Mitochondrial shaping cuts

Dual role of a GTPase conformational switch for membrane fusion by mitofusin ubiquitylation

Mitochondria are essential organelles whose function is upheld by their dynamic nature. This plasticity is mediated by large dynamin-related GTPases, called mitofusins in the case of fusion between two mitochondrial outer membranes. Fusion requires ubiquitylation, attached to K398 in the yeast mitofusin Fzo1, occurring in atypical and conserved forms. Here, modelling located ubiquitylation to α4 of the GTPase domain, a critical helix in Ras-mediated events. Structure-driven analysis revealed a dual role of K398. First, it is required for GTP-dependent dynamic changes of α4. Indeed, mutations designed to restore the conformational switch, in the absence of K398, rescued wild-type-like ubiquitylation on Fzo1 and allowed fusion. Second, K398 is needed for Fzo1 recognition by the pro-fusion factors Cdc48 and Ubp2. Finally, the atypical ubiquitylation pattern is stringently required bilaterally on both involved mitochondria. In contrast, exchange of the conserved pattern with conventional ubiquitin chains was not sufficient for fusion. In sum, α4 lysines from both small and large GTPases could generally have an electrostatic function for membrane interaction, followed by posttranslational modifications, thus driving membrane fusion events.

Small Molecule Mediated Restoration of Mitochondrial Function Augments Anti-Mycobacterial Activity of Human Macrophages Subjected to Cholesterol Induced Asymptomatic Dyslipidemia

Mycobacterium tuberculosis (M.tb) infection manifests into tuberculosis (TB) in a small fraction of the infected population that comprises the TB susceptible group. Identifying the factors potentiating susceptibility to TB persistence is one of the prime agenda of TB control programs. Recently, WHO recognized diabetes as a risk factor for TB disease progression. The closely related pathological state of metabolic imbalance, dyslipidemia, is yet another emerging risk factor involving deregulation in host immune responses. While high cholesterol levels are clinically proven condition for perturbations in cardiac health, a significant fraction of population these days suffer from borderline risk cholesterol profiles. This apparently healthy population is susceptible to various health risks placing them in the "pre-disease" range. Our study focuses on determining the role of such asymptomatic dyslipidemia as a potential risk factor for susceptibility to TB persistence. Macrophages exposed to sub-pathological levels of cholesterol for chronic period, besides impaired release of TNF-α, could not clear intracellular pathogenic mycobacteria effectively as compared to the unexposed cells. These cells also allowed persistence of opportunistic mycobacterial infection by M. avium and M. bovis BCG, indicating highly compromised immune response. The cholesterol-treated macrophages developed a foamy phenotype with a significant increase in intracellular lipid-bodies prior to M.tb infection, potentially contributing to pre-disease state for tuberculosis infection. The foamy phenotype, known to support M.tb infection, increased several fold upon infection in these cells. Additionally, mitochondrial morphology and function were perturbed, more so during infection in cholesterol treated cells. Pharmacological supplementation with small molecule M1 that restored mitochondrial structural and functional integrity limited M.tb survival more effectively in cholesterol exposed macrophages. Mechanistically, M1 molecule promoted clearance of mycobacteria by reducing total cellular lipid content and restoring mitochondrial morphology and function to its steady state. We further supported our observations by infection assays in PBMC-derived macrophages from clinically healthy volunteers with borderline risk cholesterol profiles. With these observations, we propose that prolonged exposure to sub-pathological cholesterol can lead to asymptomatic susceptibility to M.tb persistence. Use of small molecules like M1 sets yet another strategy for host-directed therapy where re-functioning of mitochondria in cholesterol abused macrophages can improve M.tb clearance.

Mitochondrial reactive oxygen species and inflammation: Molecular mechanisms, diseases and promising therapies

Over the last few decades, many different groups have been engaged in studies of new roles for mitochondria, particularly the coupling of alterations in the redox pathway with the inflammatory responses involved in different diseases, including Alzheimer's disease, Parkinson's disease, atherosclerosis, cerebral cavernous malformations, cystic fibrosis and cancer. Mitochondrial dysfunction is important in these pathological conditions, suggesting a pivotal role for mitochondria in the coordination of pro-inflammatory signaling from the cytosol and signaling from other subcellular organelles. In this regard, mitochondrial reactive oxygen species are emerging as perfect liaisons that can trigger the assembly and successive activation of large caspase-1- activating complexes known as inflammasomes. This review offers a glimpse into the mechanisms by which inflammasomes are activated by mitochondrial mechanisms, including reactive oxygen species production and mitochondrial Ca²⁺ uptake, and the roles they can play in several inflammatory pathologies.

