Oxygen dependence of respiration in coupled and uncoupled endothelial cells

Mathematical modelling of oxygen transport in a muscle-on-chip device

Muscle-on-chip devices aim to recapitulate the physiological characteristics of in vivo muscle tissue and so maintaining levels of oxygen transported to cells is essential for cell survival and for providing the normoxic conditions experienced in vivo. We use finite-element method numerical modelling to describe oxygen transport and reaction in a proposed three-dimensional muscle-on-chip bioreactor with embedded channels for muscle cells and growth medium. We determine the feasibility of ensuring adequate oxygen for muscle cell survival in a device sealed from external oxygen sources and perfused via medium channels. We investigate the effects of varying elements of the bioreactor design on oxygen transport to optimize muscle tissue yield and maintain normoxic conditions. Successful co-culturing of muscle cells with motor neurons can boost muscle tissue function and so we estimate the maximum density of seeded neurons supported by oxygen concentrations within the bioreactor. We show that an enclosed bioreactor can provide sufficient oxygen for muscle cell survival and growth. We define a more efficient arrangement of muscle and perfusion chambers that can sustain a predicted 50% increase in maximum muscle volume per perfusion vessel. A study of simulated bioreactors provides functions for predicting bioreactor designs with normoxic conditions for any size of perfusion vessel, muscle chamber and distance between chambers.

Niclosamide Is a Much More Potent Toxicant of Mitochondrial Respiration than TFM in the Invasive Sea Lamprey (Petromyzon marinus)

Invasive sea lampreys in the Laurentian Great Lakes are controlled by applying TFM (3-trifluoromethyl-4-nitrophenol) and niclosamide to streams infested with their larvae. Both agents uncouple oxidative phosphorylation in the mitochondria, but TFM specifically targets lampreys, which have a lower capacity to detoxify the lampricide. Niclosamide lacks specificity and is more potent than TFM. However, its greater potency is poorly understood. We tested the hypothesis that niclosamide is a stronger uncoupler of mitochondrial oxidative phosphorylation than TFM by measuring oxygen consumption rates in isolated liver mitochondria exposed to physiologically relevant concentrations of TFM, niclosamide, or their mixture (100 TFM:1 niclosamide) at environmentally relevant temperatures (7, 13, and 25 °C). Niclosamide increased State 4 respiration and decreased the respiratory control ratio (RCR) at much lower concentrations than TFM. Calculations of the relative EC₅₀ values, the amount of TFM or niclosamide required to decrease the RCR by 50%, indicated that niclosamide was 40-60 times more potent than TFM. Warmer temperature did not appear to decrease the sensitivity of mitochondria to niclosamide or TFM, as observed in the intact sea lamprey exposed to TFM in warmer waters. We conclude that the extreme sensitivity of mitochondria to niclosamide contributes to its greater in vivo toxicity in the whole animal.

Thriving in Oxygen While Preventing ROS Overproduction: No Two Systems Are Created Equal

From 2.5 to 2.0 billion years ago, atmospheric oxygen concentration [O₂] rose thousands of times, leading to the first mass extinction. Reactive Oxygen Species (ROS) produced by the non-catalyzed partial reduction of O₂ were highly toxic eliminating many species. Survivors developed different strategies to cope with ROS toxicity. At the same time, using O₂ as the final acceptor in respiratory chains increased ATP production manifold. Thus, both O₂ and ROS were strong drivers of evolution, as species optimized aerobic metabolism while developing ROS-neutralizing mechanisms. The first line of defense is preventing ROS overproduction and two mechanisms were developed in parallel: 1) Physiological uncoupling systems (PUS), which increase the rate of electron fluxes in respiratory systems. 2) Avoidance of excess [O₂]. However, it seems that as avoidance efficiency improved, PUSs became less efficient. PUS includes branched respiratory chains and proton sinks, which may be proton specific, the mitochondrial uncoupling proteins (UCPs) or unspecific, the mitochondrial permeability transition pore (PTP). High [O₂] avoidance also involved different strategies: 1) Cell association, as in biofilms or in multi-cellularity allowed gas-permeable organisms (oxyconformers) from bacterial to arthropods to exclude O_2. 2) Motility, to migrate from hypoxic niches. 3) Oxyregulator organisms: as early as in fish, and O₂-impermeable epithelium excluded all gases and only exact amounts entered through specialized respiratory systems. Here we follow the parallel evolution of PUS and O₂-avoidance, PUS became less critical and lost efficiency. In regard, to proton sinks, there is fewer evidence on their evolution, although UCPs have indeed drifted in function while in some species it is not clear whether PTPs exist.

Measuring Mitochondrial Function: From Organelle to Organism

Mitochondrial energy production is crucial for normal daily activities and maintenance of life. Herein, the logic and execution of two main classes of measurements are outlined to delineate mitochondrial function: ATP production and oxygen consumption. Aerobic ATP production is quantified by phosphorus magnetic resonance spectroscopy (³¹PMRS) in vivo in both human subjects and animal models using the same protocols and maintaining the same primary assumptions. Mitochondrial oxygen consumption is quantified by oxygen polarography and applied in isolated mitochondria, cultured cells, and permeabilized fibers derived from human or animal tissue biopsies. Traditionally, mitochondrial functional measures focus on maximal oxidative capacity-a flux rate that is rarely, if ever, observed outside of experimental conditions. Perhaps more physiologically relevant, both measurement classes herein focus on one principal design paradigm; submaximal mitochondrial fluxes generated by graded levels of ADP to map the function for ADP sensitivity. We propose this function defines the bioenergetic role that mitochondria fill within the myoplasm to sense and match ATP demands. Any deficit in this vital role for ATP homeostasis leads to symptoms often seen in cardiovascular and cardiopulmonary diseases, diabetes, and metabolic syndrome.

The role of oxygen intake and liver enzyme on the dynamics of damaged hepatocytes: Implications to ischaemic liver injury via a mathematical model

Ischaemic Hepatitis (IH) or Hypoxic Hepatitis (HH) also known as centrilobular liver cell necrosis is an acute liver injury characterized by a rapid increase in serum aminotransferase. The liver injury typically results from different underlying medical conditions such as cardiac failure, respiratory failure and septic shock in which the liver becomes damaged due to deprivation of either blood or oxygen. IH is a potentially lethal condition that is often preventable if diagnosed timely. The role of mechanisms that cause IH is often not well understood, making it difficult to diagnose or accurately quantify the patterns of related biomarkers. In most patients, currently, the only way to determine a case of IH is to rule out all other possible conditions for liver injuries. A better understanding of the liver's response to IH is necessary to aid in its diagnosis, measurement, and improve outcomes. The goal of this study is to identify mechanisms that can alter associated biomarkers for reducing the density of damaged hepatocytes, and thus reduce the chances of IH. We develop a mathematical model capturing dynamics of hepatocytes in the liver through the rise and fall of associated liver enzymes aspartate transaminase (AST), alanine transaminase (ALT) and lactate dehydrogenase (LDH) related to the condition of IH. The model analysis provides a novel approach to predict the level of biomarkers given variations in the systemic oxygen in the body. Using IH patient data in the US, novel model parameters are described and then estimated for the first time to capture real-time dynamics of hepatocytes in the presence and absence of IH condition. The results may allow physicians to estimate the extent of liver damage in an IH patient based on their enzyme levels and receive faster treatment on a real-time basis.

Grx2 Regulates Skeletal Muscle Mitochondrial Structure and Autophagy

Glutathione is an important antioxidant that regulates cellular redox status and is disordered in many disease states. Glutaredoxin 2 (Grx2) is a glutathione-dependent oxidoreductase that plays a pivotal role in redox control by catalyzing reversible protein deglutathionylation. As oxidized glutathione (GSSG) can stimulate mitochondrial fusion, we hypothesized that Grx2 may contribute to the maintenance of mitochondrial dynamics and ultrastructure. Here, we demonstrate that Grx2 deletion results in decreased GSH:GSSG, with a marked increase of GSSG in primary muscle cells isolated from C57BL/6 Grx2^-/- mice. The altered glutathione redox was accompanied by increased mitochondrial length, consistent with a more fused mitochondrial reticulum. Electron microscopy of Grx2^-/- skeletal muscle fibers revealed decreased mitochondrial surface area, profoundly disordered ultrastructure, and the appearance of multi-lamellar structures. Immunoblot analysis revealed that autophagic flux was augmented in Grx2^-/- muscle as demonstrated by an increase in the ratio of LC3II/I expression. These molecular changes resulted in impaired complex I respiration and complex IV activity, a smaller diameter of tibialis anterior muscle, and decreased body weight in Grx2 deficient mice. Together, these are the first results to show that Grx2 regulates skeletal muscle mitochondrial structure, and autophagy.

