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Uncoupling protein 2 (UCP2) is involved in various physiological and pathological processes such as insulin secretion, stem cell differentiation, cancer, and aging. However, its biochemical and physiological function is still under debate. Here we show that UCP2 is a metabolite transporter that regulates substrate oxidation in mitochondria. To shed light on its biochemical role, we first studied the effects of its silencing on the mitochondrial oxidation of glucose and glutamine. Compared with wild-type, UCP2-silenced human hepatocellular carcinoma (HepG2) cells, grown in the presence of glucose, showed a higher inner mitochondrial membrane potential and ATP:ADP ratio associated with a lower lactate release. Opposite results were obtained in the presence of glutamine instead of glucose. UCP2 reconstituted in lipid vesicles catalyzed the exchange of malate, oxaloacetate, and aspartate for phosphate plus a proton from opposite sides of the membrane. The higher levels of citric acid cycle intermediates found in the mitochondria of siUCP2-HepG2 cells compared with those found in wild-type cells in addition to the transport data indicate that, by exporting C4 compounds out of mitochondria, UCP2 limits the oxidation of acetyl-CoA–producing substrates such as glucose and enhances glutaminolysis, preventing the mitochondrial accumulation of C4 metabolites derived from glutamine. Our work reveals a unique regulatory mechanism in cell bioenergetics and provokes a substantial reconsideration of the physiological and pathological functions ascribed to UCP2 based on its purported uncoupling properties.

3-hydroxy-3-methyl glutaryl-CoA reductase, also known as HMGR, plays a crucial role in regulating cholesterol biosynthesis and represents the main pharmacological target of statins. In mammals, this enzyme localizes to the endoplasmic reticulum membrane. HMGR includes different regions, an integral N-terminal domain connected by a linker-region to a cytosolic C-terminal domain, the latter being responsible for enzymatic activity. The aim of this work was to design a simple strategy for cloning, expression, and purification of the catalytic C-terminal domain of the human HMGR (cf-HMGR), in order to spectrophotometrically test its enzymatic activity. The recombinant cf-HMGR protein was heterologously expressed in Escherichia coli, purified by Ni + -agarose affinity chromatography and reconstituted in its active form. MALDI mass spectrometry was adopted to monitor purification procedure as a technique orthogonal to the classical Western blot analysis. Protein identity was validated by MS and MS/MS analysis, confirming about 82% of the recombinant sequence. The specific activity of the purified and dialyzed cf-HMGR preparation was enriched about 85-fold with respect to the supernatant obtained from cell lysate. The effective, cheap, and easy method here described could be useful for screening statin-like molecules, so simplifying the search for new drugs with hypocholesterolemic effects.

The human mitochondrial carrier family (MCF) consists of 53 members. Approximately one-fifth of them are still orphans of a function. Most mitochondrial transporters have been functionally characterized by reconstituting the bacterially expressed protein into liposomes and transport assays with radiolabeled compounds. The efficacy of this experimental approach is constrained to the commercial availability of the radiolabeled substrate to be used in the transport assays. A striking example is that of N-acetylglutamate (NAG), an essential regulator of the carbamoyl synthetase I activity and the entire urea cycle. Mammals cannot modulate mitochondrial NAG synthesis but can regulate the levels of NAG in the matrix by exporting it to the cytosol, where it is degraded. The mitochondrial NAG transporter is still unknown. Here, we report the generation of a yeast cell model suitable for identifying the putative mammalian mitochondrial NAG transporter. In yeast, the arginine biosynthesis starts in the mitochondria from NAG which is converted to ornithine that, once transported into cytosol, is metabolized to arginine. The deletion of ARG8 makes yeast cells unable to grow in the absence of arginine since they cannot synthetize ornithine but can still produce NAG. To make yeast cells dependent on a mitochondrial NAG exporter, we moved most of the yeast mitochondrial biosynthetic pathway to the cytosol by expressing four E. coli enzymes, argB-E, able to convert cytosolic NAG to ornithine. Although argB-E rescued the arginine auxotrophy of arg8∆ strain very poorly, the expression of the bacterial NAG synthase (argA), which would mimic the function of a putative NAG transporter increasing the cytosolic levels of NAG, fully rescued the growth defect of arg8∆ strain in the absence of arginine, demonstrating the potential suitability of the model generated.

