Age-related impairment of mitochondrial matrix aconitase and ATP-stimulated protease in rat liver and heart

Mitochondrial Lon protease is a human stress protein

The targeted removal of damaged proteins by proteolysis is crucial for cell survival. We have shown previously that the Lon protease selectively degrades oxidized mitochondrial proteins, thus preventing their aggregation and cross-linking. We now show that the Lon protease is a stress-responsive protein that is induced by multiple stressors, including heat shock, serum starvation, and oxidative stress. Lon induction, by pretreatment with low-level stress, protects against oxidative protein damage, diminished mitochondrial function, and loss of cell proliferation induced by toxic levels of hydrogen peroxide. Blocking Lon induction with Lon siRNA also blocks this induced protection. We propose that Lon is a generalized stress-protective enzyme whose decline may contribute to the increased levels of protein damage and mitochondrial dysfunction observed in aging and age-related diseases.

Inactivation of brain mitochondrial Lon protease by peroxynitrite precedes electron transport chain dysfunction

The accumulation of oxidatively modified proteins has been shown to be a characteristic feature of many neurodegenerative disorders and its regulation requires efficient proteolytic processing. One component of the mitochondrial proteolytic system is Lon, an ATP-dependent protease that has been shown to degrade oxidatively modified aconitase in vitro and may thus play a role in defending against the accumulation of oxidized matrix proteins in mitochondria. Using an assay system that allowed us to distinguish between basal and ATP-stimulated Lon protease activity, we have shown in isolated non-synaptic rat brain mitochondria that Lon protease is highly susceptible to oxidative inactivation by peroxynitrite (ONOO(-)). This susceptibility was more pronounced with regard to ATP-stimulated activity, which was inhibited by 75% in the presence of a bolus addition of 1mM ONOO(-), whereas basal unstimulated activity was inhibited by 45%. Treatment of mitochondria with a range of peroxynitrite concentrations (10-1000 microM) revealed that a decline in Lon protease activity preceded electron transport chain (ETC) dysfunction (complex I, II-III and IV) and that ATP-stimulated activity was approximately fivefold more sensitive than basal Lon protease activity. Furthermore, supplementation of mitochondrial matrix extracts with reduced glutathione, following ONOO(-) exposure, resulted in partial restoration of basal and ATP-stimulated activity, thus suggesting possible redox regulation of this enzyme complex. Taken together these findings suggest that Lon protease may be particularly vulnerable to inactivation in conditions associated with GSH depletion and elevated oxidative stress.

Proteomic profiling of aging in the mouse heart: Altered expression of mitochondrial proteins

Using two-dimensional gel electrophoresis and liquid chromatography-tandem mass spectrometry, we have used a systems biology approach to study the molecular basis of aging of the mouse heart. We have identified 8 protein spots whose expression is up-regulated due to aging and 36 protein spots whose expression is down-regulated due to aging (p0.05 as judged by Wilcoxon Rank Sum test). Among the up-regulated proteins, we have characterized 5 protein spots and 2 of them, containing 3 different enzymes, are mitochondrial proteins. Among the down-regulated proteins, we have characterized 27 protein spots and 16 of them are mitochondrial proteins. Mitochondrial damage is believed to be a key factor in the aging process. Our current study provides molecular evidence at the level of the proteome for the alteration of structural and functional parameters of the mitochondria that contribute to impaired activity of the mouse heart due to aging.

Effects of acclimation temperature and cadmium exposure on mitochondrial aconitase and LON protease from a model marine ectotherm, Crassostrea virginica

