Human mitochondrial pyruvate carrier 2 as an autonomous membrane transporter

MULTIPLE, REDUNDANT CARBOXYLIC ACID TRANSPORTERS SUPPORT MITOCHONDRIAL METABOLISM IN PLASMODIUM FALCIPARUM

The mitochondrion of the deadliest human malaria parasite, Plasmodium falciparum, is an essential source of cellular acetyl-CoA during the asexual blood-stage of the parasite life cycle. Blocking mitochondrial acetyl-CoA synthesis leads to a hypoacetylated proteome and parasite death. We previously determined that mitochondrial acetyl-CoA is primarily synthesized from glucose-derived pyruvate by α-ketoacid dehydrogenases. Here, we asked if inhibiting the import of glycolytic pyruvate across the mitochondrial inner membrane would affect acetyl-CoA production and, thus, could be a potential target for antimalarial drug development. We selected the two predicted mitochondrial pyruvate carrier proteins, PfMPC1 (PF3D7_1340800) and PfMPC2 (PF3D7_1470400), for genetic knockout and isotopic metabolite tracing via HPLC-MS metabolomic analysis. Surprisingly, we observed that asexual blood-stage parasites could survive the loss of either or both PfMPCs with only minor growth defects, despite a substantial reduction in the amount of glucose-derived isotopic labelling into acetyl-CoA. Furthermore, genetic deletion of two additional mitochondrial carboxylic acid transporters - DTC (PF3D7_0823900, di/tricarboxylic acid carrier) and YHM2 (PF3D7_1223800, a putative citrate/α-ketoglutarate carrier protein) - only mildly affected blood-stage replication, even in the context of PfMPC deficiency. Although we observed no added impact on the incorporation of glucose carbon into acetyl-CoA in these quadruple knockout mutants, we noted a large decrease in glutamine-derived label in tricarboxylic acid cycle metabolites, suggesting that DTC and YHM2 both import glutamine derivatives into the mitochondrion. Altogether, our results demonstrate that redundant routes are used to fuel the blood-stage malaria parasite mitochondrion with imported carbon from two major sources - glucose and glutamine.

ALDH4A1 functions as an active component of the MPC complex maintaining mitochondrial pyruvate import for TCA cycle entry and tumour suppression

MPC1 and MPC2 are two well-known components of the mitochondrial pyruvate carrier (MPC) complex maintaining MPC activity to transport pyruvate into mitochondria for tricarboxylic acid (TCA) cycle entry in mammalian cells. It is currently unknown whether there is an additional MPC component crucially maintaining MPC complex activity for pyruvate mitochondrial import. Here we show that ALDH4A1, a proline-metabolizing enzyme localized in mitochondria, serves as a previously unrecognized MPC component maintaining pyruvate mitochondrial import and the TCA cycle independently of its enzymatic activity. Loss of ALDH4A1 in mammalian cells impairs pyruvate entry to mitochondria, resulting in defective TCA cycle entry. ALDH4A1 forms an active trimeric complex with MPC1-MPC2 to maintain the integrity and oligomerization of MPC1-MPC2 and facilitates pyruvate transport in an in vitro system. ALDH4A1 displays tumour suppression by maintaining MPC complex activity. Our study identifies ALDH4A1 as an essential component of MPC for pyruvate mitochondrial import, TCA cycle entry and tumour suppression.

Structure of mitochondrial pyruvate carrier and its inhibition mechanism

The mitochondrial pyruvate carrier (MPC) governs the entry of pyruvate-a central metabolite that bridges cytosolic glycolysis with mitochondrial oxidative phosphorylation-into the mitochondrial matrix1-5. It thus serves as a pivotal metabolic gatekeeper and has fundamental roles in cellular metabolism. Moreover, MPC is a key target for drugs aimed at managing diabetes, non-alcoholic steatohepatitis and neurodegenerative diseases4-6. However, despite MPC's critical roles in both physiology and medicine, the molecular mechanisms underlying its transport function and how it is inhibited by drugs have remained largely unclear. Here our structural findings on human MPC define the architecture of this vital transporter, delineate its substrate-binding site and translocation pathway, and reveal its major conformational states. Furthermore, we explain the binding and inhibition mechanisms of MPC inhibitors. Our findings provide the molecular basis for understanding MPC's function and pave the way for the development of more-effective therapeutic reagents that target MPC.

MPC2 Overexpression Drives Mitochondrial Oxidative Phosphorylation and Promotes Progression in Diffuse Large B-Cell Lymphoma

Diffuse Large B-Cell Lymphoma (DLBCL) is an aggressive form of non-Hodgkin lymphoma with heterogeneous molecular characteristics. Altered metabolism, particularly mitochondrial function, has emerged as a critical factor in cancer progression. However, the role of mitochondrial metabolism in DLBCL remains poorly understood. This study aimed to identify key mitochondrial factors associated with DLBCL progression. We analyzed transcriptomic data from multiple DLBCL datasets (GSE83632, TCGA-GTEX, GSE181063, GSE4475) using differential expression analysis, weighted gene co-expression network analysis (WGCNA), and Gene Set Enrichment Analysis (GSEA). The expression and function of the identified key factor, Mitochondrial Pyruvate Carrier 2 (MPC2), were validated using clinical samples, DLBCL cell lines, and an in vivo mouse model of xenograft. Integrative bioinformatics analysis identified MPC2 as a significantly upregulated gene in DLBCL, associated with enrichment of oxidative phosphorylation (OXPHOS) and cell cycle-related genes. MPC2 overexpression was confirmed in clinical DLBCL samples and cell lines at both mRNA and protein levels. Knockdown of MPC2 in DLBCL cells impaired mitochondrial OXPHOS, increased glycolysis, and suppressed cell proliferation, invasion, and 3D spheroid formation. In vivo, MPC2 silencing significantly reduced tumor growth in a xenograft mouse model. Our findings reveal MPC2 as a key regulator of mitochondrial function in DLBCL, promoting tumor progression through enhanced OXPHOS. This study provides new insights into the metabolic reprogramming of DLBCL and suggests MPC2 as a potential therapeutic target for this aggressive lymphoma.

Molecular basis of pyruvate transport and inhibition of the human mitochondrial pyruvate carrier

The mitochondrial pyruvate carrier transports pyruvate, produced by glycolysis from sugar molecules, into the mitochondrial matrix, as a crucial transport step in eukaryotic energy metabolism. The carrier is a drug target for the treatment of cancers, diabetes mellitus, neurodegeneration, and metabolic dysfunction-associated steatotic liver disease. We have solved the structure of the human MPC1L/MPC2 heterodimer in the inward- and outward-open states by cryo-electron microscopy, revealing its alternating access rocker-switch mechanism. The carrier has a central binding site for pyruvate, which contains an essential lysine and histidine residue, important for its ΔpH-dependent transport mechanism. We have also determined the binding poses of three chemically distinct inhibitor classes, which exploit the same binding site in the outward-open state by mimicking pyruvate interactions and by using aromatic stacking interactions.

