Choice of medium affects PBMC quantification, cell size, and downstream respiratory analysis

High-resolution respirometry (HRR) can assess peripheral blood mononuclear cell (PBMC) bioenergetics, but no standardized medium for PBMC preparation and HRR analysis exist. Here, we study the effect of four different media (MiR05, PBS, RPMI, Plasmax) on the count, size, and HRR (Oxygraph-O2k) of intact PBMCs. Remarkably, the cell count was 21 % higher when PBMCs were resuspended in MiR05 than in PBS or Plasmax, causing O2 flux underestimation during HRR due to inherent adjustments. Moreover, smaller cell size and cell aggregation was observed in MiR05. Based on our findings, we propose that Plasmax, PBS or RPMI is more suitable than MiR05 for HRR of intact PBMCs. We provide oxygen solubility factors for Plasmax and PBS and encourage further optimization of a standardized HRR protocol for intact PBMCs.

Electron transport system supercomplexes affect reactive-oxygen species production and respiration in both a hibernator (Ictidomys tridecemlineatus) and a nonhibernator (Rattus norvegicus)

Across many taxa, the complexes of the electron transport system associate with each other within the inner mitochondrial membrane to form supercomplexes (SCs). These SCs are thought to confer some selective advantage, such as increasing cellular respiratory capacity or decreasing the production of damaging reactive oxygen species (ROS). In this study, we investigate the relationship between supercomplex abundance and performance of liver mitochondria isolated from rats that do not hibernate and hibernating ground squirrels in which metabolism fluctuates substantially. We quantified the abundance of SCs (respirasomes (SCs containing CI, CIII, and CIV) or SCs containing CIII and CIV) and examined the relationship with state 3 (OXPHOS) and state 4 (LEAK) respiration rate, as well as net ROS production. We found that, in rats, state 3 and 4 respiration rate correlated negatively with respirasome abundance, but positively with CIII/CIV SC abundance. Despite the greater range of respiration rates in different hibernation stages, these relationships were similar in ground squirrels. This is, to our knowledge, the first report of differential effects of supercomplex types on mitochondrial respiration and ROS production.

Impairment of substrate-mediated mitochondrial respiration in cardiac cells by chloroquine

Chloroquine (CQ) has a long clinical history as an anti-malarial agent and also being used for the treatment of other infections and autoimmune diseases. Recently, this lysosomotropic agent and its derivatives are also been tested as adjuncts alongside conventional anti-cancer treatments in combinatorial therapies. However, their reported cardiotoxicity tends to raise concern over their indiscriminate use. Even though the influence of CQ and its derivatives on cardiac mitochondria is extensively studied in disease models, their impact on cardiac mitochondrial respiration under physiological conditions remains inconclusive. In this study, we aimed to evaluate the impact of CQ on cardiac mitochondrial respiration using both in-vitro and in-vivo model systems. Using high-resolution respirometry in isolated cardiac mitochondria from male C57BL/6 mice treated with intraperitoneal injection of 10 mg/kg/day of CQ for 14 days, CQ was found to impair substrate-mediated mitochondrial respiration in cardiac tissue. In an in-vitro model of H9C2 cardiomyoblasts, incubation with 50 µM of CQ for 24 h disrupted mitochondrial membrane potential, produced mitochondrial fragmentation, decreased mitochondrial respiration and induced superoxide generation. Altogether, our study results indicate that CQ has a deleterious impact on cardiac mitochondrial bioenergetics which in turn suggests that CQ treatment could be an added burden, especially in patients affected with diseases with underlying cardiac complications. As CQ is an inhibitor of the lysosomal pathway, the observed effect could be an outcome of the accumulation of dysfunctional mitochondria due to autophagy inhibition.

High-Resolution Respirometry for Mitochondrial Function in Rodent Brain

High-resolution mitochondrial respirometry is a modern technique that enables to measure mitochondrial respiration in various cell types. It contains chambers with oxygen sensors that measure oxygen concentration via polarography and calculate its consumption. The chamber contains plastic stoppers with injection ports that allow the injection of samples and different substrates, inhibitors, and uncoupler substances to measure mitochondrial respiration with high efficiency. These substances act on the mitochondrial electron transport chain (ETC) and help to assess the mitochondrial ATP production capacity and oxidative phosphorylation. The respirograph obtained with the help of software represents the oxygen consumption in each stage after adding different reagents.

Alterations of hepatic energy metabolism in murine models of obesity, diabetes and fatty liver diseases

BACKGROUND

Disturbed hepatic energy metabolism contributes to non-alcoholic fatty liver (NAFLD), but the development of changes over time and obesity- or diabetes-related mechanisms remained unclear.

METHODS

Two-day old male C57BL/6j mice received streptozotocin (STZ) or placebo (PLC) and then high-fat (HFD) or regular chow diet (RCD) from week 4 (W4) to either W8 or W16, yielding control [CTRL = PLC + RCD], diabetes [DIAB = STZ + RCD], obesity [OBES = PLC + HFD] and diabetes-related non-alcoholic steatohepatitis [NASH = STZ + HFD] models. Mitochondrial respiration was measured by high-resolution respirometry and insulin-sensitive glucose metabolism by hyperinsulinemic-euglycemic clamps with stable isotope dilution.

FINDINGS

NASH showed higher steatosis and NAFLD activity already at W8 and liver fibrosis at W16 (all p < 0.01 vs CTRL). Ballooning was increased in DIAB and NASH at W16 (p < 0.01 vs CTRL). At W16, insulin sensitivity was 47%, 58% and 75% lower in DIAB, NASH and OBES (p < 0.001 vs CTRL). Hepatic uncoupled fatty acid oxidation (FAO)-associated respiration was reduced in OBES at W8, but doubled in DIAB and NASH at W16 (p < 0.01 vs CTRL) and correlated with biomarkers of unfolded protein response (UPR), oxidative stress and hepatic expression of certain enzymes (acetyl-CoA carboxylase 2, Acc2; carnitine palmitoyltransferase I, Cpt1a). Tricarboxylic acid cycle (TCA)-driven respiration was lower in OBES at W8 and doubled in DIAB at W16 (p < 0.0001 vs CTRL), which positively correlated with expression of genes related to lipolysis.

INTERPRETATION

Hepatic mitochondria adapt to various metabolic challenges with increasing FAO-driven respiration, which is linked to dysfunctional UPR, systemic oxidative stress, insulin resistance and altered lipid metabolism. In a diabetes model, higher TCA-linked respiration reflected mitochondrial adaptation to greater hepatic lipid turnover.

FUNDING

Funding bodies that contributed to this study were listed in the acknowledgements section.

