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Recent studies reveal that lateral mitochondrial transfer, the movement of mitochondria from one cell to another, can affect cellular and tissue homeostasis. Most of what we know about mitochondrial transfer stems from bulk cell studies and have led to the paradigm that functional transferred mitochondria restore bioenergetics and revitalize cellular functions to recipient cells with damaged or non-functional mitochondrial networks. However, we show that mitochondrial transfer also occurs between cells with functioning endogenous mitochondrial networks, but the mechanisms underlying how transferred mitochondria can promote such sustained behavioral reprogramming remain unclear. We report that unexpectedly, transferred macrophage mitochondria are dysfunctional and accumulate reactive oxygen species in recipient cancer cells. We further discovered that reactive oxygen species accumulation activates ERK signaling, promoting cancer cell proliferation. Pro-tumorigenic macrophages exhibit fragmented mitochondrial networks, leading to higher rates of mitochondrial transfer to cancer cells. Finally, we observe that macrophage mitochondrial transfer promotes tumor cell proliferation in vivo. Collectively these results indicate that transferred macrophage mitochondria activate downstream signaling pathways in a ROS-dependent manner in cancer cells, and provide a model of how sustained behavioral reprogramming can be mediated by a relatively small amount of transferred mitochondria in vitro and in vivo., Competing Interests: CK, JC, SP, SS, DG, DB, DW, GO, JL, JJ, JR, AW, TZ, MR No competing interests declared, (© 2023, Kidwell, Casalini et al.)

Mitochondrial energetics and respiration have emerged as important factors in how cancer cells respond to or evade apoptotic signals. The study of the functional connection between these two processes may provide insight into following questions old and new: how might we target respiration or downstream signaling pathways to amplify apoptotic stress in the context of cancer therapy? Why are respiration and apoptotic regulation housed in the same organelle? Here, we briefly review mitochondrial respiration and apoptosis and then focus on how the intersection of these two processes is regulated by cytoplasmic signaling pathways such as the integrated stress response., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Neuronal excitation imposes a high demand of ATP in neurons. Most of the ATP derives primarily from pyruvate-mediated oxidative phosphorylation, a process that relies on import of pyruvate into mitochondria occuring exclusively via the mitochondrial pyruvate carrier (MPC). To investigate whether deficient oxidative phosphorylation impacts neuron excitability, we generated a mouse strain carrying a conditional deletion of MPC1, an essential subunit of the MPC, specifically in adult glutamatergic neurons. We found that, despite decreased levels of oxidative phosphorylation and decreased mitochondrial membrane potential in these excitatory neurons, mice were normal at rest. Surprisingly, in response to mild inhibition of GABA mediated synaptic activity, they rapidly developed severe seizures and died, whereas under similar conditions the behavior of control mice remained unchanged. We report that neurons with a deficient MPC were intrinsically hyperexcitable as a consequence of impaired calcium homeostasis, which reduced M-type potassium channel activity. Provision of ketone bodies restored energy status, calcium homeostasis and M-channel activity and attenuated seizures in animals fed a ketogenic diet. Our results provide an explanation for the seizures that frequently accompany a large number of neuropathologies, including cerebral ischemia and diverse mitochondriopathies, in which neurons experience an energy deficit., Competing Interests: AD, ML, SA, TM, SM, PS, ER, PM, AK, CQ, ET, JR, JN, AC, MD, CS, JM No competing interests declared, (© 2022, De La Rossa et al.)

The Connexin43 gap junction gene GJA1 has one coding exon, but its mRNA undergoes internal translation to generate N-terminal truncated isoforms of Connexin43 with the predominant isoform being only 20 kDa in size (GJA1-20k). Endogenous GJA1-20k protein is not membrane bound and has been found to increase in response to ischemic stress, localize to mitochondria, and mimic ischemic preconditioning protection in the heart. However, it is not known how GJA1-20k benefits mitochondria to provide this protection. Here, using human cells and mice, we identify that GJA1-20k polymerizes actin around mitochondria which induces focal constriction sites. Mitochondrial fission events occur within about 45 s of GJA1-20k recruitment of actin. Interestingly, GJA1-20k mediated fission is independent of canonical Dynamin-Related Protein 1 (DRP1). We find that GJA1-20k-induced smaller mitochondria have decreased reactive oxygen species (ROS) generation and, in hearts, provide potent protection against ischemia-reperfusion injury. The results indicate that stress responsive internally translated GJA1-20k stabilizes polymerized actin filaments to stimulate non-canonical mitochondrial fission which limits ischemic-reperfusion induced myocardial infarction., Competing Interests: DS, EN, RB, SV, SX, JW, JP, TH, JR, RS No competing interests declared, (© 2021, Shimura et al.)

