Import, maturation, and function of SOD1 and its copper chaperone CCS in the mitochondrial intermembrane space

A Proposed Mechanism for Neurodegeneration in Movement Disorders Characterized by Metal Dyshomeostasis and Oxidative Stress

Shared molecular pathologies between distinct neurodegenerative disorders offer unique opportunities to identify common mechanisms of neuron death, and apply lessons learned from one disease to another. Neurotoxic superoxide dismutase 1 (SOD1) proteinopathy in SOD1-associated familial amyotrophic lateral sclerosis (fALS) is recapitulated in idiopathic Parkinson disease (PD), suggesting that these two phenotypically distinct disorders share an etiological pathway, and tractable therapeutic target(s). Despite 25 years of research, the molecular determinants underlying SOD1 misfolding and toxicity in fALS remain poorly understood. The absence of SOD1 mutations in PD highlights mounting evidence that SOD1 mutations are not the sole cause of SOD1 protein misfolding occasioning oligomerization and toxicity, reinforcing the importance of non-genetic factors, including protein metallation and post-translational modification in determining SOD1 stability and function. We propose that these non-genetic factors underlie the misfolding and dysfunction of SOD1 and other proteins in both PD and fALS, constituting a shared and tractable pathway to neurodegeneration.

Protein Translocation into the Intermembrane Space and Matrix of Mitochondria: Mechanisms and Driving Forces

Mitochondria contain two aqueous subcompartments, the matrix and the intermembrane space (IMS). The matrix is enclosed by both the inner and outer mitochondrial membranes, whilst the IMS is sandwiched between the two. Proteins of the matrix are synthesized in the cytosol as preproteins, which contain amino-terminal matrix targeting sequences that mediate their translocation through translocases embedded in the outer and inner membrane. For these proteins, the translocation reaction is driven by the import motor which is part of the inner membrane translocase. The import motor employs matrix Hsp70 molecules and ATP hydrolysis to ratchet proteins into the mitochondrial matrix. Most IMS proteins lack presequences and instead utilize the IMS receptor Mia40, which facilitates their translocation across the outer membrane in a reaction that is coupled to the formation of disulfide bonds within the protein. This process requires neither ATP nor the mitochondrial membrane potential. Mia40 fulfills two roles: First, it acts as a holdase, which is crucial in the import of IMS proteins and second, it functions as a foldase, introducing disulfide bonds into newly imported proteins, which induces and stabilizes their natively folded state. For several Mia40 substrates, oxidative folding is an essential prerequisite for their assembly into oligomeric complexes. Interestingly, recent studies have shown that the two functions of Mia40 can be experimentally separated from each other by the use of specific mutants, hence providing a powerful new way to dissect the different physiological roles of Mia40. In this review we summarize the current knowledge relating to the mitochondrial matrix-targeting and the IMS-targeting/Mia40 pathway. Moreover, we discuss the mechanistic properties by which the mitochondrial import motor on the one hand and Mia40 on the other, drive the translocation of their substrates into the organelle. We propose that the lateral diffusion of Mia40 in the inner membrane and the oxidation-mediated folding of incoming polypeptides supports IMS import.

The mitochondrion: a central architect of copper homeostasis

All known eukaryotes require copper for their development and survival. The essentiality of copper reflects its widespread use as a co-factor in conserved enzymes that catalyze biochemical reactions critical to energy production, free radical detoxification, collagen deposition, neurotransmitter biosynthesis and iron homeostasis. However, the prioritized use of copper poses an organism with a considerable challenge because, in its unbound form, copper can potentiate free radical production and displace iron-sulphur clusters to disrupt protein function. Protective mechanisms therefore evolved to mitigate this challenge and tightly regulate the acquisition, trafficking and storage of copper such that the metal ion is rarely found in its free form in the cell. Findings by a number of groups over the last ten years emphasize that this regulatory framework forms the foundation of a system that is capable of monitoring copper status and reprioritizing copper usage at both the cellular and systemic levels of organization. While the identification of relevant molecular mechanisms and signaling pathways has proven to be difficult and remains a barrier to our full understanding of the regulation of copper homeostasis, mounting evidence points to the mitochondrion as a pivotal hub in this regard in both healthy and diseased states. Here, we review our current understanding of copper handling pathways contained within the organelle and consider plausible mechanisms that may serve to functionally couple their activity to that of other cellular copper handling machinery to maintain copper homeostasis.

Acylation of Superoxide Dismutase 1 (SOD1) at K122 Governs SOD1-Mediated Inhibition of Mitochondrial Respiration

In this study, we employed proteomics to identify mechanisms of posttranslational regulation on cell survival signaling proteins. We focused on Cu-Zn superoxide dismutase (SOD1), which protects cells from oxidative stress. We found that acylation of K122 on SOD1, while not impacting SOD1 catalytic activity, suppressed the ability of SOD1 to inhibit mitochondrial metabolism at respiratory complex I. We found that deacylase depletion increased K122 acylation on SOD1, which blocked the suppression of respiration in a K122-dependent manner. In addition, we found that acyl-mimicking mutations at K122 decreased SOD1 accumulation in mitochondria, initially hinting that SOD1 may inhibit respiration directly within the intermembrane space (IMS). However, surprisingly, we found that forcing the K122 acyl mutants into the mitochondria with an IMS-targeting tag did not recover their ability to suppress respiration. Moreover, we found that suppressing or boosting respiration levels toggled SOD1 in or out of the mitochondria, respectively. These findings place SOD1-mediated inhibition of respiration upstream of its mitochondrial localization. Lastly, deletion-rescue experiments show that a respiration-defective mutant of SOD1 is also impaired in its ability to rescue cells from toxicity caused by SOD1 deletion. Together, these data suggest a previously unknown interplay between SOD1 acylation, metabolic regulation, and SOD1-mediated cell survival.

Mechanisms of iron and copper-frataxin interactions

Frataxin is a mitochondrial protein whose deficiency is the cause of Friedreich's ataxia, a hereditary neurodegenerative disease. This protein plays a role in iron-sulfur cluster biosynthesis, protection against oxidative stress and iron metabolism. In an attempt to provide a better understanding of the role played by metals in its metabolic functions, the mechanisms of mitochondrial metal binding to frataxin in vitro have been investigated. A purified recombinant yeast frataxin homolog Yfh1 binds two Cu(ii) ions with a Kd1(CuII) of 1.3 × 10-7 M and a Kd2(CuII) of 3.1 × 10-4 M and a single Cu(i) ion with a higher affinity than for Cu(ii) (Kd(CuI) = 3.2 × 10-8 M). Mn(ii) forms two complexes with Yfh1 (Kd1(MnII) = 4.0 × 10-8 M; Kd2(MnII) = 4.0 × 10-7 M). Cu and Mn bind Yfh1 with higher affinities than Fe(ii). It is established for the first time that the mechanisms of the interaction of iron and copper with frataxin are comparable and involve three kinetic steps. The first step occurs in the 50-500 ms range and corresponds to a first metal uptake. This is followed by two other kinetic processes that are related to a second metal uptake and/or to a change in the conformation leading to thermodynamic equilibrium. Frataxin deficient Δyfh1 yeast cells exhibited a marked growth defect in the presence of exogenous Cu or Mn. Mitochondria from Δyfh1 strains also accumulated higher amounts of copper, suggesting a functional role of frataxin in vivo in copper homeostasis.

