Measurement of mitochondrial NADH and FAD autofluorescence in live cells

NADH Autofluorescence-A Marker on its Way to Boost Bioenergetic Research

More than 60 years ago, the idea was introduced that NADH autofluorescence could be used as a marker of cellular redox state and indirectly also of cellular energy metabolism. Fluorescence lifetime imaging microscopy of NADH autofluorescence offers a marker-free readout of the mitochondrial function of cells in their natural microenvironment and allows different pools of NADH to be distinguished within a cell. Despite its many advantages in terms of spatial resolution and in vivo applicability, this technique still requires improvement in order to be fully useful in bioenergetics research. In the present review, we give a summary of technical and biological challenges that have so far limited the spread of this powerful technology. To help overcome these challenges, we provide a comprehensible overview of biological applications of NADH imaging, along with a detailed summary of valid imaging approaches that may be used to tackle many biological questions. This review is meant to provide all scientists interested in bioenergetics with support on how to embed successfully NADH imaging in their research. © 2018 International Society for Advancement of Cytometry.

Impact of PYROXD1 deficiency on cellular respiration and correlations with genetic analyses of limb-girdle muscular dystrophy in Saudi Arabia and Sudan

Next-generation sequencing is commonly used to screen for pathogenic mutations in families with Mendelian disorders, but due to the pace of discoveries, gaps have widened for some diseases between genetic and pathophysiological knowledge. We recruited and analyzed 16 families with limb-girdle muscular dystrophy (LGMD) of Arab descent from Saudi Arabia and Sudan who did not have confirmed genetic diagnoses. The analysis included both traditional and next-generation sequencing approaches. Cellular and metabolic studies were performed on Pyroxd1 siRNA C2C12 myoblasts and controls. Pathogenic mutations were identified in eight of the 16 families. One Sudanese family of Arab descent residing in Saudi Arabia harbored a homozygous c.464A>G, p.Asn155Ser mutation in PYROXD1, a gene recently reported in association with myofibrillar myopathy and whose protein product reduces thiol residues. Pyroxd1 deficiency in murine C2C12 myoblasts yielded evidence for impairments of cellular proliferation, migration, and differentiation, while CG10721 (Pyroxd1 fly homolog) knockdown in Drosophila yielded a lethal phenotype. Further investigations indicated that Pyroxd1 does not localize to mitochondria, yet Pyroxd1 deficiency is associated with decreased cellular respiration. This study identified pathogenic mutations in half of the LGMD families from the cohort, including one in PYROXD1. Developmental impairments were demonstrated in vitro for Pyroxd1 deficiency and in vivo for CG10721 deficiency, with reduced metabolic activity in vitro for Pyroxd1 deficiency.

Strong increase in the autofluorescence of cells signals struggle for survival

Prokaryotic and eukaryotic cells exhibit an intrinsic natural fluorescence due to the presence of fluorescent cellular structural components and metabolites. Therefore, cellular autofluorescence (AF) is expected to vary with the metabolic states of cells. We examined how exposure to the different stressors changes the AF of Escherichia coli cells. We observed that bactericidal treatments increased green cellular AF, and that de novo protein synthesis was required for the observed AF increase. Excitation and emission spectra and increased expression of the genes from the flavin biosynthesis pathway, strongly suggested that flavins are major contributors to the increased AF. An increased expression of genes encoding diverse flavoproteins which are involved in energy production and ROS detoxification, indicates a cellular strategy to cope with severe stresses. An observed increase in AF under stress is an evolutionary conserved phenomenon as it occurs not only in cells from different bacterial species, but also in yeast and human cells.