Temperature and heavy metals such as cadmium (Cd) are important stressors which can strongly affect physiology of marine ectotherms in polluted estuaries. Mitochondria are among the key intracellular targets for these stressors, but the mechanisms of Cd-induced mitochondrial damage are not fully understood. In this study we determined the effects of acclimation temperature (12, 20 and 28 degrees C) and Cd exposure (0 or 50 microg L(-1) Cd) in vivo on activity and mRNA expression of a key mitochondrial enzyme, aconitase, which is known as a sensitive marker of oxidative stress, and on mRNA expression of LON protease involved in the degradation of oxidatively damaged mitochondrial proteins, in eastern oysters Crassostrea virginica. Sensitivity of mitochondrial aconitase to exposure to Cd in vitro (0 or 50 microM) was also determined in oysters acclimated to different temperatures and Cd levels. Acclimation at 28 degrees C resulted in a strong decrease in activity of mitochondrial aconitase as well as mRNA expression of aconitase and LON protease suggesting mitochondrial dysfunction at elevated temperatures. Exposure of isolated mitochondria to 50 microM Cd in vitro resulted in a 20-25% inhibition of mitochondrial aconitase reflecting oxidative damage of this enzyme. However, long-term (3-6 weeks) exposure of whole oysters to Cd had no effect on mitochondrial aconitase activity suggesting that this enzyme is well protected against Cd-induced oxidative stress in vivo. Aconitase mRNA expression was positively correlated with the enzyme activity within control and Cd-exposed groups; however, this correlation was strikingly different when compared between control and Cd-exposed oysters. The level of aconitase transcript was considerably lower (3-13-fold) in Cd-exposed oysters while the specific aconitase activities were similar in control and Cd-exposed oysters indicating regulation at the post-transcriptional level. LON protease expression was upregulated by 2-4-fold in Cd-exposed oysters suggesting an increase in mitochondrial protein degradation as a novel protective mechanism against Cd-induced mitochondrial stress. Our data indicate that mitochondrial aconitase is not a good biomarker for Cd-induced oxidative stress in oysters in vivo, because of its complex regulation at transcriptional and post-transcriptional levels, low sensitivity to Cd effects in vivo but high sensitivity to acclimation temperature that can potentially mask effects of other stressors under the field conditions.

Cardiac mitochondrial bioenergetics, oxidative stress, and aging

Mitochondria have been a central focus of several theories of aging as a result of their critical role in bioenergetics, oxidant production, and regulation of cell death. A decline in cardiac mitochondrial function coupled with the accumulation of oxidative damage to macromolecules may be causal to the decline in cardiac performance with age. In contrast, regular physical activity and lifelong caloric restriction can prevent oxidative stress, delay the onset of morbidity, increase life span, and reduce the risk of developing several pathological conditions. The health benefits of life long exercise and caloric restriction may be, at least partially, due to a reduction in the chronic amount of mitochondrial oxidant production. In addition, the available data suggest that chronic exercise may serve to enhance antioxidant enzyme activities, and augment certain repair/removal pathways, thereby reducing the amount of oxidative tissue damage. However, the characterization of age-related changes to cardiac mitochondria has been complicated by the fact that two distinct populations of mitochondria exist in the myocardium: subsarcolemmal mitochondria and interfibrillar mitochondria. Several studies now suggest the importance of studying both mitochondrial populations when attempting to elucidate the contribution of mitochondrial dysfunction to myocardial aging. The role that mitochondrial dysfunction and oxidative stress play in contributing to cardiac aging will be discussed along with the use of lifelong exercise and calorie restriction as countermeasures to aging.

Roles for the human ATP-dependent Lon protease in mitochondrial DNA maintenance

Human mitochondrial Lon is an ATP-powered proteolytic machine that specifically binds to single-stranded G-rich DNA and RNA in vitro. However, it is unknown whether Lon binds mitochondrial DNA (mtDNA) in living cells or functions in mtDNA integrity. Here, we demonstrate that Lon interacts with the mitochondrial genome in cultured cells using mtDNA immunoprecipitation (mIP). Lon associates with sites distributed primarily within one-half of the genome and preferentially with the control region for mtDNA replication and transcription. Bioinformatic analysis of mIP data revealed a G-rich consensus sequence. Consistent with these findings, in vitro experiments showed that the affinity of Lon for single-stranded DNA oligonucleotides correlates with conformity to this consensus. To examine the role of Lon in mtDNA maintenance, cells carrying an inducible short hairpin RNA for Lon depletion were used. In control and Lon-depleted cells, mtDNA copy number was essentially the same in the presence or absence of oxidative stress. However when oxidatively stressed, control cells exhibited an increased frequency of mtDNA lesions, whereas Lon-depleted cells showed little if any mtDNA damage. This suggests that oxidative mtDNA damage is permitted when Lon is present and prevented when Lon is substantially depleted. Upon oxidative stress, mIP showed reduced Lon binding to mtDNA; however binding to the control region was unaffected. It is unlikely that oxidative modification of Lon blocks its ability to bind DNA in vivo as results show that oxidized purified Lon retains sequence-specific DNA binding. Taken together, these results demonstrate that mtDNA binding is a physiological function of Lon and that cellular levels of Lon influence sensitivity to mtDNA damage. These findings suggest roles for Lon in linking protein and mtDNA quality control.

