A signalling role for 4-hydroxy-2-nonenal in regulation of mitochondrial uncoupling

Lipoxidation in cardiovascular diseases

Lipids can go through lipid peroxidation, an endogenous chain reaction that consists in the oxidative degradation of lipids leading to the generation of a wide variety of highly reactive carbonyl species (RCS), such as short-chain carbonyl derivatives and oxidized truncated phospholipids. RCS exert a wide range of biological effects due to their ability to interact and covalently bind to nucleophilic groups on other macromolecules, such as nucleic acids, phospholipids, and proteins, forming reversible and/or irreversible modifications and generating the so-called advanced lipoxidation end-products (ALEs). Lipoxidation plays a relevant role in the onset of cardiovascular diseases (CVD), mainly in the atherosclerosis-based diseases in which oxidized lipids and their adducts have been extensively characterized and associated with several processes responsible for the onset and development of atherosclerosis, such as endothelial dysfunction and inflammation. Herein we will review the current knowledge on the sources of lipids that undergo oxidation in the context of cardiovascular diseases, both from the bloodstream and tissues, and the methods for detection, characterization, and quantitation of their oxidative products and protein adducts. Moreover, lipoxidation and ALEs have been associated with many oxidative-based diseases, including CVD, not only as potential biomarkers but also as therapeutic targets. Indeed, several therapeutic strategies, acting at different levels of the ALEs cascade, have been proposed, essentially blocking ALEs formation, but also their catabolism or the resulting biological responses they induce. However, a deeper understanding of the mechanisms of formation and targets of ALEs could expand the available therapeutic strategies.

Molecular evolution of uncoupling proteins and implications for brain function

Uncoupling proteins (UCPs) belong to the mitochondrial anion carrier superfamily and catalyze important metabolic functions at the mitochondrial inner membrane. While the thermogenic role of UCP1 in brown fat of eutherian mammals is well established, the molecular functions of UCP1 in ectothermic vertebrates and of other UCP paralogs remain less clear. Here, we critically discuss the existence of brain UCPs and their potential roles. Applying phylogenetic classification of novel UCPs, we summarize the evidence for brain UCP1 among vertebrates, the role of UCP2 in specific brain areas, and the existence of brain-specific UCPs. The phylogenetic analyses and discussion on functional data should alert the scientific community that the molecular function of so-called UCP1 homologues is by far not clarified and possibly relates to neither thermogenesis nor mitochondrial uncoupling.

Lipids and lipoxidation in human brain aging. Mitochondrial ATP-synthase as a key lipoxidation target

The human brain is a target of the aging process like other cell systems of the human body. Specific regions of the human brain exhibit differential vulnerabilities to the aging process. Yet the underlying mechanisms that sustain the preservation or deterioration of neurons and cerebral functions are unknown. In this review, we focus attention on the role of lipids and the importance of the cross-regionally different vulnerabilities in human brain aging. In particular, we first consider a brief approach to the lipidomics of human brain, the relationship between lipids and lipoxidative damage, the role of lipids in human brain aging, and the specific targets of lipoxidative damage in human brain and during aging. It is proposed that the restricted set of modified proteins and the functional categories involved may be considered putative collaborative factors contributing to neuronal aging, and that mitochondrial ATP synthase is a key lipoxidative target in human brain aging.

Protein S-glutathionylation: The linchpin for the transmission of regulatory information on redox buffering capacity in mitochondria

Protein S-glutathionylation reactions are a ubiquitous oxidative modification required to control protein function in response to changes in redox buffering capacity. These reactions are rapid and reversible and are, for the most part, enzymatically mediated by glutaredoxins (GRX) and glutathione S-transferases (GST). Protein S-glutathionylation has been found to control a range of cell functions in response to different physiological cues. Although these reactions occur throughout the cell, mitochondrial proteins seem to be highly susceptible to reversible S-glutathionylation, a feature attributed to the unique physical properties of this organelle. Indeed, mitochondria contain a number of S-glutathionylation targets which includes proteins involved in energy metabolism, solute transport, reactive oxygen species (ROS) production, proton leaks, apoptosis, antioxidant defense, and mitochondrial fission and fusion. Moreover, it has been found that conjugation and removal of glutathione from proteins in mitochondria fulfills a number of important physiological roles and defects in these reactions can have some dire pathological consequences. Here, we provide an updated overview on mitochondrial protein S-glutathionylation reactions and their importance in cell functions and physiology.

The effect of inactin on kidney mitochondrial function and production of reactive oxygen species

Inactin is a long lasting anesthetic agent commonly used in rat studies, but is also shown to exert physiological effects such as reducing renal blood flow, glomerular filtration rate and depressing tubular transport capacity. The effect of inactin on isolated kidney mitochondria is unknown and may be important when studying related topics in anaesthetized animals. The aim of this study was to determine whether inactin exerts effects on mitochondrial function and production of reactive oxygen species. Kidney mitochondrial function and production of reactive oxygen after acutely (5 min) or longer (1.5 hour) anesthetizing rats with inactin was evaluated using high-resolution respirometry. The results demonstrate that inactin significantly improves respiratory control ratio, inhibits complex I in the mitochondrial respiratory chain, reduce both unregulated proton leak and time dependently reduce the regulated proton leak via uncoupling protein-2 and adenine nucleotide translocase. Inactin also contributes to increased mitochondrial hydrogen peroxide production. In conclusion, inactin exerts persistent effects on mitochondrial function and these profound effects on mitochondrial function should to be considered when studying mitochondria isolated from animals anesthesized with inactin.

Hypothalamic redox balance and leptin signaling - Emerging role of selenoproteins

The hypothalamus is the central neural site governing food intake and energy expenditure. During the past 25 years, understanding of the hypothalamic cell types, hormones, and circuitry involved in the regulation of energy metabolism has dramatically increased. It is now well established that the adipocyte-derived hormone, leptin, acts upon two distinct groups of hypothalamic neurons that comprise opposing arms of the central melanocortin system. These two cell populations are anorexigenic neurons expressing proopiomelanocortin (POMC) and orexigenic neurons that express agouti-related peptide (AGRP). Several important studies have demonstrated that reactive oxygen species and endoplasmic reticulum stress significantly impact these hypothalamic neuronal populations that regulate global energy metabolism. Reactive oxygen species and redox homeostasis are influenced by selenoproteins, an essential class of proteins that incorporate selenium co-translationally in the form of the 21st amino acid, selenocysteine. Levels of these proteins are regulated by dietary selenium intake and they are widely expressed in the brain. Of additional relevance, selenium supplementation has been linked to metabolic alterations in both animal and human studies. Recent evidence also indicates that hypothalamic selenoproteins are significant modulators of energy metabolism in both neurons and tanycytes, a population of glial-like cells lining the floor of the 3rd ventricle within the hypothalamus. This review article will summarize current understanding of the regulatory influence of redox status on hypothalamic nutrient sensing and highlight recent work revealing the importance of selenoproteins in the hypothalamus.

Uncoupling proteins: Martin Klingenberg's contributions for 40 years

The uncoupling protein (UCP1) is a proton (H⁺) transporter in the mitochondrial inner membrane. By dissipating the electrochemical H⁺ gradient, UCP1 uncouples respiration from ATP synthesis, which drives an increase in substrate oxidation via the TCA cycle flux that generates more heat. The mitochondrial uncoupling-mediated non-shivering thermogenesis in brown adipose tissue is vital primarily to mammals, such as rodents and new-born humans, but more recently additional functions in adult humans have been described. UCP1 is regulated by β-adrenergic receptors through the sympathetic nervous system and at the molecular activity level by nucleotides and fatty acid to meet thermogenesis needs. The discovery of novel UCP homologs has greatly contributed to the understanding of human diseases, such as obesity and diabetes. In this article, we review the progress made towards the molecular mechanism and function of the UCPs, in particular focusing on the influential contributions from Martin Klingenberg's laboratory. Because all members of the UCP family are potentially promising drug targets, we also present and discuss possible approaches and methods for UCP-related drug discovery.