Novel Nrf2-Inducer Prevents Mitochondrial Defects and Oxidative Stress in Friedreich's Ataxia Models

Friedreich’s Ataxia (FRDA) is an autosomal recessive neurodegenerative disorder, affecting dorsal root ganglia (DRG), cerebellar dentate nuclei and heart. It is caused by a GAA repeat expansion mutation within the frataxin gene (FXN). This impedes FXN transcription resulting in a progressive decrease of the mitochondrial protein, frataxin. Increased oxidative stress leading to a chronic depletion of endogenous antioxidants affects the survival of the cells and causes neurodegeneration. In particular, cerebellar granule neurons (CGNs) show a significant increase of reactive oxygen species (ROS), lipid peroxidation and lower level of reduced glutathione (GSH). In FRDA, one of the major pathways of oxidant scavengers, the Nrf2 antioxidant pathway, is defective. Previous studies on FRDA-like CGNs showed that the reduced level of frataxin and the oxidative stress induce mitochondrial impairments. By triggering the Nrf2 endogenous pathway pharmacologically we determined whether this could promote mitochondrial fitness and counteract oxidative stress. In this work, we sought to investigate the beneficial effect of a promising Nrf2-inducer, omaveloxolone (omav), in CGNs from two FRDA mouse models, KIKO and YG8R, and human fibroblasts from patients. We found that CGNs from both KIKO and YG8R presented Complex I deficiency and that omav was able to restore substrate availability and Complex I activity. This was also confirmed in human primary fibroblasts from FRDA patients. Although fibroblasts are not the major tissue affected, we found that they show significant differences recapitulating the disease; this is therefore an important tool to investigate patients’ pathophysiology. Interestingly, we found that patient fibroblasts had an increased level of endogenous lipid peroxidation and mitochondrial ROS (mROS), and lower GSH at rest. Omav was able to reverse this phenotype, protecting the cells against oxidative stress. By stimulating the cells with hydrogen peroxide (H₂O₂) and looking for potential mitochondrial pathophysiology, we found that fibroblasts could not maintain their mitochondrial membrane potential (ΔΨ_m). Remarkably, omav was protective to mitochondrial depolarization, promoting mitochondrial respiration and preventing cell death. Our results show that omav promotes Complex I activity and protect cells from oxidative stress. Omav could, therefore, be used as a novel therapeutic drug to ameliorate the pathophysiology of FRDA.

Mitochondrial Subtype Identification and Characterization

Healthy, functional mitochondria are central to many cellular and physiological phenomena, including aging, metabolism, and stress resistance. A key feature of healthy mitochondria is a high membrane potential (Δψ) or charge differential (i.e., proton gradient) between the matrix and inner mitochondrial membrane. Mitochondrial Δψ has been extensively characterized via flow cytometry of intact cells, which measures the average membrane potential within a cell. However, the characteristics of individual mitochondria differ dramatically even within a single cell, and thus interrogation of mitochondrial features at the organelle level is necessary to better understand and accurately measure heterogeneity. Here we describe a new flow cytometric methodology that enables the quantification and classification of mitochondrial subtypes (via their Δψ, size, and substructure) using the small animal model C. elegans. Future application of this methodology should allow research to discern the bioenergetic and mitochondrial component in a number of human disease and aging models, including, C. elegans, cultured cells, small animal models, and human biopsy samples. © 2018 by John Wiley & Sons, Inc.

Minimizing ATP depletion by oxygen scavengers for single-molecule fluorescence imaging in live cells

The stability of organic dyes against photobleaching is critical in single-molecule tracking and localization microscopy. Since oxygen accelerates photobleaching of most organic dyes, glucose oxidase is commonly used to slow dye photobleaching by depleting oxygen. As demonstrated here, pyranose-2-oxidase slows bleaching of Alexa647 dye by ∼20-fold. However, oxygen deprivation may pose severe problems for live cells by reducing mitochondrial oxidative phosphorylation and ATP production. We formulate a method to sustain intracellular ATP levels in the presence of oxygen scavengers. Supplementation with metabolic intermediates including glyceraldehyde, glutamine, and α-ketoisocaproate maintained the intracellular ATP level for at least 10 min by balancing between FADH₂ and NADH despite reduced oxygen levels. Furthermore, those metabolites supported ATP-dependent synthesis of phosphatidylinositol 4,5-bisphosphate and internalization of PAR2 receptors. Our method is potentially relevant to other circumstances that involve acute drops of oxygen levels, such as ischemic damage in the brain or heart or tissues for transplantation.

