Nature of the masking of nucleotide-binding sites in brown adipose tissue mitochondria. Involvement of endogenous adenosine triphosphate

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.

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.

UCP1 - A sophisticated energy valve

This review focuses on the biochemical work of UCP1 starting from the early observation by Ricquier and Kader in 1976. We entered this field in 1980 with the isolation of native UCP1 and then reported the amino acid sequence structure discovering a strong homology to the ADP/ATP carrier. With the isolated native UCP1 we studied structural and functional features, in particular the complex characteristics of nucleotide binding. A strong pH dependence of binding and herein the differences between diphopho- and triphopho-nucleotides were observed, resulting in the identification of residues which control binding site access by their H⁺ dissociation. Newly synthesized fluorescent nucleotide derivatives provided tools to determine a two state nucleotide binding in line with loose and tight UCP1 conformations and H⁺ transport inhibition. The slow transition between these states were a notable feature. The reconstitution of isolated UCP1 in vesicles demonstrated that UCP1 protein is in fact the uncoupling factor and not only a nucleotide controlled regulator. The H⁺ transport was shown to be electrophoretic with a linear relation to the membrane potential. The dependence of H⁺ transport on fatty acids (FA) was characterized and is elaborated here with a view of the experimental conditions of other research groups which had different views of the role of FA in H⁺ transport. Furthermore, to explain the contrast of the FA - nucleotide competition between mitochondria and reconstituted system, indirect paths for FA to relieve the inhibition in mitochondria are here proposed, such as a FA induced upward pH shift and a FA induced increase of cardiolipin level around UCP1 since cardiolipin has been found by us to relieve nucleotide binding on isolated UCP1. Recently reported patch clamp results on mitoplasts led to a reformulation of the H⁺ transport mechanism of FA in UCP1 in which bound FA shuttles with the carboxyl group between the two membrane sides along the translocation channel outward as FA^- and inward as FA^-H⁺. We propose here a modified version, where FA forms an immobile prosthetic group surrounded by the inner and outer gate of the H⁺ translocation channel. By alternating opening of the gates FA takes up H⁺ from the cytosol side and releases H⁺ to the matrix.

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.

Cold acclimation and oxygen consumption in the thymus

Mitochondrial uncoupling protein 1 is usually associated with brown adipose tissue but has recently been discovered in rat and mouse thymus. We wished to establish whether there was a thermogenic role for UCP 1 in thymus and thus examined the effect of 5 weeks cold-acclimation on rat thymus tissue abundance, thymocyte oxygen consumption, thymus mitochondrial abundance, uncoupling protein 1 expression and function. We found that thymocytes from cold-acclimated rats had oxygen consumption rates 8 times less than those from rats held at room temperature and that thymocytes from cold-acclimated rats or rats kept at room temperature were noradrenaline insensitive. In addition, we found that thymus tissue or mitochondrial abundance was not increased after cold-acclimation. However uncoupling protein 1 expression per unit mass of mitochondria was increased after cold-acclimation, as determined by immunoblotting (approximately 1.7-fold) and GDP binding (approximately 1.5-fold). Consistent with our protein expression data, we also observed an increased, state 4 (approximately 1.5-fold), GDP-inhibitable (approximately 1.3-fold) and palmitate activatable (approximately 1.6-fold) oxygen consumption rates in isolated thymus mitochondria. However, extrapolation of our data showed that cold-acclimation only increased the amount of UCP 1 per gram of thymus tissue approximately 1.2-fold. Taken together, we conclude that UCP 1 does not have a thermogenic role in thymus.

Identification of a functioning mitochondrial uncoupling protein 1 in thymus

We present evidence that rat and mouse thymi contain mitochondrial uncoupling protein (UCP 1). Reverse transcriptase-PCR detected RNA transcripts for UCP 1 in whole thymus and in thymocytes. Furthermore, using antibodies to UCP 1 the protein was also detected in mitochondria isolated from whole thymus and thymocytes but not in thymus mitochondria from UCP 1 knock-out mice. Evidence for functional UCP 1 in thymus mitochondria was obtained by a comparative analysis with the kinetics of GDP binding in mitochondria from brown adipose tissue. Both tissues showed equivalent B(max) and K(D) values. In addition, a large component of the nonphosphorylating oxygen consumption by thymus mitochondria was inhibited by GDP and subsequently stimulated by addition of nanomolar concentrations of palmitate. UCP 1 was purified from thymus mitochondria by hydroxyapatite chromatography. The isolated protein was identified by peptide mass mapping and tandem mass spectrometry by using MALDI-TOF and LC-MS/MS, respectively. We conclude that the thymus contains a functioning UCP 1 that has the capacity to regulate metabolic flux and production of reactive oxygen-containing molecules in the thymus.

