometry, confocal or time-lapse analysis related to energy production, water fluxes and cell death, employing both murine and human HCC cell lines, primary murine hepatocytes, or HCC allografts in NSG mice. Na⁺ levels of HCC cells and tissue are 8-10 times higher than that of healthy hepatocytes and livers. Monensin further increases Na⁺ levels in HCC cells and in HCC allografts but not in primary hepatocytes and in normal hepatic and extrahepatic tissue. The Na⁺ increase is associated with energy depletion, mitochondrial Na⁺ load and inhibition of O₂ consumption. The Na⁺ increase causes an enhancement of the intracellular water lifetime and death of HCC cells, and a regression and necrosis of allograft tumors, without affecting the proliferating activity of either HCCs or healthy tissues. These observations indicate that HCC cells are, unlike healthy cells, energetically incapable of compensating and surviving a pharmacologically induced Na⁺ load, highlighting Na⁺ homeostasis as druggable target for HCC therapy.

Adaptive exhaustion during prolonged intermittent hypoxia causes dysregulated skeletal muscle protein homeostasis

Nocturnal hypoxaemia, which is common in chronic obstructive pulmonary disease (COPD) patients, is associated with skeletal muscle loss or sarcopenia, which contributes to adverse clinical outcomes. In COPD, we have defined this as prolonged intermittent hypoxia (PIH) because the duration of hypoxia in skeletal muscle occurs through the duration of sleep followed by normoxia during the day, in contrast to recurrent brief hypoxic episodes during obstructive sleep apnoea (OSA). Adaptive cellular responses to PIH are not known. Responses to PIH induced by three cycles of 8 h hypoxia followed by 16 h normoxia were compared to those during chronic hypoxia (CH) or normoxia for 72 h in murine C2C12 and human inducible pluripotent stem cell-derived differentiated myotubes. RNA sequencing followed by downstream analyses were complemented by experimental validation of responses that included both unique and shared perturbations in ribosomal and mitochondrial function during PIH and CH. A sarcopenic phenotype characterized by decreased myotube diameter and protein synthesis, and increased phosphorylation of eIF2α (Ser51) by eIF2α kinase, and of GCN-2 (general controlled non-derepressed-2), occurred during both PIH and CH. Mitochondrial oxidative dysfunction, disrupted supercomplex assembly, lower activity of Complexes I, III, IV and V, and reduced intermediary metabolite concentrations occurred during PIH and CH. Decreased mitochondrial fission occurred during CH. Physiological relevance was established in skeletal muscle of mice with COPD that had increased phosphorylation of eIF2α, lower protein synthesis and mitochondrial oxidative dysfunction. Molecular and metabolic responses with PIH suggest an adaptive exhaustion with failure to restore homeostasis during normoxia. KEY POINTS: Sarcopenia or skeletal muscle loss is one of the most frequent complications that contributes to mortality and morbidity in patients with chronic obstructive pulmonary disease (COPD). Unlike chronic hypoxia, prolonged intermittent hypoxia is a frequent, underappreciated and clinically relevant model of hypoxia in patients with COPD. We developed a novel, in vitro myotube model of prolonged intermittent hypoxia with molecular and metabolic perturbations, mitochondrial oxidative dysfunction, and consequent sarcopenic phenotype. In vivo studies in skeletal muscle from a mouse model of COPD shared responses with our myotube model, establishing the pathophysiological relevance of our studies. These data lay the foundation for translational studies in human COPD to target prolonged, nocturnal hypoxaemia to prevent sarcopenia in these patients.

DRP1 mutations associated with EMPF1 encephalopathy alter mitochondrial membrane potential and metabolic programs

Mitochondria and peroxisomes are dynamic signaling organelles that constantly undergo fission, driven by the large GTPase dynamin-related protein 1 (DRP1; encoded by DNM1L). Patients with de novo heterozygous missense mutations in DNM1L present with encephalopathy due to defective mitochondrial and peroxisomal fission (EMPF1) - a devastating neurodevelopmental disease with no effective treatment. To interrogate the mechanisms by which DRP1 mutations cause cellular dysfunction, we used human-derived fibroblasts from patients who present with EMPF1. In addition to elongated mitochondrial morphology and lack of fission, patient cells display lower coupling efficiency, increased proton leak and upregulation of glycolysis. Mitochondrial hyperfusion also results in aberrant cristae structure and hyperpolarized mitochondrial membrane potential. Peroxisomes show a severely elongated morphology in patient cells, which is associated with reduced respiration when cells are reliant on fatty acid oxidation. Metabolomic analyses revealed impaired methionine cycle and synthesis of pyrimidine nucleotides. Our study provides insight into the role of mitochondrial dynamics in cristae maintenance and the metabolic capacity of the cell, as well as the disease mechanism underlying EMPF1.

The TRPV1 Receptor Is Up-Regulated by Sphingosine 1-Phosphate and Is Implicated in the Anandamide-Dependent Regulation of Mitochondrial Activity in C2C12 Myoblasts

The sphingosine 1-phosphate (S1P) and endocannabinoid (ECS) systems comprehend bioactive lipids widely involved in the regulation of similar biological processes. Interactions between S1P and ECS have not been so far investigated in skeletal muscle, where both systems are active. Here, we used murine C2C12 myoblasts to investigate the effects of S1P on ECS elements by qRT-PCR, Western blotting and UHPLC-MS. In addition, the modulation of the mitochondrial membrane potential (ΔΨm), by JC-1 and Mitotracker Red CMX-Ros fluorescent dyes, as well as levels of protein controlling mitochondrial function, along with the oxygen consumption were assessed, by Western blotting and respirometry, respectively, after cell treatment with methanandamide (mAEA) and in the presence of S1P or antagonists to endocannabinoid-binding receptors. S1P induced a significant increase in TRPV1 expression both at mRNA and protein level, while it reduced the protein content of CB2. A dose-dependent effect of mAEA on ΔΨm, mediated by TRPV1, was evidenced; in particular, low doses were responsible for increased ΔΨm, whereas a high dose negatively modulated ΔΨm and cell survival. Moreover, mAEA-induced hyperpolarization was counteracted by S1P. These findings open new dimension to S1P and endocannabinoids cross-talk in skeletal muscle, identifying TRPV1 as a pivotal target.

Evidence of impaired mitochondrial cellular bioenergetics in ocular fibroblasts derived from glaucoma patients

Glaucoma is a progressive optic neuropathy characterized by the neurodegeneration of the retinal ganglion cells (RGCs) resulting in irreversible visual impairment and eventual blindness. RGCs are extremely susceptible to mitochondrial compromise due to their marked bioenergetic requirements and morphology. There is increasing interest in therapies targeting mitochondrial health as a method of preventing visual loss in managing glaucoma. The bioenergetic profile of Tenon's ocular fibroblasts from glaucoma patients and controls was investigated using the Seahorse XF24 analyser. Impaired mitochondrial cellular bioenergetics was detected in glaucomatous ocular fibroblasts including basal respiration, maximal respiration and spare capacity. Spare respiratory capacity levels reflect mitochondrial bio-energetic adaptability in response to pathophysiological stress. Basal oxidative stress was elevated in glaucomatous Tenon's ocular fibroblasts and hydrogen peroxide (H₂O₂) induced reactive oxygen species (ROS) simulated the glaucomatous condition in normal Tenon's ocular fibroblasts. This work supports the role of therapeutic interventions to target oxidative stress or provide mitochondrial energetic support in glaucoma.