Reduced background autofluorescence for cell imaging using nanodiamonds and lanthanide chelates

Bio-imaging is a key technique in tracking and monitoring important biological processes and fundamental biomolecular interactions, however the interference of background autofluorescence with targeted fluorophores is problematic for many bio-imaging applications. This study reports on two novel methods for reducing interference with cellular autofluorescence for bio-imaging. The first method uses fluorescent nanodiamonds (FNDs), containing nitrogen vacancy centers. FNDs emit at near-infrared wavelengths typically higher than most cellular autofluorescence; and when appropriately functionalized, can be used for background-free imaging of targeted biomolecules. The second method uses europium-chelating tags with long fluorescence lifetimes. These europium-chelating tags enhance background-free imaging due to the short fluorescent lifetimes of cellular autofluorescence. In this study, we used both methods to target E-selectin, a transmembrane glycoprotein that is activated by inflammation, to demonstrate background-free fluorescent staining in fixed endothelial cells. Our findings indicate that both FND and Europium based staining can improve fluorescent bio-imaging capabilities by reducing competition with cellular autofluorescence. 30 nm nanodiamonds coated with the E-selectin antibody was found to enable the most sensitive detective of E-selectin in inflamed cells, with a 40-fold increase in intensity detected.

Mitochondrial dysfunction in Parkinsonian mesenchymal stem cells impairs differentiation

Sporadic cases account for 90–95% of all patients with Parkinson's Disease (PD). Atypical Parkinsonism comprises approximately 20% of all patients with parkinsonism. Progressive Supranuclear Palsy (PSP) belongs to the atypical parkinsonian diseases and is histopathologically classified as a tauopathy. Here, we report that mesenchymal stem cells (MSCs) derived from the bone marrow of patients with PSP exhibit mitochondrial dysfunction in the form of decreased membrane potential and inhibited NADH-dependent respiration. Furthermore, mitochondrial dysfunction in PSP-MSCs led to a significant increase in mitochondrial ROS generation and oxidative stress, which resulted in decrease of major cellular antioxidant GSH. Additionally, higher basal rate of mitochondrial degradation and lower levels of biogenesis were found in PSP-MSCs, together leading to a reduction in mitochondrial mass. This phenotype was biologically relevant to MSC stemness properties, as it heavily impaired their differentiation into adipocytes, which mostly rely on mitochondrial metabolism for their bioenergetic demand. The defect in adipogenic differentiation was detected as a significant impairment of intracellular lipid droplet formation in PSP-MSCs. This result was corroborated at the transcriptional level by a significant reduction of PPARγ and FABP4 expression, two key genes involved in the adipogenic molecular network. Our findings in PSP-MSCs provide new insights into the etiology of ‘idiopathic’ parkinsonism, and confirm that mitochondrial dysfunction is important to the development of parkinsonism, independent of the type of the cell.

•

PSP pathology leads to inhibition of mitochondrial respiration and decrease mitochondrial membrane potential.

•

Mitochondrial dysfunction in PSP-MSCs induces ROS generation and oxidative stress.

•

Higher rate of mitophagy reduces mitochondrial mass in PSP-MSCs.

•

PSP impairs differentiation properties in MSCs.

Mechanistic Analysis of Fluorescence Quenching of Reduced Nicotinamide Adenine Dinucleotide by Oxamate in Lactate Dehydrogenase Ternary Complexes

