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1.  Mitochondria distinguish granule-stored from de novo synthesized TNF secretion in human mast cells 
Background
Mast cells are immune cells derived from hematopoietic precursors that mature in the tissue microenvironment. Mast cells are critical for allergic, immune and inflammatory processes, many of which involve TNF. Mast cells uniquely store TNF in their secretory granules. Upon stimulation, mast cells rapidly (30 min) secrete beta-hexosaminidase (beta-hex) and granule-stored TNF through degranulation, but also increase TNF mRNA and release de novo synthesized TNF 24 hr later. Regulation of these two distinct pathways is poorly understood.
Methods
human LAD2 leukemic mast cells are stimulated by substance P (SP). TNF secretion and gene expression were measured by ELISA and Real-Time PCR. Live cell mitochondrial dynamics was observed under Confocal Microscopy. Cell energy consumption were measured in term of oxygen consumption rate.
Results
Here we show that granule-stored TNF is preformed and its secretion from LAD2 mast cells stimulated by SP exhibits (a) higher energy consumption and is inhibited by the mitochondrial ATP pump blocker oligomycin, (b) rapid increase of intracellular calcium levels, and (c) reversible mitochondrial translocation, from a perinuclear distribution to the cell surface, as compared to de novo synthesized TNF release induced by lipopolysaccharide (LPS). This mitochondrial translocation is confirmed using primary human umbilical cord blood-derived mast cells (hCBMCs) stimulated by an allergic trigger (IgE/streptavidin).
Conclusion
These findings indicate that unique mitochondrial functions distinguish granule-stored from newly synthesized TNF release from human mast cells, thus permitting the versatile involvement of mast cells in different biological processes.
doi:10.1159/000335178
PMCID: PMC4278585  PMID: 22555146
Lipopolysaccharide; Mast cells; Mitochondria; Substance P; TNF
2.  A bioenergetic profile of non-transformed fibroblasts uncovers a link between death-resistance and enhanced spare respiratory capacity 
Mitochondrion  2013;13(6):10.1016/j.mito.2013.09.005.
Apoptosis-resistance and metabolic imbalances are prominent features of cancer cells. We have recently reported on populations of human fibroblasts that exhibit resistance to mitochondrial-mediated apoptosis, acquired as a result of a single genotoxic exposure. The objective of the present study was to investigate the intrinsic bioenergetic profile of the death-resistant cells, as compared to the clonogenic control cells. Therefore, we analyzed the basic bioenergetic parameters including oxygen consumption and extracellular acidification rates, coupling efficiency, and spare respiratory capacity. Our data demonstrate a strong correlation between enhanced spare respiratory capacity and death-resistance, which we postulate to be indicative of the earliest stages of carcinogenesis
doi:10.1016/j.mito.2013.09.005
PMCID: PMC3837383  PMID: 24075934
Mitochondria; bioenergetics; spare respiratory capacity; death-resistance
3.  Lysosomal dysfunction and impaired autophagy underlie the pathogenesis of amyloidogenic light chain-mediated cardiotoxicity 
EMBO Molecular Medicine  2014;6(11):1493-1507.
AL amyloidosis is the consequence of clonal production of amyloidogenic immunoglobulin light chain (LC) proteins, often resulting in a rapidly progressive and fatal amyloid cardiomyopathy. Recent work has found that amyloidogenic LC directly initiate a cardio-toxic response underlying the pathogenesis of the cardiomyopathy; however, the mechanisms that contribute to this proteotoxicity remain unknown. Using human amyloidogenic LC isolated from patients with amyloid cardiomyopathy, we reveal that dysregulation of autophagic flux is critical for mediating amyloidogenic LC proteotoxicity. Restoration of autophagic flux by pharmacological intervention using rapamycin protected against amyloidogenic light chain protein-induced pathologies including contractile dysfunction and cell death at the cellular and organ level and also prolonged survival in an in vivo zebrafish model of amyloid cardiotoxicity. Mechanistically, we identify impaired lysosomal function to be the major cause of defective autophagy and amyloidogenic LC-induced proteotoxicity. Collectively, these findings detail the downstream molecular mechanisms underlying AL amyloid cardiomyopathy and highlight potential targeting of autophagy and lysosomal dysfunction in patients with amyloid cardiomyopathy.
doi:10.15252/emmm.201404190
PMCID: PMC4237473  PMID: 25319546
amyloidosis; autophagy; cardiac toxicity; lysosome; mitochondria
4.  ABCB10 Regulates Early Steps of Heme Synthesis 
Circulation research  2013;113(3):10.1161/CIRCRESAHA.113.301552.
Rationale
Heme plays a critical role in gas exchange, mitochondrial energy production and antioxidant defenses in cardiovascular system. The mitochondrial transporter ATP-binding cassette (ABC) B10 has been suggested to export heme out of the mitochondria and is required for normal hemoglobinization of erythropoietic cells and protection against ischemia-reperfusion injury in the heart; however, its primary function has not been established.