Cells harbor two systems for fatty acid synthesis, one in the cytoplasm (catalyzed by fatty acid synthase, FASN) and one in the mitochondria (mtFAS). In contrast to FASN, mtFAS is poorly characterized, especially in higher eukaryotes, with the major product(s), metabolic roles, and cellular function(s) being essentially unknown. Here we show that hypomorphic mtFAS mutant mouse skeletal myoblast cell lines display a severe loss of electron transport chain (ETC) complexes and exhibit compensatory metabolic activities including reductive carboxylation. This effect on ETC complexes appears to be independent of protein lipoylation, the best characterized function of mtFAS, as mutants lacking lipoylation have an intact ETC. Finally, mtFAS impairment blocks the differentiation of skeletal myoblasts in vitro. Together, these data suggest that ETC activity in mammals is profoundly controlled by mtFAS function, thereby connecting anabolic fatty acid synthesis with the oxidation of carbon fuels., Competing Interests: SN, AS, SR, JM, CB, SF, MJ, SL, JB, JM, YO, BN, JP, KF, JC, SG, DW, JR No competing interests declared, RD Reviewing editor, eLife, (© 2020, Nowinski et al.)

The lipids that make up biological membranes tend to be the forgotten molecules of cell biology. The paucity of data on these important entities likely reflects the difficulties of studying and understanding their biological roles, rather than revealing a lack of importance. Indeed, the lipid composition of biological membranes has a profound impact on a diverse array of cellular processes. The focus of this review is on the effects of different lipid classes on the function of mitochondria, particularly bioenergetics, in health and disease., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Although metabolic adaptations have been demonstrated to be essential for tumor cell proliferation, the metabolic underpinnings of tumor initiation are poorly understood. We found that the earliest stages of colorectal cancer (CRC) initiation are marked by a glycolytic metabolic signature, including downregulation of the mitochondrial pyruvate carrier (MPC), which couples glycolysis and glucose oxidation through mitochondrial pyruvate import. Genetic studies in Drosophila suggest that this downregulation is required because hyperplasia caused by loss of the Apc or Notch tumor suppressors in intestinal stem cells can be completely blocked by MPC overexpression. Moreover, in two distinct CRC mouse models, loss of Mpc1 prior to a tumorigenic stimulus doubled the frequency of adenoma formation and produced higher grade tumors. MPC loss was associated with a glycolytic metabolic phenotype and increased expression of stem cell markers. These data suggest that changes in cellular pyruvate metabolism are necessary and sufficient to promote cancer initiation., Competing Interests: Declaration of Interests The University of Utah has filed a patent related to the mitochondrial pyruvate carrier, of which J.R. and C.S.T. are listed as co-inventors. All other authors declare no competing interests., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Exercise capacity is a strong predictor of all-cause mortality. Skeletal muscle mitochondrial respiratory capacity, its biggest contributor, adapts robustly to changes in energy demands induced by contractile activity. While transcriptional regulation of mitochondrial enzymes has been extensively studied, there is limited information on how mitochondrial membrane lipids are regulated. Here, we show that exercise training or muscle disuse alters mitochondrial membrane phospholipids including phosphatidylethanolamine (PE). Addition of PE promoted, whereas removal of PE diminished, mitochondrial respiratory capacity. Unexpectedly, skeletal muscle-specific inhibition of mitochondria-autonomous synthesis of PE caused respiratory failure because of metabolic insults in the diaphragm muscle. While mitochondrial PE deficiency coincided with increased oxidative stress, neutralization of the latter did not rescue lethality. These findings highlight the previously underappreciated role of mitochondrial membrane phospholipids in dynamically controlling skeletal muscle energetics and function.