Killing Me Softly: Connotations to Unfolded Protein Response and Oxidative Stress in Alzheimer's Disease

This review is focused on the possible causes of mitochondrial dysfunction in AD, underlying molecular mechanisms of this malfunction, possible causes and known consequences of APP, Aβ, and hyperphosphorylated tau presence in mitochondria, and the contribution of altered lipid metabolism (nonsterol isoprenoids) to pathological processes leading to increased formation and accumulation of the aforementioned hallmarks of AD. Abnormal protein folding and unfolded protein response seem to be the outcomes of impaired glycosylation due to metabolic disturbances in geranylgeraniol intermediary metabolism. The origin and consecutive fate of APP, Aβ, and tau are emphasized on intracellular trafficking apparently influenced by inaccurate posttranslational modifications. We hypothesize that incorrect intracellular processing of APP determines protein translocation to mitochondria in AD. Similarly, without obvious reasons, the passage of Aβ and tau to mitochondria is observed. APP targeted to mitochondria blocks the activity of protein translocase complex resulting in poor import of proteins central to oxidative phosphorylation. Besides, APP, Aβ, and neurofibrillary tangles of tau directly or indirectly impair mitochondrial biochemistry and bioenergetics, with concomitant generation of oxidative/nitrosative stress. Limited protective mechanisms are inadequate to prevent the free radical-mediated lesions. Finally, neuronal loss is observed in AD-affected brains typically by pathologic apoptosis.

Upregulation of the mitochondrial Lon Protease allows adaptation to acute oxidative stress but dysregulation is associated with chronic stress, disease, and aging

The elimination of oxidatively modified proteins is a crucial process in maintaining cellular homeostasis, especially during stress. Mitochondria are protein-dense, high traffic compartments, whose polypeptides are constantly exposed to superoxide, hydrogen peroxide, and other reactive species, generated by 'electron leakage' from the respiratory chain. The level of oxidative stress to mitochondrial proteins is not constant, but instead varies greatly with numerous metabolic and environmental factors. Oxidized mitochondrial proteins must be removed rapidly (by proteolytic degradation) or they will aggregate, cross-link, and cause toxicity. The Lon Protease is a key enzyme in the degradation of oxidized proteins within the mitochondrial matrix. Under conditions of acute stress Lon is highly inducible, possibly with the oxidant acting as the signal inducer, thereby providing increased protection. It seems that under chronic stress conditions, however, Lon levels actually decline. Lon levels also decline with age and with senescence, and senescent cells even lose the ability to induce Lon during acute stress. We propose that the regulation of Lon is biphasic, in that it is up-regulated during transient stress and down-regulated during chronic stress and aging, and we suggest that the loss of Lon responsiveness may be a significant factor in aging, and in age-related diseases.

Mechanistic perspective of mitochondrial fusion: tubulation vs. fragmentation

Mitochondrial fusion is a fundamental process driven by dynamin related GTPase proteins (DRPs), in contrast to the general SNARE-dependence of most cellular fusion events. The DRPs Mfn1/Mfn2/Fzo1 and OPA1/Mgm1 are the key effectors for fusion of the mitochondrial outer and inner membranes, respectively. In order to promote fusion, these two DRPs require post-translational modifications and proteolysis. OPA1/Mgm1 undergoes partial proteolytic processing, which results in a combination between short and long isoforms. In turn, ubiquitylation of mitofusins, after oligomerization and GTP hydrolysis, promotes and positively regulates mitochondrial fusion. In contrast, under conditions of mitochondrial dysfunction, negative regulation by proteolysis on these DRPs results in mitochondrial fragmentation. This occurs by complete processing of OPA1 and via ubiquitylation and degradation of mitofusins. Mitochondrial fragmentation contributes to the elimination of damaged mitochondria by mitophagy, and may play a protective role against Parkinson's disease. Moreover, a link of Mfn2 to Alzheimer's disease is emerging and mutations in Mfn2 or OPA1 cause Charcot-Marie-Tooth type 2A neuropathy or autosomal-dominant optic atrophy. Here, we summarize our current understanding on the molecular mechanisms promoting or inhibiting fusion of mitochondrial membranes, which is essential for cellular survival and disease control. This article is part of a Special Issue entitled: Mitochondrial dynamics and physiology.