Absence of effect of the antiretrovirals Duovir and Viraday on mitochondrial bioenergetics

Most toxicity associated with antiretroviral drugs is thought to result from disruption of mitochondrial function. Unfortunately, there are no validated laboratory markers for clinically assessing the onset of mitochondrial toxicity associated with antiretroviral therapy. In a previous study on mitochondrial hepatocytes, the protease inhibitor lopimune was shown to induce mitochondrial toxicity by increasing reactive oxygen species (ROS) production and decreasing respiratory control ratio (RCR) reflecting compromised mitochondrial efficiency in adenosine triphosphate production. Mitochondrial dysfunction and ROS production were directly correlated with the expression of uncoupling protein 2 (UCP2). In the current study we aim to determine the toxicity of nucleoside or nucleotide and nonnucleoside reverse-transcriptase inhibitors, Duovir and Viraday on liver mitochondria isolated from treated mice by monitoring UCP2 expression. Our results showed that both Duovir and Viraday had no effect on mitochondrial respiration states 2, 3, 4, and on RCR. In addition, ROS generation and UCP2 expression were not affected. In conclusion, our results indicate the difference in the mechanism of action of distinct classes of antiretroviral drugs on mitochondrial functions and may associate UCP2 expression with subclinical mitochondrial damage as marker of cellular oxidative stress.

Mitochondrial Uncoupling Proteins: Subtle Regulators of Cellular Redox Signaling

SIGNIFICANCE

Mitochondria are the energetic, metabolic, redox, and information signaling centers of the cell. Substrate pressure, mitochondrial network dynamics, and cristae morphology state are integrated by the protonmotive force Δp or its potential component, ΔΨ, which are attenuated by proton backflux into the matrix, termed uncoupling. The mitochondrial uncoupling proteins (UCP1-5) play an eminent role in the regulation of each of the mentioned aspects, being involved in numerous physiological events including redox signaling. Recent Advances: UCP2 structure, including purine nucleotide and fatty acid (FA) binding sites, strongly support the FA cycling mechanism: UCP2 expels FA anions, whereas uncoupling is achieved by the membrane backflux of protonated FA. Nascent FAs, cleaved by phospholipases, are preferential. The resulting Δp dissipation decreases superoxide formation dependent on Δp. UCP-mediated antioxidant protection and its impairment are expected to play a major role in cell physiology and pathology. Moreover, UCP2-mediated aspartate, oxaloacetate, and malate antiport with phosphate is expected to alter metabolism of cancer cells.

CRITICAL ISSUES

A wide range of UCP antioxidant effects and participations in redox signaling have been reported; however, mechanisms of UCP activation are still debated. Switching off/on the UCP2 protonophoretic function might serve as redox signaling either by employing/releasing the extra capacity of cell antioxidant systems or by directly increasing/decreasing mitochondrial superoxide sources. Rapid UCP2 degradation, FA levels, elevation of purine nucleotides, decreased Mg²⁺, or increased pyruvate accumulation may initiate UCP-mediated redox signaling.

FUTURE DIRECTIONS

Issues such as UCP2 participation in glucose sensing, neuronal (synaptic) function, and immune cell activation should be elucidated. Antioxid. Redox Signal. 29, 667-714.

Hypoxia, oxidative stress and inflammation

Inflammatory Arthritis is characterized by synovial proliferation, neovascularization and leukocyte extravasation leading to joint destruction and functional disability. Efficiency of oxygen supply to the synovium is poor due to the highly dysregulated synovial microvasculature. This along with the increased energy demands of activated infiltrating immune cells and inflamed resident cells leads to an hypoxic microenvironment and mitochondrial dysfunction. This favors an increase of reactive oxygen species, leading to oxidative damage which further promotes inflammation. In this adverse microenvironment synovial cells adapt to generate energy and switch their cell metabolism from a resting regulatory state to a highly metabolically active state which allows them to produce essential building blocks to support their proliferation. This metabolic shift results in the accumulation of metabolic intermediates which act as signaling molecules that further dictate the inflammatory response. Understanding the complex interplay between hypoxia-induced signaling pathways, oxidative stress and mitochondrial function will provide better insight into the underlying mechanisms of disease pathogenesis.

Deletion of Uncoupling Protein-2 reduces renal mitochondrial leak respiration, intrarenal hypoxia and proteinuria in a mouse model of type 1 diabetes

AIM

Uncoupling protein-2 (UCP-2) can induce mitochondrial uncoupling in the diabetic kidney. Although mitochondrial uncoupling reduces oxidative stress originating from the mitochondria and can be regarded as a protective mechanism, the increased oxygen consumption occurring secondarily to increased mitochondria uncoupling, that is leak respiration, may contribute to kidney tissue hypoxia. Using UCP-2^-/- mice, we tested the hypothesis that UCP-2-mediated leak respiration is important for the development of diabetes-induced intrarenal hypoxia and proteinuria.

METHODS

Kidney function, in vivo oxygen metabolism, urinary protein leakage and mitochondrial function were determined in wild-type and UCP-2^-/- mice during normoglycaemia and 2 weeks after diabetes induction.

RESULTS

Diabetic wild-type mice displayed mitochondrial leak respiration, pronounced intrarenal hypoxia, proteinuria and increased urinary KIM-1 excretion. However, diabetic UCP-2^-/- mice did not develop increased mitochondrial leak respiration and presented with normal intrarenal oxygen levels, urinary protein and KIM-1 excretion.

CONCLUSION

Although functioning as an antioxidant system, mitochondria uncoupling is always in co-occurrence with increased oxygen consumption, that is leak respiration; a potentially detrimental side effect as it can result in kidney tissue hypoxia; an acknowledged unifying pathway to nephropathy. Indeed, this study demonstrates a novel mechanism in which UCP-2-mediated mitochondrial leak respiration is necessary for the development of diabetes-induced intrarenal tissue hypoxia and proteinuria.

Acetone fraction from Sechium edule (Jacq.) S.w. edible roots exhibits anti-endothelial dysfunction activity

ETHNOPHARMACOLOGICAL RELEVANCE

A recent ethnomedical survey on medicinal plants grown in Mexico revealed that Sechium edule (Jacq.) Sw. (Cucurbitaceae) is one of the most valued plant species to treat cardiovascular diseases, including hypertension. Fruits, young leaves, buds, stems, and tuberous roots of the plant are edible. Considering that endothelial dysfunction induced by Angiotensin II plays an important role in the pathogenesis of hypertension and is accompanied by a prooxidative condition, which in turn induces an inflammatory state, vascular remodeling, and tissue damage, and that S. edule has been reported to possess antioxidant, anti-inflammatory and antihypertensive activity, its capability to control endothelial dysfunction was also assessed.

AIM OF THE STUDY

To assess in vivo the anti-endothelial dysfunction activity of the acetone fraction (rSe-ACE) of the hydroalcoholic extract from S. edule roots.

MATERIALS AND METHODS

Endothelial dysfunction was induced in female C57BL/6 J mice by a daily intraperitoneal injection of angiotensin II for 10 weeks. Either rSe-ACE or losartan (as a control) were co-administered with angiotensin II for the same period. Blood pressure was measured at weeks 0, 5, and 10. Kidney extracts were prepared to determine IL1β, IL4, IL6, IL10, IL17, IFNγ, TNFα, and TGFβ levels by ELISA, along with the prooxidative status as assessed by the activity of antioxidant enzymes. The expression of ICAM-1 was evaluated by immunohistochemistry in kidney histological sections. Kidney and hepatic damage, as well as vascular tissue remodeling, were studied.

RESULTS

The rSe-ACE fraction administered at a dose of 10 mg/kg was able to control hypertension, as well as the prooxidative and proinflammatory status in kidney as efficiently as losartan, returning mice to normotensive levels. Additionally, the fraction was more efficient than losartan to prevent liver and kidney damage. Phytochemical characterization identified cinnamic acid as a major compound, and linoleic, palmitic, and myristic acids as the most abundant non-polar components in the mixture, previously reported to aid in the control of hypertension, inflammation, and oxidative stress, three important components of endothelial dysfunction.

IN CONCLUSION

this study demonstrated that rSe-ACE has anti-endothelial dysfunction activity in an experimental model and highlights the role of cinnamic acid and fatty acids in the observed effects.