Imaging of Tumor Hypoxia for Radiotherapy: Current Status and Future Directions

Tumor regions that are transiently or chronically undersupplied with oxygen (hypoxia) and nutrients, and enriched with acidic waste products, are common due to an abnormal and inefficient tumor vasculature, and a deviant highly glycolytic energy metabolism. There is compelling evidence that tumor hypoxia is strongly linked to poor prognosis since oxygen-deprived cells are highly resistant to therapy including radio- and chemotherapy, and survival of such cells is a primary cause of disease relapse. Despite a general improvement in cancer survival rates, hypoxia remains a formidable challenge. Recent progress in radiation delivery systems with improved spatial accuracy that allows dose escalation to hypoxic tumors or even tumor subvolumes, and the development of hypoxia-selective drugs, including bioreductive prodrugs, holds great promise for overcoming this obstacle. However, apart from one notable exception, translation of promising preclinical therapies to the clinic have largely been disappointing. A major obstacle in clinical trials on hypoxia-targeting strategies has been the lack of reliable information on tumor hypoxia, which is crucial for patient stratification into groups of those that are likely to benefit from intervention and those who are not. Further, in many newer trials on hypoxia-selective drugs the choice of cancer disease and combination therapy has not always been ideal, especially not for clinical proof of principle trials. Clearly, there is a pending need for clinical applicable methodologies that may allow us to quantify, map and monitor hypoxia. Molecular imaging may provide the information required for narrowing the gap between potential and actual patient benefit of hypoxia-targeting strategies. The grand majority of preclinical and clinical work has focused on the usefulness of PET-based assessment of hypoxia-selective tracers. Since hypoxia PET has profound inherent weaknesses, the use of other methodologies, including more indirect methods that quantifies blood flow or oxygenation-dependent flux changes through ATP-generating pathways (eg, anaerobic glycolysis) is being extensively studied. In this review, we briefly discuss established and emerging hypoxia-targeting strategies, followed by a more thorough evaluation of strengths and weaknesses of clinical applicable imaging methodologies that may guide timely treatment intensification to overcome hypoxia-driven resistance. Historically, most evidence for the linkage between hypoxia and poor outcome is based on work in the field of radiotherapy. Therefore, main emphasis in this review is on targeting and imaging of hypoxia for improved radiotherapy.

In vitro hypoxia responsiveness of [18F] FDG and [18F] FAZA retention: influence of shaking versus stagnant conditions, glass versus polystyrene substrata and cell number down-scaling

Background

In vitro experiments using radiolabeled molecules is fundamental for Positron emission tomography (PET) or single photon emission computed tomography (SPECT) tracer development and various metabolic assays, but no consensus on appropriate incubation conditions exists. Specifically, the use of shaking versus non-shaking conditions, cell number to medium volume and the choice of cell plating material may unintentionally influence cellular oxygenation and medium composition. This is problematic when testing the oxygen-dependence of tracers including ¹⁸F-fluoro-2-deoxyglucose ([¹⁸F]FDG) and hypoxia-selective 2-nitroimidazoles (e.g., ¹⁸F-fluoroazomycin-arabinoside, [¹⁸F]FAZA) or when doing prolonged experiments. The purpose of this study was to assess the influence of various experimental conditions on tracer retention.

Methods

Tumor cells were seeded in a) Glass or standard Polystyrene Petri dishes or as b) discrete droplets in polystyrene Petri dishes or on 9 mm glass coverslips positioned in glass Petri dishes. When confluent, cells were pre-equilibrated for 2 h to 21%, 0.5% or 0% O₂ and [¹⁸F] FDG or [¹⁸F] FAZA was added, followed by cell harvest and analysis of radioactivity 1 h ([¹⁸F]FDG) or 3 h ([¹⁸F]FAZA) after. Experiments were conducted with/without orbital shaking.

Results

The influence of hypoxia on tracer retention varied widely among cell lines, but shaking-induced convection did not influence uptake. In contrast, hypoxia-driven [¹⁸F] FAZA, and to some extent [¹⁸F] FDG, retention was much lower in cells grown on polyethylene than glass. Scaling-down the number of cells did not compromise accuracy.

Conclusions

Tracer retention was similar under stagnant and forced convection conditions suggesting that the former approach may be appropriate even when accurate control of oxygen and tracer availability is required. In contrast, conventional plasticware should be used with caution when studying tracers and drugs that are metabolized and retained or activated at low O₂ levels. Downscaling of cell number, by reducing the effective growth area, was feasible, without compromising accuracy.

Influence of Hyperoxic-Supplemented High-Intensity Interval Training on Hemotological and Muscle Mitochondrial Adaptations in Trained Cyclists

Background: Hyperoxia (HYPER) increases O₂ carrying capacity resulting in a higher O₂ delivery to the working muscles during exercise. Several lines of evidence indicate that lactate metabolism, power output, and endurance are improved by HYPER compared to normoxia (NORM). Since HYPER enables a higher exercise power output compared to NORM and considering the O₂ delivery limitation at exercise intensities near to maximum, we hypothesized that hyperoxic-supplemented high-intensity interval training (HIIT) would upregulate muscle mitochondrial oxidative capacity and enhance endurance cycling performance compared to training in normoxia. Methods: 23 trained cyclists, age 35.3 ± 6.4 years, body mass 75.2 ± 9.6 kg, height 179.8 ± 7.9 m, and VO₂max 4.5 ± 0.7 L min^-1 performed 6 weeks polarized and periodized endurance training on a cycle ergometer consisting of supervised HIIT sessions 3 days/week and additional low-intensity training 2 days/week. Participants were randomly assigned to either HYPER (F_IO₂ 0.30; n = 12) or NORM (F_IO₂ 0.21; n = 11) breathing condition during HIIT. Mitochondrial respiration in permeabilized fibers and isolated mitochondria together with maximal and submaximal VO₂, hematological parameters, and self-paced endurance cycling performance were tested pre- and posttraining intervention. Results: Hyperoxic training led to a small, non-significant change in performance compared to normoxic training (HYPER 6.0 ± 3.7%, NORM 2.4 ± 5.0%; p = 0.073, ES = 0.32). This small, beneficial effect on the self-paced endurance cycling performance was not explained by the change in VO₂max (HYPER 1.1 ± 3.8%, NORM 0.0 ± 3.7%; p = 0.55, ES = 0.08), blood volume and hemoglobin mass, mitochondrial oxidative phosphorylation capacity (permeabilized fibers: HYPER 27.3 ± 46.0%, NORM 16.5 ± 49.1%; p = 0.37, ES = 3.24 and in isolated mitochondria: HYPER 26.1 ± 80.1%, NORM 15.9 ± 73.3%; p = 0.66, ES = 0.51), or markers of mitochondrial content which were similar between groups post intervention. Conclusions: This study showed that 6 weeks hyperoxic-supplemented HIIT led to marginal gain in cycle performance in already trained cyclists without change in VO₂max, blood volume, hemoglobin mass, mitochondrial oxidative phosphorylation capacity, or exercise efficiency. The underlying mechanisms for the potentially meaningful performance effects of hyperoxia training remain unexplained and may raise ethical questions for elite sport.

Muscle mass and inspired oxygen influence oxygen extraction at maximal exercise: Role of mitochondrial oxygen affinity

AIM

We examined the Fick components together with mitochondrial O₂ affinity (p50_mito ) in defining O₂ extraction and O₂ uptake during exercise with large and small muscle mass during normoxia (NORM) and hyperoxia (HYPER).