The yeast mitochondrial transport of GTP and GDP is mediated by Ggc1p, a member of the mitochondrial carrier family. The physiological role of Ggc1p in S. cerevisiae is probably to transport GTP into mitochondria in exchange for GDP generated in the matrix. ggc1Δ cells exhibit lower levels of GTP and increased levels of GDP in mitochondria, are unable to grow on nonfermentable substrates and lose mtDNA. Because in yeast, succinyl-CoA ligase produces ATP instead of GTP, and the mitochondrial nucleoside diphosphate kinase is localized in the intermembrane space, Ggc1p is the only supplier of mitochondrial GTP required for the maturation of proteins containing Fe-S clusters, such as aconitase [4Fe-4S] and ferredoxin [2Fe-2S]. In this work, it was demonstrated that citrate is a regulator of purified and reconstituted Ggc1p by trans-activating unidirectional transport of GTP across the proteoliposomal membrane. It was also shown that the binding site of Ggc1p for citrate is different from the binding site for the substrate GTP. It is proposed that the citrate-induced GTP uniport (CIGU) mediated by Ggc1p is involved in the homeostasis of the guanine nucleotide pool in the mitochondrial matrix.

Members of the mitochondrial carrier (MC) protein family transport various molecules across the mitochondrial inner membrane to interlink steps of metabolic pathways and biochemical processes that take place in different compartments; i.e., are localized partly inside and outside the mitochondrial matrix. MC substrates consist of metabolites, inorganic anions (such as phosphate and sulfate), nucleotides, cofactors and amino acids. These compounds have been identified by in vitro transport assays based on the uptake of radioactively labeled substrates into liposomes reconstituted with recombinant purified MCs. By using this approach, 18 human, plant and yeast MCs for amino acids have been characterized and shown to transport aspartate, glutamate, ornithine, arginine, lysine, histidine, citrulline and glycine with varying substrate specificities, kinetics, influences of the pH gradient, and capacities for the antiport and uniport mode of transport. Aside from providing amino acids for mitochondrial translation, the transport reactions catalyzed by these MCs are crucial in energy, nitrogen, nucleotide and amino acid metabolism. In this review we dissect the transport properties, phylogeny, regulation and expression levels in different tissues of MCs for amino acids, and summarize the main structural aspects known until now about MCs. The effects of their disease-causing mutations and manipulation of their expression levels in cells are also considered as clues for understanding their physiological functions.

Education systems play a critical role in sustaining a society by equipping citizens with the mindsets and skills necessary for professional and personal success. The American K12 education system has unfortunately not kept pace though with the demands of the 21st century. Students need systemic changes that make learning more meaningful and more engaging of their existing skills and interests. Authentic learning practices, like Project-Based, Community-Based, and Work-Based Learning, make such changes by orienting instruction around topics relevant to students’ experiences and allowing students to practice their knowledge in real-world settings. Schools can encourage the adoption of authentic learning by implementing a complementary practice like portfolios. Local successes in schools using authentic learning and portfolios separately demonstrate their joint viability, but a system that combines the practices and can scale nationally has yet to be discovered.Using the local \"existence proofs\" as starting points, I developed a system architecture that addresses many known barriers to adoption, including the time/resource constraints of schools, colleges, employers and the inequitable access some students have to engaging learning experiences. This initial proposal did not, however, address constraints imposed by schools’ accountability obligations nor stakeholders’ uncertainty over their peers’ readiness to adopt the system. By investigating how federal and state policies have enacted similar transformations, I determined that authentic learning portfolios will likely require government mandates. These mandates could face pushback, however, from families concerned that the proposal would hurt their student’s college options. I also interviewed colleges to establish what changes to the proposal were needed to ensure their support and thus satisfy parents’ concerns. My findings helped refine the proposed system architecture as well as outline the next steps needed to successfully implement the proposal.

