The ATP/ADP-antiporter is involved in the uncoupling effect of fatty acids on mitochondria

Mitochondrial function in skeletal muscle contributes to reproductive endothermy in tegu lizards (Salvator merianae)

AIM

In cyclic climate variations, including seasonal changes, many animals regulate their energy demands to overcome critical transitory moments, restricting their high-demand activities to phases of resource abundance, enabling rapid growth and reproduction. Tegu lizards (Salvator merianae) are ectotherms with a robust annual cycle, being active during summer, hibernating during winter, and presenting a remarkable endothermy during reproduction in spring. Here, we evaluated whether changes in mitochondrial respiratory physiology in skeletal muscle could serve as a mechanism for the increased thermogenesis observed during the tegu's reproductive endothermy.

METHODS

We performed high-resolution respirometry and calorimetry in permeabilized red and white muscle fibers, sampled during summer (activity) and spring (high activity and reproduction), in association with citrate synthase measurements.

RESULTS

During spring, the muscle fibers exhibited increased oxidative phosphorylation. They also enhanced uncoupled respiration and heat production via adenine nucleotide translocase (ANT), but not via uncoupling proteins (UCP). Citrate synthase activity was higher during the spring, suggesting greater mitochondrial density compared to the summer. These findings were consistent across both sexes and muscle types (red and white).

CONCLUSION

The current results highlight potential cellular thermogenic mechanisms in an ectothermic reptile that contribute to transient endothermy. Our study indicates that the unique feature of transitioning to endothermy through nonshivering thermogenesis during the reproductive phase may be facilitated by higher mitochondrial density, function, and uncoupling within the skeletal muscle. This knowledge contributes significant elements to the broader picture of models for the evolution of endothermy, particularly in relation to the enhancement of aerobic capacity.

Theoretical and Experimental Study of the Interaction of Protonophore Uncouplers and Decoupling Agents with Functionally Active Mitochondria

The purpose of this work was to quantitatively characterize the effectiveness of oxidative phosphorylation uncouplers and decoupling agents in functionally active mitochondria, taking into account their content in the hydrophobic region of the inner membrane of these organelles. When conducting theoretical studies, it is accepted that uncouplers and decouplers occupy part of the volume of mitochondria to exhibit their activity, which is defined as the effective volume. The following quantities characterizing the action of these reagents are considered: (1) concentrations of reagents that cause double stimulation of mitochondrial respiration in state 4 (C 200 ); (2) effective distribution coefficient (E MW ) - the ratio of the amount of reagents in the effective volume of mitochondria and the water volume; (3) the relative amount of reagents associated with the effective volume of mitochondria (U M / U T ); (4) specific activity of reagents localized in the effective volume of mitochondria (A M ). We have developed methods for determining these values, based on an analysis of the dependence of the rate of mitochondrial respiration on the concentration of uncouplers and decoupling agents at two different concentrations of mitochondrial protein in the incubation medium. During experimental studies, we compared the effects of the classical protonophore uncouplers 2,4-dinitrophenol (DNP) and сarbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP), the natural uncouplers lauric and palmitic acids, and the natural decouplers α,ω-tetradecanedioic (TDA) and α,ω-hexadecanedioic (HDA) acids that differ both in the structure of the molecule and in the degree of solubility in lipids. Using the developed methods, we have clarified the dependence of the degree of activity of these uncouplers and decoupling agents on the distribution of their molecules between the effective volume of mitochondria and the water volume.

ATP homeostasis and signaling in plants

ATP is the primary form of energy for plants, and a shortage of cellular ATP is generally acknowledged to pose a threat to plant growth and development, stress resistance, and crop quality. The overall metabolic processes that contribute to the ATP pool, from production, dissipation, and transport to elimination, have been studied extensively. Considerable evidence has revealed that in addition to its role in energy supply, ATP also acts as a regulatory signaling molecule to activate global metabolic responses. Identification of the eATP receptor DORN1 contributed to a better understanding of how plants cope with disruption of ATP homeostasis and of the key points at which ATP signaling pathways intersect in cells or whole organisms. The functions of SnRK1α, the master regulator of the energy management network, in restoring the equilibrium of the ATP pool have been demonstrated, and the vast and complex metabolic network mediated by SnRK1α to adapt to fluctuating environments has been characterized. This paper reviews recent advances in understanding the regulatory control of the cellular ATP pool and discusses possible interactions among key regulators of ATP-pool homeostasis and crosstalk between iATP/eATP signaling pathways. Perception of ATP deficit and modulation of cellular ATP homeostasis mediated by SnRK1α in plants are discussed at the physiological and molecular levels. Finally, we suggest future research directions for modulation of plant cellular ATP homeostasis.

A model naphthenic acid decouples oxidative phosphorylation through selective inhibition of mitochondrial complex activity

The naphthenic acid fraction compound (NAFC), 3,5-dimethyladamantane-1-acetic acid, was tested for its ability to uncouple mitochondrial oxidative phosphorylation. Mitochondria isolated from rainbow trout (Oncorhynchus mykiss) liver were exposed to 3,5-dimethyladamantane-1-acetic acid in state 3 and 4 respiration, and mitochondrial membrane potential were quantified. Electron transport chain (ETC) protein complexes were isolated using pharmacological agents and inhibitors, and their activities measured. The NAFC compound completely inhibited states 3 and 4 respiration with an IC50 of 0.77 and 1.01 mM, respectively. The NAFC compound partially uncoupled mitochondrial membrane potential in state 3 and 4 respiration with an IC50 of 2.19 and 1.73 mM, respectively. The NAFC impaired the activities of ETC protein complexes with a 9.5-fold range in sensitivity. The relative inhibitory effect of the ETC protein complexes to NAFC was CIV≥CI>CIII>CII. The impairment of mitochondrial oxidative phosphorylation by adamantane 3,5-dimethyladamantane-1-acetic acid is mediated via its inhibition of ETC protein complexes.

Mitocentricity

Worldwide, interest in mitochondria is constantly growing, as evidenced by scientific statistics, and studies of the functioning of these organelles are becoming more prevalent than studies of other cellular structures. In this analytical review, mitochondria are conditionally placed in a certain cellular center, which is responsible for both energy production and other non-energetic functions, without which the existence of not only the eukaryotic cell itself, but also the entire organism is impossible. Taking into account the high multifunctionality of mitochondria, such a fundamentally new scheme of cell functioning organization, including mitochondrial management of processes that determine cell survival and death, may be justified. Considering that this issue is dedicated to the memory of V. P. Skulachev, who can be called mitocentric, due to the history of his scientific activity almost entirely aimed at studying mitochondria, this work examines those aspects of mitochondrial functioning that were directly or indirectly the focus of attention of this outstanding scientist. We list all possible known mitochondrial functions, including membrane potential generation, synthesis of Fe-S clusters, steroid hormones, heme, fatty acids, and CO2. Special attention is paid to the participation of mitochondria in the formation and transport of water, as a powerful biochemical cellular and mitochondrial regulator. The history of research on reactive oxygen species that generate mitochondria is subject to significant analysis. In the section "Mitochondria in the center of death", special emphasis is placed on the analysis of what role and how mitochondria can play and determine the program of death of an organism (phenoptosis) and the contribution made to these studies by V. P. Skulachev.

Uncoupling Protein 3 Catalyzes the Exchange of C4 Metabolites Similar to UCP2

Uncoupling protein 3 (UCP3) belongs to the mitochondrial carrier protein superfamily SLC25 and is abundant in brown adipose tissue (BAT), the heart, and muscles. The expression of UCP3 in tissues mainly dependent on fatty acid oxidation suggests its involvement in cellular metabolism and has drawn attention to its possible transport function beyond the transport of protons in the presence of fatty acids. Based on the high homology between UCP2 and UCP3, we hypothesized that UCP3 transports C4 metabolites similar to UCP2. To test this, we measured the transport of substrates against phosphate (32Pi) in proteoliposomes reconstituted with recombinant murine UCP3 (mUCP3). We found that mUCP3 mainly transports aspartate and sulfate but also malate, malonate, oxaloacetate, and succinate. The transport rates calculated from the exchange of 32Pi against extraliposomal aspartate and sulfate were 23.9 ± 5.8 and 17.5 ± 5.1 µmol/min/mg, respectively. Using site-directed mutagenesis, we revealed that mutation of R84 resulted in impaired aspartate/phosphate exchange, demonstrating its critical role in substrate transport. The difference in substrate preference between mUCP2 and mUCP3 may be explained by their different tissue expression patterns and biological functions in these tissues.