Rhomboid proteases in plants - still in square one?

Rhomboids are ubiquitous intramembrane serine proteases the sequences of which are found in nearly all sequenced genomes, including those of plants. They were molecularly characterized in a number of organisms, and were found to play a role in a variety of biological functions including signaling, development, apoptosis, mitochondrial integrity, parasite invasion and more. Although rhomboid sequences are found in plants, very little is known about their function. Here, we present the current knowledge in the rhomboids field in general, and in plant rhomboids in particular. In addition, we discuss possible physiological roles of different plant rhomboids.

Apoptosis-inducing factor: structure, function, and redox regulation

Apoptosis-inducing factor (AIF) is a flavin adenine dinucleotide-containing, NADH-dependent oxidoreductase residing in the mitochondrial intermembrane space whose specific enzymatic activity remains unknown. Upon an apoptotic insult, AIF undergoes proteolysis and translocates to the nucleus, where it triggers chromatin condensation and large-scale DNA degradation in a caspase-independent manner. Besides playing a key role in execution of caspase-independent cell death, AIF has emerged as a protein critical for cell survival. Analysis of in vivo phenotypes associated with AIF deficiency and defects, and identification of its mitochondrial, cytoplasmic, and nuclear partners revealed the complexity and multilevel regulation of AIF-mediated signal transduction and suggested an important role of AIF in the maintenance of mitochondrial morphology and energy metabolism. The redox activity of AIF is essential for optimal oxidative phosphorylation. Additionally, the protein is proposed to regulate the respiratory chain indirectly, through assembly and/or stabilization of complexes I and III. This review discusses accumulated data with respect to the AIF structure and outlines evidence that supports the prevalent mechanistic view on the apoptogenic actions of the flavoprotein, as well as the emerging concept of AIF as a redox sensor capable of linking NAD(H)-dependent metabolic pathways to apoptosis.

[Mitochondrial morphology and dynamics: actors, mechanisms and functions]

Mitochondria are dynamic organelles that continuously move, fuse and divide. Their overall morphology, ranging from a filamentous network to a collection of isolated dots, is determined by fusion-fission equilibrium, which depends on the cellular and physiological context. The machineries of fusion and fission, that are conserved throughout evolution, include three large GTPases of the dynamin-superfamily: Dnm1/DRP1 - involved in fission - as well as Fzo1/MFN and Mgm1/OPA1 - required for fusion. While the activities, mecanisms and regulations of mitochondrial fusion and fission machineries continue to be unravelled, the relevance of mitochondrial dynamics is witnessed by their impact on organelle functions, cell survival and cell differenciation, their requirement for embryonic development and their involvement in neurological diseases.

Abnormalities in mitochondrial structure in cells from patients with bipolar disorder

Postmortem, genetic, brain imaging, and peripheral cell studies all support decreased mitochondrial activity as a factor in the manifestation of Bipolar Disorder (BD). Because abnormal mitochondrial morphology is often linked to altered energy metabolism, we investigated whether changes in mitochondrial structure were present in brain and peripheral cells of patients with BD. Mitochondria from patients with BD exhibited size and distributional abnormalities compared with psychiatrically-healthy age-matched controls. Specifically, in brain, individual mitochondria profiles had significantly smaller areas, on average, in BD samples (P = 0.03). In peripheral cells, mitochondria in BD samples were concentrated proportionately more within the perinuclear region than in distal processes (P = 0.0008). These mitochondrial changes did not appear to be correlated with exposure to lithium. Also, these abnormalities in brain and peripheral cells were independent of substantial changes in the actin or tubulin cytoskeleton with which mitochondria interact. The observed changes in mitochondrial size and distribution may be linked to energy deficits and, therefore, may have consequences for cell plasticity, resilience, and survival in patients with BD, especially in brain, which has a high-energy requirement. The findings may have implications for diagnosis, if they are specific to BD, and for treatment, if they provide clues as to the underlying pathophysiology of BD.

Mitochondria: joining forces to thwart cell death

Mitochondria are highly dynamic organelles that undergo constant cycles of fusion and fission. An additional level of regulation of mitochondrial function, which is particularly important in neurons, is their active transport along microtubules. Recent evidence suggests that the mitochondrial fusion/fission machinery as well as the molecular motors responsible for their movement constitute powerful regulatory control points that directly impact metabolism and regulation of cell death. This is true for not only apoptosis, but also for excitotoxicity where calcium overload is a major component of the cell death process. In this review, we will describe the molecular mechanisms regulating fusion and fission and how this impinges on cell survival in the context of acute neuronal injury.