METHODS

Seven individuals performed 2 incremental exercise tests to exhaustion on a bicycle ergometer (BIKE) and 2 on a 1-legged knee extension ergometer (KE) in NORM or HYPER. Leg blood flow and VO₂ were determined by thermodilution and the Fick method. Maximal ADP-stimulated mitochondrial respiration (OXPHOS) and p50_mito were measured ex vivo in isolated mitochondria. Mitochondrial excess capacity in the leg was determined from OXPHOS in permeabilized fibres and muscle mass measured with magnetic resonance imaging in relation to peak leg O₂ delivery.

RESULTS

The ex vivo p50_mito increased from 0.06 ± 0.02 to 0.17 ± 0.04 kPa with varying substrate supply and O₂ flux rates from 9.84 ± 2.91 to 16.34 ± 4.07 pmol O₂ ·s^-1 ·μg^-1 respectively. O₂ extraction decreased from 83% in BIKE to 67% in KE as a function of a higher O₂ delivery and lower mitochondrial excess capacity. There was a significant relationship between O₂ extraction and mitochondrial excess capacity and p50_mito that was unrelated to blood flow and mean transit time.

CONCLUSION

O₂ extraction varies with mitochondrial respiration rate, p50_mito and O₂ delivery. Mitochondrial excess capacity maintains a low p50_mito which enhances O₂ diffusion from microvessels to mitochondria during exercise.

High-Resolution FluoRespirometry and OXPHOS Protocols for Human Cells, Permeabilized Fibers from Small Biopsies of Muscle, and Isolated Mitochondria

Protocols for High-Resolution FluoRespirometry of intact cells, permeabilized cells, permeabilized muscle fibers, isolated mitochondria, and tissue homogenates offer sensitive diagnostic tests of integrated mitochondrial function using standard cell culture techniques, small needle biopsies of muscle, and mitochondrial preparation methods. Multiple substrate-uncoupler-inhibitor titration (SUIT) protocols for analysis of oxidative phosphorylation (OXPHOS) improve our understanding of mitochondrial respiratory control and the pathophysiology of mitochondrial diseases. Respiratory states are defined in functional terms to account for the network of metabolic interactions in complex SUIT protocols with stepwise modulation of coupling control and electron transfer pathway states. A regulated degree of intrinsic uncoupling is a hallmark of oxidative phosphorylation, whereas pathological and toxicological dyscoupling is evaluated as a mitochondrial defect. The noncoupled state of maximum respiration is experimentally induced by titration of established uncouplers (CCCP, FCCP, DNP) to collapse the protonmotive force across the mitochondrial inner membrane and measure the electron transfer (ET) capacity (open-circuit operation of respiration). Intrinsic uncoupling and dyscoupling are evaluated as the flux control ratio between non-phosphorylating LEAK respiration (electron flow coupled to proton pumping to compensate for proton leaks) and ET capacity. If OXPHOS capacity (maximally ADP-stimulated O₂ flux) is less than ET capacity, the phosphorylation pathway contributes to flux control. Physiological substrate combinations supporting the NADH and succinate pathway are required to reconstitute tricarboxylic acid cycle function. This supports maximum ET and OXPHOS capacities, due to the additive effect of multiple electron supply pathways converging at the Q-junction. ET pathways with electron entry separately through NADH (pyruvate and malate or glutamate and malate) or succinate (succinate and rotenone) restrict ET capacity and artificially enhance flux control upstream of the Q-cycle, providing diagnostic information on specific ET-pathway branches. O₂ concentration is maintained above air saturation in protocols with permeabilized muscle fibers to avoid experimental O₂ limitation of respiration. Standardized two-point calibration of the polarographic oxygen sensor (static sensor calibration), calibration of the sensor response time (dynamic sensor calibration), and evaluation of instrumental background O₂ flux (systemic flux compensation) provide the unique experimental basis for high accuracy of quantitative results and quality control in High-Resolution FluoRespirometry.

Resistance Training with Co-ingestion of Anti-inflammatory Drugs Attenuates Mitochondrial Function

Aim: The current study aimed to examine the effects of resistance exercise with concomitant consumption of high vs. low daily doses of non-steroidal anti-inflammatory drugs (NSAIDs) on mitochondrial oxidative phosphorylation in skeletal muscle. As a secondary aim, we compared the effects of eccentric overload with conventional training. Methods: Twenty participants were randomized to either a group taking high doses (3 × 400 mg/day) of ibuprofen (IBU; 27 ± 5 year; n = 11) or a group ingesting a low dose (1 × 75 mg/day) of acetylsalicylic acid (ASA; 26 ± 4 year; n = 9) during 8 weeks of supervised knee extensor resistance training. Each of the subject's legs were randomized to complete the training program using either a flywheel (FW) device emphasizing eccentric overload, or a traditional weight stack machine (WS). Maximal mitochondrial oxidative phosphorylation (CI+II_P) from permeabilized skeletal muscle bundles was assessed using high-resolution respirometry. Citrate synthase (CS) activity was assessed using spectrophotometric techniques and mitochondrial protein content using western blotting. Results: After training, CI+II_P decreased (P < 0.05) in both IBU (23%) and ASA (29%) with no difference across medical treatments. Although CI+II_P decreased in both legs, the decrease was greater (interaction p = 0.015) in WS (33%, p = 0.001) compared with FW (19%, p = 0.078). CS activity increased (p = 0.027) with resistance training, with no interactions with medical treatment or training modality. Protein expression of ULK1 increased with training in both groups (p < 0.001). The increase in quadriceps muscle volume was not correlated with changes in CI+II_P (R = 0.16). Conclusion: These results suggest that 8 weeks of resistance training with co-ingestion of anti-inflammatory drugs reduces mitochondrial function but increases mitochondrial content. The observed changes were not affected by higher doses of NSAIDs consumption, suggesting that the resistance training intervention was the prime mediator of the decreased mitochondrial phosphorylation. Finally, we noted that flywheel resistance training, emphasizing eccentric overload, rescued some of the reduction in mitochondrial function seen with conventional resistance training.

The biological foundation of the genetic association of TOMM40 with late-onset Alzheimer's disease

A variable-length poly-T variant in intron 6 of the TOMM40 gene, rs10524523, is associated with risk and age-of-onset of sporadic (late-onset) Alzheimer's disease. In Caucasians, the three predominant alleles at this locus are Short (S), Long (L) or Very long (VL). On an APOE ε3/3 background, the S/VL and VL/VL genotypes are more protective than S/S. The '523 poly-T has regulatory properties, in that the VL poly-T results in higher expression than the S poly-T in luciferase expression systems. The aim of the current work was to identify effects on cellular bioenergetics of increased TOM40 protein expression. MitoTracker Green fluorescence and autophagic vesicle staining was the same in control and over-expressing cells, but TOM40 over-expression was associated with increased expression of TOM20, a preprotein receptor of the TOM complex, the mitochondrial chaperone HSPA9, and PDHE1a, and increased activities of the oxidative phosphorylation complexes I and IV and of the TCA member α-ketoglutaric acid dehydrogenase. Consistent with the complex I findings, respiration was more sensitive to inhibition by rotenone in control cells than in the TOM40 over-expressing cells. In the absence of inhibitors, total cellular ATP, the mitochondrial membrane potential, and respiration were elevated in the over-expressing cells. Spare respiratory capacity was greater in the TOM40 over-expressing cells than in the controls. TOM40 over-expression blocked Ab-elicited decreases in the mitochondrial membrane potential, cellular ATP levels, and cellular viability in the control cells. These data suggest elevated expression of TOM40 may be protective of mitochondrial function.

Evaluation of mitochondrial respiratory function in highly glycolytic glioma cells reveals low ADP phosphorylation in relation to oxidative capacity

High-grade gliomas are aggressive and intensely glycolytic tumors. In the present study, we evaluated the mitochondrial respiratory function of glioma cells (T98G and U-87MG) and fresh human glioblastoma (GBM) tissue. To this end, measurements of oxygen consumption rate (OCR) were performed under various experimental conditions. The OCR of T98G and U-87MG cells was well coupled to ADP phosphorylation based on the ratio of ATP produced per oxygen consumed of ~2.5. In agreement, the basal OCR of GBM tissue was also partially associated with ADP phosphorylation. The basal respiration of intact T98G and U-87MG cells was not limited by the supply of endogenous substrates, as indicated by the increased OCR in response to a protonophore. These cells also displayed a high affinity for oxygen, as evidenced by the values of the partial pressure of oxygen when respiration is half maximal (p ₅₀). In permeabilized glioma cells, ADP-stimulated OCR was only approximately 50% of that obtained in the presence of protonophore, revealing a significant limitation in oxidative phosphorylation (OXPHOS) relative to the activity of the electron transport system (ETS). This characteristic was maintained when the cells were grown under low glucose conditions. Flux control coefficient analyses demonstrated that the impaired OXPHOS was associated with the function of both mitochondrial ATP synthase and the adenine nucleotide translocator, but not the phosphate carrier. Altogether, these data indicate that the availability and metabolism of respiratory substrates and mitochondrial ETS are preserved in T98G and U-87MG glioma cells even though these cells possess a relatively restrained OXPHOS capability.