Direct and indirect targets of carboxyatractyloside, including overlooked toxicity toward nucleoside diphosphate kinase (NDPK) and mitochondrial H+ leak

CONTEXT

The toxicity of atractyloside/carboxyatractyloside is generally well recognized and commonly ascribed to the inhibition of mitochondrial ADP/ATP carriers, which are pivotal for oxidative phosphorylation. However, these glycosides may 'paralyze' additional target proteins.

OBJECTIVE

This review presents many facts about atractyloside/carboxyatractyloside and their plant producers, such as Xanthium spp. (Asteraceae), named cockleburs.

METHODS

Published studies and other information were obtained from databases, such as 'CABI - Invasive Species Compendium', 'PubMed', and 'The World Checklist of Vascular Plants', from 1957 to December 2022. The following major keywords were used: 'carboxyatractyloside', 'cockleburs', 'hepatotoxicity', 'mitochondria', 'nephrotoxicity', and 'Xanthium'.

RESULTS

In the third decade of the twenty first century, public awareness of the severe toxicity of cockleburs is still limited. Such toxicity is often only perceived by specialists in Europe and other continents. Interestingly, cocklebur is among the most widely distributed invasive plants worldwide, and the recognition of new European stands of Xanthium spp. is provided here. The findings arising from field and laboratory research conducted by the author revealed that (i) some livestock populations may instinctively avoid eating cocklebur while grazing, (ii) carboxyatractyloside inhibits ADP/GDP metabolism, and (iii) the direct/indirect target proteins of carboxyatractyloside are ambiguous.

CONCLUSIONS

Many aspects of the Xanthium genus still require substantial investigation/revision in the future, such as the unification of the Latin nomenclature of currently distinguished species, bur morphology status, true fruit (achene) description and biogeography of cockleburs, and a detailed description of the physiological roles of atractyloside/carboxyatractyloside and the toxicity of these glycosides, mainly toward mammals. Therefore, a more careful interpretation of atractyloside/carboxyatractyloside data, including laboratory tests using Xanthium-derived extracts and purified toxins, is needed.

Adverse bioenergetic effects of N-acyl amino acids in human adipocytes overshadow beneficial mitochondrial uncoupling

OBJECTIVE

Enhancing energy turnover via uncoupled mitochondrial respiration in adipose tissue has great potential to improve human obesity and other metabolic complications. However, the amount of human brown adipose tissue and its uncoupling protein 1 (UCP1) is low in obese patients. Recently, a class of endogenous molecules, N-acyl amino acids (NAAs), was identified as mitochondrial uncouplers in murine adipocytes, presumably acting via the adenine nucleotide translocator (ANT). Given the translational potential, we investigated the bioenergetic effects of NAAs in human adipocytes, characterizing beneficial and adverse effects, dose ranges, amino acid derivatives and underlying mechanisms.

METHOD

NAAs with neutral (phenylalanine, leucine, isoleucine) and polar (lysine) residues were synthetized and assessed in intact and permeabilized human adipocytes using plate-based respirometry. The Seahorse technology was applied to measure bioenergetic parameters, dose-dependency, interference with UCP1 and adenine nucleotide translocase (ANT) activity, as well as differences to the established chemical uncouplers niclosamide ethanolamine (NEN) and 2,4-dinitrophenol (DNP).

RESULT

NAAs with neutral amino acid residues potently induce uncoupled respiration in human adipocytes in a dose-dependent manner, even in the presence of the UCP1-inhibitor guanosine diphosphate (GDP) and the ANT-inhibitor carboxyatractylate (CAT). However, neutral NAAs significantly reduce maximal oxidation rates, mitochondrial ATP-production, coupling efficiency and reduce adipocyte viability at concentrations above 25 μM. The in vitro therapeutic index (using induced proton leak and viability as determinants) of NAAs is lower than that of NEN and DNP.

CONCLUSION

NAAs are potent mitochondrial uncouplers in human adipocytes, independent of UCP1 and ANT. However, previously unnoticed adverse effects harm adipocyte functionality, reduce the therapeutic index of NAAs in vitro and therefore question their suitability as anti-obesity agents without further chemical modifications.

The transient character of mitochondrial uncoupling by the popular fungicide fluazinam is specific for liver

The popular fungicide fluazinam is known to exhibit an unusual cyclic pattern of the protonophoric uncoupling activity in isolated rat liver mitochondria (RLM), with membrane deenergization followed by spontaneous recoupling in the minute scale, which is associated with glutathione conjugation of fluazinam catalyzed by glutathione-S-transferase (GST). Here, we compare the fluazinam effect on RLM with that on rat kidney (RKM) and heart (RHM) mitochondria by monitoring three bioenergetic parameters: oxygen consumption rate, mitochondrial membrane potential and reduction of nucleotides. Only in RLM, the uncoupling activity of fluazinam was transient, i.e. disappeared in a few minutes, whereas in RKM and RHM it was stable in this time scale. We attribute this difference to the increased activity of mitochondrial GST in liver. We report data on the detection of glutathione-fluazinam conjugates by mass-spectrometry, thin layer chromatography and capillary electrophoresis after incubation of fluazinam with RLM but not with RKM, which supports the assumption of the tissue specificity of the conjugation.

FA Sliding as the Mechanism for the ANT1-Mediated Fatty Acid Anion Transport in Lipid Bilayers

Mitochondrial adenine nucleotide translocase (ANT) exchanges ADP for ATP to maintain energy production in the cell. Its protonophoric function in the presence of long-chain fatty acids (FA) is also recognized. Our previous results imply that proton/FA transport can be best described with the FA cycling model, in which protonated FA transports the proton to the mitochondrial matrix. The mechanism by which ANT1 transports FA anions back to the intermembrane space remains unclear. Using a combined approach involving measurements of the current through the planar lipid bilayers reconstituted with ANT1, site-directed mutagenesis and molecular dynamics simulations, we show that the FA anion is first attracted by positively charged arginines or lysines on the matrix side of ANT1 before moving along the positively charged protein-lipid interface and binding to R79, where it is protonated. We show that R79 is also critical for the competitive binding of ANT1 substrates (ADP and ATP) and inhibitors (carboxyatractyloside and bongkrekic acid). The binding sites are well conserved in mitochondrial SLC25 members, suggesting a general mechanism for transporting FA anions across the inner mitochondrial membrane.

Adenine nucleotide translocase: Current knowledge in post-translational modifications, regulations and pathological implications for human diseases

Adenine nucleotide translocases (ANTs) are central to mitochondrial integrity and bioenergetic metabolism. This review aims to integrate the progresses and knowledge on ANTs over the last few years, contributing to a potential implication of ANTs for various diseases. Structures, functions, modifications, regulators and pathological implications of ANTs for human diseases are intensively demonstrated here. ANTs have four isoforms (ANT1-4), responsible for exchanging ATP/ADP, possibly composing of pro-apoptotic mPTP as a major component, and mediating FA-dependent uncoupling of proton efflux. ANT can be modified by methylation, nitrosylation and nitroalkylation, acetylation, glutathionylation, phosphorylation, carbonylation and hydroxynonenal-induced modifications. Compounds, including bongkrekic acid, atractyloside calcium, carbon monoxide, minocycline, 4-(N-(S-penicillaminylacetyl)amino) phenylarsonous acid, cardiolipin, free long-chain fatty acids, agaric acid, long chain acyl-coenzyme A esters, all have an ability to regulate ANT activities. ANT impairment leads to bioenergetic failure and mitochondrial dysfunction, contributing to pathogenesis of diseases, such as diabetes (deficiency), heart disease (deficiency), Parkinson's disease (reduction), Sengers Syndrome (decrease), cancer (isoform shifting), Alzheimer's Disease (coaggregation with Tau), Progressive External Opthalmoplegia (mutation), and Fascioscapulohumeral muscular dystrophy (overexpression). This review improves the understanding of the mechanism of ANT in pathogenesis of human diseases, and opens a window for novel therapeutic strategies targeted on ANT in diseases.

The Haves and Have-Nots: The Mitochondrial Permeability Transition Pore across Species

The demonstration that F1FO (F)-ATP synthase and adenine nucleotide translocase (ANT) can form Ca2+-activated, high-conductance channels in the inner membrane of mitochondria from a variety of eukaryotes led to renewed interest in the permeability transition (PT), a permeability increase mediated by the PT pore (PTP). The PT is a Ca2+-dependent permeability increase in the inner mitochondrial membrane whose function and underlying molecular mechanisms have challenged scientists for the last 70 years. Although most of our knowledge about the PTP comes from studies in mammals, recent data obtained in other species highlighted substantial differences that could be perhaps attributed to specific features of F-ATP synthase and/or ANT. Strikingly, the anoxia and salt-tolerant brine shrimp Artemia franciscana does not undergo a PT in spite of its ability to take up and store Ca2+ in mitochondria, and the anoxia-resistant Drosophila melanogaster displays a low-conductance, selective Ca2+-induced Ca2+ release channel rather than a PTP. In mammals, the PT provides a mechanism for the release of cytochrome c and other proapoptotic proteins and mediates various forms of cell death. In this review, we cover the features of the PT (or lack thereof) in mammals, yeast, Drosophila melanogaster, Artemia franciscana and Caenorhabditis elegans, and we discuss the presence of the intrinsic pathway of apoptosis and of other forms of cell death. We hope that this exercise may help elucidate the function(s) of the PT and its possible role in evolution and inspire further tests to define its molecular nature.

