Regulation of UCP3 by nucleotides is different from regulation of UCP1

Two-stage evolution of mammalian adipose tissue thermogenesis

Brown adipose tissue (BAT) is a heater organ that expresses thermogenic uncoupling protein 1 (UCP1) to maintain high body temperatures during cold stress. BAT thermogenesis is considered an overarching mammalian trait, but its evolutionary origin is unknown. We show that adipose tissue of marsupials, which diverged from eutherian mammals ~150 million years ago, expresses a nonthermogenic UCP1 variant governed by a partial transcriptomic BAT signature similar to that found in eutherian beige adipose tissue. We found that the reconstructed UCP1 sequence of the common eutherian ancestor displayed typical thermogenic activity, whereas therian ancestor UCP1 is nonthermogenic. Thus, mammalian adipose tissue thermogenesis may have evolved in two distinct stages, with a prethermogenic stage in the common therian ancestor linking UCP1 expression to adipose tissue and thermal stress. We propose that in a second stage, UCP1 acquired its thermogenic function specifically in eutherians, such that the onset of mammalian BAT thermogenesis occurred only after the divergence from marsupials.

Exercise Is Medicine for Nonalcoholic Fatty Liver Disease: Exploration of Putative Mechanisms

Exercise remains a key component of nonalcoholic fatty liver disease (NAFLD) treatment. The mechanisms that underpin improvements in NAFLD remain the focus of much exploration in our attempt to better understand how exercise benefits patients with NAFLD. In this review, we summarize the available scientific literature in terms of mechanistic studies which explore the role of exercise training in modulating fatty acid metabolism, reducing hepatic inflammation, and improving liver fibrosis. This review highlights that beyond simple energy expenditure, the activation of key receptors and pathways may influence the degree of NAFLD-related improvements with some pathways being sensitive to exercise type, intensity, and volume. Importantly, each therapeutic target of exercise training in this review is also the focus of previous or ongoing drug development studies in patients with nonalcoholic steatohepatitis (NASH), and even when a regulatory-agency-approved drug comes to market, exercise will likely remain an integral component in the clinical management of patients with NAFLD and NASH.

Strategies for Successful Over-Expression of Human Membrane Transport Systems Using Bacterial Hosts: Future Perspectives

Ten percent of human genes encode for membrane transport systems, which are key components in maintaining cell homeostasis. They are involved in the transport of nutrients, catabolites, vitamins, and ions, allowing the absorption and distribution of these compounds to the various body regions. In addition, roughly 60% of FDA-approved drugs interact with membrane proteins, among which are transporters, often responsible for pharmacokinetics and side effects. Defects of membrane transport systems can cause diseases; however, knowledge of the structure/function relationships of transporters is still limited. Among the expression of hosts that produce human membrane transport systems, E. coli is one of the most favorable for its low cultivation costs, fast growth, handiness, and extensive knowledge of its genetics and molecular mechanisms. However, the expression in E. coli of human membrane proteins is often toxic due to the hydrophobicity of these proteins and the diversity in structure with respect to their bacterial counterparts. Moreover, differences in codon usage between humans and bacteria hamper translation. This review summarizes the many strategies exploited to achieve the expression of human transport systems in bacteria, providing a guide to help people who want to deal with this topic.

Comparative functional analyses of UCP1 to unravel evolution, ecophysiology and mechanisms of mammalian thermogenesis

Brown adipose tissue (BAT), present in many placental mammals, provides adaptive nonshivering thermogenesis (NST) for body temperature regulation and has facilitated survival in diverse thermal niches on our planet. Intriguingly, several key details on the molecular mechanisms of NST and their potential ecophysiological adaptations are still unknown. Comparative studies at the whole animal level are unpragmatic, due to the diversity and complexity of thermoregulation among different species. We propose that the molecular evolution of mitochondrial uncoupling protein 1 (UCP1), a central component for BAT thermogenesis, represents a powerful opportunity to unravel key questions of mammalian thermoregulation. Comparative analysis of UCP1 may elucidate how its thermogenic function arose, how environmental selection has shaped protein function to support ecophysiological requirements, and how the enigmatic molecular mechanism of proton leak is governed. Several approaches for the assessment of UCP1 function in vitro have been introduced over the years. For comparative characterization of UCP1, we put forward the overexpression of UCP1 orthologues and mutated variants in a mammalian cell system as a primary strategy and discuss advantageous aspects in contrast to other experimental systems. In turn, we suggest how remaining experimental caveats can be solved by complimentary test systems before physiological consolidation in the animal model. Furthermore, we highlight the appropriate bioenergetic techniques to perform the functional analyses on UCP1. The comparative characterizations of diverse UCP1 variants may enable key insights into open questions surrounding the molecular basis of NST.

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.

UCP2 gene polymorphisms in obesity and diabetes, and the role of UCP2 in cancer

Mitochondria are the primary sites for ATP synthesis and free radical generation in organisms. Abnormal mitochondrial metabolism contributes to many diseases, including obesity, diabetes and cancer. UCP2 is an ion/anion transporter located in mitochondrial inner membrane, and has a crucial role in regulating oxidative stress, cellular metabolism, cell proliferation and cell death. Polymorphisms of the UCP2 gene have been associated with diabetes and obesity because UCP2 is involved in energy expenditure and insulin secretion. Moreover, UCP2 gene expression is often amplified in cancers, and increased UCP2 expression contributes to cancer growth, cancer metabolism, anti-apoptosis and drug resistance. The present review summarizes the latest findings of UCP2 with respect to obesity, diabetes and cancer.

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.

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.

Role of positively charged residues of the second transmembrane domain in the ion transport activity and conformation of human uncoupling protein-2

Residing at the inner mitochondrial membrane, uncoupling protein-2 (UCP2) mediates proton transport from the intermembrane space (IMS) to the mitochondrial matrix and consequently reduces the rate of ATP synthesis in the mitochondria. The ubiquitous expression of UCP2 in humans can be attributed to the protein's multiple physiological roles in tissues, including its involvement in protective mechanisms against oxidative stress, as well as glucose and lipid metabolisms. Currently, the structural properties and ion transport mechanism of UCP2 and other UCP homologues remain poorly understood. UCP2-mediated proton transport is activated by fatty acids and inhibited by di- and triphosphate purine nucleotides. UCP2 also transports chloride and some other small anions. Identification of key amino acid residues of UCP2 in its ion transport pathway can shed light on the protein's ion transport function. On the basis of our previous studies, the second transmembrane helix segment (TM2) of UCP2 exhibited chloride channel activity. In addition, it was suggested that the positively charged residues on TM2 domains of UCPs 1 and 2 were important for their chloride transport activity. On this basis, to further understand the role of these positively charged residues on the ion transport activity of UCP2, we recombinantly expressed four TM2 mutants: R76Q, R88Q, R96Q, and K104Q. The wild type UCP2 and its mutants were purified and reconstituted into liposomes, and their conformation and ion (proton and chloride) transport activity were studied. TM2 Arg residues at the matrix interface of UCP2 proved to be crucial for the protein's anion transport function, and their absence resulted in highly diminished Cl(-) transport rates. On the other hand, the two other positively charged residues of TM2, located at the UCP2-IMS interface, could participate in the salt-bridge formation in the protein and promote the interhelical tight packing in the UCP2. Absence of these residues did not influence Cl(-) transport rates, but disturbed the dense packing in UCP2 and resulted in higher UCP2-mediated proton transport rates in the presence of long chain fatty acids. Overall, the outcome of this study provides a deeper and more detailed molecular image of UCP2's ion transport mechanism.