Biology students today are taught that mitochondria are 'the powerhouse of the cell'. This gross over-simplification of their cellular role has arguably led to a paucity of knowledge concerning the many other tasks carried out by this multifunctional organelle. Mitochondrial fatty acid synthesis (mtFAS) is one such under-appreciated pathway that is crucial for mitochondrial function, although even mitochondrial experts are often surprised to learn of its existence. For many years, the only function of mtFAS was thought to be the production of lipoic acid, an important co-factor for several mitochondrial enzymes. However, recent advances have revealed a far wider role for mtFAS in mitochondrial physiology. The discovery of human patients with mutations in mtFAS enzymes has brought renewed interest in understanding the full significance of this novel mode of mitochondrial metabolic regulation. We now appreciate that mtFAS is a nutrient-sensitive pathway that provides an elegant mechanism whereby acetyl-CoA regulates its own consumption via coordination of lipoic acid synthesis and tricarboxylic acid (TCA) cycle activity, iron-sulfur (FeS) cluster biogenesis, assembly of oxidative phosphorylation complexes, and mitochondrial translation. In this minireview, we describe and build upon the important discoveries that led to our current understanding of this elegant mechanism of coordination of nutrient status and metabolism., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The electron transport chain (ETC) is an important participant in cellular energy conversion, but its biogenesis presents the cell with numerous challenges. To address these complexities, the cell utilizes ETC assembly factors, which include the LYR protein family. Each member of this family interacts with the mitochondrial acyl carrier protein (ACP), the scaffold protein upon which the mitochondrial fatty acid synthesis (mtFAS) pathway builds fatty acyl chains from acetyl-CoA. We demonstrate that the acylated form of ACP is an acetyl-CoA-dependent allosteric activator of the LYR protein family used to stimulate ETC biogenesis. By tuning ETC assembly to the abundance of acetyl-CoA, which is the major fuel of the TCA cycle and ETC, this system could provide an elegant mechanism for coordinating the assembly of ETC complexes with one another and with substrate availability., (Copyright © 2018 Elsevier Inc. All rights reserved.)

For decades, lipids were assumed to fulfill roles only in energy storage and membrane structure. Recent studies have discovered critical roles for phospholipids, sphingolipids, and sterols in many cellular pathways, including cell signaling and transcriptional regulation. Frequently, lipids from these various classes work together to achieve defined cellular outcomes. Specific mitochondrial lipids are critical for proper assembly of the electron transport chain complexes and for effective responses to mitochondrial damage, including maintenance of mitochondrial protein homeostasis, regulation of mitophagy, and induction of apoptosis. In this Minireview, we will primarily focus on mitochondrial lipid signaling mediated by lipid-protein interactions., (© 2018 by The American Society for Biochemistry and Molecular Biology, Inc.)

Vms1 translocates to damaged mitochondria in response to stress, whereupon its binding partner, Cdc48, contributes to mitochondrial protein homeostasis. Mitochondrial targeting of Vms1 is mediated by its conserved mitochondrial targeting domain (MTD), which, in unstressed conditions, is inhibited by intramolecular binding to the Vms1 leucine-rich sequence (LRS). Here, we report a 2.7 Å crystal structure of Vms1 that reveals that the LRS lies in a hydrophobic groove in the autoinhibited MTD. We also demonstrate that the oxidized sterol, ergosterol peroxide, is necessary and sufficient for Vms1 localization to mitochondria, through binding the MTD in an interaction that is competitive with binding to the LRS. These data support a model in which stressed mitochondria generate an oxidized sterol receptor that recruits Vms1 to support mitochondrial protein homeostasis., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Most differentiated cells convert glucose to pyruvate in the cytosol through glycolysis, followed by pyruvate oxidation in the mitochondria. These processes are linked by the mitochondrial pyruvate carrier (MPC), which is required for efficient mitochondrial pyruvate uptake. In contrast, proliferative cells, including many cancer and stem cells, perform glycolysis robustly but limit fractional mitochondrial pyruvate oxidation. We sought to understand the role this transition from glycolysis to pyruvate oxidation plays in stem cell maintenance and differentiation. Loss of the MPC in Lgr5-EGFP-positive stem cells, or treatment of intestinal organoids with an MPC inhibitor, increases proliferation and expands the stem cell compartment. Similarly, genetic deletion of the MPC in Drosophila intestinal stem cells also increases proliferation, whereas MPC overexpression suppresses stem cell proliferation. These data demonstrate that limiting mitochondrial pyruvate metabolism is necessary and sufficient to maintain the proliferation of intestinal stem cells.