Native UCP1 displays simple competitive kinetics between the regulators purine nucleotides and fatty acids

Elucidation of the regulation of uncoupling protein 1 (UCP1) activity in its native environment, i.e. the inner membrane of brown-fat mitochondria, has been hampered by the presence of UCP1-independent, quantitatively unresolved effects of investigated regulators on the brown-fat mitochondria themselves. Here we have utilized the availability of UCP1-ablated mice to dissect UCP1-dependent and UCP1-independent effects of regulators. Using a complex-I-linked substrate (pyruvate), we found that UCP1 can mediate a 4-fold increase in thermogenesis when stimulated with the classical positive regulator fatty acids (oleate). After demonstrating that the fatty acids act in their free form, we found that UCP1 increased fatty acid sensitivity approximately 30-fold (as compared with the 1.5-fold increase reported earlier based on nominal fatty acid values). By identifying the UCP1-mediated fraction of the response, we could conclude that the interaction between purine nucleotides (GDP) and fatty acids (oleate) unexpectedly displayed simple competitive kinetics. In GDP-inhibited mitochondria, oleate apparently acted as an activator. However, only a model in which UCP1 is inherently active (i.e."activating" fatty acids cannot be included in the model), where GDP functions as an inhibitor with a K(m) of 0.05 mm, and where oleate functions as a competitive antagonist for the GDP effect (with a K(i) of 5 nm) can fit all of the experimental data. We conclude that, when examined in its native environment, UCP1 functions as a proton (equivalent) carrier in the absence of exogenous or endogenous fatty acids.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Binding of fatty acids to the uncoupling protein from brown adipose tissue mitochondria

The uncoupling protein 1 (UCP1) is a H(+) carrier which plays a key role in heat generation in brown adipose tissue. The H(+) transport activity of UCP1 is activated by long-chain fatty acids and inhibited by purine nucleotides. While nucleotide binding has been well characterized, the interaction of fatty acid with UCP1 remains unknown. Here I demonstrate the binding of fatty acids by competition with a fluorescent nucleotide probe 2(')-O-dansyl guanosine 5(')-triphosphate (GTP), which has been shown previously to bind at the nucleotide binding site in UCP1. Fatty acids but not their esters competitively inhibit the binding of 2(')-O-dansyl GTP to UCP1. The fatty acid effect was enhanced at higher pH, suggesting the binding of fatty acid anion to UCP1. The inhibition constants K(i) were determined by fluorescence titrations for various fatty acids. Short-chain (C<8) fatty acids display no affinity, whereas medium-chain (C10-14) and unsaturated C18 fatty acids exhibit stronger affinity (K(i)=65 microM, for elaidic acid). This specificity profile agrees with previous functional data obtained in both proteoliposomes and mitochondria, suggesting a possible physiological role of this fatty acid binding site.

Uncoupling of protein-3 induces an uncontrolled uncoupling of mitochondria after expression in muscle derived L6 cells

The uncoupling proteins (UCPs) are thought to uncouple oxidative phosphorylation in the mitochondria and thus generate heat. One of the UCP isoforms, UCP3, is abundantly expressed in skeletal muscle, the major thermogenic tissue in humans. UCP3 has been overexpressed at high levels in yeast systems, where it leads to the uncoupling of cell respiration, suggesting that UCP3 may indeed be capable of dissipating the mitochondrial proton gradient. This effect, however, was recently shown to be a consequence of the high level of expression and incorrect folding of the protein and not to its intrinsic uncoupling activity. In the present study, we investigated the properties of UCP3 overexpressed in a relevant mammalian host system such as the rat myoblast L6 cell line. UCP3 was expressed in relatively low levels (< 1 microg x mg(-1) membrane protein) with the help of an adenovirus vector. Immunofluorescence microscopy of transduced L6 cells showed that UCP3 was expressed in more than 90% of the cells and that its staining pattern was characteristic for mitochondrial localization. The oxygen consumption of L6 cells under nonphosphorylating conditions increased concomitantly with the levels of UCP3 expression. However, uncoupling was associated with an inhibition of the maximal respiratory capacity of mitochondria and was not affected by purine nucleotides and free fatty acids. Moreover, recombinant UCP3 was resistant to Triton X-100 extraction under conditions that fully solubilize membrane bound proteins. Thus, UCP3 can be uniformly overexpressed in the mitochondria of a relevant muscle-derived cell line resulting in the expected increase of mitochondrial uncoupling. However, our data suggest that the protein is present in an incompetent conformation.