Pyruvate and uridine rescue the metabolic profile of OXPHOS dysfunction

Introduction

Primary mitochondrial diseases (PMD) are a large, heterogeneous group of genetic disorders affecting mitochondrial function, mostly by disrupting the oxidative phosphorylation (OXPHOS) system. Understanding the cellular metabolic re-wiring occurring in PMD is crucial for the development of novel diagnostic tools and treatments, as PMD are often complex to diagnose and most of them currently have no effective therapy.

Objectives

To characterize the cellular metabolic consequences of OXPHOS dysfunction and based on the metabolic signature, to design new diagnostic and therapeutic strategies.

Methods

In vitro assays were performed in skin-derived fibroblasts obtained from patients with diverse PMD and validated in pharmacological models of OXPHOS dysfunction. Proliferation was assessed using the Incucyte technology. Steady-state glucose and glutamine tracing studies were performed with LC-MS quantification of cellular metabolites. The therapeutic potential of nutritional supplements was evaluated by assessing their effect on proliferation and on the metabolomics profile. Successful therapies were then tested in a in vivo lethal rotenone model in zebrafish.

Results

OXPHOS dysfunction has a unique metabolic signature linked to an NAD+/NADH imbalance including depletion of TCA intermediates and aspartate, and increased levels of glycerol-3-phosphate. Supplementation with pyruvate and uridine fully rescues this altered metabolic profile and the subsequent proliferation deficit. Additionally, in zebrafish, the same nutritional treatment increases the survival after rotenone exposure.

Conclusions

Our findings reinforce the importance of the NAD+/NADH imbalance following OXPHOS dysfunction in PMD and open the door to new diagnostic and therapeutic tools for PMD.

•

OXPHOS deficiency causes a distinct metabolic profile linked to a NAD+/NADH imbalance.

•

Depleted intracellular aspartic acid is a potential biomarker for OXPHOS dysfunction.

•

Therapy with pyruvate and uridine corrects the metabolic profile of OXPHOS deficiency.

•

Pyruvate and uridine treatment increases survival in a lethal rotenone zebrafish model.

Comparable respiratory activity in attached and suspended human fibroblasts

Measurement of oxygen consumption of cultured cells is widely used for diagnosis of mitochondrial diseases, drug testing, biotechnology, and toxicology. Fibroblasts are cultured in monolayers, but physiological measurements are carried out in suspended or attached cells. We address the question whether respiration differs in attached versus suspended cells using multiwell respirometry (Agilent Seahorse XF24) and high-resolution respirometry (Oroboros O2k), respectively. Respiration of human dermal fibroblasts measured in culture medium was baseline-corrected for residual oxygen consumption and expressed as oxygen flow per cell. No differences were observed between attached and suspended cells in ROUTINE respiration of living cells and LEAK respiration obtained after inhibition of ATP synthase by oligomycin. The electron transfer capacity was higher in the O2k than in the XF24. This could be explained by a limitation to two uncoupler titrations in the XF24 which led to an underestimation compared to multiple titration steps in the O2k. A quantitative evaluation of respiration measured via different platforms revealed that short-term suspension of fibroblasts did not affect respiratory activity and coupling control. Evaluation of results obtained by different platforms provides a test for reproducibility beyond repeatability. Repeatability and reproducibility are required for building a validated respirometric database.

The Atrophic Effect of 1,25(OH)2 Vitamin D3 (Calcitriol) on C2C12 Myotubes Depends on Oxidative Stress

Dysfunctional mitochondrial metabolism has been linked to skeletal muscle loss in several physio-pathological states. Although it has been reported that vitamin D (VD) supports cellular redox homeostasis by maintaining normal mitochondrial functions, and VD deficiency often occurs in conditions associated with skeletal muscle loss, the efficacy of VD supplementation to overcome muscle wasting is debated. Investigations on the direct effects of VD metabolites on skeletal muscle using C2C12 myotubes have revealed an unexpected pro-atrophic activity of calcitriol (1,25VD), while its upstream metabolites cholecalciferol (VD3) and calcidiol (25VD) have anti-atrophic effects. Here, we investigated if the atrophic effects of 1,25VD on myotubes depend on its activity on mitochondrial metabolism. The impact of 1,25VD and its upstream metabolites VD3 and 25VD on mitochondria dynamics and the activity of C2C12 myotubes was evaluated by measuring mitochondrial content, architecture, metabolism, and reactive oxygen species (ROS) production. We found that 1,25VD induces atrophy through protein kinase C (PKC)-mediated ROS production, mainly of extramitochondrial origin. Consistent with this, cotreatment with the antioxidant N-acetylcysteine (NAC), but not with the mitochondria-specific antioxidant mitoTEMPO, was sufficient to blunt the atrophic activity of 1,25VD. In contrast, VD3 and 25VD have antioxidant properties, suggesting that the efficacy of VD supplementation might result from the balance between atrophic pro-oxidant (1,25VD) and protective antioxidant (VD3 and 25VD) metabolites.

Blocked O-GlcNAc cycling alters mitochondrial morphology, function, and mass

O-GlcNAcylation is a prevalent form of glycosylation that regulates proteins within the cytosol, nucleus, and mitochondria. The O-GlcNAc modification can affect protein cellular localization, function, and signaling interactions. The specific impact of O-GlcNAcylation on mitochondrial morphology and function has been elusive. In this manuscript, the role of O-GlcNAcylation on mitochondrial fission, oxidative phosphorylation (Oxphos), and the activity of electron transport chain (ETC) complexes were evaluated. In a cellular environment with hyper O-GlcNAcylation due to the deletion of O-GlcNAcase (OGA), mitochondria showed a dramatic reduction in size and a corresponding increase in number and total mitochondrial mass. Because of the increased mitochondrial content, OGA knockout cells exhibited comparable coupled mitochondrial Oxphos and ATP levels when compared to WT cells. However, we observed reduced protein levels for complex I and II when comparing normalized mitochondrial content and reduced linked activity for complexes I and III when examining individual ETC complex activities. In assessing mitochondrial fission, we observed increased amounts of O-GlcNAcylated dynamin-related protein 1 (Drp1) in cells genetically null for OGA and in glioblastoma cells. Individual regions of Drp1 were evaluated for O-GlcNAc modifications, and we found that this post-translational modification (PTM) was not limited to the previously characterized residues in the variable domain (VD). Additional modification sites are predicted in the GTPase domain, which may influence enzyme activity. Collectively, these results highlight the impact of O-GlcNAcylation on mitochondrial dynamics and ETC function and mimic the changes that may occur during glucose toxicity from hyperglycemia.

Breast cancer growth and proliferation is suppressed by the mitochondrial targeted furazano[3,4-b]pyrazine BAM15

BACKGROUND

Enhanced metabolic plasticity and diversification of energy production is a hallmark of highly proliferative breast cancers. This contributes to poor pharmacotherapy efficacy, recurrence, and metastases. We have previously identified a mitochondrial-targeted furazano[3,4-b]pyrazine named BAM15 that selectively reduces bioenergetic coupling efficiency and is orally available. Here, we evaluated the antineoplastic properties of uncoupling oxidative phosphorylation from ATP production in breast cancer using BAM15.

METHODS

The anticancer effects of BAM15 were evaluated in human triple-negative MDA-MB-231 and murine luminal B, ERα-negative EO771 cells as well as in an orthotopic allograft model of highly proliferative mammary cancer in mice fed a standard or high fat diet (HFD). Untargeted transcriptomic profiling of MDA-MB-231 cells was conducted after 16-h exposure to BAM15. Additionally, oxidative phosphorylation and electron transfer capacity was determined in permeabilized cells and excised tumor homogenates after treatment with BAM15.

