Expression, folding, and proton transport activity of human uncoupling protein-1 (UCP1) in lipid membranes: evidence for associated functional forms

ATP-Bound State of the Uncoupling Protein 1 (UCP1) from Molecular Simulations

The uncoupling protein 1 (UCP1) dissipates the transmembrane (TM) proton gradient in the inner mitochondrial membrane (IMM) by leaking protons across the membrane and producing heat in the process. Such a nonshivering production of heat in the brown adipose tissue can combat obesity-related diseases. UCP1-associated proton leak is activated by free fatty acids and inhibited by purine nucleotides. The mechanism of proton leak and the binding sites of the activators (fatty acids) remain unknown, while the binding site of the inhibitors (nucleotides) was described recently. Using molecular dynamics simulations, we generated a conformational ensemble of UCP1. Using metadynamics-based free energy calculations, we obtained the most likely ATP-bound conformation of UCP1. Our conformational ensemble provides a molecular basis for a breadth of prior biochemical data available for UCP1. Based on the simulations, we make the following testable predictions about the mechanisms of activation of proton leak and proton leak inhibition by ATP: (1) R277 plays the dual role of stabilizing ATP at the binding site for inhibition and acting as a proton surrogate for D28 in the absence of a proton during proton transport, (2) the binding of ATP to UCP1 is mediated by residues R84, R92, R183, and S88, (3) R92 shuttles ATP from the E191-R92 gate in the intermembrane space to the nucleotide binding site and serves to increase ATP affinity, (4) ATP can inhibit proton leak by controlling the ionization states of matrix facing lysine residues such as K269 and K56, and (5) fatty acids can bind to UCP1 from the IMM either via the cavity between TM1 and TM2 or between TM5 and TM6. Our simulations set the platform for future investigations into the proton transport and inhibition mechanisms of UCP1.

The regulatory role of adipocyte mitochondrial homeostasis in metabolism-related diseases

Adipose tissue is the most important energy storage organ in the body, maintaining its normal energy metabolism function and playing a vital role in keeping the energy balance of the body to avoid the harm caused by obesity and a series of related diseases resulting from abnormal energy metabolism. The dysfunction of adipose tissue is closely related to the occurrence of diseases related to obesity metabolism. Among various organelles, mitochondria are the main site of energy metabolism, and mitochondria maintain their quality through autophagy, biogenesis, transfer, and dynamics, which play an important role in maintaining metabolic homeostasis of adipocytes. On the other hand, mitochondria have mitochondrial genomes which are vulnerable to damage due to the lack of protective structures and their proximity to sites of reactive oxygen species generation, thus affecting mitochondrial function. Notably, mitochondria are closely related to other organelles in adipocytes, such as lipid droplets and the endoplasmic reticulum, which enhances the function of mitochondria and other organelles and regulates energy metabolism processes, thus reducing the occurrence of obesity-related diseases. This article introduces the structure and quality control of mitochondria in adipocytes and their interactions with other organelles in adipocytes, aiming to provide a new perspective on the regulation of mitochondrial homeostasis in adipocytes on the occurrence of obesity-related diseases, and to provide theoretical reference for further revealing the molecular mechanism of mitochondrial homeostasis in adipocytes on the occurrence of obesity-related diseases.

Physiological and Pathophysiological Roles of Thioredoxin Interacting Protein: A Perspective on Redox Inflammation and Metabolism

Significance: Thioredoxin interacting protein (TXNIP) is a member of the arrestin fold superfamily with important cellular functions, including cellular transport, mitochondrial energy generation, and protein cycling. It is the only arrestin-domain protein known to covalently bind to thioredoxin and plays roles in glucose metabolism, inflammation, apoptosis, and cancer. Recent Advances: The crystal structure of the TXNIP-thioredoxin complex provided details about this fascinating interaction. Recent studies showed that TXNIP is induced by endoplasmic reticulum (ER) stress, activates NLR family pyrin domain containing 3 (NLRP3) inflammasomes, and can regulate glucose transport into cells. The tumor suppressor role of TXNIP in various cancer types and the role of TXNIP in fructose absorption are now described. Critical Issues: The influence of TXNIP on redox state is more complex than its interaction with thioredoxin. Future Directions: It is incompletely understood which functions of TXNIP are thioredoxin-dependent. It is also unclear whether TXNIP binding can inhibit glucose transporters without endocytosis. TXNIP-regulated control of ER stress should also be investigated further. Antioxid. Redox Signal. 38, 442-460.

