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1.  Exercise training increases mitochondrial content and ex vivo mitochondrial function similarly in patients with type 2 diabetes and in control individuals 
Diabetologia  2010;53(8):1714-1721.
Aims/hypothesis
We previously showed that type 2 diabetic patients are characterised by compromised intrinsic mitochondrial function. Here, we examined if exercise training could increase intrinsic mitochondrial function in diabetic patients compared with control individuals.
Methods
Fifteen male type 2 diabetic patients and 14 male control individuals matched for age, BMI and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{V}{{\hbox{O}}_{2{ \max }}} $$\end{document} enrolled in a 12 week exercise intervention programme. Ex vivo mitochondrial function was assessed by high-resolution respirometry in permeabilised muscle fibres from vastus lateralis muscle. Before and after training, insulin-stimulated glucose disposal was examined during a hyperinsulinaemic–euglycaemic clamp.
Results
Diabetic patients had intrinsically lower ADP-stimulated state 3 respiration and lower carbonyl cyanide 4-(trifluoro-methoxy)phenylhydrazone (FCCP)-induced maximal oxidative respiration, both on glutamate and on glutamate and succinate, and in the presence of palmitoyl-carnitine (p < 0.05). After training, diabetic patients and control individuals showed increased state 3 respiration on the previously mentioned substrates (p < 0.05); however, an increase in FCCP-induced maximal oxidative respiration was observed only in diabetic patients (p < 0.05). The increase in mitochondrial respiration was accompanied by a 30% increase in mitochondrial content upon training (p < 0.01). After adjustment for mitochondrial density, state 3 and FCCP-induced maximal oxidative respiration were similar between groups after training. Improvements in mitochondrial respiration were paralleled by improvements in insulin-stimulated glucose disposal in diabetic patients, with a tendency for this in control individuals.
Conclusions/interpretation
We confirmed lower intrinsic mitochondrial function in diabetic patients compared with control individuals. Diabetic patients increased their mitochondrial content to the same extent as control individuals and had similar intrinsic mitochondrial function, which occurred parallel with improved insulin sensitivity.
doi:10.1007/s00125-010-1764-2
PMCID: PMC2892060  PMID: 20422397
Type 2 diabetes; Insulin resistance; Mitochondrial dysfunction; Exercise training
2.  Muscle Mitochondrial ATP Synthesis and Glucose Transport/Phosphorylation in Type 2 Diabetes 
PLoS Medicine  2007;4(5):e154.
Background
Muscular insulin resistance is frequently characterized by blunted increases in glucose-6-phosphate (G-6-P) reflecting impaired glucose transport/phosphorylation. These abnormalities likely relate to excessive intramyocellular lipids and mitochondrial dysfunction. We hypothesized that alterations in insulin action and mitochondrial function should be present even in nonobese patients with well-controlled type 2 diabetes mellitus (T2DM).
Methods and Findings
We measured G-6-P, ATP synthetic flux (i.e., synthesis) and lipid contents of skeletal muscle with 31P/1H magnetic resonance spectroscopy in ten patients with T2DM and in two control groups: ten sex-, age-, and body mass-matched elderly people; and 11 younger healthy individuals. Although insulin sensitivity was lower in patients with T2DM, muscle lipid contents were comparable and hyperinsulinemia increased G-6-P by 50% (95% confidence interval [CI] 39%–99%) in all groups. Patients with diabetes had 27% lower fasting ATP synthetic flux compared to younger controls (p = 0.031). Insulin stimulation increased ATP synthetic flux only in controls (younger: 26%, 95% CI 13%–42%; older: 11%, 95% CI 2%–25%), but failed to increase even during hyperglycemic hyperinsulinemia in patients with T2DM. Fasting free fatty acids and waist-to-hip ratios explained 44% of basal ATP synthetic flux. Insulin sensitivity explained 30% of insulin-stimulated ATP synthetic flux.
Conclusions
Patients with well-controlled T2DM feature slightly lower flux through muscle ATP synthesis, which occurs independently of glucose transport /phosphorylation and lipid deposition but is determined by lipid availability and insulin sensitivity. Furthermore, the reduction in insulin-stimulated glucose disposal despite normal glucose transport/phosphorylation suggests further abnormalities mainly in glycogen synthesis in these patients.
Michael Roden and colleagues report that even patients with well-controlled insulin-resistant type 2 diabetes have altered mitochondrial function.
Editors' Summary
Background.
Diabetes mellitus is an increasingly common chronic disease characterized by high blood sugar (glucose) levels. In normal individuals, blood sugar levels are maintained by the hormone insulin. Insulin is released by the pancreas when blood glucose levels rise after eating (glucose is produced by the digestion of food) and “instructs” insulin-responsive muscle and fat cells to take up glucose from the bloodstream. The cells then use glucose as a fuel or convert it into glycogen, a storage form of glucose. In type 2 diabetes, the commonest type of diabetes, the muscle and fat cells become nonresponsive to insulin (a condition called insulin resistance) and consequently blood glucose levels rise. Over time, this hyperglycemia increases the risk of heart attacks, kidney failure, and other life-threatening complications.
Why Was This Study Done?
Insulin resistance is often an early sign of type 2 diabetes, sometimes predating its development by many years, so understanding its causes might provide clues about how to stop the global diabetes epidemic. One theory is that mitochondria—cellular structures that produce the energy (in the form of a molecule called ATP) needed to keep cells functioning—do not work properly in people with insulin resistance. Mitochondria change (metabolize) fatty acids into energy, and recent studies have revealed that fat accumulation caused by poorly regulated fatty acid metabolism blocks insulin signaling, thus causing insulin resistance. Other studies using magnetic resonance spectroscopy (MRS) to study mitochondrial function noninvasively in human muscle indicate that mitochondria are dysfunctional in people with insulin resistance by showing that ATP synthesis is impaired in such individuals. In this study, the researchers have examined both baseline and insulin-stimulated mitochondrial function in nonobese patients with well-controlled type 2 diabetes and in normal controls to discover more about the relationship between mitochondrial dysfunction and insulin resistance.
What Did the Researchers Do and Find?
The researchers determined the insulin sensitivity of people with type 2 diabetes and two sets of people (the “controls”) who did not have diabetes: one in which the volunteers were age-matched to the people with diabetes, and the other containing younger individuals (insulin resistance increases with age). To study insulin sensitivity in all three groups, the researchers used a “hyperinsulinemic–euglycemic clamp.” For this, after an overnight fast, the participants' insulin levels were kept high with a continuous insulin infusion while blood glucose levels were kept normal using a variable glucose infusion. In this situation, the glucose infusion rate equals glucose uptake by the body and therefore measures tissue sensitivity to insulin. Before and during the clamp, the researchers used MRS to measure glucose-6-phosphate (an indicator of how effectively glucose is taken into cells and phosphorylated), ATP synthesis, and the fat content of the participants' muscle cells. Insulin sensitivity was lower in the patients with diabetes than in the controls, but muscle lipid content was comparable and hyperinsulinemia increased glucose-6-phosphate levels similarly in all the groups. Patients with diabetes and the older controls had lower fasting ATP synthesis rates than the young controls and, whereas insulin stimulation increased ATP synthesis in all the controls, it had no effect in the patients with diabetes. In addition, fasting blood fatty acid levels were inversely related to basal ATP synthesis, whereas insulin sensitivity was directly related to insulin-stimulated ATP synthesis.
What Do These Findings Mean?