Trends in thermostability provide information on the nature of substrate, inhibitor, and lipid interactions with mitochondrial carriers

Mitochondrial carriers, including uncoupling proteins, are unstable in detergents, which hampers structural and mechanistic studies. To investigate carrier stability, we have purified ligand-free carriers and assessed their stability with a fluorescence-based thermostability assay that monitors protein unfolding with a thiol-reactive dye. We find that mitochondrial carriers from both mesophilic and thermophilic organisms exhibit poor stability in mild detergents, indicating that instability is inherent to the protein family. Trends in the thermostability of yeast ADP/ATP carrier AAC2 and ovine uncoupling protein UCP1 allow optimal conditions for stability in detergents to be established but also provide mechanistic insights into the interactions of lipids, substrates, and inhibitors with these proteins. Both proteins exhibit similar stability profiles across various detergents, where stability increases with the size of the associated detergent micelle. Detailed analysis shows that lipids stabilize carriers indirectly by increasing the associated detergent micelle size, but cardiolipin stabilizes by direct interactions as well. Cardiolipin reverses destabilizing effects of ADP and bongkrekic acid on AAC2 and enhances large stabilizing effects of carboxyatractyloside, revealing that this lipid interacts in the m-state and possibly other states of the transport cycle, despite being in a dynamic interface. Fatty acid activators destabilize UCP1 in a similar way, which can also be prevented by cardiolipin, indicating that they interact like transport substrates. Our controls show that carriers can be soluble but unfolded in some commonly used detergents, such as the zwitterionic Fos-choline-12, which emphasizes the need for simple validation assays like the one used here.

Brown fat in a protoendothermic mammal fuels eutherian evolution

Endothermy has facilitated mammalian species radiation, but the sequence of events leading to sustained thermogenesis is debated in multiple evolutionary models. Here we study the Lesser hedgehog tenrec (Echinops telfairi), a phylogenetically ancient, ‘protoendothermic’ eutherian mammal, in which constantly high body temperatures are reported only during reproduction. Evidence for nonshivering thermogenesis is found in vivo during periodic ectothermic–endothermic transitions. Anatomical studies reveal large brown fat-like structures in the proximity of the reproductive organs, suggesting physiological significance for parental care. Biochemical analysis demonstrates high mitochondrial proton leak catalysed by an uncoupling protein 1 ortholog. Strikingly, bioenergetic profiling of tenrec uncoupling protein 1 reveals similar thermogenic potency as modern mouse uncoupling protein 1, despite the large phylogenetic distance. The discovery of functional brown adipose tissue in this ‘protoendothermic’ mammal links nonshivering thermogenesis directly to the roots of eutherian evolution, suggesting physiological importance prior to sustained body temperatures and migration to the cold.

Endothermy facilitated mammalian species radiation, but the events leading to sustained thermogenesis are not clear. Here, the authors report functional brown adipose tissue in a protoendothermic mammal, linking nonshivering thermogenesis directly to the roots of eutherian endothermic evolution.

Reconstitution in liposomes of the functionally active human OCTN1 (SLC22A4) transporter overexpressed in Escherichia coli

The hOCTN1 (human organic cation transporter 1) overexpressed in Escherichia coli and purified by Ni-chelating chromatography has been reconstituted in liposomes by detergent removal with a batch-wise procedure. The reconstitution was optimized with respect to the protein concentration, the detergent/phospholipid ratio and the time of incubation with Amberlite XAD-4 resin. Time-dependent [14C]tetraethylammonium, [3H]carnitine or [3H]ergothioneine uptake was measured in proteoliposomes with activities ratios of 8:1.3:1 respectively. Optimal activity was found at pH 8.0. The transport depended on intraliposomal ATP. [14C]tetraethylammonium transport was inhibited by several compounds. The most effective were acetyl-choline and γ-butyrobetaine, followed by acetylcarnitine and tetramethylammonium. Reagents such as pyridoxal 5-phosphate, MTSES [sodium (2-sulfonatoethyl) methanethiosulfonate] and mercurials strongly inhibited the transport. From kinetic analysis of tetraethylammonium transport a Km of 0.77 mM was calculated. Acetylcholine and γ-butyrobetaine behaved as competitive inhibitors of TEA (tetraethylammonium) transport with Ki values of 0.44 and 0.63 mM respectively.

A comparative study on conformation and ligand binding of the neuronal uncoupling proteins

Mitochondrial uncoupling proteins of the nervous system (UCPs 2, 4, and 5) have potential roles in the function and protection of the central nervous system (CNS). In the absence of structural information, conformations of the hexahistidine-tagged versions of all five human UCPs in liposomes were investigated for the first time, using far- and near-UV CD and fluorescence spectroscopy. Highly pure UCPs 1-5 were reconstituted in detergents and stable small unilamellar vesicles, appropriate for spectroscopic studies. All UCPs formed dominantly helical conformations in negatively charged phospholipid vesicles (palmitoyloleoylphosphatidylcholine/palmitoyloleoylphosphatidylglycerol, 7:3 molar ratio). UCPs 2 and 5 exhibited comparable helical conformations with possible association in lipid bilayers, whereas UCP4 had a different helical profile that can be related to its less associated form. Interaction of reconstituted UCPs with GDP and GTP, inhibitors of the prototypic UCP1, was detected by near-UV CD and fluorescence spectroscopy, utilizing the sensitivity of these techniques to microenvironments around Trp residues close to the nucleotide binding site. Binding of UCP4 to purine nucleotides was also different from other UCPs. Binding of fatty acids, activators of proton transport in UCPs, to UCPs could not be unambiguously detected, implying a nonbinding conformation/orientation of the proteoliposomes. Interaction of CoA with UCPs was comparable to nucleotide binding, suggesting a possible binding of this molecule at the nucleotide binding site. Despite dissimilar primary sequences, neuronal UCPs share common structural and functional properties with UCPs 1 and 3, supporting a common physiological role in addition to their specific roles in the CNS.

Wanderings in bioenergetics and biomembranes

Having worked for 55 years in the center and at the fringe of bioenergetics, my major research stations are reviewed in the following wanderings: from microsomes to mitochondria, from NAD to CoQ, from reversed electron transport to reversed oxidative phosphorylation, from mitochondrial hydrogen transfer to phosphate transfer pathways, from endogenous nucleotides to mitochondrial compartmentation, from transport to mechanism, from carrier to structure, from coupling by AAC to uncoupling by UCP, and from specific to general transport laws. These wanderings are recalled with varying emphasis paid to the covered science stations.

