Isolation of the GDP binding protein from brown adipose tissue mitochondria of several animals and amino acid composition study in rat

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.

The heat is on: Molecular mechanisms of drug-induced hyperthermia

Thermoregulation is an essential homeostatic process in which critical mechanisms of heat production and dissipation are controlled centrally in large part by the hypothalamus and peripherally by activation of the sympathetic nervous system. Drugs that disrupt the components of this highly orchestrated multi-organ process can lead to life-threatening hyperthermia. In most cases, hyperthermic agents raise body temperature by increasing the central and peripheral release of thermoregulatory neurotransmitters that ultimately lead to heat production in thermogenic effector organs skeletal muscle (SKM) and brown adipose tissue (BAT). In many cases hyperthermic drugs also decrease heat dissipation through peripheral changes in blood flow. Drug-induced heat production is driven by the stimulation of mechanisms that normally regulate the adaptive thermogenic responses including both shivering and non-shivering thermogenesis (NST) mechanisms. Modulation of the mitochondrial electrochemical proton/pH gradient by uncoupling protein 1 (UCP1) in BAT is the most well characterized mechanism of NST in response to cold, and may contribute to thermogenesis induced by sympathomimetic agents, but this is far from established. However, the UCP1 homologue, UCP3, and the ryanodine receptor (RYR1) are established mediators of toxicant-induced hyperthermia in SKM. Defining the molecular mechanisms that orchestrate drug-induced hyperthermia will be essential in developing treatment modalities for thermogenic illnesses. This review will briefly summarize mechanisms of thermoregulation and provide a survey of pharmacologic agents that can lead to hyperthermia. We will also provide an overview of the established and candidate molecular mechanisms that regulate the actual thermogenic processes in heat effector organs BAT and SKM.

The mitochondrial oxoglutarate carrier: from identification to mechanism

The 2-oxoglutarate carrier (OGC) belongs to the mitochondrial carrier protein family whose members are responsible for the exchange of metabolites, cofactors and nucleotides between the cytoplasm and mitochondrial matrix. Initially, OGC was characterized by determining substrate specificity, kinetic parameters of transport, inhibitors and molecular probes that form covalent bonds with specific residues. It was shown that OGC specifically transports oxoglutarate and certain carboxylic acids. The substrate specificity combination of OGC is unique, although many of its substrates are also transported by other mitochondrial carriers. The abundant recombinant expression of bovine OGC in Escherichia coli and its ability to functionally reconstitute into proteoliposomes made it possible to deduce the individual contribution of each and every residue of OGC to the transport activity by a complete set of cys-scanning mutants. These studies give experimental support for a substrate binding site constituted by three major contact points on the even-numbered α-helices and identifies other residues as important for transport function through their crucial positions in the structure for conserved interactions and the conformational changes of the carrier during the transport cycle. The results of these investigations have led to utilize OGC as a model protein for understanding the transport mechanism of mitochondrial carriers.

Adaptive activation of thyroid hormone and energy expenditure

The mechanisms by which thyroid hormone accelerates energy expenditure are poorly understood. In the brown adipose tissue (BAT), activation of thyroid hormone by type 2 iodothyronine deiodinase (D2) has been known to play a role in adaptive energy expenditure during cold exposure in human newborns and other small mammals. Although BAT is not present in significant amounts in normal adult humans, recent studies have found substantial amounts of D2 in skeletal muscle, a metabolically relevant tissue in humans. This article reviews current biological knowledge about D2 and adaptive T3 production and their roles in energy expenditure.

The role of the sympathetic nervous system and uncoupling proteins in the thermogenesis induced by 3,4-methylenedioxymethamphetamine

Body temperature regulation involves a homeostatic balance between heat production and dissipation. Sympathetic agents such as 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) can disrupt this balance and as a result produce an often life-threatening hyperthermia. The hyperthermia induced by MDMA appears to result from the activation of the sympathetic nervous system (SNS) and the hypothalamic-pituitary-thyroid/adrenal axis. Norepinephrine release mediated by MDMA creates a double-edged sword of heat generation through activation of uncoupling protein (UCP3) along with alpha1- and beta3-adrenoreceptors and loss of heat dissipation through SNS-mediated vasoconstriction. This review examines cellular mechanisms involved in MDMA-induced thermogenesis from UCP activation to vasoconstriction and how these mechanisms are related to other thermogenic conditions and potential treatment modalities.

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.

Rapid increase of mitochondrial uncoupling protein and its mRNA in stimulated brown adipose tissue

The increase in mitochondrial uncoupling protein in brown adipose tissue during acute stimulation by exposure of animals to cold was examined. Uncoupling protein level increased during the first hours of tissue stimulation. Use of a cDNA probe shows that synthesis of uncoupling protein mRNA was quickly stimulated. Animals treated with propranolol exhibited neither increase in uncoupling protein mRNA nor increase in the protein itself.