Fluorescence of Reduced Nicotinamide Adenine Dinucleotide (NADH) is extensively employed in studies of oxidoreductases. A substantial amount of static and kinetic work has focused on the binding of pyruvate or substrate mimic oxamate to the binary complex of lactate dehydrogenase (LDH)-NADH where substantial fluorescence quenching is typically observed. However, the quenching mechanism is not well understood limiting structural interpretation. Based on time-dependent density functional theory (TDDFT) computations with cam-B3LYP functional in conjunction with the analysis of previous experimental results, we propose that bound oxamate acts as an electron acceptor in the quenching of fluorescence of NADH in the ternary complex, where a charge transfer (CT) state characterized by excitation from the highest occupied molecular orbital (HOMO) of the nicotinamide moiety of NADH to the lowest unoccupied molecular orbital (LUMO) of oxamate exists close to the locally excited (LE) state involving only the nicotinamide moiety. Efficient quenching in the encounter complex like in pig heart LDH requires that oxamate forms a salt bridge with Arg-171 and hydrogen bonds with His-195, Thr-246 and Asn-140. Further structural rearrangement and loop closure, which also brings about another hydrogen bond between oxamate and Arg-109, will increase the rate of fluorescence quenching as well.

NADH autofluorescence, a new metabolic biomarker for cancer stem cells: Identification of Vitamin C and CAPE as natural products targeting "stemness"

Here, we assembled a broad molecular “tool-kit” to interrogate the role of metabolic heterogeneity in the propagation of cancer stem-like cells (CSCs). First, we subjected MCF7 cells to “metabolic fractionation” by flow cytometry, using fluorescent mitochondrial probes to detect PCG1α activity, as well ROS and hydrogen-peroxide (H2O2) production; NADH levels were also monitored by auto-fluorescence. Then, the various cell populations were functionally assessed for “stem cell activity”, using the mammosphere assay (3D-spheroids). Our results indicate that a sub-population of MCF7 cells, with increased PGC1α activity, high mitochondrial ROS/H2O2 production and high NADH levels, all form mammospheres with a higher efficiency. Thus, it appears that mitochondrial oxidative stress and the anti-oxidant response both contribute to the promotion of mitochondrial biogenesis and oxidative metabolism in CSCs. Further validation was provided by using specific inhibitors to target metabolic processes (the NAD+ salvage pathway, glycolysis, mitochondrial protein synthesis and OXPHOS), significantly reducing CSC propagation. As a consequence, we have now identified a variety of clinically-approved drugs (stiripentol), natural products (caffeic acid phenyl ester (CAPE), ascorbic acid, silibinin) and experimental pharmaceuticals (actinonin, FK866, 2-DG), that can be used to effectively inhibit CSC activity. We discuss the use of CAPE (derived from honey-bee propolis) and Vitamin C, as potential natural therapeutic modalities. In this context, Vitamin C was ∼10 times more potent than 2-DG for the targeting of CSCs. Similarly, stiripentol was between 50 to 100 times more potent than 2-DG.

Glucose 6 phosphatase dehydrogenase (G6PD) and neurodegenerative disorders: Mapping diagnostic and therapeutic opportunities

Glucose 6 phosphate dehydrogenase (G6PD) is a key and rate limiting enzyme in the pentose phosphate pathway (PPP). The physiological significance of enzyme is providing reduced energy to specific cells like erythrocyte by maintaining co-enzyme nicotinamide adenine dinucleotide phosphate (NADPH). There are preponderance research findings that demonstrate the enzyme (G6PD) role in the energy balance, and it is associated with blood-related diseases and disorders, primarily the anemia resulted from G6PD deficiency. The X-linked genetic deficiency of G6PD and associated non-immune hemolytic anemia have been studied widely across the globe. Recent advancement in biology, more precisely neuroscience has revealed that G6PD is centrally involved in many neurological and neurodegenerative disorders. The neuroprotective role of the enzyme (G6PD) has also been established, as well as the potential of G6PD in oxidative damage and the Reactive Oxygen Species (ROS) produced in cerebral ischemia. Though G6PD deficiency remains a global health issue, however, a paradigm shift in research focusing the potential of the enzyme in neurological and neurodegenerative disorders will surely open a new avenue in diagnostics and enzyme therapeutics. Here, in this study, more emphasis was made on exploring the role of G6PD in neurological and inflammatory disorders as well as non-immune hemolytic anemia, thus providing diagnostic and therapeutic opportunities.

