Mitochondrial uncoupling proteins in unicellular eukaryotes

Uncoupling proteins (UCPs) are members of the mitochondrial anion carrier protein family that are present in the mitochondrial inner membrane and mediate free fatty acid (FFA)-activated, purine nucleotide (PN)-inhibited proton conductance. Since 1999, the presence of UCPs has been demonstrated in some non-photosynthesising unicellular eukaryotes, including amoeboid and parasite protists, as well as in non-fermentative yeast and filamentous fungi. In the mitochondria of these organisms, UCP activity is revealed upon FFA-induced, PN-inhibited stimulation of resting respiration and a decrease in membrane potential, which are accompanied by a decrease in membranous ubiquinone (Q) reduction level. UCPs in unicellular eukaryotes are able to divert energy from oxidative phosphorylation and thus compete for a proton electrochemical gradient with ATP synthase. Our recent work indicates that membranous Q is a metabolic sensor that might utilise its redox state to release the PN inhibition of UCP-mediated mitochondrial uncoupling under conditions of phosphorylation and resting respiration. The action of reduced Q (QH₂) could allow higher or complete activation of UCP. As this regulatory feature was demonstrated for microorganism UCPs (A. castellanii UCP), plant and mammalian UCP1 analogues, and UCP1 in brown adipose tissue, the process could involve all UCPs. Here, we discuss the functional connection and physiological role of UCP and alternative oxidase, two main energy-dissipating systems in the plant-type mitochondrial respiratory chain of unicellular eukaryotes, including the control of cellular energy balance as well as preventive action against the production of reactive oxygen species.     

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.     

Fatty Acid Interaction with Mitochondrial Uncoupling Proteins

The phenomena of fatty acid interaction with mitochondrial integral membrane proteins, namelyuncoupling proteins (UCPs), are reviewed to emphasize the fatty acid cycling mechanism thathas been suggested to explain the UCP function. Fatty acid-induced uncoupling is suggestedto serve in bioenergetic systems, to set the optimum efficiency, and to tune the degree ofcoupling of oxidative phosphorylation. Fatty acid interaction with the classic uncouplingprotein (UCP1) from mitochondria of thermogenic brown adipose tissue (BAT) is well known.UCP1 is considered to mediate purine nucleotide-sensitive uniport of monovalent unipolaranions, including anionic fatty acids. The return of protonated fatty acid leads to H⁺ uniportand uncoupling. Experiments supporting this mechanism are also reviewed for plant uncouplingmitochondrial protein (PUMP) and ADP/ATP carrier. The fatty acid cycling mechanism ispredicted, as well for the recently discovered uncoupling proteins, UCP2 and UCP3.     

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.     

Functional Characterization of a Drosophila Mitochondrial Uncoupling Protein

Sequence alignment of conserved signature motifs predicts the existence of the uncoupling protein 5 (UCP5)/brain mitochondrial carrier protein (BMCP1) homologue in Drosophila melanogaster. Here we demonstrate the functional characterization of the Drosophila melanogaster UCP5 protein (DmUCP5) in the heterologous yeast system, the first insect UCP reported to date. We show that physiological levels of DmUCP5 expression are responsible for an increase in state 4 respiration rates and a decrease in mitochondrial membrane potential. Furthermore, similar to UCP1, UCP2, and UCP3, the uncoupling activity of DmUCP5 is augmented by fatty acids and inhibited by the purine nucleotide GDP. Thus, DmUCP5 shares the mechanisms known to regulate the UCPs characterized to date. A lack of growth inhibition observed in DmUCP5 expressing yeast is consistent with the notion that physiological uncoupling has a minimal effect on cell growth. Finally, semiquantitative RT-PCR analysis shows a distinctive pattern of DmUCP5 expression predominantly localized in the adult head, similar to the expression pattern of its mammalian homologues. The conserved regulation of the expression of this gene from mammals to fruit flies suggests a role for UCP5 in the brain.     

Mitochondrial uncoupling proteins and phylogenesis--UCP4 as the ancestral uncoupling protein

We searched for the previously defined uncoupling protein (UCP) signatures [Jezek, P. and Urbánková, E. (2000) IUBMB Life 49, 63-70] in genomes of Drosophila melanogaster, Caenorhabditis elegans, Dictyostelium discoideum, and Arabidopsis thaliana. We identified four UCPs in Drosophila and one in Caenorhabditis or Dictyostelium as close relatives of human UCP4 (BMCP), but distant from UCP1, UCP2, UCP3, and two plant UCPs of Arabidopsis. But the third Arabidopsis UCP is the closest UCP4 relative. This suggests that UCP4 represents the ancestral UCP from which other mammalian and plant UCPs diverged. Speculations on UCP4 participation in apoptosis are thus supported by its early phylogenetic occurrence.     