Bile Acids as Inducers of Protonophore and Ionophore Permeability of Biological and Artificial Membranes

It is now generally accepted that the role of bile acids in the organism is not limited to their participation in the process of food digestion. Indeed, bile acids are signaling molecules and being amphiphilic compounds, are also capable of modifying the properties of cell membranes and their organelles. This review is devoted to the analysis of data on the interaction of bile acids with biological and artificial membranes, in particular, their protonophore and ionophore effects. The effects of bile acids were analyzed depending on their physicochemical properties: namely the structure of their molecules, indicators of the hydrophobic-hydrophilic balance, and the critical micelle concentration. Particular attention is paid to the interaction of bile acids with the powerhouse of cells, the mitochondria. It is of note that bile acids, in addition to their protonophore and ionophore actions, can also induce Ca²⁺-dependent nonspecific permeability of the inner mitochondrial membrane. We consider the unique action of ursodeoxycholic acid as an inducer of potassium conductivity of the inner mitochondrial membrane. We also discuss a possible relationship between this K⁺ ionophore action of ursodeoxycholic acid and its therapeutic effects.

Mitochondrial ATP Synthase and Mild Uncoupling by Butyl Ester of Rhodamine 19, C4R1

The homeostasis of the transmembrane potential of hydrogen ions in mitochondria is a prerequisite for the normal mitochondrial functioning. However, in different pathological conditions it is advisable to slightly reduce the membrane potential, while maintaining it at levels sufficient to produce ATP that will ensure the normal functioning of the cell. A number of chemical agents have been found to provide mild uncoupling; however, natural proteins residing in mitochondrial membrane can carry this mission, such as proteins from the UCP family, an adenine nucleotide translocator and a dicarboxylate carrier. In this study, we demonstrated that the butyl ester of rhodamine 19, C4R1, binds to the components of the mitochondrial ATP synthase complex due to electrostatic interaction and has a good uncoupling effect. The more hydrophobic derivative C12R1 binds poorly to mitochondria with less uncoupling activity. Mass spectrometry confirmed that C4R1 binds to the β-subunit of mitochondrial ATP synthase and based on molecular docking, a C4R1 binding model was constructed suggesting the binding site on the interface between the α- and β-subunits, close to the anionic amino acid residues of the β-subunit. The association of the uncoupling effect with binding suggests that the ATP synthase complex can provide induced uncoupling.

Membrane Lipid Reshaping Underlies Oxidative Stress Sensing by the Mitochondrial Proteins UCP1 and ANT1

Oxidative stress and ROS are important players in the pathogenesis of numerous diseases. In addition to directly altering proteins, ROS also affects lipids with negative intrinsic curvature such as phosphatidylethanolamine (PE), producing PE adducts and lysolipids. The formation of PE adducts potentiates the protonophoric activity of mitochondrial uncoupling proteins, but the molecular mechanism remains unclear. Here, we linked the ROS-mediated change in lipid shape to the mechanical properties of the membrane and the function of uncoupling protein 1 (UCP1) and adenine nucleotide translocase 1 (ANT1). We show that the increase in the protonophoric activity of both proteins occurs due to the decrease in bending modulus in lipid bilayers in the presence of lysophosphatidylcholines (OPC and MPC) and PE adducts. Moreover, MD simulations showed that modified PEs and lysolipids change the lateral pressure profile of the membrane in the same direction and by the similar amplitude, indicating that modified PEs act as lipids with positive intrinsic curvature. Both results indicate that oxidative stress decreases stored curvature elastic stress (SCES) in the lipid bilayer membrane. We demonstrated that UCP1 and ANT1 sense SCES and proposed a novel regulatory mechanism for the function of these proteins. The new findings should draw the attention of the scientific community to this important and unexplored area of redox biochemistry.

Inhibition of respiratory complex I by 6-ketocholestanol: Relevance to recoupling action in mitochondria

6-Ketocholestanol (kCh) is known as a mitochondrial recoupler, i.e. it abolishes uncoupling of mitochondria by such potent agents as carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and 3,5-di(tert-butyl)-4-hydroxybenzylidenemalononitril (SF6847) [Starkov et al., 1997]. Here, we report data on the kCh-induced inhibition of both NADH-oxidase and NADH-ubiquinone oxidoreductase activities of the respiratory complex I in bovine heart submitochondrial particles (SMP). Based on the absence of such inhibition with hexaammineruthenium (III) (HAR) as the complex I electron acceptor, the kCh effect could be associated with the ubiquinone-binding centre of this respiratory enzyme. In isolated rat liver mitochondria (RLM), kCh inhibited oxygen consumption with the glutamate/malate, substrates of NAD-linked dehydrogenases, while no inhibition of RLM respiration was observed with succinate, in agreement with the absence of the kCh effect on the succinate oxidase activity in SMP. Three kCh analogs (cholesterol, 6α-hydroxycholesterol, and 5α,6α-epoxycholesterol) exhibited no effect on the NADH oxidase activities in both SMP and RLM. Importantly, the kCh analogs were ineffective in the recoupling of RLM treated with CCCP or SF6847. Therefore, interaction of kCh with the complex I may be involved in the kCh-mediated mitochondrial recoupling.

The multiple facets of mitochondrial regulations controlling cellular thermogenesis

Understanding temperature production and regulation in endotherm organisms becomes a crucial challenge facing the increased frequency and intensity of heat strokes related to global warming. Mitochondria, located at the crossroad of metabolism, respiration, Ca²⁺ homeostasis, and apoptosis, were recently proposed to further act as cellular radiators, with an estimated inner temperature reaching 50 °C in common cell lines. This inner thermogenesis might be further exacerbated in organs devoted to produce consistent efforts as muscles, or heat as brown adipose tissue, in response to acute solicitations. Consequently, pathways promoting respiratory chain uncoupling and mitochondrial activity, such as Ca²⁺ fluxes, uncoupling proteins, futile cycling, and substrate supplies, provide the main processes controlling heat production and cell temperature. The mitochondrial thermogenesis might be further amplified by cytoplasmic mechanisms promoting the over-consumption of ATP pools. Considering these new thermic paradigms, we discuss here all conventional wisdoms linking mitochondrial functions to cellular thermogenesis in different physiological conditions.

Targeting skeletal muscle mitochondrial health in obesity

Metabolic demands of skeletal muscle are substantial and are characterized normally as highly flexible and with a large dynamic range. Skeletal muscle composition (e.g., fiber type and mitochondrial content) and metabolism (e.g., capacity to switch between fatty acid and glucose substrates) are altered in obesity, with some changes proceeding and some following the development of the disease. Nonetheless, there are marked interindividual differences in skeletal muscle composition and metabolism in obesity, some of which have been associated with obesity risk and weight loss capacity. In this review, we discuss related molecular mechanisms and how current and novel treatment strategies may enhance weight loss capacity, particularly in diet-resistant obesity.