Stimulus-Secretion Coupling Mechanisms of Glucose-Induced Insulin Secretion: Biochemical Discrepancies Among the Canonical, ADP Privation, and GABA-Shunt Models

Integration of old and recent experimental data consequences is needed to correct and help improve the hypothetical mechanism responsible for the stimulus-secretion coupling mechanism of glucose-induced insulin secretion. The main purpose of this review is to supply biochemical considerations about some of the metabolic pathways implicated in the process of insulin secretion. It is emphasized that glucose β-cells' threshold to activate secretion (5 mM) might depend on the predominance of anaerobic glycolysis at this basal glucose concentration. This argues against the predominance of phosphoenolpyruvate (PEP) over mitochondrial pyruvate oxidation for the initiation of insulin secretion. Full quantitative and qualitative reproduction, except the threshold effect, of glucose-induced insulin release by a permeable methylated analog of succinic acid indicates that mitochondrial metabolism is enough for sustained insulin secretion. Mitochondrial PEP generation is skipped if the GABA-shunt pathway is exclusively coupled to the citric acid cycle, as proposed in the "GABA-shunt" model of stimulus-secretion coupling. Strong or maintained depolarization by KCl or sulfonylureas might induce the opening of β-cells Cx36 hemichannels, allowing the loss of adenine nucleotides and other metabolites, mimicking the effect of an excessive mitochondrial ATP demand. A few alterations of OxPhos (Oxidative Phosphorylation) regulation in human T2D islets have been described, but the responsible mechanism(s) is (are) not yet known. Finally, some experimental data arguing as proof of the relative irrelevance of the mitochondrial function in the insulin secretion coupling mechanism for the initiation and/or sustained stimulation of hormone release are discussed.

The Role of Mitochondrial Pyruvate Carrier in Neurological Disorders

The mitochondrial pyruvate carrier (MPC) is a specific protein complex located in the inner mitochondrial membrane. Comprising a heterodimer of two homodimeric membrane proteins, mitochondrial pyruvate carrier 1 and mitochondrial pyruvate carrier 2, MPC connects cytoplasmic metabolism to mitochondrial metabolism by transferring pyruvate from the cytoplasm to the mitochondria. The nervous system requires substantial energy to maintain its function, and the mitochondrial energy supply is closely linked to neurological function. Mitochondrial dysfunction can induce or exacerbate intracerebral pathologies. MPC influences mitochondrial function due to its specific role as a pyruvate transporter. However, recent studies on MPC and mitochondrial dysfunction in neurological disorders have yielded controversial results, and the underlying mechanisms remain unclear. In this brief review, we provide an overview of the structure and function of MPC. We further discuss the potential mechanisms and feasibility of targeting MPC in treating Parkinson's disease, Alzheimer's disease, and cerebral ischemia/hypoxia injury. This review aims to offer insights into MPC as a target for clinical treatment.

MULTIPLE, REDUNDANT CARBOXYLIC ACID TRANSPORTERS SUPPORT MITOCHONDRIAL METABOLISM IN PLASMODIUM FALCIPARUM

The mitochondrion of the deadliest human malaria parasite, Plasmodium falciparum, is an essential source of cellular acetyl-CoA during the asexual blood-stage of the parasite life cycle. Blocking mitochondrial acetyl-CoA synthesis leads to a hypoacetylated proteome and parasite death. We previously determined that mitochondrial acetyl-CoA is primarily synthesized from glucose-derived pyruvate by α-ketoacid dehydrogenases. Here, we asked if inhibiting the import of glycolytic pyruvate across the mitochondrial inner membrane would affect acetyl-CoA production and, thus, could be a potential target for antimalarial drug development. We selected the two predicted mitochondrial pyruvate carrier proteins ( Pf MPC1 and Pf MPC2) for genetic knockout and isotopic metabolite tracing via HPLC-MS metabolomic analysis. Surprisingly, we observed that asexual blood-stage parasites could survive the loss of either or both Pf MPCs with only minor growth defects, despite a substantial reduction in the amount of glucose-derived isotopic labelling into acetyl-CoA. Furthermore, genetic deletion of two additional mitochondrial carboxylic acid transporters - DTC (di/tricarboxylic acid carrier) and YHM2 (a putative citrate/α-ketoglutarate carrier protein) - only mildly affected asexual blood-stage replication, even in the context of Pf MPC deficiency. Although we observed no added impact on the incorporation of glucose carbon into acetyl-CoA in these quadruple knockout mutants, we noted a large decrease in glutamine-derived label in tricarboxylic acid cycle metabolites, suggesting that DTC and YHM2 both import glutamine derivatives into the mitochondrion. Altogether, our results expose redundant routes used to fuel the blood-stage malaria parasite mitochondrion with imported carbon from two major sources - glucose and glutamine.

SIGNIFICANCE

The mitochondrion of malaria parasites generates key molecules, such as acetyl-CoA, that are required for numerous cellular processes. To support mitochondrial biosynthetic pathways, the parasites must transport carbon sources into this organelle. By studying how the mitochondrion obtains pyruvate, a molecule derived from glucose, we have uncovered redundant carbon transport systems that ensure parasite survival in red blood cells. This metabolic redundancy poses a challenge for drug development, as it enables the parasite to adapt and survive by relying on alternative pathways when one is disrupted.

Computational structural prediction and chemical inhibition of the human mitochondrial pyruvate carrier protein heterodimer complex

The mitochondrial pyruvate carrier (MPC) plays a role in numerous diseases including neurodegeneration, metabolically dependent cancers, and the development of insulin resistance. Several previous studies in genetic mouse models or with existing inhibitors suggest that inhibition of the MPC could be used as a viable therapeutic strategy in these diseases. However, the MPC's structure is unknown, making it difficult to screen for and develop therapeutically viable inhibitors. Currently known MPC inhibitors would make for poor drugs due to their poor pharmacokinetic properties, or in the case of the thiazolidinediones (TZDs), off-target specificity for peroxisome-proliferator activated receptor gamma (PPARγ) leads to unwanted side effects. In this study, we develop several structural models for the MPC heterodimer complex and investigate the chemical interactions required for the binding of these known inhibitors to MPC and PPARγ. Based on these models, the MPC most likely takes on outward-facing (OF) and inward-facing (IF) conformations during pyruvate transport, and inhibitors likely plug the carrier to inhibit pyruvate transport. Although some chemical interactions are similar between MPC and PPARγ binding, there is likely enough difference to reduce PPARγ specificity for future development of novel, more specific MPC inhibitors.

Loss of mitochondrial pyruvate carrier 1 supports proline-dependent proliferation and collagen biosynthesis in ovarian cancer

The pyruvate transporter MPC1 (mitochondrial pyruvate carrier 1) acts as a tumour-suppressor, loss of which correlates with a pro-tumorigenic phenotype and poor survival in several tumour types. In high-grade serous ovarian cancers (HGSOC), patients display copy number loss of MPC1 in around 78% of cases and reduced MPC1 mRNA expression. To explore the metabolic effect of reduced expression, we demonstrate that depleting MPC1 in HGSOC cell lines drives expression of key proline biosynthetic genes; PYCR1, PYCR2 and PYCR3, and biosynthesis of proline. We show that altered proline metabolism underpins cancer cell proliferation, reactive oxygen species (ROS) production, and type I and type VI collagen formation in ovarian cancer cells. Furthermore, exploring The Cancer Genome Atlas, we discovered the PYCR3 isozyme to be highly expressed in a third of HGSOC patients, which was associated with more aggressive disease and diagnosis at a younger age. Taken together, our study highlights that targeting proline metabolism is a potential therapeutic avenue for the treatment of HGSOC.