The role of attenuated redox and heat shock protein responses in the age-related decline in skeletal muscle mass and function

The loss of muscle mass and weakness that accompanies ageing is a major contributor to physical frailty and loss of independence in older people. A failure of muscle to adapt to physiological stresses such as exercise is seen with ageing and disruption of redox regulated processes and stress responses are recognized to play important roles in theses deficits. The role of redox regulation in control of specific stress responses, including the generation of heat shock proteins (HSPs) by muscle appears to be particularly important and affected by ageing. Transgenic and knockout studies in experimental models in which redox and HSP responses were modified have demonstrated the importance of these processes in maintenance of muscle mass and function during ageing. New data also indicate the potential of these processes to interact with and influence ageing in other tissues. In particular the roles of redox signalling and HSPs in regulation of inflammatory pathways appears important in their impact on organismal ageing. This review will briefly indicate the importance of this area and demonstrate how an understanding of the manner in which redox and stress responses interact and how they may be controlled offers considerable promise as an approach to ameliorate the major functional consequences of ageing of skeletal muscle (and potentially other tissues) in man.

Sex specific activation of the ERα axis of the mitochondrial UPR (UPRmt) in the G93A-SOD1 mouse model of familial ALS

The mitochondrial unfolded protein response (UPRmt) is a transcriptional program aimed at restoring proteostasis in mitochondria. Upregulation of mitochondrial matrix proteases and heat shock proteins was initially described. Soon thereafter, a distinct UPRmt induced by misfolded proteins in the mitochondrial intermembrane space (IMS) and mediated by the estrogen receptor alpha (ERα), was found to upregulate the proteasome and the IMS protease OMI. However, the IMS-UPRmt was never studied in a neurodegenerative disease in vivo. Thus, we investigated the IMS-UPRmt in the G93A-SOD1 mouse model of familial ALS, since mutant SOD1 is known to accumulate in the IMS of neural tissue and cause mitochondrial dysfunction. As the ERα is most active in females, we postulated that a differential involvement of the IMS-UPRmt could be linked to the longer lifespan of females in the G93A-SOD1 mouse. We found a significant sex difference in the IMS-UPRmt, because the spinal cords of female, but not male, G93A-SOD1 mice showed elevation of OMI and proteasome activity. Then, using a mouse in which G93A-SOD1 was selectively targeted to the IMS, we demonstrated that the IMS-UPRmt could be specifically initiated by mutant SOD1 localized in the IMS. Furthermore, we showed that, in the absence of ERα, G93A-SOD1 failed to activate OMI and the proteasome, confirming the ERα dependence of the response. Taken together, these results demonstrate the IMS-UPRmt activation in SOD1 familial ALS, and suggest that sex differences in the disease phenotype could be linked to differential activation of the ERα axis of the IMS-UPRmt.

Activation of Mitochondrial Protein Phosphatase SLP2 by MIA40 Regulates Seed Germination

Reversible protein phosphorylation catalyzed by protein kinases and phosphatases represents the most prolific and well-characterized posttranslational modification known. Here, we demonstrate that Arabidopsis (Arabidopsis thaliana) Shewanella-like protein phosphatase 2 (AtSLP2) is a bona fide Ser/Thr protein phosphatase that is targeted to the mitochondrial intermembrane space (IMS) where it interacts with the mitochondrial oxidoreductase import and assembly protein 40 (AtMIA40), forming a protein complex. Interaction with AtMIA40 is necessary for the phosphatase activity of AtSLP2 and is dependent on the formation of disulfide bridges on AtSLP2. Furthermore, by utilizing atslp2 null mutant, AtSLP2 complemented and AtSLP2 overexpressing plants, we identify a function for the AtSLP2-AtMIA40 complex in negatively regulating gibberellic acid-related processes during seed germination. Results presented here characterize a mitochondrial IMS-localized protein phosphatase identified in photosynthetic eukaryotes as well as a protein phosphatase target of the highly conserved eukaryotic MIA40 IMS oxidoreductase.

Cysteine Modifications in the Pathogenesis of ALS

Several proteins are found misfolded and aggregated in sporadic and genetic forms of amyotrophic lateral sclerosis (ALS). These include superoxide dismutase (SOD1), transactive response DNA-binding protein (TDP-43), fused in sarcoma/translocated in liposarcoma protein (FUS/TLS), p62, vasolin-containing protein (VCP), Ubiquilin-2 and dipeptide repeats produced by unconventional RAN-translation of the GGGGCC expansion in C9ORF72. Up to date, functional studies have not yet revealed a common mechanism for the formation of such diverse protein inclusions. Consolidated studies have demonstrated a fundamental role of cysteine residues in the aggregation process of SOD1 and TDP43, but disturbance of protein thiols homeostatic factors such as protein disulfide isomerases (PDI), glutathione, cysteine oxidation or palmitoylation might contribute to a general aberration of cysteine residues proteostasis in ALS. In this article we review the evidence that cysteine modifications may have a central role in many, if not all, forms of this disease.

An Adaptation to Low Copper in Candida albicans Involving SOD Enzymes and the Alternative Oxidase

In eukaryotes, the Cu/Zn superoxide dismutase (SOD1) is a major cytosolic cuproprotein with a small fraction residing in the mitochondrial intermembrane space (IMS) to protect against respiratory superoxide. Curiously, the opportunistic human fungal pathogen Candida albicans is predicted to express two cytosolic SODs including Cu/Zn containing SOD1 and manganese containing SOD3. As part of a copper starvation response, C. albicans represses SOD1 and induces the non-copper alternative SOD3. While both SOD1 and SOD3 are predicted to exist in the same cytosolic compartment, their potential role in mitochondrial oxidative stress had yet to be investigated. We show here that under copper replete conditions, a fraction of the Cu/Zn containing SOD1 localizes to the mitochondrial IMS to guard against mitochondrial superoxide. However in copper starved cells, localization of the manganese containing SOD3 is restricted to the cytosol leaving the mitochondrial IMS devoid of SOD. We observe that during copper starvation, an alternative oxidase (AOX) form of respiration is induced that is not coupled to ATP synthesis but maintains mitochondrial superoxide at low levels even in the absence of IMS SOD. Surprisingly, the copper-dependent cytochrome c oxidase (COX) form of respiration remains high with copper starvation. We provide evidence that repression of SOD1 during copper limitation serves to spare copper for COX and maintain COX respiration. Overall, the complex copper starvation response of C. albicans involving SOD1, SOD3 and AOX minimizes mitochondrial oxidative damage whilst maximizing COX respiration essential for fungal pathogenesis.

Role of reactive oxygen species in age-related neuromuscular deficits

Although it is now clear that reactive oxygen species (ROS) are not the key determinants of longevity, a number of studies have highlighted the key role that these species play in age-related diseases and more generally in determining individual health span. Age-related loss of skeletal muscle mass and function is a key contributor to physical frailty in older individuals and our current understanding of the key areas in which ROS contribute to age-related deficits in muscle is through defective redox signalling and key roles in maintenance of neuromuscular integrity. This topical review will describe how ROS stimulate adaptations to contractile activity in muscle that include up-regulation of short-term stress responses, an increase in mitochondrial biogenesis and an increase in some catabolic processes. These adaptations occur through stimulation of redox-regulated processes that lead to the activation of transcription factors such as NF-κB, AP-1 and HSF1 which mediate changes in gene expression. They are attenuated during ageing and this appears to occur through an age-related increase in mitochondrial hydrogen peroxide production. The potential for redox-mediated cross-talk between motor neurons and muscle is also described to illustrate how ROS released from muscle fibres during exercise may help maintain the integrity of axons and how the degenerative changes in neuromuscular structure that occur with ageing may contribute to mitochondrial ROS generation in skeletal muscle fibres.