Functional characterization of the mitochondrial uncoupling proteins from the white shrimp Litopenaeus vannamei

Mitochondrial uncoupling proteins (UCPs) play an essential role in dissipating the proton gradient and controlling the mitochondrial inner membrane potential. When active, UCPs promote proton leak across the inner membrane, oxidative phosphorylation uncoupling, oxygen uptake increase and decrease the ATP synthesis. Invertebrates possess only isoforms UCP4 and UCP5, however, the role of these proteins is not clear in most species since it may depend on the physiological needs of each animal. This study presents the first functional characterization of crustacean uncoupling proteins from the white shrimp Litopenaeus vannamei LvUCP4 and LvUCP5. Free radicals production in various shrimp organs/tissues was first evaluated, and mitochondria were isolated from shrimp pleopods. The oxygen consumption rate, membrane potential and proton transport of the isolated non-phosphorylating mitochondria were used to determine LvUCPs activation/inhibition. Results indicate that UCPs activity is stimulated in the presence of 4-hydroxyl-2-nonenal (HNE) and myristic acid, and inhibited by the purine nucleotide GDP. A hypoxia/re-oxygenation assay was conducted to determine whether UCPs participate in shrimp mitochondria response to oxidative stress. Isolated mitochondria from shrimp at re-oxygenation produced large quantities of hydrogen peroxide and higher levels of both LvUCPs were immunodetected. Results suggest that, besides the active response of the shrimp antioxidant system, UCP-like activity is activated after hypoxia exposure and during re-oxygenation. LvUCPs may represent a mild uncoupling mechanism, which may be activated before the antioxidant system of cells, to early control reactive oxygen species production and oxidative damage in shrimp.     

Functional characterization of UCP1 in mammalian HEK293 cells excludes mitochondrial uncoupling artefacts and reveals no contribution to basal proton leak

Mechanistic studies on uncoupling proteins (UCPs) not only are important to identify their cellular function but also are pivotal to identify potential drug targets to manipulate mitochondrial energy transduction. So far, functional and comparative studies of uncoupling proteins in their native environment are hampered by different mitochondrial, cellular and genetic backgrounds. Artificial systems such as yeast ectopically expressing UCPs or liposomes with reconstituted UCPs were employed to address crucial mechanistic questions but these systems also produced inconsistencies with results from native mitochondria. We here introduce a novel mammalian cell culture system (Human Embryonic Kidney 293 - HEK293) to study UCP1 function. Stably transfected HEK293 cell lines were derived that contain mouse UCP1 at concentrations comparable to tissue mitochondria. In this cell-based test system UCP1 displays native functional behaviour as it can be activated with fatty acids (palmitate) and inhibited with purine nucleotides guanosine-diphosphate (GDP). The catalytic centre activity of the UCP1 homodimer in HEK293 is comparable to activities in brown adipose tissue supporting functionality of UCP1. Importantly, at higher protein levels than in yeast mitochondria, UCP1 in HEK293 cell mitochondria is fully inhibitable and does not contribute to basal proton conductance, thereby emphasizing the requirement of UCP1 activation for therapeutic purposes. These findings and resulting analysis on UCP1 characteristics demonstrate that the mammalian HEK293 cell system is suitable for mechanistic and comparative functional studies on UCPs and provides a non-confounding mitochondrial, cellular and genetic background.     

解偶联蛋白与肥胖及2型糖尿病发病的关系

解偶联蛋白（UCPs）是线粒体内膜上的一种转运蛋白,它能够降低线粒体内膜上的质子梯度,使底物氧化和ADP磷酸化解偶联,减少ATP的产生。基于其功能,解偶联蛋白基因被视为肥胖病及2型糖尿病的重要候选基因。UCP同系物过表达的遗传工程小鼠表现出对饮食导致的肥胖具有耐受性,同时UCP2基因3＇非翻译区的45bp插入／缺失以及UCP3基因C-55T多态与肥胖表型的相关性等研究结论支持这一假说。本文对UCP基因与多基因控制的肥胖病及2型糖尿病发病的相关研究进行综述和讨论。 Abstract:Uncoupling proteins (UCPs) are mitochondria carrier proteins,which are able to dissipate the proton gradient of the inner mitochondria membrane.The uncoupling procedure reduces the amount of ATP generated through an oxidation of fuels.Therefore,UCPs are suggested as candidate genes for human obesity or type II diabetes mellitus.Experimental evidences,that genetically engineered mice over expressing different UCP homologues were resistant to diet-induced obesity and 45bp insertion polymorphism in the UCP2 3＇untranslated region and C-55T in UCP3 promoter region were associated with obesity related phenotype,supported the hypothesis.The roles of UCP genes in polygenic obesity and type II diabetes are evaluated and discussed in this paper.     