Improved approach for normothermic machine perfusion of cold stored kidney grafts

Normothermic machine perfusion can decrease reperfusion injury in renal transplantation. Clinical procurement logistics include retrieval and initial transport of the graft using static cold storage. Therefore, use and benefits of brief normothermic reconditioning by machine perfusion should be investigated in the initially cold preserved graft. Porcine kidneys (6 per group) were retrieved 20 min after cardiac standstill. After 20 h of static cold preservation some grafts were put on a machine perfusion circuit and normothermically perfused for 2 h at 35°C (NMP). Another group was subjected to controlled oxygenated rewarming (COR), starting perfusion at 8°C and elevating temperature and pressure slowly up to 35°C and 75 mmHg during the first 90 min of 2 h perfusion. Control kidneys were only cold stored (CS). Post implant graft function was evaluated afterwards in an established in vitro reperfusion model. During graft reconditioning, COR reduced oxygen free radical production and formation of 4-hydroxy-2-nonenal (HNE), an activator of mitochondrial uncoupling proteins, in comparison to NMP. Upon reperfusion, NMP only led to a slight improvement of renal function (clearance of creatinine, fractional excretion of Na and glucose) compared to controls. But 2-3 fold improvements of renal function were seen after COR, which also significantly improved aerobic efficiency (total Na absorption/VO₂) upon reperfusion. A slow and controlled increase in temperature up to normothermia improves mitochondrial recovery and oxygen utilization efficiency, resulting in better functional recovery, possibly through a more mild and adapted increase of cellular metabolism.

Oxidative stress impairs energy metabolism in primary cells and synovial tissue of patients with rheumatoid arthritis

Background

In this study, we examined the effect of oxidative stress on cellular energy metabolism and pro-angiogenic/pro-inflammatory mechanisms of primary rheumatoid arthritis synovial fibroblast cells (RASFC) and human umbilical vein endothelial cells (HUVEC).

Methods

Primary RASFC and HUVEC were cultured with the oxidative stress inducer 4-hydroxy-2-nonenal (4-HNE), and extracellular acidification rate, oxygen consumption rate, mitochondrial function and pro-angiogenic/pro-inflammatory mechanisms were assessed using the Seahorse analyser, complex I–V activity assays, random mutation mitochondrial capture assays, enzyme-linked immunosorbent assays and functional assays, including angiogenic tube formation, migration and invasion. Expression of angiogenic growth factors in synovial tissue (ST) was assessed by IHC in patients with rheumatoid arthritis (RA) undergoing arthroscopy before and after administration of tumour necrosis factor inhibitors (TNFi).

Results

In RASFC and HUVEC, 4-HNE-induced oxidative stress reprogrammed energy metabolism by inhibiting mitochondrial basal, maximal and adenosine triphosphate-linked respiration and reserve capacity, coupled with the reduced enzymatic activity of oxidative phosphorylation complexes III and IV. In contrast, 4-HNE elevated basal glycolysis, glycolytic capacity and glycolytic reserve, paralleled by an increase in mitochondrial DNA mutations and reactive oxygen species. 4-HNE activated pro-angiogenic responses of RASFC, which subsequently altered HUVEC invasion and migration, angiogenic tube formation and the release of pro-angiogenic mediators. In vivo markers of angiogenesis (vascular endothelial growth factor, angiopoietin 2 [Ang2], tyrosine kinase receptor [Tie2]) were significantly associated with oxidative damage and oxygen metabolism in the inflamed synovium. Significant reduction in ST vascularity and Ang2/Tie2 expression was demonstrated in patients with RA before and after administration of TNFi.

Conclusions

Oxidative stress promotes metabolism in favour of glycolysis, an effect that may contribute to acceleration of inflammatory mechanisms and subsequent dysfunctional angiogenesis in RA.

Electronic supplementary material

The online version of this article (10.1186/s13075-018-1592-1) contains supplementary material, which is available to authorized users.

Inhibition of mammalian target of rapamycin decreases intrarenal oxygen availability and alters glomerular permeability

An increased kidney oxygen consumption causing tissue hypoxia has been suggested to be a common pathway toward chronic kidney disease. The mammalian target of rapamycin (mTOR) regulates cell proliferation and mitochondrial function. mTOR inhibitors (e.g., rapamycin) are used clinically to prevent graft rejection. mTOR has been identified as a key player in diabetes, which has stimulated the use of mTOR inhibitors to counter diabetic nephropathy. However, the effect of mTOR inhibition on kidney oxygen consumption is unknown. Therefore, we investigated the effects of mTOR inhibition on in vivo kidney function, oxygen homeostasis, and glomerular permeability. Control and streptozotocin-induced diabetic rats were chronically treated with rapamycin, and the functional consequences were studied 14 days thereafter. In both groups, mTOR inhibition induced mitochondrial uncoupling, resulting in increased total kidney oxygen consumption and decreased intrarenal oxygen availability. Concomitantly, mTOR inhibition induced tubular injury, as estimated from urinary excretion of kidney injury molecule-1 (KIM-1) and reduced urinary protein excretion. The latter corresponded to reduced sieving coefficient for large molecules. In conclusion, mTOR inhibition induces mitochondrial dysfunction leading to decreased oxygen availability in normal and diabetic kidneys, which translates into increased KIM-1 in the urine. Reduced proteinuria after mTOR inhibition is an effect of reduced glomerular permeability for large molecules. Since hypoxia has been suggested as a common pathway in the development of chronic kidney disease, mTOR inhibition to patients with preexisting nephropathy should be used with caution, since it may accelerate the progression of the disease.

Skeletal muscle bioenergetics in aging and heart failure

Changes in mitochondrial capacity and quality play a critical role in skeletal and cardiac muscle dysfunction. In vivo measurements of mitochondrial capacity provide a clear link between physical activity and mitochondrial function in aging and heart failure, although the cause and effect relationship remains unclear. Age-related decline in mitochondrial quality leads to mitochondrial defects that affect redox, calcium, and energy-sensitive signaling by altering the cellular environment that can result in skeletal muscle dysfunction independent of reduced mitochondrial capacity. This reduced mitochondrial quality with age is also likely to sensitize skeletal muscle mitochondria to elevated angiotensin or beta-adrenergic signaling associated with heart failure. This synergy between aging and heart failure could further disrupt cell energy and redox homeostasis and contribute to exercise intolerance in this patient population. Therefore, the interaction between aging and heart failure, particularly with respect to mitochondrial dysfunction, should be a consideration when developing strategies to improve quality of life in heart failure patients. Given the central role of the mitochondria in skeletal and cardiac muscle dysfunction, mitochondrial quality may provide a common link for targeted interventions in these populations.

Male Mice Lacking NLRX1 Are Partially Protected From High-Fat Diet-Induced Hyperglycemia

Nod-like receptor (NLR)X1 is an NLR family protein that localizes to the mitochondrial matrix and modulates reactive oxygen species production, possibly by directly interacting with the electron transport chain. Recent work demonstrated that cells lacking NLRX1 have higher oxygen consumption but lower levels of adenosine triphosphate, suggesting that NLRX1 might prevent uncoupling of oxidative phosphorylation. We therefore hypothesized that NLRX1 might regulate whole-body energy metabolism through its effect on mitochondria. Male NLRX1 whole-body knockout (KO) mice and wild-type (WT) C57BL/6N controls were fed a low-fat or a high-fat (HF) diet for 16 weeks from weaning. Contrary to this hypothesis, there were no differences in body weight, adiposity, energy intake, or energy expenditure between HF-fed KO and WT mice, but instead HF KO mice were partially protected from the development of diet-induced hyperglycemia. Additionally, HF KO mice did not present with hyperinsulinemia during the glucose tolerance test, as did HF WT mice. There were no genotype differences in insulin tolerance, which led us to consider a pancreatic phenotype. Histology revealed that KO mice were protected from HF-induced pancreatic lipid accumulation, suggesting a potential role for NLRX1 in pancreatic dysfunction during the development diet-induced type 2 diabetes mellitus. Hence, NLRX1 depletion partially protects against postabsorptive hyperglycemia in obesity that may be linked to the prevention of pancreatic lipid accumulation. Although the actual mechanisms restoring glucose and insulin dynamics remain unknown, NLRX1 emerges as a potentially interesting target to inhibit for the prevention of type 2 diabetes mellitus.