The activity of pyruvate carrier in a reconstituted system: substrate specificity and inhibitor sensitivity

The pyruvate carrier, of molecular mass 34 kDa, was purified from mitochondria isolated from rat liver, rat brain, and bovine heart, by affinity chromatography on immobilized 2-cyano-4-hydroxycinnamate. Its activity after reconstitution in phosphatidylcholine vesicles was measured either as uptake of [1-14C]pyruvate or as exchange with different 2-oxoacids. All preparations exhibited similar apparent Km values for pyruvate, but somewhat different V(max) values. The ability to exchange different anions of physiological significance, including branched-chain 2-oxoacids, confirmed the known substrate specificity described for the pyruvate carrier in mitochondria. The sensitivity of pyruvate transport toward phenylglyoxal suggested an important role of arginyl residues in the transport activity, while a role of lysyl and histidyl residues was not confirmed.

Stable expression of functional mitochondrial uncoupling protein in Chinese hamster ovary cells

The mitochondrial uncoupling protein (UCP) is a membranous proton carrier exclusively synthesized in brown adipocytes. The cDNA for the rat UCP was placed in an expression vector and transfected into mammalian cells. Its expression was tested in transiently transfected CHO cells. In these cells the UCP was detected in mitochondria by using antibodies. Permanent expression of the UCP was achieved in stable transformed CHO cell lines. In these cells the UCP was characterized in mitochondrial membranes, by using antibodies and hydroxyapatite purification. The protein expressed in CHO cells displayed the functional characteristics of brown adipocyte UCP. It induced the uncoupling of respiration in isolated CHO mitochondria. The membrane potential of transformed mitochondria was also significantly lowered, as a result of the proton translocating activity of the UCP. GDP is known to inhibit the proton pathway in brown fat mitochondria. Addition of GDP to CHO mitochondria containing UCP resulted in a recoupling of respiration and an increase in membrane potential. Thus we conclude that functional UCP is expressed in CHO cells and that the insertion of the UCP alone in any mitochondria is sufficient to induce the uncoupling of respiration. This approach should allow studies on the structure-function relationship of the UCP and of several other related mitochondrial carriers.

Reconstitution of purified brown adipose tissue mitochondria uncoupling protein: demonstration of separate identity of nucleotide binding and proton translocation sites by chemical probes

The reaction of two arginine-modifying reagents, phenylglyoxal and 2,3-butanedione, with a highly purified uncoupling protein of brown adipose tissue mitochondria decreased the GDP binding to the uncoupling protein. This inhibition was irreversible and dependent on time and concentration of the reagent. Complete inhibition of GDP binding by both reagents establishes that arginine is one of the critical amino acid residues involved in the binding of GDP. Reconstitution of the uncoupling protein (both unmodified and modified) into phospholipid vesicles by our procedure showed no effect of phenylglyoxal modification on the H+ conductance, thus demonstrating that the proton translocation site is structurally different and distinct from the GDP binding site.

Changes in GDP binding to brown adipose tissue mitochondria and the uncoupling protein

Incubation in vitro of brown adipose tissue (BAT) mitochondria with divalent cations, spermine, or alkaline phosphatase led to a marked increase in the binding of [3H]GDP. The effect of Mg2+ appeared to be the most specific and led to the largest increase in GDP binding. A simplified method was developed for measuring GDP binding to purified uncoupling protein from rat BAT mitochondria. Application of this method indicates that uncoupling protein from cold-acclimated rats binds twice as much GDP as uncoupling protein from cold-acclimated rats that were briefly returned to thermoneutrality, paralleling changes in GDP binding to the mitochondria. Incubation of BAT mitochondria with Mg2+ led to a smaller increase in GDP binding to the subsequently purified uncoupling protein, suggesting that divalent cations may somehow participate in the regulation of the activity of the uncoupling protein.

Limitations in the use of the enzyme-linked immunosorbent assay (ELISA) for identification and quantification of thermogenin

The use of immunological assays, ELISA and RIA, for the identification and quantification of thermogenin (the brown adipose tissue-specific, GDP-binding, 32 kDa uncoupling protein) raises doubts regarding the exclusive occurrence of thermogenin in brown adipose tissue. Weak reactions between mitochondria from rat liver, rat skeletal and heart muscle and hamster white adipose and thermogenin antibodies have been observed (Cannon et al., 1982; Lean et al., 1983; Hansen et al., 1984). In order to study whether these reactions were due to thermogenin in tissues other than brown adipose tissue (BAT) or due to non-specific binding of thermogenin antibodies, a protein from rat liver mitochondria and a protein from tubifex mitochondria were isolated by the same procedure as thermogenin. The 2 proteins had almost the same molecular weight as thermogenin and reacted with thermogenin antibodies in ELISA and dot-blotting, but did not bind GDP and had an amino acid composition different from that of thermogenin. It is concluded that the weak reactions seen between thermogenin antibodies and mitochondria from different tissues other than BAT are due to non-specific binding, and that antibody cross-reactivity alone is unsuitable for the identification of thermogenin.