RESULTS

BAM15 increased proton leak and over time, diminished cell proliferation, migration, and ATP production in both MDA-MB-231 and EO771 cells. Additionally, BAM15 decreased mitochondrial membrane potential, while inducing apoptosis and reactive oxygen species accumulation in MDA-MB-231 and EO771 cells. Untargeted transcriptomic profiling of MDA-MB-231 cells further revealed inhibition of signatures associated with cell survival and energy production by BAM15. In lean mice, BAM15 lowered body weight independent of food intake and slowed tumor progression compared to vehicle-treated controls. In HFD mice, BAM15 reduced tumor growth relative to vehicle and calorie-restricted weight-matched controls mediated in part by impaired cell proliferation, mitochondrial respiratory function, and ATP production. LC-MS/MS profiling of plasma and tissues from BAM15-treated animals revealed distribution of BAM15 in adipose, liver, and tumor tissue with low abundance in skeletal muscle.

CONCLUSIONS

Collectively, these data indicate that mitochondrial uncoupling may be an effective strategy to limit proliferation of aggressive forms of breast cancer. More broadly, these findings highlight the metabolic vulnerabilities of highly proliferative breast cancers which may be leveraged in overcoming poor responsiveness to existing therapies.

Dynamin-related protein 1 regulates substrate oxidation in skeletal muscle by stabilizing cellular and mitochondrial calcium dynamics

Mitochondria undergo continuous cycles of fission and fusion to promote inheritance, regulate quality control, and mitigate organelle stress. More recently, this process of mitochondrial dynamics has been demonstrated to be highly sensitive to nutrient supply, ultimately conferring bioenergetic plasticity to the organelle. However, whether regulators of mitochondrial dynamics play a causative role in nutrient regulation remains unclear. In this study, we generated a cellular loss-of-function model for dynamin-related protein 1 (DRP1), the primary regulator of outer membrane mitochondrial fission. Loss of DRP1 (shDRP1) resulted in extensive ultrastructural and functional remodeling of mitochondria, characterized by pleomorphic enlargement, increased electron density of the matrix, and defective NADH and succinate oxidation. Despite increased mitochondrial size and volume, shDRP1 cells exhibited reduced cellular glucose uptake and mitochondrial fatty acid oxidation. Untargeted transcriptomic profiling revealed severe downregulation of genes required for cellular and mitochondrial calcium homeostasis, which was coupled to loss of ATP-stimulated calcium flux and impaired substrate oxidation stimulated by exogenous calcium. The insights obtained herein suggest that DRP1 regulates substrate oxidation by altering whole-cell and mitochondrial calcium dynamics. These findings are relevant to the targetability of mitochondrial fission and have clinical relevance in the identification of treatments for fission-related pathologies such as hereditary neuropathies, inborn errors in metabolism, cancer, and chronic diseases.

CHCHD2 harboring Parkinson's disease-linked T61I mutation precipitates inside mitochondria and induces precipitation of wild-type CHCHD2

The T61I mutation in coiled-coil-helix-coiled-coil-helix domain containing 2 (CHCHD2), a protein residing in the mitochondrial intermembrane space (IMS), causes an autosomal dominant form of Parkinson's disease (PD), but the underlying pathogenic mechanisms are not well understood. Here, we compared the subcellular localization and solubility of wild-type (WT) and T61I mutant CHCHD2 in human cells. We found that mitochondrial targeting of both WT and T61I CHCHD2 depended on the four cysteine residues in the C-terminal coiled-coil-helix-coiled-coil-helix (CHCH) domain but not on the N-terminal predicted mitochondrial targeting sequence. The T61I mutation did not interfere with mitochondrial targeting of the mutant protein but induced its precipitation in the IMS. Moreover, T61I CHCHD2 induced increased mitochondrial production of reactive oxygen species and apoptosis, which was prevented by treatment with anti-oxidants. Retention of T61I CHCHD2 in the cytosol through mutation of the cysteine residues in the CHCH domain prevented its precipitation as well as its apoptosis-inducing effect. Importantly, T61I CHCHD2 potently impaired the solubility of WT CHCHD2. In conclusion, our data show that the T61I mutation renders mutant CHCHD2 insoluble inside mitochondria, suggesting loss of function of the mutant protein. In addition, T61I CHCHD2 exerts a dominant-negative effect on the solubility of WT CHCHD2, explaining the dominant inheritance of this form of PD.

Lipids activate skeletal muscle mitochondrial fission and quality control networks to induce insulin resistance in humans

BACKGROUND AND AIMS

A diminution in skeletal muscle mitochondrial function due to ectopic lipid accumulation and excess nutrient intake is thought to contribute to insulin resistance and the development of type 2 diabetes. However, the functional integrity of mitochondria in insulin-resistant skeletal muscle remains highly controversial.

METHODS

19 healthy adults (age:28.4 ± 1.7 years; BMI:22.7 ± 0.3 kg/m²) received an overnight intravenous infusion of lipid (20% Intralipid) or saline followed by a hyperinsulinemic-euglycemic clamp to assess insulin sensitivity using a randomized crossover design. Skeletal muscle biopsies were obtained after the overnight lipid infusion to evaluate activation of mitochondrial dynamics proteins, ex-vivo mitochondrial membrane potential, ex-vivo oxidative phosphorylation and electron transfer capacity, and mitochondrial ultrastructure.

RESULTS

Overnight lipid infusion increased dynamin related protein 1 (DRP1) phosphorylation at serine 616 and PTEN-induced kinase 1 (PINK1) expression (P = 0.003 and P = 0.008, respectively) in skeletal muscle while reducing mitochondrial membrane potential (P = 0.042). The lipid infusion also increased mitochondrial-associated lipid droplet formation (P = 0.011), the number of dilated cristae, and the presence of autophagic vesicles without altering mitochondrial number or respiratory capacity. Additionally, lipid infusion suppressed peripheral glucose disposal (P = 0.004) and hepatic insulin sensitivity (P = 0.014).

CONCLUSIONS

These findings indicate that activation of mitochondrial fission and quality control occur early in the onset of insulin resistance in human skeletal muscle. Targeting mitochondrial dynamics and quality control represents a promising new pharmacological approach for treating insulin resistance and type 2 diabetes.

CLINICAL TRIAL REGISTRATION

NCT02697201, ClinicalTrials.gov.

[New possibilities in diagnosis of hereditary optic neuropathies]

The study analyses data from clinical and genetic examination of 114 patients, as well as examination of cytological skin fibroblasts of 20 patients with hereditary optic neuropathy (HON). The clinical examination revealed HON symptoms in all study patients, primary damage of the retinal ganglion cells accompanied by swelling of the peripapillary retinal nerve fiber layer (RNFL) in the acute stage of the disease was observed in 47% of cases. MtDNA mutations that cause the development of Leber hereditary optic neuropathy (LHON) were detected in 73% of cases, including three frequent mutations in 59% of cases, rare and candidate mutations - in 14% of cases; nDNA mutations associated with autosomal dominant optic neuropathy (ADON) - in 6.1% of cases; mutations in the DNAJC30 nDNA gene that caused autosomal recessive optic neuropathy (ARON) - in 21% of cases. Among patients with a clinical picture of LHON, mtDNA mutations were found in 77.6% of cases, while mutations of the DNAJC30 gene of nDNA - in 22.4% of cases. Cytological studies using high-resolution respirometry confirmed the presence of mitochondrial dysfunction not only in the cells of patients harboring pathogenic mutations, but also of those harboring candidate mutations. An algorithm for clinical and genetic verification of HON together with a set of cytological studies allows identification of the mitochondrial genesis of the disease and is indispensable in confirming the pathogenicity of new or candidate mutations.