Due to their varied metabolic and signalling roles, mitochondria are important in mediating cell behaviour. By altering mitochondrial function, two studies now identify metabolite-induced epigenetic changes that have profound effects on haematopoietic stem cell fate and function.

Mitochondrial fatty acid synthesis (FASII) and iron sulfur cluster (FeS) biogenesis are both vital biosynthetic processes within mitochondria. In this study, we demonstrate that the mitochondrial acyl carrier protein (ACP), which has a well-known role in FASII, plays an unexpected and evolutionarily conserved role in FeS biogenesis. ACP is a stable and essential subunit of the eukaryotic FeS biogenesis complex. In the absence of ACP, the complex is destabilized resulting in a profound depletion of FeS throughout the cell. This role of ACP depends upon its covalently bound 4'-phosphopantetheine (4-PP)-conjugated acyl chain to support maximal cysteine desulfurase activity. Thus, it is likely that ACP is not simply an obligate subunit but also exploits the 4-PP-conjugated acyl chain to coordinate mitochondrial fatty acid and FeS biogenesis., Competing Interests: The authors declare that no competing interests exist.

Succinate dehydrogenase (or complex II; SDH) is a heterotetrameric protein complex that links the tribarboxylic acid cycle with the electron transport chain. SDH is composed of four nuclear-encoded subunits that must translocate independently to the mitochondria and assemble into a mature protein complex embedded in the inner mitochondrial membrane. Recently, it has become clear that failure to assemble functional SDH complexes can result in cancer and neurodegenerative syndromes. The effort to thoroughly elucidate the SDH assembly pathway has resulted in the discovery of four subunit-specific assembly factors that aid in the maturation of individual subunits and support the assembly of the intact complex. This review will focus on these assembly factors and assess the contribution of each factor to the assembly of SDH. Finally, we propose a model of the SDH assembly pathway that incorporates all extant data.

Succinate dehydrogenase (SDH) occupies a central place in cellular energy production, linking the tricarboxylic cycle with the electron transport chain. As a result, a subset of cancers and neuromuscular disorders result from mutations affecting any of the four SDH structural subunits or either of two known SDH assembly factors. Herein we characterize an evolutionarily conserved SDH assembly factor designated Sdh8/SDHAF4, using yeast, Drosophila, and mammalian cells. Sdh8 interacts specifically with the catalytic Sdh1 subunit in the mitochondrial matrix, facilitating its association with Sdh2 and the subsequent assembly of the SDH holocomplex. These roles for Sdh8 are critical for preventing motility defects and neurodegeneration in Drosophila as well as the excess ROS generated by free Sdh1. These studies provide insights into the mechanisms by which SDH is assembled and raise the possibility that some forms of neuromuscular disease may be associated with mutations that affect this SDH assembly factor., (Copyright © 2014 Elsevier Inc. All rights reserved.)

The majority of ER-targeted tail-anchored (TA) proteins are inserted into membranes by the Guided Entry of Tail-anchored protein (GET) system. Disruption of this system causes a subset of TA proteins to mislocalize to mitochondria. We show that the AAA+ ATPase Msp1 limits the accumulation of mislocalized TA proteins on mitochondria. Deletion of MSP1 causes the Pex15 and Gos1 TA proteins to accumulate on mitochondria when the GET system is impaired. Likely as a result of failing to extract mislocalized TA proteins, yeast with combined mutation of the MSP1 gene and the GET system exhibit strong synergistic growth defects and severe mitochondrial damage, including loss of mitochondrial DNA and protein and aberrant mitochondrial morphology. Like yeast Msp1, human ATAD1 limits the mitochondrial mislocalization of PEX26 and GOS28, orthologs of Pex15 and Gos1, respectively. GOS28 protein level is also increased in ATAD1(-/-) mouse tissues. Therefore, we propose that yeast Msp1 and mammalian ATAD1 are conserved members of the mitochondrial protein quality control system that might promote the extraction and degradation of mislocalized TA proteins to maintain mitochondrial integrity., (© 2014 The Authors.)