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 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.

Evidence of UCP1-independent regulation of norepinephrine-induced thermogenesis in brown fat

To study the thermal response of interscapular brown fat (IBF) to norepinephrine (NE), urethan-anesthetized rats (1.2 g/kg ip) maintained at 28-30 degrees C received a constant venous infusion of NE (0-2 x 10(4) pmol/min) over a period of 60 min. IBF temperatures (T(IBF)) were recorded with a small thermistor fixed under the IBF pad. Data were plotted against time and expressed as maximal variation (Deltat degrees C). Saline-injected rats showed a decrease in T(IBF) of approximately 0.6 degrees C. NE infusion increased T(IBF) by a maximum of approximately 3.0 degrees C at a dose of 10(4) pmol x min(-1) x 100 g body wt(-1). Surgically thyroidectomized (Tx) rats kept on 0.05% methimazole showed a flat response to NE. Treatment with thyroxine (T(4), 0.8 microg x 100 g(-1) x day(-1)) for 2-15 days normalized mitochondrial UCP1 (Western blotting) and IBF thermal response to NE, whereas iopanoic acid (5 mg x 100 g body wt(-1) x day(-1)) blocked the effects of T(4). Treatment with 3,5, 3'-triiodothyronine (T(3), 0.6 microg x 100 g body wt(-1) x day(-1)) for up to 15 days did not normalize UCP1 levels. However, these animals showed a normal IBF thermal response to NE. Cold exposure for 5 days or feeding a cafeteria diet for 20 days increased UCP1 levels by approximately 3.5-fold. Nevertheless, the IBF thermal response was only greater than that of controls when maximal doses of NE (2 x 10(4) pmol/min and higher) were used.

CONCLUSIONS

1) hypothyroidism is associated with a blunted IBF thermal response to NE; 2) two- to fourfold changes in mitochondrial UCP1 concentration are not necessarily translated into heat production during NE infusion.

In the uncoupling protein (UCP-1) His-214 is involved in the regulation of purine nucleoside triphosphate but not diphosphate binding

The nucleotide binding to uncoupling protein (UCP-1) of brown adipose tissue is regulated by pH. The binding pocket of the nucleotide phosphate moiety has been proposed to be controlled by the protonization of a carboxyl group (pK approximately 4.5) for both nucleoside diphosphates (NDP) and nucleoside triphosphates (NTP) (identified as Glu-190) and of a histidine (pK approximately 7. 2) for NTP only. Here we identify His-214 as a pH sensor specific for NTP binding only. In reconstituted UCP-1 from hamster, DEPC diminishes binding of NTP but not of NDP. It also prevents inhibition of H+ transport by NTP but not by NDP. Hamster UCP-1 expressed in Saccharomyces cerevisiae was mutated to H214N resulting in only moderate change of the binding affinity for NTP (GTP) but a 10-fold affinity decrease with the bulkier substituent in H214W, whereas the affinity for NDP (ADP) was largely unchanged. The steep decrease with pH of the binding affinity for NTP in wild type (from pH 6.0 to 7.5) was much flatter in the mutants. Also, the pH dependence of binding and dissociation rates was diminished in these mutants. The transport of H+ and Cl- was not affected. Thus, His-214 is only involved in nucleotide binding, whereas, as previously shown, His-145 and His-147 are involved only in H+ transport. The results validate the earlier proposal of a histidine regulating the NTP binding in addition to a carboxyl group controlling both NTP and NDP binding. It is proposed that His-214 protrudes into the binding pocket for the gamma-phosphate thus inhibiting NTP binding and that His214H+ is retracted by a background -CO2- group to give way for the gamma-phosphate.

Uncoupling protein mRNA, mitochondrial GTP-binding, and T4 5'-deiodinase activity of brown adipose tissue in Daurian ground squirrel during hibernation and arousal

The mRNA level of uncoupling protein (UCP) specific for brown adipose tissue (BAT) in Daurian ground squirrel, was detected by using a [32P]-labeled oligonucleotide probe. The UCP concentration in mitochondria was indirectly determined by titration with its specific ligand [H3]-labeled GTP. Type II T4 5'-deiodinase of BAT was assayed concomitantly. We found two species of mRNA for UCP with lengths of about 1.9 and 1.5 kb, respectively, both occurring in almost the same concentration. UCP mRNA content was elevated significantly during hibernation, but the UCP concentration did not change compared with that of nonhibernating controls kept at room temperature. When hibernating squirrels were aroused, the UCP mRNA remained at the elevated level as during hibernation, but the UCP concentration increased in comparison with that of nonhibernating controls or during hibernating. Changes in T4 5'-deiodinase activity in BAT were similar to the variations of the UCP mRNA level. These results suggest that the activation of T4 5'-deiodinase in BAT may be an important factor for the up-regulation and maintenance of UCP mRNA content needed for the synthesis of sufficient UCP to acquire the thermogenic capacity for arousal from hibernation.