Antibiotics administration alleviates the high fat diet-induced obesity through altering the lipid metabolism in young mice

Currently, there is a global trend of rapid increase in obesity, especially among adolescents. The antibiotics cocktails (ABX) therapy is commonly used as an adjunctive treatment for gut microbiota related diseases, including obesity. However, the effects of broad-spectrum antibiotics alone on young obese hosts have rarely been reported. In the present study, the 3-week-old C57BL/6J male mice fed a high-fat diet (HFD) were intragastric administration with ampicillin, vancomycin, metronidazole or neomycin for 30 days. The lipid metabolites in plasma were assessed by biochemical assay kits, and genes related to lipid metabolite in the white adipose were assessed by qPCR. To further analyze the underlying mechanisms, the expression of genes related to lipid metabolism, inflammatory reactions and oxidative stress in the liver were determined by qPCR assay. In addition, the expression of oxidative damage-associated proteins in the liver were detected by western blot. The results showed that oral antibiotics exposure could reduce body weight and fat index in HFD-fed mice, concurrent with the increase of white adipose lipolysis genes and the decrease of hepatic lipogenic genes. Furthermore, antibiotics treatment could clearly reverse the HFD-induced elevation of oxidative damage-related proteins in the liver. Together, these findings will provide valuable clues into the effects of antibiotics on obesity.

Browning of the white adipose tissue regulation: new insights into nutritional and metabolic relevance in health and diseases

Adipose tissues are dynamic tissues that play crucial physiological roles in maintaining health and homeostasis. Although white adipose tissue and brown adipose tissue are currently considered key endocrine organs, they differ functionally and morphologically. The existence of the beige or brite adipocytes, cells displaying intermediary characteristics between white and brown adipocytes, illustrates the plastic nature of the adipose tissue. These cells are generated through white adipose tissue browning, a process associated with augmented non-shivering thermogenesis and metabolic capacity. This process involves the upregulation of the uncoupling protein 1, a molecule that uncouples the respiratory chain from Adenosine triphosphate synthesis, producing heat. β-3 adrenergic receptor system is one important mediator of white adipose tissue browning, during cold exposure. Surprisingly, hyperthermia may also induce beige activation and white adipose tissue beiging. Physical exercising copes with increased levels of specific molecules, including Beta-Aminoisobutyric acid, irisin, and Fibroblast growth factor 21 (FGF21), which induce adipose tissue browning. FGF21 is a stress-responsive hormone that interacts with beta-klotho. The central roles played by hormones in the browning process highlight the relevance of the individual lifestyle, including circadian rhythm and diet. Circadian rhythm involves the sleep-wake cycle and is regulated by melatonin, a hormone associated with UCP1 level upregulation. In contrast to the pro-inflammatory and adipose tissue disrupting effects of the western diet, specific food items, including capsaicin and n-3 polyunsaturated fatty acids, and dietary interventions such as calorie restriction and intermittent fasting, favor white adipose tissue browning and metabolic efficiency. The intestinal microbiome has also been pictured as a key factor in regulating white tissue browning, as it modulates bile acid levels, important molecules for the thermogenic program activation. During embryogenesis, in which adipose tissue formation is affected by Bone morphogenetic proteins that regulate gene expression, the stimuli herein discussed influence an orchestra of gene expression regulators, including a plethora of transcription factors, and chromatin remodeling enzymes, and non-coding RNAs. Considering the detrimental effects of adipose tissue browning and the disparities between adipose tissue characteristics in mice and humans, further efforts will benefit a better understanding of adipose tissue plasticity biology and its applicability to managing the overwhelming burden of several chronic diseases.

Determination of tissue contributions to the circulating lipid pool in cold exposure via systematic assessment of lipid profiles

Plasma lipid levels are altered in chronic conditions such as type 2 diabetes and cardiovascular disease as well as during acute stresses such as fasting and cold exposure. Advances in MS-based lipidomics have uncovered a complex plasma lipidome of more than 500 lipids that serve functional roles, including as energy substrates and signaling molecules. This plasma lipid pool is maintained through regulation of tissue production, secretion, and uptake. A major challenge in understanding the lipidome complexity is establishing the tissues of origin and uptake for various plasma lipids, which is valuable for determining lipid functions. Using cold exposure as an acute stress, we performed global lipidomics on plasma and in nine tissues that may contribute to the circulating lipid pool. We found that numerous species of plasma acylcarnitines (ACars) and ceramides (Cers) were significantly altered upon cold exposure. Through computational assessment, we identified the liver and brown adipose tissue as major contributors and consumers of circulating ACars, in agreement with our previous work. We further identified the kidney and intestine as novel contributors to the circulating ACar pool and validated these findings with gene expression analysis. Regression analysis also identified that the brown adipose tissue and kidney are interactors with the plasma Cer pool. Taken together, these studies provide an adaptable computational tool to assess tissue contribution to the plasma lipid pool. Our findings have further implications in understanding the function of plasma ACars and Cers, which are elevated in metabolic diseases.