Combined effects of salinity and intermittent hypoxia on mitochondrial capacity and reactive oxygen species efflux in the Pacific oyster, Crassostrea gigas

Coastal environments commonly experience fluctuations in salinity and hypoxia-reoxygenation (H/R) stress that can negatively affect mitochondrial functions of marine organisms. Although intertidal bivalves are adapted to these conditions, the mechanisms that sustain mitochondrial integrity and function are not well understood. We determined the rates of respiration and reactive oxygen species (ROS) efflux in the mitochondria of oysters, Crassostrea gigas, acclimated to high (33 psu) or low (15 psu) salinity, and exposed to either normoxic conditions (control; 21% O2) or short-term hypoxia (24 h at <0.01% O2) and subsequent reoxygenation (1.5 h at 21% O2). Further, we exposed isolated mitochondria to anoxia in vitro to assess their ability to recover from acute (∼10 min) oxygen deficiency (<0.01% O2). Our results showed that mitochondria of oysters acclimated to high or low salinity did not show severe damage and dysfunction during H/R stress, consistent with the hypoxia tolerance of C. gigas. However, acclimation to low salinity led to improved mitochondrial performance and plasticity, indicating that 15 psu might be closer to the metabolic optimum of C. gigas than 33 psu. Thus, acclimation to low salinity increased mitochondrial oxidative phosphorylation rate and coupling efficiency and stimulated mitochondrial respiration after acute H/R stress. However, elevated ROS efflux in the mitochondria of low-salinity-acclimated oysters after acute H/R stress indicates a possible trade-off of higher respiration. The high plasticity and stress tolerance of C. gigas mitochondria may contribute to the success of this invasive species and facilitate its further expansion into brackish regions such as the Baltic Sea.

Cryopreserved placental biopsies maintain mitochondrial activity for high-resolution respirometry

BACKGROUND

High-resolution respirometry (HRR) of human biopsies can provide useful metabolic, diagnostic, and mechanistic insights for clinical research and comparative medical studies. Fresh tissues analysis offers the potential best condition, the drawback being the need to use them shortly after dissection for mitochondrial respiratory experiments. The development of effective long-term storage protocols for biopsies that allow the assessment of key Electron Transport System (ETS) parameters at later stages is thus a major need.

METHODS

We optimised a cryopreservation protocol that preserves mitochondrial membranes intactness, otherwise affected by direct tissue freezing. The protocol is based on a gradual freezing step from on-ice to liquid nitrogen and - 80 °C storage using a specific DMSO-based buffer.

RESULTS

Placenta is a suitable tissue to design and test the effectiveness of long-term storage protocols being metabolically active foetal tissue with mitochondrial dysfunctions contributing to placental disease and gestational disorders. Here we designed and tested the effectiveness of the cryopreservation protocol using human placenta biopsies; we measured the ETS activity by HRR of placenta specimens comparing fresh, cryopreserved, and snap frozen conditions.

CONCLUSIONS

By this protocol, Oxygen Consumption Rate (OCR) measurements of fresh and cryopreserved placental specimens are comparable whereas snap frozen procedure impairs mitochondrial activity.

Methylmalonic Acid Impairs Cell Respiration and Glutamate Uptake in C6 Rat Glioma Cells: Implications for Methylmalonic Acidemia

Methylmalonic acidemia is an organic acidemia caused by deficient activity of L-methylmalonyl-CoA mutase or its cofactor cyanocobalamin and it is biochemically characterized by an accumulation of methylmalonic acid (MMA) in tissue and body fluids of patients. The main clinical manifestations of this disease are neurological and observable symptoms during metabolic decompensation are encephalopathy, cerebral atrophy, coma, and seizures, which commonly appear in newborns. This study aimed to investigate the toxic effects of MMA in a glial cell line presenting astrocytic features. Astroglial C6 cells were exposed to MMA (0.1-10 mM) for 24 or 48 h and cell metabolic viability, glucose consumption, and oxygen consumption rate, as well as glutamate uptake and ATP content were analyzed. The possible preventive effects of bezafibrate were also evaluated. MMA significantly reduced cell metabolic viability after 48-h period and increased glucose consumption during the same period of incubation. Regarding the energy homeostasis, MMA significantly reduced respiratory parameters of cells after 48-h exposure, indicating that cell metabolism is compromised at resting and reserve capacity state, which might influence the cell capacity to meet energetic demands. Glutamate uptake and ATP content were also compromised after exposure to MMA, which can be influenced energy metabolism impairment, affecting the functionality of the astroglial cells. Our findings suggest that these effects could be involved in the pathophysiology of neurological dysfunction of this disease. Methylmalonic acid compromises mitochondrial functioning leading to reduced ATP production and reduces glutamate uptake by C6 astroglial cells.

Effects of cigarette smoke on in situ mitochondrial substrate oxidation of slow- and fast-twitch skeletal muscles

Epidemiological and clinical evidence suggests that cigarette smoke exposure alters glucose and fatty acid metabolism, leading to greater susceptibility to metabolic disorders. However, the effects of cigarette smoke exposure on mitochondrial substrate oxidation in the skeletal muscle are still poorly understood. Accordingly, this study aimed to examine the acute effects of cigarette smoke on mitochondrial respiratory capacity, sensitivity, and concurrent utilization of palmitoylcarnitine (PC), a long-chain fatty acid, and pyruvate, a product of glycolysis, in permeabilized gastrocnemius and soleus muscle fibers exposed to an acute (1 h) dose (4 %) of cigarette smoke concentrate. Cigarette smoke decreased both mitochondrial respiratory capacity (CONTROL: 50.4 ± 11.8 pmol_O2/s/mg_wt and SMOKE: 22.3 ± 4.4 pmol_O2/s/mg_wt, p < 0.01) and sensitivity for pyruvate (CONTROL: 0.10 ± 0.04 mM and SMOKE: 0.11 ± 0.04 mM, p < 0.01) in the gastrocnemius muscle. In the soleus, only the sensitivity for pyruvate-stimulated mitochondrial respiration trended toward a decrease (CONTROL: 0.11 ± 0.04 mM and SMOKE: 0.23 ± 0.15 mM, p = 0.08). In contrast, cigarette smoke did not significantly alter palmitoylcarnitine-stimulated mitochondrial respiration in either muscle. In the control condition, pyruvate-supported respiration was inhibited by the concurrent addition of palmitoylcarnitine in the fast-twitch gastrocnemius muscle (-27.1 ± 19.7 %, p < 0.05), but not in the slow-twitch soleus (-9.2 ± 17.0 %). With cigarette smoke, the addition of palmitoylcarnitine augmented the maximal respiration rate stimulated by the concurrent addition of pyruvate in the gastrocnemius (+18.5 ± 39.3 %, p < 0.05). However, cigarette smoke still significantly impaired mitochondrial respiratory capacity with combined substrates compared to control (p < 0.05). Our findings underscore that cigarette smoke directly impairs mitochondrial respiration of carbohydrate-derived substrates and is a primary mechanism underlying cigarette smoke-induced muscle dysfunction, which leads to a vicious cycle involving excess glucose conversion into fatty acids and lipotoxicity.