Mitochondria are crucial for many cellular functions. In this issue of Cell Reports, studies from Lanning et al. and Wolf and Mootha describe RNAi approaches to screening the mitochondrial proteome. Unexpectedly, they uncover key roles for two poorly characterized mitochondrial proteins: AK4 and FASTKD4. These studies provide examples of the power of forward genetic screens, even when screening a subset of genes, in deciphering functions of previously mysterious mitochondrial proteins., (Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.)

Stemming from the pioneering studies of bioenergetics in the 1950s, 1960s, and 1970s, mitochondria have become ingrained in the collective psyche of scientists as the "powerhouses" of the cell. While this remains a worthy moniker, more recent efforts have revealed that these organelles are home to a vast array of metabolic and signaling processes and possess a proteomic landscape that is both highly varied and largely uncharted. As mitochondrial dysfunction is increasingly being implicated in a spectrum of human diseases, it is imperative that we construct a more complete framework of these organelles by systematically defining the functions of their component parts. Powerful new approaches in biochemistry and systems biology are helping to fill in the gaps.

Mitochondrial dysfunction is associated with the development of many age-related human diseases. Therefore recognizing and correcting the early signs of malfunctioning mitochondria is of critical importance for cellular welfare and survival. We previously demonstrated that VCP/Cdc48-associated mitochondrial stress responsive 1 (Vms1) is a component of a mitochondrial surveillance system that mediates the stress-responsive degradation of mitochondrial proteins by the proteasome. Here we propose novel mechanisms through which Vms1 monitors the status of mitochondria and is recruited to damaged or stressed mitochondria. We find that Vms1 contains a highly conserved region that is necessary and sufficient for mitochondrial targeting (the mitochondrial targeting domain [MTD]). Of interest, MTD-mediated mitochondrial targeting of Vms1 is negatively regulated by a direct interaction with the Vms1 N-terminus. Using laser-induced generation of mitochondrial reactive oxygen species, we also show that Vms1 is preferentially recruited to mitochondria subjected to oxidative stress. These studies define cellular and biochemical mechanisms by which Vms1 locali-zation to mitochondria is controlled to enable an efficient protein quality control system.

Pyruvate constitutes a critical branch point in cellular carbon metabolism. We have identified two proteins, Mpc1 and Mpc2, as essential for mitochondrial pyruvate transport in yeast, Drosophila, and humans. Mpc1 and Mpc2 associate to form an ~150-kilodalton complex in the inner mitochondrial membrane. Yeast and Drosophila mutants lacking MPC1 display impaired pyruvate metabolism, with an accumulation of upstream metabolites and a depletion of tricarboxylic acid cycle intermediates. Loss of yeast Mpc1 results in defective mitochondrial pyruvate uptake, and silencing of MPC1 or MPC2 in mammalian cells impairs pyruvate oxidation. A point mutation in MPC1 provides resistance to a known inhibitor of the mitochondrial pyruvate carrier. Human genetic studies of three families with children suffering from lactic acidosis and hyperpyruvatemia revealed a causal locus that mapped to MPC1, changing single amino acids that are conserved throughout eukaryotes. These data demonstrate that Mpc1 and Mpc2 form an essential part of the mitochondrial pyruvate carrier.