Objective
The aim of this study was to identify the function of ABCB10 in heme synthesis in cardiac cells.
Methods and Results
Knockdown of ABCB10 in cardiac myoblasts significantly reduced heme levels and the activities of heme-containing proteins, while supplementation with δ-aminolevulinic acid (ALA) reversed these defects. Overexpression of mitochondrial ALA synthase 2 (ALAS2), the rate-limiting enzyme upstream of ALA export, failed to restore heme levels in cells with ABCB10 downregulation. ABCB10 and heme levels were increased by hypoxia, and reversal of ABCB10 upregulation caused oxidative stress and cell death. Moreover, ABCB10 knockdown in neonatal rat cardiomyocytes (NRCM) resulted in a significant delay of calcium removal from the cytoplasm, suggesting a relaxation defect. Finally, ABCB10 expression and heme levels were altered in failing human hearts and mice with ischemic cardiomyopathy.
Conclusions
ABCB10 plays a critical role in heme synthesis pathway by facilitating ALA production or export from the mitochondria. In contrast to previous reports, we show that ABCB10 is not a heme exporter, and instead is required for the early mitochondrial steps of heme biosynthesis.
doi:10.1161/CIRCRESAHA.113.301552
PMCID: PMC3817742  PMID: 23720443
δ-aminolevulinic acid; heme; mitochondria; ATP-binding cassette transporters; ischemic cardiomyopathy
5.  Relation of Mitochondrial Oxygen Consumption in Peripheral Blood Mononuclear Cells to Vascular Function in Type 2 Diabetes Mellitus 
Recent studies have shown mitochondrial dysfunction and increased production of reactive oxygen species in peripheral blood mononuclear cells (PBMC’s) and endothelial cells from patients with diabetes mellitus. Mitochondria oxygen consumption is coupled to ATP production and also occurs in an uncoupled fashion during formation of reactive oxygen species by components of the electron transport chain and other enzymatic sites. We therefore hypothesized that diabetes would be associated with higher total and uncoupled oxygen consumption in PBMC’s that would correlate with endothelial dysfunction. We developed a method to measure oxygen consumption in freshly isolated PBMC’s and applied it to 26 patients with type 2 diabetes mellitus and 28 non-diabetic controls. Basal (192±47 vs. 161±44 pMoles/min, P=0.01), uncoupled (64±16 vs. 53±16 pMoles/min, P=0.007), and maximal (795±87 vs. 715±128 pMoles/min, P=0.01) oxygen consumption rates were higher in diabetic patients compared to controls. There were no significant correlations between oxygen consumption rates and endothelium-dependent flow-mediated dilation measured by vascular ultrasound. Non-endothelium-dependent nitroglycerin-mediated dilation was lower in diabetics (10.1±6.6 vs. 15.8±4.8%, P=0.03) and correlated with maximal oxygen consumption (R= −0.64, P=0.001). In summary, we found that diabetes mellitus is associated with a pattern of mitochondrial oxygen consumption consistent with higher production of reactive oxygen species. The correlation between oxygen consumption and nitroglycerin-mediated dilation may suggest a link between mitochondrial dysfunction and vascular smooth muscle cell dysfunction that merits further study. Finally, the described method may have utility for assessment of mitochondrial function in larger scale observational and interventional studies in humans.
doi:10.1177/1358863X14521315
PMCID: PMC3932629  PMID: 24558030
mitochondria; oxygen consumption; diabetes mellitus; blood mononuclear cells
6.  Dual role of proapoptotic BAD in insulin secretion and beta cell survival 
Nature medicine  2008;14(2):144-153.
The proapoptotic BCL-2 family member BAD resides in a glucokinase-containing complex that regulates glucose-driven mitochondrial respiration. Here, we present genetic evidence of a physiologic role for BAD in glucose-stimulated insulin secretion by beta cells. This novel function of BAD is specifically dependent upon the phosphorylation of its BH3 sequence, previously defined as an essential death domain. We highlight the pharmacologic relevance of phosphorylated BAD BH3 by using cell-permeable, hydrocarbon-stapled BAD BH3 helices that target glucokinase, restore glucose-driven mitochondrial respiration and correct the insulin secretory response in Bad-deficient islets. Our studies uncover an alternative target and function for the BAD BH3 domain and emphasize the therapeutic potential of phosphorylated BAD BH3 mimetics in selectively restoring beta cell function. Furthermore, we show that BAD regulates the physiologic adaptation of beta cell mass during high-fat feeding. Our findings provide genetic proof of the bifunctional activities of BAD in both beta cell survival and insulin secretion.
doi:10.1038/nm1717
PMCID: PMC3918232  PMID: 18223655
7.  Correction: A Novel High-Throughput Assay for Islet Respiration Reveals Uncoupling of Rodent and Human Islets 
PLoS ONE  2013;8(12):10.1371/annotation/f21efac9-2906-4a2f-ad22-befcb7714dd0.