Considerations for using isolated cell systems to understand cardiac metabolism and biology

Changes in myocardial metabolic activity are fundamentally linked to cardiac health and remodeling. Primary cardiomyocytes, induced pluripotent stem cell-derived cardiomyocytes, and transformed cardiomyocyte cell lines are common models used to understand how (patho)physiological conditions or stimuli contribute to changes in cardiac metabolism. These cell models are helpful also for defining metabolic mechanisms of cardiac dysfunction and remodeling. Although technical advances have improved our capacity to measure cardiomyocyte metabolism, there is often heterogeneity in metabolic assay protocols and cell models, which could hinder data interpretation and discernment of the mechanisms of cardiac (patho)physiology. In this review, we discuss considerations for integrating cardiomyocyte cell models with techniques that have become relatively common in the field, such as respirometry and extracellular flux analysis. Furthermore, we provide overviews of metabolic assays that complement XF analyses and that provide information on not only catabolic pathway activity, but biosynthetic pathway activity and redox status as well. Cultivating a more widespread understanding of the advantages and limitations of metabolic measurements in cardiomyocyte cell models will continue to be essential for the development of coherent metabolic mechanisms of cardiac health and pathophysiology.

BAM15-mediated mitochondrial uncoupling protects against obesity and improves glycemic control

Obesity is a leading cause of preventable death worldwide. Despite this, current strategies for the treatment of obesity remain ineffective at achieving long-term weight control. This is due, in part, to difficulties in identifying tolerable and efficacious small molecules or biologics capable of regulating systemic nutrient homeostasis. Here, we demonstrate that BAM15, a mitochondrially targeted small molecule protonophore, stimulates energy expenditure and glucose and lipid metabolism to protect against diet-induced obesity. Exposure to BAM15 in vitro enhanced mitochondrial respiratory kinetics, improved insulin action, and stimulated nutrient uptake by sustained activation of AMPK. C57BL/6J mice treated with BAM15 were resistant to weight gain. Furthermore, BAM15-treated mice exhibited improved body composition and glycemic control independent of weight loss, effects attributable to drug targeting of lipid-rich tissues. We provide the first phenotypic characterization and demonstration of pre-clinical efficacy for BAM15 as a pharmacological approach for the treatment of obesity and related diseases.

Oxidative stress mediates ethanol-induced skeletal muscle mitochondrial dysfunction and dysregulated protein synthesis and autophagy

Protein synthesis and autophagy are regulated by cellular ATP content. We tested the hypothesis that mitochondrial dysfunction, including generation of reactive oxygen species (ROS), contributes to impaired protein synthesis and increased proteolysis resulting in tissue atrophy in a comprehensive array of models. In myotubes treated with ethanol, using unbiased approaches, we identified defects in mitochondrial electron transport chain components, endogenous antioxidants, and enzymes regulating the tricarboxylic acid (TCA) cycle. Using high sensitivity respirometry, we observed impaired cellular respiration, decreased function of complexes I, II, and IV, and a reduction in oxidative phosphorylation in ethanol-treated myotubes and muscle from ethanol-fed mice. These perturbations resulted in lower skeletal muscle ATP content and redox ratio (NAD+/NADH). Ethanol also caused a leak of electrons, primarily from complex III, with generation of mitochondrial ROS and reverse electron transport. Oxidant stress with lipid peroxidation (thiobarbituric acid reactive substances) and protein oxidation (carbonylated proteins) were increased in myotubes and skeletal muscle from mice and humans with alcoholic liver disease. Ethanol also impaired succinate oxidation in the TCA cycle with decreased metabolic intermediates. MitoTEMPO, a mitochondrial specific antioxidant, reversed ethanol-induced mitochondrial perturbations (including reduced oxygen consumption, generation of ROS and oxidative stress), increased TCA cycle intermediates, and reversed impaired protein synthesis and the sarcopenic phenotype. We show that ethanol causes skeletal muscle mitochondrial dysfunction, decreased protein synthesis, and increased autophagy, and that these perturbations are reversed by targeting mitochondrial ROS.

Three rare pathogenic mtDNA substitutions in LHON patients with low heteroplasmy

In this article we present clinical, molecular and biochemical investigations of three patients with LHON caused by rare point substitutions in mtDNA. One patient harbours the known mtDNA mutation (m.13513 G>A), the others have new variants (m.13379 A>G in MT-ND5 gene and m.14597 A>G in MT-ND6 gene, which has never been previously associated with LHON). NGS analysis of a whole mtDNA derived from patient's blood revealed a low mutation load (24%, 47%, 23% respectively). Our data, including family segregation analysis, measurement of reactive oxygen species (ROS) production and cytotoxic effect of paraquat and high-resolution respirometry, showed that nucleotide variant m.14597 A>G can be classified as pathogenic mutation.

Oxygraphy Versus Enzymology for the Biochemical Diagnosis of Primary Mitochondrial Disease

Primary mitochondrial disease (PMD) is a large group of genetic disorders directly affecting mitochondrial function. Although next generation sequencing technologies have revolutionized the diagnosis of these disorders, biochemical tests remain essential and functional confirmation of the critical genetic diagnosis. While enzymological testing of the mitochondrial oxidative phosphorylation (OXPHOS) complexes remains the gold standard, oxygraphy could offer several advantages. To this end, we compared the diagnostic performance of both techniques in a cohort of 34 genetically defined PMD patient fibroblast cell lines. We observed that oxygraphy slightly outperformed enzymology for sensitivity (79 ± 17% versus 68 ± 15%, mean and 95% CI), and had a better discriminatory power, identifying 58 ± 17% versus 35 ± 17% as “very likely” for oxygraphy and enzymology, respectively. The techniques did, however, offer synergistic diagnostic prediction, as the sensitivity rose to 88 ± 11% when considered together. Similarly, the techniques offered varying defect specific information, such as the ability of enzymology to identify isolated OXPHOS deficiencies, while oxygraphy pinpointed PDHC mutations and captured POLG mutations that were otherwise missed by enzymology. In summary, oxygraphy provides useful information for the diagnosis of PMD, and should be considered in conjunction with enzymology for the diagnosis of PMD.

Relationships between expression of BCS1L, mitochondrial bioenergetics, and fatigue among patients with prostate cancer

Introduction: Cancer-related fatigue (CRF) is the most debilitating symptom with the greatest adverse side effect on quality of life. The etiology of this symptom is still not understood. The purpose of this study was to examine the relationship between mitochondrial gene expression, mitochondrial oxidative phosphorylation, electron transport chain complex activity, and fatigue in prostate cancer patients undergoing radiotherapy (XRT), compared to patients on active surveillance (AS).

Methods: The study used a matched case–control and repeated-measures research design. Fatigue was measured using the revised Piper Fatigue Scale from 52 patients with prostate cancer. Mitochondrial oxidative phosphorylation, electron-transport chain enzymatic activity, and BCS1L gene expression were determined using patients’ peripheral mononuclear cells. Data were collected at three time points and analyzed using repeated measures ANOVA.

Results: The fatigue score was significantly different over time between patients undergoing XRT and AS (P<0.05). Patients undergoing XRT experienced significantly increased fatigue at day 21 and day 42 of XRT (P<0.01). Downregulated mitochondrial gene (BC1, ubiquinol-cytochrome c reductase, synthesis-like, BCS1L, P<0.05) expression, decreased OXPHOS-complex III oxidation (P<0.05), and reduced activity of complex III were observed over time in patients with XRT. Moreover, increased fatigue was significantly associated with downregulated BCS1L and decreased complex III oxidation in patients undergoing XRT.

Conclusion: Our results suggest that BCS1L and complex III in mitochondrial mononuclear cells are potential biomarkers and feasible therapeutic targets for acute XRT-induced fatigue in this clinical population.