Alkyl esters of 7-hydroxycoumarin-3-carboxylic acid as potent tissue-specific uncouplers of oxidative phosphorylation: Involvement of ATP/ADP translocase in mitochondrial uncoupling

An impressive body of evidence has been accumulated now on sound beneficial effects of mitochondrial uncouplers in struggling with the most dangerous pathologies such as cancer, infective diseases, neurodegeneration and obesity. To increase their efficacy while gaining further insight in the mechanism of the uncoupling action has been remaining a challenge. Encouraged by our previous promising results on lipophilic derivatives of 7-hydroxycoumarin-4-acetic acid (UB-4 esters), here, we use a 7-hydroxycoumarin-3-carboxylic acid scaffold to synthesize a new series of 7-hydroxycoumarin (umbelliferone, UB)-derived uncouplers of oxidative phosphorylation - alkyl esters of umbelliferone-3-carboxylic acid (UB-3 esters) with varying carbon chain length. Compared to the UB-4 derivatives, UB-3 esters proved to be stronger uncouplers: the most effective of them caused a pronounced increase in the respiration rate of isolated rat heart mitochondria (RHM) at submicromolar concentrations. Both of these series of UB derivatives exhibited a striking difference between their uncoupling patterns in mitochondria isolated from liver and heart or kidney, namely: a pronounced but transient decrease in membrane potential, followed by its recovery, was observed after the addition of these compounds to isolated rat liver mitochondria (RLM), while the depolarization of RHM and rat kidney mitochondria (RKM) was rather stable under the same conditions. Interestingly, partial reversal of this depolarization in RHM and RKM was caused by carboxyatractyloside, an inhibitor of ATP/ADP translocase, thereby pointing to the involvement of this mitochondrial membrane protein in the uncoupling activity of both UB-3 and UB-4 esters. The fast membrane potential recovery in RLM uncoupled by the addition of the UB esters was apparently associated with hydrolysis of these compounds, catalyzed by mitochondrial aldehyde dehydrogenase (ALDH2), being in high abundance in liver compared to other tissues. Protonophoric properties of the UB derivatives in isolated mitochondria were confirmed by measurements of RHM swelling in the presence of potassium acetate. In model bilayer lipid membranes (BLM), proton-carrying activity of UB-3 esters was demonstrated by measuring fluorescence response of the pH-dependent dye pyranine in liposomes. Electrophysiological experiments on identified neurons from Lymnaea stagnalis demonstrated low neurotoxicity of UB-3 esters. Resazurin-based cell viability assay showed low toxicity of UB-3 esters to HEK293 cells and primary human fibroblasts. Thus, the present results enable us to consider UB-3 esters as effective tissue-specific protonophoric mitochondrial uncouplers.

Mitochondrial uncouplers induce proton leak by activating AAC and UCP1

Mitochondria generate heat due to H+ leak (IH) across their inner membrane1. IH results from the action of long-chain fatty acids on uncoupling protein 1 (UCP1) in brown fat2-6 and ADP/ATP carrier (AAC) in other tissues1,7-9, but the underlying mechanism is poorly understood. As evidence of pharmacological activators of IH through UCP1 and AAC is lacking, IH is induced by protonophores such as 2,4-dinitrophenol (DNP) and cyanide-4-(trifluoromethoxy) phenylhydrazone (FCCP)10,11. Although protonophores show potential in combating obesity, diabetes and fatty liver in animal models12-14, their clinical potential for treating human disease is limited due to indiscriminately increasing H+ conductance across all biological membranes10,11 and adverse side effects15. Here we report the direct measurement of IH induced by DNP, FCCP and other common protonophores and find that it is dependent on AAC and UCP1. Using molecular structures of AAC, we perform a computational analysis to determine the binding sites for protonophores and long-chain fatty acids, and find that they overlap with the putative ADP/ATP-binding site. We also develop a mathematical model that proposes a mechanism of uncoupler-dependent IH through AAC. Thus, common protonophoric uncouplers are synthetic activators of IH through AAC and UCP1, paving the way for the development of new and more specific activators of these two central mediators of mitochondrial bioenergetics.

Mitochondrial Respiration-Dependent ANT2-UCP2 Interaction

Adenine nucleotide translocases (ANTs) and uncoupling proteins (UCPs) are known to facilitate proton leak across the inner mitochondrial membrane. However, it remains to be unravelled whether UCP2/3 contribute to significant amount of proton leak in vivo. Reports are indicative of UCP2 dependent proton-coupled efflux of C4 metabolites from the mitochondrial matrix. Previous studies have suggested that UCP2/3 knockdown (KD) contributes to increased ANT-dependent proton leak. Here we investigated the hypothesis that interaction exists between the UCP2 and ANT2 proteins, and that such interaction is regulated by the cellular metabolic demand. Protein-protein interaction was evaluated using reciprocal co-immunoprecipitation and in situ proximity ligation assay. KD of ANT2 and UCP2 was performed by siRNA in human embryonic kidney cells 293A (HEK293A) cells. Mitochondrial and cellular respiration was measured by high-resolution respirometry. ANT2-UCP2 interaction was demonstrated, and this was dependent on cellular metabolism. Inhibition of ATP synthase promoted ANT2-UCP2 interaction whereas high cellular respiration, induced by adding the mitochondrial uncoupler FCCP, prevented interaction. UCP2 KD contributed to increased carboxyatractyloside (CATR) sensitive proton leak, whereas ANT2 and UCP2 double KD reduced CATR sensitive proton leak, compared to UCP2 KD. Furthermore, proton leak was reduced in double KD compared to UCP2 KD. In conclusion, our results show that there is an interaction between ANT2-UCP2, which appears to be dynamically regulated by mitochondrial respiratory activity. This may have implications in the regulation of mitochondrial efficiency or cellular substrate utilization as increased activity of UCP2 may promote a switch from glucose to fatty acid metabolism.

Establishing the potency of N-acyl amino acids versus conventional fatty acids as thermogenic uncouplers in cells and mitochondria from different tissues

The possibility that N-acyl amino acids could function as brown or brite/beige adipose tissue-derived lipokines that could induce UCP1-independent thermogenesis by uncoupling mitochondrial respiration in several peripheral tissues is of significant physiological interest. To quantify the potency of N-acyl amino acids versus conventional fatty acids as thermogenic inducers, we have examined the affinity and efficacy of two pairs of such compounds: oleate versus N-oleoyl-leucine and arachidonate versus N-arachidonoyl-glycine in cells and mitochondria from different tissues. We found that in cultures of the muscle-derived L6 cell line, as well as in primary cultures of murine white, brite/beige and brown adipocytes, the N-acyl amino acids were proficient uncouplers but that they did not systematically display higher affinity or potency than the conventional fatty acids, and they were not as efficient uncouplers as classical protonophores (FCCP). Higher concentrations of the N-acyl amino acids (as well as of conventional fatty acids) were associated with signs of deleterious effects on the cells. In liver mitochondria, we found that the N-acyl amino acids uncoupled similarly to conventional fatty acids, thus apparently via activation of the adenine nucleotide transporter-2. In brown adipose tissue mitochondria, the N-acyl amino acids were able to activate UCP1, again similarly to conventional fatty acids. We thus conclude that the formation of the acyl-amino acid derivatives does not confer upon the corresponding fatty acids an enhanced ability to induce thermogenesis in peripheral tissues, and it is therefore unlikely that the N-acyl amino acids are of specific physiological relevance as UCP1-independent thermogenic compounds.

Mitochondrial H+ Leak and Thermogenesis

Mitochondria of all tissues convert various metabolic substrates into two forms of energy: ATP and heat. Historically, the primary focus of research in mitochondrial bioenergetics was on the mechanisms of ATP production, while mitochondrial thermogenesis received significantly less attention. Nevertheless, mitochondrial heat production is crucial for the maintenance of body temperature, regulation of the pace of metabolism, and prevention of oxidative damage to mitochondria and the cell. In addition, mitochondrial thermogenesis has gained significance as a pharmacological target for treating metabolic disorders. Mitochondria produce heat as the result of H⁺ leak across their inner membrane. This review provides a critical assessment of the current field of mitochondrial H⁺ leak and thermogenesis, with a focus on the molecular mechanisms involved in the function and regulation of uncoupling protein 1 and the ADP/ATP carrier, the two proteins that mediate mitochondrial H⁺ leak.

Elovl2-Ablation Leads to Mitochondrial Membrane Fatty Acid Remodeling and Reduced Efficiency in Mouse Liver Mitochondria

The fatty acid elongase elongation of very long-chain fatty acids protein 2 (ELOVL2) controls the elongation of polyunsaturated fatty acids (PUFA) producing precursors for omega-3, docosahexaenoic acid (DHA), and omega-6, docosapentaenoic acid (DPAn-6) in vivo. Expectedly, Elovl2-ablation drastically reduced the DHA and DPAn-6 in liver mitochondrial membranes. Unexpectedly, however, total PUFAs levels decreased further than could be explained by Elovl2 ablation. The lipid peroxidation process was not involved in PUFAs reduction since malondialdehyde-lysine (MDAL) and other oxidative stress biomarkers were not enhanced. The content of mitochondrial respiratory chain proteins remained unchanged. Still, membrane remodeling was associated with the high voltage-dependent anion channel (VDAC) and adenine nucleotide translocase 2 (ANT2), a possible reflection of the increased demand on phospholipid transport to the mitochondria. Mitochondrial function was impaired despite preserved content of the respiratory chain proteins and the absence of oxidative damage. Oligomycin-insensitive oxygen consumption increased, and coefficients of respiratory control were reduced by 50%. The mitochondria became very sensitive to fatty acid-induced uncoupling and permeabilization, where ANT2 is involved. Mitochondrial volume and number of peroxisomes increased as revealed by transmission electron microscopy. In conclusion, the results imply that endogenous DHA production is vital for the normal function of mouse liver mitochondria and could be relevant not only for mice but also for human metabolism.