Mitochondrial accumulation of APP and Abeta: significance for Alzheimer disease pathogenesis

Accumulating evidence suggest that alterations in energy metabolism are among the earliest events that occur in the Alzheimer disease (AD) affected brain. Energy consumption is drastically decreased in the AD-affected regions of cerebral cortex and hippocampus pointing towards compromised mitochondrial function of neurons within specific brain regions. This is accompanied by an elevated production of reactive oxygen species contributing to increased rates of neuronal loss in the AD-affected brain regions. In this review, we will discuss the role of mitochondrial function and dysfunction in AD. We will focus on the consequences of amyloid precursor protein and amyloid-β peptide accumulation in mitochondria and their involvement in AD pathogenesis.

Rhomboids: 7 years of a new protease family

Drosophila Rhomboid-1 was discovered to be the first known intramembrane serine protease about 7 years ago. The study of the rhomboid-like family has since blossomed, and the purpose of this review is to take stock of where the field is, and how it may progress in the next few years. Three major themes are the increasing understanding of the biological roles of rhomboids, the detailed information we now have about their function and mechanism, and the promising leads they offer as medical targets.

Multiple pathways influence mitochondrial inheritance in budding yeast

Yeast mitochondria form a branched tubular network. Mitochondrial inheritance is tightly coupled with bud emergence, ensuring that daughter cells receive mitochondria from mother cells during division. Proteins reported to influence mitochondrial inheritance include the mitochondrial rho (Miro) GTPase Gem1p, Mmr1p, and Ypt11p. A synthetic genetic array (SGA) screen revealed interactions between gem1Delta and deletions of genes that affect mitochondrial function or inheritance, including mmr1Delta. Synthetic sickness of gem1Delta mmr1Delta double mutants correlated with defective mitochondrial inheritance by large buds. Additional studies demonstrated that GEM1, MMR1, and YPT11 each contribute to mitochondrial inheritance. Mitochondrial accumulation in buds caused by overexpression of either Mmr1p or Ypt11p did not depend on Gem1p, indicating these three proteins function independently. Physical linkage of mitochondria with the endoplasmic reticulum (ER) has led to speculation that distribution of these two organelles is coordinated. We show that yeast mitochondrial inheritance is not required for inheritance or spreading of cortical ER in the bud. Moreover, Ypt11p overexpression, but not Mmr1p overexpression, caused ER accumulation in the bud, revealing a potential role for Ypt11p in ER distribution. This study demonstrates that multiple pathways influence mitochondrial inheritance in yeast and that Miro GTPases have conserved roles in mitochondrial distribution.

Towards the control of intracellular protein turnover: mitochondrial Lon protease inhibitors versus proteasome inhibitors

Cellular protein homeostasis results from the combination of protein biogenesis processes and protein quality control mechanisms, which contribute to the functional state of cells under normal and stress conditions. Proteolysis constitutes the final step by which short-lived, misfolded and damaged intracellular proteins are eliminated. Protein turnover and oxidatively modified protein degradation are mainly achieved by the proteasome in the cytosol and nucleus of eukaryotic cells while several ATP-dependent proteases including the matrix protease Lon take part in the mitochondrial protein degradation. Moreover, Lon protease seems to play a major role in the elimination of oxidatively modified proteins in the mitochondrial matrix. Specific inhibitors are commonly used to assess cellular functions of proteolytic systems as well as to identify their protein substrates. Here, we present and discuss known proteasome and Lon protease inhibitors. To date, very few inhibitors of Lon have been described and no specific inhibitors of this protease are available. The current knowledge on both catalytic mechanisms and inhibitors of these two proteases is first described and attempts to define specific non-peptidic inhibitors of the human Lon protease are presented.

Regulation of mitochondrial fusion and division

In many organisms, ranging from yeast to humans, mitochondria fuse and divide to change their morphology in response to a multitude of signals. During the past decade, work using yeast and mammalian cells has identified much of the machinery required for fusion and division, including the dynamin-related GTPases--mitofusins (Fzo1p in yeast) and OPA1 (Mgm1p in yeast) for fusion and Drp1 (Dnm1p) for division. However, the mechanisms by which cells regulate these dynamic processes have remained largely unknown. Recent studies have uncovered regulatory mechanisms that control the activity, assembly, distribution and stability of the key components for mitochondrial fusion and division. In this review, we discuss how mitochondrial dynamics are controlled and how these events are coordinated with cell growth, mitosis, apoptosis and human diseases.