The receptor for advanced glycation end products (RAGE) is involved in mitochondrial function and cigarette smoke-induced oxidative stress

The mechanisms underlying muscle dysfunction with Chronic Obstructive Pulmonary Disease (COPD) are poorly understood. Indirect evidence has recently suggested a role of Advanced Glycation End Products (AGEs) and their receptor (RAGE) in the pathophysiology of COPD. Accordingly, this study aimed to examine the redox balance and mitochondrial alterations in the skeletal muscle of a mouse model deficient in the receptor for AGE (RAGE-KO) and wild-type C57BL/6 exposed to cigarette smoke for 8-months using immunoblotting, spectrophotometry, and high-resolution respirometry. Cigarette smoke exposure increased by two-fold 4-HNE levels (P < 0.001), a marker of oxidative stress, and markedly downregulated contractile proteins, mitochondrial respiratory complexes, and uncoupling proteins levels (P < 0.001). Functional alterations with cigarette smoke exposure included a greater reliance on complex-I supported respiration (P < 0.01) and lower relative respiratory capacity for fatty acid (P < 0.05). RAGE knockout resulted in 47% lower 4-HNE protein levels than the corresponding WT control mice exposed to cigarette smoke (P < 0.05), which was partly attributed to increased Complex III protein levels. Independent of cigarette smoke exposure, RAGE KO decreased mitochondrial specific maximal respiration (P < 0.05), resulting in a compensatory increase in mitochondrial content measured by citrate synthase activity (P < 0.001) such that muscle respiratory capacity remained unaltered. Together, these findings suggest that knockout of RAGE protected the skeletal muscle against oxidative damage induced by 8 months of cigarette smoke exposure. In addition, this study supports a role for RAGE in regulating mitochondrial content and function and can thus serve as a potential therapeutic target.

Caffeine improves mitochondrial function in PINK1B9-null mutant Drosophila melanogaster

Mitochondrial dysfunction plays a central role in Parkinson's disease (PD) and can be triggered by xenobiotics and mutations in mitochondrial quality control genes, such as the PINK1 gene. Caffeine has been proposed as a secondary treatment to relieve PD symptoms mainly by its antagonistic effects on adenosine receptors (ARs). Nonetheless, the potential protective effects of caffeine on mitochondrial dysfunction could be a strategy in PD treatment but need further investigation. In this study, we used high-resolution respirometry (HRR) to test caffeine's effects on mitochondrial dysfunction in PINK1^B9-null mutants of Drosophila melanogaster. PINK1 loss-of-function induced mitochondrial dysfunction in PINK1^B9-null flies observed by a decrease in O₂ flux related to oxidative phosphorylation (OXPHOS) and electron transfer system (ETS), respiratory control ratio (RCR) and ATP synthesis compared to control flies. Caffeine treatment improved OXPHOS and ETS in PINK^B9-null mutant flies, increasing the mitochondrial O₂ flux compared to untreated PINK^B9-null mutant flies. Moreover, caffeine treatment increased O₂ flux coupled to ATP synthesis and mitochondrial respiratory control ratio (RCR) in PINK 1^B9-null mutant flies. The effects of caffeine on respiratory parameters were abolished by rotenone co-treatment, suggesting that caffeine exerts its beneficial effects mainly by stimulating the mitochondrial complex I (CI). In conclusion, we demonstrate that caffeine may improve mitochondrial function by increasing mitochondrial OXPHOS and ETS respiration in the PD model using PINK1 loss-of-function mutant flies.

Measurement of Fatty Acid Oxidation by High-Resolution Respirometry: Special Considerations for Analysis of Skeletal and Cardiac Muscle and Adipose Tissue

High-resolution respirometry is a state-of-the-art approach for the quantitation of mitochondrial function. Isolated mitochondria, cultured cells, or tissues/fibers are suspended in oxygenated respiration medium within a closed chamber and substrates or inhibitors added in a stepwise manner. The dissolved oxygen concentration decreases as aerobic metabolism in the specimen proceeds, recorded by an oxygen sensor within the chamber to give a quantifiable measure of oxygen consumption by the sample. Measuring oxygen consumption using a variety of respiratory substrates or respiratory complex-targeted inhibitors enables multiple respiratory pathways to be interrogated to determine the functional capacity of the mitochondria in real time. Using a substrate-uncoupler-inhibitor titration (SUIT) protocol, we have developed a method which makes use of differing chain length fatty acids to derive a measure of fatty acid-stimulated respiration through β-oxidation in a variety of tissue types including skeletal and cardiac muscles and brown and white adipose tissues. This report provides technical details of the protocol, and the adaptations employed, to generate robust analysis of mitochondrial fatty acid β-oxidation.

Impairment of mitochondrial respiration in platelets and placentas: a pilot study in preeclamptic pregnancies

Preeclampsia (PE) is a major complication of pregnancy with partially elucidated pathophysiology. Placental mitochondrial dysfunction has been increasingly studied as major pathomechanism in both early- and late-onset PE. Impairment of mitochondrial respiration in platelets has recently emerged as a peripheral biomarker that may mirror organ mitochondrial dysfunction in several acute and chronic pathologies. The present study was purported to assess mitochondrial respiratory dys/function in both platelets and placental mitochondria in PE pregnancies. To this aim, a high-resolution respirometry SUIT (Substrate-Uncoupler-Inhibitor-Titration) protocol was adapted to assess complex I (glutamate + malate)- and complex II (succinate)-supported respiration. A decrease in all respiratory parameters (basal, coupled, and maximal uncoupled respiration) in peripheral platelets was found in preeclamptic as compared to healthy pregnancies. At variance, placental mitochondria showed a dichotomous behavior in preeclampsia in relation to the fetal birth weight. PE pregnancies with fetal growth restriction were associated with decreased in coupled respiration (oxidative phosphorylation/OXPHOS capacity) and maximal uncoupled respiration (electron transfer/ET capacity). At variance, these respiratory parameters were increased for both complex I- and II-supported respiration in PE pregnancies with normal weight fetuses. Large randomized controlled clinical studies are needed in order to advance our understanding of mitochondrial adaptive vs. pathological changes in preeclampsia.