UCP1 ectopically expressed in murine muscle displays native function and mitigates mitochondrial superoxide production

Mitochondrial uncoupling in skeletal muscle has raised a major interest as a therapeutic target for treatment of obesity, insulin sensitivity, and age-related disease. These physiological effects could be demonstrated in several mouse models ectopically expressing uncoupling protein 1 (UCP1). Here, we investigated whether UCP1 expressed under the control of the human skeletal actin (HSA) promoter in mouse skeletal muscle can be regulated, and whether it affects mitochondrial superoxide production. We show that the skeletal muscle UCP1 can be fully inhibited by a purine nucleotide (GDP) and reactivated by fatty acids (palmitate). During mitochondrial resting state (State 4), mitochondrial superoxide production is about 76% lower in transgenic mice. We suggest that this reduction is due to uncoupling activity as the administration of GDP restores superoxide production to wildtype levels. Our study confirms native behaviour of UCP1 in skeletal muscle and demonstrates beneficial effects on prevention of mitochondrial reactive oxygen species production which may reduce age-related deleterious processes.

UCP2, not a physiologically relevant uncoupler but a glucose sparing switch impacting ROS production and glucose sensing

In mammals the two proteins UCP2 and UCP3 are highly similar to the mitochondrial uncoupling protein found in the brown adipose tissue (UCP1). Accordingly, it was proposed that UCP2 and UCP3 are also uncoupling proteins i.e. protonophores with impact on mitochondrial ROS production and glucose signaling. However, it appears now impossible to explain the physiological relevance of the new UCPs uniquely by their uncoupling activity as observed in vitro. Therefore, we propose a metabolic hypothesis in which UCP2 acts through a transport distinct of the proton transport. A consequence of this transport activity would be a decrease of the mitochondrial oxidation of the pyruvate originating from glucose. This would put UCP2 and UCP3 in a crucial position to influence cellular metabolism. The tight control exerted on UCP2 expression appears consistent with it. In this hypothesis, UCP2/3 would allow a cell to remain glycolytic within an aerobic organism. This tallies with the high expression level of UCP2 or UCP3 in glycolytic cells. The metabolic hypothesis would explain the spectacular modifications associated with UCP2 manipulation as well as the uncoupling activity usually called for and which in fact remains elusive in vivo.

Dynamic regulation of uncoupling protein 2 content in INS-1E insulinoma cells

Uncoupling protein 2 (UCP2) regulates glucose-stimulated insulin secretion in pancreatic beta-cells. UCP2 content, measured by calibrated immunoblot in INS-1E insulinoma cells (a pancreatic beta-cell model) grown in RPMI medium, and INS-1E mitochondria, was 2.0 ng/million cells (7.9 ng/mg mitochondrial protein). UCP2 content was lower in cells incubated without glutamine and higher in cells incubated with 20 mM glucose, and varied from 1.0–4.4 ng/million cells (2.7–14.5 ng/mg mitochondrial protein). This dynamic response to nutrients was achieved by varied expression rates against a background of a very short UCP2 protein half-life of about 1 h.

Avian UCP: the killjoy in the evolution of the mitochondrial uncoupling proteins

The understanding of mitochondrial functioning is of prime importance since it combines the production of energy as adenosine triphosphate (ATP) with an efficient chain of redox reactions, but also with the unavoidable production of reactive oxygen species (ROS) involved in aging. Mitochondrial respiration may be uncoupled from ATP synthesis by a proton leak induced by the thermogenic uncoupling protein 1 (UCP1). Mild uncoupling activity, as proposed for UCP2, UCP3, and avian UCP could theoretically control ROS production, but the nature of their transport activities is far from being definitively understood. The recent discovery of a UCP1 gene in fish has balanced the evolutionary view of uncoupling protein history. The thermogenic proton transport of mammalian UCP1 seems now to be a late evolutionary characteristic and the hypothesis that ancestral UCPs may carry other substrates is tempting. Using in silico genome analyses among taxa and a biochemical approach, we present a detailed phylogenetic analysis of UCPs and investigate whether avian UCP is a good candidate for pleiotropic mitochondrial activities, knowing that only one UCP has been characterized in the avian genome, unlike all other vertebrates. We show, here, that the avian class seems to be the only vertebrate lineage lacking two of the UCP1/2/3 homologues present in fish and mammals. We suggest, based on phylogenetic evidence and synteny of the UCP genes, that birds have lost UCP1 and UCP2. The phylogeny also supports the history of two rounds of duplication during vertebrate evolution. The avian uncoupling protein then represents a unique opportunity to explore how UCPs' activities are controlled, but also to understand why birds exhibit such a particular relationship between high metabolism and slow rate of aging.

Mechanisms responsible for enhanced fatty acid utilization by perfused hearts from type 2 diabetic db/db mice

The aim of this study was to determine the biochemical mechanism(s) responsible for enhanced FA utilization (oxidation and esterification) by perfused hearts from type 2 diabetic db/db mice. The plasma membrane content of fatty acid transporters FAT/CD36 and FABPpm was elevated in db/db hearts. Mitochondrial mechanisms that could contribute to elevated rates of FA oxidation were also examined. Carnitine palmitoyl transferase-1 activity was unchanged in mitochondria from db/db hearts, and sensitivity to inhibition by malonyl-CoA was unchanged. Malonyl-CoA content was elevated and AMP kinase activity was decreased in db/db hearts, opposite to what would be expected in hearts exhibiting elevated rates of FA oxidation. Uncoupling protein-3 expression was unchanged in mitochondria from db/db hearts. Therefore, enhanced FA utilization in db/db hearts is most likely due to increased FA uptake caused by increased plasma membrane content of FA transporters; the mitochondrial mechanisms examined do not contribute to elevated FA oxidation observed in db/db hearts.

UCP2, UCP3, avUCP, what do they do when proton transport is not stimulated? Possible relevance to pyruvate and glutamine metabolism

Uncoupling proteins (UCPs) are specialized members of the mitochondrial transporter family. They allow passive proton transport through the mitochondrial inner membrane. This activity leads to uncoupling of mitochondrial respiration and to energy waste, which is well documented with UCP1 in brown adipose tissue. The uncoupling activity of the new UCPs (discovered after 1997), such as UCP2 and UCP3 in mammals or avUCP in birds, is more difficult to characterize. However, extensive data support the idea that the new UCPs are involved in the control of reactive oxygen species (ROS) generation. This fits with the hypothesis that mild uncoupling caused by the UCPs prevents ROS production. Activators and inhibitors regulate the proton transport activity of the UCPs. In the absence of activators of proton transport, the UCP allows the permeation of other ions. We suggest that this activity has physiological significance and, for example, UCP3 expressed in glycolytic muscle fibres may be a passive pyruvate transporter ensuring equilibrium between glycolysis and oxidative phosphorylation. Induction of UCP2 expression by glutamine strengthens the proposal that new UCPs could act to determine the choice of mitochondrial substrate. This would obviously have an impact on mitochondrial bioenergetics and ROS production.

