BBA - Molecular Basis of Disease (v.1762, #2)

Special issue: Mitochondria in diseases and therapeutics by Shey-Shing Sheu; John J. Lemasters (139).

This review summarizes recent findings from electron tomography about the three-dimensional shape of mitochondrial membranes and its possible influence on a range of mitochondrial functions. The inner membrane invaginations called cristae are pleomorphic, typically connected by narrow tubular junctions of variable length to the inner boundary membrane. This design may restrict intra-mitochondrial diffusion of metabolites such as ADP, and of soluble proteins such as cytochrome c. Tomographic images of a variety of mitochondria suggest that inner membrane topology reflects a balance between membrane fusion and fission. Proteins that can affect cristae morphology include tBid, which triggers cytochrome c release in apoptosis, and the dynamin-like protein Mgm1, involved in inter-mitochondrial membrane fusion. In frozen-hydrated rat-liver mitochondria, the space between the inner and outer membranes contains 10–15 nm particles that may represent macromolecular complexes involved in activities that span the two membranes.
Keywords: Mitochondria; Electron microscopy; Electron tomography; Membrane topology; Bioenergetics; Apoptosis;

Mitochondrial contact sites: Their role in energy metabolism and apoptosis by Dieter G. Brdiczka; Dmitry B. Zorov; Shey-Shing Sheu (148-163).
The energy metabolism of the failing heart is characterised by a 30% decrease of the total adenine nucleotides content and what may be more important by a 60% loss of creatine and creatine phosphate [J.S. Ingwall, R.G. Weiss, Is the failing heart energy starved? On using chemical energy to support cardiac function, Circ. Res. 95 (2004) 35–145]. Besides the effect of these changes on the energy supply, failing heart is known to be more vulnerable to Ca2+ overload and apoptosis-inducing processes. Recent studies have pointed to the critical role of mitochondrial contact sites in controlling both the mitochondrial energy metabolism and Ca2+ homeostasis. This review focuses on the structure and function of protein complexes in mitochondrial contact sites and their regulatory role in the cellular bioenergetics, intra- and extra-mitochondrial Ca2+ levels, and release of apoptosis-inducing factors. Firstly, we review the compositions of different contact sites following by the discussion of experimental data obtained with isolated and reconstituted voltage-dependent anion channel−adenine nucleotide translocase complexes and consequences of the complex disassembling. Furthermore, we describe experiments involving the complex-stabilizing conditions in vitro and in intact cells. At the end, we discuss unsolved problems and opportunities for clinical application of the complex-stabilizing factors.
Keywords: Adenine nucleotide translocator (ANT); Voltage-dependent anion channel (VDAC); Hexokinase; Creatine kinase; Permeability transition pore; Protein kinase A; Protein kinase B; AMP Kinase; Chronic heart failure;

Mitochondrial creatine kinase in human health and disease by Uwe Schlattner; Malgorzata Tokarska-Schlattner; Theo Wallimann (164-180).
Mitochondrial creatine kinase (MtCK), together with cytosolic creatine kinase isoenzymes and the highly diffusible CK reaction product, phosphocreatine, provide a temporal and spatial energy buffer to maintain cellular energy homeostasis. Mitochondrial proteolipid complexes containing MtCK form microcompartments that are involved in channeling energy in form of phosphocreatine rather than ATP into the cytosol. Under situations of compromised cellular energy state, which are often linked to ischemia, oxidative stress and calcium overload, two characteristics of mitochondrial creatine kinase are particularly relevant: its exquisite susceptibility to oxidative modifications and the compensatory up-regulation of its gene expression, in some cases leading to accumulation of crystalline MtCK inclusion bodies in mitochondria that are the clinical hallmarks for mitochondrial cytopathies. Both of these events may either impair or reinforce, respectively, the functions of mitochondrial MtCK complexes in cellular energy supply and protection of mitochondria form the so-called permeability transition leading to apoptosis or necrosis.
Keywords: Apoptosis; Cardiomyopathy; Crystalline intra-mitochondrial inclusion; Energy metabolism; Neurodegenerative disease; Oxidative damage;

Despite a detailed understanding of their metabolism, mitochondria often behave anomalously. In particular, global suppression of mitochondrial metabolism and metabolite exchange occurs in apoptosis, ischemia and anoxia, cytopathic hypoxia of sepsis and multiple organ failure, alcoholic liver disease, aerobic glycolysis in cancer cells (Warburg effect) and unstimulated pancreatic beta cells. Here, we propose that closure of voltage-dependent anion channels (VDAC) in the mitochondrial outer membrane accounts for global mitochondrial suppression. In anoxia, cytopathic hypoxia and ethanol treatment, reactive oxygen and nitrogen species, cytokines, kinase cascades and increased NADH act to inhibit VDAC conductance and promote selective oxidation of membrane-permeable respiratory substrates like short chain fatty acids and acetaldehyde. In cancer cells, highly expressed hexokinase binds to and inhibits VDAC to suppress mitochondrial function while stimulating glycolysis, but an escape mechanism intervenes when glucose-6-phosphate accumulates and dissociates hexokinase from VDAC. Similarly, glucokinase binds mitochondria of insulin-secreting beta cells, possibly blocking VDAC and suppressing mitochondrial function. We propose that glucose metabolism leads to glucose-6-phosphate-dependent unbinding of glucokinase, relief of VDAC inhibition, release of ATP from mitochondria and ATP-dependent insulin release. In support of the overall proposal, ethanol treatment of isolated rat hepatocytes inhibited mitochondrial respiration and accessibility to adenylate kinase in the intermembrane space, effects that were overcome by digitonin permeabilization of the outer membrane. Overall, these considerations suggest that VDAC is a dynamic regulator, or governator, of global mitochondrial function both in health and disease.
Keywords: Anoxia; Apoptosis; Beta cell; Cytopathic hypoxia; Ethanol; Glycolysis; Governator; Mitochondria; VDAC; Warburg effect;