NAD(P)H and Flavin Autofluorescence Correlation with ATP in Rat Livers with Different Metabolic Steady-State Conditions

The monitoring of NAD(P)H and flavin autofluorescence (AF) is at the basis of numerous investigations on energy metabolism. Nevertheless, the ability of these AF biomarkers to accurately represent the energy currency, ATP, is poorly explored. Here, we focused on the AF/ATP correlation in lean and fatty livers with different steady-state metabolic conditions, achieved after organ isolation, preservation and recovery, in a likely dependence on both liver intrinsic metabolic features and externally induced perturbations. Within these eventual, various conditions, a significant correlation was detected between liver NAD(P)H and flavin AF, measured via fiber-optic probe, and biochemical ATP data, strengthening AF as biomarker of energy metabolism in steady-state conditions for wide-ranging experimental and diagnostic applications.

iPSC-derived neuronal models of PANK2-associated neurodegeneration reveal mitochondrial dysfunction contributing to early disease

Mutations in PANK2 lead to neurodegeneration with brain iron accumulation. PANK2 has a role in the biosynthesis of coenzyme A (CoA) from dietary vitamin B5, but the neuropathological mechanism and reasons for iron accumulation remain unknown. In this study, atypical patient-derived fibroblasts were reprogrammed into induced pluripotent stem cells (iPSCs) and subsequently differentiated into cortical neuronal cells for studying disease mechanisms in human neurons. We observed no changes in PANK2 expression between control and patient cells, but a reduction in protein levels was apparent in patient cells. CoA homeostasis and cellular iron handling were normal, mitochondrial function was affected; displaying activated NADH-related and inhibited FADH-related respiration, resulting in increased mitochondrial membrane potential. This led to increased reactive oxygen species generation and lipid peroxidation in patient-derived neurons. These data suggest that mitochondrial deficiency is an early feature of the disease process and can be explained by altered NADH/FADH substrate supply to oxidative phosphorylation. Intriguingly, iron chelation appeared to exacerbate the mitochondrial phenotype in both control and patient neuronal cells. This raises caution for the use iron chelation therapy in general when iron accumulation is absent.

Monomeric Alpha-Synuclein Exerts a Physiological Role on Brain ATP Synthase

Misfolded α-synuclein is a key factor in the pathogenesis of Parkinson's disease (PD). However, knowledge about a physiological role for the native, unfolded α-synuclein is limited. Using brains of mice lacking α-, β-, and γ-synuclein, we report that extracellular monomeric α-synuclein enters neurons and localizes to mitochondria, interacts with ATP synthase subunit α, and modulates ATP synthase function. Using a combination of biochemical, live-cell imaging and mitochondrial respiration analysis, we found that brain mitochondria of α-, β-, and γ-synuclein knock-out mice are uncoupled, as characterized by increased mitochondrial respiration and reduced mitochondrial membrane potential. Furthermore, synuclein deficiency results in reduced ATP synthase efficiency and lower ATP levels. Exogenous application of low unfolded α-synuclein concentrations is able to increase the ATP synthase activity that rescues the mitochondrial phenotypes observed in synuclein deficiency. Overall, the data suggest that α-synuclein is a previously unrecognized physiological regulator of mitochondrial bioenergetics through its ability to interact with ATP synthase and increase its efficiency. This may be of particular importance in times of stress or PD mutations leading to energy depletion and neuronal cell toxicity.

SIGNIFICANCE STATEMENT

Misfolded α-synuclein aggregations in the form of Lewy bodies have been shown to be a pathological hallmark in histological staining of Parkinson's disease (PD) patient brains. It is known that misfolded α-synuclein is a key driver in PD pathogenesis, but the physiological role of unfolded monomeric α-synuclein remains unclear. Using neuronal cocultures and isolated brain mitochondria of α-, β-, and γ-synuclein knock-out mice and monomeric α-synuclein, this current study shows that α-synuclein in its unfolded monomeric form improves ATP synthase efficiency and mitochondrial function. The ability of monomeric α-synuclein to enhance ATP synthase efficiency under physiological conditions may be of importance when α-synuclein undergoes the misfolding and aggregation reported in PD.