These findings indicate that the impairment of muscle mitochondrial ATP synthesis in fasting conditions and after insulin stimulation in people with diabetes is not due to impaired glucose transport/phosphorylation or fat deposition in the muscles. Instead, it seems to be determined by lipid availability and insulin sensitivity. These results add to the evidence suggesting that mitochondrial function is disrupted in type 2 diabetes and in insulin resistance, but also suggest that there may be abnormalities in glycogen synthesis. More work is needed to determine the exact nature of these abnormalities and to discover whether they can be modulated to prevent the development of insulin resistance and type 2 diabetes. For now, though, these findings re-emphasize the need for people with type 2 diabetes or insulin resistance to reduce their food intake to compensate for the reduced energy needs of their muscles and to exercise to increase the ATP-generating capacity of their muscles. Both lifestyle changes could improve their overall health and life expectancy.
Additional Information.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040154.
The MedlinePlus encyclopedia has pages on diabetes
The US National Institute of Diabetes and Digestive and Kidney Diseases provides information for patients on diabetes and insulin resistance
The US Centers for Disease Control and Prevention has information on diabetes for patients and professionals
American Diabetes Association provides information for patients on diabetes and insulin resistance
Diabetes UK has information for patients and professionals on diabetes
doi:10.1371/journal.pmed.0040154
PMCID: PMC1858707  PMID: 17472434
3.  Patients with type 2 diabetes have normal mitochondrial function in skeletal muscle 
Diabetologia  2007;50(4):790-796.
Aims/hypothesis
Insulin resistance and type 2 diabetes are associated with mitochondrial dysfunction. The aim of the present study was to test the hypothesis that oxidative phosphorylation and electron transport capacity are diminished in the skeletal muscle of type 2 diabetic subjects, as a result of a reduction in the mitochondrial content.
Materials and methods
The O2 flux capacity of permeabilised muscle fibres from biopsies of the quadriceps in healthy subjects (n = 8; age 58 ± 2 years [mean±SEM]; BMI 28 ± 1 kg/m2; fasting plasma glucose 5.4 ± 0.2 mmol/l) and patients with type 2 diabetes (n = 11; age 62 ± 2 years; BMI 32 ± 2 kg/m2; fasting plasma glucose 9.0 ± 0.8 mmol/l) was measured by high-resolution respirometry.
Results
O2 flux expressed per mg of muscle (fresh weight) during ADP-stimulated state 3 respiration was lower (p < 0.05) in patients with type 2 diabetes in the presence of complex I substrate (glutamate) (31 ± 2 vs 43 ± 3 pmol O2 s−1 mg−1) and in response to glutamate + succinate (parallel electron input from complexes I and II) (63 ± 3 vs 85 ± 6 pmol s−1 mg−1). Further increases in O2 flux capacity were observed in response to uncoupling by FCCP, but were again lower (p < 0.05) in type 2 diabetic patients than in healthy control subjects (86 ± 4 vs 109 ± 8 pmol s−1 mg−1). However, when O2 flux was normalised for mitochondrial DNA content or citrate synthase activity, there were no differences in oxidative phosphorylation or electron transport capacity between patients with type 2 diabetes and healthy control subjects.
Conclusions/interpretation
Mitochondrial function is normal in type 2 diabetes. Blunting of coupled and uncoupled respiration in type 2 diabetic patients can be attributed to lower mitochondrial content.
doi:10.1007/s00125-007-0594-3
PMCID: PMC1820754  PMID: 17334651
Diabetes; Mitochondria; Skeletal muscle
4.  Restoration of Muscle Mitochondrial Function and Metabolic Flexibility in Type 2 Diabetes by Exercise Training Is Paralleled by Increased Myocellular Fat Storage and Improved Insulin Sensitivity 
Diabetes  2009;59(3):572-579.
OBJECTIVE
Mitochondrial dysfunction and fat accumulation in skeletal muscle (increased intramyocellular lipid [IMCL]) have been linked to development of type 2 diabetes. We examined whether exercise training could restore mitochondrial function and insulin sensitivity in patients with type 2 diabetes.
RESEARCH DESIGN AND METHODS
Eighteen male type 2 diabetic and 20 healthy male control subjects of comparable body weight, BMI, age, and Vo2max participated in a 12-week combined progressive training program (three times per week and 45 min per session). In vivo mitochondrial function (assessed via magnetic resonance spectroscopy), insulin sensitivity (clamp), metabolic flexibility (indirect calorimetry), and IMCL content (histochemically) were measured before and after training.
RESULTS
Mitochondrial function was lower in type 2 diabetic compared with control subjects (P = 0.03), improved by training in control subjects (28% increase; P = 0.02), and restored to control values in type 2 diabetic subjects (48% increase; P < 0.01). Insulin sensitivity tended to improve in control subjects (delta Rd 8% increase; P = 0.08) and improved significantly in type 2 diabetic subjects (delta Rd 63% increase; P < 0.01). Suppression of insulin-stimulated endogenous glucose production improved in both groups (−64%; P < 0.01 in control subjects and −52% in diabetic subjects; P < 0.01). After training, metabolic flexibility in type 2 diabetic subjects was restored (delta respiratory exchange ratio 63% increase; P = 0.01) but was unchanged in control subjects (delta respiratory exchange ratio 7% increase; P = 0.22). Starting with comparable pretraining IMCL levels, training tended to increase IMCL content in type 2 diabetic subjects (27% increase; P = 0.10), especially in type 2 muscle fibers.
CONCLUSIONS
Exercise training restored in vivo mitochondrial function in type 2 diabetic subjects. Insulin-mediated glucose disposal and metabolic flexibility improved in type 2 diabetic subjects in the face of near–significantly increased IMCL content. This indicates that increased capacity to store IMCL and restoration of improved mitochondrial function contribute to improved muscle insulin sensitivity.
doi:10.2337/db09-1322
PMCID: PMC2828651  PMID: 20028948
5.  Prolonged Fasting Identifies Skeletal Muscle Mitochondrial Dysfunction as Consequence Rather Than Cause of Human Insulin Resistance 
Diabetes  2010;59(9):2117-2125.
OBJECTIVE
Type 2 diabetes and insulin resistance have been associated with mitochondrial dysfunction, but it is debated whether this is a primary factor in the pathogenesis of the disease. To test the concept that mitochondrial dysfunction is secondary to the development of insulin resistance, we employed the unique model of prolonged fasting in humans. Prolonged fasting is a physiologic condition in which muscular insulin resistance develops in the presence of increased free fatty acid (FFA) levels, increased fat oxidation and low glucose and insulin levels. It is therefore anticipated that skeletal muscle mitochondrial function is maintained to accommodate increased fat oxidation unless factors secondary to insulin resistance exert negative effects on mitochondrial function.
RESEARCH DESIGN AND METHODS
While in a respiration chamber, twelve healthy males were subjected to a 60 h fast and a 60 h normal fed condition in a randomized crossover design. Afterward, insulin sensitivity was assessed using a hyperinsulinemic-euglycemic clamp, and mitochondrial function was quantified ex vivo in permeabilized muscle fibers using high-resolution respirometry.
RESULTS
Indeed, FFA levels were increased approximately ninefold after 60 h of fasting in healthy male subjects, leading to elevated intramuscular lipid levels and decreased muscular insulin sensitivity. Despite an increase in whole-body fat oxidation, we observed an overall reduction in both coupled state 3 respiration and maximally uncoupled respiration in permeabilized skeletal muscle fibers, which could not be explained by changes in mitochondrial density.