Comparative proteomics of saliva of healthy and gingivitis individuals from Rio de Janeiro

PURPOSE

In this work, we identified human and bacterial proteomes in the saliva from volunteers with gingivitis or healthy.

EXPERIMENTAL DESIGN

The reported population consisted of 18 volunteers (six with gingivitis and 12 healthy controls). Proteomics characterization was performed using a quantitative mass spectrometry method.

RESULTS

A total of 74 human and 116 bacterial proteins were identified in saliva. The major functional category that was modified in the human proteome was the immune response, followed by transport and protease inhibition. In the bacterial proteome, most of the proteins identified were from the Fusobacteria phylum, followed by Chlamydiae and Spirochaetes.

CONCLUSIONS AND CLINICAL RELEVANCE

We observed statistically relevant differences in the data between the groups. The 15 most important human proteins affecting the variation between case and control groups included cystatin S, alpha amylase, lactotransferrin, and negative elongation factor E. We found that bacterial proteins from Porphyromonas gingivalis and Fusobacterium nucleatum subsp. nucleatum related to the red and orange complexes were closely correlated with the occurrence of periodontal diseases.

Fifty years of the mitochondrial pyruvate carrier: New insights into its structure, function, and inhibition

The mitochondrial pyruvate carrier (MPC) resides in the mitochondrial inner membrane, where it links cytosolic and mitochondrial metabolism by transporting pyruvate produced in glycolysis into the mitochondrial matrix. Due to its central metabolic role, it has been proposed as a potential drug target for diabetes, non-alcoholic fatty liver disease, neurodegeneration, and cancers relying on mitochondrial metabolism. Little is known about the structure and mechanism of MPC, as the proteins involved were only identified a decade ago and technical difficulties concerning their purification and stability have hindered progress in functional and structural analyses. The functional unit of MPC is a hetero-dimer comprising two small homologous membrane proteins, MPC1/MPC2 in humans, with the alternative complex MPC1L/MPC2 forming in the testis, but MPC proteins are found throughout the tree of life. The predicted topology of each protomer consists of an amphipathic helix followed by three transmembrane helices. An increasing number of inhibitors are being identified, expanding MPC pharmacology and providing insights into the inhibitory mechanism. Here, we provide critical insights on the composition, structure, and function of the complex and we summarize the different classes of small molecule inhibitors and their potential in therapeutics.

Pyruvate metabolism controls chromatin remodeling during CD4+ T cell activation

Upon antigen-specific T cell receptor (TCR) engagement, human CD4+ T cells proliferate and differentiate, a process associated with rapid transcriptional changes and metabolic reprogramming. Here, we show that the generation of extramitochondrial pyruvate is an important step for acetyl-CoA production and subsequent H3K27ac-mediated remodeling of histone acetylation. Histone modification, transcriptomic, and carbon tracing analyses of pyruvate dehydrogenase (PDH)-deficient T cells show PDH-dependent acetyl-CoA generation as a rate-limiting step during T activation. Furthermore, T cell activation results in the nuclear translocation of PDH and its association with both the p300 acetyltransferase and histone H3K27ac. These data support the tight integration of metabolic and histone-modifying enzymes, allowing metabolic reprogramming to fuel CD4+ T cell activation. Targeting this pathway may provide a therapeutic approach to specifically regulate antigen-driven T cell activation.

iTRAQ-based comparative proteomics reveal an enhancing role of PRDX6 in the freezability of Mediterranean buffalo sperm

BACKGROUND

Semen cryopreservation is a critical tool for breed improvement and preservation of biodiversity. However, instability of sperm freezability affects its application. The Mediterranean buffalo is one of the river-type buffaloes with the capacity for high milk production. Until now, there is no specific cryopreservation system for Mediterranean buffalo, which influences the promotion of excellent cultivars. To improve the semen freezing extender used in cryopreservation of Mediterranean buffalo, different protein datasets relating to freezability sperm were analyzed by iTRAQ-based proteomics. This study will be beneficial for further understanding the sperm freezability mechanism and developing new cryopreservation strategy for buffalo semen.

RESULTS

2652 quantified proteins were identified, including 248 significantly differentially expressed proteins (DEP). Gene Ontology (GO) analysis indicated that many these were mitochondrial proteins, enriched in the molecular function of phospholipase A2 activity and enzyme binding, and biological processes of regulation of protein kinase A signaling and motile cilium assembly. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis identified 17 significant pathways, including oxidative phosphorylation (OXPHOS). Furthermore, 7 DEPs were verified using parallel reaction monitoring or western blot, which confirmed the accuracy of the iTRAQ data. Peroxiredoxin 6 (PRDX6), which expressed 1.72-fold higher in good freezability ejaculate (GFE) compared to poor freezability ejaculate (PFE) sperms, was selected to explore the function in sperm freezability by adding recombinant PRDX6 protein into the semen freezing extender. The results showed that the motility, mitochondrial function and in vitro fertilization capacity of frozen-thawed sperm were significantly increased, while the oxidation level was significantly decreased when 0.1 mg/L PRDX6 was added compared with blank control.

CONCLUSIONS

Above results revealed the metabolic pattern of freezability of Mediterranean buffalo sperms was negatively associated with OXPHOS, and PRDX6 had protective effect on cryo-damage of frozen-thawed sperms.

NMR and Patch-Clamp Characterization of Yeast Mitochondrial Pyruvate Carrier Complexes

The mitochondrial pyruvate carrier (Mpc) plays an indispensable role in the transport of pyruvates across the mitochondrial inner membrane. Despite the two distinct homologous proteins, Mpc1 and Mpc2, were identified in 2012, there are still controversies on the basic functional units and oligomeric state of Mpc complexes. In this study, yeast Mpc1 and Mpc2 proteins were expressed in a prokaryotic heterologous system. Both homo- and hetero-dimers were successfully reconstituted in mixed detergents. Interactions among Mpc monomers were recorded utilizing paramagnetic relaxation enhancement (PRE) nuclear magnetic resonance (NMR) methods. By single-channel patch-clamp assays, we discovered that both the Mpc1-Mpc2 hetero-dimer and Mpc1 homo-dimer are able to transport K⁺ ions. Furthermore, the Mpc1-Mpc2 hetero-dimer demonstrated the ability to transport pyruvates, at a rate significantly higher than that of the Mpc1 homo-dimer, indicating that it could be the basic functional unit of Mpc complexes. Our findings provide valuable insights for further structural determination and the study of the transport mechanism of Mpc complexes.