Mitochondrial uncoupling proteins in the CNS: in support of function and survival

Mitochondrial uncoupling mediated by uncoupling protein 1 (UCP1) is classically associated with non-shivering thermogenesis by brown fat. Recent evidence indicates that UCP family proteins are also present in selected neurons. Unlike UCP1, these proteins (UCP2, UCP4 and BMCP1/UCP5) are not constitutive uncouplers and are not crucial for non-shivering thermogenesis. However, they can be activated by free radicals and free fatty acids, and their activity has a profound influence on neuronal function. By regulating mitochondrial biogenesis, calcium flux, free radical production and local temperature, neuronal UCPs can directly influence neurotransmission, synaptic plasticity and neurodegenerative processes. Insights into the regulation and function of these proteins offer unsuspected avenues for a better understanding of synaptic transmission and neurodegeneration.

Uncoupling protein 3 protects aconitase against inactivation in isolated skeletal muscle mitochondria

Mitochondrial uncoupling proteins only catalyse proton transport when they are activated. Activators include superoxide and reactive alkenals, suggesting new physiological functions for UCP2 and UCP3: their activation by superoxide when protonmotive force is high causes mild uncoupling, which lowers protonmotive force and attenuates superoxide generation by the electron transport chain. This feedback loop acts to prevent excessive mitochondrial superoxide production. Superoxide inactivates aconitase in the mitochondrial matrix, so aconitase activity provides a sensitive measure of the effects of UCPs on matrix superoxide. We find that inhibition of UCP3 in isolated skeletal muscle mitochondria by GDP decreases aconitase activity by 25% after 20 min incubation. The GDP effect is absent in skeletal muscle mitochondria from UCP3 knockout mice, showing that it is mediated by UCP3. Protection of aconitase by UCP3 in the absence of nucleotides does not require added fatty acids. The purine nucleoside diphosphates and triphosphates cause aconitase inactivation, but the monophosphates and CDP do not, consistent with the known nucleotide specificity of UCP3. The IC(50) for GDP is about 100 microM. These findings support the proposal that UCP3 attenuates endogenous radical production by the mitochondrial electron transport chain at high protonmotive force.

Physiological functions of the mitochondrial uncoupling proteins UCP2 and UCP3

Evidence for the physiological functions of UCP2 and UCP3 is critically reviewed. They do not mediate adaptive thermogenesis, but they may be significantly thermogenic under specific pharmacological conditions. There is strong evidence that the mild regulated uncoupling they cause attenuates mitochondrial ROS production, protects against cellular damage, and diminishes insulin secretion. Evidence that they export fatty acids physiologically is weak. UCP2 and UCP3 are important potential targets for treatment of aging, degenerative diseases, diabetes, and perhaps obesity.

The reactions catalysed by the mitochondrial uncoupling proteins UCP2 and UCP3

The mitochondrial uncoupling proteins UCP2 and UCP3 may be important in attenuating mitochondrial production of reactive oxygen species, in insulin signalling (UCP2), and perhaps in thermogenesis and other processes. To understand their physiological roles, it is necessary to know what reactions they are able to catalyse. We critically examine the evidence for proton transport and anion transport by UCP2 and UCP3. There is good evidence that they increase mitochondrial proton conductance when activated by superoxide, reactive oxygen species derivatives such as hydroxynonenal, and other alkenals or their analogues. However, they do not catalyse proton leak in the absence of such acute activation. They can also catalyse export of fatty acid and other anions, although the relationship of anion transport to proton transport remains controversial.

Hydroxynonenal and uncoupling proteins: a model for protection against oxidative damage

In this mini review we summarize recent studies from our laboratory that show the involvement of superoxide and the lipid peroxidation product 4-hydroxynonenal in the regulation of mitochondrial uncoupling. Superoxide produced during mitochondrial respiration is a major cause of the cellular oxidative damage that may underlie degenerative diseases and ageing. Superoxide production is very sensitive to the magnitude of the mitochondrial protonmotive force, so can be strongly decreased by mild uncoupling. Superoxide is able to give rise to other reactive oxygen species, which elicit deleterious effects primarily by oxidizing intracellular components, including lipids, DNA and proteins. Superoxide-induced lipid peroxidation leads to the production of reactive aldehydes, including 4-hydroxynonenal. These aldehydic lipid peroxidation products are in turn able to modify proteins such as mitochondrial uncoupling proteins and the adenine nucleotide translocase, converting them into active proton transporters. This activation induces mild uncoupling and so diminishes mitochondrial superoxide production, hence protecting against disease and oxidative damage at the expense of energy production.

The plant uncoupling protein homologues: a new family of energy-dissipating proteins in plant mitochondria

Uncoupling proteins (UCPs) form a subfamily within the mitochondrial carrier protein family, which catalyze a free fatty acid-mediated proton recycling and can modulate the tightness of coupling between mitochondrial respiration and ATP synthesis. As in mammalian tissues, UCPs are rather ubiquitous in the plant kingdom and widespread in plant tissues in which they could have various physiological roles, such as heat production or protection against free oxygen radicals. The simultaneous occurrence in plant mitochondria of two putative energy-dissipating systems, namely UCP which dissipates the proton motive force, and alternative oxidase (AOX) which dissipates the redox potential, raises the question of their functional interactions.

Reconstitution of recombinant uncoupling proteins: UCP1, -2, and -3 have similar affinities for ATP and are unaffected by coenzyme Q10

The successful development of recombinant expression and reconstitution protocols has enabled a detailed study of the transport properties and regulation of the uncoupling proteins (UCP). We optimized conditions of isolation and refolding of bacterially expressed uncoupling proteins and reexamined the transport properties and regulation of bacterially expressed UCP1, -2, and -3 reconstituted in liposomes. We show for the first time that ATP inhibits UCP1, -2, and -3 with similar affinities. The Ki values for ATP inhibition were 50 microm (UCP1), 70 microm (UCP2), and 120 microm (UCP3) at pH 7.2. These affinities for ATP are similar to those obtained with native UCP1 isolated from brown adipose tissue mitochondria (Ki = 65 microm at pH 7.2). The Vmax values for proton transport were also similar among the UCPs, ranging from 8 to 20 micromol.min(-1).mg(-1), depending on experimental conditions. We also examined the effect of coenzyme Q on fatty acid-catalyzed proton flux in liposomes containing recombinant UCP1, -2, and -3. We found that coenzyme Q had no effect on the fatty acid-dependent proton transport catalyzed by any of the UCPs nor did it affect nucleotide regulation of the UCPs. We conclude that coenzyme Q is not a cofactor of UCP-mediated proton transport.