Methylglyoxal disrupts the functionality of rat liver mitochondria

Methylglyoxal (MG) is a reactive metabolite derived from different physiological pathways. Its production can be harmful to cells via glycation reactions of lipids, DNA, and proteins. But, the effects of MG on mitochondrial functioning and bioenergetic responses are still elusive. Then, the effects of MG on key parameters of mitochondrial functionality were examined here. Isolated rat liver mitochondria were exposed to 0.1-10 mM of MG to determine its toxicity in the mitochondrial viability, membrane potential (Δψm), swelling and the superoxide (O2•-) production. Besides, mitochondrial oxidative phosphorylation parameters were analyzed by high-resolution respiratory (HRR) assay. In this set of experiments, routine state, PM state (pyruvate/malate), oxidative phosphorylation (OXPHOS), LEAK respiration, electron transport system (ETS) and oxygen residual (ROX) states were evaluated. HRR showed that PM state, OXPHOS CI-Linked, LEAK respiration, ETS CI/CII-Linked and ETS CII-Linked/ROX were significantly inhibited by MG exposure. MG also inhibited the complex II activity, and decreased Δψm and the viability of mitochondria. Taken together, our data indicates that MG is an inductor of mitochondrial dysfunctions and impairs important steps of respiratory chain, effects that can alter bioenergetics responses.

Mitochondrial capacity and reactive oxygen species production during hypoxia and reoxygenation in the ocean quahog, Arctica islandica

Oxygen fluctuations are common in marine waters, and hypoxia-reoxygenation (H-R) stress can negatively affect mitochondrial metabolism. The long-lived ocean quahog, Arctica islandica, is known for its hypoxia tolerance associated with metabolic rate depression, yet the mechanisms that sustain mitochondrial function during oxygen fluctuations are not well understood. We used top-down metabolic control analysis (MCA) to determine aerobic capacity and control over oxygen flux in the mitochondria of quahogs exposed to short-term hypoxia (24 h <0.01% O2) and subsequent reoxygenation (1.5 h 21% O2) compared with normoxic control animals (21% O2). We demonstrated that flux capacity of the substrate oxidation and proton leak subsystems were not affected by hypoxia, while the capacity of the phosphorylation subsystem was enhanced during hypoxia associated with a depolarization of the mitochondrial membrane. Reoxygenation decreased the oxygen flux capacity of all three mitochondrial subsystems. Control over oxidative phosphorylation (OXPHOS) respiration was mostly exerted by substrate oxidation regardless of H-R stress, whereas control by the proton leak subsystem of LEAK respiration increased during hypoxia and returned to normoxic levels during reoxygenation. During hypoxia, reactive oxygen species (ROS) efflux was elevated in the LEAK state, whereas it was suppressed in the OXPHOS state. Mitochondrial ROS efflux returned to normoxic control levels during reoxygenation. Thus, mitochondria of A. islandica appear robust to hypoxia by maintaining stable substrate oxidation and upregulating phosphorylation capacity, but remain sensitive to reoxygenation. This mitochondrial phenotype might reflect adaptation of A. islandica to environments with unpredictable oxygen fluctuations and its behavioural preference for low oxygen levels.

High-resolution respirometry for evaluation of mitochondrial function on brain and heart homogenates in a rat model of cardiac arrest and cardiopulmonary resuscitation

The physiology and physiopathology process of mitochondrial function following cardiac arrest remains poorly understood. We aimed to assess mitochondrial respiratory function on the heart and brain homogenates from cardiac arrest rats. The expression level of SIRT1/PGC-1α pathway was measured by immunoblotting. 30 rats were assigned to the CA group and the sham group. Rats of CA were subjected to 6 min of untreated ventricular fibrillation (VF) followed by 8 min of cardiopulmonary resuscitation (CPR). Mitochondrial respiratory function was compromised following CA and I/R injury, as indicated by CIL (451.46 ± 71.48 vs. 909.91 ± 5.51 pmol/min*mg for the heart and 464.14 ± 8.22 vs. 570.53 ± 56.33 pmol/min*mg for the brain), CI (564.04 ± 64.34 vs. 2729.52 ± 347.39 pmol/min*mg for the heart and 726.07 ± 85.78 vs. 1762.82 ± 262.04 pmol/min*mg for the brain), RCR (1.88 ± 0.46 vs. 3.57 ± 0.38 for the heart and 2.05 ± 0.19 vs. 3.49 ± 0.19, for the brain) and OXPHOS coupling efficiency (0.45 ± 0.11 vs. 0.72 ± 0.03 for the heart and 0.52 ± 0.05 vs. 0.71 ± 0.01 for the brain). However, routine respiration was lower in the heart and comparable in the brain after CA. CIV did not change in the heart but was enhanced in the brain. Furthermore, both SIRT1 and PGC-1α were downregulated concurrently in the heart and brain. The mitochondrial respiratory function was compromised following CA and I/R injury, and the major affected respiratory state is complex I-linked respiration. Furthermore, the heart and the brain respond differently to the global I/R injury after CA in mitochondrial respiratory function. Inhibition of the SIRT1/PGC-1α pathway may be a major contributor to the impaired mitochondrial respiratory function.

Measurement of Mitochondrial Respiration in Platelets

Platelet mitochondria can be used in the study of mitochondrial dysfunction in various complex diseases and can help in finding biological markers for diagnosing the disease, monitoring its course and the effects of treatment. The aim of this chapter was to describe in detail the method of measuring mitochondrial respiration in platelets using high-resolution respirometry. The described method was successfully used for the study of mitochondrial dysfunction in neuropsychiatric diseases.

Assessment of the Effects of Drugs on Mitochondrial Respiration

Mitochondria are targets of newly synthesized drugs and being tested for the treatment of various diseases caused or accompanied by disruption of cellular bioenergetics. In drug development, it is necessary to test for drug-induced changes in mitochondrial enzyme activity that may be related to therapeutic or adverse drug effects. Measurement of drug effect on mitochondrial oxygen consumption kinetics and/or protective effects of drugs against calcium-induced inhibition of the mitochondrial respiration can be used for the study mitochondrial toxicity and neuroprotective effects of drugs. Supposing that the drug-induced inhibition of the mitochondrial respiratory rate and/or individual mitochondrial complexes is associated with adverse drug effects, the effects of drugs on mitochondrial respiration in isolated mitochondria allow selection of novel molecules that are relatively safe for mitochondrial toxicity.