A novel null mutation in the pyruvate dehydrogenase phosphatase catalytic subunit gene (PDP1) causing pyruvate dehydrogenase complex deficiency

Congenital lactic acidosis due to pyruvate dehydrogenase phosphatase (PDP) deficiency is very rare. PDP regulates pyruvate dehydrogenase complex (PDC) and defective PDP leads to PDC deficiency. We report a case with functional PDC deficiency with low activated (+dichloroacetate) and inactivated (+fluoride) PDC activities in lymphocytes and fibroblasts, normal activity of other mitochondrial enzymes in fibroblasts, and novel biallelic frameshift mutation in the PDP1 gene, c.575dupT (p.L192FfsX5), with absent PDP1 product in fibroblasts. Unexpectedly, the patient also had low branched-chain 2-ketoacid dehydrogenase (BCKDH) activity in fibroblasts with slight elevation of branched-chain amino acids in plasma and ketoacids in urine but with no pathogenic mutations in the enzymes of BCKDH, which could suggest shared regulatory function of PDC and BCKDH in fibroblasts, potentially in other tissues or cell types as well, but this remains to be determined. The clinical presentation of this patient overlaps that of other patients with primary-specific PDC deficiency, with neonatal/infantile and childhood lactic acidosis, normal lactate to pyruvate ratio, elevated plasma alanine, delayed psychomotor development, epileptic encephalopathy, feeding difficulties, and hypotonia. This patient exhibited marked improvement of overall development following initiation of ketogenic diet at 31 months of age. To the best of our knowledge, this is the fourth case of functional PDC deficiency with a defined mutation in PDP1.

SYNOPSIS

Pyruvate dehydrogenase phosphatase (PDP) regulates pyruvate dehydrogenase complex (PDC) and defective PDP due to PDP1 mutations leads to PDC deficiency and congenital lactic acidosis.

Berberine hydrochloride protects against cytokine-induced inflammation through multiple pathways in undifferentiated C2C12 myoblast cells

Obesity is associated with skeletal muscle insulin resistance and the development of metabolic syndrome. Undifferentiated skeletal muscle cells are sensitive to oxidative stress. Berberine hydrochloride (BBR) improves insulin resistance and exhibits anti-inflammatory properties. However, the underlying mechanism and the cell signaling pathways involved remain largely elusive. We therefore investigated the anti-inflammatory effects of BBR and the signaling pathways using skeletal C2C12 myoblast cells. Undifferentiated C2C12 myoblast cells were treated with interleukin-1β alone or in combination with tumor necrosis factor-α in the presence or absence of BBR. We found that BBR reduced the cytokine-induced expression of inducible nitric oxide synthase and stress-related kinases including p-38 mitogen-activated protein kinase, nuclear factor kappa B (NF-κB), and stress-activated protein kinases/Jun amino-terminal kinases (SAPK/JNK) in C2C12 myoblast cells. Furthermore, BBR reversed cytokine-mediated suppression of AMP-activated protein kinase (AMPKα), sirtuin-1 (SIRT-1), and PPAR-γ coactivator-1α (PGC-1α). In addition, cytokine-induced reduction of mitochondrial marker proteins and function were rescued after BBR treatment. Catalase, an antioxidant enzyme, was elevated after BBR treatment. Our results demonstrate that BBR ameliorates cytokine-induced inflammation. The anti-inflammatory effect of BBR in skeletal progenitor cells is mediated through pathways including activation of the AMPKα-SIRT-1-PGC-1α, inhibition of the mitogen-activated protein kinase 4 (MKK4)-SAPK/JNK-C-JUN, as well as protection of mitochondrial bioenergetics. BBR may be a potential medication for metabolic syndrome.

A novel variant m.641A>T in the mitochondrial MT-TF gene is associated with epileptic encephalopathy in adolescent

We present a 14-year-old girl with loss of motor functions, tetraplegia, epilepsy and nystagmus, caused by a novel heteroplasmic m.641A>T transition in an evolutionary conserved region of mitochondrial genome, affecting the aminoacyl stem of mitochondrial tRNA-Phe. In silico prediction, respirometry, Western blot and enzymatic analyses in skin fibroblasts support the pathogenicity of the m.641A>T substitution. This is the 18th MT-TF point mutation associated with a mitochondrial disorder. The onset and the severity of the disease, however, is unique in this case and broadens the clinical picture related to mutations of mitochondrial tRNA-Phe.

Mutations in the mitochondrial ribosomal protein MRPS22 lead to primary ovarian insufficiency

Primary ovarian insufficiency (POI) is characterized by amenorrhea and loss or dysfunction of ovarian follicles prior to the age of 40. POI has been associated with autosomal recessive mutations in genes involving hormonal signaling and folliculogenesis, however, the genetic etiology of POI most often remains unknown. Here we report MRPS22 homozygous missense variants c.404G>A (p.R135Q) and c.605G>A (p.R202H) identified in four females from two independent consanguineous families as a novel genetic cause of POI in adolescents. Both missense mutations identified in MRPS22 are rare, occurred in highly evolutionarily conserved residues, and are predicted to be deleterious to protein function. In contrast to prior reports of mutations in MRPS22 associated with severe mitochondrial disease, the POI phenotype is far less severe. Consistent with this genotype-phenotype correlation, mitochondrial defects in oxidative phosphorylation or rRNA levels were not detected in fibroblasts derived from the POI patients, suggesting a non-bioenergetic or tissue-specific mitochondrial defect. Furthermore, we demonstrate in a Drosophila model that mRpS22 deficiency specifically in somatic cells of the ovary had no effect on fertility, whereas flies with mRpS22 deficiency specifically in germ cells were infertile and agametic, demonstrating a cell autonomous requirement for mRpS22 in germ cell development. These findings collectively identify that MRPS22, a component of the small mitochondrial ribosome subunit, is critical for ovarian development and may therefore provide insight into the pathophysiology and treatment of ovarian dysfunction.

Determining Compartment-Specific Metabolic Fluxes

In this chapter, we present an experimental protocol for the targeted metabolic profiling of full cells and mitochondria in selectively permeabilized cells. Mitochondria of adherent cell cultures are made accessible by the addition of digitonin-a compound that selectively permeabilizes the cytosolic membrane without affecting mitochondrial integrity. The generated in situ mitochondria are subsequently used in a stable isotope labeling assay in which their metabolic fluxes can be analyzed without any interfering influence originating from cytosolic components. The protocol is complemented by oxygen consumption measurements of permeabilized cells on a Seahorse XF instrument. The additional data on mitochondrial respiration can be used to validate the functionality of mitochondria in the applied setup but are also a valuable add-on to the stable isotope labeling data.

Distinction of sporadic and familial forms of ALS based on mitochondrial characteristics

Bioenergetic failure, oxidative stress, and changes in mitochondrial morphology are common pathologic hallmarks of amyotrophic lateral sclerosis (ALS) in several cellular and animal models. Disturbed mitochondrial physiology has serious consequences for proper functioning of the cell, leading to the chronic mitochondrial stress. Mitochondria, being in the center of cellular metabolism, play a pivotal role in adaptation to stress conditions. We found that mitochondrial dysfunction and adaptation processes differ in primary fibroblasts derived from patients diagnosed with either sporadic or familial forms of ALS. The evaluation of mitochondrial parameters such as the mitochondrial membrane potential, the oxygen consumption rate, the activity and levels of respiratory chain complexes, and the levels of ATP, reactive oxygen species, and Ca²⁺ show that the bioenergetic properties of mitochondria are different in sporadic ALS, familial ALS, and control groups. Comparative statistical analysis of the data set (with use of principal component analysis and support vector machine) identifies and distinguishes 3 separate groups despite the small number of investigated cell lines and high variability in measured parameters. These findings could be a first step in development of a new tool for predicting sporadic and familial forms of ALS and could contribute to knowledge of its pathophysiology.-Walczak, J., Dębska-Vielhaber, G., Vielhaber, S., Szymański, J., Charzyńska, A., Duszyński, J., Szczepanowska, J. Distinction of sporadic and familial forms of ALS based on mitochondrial characteristics.