Regulation of the mitochondrial apoptosis-induced channel, MAC, by BCL-2 family proteins by Laurent M. Dejean; Sonia Martinez-Caballero; Stephen Manon; Kathleen W. Kinnally (191-201).
Programmed cell death or apoptosis is central to many physiological processes and pathological conditions such as organogenesis, tissue homeostasis, cancer, and neurodegenerative diseases. Bcl-2 family proteins tightly control this cell death program by regulating the permeabilization of the mitochondrial outer membrane and, hence, the release of cytochrome c and other pro-apoptotic factors. Control of the formation of the mitochondrial apoptosis-induced channel, or MAC, is central to the regulation of apoptosis by Bcl-2 family proteins. MAC is detected early in apoptosis by patch clamping the mitochondrial outer membrane. The focus of this review is on the regulation of MAC activity by Bcl-2 family proteins. The role of MAC as the putative cytochrome c release channel during early apoptosis and insights concerning its molecular composition are also discussed.
Keywords: MAC; Mitochondrial apoptosis-induced channel; Apoptosis; Patch clamp; Bcl-2; Bax;

Tissue protection mediated by mitochondrial K+ channels by Heberty T.F. Facundo; Maynara Fornazari; Alicia J. Kowaltowski (202-212).
Two distinct K+ uniporters have been described in mitochondria, ATP-sensitive and Ca2+-activated. Both are capable of protecting tissues against ischemia and other forms of injury when active. These findings indicate a central role for mitochondrial K+ uptake in tissue protection. This review describes the characteristics of mitochondrial K+ uniport, physiological consequences of this transport, forms of tissue damage in which K+ channels are implicated and possible mechanisms through which protection occurs.
Keywords: Mitochondria; Potassium; Calcium; Ischemia; Preconditioning; Reactive oxygen;

Proteomic analysis of succinate dehydrogenase and ubiquinol-cytochrome c reductase (Complex II and III) isolated by immunoprecipitation from bovine and mouse heart mitochondria by Birgit Schilling; James Murray; Chris B. Yoo; Richard H. Row; Michael P. Cusack; Roderick A. Capaldi; Bradford W. Gibson (213-222).
The oxidative phosphorylation system (OXPHOS) consists of five multi-enzyme complexes, Complexes I–V, and is a key component of mitochondrial function relating to energy production, oxidative stress, cell signaling and apoptosis. Defects or a reduction in activity in various components that make up the OXPHOS enzymes can cause serious diseases, including neurodegenerative disease and various metabolic disorders. Our goal is to develop techniques that are capable of rapid and in-depth analysis of all five OXPHOS complexes. Here, we describe a mild, micro-scale immunoisolation and mass spectrometric/proteomic method for the characterization of Complex II (succinate dehydrogenase) and Complex III (ubiquinol-cytochrome c reductase) from bovine and rodent heart mitochondria. Extensive protein sequence coverage was obtained after immunocapture, 1D SDS PAGE separation and mass spectrometric analysis for a majority of the 4 and 11 subunits, respectively, that make up Complexes II and III. The identification of several posttranslational modifications, including the covalent FAD modification of flavoprotein subunit 1 from Complex II, was possible due to high mass spectrometric sequence coverage.
Keywords: Mitochondria; Succinate dehydrogenase; Ubiquinol-cytochrome c reductase; Posttranslational modification; Neurodegenerative disease;

Mitochondrial dysfunction in cardiac ischemia–reperfusion injury: ROS from complex I, without inhibition by Andrew J. Tompkins; Lindsay S. Burwell; Stanley B. Digerness; Corinne Zaragoza; William L. Holman; Paul S. Brookes (223-231).
A key pathologic event in cardiac ischemia reperfusion (I–R) injury is mitochondrial energetic dysfunction, and several studies have attributed this to complex I (CxI) inhibition. In isolated perfused rat hearts, following I–R, we found that CxI-linked respiration was inhibited, but isolated CxI enzymatic activity was not. Using the mitochondrial thiol probe iodobutyl-triphenylphosphonium in conjunction with proteomic tools, thiol modifications were identified in several subunits of the matrix-facing 1α sub-complex of CxI. These thiol modifications were accompanied by enhanced ROS generation from CxI, but not complex III. Implications for the pathology of cardiac I–R injury are discussed.
Keywords: ROS; Ischemia; Mitochondria; Complex I; Nitric oxide;