Measurement of β-oxidation capacity of biological samples by respirometry: a review of principles and substrates

Oxidation of fatty acids is a major source of energy in the heart, liver, and skeletal muscle. It can be measured accurately using respirometry in isolated mitochondria, intact cells, and permeabilized cells or tissues. This technique directly measures the rate of oxygen consumption or flux at various respiratory states when appropriate substrates, uncouplers, and inhibitors are used. Acylcarnitines such as palmitoylcarnitine or octanoylcarnitine are the commonly used substrates. The β-oxidation pathway is prone to feedforward inhibition resulting from accumulation of short-chain acyl-CoA and depletion of CoA, but inclusion of malate or carnitine prevents accumulation of these intermediaries and CoA depletion.

Underestimation of the Maximal Capacity of the Mitochondrial Electron Transport System in Oligomycin-Treated Cells

The maximal capacity of the mitochondrial electron transport system (ETS) in intact cells is frequently estimated by promoting protonophore-induced maximal oxygen consumption preceded by inhibition of oxidative phosphorylation by oligomycin. In the present study, human glioma (T98G and U-87MG) and prostate cancer (PC-3) cells were titrated with different concentrations of the protonophore CCCP to induce maximal oxygen consumption rate (OCR) within respirometers in a conventional growth medium. The results demonstrate that the presence of oligomycin or its A-isomer leads to underestimation of maximal ETS capacity. In the presence of oligomycin, the spare respiratory capacity (SRC), i.e., the difference between the maximal and basal cellular OCR, was underestimated by 25 to 45%. The inhibitory effect of oligomycin on SRC was more pronounced in T98G cells and was observed in both suspended and attached cells. Underestimation of SRC also occurred when oxidative phosphorylation was fully inhibited by the ATP synthase inhibitor citreoviridin. Further experiments indicated that oligomycin cannot be replaced by the adenine nucleotide translocase inhibitors bongkrekic acid or carboxyatractyloside because, although these compounds have effects in permeabilized cells, they do not inhibit oxidative phosphorylation in intact cells. We replaced CCCP by FCCP, another potent protonophore and similar results were observed. Lower maximal OCR and SRC values were obtained with the weaker protonophore 2,4-dinitrophenol, and these parameters were not affected by the presence of oligomycin. In permeabilized cells or isolated brain mitochondria incubated with respiratory substrates, only a minor inhibitory effect of oligomycin on CCCP-induced maximal OCR was observed. We conclude that unless a previously validated protocol is employed, maximal ETS capacity in intact cells should be estimated without oligomycin. The inhibitory effect of an ATP synthase blocker on potent protonophore-induced maximal OCR may be associated with impaired metabolism of mitochondrial respiratory substrates.

Oxygen delivery from hyperbarically loaded microtanks extends cell viability in anoxic environments

Oxygen diffusion limitations within nascent tissue engineered (TE) grafts lead to the development of hypoxic regions, cell death, and graft failure. Previous efforts have been made to deliver oxygen within TE scaffolds, including peroxide-doping, perfluorocarbons, and hyperbaric oxygen therapy, to mitigate these effects and help maintain post transplantation cell viability, but these have suffered from significant drawbacks. Here we present a novel approach utilizing polymeric hollow-core microspheres that can be hyperbarically loaded with oxygen and subsequently provide prolonged oxygen delivery. These oxygen carriers are termed, microtanks. With an interest in orthopedic applications, we combined microtanks within polycaprolactone to form solid phase constructs with oxygen delivery capabilities. The mathematical laws governing oxygen delivery from microtank-loaded constructs are developed along with empirical validation. Constructs achieved periods of oxygen delivery out to 6 days, which was shown to prolong the survival of human adipose derived stem cells (hASCs) and human umbilical vein endothelial cells (HUVECs) as well as to enhance their cellular morphology under anoxic conditions. The results of this study suggest the microtank approach may be a feasible means of maintaining cell viability in TE scaffolds during the critical period of vascularization in vivo.

Alginate lyase: Review of major sources and classification, properties, structure-function analysis and applications

Alginate lyases catalyze the degradation of alginate, a complex copolymer of α-L-guluronate and its C5 epimer β-D-mannuronate. The enzymes have been isolated from various kinds of organisms with different substrate specificities, including algae, marine mollusks, marine and terrestrial bacteria, and some viruses and fungi. With the progress of structural biology, many kinds of alginate lyases of different polysaccharide lyases families have been characterized by obtaining crystal structures, and the catalytic mechanism has also been elucidated. Combined with various studies, we summarized the source, classification and properties of the alginate lyases from different polysaccharide lyases families. The relationship between substrate specificity and protein sequence was also investigated.

Programming and regulation of metabolic homeostasis

Evidence is presented that the rate and equilibrium constants in mitochondrial oxidative phosphorylation set and maintain metabolic homeostasis in eukaryotic cells. These internal constants determine the energy state ([ATP]/[ADP][Pi]), and the energy state maintains homeostasis through a bidirectional sensory/signaling control network that reaches every aspect of cellular metabolism. The energy state is maintained with high precision (to ∼1 part in 10(10)), and the control system can respond to transient changes in energy demand (ATP utilization) of more than 100 times the resting rate. Epigenetic and environmental factors are able to "fine-tune" the programmed set point over a narrow range to meet the special needs associated with cell differentiation and chronic changes in metabolic requirements. The result is robust across-platform control of metabolism, which is essential to cellular differentiation and the evolution of complex organisms. A model of oxidative phosphorylation is presented, for which the steady-state rate expression has been derived and computer programmed. The behavior of oxidative phosphorylation predicted by the model is shown to fit the experimental data available for isolated mitochondria as well as for cells and tissues. This includes measurements from several different mammalian tissues as well as from insect flight muscle and plants. The respiratory chain and oxidative phosphorylation is remarkably similar for all higher plants and animals. This is consistent with the efficient synthesis of ATP and precise control of metabolic homeostasis provided by oxidative phosphorylation being a key to cellular differentiation and the evolution of structures with specialized function.

Mitochondrial cytochrome c oxidase: mechanism of action and role in regulating oxidative phosphorylation

Mitochondrial oxidative phosphorylation has a central role in eukaryotic metabolism, providing the energy (ATP) required for survival. Regulation of this important pathway is, however, still not understood, largely due to limitations in the ability to measure the essential metabolites, including oxygen (pO2, oxygen pressure), ADP, and AMP. In addition, neither the mechanism of oxygen reduction by mitochondrial cytochrome c oxidase nor how its rate is controlled is understood, although this enzyme determines the rate of oxygen consumption and thereby the rate of ATP synthesis. Cytochrome c oxidase is responsible for reduction of molecular oxygen to water using reducing equivalents donated by cytochrome c and for site 3 energy coupling in oxidative phosphorylation. A mechanism-based model of the cytochrome c oxidase reaction is presented in which transfer of reducing equivalents from the lower- to the higher-potential region of the coupling site occurs against an opposing energy barrier, Q. The steady-state rate equation is fitted to data for the dependence of mitochondrial respiratory rate on cytochrome c reduction, oxygen pressure (pO2), and [ATP]/[ADP][Pi] at pH 6.5 to 8.35 (where Pi is inorganic phosphate). The fit of the rate expression to the experimental data is very good for all experimental conditions. Levels of the intermediates in oxygen reduction in the oxidase reaction site have been calculated. An intermediate in the reaction, tentatively identified as peroxide, bridged between the iron and copper atoms of the reaction site has a central role in coupling mitochondrial respiration to the [ATP]/[ADP][Pi].