The Mitochondrial Pyruvate Carrier Coupling Glycolysis and the Tricarboxylic Acid Cycle Is Required for the Asexual Reproduction of Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite capable of infecting humans and animals. The organism has extraordinary metabolic resilience that allows it to establish parasitism in varied nutritional milieus of diverse host cells. Our earlier work has shown that, despite flexibility in the usage of glucose and glutamine as the major carbon precursors, the production of pyruvate by glycolytic enzymes is central to the parasite's growth. Pyruvate is metabolized in a number of subcellular compartments, including the mitochondrion, apicoplast, and cytosol. With the objective of examining the mechanism and importance of the mitochondrial pool of pyruvate imported from the cytosol, we identified the conserved mitochondrial pyruvate carrier (MPC) complex, consisting of two subunits, MPC1 and MPC2, in T. gondii. The two parasite proteins could complement a yeast mutant deficient in growth on leucine and valine. Genetic ablation of either one or both subunits reduced the parasite's growth, mimicking the deletion of branched-chain ketoacid dehydrogenase (BCKDH), which has been reported to convert pyruvate into acetyl-coenzyme A (CoA) in the mitochondrion. Metabolic labeling of the MPC mutants by isotopic glucose revealed impaired synthesis of acetyl-CoA, correlating with a global decrease in carbon flux through glycolysis and the tricarboxylic acid (TCA) cycle. Disruption of MPC proteins exerted only a modest effect on the parasite's virulence in mice, further highlighting its metabolic flexibility. In brief, our work reveals the modus operandi of pyruvate transport from the cytosol to the mitochondrion in the parasite, providing the missing link between glycolysis and the TCA cycle in T. gondii. IMPORTANCE T. gondii is a zoonotic parasite capable of infecting many warm-blooded organisms, including humans. Among others, a feature that allows it to parasitize multiple hosts is its exceptional metabolic plasticity. Although T. gondii can utilize different carbon sources, pyruvate homeostasis is critical for parasite growth. Pyruvate is produced primarily in the cytosol but metabolized in other organelles, such as the mitochondrion and apicoplast. The mechanism of import and physiological significance of pyruvate in these organelles remains unclear. Here, we identified the transporter of cytosol-derived pyruvate into the mitochondrion and studied its constituent subunits and their relevance. Our results show that cytosolic pyruvate is a major source of acetyl-CoA in the mitochondrion and that the mitochondrial pyruvate transporter is needed for optimal parasite growth. The mutants lacking the transporter are viable and virulent in a mouse model, underscoring the metabolic plasticity in the parasite's mitochondrion.

Exogenous succinate impacts mouse brown adipose tissue mitochondrial proteome and potentiates body mass reduction induced by liraglutide

Obesity is one of the leading noncommunicable diseases in the world. Despite intense efforts to develop strategies to prevent and treat obesity, its prevalence continues to rise worldwide. A recent study has shown that the tricarboxylic acid intermediate succinate increases body energy expenditure by promoting brown adipose tissue thermogenesis through the activation of uncoupling protein-1; this has generated interest surrounding its potential usefulness as an approach to treat obesity. It is currently unknown how succinate impacts brown adipose tissue protein expression, and how exogenous succinate impacts body mass reduction promoted by a drug approved to treat human obesity, the glucagon-like-1 receptor agonist, liraglutide. In the first part of this study, we used bottom-up shotgun proteomics to determine the acute impact of exogenous succinate on the brown adipose tissue. We show that succinate rapidly affects the expression of 177 brown adipose tissue proteins, which are mostly associated with mitochondrial structure and function. In the second part of this study, we performed a short-term preclinical pharmacological intervention, treating diet-induced obese mice with a combination of exogenous succinate and liraglutide. We show that the combination was more efficient than liraglutide alone in promoting body mass reduction, food energy efficiency reduction, food intake reduction, and an increase in body temperature. Using serum metabolomics analysis, we showed that succinate, but not liraglutide, promoted a significant increase in the blood levels of several medium and long-chain fatty acids. In conclusion, exogenous succinate promotes rapid changes in brown adipose tissue mitochondrial proteins, and when used in association with liraglutide, increases body mass reduction.NEW & NOTEWORTHY Exogenous succinate induces major changes in brown adipose tissue protein expression affecting particularly mitochondrial respiration and structural proteins. When given exogenously in drinking water, succinate mitigates body mass gain in a rodent model of diet-induced obesity; in addition, when given in association with the glucagon-like peptide-1 receptor agonist, liraglutide, succinate increases body mass reduction promoted by liraglutide alone.

The Hepatic Mitochondrial Pyruvate Carrier as a Regulator of Systemic Metabolism and a Therapeutic Target for Treating Metabolic Disease

Pyruvate sits at an important metabolic crossroads of intermediary metabolism. As a product of glycolysis in the cytosol, it must be transported into the mitochondrial matrix for the energy stored in this nutrient to be fully harnessed to generate ATP or to become the building block of new biomolecules. Given the requirement for mitochondrial import, it is not surprising that the mitochondrial pyruvate carrier (MPC) has emerged as a target for therapeutic intervention in a variety of diseases characterized by altered mitochondrial and intermediary metabolism. In this review, we focus on the role of the MPC and related metabolic pathways in the liver in regulating hepatic and systemic energy metabolism and summarize the current state of targeting this pathway to treat diseases of the liver. Available evidence suggests that inhibiting the MPC in hepatocytes and other cells of the liver produces a variety of beneficial effects for treating type 2 diabetes and nonalcoholic steatohepatitis. We also highlight areas where our understanding is incomplete regarding the pleiotropic effects of MPC inhibition.

Role of Mitochondrial Transporters on Metabolic Rewiring of Pancreatic Adenocarcinoma: A Comprehensive Review

Pancreatic cancer is among the deadliest cancers worldwide and commonly presents as pancreatic ductal adenocarcinoma (PDAC). Metabolic reprogramming is a hallmark of PDAC. Glucose and glutamine metabolism are extensively rewired in order to fulfil both energetic and synthetic demands of this aggressive tumour and maintain favorable redox homeostasis. The mitochondrial pyruvate carrier (MPC), the glutamine carrier (SLC1A5_Var), the glutamate carrier (GC), the aspartate/glutamate carrier (AGC), and the uncoupling protein 2 (UCP2) have all been shown to influence PDAC cell growth and progression. The expression of MPC is downregulated in PDAC and its overexpression reduces cell growth rate, whereas the other four transporters are usually overexpressed and the loss of one or more of them renders PDAC cells unable to grow and proliferate by altering the levels of crucial metabolites such as aspartate. The aim of this review is to comprehensively evaluate the current experimental evidence about the function of these carriers in PDAC metabolic rewiring. Dissecting the precise role of these transporters in the context of the tumour microenvironment is necessary for targeted drug development.

Heterologous (Over) Expression of Human SoLute Carrier (SLC) in Yeast: A Well-Recognized Tool for Human Transporter Function/Structure Studies

For more than 20 years, yeast has been a widely used system for the expression of human membrane transporters. Among them, more than 400 are members of the largest transporter family, the SLC superfamily. SLCs play critical roles in maintaining cellular homeostasis by transporting nutrients, ions, and waste products. Based on their involvement in drug absorption and in several human diseases, they are considered emerging therapeutic targets. Despite their critical role in human health, a large part of SLCs' is 'orphans' for substrate specificity or function. Moreover, very few data are available concerning their 3D structure. On the basis of the human health benefits of filling these knowledge gaps, an understanding of protein expression in systems that allow functional production of these proteins is essential. Among the 500 known yeast species, S. cerevisiae and P. pastoris represent those most employed for this purpose. This review aims to provide a comprehensive state-of-the-art on the attempts of human SLC expression performed by exploiting yeast. The collected data will hopefully be useful for guiding new attempts in SLCs expression with the aim to reveal new fundamental data that could lead to potential effects on human health.