Mitochondrial criticality: A new concept at the turning point of life or death by Miguel Antonio Aon; Sonia Cortassa; Fadi Gabriel Akar; Brian O'Rourke (232-240).
A variety of stressors can cause the collapse of mitochondrial membrane potential (ΔΨ m), but the events leading up to this catastrophic cellular event are not well understood at the mechanistic level. Based on our recent studies of oscillations in mitochondrial energetics, we have coined the term “mitochondrial criticality” to describe the state in which the mitochondrial network of cardiomyocytes becomes very sensitive to small perturbations in reactive oxygen species (ROS), resulting in the scaling of local mitochondrial uncoupling and ΔΨ m loss to the whole cell, and the myocardial syncytium. At the point of criticality, the dynamics of the mitochondrial network bifurcate to oscillatory behavior. These energetic changes are translated into effects on the electrical excitability of the cell, inducing dramatic changes in the morphology and the threshold for activating an action potential. Emerging evidence suggests that this mechanism, by creating spatial and temporal heterogeneity of excitability in the heart during ischemia and reperfusion, underlies the genesis of potentially lethal cardiac arrhythmias.
Keywords: Mitochondrial oscillation; Oxidative stress; Inner membrane anion channel; Ischemia–reperfusion injury; Arrhythmias;

Visualizing common deletion of mitochondrial DNA-augmented mitochondrial reactive oxygen species generation and apoptosis upon oxidative stress by Tsung-I Peng; Pei-Ru Yu; Jing-Yi Chen; Hung-Li Wang; Hong-Yeuh Wu; Yau-Huei Wei; Mei-Jie Jou (241-255).
Common deletion (CD) 4977 bp of mitochondrial DNA (mtDNA) disrupt specifically mitochondrial complex I, IV and V on the electron transport chain (ETC) and is closely associated with wide spectrums of clinical manifestations. To quantitatively investigate how CD-induced ETC defect alters mitochondrial reactive oxygen species (mROS) generation as well as down stream apoptotic signaling, we employed an established array of human CD cytoplasmic hybrids (cybrids) harboring 0%–80% of CD. Pathological effects of CD on the mitochondria were visualized at single cell level by the application of fluorescent probes coupled with conventional and multiphoton imaging microscopy. Intriguingly, we observed CD-augmented mROS generation omitted “threshold effect”. CD-augmented mROS generation was associated with depolarized mitochondrial membrane potential (ΔΨ m). Upon oxidative stress, the amount of CD-augmented mROS generation was greatly enhanced to cause pathological apoptotic deterioration including opening of the mitochondrial permeability transition, cytochrome c release, phosphatidylserine externalization and DNA fragmentation. In addition, heterogeneous mitochondrial dysfunctions were found in cybrids containing 80% of CD (D cybrids), i.e., low sensitive-D (LS-D, roughly 80%) and a super sensitive-D (SS-D, 20%). As compared to LS-D, SS-D had higher resting mROS level but slightly hyperpolarized ΔΨ m. Upon H2O2 treatment, much faster generation of mROS was observed which induced a faster depolarization of ΔΨ m and later apoptotic deterioration in SS-D. We proposed a dose-dependent, feed-forward and self-accelerating vicious cycle of mROS production might be initiated in CD-induced ETC defect without threshold effect. As CD-augmented mROS generation is obligated to cause an enhanced pathological apoptosis, precise detection of CD-augmented mROS generation and their degree of heterogeneity in single cells may serve as sensitive pathological indexes for early diagnosis, prognosis and treatment of CD-associated diseases.
Keywords: Apoptosis; Electron transport chain; Mitochondrial DNA mutation; Mitochondrial membrane potential; Mitochondrial permeability transition pore; Mitochondrial reactive oxygen specie;

Targeting antioxidants to mitochondria: A new therapeutic direction by Shey-Shing Sheu; Dhananjaya Nauduri; M.W. Anders (256-265).
Mitochondria play an important role in controlling the life and death of a cell. Consequently, mitochondrial dysfunction leads to a range of human diseases such as ischemia–reperfusion injury, sepsis, and diabetes. Although the molecular mechanisms responsible for mitochondria-mediated disease processes are not fully elucidated yet, the oxidative stress appears to be critical. Accordingly, strategies are being developed for the targeted delivery of antioxidants to mitochondria. In this review, we shall briefly discuss cellular reactive oxygen species metabolism and its role in pathophysiology; the currently existing antioxidants and possible reasons why they are not effective in ameliorating oxidative stress-mediated diseases; and recent developments in mitochondrially targeted antioxidants and their future promise for disease treatment.
Keywords: Reactive oxygen species; Antioxidant; Mitochondria; Mitochondria membrane potential; Oxidative stress;

Acknowledgment (266-268).