A screening method for binding synthetic metallo-complexes to haem proteins

The introduction of a second coordination sphere, in the form of a protein scaffold, to synthetic catalysts can be beneficial for their reactivity and substrate selectivity. Here we present semi-native polyacrylamide gel electrophoresis (semi-native PAGE) as a rapid screening method for studying metal complex-protein interactions. Such a screening is generally performed using electron spray ionization mass spectrometry (ESI-MS) and/or UV-Vis spectroscopy. Semi-native PAGE analysis has the advantage that it does not rely on spectral changes of the metal complex upon protein interaction and can be applied for high-throughput screening and optimization of complex binding. In semi-native PAGE non-denatured protein samples are loaded on a gel containing sodium dodecyl sulphate (SDS), leading to separation based on differences in structural stability. Semi-native PAGE gel runs of catalyst-protein mixtures were compared to gel runs obtained with native and denaturing PAGE. ESI-MS was additionally realised to confirm protein-complex binding. The general applicability of semi-native PAGE was investigated by screening the binding of various cobalt- and ruthenium-based compounds to three types of haem proteins.

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.

Uncoupling Proteins and Regulated Proton Leak in Mitochondria

Higher concentration of protons in the mitochondrial intermembrane space compared to the matrix results in an electrochemical potential causing the back flux of protons to the matrix. This proton transport can take place through ATP synthase complex (leading to formation of ATP) or can occur via proton transporters of the mitochondrial carrier superfamily and/or membrane lipids. Some mitochondrial proton transporters, such as uncoupling proteins (UCPs), transport protons as their general regulating function; while others are symporters or antiporters, which use the proton gradient as a driving force to co-transport other substrates across the mitochondrial inner membrane (such as phosphate carrier, a symporter; or aspartate/glutamate transporter, an antiporter). Passage (or leakage) of protons across the inner membrane to matrix from any route other than ATP synthase negatively impacts ATP synthesis. The focus of this review is on regulated proton transport by UCPs. Recent findings on the structure and function of UCPs, and the related research methodologies, are also critically reviewed. Due to structural similarity of members of the mitochondrial carrier superfamily, several of the known structural features are potentially expandable to all members. Overall, this report provides a brief, yet comprehensive, overview of the current knowledge in the field.

Perirenal Adipose Tissue from Healthy Donor: Characteristics and Promise as Potential Therapeutic Cell Source

Perirenal adipose tissue, one of the fat masses surrounding the kidneys, can be obtained from healthy donors during a kidney transplant. Perirenal adipose tissue has only ever been known as a connective tissue to protect the kidneys and renal blood vessels from external physical stimulation. Yet, recently, as adipose tissue has begun to be considered an endocrine organ, and perirenal adipose tissue is now regarded to have a direct effect on metabolic diseases. The characteristics of perirenal adipose tissue from a healthy donor are that: (1) There are a large number of brown adipose cells (70–80% of the total), (2) Most of the brown adipose cells are inactive in the resting cell cycle, (3) Activating factors are constant low-temperature exposure, hormones, metastasis factors, and environmental factors, (4) Anatomically, a large number of brown adipose cells are distributed close to the adrenal glands, (5) Beige cells, produced by converting white adipocytes to brown-like adipocytes, are highly active, (6) Activated cells secrete BATokines, and (7) Energy consumption efficiency is high. Despite these advantages, all of the perirenal adipose tissue from a healthy donor is incinerated as medical waste. With a view to its use, this review discusses the brown adipocytes and beige cells in perirenal adipose tissue from a healthy donor, and proposes opportunities for their clinical application.