doi:10.1371/annotation/f21efac9-2906-4a2f-ad22-befcb7714dd0
PMCID: PMC3855804
8.  Metabolic Master Regulators: Sharing Information among Multiple Systems 
Obesity and diabetes are caused by defects in metabolically sensitive tissues. Attention has been paid to insulin resistance as the key relevant pathosis, with a detailed focus on signal transduction pathways in metabolic tissues. Evidence exists to support an important role for each tissue in metabolic homeostasis and a potential causative role in both diabetes and obesity. The redox metabolome, that coordinates tissue responses and reflects shared control and regulation, is our focus. Consideration is given to the possibility that pathosis results from contributions of all relevant tissues, by virtue of a circulating communication system. Validation of this model would support simultaneous regulation of all collaborating metabolic organs through changes in the circulation, regardless of whether change was initiated exogenously or by a single organ.
doi:10.1016/j.tem.2012.07.006
PMCID: PMC3502692  PMID: 22939743
9.  Organellar vs Cellular Control of Mitochondrial Dynamics 
Seminars in cell & developmental biology  2010;21(6):10.1016/j.semcdb.2010.01.003.
Mitochondrial dynamics, the fusion and fission of individual mitochondrial units, is critical to the exchange of the metabolic, genetic and proteomic contents of individual mitochondria. In this regard, fusion and fission events have been shown to modulate mitochondrial bioenergetics, as well as several cellular processes including fuel sensing, ATP production, autophagy, apoptosis, and the cell cycle. Regulation of the dynamic events of fusion and fission occur at two redundant and interactive levels. Locally, the microenvironment of the individual mitochondrion can alter its ability to fuse, divide or move through the cell. Globally, nuclear-encoded processes and cellular ionic and second messenger systems can alter or activate mitochondrial proteins, regulate mitochondrial dynamics and concomitantly change the condition of the mitochondrial population. In this review we investigate the different global and local signals that control mitochondrial biology. This discussion is carried out to clarify the different signals that impact the status of the mitochondrial population.
doi:10.1016/j.semcdb.2010.01.003
PMCID: PMC3808978  PMID: 20079451
Mitochondrial Dynamics; Cell Cycle; Autophagy; Fusion; Fission; mtDNA; Mitochondrial Movement; Bioenergetics; Mitophagy
10.  Mitochondrial ABC transporters function: the role of ABCB10 (ABC-me) as a novel player in cellular handling of reactive oxygen species 
Biochimica et biophysica acta  2012;1823(10):10.1016/j.bbamcr.2012.07.013.
Mitochondria are one of the major sources of reactive oxygen species (ROS) in the cell. When exceeding the capacity of antioxidant mechanisms, ROS production may lead to different pathologies, such as ischemia-reperfusion injury, neurodegeneration, anemia and ageing. As a consequence of the endosymbiotic origin of mitochondria, eukaryotic cells have developed different transport mechanisms that coordinate mitochondrial function with other cellular compartments. Four mitochondrial ATP-binding cassette (ABC) transporters have been described to date in mammals: ABCB6, ABCB8, ABCB7 and ABCB10. ABCB10 is located in the inner mitochondrial membrane forming homodimers, with the ATP binding domain facing the mitochondrial matrix. ABCB10 expression is highly induced during erythroid differentiation and its overexpression increases hemoglobin synthesis in erythroid cells. However, ABCB10 is also expressed in nonerythroid tissues, suggesting a role not directly related to hemoglobin synthesis. Recent evidence points toward ABCB10 as an important player in the protection from oxidative stress in mammals. In this regard, ABCB10 is required for normal erythropoiesis and cardiac recovery after ischemia-reperfusion, processes intimately related to mitochondrial ROS generation. Here, we review the current knowledge on mitochondrial ABC transporters and ABCB10 and discuss the potential mechanisms by which ABCB10 and its transport activity may regulate oxidative stress. We discuss ABCB10 as a potential therapeutic target for diseases in which increased mitochondrial ROS production and oxidative stress play a major role.
doi:10.1016/j.bbamcr.2012.07.013
PMCID: PMC3808996  PMID: 22884976
mitochondria; oxidative stress; ABCB10; ABC-me; erythropoiesis; ischemia-reperfusion
11.  The CB1 Antagonist Rimonabant Decreases Insulin Hypersecretion in Rat Pancreatic Islets 
Obesity (Silver Spring, Md.)  2009;17(10):10.1038/oby.2009.234.