Partial purification and functional reconstitution of GDP sensitive brown adipose tissue mitochondria uncoupling protein using octyl glucoside

A partial purification of the uncoupling protein of brown adipose tissue mitochondria (BATM) was achieved by extraction of BATM with 40 mM octyl glucoside, followed by affinity chromatography on ATP-agarose. The isolated protein was functionally reconstituted into liposomes using octyl glucoside dialysis. Proteoliposomes containing the uncoupling protein had an increased proton or chloride conductance when subjected to a valinomycin-induced potassium diffusion potential. The increased ion conductance was consistently found to be inhibited by 200 microM GDP.

The possible proton translocating activity of the mitochondrial uncoupling protein of brown adipose tissue. Reconstitution studies in liposomes

Loose coupling of thermogenic mitochondria of brown adipose tissue is related to a high proton (or hydroxyl) conductance of the inner membrane and to the presence of a unique 32 kDa uncoupling protein. Reconstitution experiments of the purified protein in liposomes are reported which suggest that this component could form proton channels in the membrane.

The regulation of the proton conductance of brown fat mitochondria. Identification of functional and non-functional nucleotide-binding sites

The binding of purine nucleotides to intact brown fat mitochondria is re-examined. In addition to the previously reported high affinity binding site, a low-affinity site is found, which requires several minutes to saturate. Only the high affinity site is functional in regulating the proton and halide permeabilities of the mitochondria. The low affinity site can introduce errors in the use of nucleotide binding to quantitate the Mr 32000 uncoupling protein unique to these mitochondria.

Development and activation of brown fat in rats with pheochromocytoma PC 12 tumors

To study the neurohormonal determinism of cellular growth and differentiation and mitochondrial development in brown adipose tissue (BAT), this organ was analyzed in rats bearing uncloned or cloned (PC 12) pheochromocytoma; comparison was made with cold adaptation. Both uncloned and PC 12 tumors induced an enlargement of tissue weight and DNA total content, although smaller than during cold adaptation. The following striking modifications were observed in rats bearing PC 12 tumors: strong vasodilation, increase in protein and phospholipid percentage, alteration of the fatty acid composition of phospholipids, increase in mitochondrial protein, large increase of the GDP binding to isolated mitochondria, and marked rise in specific amount of 32,000-dalton uncoupling protein (ascertained using immunological approach). It is concluded that secretions of PC 12 tumors can induce the same alterations in BAT as does the sympathetic system during cold adaptation of animals. An important contribution of norepinephrine to these effects is evidenced, but a specific function of other trophic factors secreted by PC 12 cells and by sympathetic nerves can be postulated.

Mitochondria of adult human brown adipose tissue contain a 32 000-Mr uncoupling protein

Using both immunohistological study and photoaffinity labelling with radioactive azido-ATP, evidence is presented that the mitochondrial membranes of the brown adipose tissue of the human adult contain a 32 000-Mr uncoupling protein, which is probably similar to the uncoupling protein of BAT of rodents.

An immunological study of the uncoupling protein of brown adipose tissue mitochondria

1. Ewes were injected with purified 32,000-Mr uncoupling protein from mitochondria of brown adipose tissue of cold-adapted rats in order to raise antibodies. 2. The existence of antibodies in the plasma of ewes and the cross-reactivity of plasmas were demonstrated and studied by 125I-labelled antigen-antibody reaction, double immunodiffusion, the inhibition of GDP binding to the 32,000 Mr protein and by immunohistochemistry. 3. The antibodies raised against the homogeneous protein yielded a single immunoprecipitation band with detergent-solubilized mitochondrial membranes of brown adipose tissue from rat, hamster, guinea-pig, rabbit and with the purified uncoupling protein of these animals. No immunoprecipitation was obtained with the protein purified from brown adipose tissue of term lamb foetus. 4. The GDP-binding activity of the uncoupling protein (isolated or in solubilized membranes) was largely inhibited by the antiserum. 5. The anti-(rat uncoupling protein) could not cross-react with solubilized membranes from liver or muscle, nor with the purified beef heart or rat liver ADP/ATP translocator.