Mitochondrial dynamics and quality control are altered in a hepatic cell culture model of cancer cachexia

Hepatic mitochondrial function loss is associated with cancer cachexia pathology in vivo. Here, we examined if hepatic mitochondrial defects observed in vivo in the cachexic liver also recapitulate during the in vitro treatment of mouse hepatocytes with tumor conditioned media. In vitro experiments were combined with proteome-wide expression analysis of cachexic liver tissue curated for mitochondrial dynamics and quality control proteins, to determine the fidelity of hepatic mitochondrial maladaptation in cancer cachexia pathology. AML12 hepatocytes were exposed to colon-26 (C26) and Lewis lung carcinoma (LLC) conditioned media for 6-72 h and assayed for cell viability, membrane potential, respiratory function, H₂O₂ production, total ROS/RNS, and mitochondrial dynamics and quality control proteins by immunoblotting. Liver tissue from cachexic C26 mice was analyzed by TMT-based quantitative proteomics for in vivo comparison. Cell viability, membrane potential, H₂O₂ production, total ROS/RNS, and respiration were decreased 48-72 h after exposure to C26 and/or LLC. Protein expression of treated hepatocytes and cachexic liver tissue showed altered mitochondrial dynamics and quality control, in a manner that suggests limited fusion and content mixing, but also impaired ability to fragment and clear damaged mitochondria. Two strategies to maintain mitochondrial health, therefore, may not be functioning sufficiently in the cachexic liver. Together these findings imply adverse effects of C26 and LLC exposure on hepatocyte health, due to impaired mitochondrial function and remodeling. Exposure of mouse hepatocytes to tumor conditioned media models aspects of cachexic liver mitochondria dysfunction in vivo and validates the importance of hepatic mitochondrial maladaptation in cancer cachexia pathology.

Relevance of Mitochondrial Dysfunction in the Reserpine-Induced Experimental Fibromyalgia Model

Fibromyalgia (FM) is one of the most common musculoskeletal pain conditions. Although the aetiology of FM is still unknown, mitochondrial dysfunction and the overproduction of reactive oxygen intermediates (ROI) are common characteristics in its pathogenesis. The reserpine experimental model can induce FM-related symptoms in rodents by depleting biogenic amines. However, it is unclear whether reserpine causes other pathophysiologic characteristics of FM. So far, no one has investigated the relevance of mitochondrial dysfunction in the reserpine-induced experimental FM model using protection- and insult-based mitochondrial modulators. Reserpine (1 mg/kg) was subcutaneously injected once daily for three consecutive days in male Swiss mice. We carried out analyses of reserpine-induced FM-related symptoms, and their modulation by using mitochondrial insult on ATP synthesis (oligomycin; 1 mg/kg, intraperitoneally) or mitochondrial protection (coenzyme Q10; 150 mg/kg/5 days, orally). We also evaluated the effect of reserpine on mitochondrial function using high-resolution respirometry and oxidative status. Reserpine caused nociception, loss in muscle strength, and anxiety- and depressive-like behaviours in mice that were consistent with clinical symptoms of FM, without inducing body weight and temperature alterations or motor impairment. Reserpine-induced FM-related symptoms were increased by oligomycin and reduced by coenzyme Q10 treatment. Reserpine caused mitochondrial dysfunction by negatively modulating the electron transport system and mitochondrial respiration (ATP synthesis) mainly in oxidative muscles and the spinal cord. These results support the role of mitochondria in mediating oxidative stress and FM symptoms in this model. In this way, reserpine-inducing mitochondrial dysfunction and increased production of ROI contribute to the development and maintenance of nociceptive, fatigue, and depressive-like behaviours.

Guanosine protects against behavioural and mitochondrial bioenergetic alterations after mild traumatic brain injury

Traumatic brain injury (TBI) constitutes a heterogeneous cerebral insult induced by traumatic biomechanical forces. Mitochondria play a critical role in brain bioenergetics, and TBI induces several consequences related with oxidative stress and excitotoxicity clearly demonstrated in different experimental model involving TBI. Mitochondrial bioenergetics alterations can present several targets for therapeutics which could help reduce secondary brain lesions such as neuropsychiatric problems, including memory loss and motor impairment. Guanosine (GUO), an endogenous neuroprotective nucleoside, affords the long-term benefits of controlling brain neurodegeneration, mainly due to its capacity to activate the antioxidant defense system and maintenance of the redox system. However, little is known about the exact protective mechanism exerted by GUO on mitochondrial bioenergetics disruption induced by TBI. Thus, the aim of this study was to investigate the effects of GUO in brain cortical and hippocampal mitochondrial bioenergetics in the mild TBI model. Additionally, we aimed to assess whether mitochondrial damage induced by TBI may be related to behavioral alterations in rats. Our findings showed that 24 h post-TBI, GUO treatment promotes an adaptive response of mitochondrial respiratory chain increasing oxygen flux which it was able to protect against the uncoupling of oxidative phosphorylation (OXPHOS) induced by TBI, restored the respiratory electron transfer system (ETS) established with an uncoupler. Guanosine treatment also increased respiratory control ratio (RCR), an indicator of the state of mitochondrial coupling, which is related to the mitochondrial functionality. In addition, mitochondrial bioenergetics failure was closely related with locomotor, exploratory and memory impairments. The present study suggests GUO treatment post mild TBI could increase GDP endogenous levels and consequently increasing ATP levels promotes an increase of RCR increasing OXPHOS and in substantial improve mitochondrial respiration in different brain regions, which, in turn, could promote an improvement in behavioral parameters associated to the mild TBI. These findings may contribute to the development of future therapies with a target on failure energetic metabolism induced by TBI.

Measurement of Energy States of the Trypanosomatid Mitochondrion

The evaluation of mitochondrial functionality is critical to interpret most biological data at the (eukaryotic) cellular level. For example, metabolism, cell cycle, epigenetic regulation, cell death mechanisms, autophagy, differentiation, and response redox imbalance are dependent on the mitochondrial state. In case of parasitic organisms, such as trypanosomatids, it is very often important to have information on mitochondrial functionality in order to assess the mechanisms of actions of drugs being proposed for therapy. In this chapter we present a set of methods that together allow to evaluate with some precision the mitochondrial functionality in Trypanosoma cruzi and Trypanosoma brucei. We discuss how to determine O2 consumption, mitochondrial inner membrane potential, ATP production, and the endogenous production of reactive oxygen species.

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.

Guanosine protects against Ca2+-induced mitochondrial dysfunction in rats

Mitochondria play an important role in cell life and in the regulation of cell death. In addition, mitochondrial dysfunction contributes to a wide range of neuropathologies. The nucleoside Guanosine (GUO) is an endogenous molecule, presenting antioxidant properties, possibly due to its direct scavenging ability and/or from its capacity to activate the antioxidant defense system. GUO demonstrate a neuroprotective effect due to the modulation of the glutamatergic system and maintenance of the redox system. Thus, considering the few studies focused on the direct effects of GUO on mitochondrial bioenergetics, we designed a study to evaluate the in vitro effects of GUO on rat mitochondrial function, as well as against Ca²⁺-induced impairment. Our results indicate that GUO prevented mitochondrial dysfunction induced by Ca²⁺ misbalance, once GUO was able to reduce mitochondrial swelling in the presence of Ca²⁺, as well as ROS production and hydrogen peroxide levels, and to increase manganese superoxide dismutase activity, oxidative phosphorylation and tricarboxylic acid cycle activities. Our study indicates for the first time that GUO could direct prevent the mitochondrial damage induced by Ca²⁺ and that these effects were not related to its scavenging properties. Our data indicates that GUO could be included as a new pharmacological strategy for diseases linked to mitochondrial dysfunction.