Disulfiram anti-cancer efficacy without copper overload is enhanced by extracellular H2O2 generation: antagonism by tetrathiomolybdate

Highlights

Background

Cu/Zn superoxide dismutases (SODs) like the extracellular SOD3 and cytoplasmic SOD1 regulate cell proliferation by generating hydrogen peroxide (H₂O₂). This pro-oxidant inactivates essential cysteine residues in protein tyrosine phosphatases (PTP) helping receptor tyrosine kinase activation by growth factor signaling, and further promoting downstream MEK/ERK linked cell proliferation. Disulfiram (DSF), currently in clinical cancer trials is activated by copper chelation, being potentially capable of diminishing the copper dependent activation of MEK1/2 and SOD1/SOD3 and promoting reactive oxygen species (ROS) toxicity. However, copper (Cu) overload may occur when co-administered with DSF, resulting in toxicity and mutagenicity against normal tissue, through generation of the hydroxyl radical (•OH) by the Fenton reaction.

Purpose

To investigate: a) whether sub-toxic DSF efficacy can be increased without Cu overload against human melanoma cells with unequal BRAF(V600E) mutant status and Her2-overexpressing SKBR3 breast cancer cells, by increasing H₂O₂from exogenous SOD; b) to compare the anti-tumor efficacy of DSF with that of another clinically used copper chelator, tetrathiomolybdate (TTM)

Results

a) without copper supplementation, exogenous SOD potentiated sub-toxic DSF toxicity antagonized by sub-toxic TTM or by the anti-oxidant N-acetylcysteine; b) exogenous glucose oxidase, another H₂O₂ generator resembled exogenous SOD in potentiating sub-toxic DSF.

Conclusions

potentiation of sub-lethal DSF toxicity by extracellular H₂O₂ against the human tumor cell lines investigated, only requires basal Cu and increased ROS production, being unrelated to non-specific or TTM copper chelator sequestration.

Significance

These findings emphasize the relevance of extracellular H₂O₂ as a novel mechanism to improve disulfiram anticancer effects minimizing copper toxicity.

Proteomics of human mitochondria

Proteomics have passed through a tremendous development in the recent years by the development of ever more sensitive, fast and precise mass spectrometry methods. The dramatically increased research in the biology of mitochondria and their prominent involvement in all kinds of diseases and ageing has benefitted from mitochondrial proteomics. We here review substantial findings and progress of proteomic analyses of human cells and tissues in the recent past. One challenge for investigations of human samples is the ethically and medically founded limited access to human material. The increased sensitivity of mass spectrometry technology aids in lowering this hurdle and new approaches like generation of induced pluripotent cells from somatic cells allow to produce patient-specific cellular disease models with great potential. We describe which human sample types are accessible, review the status of the catalog of human mitochondrial proteins and discuss proteins with dual localization in mitochondria and other cellular compartments. We describe the status and developments of pertinent mass spectrometric strategies, and the use of databases and bioinformatics. Using selected illustrative examples, we draw a picture of the role of proteomic analyses for the many disease contexts from inherited disorders caused by mutation in mitochondrial proteins to complex diseases like cancer, type 2 diabetes and neurodegenerative diseases. Finally, we speculate on the future role of proteomics in research on human mitochondria and pinpoint fields where the evolving technologies will be exploited.

Alpha-synuclein-induced oxidative stress correlates with altered superoxide dismutase and glutathione synthesis in human neuroblastoma SH-SY5Y cells

Alpha-synuclein (α-syn) is a major component of Lewy bodies found in sporadic and inherited forms of Parkinson's disease (PD). Mutations in the gene encoding α-syn and duplications and triplications of wild-type (WT) α-syn have been associated with PD. Several mechanisms have been implicated in the degeneration of dopaminergic neurons in PD, including oxidative stress and mitochondrial dysfunction. Here we defined the occurrence of oxidative stress in SH-SY5Y cells overexpressing WT α-syn in a doxycycline (Dox) regulated manner, before and after exposure to iron (500 µM), and determined the changes in proteins involved in the intracellular antioxidant defense system. Data evidenced an increase in caspase-3 activation and diminished reducing capacity of -Dox cells, associated with decreased activity of mitochondria complex I and reduced mitochondrial transcription factor A (TFAM) levels in these cells. Furthermore, total and mitochondrial reactive oxygen species levels were higher under basal conditions in cells overexpressing α-syn (-Dox) and this increase was apparently correlated with diminished levels and activities of SOD1 and SOD2 in -Dox cells. Moreover, both reduced and oxidized glutathione levels were diminished in -Dox cells under basal conditions, concomitantly with decreased activity of GCL and reduced protein levels of GCLc. The effects caused by iron (500 µM) were mostly independent of α-syn expression and triggered different antioxidant responses to possibly counterbalance higher levels of free radicals. Overall, data suggest that overexpression of α-syn modifies the antioxidant capacity of SH-SY5Y cells due to altered activity and protein levels of SOD1 and SOD2, and decreased glutathione pool.

Reactive oxygen species in sarcopenia: Should we focus on excess oxidative damage or defective redox signalling?

Physical frailty in the elderly is driven by loss of muscle mass and function and hence preventing this is the key to reduction in age-related physical frailty. Our current understanding of the key areas in which ROS contribute to age-related deficits in muscle is through increased oxidative damage to cell constituents and/or through induction of defective redox signalling. Recent data have argued against a primary role for ROS as a regulator of longevity, but studies have persistently indicated that aspects of the aging phenotype and age-related disorders may be mediated by ROS. There is increasing interest in the effects of defective redox signalling in aging and some studies now indicate that this process may be important in reducing the integrity of the aging neuromuscular system. Understanding how redox-signalling pathways are altered by aging and the causes of the defective redox homeostasis seen in aging muscle provides opportunities to identify targeted interventions with the potential to slow or prevent age-related neuromuscular decline with a consequent improvement in quality of life for older people.

Behind the Link between Copper and Angiogenesis: Established Mechanisms and an Overview on the Role of Vascular Copper Transport Systems

Angiogenesis critically sustains the progression of both physiological and pathological processes. Copper behaves as an obligatory co-factor throughout the angiogenic signalling cascades, so much so that a deficiency causes neovascularization to abate. Moreover, the progress of several angiogenic pathologies (e.g. diabetes, cardiac hypertrophy and ischaemia) can be tracked by measuring serum copper levels, which are being increasingly investigated as a useful prognostic marker. Accordingly, the therapeutic modulation of body copper has been proven effective in rescuing the pathological angiogenic dysfunctions underlying several disease states. Vascular copper transport systems profoundly influence the activation and execution of angiogenesis, acting as multi-functional regulators of apparently discrete pro-angiogenic pathways. This review concerns the complex relationship among copper-dependent angiogenic factors, copper transporters and common pathological conditions, with an unusual accent on the multi-faceted involvement of the proteins handling vascular copper. Functions regulated by the major copper transport proteins (CTR1 importer, ATP7A efflux pump and metallo-chaperones) include the modulation of endothelial migration and vascular superoxide, known to activate angiogenesis within a narrow concentration range. The potential contribution of prion protein, a controversial regulator of copper homeostasis, is discussed, even though its angiogenic involvement seems to be mainly associated with the modulation of endothelial motility and permeability.