Effect of 12, 24 and 72 hours fasting in thermogenic parameters of rat brown adipose tissue mitochondrial subpopulations

The aim of the present work was to study the effects of various durations of fasting (12, 24 and 72 hours) on brown adipose tissue (BAT) thermogenic parameters--cytrochrome-c-oxidase (COX) activity, GDP-binding activity and uncoupling protein (UCP1) content--and also on morphological features of different mitochondrial subpopulations, obtained by differential centrifugation--M1 (1000 g), M3 (3000 g) and M15 (15,000 g) fractions. The mitochondrial subpopulations showed morphological differences and a different distribution of UCP1 levels and of GDP-binding in all experimental groups. Starvation induced a decrease in the average size for all mitochondrial subtypes. The main changes induced by fasting in thermogenic parameters were observed in the M15 subtype. After the first 24h of starvation, there was a significant decrease of UCP1 levels only in the lightest mitochondrial subpopulation. However, the 72h fasted situation reflected a tendency to increase UCP1 content and UCP1/COX ratio together with a significant decrease of GDP-binding/UCP1 ratio, thus indicating more masked GDP-binding sites. Important fasting-induced changes in both morphological and biochemical parameters in BAT mitochondrial subtypes reflect their role in the physiological response of BAT to starvation.

Slow-phase kinetics of nucleotide binding to the uncoupling protein from brown adipose tissue mitochondria

The kinetics of nucleotide binding to the uncoupling protein (UCP) from brown adipose tissue mitochondria were studied with a filter binding method. Fast and slow phases of binding were observed, corresponding to the two-stage binding model based on equilibrium binding studies (Huang, S. G., and Klingenberg, M. (1996) Biochemistry 35, 7846-7854) (Reaction 1). [reaction: see text] Although this method determines total binding, only the slow phase can be resolved. The fast unresolved phase represents the formation of the initial loose UCP-nucleotide complex (UN; Kd approximately 2 microM), whereas the slow phase reflects the tight binding (U*N) associated with a conformational change induced by the bound nucleotide. Best fits of the binding data yielded, for the slow phase, k+1 values of 3.0 x 10(-3) s-1 for GTP, 4.8 x 10(-3) s-1 for ATP, 0.13 s-1 for GDP, and >0.7 s-1 for ADP and dissociation rate constants (k-1) of 0.10 x 10(-3) s-1 for GTP, 0.58 x 10(-3) s-1 for ATP, 8.8 x 10(-3) s-1 for GDP, and >0.3 s-1 for ADP at pH 6.7 and 4 degrees C. The rates were fairly pH- and temperature-dependent. The distribution constant Kc' (=k+1/k-1) between the tight and loose complexes ranged between 2 and 30, suggesting formation of 71-97% of the tight complex at equilibrium. The Kc' decreases with increasing pH, indicating a progressively less tight complex population. Anions (SO42-) form a loose complex with UCP, thus affecting the initial association step, but not the subsequent transition step. While the kinetic constants were verified by dilution and chase experiments as well as in mass action plots, they were further corroborated with data obtained by fluorescence competition measurements. Taken together, our results show that nucleotide binding to UCP occurs via a two-stage mechanism in which the initial loose complex rearranges slowly into a tight complex.

The structure and function of the brown fat uncoupling protein UCP1: current status

The uncoupling protein of brown adipose tissue (UCP1) is a transporter that allows the dissipation as heat of the proton gradient generated by the respiratory chain. The discovery of new UCPs in other mammalian tissues and even in plants suggests that the proton permeability of the mitochondrial inner membrane can be regulated and its control is exerted by specialised proteins. The UCP1 is regulated both at the gene and the mitochondrial level to ensure a high thermogenic capacity to the tissue. The members of the mitochondrial transporter family, which includes the UCPs, present two behaviours with carrier and channel transport modes. It has been proposed that this property reflects a functional organization in two domains: a channel and a gating domain. Mounting evidence suggest that the matrix loops contribute to the formation of the gating domain and thus they are determinants to the control of transport activity.