Type 2 diabetes and obesity are characterized by elevated nocturnal circulating free fatty acids, elevated basal insulin secretion, and blunted glucose-stimulated insulin secretion (GSIS). The CB1 receptor antagonist, Rimonabant, has been shown to improve glucose tolerance and insulin sensitivity in vivo but its direct effect on islets has been unclear. Islets from lean littermates and obese Zucker (ZF) and Zucker Diabetic Fatty (ZDF) rats were incubated for 24 h in vitro and exposed to 11 mmol/l glucose and 0.3 mmol/l palmitate (GL) with or without Rimonabant. Insulin secretion was determined at basal (3 mmol/l) or stimulatory (15 mmol/l) glucose concentrations. As expected, basal secretion was significantly elevated in islets from obese or GL-treated lean rats whereas the fold increase in GSIS was diminished. Rimonabant decreased basal hypersecretion in islets from obese rats and GL-treated lean rats without decreasing the fold increase in GSIS. However, it decreased GSIS in islets from lean rats without affecting basal secretion. These findings indicate that Rimonabant has direct effects on islets to reduce insulin secretion when secretion is elevated above normal levels by diet or in obesity. In contrast, it appears to decrease stimulated secretion in islets from lean animals but not in obese or GL-exposed islets.
doi:10.1038/oby.2009.234
PMCID: PMC3808997  PMID: 19644453
12.  Mitochondrial Fusion, Fission and Autophagy as a Quality Control Axis: The Bioenergetic View 
Biochimica et biophysica acta  2008;1777(9):10.1016/j.bbabio.2008.05.001.
The mitochondrial life cycle consists of frequent fusion and fission events. Ample experimental and clinical data demonstrate that inhibition of either fusion or fission result in deterioration of mitochondrial bioenergetics. While fusion may benefit mitochondrial function by allowing the spreading of metabolites, protein and DNA throughout the network, the functional benefit of fission is not as intuitive. Remarkably, studies that track individual mitochondria through fusion and fission found that the two events are paired and that fusion triggers fission. On average each mitochondrion would go though ~5 fusion:fission cycles every hour. Measurement of Δψm during single fusion and fission events demonstrate that fission may yield uneven daughter mitochondria where the depolarized daughter less likely to become involved in a subsequent fusion and is more likely to be targeted by autophagy. Based on these observations we propose a mechanism by which the integration of mitochondrial fusion, fission and autophagy form a quality maintenance mechanism. According to this hypothesis pairs of fusion and fission allow for the reorganization and sequestration of damaged mitochondrial components into daughter mitochondria that are segregated from the networking pool and then becoming eliminated by autophagy.
doi:10.1016/j.bbabio.2008.05.001
PMCID: PMC3809017  PMID: 18519024
Mitochondria; Membrane potential; Fusion; Fission; Autophagy
13.  Association of Genetic Variation in the Mitochondrial Genome with Blood Pressure and Metabolic Traits 
Hypertension  2012;60(4):949-956.
Elevated blood pressure (BP) is a major risk factor for cardiovascular disease. Several studies have noted a consistent maternal effect on BP; consequently, mitochondrial DNA (mtDNA) variation has become an additional target of investigation of the missing BP heritability. Analyses of common mtDNA polymorphisms, however, have not found evidence of association with hypertension. To explore associations of relatively rare (frequency < 5%) mtDNA variants with BP, we identified uncommon/rare variants through sequencing the entire mitochondrial genome in 32 unrelated individuals with extreme-high BP in the Framingham Heart Study (FHS) and genotyped 40 mtSNPs in 7,219 FHS participants. The nonsynonymous mtSNP 5913G>A (Asp4Asn) in the cytochrome c oxidase subunit 1 of Complex IV demonstrated significant associations with BP and fasting blood glucose (FBG) levels. Individuals with the rare 5913A allele had, on average, 7 mm Hg higher systolic BP at baseline (Pempirical = 0.05) and 17 mg/dL higher mean FBG over 25 years of follow up (Pempirical = 0.009). Significant associations with FBG levels were also detected for nonsynonymous mtSNP 3316G>A (Ala4Thr) in the NADH dehydrogenase subunit 1 of Complex I. On average, individuals with rare allele 3316A had 17 and 25 mg/dL higher FBG at baseline (Pempirical = 0.01) and over 25 years of follow up (Pempirical = 0.007). Our findings provide the first evidence of putative association of variants in the mitochondrial genome with SBP and FBG in the general population. Replication in independent samples, however, is needed to confirm these putative associations.
doi:10.1161/HYPERTENSIONAHA.112.196519
PMCID: PMC3753106  PMID: 22949535
Mitochondrial genome; Association study; Genetics; Hypertension; Diabetes
14.  MitoTimer probe reveals the impact of autophagy, fusion, and motility on subcellular distribution of young and old mitochondrial protein and on relative mitochondrial protein age 
Autophagy  2013;9(11):1887-1896.