Voltage-Induced Misfolding of Zinc-Replete ALS Mutant Superoxide Dismutase-1

The monomerization of Cu, Zn superoxide dismutase (SOD1) is an early step along pathways of misfolding linked to amyotrophic lateral sclerosis (ALS). Monomerization requires the reversal of two post-translational modifications that are thermodynamically favorable: (i) dissociation of active-site metal ions and (ii) reduction of intramolecular disulfide bonds. This study found, using amide hydrogen/deuterium (H/D) exchange, capillary electrophoresis, and lysine-acetyl protein charge ladders, that ALS-linked A4V SOD1 rapidly monomerizes and partially unfolds in an external electric field (of physiological strength), without loss of metal ions, exposure to disulfide-reducing agents, or Joule heating. Voltage-induced monomerization was not observed for metal-free A4V SOD1, metal-free WT SOD1, or metal-loaded WT SOD1. Computational modeling suggested a mechanism for this counterintuitive effect: subunit macrodipoles of dimeric SOD1 are antiparallel and amplified 2-fold by metal coordination, which increases torque at the dimer interface as subunits rotate to align with the electric field.

The Features of Copper Metabolism in the Rat Liver during Development

Strong interest in copper homeostasis is due to the fact that copper is simultaneously a catalytic co-factor of the vital enzymes, a participant in signaling, and a toxic agent provoking oxidative stress. In mammals, during development copper metabolism is conformed to two types. In embryonic type copper metabolism (ETCM), newborns accumulate copper to high level in the liver because its excretion via bile is blocked; and serum copper concentration is low because ceruloplasmin (the main copper-containing protein of plasma) gene expression is repressed. In the late weaning, the ETCM switches to the adult type copper metabolism (ATCM), which is manifested by the unlocking of copper excretion and the induction of ceruloplasmin gene activity. The considerable progress has been made in the understanding of the molecular basis of copper metabolic turnover in the ATCM, but many aspects of the copper homeostasis in the ETCM remain unclear. The aim of this study was to investigate the copper metabolism during transition from the ETCM (up to 12-days-old) to the ATCM in the rats. It was shown that in the liver, copper was accumulated in the nuclei during the first 5 days of life, and then it was re-located to the mitochondria. In parallel with the mitochondria, copper bulk bound with cytosolic metallothionein was increased. All compartments of the liver cells rapidly lost most of their copper on the 13^th day of life. In newborns, serum copper concentration was low, and its major fraction was associated with holo-Cp, however, a small portion of copper was bound to extracellular metallothionein and a substance that was slowly eluted during gel-filtration. In adults, serum copper concentration increased by about a factor of 3, while metallothionein-bound copper level decreased by a factor of 2. During development, the expression level of Cp, Sod1, Cox4i1, Atp7b, Ctr1, Ctr2, Cox17, and Ccs genes was significantly increased, and metallothionein was decreased. Atp7a gene’s activity was fully repressed. The copper routes in newborns are discussed.

Reactive oxygen species and mitochondria: A nexus of cellular homeostasis

Reactive oxygen species (ROS) are integral components of multiple cellular pathways even though excessive or inappropriately localized ROS damage cells. ROS function as anti-microbial effector molecules and as signaling molecules that regulate such processes as NF-kB transcriptional activity, the production of DNA-based neutrophil extracellular traps (NETs), and autophagy. The main sources of cellular ROS are mitochondria and NADPH oxidases (NOXs). In contrast to NOX-generated ROS, ROS produced in the mitochondria (mtROS) were initially considered to be unwanted by-products of oxidative metabolism. Increasing evidence indicates that mtROS have been incorporated into signaling pathways including those regulating immune responses and autophagy. As metabolic hubs, mitochondria facilitate crosstalk between the metabolic state of the cell with these pathways. Mitochondria and ROS are thus a nexus of multiple pathways that determine the response of cells to disruptions in cellular homeostasis such as infection, sterile damage, and metabolic imbalance. In this review, we discuss the roles of mitochondria in the generation of ROS-derived anti-microbial effectors, the interplay of mitochondria and ROS with autophagy and the formation of DNA extracellular traps, and activation of the NLRP3 inflammasome by ROS and mitochondria.

SOD1 misplacing and mitochondrial dysfunction in amyotrophic lateral sclerosis pathogenesis

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease presenting as sporadic (sALS) or familial (fALS) forms. Even if the list of the genes underlining ALS greatly expanded, defects in superoxide dismutase 1 (SOD1), encoding the copper/zinc SOD1, still remain a major cause of fALS and are likely involved also in apparently sporadic presentations. The pathogenesis of ALS is still unknown, but several lines of evidence indicate that the mitochondrial accumulation of mutant SOD1 is an important mechanism of mitochondrial dysfunction, leading to motor neuron pathology and death. The intramitochondrial localization of mutant SOD1 is debated. Mutant SOD1 might accumulate inside the intermembrane space (IMS), overriding the physiological retention regulated by the copper chaperone for superoxide dismutase (CCS). On the other hand, misfolded SOD1 might deposit onto the outer mitochondrial membrane (OMM), clumping the transport across mitochondrial membranes and engaging mitochondrial-dependent cell apoptosis. The elucidation of the mechanisms ruling SOD1 localization and misplacing might shed light on peculiar ALS features such as cell selectivity and late onset. More importantly, these studies might disclose novel targets for therapeutic intervention in familial ALS as well as non-genetic forms. Finally, pharmacological or genetic manipulation aimed to prevent or counteract the intracellular shifting of mutant SOD1 could be effective for other neurodegenerative disorders featuring the toxic accumulation of misfolded proteins.

Redox regulation of muscle adaptations to contractile activity and aging

Superoxide and nitric oxide are generated by skeletal muscle, and these species are increased by contractile activity. Mitochondria have long been assumed to play the primary role in generation of superoxide in muscle, but recent studies indicate that, during contractile activity, membrane-localized NADPH oxidase(s) rapidly generate(s) superoxide that plays a role in redox signaling. This process is important in upregulation of rapid and specific cytoprotective responses that aid maintenance of cell viability following contractile activity, but the overall extent to which redox signaling contributes to regulation of muscle metabolism and homeostasis following contractile activity is currently unclear, as is identification of key redox-sensitive protein targets involved in these processes. Reactive oxygen and nitrogen species have also been implicated in the loss of muscle mass and function that occurs with aging, although recent work has questioned whether oxidative damage plays a key role in these processes. A failure of redox signaling occurs in muscle during aging and may contribute to the age-related loss of muscle fibers. Whether such changes in redox signaling reflect primary age-related changes or are secondary to the fundamental mechanisms is unclear. For instance, denervated muscle fibers within muscles from aged rodents or humans appear to generate large amounts of mitochondrial hydrogen peroxide that could influence adjacent innervated fibers. Thus, in this instance, a "secondary" source of reactive oxygen species may be potentially generated as a result of a primary age-related pathology (loss of neurons), but, nevertheless, may contribute to loss of muscle mass and function during aging.

Targeting mitochondrial metal dyshomeostasis for the treatment of neurodegeneration

Mitochondrial impairment and metal dyshomeostasis are suggested to be associated with many neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and Friedreich's ataxia. Treatments aimed at restoring metal homeostasis are highly effective in models of these diseases, and clinical trials hold promise. However, in general, the effect of these treatments on mitochondrial metal homeostasis is unclear, and the contribution of mitochondrial metal dyshomeostasis to disease pathogenesis requires further investigation. This review describes the role of metals in mitochondria in health, how mitochondrial metals are disrupted in neurodegenerative diseases, and potential therapeutics aimed at restoring mitochondrial metal homeostasis and function.