Fifty Years of Research on Protonophores: Mitochondrial Uncoupling As a Basis for Therapeutic Action

Protonophores are compounds capable of electrogenic transport of protons across membranes. Protonophores have been intensively studied over the past 50 years owing to their ability to uncouple oxidation and phosphorylation in mitochondria and chloroplasts. The action mechanism of classical uncouplers, such as DNP and CCCP, in mitochondria is believed to be related to their protonophoric activity; i.e., their ability to transfer protons across the lipid part of the mitochondrial membrane. Given the recently revealed deviations in the correlation between the protonophoric activity of some uncouplers and their ability to stimulate mitochondrial respiration, this review addresses the involvement of some proteins of the inner mitochondrial membrane, such as the ATP/ADP antiporter, dicarboxylate carrier, and ATPase, in the uncoupling process. However, these deviations do not contradict the Mitchell theory but point to a more complex nature of the interaction of DNP, CCCP, and other uncouplers with mitochondrial membranes. Therefore, a detailed investigation of the action mechanism of uncouplers is required for a more successful pharmacological use, including their antibacterial, antiviral, anticancer, as well as cardio-, neuro-, and nephroprotective effects.

Revisiting the contribution of mitochondrial biology to the pathophysiology of skeletal muscle insulin resistance

While the etiology of type 2 diabetes is multifaceted, the induction of insulin resistance in skeletal muscle is a key phenomenon, and impairments in insulin signaling in this tissue directly contribute to hyperglycemia. Despite the lack of clarity regarding the specific mechanisms whereby insulin signaling is impaired, the key role of a high lipid environment within skeletal muscle has been recognized for decades. Many of the proposed mechanisms leading to the attenuation of insulin signaling - namely the accumulation of reactive lipids and the pathological production of reactive oxygen species (ROS), appear to rely on this high lipid environment. Mitochondrial biology is a central component to these processes, as these organelles are almost exclusively responsible for the oxidation and metabolism of lipids within skeletal muscle and are a primary source of ROS production. Classic studies have suggested that reductions in skeletal muscle mitochondrial content and/or function contribute to lipid-induced insulin resistance; however, in recent years the role of mitochondria in the pathophysiology of insulin resistance has been gradually re-evaluated to consider the biological effects of alterations in mitochondrial content. In this respect, while reductions in mitochondrial content are not required for the induction of insulin resistance, mechanisms that increase mitochondrial content are thought to enhance mitochondrial substrate sensitivity and submaximal adenosine diphosphate (ADP) kinetics. Thus, this review will describe the central role of a high lipid environment in the pathophysiology of insulin resistance, and present both classic and contemporary views of how mitochondrial biology contributes to insulin resistance in skeletal muscle.

Modulation and Pharmacology of the Mitochondrial Permeability Transition: A Journey from F-ATP Synthase to ANT

The permeability transition (PT) is an increased permeation of the inner mitochondrial membrane due to the opening of the PT pore (PTP), a Ca²⁺-activated high conductance channel involved in Ca²⁺ homeostasis and cell death. Alterations of the PTP have been associated with many pathological conditions and its targeting represents an incessant challenge in the field. Although the modulation of the PTP has been extensively explored, the lack of a clear picture of its molecular nature increases the degree of complexity for any target-based approach. Recent advances suggest the existence of at least two mitochondrial permeability pathways mediated by the F-ATP synthase and the ANT, although the exact molecular mechanism leading to channel formation remains elusive for both. A full comprehension of this to-pore conversion will help to assist in drug design and to develop pharmacological treatments for a fine-tuned PT regulation. Here, we will focus on regulatory mechanisms that impinge on the PTP and discuss the relevant literature of PTP targeting compounds with particular attention to F-ATP synthase and ANT.

Pioglitazone Is a Mild Carrier-Dependent Uncoupler of Oxidative Phosphorylation and a Modulator of Mitochondrial Permeability Transition

Pioglitazone (PIO) is an insulin-sensitizing antidiabetic drug, which normalizes glucose and lipid metabolism but may provoke heart and liver failure and chronic kidney diseases. Both therapeutic and adverse effects of PIO can be accomplished through mitochondrial targets. Here, we explored the capability of PIO to modulate the mitochondrial membrane potential (ΔΨ_m) and the permeability transition pore (mPTP) opening in different models in vitro. ΔΨ_m was measured using tetraphenylphosphonium and the fluorescent dye rhodamine 123. The coupling of oxidative phosphorylation was estimated polarographically. The transport of ions and solutes across membranes was registered by potentiometric and spectral techniques. We found that PIO decreased ΔΨ_m in isolated mitochondria and intact thymocytes and the efficiency of ADP phosphorylation, particularly after the addition of Ca²⁺. The presence of the cytosolic fraction mitigated mitochondrial depolarization but made it sustained. Carboxyatractyloside diminished the PIO-dependent depolarization. PIO activated proton transport in deenergized mitochondria but not in artificial phospholipid vesicles. PIO had no effect on K⁺ and Ca²⁺ inward transport but drastically decreased the mitochondrial Ca²⁺-retention capacity and protective effects of adenine nucleotides against mPTP opening. Thus, PIO is a mild, partly ATP/ADP-translocase-dependent, uncoupler and a modulator of ATP production and mPTP sensitivity to Ca²⁺ and adenine nucleotides. These properties contribute to both therapeutic and adverse effects of PIO.

Mitochondrial Uncoupling Proteins (UCP1-UCP3) and Adenine Nucleotide Translocase (ANT1) Enhance the Protonophoric Action of 2,4-Dinitrophenol in Mitochondria and Planar Bilayer Membranes

2,4-Dinitrophenol (DNP) is a classic uncoupler of oxidative phosphorylation in mitochondria which is still used in “diet pills”, despite its high toxicity and lack of antidotes. DNP increases the proton current through pure lipid membranes, similar to other chemical uncouplers. However, the molecular mechanism of its action in the mitochondria is far from being understood. The sensitivity of DNP’s uncoupling action in mitochondria to carboxyatractyloside, a specific inhibitor of adenine nucleotide translocase (ANT), suggests the involvement of ANT and probably other mitochondrial proton-transporting proteins in the DNP’s protonophoric activity. To test this hypothesis, we investigated the contribution of recombinant ANT1 and the uncoupling proteins UCP1-UCP3 to DNP-mediated proton leakage using the well-defined model of planar bilayer lipid membranes. All four proteins significantly enhanced the protonophoric effect of DNP. Notably, only long-chain free fatty acids were previously shown to be co-factors of UCPs and ANT1. Using site-directed mutagenesis and molecular dynamics simulations, we showed that arginine 79 of ANT1 is crucial for the DNP-mediated increase of membrane conductance, implying that this amino acid participates in DNP binding to ANT1.

Neuroprotective Potential of Mild Uncoupling in Mitochondria. Pros and Cons

There has been an explosion of interest in the use of uncouplers of oxidative phosphorylation in mitochondria in the treatment of several pathologies, including neurological ones. In this review, we analyzed all the mechanisms associated with mitochondrial uncoupling and the metabolic and signaling cascades triggered by uncouplers. We provide a full set of positive and negative effects that should be taken into account when using uncouplers in experiments and clinical practice.

Nonphosphorylating Oxidation in Mitochondria and Related Processes

The mechanism of oxidative phosphorylation and its regulation remain one of the main problems of bioenergetics. Efficiency of the mitochondrial energization is determined by the relationship between the rate of generation of electrochemical potential of hydrogen ions and the rate of its expenditure on the synthesis of ATP and the use of ATP in endergonic reactions. Uncoupling (partial or complete), which occurs in the process of uncontrolled and controlled leakage of ions through the inner mitochondrial membrane, on the one hand leads to the decrease in the relative synthesis of ATP, and on the other, being consistent with the law of conservation of energy, leads to the formation of heat, generation of which is an essential function of the organism. In addition to increased thermogenesis, the increase of non-phosphorylating oxidation of various substrates is accompanied by the decrease in transmembrane potential, production of reactive oxygen species, and activation of oxygen consumption, water and carbon dioxide production, increase in the level of intracellular ADP and acidification of the cytosol. In this analysis, each of these factors will be considered separately for its role in regulating metabolism.