Epoxycyclohexenedione-Type Compounds Make Up a New Class of Inhibitors of the Bovine Mitochondrial ADP/ATP Carrier

Through the extensive screening of our chemical library, we found epoxycyclohexenedione (ECHD)-type compounds (AMM-59 and -120) as unique inhibitors of the bovine heart mitochondrial ADP/ATP carrier (AAC). This study investigated the mechanism of inhibition of AAC by ECHDs using submitochondrial particles (SMPs). Proteomic analyses of ECHD-bound AAC as well as biochemical characterization using different SH reagents showed that ECHDs inhibit the function of AAC by covalently binding primarily to Cys57 and secondarily to Cys160. Interestingly, AAC remarkably aggregated in SMPs upon being incubated with high concentrations of ECHDs for a long period of time. This aggregation was observed under both oxidative and reductive conditions of the sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of SMP proteins, indicating that aggregation is not caused by intermolecular S-S linkages. ECHDs are the first chemicals, to the best of our knowledge, to induce prominent structural alteration in AAC without forming intermolecular S-S linkages. When all solvent-accessible cysteines (Cys57, Cys160, and Cys257) were previously modified by N-ethylmaleimide, the aggregation of AAC was completely suppressed. In contrast, when Cys57 or Cys160 is selectively modified by a SH reagent, the covalent binding of ECHDs to a residual free residue of the two cysteines is sufficient to induce aggregation. The aggregation-inducing ability of another ECHD analogue (AMM-124), which has an alkyl chain that is shorter than those of AMM-59 and -120, was significantly less efficient than that of the two compounds. On the basis of these results, the mechanism underlying the aggregation of AAC induced by ECHDs is discussed.

Chronic Heat Stress Impairs the Quality of Breast-Muscle Meat in Broilers by Affecting Redox Status and Energy-Substance Metabolism

We investigated the molecular mechanisms by which chronic heat stress impairs the breast-meat quality of broilers. Broilers were assigned to three groups: the normal control (NC) group, heat-stress (HS) group, and pair-fed (PF) group. After 7 days of heat exposure (32 °C), the high temperature had caused oxidative stress; elevated the activity of citrate synthase (CS), the mRNA expression of M-CPT1, and the phosphorylation level of AMPKα; and reduced the mRNA expression of avUCP. After 14 days of heat exposure, the heat stress had increased the lightness and drip loss and decreased the pH and shear force of the breast meat. Additionally, the heat exposure had increased the mRNA expressions of FAS, ACC, and PDK4; the content of lipids; and the activities of lactic dehydrogenase and pyruvate kinase, and it had decreased the mRNA expression of M-CPT1 and the activity of CS. In conclusion, chronic heat stress impairs meat quality by causing mitochondria to malfunction and affecting energy-substance aerobic metabolism, resulting in increased glycolysis and intramuscular fat deposition.

Melatonin Efficacy in Obese Leptin-Deficient Mice Heart

Cardiomyocytes are particularly sensitive to oxidative damage due to the link between mitochondria and sarcoplasmic reticulum necessary for calcium flux and contraction. Melatonin, important indoleamine secreted by the pineal gland during darkness, also has important cardioprotective properties. We designed the present study to define morphological and ultrastructural changes in cardiomyocytes and mainly in mitochondria of an animal model of obesity (ob/ob mice), when treated orally or not with melatonin at 100 mg/kg/day for 8 weeks (from 5 up to 13 week of life). We observed that ob/ob mice mitochondria in sub-sarcolemmal and inter-myofibrillar compartments are often devoid of cristae with an abnormally large size, which are called mega-mitochondria. Moreover, in ob/ob mice the hypertrophic cardiomyocytes expressed high level of 4hydroxy-2-nonenal (4HNE), a marker of lipid peroxidation but scarce degree of mitofusin2, indicative of mitochondrial sufferance. Melatonin oral supplementation in ob/ob mice restores mitochondrial cristae, enhances mitofusin2 expression and minimizes 4HNE and p62/SQSTM1, an index of aberrant autophagic flux. At pericardial fat level, adipose tissue depot strictly associated with myocardium infarction, melatonin reduces adipocyte hypertrophy and inversely regulates 4HNE and adiponectin expressions. In summary, melatonin might represent a safe dietary adjuvant to hamper cardiac mitochondria remodeling and the hypoxic status that occur in pre-diabetic obese mice at 13 weeks of life.

Progress in understanding the molecular oxygen paradox - function of mitochondrial reactive oxygen species in cell signaling

The molecular oxygen (O2) paradox was coined to describe its essential nature and toxicity. The latter characteristic of O2 is associated with the formation of reactive oxygen species (ROS), which can damage structures vital for cellular function. Mammals are equipped with antioxidant systems to fend off the potentially damaging effects of ROS. However, under certain circumstances antioxidant systems can become overwhelmed leading to oxidative stress and damage. Over the past few decades, it has become evident that ROS, specifically H2O2, are integral signaling molecules complicating the previous logos that oxyradicals were unfortunate by-products of oxygen metabolism that indiscriminately damage cell structures. To avoid its potential toxicity whilst taking advantage of its signaling properties, it is vital for mitochondria to control ROS production and degradation. H2O2 elimination pathways are well characterized in mitochondria. However, less is known about how H2O2 production is controlled. The present review examines the importance of mitochondrial H2O2 in controlling various cellular programs and emerging evidence for how production is regulated. Recently published studies showing how mitochondrial H2O2 can be used as a secondary messenger will be discussed in detail. This will be followed with a description of how mitochondria use S-glutathionylation to control H2O2 production.

Nutrient overload, lipid peroxidation and pancreatic beta cell function

Since the landmark discovery of α,β-unsaturated 4-hydroxyalkenals by Esterbauer and colleagues most studies have addressed the consequences of the tendency of these lipid peroxidation products to form covalent adducts with macromolecules and modify cellular functions. Many studies describe detrimental and cytotoxic effects of 4-hydroxy-2E-nonenal (4-HNE) in myriad tissues and organs and many pathologies. Other studies similarly assigned unfavorable effects to 4-hydroxy-2E-hexenal (4-HHE) and 4-hydroxy-2E,6Z-dodecadienal (4-HDDE). Nutrient overload (e.g., hyperglycemia, hyperlipidemia) modifies lipid metabolism in cells and promotes lipid peroxidation and the generation of α,β-unsaturated 4-hydroxyalkenals. Advances glycation- and lipoxidation end products (AGEs and ALEs) have been associated with the development of insulin resistance and pancreatic beta cell dysfunction and the etiology of type 2 diabetes and its peripheral complications. Less acknowledged are genuine signaling properties of 4-hydroxyalkenals in hormetic processes that provide defense against the consequences of nutrient overload. This review addresses recent findings on such lipohormetic mechanisms that are associated with lipid peroxidation in pancreatic beta cells. This article is part of a Special Issue entitled SI: LIPID OXIDATION PRODUCTS, edited by Giuseppe Poli.

Regulation of autophagy, mitochondrial dynamics, and cellular bioenergetics by 4-hydroxynonenal in primary neurons

The production of reactive species contributes to the age-dependent accumulation of dysfunctional mitochondria and protein aggregates, all of which are associated with neurodegeneration. A putative mediator of these effects is the lipid peroxidation product 4-hydroxynonenal (4-HNE), which has been shown to inhibit mitochondrial function, and accumulate in the postmortem brains of patients with neurodegenerative diseases. This deterioration in mitochondrial quality could be due to direct effects on mitochondrial proteins, or through perturbation of the macroautophagy/autophagy pathway, which plays an essential role in removing damaged mitochondria. Here, we use a click chemistry-based approach to demonstrate that alkyne-4-HNE can adduct to specific mitochondrial and autophagy-related proteins. Furthermore, we found that at lower concentrations (5-10 μM), 4-HNE activates autophagy, whereas at higher concentrations (15 μM), autophagic flux is inhibited, correlating with the modification of key autophagy proteins at higher concentrations of alkyne-4-HNE. Increasing concentrations of 4-HNE also cause mitochondrial dysfunction by targeting complex V (the ATP synthase) in the electron transport chain, and induce significant changes in mitochondrial fission and fusion protein levels, which results in alterations to mitochondrial network length. Finally, inhibition of autophagy initiation using 3-methyladenine (3MA) also results in a significant decrease in mitochondrial function and network length. These data show that both the mitochondria and autophagy are critical targets of 4-HNE, and that the proteins targeted by 4-HNE may change based on its concentration, persistently driving cellular dysfunction.

Mitochondrial reactive oxygen species and adipose tissue thermogenesis: Bridging physiology and mechanisms

Brown and beige adipose tissues can catabolize stored energy to generate heat, relying on the principal effector of thermogenesis: uncoupling protein 1 (UCP1). This unique capability could be leveraged as a therapy for metabolic disease. Numerous animal and cellular models have now demonstrated that mitochondrial reactive oxygen species (ROS) signal to support adipocyte thermogenic identity and function. Herein, we contextualize these findings within the established principles of redox signaling and mechanistic studies of UCP1 function. We provide a framework for understanding the role of mitochondrial ROS signaling in thermogenesis together with testable hypotheses for understanding mechanisms and developing therapies.