Mitochondrial dynamic abnormalities in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disease characterized by progressive loss of motor neurons in the brainstem and spinal cord. Currently, there is no cure or effective treatment for ALS and the cause of disease is unknown in the majority of ALS cases. Neuronal mitochondria dysfunction is one of the earliest features of ALS. Mitochondria are highly dynamic organelles that undergo continuous fission, fusion, trafficking and turnover, all of which contribute to the maintenance of mitochondrial function. Abnormal mitochondrial dynamics have been repeatedly reported in ALS and increasing evidence suggests altered mitochondrial dynamics as possible pathomechanisms underlying mitochondrial dysfunction in ALS. Here, we provide an overview of mitochondrial dysfunction and dynamic abnormalities observed in ALS, and discuss the possibility of targeting mitochondrial dynamics as a novel therapeutic approach for ALS.

Miro1 deficiency in amyotrophic lateral sclerosis

Proper transportation of mitochondria to sites with high energy demands is critical for neuronal function and survival. Impaired mitochondrial movement has been repeatedly reported in motor neurons of amyotrophic lateral sclerosis (ALS) patients and indicated as an important mechanism contributing to motor neuron degeneration in ALS. Miro1, a RhoGTPase also referred to as Rhot1, is a key regulator of mitochondrial movement linking mitochondria and motor proteins. In this study, we investigated whether the expression of Miro1 was altered in ALS patients and ALS animal models. Immunoblot analysis revealed that Miro1 was significantly reduced in the spinal cord tissue of ALS patients. Consistently, the decreased expression of Miro1 was also noted only in the spinal cord, and not in the brain tissue of transgenic mice expressing ALS-associated SOD1 G93A or TDP-43 M337V. Glutamate excitotoxicity is one of the major pathophysiological mechanisms implicated in the pathogenesis of ALS, and we found that excessive glutamate challenge lead to significant reduction of Miro1 expression in spinal cord motor neurons both in vitro and in mice. Taken together, these findings show Miro1 deficiency in ALS patients and ALS animal models and suggest glutamate excitotoxicity as a likely cause of Miro1 deficiency.

Oxidative folding: recent developments

Disulfide bond formation in proteins is an effective tool of both structure stabilization and redox regulation. The prokaryotic periplasm and the endoplasmic reticulum of eukaryotes were long considered as the only compartments for enzyme mediated formation of stable disulfide bonds. Recently, the mitochondrial intermembrane space has emerged as the third protein-oxidizing compartment. The classic view on the mechanism of oxidative folding in the endoplasmic reticulum has also been reshaped by new observations. Moreover, besides the structure stabilizing function, reversible disulfide bridge formation in some proteins of the endoplasmic reticulum, seems to play a regulatory role. This review briefly summarizes the present knowledge of the redox systems supporting oxidative folding, emphasizing recent developments.

Mitochondrial hormesis and diabetic complications

The concept that excess superoxide production from mitochondria is the driving, initial cellular response underlying diabetes complications has been held for the past decade. However, results of antioxidant-based trials have been largely negative. In the present review, the data supporting mitochondrial superoxide as a driving force for diabetic kidney, nerve, heart, and retinal complications are reexamined, and a new concept for diabetes complications--mitochondrial hormesis--is presented. In this view, production of mitochondrial superoxide can be an indicator of healthy mitochondria and physiologic oxidative phosphorylation. Recent data suggest that in response to excess glucose exposure or nutrient stress, there is a reduction of mitochondrial superoxide, oxidative phosphorylation, and mitochondrial ATP generation in several target tissues of diabetes complications. Persistent reduction of mitochondrial oxidative phosphorylation complex activity is associated with the release of oxidants from nonmitochondrial sources and release of proinflammatory and profibrotic cytokines, and a manifestation of organ dysfunction. Restoration of mitochondrial function and superoxide production via activation of AMPK has now been associated with improvement in markers of renal, cardiovascular, and neuronal dysfunction with diabetes. With this Perspective, approaches that stimulate AMPK and PGC1α via exercise, caloric restriction, and medications result in stimulation of mitochondrial oxidative phosphorylation activity, restore physiologic mitochondrial superoxide production, and promote organ healing.

A mitochondrial superoxide theory for oxidative stress diseases and aging

Fridovich identified CuZnSOD in 1969 and manganese superoxide dismutase (MnSOD) in 1973, and proposed ”the Superoxide Theory,” which postulates that superoxide (O₂^•−) is the origin of most reactive oxygen species (ROS) and that it undergoes a chain reaction in a cell, playing a central role in the ROS producing system. Increased oxidative stress on an organism causes damage to cells, the smallest constituent unit of an organism, which can lead to the onset of a variety of chronic diseases, such as Alzheimer’s, Parkinson’s, amyotrophic lateral sclerosis and other neurological diseases caused by abnormalities in biological defenses or increased intracellular reactive oxygen levels. Oxidative stress also plays a role in aging. Antioxidant systems, including non-enzyme low-molecular-weight antioxidants (such as, vitamins A, C and E, polyphenols, glutathione, and coenzyme Q₁₀) and antioxidant enzymes, fight against oxidants in cells. Superoxide is considered to be a major factor in oxidant toxicity, and mitochondrial MnSOD enzymes constitute an essential defense against superoxide. Mitochondria are the major source of superoxide. The reaction of superoxide generated from mitochondria with nitric oxide is faster than SOD catalyzed reaction, and produces peroxynitrite. Thus, based on research conducted after Fridovich’s seminal studies, we now propose a modified superoxide theory; i.e., superoxide is the origin of reactive oxygen and nitrogen species (RONS) and, as such, causes various redox related diseases and aging.

Mitochondrial metals as a potential therapeutic target in neurodegeneration

Transition metals are critical for enzyme function and protein folding, but in excess can mediate neurotoxic oxidative processes. As mitochondria are particularly vulnerable to oxidative damage due to radicals generated during ATP production, mitochondrial biometal homeostasis must therefore be tightly controlled to safely harness the redox potential of metal enzyme cofactors. Dysregulation of metal functions is evident in numerous neurological disorders including Alzheimer's disease, stroke, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and Friedrich's ataxia. This review describes the mitochondrial metal defects in these disorders and highlights novel metal-based therapeutic approaches that target mitochondrial metal homeostasis in neurological disorders.

Exploring new pathways of neurodegeneration in ALS: the role of mitochondria quality control

Neuronal cells are highly dependent on mitochondria, and mitochondrial dysfunction is associated with neurodegenerative diseases. As perturbed mitochondrial function renders neurons extremely sensitive to a wide variety of insults, such as oxidative stress and bioenergetic defects, mitochondrial defects can profoundly affect neuronal fate. Several studies have linked ALS with mitochondrial dysfunction, stemming from observations of mitochondrial abnormalities, both in patients and in cellular and mouse models of familial forms of ALS. Mitochondrial changes have been thoroughly investigated in mutants of superoxide dismutase 1 (SOD1), one of the most common causes of familial ALS, for which excellent cellular and animal models are available, but recently evidence is emerging also in other forms of ALS, both familial and sporadic. Mitochondrial defects in ALS involve many critical physiopathological processes, from defective bioenergetics to abnormal calcium homeostasis, altered morphology and impaired trafficking. In this review, we summarize established evidence of mitochondrial dysfunction in ALS, especially in SOD1 mutant models of familial ALS. The main focus of the review is on defective mitochondrial quality control (MQC) in ALS. MQC operates at multiple levels to clear damaged proteins through proteostasis and to eliminate irreparably damaged organelles through mitophagy. However, since ALS motor neurons progressively accumulate damaged mitochondria, it is plausible that the MQC is ineffective or overwhelmed by excessive workload imposed by the chronic and extensive mitochondrial damage. This article is part of a Special Issue entitled ALS complex pathogenesis.