Mitochondrial cytochrome c oxidase and control of energy metabolism: measurements in suspensions of isolated mitochondria

Cytochrome c oxidase is the enzyme responsible for oxygen consumption by mitochondrial oxidative phosphorylation and coupling site 3 of oxidative phosphorylation. In this role it determines the cellular rate of ATP synthesis by oxidative phosphorylation and is the key to understanding how energy metabolism is regulated. Four electrons are required for the reduction of oxygen to water, and these are provided by the one-electron donor, cytochrome c. The rate of oxygen consumption (ATP synthesis) is dependent on the fraction of cytochrome c reduced (fred), oxygen pressure (pO2), energy state ([ATP]/[ADP][Pi]), and pH. In coupled mitochondria (high energy state) and pO2 >60 torr, the rate increases in an exponential-like fashion with increasing fred. When the dependence on fred is fitted to the equation rate = A: (fred)(b), A: decreased from 100 to near 20, and B: increased from 1.3 to 4 as the pH of the medium increased from 6.5 to 8.3. During oxygen depletion from the medium fred progressively increases and the rate of respiration decreases. The respiratory rate falls to ½ (P50) by about 1.5 torr, at which point fred is substantially increased. The metabolically relevant dependence on pO2 is obtained by correcting for the increase in fred, in which case the P50 is 12 torr. Adding an uncoupler of oxidative phosphorylation eliminates the dependence of the cytochrome c oxidase activity on pH and energy state. The respiratory rate becomes proportional to fred and the P50 decreases to less than 1 torr.

An in vitro hepatic zonation model with a continuous oxygen gradient in a microdevice

In a hepatic lobule, different sets of metabolic enzymes are expressed in the periportal (PP) and pericentral (PC) regions, forming a functional zonation, and the oxygen gradient is considered a determinant of zone formation. It is desirable to reproduce lobular microenvironment in vitro, but incubation of primary hepatocytes in conventional culture dishes has been limited at fixed oxygen concentrations due to technical difficulties. We designed a cell culture microdevice with an oxygen gradient to reproduce the hepatic microenvironment in vitro. The oxygen gradient during cell culture was monitored using a laser-assisted phosphorescence quenching method, and the cellular oxygen consumption rate could be estimated from changes in the gradient. Culture medium was continuously exchanged through microchannels installed in the device to maintain the oxygen gradient for a long term without transient hyper-oxygenation. The oxygen consumption rates of hepatocytes at 70.0mmHg and 31.4mmHg of partial oxygen pressure, which correspond to PP and PC regions in the microdevice, were 3.67×10(-10) and 3.15×10(-10)mol/s/10(6) cells, respectively. Antimycin A changed the oxygen gradient profile, indicating that cellular respiration can be estimated during cell culture. RT-PCR analysis of hepatocytes cultured under the oxygen gradient showed that mRNA expression of PEPCK and GK significantly increased in culture areas corresponding to PP and PC regions, respectively. These results indicate that the developed microdevice can reproduce the hepatic lobular microenvironment. The oxygen gradient in the microdevice can be closely controlled by changing the sizes of gas channels and the ambient oxygen concentration around the device; therefore, it could be expected to mimic the oxygen gradient of various organs, and it may be applicable to other pathological models.

Bang-bang model for regulation of local blood flow

The classical model of metabolic regulation of blood flow in muscle tissue implies the maintenance of basal tone in arterioles of resting muscle and their dilation in response to exercise and/or tissue hypoxia via the evoked production of vasodilator metabolites by myocytes. A century-long effort to identify specific metabolites responsible for explaining active and reactive hyperemia has not been successful. Furthermore, the metabolic theory is not compatible with new knowledge on the role of physiological radicals (e.g., nitric oxide, NO, and superoxide anion, O2 (-) ) in the regulation of microvascular tone. We propose a model of regulation in which muscle contraction and active hyperemia are considered the physiologically normal state. We employ the "bang-bang" or "on/off" regulatory model which makes use of a threshold and hysteresis; a float valve to control the water level in a tank is a common example of this type of regulation. Active bang-bang regulation comes into effect when the supply of oxygen and glucose exceeds the demand, leading to activation of membrane NADPH oxidase, release of O2 (-) into the interstitial space and subsequent neutralization of the interstitial NO. Switching arterioles on/off when local blood flow crosses the threshold is realized by a local cell circuit with the properties of a bang-bang controller, determined by its threshold, hysteresis, and dead-band. This model provides a clear and unambiguous interpretation of the mechanism to balance tissue demand with a sufficient supply of nutrients and oxygen.

Bariatric surgery rapidly improves mitochondrial respiration in morbidly obese patients

BACKGROUND

Obesity and its attendant comorbidities are an emerging epidemic. Chronic metabolic inflammation (metainflammation) is thought to precipitate obesity-associated morbidities; however, its mechanistic progression is poorly understood. Moreover, although interventions such as diet, exercise, and bariatric surgery can control body weight, their effects on metainflammation are also poorly understood. Recently, metainflammation and the pathobiology of obesity have been linked to mitochondrial dysfunction. Herein we examined the effects of bariatric surgery on mitochondrial respiration as an index of resolving metainflammation in morbidly obese patients.

METHODS

This institutional review board-approved study involved morbidly obese patients (body mass index > 35 kg/m(2)) undergoing sleeve gastrectomy or Roux-en-Y gastric bypass. Mitochondrial respiration was assessed in peripheral blood monocytes and in skeletal muscle samples before surgery and at 12 weeks after surgery. Patient biometrics, homeostasis model assessment-estimated insulin resistance (HOMA-IR) score, C-reactive protein, and lipid profile were analyzed.

RESULTS

Twenty patients were enrolled and showed an average percent excess body weight loss of 30.3% weight loss at 12 weeks after surgery. Average HOMA-IR score decreased from 3.0 to 1.2 in insulin-resistant patients. C-reactive protein, an index of metainflammation, showed a modest decrease. Lipid profile remained stable. Intriguingly, mitochondrial basal and maximal respiration rates in peripheral blood monocytes increased after surgery. Basal rates of skeletal muscle mitochondrial respiration were unchanged, but the maximal respiration rate trended toward an increase after surgery.

CONCLUSIONS

Cellular and tissue mitochondrial respiration increased in a morbidly obese patient cohort after laparoscopic bariatric surgery. These changes were consistent in patients with postsurgical weight loss. Importantly, no significant changes or improvements occurred in canonical indices used to assess recovery after bariatric surgery over this short time course. Thus, increased mitochondrial respiration may represent a novel biomarker of early improvement and positive outcome after surgery in morbidly obese patients.

Glucocorticoid modulation of mitochondrial function in hepatoma cells requires the mitochondrial fission protein Drp1

AIMS

Glucocorticoids, such as dexamethasone, enhance hepatic energy metabolism and gluconeogenesis partly through changes in mitochondrial function. Mitochondrial function is influenced by the balance between mitochondrial fusion and fission events. However, whether glucocorticoids modulate mitochondrial function through the regulation of mitochondrial dynamics is currently unknown.

RESULTS

Here, we report that the effects of dexamethasone on mitochondrial function and gluconeogenesis in hepatoma cells are dependent on the mitochondrial fission protein dynamin-related protein 1 (Drp1). Dexamethasone increased routine oxygen consumption, maximal respiratory capacity, superoxide anion, proton leak, and gluconeogenesis in hepatoma cells. Under these conditions, dexamethasone altered mitochondrial morphology, which was paralleled by a large increase in Drp1 expression, and reduced mitofusin 1 (Mfn1) and Mfn2. In vivo dexamethasone treatment also enhanced Drp1 expression in mouse liver. On the basis of these observations, we analyzed the dependence on the Drp1 function of dexamethasone effects on mitochondrial respiration and gluconeogenesis. We show that the increase in mitochondrial respiration and gluconeogenesis induced by dexamethasone are hampered by the inhibition of Drp1 function.

INNOVATION

Our findings provide the first evidence that the effects of glucocorticoids on hepatic metabolism require the mitochondrial fission protein Drp1.

CONCLUSION

In summary, we demonstrate that the mitochondrial effects of dexamethasone both on mitochondrial respiration and on the gluconeogenic pathway depend on Drp1.