Evaluation of mitochondrial function in chronic myofascial trigger points - a prospective cohort pilot study using high-resolution respirometry

Background

Myofascial trigger points (MTrPs) are hyperirritable areas in the fascia of the affected muscle, possibly related to mitochondrial impairment. They can result in pain and hypoxic areas within the muscle. This pilot study established a minimally invasive biopsy technique to obtain high-quality MTrP tissue samples to evaluate mitochondrial function via high-resolution respirometry. Secondary objectives included the feasibility and safety of the biopsy procedure.

Methods

Twenty healthy males participated in this study, 10 with a diagnosis of myofascial pain in the musculus (m.) trapezius MTrP (TTP group) and 10 with a diagnosis of myofascial pain in the m. gluteus medius (GTP group). Each participant had 2 muscle biopsies taken in one session. The affected muscle was biopsied followed by a biopsy from the m. vastus lateralis to be used as a control. Measurements of oxygen consumption were carried out using high-resolution respirometry.

Results

Mitochondrial respiration was highest in the GTP group compared to the TTP group and the control muscle whereas no differences were observed between the GTP and the control muscle. When normalizing respiration to an internal reference state, there were no differences between muscle groups. None of the participants had hematomas or reported surgical complications. Patient-reported pain was minimal for all 3 groups. All participants reported a low procedural burden.

Conclusions

This pilot study used a safe and minimally invasive technique for obtaining biopsies from MTrPs suitable for high-resolution respirometry analysis of mitochondrial function. The results suggest that there are no qualitative differences in mitochondrial function of MTrPs of the trapezius and gluteus medius muscles compared to the vastus lateralis control muscle, implying that alterations of mitochondrial function do not appear to have a role in the development of MTrPs.

Trial registration

Registered as No. 20131128–850 at the Coordinating Center for Clinical Studies of the Medical University of Innsbruck, trial registration date: 28th November 2013 and retrospectively registered on 11th of October 2018 at ClinicalTrials.gov with the ID NCT03704311.

6-Hydroxydopamine induces different mitochondrial bioenergetics response in brain regions of rat

Mitochondrial dysfunction has been demonstrated to have a central role in Parkinson Disease (PD) pathophysiology. Some studies have indicated that PD causes an impairment in mitochondrial bioenergetics; however, the effects of PD on brain-region specific bioenergetics was never investigated before. This study aimed to evaluate mitochondrial bioenergetics in different rat brain structures in an in vitro model of PD using 6-OHDA. Rat brain slices of hippocampus, striatum, and cortex were exposed to 6-OHDA (100 μM) for 1 h and mitochondrial bioenergetic parameters, peroxide production, lactate dehydrogenase (LDH) and citrate synthase (CS) activities were analyzed. Hippocampus slices exposed to 6-OHDA presented increased peroxide production but, no mitochondrial adaptive response against 6-OHDA damage. Cortex slices exposed to 6-OHDA presented increased oxygen flux related to oxidative phosphorylation and energetic pathways exchange demonstrated by the increase in LDH activity, suggesting a mitochondrial compensatory response. Striatum slices exposed to 6-OHDA presented a decrease of oxidative phosphorylation and decrease of oxygen flux related to ATP-synthase indicating an impairment in the respiratory chain. The co-incubation of 6-OHDA with n-acetylcysteine (NAC) abolished the effects of 6-OHDA on mitochondrial function in all brain regions tested, indicating that the increased reactive oxygen species (ROS) production is responsible for the alterations observed in mitochondrial bioenergetics. The present results indicate a brain-region specific response against 6-OHDA, providing new insights into brain mitochondrial bioenergetic function in PD. These findings may contribute to the development of future therapies with a target on energy metabolism.

Altered mitochondrial oxidative phosphorylation capacity in horses suffering from polysaccharide storage myopathy

Polysaccharide storage myopathy (PSSM) is a widely described cause of exertional rhabdomyolysis in horses. Mitochondria play a central role in cellular energetics and are involved in human glycogen storage diseases but their role has been overlooked in equine PSSM. We hypothesized that the mitochondrial function is impaired in the myofibers of PSSM-affected horses. Nine horses with a history of recurrent exercise-associated rhabdomyolysis were tested for the glycogen synthase 1 gene (GYS1) mutation: 5 were tested positive (PSSM group) and 4 were tested negative (horses suffering from rhabdomyolysis of unknown origin, RUO group). Microbiopsies were collected from the gluteus medius (gm) and triceps brachii (tb) muscles of PSSM, RUO and healthy controls (HC) horses and used for histological analysis and for assessment of oxidative phosphorylation (OXPHOS) using high-resolution respirometry. The modification of mitochondrial respiration between HC, PSSM and RUO horses varied according to the muscle and to substrates feeding OXPHOS. In particular, compared to HC horses, the gm muscle of PSSM horses showed decreased OXPHOS- and electron transfer (ET)-capacities in presence of glutamate&malate&succinate. RUO horses showed a higher OXPHOS-capacity (with glutamate&malate) and ET-capacity (with glutamate&malate&succinate) in both muscles in comparison to the PSSM group. When expressed as ratios, our results highlighted a higher contribution of the NADH pathway (feeding electrons into Complex I) to maximal OXPHOS or ET-capacity in both rhabdomyolysis groups compared to the HC. Specific modifications in mitochondrial function might contribute to the pathogenesis of PSSM and of other types of exertional rhabdomyolyses.

Respiratory analysis of coupled mitochondria in cryopreserved liver biopsies

The aim of this work was to develop a cryopreservation method of small liver biopsies for in situ mitochondrial function assessment. Herein we describe a detailed protocol for tissue collection, cryopreservation, high-resolution respirometry using complex I and II substrates, calculation and interpretation of respiratory parameters. Liver biopsies from cow and rat were sequentially frozen in a medium containing dimethylsulfoxide as cryoprotectant and stored for up to 3 months at -80 °C. Oxygen consumption rate studies of fresh and cryopreserved samples revealed that most respiratory parameters remained unchanged. Additionally, outer mitochondrial membrane integrity was assessed adding cytochrome c, proving that our cryopreservation method does not harm mitochondrial structure. In sum, we present a reliable way to cryopreserve small liver biopsies without affecting mitochondrial function. Our protocol will enable the transport and storage of samples, extending and facilitating mitochondrial function analysis of liver biopsies.