Pathogenic p62/SQSTM1 mutations impair energy metabolism through limitation of mitochondrial substrates

Abnormal mitochondrial function has been found in patients with frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Mutations in the p62 gene (also known as SQSTM1) which encodes the p62 protein have been reported in both disorders supporting the idea of an ALS/FTD continuum. In this work the role of p62 in energy metabolism was studied in fibroblasts from FTD patients carrying two independent pathogenic mutations in the p62 gene, and in a p62-knock-down (p62 KD) human dopaminergic neuroblastoma cell line (SH-SY5Y). We found that p62 deficiency is associated with inhibited complex I mitochondrial respiration due to lack of NADH for the electron transport chain. This deficiency was also associated with increased levels of NADPH reflecting a higher activation of pentose phosphate pathway as this is accompanied with higher cytosolic reduced glutathione (GSH) levels. Complex I inhibition resulted in lower mitochondrial membrane potential and higher cytosolic ROS production. Pharmacological activation of transcription factor Nrf2 increased mitochondrial NADH levels and restored mitochondrial membrane potential in p62-deficient cells. Our results suggest that the phenotype is caused by a loss-of-function effect, because similar alterations were found both in the mutant fibroblasts and the p62 KD model. These findings highlight the implication of energy metabolism in pathophysiological events associated with p62 deficiency.

Mitochondrial hyperpolarization in iPSC-derived neurons from patients of FTDP-17 with 10+16 MAPT mutation leads to oxidative stress and neurodegeneration

Tau protein inclusions are a frequent hallmark of a variety of neurodegenerative disorders. The 10+16 intronic mutation in MAPT gene, encoding tau, causes frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), by altering the splicing of the gene and inducing an increase in the production of 4R tau isoforms, which are more prone to aggregation. However, the molecular mechanisms linking increased 4R tau to neurodegeneration are not well understood. Here, we have used iPSC-derived neurons from patients of FTDP-17 carrying the 10+16 mutation to study the molecular mechanisms underlying neurodegeneration. We show that mitochondrial function is altered in the neurons of the patients. We found that FTDP-17 neurons present an increased mitochondrial membrane potential, which is partially maintained by the F1Fo ATPase working in reverse mode. The 10+16 MAPT mutation is also associated with lower mitochondrial NADH levels, partially supressed complex I-driven respiration, and lower ATP production by oxidative phosphorylation, with cells relying on glycolysis to maintain ATP levels. Increased mitochondrial membrane potential in FTDP-17 neurons leads to overproduction of the ROS in mitochondria, which in turn causes oxidative stress and cell death. Mitochondrial ROS overproduction in these cells is a major trigger for neuronal cell death and can be prevented by mitochondrial antioxidants.

Calcium-Induced Mitochondrial Permeability Transitions: Parameters of Ca2+ Ion Interactions with Mitochondria and Effects of Oxidative Agents

We evaluated the parameters of Ca²⁺-induced mitochondrial permeability transition (MPT) pore formations, Ca²⁺ binding constants, stoichiometry, energy of activation, and the effect of oxidative agents, tert-butyl hydroperoxide (tBHP), and hypochlorous acid (HOCl), on Ca²⁺ -mediated process in rat liver mitochondria. From the Hill plot of the dependence of MPT rate on Ca²⁺ concentration, we determined the order of interaction of Ca²⁺ ions with the mitochondrial sites, n = 3, and the apparent K_d = 60 ± 12 µM. We also found the apparent Michaelis-Menten constant, K_m, for Ca²⁺ interactions with mitochondria to be equal to 75 ± 20 µM, whereas that in the presence of 300 µM tBHP was 120 ± 20 µM. Using the Arrhenius plots of the temperature dependences of apparent mitochondrial swelling rate at various Ca²⁺ concentrations, we calculated the activation energy of the MPT process. ΔE_a was 130 ± 20 kJ/mol at temperatures below the break point of the Arrhenius plot (30-34 °C) and 50 ± 9 kJ/mol at higher temperatures. Ca²⁺ ions induced rapid mitochondrial NADH depletion and membrane depolarization. Prevention of the pore formation by cyclosporin A inhibited Ca²⁺-dependent mitochondrial depolarization and Mg²⁺ ions attenuated the potential dissipation. tBHP (10-150 µM) dose-dependently enhanced the rate of MPT opening, whereas the effect of HOCl on MPT depended on the ratio of HOCl/Ca²⁺. The apparent K_m of tBHP interaction with mitochondria in the swelling reaction was found to be K_m = 11 ± 3 µM. The present study provides evidence that three calcium ions interact with mitochondrial site with high affinity during MPT. Ca²⁺-induced MPT pore formations due to mitochondrial membrane protein denaturation resulted in membrane potential dissipation. Oxidants with different mechanisms, tBHP and HOCl, reduced mitochondrial membrane potential and oxidized mitochondrial NADH in EDTA-free medium and had an effect on Ca²⁺-induced MPT onset.