CONCLUSIONS
These findings confirm that the insulin-resistant state has secondary negative effects on mitochondrial function. Given the low insulin and glucose levels after prolonged fasting, hyperglycemia and insulin action per se can be excluded as underlying mechanisms, pointing toward elevated plasma FFA and/or intramuscular fat accumulation as possible causes for the observed reduction in mitochondrial capacity.
doi:10.2337/db10-0519
PMCID: PMC2927932  PMID: 20573749
6.  Mitochondrial Dysfunction in Diabetes: From Molecular Mechanisms to Functional Significance and Therapeutic Opportunities 
Antioxidants & Redox Signaling  2010;12(4):537-577.
Abstract
Given their essential function in aerobic metabolism, mitochondria are intuitively of interest in regard to the pathophysiology of diabetes. Qualitative, quantitative, and functional perturbations in mitochondria have been identified and affect the cause and complications of diabetes. Moreover, as a consequence of fuel oxidation, mitochondria generate considerable reactive oxygen species (ROS). Evidence is accumulating that these radicals per se are important in the pathophysiology of diabetes and its complications. In this review, we first present basic concepts underlying mitochondrial physiology. We then address mitochondrial function and ROS as related to diabetes. We consider different forms of diabetes and address both insulin secretion and insulin sensitivity. We also address the role of mitochondrial uncoupling and coenzyme Q. Finally, we address the potential for targeting mitochondria in the therapy of diabetes. Antioxid. Redox Signal. 12, 537–577.
Introduction
Basic Physiology
Electron transport
Reactive oxygen species and mitochondria
Mitochondrial nitric oxide
Role of calcium and the mitochondrial permeability transition pore
Assessing Mitochondrial Function
Respiration and potential
ATP production and the proton leak
ROS production by isolated mitochondria
Site specificity of mitochondrial superoxide production
Mitochondrial ROS production in intact cells
Oxidative damage to mitochondria in intact cells
Mitochondrial Metabolism and Diabetes
General considerations
Mitochondrial diabetes
Type 1 and type 2 diabetes
Mitochondrial number and morphology
Fission/fusion
Mitochondrial biogenesis
Mitochondrial function in type 2 diabetes and insulin-resistant states
Is mitochondrial impairment a cause of insulin resistance?
Mitochondrial respiratory coupling and insulin release
Mitochondrial function in insulin-deficient diabetes
Diabetes and mitochondrial function in non–insulin-sensitive tissues
Mitochondria and cell-fuel selectivity
Diabetic cardiomyopathy and mitochondrial function
Summary
Mitochondrial ROS and Diabetes
ROS production and the cause of diabetes
Oxidative damage and pancreatic islet β cells
ROS and oxidative damage in insulin-sensitive target tissues
ROS and the complications of diabetes
Non–insulin-sensitive tissues (retina, renal, neural cells)
ROS and vascular cells
Summary
Mitochondrial Membrane Potential and Diabetes
Role of uncoupling proteins
Does membrane potential actually protect against superoxide production?
Summary
Coenzyme Q and Diabetes
Therapeutic Implications
Improving mitochondrial metabolism
Lifestyle modification
Pharmacologic intervention
Controlling ROS production and oxidative damage
Mitochondria-targeted antioxidants
Metabolic effects of mitochondria-targeted antioxidants
Mitochondria-targeted antioxidant peptides
Targeting superoxide
Summary
doi:10.1089/ars.2009.2531
PMCID: PMC2824521  PMID: 19650713
7.  Temporal increase of platelet mitochondrial respiration is negatively associated with clinical outcome in patients with sepsis 
Critical Care  2010;14(6):R214.
Introduction
Mitochondrial dysfunction has been suggested as a contributing factor to the pathogenesis of sepsis-induced multiple organ failure. Also, restoration of mitochondrial function, known as mitochondrial biogenesis, has been implicated as a key factor for the recovery of organ function in patients with sepsis. Here we investigated temporal changes in platelet mitochondrial respiratory function in patients with sepsis during the first week after disease onset.
Methods
Platelets were isolated from blood samples taken from 18 patients with severe sepsis or septic shock within 48 hours of their admission to the intensive care unit. Subsequent samples were taken on Day 3 to 4 and Day 6 to 7. Eighteen healthy blood donors served as controls. Platelet mitochondrial function was analyzed by high-resolution respirometry. Endogenous respiration of viable, intact platelets suspended in their own plasma or phosphate-buffered saline (PBS) glucose was determined. Further, in order to investigate the role of different dehydrogenases and respiratory complexes as well as to evaluate maximal respiratory activity of the mitochondria, platelets were permeabilized and stimulated with complex-specific substrates and inhibitors.
Results
Platelets suspended in their own septic plasma exhibited increased basal non-phosphorylating respiration (state 4) compared to controls and to platelets suspended in PBS glucose. In parallel, there was a substantial increase in respiratory capacity of the electron transfer system from Day 1 to 2 to Day 6 to 7 as well as compared to controls in both intact and permeabilized platelets oxidizing Complex I and/or II-linked substrates. No inhibition of respiratory complexes was detected in septic patients compared to controls. Non-survivors, at 90 days, had a more elevated respiratory capacity at Day 6 to 7 as compared to survivors. Cytochrome c increased over the time interval studied but no change in mitochondrial DNA was detected.
Conclusions
The results indicate the presence of a soluble plasma factor in the initial stage of sepsis inducing uncoupling of platelet mitochondria without inhibition of the electron transfer system. The mitochondrial uncoupling was paralleled by a gradual and substantial increase in respiratory capacity. This may reflect a compensatory response to severe sepsis or septic shock, that was most pronounced in non-survivors, likely correlating to the severity of the septic insult.
doi:10.1186/cc9337
PMCID: PMC3219983  PMID: 21106065
8.  High Fat Diet-Induced Changes in Mouse Muscle Mitochondrial Phospholipids Do Not Impair Mitochondrial Respiration Despite Insulin Resistance 
PLoS ONE  2011;6(11):e27274.
Background
Type 2 diabetes mellitus and muscle insulin resistance have been associated with reduced capacity of skeletal muscle mitochondria, possibly as a result of increased intake of dietary fat. Here, we examined the hypothesis that a prolonged high-fat diet consumption (HFD) increases the saturation of muscle mitochondrial membrane phospholipids causing impaired mitochondrial oxidative capacity and possibly insulin resistance.
Methodology
C57BL/6J mice were fed an 8-week or 20-week low fat diet (10 kcal%; LFD) or HFD (45 kcal%). Skeletal muscle mitochondria were isolated and fatty acid (FA) composition of skeletal muscle mitochondrial phospholipids was analyzed by thin-layer chromatography followed by GC. High-resolution respirometry was used to assess oxidation of pyruvate and fatty acids by mitochondria. Insulin sensitivity was estimated by HOMA-IR.
Principal Findings
At 8 weeks, mono-unsaturated FA (16∶1n7, 18∶1n7 and 18∶1n9) were decreased (−4.0%, p<0.001), whereas saturated FA (16∶0) were increased (+3.2%, p<0.001) in phospholipids of HFD vs. LFD mitochondria. Interestingly, 20 weeks of HFD descreased mono-unsaturated FA while n-6 poly-unsaturated FA (18∶2n6, 20∶4n6, 22∶5n6) showed a pronounced increase (+4.0%, p<0.001). Despite increased saturation of muscle mitochondrial phospholipids after the 8-week HFD, mitochondrial oxidation of both pyruvate and fatty acids were similar between LFD and HFD mice. After 20 weeks of HFD, the increase in n-6 poly-unsaturated FA was accompanied by enhanced maximal capacity of the electron transport chain (+49%, p = 0.002) and a tendency for increased ADP-stimulated respiration, but only when fuelled by a lipid-derived substrate. Insulin sensitivity in HFD mice was reduced at both 8 and 20 weeks.