Mechanisms of NFATc3 activation by increased superoxide and reduced hydrogen peroxide in pulmonary arterial smooth muscle

We recently demonstrated increased superoxide (O2(·-)) and decreased H2O2 levels in pulmonary arteries of chronic hypoxia-exposed wild-type and normoxic superoxide dismutase 1 (SOD1) knockout mice. We also showed that this reciprocal change in O2(·-) and H2O2 is associated with elevated activity of nuclear factor of activated T cells isoform c3 (NFATc3) in pulmonary arterial smooth muscle cells (PASMC). This suggests that an imbalance in reactive oxygen species levels is required for NFATc3 activation. However, how such imbalance activates NFATc3 is unknown. This study evaluated the importance of O2(·-) and H2O2 in the regulation of NFATc3 activity. We tested the hypothesis that an increase in O2(·-) enhances actin cytoskeleton dynamics and a decrease in H2O2 enhances intracellular Ca(2+) concentration, contributing to NFATc3 nuclear import and activation in PASMC. We demonstrate that, in PASMC, endothelin-1 increases O2(·-) while decreasing H2O2 production through the decrease in SOD1 activity without affecting SOD protein levels. We further demonstrate that O2(·-) promotes, while H2O2 inhibits, NFATc3 activation in PASMC. Additionally, increased O2(·-)-to-H2O2 ratio activates NFATc3, even in the absence of a Gq protein-coupled receptor agonist. Furthermore, O2(·-)-dependent actin polymerization and low intracellular H2O2 concentration-dependent increases in intracellular Ca(2+) concentration contribute to NFATc3 activation. Together, these studies define important and novel regulatory mechanisms of NFATc3 activation in PASMC by reactive oxygen species.

Regulation of CuZnSOD and its redox signaling potential: implications for amyotrophic lateral sclerosis

SIGNIFICANCE

Molecular oxygen is a Janus-faced electron acceptor for biological systems, serving as a reductant for respiration, or as the genesis for oxygen-derived free radicals that damage macromolecules. Superoxide is well known to perturb nonheme iron proteins, including Fe/S proteins such as aconitase and succinate dehydrogenase, as well as other enzymes containing labile iron such as the prolyl hydroxylase domain-containing family of enzymes; whereas hydrogen peroxide is more specific for two-electron reactions with thiols on glutathione, glutaredoxin, thioredoxin, and the peroxiredoxins.

RECENT ADVANCES

Over the past two decades, familial cases of amyotrophic lateral sclerosis (ALS) have been shown to have an association with commonly altered superoxide dismutase 1 (SOD1) activity, expression, and protein structure. This has led to speculation that an altered redox balance may have a role in creating the ALS phenotype.

CRITICAL ISSUES

While SOD1 alterations in familial ALS are manifold, they generally create perturbations in the flux of electrons. The nexus of SOD1 between one- and two-electron signaling processes places it at a key signaling regulatory checkpoint for governing cellular responses to physiological and environmental cues.

FUTURE DIRECTIONS

The manner in which ALS-associated mutations adjust SOD1's role in controlling the flow of electrons between one- and two-electron signaling processes remains obscure. Here, we discuss the ways in which SOD1 mutations influence the form and function of copper zinc SOD, the consequences of these alterations on free radical biology, and how these alterations might influence cell signaling during the onset of ALS.

The use of the Cre/loxP system to study oxidative stress in tissue-specific manganese superoxide dismutase knockout models

SIGNIFICANCE

Respiring mitochondria are a significant site for reactions involving reactive oxygen and nitrogen species that contribute to irreversible cellular, structural, and functional damage leading to multiple pathological conditions. Manganese superoxide dismutase (MnSOD) is a critical component of the antioxidant system tasked with protecting the oxidant-sensitive mitochondrial compartment from oxidative stress. Since global knockout of MnSOD results in significant cardiac and neuronal damage leading to early postnatal lethality, this approach has limited use for studying the mechanisms of oxidant stress and the development of disease in specific tissues lacking MnSOD. To circumvent this problem, a number of investigators have employed the Cre/loxP system to precisely knockout MnSOD in individual tissues.

RECENT ADVANCES

Multiple tissue and organ-specific Cre-expressing mice have been generated, which greatly enhance the specificity of MnSOD knockout in tissues and organ systems that were once difficult, if not impossible to study.

CRITICAL ISSUES

Evaluating the contribution of MnSOD deficiency to oxidant-mediated mitochondrial damage requires careful consideration of the promoter system used for creating the tissue-specific knockout animal, in addition to the collection and interpretation of multiple indices of oxidative stress and damage.

FUTURE DIRECTIONS

Expanded use of well-characterized tissue-specific promoter elements and inducible systems to drive the Cre/loxP recombinational events will lead to a spectrum of MnSOD tissue knockout models, and a clearer understanding of the role of MnSOD in preventing mitochondrial dysfunction in human disease.

SOD1 oxidation and formation of soluble aggregates in yeast: relevance to sporadic ALS development

Misfolding and aggregation of copper–zinc superoxide dismutase (Sod1) are observed in neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). Mutations in Sod1 lead to familial ALS (FALS), which is a late-onset disease. Since oxidative damage to proteins increases with age, it had been proposed that oxidation of Sod1 mutants may trigger their misfolding and aggregation in FALS. However, over 90% of ALS cases are sporadic (SALS) with no obvious genetic component. We hypothesized that oxidation could also trigger the misfolding and aggregation of wild-type Sod1 and sought to confirm this in a cellular environment. Using quiescent, stationary-phase yeast cells as a model for non-dividing motor neurons, we probed for post-translational modification (PTM) and aggregation of wild-type Sod1 extracted from these cells. By size-exclusion chromatography (SEC), we isolated two populations of Sod1 from yeast: a low-molecular weight (LMW) fraction that is catalytically active and a catalytically inactive, high-molecular weight (HMW) fraction. High-resolution mass spectrometric analysis revealed that LMW Sod1 displays no PTMs but HMW Sod1 is oxidized at Cys146 and His71, two critical residues for the stability and folding of the enzyme. HMW Sod1 is also oxidized at His120, a copper ligand, which will promote loss of this catalytic metal cofactor essential for SOD activity. Monitoring the fluorescence of a Sod1-green-fluorescent-protein fusion (Sod1-GFP) extracted from yeast chromosomally expressing this fusion, we find that HMW Sod1-GFP levels increase up to 40-fold in old cells. Thus, we speculate that increased misfolding and inclusion into soluble aggregates is a consequence of elevated oxidative modifications of wild-type Sod1 as cells age. Our observations argue that oxidative damage to wild-type Sod1 initiates the protein misfolding mechanisms that give rise to SALS.

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Key Sod1 catalytic and structure-stabilizing residues (Cys146, His120, His71) are oxidized in stationary-phase yeast.

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Oxidized Sod1 is isolated in an inactive, high-molecular-weight, soluble aggregate.

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Sod1 with native mass isolated from the same samples is not oxidized and is catalytically active.

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Our results argue that oxidation triggers the formation of soluble Sod1-containing aggregates that may contribute to sporadic ALS development.

Role of reactive oxygen species in the defective regeneration seen in aging muscle

The ability of muscles to regenerate successfully following damage diminishes with age and this appears to be a major contributor to the development of muscle weakness and physical frailty. Successful muscle regeneration is dependent on appropriate reinnervation of regenerating muscle. Age-related changes in the interactions between nerve and muscle are poorly understood but may play a major role in the defective regeneration. During aging there is defective redox homeostasis and an accumulation of oxidative damage in nerve and muscle that may contribute to defective regeneration. The aim of this review is to summarise the evidence that abnormal reactive oxygen species (ROS) generation in nerve and/or muscle may be responsible for the defective regeneration that contributes to the degeneration of skeletal muscle observed during aging. Identifying the importance of ROS generation in skeletal muscle during aging could have fundamental implications for interventions to prevent muscle degeneration and treatments to reverse the age-related decline in muscle mass and function.