Acute mitochondrial inhibition by mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) 1/2 inhibitors regulates proliferation

The Ras-MEK1/2-ERK1/2 kinase signaling pathway regulates proliferation, survival, and differentiation and, because it is often aberrant in tumors, is a popular target for small molecule inhibition. A novel metabolic analysis that measures the real-time oxidation state of NAD(H) and the hemes of the electron transport chain and oxygen consumption within intact, living cells found that structurally distinct MEK1/2 inhibitors had an immediate, dose-dependent effect on mitochondrial metabolism. The inhibitors U0126, MIIC and PD98059 caused NAD(H) reduction, heme oxidation, and decreased oxygen consumption, characteristic of complex I inhibition. PD198306, an orally active MEK1/2 inhibitor, acted as an uncoupler. Each MEK1/2 inhibitor depleted phosphorylated ERK1/2 and inhibited proliferation, but the most robust antiproliferative effects always correlated with the metabolic failure which followed mitochondrial inhibition rather than inhibition of MEK1/2. This warrants rethinking the role of ERK1/2 in proliferation and emphasizes the importance of mitochondrial function in this process.

Oxygen dependence of respiration in rat spinotrapezius muscle in situ

The oxygen dependence of respiration in striated muscle in situ was studied by measuring the rate of decrease of interstitial Po(2) [oxygen disappearance curve (ODC)] following rapid arrest of blood flow by pneumatic tissue compression, which ejected red blood cells from the muscle vessels and made the ODC independent from oxygen bound to hemoglobin. After the contribution of photo-consumption of oxygen by the method was evaluated and accounted for, the corrected ODCs were converted into the Po(2) dependence of oxygen consumption, Vo(2), proportional to the rate of Po(2) decrease. Fitting equations obtained from a model of heterogeneous intracellular Po(2) were applied to recover the parameters describing respiration in muscle fibers, with a predicted sigmoidal shape for the dependence of Vo(2) on Po(2). This curve consists of two regions connected by the point for critical Po(2) of the cell (i.e., Po(2) at the sarcolemma when the center of the cell becomes anoxic). The critical Po(2) was below the Po(2) for half-maximal respiratory rate (P(50)) for the cells. In six muscles at rest, the rate of oxygen consumption was 139 ± 6 nl O(2)/cm(3)·s and mitochondrial P(50) was k = 10.5 ± 0.8 mmHg. The range of Po(2) values inside the muscle fibers was found to be 4-5 mmHg at the critical Po(2). The oxygen dependence of respiration can be studied in thin muscles under different experimental conditions. In resting muscle, the critical Po(2) was substantially lower than the interstitial Po(2) of 53 ± 2 mmHg, a finding that indicates that Vo(2) under this circumstance is independent of oxygen supply and is discordant with the conventional hypothesis of metabolic regulation of the oxygen supply to tissue.

High-resolution respirometry: OXPHOS protocols for human cells and permeabilized fibers from small biopsies of human muscle

Protocols for high-resolution respirometry (HRR) of intact cells, permeabilized cells, and permeabilized muscle fibers offer sensitive diagnostic tests of integrated mitochondrial function using standard cell culture techniques and small needle biopsies of muscle. Multiple substrate-uncoupler-inhibitor titration (SUIT) protocols for analysis of oxidative phosphorylation improve our understanding of mitochondrial respiratory control and the pathophysiology of mitochondrial diseases. Respiratory states are defined in functional terms to account for the network of metabolic interactions in complex SUIT protocols with stepwise modulation of coupling and substrate control. A regulated degree of intrinsic uncoupling is a hallmark of oxidative phosphorylation, whereas pathological and toxicological dyscoupling is evaluated as a mitochondrial defect. The noncoupled state of maximum respiration is experimentally induced by titration of established uncouplers (FCCP, DNP) to collapse the proton gradient across the mitochondrial inner membrane and measure the capacity of the electron transfer system (ETS, open-circuit operation of respiration). Intrinsic uncoupling and dyscoupling are evaluated as the flux control ratio between nonphosphorylating LEAK respiration (electron flow coupled to proton pumping to compensate for proton leaks) and ETS capacity. If OXPHOS capacity (maximally ADP-stimulated oxygen flux) is less than ETS capacity, the phosphorylation system contributes to flux control. Physiological Complex I + II substrate combinations are required to reconstitute TCA cycle function. This supports maximum ETS and OXPHOS capacities, due to the additive effect of multiple electron supply pathways converging at the Q-junction. Substrate control with electron entry separately through Complex I (pyruvate + malate or glutamate + malate) or Complex II (succinate + rotenone) restricts ETS capacity and artificially enhances flux control upstream of the Q-cycle, providing diagnostic information on specific branches of the ETS. Oxygen levels are maintained above air saturation in protocols with permeabilized muscle fibers to avoid experimental oxygen limitation of respiration. Standardized two-point calibration of the polarographic oxygen sensor (static sensor calibration), calibration of the sensor response time (dynamic sensor calibration), and evaluation of instrumental background oxygen flux (systemic flux compensation) provide the unique experimental basis for high accuracy of quantitative results and quality control in HRR.

Endogenous myoglobin in breast cancer is hypoxia-inducible by alternative transcription and functions to impair mitochondrial activity: a role in tumor suppression?

Recently, immunohistochemical analysis of myoglobin (MB) in human breast cancer specimens has revealed a surprisingly widespread expression of MB in this nonmuscle context. The positive correlation with hypoxia-inducible factor 2α (HIF-2α) and carbonic anhydrase IX suggested that oxygen regulates myoglobin expression in breast carcinomas. Here, we report that MB mRNA and protein levels are robustly induced by prolonged hypoxia in breast cancer cell lines, in part via HIF-1/2-dependent transactivation. The hypoxia-induced MB mRNA originated from a novel alternative transcription start site 6 kb upstream of the ATG codon. MB regulation in normal and tumor tissue may thus be fundamentally different. Functionally, the knockdown of MB in MDA-MB468 breast cancer cells resulted in an unexpected increase of O(2) uptake and elevated activities of mitochondrial enzymes during hypoxia. Silencing of MB transcription attenuated proliferation rates and motility capacities of hypoxic cancer cells and, surprisingly, also fully oxygenated breast cancer cells. Endogenous MB in cancer cells is apparently involved in controlling oxidative cell energy metabolism, contrary to earlier findings on mouse heart, where the targeted disruption of the Mb gene did not effect myocardial energetics and O(2) consumption. This control function of MB seemingly impacts mitochondria and influences cell proliferation and motility, but it does so in ways not directly related to the facilitated diffusion or storage of O(2). Hypothetically, the mitochondrion-impairing role of MB in hypoxic cancer cells is part of a novel tumor-suppressive function.

Unforeseen decreases in dissolved oxygen levels affect tube formation kinetics in collagen gels

The availability of oxygen (O(2)) is a critical parameter affecting vascular tube formation. In this study, we hypothesize that dissolved oxygen (DO) levels in collagen gels change during the three-dimensional (3D) culture of human umbilical vein endothelial cells (HUVECs) in atmospheric conditions and that such changes affect the kinetics of tube formation through the production of reactive oxygen species (ROS). We demonstrate a decrease in O(2) tension during 3D cultures of HUVECs. Noninvasive measurements of DO levels during culture under atmospheric conditions revealed a profound decrease that reached as low as 2% O(2) at the end of 24 h. After media replacement, DO levels rose rapidly and equilibrated at ∼15% O(2), creating a reoxygenated environment. To accurately estimate DO gradients in 3D collagen gels, we developed a 3D mathematical model and determined the Michaelis-Menten parameters, V(max) and K(m), of HUVECs in collagen gels. We detected an increase in ROS levels throughout the culture period. Using diphenyliodonium to inhibit ROS production resulted in the complete inhibition of tube formation. Interference RNA studies further showed that hypoxia-inducible factors (HIFs)-1α and -2α are not involved in the formation of 3D tubes in collagen gels. We conclude that ROS affect the tubulogenesis process through HIFα-independent pathways, where the levels of ROS are influenced by the uncontrolled variations in DO levels. This study is the first demonstration of the critical and unexpected role of O(2) during 3D in vitro culture models of tubulogenesis in atmospheric conditions.