Analysis of respiratory capacity in brain tissue preparations: high-resolution respirometry for intact hippocampal slices

The evaluation of mitochondrial function provides the basis for the study of brain bioenergetics. However, analysis of brain mitochondrial respiration has been hindered by the low yield associated with mitochondria isolation procedures. Furthermore, isolating mitochondria or cells results in loss of the inherent complexity of the central nervous system. High-resolution respirometry (HRR), is a valuable tool to study mitochondrial function and has been used in diverse biological preparations ranging from isolated mitochondria to tissue homogenates and permeabilized tissue biopsies. Here we describe a novel methodology for evaluation of mitochondrial respiration using tissue preparations from the central nervous system, namely acute hippocampal slices from rodents, with HRR. By using acute intact hippocampal slices, tissue cytoarchitecture, intercellular communication and connectivity are preserved. Mitochondrial respiration was evaluated by using an adapted substrate-uncoupler-inhibitor titration (SUIT) protocol and the expected responses were observed. This methodology can be used to detect differences in mitochondrial function at the oxidative phosphorylation level and for studies with different brain oxidative substrates in physiological and neuropathological settings, by using a system that better represents the in vivo conditions than isolated mitochondria and/or cells.

Mitochondrial energy metabolism and signalling in human glioblastoma cell lines with different PTEN gene status

Glioblastomas epidemiology and aggressiveness demand for a well characterization of biochemical mechanisms of the cells. The discovery of oxidative tumours related to chemoresistance is changing the prevalent view of dysfunctional mitochondria in cancer cells. Thus, glioblastomas metabolism is now an area of intense research, wherein was documented a high heterogeneity in energy metabolism and in particular in mitochondrial OxPhos. We report results gained by investigating mitochondrial OxPhos and bioenergetics, in a model of three human glioblastoma cell lines characterized by a different PTEN gene status. Functional data are analysed in relation to the expression levels of some main transcription factors and signalling proteins, which can be involved in the regulation of mitochondrial biogenesis and activity. Collectively, our observations indicate for the three cell lines a similar bioenergetic phenotype maintaining a certain degree of mitochondrial oxidative activity, with some difference for PTEN-wild type SF767 cells respect to PTEN-deleted A172 and U87MG characterized by a loss-of-function point mutation of PTEN. SF767 has lower ATP content and higher ADP/ATP ratio, higher AMPK activating-phosphorylation evoking energy impairment, higher OxPhos complexes and PGC1α-Sirt3-p53 protein abundance, in line with a higher respiration. Finally, SF767 shows a similar mitochondrial energy supply, but higher non-phosphorylating respiration linked to dissipation of protonmotive force. Intriguingly, it is now widely accepted that a regulated mitochondrial proton leak attenuate ROS generation and in tumours may be at the base of pro-survival advantage and chemoresistance.

Mitochondrial capacity, oxidative damage and hypoxia gene expression are associated with age-related division of labor in honey bee (Apis mellifera L.) workers

During adult life, honey bee workers undergo a succession of behavioral states. Nurse bees perform tasks inside the nest, and when they are about 2-3 weeks old they initiate foraging. This switch is associated with alterations in diet, and with the levels of juvenile hormone and vitellogenin circulating in hemolymph. It is not clear whether this behavioral maturation involves major changes at the cellular level, such as mitochondrial activity and the redox environment in the head, thorax and abdomen. Using high-resolution respirometry, biochemical assays and RT-qPCR, we evaluated the association of these parameters with this behavioral change. We found that tissues from the head and abdomen of nurses have a higher oxidative phosphorylation capacity than those of foragers, while for the thorax we found the opposite situation. As higher mitochondrial activity tends to generate more H₂O₂, and H₂O₂ is known to stabilize HIF-1α, this would be expected to stimulate hypoxia signaling. The positive correlation that we observed between mitochondrial activity and hif-1α gene expression in abdomen and head tissue of nurses would be in line with this hypothesis. Higher expression of antioxidant enzyme genes was observed in foragers, which could explain their low levels of protein carbonylation. No alterations were seen in nitric oxide (NO) levels, suggesting that NO signaling is unlikely to be involved in behavioral maturation. We conclude that the behavioral change seen in honey bee workers is reflected in differential mitochondrial activities and redox parameters, and we consider that this can provide insights into the underlying aging process.

Heterologous expression of the Crassostrea gigas (Pacific oyster) alternative oxidase in the yeast Saccharomyces cerevisiae

Alternative oxidase (AOX) is a terminal oxidase within the inner mitochondrial membrane (IMM) present in many organisms where it functions in the electron transport system (ETS). AOX directly accepts electrons from ubiquinol and is therefore capable of bypassing ETS Complexes III and IV. The human genome does not contain a gene coding for AOX, so AOX expression has been suggested as a gene therapy for a range of human mitochondrial diseases caused by genetic mutations that render Complex III and/or IV dysfunctional. An effective means of screening mutations amenable to AOX treatment remains to be devised. We have generated such a tool by heterologously expressing AOX from the Pacific oyster (Crassostrea gigas) in the yeast Saccharomyces cerevisiae under the control of a galactose promoter. Our results show that this animal AOX is monomeric and is correctly targeted to yeast mitochondria. Moreover, when expressed in yeast, Pacific oyster AOX is a functional quinol oxidase, conferring cyanide-resistant growth and myxothiazol-resistant oxygen consumption to yeast cells and isolated mitochondria. This system represents a high-throughput screening tool for determining which Complex III and IV genetic mutations in yeast will be amenable to AOX gene therapy. As many human genes are orthologous to those found in yeast, our invention represents an efficient and cost-effective way to evaluate viable research avenues. In addition, this system provides the opportunity to learn more about the localization, structure, and regulation of AOXs from animals that are not easily reared or manipulated in the lab.

Mitochondrial function is altered in horse atypical myopathy

Equine atypical myopathy in Europe is a fatal rhabdomyolysis syndrome that results from the ingestion of hypoglycin A contained in seeds and seedlings of Acer pseudoplatanus (sycamore maple). Acylcarnitine concentrations in serum and muscle OXPHOS capacity were determined in 15 atypical myopathy cases. All but one acylcarnitine were out of reference range and mitochondrial respiratory capacity was severely decreased up to 49% as compared to 10 healthy controls. The hallmark of atypical myopathy thus consists of a severe alteration in the energy metabolism including a severe impairment in muscle mitochondrial respiration that could contribute to its high death rate.