Selective detection of UCP 3 expression in skeletal muscle: effect of thyroid status and temperature acclimation

A novel peptide antibody to UCP 3 is characterized which is sensitive and discriminatory for UCP 3 over UCP 2, UCP 1 and other mitochondrial transporters. The peptide antibody detects UCP 3 expression in E. coli, COS cells and yeast expression systems. The peptide antibody detects a single approximately 33 kDa protein band in mitochondria from isolated rat skeletal muscle, mouse and rat brown adipose tissue, and in whole muscle groups (soleus and extensor digitorum longus) from mice. No 33 kDa band is detectable in isolated mitochondria from liver, heart, brain, kidney and lungs of rats, or gastrocnemius mitochondria from UCP 3 knock-out mice. From our data, we conclude that the peptide antibody is detecting UCP 3 in skeletal muscle, skeletal muscle mitochondria and brown adipose tissue mitochondria. It is also noteworthy that the peptide antibody can detect human, mouse and rat forms of UCP 3. Using the UCP 3 peptide antibody, we confirm and quantify the increased (2.8-fold) UCP 3 expression observed in skeletal muscle mitochondria isolated from 48-h-starved rats. We show that UCP 3 expression is increased (1.6-fold) in skeletal muscle of rats acclimated over 8 weeks to 8 degrees C and that UCP 3 expression is decreased (1.4-fold) in rats acclimated to 30 degrees C. Furthermore, UCP 3 expression is increased (2.3-fold) in skeletal muscle from hyperthyroid rats compared to euthyroid controls. In addition, we show that UCP 3 expression is only coincident with the mitochondrial fraction of skeletal muscle homogenates and not peroxisomal, nuclear or cytosolic and microsomal fractions.

Activating omega-6 polyunsaturated fatty acids and inhibitory purine nucleotides are high affinity ligands for novel mitochondrial uncoupling proteins UCP2 and UCP3

UCP2 (the lowest Km values: 20 and 29 microm, respectively) for omega-6 polyunsaturated FAs (PUFAs), all-cis-8,11,14-eicosatrienoic and all-cis-6,9,12-octadecatrienoic acids, which are also the most potent agonists of the nuclear PPARbeta receptor in the activation of UCP2 transcription. omega-3 PUFA, cis-5,8,11,14,17-eicosapentaenoic acid had lower affinity (Km, 50 microm), although as an omega-6 PUFA, arachidonic acid exhibited the same low affinity as lauric acid (Km, approximately 200 microm). These findings suggest a possible dual role of some PUFAs in activating both UCPn expression and uncoupling activity. UCP2 (UCP3)-dependent H+ translocation activated by all tested FAs was inhibited by purine nucleotides with apparent affinity to UCP2 (reciprocal Ki) decreasing in order: ADP > ATP approximately GTP > GDP >> AMP. Also [3H]GTP ([3H]ATP) binding to isolated Escherichia coli (Kd, approximately 5 microm) or yeast-expressed UCP2 (Kd, approximately 1.5 microm) or UCP3 exhibited high affinity, similar to UCP1. The estimated number of [3H]GTP high affinity (Kd, <0.4 microm) binding sites was (in pmol/mg of protein) 182 in lung mitochondria, 74 in kidney, 28 in skeletal muscle, and approximately 20 in liver mitochondria. We conclude that purine nucleotides must be the physiological inhibitors of UCPn-mediated uncoupling in vivo.

Superoxide activates mitochondrial uncoupling protein 2 from the matrix side. Studies using targeted antioxidants

Superoxide activates nucleotide-sensitive mitochondrial proton transport through the uncoupling proteins UCP1, UCP2, and UCP3 (Echtay, K. S., et al. (2002) Nature 415, 1482-1486). Two possible mechanisms were proposed: direct activation of the UCP proton transport mechanism by superoxide or its products and a cycle of hydroperoxyl radical entry coupled to UCP-catalyzed superoxide anion export. Here we provide evidence for the first mechanism and show that superoxide activates UCP2 in rat kidney mitochondria from the matrix side of the mitochondrial inner membrane: (i) Exogenous superoxide inhibited matrix aconitase, showing that external superoxide entered the matrix. (ii) Superoxide-induced uncoupling was abolished by low concentrations of the mitochondrially targeted antioxidants 10-(6'-ubiquinonyl)decyltriphenylphosphonium (mitoQ) or 2-[2-(triphenylphosphonio)ethyl]-3,4-dihydro-2,5,7,8-tetramethyl-2H-1-benzopyran-6-ol bromide (mitoVit E), which are ubiquinone (Q) or tocopherol derivatives targeted to the matrix by covalent attachment to triphenylphosphonium cation. However, superoxide-induced uncoupling was not affected by similar concentrations of the nontargeted antioxidants Q(o), Q(1), decylubiquinone, vitamin E, or 6-hydroxy-2,5,7,8-tetramethylchroman 2-carboxylic acid (TROLOX) or of the mitochondrially targeted but redox-inactive analogs decyltriphenylphosphonium or 4-chlorobutyltriphenylphosphonium. Thus matrix superoxide appears to be necessary for activation of UCP2 by exogenous superoxide. (iii) When the reduced to oxidized ratio of mitoQ accumulated by mitochondria was increased by inhibiting cytochrome oxidase, it induced nucleotide-sensitive uncoupling that was not inhibited by external superoxide dismutase. Under these conditions quinols are known to produce superoxide, and because mitoQ is localized within the mitochondrial matrix this suggests that production of superoxide in the matrix was sufficient to activate UCP2. Furthermore, the superoxide did not need to be exported or to cycle across the inner membrane to cause uncoupling. We conclude that superoxide (or its products) exerts its uncoupling effect by activating the proton transport mechanism of uncoupling proteins at the matrix side of the mitochondrial inner membrane.

Alteration of plant mitochondrial proton conductance by free fatty acids. Uncoupling protein involvement

We characterized the uncoupling activity of the plant uncoupling protein from Solanum tuberosum (StUCP) using mitochondria from intact potato tubers or from yeast (Saccharomyces cerevisiae) expressing the StUCP gene. Compared with mitochondria from transfected yeast, StUCP is present at very low levels in intact potato mitochondrial membranes (at least thirty times lower) as shown by immunodetection with anti-UCP1 antibodies. Under conditions that ruled out undesirable effects of nucleotides and free fatty acids on uncoupling activity measurement in plant mitochondria, the linoleic acid-induced depolarization in potato mitochondria was insensitive to the nucleotides ATP, GTP, or GDP. In addition, sensitivity to linoleic acid was similar in potato and in control yeast mitochondria, suggesting that uncoupling occurring in potato mitochondria was because of a UCP-independent proton diffusion process. By contrast, yeast mitochondria expressing StUCP exhibited a higher sensitivity to free fatty acids than those from the control yeast and especially a marked proton conductance in the presence of low amounts of linoleic acid. However, this fatty acid-induced uncoupling was also insensitive to nucleotides. Altogether, these results suggest that uncoupling of oxidative phosphorylation and heat production cannot be the dominant feature of StUCP expressed in native potato tissues. However, it could play a role in preventing reactive oxygen species production as proposed for mammalian UCP2 and UCP3.