To study mitochondrial protein age dynamics, we targeted a time-sensitive fluorescent protein, MitoTimer, to the mitochondrial matrix. Mitochondrial age was revealed by the integrated portions of young (green) and old (red) MitoTimer protein. Mitochondrial protein age was dependent on turnover rates as pulsed synthesis, decreased import, or autophagic inhibition all increased the proportion of aged MitoTimer protein. Mitochondrial fusion promotes the distribution of young mitochondrial protein across the mitochondrial network as cells lacking essential fusion genes Mfn1 and Mfn2 displayed increased heterogeneity in mitochondrial protein age. Experiments in hippocampal neurons illustrate that the distribution of older and younger mitochondrial protein within the cell is determined by subcellular spatial organization and compartmentalization of mitochondria into neurites and soma. This effect was altered by overexpression of mitochondrial transport protein, RHOT1/MIRO1. Collectively our data show that distribution of young and old protein in the mitochondrial network is dependent on turnover, fusion, and transport.
doi:10.4161/auto.26503
PMCID: PMC4028338  PMID: 24149000
MitoTimer; aging; mitochondrial dynamics; autophagy; RHOT1
15.  Bactericidal Antibiotics Induce Mitochondrial Dysfunction and Oxidative Damage in Mammalian Cells 
Science translational medicine  2013;5(192):192ra85.
Prolonged antibiotic treatment can lead to detrimental side effects in patients, including ototoxicity, nephrotoxicity, and tendinopathy, yet the mechanisms underlying the effects of antibiotics in mammalian systems remain unclear. It has been suggested that bactericidal antibiotics induce the formation of toxic reactive oxygen species (ROS) in bacteria. We show that clinically relevant doses of bactericidal antibiotics—quinolones, aminoglycosides, and β-lactams—cause mitochondrial dysfunction and ROS overproduction in mammalian cells. We demonstrate that these bactericidal antibiotic–induced effects lead to oxidative damage to DNA, proteins, and membrane lipids. Mice treated with bactericidal antibiotics exhibited elevated oxidative stress markers in the blood, oxidative tissue damage, and up-regulated expression of key genes involved in antioxidant defense mechanisms, which points to the potential physiological relevance of these antibiotic effects. The deleterious effects of bactericidal antibiotics were alleviated in cell culture and in mice by the administration of the antioxidant N-acetyl-L-cysteine or prevented by preferential use of bacteriostatic antibiotics. This work highlights the role of antibiotics in the production of oxidative tissue damage in mammalian cells and presents strategies to mitigate or prevent the resulting damage, with the goal of improving the safety of antibiotic treatment in people.
doi:10.1126/scitranslmed.3006055
PMCID: PMC3760005  PMID: 23825301
16.  Telomere dysfunction induces metabolic and mitochondrial compromise 
Nature  2011;470(7334):359-365.
Telomere dysfunction activates p53-mediated cellular growth arrest, senescence and apoptosis to drive progressive atrophy and functional decline in high-turnover tissues. The broader adverse impact of telomere dysfunction across many tissues including more quiescent systems prompted transcriptomic network analyses to identify common mechanisms operative in haematopoietic stem cells, heart and liver. These unbiased studies revealed profound repression of peroxisome proliferator-activated receptor gamma, coactivator 1 alpha and beta (PGC-1α and PGC-1β, also known as Ppargc1a and Ppargc1b, respectively) and the downstream network in mice null for either telomerase reverse transcriptase (Tert) or telomerase RNA component (Terc) genes. Consistent with PGCs as master regulators of mitochondrial physiology and metabolism, telomere dysfunction is associated with impaired mitochondrial biogenesis and function, decreased gluconeogenesis, cardiomyopathy, and increased reactive oxygen species. In the setting of telomere dysfunction, enforced Tert or PGC-1α expression or germline deletion of p53 (also known as Trp53) substantially restores PGC network expression, mitochondrial respiration, cardiac function and gluconeogenesis. We demonstrate that telomere dysfunction activates p53 which in turn binds and represses PGC-1α and PGC-1β promoters, thereby forging a direct link between telomere and mitochondrial biology. We propose that this telomere–p53–PGC axis contributes to organ and metabolic failure and to diminishing organismal fitness in the setting of telomere dysfunction.
doi:10.1038/nature09787
PMCID: PMC3741661  PMID: 21307849
17.  Optimal Dynamics for Quality Control in Spatially Distributed Mitochondrial Networks 
PLoS Computational Biology  2013;9(7):e1003108.
Recent imaging studies of mitochondrial dynamics have implicated a cycle of fusion, fission, and autophagy in the quality control of mitochondrial function by selectively increasing the membrane potential of some mitochondria at the expense of the turnover of others. This complex, dynamical system creates spatially distributed networks that are dependent on active transport along cytoskeletal networks and on protein import leading to biogenesis. To study the relative impacts of local interactions between neighboring mitochondria and their reorganization via transport, we have developed a spatiotemporal mathematical model encompassing all of these processes in which we focus on the dynamics of a health parameter meant to mimic the functional state of mitochondria. In agreement with previous models, we show that both autophagy and the generation of membrane potential asymmetry following a fusion/fission cycle are required for maintaining a healthy mitochondrial population. This health maintenance is affected by mitochondrial density and motility primarily through changes in the frequency of fusion events. Health is optimized when the selectivity thresholds for fusion and fission are matched, providing a mechanistic basis for the observed coupling of the two processes through the protein OPA1. We also demonstrate that the discreteness of the components exchanged during fusion is critical for quality control, and that the effects of limiting total amounts of autophagy and biogenesis have distinct consequences on health and population size, respectively. Taken together, our results show that several general principles emerge from the complexity of the quality control cycle that can be used to focus and interpret future experimental studies, and our modeling framework provides a road-map for deconstructing the functional importance of local interactions in communities of cells as well as organelles.