Biphasic Proton Transport Mechanism for Uncoupling Proteins

It has been suggested that uncoupling proteins (UCPs) transport protons via interconversion between two conformational states: one in the "cytoplasmic state" and the other in the "matrix state". Matrix and cytoplasmic salt-bridge networks are key controllers of these states. This study proposes a mechanism for proton transport in tetrameric UCP2, with focus on the role of the matrix network. Eleven mutants were prepared to disrupt (K → Q or D → N mutations) or alter (K → D and D → K mutations) the salt-bridges in the matrix network. Proteins were recombinantly expressed in Escherichia coli membrane, reconstituted in model lipid membranes, and their structures and functions were analyzed by gel electrophoresis, circular dichroism spectroscopy, fluorescence assays, as well as molecular dynamics simulations. It is shown that the UCP2 matrix network contains five salt-bridges (rather than the previously reported three), and the matrix network can regulate the proton transport by holding the protein's transmembrane helices in close proximity, limiting the movement of the activator fatty acid(s). A biphasic two-state molecular model is proposed for proton transport in tetrameric (a dimer of stable dimers) UCP2, in which all the monomers are functional, and monomers in each dimer are in the same transport mode. Purine nucleotide (e.g., ATP) can occlude the internal pore of the monomeric units of UCP tetramers via interacting with positive residues at or in the proximity of the matrix network (K38, K141, K239, R88, R185, and R279) and prevent switching between cytoplasmic and matrix states, thus inhibiting the proton transport. This study provides new insights into the mechanism of proton transport and regulation in UCPs.

Excess dietary carbohydrate affects mitochondrial integrity as observed in brown adipose tissue

Hyperglycemia affects over 400 million individuals worldwide. The detrimental health effects are well studied at the tissue level, but the in vivo effects at the organelle level are poorly understood. To establish such an in vivo model, we used mice lacking TXNIP, a negative regulator of glucose uptake. Examining mitochondrial function in brown adipose tissue, we find that TXNIP KO mice have a lower content of polyunsaturated fatty acids (PUFAs) in their membrane lipids, which affects mitochondrial integrity and electron transport chain efficiency and ultimately results in lower mitochondrial heat output. This phenotype can be rescued by a ketogenic diet, confirming the usefulness of this model and highlighting one facet of early cellular damage caused by excess glucose influx.

Functional Oligomeric Forms of Uncoupling Protein 2: Strong Evidence for Asymmetry in Protein and Lipid Bilayer Systems

Stoichiometry of uncoupling proteins (UCPs) and their coexistence as functional monomeric and associated forms in lipid membranes remain intriguing open questions. In this study, tertiary and quaternary structures of UCP2 were analyzed experimentally and through molecular dynamics (MD) simulations. UCP2 was overexpressed in the inner membrane of Escherichia coli, then purified and reconstituted in lipid vesicles. Structure and proton transport function of UCP2 were characterized by circular dichroism (CD) spectroscopy and fluorescence methods. Findings suggest a tetrameric functional form for UCP2. MD simulations conclude that tetrameric UCP2 is a dimer of dimers, is more stable than its monomeric and dimeric forms, is asymmetrical and induces asymmetry in the membrane's lipid structure, and a biphasic on-off switch between the dimeric units is its possible mode of transport. MD simulations also show that the water density inside the UCP2 monomer is asymmetric, with the cytoplasmic side having a higher water density and a wider radius. In contrast, the structurally comparable adenosine 5'-diphosphate (ADP)/adenosine 5'-triphosphate (ATP) carrier (AAC1) did not form tetramers, implying that tetramerization cannot be generalized to all mitochondrial carriers.

The Effect of Hydro-Alcoholic Extract of Nigella sativa on Bmp7 and Bmp8b Expression in Rats Fed with a High-Fat Diet

Background: Bone morphogenetic protein7 (BMP7) and bone morphogenetic protein 8b (BMP8b) can induce browning of white adipose tissue. Objectives: The present study aimed to investigate the antioxidative effects of hydro-alcoholic extract of Nigella sativa on the repair of oxidative damage caused by a high-fat diet. Also, Bmp7 and Bmp8b gene expressions were investigated on white adipose tissue of the rats and then compared with metformin effects. Methods: Eighty rats were divided into two groups of prevention and treatment; then each set was divided into four sub-groups based on the administered diet (i.e., ordinary, fat, metformin, and extract of Nigella sativa). Lipid profile, paraoxonase1, malondialdehyde (MDA), HDL, and antioxidant capacity were measured in serum samples, and relative Bmp7 and Bmp8b gene expressions were calculated in white adipose tissue. Results: For both prevention and treatment sets, the weight of rats who received a high-fat diet decreased more compared to those in the normal diet group. The weight of rats who received metformin or nigella extract was also decreased compared to the high-fat diet group. MDA was also increased, but total antioxidant capacity and catalase were decreased in rats of the high-fat diet group compared to the normal diet group. MDA was also declined in nigella receiving rats, but liver PON1 activity, total antioxidant capacity, and catalase were increased, compared to the second group (P < 0.05). In the prevention and treatment set, Bmp8b gene expression was increased in the metformin and Nigella sativa groups, whereas it was decreased among those who received a high-fat diet. Bmp7 gene expression was decreased in the high-fat diet set, but metformin and Nigella sativa extract didn’t influence Bmp7 gene expression. Conclusions: This study demonstrated that Nigella sativa extract has a protective role against oxidative stress in a high-fat diet.