Progressive mitochondrial failure is tightly associated with the onset of many age-related human pathologies. This tight connection results from the double-edged sword of mitochondrial respiration, which is responsible for generating both ATP and ROS, as well as from risks that are inherent to mitochondrial biogenesis. To prevent and treat these diseases, a precise understanding of the mechanisms that maintain functional mitochondria is necessary. Mitochondrial protein quality control is one of the mechanisms that protect mitochondrial integrity, and increasing evidence implicates the cytosolic ubiquitin/proteasome system (UPS) as part of this surveillance network. In this review, we will discuss our current understanding of UPS-dependent mitochondrial protein degradation, its roles in diseases progression, and insights into future studies., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Mitochondria perform multiple functions critical to the maintenance of cellular homoeostasis and their dysfunction leads to disease. Several lines of evidence suggest the presence of a MAD (mitochondria-associated degradation) pathway that regulates mitochondrial protein quality control. Internal mitochondrial proteins may be retrotranslocated to the OMM (outer mitochondrial membrane), multiple E3 ubiquitin ligases reside at the OMM and inhibition of the proteasome causes accumulation of ubiquitinated proteins at the OMM. Reminiscent of ERAD [ER (endoplasmic reticulum)-associated degradation], Cdc48 (cell division cycle 42)/p97 is recruited to stressed mitochondria, extracts ubiquitinated proteins from the OMM and presents ubiquitinated proteins to the proteasome for degradation. Recent research has provided mechanistic insights into the interaction of the UPS (ubiquitin-proteasome system) with the OMM. In yeast, Vms1 [VCP (valosin-containing protein) (p97)/Cdc48-associated mitochondrial-stress-responsive 1] protein recruits Cdc48/p97 to the OMM. In mammalian systems, the E3 ubiquitin ligase parkin regulates the recruitment of Cdc48/p97 to mitochondria, subsequent mitochondrial protein degradation and mitochondrial autophagy. Disruption of the Vms1 or parkin systems results in the hyper-accumulation of ubiquitinated proteins at mitochondria and subsequent mitochondrial dysfunction. The emerging MAD pathway is important for the maintenance of cellular and therefore organismal viability.

Mammalian mitochondria contain about 1100 proteins, nearly 300 of which are uncharacterized. Given the well-established role of mitochondrial defects in human disease, functional characterization of these proteins may shed new light on disease mechanisms. Starting with yeast as a model system, we investigated an uncharacterized but highly conserved mitochondrial protein (named here Sdh5). Both yeast and human Sdh5 interact with the catalytic subunit of the succinate dehydrogenase (SDH) complex, a component of both the electron transport chain and the tricarboxylic acid cycle. Sdh5 is required for SDH-dependent respiration and for Sdh1 flavination (incorporation of the flavin adenine dinucleotide cofactor). Germline loss-of-function mutations in the human SDH5 gene, located on chromosome 11q13.1, segregate with disease in a family with hereditary paraganglioma, a neuroendocrine tumor previously linked to mutations in genes encoding SDH subunits. Thus, a mitochondrial proteomics analysis in yeast has led to the discovery of a human tumor susceptibility gene.

Mitochondrial membrane potential directly powers many critical functions of mitochondria, including ATP production, mitochondrial protein import, and metabolite transport. Its loss is a cardinal feature of aging and mitochondrial diseases, and cells closely monitor membrane potential as an indicator of mitochondrial health. Given its central importance, it is logical that cells would modulate mitochondrial membrane potential in response to demand and environmental cues, but there has been little exploration of this question. We report that loss of the Sit4 protein phosphatase in yeast increases mitochondrial membrane potential, both by inducing the electron transport chain and the phosphate starvation response. Indeed, a similarly elevated mitochondrial membrane potential is also elicited simply by phosphate starvation or by abrogation of the Pho85-dependent phosphate sensing pathway. This enhanced membrane potential is primarily driven by an unexpected activity of the ADP/ATP carrier. We also demonstrate that this connection between phosphate limitation and enhancement of mitochondrial membrane potential is observed in primary and immortalized mammalian cells as well as in Drosophila. These data suggest that mitochondrial membrane potential is subject to environmental stimuli and intracellular signaling regulation and raise the possibility for therapeutic enhancement of mitochondrial function even in defective mitochondria.

Thermogenesis by uncoupling protein 1 (UCP1) is one of the primary mechanisms by which brown adipose tissue (BAT) increases energy expenditure. UCP1 resides in the inner mitochondrial membrane (IMM), where it dissipates membrane potential independent of adenosine triphosphate (ATP) synthase. Here, we provide evidence that phosphatidylethanolamine (PE) modulates UCP1-dependent proton conductance across the IMM to modulate thermogenesis. Mitochondrial lipidomic analyses revealed PE as a signature molecule whose abundance bidirectionally responds to changes in thermogenic burden. Reduction in mitochondrial PE by deletion of phosphatidylserine decarboxylase (PSD) made mice cold intolerant and insensitive to β3 adrenergic receptor agonist-induced increase in whole-body oxygen consumption. High-resolution respirometry and fluorometry of BAT mitochondria showed that loss of mitochondrial PE specifically lowers UCP1-dependent respiration without compromising electron transfer efficiency or ATP synthesis. These findings were confirmed by a reduction in UCP1 proton current in PE-deficient mitoplasts. Thus, PE performs a previously unknown role as a temperature-responsive rheostat that regulates UCP1-dependent thermogenesis.