The mitochondrial pyruvate carrier at the crossroads of intermediary metabolism

Pyruvate metabolism, a central nexus of carbon homeostasis, is an evolutionarily conserved process and aberrant pyruvate metabolism is associated with and contributes to numerous human metabolic disorders including diabetes, cancer, and heart disease. As a product of glycolysis, pyruvate is primarily generated in the cytosol before being transported into the mitochondrion for further metabolism. Pyruvate entry into the mitochondrial matrix is a critical step for efficient generation of reducing equivalents and ATP and for the biosynthesis of glucose, fatty acids, and amino acids from pyruvate. However, for many years, the identity of the carrier protein(s) that transported pyruvate into the mitochondrial matrix remained a mystery. In 2012, the molecular-genetic identification of the mitochondrial pyruvate carrier (MPC), a heterodimeric complex composed of protein subunits MPC1 and MPC2, enabled studies that shed light on the many metabolic and physiological processes regulated by pyruvate metabolism. A better understanding of the mechanisms regulating pyruvate transport and the processes affected by pyruvate metabolism may enable novel therapeutics to modulate mitochondrial pyruvate flux to treat a variety of disorders. Herein, we review our current knowledge of the MPC, discuss recent advances in the understanding of mitochondrial pyruvate metabolism in various tissue and cell types, and address some of the outstanding questions relevant to this field.

Key features of inhibitor binding to the human mitochondrial pyruvate carrier hetero-dimer

OBJECTIVE

The mitochondrial pyruvate carrier (MPC) has emerged as a promising drug target for metabolic disorders, including non-alcoholic steatohepatitis and diabetes, metabolically dependent cancers and neurodegenerative diseases. A range of structurally diverse small molecule inhibitors have been proposed, but the nature of their interaction with MPC is not understood, and the composition of the functional human MPC is still debated. The goal of this study was to characterise the human MPC protein in vitro, to understand the chemical features that determine binding of structurally diverse inhibitors and to develop novel higher affinity ones.

METHODS

We recombinantly expressed and purified human MPC hetero-complexes and studied their composition, transport and inhibitor binding properties by establishing in vitro transport assays, high throughput thermostability shift assays and pharmacophore modeling.

RESULTS

We determined that the functional unit of human MPC is a hetero-dimer. We compared all different classes of MPC inhibitors to find that three closely arranged hydrogen bond acceptors followed by an aromatic ring are shared characteristics of all inhibitors and represent the minimal requirement for high potency. We also demonstrated that high affinity binding is not attributed to covalent bond formation with MPC cysteines, as previously proposed. Following the basic pharmacophore properties, we identified 14 new inhibitors of MPC, one outperforming compound UK5099 by tenfold. Two are the commonly prescribed drugs entacapone and nitrofurantoin, suggesting an off-target mechanism associated with their adverse effects.

CONCLUSIONS

This work defines the composition of human MPC and the essential MPC inhibitor characteristics. In combination with the functional assays we describe, this new understanding will accelerate the development of clinically relevant MPC modulators.

Metabolic evidence for distinct pyruvate pools inside plant mitochondria

The majority of the pyruvate inside plant mitochondria is either transported into the matrix from the cytosol via the mitochondria pyruvate carrier (MPC) or synthesized in the matrix by alanine aminotransferase (AlaAT) or NAD-malic enzyme (NAD-ME). Pyruvate from these origins could mix into a single pool in the matrix and contribute indistinguishably to respiration via the pyruvate dehydrogenase complex (PDC), or these molecules could maintain a degree of independence in metabolic regulation. Here we demonstrate that feeding isolated mitochondria with uniformly labelled ¹³C-pyruvate and unlabelled malate enables the assessment of pyruvate contribution from different sources to intermediate production in the tricarboxylic acid cycle. Imported pyruvate was the preferred source for citrate production even when the synthesis of NAD-ME-derived pyruvate was optimized. Genetic or pharmacological elimination of MPC activity removed this preference and allowed an equivalent amount of citrate to be generated from the pyruvate produced by NAD-ME. Increasing the mitochondrial pyruvate pool size by exogenous addition affected only metabolites from pyruvate transported by MPC, whereas depleting the pyruvate pool size by transamination to alanine affected only metabolic products derived from NAD-ME. PDC was more membrane-associated than AlaAT and NAD-ME, suggesting that the physical organization of metabolic machinery may influence metabolic rates. Together, these data reveal that the respiratory substrate supply in plants involves distinct pyruvate pools inside the matrix that can be flexibly mixed on the basis of the rate of pyruvate transport from the cytosol. These pools are independently regulated and contribute differentially to organic acid export from plant mitochondria.

Structural Insights into the Human Mitochondrial Pyruvate Carrier Complexes

Pyruvate metabolism requires the mitochondrial pyruvate carrier (MPC) proteins to transport pyruvate from the intermembrane space through the inner mitochondrial membrane to the mitochondrial matrix. The lack of the atomic structures of MPC hampers the understanding of the functional states of MPC and molecular interactions with the substrate or inhibitor. Here, we develop the de novo models of human MPC complexes and characterize the conformational dynamics of the MPC heterodimer formed by MPC1 and MPC2 (MPC1/2) by computational simulations. Our results reveal that functional MPC1/2 prefers to adopt an inward-open conformation, with the carrier open to the matrix side, whereas the outward-open states are less populated. The energy barrier for pyruvate transport in MPC1/2 is low enough, and the inhibitor UK5099 blocks the pyruvate transport by stably binding to MPC1/2. Notably, consistent with experimental results, the MPC1 L79H mutation significantly alters the conformations of MPC1/2 and thus fails for substrate transport. However, the MPC1 R97W mutation seems to retain the transport activity. The present de novo models of MPC complexes provide structural insights into the conformational states of MPC complexes and mechanistic understanding of interactions between the substrate/inhibitor and MPC proteins.

Dimming the Powerhouse: Mitochondrial Dysfunction in the Liver and Skeletal Muscle of Intrauterine Growth Restricted Fetuses

Intrauterine growth restriction (IUGR) of the fetus, resulting from placental insufficiency (PI), is characterized by low fetal oxygen and nutrient concentrations that stunt growth rates of metabolic organs. Numerous animal models of IUGR recapitulate pathophysiological conditions found in human fetuses with IUGR. These models provide insight into metabolic dysfunction in skeletal muscle and liver. For example, cellular energy production and metabolic rate are decreased in the skeletal muscle and liver of IUGR fetuses. These metabolic adaptations demonstrate that fundamental processes in mitochondria, such as substrate utilization and oxidative phosphorylation, are tempered in response to low oxygen and nutrient availability. As a central metabolic organelle, mitochondria coordinate cellular metabolism by coupling oxygen consumption to substrate utilization in concert with tissue energy demand and accretion. In IUGR fetuses, reducing mitochondrial metabolic capacity in response to nutrient restriction is advantageous to ensure fetal survival. If permanent, however, these adaptations may predispose IUGR fetuses toward metabolic diseases throughout life. Furthermore, these mitochondrial defects may underscore developmental programming that results in the sequela of metabolic pathologies. In this review, we examine how reduced nutrient availability in IUGR fetuses impacts skeletal muscle and liver substrate catabolism, and discuss how enzymatic processes governing mitochondrial function, such as the tricarboxylic acid cycle and electron transport chain, are regulated. Understanding how deficiencies in oxygen and substrate metabolism in response to placental restriction regulate skeletal muscle and liver metabolism is essential given the importance of these tissues in the development of later lifer metabolic dysfunction.