Integrated mitochondrial function and cancer-related fatigue in men with prostate cancer undergoing radiation therapy

Introduction

Fatigue experienced by cancer patients is one of the most common symptoms with the greatest adverse effect on quality of life, but arguably the least understood. The purpose of this study was to explore changes in integrated mitochondrial function and fatigue in non-metastatic prostate cancer patients receiving localized radiation therapy (XRT).

Materials and methods

We proposed a mitochondrial bioenergetics mechanism of radiation-induced fatigue linking impaired oxidative phosphorylation (OXPHOS) through complex III and decreased adenosine triphosphate (ATP) production as consequences of XRT. Integrated mitochondrial function was measured as mitochondrial OXPHOS from patients’ peripheral blood mononuclear cells. Fatigue was measured using the revised Piper Fatigue Scale. Data were collected before (day 0) and at day 21 of XRT.

Results

At day 21 of XRT, fatigue symptom intensified in 15 prostate cancer patients (P<0.05). Mitochondrial OXPHOS complex III-linked and uncoupled complex III rates were significantly decreased in mononuclear cells at day 21 during XRT compared to that before XRT (P<0.05). Additionally, increased fatigue appeared to be associated with decreased OXPHOS complex III-linked respiration in patients undergoing XRT.

Conclusion

Fatigue was associated with OXPHOS complex III-linked oxidation and a defect in oxidation starting at complex III in mononuclear cell mitochondria was revealed at day 21 of XRT in 15 prostate cancer patients. Complex III is a potential target for pharmacological and, in particular, nutraceutical interventions, eg, Q10, for design of interventions for CRF.

Intracellular calcium dysregulation in autism spectrum disorder: An analysis of converging organelle signaling pathways

Autism spectrum disorder (ASD) is a group of complex, neurological disorders that affect early cognitive, social, and verbal development. Our understanding of ASD has vastly improved with advances in genomic sequencing technology and genetic models that have identified >800 loci with variants that increase susceptibility to ASD. Although these findings have confirmed its high heritability, the underlying mechanisms by which these genes produce the ASD phenotypes have not been defined. Current efforts have begun to "functionalize" many of these variants and envisage how these susceptibility factors converge at key biochemical and biophysical pathways. In this review, we discuss recent work on intracellular calcium signaling in ASD, including our own work, which begins to suggest it as a compelling candidate mechanism in the pathophysiology of autism and a potential therapeutic target. We consider how known variants in the calcium signaling genomic architecture of ASD may exert their deleterious effects along pathways particularly involving organelle dysfunction including the endoplasmic reticulum (ER), a major calcium store, and the mitochondria, a major calcium ion buffer, and theorize how many of these pathways intersect.

Analyzing mitochondrial function in human peripheral blood mononuclear cells

Mitochondrial oxidative phosphorylation (OXPHOS) is responsible for producing most of the adenosine triphosphate required by eukaryotic cells. Lymphocytes make up the majority of the peripheral blood mononuclear cells. Peripheral blood mononuclear cells are readily obtainable, providing an ideal sample to monitor systemic changes and understand molecular signaling mechanisms in disease processes. Mitochondrial energy metabolism of lymphocyte has been used to screen for OXPHOS disorders. While there are increasing studies of lymphocyte OXPHOS, few studies examined activity of electron transport chain of lymphocyte mitochondria. We present an optimal protocol to harvest fresh peripheral blood mononuclear cells from human whole blood, determine integrated mitochondrial function, and analyze electron transport chain complex activity. Analyzing integrated mitochondrial function using OXPHOS provides data to uncover defects in the transport of substrates into the mitochondria, generation of reducing equivalents, the electron transport chain, and coupling to the production of adenosine triphosphate. The optimal conditions to harvest peripheral blood mononuclear cells were using blood anticoagulated with ethylenediaminetetraacetic acid, processed utilizing Lymphoprep™, and washed in phosphate buffered saline, all at room temperature. Using isolated peripheral blood mononuclear cells, integrated mitochondrial function and the activities of electron transport chain were determined.

[High resolution respirometry in diagnostic of mitochondrial disorders caused by mitochondrial complex I deficiency]

Complex I (CI) deficiency is one of the most common defects in the OXPHOS system; it represents more than 30% cases of mitochondrial diseases. The group is characterized by clinical and genetic heterogeneity and comprise several nosological forms. The most prevalent phenotypes for CI are LHON and Leigh syndrome. In this study we have analyzed skin fibroblasts from 11 patients with mutations in mtDNA, which cause LHON or Leigh-like phenotypes: m.11778 G>A (n=3), m.3460 A>G (n=2), m.3635 G>A (n=1), m.3308 T>G (n=2), m.3472 T>C (n=1) and 2 patients with earlier unknown substitutions m.3945 C>A and m.14441T>C. High-resolution respirometry (HRR) on the Oxygraph-2k instrument ("Oroboros corp.", Austria) was performed for complex analysis of the mitochondrial respiratory function in intact and permeabilized fibroblasts of patients and healthy controls. Flux control rations in intact cells R/E, (R-L)/E (p<0.05) were raised compared to the control. Rates of R, E, L normalized on the CS were statistically varied between patients and controls. In permeabilized fibroblasts we observed differences in CII/E, Rot/E, R/CII, CI/CII (p<0.05) between groups. These data highlight the dysfunction of the OXPHOS system and particularly CI. Increased citrate synthase level and decreased CI/CII ratio indicate compensatory metabolic response to respiratory chain dysfunction. Our results show applicability of HRR in revealing the biochemical abnormalities of complex I in fibroblasts of patients with LHON and Leigh-like syndrome. We also suggest HRR to be a useful method for inspection of other mutations causing complex I deficiency.

Parkinson's disease-associated pathogenic VPS35 mutation causes complex I deficits

Defect in the complex I of the mitochondrial electron-transport chain is a characteristic of Parkinson's disease (PD) which is thought to play a critical role in the disease pathogenesis. Mutations in vacuolar protein sorting 35 (VPS35) cause autosomal dominant PD and we recently demonstrated that pathogenic VPS35 mutations cause mitochondrial damage through enhanced mitochondrial fragmentation. In this study, we aimed to determine whether pathogenic VPS35 mutation impacts the activity of complex I and its underlying mechanism. Indeed, VPS35 D620N mutation led to decreased enzymatic activity and respiratory defects in complex I and II in patient fibroblasts. While no changes in the expression of the complex I and II subunits were noted, the level of assembled complex I and II as well as the supercomplex was significantly reduced in D620N fibroblasts. Importantly, inhibition of mitochondrial fission rescued the contents of assembled complexes as well as the functional defects in complex I and II. Overall, these results suggest that VPS35 D620N mutation-induced excessive mitochondrial fission leads to the defects in the assembled complex I and supercomplex and causes bioenergetics deficits.