Dietary inorganic nitrate improves mitochondrial efficiency in humans

Nitrate, an inorganic anion abundant in vegetables, is converted in vivo to bioactive nitrogen oxides including NO. We recently demonstrated that dietary nitrate reduces oxygen cost during physical exercise, but the mechanism remains unknown. In a double-blind crossover trial we studied the effects of a dietary intervention with inorganic nitrate on basal mitochondrial function and whole-body oxygen consumption in healthy volunteers. Skeletal muscle mitochondria harvested after nitrate supplementation displayed an improvement in oxidative phosphorylation efficiency (P/O ratio) and a decrease in state 4 respiration with and without atractyloside and respiration without adenylates. The improved mitochondrial P/O ratio correlated to the reduction in oxygen cost during exercise. Mechanistically, nitrate reduced the expression of ATP/ADP translocase, a protein involved in proton conductance. We conclude that dietary nitrate has profound effects on basal mitochondrial function. These findings may have implications for exercise physiology- and lifestyle-related disorders that involve dysfunctional mitochondria.

A noninvasive technique for the measurement of the energetic state of free-suspension mammalian cells

A perfusion small-scale bioreactor allowing on-line monitoring of the cell energetic state was developed for free-suspension mammalian cells. The bioreactor was designed to perform in vivo nuclear magnetic resonance (NMR) spectroscopy, which is a noninvasive and nondestructive method that permits the monitoring of intracellular nutrient concentrations, metabolic precursors and intermediates, as well as metabolites and energy shuttles, such as ATP, ADP, and NADPH. The bioreactor was made of a 10-mm NMR tube following a fluidized bed design. Perfusion flow rate allowing for adequate oxygen supply was found to be above 0.79 mL min(-1) for high-density cell suspensions (10(8) cells). Chinese hamster ovary (CHO) cells were studied here as model system. Hydrodynamic studies using coloration/decoloration and residence time distribution measurements were realized to perfect bioreactor design as well as to determine operating conditions bestowing adequate homogeneous mixing and cell retention in the NMR reading zone. In vivo (31)P NMR was performed and demonstrated the small-scale bioreactor platform ability to monitor the cell physiological behavior for 30-min experiments.

Cell respiration under hypoxia: facts and artefacts in mitochondrial oxygen kinetics

When oxygen supply to tissues is limiting, mitochondrial respiration and ATP production are compromised. To assess the bioenergetic consequences under normoxia and hypoxia, quantitative evaluation of mitochondrial oxygen kinetics is required. Using high-resolution respirometry, the "apparent K (m)" for oxygen or p (50) of respiration in 32D cells was determined at 0.05 +/- 0.01 kPa (0.4 mmHg, 0.5 microM, 0.25% air saturation). Close agreement with p (50) of isolated mitochondria indicates that intracellular gradients are small in small cells at routine activity. At intracellular p (O2) <2 kPa (15 mmHg, 10% air saturation) in various tissues under normoxia, respiration is limited by >2% with a p (50) of 0.05 kPa. Over-estimation of p (50) at 0.4 kPa (3 mmHg) would imply significant (>17%) oxygen limitation of respiration under intracellular normoxia. Based on a critical review, we conclude that p (50) ranges from 0.01 to 0.10 kPa in mitochondria and small cells in the absence of inhibitors of cytochrome c oxidase, whereas experimental artefacts explain the controversial >200-fold range of p (50) in the literature on mitochondrial oxygen kinetics.

Respiratory response of malignant and placental cells to changes in oxygen concentration

Malignant cells and foetal tissues are exposed to low oxygen partial pressure (pO2) in situ due to the limited supply of oxygenated blood. Whether these cells have adapted to low pO2 or live under constant constraint is not clear. Herein, we compared the respiratory responses of different malignant cell types, maternal and foetal placental leucocytes, and benign cells by incubating them under a gradient of pO2, from saturation to hypoxia, in a high resolution respirometer. The malignant cells and foetal leucocytes showed higher rates of mitochondrial oxygen uptake compared to the benign cells and maternal leucocytes, respectively. On the other hand, the mitochondrial oxygen uptake rates of the hypoxia adapted cells declined faster than the other cell types during the onset of hypoxia, probably suggesting conformance of aerobic metabolism to the local oxygen concentration. The O2 consumption rate per million cells (JO2) of the malignant cells declined only when the O2 concentration ([O2]) decreased to values<or=10 microM. On the other hand, the JO2 of the benign cells declined with the decrease in [O2] from 200 to 40 microM and <or=10 microM. In the [O2] ranges outside these values the JO2 remained constant regardless of the decline of [O2] in the medium. The JO2 of foetal leucocytes and malignant cells responded to the change in [O2] in a similar manner, and may indicate comparable mechanisms of adaptation to hypoxia.

Influence of cryopreservation on mitochondrial functions in equine spermatozoa

Cryopreservation of spermatozoa is of essential importance for artificial insemination and breeding programs in horses. Besides other factors, spermatozoal motility depends on mitochondrial energy metabolism. Based on changes of single mitochondrial functions it has been suggested that mitochondrial damage during cryopreservation could be a major reason for diminished post thaw semen quality. However, it is still unclear to which extent this influences the whole bioenergetic performance of mitochondria and whether this plays a role during routine cryopreservation procedures. Therefore, it was the aim of this study to compare changes in mitochondrial bioenergetics in spermatozoa during shock freezing and routine cryopreservation. Mitochondrial integrity in spermatozoa was studied by determination of oxygen consumption, mitochondrial membrane potential, and the oxidation of externally added cytochrome c(2+). Shock freezing of spermatozoa resulted in an irreversible loss of mitochondrial functions. However, respiration difference of uncoupled minus resting state and routine respiration also decreased by 48+/-14 and 58+/-6% (p<0.05), respectively, after routine cryopreservation. This was accompanied by a decline in the mitochondrial membrane potential to 83+/-4% (p<0.05) and spermatozoal motility to 56+/-11% (p<0.05) of pre-freezing values. In contrast, the oxidation rates of externally added cytochrome c(2+) by cytochrome c oxidase slightly increased by 26+/-14% (p<0.1) suggesting a partial rupture of cellular and outer mitochondrial membranes. Our data indicate that also widely used cryopreservation protocols for equine spermatozoa need adjustment to optimize post thaw mitochondrial functions.

Electron paramagnetic resonance oximetry as a quantitative method to measure cellular respiration: a consideration of oxygen diffusion interference

Electron paramagnetic resonance (EPR) oximetry is being widely used to measure the oxygen consumption of cells, mitochondria, and submitochondrial particles. However, further improvement of this technique, in terms of data analysis, is required to use it as a quantitative tool. Here, we present a new approach for quantitative analysis of cellular respiration using EPR oximetry. The course of oxygen consumption by cells in suspension has been observed to have three distinct zones: pO(2)-independent respiration at higher pO(2) ranges, pO(2)-dependent respiration at low pO(2) ranges, and a static equilibrium with no change in pO(2) at very low pO(2) values. The approach here enables one to comprehensively analyze all of the three zones together-where the progression of O(2) diffusion zones around each cell, their overlap within time, and their potential impact on the measured pO(2) data are considered. The obtained results agree with previously established methods such as high-resolution respirometry measurements. Additionally, it is also demonstrated how the diffusion limitations can depend on cell density and consumption rate. In conclusion, the new approach establishes a more accurate and meaningful model to evaluate the EPR oximetry data on cellular respiration to quantify related parameters using EPR oximetry.