Causative role of oxidative stress in a Drosophila model of Friedreich ataxia

Friedreich ataxia (FA), the most common form of hereditary ataxia, is caused by a deficit in the mitochondrial protein frataxin. While several hypotheses have been suggested, frataxin function is not well understood. Oxidative stress has been suggested to play a role in the pathophysiology of FA, but this view has been recently questioned, and its link to frataxin is unclear. Here, we report the use of RNA interference (RNAi) to suppress the Drosophila frataxin gene (fh) expression. This model system parallels the situation in FA patients, namely a moderate systemic reduction of frataxin levels compatible with normal embryonic development. Under these conditions, fh-RNAi flies showed a shortened life span, reduced climbing abilities, and enhanced sensitivity to oxidative stress. Under hyperoxia, fh-RNAi flies also showed a dramatic reduction of aconitase activity that seriously impairs the mitochondrial respiration while the activities of succinate dehydrogenase, respiratory complex I and II, and indirectly complex III and IV are normal. Remarkably, frataxin overexpression also induced the oxidative-mediated inactivation of mitochondrial aconitase. This work demonstrates, for the first time, the essential function of frataxin in protecting aconitase from oxidative stress-dependent inactivation in a multicellular organism. Moreover our data support an important role of oxidative stress in the progression of FA and suggest a tissue-dependent sensitivity to frataxin imbalance. We propose that in FA, the oxidative mediated inactivation of aconitase, which occurs normally during the aging process, is enhanced due to the lack of frataxin.

Maintenance of proteins and aging: the role of oxidized protein repair

According to the free radical theory of aging proposed by Denham Harman (Journal of Gerontology 1956, 11, pp. 298-300), the continuous oxidative damage to cellular components over an organism's life span is a causal factor of the aging process. The age-related build-up of oxidized protein is therefore resulting from increased protein oxidative damage and/or decreased elimination of oxidized proteins. In this mini-review, we will address the fate, during aging, of the protein maintenance systems that are involved in the degradation of irreversibly oxidized proteins and in the repair of reversible protein oxidative damage with a special focus on the methionine sulfoxide reductases system. Since these protein degradation and repair systems have been found to be impaired with age, it is proposed that not only failure of redox homeostasis but, as importantly, failure of protein maintenance are critical factors in the aging process.

Metabolic regulation of citrate and iron by aconitases: role of iron–sulfur cluster biogenesis

Iron and citrate are essential for the metabolism of most organisms, and regulation of iron and citrate biology at both the cellular and systemic levels is critical for normal physiology and survival. Mitochondrial and cytosolic aconitases catalyze the interconversion of citrate and isocitrate, and aconitase activities are affected by iron levels, oxidative stress and by the status of the Fe-S cluster biogenesis apparatus. Assembly and disassembly of Fe-S clusters is a key process not only in regulating the enzymatic activity of mitochondrial aconitase in the citric acid cycle, but also in controlling the iron sensing and RNA binding activities of cytosolic aconitase (also known as iron regulatory protein IRP1). This review discusses the central role of aconitases in intermediary metabolism and explores how iron homeostasis and Fe-S cluster biogenesis regulate the Fe-S cluster switch and modulate intracellular citrate flux.

Protein degradation by the 26S proteasome system in the normal and stressed myocardium

The 26S proteasome is a multicatalytic threonine protease complex responsible for degradation of the majority of proteins in eukaryotic cells. In the last two decades, the ubiquitin proteasome system (UPS) has been increasingly recognized as an integral component in numerous biologic processes including cell proliferation, adaptation to stress, and cell death. The turnover of intracellular proteins inevitably affects the contributions of these molecules to cellular networks and pathways in any given tissue or organ, including the myocardium. Perturbations in the protein-degradation process have been shown to affect protein turnover and thereby affect the cardiac cell functions that these molecules are designated to carry out, engendering diseased cardiac phenotypes. Recent studies have implicated the role of proteasomes in stressed cardiac phenotypes including postischemia-reperfusion injury and cardiac remodeling (e.g., heart failure). The 26S proteasomes also appear to be susceptible to modulation by stresses (e.g., reactive oxygen species). This review focuses on roles of the 26S proteasome system in protein degradation; it provides an overview of the progress made in cardiac proteasome research as well as a discussion of recent controversies regarding the UPS system in diseased cardiac phenotypes.