GDP and carboxyatractylate inhibit 4-hydroxynonenal-activated proton conductance to differing degrees in mitochondria from skeletal muscle and heart

The lipid peroxidation product 4-hydroxynonenal (HNE) increases the proton conductance of the inner mitochondrial membrane through effects on uncoupling proteins (UCPs) and the adenine nucleotide translocase (ANT); however, the relative contribution of the two carriers to these effects is unclear. To clarify this we isolated mitochondria from skeletal muscle and heart of wild-type and Ucp3 knockout (Ucp3KO) mice. To increase UCP3 expression, some mice were i.p. injected with LPS (12 mg/kg body weight). In spite of the increased UCP3 expression levels, basal proton conductance did not change. HNE increased the proton conductance of skeletal muscle and heart mitochondria. In skeletal muscle, this increase was lower in Ucp3KO mice and higher in LPS-treated wild-type mice, and was partially abolished by GDP (UCPs inhibitor) and completely abolished by carboxyatractylate (ANT inhibitor) or addition of both inhibitors. GDP had no effect on HNE-induced conductance in heart mitochondria, but carboxyatractylate or administration of both inhibitors had a partial effect. GDP-mediated inhibition of HNE-activated proton conductance in skeletal muscle mitochondria was not observed in Ucp3KO mice, indicating that GDP is specific for UCP3, at least in muscle. Carboxyatractylate was able to inhibit UCP3, probably through an indirect mechanism. Our results are consistent with the conclusion that, in skeletal muscle, HNE-induced increase in proton conductance is mediated by UCP3 (30%) and ANT, whereas in the heart the increase is mediated by ANT and other carriers, possibly including UCP3.     

Links between fatty acids and expression of UCP2 and UCP3 mRNAs

Physiological and pathological states that are associated with elevated plasma fatty acids (FAs) increase uncoupling protein 2 (UCP2) mRNA in white adipose tissue and UCP3 mRNA in skeletal muscle and heart. A direct effect of unsaturated fatty acids from all classes has been shown in various cultured cells. There is evidence that FAs could induce expression of UCPs by acting as ligands for peroxisome proliferator-activated receptors, influencing the function of sterol responsive element binding protein or activating 5'-AMP-activated protein kinase. Oleic acid has been shown to stimulate the activity of the promoter regions of UCP2 and UCP3 genes and the FA responsive regions are beginning to be characterised.     

Toward understanding the mechanism of ion transport activity of neuronal uncoupling proteins UCP2, UCP4, and UCP5

Neuronal uncoupling proteins (UCP2, UCP4, and UCP5) have crucial roles in the function and protection of the central nervous system (CNS). Extensive biochemical studies of UCP2 have provided ample evidence of its participation in proton and anion transport. To date, functional studies of UCP4 and UCP5 are scarce. In this study, we show for the first time that, despite a low level of amino acid sequence identity with the previously characterized UCPs (UCP1-UCP3), UCP4 and UCP5 share their functional properties. Recombinantly expressed in Escherichia coli, UCP2, UCP4, and UCP5 were isolated and reconstituted into liposome systems, where their conformations and ion (proton and chloride) transport properties were examined. All three neuronal UCPs are able to transport protons across lipid membranes with characteristics similar to those of the archetypal protein UCP1, which is activated by fatty acids and inhibited by purine nucleotides. Neuronal UCPs also exhibit transmembrane chloride transport activity. Circular dichroism spectroscopy shows that these three transporters exist in different conformations. In addition, their structures and functions are differentially modulated by the mitochondrial lipid cardiolipin. In total, this study supports the existence of general conformational and ion transport features in neuronal UCPs. On the other hand, it also emphasizes the subtle structural and functional differences between UCPs that could distinguish their physiological roles. Differentiation between structure-function relationships of neuronal UCPs is essential for understanding their physiological functions in the CNS.     

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.     

Uncoupling proteins UCP2 and UCP3 from mitochondria of liver and skeletal muscle of ground squirrel Spermophilus undulatus do not transport pyruvate in contrast to UCP1 from brown adipose tissue