Mitochondrial Pyruvate Carrier Subunits Are Essential for Pyruvate-Driven Respiration, Infectivity, and Intracellular Replication of Trypanosoma cruzi

Trypanosoma cruzi is the causative agent of Chagas disease. Pyruvate is the end product of glycolysis, and its transport into the mitochondrion is mediated by the mitochondrial pyruvate carrier (MPC) subunits.

Pyruvate is the final metabolite of glycolysis and can be converted into acetyl coenzyme A (acetyl-CoA) in mitochondria, where it is used as the substrate for the tricarboxylic acid cycle. Pyruvate availability in mitochondria depends on its active transport through the heterocomplex formed by the mitochondrial pyruvate carriers 1 and 2 (MPC1/MPC2). We report here studies on MPC1/MPC2 of Trypanosoma cruzi, the etiologic agent of Chagas disease. Endogenous tagging of T. cruziMPC1 (TcMPC1) and TcMPC2 with 3×c-Myc showed that both encoded proteins colocalize with MitoTracker to the mitochondria of epimastigotes. Individual knockout (KO) of TcMPC1 and TcMPC2 genes using CRISPR/Cas9 was confirmed by PCR and Southern blot analyses. Digitonin-permeabilized TcMPC1-KO and TcMPC2-KO epimastigotes showed reduced O₂ consumption rates when pyruvate, but not succinate, was used as the mitochondrial substrate, while α-ketoglutarate increased their O₂ consumption rates due to an increase in α-ketoglutarate dehydrogenase activity. Defective mitochondrial pyruvate import resulted in decreased Ca²⁺ uptake. The inhibitors UK5099 and malonate impaired pyruvate-driven oxygen consumption in permeabilized control cells. Inhibition of succinate dehydrogenase by malonate indicated that pyruvate needs to be converted into succinate to increase respiration. TcMPC1-KO and TcMPC2-KO epimastigotes showed little growth differences in standard or low-glucose culture medium. However, the ability of trypomastigotes to infect tissue culture cells and replicate as intracellular amastigotes was decreased in TcMPC-KOs. Overall, T. cruzi MPC1 and MPC2 are essential for cellular respiration in the presence of pyruvate, invasion of host cells, and replication of amastigotes.

Heart failure-emerging roles for the mitochondrial pyruvate carrier

The mitochondrial pyruvate carrier (MPC) is the entry point for the glycolytic end-product pyruvate to the mitochondria. MPC activity, which is controlled by its abundance and post-translational regulation, determines whether pyruvate is oxidised in the mitochondria or metabolised in the cytosol. MPC serves as a crucial metabolic branch point that determines the fate of pyruvate in the cell, enabling metabolic adaptations during health, such as exercise, or as a result of disease. Decreased MPC expression in several cancers limits the mitochondrial oxidation of pyruvate and contributes to lactate accumulation in the cytosol, highlighting its role as a contributing, causal mediator of the Warburg effect. Pyruvate is handled similarly in the failing heart where a large proportion of it is reduced to lactate in the cytosol instead of being fully oxidised in the mitochondria. Several recent studies have found that the MPC abundance was also reduced in failing human and mouse hearts that were characterised by maladaptive hypertrophic growth, emulating the anabolic scenario observed in some cancer cells. In this review we discuss the evidence implicating the MPC as an important, perhaps causal, mediator of heart failure progression.

The Importance of Mitochondrial Pyruvate Carrier in Cancer Cell Metabolism and Tumorigenesis

Simple Summary

The characteristic metabolic hallmark of cancer cells is the massive catabolism of glucose by glycolysis, even under aerobic conditions—the so-called Warburg effect. Although energetically unfavorable, glycolysis provides “building blocks” to sustain the unlimited growth of malignant cells. Aberrant glycolysis is also responsible for lactate accumulation and acidosis in the tumor milieu, which fosters hypoxia and immunosuppression. One of the mechanisms used by cancer cells to increase glycolytic flow is the negative regulation of the proteins that conform the mitochondrial pyruvate carrier (MPC) complex, which transports pyruvate into the mitochondrial matrix to be metabolized in the tricarboxylic acid (TCA) cycle. Evidence suggests that MPC downregulation in tumor cells impacts many aspects of tumorigenesis, including cancer cell-intrinsic (proliferation, invasiveness, stemness, resistance to therapy) and -extrinsic (angiogenesis, anti-tumor immune activity) properties. In many cancers, but not in all, MPC downregulation is associated with poor survival. MPC regulation is therefore central to tackling glycolysis in tumors.

Abstract

Pyruvate is a key molecule in the metabolic fate of mammalian cells; it is the crossroads from where metabolism proceeds either oxidatively or ends with the production of lactic acid. Pyruvate metabolism is regulated by many enzymes that together control carbon flux. Mitochondrial pyruvate carrier (MPC) is responsible for importing pyruvate from the cytosol to the mitochondrial matrix, where it is oxidatively phosphorylated to produce adenosine triphosphate (ATP) and to generate intermediates used in multiple biosynthetic pathways. MPC activity has an important role in glucose homeostasis, and its alteration is associated with diabetes, heart failure, and neurodegeneration. In cancer, however, controversy surrounds MPC function. In some cancers, MPC upregulation appears to be associated with a poor prognosis. However, most transformed cells undergo a switch from oxidative to glycolytic metabolism, the so-called Warburg effect, which, amongst other possibilities, is induced by MPC malfunction or downregulation. Consequently, impaired MPC function might induce tumors with strong proliferative, migratory, and invasive capabilities. Moreover, glycolytic cancer cells secrete lactate, acidifying the microenvironment, which in turn induces angiogenesis, immunosuppression, and the expansion of stromal cell populations supporting tumor growth. This review examines the latest findings regarding the tumorigenic processes affected by MPC.

Insights on the Quest for the Structure-Function Relationship of the Mitochondrial Pyruvate Carrier

Simple Summary

The atomic structure of a biological macromolecule determines its function. Discovering how one or more amino acid chains fold and interact to form a protein complex is critical, from understanding the most fundamental cellular processes to developing new therapies. However, this is far from a straightforward task, especially when studying a membrane protein. The functional link between the oligomeric state and complex composition of the proteins involved in the active mitochondrial transport of cytosolic pyruvate is a decades-old question but remains urgent. We present a brief historical review beginning with the identification of the so-called mitochondrial pyruvate carrier (MPC) proteins, followed by a rigorous conceptual analysis of technical approaches in more recent biochemical studies that seek to isolate and reconstitute the functional MPC complex(es) in vitro. We correlate these studies with early kinetic observations and current experimental and computational knowledge to assess their main contributions, identify gaps, resolve ambiguities, and better define the research goal.