The basal proton conductance of skeletal muscle mitochondria from transgenic mice overexpressing or lacking uncoupling protein-3

The ability of native uncoupling protein-3 (UCP3) to uncouple mitochondrial oxidative phosphorylation is controversial. We measured the expression level of UCP3 and the proton conductance of skeletal muscle mitochondria isolated from transgenic mice overexpressing human UCP3 (UCP3-tg) and from UCP3 knockout (UCP3-KO) mice. The concentration of UCP3 in UCP3-tg mitochondria was approximately 3 microg/mg protein, approximately 20-fold higher than the wild type value. UCP3-tg mitochondria had increased nonphosphorylating respiration rates, decreased respiratory control, and approximately 4-fold increased proton conductance compared with the wild type. However, this increased uncoupling in UCP3-tg mitochondria was not caused by native function of UCP3 because it was not proportional to the increase in UCP3 concentration and was neither activated by superoxide nor inhibited by GDP. UCP3 was undetectable in mitochondria from UCP3-KO mice. Nevertheless, UCP3-KO mitochondria had unchanged respiration rates, respiratory control ratios, and proton conductance compared with the wild type under a variety of assay conditions. We conclude that uncoupling in UCP3-tg mice is an artifact of transgenic expression, and that UCP3 does not catalyze the basal proton conductance of skeletal muscle mitochondria in the absence of activators such as superoxide.

A significant portion of mitochondrial proton leak in intact thymocytes depends on expression of UCP2

The uncoupling protein homologue UCP2 is expressed in a variety of mammalian cells. It is thought to be an uncoupler of oxidative phosphorylation. Uncoupling proteins previously have been shown to be capable of translocating protons across phospholipid bilayers in proteoliposome systems. Furthermore, studies in mitochondria from yeast overexpressing the proteins have led to suggestions that they may act as uncouplers in cells. However, this issue is controversial, and to date, definitive experimental evidence is lacking as to whether UCP2 mediates part or all of the basal mitochondrial proton leak in mammalian cells in situ. In the present study, by using thymocytes isolated from UCP2-deficient and wild-type (WT) mice, we addressed the question whether UCP2 is directly involved in catalyzing proton leak in intact cells. Over a range of mitochondrial membrane potentials (DeltaPsi(m)), proton leak activity was lower in thymocytes from UCP2-deficient mice compared with WT mice. At physiological levels of DeltaPsi(m), a significant portion (50%) of basal proton leak in resting cells depended on UCP2. Of note, proton leak in whole cells from WT mice, but not UCP2-deficient mice, responded to stimulation by 4-[(E)-2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-napthalenyl)-1-propenyl]benzoic acid (TTNPB), a known activator of UCP2 activity. Consistent with the observed changes in proton leak, DeltaPsi(m) and ATP levels were increased in untreated thymocytes from UCP2-deficient mice. Interestingly, resting respiration was unaltered, suggesting that UCP2 function in resting cells may be concerned with the control of ATP production rather than substrate oxidation. This study establishes that UCP2, expressed at endogenous levels, mediates proton leak in intact cells.

Mitochondrial uncoupling as a target for drug development for the treatment of obesity

Mitochondrial proton cycling is responsible for a significant proportion of basal or standard metabolic rate, so further uncoupling of mitochondria may be a good way to increase energy expenditure and represents a good pharmacological target for the treatment of obesity. Uncoupling by 2,4-dinitrophenol has been used in this way in the past with notable success, and some of the effects of thyroid hormone treatment to induce weight loss may also be due to uncoupling. Diet can alter the pattern of phospholipid fatty acyl groups in the mitochondrial membrane, and this may be a route to uncoupling in vivo. Energy expenditure can be increased by stimulating the activity of uncoupling protein 1 (UCP1) in brown adipocytes either directly or through beta 3-adrenoceptor agonists. UCP2 in a number of tissues, UCP3 in skeletal muscle and the adenine nucleotide translocase have also been proposed as possible drug targets. Specific uncoupling of muscle or brown adipocyte mitochondria remains an attractive target for the development of antiobesity drugs.

A mitochondrial uncoupling artifact can be caused by expression of uncoupling protein 1 in yeast

Uncoupling protein 1 (UCP1) from mouse was expressed in yeast and the specific (GDP-inhibitable) and artifactual (GDP-insensitive) effects on mitochondrial uncoupling were assessed. UCP1 provides a GDP-inhibitable model system to help interpret the uncoupling effects of high expression in yeast of other members of the mitochondrial carrier protein family, such as the UCP1 homologues UCP2 and UCP3. Yeast expressing UCP1 at modest levels (approx. 1 microg/mg of mitochondrial protein) showed no growth defect, normal rates of chemically uncoupled respiration and an increased non-phosphorylating proton conductance that was completely GDP-sensitive. The catalytic-centre activity of UCP1 in these yeast mitochondria was similar to that in mammalian brown-adipose-tissue mitochondria. However, yeast expressing UCP1 at higher levels (approx. 11 microg/mg of mitochondrial protein) showed a growth defect. Their mitochondria had depressed chemically uncoupled respiration rates and an increased proton conductance that was partly GDP-insensitive. Thus, although UCP1 shows native behaviour at modest levels of expression in yeast, higher levels (or rates) of expression can lead to an uncoupling that is not a physiological property of the native protein and is therefore artifactual. This observation might be important in the interpretation of results from experiments in which the functions of UCP1 homologues are verified by their ability to uncouple yeast mitochondria.

Physiological levels of mammalian uncoupling protein 2 do not uncouple yeast mitochondria

We assessed the ability of human uncoupling protein 2 (UCP2) to uncouple mitochondrial oxidative phosphorylation when expressed in yeast at physiological and supraphysiological levels. We used three different inducible UCP2 expression constructs to achieve mitochondrial UCP2 expression levels in yeast of 33, 283, and 4100 ng of UCP2/mg of mitochondrial protein. Yeast mitochondria expressing UCP2 at 33 or 283 ng/mg showed no increase in proton conductance, even in the presence of various putative effectors, including palmitate and all-trans-retinoic acid. Only when UCP2 expression in yeast mitochondria was increased to 4 microg/mg, more than an order of magnitude greater than the highest known physiological concentration, was proton conductance increased. This increased proton conductance was not abolished by GDP. At this high level of UCP2 expression, an inhibition of substrate oxidation was observed, which cannot be readily explained by an uncoupling activity of UCP2. Quantitatively, even the uncoupling seen at 4 microgram/mg was insufficient to account for the basal proton conductance of mammalian mitochondria. These observations suggest that uncoupling of yeast mitochondria by UCP2 is an overexpression artifact leading to compromised mitochondrial integrity.

Uncoupling protein1 mRNA, mitochondrial GTP-binding, and T4 5'-deiodinase of brown adipose tissue in euthermic Daurian ground squirrel during cold exposure

Regulation of thermogenic activity and uncoupling protein1 (UCP1) expression in brown adipose tissue (BAT) were studied in euthermic Daurian ground squirrel after acute and chronic cold exposure at 4 degrees C. The UCP1 concentration was indirectly determined by titration with its specific ligand [3H]-labeled GTP, and Ucp1 mRNA was detected by using a [32P]-labeled antisense oligonucleotide probe. Both acute and chronic cold exposure stimulated up-regulation of Ucp1 mRNA. Although UCP1 concentration is not significantly increased after 24 h of cold exposure, it is markedly elevated by 75% in squirrels after 4-week cold adaptation compared with controls raised at 22 degrees C. Changes in T4 5'-deiodinase activity were closely associated with variations of Ucp1 mRNA level. Ucp1 gene expression is significantly affected by cold exposure in BAT from euthermic Daurian ground squirrels. In addition, the activation of T4 5'-deiodinase may be an important regulatory factor in cold-induced Ucp1 expression.