Secretory phospholipase A2 group IIA modulates insulin sensitivity and metabolism

Secretory phospholipase A₂ group IIA (PLA2G2A) is a member of a family of secretory phospholipases that have been implicated in inflammation, atherogenesis, and antibacterial actions. Here, we evaluated the role of PLA2G2A in the metabolic response to a high fat diet. C57BL/6 (BL/6) mice do not express PLA2g2a due to a frameshift mutation. We fed BL/6 mice expressing the human PLA2G2A gene (IIA+ mice) a fat diet and assessed the physiologic response. After 10 weeks on the high fat diet, the BL/6 mice were obese, but the IIA+ mice did not gain weight or accumulate lipid. The lean mass in chow- and high fat-fed IIA+ mice was constant and similar to the BL/6 mice on a chow diet. Surprisingly, the IIA+ mice had an elevated metabolic rate, which was not due to differences in physical activity. The IIA+ mice were more insulin sensitive and glucose tolerant than the BL/6 mice, even when the IIA+ mice were provided the high fat diet. The IIA+ mice had increased expression of uncoupling protein 1 (UCP1), sirtuin 1 (SIRT1), and PPARγ coactivator 1α (PGC-1α) in brown adipose tissue (BAT), suggesting that PLA2G2A activates mitochondrial uncoupling in BAT. Our data indicate that PLA2G2A has a previously undiscovered impact on insulin sensitivity and metabolism.

Uncoupling protein 2 regulates daily rhythms of insulin secretion capacity in MIN6 cells and isolated islets from male mice

Objective

Upregulation of uncoupling protein 2 (UCP2) is associated with impaired glucose-stimulated insulin secretion (GSIS), which is thought to be an important contributor to pathological β cell failure in obesity and type 2 diabetes (T2D); however, the physiological function of UCP2 in the β cell remains undefined. It has been suggested, but not yet tested, that UCP2 plays a physiological role in β cells by coordinating insulin secretion capacity with anticipated fluctuating nutrient supply, such that upregulation of UCP2 in the inactive/fasted state inhibits GSIS as a mechanism to prevent hypoglycemia. Therefore, we hypothesized that daily cycles of GSIS capacity are dependent on rhythmic and predictable patterns of Ucp2 gene expression such that low Ucp2 in the active/fed phase promotes maximal GSIS capacity, whereas elevated Ucp2 expression in the inactive/fasted phase supresses GSIS capacity. We further hypothesized that rhythmic Ucp2 expression is required for the maintenance of glucose tolerance over the 24 h cycle.

Methods

We used synchronized MIN6 clonal β cells and isolated mouse islets from wild type (C57BL6) and mice with β cell knockout of Ucp2 (Ucp2-βKO; and respective Ins2-cre controls) to determine the endogenous expression pattern of Ucp2 over 24 h and its impact on GSIS capacity and glucose tolerance over 24 h.

Results

A dynamic pattern of Ucp2 mRNA expression was observed in synchronized MIN6 cells, which showed a reciprocal relationship with GSIS capacity in a time-of-day-specific manner. GSIS capacity was suppressed in islets isolated from wild type and control mice during the light/inactive phase of the daily cycle; a suppression that was dependent on Ucp2 in the β cell and was lost in islets isolated from Ucp2-βKO mice or wild type islets treated with a UCP2 inhibitor. Finally, suppression of GSIS capacity by UCP2 in the light phase was required for the maintenance of normal patterns of glucose tolerance.

Conclusions

Our study suggests that Ucp2/UCP2 in the β cell is part of an important, endogenous, metabolic regulator that controls the temporal capacity of GSIS over the course of the day/night cycle, which, in turn, regulates time-of-day glucose tolerance. Targeting Ucp2/UCP2 as a therapeutic in type 2 diabetes or any other metabolic condition must take into account the rhythmic nature of its expression and its impact on glucose tolerance over 24 h, specifically during the inactive/fasted phase.

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Ucp2 mRNA expression in MIN6 β cells and isolated islets is dynamic and rhythmic over 24 h.

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Daily cycles of glucose-stimulated insulin secretion capacity are dependent on rhythmic Ucp2 expression and UCP2 activity.

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Loss of rhythmic Ucp2 mRNA expression triggers glucose intolerance only in the light/inactive phase of the daily cycle.

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UCP2 is part of an endogenous diurnal metabolic regulator that coordinates islet function with the daily cycle of fasting and feeding.

Mitochondrial Reactive Oxygen Species and Kidney Hypoxia in the Development of Diabetic Nephropathy

The underlying mechanisms in the development of diabetic nephropathy are currently unclear and likely consist of a series of dynamic events from the early to late stages of the disease. Diabetic nephropathy is currently without curative treatments and it is acknowledged that even the earliest clinical manifestation of nephropathy is preceded by an established morphological renal injury that is in turn preceded by functional and metabolic alterations. An early manifestation of the diabetic kidney is the development of kidney hypoxia that has been acknowledged as a common pathway to nephropathy. There have been reports of altered mitochondrial function in the diabetic kidney such as altered mitophagy, mitochondrial dynamics, uncoupling, and cellular signaling through hypoxia inducible factors and AMP-kinase. These factors are also likely to be intertwined in a complex manner. In this review, we discuss how these pathways are connected to mitochondrial production of reactive oxygen species (ROS) and how they may relate to the development of kidney hypoxia in diabetic nephropathy. From available literature, it is evident that early correction and/or prevention of mitochondrial dysfunction may be pivotal in the prevention and treatment of diabetic nephropathy.

Mitochondrial carriers in inflammation induced by bacterial endotoxin and cytokines

Significant metabolic changes occur in the shift from resting to activated cellular status in inflammation. Thus, changes in expression of a large number of genes and extensive metabolic reprogramming gives rise to acquisition of new functions (e.g. production of cytokines, intermediates for biosynthesis, lipid mediators, PGE, ROS and NO). In this context, mitochondrial carriers, which catalyse the transport of solute across mitochondrial membrane, change their expression to transport mitochondrially produced molecules, among which citrate and succinate, to be used as intracellular signalling molecules in inflammation. This review summarises the mitochondrial carriers studied so far that are, directly or indirectly, involved in inflammation.

The molecular features of uncoupling protein 1 support a conventional mitochondrial carrier-like mechanism

Uncoupling protein 1 (UCP1) is an integral membrane protein found in the mitochondrial inner membrane of brown adipose tissue, and facilitates the process of non-shivering thermogenesis in mammals. Its activation by fatty acids, which overcomes its inhibition by purine nucleotides, leads to an increase in the proton conductance of the inner mitochondrial membrane, short-circuiting the mitochondrion to produce heat rather than ATP. Despite 40 years of intense research, the underlying molecular mechanism of UCP1 is still under debate. The protein belongs to the mitochondrial carrier family of transporters, which have recently been shown to utilise a domain-based alternating-access mechanism, cycling between a cytoplasmic and matrix state to transport metabolites across the inner membrane. Here, we review the protein properties of UCP1 and compare them to those of mitochondrial carriers. UCP1 has the same structural fold as other mitochondrial carriers and, in contrast to past claims, is a monomer, binding one purine nucleotide and three cardiolipin molecules tightly. The protein has a single substrate binding site, which is similar to those of the dicarboxylate and oxoglutarate carriers, but also contains a proton binding site and several hydrophobic residues. As found in other mitochondrial carriers, UCP1 has two conserved salt bridge networks on either side of the central cavity, which regulate access to the substrate binding site in an alternating way. The conserved domain structures and mobile inter-domain interfaces are consistent with an alternating access mechanism too. In conclusion, UCP1 has retained all of the key features of mitochondrial carriers, indicating that it operates by a conventional carrier-like mechanism.