Abstract

The molecular identity of the mitochondrial pyruvate carrier (MPC) was presented in 2012, forty years after the active transport of cytosolic pyruvate into the mitochondrial matrix was first demonstrated. An impressive amount of in vivo and in vitro studies has since revealed an unexpected interplay between one, two, or even three protein subunits defining different functional MPC assemblies in a metabolic-specific context. These have clear implications in cell homeostasis and disease, and on the development of future therapies. Despite intensive efforts by different research groups using state-of-the-art computational tools and experimental techniques, MPCs’ structure-based mechanism remains elusive. Here, we review the current state of knowledge concerning MPCs’ molecular structures by examining both earlier and recent studies and presenting novel data to identify the regulatory, structural, and core transport activities to each of the known MPC subunits. We also discuss the potential application of cryogenic electron microscopy (cryo-EM) studies of MPC reconstituted into nanodiscs of synthetic copolymers for solving human MPC2.

The Multifaceted Pyruvate Metabolism: Role of the Mitochondrial Pyruvate Carrier

Pyruvate, the end product of glycolysis, plays a major role in cell metabolism. Produced in the cytosol, it is oxidized in the mitochondria where it fuels the citric acid cycle and boosts oxidative phosphorylation. Its sole entry point into mitochondria is through the recently identified mitochondrial pyruvate carrier (MPC). In this review, we report the latest findings on the physiology of the MPC and we discuss how a dysfunctional MPC can lead to diverse pathologies, including neurodegenerative diseases, metabolic disorders, and cancer.

Lower oxygen consumption and Complex I activity in mitochondria isolated from skeletal muscle of fetal sheep with intrauterine growth restriction

Fetal sheep with placental insufficiency-induced intrauterine growth restriction (IUGR) have lower hindlimb oxygen consumption rates (OCRs), indicating depressed mitochondrial oxidative phosphorylation capacity in their skeletal muscle. We hypothesized that OCRs are lower in skeletal muscle mitochondria from IUGR fetuses, due to reduced electron transport chain (ETC) activity and lower abundances of tricarboxylic acid (TCA) cycle enzymes. IUGR sheep fetuses (n = 12) were created with mid-gestation maternal hyperthermia and compared with control fetuses (n = 12). At 132 ± 1 days of gestation, biceps femoris muscles were collected, and the mitochondria were isolated. Mitochondria from IUGR muscle have 47% lower State 3 (Complex I-dependent) OCRs than controls, whereas State 4 (proton leak) OCRs were not different between groups. Furthermore, Complex I, but not Complex II or IV, enzymatic activity was lower in IUGR fetuses compared with controls. Proteomic analysis (n = 6/group) identified 160 differentially expressed proteins between groups, with 107 upregulated and 53 downregulated mitochondria proteins in IUGR fetuses compared with controls. Although no differences were identified in ETC subunit protein abundances, abundances of key TCA cycle enzymes [isocitrate dehydrogenase (NAD+) 3 noncatalytic subunit β (IDH3B), succinate-CoA ligase ADP-forming subunit-β (SUCLA2), and oxoglutarate dehydrogenase (OGDH)] were lower in IUGR mitochondria. IUGR mitochondria had a greater abundance of a hypoxia-inducible protein, NADH dehydrogenase 1α subcomplex 4-like 2, which is known to incorporate into Complex I and lower Complex I-mediated NADH oxidation. Our findings show that mitochondria from IUGR skeletal muscle adapt to hypoxemia and hypoglycemia by lowering Complex I activity and TCA cycle enzyme concentrations, which together, act to lower OCR and NADH production/oxidation in IUGR skeletal muscle.

Characteristic Analysis of Homo- and Heterodimeric Complexes of Human Mitochondrial Pyruvate Carrier Related to Metabolic Diseases

Human mitochondrial pyruvate carriers (hMPCs), which are required for the uptake of pyruvate into mitochondria, are associated with several metabolic diseases, including type 2 diabetes and various cancers. Yeast MPC was recently demonstrated to form a functional unit of heterodimers. However, human MPC-1 (hMPC-1) and MPC-2 (hMPC-2) have not yet been individually isolated for their detailed characterization, in particular in terms of their structural and functional properties, namely, whether they exist as homo- or heterodimers. In this study, hMPC-1 and hMPC-2 were successfully isolated in micelles and they formed stable homodimers. However, the heterodimer state was found to be dominant when both hMPC-1 and hMPC-2 were present. In addition, as heterodimers, the molecules exhibited a higher binding capacity to both substrates and inhibitors, together with a larger structural stability than when they existed as homodimers. Taken together, our results demonstrated that the hetero-dimerization of hMPCs is the main functional unit of the pyruvate metabolism, providing a structural insight into the transport mechanisms of hMPCs.

Mitochondrial pyruvate and fatty acid flux modulate MICU1-dependent control of MCU activity

The tricarboxylic acid (TCA) cycle converts the end products of glycolysis and fatty acid β-oxidation into the reducing equivalents NADH and FADH2 Although mitochondrial matrix uptake of Ca2+ enhances ATP production, it remains unclear whether deprivation of mitochondrial TCA substrates alters mitochondrial Ca2+ flux. We investigated the effect of TCA cycle substrates on MCU-mediated mitochondrial matrix uptake of Ca2+, mitochondrial bioenergetics, and autophagic flux. Inhibition of glycolysis, mitochondrial pyruvate transport, or mitochondrial fatty acid transport triggered expression of the MCU gatekeeper MICU1 but not the MCU core subunit. Knockdown of mitochondrial pyruvate carrier (MPC) isoforms or expression of the dominant negative mutant MPC1R97W resulted in increased MICU1 protein abundance and inhibition of MCU-mediated mitochondrial matrix uptake of Ca2+ We also found that genetic ablation of MPC1 in hepatocytes and mouse embryonic fibroblasts resulted in reduced resting matrix Ca2+, likely because of increased MICU1 expression, but resulted in changes in mitochondrial morphology. TCA cycle substrate-dependent MICU1 expression was mediated by the transcription factor early growth response 1 (EGR1). Blocking mitochondrial pyruvate or fatty acid flux was linked to increased autophagy marker abundance. These studies reveal a mechanism that controls the MCU-mediated Ca2+ flux machinery and that depends on TCA cycle substrate availability. This mechanism generates a metabolic homeostatic circuit that protects cells from bioenergetic crisis and mitochondrial Ca2+ overload during periods of nutrient stress.

A highly responsive pyruvate sensor reveals pathway-regulatory role of the mitochondrial pyruvate carrier MPC

Mitochondria generate ATP and building blocks for cell growth and regeneration, using pyruvate as the main substrate. Here we introduce PyronicSF, a user-friendly GFP-based sensor of improved dynamic range that enables real-time subcellular quantitation of mitochondrial pyruvate transport, concentration and flux. We report that cultured mouse astrocytes maintain mitochondrial pyruvate in the low micromolar range, below cytosolic pyruvate, which means that the mitochondrial pyruvate carrier MPC is poised to exert ultrasensitive control on the balance between respiration and anaplerosis/gluconeogenesis. The functionality of the sensor in living tissue is demonstrated in the brain of Drosophila melanogaster larvae. Mitochondrial subpopulations are known to coexist within a given cell, which differ in their morphology, mobility, membrane potential, and vicinity to other organelles. The present tool can be used to investigate how mitochondrial diversity relates to metabolism, to study the role of MPC in disease, and to screen for small-molecule MPC modulators.