Senescence-associated changes in respiration and oxidative phosphorylation in primary human fibroblasts

Limitation of lifespan in replicative senescence is related to oxidative stress, which is probably both the cause and consequence of impaired mitochondrial respiratory function. The respiration of senescent human diploid fibroblasts was analysed by high-resolution respirometry. To rule out cell-cycle effects, proliferating and growth-arrested young fibroblasts were used as controls. Uncoupled respiration, as normalized to citrate synthase activity, remained unchanged, reflecting a constant capacity of the respiratory chain. Oligomycin-inhibited respiration, however, was significantly increased in mitochondria of senescent cells, indicating a lower coupling of electron transport with phosphorylation. In contrast, growth-arrested young fibroblasts exhibited a higher coupling state compared with proliferating controls. In intact cells, partial uncoupling may lead to either decreased oxidative ATP production or a compensatory increase in routine respiration. To distinguish between these alternatives, we subtracted oligomycin-inhibited respiration from routine respiration, which allowed us to determine the part of respiratory activity coupled with ATP production. Despite substantial differences in the respiratory control ratio, ranging from 4 to 11 in the different experimental groups, a fixed proportion of respiratory capacity was maintained for coupled oxidative phosphorylation in all the experimental groups. This finding indicates that the senescent cells fully compensate for increased proton leakage by enhanced electron-transport activity in the routine state. These results provide a new insight into age-associated defects in mitochondrial function and compensatory mechanisms in intact cells.

Decreased affinity for oxygen of cytochrome-coxidase in Leigh syndrome caused bySURF1mutations

Mutations in the gene SURF1 prevent synthesis of cytochrome- c oxidase (COX)-specific assembly protein and result in a fatal neurological disorder, Leigh syndrome. Because this severe COX deficiency presents with barely detectable changes of cellular respiratory rates under normoxic conditions, we analyzed the respiratory response to low oxygen in cultured fibroblasts harboring SURF1 mutations with high-resolution respirometry. The oxygen kinetics was quantified by the partial pressure of oxygen (Po2) at half-maximal respiration rate (P50) in intact coupled cells and in digitonin-permeabilized uncoupled cells. In both cases, the P50in patients was elevated 2.1- and 3.3-fold, respectively, indicating decreased affinity of COX for oxygen. These results suggest that at physiologically low intracellular Po2, the depressed oxygen affinity may lead in vivo to limitations of respiration, resulting in impaired energy provision in Leigh syndrome patients.

Measurement of oxygen consumption in mouse aortic endothelial cells using a microparticulate oximetry probe

The purpose of this study was to determine the rate of oxygen consumption in mouse aortic endothelial cells (MAECs) and to determine the effect of a variety of inhibitors and stimulators of oxygen consumption measured by electron paramagnetic resonance (EPR) spectroscopy utilizing a new particulate oximetry probe. We have previously demonstrated that the octa-n-butoxy derivative of naphthalocyanine neutral radical (LiNc-BuO) enables accurate, precise, and reproducible measurements of pO(2) in cellular suspensions. In the current study, we carried out measurements to provide an accurate determination of pO(2) in small volume with less number of cells (20,000 cells) that has not been possible with other techniques. To establish the reliability of this method, agents such as menadione, lipopolysaccharide (LPS), potassium cyanide, rotenone, and diphenyleneiodonium chloride (DPI) were used to modulate the oxygen consumption rate in the cells. We observed an increase in oxygen consumption by the cells upon treatment with menadione and LPS, whereas treatment with cyanide, rotenone, and DPI inhibited oxygen consumption. This study clearly demonstrated the utilization of EPR spectrometry with LiNc-BuO probe for determination of oxygen concentration in cultured cells.

Monitoring cytochrome redox changes in the mitochondria of intact cells using multi-wavelength visible light spectroscopy

We have developed an optical system based on visible light spectroscopy for the continuous study of changes in the redox states of mitochondrial cytochromes in intact mammalian cells. Cells are suspended in a closed incubation chamber in which oxygen and nitric oxide (NO) concentrations can be monitored during respiration. Simultaneously the cells are illuminated with a broad-band tungsten-halogen light source. Emergent light in the visible region (from 490-650 nm) is detected using a spectrophotometer and charge-coupled device camera system. Intensity spectra are then converted into changes in optical attenuation from a 'steady-state' baseline. The oxidised-minus-reduced absorption spectra of the mitochondrial cytochromes are fitted to the attenuation spectra using a multi-wavelength least-squares algorithm. Thus, the system can measure changes in the redox states of the cytochromes during cellular respiration. Here we describe this novel methodology and demonstrate its validity by monitoring the action of known respiratory chain inhibitors, including the endogenous signalling molecule NO, on cytochrome redox states in human leukocytes.

Oxygen regulation and limitation to cellular respiration in mouse skeletal muscle in vivo

In skeletal muscle, intracellular Po2 can fall to as low as 2-3 mmHg. This study tested whether oxygen regulates cellular respiration in this range of oxygen tensions through direct coupling between phosphorylation potential and intracellular Po2. Oxygen may also behave as a simple substrate in cellular respiration that is near saturating levels over most of the physiological range. A novel optical spectroscopic method was used to measure tissue oxygen consumption (Mo2) and intracellular Po2 using the decline in hemoglobin and myoglobin saturation in the ischemic hindlimb muscle of Swiss-Webster mice. 31P magnetic resonance spectroscopic determinations yielded phosphocreatine concentration ([PCr]) and pH in the same muscle volume. Intracellular Po2 fell to <2 mmHg during the ischemic period without a change in the muscle [PCr] or pH. The constant phosphorylation state despite the decline in intracellular Po2 rejects the hypothesis that direct coupling between these two variables results in a regulatory role for oxygen in cellular respiration. A second set of experiments tested the relationship between intracellular Po2 and Mo2. In vivo Mo2 in mouse skeletal muscle was increased by systemic treatment with 2 and 4 mg/kg body wt 2,4-dinitrophenol to partially uncouple mitochondria. Mo2 was not dependent on intracellular Po2 above 3 mmHg in the three groups despite a threefold increase in Mo2. These results indicate that Mo2 and the phosphorylation state of the cell are independent of intracellular Po2 throughout the physiological range of oxygen tensions. Therefore, we reject a regulatory role for oxygen in cellular respiration and conclude that oxygen acts as a simple substrate for respiration under physiological conditions.

Biphasic oxygen kinetics of cellular respiration and linear oxygen dependence of antimycin A inhibited oxygen consumption

Oxygen kinetics in fibroblasts was biphasic. This was quantitatively explained by a major mitochondrial hyperbolic component in the low-oxygen range and a linear increase of rotenone- and antimycin A-inhibited oxygen consumption in the high-oxygen range. This suggests an increased production of reactive oxygen species and oxidative stress at elevated, air-level oxygen concentrations. The high oxygen affinity of mitochondrial respiration provides the basis for the maintenance of a high aerobic scope at physiological low-oxygen levels, whereas further pronounced depression of oxygen pressure induces energetic stress under hypoxia.

Oxygen Conformance of Cellular Respiration

Oxygen pressure declines from normoxic air-level to the microenvironment of mitochondria where cytochrome c oxidase (COX) reduces oxygen to water at oxygen levels as low as 0.3 kPa (2 Torr; 3 microM; 1.5 % air saturation). Intracellular hypoxia is defined as (1) local oxygen pressure below normoxic reference states, or (2) limitation of mitochondrial respiration by oxygen levels below kinetic saturation, resulting in oxyconformance. High-resolution respirometry provides the methodology to measure mitochondrial and cellular oxygen kinetics in the relevant low oxygen range < 1 kPa (7.5 mmHg; 9-10 microM; 5% air saturation). Respiration of isolated heart mitochondria follows hyperbolic oxygen kinetics with half-saturating oxygen pressure, p50, of 0.04 kPa (0.3 Torr; 0.4 microM) in ADP-stimulated state 3. Thus mitochondrial respiration proceeds at 90% of its hyperbolic maximum at the p50 of myoglobin, suggesting the possibility of a small but significant oxygen limitation even under normoxia in active muscle. Any impairment of oxygen delivery, therefore, induces oxyconformance. In addition, a shift of mitochondrial oxygen kinetics to the right, particularly by competitive inhibition of COX by NO, causes a further depression of respiration and a compensatory increase of local oxygen pressure. Above 1 kPa, mitochondrial oxygen uptake increases above hyperbolic saturation, which is probably due to oxygen radical production rather than the kinetics of COX. In cultured cells, the pronounced oxygen uptake above mitochondrial saturation at air-level oxygen pressure cannot be inhibited by rotenone and antimycin A, amounting to > 20 % of routine respiration in fibroblasts. Biochemical models of oxyconformance of COX are evaluated relative to patterns of intracellular oxygen distribution in the tissue and enzyme turnover in vivo, considering the kinetic effects of COX excess capacity on flux through the mitochondrial electron transport chain.