Author Summary
Mitochondria are the powerhouses of eukaryotic cells, oxidizing glucose to produce ATP. Most cells harbor tens to hundreds of mitochondria in a constant state of flux, in which they fuse with one another, undergo fission, import proteins to grow larger, and eventually are recycled by autophagy. These dynamic processes depend on the electrical potential that is maintained across the mitochondrial inner membrane and powers the production of both ATP and detrimental reactive oxygen species. How do mitochondria maintain high membrane potential in the face of damage due to reactive oxygen species? Here, we develop a model to study how the reorganization of mitochondrial networks in space and time due to fusion, fission, and the experimentally observed development of membrane potential asymmetry after fission affect overall mitochondrial health. We show that health, which is a proxy for the mitochondrial membrane potential, is dominated by how density and motility affect the frequency of fusion events, and that several simple rules for the system kinetics lead to optimal quality control. This model predicts general behaviors that can be applied to specific studies of mitochondrial dynamics in a wide variety of cell types, and provides a framework for deconstructing complex organellar organization and their function in human disease.
doi:10.1371/journal.pcbi.1003108
PMCID: PMC3708874  PMID: 23874166
18.  Mitochondrial autophagy in cells with mtDNA mutations results from synergistic loss of transmembrane potential and mTORC1 inhibition 
Human Molecular Genetics  2011;21(5):978-990.
Autophagy has emerged as a key cellular process for organellar quality control, yet this pathway apparently fails to eliminate mitochondria containing pathogenic mutations in mitochondrial DNA (mtDNA) in patients with a variety of human diseases. In order to explore how mtDNA-mediated mitochondrial dysfunction interacts with endogenous autophagic pathways, we examined autophagic status in a panel of human cytoplasmic hybrid (cybrid) cell lines carrying a variety of pathogenic mtDNA mutations. We found that both genetic- and chemically induced loss of mitochondrial transmembrane potential (Δψm) caused recruitment of the pro-mitophagic factor Parkin to mitochondria. Strikingly, however, the loss of Δψm alone was insufficient to prompt delivery of mitochondria to the autophagosome (mitophagy). We found that mitophagy could be induced following treatment with the mTORC1 inhibitor rapamycin in cybrids carrying either large-scale partial deletions of mtDNA or complete depletion of mtDNA. Further, we found that the level of endogenous Parkin is a crucial determinant of mitophagy. These results suggest a two-hit model, in which the synergistic induction of both (i) mitochondrial recruitment of Parkin following the loss of Δψm and (ii) mTORC1-controlled general macroautophagy is required for mitophagy. It appears that mitophagy can be accomplished by the endogenous autophagic machinery, but requires the full engagement of both of these pathways.
doi:10.1093/hmg/ddr529
PMCID: PMC3277306  PMID: 22080835
19.  Anti-telomerase therapy provokes ALT and mitochondrial adaptive mechanisms in cancer 
Cell  2012;148(4):651-663.
SUMMARY
To assess telomerase as a cancer therapeutic target and determine adaptive mechanisms to telomerase inhibition, we modeled telomerase reactivation and subsequent extinction in T-cell lymphomas arising in Atm-/- mice engineered with an inducible telomerase reverse transcriptase allele. Telomerase reactivation in the setting of telomere dysfunction enabled full malignant progression with alleviation of telomere dysfunction-induced checkpoints. These cancers possessed copy number alterations targeting key loci in human T-cell lymphomagenesis. Upon telomerase extinction, tumor growth eventually slowed with re-instatement of telomere dysfunction-induced checkpoints, yet growth subsequently resumed as tumors acquired Alternative Lengthening of Telomeres (ALT) and aberrant transcriptional networks centering on mitochondrial biology and oxidative defense. ALT+ tumors acquired amplification/overexpression of PGC-1β, a master regulator of mitochondrial biogenesis and function, and they showed marked sensitivity to PGC-1β or SOD2 knock-down. Genetic modeling of telomerase extinction reveals vulnerabilities that motivate coincidental inhibition of mitochondrial maintenance and oxidative defense mechanisms to enhance anti-telomerase cancer therapy.
doi:10.1016/j.cell.2011.12.028
PMCID: PMC3286017  PMID: 22341440
20.  Defective Mitochondrial Morphology and Bioenergetic Function in Mice Lacking the Transcription Factor Yin Yang 1 in Skeletal Muscle 
Molecular and Cellular Biology  2012;32(16):3333-3346.