Short-term intensified training temporarily impairs mitochondrial respiratory capacity in elite endurance athletes

The maintenance of healthy and functional mitochondria is the result of a complex mitochondrial turnover and herein quality-control program that includes both mitochondrial biogenesis and autophagy of mitochondria. The aim of this study was to examine the effect of an intensified training load on skeletal muscle mitochondrial quality control in relation to changes in mitochondrial oxidative capacity, maximal oxygen consumption, and performance in highly trained endurance athletes. Elite endurance athletes (n = 27) performed high-intensity interval exercise followed by moderate-intensity continuous exercise 3 days per week for 4 wk in addition to their usual volume of training. Mitochondrial oxidative capacity, abundance of mitochondrial proteins, markers of autophagy, and antioxidant capacity of skeletal muscle were assessed in skeletal muscle biopsies before and after the intensified training period. The intensified training period increased several autophagy markers suggesting an increased turnover of mitochondrial and cytosolic proteins. In permeabilized muscle fibers, mitochondrial respiration was ∼20% lower after training although some markers of mitochondrial density increased by 5%-50%, indicative of a reduced mitochondrial quality by the intensified training intervention. The antioxidative proteins UCP3, ANT1, and SOD2 were increased after training, whereas we found an inactivation of aconitase. In agreement with the lower aconitase activity, the amount of mitochondrial LON protease that selectively degrades oxidized aconitase was doubled. Together, this suggests that mitochondrial respiratory function is impaired during the initial recovery from a period of intensified endurance training whereas mitochondrial quality control is slightly activated in highly trained skeletal muscle.NEW & NOTEWORTHY We show that mitochondrial respiration is temporarily impaired after a period of intensified exercise training in elite athletes. In parallel, proteins involved in the antioxidative response including SOD2, UCP3, and ANT2 were upregulated, whereas mitochondrial biogenesis was slightly activated. Despite the mitochondrial respiratory impairments, physical performance was improved a few days after the intense training period.

How the brain fights fatty acids' toxicity

Neurons spurn hydrogen-rich fatty acids for energizing oxidative ATP synthesis, contrary to other cells. This feature has been mainly attributed to a lower yield of ATP per reduced oxygen, as compared to glucose. Moreover, the use of fatty acids as hydrogen donor is accompanied by severe β-oxidation-associated ROS generation. Neurons are especially susceptible to detrimental activities of ROS due to their poor antioxidative equipment. It is also important to note that free fatty acids (FFA) initiate multiple harmful activities inside the cells, particularly on phosphorylating mitochondria. Several processes enhance FFA-linked lipotoxicity in the cerebral tissue. Thus, an uptake of FFA from the circulation into the brain tissue takes place during an imbalance between energy intake and energy expenditure in the body, a situation similar to that during metabolic syndrome and fat-rich diet. Traumatic or hypoxic brain injuries increase hydrolytic degradation of membrane phospholipids and, thereby elevate the level of FFA in neural cells. Accumulation of FFA in brain tissue is markedly associated with some inherited neurological disorders, such as Refsum disease or X-linked adrenoleukodystrophy (X-ALD). What are strategies protecting neurons against FFA-linked lipotoxicity? Firstly, spurning the β-oxidation pathway in mitochondria of neurons. Secondly, based on a tight metabolic communication between neurons and astrocytes, astrocytes donate metabolites to neurons for synthesis of antioxidants. Further, neuronal autophagy of ROS-emitting mitochondria combined with the transfer of degradation-committed FFA for their disposal in astrocytes, is a potent protective strategy against ROS and harmful activities of FFA. Finally, estrogens and neurosteroids are protective as triggers of ERK and PKB signaling pathways, consequently initiating the expression of various neuronal survival genes via the formation of cAMP response element-binding protein (CREB).

Antioxidant Synergy of Mitochondrial Phospholipase PNPLA8/iPLA2γ with Fatty Acid-Conducting SLC25 Gene Family Transporters

Patatin-like phospholipase domain-containing protein PNPLA8, also termed Ca²⁺-independent phospholipase A2γ (iPLA2γ), is addressed to the mitochondrial matrix (or peroxisomes), where it may manifest its unique activity to cleave phospholipid side-chains from both sn-1 and sn-2 positions, consequently releasing either saturated or unsaturated fatty acids (FAs), including oxidized FAs. Moreover, iPLA2γ is directly stimulated by H₂O₂ and, hence, is activated by redox signaling or oxidative stress. This redox activation permits the antioxidant synergy with mitochondrial uncoupling proteins (UCPs) or other SLC25 mitochondrial carrier family members by FA-mediated protonophoretic activity, termed mild uncoupling, that leads to diminishing of mitochondrial superoxide formation. This mechanism allows for the maintenance of the steady-state redox status of the cell. Besides the antioxidant role, we review the relations of iPLA2γ to lipid peroxidation since iPLA2γ is alternatively activated by cardiolipin hydroperoxides and hypothetically by structural alterations of lipid bilayer due to lipid peroxidation. Other iPLA2γ roles include the remodeling of mitochondrial (or peroxisomal) membranes and the generation of specific lipid second messengers. Thus, for example, during FA β-oxidation in pancreatic β-cells, H₂O₂-activated iPLA2γ supplies the GPR40 metabotropic FA receptor to amplify FA-stimulated insulin secretion. Cytoprotective roles of iPLA2γ in the heart and brain are also discussed.

Mitochondrial proton leaks and uncoupling proteins

Non-shivering thermogenesis in brown adipose tissue is mediated by uncoupling protein 1 (UCP1), which provides a carefully regulated proton re-entry pathway across the mitochondrial inner membrane operating in parallel to the ATP synthase and allowing respiration, and hence thermogenesis, to be released from the constraints of respiratory control. In the 40 years since UCP1 was first described, an extensive, and frequently contradictory, literature has accumulated, focused on the acute physiological regulation of the protein by fatty acids, purine nucleotides and possible additional factors. The purpose of this review is to examine, in detail, the experimental evidence underlying these proposed mechanisms. Emphasis will be placed on the methodologies employed and their relation to the physiological constraints under which the protein functions in the intact cell. The nature of the endogenous, UCP1-independent, proton leak will also be discussed. Finally, the troubled history of the putative novel uncoupling proteins, UCP2 and UCP3, will be evaluated.

ANT1 Activation and Inhibition Patterns Support the Fatty Acid Cycling Mechanism for Proton Transport

Adenine nucleotide translocase (ANT) is a well-known mitochondrial exchanger of ATP against ADP. In contrast, few studies have shown that ANT also mediates proton transport across the inner mitochondrial membrane. The results of these studies are controversial and lead to different hypotheses about molecular transport mechanisms. We hypothesized that the H⁺-transport mediated by ANT and uncoupling proteins (UCP) has a similar regulation pattern and can be explained by the fatty acid cycling concept. The reconstitution of purified recombinant ANT1 in the planar lipid bilayers allowed us to measure the membrane current after the direct application of transmembrane potential ΔΨ, which would correspond to the mitochondrial states III and IV. Experimental results reveal that ANT1 does not contribute to a basal proton leak. Instead, it mediates H⁺ transport only in the presence of long-chain fatty acids (FA), as already known for UCPs. It depends on FA chain length and saturation, implying that FA’s transport is confined to the lipid-protein interface. Purine nucleotides with the preference for ATP and ADP inhibited H⁺ transport. Specific inhibitors of ATP/ADP transport, carboxyatractyloside or bongkrekic acid, also decreased proton transport. The H⁺ turnover number was calculated based on ANT1 concentration determined by fluorescence correlation spectroscopy and is equal to 14.6 ± 2.5 s⁻¹. Molecular dynamic simulations revealed a large positively charged area at the protein/lipid interface that might facilitate FA anion’s transport across the membrane. ANT’s dual function—ADP/ATP and H⁺ transport in the presence of FA—may be important for the regulation of mitochondrial membrane potential and thus for potential-dependent processes in mitochondria. Moreover, the expansion of proton-transport modulating drug targets to ANT1 may improve the therapy of obesity, cancer, steatosis, cardiovascular and neurodegenerative diseases.

Molecular Dynamics Simulations of Mitochondrial Uncoupling Protein 2

Molecular dynamics (MD) simulations of uncoupling proteins (UCP), a class of transmembrane proteins relevant for proton transport across inner mitochondrial membranes, represent a complicated task due to the lack of available structural data. In this work, we use a combination of homology modelling and subsequent microsecond molecular dynamics simulations of UCP2 in the DOPC phospholipid bilayer, starting from the structure of the mitochondrial ATP/ADP carrier (ANT) as a template. We show that this protocol leads to a structure that is impermeable to water, in contrast to MD simulations of UCP2 structures based on the experimental NMR structure. We also show that ATP binding in the UCP2 cavity is tight in the homology modelled structure of UCP2 in agreement with experimental observations. Finally, we corroborate our results with conductance measurements in model membranes, which further suggest that the UCP2 structure modeled from ANT protein possesses additional key functional elements, such as a fatty acid-binding site at the R60 region of the protein, directly related to the proton transport mechanism across inner mitochondrial membranes.