The colorful versatility of adipocytes: white-to-brown transdifferentiation and its therapeutic potential in humans

Brown and brite adipocytes contribute to energy expenditure through nonshivering thermogenesis. Though these cell types are thought to arise primarily from the de novo differentiation of precursor cells, their abundance is also controlled through the transdifferentiation of mature white adipocytes. Here, we review recent advances in our understanding of the regulation of white-to-brown transdifferentiation, as well as the conversion of brown and brite adipocytes to dormant, white-like fat cells. Converting mature white adipocytes into brite cells or reactivating dormant brown and brite adipocytes has emerged as a strategy to ameliorate human metabolic disorders. We analyze the evidence of learning from mice and how they translate to humans to ultimately scrutinize the relevance of this concept. Moreover, we estimate that converting a small percentage of existing white fat mass in obese subjects into active brite adipocytes could be sufficient to achieve meaningful benefits in metabolism. In conclusion, novel browning agents have to be identified before adipocyte transdifferentiation can be realized as a safe and efficacious therapy.

Association of uncoupling protein (Ucp) gene polymorphisms with cardiometabolic diseases

The hereditary aspect of obesity is a major focus of modern medical genetics. The genetic background is known to determine a higher-than-average prevalence of obesity in certain regions, like Oceania. There is evidence that dysfunction of brown adipose tissue (BAT) may be a risk factor for obesity and type 2 diabetes (T2D). A significant number of studies in the field focus on the UCP family. The Ucp genes code for electron transport carriers. UCP1 (thermogenin) is the most abundant protein of the UCP superfamily and is expressed in BAT, contributing to its capability of generating heat. Single nucleotide polymorphisms (SNPs) of Ucp1-Ucp3 were recently associated with risk of cardiometabolic diseases. This review covers the main Ucp SNPs A-3826G, A-1766G, A-112C, Met229Leu, Ala64Thr (Ucp1), Ala55Val, G-866A (Ucp2), and C-55 T (Ucp3), which may be associated with the development of obesity, disturbance in lipid metabolism, T2D, and cardiovascular diseases.

Lipid Regulators of Thermogenic Fat Activation

The global prevalence of obesity continues to increase, suggesting a need for alternative treatment approaches. Targeting brown fat function to promote energy expenditure represents one such approach. Brown adipocytes and the related beige adipocytes oxidize fatty acids and glucose to generate heat and are activated by cold exposure or consumption of high-calorie diets. Alternative, more practical means to activate thermogenic fat are needed. Here, we review emerging data suggesting new roles for lipids in activating thermogenesis that extend beyond their serving as a fuel source for heat generation. Lipids have also been implicated in mediating interorgan communication, crosstalk between organelles, and cellular signaling regulating thermogenesis. Understanding how lipids regulate thermogenesis could identify innovative therapeutic interventions for obesity.

Effects of Nigella sativa Extracts on the Lipid Profile and Uncoupling Protein-1 Gene Expression in Brown Adipose Tissue of Mice

Background:

Uncoupling protein-1 (UCP-1) is the index protein of the brown adipose tissue (BAT), used in the obesity studies. We evaluated the effects of thymoquinone (TQ), hydroalcoholic, and hexane extracts of Nigella sativa, on the UCP-1 gene expression in BAT, and also on the recovery from oxidative stress, due to a high-fat diet.

Materials and Methods:

Fifty mice were divided into five groups: the first group was fed with a usual diet and the second, third, fourth, and fifth groups with a high-fat diet, hydroalcoholic extract, hexane extract, and TQ, respectively. After completing the course, the lipid profile, paraoxonase 1 (PON1), serum total antioxidant capacity (TAC), and malondialdehyde (MDA) were measured. UCP-1 expression in BAT was evaluated at the gene and protein level.

Results:

The weight of mice, receiving TQ, hydroalcoholic, and hexane extracts, was decreased (P < 0.05), compared to the second group (P < 0.05). MDA was increased in the second group, compared to the first group (P < 0.05); however, TAC, liver catalase enzyme, and PON1 were decreased (P < 0.05). Furthermore, MDA of the third, fourth, and fifth groups had decreased, and the activity of PON1, liver catalase enzyme, and the amount of TAC was increased (P < 0.05). UCP-1 expression of the third and fourth groups was increased, compared to the second group (P < 0.05).