Real-time and high-throughput analysis of mitochondrial metabolic states in living cells using genetically encoded NAD+/NADH sensors

Mitochondria are central organelles that regulate cellular bioenergetics, biosynthesis, and signaling processes. NADH, a key player in cell metabolism, is often considered as a marker of mitochondrial function. However, traditional methods for NADH measurements are either destructive or unable to distinguish between NADH and NADPH. In contrast to traditional methods, genetically encoded NADH sensors can be used for the real-time tracking and quantitative measurement of subcellular NADH levels in living cells. Therefore, these sensors provide innovative tools and address the limitations of current techniques. We herein summarize the properties of different types of recently developed NADH biosensors, discuss their advantages and disadvantages, and focus on the high-throughput analysis of mitochondrial function by using highly responsive NAD⁺/NADH sensors.

Carbon monoxide shifts energetic metabolism from glycolysis to oxidative phosphorylation in endothelial cells

Carbon monoxide (CO) modulates mitochondrial respiration, but the mechanisms involved are not completely understood. The aim of the present study was to investigate the acute effects of CO on bioenergetics and metabolism in intact EA.hy926 endothelial cells using live cell imaging techniques. Our findings indicate that CORM-401, a compound that liberates CO, reduces ATP production from glycolysis, and induces a mild mitochondrial depolarization. In addition, CO from CORM-401 increases mitochondrial calcium and activates complexes I and II. The subsequent increase in mitochondrial respiration leads to ATP production through oxidative phosphorylation. Thus, our results show that nonactivated endothelial cells rely primarily on glycolysis, but in the presence of CO, mitochondrial Ca²⁺ increases and activates respiration that shifts the metabolism of endothelial cells from glycolysis- to oxidative phosphorylation-dependent ATP production.

Deficiency of Parkinson's disease-related gene Fbxo7 is associated with impaired mitochondrial metabolism by PARP activation

The Parkinson's disease (PD)-related protein F-box only protein 7 (Fbxo7) is the substrate-recognition component of the Skp1-Cullin-F-box protein E3 ubiquitin ligase complex. We have recently shown that PD-associated mutations in Fbxo7 disrupt mitochondrial autophagy (mitophagy), suggesting a role for Fbxo7 in modulating mitochondrial homeostasis. Here we report that Fbxo7 deficiency is associated with reduced cellular NAD⁺ levels, which results in increased mitochondrial NADH redox index and impaired activity of complex I in the electron transport chain. Under these conditions of compromised respiration, mitochondrial membrane potential and ATP contents are reduced, and cytosolic reactive oxygen species (ROS) production is increased. ROS activates poly (ADP-ribose) polymerase (PARP) activity in Fbxo7-deficient cells. PARP inhibitor restores cellular NAD⁺ content and redox index and ATP pool, suggesting that PARP overactivation is cause of decreased complex I-driven respiration. These findings bring new insight into the mechanism of Fbxo7 deficiency, emphasising the importance of mitochondrial dysfunction in PD.