Uncoupling protein 2, in vivo distribution, induction upon oxidative stress, and evidence for translational regulation

Uncoupling protein 2 (UCP2) belongs to the mitochondrial anion carrier family and partially uncouples respiration from ATP synthesis when expressed in recombinant yeast mitochondria. We generated a highly sensitive polyclonal antibody against human UCP2. Its reactivity toward mitochondrial proteins was compared between wild type and ucp2(-/-) mice, leading to non-ambiguous identification of UCP2. We detected UCP2 in spleen, lung, stomach, and white adipose tissue. No UCP2 was detected in heart, skeletal muscle, liver, and brown adipose tissue. The level of UCP2 in spleen mitochondria is less than 1% of the level of UCP1 in brown adipose tissue mitochondria. Starvation and LPS treatments increase UCP2 level up to 12 times in lung and stomach, which supports the hypothesis that UCP2 responds to oxidative stress situations. Stimulation of the UCP2 expression occurs without any change in UCP2 mRNA levels. This is explained by translational regulation of the UCP2 mRNA. We have shown that an upstream open reading frame located in exon two of the ucp2 gene strongly inhibits the expression of the protein. This further level of regulation of the ucp2 gene provides a mechanism by which expression can be strongly and rapidly induced under stress conditions.

Uncoupling proteins: the issues from a biochemist point of view

The functional characteristics of uncoupling proteins (UCP) are reviewed, with the main focus on the results with isolated and reconstituted proteins. UCP1 from brown adipose tissue, the paradigm of the UCP subfamily, is treated in more detail. The issues addressed are the role and mechanism of fatty acids, the nucleotide binding, the regulation by pH and the identification by mutagenesis of residues involved in these functions. The transport and regulatory functions of UCP2 and 3 are reviewed in comparison to UCP1. The inconsistencies of a proposed nucleotide insensitive H(+) transport by these UCPs as concluded from the expression in yeast and Escherichia coli are elucidated. In both expression system UCP 2 and 3 are not in or cannot be converted to a functionally native state and thus also for these UCPs a nucleotide regulated H (+) transport is postulated.

Mitochondrial proton leak: a role for uncoupling proteins 2 and 3?

In mitochondria ATP synthesis is not perfectly coupled to oxygen consumption due to proton leak across the mitochondrial inner membrane. Quantitative studies have shown that proton leak contributes to approximately 25% of the resting oxygen consumption of mammals. Proton leak plays a role in accounting for differences in basal metabolic rate. Thyroid studies, body mass studies, phylogenic studies and obesity studies have all shown that increased mass-specific metabolic rate is linked to increased mitochondrial proton leak. The mechanism of the proton leak is unclear. Evidence suggests that proton leak occurs by a non-specific diffusion process across the mitochondrial inner membrane. However, the high degree of sequence homology of the recently cloned uncoupling proteins UCP 2 and UCP 3 to brown adipose tissue UCP 1, and their extensive tissue distribution, suggest that these novel uncoupling proteins play a role in proton leak. Early indications from reconstitution experiments and several in vitro expression studies suggest that the novel uncoupling proteins uncouple mitochondria. Furthermore, mice overexpressing UCP 3 certainly show a phenotype consistent with increased metabolism. The evidence for a role for these novel UCPs in mitochondrial proton leak is reviewed.

Mitochondrial proton leak and the uncoupling protein 1 homologues

Mitochondrial proton leak is the largest single contributor to the standard metabolic rate (SMR) of a rat, accounting for about 20% of SMR. Yet the mechanisms by which proton leak occurs are incompletely understood. The available evidence suggests that both phospholipids and proteins in the mitochondrial inner membrane are important determinants of proton conductance. The uncoupling protein 1 homologues (e.g. UCP2, UCP3) may play a role in mediating proton leak, but it is unlikely they account for all of the observed proton conductance. Experimental data regarding the functions of these proteins include important ambiguities and contradictions which must be addressed before their function can be confirmed. The physiological role of the proton leak, and of the uncoupling protein 1 homologues, remains similarly unclear.

Uncoupling proteins 2 and 3 are highly active H(+) transporters and highly nucleotide sensitive when activated by coenzyme Q (ubiquinone)

Based on the discovery of coenzyme Q (CoQ) as an obligatory cofactor for H(+) transport by uncoupling protein 1 (UCP1) [Echtay, K. S., Winkler, E. & Klingenberg, M. (2000) Nature (London) 408, 609-613] we show here that UCP2 and UCP3 are also highly active H(+) transporters and require CoQ and fatty acid for H(+) transport, which is inhibited by low concentrations of nucleotides. CoQ is proposed to facilitate injection of H(+) from fatty acid into UCP. Human UCP2 and 3 expressed in Escherichia coli inclusion bodies are solubilized, and by exchange of sarcosyl against digitonin, nucleotide binding as measured with 2'-O-[5-(dimethylamino)naphthalene-1-sulfonyl]-GTP can be restored. After reconstitution into vesicles, Cl(-) but no H(+) are transported. The addition of CoQ initiates H(+) transport in conjunction with fatty acids. This increase is fully sensitive to nucleotides. The rates are as high as with reconstituted UCP1 from mitochondria. Maximum activity is at a molar ratio of 1:300 of CoQ:phospholipid. In UCP2 as in UCP1, ATP is a stronger inhibitor than ADP, but in UCP3 ADP inhibits more strongly than ATP. Thus UCP2 and UCP3 are regulated differently by nucleotides, in line with their different physiological contexts. These results confirm the regulation of UCP2 and UCP3 by the same factors CoQ, fatty acids, and nucleotides as UCP1. They supersede reports that UCP2 and UCP3 may not be H(+) transporters.