Conclusion:

The results suggest that TQ, hydroalcoholic, and hexane extracts of N. sativa have a protective and therapeutic role in the oxidative stress, caused by high-fat diets. The hydroalcoholic and hexane extracts can induce weight loss, by positively affecting UCP-1, at the gene and protein level.

Cardiolipin Synthesis in Brown and Beige Fat Mitochondria Is Essential for Systemic Energy Homeostasis

Activation of energy expenditure in thermogenic fat is a promising strategy to improve metabolic health, yet the dynamic processes that evoke this response are poorly understood. Here we show that synthesis of the mitochondrial phospholipid cardiolipin is indispensable for stimulating and sustaining thermogenic fat function. Cardiolipin biosynthesis is robustly induced in brown and beige adipose upon cold exposure. Mimicking this response through overexpression of cardiolipin synthase (Crls1) enhances energy consumption in mouse and human adipocytes. Crls1 deficiency in thermogenic adipocytes diminishes inducible mitochondrial uncoupling and elicits a nuclear transcriptional response through endoplasmic reticulum stress-mediated retrograde communication. Cardiolipin depletion in brown and beige fat abolishes adipose thermogenesis and glucose uptake, which renders animals insulin resistant. We further identify a rare human CRLS1 variant associated with insulin resistance and show that adipose CRLS1 levels positively correlate with insulin sensitivity. Thus, adipose cardiolipin has a powerful impact on organismal energy homeostasis through thermogenic fat bioenergetics.

Inhibition of mitochondrial UCP1 and UCP3 by purine nucleotides and phosphate

Mitochondrial membrane uncoupling protein 3 (UCP3) is not only expressed in skeletal muscle and heart, but also in brown adipose tissue (BAT) alongside UCP1, which facilitates a proton leak to support non-shivering thermogenesis. In contrast to UCP1, the transport function and molecular mechanism of UCP3 regulation are poorly investigated, although it is generally agreed upon that UCP3, analogous to UCP1, transports protons, is activated by free fatty acids (FFAs) and is inhibited by purine nucleotides (PNs). Because the presence of two similar uncoupling proteins in BAT is surprising, we hypothesized that UCP1 and UCP3 are differently regulated, which may lead to differences in their functions. By combining atomic force microscopy and electrophysiological measurements of recombinant proteins reconstituted in planar bilayer membranes, we compared the level of protein activity with the bond lifetimes between UCPs and PNs. Our data revealed that, in contrast to UCP1, UCP3 can be fully inhibited by all PNs and IC50 increases with a decrease in PN-phosphorylation. Experiments with mutant proteins demonstrated that the conserved arginines in the PN-binding pocket are involved in the inhibition of UCP1 and UCP3 to different extents. Fatty acids compete with all PNs bound to UCP1, but only with ATP bound to UCP3. We identified phosphate as a novel inhibitor of UCP3 and UCP1, which acts independently of PNs. The differences in molecular mechanisms of the inhibition between the highly homologous transporters UCP1 and UCP3 indicate that UCP3 has adapted to fulfill a different role and possibly another transport function in BAT.

The molecular features of uncoupling protein 1 support a conventional mitochondrial carrier-like mechanism

Uncoupling protein 1 (UCP1) is an integral membrane protein found in the mitochondrial inner membrane of brown adipose tissue, and facilitates the process of non-shivering thermogenesis in mammals. Its activation by fatty acids, which overcomes its inhibition by purine nucleotides, leads to an increase in the proton conductance of the inner mitochondrial membrane, short-circuiting the mitochondrion to produce heat rather than ATP. Despite 40 years of intense research, the underlying molecular mechanism of UCP1 is still under debate. The protein belongs to the mitochondrial carrier family of transporters, which have recently been shown to utilise a domain-based alternating-access mechanism, cycling between a cytoplasmic and matrix state to transport metabolites across the inner membrane. Here, we review the protein properties of UCP1 and compare them to those of mitochondrial carriers. UCP1 has the same structural fold as other mitochondrial carriers and, in contrast to past claims, is a monomer, binding one purine nucleotide and three cardiolipin molecules tightly. The protein has a single substrate binding site, which is similar to those of the dicarboxylate and oxoglutarate carriers, but also contains a proton binding site and several hydrophobic residues. As found in other mitochondrial carriers, UCP1 has two conserved salt bridge networks on either side of the central cavity, which regulate access to the substrate binding site in an alternating way. The conserved domain structures and mobile inter-domain interfaces are consistent with an alternating access mechanism too. In conclusion, UCP1 has retained all of the key features of mitochondrial carriers, indicating that it operates by a conventional carrier-like mechanism.