Lethal neonatal case and review of primary short-chain enoyl-CoA hydratase (SCEH) deficiency associated with secondary lymphocyte pyruvate dehydrogenase complex (PDC) deficiency

Mutations in ECHS1 result in short-chain enoyl-CoA hydratase (SCEH) deficiency which mainly affects the catabolism of various amino acids, particularly valine. We describe a case compound heterozygous for ECHS1 mutations c.836T>C (novel) and c.8C>A identified by whole exome sequencing of proband and parents. SCEH deficiency was confirmed with very low SCEH activity in fibroblasts and nearly absent immunoreactivity of SCEH. The patient had a severe neonatal course with elevated blood and cerebrospinal fluid lactate and pyruvate concentrations, high plasma alanine and slightly low plasma cystine. 2-Methyl-2,3-dihydroxybutyric acid was markedly elevated as were metabolites of the three branched-chain α-ketoacids on urine organic acids analysis. These urine metabolites notably decreased when lactic acidosis decreased in blood. Lymphocyte pyruvate dehydrogenase complex (PDC) activity was deficient, but PDC and α-ketoglutarate dehydrogenase complex activities in cultured fibroblasts were normal. Oxidative phosphorylation analysis on intact digitonin-permeabilized fibroblasts was suggestive of slightly reduced PDC activity relative to control range in mitochondria. We reviewed 16 other cases with mutations in ECHS1 where PDC activity was also assayed in order to determine how common and generalized secondary PDC deficiency is associated with primary SCEH deficiency. For reasons that remain unexplained, we find that about half of cases with primary SCEH deficiency also exhibit secondary PDC deficiency. The patient died on day-of-life 39, prior to establishing his diagnosis, highlighting the importance of early and rapid neonatal diagnosis because of possible adverse effects of certain therapeutic interventions, such as administration of ketogenic diet, in this disorder. There is a need for better understanding of the pathogenic mechanisms and phenotypic variability in this relatively recently discovered disorder.

Succinyl-CoA synthetase (SUCLA2) deficiency in two siblings with impaired activity of other mitochondrial oxidative enzymes in skeletal muscle without mitochondrial DNA depletion

Mutations in SUCLA2 result in succinyl-CoA ligase (ATP-forming) or succinyl-CoA synthetase (ADP-forming) (A-SCS) deficiency, a mitochondrial tricarboxylic acid cycle disorder. The phenotype associated with this gene defect is largely encephalomyopathy. We describe two siblings compound heterozygous for SUCLA2 mutations, c.985A>G (p.M329V) and c.920C>T (p.A307V), with parents confirmed as carriers of each mutation. We developed a new LC-MS/MS based enzyme assay to demonstrate the decreased SCS activity in the siblings with this unique genotype. Both siblings shared bilateral progressive hearing loss, encephalopathy, global developmental delay, generalized myopathy, and dystonia with choreoathetosis. Prior to diagnosis and because of lactic acidosis and low activity of muscle pyruvate dehydrogenase complex (PDC), sibling 1 (S1) was placed on dichloroacetate, while sibling 2 (S2) was on a ketogenic diet. S1 developed severe cyclic vomiting refractory to therapy, while S2 developed Leigh syndrome, severe GI dysmotility, intermittent anemia, hypogammaglobulinemia and eventually succumbed to his disorder. The mitochondrial DNA contents in skeletal muscle (SM) were normal in both siblings. Pyruvate dehydrogenase complex, ketoglutarate dehydrogenase complex, and several mitochondrial electron transport chain (ETC) activities were low or at the low end of the reference range in frozen SM from S1 and/or S2. In contrast, activities of PDC, other mitochondrial enzymes of pyruvate metabolism, ETC and, integrated oxidative phosphorylation, in skin fibroblasts were not significantly impaired. Although we show that propionyl-CoA inhibits PDC, it does not appear to account for decreased PDC activity in SM. A better understanding of the mechanisms of phenotypic variability and the etiology for tissue-specific secondary deficiencies of mitochondrial enzymes of oxidative metabolism, and independently mitochondrial DNA depletion (common in other cases of A-SCS deficiency), is needed given the implications for control of lactic acidosis and possible clinical management.

Mutations in the accessory subunit NDUFB10 result in isolated complex I deficiency and illustrate the critical role of intermembrane space import for complex I holoenzyme assembly

An infant presented with fatal infantile lactic acidosis and cardiomyopathy, and was found to have profoundly decreased activity of respiratory chain complex I in muscle, heart and liver. Exome sequencing revealed compound heterozygous mutations in NDUFB10, which encodes an accessory subunit located within the PD part of complex I. One mutation resulted in a premature stop codon and absent protein, while the second mutation replaced the highly conserved cysteine 107 with a serine residue. Protein expression of NDUFB10 was decreased in muscle and heart, and less so in the liver and fibroblasts, resulting in the perturbed assembly of the holoenzyme at the 830 kDa stage. NDUFB10 was identified together with three other complex I subunits as a substrate of the intermembrane space oxidoreductase CHCHD4 (also known as Mia40). We found that during its mitochondrial import and maturation NDUFB10 transiently interacts with CHCHD4 and acquires disulfide bonds. The mutation of cysteine residue 107 in NDUFB10 impaired oxidation and efficient mitochondrial accumulation of the protein and resulted in degradation of non-imported precursors. Our findings indicate that mutations in NDUFB10 are a novel cause of complex I deficiency associated with a late stage assembly defect and emphasize the role of intermembrane space proteins for the efficient assembly of complex I.

High-throughput screen detects calcium signaling dysfunction in typical sporadic autism spectrum disorder

Autism spectrum disorder (ASD) is a heterogeneous group of neurodevelopmental disorders without any defined uniting pathophysiology. Ca²⁺ signaling is emerging as a potential node in the genetic architecture of the disorder. We previously reported decreased inositol trisphosphate (IP₃)-mediated Ca²⁺ release from the endoplasmic reticulum in several rare monogenic syndromes highly comorbid with autism – fragile X and tuberous sclerosis types 1 and 2 syndromes. We now extend those findings to a cohort of subjects with sporadic ASD without any known mutations. We developed and applied a high throughput Fluorometric Imaging Plate Reader (FLIPR) assay to monitor agonist-evoked Ca²⁺ signals in human primary skin fibroblasts. Our results indicate that IP₃ -mediated Ca²⁺ release from the endoplasmic reticulum in response to activation of purinergic receptors is significantly depressed in subjects with sporadic as well as rare syndromic forms of ASD. We propose that deficits in IP₃-mediated Ca²⁺ signaling represent a convergent hub function shared across the spectrum of autistic disorders – whether caused by rare highly penetrant mutations or sporadic forms – and holds promise as a biomarker for diagnosis and novel drug discovery.

Hyperammonaemia-induced skeletal muscle mitochondrial dysfunction results in cataplerosis and oxidative stress

KEY POINTS

Hyperammonaemia occurs in hepatic, cardiac and pulmonary diseases with increased muscle concentration of ammonia. We found that ammonia results in reduced skeletal muscle mitochondrial respiration, electron transport chain complex I dysfunction, as well as lower NAD⁺ /NADH ratio and ATP content. During hyperammonaemia, leak of electrons from complex III results in oxidative modification of proteins and lipids. Tricarboxylic acid cycle intermediates are decreased during hyperammonaemia, and providing a cell-permeable ester of αKG reversed the lower TCA cycle intermediate concentrations and increased ATP content. Our observations have high clinical relevance given the potential for novel approaches to reverse skeletal muscle ammonia toxicity by targeting the TCA cycle intermediates and mitochondrial ROS.