Uncoupling proteins (UCPs) form a distinct subfamily of the mitochondrial carrier family (MCF) SLC25. Four UCPs, DmUCP4A-C and DmUCP5, have been identified in Drosophila melanogaster on the basis of their sequence homology with mammalian UCP4 and UCP5. In a Parkinson’s disease model, DmUCP4A showed a protective role against mitochondrial dysfunction, by increasing mitochondrial membrane potential and ATP synthesis. To date, DmUCP4A is still an orphan of a biochemical function, although its possible involvement in mitochondrial uncoupling has been ruled out. Here, we show that DmUCP4A expressed in bacteria and reconstituted in phospholipid vesicles catalyzes a unidirectional transport of aspartate, which is saturable and inhibited by mercurials and other mitochondrial carrier inhibitors to various degrees. Swelling experiments carried out in yeast mitochondria have demonstrated that the unidirectional transport of aspartate catalyzed by DmUCP4 is not proton-coupled. The biochemical function of DmUCP4A has been further confirmed in a yeast cell model, in which growth has required an efflux of aspartate from mitochondria. Notably, DmUCP4A is the first UCP4 homolog from any species to be biochemically characterized. In Drosophila melanogaster, DmUCP4A could be involved in the transport of aspartate from mitochondria to the cytosol, in which it could be used for protein and nucleotide synthesis, as well as in the biosynthesis of ß-alanine and N-acetylaspartate, which play key roles in signal transmission in the central nervous system.

Mutations in the SLC25A38 gene are responsible for the second most common form of congenital sideroblastic anemia (CSA), a severe condition for which no effective treatment exists. We developed and characterized a K562 erythroleukemia cell line with markedly reduced expression of the SLC25A38 protein (A38-low cells). This model successfully recapitulated the main features of CSA, including reduced heme content and mitochondrial respiration, increase in mitochondrial iron, ROS levels and sensitivity to oxidative stress. Notably, our study uncovered a new role for extracellular pyridoxal 5′-phosphate (PLP) and other P2 receptor antagonists in rescuing the altered parameters of A38-low cells (for example, the heme content of the A38-low cells was increased from about 50% to about 80% by the P2 receptor antagonists treatment compared with the value of the controls). These findings suggest that targeting P2 receptors could represent a promising therapeutic approach for SLC25A38-associated CSA.

The oncogenic KRAS mutation has a critical role in the initiation of human pancreatic ductal adenocarcinoma (PDAC) since it rewires glutamine metabolism to increase reduced nicotinamide adenine dinucleotide phosphate (NADPH) production, balancing cellular redox homeostasis with macromolecular synthesis 1 , 2 . Mitochondrial glutamine-derived aspartate must be transported into the cytosol to generate metabolic precursors for NADPH production 2 . The mitochondrial transporter responsible for this aspartate efflux has remained elusive. Here, we show that mitochondrial uncoupling protein 2 (UCP2) catalyses this transport and promotes tumour growth. UCP2-silenced KRAS mut cell lines display decreased glutaminolysis, lower NADPH/NADP + and glutathione/glutathione disulfide ratios and higher reactive oxygen species levels compared to wild-type counterparts. UCP2 silencing reduces glutaminolysis also in KRAS WT PDAC cells but does not affect their redox homeostasis or proliferation rates. In vitro and in vivo, UCP2 silencing strongly suppresses KRAS mut PDAC cell growth. Collectively, these results demonstrate that UCP2 plays a vital role in PDAC, since its aspartate transport activity connects the mitochondrial and cytosolic reactions necessary for KRAS mut rewired glutamine metabolism 2 , and thus it should be considered a key metabolic target for the treatment of this refractory tumour. UCP2 is shown in yeast and mammalian cells to transport aspartate out of mitochondria, thus enabling KRAS -mutated pancreatic ductal adenocarcinoma cells to perform glutaminolysis to support cancer growth.

Defects of the Fe/S cluster biosynthesis represent a subgroup of diseases affecting the mitochondrial energy metabolism. In the last years, mutations in four genes ( NFU1 , BOLA3 , ISCA2 and IBA57 ) have been related to a new group of multiple mitochondrial dysfunction syndromes characterized by lactic acidosis, hyperglycinemia, multiple defects of the respiratory chain complexes, and impairment of four lipoic acid-dependent enzymes: α-ketoglutarate dehydrogenase complex, pyruvic dehydrogenase, branched-chain α-keto acid dehydrogenase complex and the H protein of the glycine cleavage system. Few patients have been reported with mutations in IBA57 and with variable clinical phenotype. Herein, we describe four unrelated patients carrying novel mutations in IBA57 . All patients presented with combined or isolated defect of complex I and II. Clinical features varied widely, ranging from fatal infantile onset of the disease to acute and severe psychomotor regression after the first year of life. Brain MRI was characterized by cavitating leukodystrophy. The identified mutations were never reported previously and all had a dramatic effect on IBA57 stability. Our study contributes to expand the array of the genotypic variation of IBA57 and delineates the leukodystrophic pattern of IBA57 deficient patients.