Conclusions/Interpretation
Our findings do not support the concept that prolonged HF feeding leads to increased saturation of skeletal muscle mitochondrial phospholipids resulting in a decrease in mitochondrial fat oxidative capacity and (muscle) insulin resistance.
doi:10.1371/journal.pone.0027274
PMCID: PMC3225362  PMID: 22140436
9.  Relationships between Mitochondrial Function and Metabolic Flexibility in Type 2 Diabetes Mellitus 
PLoS ONE  2013;8(2):e51648.
Introduction
Mitochondrial dysfunction, lipid accumulation, insulin resistance and metabolic inflexibility have been implicated in the etiology of type 2 diabetes (T2D), yet their interrelationship remains speculative. We investigated these interrelationships in a group of T2D and obese normoglycemic control subjects.
Methods
49 non-insulin dependent male T2D patients and 54 male control subjects were enrolled, and a hyperinsulinemic-euglycemic clamp and indirect calorimetry were performed. A muscle biopsy was taken and intramyocellular lipid (IMCL) was measured. In vivo mitochondrial function was measured by PCr recovery in 30 T2D patients and 31 control subjects.
Results
Fasting NEFA levels were significantly elevated in T2D patients compared with controls, but IMCL was not different. Mitochondrial function in T2D patients was compromised by 12.5% (p<0.01). Whole body glucose disposal (WGD) was higher at baseline and lower after insulin stimulation. Metabolic flexibility (ΔRER) was lower in the type 2 diabetic patients (0.050±0.033 vs. 0.093±0.050, p<0.01). Mitochondrial function was the sole predictor of basal respiratory exchange ratio (RER) (R2 = 0.18, p<0.05); whereas WGD predicted both insulin-stimulated RER (R2 = 0.29, p<0.001) and metabolic flexibility (R2 = 0.40, p<0.001).
Conclusions
These results indicate that defects in skeletal muscle in vivo mitochondrial function in type 2 diabetic patients are only reflected in basal substrate oxidation and highlight the importance of glucose disposal rate as a determinant of substrate utilization in response to insulin.
doi:10.1371/journal.pone.0051648
PMCID: PMC3572106  PMID: 23418416
10.  Chronic Antidiabetic Sulfonylureas In Vivo: Reversible Effects on Mouse Pancreatic β-Cells 
PLoS Medicine  2008;5(10):e206.
Background
Pancreatic β-cell ATP-sensitive potassium (KATP) channels are critical links between nutrient metabolism and insulin secretion. In humans, reduced or absent β-cell KATP channel activity resulting from loss-of-function KATP mutations induces insulin hypersecretion. Mice with reduced KATP channel activity also demonstrate hyperinsulinism, but mice with complete loss of KATP channels (KATP knockout mice) show an unexpected insulin undersecretory phenotype. Therefore we have proposed an “inverse U” hypothesis to explain the response to enhanced excitability, in which excessive hyperexcitability drives β-cells to insulin secretory failure without cell death. Many patients with type 2 diabetes treated with antidiabetic sulfonylureas (which inhibit KATP activity and thereby enhance insulin secretion) show long-term insulin secretory failure, which we further suggest might reflect a similar progression.
Methods and Findings
To test the above hypotheses, and to mechanistically investigate the consequences of prolonged hyperexcitability in vivo, we used a novel approach of implanting mice with slow-release sulfonylurea (glibenclamide) pellets, to chronically inhibit β-cell KATP channels. Glibenclamide-implanted wild-type mice became progressively and consistently diabetic, with significantly (p < 0.05) reduced insulin secretion in response to glucose. After 1 wk of treatment, these mice were as glucose intolerant as adult KATP knockout mice, and reduction of secretory capacity in freshly isolated islets from implanted animals was as significant (p < 0.05) as those from KATP knockout animals. However, secretory capacity was fully restored in islets from sulfonylurea-treated mice within hours of drug washout and in vivo within 1 mo after glibenclamide treatment was terminated. Pancreatic immunostaining showed normal islet size and α-/β-cell distribution within the islet, and TUNEL staining showed no evidence of apoptosis.
Conclusions
These results demonstrate that chronic glibenclamide treatment in vivo causes loss of insulin secretory capacity due to β-cell hyperexcitability, but also reveal rapid reversibility of this secretory failure, arguing against β-cell apoptosis or other cell death induced by sulfonylureas. These in vivo studies may help to explain why patients with type 2 diabetes can show long-term secondary failure to secrete insulin in response to sulfonylureas, but experience restoration of insulin secretion after a drug resting period, without permanent damage to β-cells. This finding suggests that novel treatment regimens may succeed in prolonging pharmacological therapies in susceptible individuals.
In a mouse study aiming to understand why long-term treatment for type 2 diabetes with sulfonylureas eventually fails, Colin Nichols and Maria Remedi suggest that slow restoration of insulin secretion may be possible after a drug-resting period.
Editors' Summary
Background.
Diabetes is an increasingly common chronic disease characterized by high blood sugar (glucose) levels. In normal people, blood sugar levels are controlled by the hormone insulin. Insulin is released by β-cells in the pancreas when blood glucose levels rise after eating (glucose is produced by the digestion of food). In fasting people, membrane proteins called ATP-sensitive potassium (KATP) channels keep the β-cell in a “hyperpolarized” state in which they do not secrete insulin. After a meal, glucose enters the β-cell where its chemical breakdown converts ADP into ATP (the molecule that provides the energy that drives cellular processes). The increased ratio of ATP to ADP closes the KATP channels, “depolarizes” the β-cells, and allows the entry of calcium ions, which trigger insulin release. The released insulin then “instructs” insulin-responsive muscle and fat cells to take up glucose from the bloodstream. In type 2 diabetes, the commonest type of diabetes, the muscle and fat cells gradually become nonresponsive to insulin and consequently blood glucose levels rise. Over time, this hyperglycemia increases the risk of heart attacks, kidney failure, and other life-threatening complications. On average, people with diabetes die 5–10 y younger than people without diabetes.
Why Was This Study Done?
People with type 2 diabetes are often initially treated with drugs called sulfonylureas (for example, glibenclamide). Sulfonylureas help to reduce blood glucose levels by inhibiting (in effect, closing) the KATP channels, which enhances insulin secretion. Unfortunately, after patients have been treated for several years with sulfonylureas, their β-cells often stop secreting insulin and the patients then have to inject insulin to control their blood sugar levels. The mechanism by which chronic sulfonylurea treatment affects β-cell behavior is poorly understood, which means that it is hard to improve this antidiabetes treatment. Mice that have been genetically altered so that they have no KATP channels (KATP knockout mice) also rapidly lose their ability to secrete insulin, although they secrete unusually large amounts at birth. This suggests that permanent membrane depolarization (β-cell hyperexcitability) may cause insulin secretory failure. In this study, the researchers investigate whether this mechanism might be responsible for sulfonylurea-induced loss of insulin secretion.
What Did the Researchers Do and Find?