The formation, distribution, and maintenance of functional mitochondria are achieved through dynamic processes that depend strictly on the transcription of nuclear genes encoding mitochondrial proteins. A large number of these mitochondrial genes contain binding sites for the transcription factor Yin Yang 1 (YY1) in their proximal promoters, but the physiological relevance is unknown. We report here that skeletal-muscle-specific YY1 knockout (YY1mKO) mice have severely defective mitochondrial morphology and oxidative function associated with exercise intolerance, signs of mitochondrial myopathy, and short stature. Gene set enrichment analysis (GSEA) revealed that the top pathways downregulated in YY1mKO mice were assigned to key metabolic and regulatory mitochondrial genes. This analysis was consistent with a profound decrease in the level of mitochondrial proteins and oxidative phosphorylation (OXPHOS) bioenergetic function in these mice. In contrast to the finding for wild-type mice, inactivation of the mammalian target of rapamycin (mTOR) did not suppress mitochondrial genes in YY1mKO mice. Mechanistically, mTOR-dependent phosphorylation of YY1 resulted in a strong interaction between YY1 and the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1α (PGC1α), a major regulator of mitochondrial function. These results underscore the important role of YY1 in the maintenance of mitochondrial function and explain how its inactivation might contribute to exercise intolerance and mitochondrial myopathies.
doi:10.1128/MCB.00337-12
PMCID: PMC3434543  PMID: 22711985
21.  Testosterone Plus Low-Intensity Physical Training in Late Life Improves Functional Performance, Skeletal Muscle Mitochondrial Biogenesis, and Mitochondrial Quality Control in Male Mice 
PLoS ONE  2012;7(12):e51180.
Testosterone supplementation increases muscle mass in older men but has not been shown to consistently improve physical function and activity. It has been hypothesized that physical exercise is required to induce the adaptations necessary for translation of testosterone-induced muscle mass gain into functional improvements. However, the effects of testosterone plus low intensity physical exercise training (T/PT) on functional performance and bioenergetics are unknown. In this pilot study, we tested the hypothesis that combined administration of T/PT would improve functional performance and bioenergetics in male mice late in life more than low-intensity physical training alone. 28-month old male mice were randomized to receive T/PT or vehicle plus physical training (V/PT) for 2 months. Compare to V/PT control, administration of T/PT was associated with improvements in muscle mass, grip strength, spontaneous physical movements, and respiratory activity. These changes were correlated with increased mitochondrial DNA copy number and expression of markers for mitochondrial biogenesis. Mice receiving T/PT also displayed increased expression of key elements for mitochondrial quality control, including markers for mitochondrial fission-and-fusion and mitophagy. Concurrently, mice receiving T/PT also displayed increased expression of markers for reduced tissue oxidative damage and improved muscle quality. Conclusion: Testosterone administered with low-intensity physical training improves grip strength, spontaneous movements, and respiratory activity. These functional improvements were associated with increased muscle mitochondrial biogenesis and improved mitochondrial quality control.
doi:10.1371/journal.pone.0051180
PMCID: PMC3519841  PMID: 23240002
22.  β-Cell Uncoupling Protein 2 Regulates Reactive Oxygen Species Production, Which Influences Both Insulin and Glucagon Secretion 
Diabetes  2011;60(11):2710-2719.
OBJECTIVE
The role of uncoupling protein 2 (UCP2) in pancreatic β-cells is highly debated, partly because of the broad tissue distribution of UCP2 and thus limitations of whole-body UCP2 knockout mouse models. To investigate the function of UCP2 in the β-cell, β-cell–specific UCP2 knockout mice (UCP2BKO) were generated and characterized.
RESEARCH DESIGN AND METHODS
UCP2BKO mice were generated by crossing loxUCP2 mice with mice expressing rat insulin promoter-driven Cre recombinase. Several in vitro and in vivo parameters were measured, including respiration rate, mitochondrial membrane potential, islet ATP content, reactive oxygen species (ROS) levels, glucose-stimulated insulin secretion (GSIS), glucagon secretion, glucose and insulin tolerance, and plasma hormone levels.
RESULTS
UCP2BKO β-cells displayed mildly increased glucose-induced mitochondrial membrane hyperpolarization but unchanged rates of uncoupled respiration and islet ATP content. UCP2BKO islets had elevated intracellular ROS levels that associated with enhanced GSIS. Surprisingly, UCP2BKO mice were glucose-intolerant, showing greater α-cell area, higher islet glucagon content, and aberrant ROS-dependent glucagon secretion under high glucose conditions.