A Chemical Proteomic Probe for the Mitochondrial Pyruvate Carrier Complex

Target engagement assays are crucial for establishing the mechanism-of-action of small molecules in living systems. Integral membrane transporters can present a challenging protein class for assessing cellular engagement by small molecules. The chemical proteomic discovery of alpha-chloroacetamide (αCA) compounds that covalently modify cysteine-54 (C54) of the MPC2 subunit of the mitochondrial pyruvate carrier (MPC) is presented. This finding is used to create an alkyne-modified αCA, YY4-yne, that serves as a cellular engagement probe for MPC2 in click chemistry-enabled western blotting or global mass spectrometry-based proteomic experiments. Studies with YY4-yne revealed that UK-5099, an alpha-cyanocinnamate inhibitor of the MPC complex, engages MPC2 with remarkable selectivity in human cells. These findings support a model where UK-5099 inhibits the MPC complex by binding to C54 of MPC2 in a covalent reversible manner that can be quantified in cells using the YY4-yne probe.

New drugs for NAFLD: lessons from basic models to the clinic

The term nonalcoholic fatty liver disease (NAFLD) comprises a spectrum of increasingly harmful conditions ranging from nonalcoholic fatty liver (NAFL) to nonalcoholic steatohepatitis (NASH) to liver fibrosis and end-stage cirrhosis. NAFLD is the currently most common form of chronic liver disease in both adults and children worldwide. As NAFLD evolves as a global pandemic alongside the still growing prevalence of metabolic syndrome, obesity, and diabetes, it is inevitable to develop effective counterstrategies. Over the last decades, great effort has been dedicated to the understanding of the pathogenesis of NAFLD. This includes the development of an array of models for NAFLD, ranging from advanced in vitro (primary cells, 3D cultures, biochip, spheroids, organoids) to in vivo rodent models (particularly in mice). Based on these approaches novel therapies have been proposed and subsequently evaluated for patients with advanced forms of NAFLD, in particular those with NASH and liver fibrosis or cirrhosis. In this review, we delineate the current understanding of disease pathophysiology and depict how novel therapeutic strategies aim to exploit these different mechanisms to ameliorate, treat, or stop progression of NASH. We also discuss obstacles and chances along the way from basic models to promising clinical treatment options.

Targeting the Mitochondrial Pyruvate Carrier for Neuroprotection

The mitochondrial pyruvate carriers mediate pyruvate import into the mitochondria, which is key to the sustenance of the tricarboxylic cycle and oxidative phosphorylation. However, inhibition of mitochondria pyruvate carrier-mediated pyruvate transport was recently shown to be beneficial in experimental models of neurotoxicity pertaining to the context of Parkinson's disease, and is also protective against excitotoxic neuronal death. These findings attested to the metabolic adaptability of neurons resulting from MPC inhibition, a phenomenon that has also been shown in other tissue types. In this short review, I discuss the mechanism and potential feasibility of mitochondrial pyruvate carrier inhibition as a neuroprotective strategy in neuronal injury and neurodegenerative diseases.

Altered Gene Expression along the Glycolysis-Cholesterol Synthesis Axis Is Associated with Outcome in Pancreatic Cancer

PURPOSE

Identification of clinically actionable molecular subtypes of pancreatic ductal adenocarcinoma (PDAC) is key to improving patient outcome. Intertumoral metabolic heterogeneity contributes to cancer survival and the balance between distinct metabolic pathways may influence PDAC outcome. We hypothesized that PDAC can be stratified into prognostic metabolic subgroups based on alterations in the expression of genes involved in glycolysis and cholesterol synthesis.

EXPERIMENTAL DESIGN

We performed bioinformatics analysis of genomic, transcriptomic, and clinical data in an integrated cohort of 325 resectable and nonresectable PDAC. The resectable datasets included retrospective The Cancer Genome Atlas (TCGA) and the International Cancer Genome Consortium (ICGC) cohorts. The nonresectable PDAC cohort studies included prospective COMPASS, PanGen, and BC Cancer Personalized OncoGenomics program (POG).

RESULTS

On the basis of the median normalized expression of glycolytic and cholesterogenic genes, four subgroups were identified: quiescent, glycolytic, cholesterogenic, and mixed. Glycolytic tumors were associated with the shortest median survival in resectable (log-rank test P = 0.018) and metastatic settings (log-rank test P = 0.027). Patients with cholesterogenic tumors had the longest median survival. KRAS and MYC-amplified tumors had higher expression of glycolytic genes than tumors with normal or lost copies of the oncogenes (Wilcoxon rank sum test P = 0.015). Glycolytic tumors had the lowest expression of mitochondrial pyruvate carriers MPC1 and MPC2. Glycolytic and cholesterogenic gene expression correlated with the expression of prognostic PDAC subtype classifier genes.

CONCLUSIONS

Metabolic classification specific to glycolytic and cholesterogenic pathways provides novel biological insight into previously established PDAC subtypes and may help develop personalized therapies targeting unique tumor metabolic profiles.See related commentary by Mehla and Singh, p. 6.

The yeast mitochondrial pyruvate carrier is a hetero-dimer in its functional state

The mitochondrial pyruvate carrier (MPC) is critical for cellular homeostasis, as it is required in central metabolism for transporting pyruvate from the cytosol into the mitochondrial matrix. MPC has been implicated in many diseases and is being investigated as a drug target. A few years ago, small membrane proteins, called MPC1 and MPC2 in mammals and Mpc1, Mpc2 and Mpc3 in yeast, were proposed to form large protein complexes responsible for this function. However, the MPC complexes have never been isolated and their composition, oligomeric state and functional properties have not been defined. Here, we identify the functional unit of MPC from Saccharomyces cerevisiae In contrast to earlier hypotheses, we demonstrate that MPC is a hetero-dimer, not a multimeric complex. When not engaged in hetero-dimers, the yeast Mpc proteins can also form homo-dimers that are, however, inactive. We show that the earlier described substrate transport properties and inhibitor profiles are embodied by the hetero-dimer. This work provides a foundation for elucidating the structure of the functional complex and the mechanism of substrate transport and inhibition.

Characterisation of sensor kinase by CD spectroscopy: golden rules and tips

This is a review that describes the golden rules and tips on how to characterise the molecular interactions of membrane sensor kinase proteins with ligands using mainly circular dichroism (CD) spectroscopy. CD spectroscopy is essential for this task as any conformational change observed in the far-UV (secondary structures (α-helix, β-strands, poly-proline of type II, β-turns, irregular and folding) and near-UV regions [local environment of the aromatic side-chains of amino acid residues (Phe, Tyr and Trp) and ligands (drugs) and prosthetic groups (porphyrins, cofactors and coenzymes (FMN, FAD, NAD))] upon ligand addition to the protein can be used to determine qualitatively and quantitatively ligand-binding interactions. Advantages of using CD versus other techniques will be discussed. The difference CD spectra of the protein-ligand mixtures calculated subtracting the spectra of the ligand at various molar ratios can be used to determine the type of conformational changes induced by the ligand in terms of the estimated content of the various elements of protein secondary structure. The highly collimated microbeam and high photon flux of Diamond Light Source B23 beamline for synchrotron radiation circular dichroism (SRCD) enable the use of minimal amount of membrane proteins (7.5 µg for a 0.5 mg/ml solution) for high-throughput screening. Several examples of CD titrations of membrane proteins with a variety of ligands are described herein including the protocol tips that would guide the choice of the appropriate parameters to conduct these titrations by CD/SRCD in the best possible way.