Physiological role of mitochondrial uncoupling proteins UCP2 and UCP3, homologous to UCP1 from brown adipose tissue, is unclear. It was proposed recently that UCP2 and UCP3 are metabolic triggers that switch oxidation of glucose to oxidation of fatty acids, exporting pyruvate from mitochondria. In the present study we tried to verify this hypothesis using ground squirrels (Spermophilus undulatus), since expression of all UCPs in different tissues increases during winter season, and UCP1 is abundant in brown fat. We confirmed the possibility of nonspecific transport of pyruvate through UCP1 in brown fat mitochondria and tried to identify similar transport in liver and skeletal muscle mitochondria where UCP2 and UCP3 are expressed. Transport of pyruvate mediated by UCP1 in mitochondria of brown fat was observed using valinomycin-induced swelling of non-respiring mitochondria in 55 mM potassium pyruvate and was inhibited by GDP. In contrast, mitochondria of liver and skeletal muscles in similar conditions did not exhibit electrogenic transport of pyruvate anions that could be related to functioning of UCP2 and UCP3. At the same time, functioning of pyruvate carrier was detected in these mitochondria by nigericin-induced passive swelling or valinomycin-induced active swelling in potassium pyruvate that was inhibited by α-CHC, a specific inhibitor of the pyruvate carrier. Thus, our results suggest that in contrast to UCP1 of brown fat, UCP2 and UCP3 from intact liver and skeletal muscle mitochondria of winter active ground squirrels are unable to carry out pyruvate transport.     

Molecular cloning of lamprey uncoupling protein and assessment of its uncoupling activity using a yeast heterologous expression system

We report the molecular cloning of a novel cDNA fragment from lamprey encoding a 313-amino acid protein that is highly homologous to human uncoupling proteins (UCP). We therefore named the protein lamprey UCP. This lamprey UCP, rat UCP1, human UCP2, and human mitochondrial oxoglutarate carrier were individually expressed in Saccharomyces cerevisiae and the recombinant yeast mitochondria were isolated and assayed for the state 4 respiration rate and proton leak. Only UCP1 showed a strong (3.6-fold increase of the ratio of mitochondrial state 4 respiration rate to FCCP-stimulated fully uncoupled respiration rate) and GDP-inhibitable uncoupling activity, while the uncoupling activities of both UCP2 and lamprey UCP were relatively weak (1.5-fold and 1.4-fold, respectively) and GDP-insensitive. The oxoglutarate carrier had no effect on the studied parameters. In conclusion, the lamprey UCP has a mild, unregulated uncoupling activity in the yeast system, which resembles UCP2, but not UCP1.     

Redox State of Endogenous Coenzyme Q Modulates the Inhibition of Linoleic Acid-Induced Uncoupling by Guanosine Triphosphate in Isolated Skeletal Muscle Mitochondria

The skeletal muscle mitochondria contain two isoforms of uncoupling protein, UCP2 and mainly UCP3, which had been shown to be activated by free fatty acids and inhibited by purine nucleotides in reconstituted systems. On the contrary in isolated mitochondria, the protonophoretic action of muscle UCPs had failed to be demonstrated in the absence of superoxide production. We showed here for the first time that muscle UCPs were activated in state 3 respiration by linoleic acid and dissipated energy from oxidative phosphorylation by decreasing the ADP/O ratio. The efficiency of UCPs in mitochondrial uncoupling increased when the state 3 respiratory rate decreased. The inhibition of the linoleic acid-induced uncoupling by a purine nucleotide (GTP), was not observed in state 4 respiration, in uninhibited state 3 respiration, as well as in state 3 respiration inhibited by complex III inhibitors. On the contrary, the progressive inhibition of state 3 respiration by n -butyl malonate, which inhibits the uptake of succinate, led to a full inhibitory effect of GTP. Therefore, as the inhibitory effect of GTP was observed only when the reduced state of coenzyme Q was decreased, we propose that the coenzyme Q redox state could be a metabolic sensor that modulates the purine nucleotide inhibition of FFA-activated UCPs in muscle mitochondria.     

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

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

Cell-free production and characterisation of human uncoupling protein 1–3

The uncoupling proteins (UCPs) leak protons across the inner mitochondrial membrane, thus uncoupling the proton gradient from ATP synthesis. The main known physiological role for this is heat generation by UCP1 in brown adipose tissue. However, UCPs are also believed to be important for protection against reactive oxygen species, fine-tuning of metabolism and have been suggested to be involved in disease states such as obesity, diabetes and cancer.Structural studies of UCPs have long been hampered by difficulties in sample preparation with neither expression in yeast nor refolding from inclusion bodies in E. coli yielding sufficient amounts of pure and stable protein. In this study, we have developed a protocol for cell-free expression of human UCP1, 2 and 3, resulting in 1 mg pure protein per 20 mL of expression media. Lauric acid, a natural UCP ligand, significantly improved protein thermal stability and was therefore added during purification. Secondary structure characterisation using circular dichroism spectroscopy revealed the proteins to consist of mostly α-helices, as expected. All three UCPs were able to bind GDP, a well-known physiological inhibitor, as shown by the Fluorescence Resonance Energy Transfer (FRET) technique, suggesting that the proteins are in a natively folded state.