Brown adipose tissue (BAT) is composed of thermogenic cells that convert chemical energy into heat to maintain a constant body temperature and counteract metabolic disease. The metabolic adaptations required for thermogenesis are not fully understood. Here, we explore how steady state levels of metabolic intermediates are altered in brown adipose tissue in response to cold exposure. Transcriptome and metabolome analysis revealed changes in pathways involved in amino acid, glucose, and TCA cycle metabolism. Using isotopic labeling experiments, we found that activated brown adipocytes increased labeling of pyruvate and TCA cycle intermediates from U13C-glucose. Although glucose oxidation has been implicated as being essential for thermogenesis, its requirement for efficient thermogenesis has not been directly tested. We show that mitochondrial pyruvate uptake is essential for optimal thermogenesis, as conditional deletion of Mpc1 in brown adipocytes leads to impaired cold adaptation. Isotopic labeling experiments using U13C-glucose showed that loss of MPC1 led to impaired labeling of TCA cycle intermediates. Loss of MPC1 in BAT increased 3-hydroxybutyrate levels in blood and BAT in response to the cold, suggesting that ketogenesis provides an alternative fuel source to compensate. Collectively, these studies highlight that complete glucose oxidation is essential for optimal brown fat thermogenesis.

The budding yeast Saccharomyces cerevisiae has served as a remarkable model organism for numerous seminal discoveries in biology. This paradigm extends to the mitochondria, a central hub for cellular metabolism, where studies in yeast have helped to reinvigorate the field and launch an exciting new era in mitochondrial biology. Here we discuss a few recent examples in which yeast research has laid a foundation for our understanding of evolutionarily conserved mitochondrial processes and functions, from key factors and pathways involved in the assembly of oxidative phosphorylation (OXPHOS) complexes to metabolite transport, lipid metabolism, and interorganelle communication. We also highlight new areas of yeast mitochondrial biology that are likely to aid in our understanding of the mitochondrial etiology of disease in the future. [ABSTRACT FROM AUTHOR]

Summary: The complexes of the electron transport chain associate into large macromolecular assemblies, which are believed to facilitate efficient electron flow. We have identified a conserved mitochondrial protein, named respiratory supercomplex factor 1 (Rcf1—Yml030w), that is required for the normal assembly of respiratory supercomplexes. We demonstrate that Rcf1 stably and independently associates with both Complex III and Complex IV of the electron transport chain. Deletion of the RCF1 gene caused impaired respiration, probably as a result of destabilization of respiratory supercomplexes. Consistent with the hypothetical function of these respiratory assemblies, loss of RCF1 caused elevated mitochondrial oxidative stress and damage. Finally, we show that knockdown of HIG2A, a mammalian homolog of RCF1, causes impaired supercomplex formation. We suggest that Rcf1 is a member of an evolutionarily conserved protein family that acts to promote respiratory supercomplex assembly and activity. [ABSTRACT FROM AUTHOR]

Abstract: Succinate dehydrogenase (or Electron Transport Chain Complex II) has been the subject of a focused but significant renaissance. This complex, which has been the least studied of the mitochondrial respiratory complexes has seen renewed interest due to the discovery of its role in human disease. Under this heightened scrutiny, the succinate dehydrogenase complex has proven to be a fascinating machine, whose regulation and assembly requires additional factors that are beginning to be discovered. Mutations in these factors and in the structural subunits of the complex itself cause a variety of human diseases. The mechanisms underlying the pathogenesis of SDH mutations is beginning to be understood. [Copyright &y& Elsevier]

Although the mitochondrial electron transport chain (ETC) is best known for its role in ATP synthesis, two studies, Sullivan et al. and Birsoy et al., conclude that its only essential function in proliferating cells is making aspartate (D). [ABSTRACT FROM AUTHOR]