Pros and cons for the evidence of adaptive non-shivering thermogenesis in marsupials

The thermogenic mechanisms supporting endothermy are still not fully understood in all major mammalian subgroups. In placental mammals, brown adipose tissue currently represents the most accepted source of adaptive non-shivering thermogenesis. Its mitochondrial protein UCP1 (uncoupling protein 1) catalyzes heat production, but the conservation of this mechanism is unclear in non-placental mammals and lost in some placentals. Here, we review the evidence for and against adaptive non-shivering thermogenesis in marsupials, which diverged from placentals about 120-160 million years ago. We critically discuss potential mechanisms that may be involved in the heat-generating process among marsupials.

The effect of mitochondrial calcium uniporter and cyclophilin D knockout on resistance of brain mitochondria to Ca2+-induced damage

The mitochondrial calcium uniporter (MCU) and cyclophilin D (CyD) are key players in induction of the permeability transition pore (PTP), which leads to mitochondrial depolarization and swelling, the major signs of Ca2+-induced mitochondrial damage. Mitochondrial depolarization inhibits ATP production, whereas swelling results in the release of mitochondrial pro-apoptotic proteins. The extent to which simultaneous deletion of MCU and CyD inhibits PTP induction and prevents damage of brain mitochondria is not clear. Here, we investigated the effects of MCU and CyD deletion on the propensity for PTP induction using mitochondria isolated from the brains of MCU-KO, CyD-KO, and newly created MCU/CyD-double knockout (DKO) mice. Neither deletion of MCU nor of CyD affected respiration or membrane potential in mitochondria isolated from the brains of these mice. Mitochondria from MCU-KO and MCU/CyD-DKO mice displayed reduced Ca2+ uptake and diminished extent of PTP induction. The Ca2+ uptake by mitochondria from CyD-KO mice was increased compared with mitochondria from WT mice. Deletion of CyD prevented mitochondrial swelling and resulted in transient depolarization in response to Ca2+, but it did not prevent Ca2+-induced delayed mitochondrial depolarization. Mitochondria from MCU/CyD-DKO mice did not swell in response to Ca2+, but they did exhibit mild sustained depolarization. Dibucaine, an inhibitor of the Ca2+-activated mitochondrial phospholipase A2, attenuated and bovine serum albumin completely eliminated the sustained depolarization. This suggests the involvement of phospholipase A2 and free fatty acids. Thus, in addition to induction of the classical PTP, alternative deleterious mechanisms may contribute to mitochondrial damage following exposure to elevated Ca2+.

Skeletal muscle metabolism in sea-acclimatized king penguins. II. Improved efficiency of mitochondrial bioenergetics

At fledging, juvenile king penguins (Aptenodytes patagonicus) must overcome the tremendous energetic constraints imposed by their marine habitat, including during sustained extensive swimming activity and deep dives in cold seawater. Both endurance swimming and skeletal muscle thermogenesis require high mitochondrial respiratory capacity while the submerged part of dive cycles repeatedly and greatly reduces oxygen availability, imposing a need for solutions to conserve oxygen. The aim of the present study was to determine in vitro whether skeletal muscle mitochondria become more 'thermogenic' to sustain heat production or more 'economical' to conserve oxygen in sea-acclimatized immature penguins (hereafter 'immatures') compared with terrestrial juveniles. Rates of mitochondrial oxidative phosphorylation were measured in permeabilized fibers and mitochondria from the pectoralis muscle. Mitochondrial ATP synthesis and coupling efficiency were measured in isolated muscle mitochondria. The mitochondrial activities of respiratory chain complexes and citrate synthase were also assessed. The results showed that respiration, ATP synthesis and respiratory chain complex activities in pectoralis muscles were increased by sea acclimatization. Furthermore, muscle mitochondria were on average 30-45% more energy efficient in sea-acclimatized immatures than in pre-fledging juveniles, depending on the respiratory substrate used (pyruvate, palmitoylcarnitine). Hence sea acclimatization favors the development of economical management of oxygen, decreasing the oxygen needed to produce a given amount of ATP. This mitochondrial phenotype may improve dive performance during the early marine life of king penguins, by extending their aerobic dive limit.

Protonophoric action of BAM15 on planar bilayers, liposomes, mitochondria, bacteria and neurons

Small molecules capable of uncoupling respiration and ATP synthesis in mitochondria are protective towards various cell malfunctions. Recently (2-fluorophenyl){6-[(2-fluorophenyl)amino](1,2,5-oxadiazolo[3,4-e]pyrazin-5-yl)}amine (BAM15), a new compound of this type, has become popular as a potent mitochondria-selective depolarizing agent producing minimal adverse effects. To validate protonophoric mechanism of BAM15 action, we examined its behavior in bilayer lipid membranes (BLM). BAM15 proved to be a typical anionic protonophore with the activity on planar membranes being suppressed upon decreasing membrane dipole potential. In both planar BLM and liposomes, BAM15 induced proton conductance with the potency close to that of the classical protonophoric uncoupler carbonyl cyanide m-chlorophenyl hydrazone (CCCP). In isolated rat liver mitochondria (RLM), BAM15 caused membrane potential collapse, increased respiration rate and induced Ca²⁺ efflux at concentrations slightly higher than those for CCCP. Surprisingly, the uncoupling action of BAM15 on isolated RLM, in contrast to that of CCCP, was partially reversed by carboxyatractyloside (CATR), an inhibitor of adenine nucleotide translocase, thereby indicating involvement of this protein in the BAM15-induced uncoupling. BAM15 inhibited growth of Bacillus subtilis at micromolar concentrations. In electrophysiological experiments on molluscan neurons, BAM15 caused plasma membrane depolarization and suppression of electrical activity, but the effect developed more slowly than that of CCCP.

Bicarbonate suppresses mitochondrial membrane depolarization induced by conventional uncouplers

Bicarbonate has been known to modulate activities of various mitochondrial enzymes such as ATPase and soluble adenylyl cyclase. Here, we found that the ability of conventional protonophoric uncouplers, such as 2,4-dinitrophenol (DNP), carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP) and carbonyl cyanide m-chlorophenyl hydrazone (CCCP), but not that of the new popular uncoupler BAM15, to decrease mitochondrial membrane potential was significantly diminished in the presence of millimolar concentrations of bicarbonate. Thus, the depolarizing activity of DNP and FCCP in mitochondria could be sensitive to the local concentration of bicarbonate in cells and tissues. However, bicarbonate could not restore the ATP synthesis suppressed by DNP or CCCP in mitochondria. Bicarbonate neither altered the depolarizing action of DNP and FCCP on proteoliposomes with reconstituted cytochrome c oxidase, nor affected the protonophoric activity of DNP and FCCP in artificial lipid membranes as measured with pyranine-loaded liposomes, thereby showing that the bicarbonate-induced reversal of the depolarizing action of DNP and FCCP on mitochondria did not result from direct interaction of bicarbonate with the uncouplers.

A 20/20 view of ANT function in mitochondrial biology and necrotic cell death

The adenosine nucleotide translocase (ANT) family of proteins are inner mitochondrial membrane proteins involved in energy homeostasis and cell death. The primary function of ANT proteins is to exchange cytosolic ADP with matrix ATP, facilitating the export of newly synthesized ATP to the cell while providing new ADP substrate to the mitochondria. As such, the ANT proteins are central to maintaining energy homeostasis in all eukaryotic cells. Evidence also suggests that the ANTs constitute a pore-forming component of the mitochondrial permeability transition pore (MPTP), a structure that forms in the inner mitochondrial membrane that is thought to underlie regulated necrotic cell death. Additionally, emerging studies suggest that ANT proteins are also critical for mitochondrial uncoupling and for promoting mitophagy. Thus, the ANTs are multifunctional proteins that are poised to participate in several aspects of mitochondrial biology and the greater regulation of cell death, which will be discussed here.

Calcium overload decreases net free radical emission in cardiac mitochondria

Elevated calcium and reactive oxygen species (ROS) are responsible for the bulk of cell death occurring in a variety of clinical settings that include acute coronary events, cerebrovascular accidents, and acute kidney injury. It is commonly believed that calcium and ROS participate in a viscous cycle during these events. However, the precise feedback mechanisms are unknown. We quantitatively demonstrate in this study that, on the contrary, calcium does not stimulate free radical production but suppresses it. Isolated mitochondria from guinea pig hearts were energized with a variety of substrates and exposed to calcium concentrations designed to induce moderate calcium overload conditions associated with ischemia/reperfusion injury but do not elicit the well-known mitochondrial permeability transition phenomenon. Metabolic function and free radical emission were simultaneously quantified using high-resolution respirometry and fluorimetry. Membrane potential, high amplitude swelling, and calcium dynamics were also quantified in parallel. Our results reveal that calcium overload does not lead to excessive ROS emission but does decrease ADP stimulated respiration rates for NADH-dependent pathways. Moreover, we developed an empirical model of mitochondrial free radical homeostasis to identify the processes that are different for each substrate and calcium condition. In summary, we show that in healthy guinea pig mitochondria, calcium uptake and free radical generation do not contribute to a viscous cycle and that the relationship between net free radical production and oxygen concentration is hyperbolic. Altogether, these results lay out an important foundation necessary to quantitatively determine the role of calcium in IR injury and ROS production.