The researchers implanted slowly releasing pellets of glibenclamide into wild-type mice and then monitored their blood glucose levels and glucose tolerance (the speed of glucose removal from the blood after a glucose “meal”) for up to 128 d; the pellets released drug for 90 d. The glibenclamide-implanted mice progressively developed diabetes, lost the ability to secrete insulin in response to glucose and, after 1 wk of treatment, were as glucose intolerant as adult KATP knockout mice. Compared to freshly isolated β-cells from untreated wild-type mice, glucose-stimulated insulin secretion by β-cells isolated from glibenclamide-treated wild-type mice and from KATP knockout mice was reduced to a similar degree. However, the secretory capacity of β-cells isolated from the glibenclamide-treated wild-type mice was restored to normal within hours of drug washout and was normal in β-cells isolated from treated mice 1 mo after exhaustion of the slow-release pellets. Consistent with this result, there was no obvious β-cell death in the glibenclamide-treated mice.
What Do These Findings Mean?
Although findings from animal studies do not always reflect what happens in people, these findings suggest that insulin secretion might sometimes fail in people who take sulfonylureas for a long time, because these drugs cause β-cell hyperexcitability. The finding that the secretory failure caused by sulfonylurea treatment is reversible is important because it suggests that short-acting sulfonylureas might be re-evaluated to see whether they could delay sulfonylurea-induced failure of the insulin secretory response by providing the pancreatic β-cells with periods when they are not depolarized. This finding (and the absence of β-cells death in the glibenclamide-treated mice) also suggests that there may be a way to reverse the loss of the insulin secretory response in patients who have taken sulfonylureas for a long time. Both approaches could help patients with diabetes delay or even avoid the need for insulin injections.
Additional Information.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0050206.
This study is further discussed in a PLoS Medicine Perspective by Renstrom and colleagues
The MedlinePlus encyclopedia provides information for patients about diabetes (in English and Spanish)
The US National Diabetes Information Clearinghouse provides information on all aspects of diabetes (in English and Spanish)
The International Diabetes Federation also provides comprehensive information about diabetes
Wikipedia has pages on KATP channels and on sulfonylurea drugs (note: Wikipedia is a free online encyclopedia that anyone can edit; available in several languages)
doi:10.1371/journal.pmed.0050206
PMCID: PMC2573909  PMID: 18959471
11.  Hepatic mitochondrial function in ketogenic states. Diabetes, starvation, and after growth hormone administration. 
Journal of Clinical Investigation  1975;55(6):1237-1244.
The study was designed to evaluate hepatic mitochondrial function during ketotic states. The ketogenic models studied were streptozotocin-induced diabetic ketoacidosis, 48 h of starvation, and after growth hormone administration. In the last-mentioned model we observed increased free fatty acids but not ketonemia. Oxidative phosphorylation was measured using the citric acid cycle substrates pyruvate and succinate, the amino acid glutamate, a ketone body beta-hydroxybutyrate, and a long-chain fatty acid palmitoyl-l-carnitine. State 3 (ADP stimulated) and state 4 (ADP limited) respiration, respiratory control ratio (state 3/state 4), and the ADP/O ratios were normal in the controls and the experimental groups. Uncoupled respiration produced by dinitrophenol with a variety of substrates was unchanged in the experimental groups compared to the controls. Fatty acid oxidation was studied in detail. The rate of utilization of palmitoyl-l-carnitine by controls or experimental groups did not depend on the product formed (citrate, acetoacetate). No significant changes were observed in the oxidation of palmitoyl-CoA (+ carnitine) or with an intermediate-chain fatty acid hexanoate. The specific activity of hepatic mitochondria carnitine palmitoyltransferase did not change in any of the three experimental groups. It is concluded that during diabetic ketoacidosis, starvation, and growth hormone administration, there is (a) no alteration in hepatic mitochondrial function; (b) no change in the intrinsic capacity of hepatic mitochondria to oxidize fatty acids; and (c) no change in the specific activity of mitochondrial carnitine palmitoyltransferase. The mechanism by which the body restrains flux through the mitochondrial oxidative machinery remains to be fully determined.
PMCID: PMC301878  PMID: 124319
12.  Short-Term Exercise Training Does Not Stimulate Skeletal Muscle ATP Synthesis in Relatives of Humans With Type 2 Diabetes 
Diabetes  2009;58(6):1333-1341.
OBJECTIVE
We tested the hypothesis that short-term exercise training improves hereditary insulin resistance by stimulating ATP synthesis and investigated associations with gene polymorphisms.
RESEARCH DESIGN AND METHODS
We studied 24 nonobese first-degree relatives of type 2 diabetic patients and 12 control subjects at rest and 48 h after three bouts of exercise. In addition to measurements of oxygen uptake and insulin sensitivity (oral glucose tolerance test), ectopic lipids and mitochondrial ATP synthesis were assessed using1H and31P magnetic resonance spectroscopy, respectively. They were genotyped for polymorphisms in genes regulating mitochondrial function, PPARGC1A (rs8192678) and NDUFB6 (rs540467).
RESULTS
Relatives had slightly lower (P = 0.012) insulin sensitivity than control subjects. In control subjects, ATP synthase flux rose by 18% (P = 0.0001), being 23% higher (P = 0.002) than that in relatives after exercise training. Relatives responding to exercise training with increased ATP synthesis (+19%, P = 0.009) showed improved insulin sensitivity (P = 0.009) compared with those whose insulin sensitivity did not improve. A polymorphism in the NDUFB6 gene from respiratory chain complex I related to ATP synthesis (P = 0.02) and insulin sensitivity response to exercise training (P = 0.05). ATP synthase flux correlated with O2uptake and insulin sensitivity.
CONCLUSIONS
The ability of short-term exercise to stimulate ATP production distinguished individuals with improved insulin sensitivity from those whose insulin sensitivity did not improve. In addition, the NDUFB6 gene polymorphism appeared to modulate this adaptation. This finding suggests that genes involved in mitochondrial function contribute to the response of ATP synthesis to exercise training.
doi:10.2337/db08-1240
PMCID: PMC2682667  PMID: 19265027
13.  Cardiac contractile function and mitochondrial respiration in diabetes-related mouse models 
Cardiovascular Diabetology  2014;13(1):118.
Background
Pathophysiological processes underlying diabetic-related cardiomyopathies are complex. Mitochondria dysfunction is often described as a cause of cardiac impairment but its extent may depend on the type of experimental diabetes. Here we proposed to compare drug- or diet-induced models of diabetes in terms of metabolic features, cardiac and mitochondrial functions.
Methods
Mice were fed with regular chow or fat-enriched diet. After three weeks, they received either citrate or streptozotocin injections for five consecutive days. Metabolic parameters, myocardial contractile function and mitochondrial respiration were measured after three more weeks. Fat mass volumes were assessed by magnetic resonance imaging. Oral glucose tolerance test, insulin tolerance test, triglyceride and adipocytokine quantification were evaluated to establish metabolic profiles. Cardiac function was assessed ex vivo onto a Langendorff column. Isolated cardiac mitochondria respiration was obtained using high-resolution oxygraphy.
Results
Mice fed with the fat-enriched regimen presented abdominal obesity, increased blood glucose, elevated leptin level, glucose intolerance, and insulin resistance. Mice treated with streptozotocin, independently of the regimen, lost their capacity to release insulin in response to glucose ingestion. Mice fed with regular chow diet and injected with streptozotocin developed cardiac dysfunction without mitochondrial respiration defect. However, both groups of high-fat diet fed mice developed cardiac alterations associated with reduction in mitochondrial oxygen consumption, despite an increase in mitochondrial biogenesis signalling.