Recently, three groups, Girardi et al. , Kory et al. , and Luongo et al. , independently identified solute carrier (SLC) 25A51 as the long-sought, major mitochondrial NAD+ transporter in mammalian cells. These studies not only deorphan an uncharacterized transporter of the SLC25A family, but also shed light on other aspects of NAD+ biology. [ABSTRACT FROM AUTHOR]

The tumor suppressor gene PTEN is the second most commonly deleted gene in cancer. Such deletions often include portions of the chromosome 10q23 locus beyond the bounds of PTEN itself, which frequently disrupts adjacent genes. Coincidental loss of PTEN-adjacent genes might impose vulnerabilities that could either affect patient outcome basally or be exploited therapeutically. Here, we describe how the loss of ATAD1, which is adjacent to and frequently co-deleted with PTEN, predisposes cancer cells to apoptosis triggered by proteasome dysfunction and correlates with improved survival in cancer patients. ATAD1 directly and specifically extracts the pro-apoptotic protein BIM from mitochondria to inactivate it. Cultured cells and mouse xenografts lacking ATAD1 are hypersensitive to clinically used proteasome inhibitors, which activate BIM and trigger apoptosis. This work furthers our understanding of mitochondrial protein homeostasis and could lead to new therapeutic options for the hundreds of thousands of cancer patients who have tumors with chromosome 10q23 deletion. [ABSTRACT FROM AUTHOR]

Compared to their differentiated progeny, stem cells are often characterized by distinct metabolic landscapes that emphasize anaerobic glycolysis and a lower fraction of mitochondrial carbohydrate oxidation. Until recently, the metabolic program of stem cells had been thought to be a byproduct of the environment, rather than an intrinsic feature determined by the cell itself. However, new studies highlight the impact of metabolic behavior on the maintenance and function of intestinal stem cells and hair follicle stem cells. This Review summarizes and discusses the evidence that metabolism is not a mere consequence of, but rather influential on stem cell fate. [ABSTRACT FROM AUTHOR]

Summary: We show that Ydr049 (renamed VCP/Cdc48-associated mitochondrial stress-responsive—Vms1), a member of an unstudied pan-eukaryotic protein family, translocates from the cytosol to mitochondria upon mitochondrial stress. Cells lacking Vms1 show progressive mitochondrial failure, hypersensitivity to oxidative stress, and decreased chronological life span. Both yeast and mammalian Vms1 stably interact with Cdc48/VCP/p97, a component of the ubiquitin/proteasome system with a well-defined role in endoplasmic reticulum-associated protein degradation (ERAD), wherein misfolded ER proteins are degraded in the cytosol. We show that oxidative stress triggers mitochondrial localization of Cdc48 and this is dependent on Vms1. When this system is impaired by mutation of Vms1, ubiquitin-dependent mitochondrial protein degradation, mitochondrial respiratory function, and cell viability are compromised. We demonstrate that Vms1 is a required component of an evolutionarily conserved system for mitochondrial protein degradation, which is necessary to maintain mitochondrial, cellular, and organismal viability. [ABSTRACT FROM AUTHOR]

Mammalian mitochondria contain about 1100 proteins, nearly 300 of which are uncharacterized. Given the well-established role of mitochondrial defects in human disease, functional characterization of these proteins may shed new tight on disease mechanisms. Starting with yeast as a model system, we investigated an uncharacterized but highly conserved mitochondrial protein (named here Sdh5). Both yeast and human Sdh5 interact with the catalytic subunit of the succinate dehydrogenase (SDH) complex, a component of both the electron transport chain and the tricarboxylic acid cycle. Sdh5 is required for SDH-dependent respiration and for Sdh1 flavination (incorporation of the flavin adenine dinucleotide cofactor). Germline loss-of-function mutations in the human SDH5 gene, located on chromosome 11q13.1, segregate with disease in a family with hereditary paraganglioma, a neuroendocrine tumor previously linked to mutations in genes encoding SDH subunits. Thus, a mitochondrial proteomics analysis in yeast has led to the discovery of a human tumor susceptibility gene. [ABSTRACT FROM AUTHOR]