Mitochondrial Uncoupling: A Key Controller of Biological Processes in Physiology and Diseases

Mitochondrial uncoupling can be defined as a dissociation between mitochondrial membrane potential generation and its use for mitochondria-dependent ATP synthesis. Although this process was originally considered a mitochondrial dysfunction, the identification of UCP-1 as an endogenous physiological uncoupling protein suggests that the process could be involved in many other biological processes. In this review, we first compare the mitochondrial uncoupling agents available in term of mechanistic and non-specific effects. Proteins regulating mitochondrial uncoupling, as well as chemical compounds with uncoupling properties are discussed. Second, we summarize the most recent findings linking mitochondrial uncoupling and other cellular or biological processes, such as bulk and specific autophagy, reactive oxygen species production, protein secretion, cell death, physical exercise, metabolic adaptations in adipose tissue, and cell signaling. Finally, we show how mitochondrial uncoupling could be used to treat several human diseases, such as obesity, cardiovascular diseases, or neurological disorders.

Important Trends in UCP3 Investigation

Membrane uncoupling protein 3 (UCP3), a member of the mitochondrial uncoupling protein family, was discovered in 1997. UCP3's properties, such as its high homology to other mitochondrial carriers, especially to UCP2, its short lifetime and low specificity of UCP3 antibodies, have hindered progress in understanding its biological function and transport mechanism over decades. The abundance of UCP3 is highest in murine brown adipose tissue (BAT, 15.0 pmol/mg protein), compared to heart (2.7 pmol/mg protein) and the gastrocnemius muscle (1.7 pmol/mg protein), but it is still 400-fold lower than the abundance of UCP1, a biomarker for BAT. Investigation of UCP3 reconstituted in planar bilayer membranes revealed that it transports protons only when activated by fatty acids (FA). Although purine nucleotides (PN) inhibit UCP3-mediated transport, the molecular mechanism differs from that of UCP1. It remains a conundrum that two homologous proton-transporting proteins exist within the same tissue. Recently, we proposed that UCP3 abundance directly correlates with the degree of FA β-oxidation in cell metabolism. Further development in this field implies that UCP3 may have dual function in transporting substrates, which have yet to be identified, alongside protons. Evaluation of the literature with respect to UCP3 is a complex task because (i) UCP3 features are often extrapolated from its "twin" UCP2 without additional proof, and (ii) the specificity of antibodies against UCP3 used in studies is rarely evaluated. In this review, we primarily focus on recent findings obtained for UCP3 in biological and biomimetic systems.

Glutamine regulates mitochondrial uncoupling protein 2 to promote glutaminolysis in neuroblastoma cells

Mitochondrial uncoupling protein 2 (UCP2) is highly abundant in rapidly proliferating cells that utilize aerobic glycolysis, such as stem cells, cancer cells, and cells of the immune system. However, the function of UCP2 has been a longstanding conundrum. Considering the strict regulation and unusually short life time of the protein, we propose that UCP2 acts as a "signaling protein" under nutrient shortage in cancer cells. We reveal that glutamine shortage induces the rapid and reversible downregulation of UCP2, decrease of the metabolic activity and proliferation of neuroblastoma cells, that are regulated by glutamine per se but not by glutamine metabolism. Our findings indicate a very rapid (within 1 h) metabolic adaptation that allows the cell to survive by either shifting its metabolism to the use of the alternative fuel glutamine or going into a reversible, more quiescent state. The results imply that UCP2 facilitates glutamine utilization as an energetic fuel source, thereby providing metabolic flexibility during glucose shortage. The targeting UCP2 by drugs to intervene with cancer cell metabolism may represent a new strategy for treatment of cancers resistant to other therapies.

Knockdown of Ant2 Reduces Adipocyte Hypoxia And Improves Insulin Resistance in Obesity

Decreased adipose tissue oxygen tension and increased HIF-1α expression can trigger adipose tissue inflammation and dysfunction in obesity. Our current understanding of obesity-associated decreased adipose tissue oxygen tension is mainly focused on changes in oxygen supply and angiogenesis. Here, we demonstrate that increased adipocyte O2 demand, mediated by ANT2 activity, is the dominant cause of adipocyte hypoxia. Deletion of adipocyte Ant2 improves obesity-induced intracellular adipocyte hypoxia by decreasing obesity-induced adipocyte oxygen demand, without effects on mitochondrial number or mass, or oligomycin-sensitive respiration. This led to decreased adipose tissue HIF-1α expression and inflammation with improved glucose tolerance and insulin resistance in both a preventative or therapeutic setting. Our results suggest that ANT2 may be a target for the development of insulin sensitizing drugs and that ANT2 inhibition might have clinical utility.

Carborane derivatives of 1,2,3-triazole depolarize mitochondria by transferring protons through the lipid part of membranes

Boron containing polyhedra (carboranes) are three-dimensional delocalized aromatic systems. These structures have been shown to transport protons through lipid membranes and mitochondria. Conjugation of carboranes to various organic moieties is aimed at obtaining biologically active compounds with novel properties. Taking advantage of 1,2,3-triazoles as the scaffolds valuable in medicinal chemistry, we synthesized 1-(o-carboranylmethyl)-4-pentyl-1,2,3-triazole (c-triazole) and 1-(o-carboranylmethyl)-4-pentyl-1,2,3-triazolium iodide (c-triazolium). Both compounds interacted with model lipid membranes and exhibited a proton carrying activity in planar bilayers and liposomes in a concentration- and pH-dependent manner. Importantly, mechanisms of the protonophoric activity differed; namely, protonation-deprotonation reactions of the triazole and the o-carborane moieties were involved in the transport cycles of c-triazole and c-triazolium, respectively. At micromolar concentrations, c-triazole and c-triazolium stimulated respiration of isolated rat liver mitochondria and depolarized their membrane potential, with c-triazole being more potent. In living K562 (human chronic myelogenous leukemia) cells, both c-triazolium and c-triazole altered the mitochondrial membrane potential as determined by a decreased intracellular accumulation of the potential-dependent dye tetramethylrhodamine ethyl ester. Finally, cell viability testing demonstrated a cytotoxic potency of c-triazolium and, to a lesser extent, of c-triazole against K562 cells, whereas non-malignant fibroblasts were much less sensitive. In all tests, the reference boron-free benzyl-4-pentyl-1,2,3-triazole showed little-to-no effects. These results demonstrated that carboranyltriazoles carry protons across biological membranes, a property potentially important in anticancer drug design.

Does Oxidation of Mitochondrial Cardiolipin Trigger a Chain of Antiapoptotic Reactions?

Oxidative stress causes selective oxidation of cardiolipin (CL), a four-tail lipid specific for the inner mitochondrial membrane. Interaction with oxidized CL transforms cytochrome c into peroxidase capable of oxidizing even more CL molecules. Ultimately, this chain of events leads to the pore formation in the outer mitochondrial membrane and release of mitochondrial proteins, including cytochrome c, into the cytoplasm. In the cytoplasm, cytochrome c promotes apoptosome assembly that triggers apoptosis (programmed cell death). Because of this amplification cascade, even an occasional oxidation of a single CL molecule by endogenously formed reactive oxygen species (ROS) might cause cell death, unless the same CL oxidation triggers a separate chain of antiapoptotic reactions that would prevent the CL-mediated apoptotic cascade. Here, we argue that the key function of CL in mitochondria and other coupling membranes is to prevent proton leak along the interface of interacting membrane proteins. Therefore, CL oxidation should increase proton permeability through the CL-rich clusters of membrane proteins (CL islands) and cause a drop in the mitochondrial membrane potential (MMP). On one hand, the MMP drop should hinder ROS generation and further CL oxidation in the entire mitochondrion. On the other hand, it is known to cause rapid fission of the mitochondrial network and formation of many small mitochondria, only some of which would contain oxidized CL islands. The fission of mitochondrial network would hinder apoptosome formation by preventing cytochrome c release from healthy mitochondria, so that slowly working protein quality control mechanisms would have enough time to eliminate mitochondria with the oxidized CL. Because of these two oppositely directed regulatory pathways, both triggered by CL oxidation, the fate of the cell appears to be determined by the balance between the CL-mediated proapoptotic and antiapoptotic reactions. Since this balance depends on the extent of CL oxidation, mitochondria-targeted antioxidants might be able to ensure cell survival in many pathologies by preventing CL oxidation.