Ultra-rapid cardiotoxicity of the hepatitis C virus protease inhibitor BILN 2061 in the urokinase-type plasminogen activator mouse

BACKGROUND & AIMS

Because current therapies for chronic hepatitis C virus (HCV) infections are suboptimal and associated with severe side effects, novel treatment options are needed. A small animal model has recently been developed to study HCV infections. To examine the usefulness of this human liver-urokinase-type plasminogen activator (uPA)(+/+) severe combined immune deficient (SCID) mouse for the development of HCV-targeted drugs, we evaluated the antiviral efficacy and safety of an HCV NS3-protease inhibitor, BILN 2061.

METHODS

BILN 2061 was orally administered at clinical range doses for 4 days to SCID mice that differed in the presence of HCV infection, human hepatocyte grafts, and uPA zygosity. Treatment outcome was evaluated clinically, virologically, and morphologically. Using standard high-performance liquid chromatography-ultraviolet (HPLC-UV) methods and mass spectrometry, single-dose pharmacokinetics and multiple-dose drug exposures were analyzed. The (13)C-aminopyrine breath test was applied to compare in vivo liver function.

RESULTS

A 4-day treatment with BILN 2061 of HCV genotype-1b infected chimeric animals reduced the viral load by >100-fold, but concomitant clinical and ultrastructural signs of cardiotoxicity appeared. BILN 2061 administration to uPA-transgenic mice induced mitochondrial swelling with aberrant cristae in cardiomyocytes, but not in skeletal muscle. Because both drug accumulation and liver function were identical in affected uPA-transgenic and nontransgenic SCID mice without cardiac involvement, the urokinase plasminogen activator transgene itself appears to be implicated.

CONCLUSIONS

The human liver-uPA(+/+)SCID mouse is an interesting small animal model to evaluate the preclinical safety and efficacy of new antiviral compounds against HCV. The uPA-transgene increases the susceptibility of mice to BILN 2061-induced cardiotoxicity.

Protein degradation within mitochondria: versatile activities of AAA proteases and other peptidases

Cell survival depends on essential processes in mitochondria. Various proteases within these organelles regulate mitochondrial biogenesis and ensure the complete degradation of excess or damaged proteins. Many of these proteases are highly conserved and ubiquitous in eukaryotic cells. They can be assigned to three functional classes: processing peptidases, which cleave off mitochondrial targeting sequences of nuclearly encoded proteins and process mitochondrial proteins with regulatory functions; ATP-dependent proteases, which either act as processing peptidases with regulatory functions or as quality-control enzymes degrading non-native polypeptides to peptides; and oligopeptidases, which degrade these peptides and mitochondrial targeting sequences to amino acids. Disturbances of protein degradation within mitochondria cause severe phenotypes in various organisms and can lead to the induction of apoptotic programmes and cell-specific neurodegeneration in mammals. After an overview of the proteolytic system of mitochondria, we will focus on versatile functions of ATP-dependent AAA proteases in the inner membrane. These conserved proteolytic machines conduct protein quality surveillance of mitochondrial inner membrane proteins, mediate vectorial protein dislocation from membranes, and, acting as processing enzymes, control ribosome assembly, mitochondrial protein synthesis, and mitochondrial fusion. Implications of these functions for cell-specific axonal degeneration in hereditary spastic paraplegia will be discussed.

Regulation of various proteolytic pathways by insulin and amino acids in human fibroblasts

Intracellular protein degradation is a regulated process with several proteolytic pathways. Although regulation of macroautophagy has been investigated in some detail in hepatocytes and in few other cells, less is known on this regulation in other cells and proteolytic pathways. We show that in human fibroblasts insulin and amino acids reduce protein degradation by different signalling pathways and that this inhibition proceeds in part via the mammalian target of rapamycin, especially with amino acids, which probably increase lysosomal pH. Moreover, the regulatory amino acids (Phe, Arg, Met, Tyr, Trp and Cys) are partially different from other cells. Finally, and in addition to macroautophagy, insulin and amino acids modify, to different extents and sometimes in opposite directions, the activities of other proteolytic pathways.