Salsalate (Salicylate) Uncouples Mitochondria, Improves Glucose Homeostasis, and Reduces Liver Lipids Independent of AMPK-β1

Salsalate is a prodrug of salicylate that lowers blood glucose in patients with type 2 diabetes (T2D) and reduces nonalcoholic fatty liver disease (NAFLD) in animal models; however, the mechanism mediating these effects is unclear. Salicylate directly activates AMPK via the β1 subunit, but whether salsalate requires AMPK-β1 to improve T2D and NAFLD has not been examined. Therefore, wild-type (WT) and AMPK-β1-knockout (AMPK-β1KO) mice were treated with a salsalate dose resulting in clinically relevant serum salicylate concentrations (∼1 mmol/L). Salsalate treatment increased VO₂, lowered fasting glucose, improved glucose tolerance, and led to an ∼55% reduction in liver lipid content. These effects were observed in both WT and AMPK-β1KO mice. To explain these AMPK-independent effects, we found that salicylate increases oligomycin-insensitive respiration (state 4o) and directly increases mitochondrial proton conductance at clinical concentrations. This uncoupling effect is tightly correlated with the suppression of de novo lipogenesis. Salicylate is also able to stimulate brown adipose tissue respiration independent of uncoupling protein 1. These data indicate that the primary mechanism by which salsalate improves glucose homeostasis and NAFLD is via salicylate-driven mitochondrial uncoupling.

The conserved regulation of mitochondrial uncoupling proteins: From unicellular eukaryotes to mammals

Uncoupling proteins (UCPs) belong to the mitochondrial anion carrier protein family and mediate regulated proton leak across the inner mitochondrial membrane. Free fatty acids, aldehydes such as hydroxynonenal, and retinoids activate UCPs. However, there are some controversies about the effective action of retinoids and aldehydes alone; thus, only free fatty acids are commonly accepted positive effectors of UCPs. Purine nucleotides such as GTP inhibit UCP-mediated mitochondrial proton leak. In turn, membranous coenzyme Q may play a role as a redox state-dependent metabolic sensor that modulates the complete activation/inhibition of UCPs. Such regulation has been observed for UCPs in microorganisms, plant and animal UCP1 homologues, and UCP1 in mammalian brown adipose tissue. The origin of UCPs is still under debate, but UCP homologues have been identified in all systematic groups of eukaryotes. Despite the differing levels of amino acid/DNA sequence similarities, functional studies in unicellular and multicellular organisms, from amoebae to mammals, suggest that the mechanistic regulation of UCP activity is evolutionarily well conserved. This review focuses on the regulatory feedback loops of UCPs involving free fatty acids, aldehydes, retinoids, purine nucleotides, and coenzyme Q (particularly its reduction level), which may derive from the early stages of evolution as UCP first emerged.

Uncoupling Protein, UCP-4 May Be Involved in Neuronal Defects During Aging and Resistance to Pathogens in Caenorhabditis elegans

Uncoupling proteins (UCPs) are mitochondrial inner membrane proteins that function to dissipate proton motive force and mitochondrial membrane potential. One UCP has been identified in Caenorhabditis elegans (C. elegans), namely UCP-4. In this study, we examined its expression and localization using a GFP marker in C. elegans. ucp-4 was expressed throughout the body from early embryo to aged adult and UCP-4 was localized in the mitochondria. It is known that increased mitochondrial membrane protential leads to a reactive oxygen species (ROS) increase, which is associated with age-related diseases, including neurodegenerative diseases in humans. A ucp-4 mutant showed increased mitochondrial membrane protential in association with increased neuronal defects during aging, and the neurons of ucp-4 overexpressing animals showed decreased neuronal defects during aging. These results suggest that UCP-4 may be involved in neuroprotection during aging via relieving mitochondrial membrane protential. We also investigated the relationship between UCP-4 and innate immunity because increased ROS can affect innate immunity. ucp-4 mutant displayed increased resistance to the pathogen Staphylococcus aureus compared to wild type. The enhanced immunity in the ucp-4 mutant could be related to increased mitochondrial membrane protential, presumably followed by increased ROS. In summary, UCP-4 might have an important role in neuronal aging and innate immune responses through mediating mitochondrial membrane protential.

Cell Death and Heart Failure in Obesity: Role of Uncoupling Proteins

Metabolic diseases such as obesity, metabolic syndrome, and type II diabetes are often characterized by increased reactive oxygen species (ROS) generation in mitochondrial respiratory complexes, associated with fat accumulation in cardiomyocytes, skeletal muscle, and hepatocytes. Several rodents studies showed that lipid accumulation in cardiac myocytes produces lipotoxicity that causes apoptosis and leads to heart failure, a dynamic pathological process. Meanwhile, several tissues including cardiac tissue develop an adaptive mechanism against oxidative stress and lipotoxicity by overexpressing uncoupling proteins (UCPs), specific mitochondrial membrane proteins. In heart from rodent and human with obesity, UCP2 and UCP3 may protect cardiomyocytes from death and from a state progressing to heart failure by downregulating programmed cell death. UCP activation may affect cytochrome c and proapoptotic protein release from mitochondria by reducing ROS generation and apoptotic cell death. Therefore the aim of this review is to discuss recent findings regarding the role that UCPs play in cardiomyocyte survival by protecting against ROS generation and maintaining bioenergetic metabolism homeostasis to promote heart protection.

Role of ROS and RNS Sources in Physiological and Pathological Conditions

There is significant evidence that, in living systems, free radicals and other reactive oxygen and nitrogen species play a double role, because they can cause oxidative damage and tissue dysfunction and serve as molecular signals activating stress responses that are beneficial to the organism. Mitochondria have been thought to both play a major role in tissue oxidative damage and dysfunction and provide protection against excessive tissue dysfunction through several mechanisms, including stimulation of opening of permeability transition pores. Until recently, the functional significance of ROS sources different from mitochondria has received lesser attention. However, the most recent data, besides confirming the mitochondrial role in tissue oxidative stress and protection, show interplay between mitochondria and other ROS cellular sources, so that activation of one can lead to activation of other sources. Thus, it is currently accepted that in various conditions all cellular sources of ROS provide significant contribution to processes that oxidatively damage tissues and assure their survival, through mechanisms such as autophagy and apoptosis.

Association between heat stress and oxidative stress in poultry; mitochondrial dysfunction and dietary interventions with phytochemicals

Heat as a stressor of poultry has been studied extensively for many decades; it affects poultry production on a worldwide basis and has significant impact on well-being and production. More recently, the involvement of heat stress in inducing oxidative stress has received much interest. Oxidative stress is defined as the presence of reactive species in excess of the available antioxidant capacity of animal cells. Reactive species can modify several biologically cellular macromolecules and can interfere with cell signaling pathways. Furthermore, during the last decade, there has been an ever-increasing interest in the use of a wide array of natural feed-delivered phytochemicals that have potential antioxidant properties for poultry. In light of this, the current review aims to (1) summarize the mechanisms through which heat stress triggers excessive superoxide radical production in the mitochondrion and progresses into oxidative stress, (2) illustrate that this pathophysiology is dependent on the intensity and duration of heat stress, (3) present different nutritional strategies for mitigation of mitochondrial dysfunction, with particular focus on antioxidant phytochemicals. Oxidative stress that occurs with heat exposure can be manifest in all parts of the body; however, mitochondrial dysfunction underlies oxidative stress. In the initial phase of acute heat stress, mitochondrial substrate oxidation and electron transport chain activity are increased resulting in excessive superoxide production. During the later stage of acute heat stress, down-regulation of avian uncoupling protein worsens the oxidative stress situation causing mitochondrial dysfunction and tissue damage. Typically, antioxidant enzyme activities are upregulated. Chronic heat stress, however, leads to downsizing of mitochondrial metabolic oxidative capacity, up-regulation of avian uncoupling protein, a clear alteration in the pattern of antioxidant enzyme activities, and depletion of antioxidant reserves. Some phytochemicals, such as various types of flavonoids and related compounds, were shown to be beneficial in chronic heat-stressed poultry, but were less or not effective in non-heat-stressed counterparts. This supports the contention that antioxidant phytochemicals have potential under challenging conditions. Though substantial progress has been made in our understanding of the association between heat stress and oxidative stress, the means by which phytochemicals can alleviate oxidative stress have been sparsely explored.

Role of Protein Carbonylation in Skeletal Muscle Mass Loss Associated with Chronic Conditions

Muscle dysfunction, characterized by a reductive remodeling of muscle fibers, is a common systemic manifestation in highly prevalent conditions such as chronic heart failure (CHF), chronic obstructive pulmonary disease (COPD), cancer cachexia, and critically ill patients. Skeletal muscle dysfunction and impaired muscle mass may predict morbidity and mortality in patients with chronic diseases, regardless of the underlying condition. High levels of oxidants may alter function and structure of key cellular molecules such as proteins, DNA, and lipids, leading to cellular injury and death. Protein oxidation including protein carbonylation was demonstrated to modify enzyme activity and DNA binding of transcription factors, while also rendering proteins more prone to proteolytic degradation. Given the relevance of protein oxidation in the pathophysiology of many chronic conditions and their comorbidities, the current review focuses on the analysis of different studies in which the biological and clinical significance of the modifications induced by reactive carbonyls on proteins have been explored so far in skeletal muscles of patients and animal models of chronic conditions such as COPD, disuse muscle atrophy, cancer cachexia, sepsis, and physiological aging. Future research will elucidate the specific impact and sites of reactive carbonyls on muscle protein content and function in human conditions.