ABSTRACT

Ammonia is a cytotoxic metabolite that is removed primarily by hepatic ureagenesis in humans. Hyperammonaemia occurs in advanced hepatic, cardiac and pulmonary disease, and in urea cycle enzyme deficiencies. Increased skeletal muscle ammonia uptake and metabolism are the major mechanism of non-hepatic ammonia disposal. Non-hepatic ammonia disposal occurs in the mitochondria via glutamate synthesis from α-ketoglutarate resulting in cataplerosis. We show skeletal muscle mitochondrial dysfunction during hyperammonaemia in a comprehensive array of human, rodent and cellular models. ATP synthesis, oxygen consumption, generation of reactive oxygen species with oxidative stress, and tricarboxylic acid (TCA) cycle intermediates were quantified. ATP content was lower in the skeletal muscle from cirrhotic patients, hyperammonaemic portacaval anastomosis rat, and C2C12 myotubes compared to appropriate controls. Hyperammonaemia in C2C12 myotubes resulted in impaired intact cell respiration, reduced complex I/NADH oxidase activity and electron leak occurring at complex III of the electron transport chain. Consistently, lower NAD⁺ /NADH ratio was observed during hyperammonaemia with reduced TCA cycle intermediates compared to controls. Generation of reactive oxygen species resulted in increased content of skeletal muscle carbonylated proteins and thiobarbituric acid reactive substances during hyperammonaemia. A cell-permeable ester of α-ketoglutarate reversed the low TCA cycle intermediates and ATP content in myotubes during hyperammonaemia. However, the mitochondrial antioxidant MitoTEMPO did not reverse the lower ATP content during hyperammonaemia. We provide for the first time evidence that skeletal muscle hyperammonaemia results in mitochondrial dysfunction and oxidative stress. Use of anaplerotic substrates to reverse ammonia-induced mitochondrial dysfunction is a novel therapeutic approach.

Inactivation of 3-hydroxybutyrate dehydrogenase 2 delays zebrafish erythroid maturation by conferring premature mitophagy

Mitochondria are the site of iron utilization, wherein imported iron is incorporated into heme or iron-sulfur clusters. Previously, we showed that a cytosolic siderophore, which resembles a bacterial siderophore, facilitates mitochondrial iron import in eukaryotes, including zebrafish. An evolutionarily conserved 3-hydroxy butyrate dehydrogenase, 3-hydroxy butyrate dehydrogenase 2 (Bdh2), catalyzes a rate-limiting step in the biogenesis of the eukaryotic siderophore. We found that inactivation of bdh2 in developing zebrafish embryo results in heme deficiency and delays erythroid maturation. The basis for this erythroid maturation defect is not known. Here we show that bdh2 inactivation results in mitochondrial dysfunction and triggers their degradation by mitophagy. Thus, mitochondria are prematurely lost in bdh2-inactivated erythrocytes. Interestingly, bdh2-inactivated erythroid cells also exhibit genomic alterations as indicated by transcriptome analysis. Reestablishment of bdh2 restores mitochondrial function, prevents premature mitochondrial degradation, promotes erythroid development, and reverses altered gene expression. Thus, mitochondrial communication with the nucleus is critical for erythroid development.

Desmin common mutation is associated with multi-systemic disease manifestations and depletion of mitochondria and mitochondrial DNA

Desmin (DES) is a major muscle scaffolding protein that also functions to anchor mitochondria. Pathogenic DES mutations, however, have not previously been recognized as a cause of multi-systemic mitochondrial disease. Here, we describe a 45-year-old man who presented to The Children's Hospital of Philadelphia Mitochondrial-Genetics Diagnostic Clinic for evaluation of progressive cardiac, neuromuscular, gastrointestinal, and mood disorders. Muscle biopsy at age 45 was remarkable for cytoplasmic bodies, as well as ragged red fibers and SDH positive/COX negative fibers that were suggestive of a mitochondrial myopathy. Muscle also showed significant reductions in mitochondrial content (16% of control mean for citrate synthase activity) and mitochondrial DNA (35% of control mean). His family history was significant for cardiac conduction defects and myopathy in multiple maternal relatives. Multiple single gene and panel-based sequencing studies were unrevealing. Whole exome sequencing identified a known pathogenic p.S13F mutation in DES that had previously been associated with desmin-related myopathy. Desmin-related myopathy is an autosomal dominant disorder characterized by right ventricular hypertrophic cardiomyopathy, myopathy, and arrhythmias. However, neuropathy, gastrointestinal dysfunction, and depletion of both mitochondria and mitochondrial DNA have not previously been widely recognized in this disorder. Recognition that mitochondrial dysfunction occurs in desmin-related myopathy clarifies the basis for the multi-systemic manifestations, as are typical of primary mitochondrial disorders. Understanding the mitochondrial pathophysiology of desmin-related myopathy highlights the possibility of new therapies for this otherwise untreatable and often fatal class of disease. We postulate that drug treatments aimed at improving mitochondrial biogenesis or reducing oxidative stress may be effective therapies to ameliorate the effects of desmin-related disease.

Comprehensive measurement of respiratory activity in permeabilized cells using extracellular flux analysis

Extracellular flux (XF) analysis has become a mainstream method for measuring mitochondrial function in cells and tissues. Although this technique is commonly used to measure bioenergetics in intact cells, we outline here a detailed XF protocol for measuring respiration in permeabilized cells. Cells are permeabilized using saponin (SAP), digitonin (DIG) or recombinant perfringolysin O (rPFO) (XF-plasma membrane permeabilizer (PMP) reagent), and they are provided with specific substrates to measure complex I- or complex II-mediated respiratory activity, complex III+IV respiratory activity or complex IV activity. Medium- and long-chain acylcarnitines or glutamine may also be provided for measuring fatty acid (FA) oxidation or glutamine oxidation, respectively. This protocol uses a minimal number of cells compared with other protocols and does not require isolation of mitochondria. The results are highly reproducible, and mitochondria remain well coupled. Collectively, this protocol provides comprehensive and detailed information regarding mitochondrial activity and efficiency, and, after preparative steps, it takes 6-8 h to complete.

Mitochondrial fission induced by platelet-derived growth factor regulates vascular smooth muscle cell bioenergetics and cell proliferation

Vascular smooth muscle cells (VSMCs) develop a highly proliferative and synthetic phenotype in arterial diseases. Because such phenotypic changes are likely integrated with the energetic state of the cell, we hypothesized that changes in cellular metabolism regulate VSMC plasticity. VSMCs were exposed to platelet-derived growth factor-BB (PDGF) and changes in mitochondrial morphology, proliferation, contractile protein expression, and mitochondrial metabolism were examined. Exposure of VSMCs to PDGF resulted in mitochondrial fragmentation and a 50% decrease in the abundance of mitofusin 2. Synthetic VSMCs demonstrated a 20% decrease in glucose oxidation, which was accompanied by an increase in fatty acid oxidation. Results of mitochondrial function assays in permeabilized cells showed few changes due to PDGF treatment in mitochondrial respiratory chain capacity and coupling. Treatment of VSMCs with Mdivi-1—an inhibitor of mitochondrial fission—inhibited PDGF-induced mitochondrial fragmentation by 50% and abolished increases in cell proliferation; however, it failed to prevent PDGF-mediated activation of autophagy and removal of contractile proteins. In addition, treatment with Mdivi-1 reversed changes in fatty acid and glucose oxidation associated with the synthetic phenotype. These results suggest that changes in mitochondrial morphology and bioenergetics underlie the hyperproliferative features of the synthetic VSMC phenotype, but do not affect the degradation of contractile proteins. Mitochondrial fragmentation occurring during the transition to the synthetic phenotype could be a therapeutic target for hyperproliferative vascular disorders.

•

PDGF promotes mitochondrial fragmentation in vascular smooth muscle cells.

•

PDGF increases metabolic reliance on fatty acids.

•

Mitochondrial fragmentation regulates proliferation and bioenergetics.

•

PDGF-induced bioenergetic and autophagic responses regulate de-differentiation.