Magic angle spinning 31P NMR spectroscopy reveals two essentially identical ionization states for the cardiolipin phosphates in phospholipid liposomes

Specific membrane lipid composition is crucial for optimized structural and functional organization of biological membranes. Cardiolipin is a unique phospholipid and important component of the inner mitochondrial membrane. It is involved in energy metabolism, inner mitochondrial membrane transport, regulation of multiple metabolic reactions and apoptotic cell death. The physico-chemical properties of cardiolipin have been studied extensively but despite all these efforts there is still lingering controversy regarding the ionization of the two phosphate groups of cardiolipin. Results obtained in the 1990s and early 2000s suggested that cardiolipin has two disparate pKa values where one of the protons was proposed to be stabilized by an intramolecular hydrogen bond. This has led to extensive speculations on the roles of these two putative ionization states of cardiolipin in mitochondria. More recently the notion of two pKa values has been challenged and rejected by several groups. These studies relied on external measurements of proton adsorption or electrophoretic mobility of membranes but did not take into account the low pH phase behavior and chemical stability of cardiolipin. Here we used 31P NMR to show that in the physiologically relevant membrane phospholipid environment, cardiolipin carries two negative charges at physiological pH. We additionally demonstrate the pH dependent phase behavior and chemical stability of cardiolipin containing membranes.

Design of a peptidic inhibitor that targets the dimer interface of a prototypic galectin

Galectins are small soluble lectins that bind β-galactosides via their carbohydrate recognition domain (CRD). Their ability to dimerize is critical for the crosslinking of glycoprotein receptors and subsequent cellular signaling. This is particularly important in their immunomodulatory role via the induction of T-cell apoptosis. Because galectins play a central role in many pathologies, including cancer, they represent valuable therapeutic targets. At present, most inhibitors have been directed towards the CRD, a challenging task in terms of specificity given the high structural homology of the CRD among galectins. Such inhibitors are not effective at targeting CRD-independent functions of galectins. Here, we report a new class of galectin inhibitors that specifically binds human galectin-7 (hGal-7), disrupts the formation of homodimers, and inhibits the pro-apoptotic activity of hGal-7 on Jurkat T cells. In addition to representing a new means to achieve specificity when targeting galectins, such inhibitors provide a promising alternative to more conventional galectin inhibitors that target the CRD with soluble glycans or other small molecular weight allosteric inhibitors.

Women have more potential to induce browning of perirenal adipose tissue than men

OBJECTIVE

Brown adipose tissue (BAT) can generate heat by burning fatty acids, a process mediated by uncoupling protein 1 (UCP1). White adipose tissue (WAT) depots can gain BAT-like properties, and various studies have suggested that females have more active BAT or BAT-like WAT. We studied sex differences in BAT-like properties of human perirenal adipose tissue.

METHODS

Perirenal and subcutaneous adipose tissue was obtained from 20 male and 24 female healthy live kidney donors. Mesenchymal stem cells (MSCs), adipocyte precursor cells, were isolated from these depots to study whether intrinsic factors control BAT-like properties of the adipose tissue depots.

RESULTS

When average outside temperature a week before harvesting was below 11°C, brown-like adipocytes expressing UCP1 were present in perirenal adipose tissue of women, but not of men. MSCs derived from perirenal adipose tissue expressed significantly more UCP1 when from female origin compared to male origin (P = 0.009). However, UCP1 protein content and oxygen consumption rate did not differ between adipocytes derived from male and female perirenal MSCs.

CONCLUSIONS

Female perirenal adipose tissue has a higher potency to gain BAT-like properties than male perirenal adipose tissue. The degree of gaining BAT-like properties depends on sex-specific intrinsic factors and environmental triggers such as temperature.

A biophysical study on molecular physiology of the uncoupling proteins of the central nervous system

Mitochondrial inner membrane uncoupling proteins (UCPs) facilitate transmembrane (TM) proton flux and consequently reduce the membrane potential and ATP production. It has been proposed that the three neuronal human UCPs (UCP2, UCP4 and UCP5) in the central nervous system (CNS) play significant roles in reducing cellular oxidative stress. However, the structure and ion transport mechanism of these proteins remain relatively unexplored. Recently, we reported a novel expression system for obtaining functionally folded UCP1 in bacterial membranes and applied this system to obtain highly pure neuronal UCPs in high yields. In the present study, we report on the structure and function of the three neuronal UCP homologues. Reconstituted neuronal UCPs were dominantly helical in lipid membranes and transported protons in the presence of physiologically-relevant fatty acid (FA) activators. Under similar conditions, all neuronal UCPs also exhibited chloride transport activities that were partially inhibited by FAs. CD, fluorescence and MS measurements and semi-native gel electrophoresis collectively suggest that the reconstituted proteins self-associate in the lipid membranes. Based on SDS titration experiments and other evidence, a general molecular model for the monomeric, dimeric and tetrameric functional forms of UCPs in lipid membranes is proposed. In addition to their shared structural and ion transport features, neuronal UCPs differ in their conformations and proton transport activities (and possibly mechanism) in the presence of different FA activators. The differences in FA-activated UCP-mediated proton transport could serve as an essential factor in understanding and differentiating the physiological roles of UCP homologues in the CNS.