Structure-function relationships in UCP1, UCP2 and chimeras: EPR analysis and retinoic acid activation of UCP2

Uncoupling proteins (UCPs) are composed of three repeated domains of approximately 100 amino acids each. We have used chimeras of UCP1 and UCP2, and electron paramagnetic resonance (EPR), to investigate domain specific properties of these UCPs. Questions include: are the effects of nucleotide binding on proton transport solely mediated by amino acids in the third C-terminal domain, and are the amino acids in the first two domains involved in retinoic or fatty acid activation? We first confirmed that our reconstitution system produced UCP1 that exhibited known properties, such as activation by fatty acids and inhibition of proton transport by purine nucleotides. Our results confirm the observations reported for recombinant yeast that retinoic acid, but not fatty acids known to activate UCP1, activates proton transport by UCP2 and that this activation is insensitive to nucleotide inhibition. We constructed chimeras in which the last domains of UCP1 or UCP2 were switched and tested for activation by fatty acids or retinoic acid and inhibition by nucleotides. U1U2 is composed of mUCP1 (amino acids 1-198) and hUCP2 (amino acids 211-309). Fatty acids activated proton transport of U1U2 and GTP mediated inhibition. In the other chimeric construct U2U1, hUCP2 (amino acids 1-210) and mUCP1 (amino acids 199-307), retinoic acid still acted as an activator, but no inhibition was observed with GTP. Using EPR, a method well suited to the analysis of the structure of membrane proteins such as UCPs, we confirmed that UCP2 binds nucleotides. The EPR data show large structural changes in UCP1 and UCP2 on exposure to ATP, implying that a putative nucleotide-binding site is present on UCP2. EPR analysis also demonstrated changes in conformation of UCP1/UCP2 chimeras following exposure to purine nucleotides. These data demonstrate that a nucleotide-binding site is present in the C-terminal domain of UCP2. This domain was able to inhibit proton transport only when fused to the N-terminal part of UCP1 (chimera U1U2). Thus, residues involved in nucleotide inhibition of proton transport are located in the two first carrier motifs of UCP1. While these results are consistent with previously reported effects of the C-terminal domain on nucleotide binding, they also demonstrate that interactions with the N-terminal domains are necessary to inhibit proton transport. Finally, the results suggest that proteins such as UCP2 may transport protons even though they are not responsible for basal or cold-induced thermogenesis.

Nucleotide effects on liver and muscle mitochondrial non-phosphorylating respiration and membrane potential

Uncoupling protein-1 homologs are hypothesized to mediate mitochondrial proton leak. To test this hypothesis, we determined the effects of ATP and other nucleotides on liver and skeletal muscle mitochondrial non-phosphorylating respiration (VO(2)), membrane potential, FCCP-stimulated respiratory control ratios, and swelling. Neither ATP nor CTP affected liver or muscle proton leak, but both inhibited the respiratory chain. Unexpectedly, CMP stimulated liver proton leak (EC(50) approximately 4.4+/-0.5 mM). Using CMP chromatography, we identified two proteins (M(r)=31.2 and 32.6 kDa) from liver mitochondria that are similar in size to members of the mitochondrial carrier protein family. We conclude (a) liver and muscle mitochondrial proton leak is insensitive to ATP and CTP, and (b) CMP activates a leak in liver mitochondria. The CMP-inducible leak may be mediated by a 30-32 kDa protein. Based on the high concentrations required, CMP is unlikely to be a physiologically important leak regulator. Nonetheless, our results show that tissues other than brown fat have inducible leaks that may be protein-mediated.

Coenzyme Q is an obligatory cofactor for uncoupling protein function

Uncoupling proteins (UCPs) are thought to be intricately controlled uncouplers that are responsible for the futile dissipation of mitochondrial chemiosmotic gradients, producing heat rather than ATP. They occur in many animal and plant cells and form a subfamily of the mitochondrial carrier family. Physiological uncoupling of oxidative phosphorylation must be strongly regulated to avoid deterioration of the energy supply and cell death, which is caused by toxic uncouplers. However, an H+ transporting uncoupling function is well established only for UCP1 from brown adipose tissue, and the regulation of UCP1 by fatty acids, nucleotides and pH remains controversial. The failure of UCP1 expressed in Escherichia coli inclusion bodies to carry out fatty-acid-dependent H+ transport activity inclusion bodies made us seek a native UCP cofactor. Here we report the identification of coenzyme Q (ubiquinone) as such a cofactor. On addition of CoQ10 to reconstituted UCP1 from inclusion bodies, fatty-acid-dependent H+ transport reached the same rate as with native UCP1. The H+ transport was highly sensitive to purine nucleotides, and activated only by oxidized but not reduced CoQ. H+ transport of native UCP1 correlated with the endogenous CoQ content.

The bulk of UCP3 expressed in yeast cells is incompetent for a nucleotide regulated H+ transport

The impact of uncoupling protein (UCP) 1, UCP3 and UCP3s expressed in yeast on oxidative phosphorylation, membrane potential and H+ transport is determined. Intracellular ATP synthesis is inhibited by UCP3, much more than by UCP1, while similar levels of UCP3 and UCP1 exist in the mitochondrial fractions. Measurements of membrane potential and H+ efflux in isolated mitochondria show that, different from UCP1, with UCP3 and UCP3s there is a priori a preponderant uncoupling not inhibited by GDP. The results are interpreted to show that UCP3 and UCP3s in yeast mitochondria are in a deranged state causing uncontrolled uncoupling, which does not represent their physiological function.

Lack of obesity and normal response to fasting and thyroid hormone in mice lacking uncoupling protein-3

Uncoupling protein-3 (UCP3) is a mitochondrial protein that can diminish the mitochondrial membrane potential. Levels of muscle Ucp3 mRNA are increased by thyroid hormone and fasting. Ucp3 has been proposed to influence metabolic efficiency and is a candidate obesity gene. We have produced a Ucp3 knockout mouse to test these hypotheses. The Ucp3 (-/-) mice had no detectable immunoreactive UCP3 by Western blotting. In mitochondria from the knockout mice, proton leak was greatly reduced in muscle, minimally reduced in brown fat, and not reduced at all in liver. These data suggest that UCP3 accounts for much of the proton leak in skeletal muscle. Despite the lack of UCP3, no consistent phenotypic abnormality was observed. The knockout mice were not obese and had normal serum insulin, triglyceride, and leptin levels, with a tendency toward reduced free fatty acids and glucose. Knockout mice showed a normal circadian rhythm in body temperature and motor activity and had normal body temperature responses to fasting, stress, thyroid hormone, and cold exposure. The base-line metabolic rate and respiratory exchange ratio were the same in knockout and control mice, as were the effects of fasting, a beta3-adrenergic agonist (CL316243), and thyroid hormone on these parameters. The phenotype of Ucp1/Ucp3 double knockout mice was indistinguishable from Ucp1 single knockout mice. These data suggest that Ucp3 is not a major determinant of metabolic rate but, rather, has other functions.

Uncoupling protein homologs: emerging views of physiological function

The widespread occurrence of excess weight and related diseases demands that efforts be made to understand energy expenditure from the gene to the whole animal. For some time, it has been understood that mitochondrial oxidation of fuels generates an electrochemical gradient via outward pumping of protons by the electron transport chain. ATP production via F(1)F(0) ATP synthase is then facilitated by the inward flux of protons down the gradient. There is a growing appreciation that a significant portion of the metabolic rate of endotherms is attributable to counteracting "proton leak" (uncoupling), wherein a flux of protons down the electrochemical gradient generates heat independently of ATP production. Proton leak is especially apparent in thermogenic brown adipose tissue, which expresses a tissue-specific uncoupling protein (UCP1). The recent discovery of widely expressed putative UCP1 homologs [UCP2, UCP3, UCP4, UCP5/brain mitochondrial carrier protein-1 (BMCP1)] raised the possibility that innate proton leak and metabolic rate are regulated by UCP1-like proteins. On the basis of current published data, one may not exclude the possibility that UCP homologs influence metabolic rate.