Conclusions
We explored three animal models mimicking type 1 and 2 diabetes. While cardiac dysfunction was present in the three groups of mice, mitochondrial respiration impairment was only obvious in models reproducing features of type 2 diabetes.
doi:10.1186/s12933-014-0118-7
PMCID: PMC4243842  PMID: 25142225
Metabolic syndrome; Diabetes; Heart; Mitochondria; Respiration; Streptozotocin; High-fat diet
14.  Metformin Impairs Mitochondrial Function in Skeletal Muscle of Both Lean and Diabetic Rats in a Dose-Dependent Manner 
PLoS ONE  2014;9(6):e100525.
Metformin is a widely prescribed drug for the treatment of type 2 diabetes. Previous studies have demonstrated in vitro that metformin specifically inhibits Complex I of the mitochondrial respiratory chain. This seems contraindicative since muscle mitochondrial dysfunction has been linked to the pathogenesis of type 2 diabetes. However, its significance for in vivo skeletal muscle mitochondrial function has yet to be elucidated. The aim of this study was to assess the effects of metformin on in vivo and ex vivo skeletal muscle mitochondrial function in a rat model of diabetes. Healthy (fa/+) and diabetic (fa/fa) Zucker diabetic fatty rats were treated by oral gavage with metformin dissolved in water (30, 100 or 300 mg/kg bodyweight/day) or water as a control for 2 weeks. After 2 weeks of treatment, muscle oxidative capacity was assessed in vivo using 31P magnetic resonance spectroscopy and ex vivo by measuring oxygen consumption in isolated mitochondria using high-resolution respirometry. Two weeks of treatment with metformin impaired in vivo muscle oxidative capacity in a dose-dependent manner, both in healthy and diabetic rats. Whereas a dosage of 30 mg/kg/day had no significant effect, in vivo oxidative capacity was 21% and 48% lower after metformin treatment at 100 and 300 mg/kg/day, respectively, independent of genotype. High-resolution respirometry measurements demonstrated a similar dose-dependent effect of metformin on ex vivo mitochondrial function. In conclusion, metformin compromises in vivo and ex vivo muscle oxidative capacity in Zucker diabetic fatty rats in a dose-dependent manner.
doi:10.1371/journal.pone.0100525
PMCID: PMC4065055  PMID: 24950069
15.  Increased platelet storage time is associated with mitochondrial dysfunction and impaired platelet function 
The Journal of surgical research  2013;184(1):10.1016/j.jss.2013.05.097.
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. FACS was performed to analyze mitochondrial content, mitochondrial reactive oxygen species (ROS), the expression of P-selectin (both before and after challenge with thrombin receptor activating peptide (SFLLRN)), and apoptosis. Data was analyzed using ANOVA and Pearson’s correlation (p<0.05 significant).
Results
Mitochondrial respiration decreased significantly in platelets stored longer than 2 days (p<0.05). Platelets also demonstrated a persistent decrease in response to stimulation with SFLLRN by the 3rd day of storage (p<0.05) as well as an increase in mitochondrial ROS and apoptosis (p<0.05). Mitochondrial respiration significantly correlated with platelet capacity to activate (r=0.8, p<0.05).
Conclusion
Platelet mitochondrial respiratory function and activation response decrease significantly in platelets stored for 3 or more days. Because platelet transfusions almost universally occur between the 3rd and 5th day of storage, our findings may have significant clinical importance and warrant further in vivo analysis.
doi:10.1016/j.jss.2013.05.097
PMCID: PMC3879371  PMID: 23830370
16.  Substrate-Specific Derangements in Mitochondrial Metabolism and Redox Balance in Atrium of Type 2 Diabetic Human Heart 
Objective
This aim of this study was to determine the impact of diabetes on oxidant balance and mitochondrial metabolism of carbohydrate- and lipid-based substrates in myocardium of type 2 diabetic patients.
Background
Heart failure represents a major cause of death among diabetics, and it has been proposed that derangements in cardiac metabolism and oxidative stress may underlie the progression of this co-morbidity, but scarce evidence exists in support of this mechanism in humans.
Methods
Mitochondrial O2 consumption and H2O2 emission were measured in permeabilized myofibers prepared from samples of right atrial appendage obtained from non-diabetic (n=13) and diabetic (n=11) patients undergoing non-emergent coronary artery bypass graft surgery.
Results
Mitochondria in atrial tissue of type 2 diabetic individuals display a sharply decreased capacity for glutamate and fatty acid-supported respiration, in addition to an increased content of myocardial triglycerides, as compared to non-diabetics. Furthermore, diabetics display an increased mitochondrial H2O2 emission during oxidation of carbohydrate- and lipid-based substrates, depleted glutathione, and evidence of persistent oxidative stress in their atrial tissue.
Conclusions
These findings are the first to directly investigate the effects of type 2 diabetes on a panoply of mitochondrial functions in the human myocardium using cellular and molecular approaches, and they demonstrate that mitochondria in diabetic human heart have specific impairments in maximal capacity to oxidize fatty acids and glutamate, yet increased mitochondrial H2O2 emission, providing insight into the role of mitochondrial dysfunction and oxidative stress in the pathogenesis of heart failure in diabetic patients.
doi:10.1016/j.jacc.2009.07.031
PMCID: PMC2800130  PMID: 19892241
human heart; mitochondria; diabetes mellitus; lipids; oxidative stress
17.  A high fat diet increases mitochondrial fatty acid oxidation and uncoupling to decrease efficiency in rat heart 
Basic Research in Cardiology  2011;106(3):447-457.
Elevated levels of cardiac mitochondrial uncoupling protein 3 (UCP3) and decreased cardiac efficiency (hydraulic power/oxygen consumption) with abnormal cardiac function occur in obese, diabetic mice. To determine whether cardiac mitochondrial uncoupling occurs in non-genetic obesity, we fed rats a high fat diet (55% kcal from fat) or standard laboratory chow (7% kcal from fat) for 3 weeks, after which we measured cardiac function in vivo using cine MRI, efficiency in isolated working hearts and respiration rates and ADP/O ratios in isolated interfibrillar mitochondria; also, measured were medium chain acyl-CoA dehydrogenase (MCAD) and citrate synthase activities plus uncoupling protein 3 (UCP3), mitochondrial thioesterase 1 (MTE-1), adenine nucleotide translocase (ANT) and ATP synthase protein levels. We found that in vivo cardiac function was the same for all rats, yet oxygen consumption was 19% higher in high fat-fed rat hearts, therefore, efficiency was 21% lower than in controls. We found that mitochondrial fatty acid oxidation rates were 25% higher, and MCAD activity was 23% higher, in hearts from rats fed the high fat diet when compared with controls. Mitochondria from high fat-fed rat hearts had lower ADP/O ratios than controls, indicating increased respiratory uncoupling, which was ameliorated by GDP, a UCP3 inhibitor. Mitochondrial UCP3 and MTE-1 levels were both increased by 20% in high fat-fed rat hearts when compared with controls, with no significant change in ATP synthase or ANT levels, or citrate synthase activity. We conclude that increased cardiac oxygen utilisation, and thereby decreased cardiac efficiency, occurs in non-genetic obesity, which is associated with increased mitochondrial uncoupling due to elevated UCP3 and MTE-1 levels.
doi:10.1007/s00395-011-0156-1
PMCID: PMC3071466  PMID: 21318295
Cardiac efficiency; Oxygen consumption; Free fatty acids; Uncoupling; High fat diet; Mitochondria
18.  Mitochondrial reserve capacity in endothelial cells: the impact of nitric oxide and reactive oxygen species 
Free radical biology & medicine  2010;48(7):905-914.