Cardiolipin is a key determinant for mtDNA stability and segregation during mitochondrial stress

Mitochondria play a key role in adaptation during stressing situations. Cardiolipin, the main anionic phospholipid in mitochondrial membranes, is expected to be a determinant in this adaptive mechanism since it modulates the activity of most membrane proteins. Here, we used Saccharomyces cerevisiae subjected to conditions that affect mitochondrial metabolism as a model to determine the possible role of cardiolipin in stress adaptation. Interestingly, we found that thermal stress promotes a 30% increase in the cardiolipin content and modifies the physical state of mitochondrial membranes. These changes have effects on mtDNA stability, adapting cells to thermal stress. Conversely, this effect is cardiolipin-dependent since a cardiolipin synthase-null mutant strain is unable to adapt to thermal stress as observed by a 60% increase of cells lacking mtDNA (ρ0). Interestingly, we found that the loss of cardiolipin specifically affects the segregation of mtDNA to daughter cells, leading to a respiratory deficient phenotype after replication. We also provide evidence that mtDNA physically interacts with cardiolipin both in S. cerevisiae and in mammalian mitochondria. Overall, our results demonstrate that the mitochondrial lipid cardiolipin is a key determinant in the maintenance of mtDNA stability and segregation.

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.

Folding and self-association of atTic20 in lipid membranes: implications for understanding protein transport across the inner envelope membrane of chloroplasts

Background

The Arabidopsis thaliana protein atTic20 is a key component of the protein import machinery at the inner envelope membrane of chloroplasts. As a component of the TIC complex, it is believed to form a preprotein-conducting channel across the inner membrane.

Results

We report a method for producing large amounts of recombinant atTic20 using a codon-optimized strain of E. coli coupled with an autoinduction method of protein expression. This method resulted in the recombinant protein being directed to the bacterial membrane without the addition of a bacterial targeting sequence. Using biochemical and biophysical approaches, we were able to demonstrate that atTic20 homo-oligomerizes in vitro when solubilized in detergents or reconstituted into liposomes. Furthermore, we present evidence that the extramembranous N-terminus of the mature protein displays characteristics that are consistent with it being an intrinsically disordered protein domain.

Conclusion

Our work strengthens the hypothesis that atTic20 functions similarly to other small α-helical integral membrane proteins, such as Tim23, that are involved in protein transport across membranes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12858-014-0029-y) contains supplementary material, which is available to authorized users.

The C-terminus of Bienertia sinuspersici Toc159 contains essential elements for its targeting and anchorage to the chloroplast outer membrane

Most nucleus-encoded chloroplast proteins rely on an N-terminal transit peptide (TP) as a post-translational sorting signal for directing them to the organelle. Although Toc159 is known to be a receptor for specific preprotein TPs at the chloroplast surface, the mechanism for its own targeting and integration into the chloroplast outer membrane is not completely understood. In a previous study, we identified a novel TP-like sorting signal at the C-terminus (CT) of a Toc159 homolog from the single-cell C4 species, Bienertia sinuspersici. In the current study, we have extended our understanding of the sorting signal using transient expression of fluorescently-tagged fusion proteins of variable-length, and with truncated and swapped versions of the CT. As was shown in the earlier study, the 56 residues of the CT contain crucial sorting information for reversible interaction of the receptor with the chloroplast envelope. Extension of this region to 100 residues in the current study stabilized the interaction via membrane integration, as demonstrated by more prominent plastid-associated signals and resistance of the fusion protein to alkaline extraction. Despite a high degree of sequence similarity, the plastid localization signals of the equivalent CT regions of Arabidopsis thaliana Toc159 homologs were not as strong as that of the B. sinuspersici counterparts. Together with computational and circular dichroism analyses of the CT domain structures, our data provide insights into the critical elements of the CT for the efficient targeting and anchorage of Toc159 receptors to the dimorphic chloroplasts in the single-cell C4 species.