Mitochondrial DNA instability in cells lacking aconitase correlates with iron citrate toxicity

Aconitase, the second enzyme of the tricarboxylic acid cycle encoded by ACO1 in the budding yeast Saccharomyces cerevisiae, catalyzes the conversion of citrate to isocitrate. aco1Δ results in mitochondrial DNA (mtDNA) instability. It has been proposed that Aco1 binds to mtDNA and mediates its maintenance. Here we propose an alternative mechanism to account for mtDNA loss in aco1Δ mutant cells. We found that aco1Δ activated the RTG pathway, resulting in increased expression of genes encoding citrate synthase. By deleting RTG1, RTG3, or genes encoding citrate synthase, mtDNA instability was prevented in aco1Δ mutant cells. Increased activity of citrate synthase leads to iron accumulation in the mitochondria. Mutations in MRS3 and MRS4, encoding two mitochondrial iron transporters, also prevented mtDNA loss due to aco1Δ. Mitochondria are the main source of superoxide radicals, which are converted to H₂O₂ through two superoxide dismutases, Sod1 and Sod2. H₂O₂ in turn reacts with Fe²⁺ to generate very active hydroxyl radicals. We found that loss of Sod1, but not Sod2, prevents mtDNA loss in aco1Δ mutant cells. We propose that mtDNA loss in aco1Δ mutant cells is caused by the activation of the RTG pathway and subsequent iron citrate accumulation and toxicity.

Quantitative proteomic profiling of tumor cell response to telomere dysfunction using isotope-coded protein labeling (ICPL) reveals interaction network of candidate senescence markers

Telomerase inhibition causes progressive telomere shortening and cellular senescence, which constitutes a universal barrier to tumor growth and therefore an attractive target for tumor therapy. To expand our previous studies, we investigated the global effects of telomere dysfunction on the proteome of tumor cells in order to find novel senescence biomarkers. Telomerase-deficient HCT-116 cell clones were analyzed by a quantitative proteomic approach using isotope-coded protein labeling (ICPL) and nanoflow-HPLC-MS/MS. Stringent reduction of the extensive proteomic data from this tumor cell model revealed a list of 59 markers including proteins identified in our former studies and a number of novel proteins involved in tumorigenesis and metastasis such as SFN, S100A4, ANXA2, and LGALS1. A loss of the chromatin protein HMGB2 was demonstrated not only in various telomerase-inhibited clones of different tumor cell lines, but also in normal human fibroblasts undergoing replicative senescence and in aging telomerase knockout mice. Impressively, a coherent and dense network of protein-protein interactions for the bulk of the markers and their implementation in signaling pathways involving key regulators for tumorigenesis were revealed. These results have an impact on the understanding of telomere- and senescence-related signal transduction in tumor cells in consideration of the general lack of senescence markers.

BIOLOGICAL SIGNIFICANCE

Induction of cellular senescence constitutes a potent concept for tumor therapy which interferes with immortalization and additional hallmarks of cancer. The application of a powerful quantitative proteomic approach using isotope-coded protein labeling to an approved model for senescence represented by telomerase inhibited tumor cells led to the identification of novel candidate biomarkers for telomere dysfunction and replicative senescence. Thereby, the identified markers not only fit in the context of the investigated processes with a relevance for additional hallmarks of cancer but are also involved in a strong interaction network and integrated in canonical pathways centered around key cancer-relevant proteins. These potential markers alone or in combination will significantly extend the view on telomere-associated signal transduction in tumor cells and contribute to the field of cellular senescence and aging in consideration of the general lack of biomarkers in this regard.

Mia40 and MINOS act in parallel with Ccs1 in the biogenesis of mitochondrial Sod1

Superoxide dismutase 1 (Sod1) is a major superoxide-scavenging enzyme in the eukaryotic cell, and is localized in the cytosol and intermembrane space of mitochondria. Sod1 requires its specific chaperone Ccs1 and disulfide bond formation in order to be retained in the intermembrane space. Our study identified a pool of Sod1 that is present in the reduced state in mitochondria that lack Ccs1. We created yeast mutants with mutations in highly conserved amino acid residues corresponding to human mutations that cause amyotrophic lateral sclerosis, and found that some of the mutant proteins were present in the reduced state. These mutant variants of Sod1 were efficiently localized in mitochondria. Localization of the reduced, Ccs1-independent forms of Sod1 relied on Mia40, an essential component of the mitochondrial intermembrane space import and assembly pathway that is responsible for the biogenesis of intermembrane space proteins. Furthermore, the mitochondrial inner membrane organizing system (MINOS), which is responsible for mitochondrial membrane architecture, differentially modulated the presence of reduced Sod1 in mitochondria. Thus, we identified novel mitochondrial players that are possibly involved in pathological conditions caused by changes in the biogenesis of Sod1.

Cytosolic caspases mediate mislocalised SOD2 depletion in an in vitro model of chronic prion infection

Oxidative stress as a contributor to neuronal death during prion infection is supported by the fact that various oxidative damage markers accumulate in the brain during the course of this disease. The normal cellular substrate of the causative agent, the prion protein, is also linked with protective functions against oxidative stress. Our previous work has found that, in chronic prion infection, an apoptotic subpopulation of cells exhibit oxidative stress and the accumulation of oxidised lipid and protein aggregates with caspase recruitment. Given the likely failure of antioxidant defence mechanisms within apoptotic prion-infected cells, we aimed to investigate the role of the crucial antioxidant pathway components, superoxide dismutases (SOD) 1 and 2, in an in vitro model of chronic prion infection. Increased total SOD activity, attributable to SOD1, was found in the overall population coincident with a decrease in SOD2 protein levels. When apoptotic cells were separated from the total population, the induction of SOD activity in the infected apoptotic cells was lost, with activity reduced back to levels seen in mock-infected control cells. In addition, mitochondrial superoxide production was increased and mitochondrial numbers decreased in the infected apoptotic subpopulation. Furthermore, a pan-caspase probe colocalised with SOD2 outside of mitochondria within cytosolic aggregates in infected cells and inhibition of caspase activity was able to restore cellular levels of SOD2 in the whole unseparated infected population to those of mock-infected control cells. Our results suggest that prion propagation exacerbates an apoptotic pathway whereby mitochondrial dysfunction follows mislocalisation of SOD2 to cytosolic caspases, permitting its degradation. Eventually, cellular capacity to maintain oxidative homeostasis is overwhelmed, thus resulting in cell death.

NFAT is required for spontaneous pulmonary hypertension in superoxide dismutase 1 knockout mice

Elevated reactive oxygen species are implicated in pulmonary hypertension (PH). Superoxide dismutase (SOD) limits superoxide bioavailability, and decreased SOD activity is associated with PH. A decrease in SOD activity is expected to increase superoxide and reduce hydrogen peroxide levels. Such an imbalance of superoxide/hydrogen peroxide has been implicated as a mediator of nuclear factor of activated T cells (NFAT) activation in epidermal cells. We have shown that NFATc3 is required for chronic hypoxia-induced PH. However, it is unknown whether NFATc3 is activated in the pulmonary circulation in a mouse model of decreased SOD1 activity and whether this leads to PH. Therefore, we hypothesized that an elevated pulmonary arterial superoxide/hydrogen peroxide ratio activates NFATc3, leading to PH. We found that SOD1 knockout (KO) mice have elevated pulmonary arterial wall superoxide and decreased hydrogen peroxide levels compared with wild-type (WT) littermates. Right ventricular systolic pressure (RVSP) was elevated in SOD1 KO and was associated with pulmonary arterial remodeling. Vasoreactivity to endothelin-1 was also greater in SOD1 KO vs. WT mice. NFAT activity and NFATc3 nuclear localization were increased in pulmonary arteries from SOD1 KO vs. WT mice. Administration of A-285222 (selective NFAT inhibitor) decreased RVSP, arterial wall thickness, vasoreactivity, and NFAT activity in SOD1 KO mice to WT levels. The SOD mimetic, tempol, also reduced NFAT activity, NFATc3 nuclear localization, and RVSP to WT levels. These findings suggest that an elevated superoxide/hydrogen peroxide ratio activates NFAT in pulmonary arteries, which induces vascular remodeling and increases vascular reactivity leading to PH.