UCPs, at the interface between bioenergetics and metabolism

The first member of the uncoupling protein (UCP) family, brown adipose tissue uncoupling protein 1 (UCP1), was identified in 1976. Twenty years later, two closely related proteins, UCP2 and UCP3, were described in mammals. Homologs of these proteins exist in other organisms, including plants. Uncoupling refers to a deterioration of energy conservation between substrate oxidation and ADP phosphorylation. Complete energy conservation loss would be fatal but fine-tuning can be beneficial for processes such as thermogenesis, redox control, and prevention of mitochondrial ROS release. The coupled/uncoupled state of mitochondria is related to the permeability of the inner membrane and the proton transport mediated by activated UCPs underlies the uncoupling activity of these proteins. Proton transport by UCP1 is activated by fatty acids and this ensures thermogenesis. In vivo in absence of this activation UCP1 remains inhibited with no transport activity. A similar situation now seems unlikely for UCP2 and UCP3 and while activation of their proton transport has been described its physiological relevance remains uncertain and their influence can be envisaged as a result of another transport pathway that takes place in the absence of activation. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

The 12-day thermoregulatory metamorphosis of Red-winged Blackbirds (Agelaius phoeniceus)

We examined development of endothermy in altricial Red-winged Blackbirds (Agelaius phoeniceus) by measuring oxygen consumption [Formula: see text], body temperature and ventilation at ambient temperatures from 35 to 15 °C. Mitochondrial respiration of permeabilized skeletal muscle was also measured from breast (pectoralis) and thigh (femorotibialis) muscles. Animals were studied from the first day of hatching through fledging (12 days post-hatch, dph). Nestling whole-body metabolic rate began to show an endothermic response to cold temperature midway between hatching and fledging. Nestlings less than 5 dph were unable to maintain elevated [Formula: see text] and body temperature when exposed to gradually decreasing temperature, whereas 7 dph nestlings maintained [Formula: see text] until ~25 °C, after which [Formula: see text] decreased. From 10 dph to fledging, animals maintained elevated [Formula: see text] and body temperature when exposed to gradual cooling; full endothermic capacity was achieved. Ventilation followed a similar developmental trend to that of [Formula: see text], with increases in 10 dph fledglings occurring in tidal volume rather than ventilation frequency. LEAK respiration and oxidative phosphorylation (OXPHOS) through complex I of breast muscle mitochondria increased significantly after 3 dph. Expression of avUCP and PCG-1α mRNA increased significantly at 3 dph and remained elevated in both skeletal muscle types. Increased metabolic capacity at the cellular level occurred prior to that of the whole animal. This change in whole animal metabolic capacity increased steadily upon hatching as evidenced by the shift of metabolic rate from an ectothermic to endothermic phenotype and the increase of mitochondrial OXPHOS activity of the shivering muscles of this altricial avian species.

The Complex Relationship of Extracorporeal Membrane Oxygenation and Acute Kidney Injury: Causation or Association?

Extracorporeal membrane oxygenation (ECMO) is a modified cardiopulmonary bypass (CPB) circuit capable of providing prolonged cardiorespiratory support. Recent advancement in ECMO technology has resulted in increased utilisation and clinical application. It can be used as a bridge-to-recovery, bridge-to-bridge, bridge-to-transplant, or bridge-to-decision. ECMO can restitute physiology in critically ill patients, which may minimise the risk of progressive multiorgan dysfunction. Alternatively, iatrogenic complications of ECMO clearly contribute to worse outcomes. These factors affect the risk : benefit ratio of ECMO which ultimately influence commencement/timing of ECMO. The complex interplay of pre-ECMO, ECMO, and post-ECMO pathophysiological processes are responsible for the substantial increased incidence of ECMO-associated acute kidney injury (EAKI). The development of EAKI significantly contributes to morbidity and mortality; however, there is a lack of evidence defining a potential benefit or causative link between ECMO and AKI. This area warrants investigation as further research will delineate the mechanisms involved and subsequent strategies to minimise the risk of EAKI. This review summarizes the current literature of ECMO and AKI, considers the possible benefits and risks of ECMO on renal function, outlines the related pathophysiology, highlights relevant investigative tools, and ultimately suggests an approach for future research into this under investigated area of critical care.

Modification of platelet proteins by 4-hydroxynonenal: Potential Mechanisms for inhibition of aggregation and metabolism

Platelet aggregation is an essential response to tissue injury and is associated with activation of pro-oxidant enzymes, such as cyclooxygenase, and is also a highly energetic process. The two central energetic pathways in the cell, glycolysis and mitochondrial oxidative phosphorylation, are susceptible to damage by reactive lipid species. Interestingly, how platelet metabolism is affected by the oxidative stress associated with aggregation is largely unexplored. To address this issue, we examined the response of human platelets to 4-hydroxynonenal (4-HNE), a reactive lipid species which is generated during thrombus formation and during oxidative stress. Elevated plasma 4-HNE has been associated with renal failure, septic shock and cardiopulmonary bypass surgery. In this study, we found that 4-HNE decreased thrombin stimulated platelet aggregation by approximately 60%. The metabolomics analysis demonstrated that underlying our previous observation of a stimulation of platelet energetics by thrombin glycolysis and TCA (Tricarboxylic acid) metabolites were increased. Next, we assessed the effect of both 4-HNE and alkyne HNE (A-HNE) on bioenergetics and targeted metabolomics, and found a stimulatory effect on glycolysis, associated with inhibition of bioenergetic parameters. In the presence of HNE and thrombin glycolysis was further stimulated but the levels of the TCA metabolites were markedly suppressed. Identification of proteins modified by A-HNE followed by click chemistry and mass spectrometry revealed essential targets in platelet activation including proteins involved in metabolism, adhesion, cytoskeletal reorganization, aggregation, vesicular transport, protein folding, antioxidant proteins, and small GTPases. In summary, the biological effects of 4-HNE can be more effectively explained in platelets by the integrated effects of the modification of an electrophile responsive proteome rather than the isolated effects of candidate proteins.

Methionine deficiency leads to hepatic fat accretion via impairment of fatty acid import by carnitine palmitoyltransferase I

1. To clarify the underlying mechanism of hepatic fat accretion due to methionine (Met) deficiency in broiler chickens, the present study investigated the effect of Met deficiency on the hepatic carnitine palmitoyltransferase (CPT) system, which imports fatty acids into mitochondria. 2. Fifteen-d-old male meat-type chickens were fed on either a control diet (containing 0.52 g/100 g Met) or a Met-deficient diet (containing 0.27 g Met/100 g). After a 10-d feeding period, the birds were killed by decapitation and their livers excised to determine hepatic CPT1 and CPT2 mRNA levels and for the related hepatic fatty acid-supported mitochondrial respiration to be measured. 3. Met deficiency decreased body weight gain and feed efficiency and increased hepatic lipid content compared to the control group. Whereas the hepatic CPT2 mRNA level in the Met-deficient group remained unchanged compared to that of the control group, the CPT1 mRNA level was decreased in the Met-deficient group and CPT1-dependent hepatic mitochondrial respiration was impaired. 4. Our results suggest that the hepatic lipid accretion that occurs in response to Met deficiency might be attributable to the impairment of CPT1-mediated fatty acid import into mitochondria.