CONCLUSIONS
Using a novel β-cell–specific UCP2KO mouse model, we have shed light on UCP2 function in primary β-cells. UCP2 does not behave as a classical metabolic uncoupler in the β-cell, but has a more prominent role in the regulation of intracellular ROS levels that contribute to GSIS amplification. In addition, β-cell UCP2 contributes to the regulation of intraislet ROS signals that mediate changes in α-cell morphology and glucagon secretion.
doi:10.2337/db11-0132
PMCID: PMC3198081  PMID: 21984579
23.  Altered Mitochondrial Dynamics Contributes to Endothelial Dysfunction in Diabetes Mellitus 
Circulation  2011;124(4):444-453.
Background
Endothelial dysfunction contributes to the development of atherosclerosis in patients with diabetes mellitus, but the mechanisms of endothelial dysfunction in this setting are incompletely understood. Recent studies have shown altered mitochondrial dynamics in diabetes mellitus with increased mitochondrial fission and production of reactive oxygen species (ROS). We investigated the contribution of altered dynamics to endothelial dysfunction in diabetes.
Methods and Results
We observed mitochondrial fragmentation (P=0.002) and increased expression of fission-1 protein (Fis1, P<0.0001) in venous endothelial cells freshly isolated from patients with diabetes mellitus (n=10) compared to healthy controls (n=9). In cultured human aortic endothelial cells exposed to 30 mM glucose, we observed a similar loss of mitochondrial networks and increased expression of Fis1 and dynamin-related protein-1 (Drp1), proteins required for mitochondrial fission. Altered mitochondrial dynamics was associated with increased mitochondrial ROS production and a marked impairment of agonist-stimulated activation of endothelial nitric oxide synthase (eNOS) and cGMP production. Silencing Fis1 or DRP1 expression with siRNA blunted high glucose-induced alterations in mitochondrial networks, ROS production, eNOS activation, and cGMP production. An intracellular ROS scavenger provided no additional benefit, suggesting that increased mitochondrial fission may impair endothelial function via increased ROS.
Conclusions
These findings implicate increased mitochondrial fission as a contributing mechanism for endothelial dysfunction in diabetic states.
doi:10.1161/CIRCULATIONAHA.110.014506
PMCID: PMC3149100  PMID: 21747057
endothelium; mitochondria; mitochondrial dynamics; reactive oxygen species
24.  The Interplay Between Mitochondrial Dynamics and Mitophagy 
Antioxidants & Redox Signaling  2011;14(10):1939-1951.
Abstract
Mitochondrial dynamics and mitophagy are recognized as two critical processes underlying mitochondrial homeostasis. Morphological and bioenergetic characterization of the life cycle of an individual mitochondrion reveals several points where fusion, fission, and mitophagy interact. Mitochondrial fission can produce an impaired daughter unit that will be targeted by the autophagic machinery. Mitochondrial fusion, on the other hand, may serve to dilute impaired respiratory components and thereby prevent their removal. The inverse dependency of fusion and mitophagy on membrane potential allows them to act as complementary rather than competitive fates of the daughter mitochondrion after a fission event. We discuss the interplay between mitochondrial dynamics and mitophagy in different tissues and in different disease models under both stress-induced and steady-state conditions. Antioxid. Redox Signal. 14, 1939–1951.
doi:10.1089/ars.2010.3779
PMCID: PMC3078508  PMID: 21128700
25.  A Novel High-Throughput Assay for Islet Respiration Reveals Uncoupling of Rodent and Human Islets 
PLoS ONE  2012;7(5):e33023.
Background
The pancreatic beta cell is unique in its response to nutrient by increased fuel oxidation. Recent studies have demonstrated that oxygen consumption rate (OCR) may be a valuable predictor of islet quality and long term nutrient responsiveness. To date, high-throughput and user-friendly assays for islet respiration are lacking. The aim of this study was to develop such an assay and to examine bioenergetic efficiency of rodent and human islets.
Methodology/Principal Findings
The XF24 respirometer platform was adapted to islets by the development of a 24-well plate specifically designed to confine islets. The islet plate generated data with low inter-well variability and enabled stable measurement of oxygen consumption for hours. The F1F0 ATP synthase blocker oligomycin was used to assess uncoupling while rotenone together with myxothiazol/antimycin was used to measure the level of non-mitochondrial respiration. The use of oligomycin in islets was validated by reversing its effect in the presence of the uncoupler FCCP. Respiratory leak averaged to 59% and 49% of basal OCR in islets from C57Bl6/J and FVB/N mice, respectively. In comparison, respiratory leak of INS-1 cells and C2C12 myotubes was measured to 38% and 23% respectively. Islets from a cohort of human donors showed a respiratory leak of 38%, significantly lower than mouse islets.
Conclusions/Significance
The assay for islet respiration presented here provides a novel tool that can be used to study islet mitochondrial function in a relatively high-throughput manner. The data obtained in this study shows that rodent islets are less bioenergetically efficient than human islets as well as INS1 cells.
doi:10.1371/journal.pone.0033023
PMCID: PMC3351473  PMID: 22606219

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