Iron and copper in mitochondrial diseases

Transition metals are frequently used as cofactors for enzymes and oxygen-carrying proteins that take advantage of their propensity to gain and lose single electrons. Metals are particularly important in mitochondria, where they play essential roles in the production of ATP and detoxification of reactive oxygen species. At the same time, transition metals (particularly Fe and Cu) can promote the formation of harmful radicals, necessitating meticulous control of metal concentration and subcellular compartmentalization. We summarize our current understanding of Fe and Cu in mammalian mitochondrial biology and discuss human diseases associated with aberrations in mitochondrial metal homeostasis.

Maintenance of mitochondrial genomic integrity in the absence of manganese superoxide dismutase in mouse liver hepatocytes

Manganese superoxide dismutase, encoded by the Sod2 gene, is a ubiquitously expressed mitochondrial antioxidant enzyme that is essential for mammalian life. Mice born with constitutive genetic knockout of Sod2 do not survive the neonatal stage, which renders the longitudinal study of the biochemical and metabolic effects of Sod2 loss difficult. However, multiple studies have demonstrated that tissue-specific knockout of Sod2 in murine liver yields no observable gross pathology or injury to the mouse. We hypothesized that Sod2 loss may have sub-pathologic effects on liver biology, including the acquisition of reactive oxygen species-mediated mitochondrial DNA mutations. To evaluate this, we established and verified a hepatocyte-specific knockout of Sod2 in C57/B6 mice using Cre-LoxP recombination technology. We utilized deep sequencing to identify possible mutations in Sod2^−/− mitochondrial DNA as compared to wt, and both RT-PCR and traditional biochemical assays to evaluate baseline differences in redox-sensitive pathways in Sod2^−/− hepatocytes. Surprisingly, no mutations in Sod2^−/− mitochondrial DNA were detected despite measurable increases in dihydroethidium staining in situ and concomitant decreases in complex II activity indicative of elevated superoxide in the Sod2^−/− hepatocytes. In contrast, numerous compensatory alterations in gene expression were identified that suggest hepatocytes have a remarkable capacity to adapt and overcome the loss of Sod2 through transcriptional means. Taken together, these results suggest that murine hepatocytes have a large reserve capacity to cope with the presence of additional mitochondrial reactive oxygen species.

Oxidative folding in the mitochondrial intermembrane space in human health and disease

Oxidative folding in the mitochondrial intermembrane space (IMS) is a key cellular event associated with the folding and import of a large and still undetermined number of proteins. This process is catalyzed by an oxidoreductase, Mia40 that is able to recognize substrates with apparently little or no homology. Following substrate oxidation, Mia40 is reduced and must be reoxidized by Erv1/Alr1 that consequently transfers the electrons to the mitochondrial respiratory chain. Although our understanding of the physiological relevance of this process is still limited, an increasing number of pathologies are being associated with the impairment of this pathway; especially because oxidative folding is fundamental for several of the proteins involved in defense against oxidative stress. Here we review these aspects and discuss recent findings suggesting that oxidative folding in the IMS is modulated by the redox state of the cell.

Atp23 biogenesis reveals a chaperone-like folding activity of Mia40 in the IMS of mitochondria

Mia40 is a recently identified oxidoreductase in the intermembrane space (IMS) of mitochondria that mediates protein import in an oxidation-dependent reaction. Substrates of Mia40 that were identified so far are of simple structure and receive one or two disulphide bonds. Here we identified the protease Atp23 as a novel substrate of Mia40. Atp23 contains ten cysteine residues which are oxidized during several rounds of interaction with Mia40. In contrast to other Mia40 substrates, oxidation of Atp23 is not essential for its import; an Atp23 variant in which all ten cysteine residues were replaced by serine residues still accumulates in mitochondria in a Mia40-dependent manner. In vitro Mia40 can mediate the folding of wild-type Atp23 and prevents its aggregation. In these reactions, the hydrophobic substrate-binding pocket of Mia40 was found to be essential for its chaperone-like activity. Thus, Mia40 plays a much broader role in import and folding of polypeptides than previously expected and can serve as folding factor for proteins with complex disulphide patterns as well as for cysteine-free polypeptides.

Mitochondrial protein import: common principles and physiological networks

Most mitochondrial proteins are encoded in the nucleus. They are synthesized as precursor forms in the cytosol and must be imported into mitochondria with the help of different protein translocases. Distinct import signals within precursors direct each protein to the mitochondrial surface and subsequently onto specific transport routes to its final destination within these organelles. In this review we highlight common principles of mitochondrial protein import and address different mechanisms of protein integration into mitochondrial membranes. Over the last years it has become clear that mitochondrial protein translocases are not independently operating units, but in fact closely cooperate with each other. We discuss recent studies that indicate how the pathways for mitochondrial protein biogenesis are embedded into a functional network of various other physiological processes, such as energy metabolism, signal transduction, and maintenance of mitochondrial morphology. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Mechanisms of altered redox regulation in neurodegenerative diseases--focus on S--glutathionylation

SIGNIFICANCE

Neurodegenerative diseases are characterized by progressive loss of neurons. A common feature is oxidative stress, which arises when reactive oxygen species (ROS) and/or reactive nitrogen species (RNS) exceed amounts required for normal redox signaling. An imbalance in ROS/RNS alters functionality of cysteines and perturbs thiol-disulfide homeostasis. Many cysteine modifications may occur, but reversible protein mixed disulfides with glutathione (GSH) likely represents the common steady-state derivative due to cellular abundance of GSH and ready conversion of cysteine-sulfenic acid and S-nitrosocysteine precursors to S-glutathionylcysteine disulfides. Thus, S-glutathionylation acts in redox signal transduction and serves as a protective mechanism against irreversible cysteine oxidation. Reversal of protein-S-glutathionylation is catalyzed specifically by glutaredoxin which thereby plays a critical role in cellular regulation. This review highlights the role of oxidative modification of proteins, notably S-glutathionylation, and alterations in thiol homeostatic enzyme activities in neurodegenerative diseases, providing insights for therapeutic intervention.

RECENT ADVANCES

Recent studies show that dysregulation of redox signaling and sulfhydryl homeostasis likely contributes to onset/progression of neurodegeneration. Oxidative stress alters the thiol-disulfide status of key proteins that regulate the balance between cell survival and cell death.

CRITICAL ISSUES

Much of the current information about redox modification of key enzymes and signaling intermediates has been gleaned from studies focused on oxidative stress situations other than the neurodegenerative diseases.

FUTURE DIRECTIONS

The findings in other contexts are expected to apply to understanding neurodegenerative mechanisms. Identification of selectively glutathionylated proteins in a quantitative fashion will provide new insights about neuropathological consequences of this oxidative protein modification.