A PGC-1α-Mediated Transcriptional Network Maintains Mitochondrial Redox and Bioenergetic Homeostasis against Doxorubicin-Induced Toxicity in Human Cardiomyocytes: Implementation of TT21C

Chemical toxicity testing is fast moving in a direction that relies increasingly on cell-basedin vitroassays anchored on toxicity pathways according to the toxicity testing in the 21st century vision. Identifying points of departure (POD) via these assays and revealing their mechanistic underpinnings via computational modeling of the relevant pathways are critical and challenging steps. Here we used doxorubicin (DOX) as a prototype chemical to study mitochondrial toxicity in human AC16 cells. Mitochondrial toxicity has been linked to cardiovascular risk of DOX, which has limited its clinical use as an antitumor drug. Ourin vitrostudy revealed a well-defined POD concentration of DOX below which adaptive induction of proliferator-activated receptor-γ coactivator-1α (PGC-1α) -mediated mitochondrial genes, including NRF-1, MnSOD, UCP2, and COX1, concurred with negligible changes in mitochondrial superoxide and cytotoxicity. At higher DOX concentrations adversity became significant with elevated superoxide and suppressed ATP levels. A computational model was formulated to simulate the PGC-1α-mediated transcriptional network comprising multiple negative feedback loops that underlie redox and bioenergetics homeostasis in the mitochondrion. The model recapitulated the transition phase from adaptive to adverse responses, supporting the notion that saturated induction of PGC-1α-mediated gene network underpins POD. The model further predicts (follow-up experiments verified) that silencing PGC-1α compromises the adaptive function of the transcriptional network, leading to disruption of mitochondria and cytotoxicity at lower DOX concentrations. In summary, our study demonstrates that combining pathway-focusedin vitroassays and computational simulation of relevant biochemical network is synergistic for understanding dose-response behaviors in the low-dose region and identifying POD.

Interplay between TDP-43 and docosahexaenoic acid-related processes in amyotrophic lateral sclerosis

BACKGROUND

Docosahexaenoic acid (DHA), a key lipid in nervous system homeostasis, is depleted in the spinal cord of sporadic amyotrophic lateral sclerosis (sALS) patients. However, the basis for such loss was unknown.

METHODS

DHA synthetic machinery was evaluated in spinal cord samples from ALS patients and controls by immunohistochemistry and western blot. Further, lipid composition was measured in organotypic spinal cord cultures by gas chromatography and liquid chromatography coupled to mass spectrometry. In these samples, mitochondrial respiratory functions were measured by high resolution respirometry. Finally, Neuro2-A and stem cell-derived human neurons were used for evaluating mechanistic relationships between TDP-43 aggregation, oxidative stress and cellular changes in DHA-related proteins.

RESULTS

ALS is associated to changes in the spinal cord distribution of DHA synthesis enzymatic machinery comparing ten ALS cases and eight controls. We found increased levels of desaturases (ca 95% increase, p<0.001), but decreased amounts of DHA-related β-oxidation enzymes in ALS samples (40% decrease, p<0.05). Further, drebrin, a DHA-dependent synaptic protein, is depleted in spinal cord samples from ALS patients (around 40% loss, p<0.05). In contrast, chronic excitotoxicity in spinal cord increases DHA acid amount, with both enhanced concentrations of neuroprotective docosahexaenoic acid-derived resolvin D, and higher lipid peroxidation-derived molecules such as 8-iso-prostaglandin-F2-α (8-iso-PGF2α) levels. Since α-tocopherol improved mitochondrial respiratory function and motor neuron survival in these conditions, it is suggested that oxidative stress could boost motor neuron loss. Cell culture and metabolic flux experiments, showing enhanced expression of desaturases (FADS2) and β-oxidation enzymes after H2O2 challenge suggest that DHA production can be an initial response to oxidative stress, driven by TDP-43 aggregation and drebrin loss. Interestingly, these changes were dependent on cell type used, since human neurons exhibited losses of FADS2 and drebrin after oxidative stress. These features (drebrin loss and FADS2 alterations) were also produced by transfection by aggregation prone C-terminal fragments of TDP-43.

CONCLUSIONS

sALS is associated with tissue-specific DHA-dependent synthetic machinery alteration. Furthermore, excitotoxicity sinergizes with oxidative stress to increase DHA levels, which could act as a response over stress, involving the expression of DHA synthetic enzymes. Later on, this allostatic overload could exacerbate cell stress by contributing to TDP-43 aggregation. This, at its turn, could blunt this protective response, overall leading to DHA depletion and neuronal dysfunction.

Neurotransmitters in the vestibular system

Neuronal networks that are linked to the peripheral vestibular system contribute to gravitoinertial sensation, balance control, eye movement control, and autonomic function. Ascending connections to the limbic system and cerebral cortex are also important for motion perception and threat recognition, and play a role in comorbid balance and anxiety disorders. The vestibular system also shows remarkable plasticity, termed vestibular compensation. Activity in these networks is regulated by an interaction between: (1) intrinsic neurotransmitters of the inner ear, vestibular nerve, and vestibular nuclei; (2) neurotransmitters associated with thalamocortical and limbic pathways that receive projections originating in the vestibular nuclei; and (3) locus coeruleus and raphe (serotonergic and nonserotonergic) projections that influence the latter components. Because the ascending vestibular interoceptive and thalamocortical pathways include networks that influence a broad range of stress responses (endocrine and autonomic), memory consolidation, and cognitive functions, common transmitter substrates provide a basis for understanding features of acute and chronic vestibular disorders.

Hypothesis on Skeletal Muscle Aging: Mitochondrial Adenine Nucleotide Translocator Decreases Reactive Oxygen Species Production While Preserving Coupling Efficiency

Mitochondrial membrane potential is the major regulator of mitochondrial functions, including coupling efficiency and production of reactive oxygen species (ROS). Both functions are crucial for cell bioenergetics. We previously presented evidences for a specific modulation of adenine nucleotide translocase (ANT) appearing during aging that results in a decrease in membrane potential - and therefore ROS production-but surprisingly increases coupling efficiency under conditions of low ATP turnover. Careful study of the bioenergetic parameters (oxidation and phosphorylation rates, membrane potential) of isolated mitochondria from skeletal muscles (gastrocnemius) of aged and young rats revealed a remodeling at the level of the phosphorylation system, in the absence of alteration of the inner mitochondrial membrane (uncoupling) or respiratory chain complexes regulation. We further observed a decrease in mitochondrial affinity for ADP in aged isolated mitochondria, and higher sensitivity of ANT to its specific inhibitor atractyloside. This age-induced modification of ANT results in an increase in the ADP concentration required to sustain the same ATP turnover as compared to young muscle, and therefore in a lower membrane potential under phosphorylating-in vivo-conditions. Thus, for equivalent ATP turnover (cellular ATP demand), coupling efficiency is even higher in aged muscle mitochondria, due to the down-regulation of inner membrane proton leak caused by the decrease in membrane potential. In the framework of the radical theory of aging, these modifications in ANT function may be the result of oxidative damage caused by intra mitochondrial ROS and may appear like a virtuous circle where ROS induce a mechanism that reduces their production, without causing uncoupling, and even leading in improved efficiency. Because of the importance of ROS as therapeutic targets, this new mechanism deserves further studies.

Mitochondrial oxidative metabolism and uncoupling proteins in the failing heart

Despite significant progress in cardiovascular medicine, myocardial ischemia and infarction, progressing eventually to the final end point heart failure (HF), remain the leading cause of morbidity and mortality in the USA. HF is a complex syndrome that results from any structural or functional impairment in ventricular filling or blood ejection. Ultimately, the heart's inability to supply the body's tissues with enough blood may lead to death. Mechanistically, the hallmarks of the failing heart include abnormal energy metabolism, increased production of reactive oxygen species (ROS) and defects in excitation-contraction coupling. HF is a highly dynamic pathological process, and observed alterations in cardiac metabolism and function depend on the disease progression. In the early stages, cardiac remodeling characterized by normal or slightly increased fatty acid (FA) oxidation plays a compensatory, cardioprotective role. However, upon progression of HF, FA oxidation and mitochondrial oxidative activity are decreased, resulting in a significant drop in cardiac ATP levels. In HF, as a compensatory response to decreased oxidative metabolism, glucose uptake and glycolysis are upregulated, but this upregulation is not sufficient to compensate for a drop in ATP production. Elevated mitochondrial ROS generation and ROS-mediated damage, when they overwhelm the cellular antioxidant defense system, induce heart injury and contribute to the progression of HF. Mitochondrial uncoupling proteins (UCPs), which promote proton leak across the inner mitochondrial membrane, have emerged as essential regulators of mitochondrial membrane potential, respiratory activity and ROS generation. Although the physiological role of UCP2 and UCP3, expressed in the heart, has not been clearly established, increasing evidence suggests that these proteins by promoting mild uncoupling could reduce mitochondrial ROS generation and cardiomyocyte apoptosis and ameliorate thereby myocardial function. Further investigation on the alterations in cardiac UCP activity and regulation will advance our understanding of their physiological roles in the healthy and diseased heart and also may facilitate the development of novel and more efficient therapies.