Age-dependent changes in the glutamate-nitric oxide pathway in the hippocampus of the triple transgenic model of Alzheimer's disease: implications for neurometabolic regulation

Age-dependent changes in nitric oxide ((•)NO) concentration dynamics may play a significant role in both decaying synaptic and metabolic functions in Alzheimer's disease (AD). This neuromodulator acts presynaptically to increase vesicle release and glutamatergic transmission and also regulates mitochondrial function. Under conditions of altered intracellular redox environment, (•)NO may react and produce reactive species such as peroxynitrite. Using the triple transgenic mouse model of AD (3xTgAD), we investigated age-dependent changes in the glutamate-(•)NO axis in the hippocampus. Direct measurement of (•)NO concentration dynamics revealed a significant increase in N-methyl-D-aspartate type receptor-evoked peak (•)NO in the 3xTgAD model at an early age. Aging produced a decrease in peak (•)NO accompanied by significant decrease in production and decay rates in the transgenic model. Evaluation of energy metabolism revealed age-dependent decrease in basal oxygen consumption rate, a general decrease in mitochondrial oxidative phosphorylation parameters, and loss in mitochondrial sparing capacity in both genotypes. Finally, we observed age-dependent increase in 3-nitrotyrosine residues in the hippocampus, consistent with a putative shift in (•)NO bioactivity toward oxidative chemistry associated with neurotoxicity.

Mitochondrial structure and dynamics as critical factors in honey bee (Apis mellifera L.) caste development

The relationship between nutrition and phenotype is an especially challenging question in cases of facultative polyphenism, like the castes of social insects. In the honey bee, Apis mellifera, unexpected modifications in conserved signaling pathways revealed the hypoxia response as a possible mechanism underlying the regulation of body size and organ growth. Hence, the current study was designed to investigate possible causes of why the three hypoxia core genes are overexpressed in worker larvae. Parting from the hypothesis that this has an endogenous cause and is not due to differences in external oxygen levels we investigated mitochondrial numbers and distribution, as well as mitochondrial oxygen consumption rates in fat body cells of queen and worker larvae during the caste fate-critical larval stages. By immunofluorescence and electron microscopy we found higher densities of mitochondria in queen larval fat body, a finding further confirmed by a citrate synthase assay quantifying mitochondrial functional units. Oxygen consumption measurements by high-resolution respirometry revealed that queen larvae have higher maximum capacities of ATP production at lower physiological demand. Finally, the expression analysis of mitogenesis-related factors showed that the honey bee TFB1 and TFB2 homologs, and a nutritional regulator, ERR, are overexpressed in queen larvae. These results are strong evidence that the differential nutrition of queen and worker larvae by nurse bees affects mitochondrial dynamics and functionality in the fat body of these larvae, hence explaining their differential hypoxia response.

Determination of muscle mitochondrial respiratory capacity in Standardbred racehorses as an aid to predicting exertional rhabdomyolysis

This prospective cohort study evaluated the potential of high-resolution respirometry applied to permeabilized muscle fibers for fitness evaluation in French Standardbred racehorses. Fitness evaluation by means of respirometric parameters did not correlate with racing performance registered over the following racing season. However, altered mitochondrial energy metabolism was associated with higher risk of developing exertional rhabdomyolysis, a common cause of exercise intolerance in racehorses. These data represent a first step towards establishing reference values for muscle OXPHOS capacity in this breed.

Use of safranin for the assessment of mitochondrial membrane potential by high-resolution respirometry and fluorometry

The mitochondrial transmembrane potential (Δψmt or mtMP) is directly influenced by oxidative phosphorylation (OXPHOS). The exact nature of the interactions between respiration (flux) and mtMP (force) under various physiological and pathological conditions remains unclear, partially due to methodological limitations. Here, we describe a combination of high-resolution respirometry and fluorometry based on the OROBOROS Oxygraph-2k and the widely applied mtMP indicator safranin. The analysis of OXPHOS in mouse brain homogenates revealed that, at commonly applied concentrations, safranin inhibits Complex I-driven OXPHOS capacity, primarily targeting the phosphorylation system, but has no effects on LEAK respiration. Conversely, Complex II-driven OXPHOS capacity was inhibited by <20% by safranin concentrations normally used for mtMP monitoring. The mtMP was higher in the LEAK state without adenylates than at identical LEAK respiration after ADP stimulation and Complex V inhibition with oligomycin. The maximal electron transfer system (ETS) capacity was reached in uncoupler titrations before the mtMP fully collapsed, whereas respiration was inhibited at increasing uncoupler concentrations, resulting in the progressive reduction of mtMP. In a pharmacologically induced state of Complex II dysfunction, mtMP was rather insensitive to the inhibition of OXPHOS to 50% of its normal capacity, but robustly responded to inhibitors when respiration was limited by substrate depletion. The optimal concentration of uncoupler supporting maximal ETS capacity varied as a function of pharmacological intervention. Taken together, the combined measurement of respiration and mtMP greatly enhances the informative potential of OXPHOS studies. The respirometric validation of inhibitory and uncoupling effects is mandatory for any fluorophore employed to assess mtMP in any respiratory state, tissue type, and pathophysiological condition. The methodological issues analyzed herein are relevant for the study of mitochondrial respiration in a wide variety of setting, including cancer cell metabolism.

Increased platelet storage time is associated with mitochondrial dysfunction and impaired platelet function

BACKGROUND

Hemorrhagic shock is a leading cause of death following severe trauma, and platelet transfusions are frequently necessary to achieve hemostasis. Platelets, however, require special storage conditions, and storage time has been associated with loss of platelet quality. We hypothesized that standard storage conditions have a deleterious effect on platelet mitochondrial function and platelet activation.

MATERIALS AND METHODS

Platelet donations were collected from healthy donors (n = 5) and stored in gas-permeable collection bags according to American Association of Blood Bank recommendations. Platelet units were sampled from day of collection (day 0) until day 7. High-resolution respirometry was used to assess baseline mitochondrial respiration, maximal oxygen utilization, and individual mitochondrial complex-dependent respiration. Fluorescence-activated cell sorting was performed to analyze mitochondrial content, mitochondrial reactive oxygen species, the expression of P-selectin (both before and after challenge with thrombin receptor-activating peptide), and apoptosis. Data were analyzed using analysis of variance and Pearson correlation (P < 0.05 significant).

RESULTS

Mitochondrial respiration decreased significantly in platelets stored longer than 2 d (P < 0.05). Platelets also demonstrated a persistent decrease in response to stimulation with thrombin receptor-activating peptide by the third day of storage (P < 0.05) as well as an increase in mitochondrial reactive oxygen species and apoptosis (P < 0.05). Mitochondrial respiration significantly correlated with platelet capacity to activate (r = 0.8, P < 0.05).

CONCLUSIONS

Platelet mitochondrial respiratory function and activation response decrease significantly in platelets stored for 3 d or more. Because platelet transfusions almost universally occur between the third and fifth day of storage, our findings may have significant clinical importance and warrant further in vivo analysis.