The endothelium is not considered to be a major energy requiring organ, but nevertheless endothelial cells have an extensive mitochondrial network. This suggests that mitochondrial function may be important in response to stress and signaling in these cells. In this study, we used extracellular flux analysis to measure mitochondrial function in adherent bovine aortic endothelial cells (BAEC). Under basal conditions, BAEC use only ~35% of their maximal respiratory capacity. We calculate that this represents an intermediate respiratory State between States 3 and 4 which we define as Stateapparent equal to 3.64. Interestingly, the apparent respiratory control ratio (maximal mitochondrial oxygen consumption/non-ADP linked respiration) in these cells is on the order of 23 which is substantially higher than that which is frequently obtained with isolated mitochondria. These results suggest that mitochondria in endothelial cells are highly coupled and possess a considerable bioenergetic reserve. Since endothelial cells are exposed to both reactive oxygen and nitrogen species (ROS/RNS) in the course of vascular disease, we hypothesized that this reserve capacity is important in responding to oxidative stress. To test this, we exposed BAEC to NO or ROS alone or in combination. We found that exposure to non-toxic concentrations of NO or low levels of hydrogen peroxide generated from 2,3-dimethoxy-1,4-napthoquinone (DMNQ) had little impact on basal mitochondrial function but both treatments reversibly decreased mitochondrial reserve capacity. However, combined NO and DMNQ treatment resulted in an irreversible loss of reserve capacity and was associated with cell death. These data are consistent with a critical role of mitochondrial reserve capacity in endothelial cells in responding to oxidative stress.
doi:10.1016/j.freeradbiomed.2010.01.015
PMCID: PMC2860730  PMID: 20093177
Atherosclerosis; extracellular flux; reactive nitrogen species; oxidative stress; respiration
19.  Simvastatin impairs ADP-stimulated respiration and increases mitochondrial oxidative stress in primary human skeletal myotubes 
Free radical biology & medicine  2011;52(1):198-207.
Statins, the widely prescribed cholesterol-lowering drugs for the treatment of cardiovascular disease, cause adverse skeletal muscle side effects ranging from fatigue to fatal rhabdomyolysis. The purpose of this study was to determine the effects of simvastatin on mitochondrial respiration, oxidative stress, and cell death in differentiated primary human skeletal muscle cells (i.e. myotubes). Simvastatin induced a dose dependent decrease in viability of proliferating and differentiating primary human muscle precursor cells, and a similar dose-dependent effect was noted in differentiated myoblasts and myotubes. Additionally, there were decreases in myotube number and size following 48 h of simvastatin treatment (5 µM). In permeabilized myotubes, maximal ADP-stimulated oxygen consumption, supported by palmitoyl-carnitine + malate (PCM, complex I and II substrates) and glutamate + malate (GM, complex I substrates), was 32–37% lower (P<0.05) in simvastatin treated (5 µM) vs. control myotubes, providing evidence of impaired respiration at complex I. Mitochondrial superoxide and hydrogen peroxide generation were significantly greater in the simvastatin treated human skeletal myotube cultures compared to control. In addition, simvastatin markedly increased protein levels of Bax (pro-apoptotic, +53%) and Bcl-2 (anti-apoptotic, +100%, P<0.05), mitochondrial PTP opening (+44%, P<0.05), and TUNEL-positive nuclei in human skeletal myotubes, demonstrating up-regulation of mitochondrial-mediated myonuclear apoptotic mechanisms. These data demonstrate that simvastatin induces myotube atrophy and cell loss associated with impaired ADP-stimulated maximal mitochondrial respiratory capacity, mitochondrial oxidative stress, and apoptosis in primary human skeletal myotubes, suggesting mitochondrial dysfunction may underlie human statin-induced myopathy.
doi:10.1016/j.freeradbiomed.2011.10.449
PMCID: PMC3313473  PMID: 22080086
simvastatin; mitochondrial respiration; superoxide; hydrogen peroxide; oxidative stress; apoptosis; mitochondria; human skeletal muscle
20.  Oxidative Stress Induces Mitochondrial Dysfunction in a Subset of Autism Lymphoblastoid Cell Lines in a Well-Matched Case Control Cohort 
PLoS ONE  2014;9(1):e85436.
There is increasing recognition that mitochondrial dysfunction is associated with the autism spectrum disorders. However, little attention has been given to the etiology of mitochondrial dysfunction or how mitochondrial abnormalities might interact with other physiological disturbances associated with autism, such as oxidative stress. In the current study we used respirometry to examine reserve capacity, a measure of the mitochondrial ability to respond to physiological stress, in lymphoblastoid cell lines (LCLs) derived from children with autistic disorder (AD) as well as age and gender-matched control LCLs. We demonstrate, for the first time, that LCLs derived from children with AD have an abnormal mitochondrial reserve capacity before and after exposure to increasingly higher concentrations of 2,3-dimethoxy-1,4-napthoquinone (DMNQ), an agent that increases intracellular reactive oxygen species (ROS). Specifically, the AD LCLs exhibit a higher reserve capacity at baseline and a sharper depletion of reserve capacity when ROS exposure is increased, as compared to control LCLs. Detailed investigation indicated that reserve capacity abnormalities seen in AD LCLs were the result of higher ATP-linked respiration and maximal respiratory capacity at baseline combined with a marked increase in proton leak respiration as ROS was increased. We further demonstrate that these reserve capacity abnormalities are driven by a subgroup of eight (32%) of 25 AD LCLs. Additional investigation of this subgroup of AD LCLs with reserve capacity abnormalities revealed that it demonstrated a greater reliance on glycolysis and on uncoupling protein 2 to regulate oxidative stress at the inner mitochondria membrane. This study suggests that a significant subgroup of AD children may have alterations in mitochondrial function which could render them more vulnerable to a pro-oxidant microenvironment derived from intrinsic and extrinsic sources of ROS such as immune activation and pro-oxidant environmental toxicants. These findings are consistent with the notion that AD is caused by a combination of genetic and environmental factors.
doi:10.1371/journal.pone.0085436
PMCID: PMC3885720  PMID: 24416410
21.  Adaptation of Microplate-based Respirometry for Hippocampal Slices and Analysis of Respiratory Capacity 
Journal of neuroscience research  2011;89(12):1979-1988.