Mitochondrial protein oxidation and degradation in response to oxidative stress and aging

Mitochondria are a major source of intracellular reactive oxygen species (ROS), the production of which increases with age. These organelles are also targets of oxidative damage. The deleterious effects of ROS may be responsible for impairment of mitochondrial function observed during various pathophysiological states associated with oxidative stress and aging. An important factor for protein maintenance in the presence of oxidative stress is enzymatic reversal of oxidative modifications and/or protein degradation. Failure of these protein maintenance systems is likely a critical component of the aging process. Mitochondrial matrix proteins are sensitive to oxidative inactivation and oxidized proteins are known to accumulate during aging. The ATP-stimulated mitochondrial Lon protease is a highly conserved protease found in prokaryotes and the mitochondrial compartment of eukaryotes and is believed to play an important role in the degradation of oxidized mitochondrial matrix proteins. Age-dependent declines in the activity and regulation of this proteolytic system may underlie accumulation of oxidatively modified and dysfunctional protein and loss in mitochondrial viability.

Oxidized protein degradation and repair in ageing and oxidative stress

Cellular ageing is characterized by the accumulation of oxidatively modified proteins which may be due to increased protein damage and/or decreased elimination of oxidized protein. Since the proteasome is in charge of protein turnover and removal of oxidized protein, its fate during ageing and upon oxidative stress has received special attention, and evidence has been provided for an age-related impairment of proteasome function. However, proteins when oxidized at the level of sulfur-containing amino acids can also be repaired. Therefore, the fate of the methionine sulfoxide reductase system during ageing has also been addressed as well as its role in protection against oxidative stress.

Comparative Profiling of the Mammalian Mitochondrial Proteome: Multiple Aconitase-2 Isoforms IncludingN-formylkynurenine Modifications as Part of a Protein Biomarker Signature for Reactive Oxidative Species

The activity of mitochondria induces, as a byproduct, a variety of post-translational modifications in associated proteins, which have functional downstream consequences for processes such as apoptosis, autophagy, and plasticity; e.g., reactive oxygen species (ROS), which induce N-formyl-kynurenine from oxidized tryptophans in certain mitochondrial proteins which are localized in close spatial proximity to their source. This type of fast molecular changes has profound influence on cell death and survival with implications in a number of pathologies. The quantitative and differential analysis of bovine heart mitochondria by four 2D-PAGE methods, including 2D-PAGE with high-resolution IEF as first dimension, revealed that due to limited resolution, those methods employing blue native-, tricine-urea-, and 16-BAC-PAGE as the first dimension are less applicable for the differential quantitative analysis of redundant protein spots which might give insight into post-translational modifications that are relevant in age- and stress-related changes. Moreover, 2D-PAGE with high resolution IEF was able to resolve a surprisingly large number of membrane proteins from mitochondrial preparations. For aconitase-2, an enzyme playing an important role in mitochondrial aging, a more thorough molecular analysis of all separable isoforms was performed, leading to the identification of two particular N-formylkynurenine modifications. Next to protein redundancy, native protein-protein interactions, with the potential of relating certain post-translational modification patterns to distinct oligomeric states, e.g., oxidative phosphorylation super complexes, might provide novel and (patho-) physiologically relevant information. Among proteins identified, 14 new proteins (GenBank entries), previously not associated with mitochondria, were found.

Proteasome function in aging and oxidative stress: implications in protein maintenance failure

Damage to cellular components by reactive oxygen species is believed to be an important factor contributing to the aging process. Likewise, the progressive failure of maintenance and repair is believed to be a major cause of biological aging. Cellular aging is characterized by the accumulation of oxidatively modified proteins, a process that results, at least in part, from impaired protein turnover. Indeed, oxidized protein buildup with age may be due to increased protein damage, decreased elimination of oxidized protein (i.e., repair and degradation), or a combination of both mechanisms. Since the proteasome has been implicated in both general protein turnover and the removal of oxidized protein, the fate of the proteasome during aging has recently received considerable attention, and evidence has been provided for impaired proteasome function with age in different cellular systems. The present review will mainly address age-related changes in proteasome structure and function in relation to the impact of oxidative stress on the proteasome and the accumulation of oxidized protein. Knowledge of molecular mechanisms involved in the decline of proteasome function during aging and in oxidative stress is expected to provide new insight that will be useful in defining antiaging strategies aimed at preserving this critical function.