Multiple neurodegenerative disorders are associated with altered mitochondrial bioenergetics. Although mitochondrial O2 consumption is frequently measured in isolated mitochondria, isolated synaptic nerve terminals (synaptosomes), or cultured cells, the absence of mature brain circuitry is a remaining limitation. Here we describe the development of a method that adapts the Seahorse Extracellular Flux Analyzer (XF24) for the microplate-based measurement of hippocampal slice O2 consumption. As a first evaluation of the technique, we compared whole slice bioenergetics to previous measurements made with synaptosomes or cultured neurons. We found that mitochondrial respiratory capacity and O2 consumption coupled to ATP synthesis could be estimated in cultured or acute hippocampal slices with preserved neural architecture. Mouse organotypic hippocampal slices oxidizing glucose displayed mitochondrial O2 consumption that was well-coupled, as determined by the sensitivity to the ATP synthase inhibitor oligomycin. However stimulation of respiration by uncoupler was modest (<120% of basal respiration) compared to previous measurements in cells or synaptosomes, although enhanced slightly (to ~150% of basal respiration) by the acute addition of the mitochondrial complex I-linked substrate pyruvate. These findings suggest a high basal utilization of respiratory capacity in slices and a limitation of glucose-derived substrate for maximal respiration. The improved throughput of microplate-based hippocampal respirometry over traditional O2 electrode-based methods is conducive to neuroprotective drug screening. When coupled with cell type-specific pharmacology or genetic manipulations, the ability to efficiently measure O2 consumption from whole slices should advance our understanding of mitochondrial roles in physiology and neuropathology.
doi:10.1002/jnr.22650
PMCID: PMC3144988  PMID: 21520220
mitochondria; bioenergetics; oxygen; respiration; pyruvate
22.  Desmin Cytoskeleton Linked to Muscle Mitochondrial Distribution and Respiratory Function 
The Journal of Cell Biology  2000;150(6):1283-1298.
Ultrastructural studies have previously suggested potential association of intermediate filaments (IFs) with mitochondria. Thus, we have investigated mitochondrial distribution and function in muscle lacking the IF protein desmin. Immunostaining of skeletal muscle tissue sections, as well as histochemical staining for the mitochondrial marker enzymes cytochrome C oxidase and succinate dehydrogenase, demonstrate abnormal accumulation of subsarcolemmal clumps of mitochondria in predominantly slow twitch skeletal muscle of desmin-null mice. Ultrastructural observation of desmin-null cardiac muscle demonstrates in addition to clumping, extensive mitochondrial proliferation in a significant fraction of the myocytes, particularly after work overload. These alterations are frequently associated with swelling and degeneration of the mitochondrial matrix. Mitochondrial abnormalities can be detected very early, before other structural defects become obvious. To investigate related changes in mitochondrial function, we have analyzed ADP-stimulated respiration of isolated muscle mitochondria, and ADP-stimulated mitochondrial respiration in situ using saponin skinned muscle fibers. The in vitro maximal rates of respiration in isolated cardiac mitochondria from desmin-null and wild-type mice were similar. However, mitochondrial respiration in situ is significantly altered in desmin-null muscle. Both the maximal rate of ADP-stimulated oxygen consumption and the dissociation constant (Km) for ADP are significantly reduced in desmin-null cardiac and soleus muscle compared with controls. Respiratory parameters for desmin-null fast twitch gastrocnemius muscle were unaffected. Additionally, respiratory measurements in the presence of creatine indicate that coupling of creatine kinase and the adenine translocator is lost in desmin-null soleus muscle. This coupling is unaffected in cardiac muscle from desmin-null animals. All of these studies indicate that desmin IFs play a significant role in mitochondrial positioning and respiratory function in cardiac and skeletal muscle.
PMCID: PMC2150713  PMID: 10995435
intermediate filaments; mitochondrial proliferation; cardiomyopathy; respiration; desmin
23.  Two Weeks of Metformin Treatment Enhances Mitochondrial Respiration in Skeletal Muscle of AMPK Kinase Dead but Not Wild Type Mice 
PLoS ONE  2013;8(1):e53533.
Metformin is used as an anti-diabetic drug. Metformin ameliorates insulin resistance by improving insulin sensitivity in liver and skeletal muscle. Reduced mitochondrial content has been reported in type 2 diabetic muscles and it may contribute to decreased insulin sensitivity characteristic for diabetic muscles. The molecular mechanism behind the effect of metformin is not fully clarified but inhibition of complex I in the mitochondria and also activation of the 5′AMP activated protein kinase (AMPK) has been reported in muscle. Furthermore, both AMPK activation and metformin treatment have been associated with stimulation of mitochondrial function and biogenesis. However, a causal relationship in skeletal muscle has not been investigated. We hypothesized that potential effects of in vivo metformin treatment on mitochondrial function and protein expressions in skeletal muscle are dependent upon AMPK signaling. We investigated this by two weeks of oral metformin treatment of muscle specific kinase dead α2 (KD) AMPK mice and wild type (WT) littermates. We measured mitochondrial respiration and protein activity and expressions of key enzymes involved in mitochondrial carbohydrate and fat metabolism and oxidative phosphorylation. Mitochondrial respiration, HAD and CS activity, PDH and complex I-V and cytochrome c protein expression were all reduced in AMPK KD compared to WT tibialis anterior muscles. Surprisingly, metformin treatment only enhanced respiration in AMPK KD mice and thereby rescued the respiration defect compared to the WT mice. Metformin did not influence protein activities or expressions in either WT or AMPK KD mice.
We conclude that two weeks of in vivo metformin treatment enhances mitochondrial respiration in the mitochondrial deficient AMPK KD but not WT mice. The improvement seems to be unrelated to AMPK, and does not involve changes in key mitochondrial proteins.
doi:10.1371/journal.pone.0053533
PMCID: PMC3544921  PMID: 23341947
24.  Skeletal Muscle Mitochondrial Energetics Are Associated With Maximal Aerobic Capacity and Walking Speed in Older Adults 
Background.
Lower ambulatory performance with aging may be related to a reduced oxidative capacity within skeletal muscle. This study examined the associations between skeletal muscle mitochondrial capacity and efficiency with walking performance in a group of older adults.
Methods.
Thirty-seven older adults (mean age 78 years; 21 men and 16 women) completed an aerobic capacity (VO2 peak) test and measurement of preferred walking speed over 400 m. Maximal coupled (State 3; St3) mitochondrial respiration was determined by high-resolution respirometry in saponin-permeabilized myofibers obtained from percutanous biopsies of vastus lateralis (n = 22). Maximal phosphorylation capacity (ATPmax) of vastus lateralis was determined in vivo by 31P magnetic resonance spectroscopy (n = 30). Quadriceps contractile volume was determined by magnetic resonance imaging. Mitochondrial efficiency (max ATP production/max O2 consumption) was characterized using ATPmax per St3 respiration (ATPmax/St3).
Results.
In vitro St3 respiration was significantly correlated with in vivo ATPmax (r 2 = .47, p = .004). Total oxidative capacity of the quadriceps (St3*quadriceps contractile volume) was a determinant of VO2 peak (r 2 = .33, p = .006). ATPmax (r 2 = .158, p = .03) and VO2 peak (r 2 = .475, p < .0001) were correlated with preferred walking speed. Inclusion of both ATPmax/St3 and VO2 peak in a multiple linear regression model improved the prediction of preferred walking speed (r 2 = .647, p < .0001), suggesting that mitochondrial efficiency is an important determinant for preferred walking speed.
Conclusions.
Lower mitochondrial capacity and efficiency were both associated with slower walking speed within a group of older participants with a wide range of function. In addition to aerobic capacity, lower mitochondrial capacity and efficiency likely play roles in slowing gait speed with age.
doi:10.1093/gerona/gls196
PMCID: PMC3593613  PMID: 23051977
Muscle; Mitochondria; Aging; Walking speed.
25.  Glucocorticoid Modulation of Mitochondrial Function in Hepatoma Cells Requires the Mitochondrial Fission Protein Drp1 
Antioxidants & Redox Signaling  2013;19(4):366-378.
Abstract
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. Antioxid. Redox Signal. 19, 366–378.
doi:10.1089/ars.2011.4269
PMCID: PMC3700019  PMID: 22703557

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