Impact of Mitochondrial Architecture, Function, Redox Homeostasis, and Quality Control on Organismic Aging: Lessons from a Fungal Model System

Significance: Mitochondria are eukaryotic organelles with various essential functions. They are both the source and the targets of reactive oxygen species (ROS). Different branches of a mitochondrial quality control system (mQCS), such as ROS balancing, degradation of damaged proteins, or whole mitochondria, can mitigate the adverse effects of ROS stress. However, the capacity of mQCS is limited. Overwhelming this capacity leads to dysfunctions and aging. Strategies to interfere into mitochondria-dependent human aging with the aim to increase the healthy period of life, the health span, rely on the precise knowledge of mitochondrial functions. Experimental models such as Podospora anserina, a filamentous fungus with a clear mitochondrial aging etiology, proved to be instrumental to reach this goal. Recent Advances: Investigations of the P. anserina mQCS revealed that it is constituted by a complex network of different branches. Moreover, mitochondrial architecture and lipid homeostasis emerged to affect aging. Critical Issues: The regulation of the mQCS is only incompletely understood. Details about the involved signaling molecules and interacting pathways remain to be elucidated. Moreover, most of the currently generated experimental data were generated in well-controlled experiments that do not reflect the constantly changing natural life conditions and bear the danger to miss relevant aspects leading to incorrect conclusions. Future Directions: In P. anserina, the precise impact of redox signaling as well as of molecular damaging for aging remains to be defined. Moreover, natural fluctuation of environmental conditions needs to be considered to generate a realistic picture of aging mechanisms as they developed during evolution.

Structure and the mode of activity of Lon proteases from diverse organisms

Lon proteases, members of the AAA⁺ superfamily of enzymes, are key components of the protein quality control system in bacterial cells, as well as in the mitochondria and other specialized organelles of higher organisms. These enzymes have been subject of extensive biochemical and structural investigations, resulting in 72 crystal and solution structures, including structures of the individual domains, multi-domain constructs, and full-length proteins. However, interpretation of the latter structures still leaves some questions unanswered. Based on their amino acid sequence and details of their structure, Lon proteases can be divided into at least three subfamilies, designated as LonA, LonB, and LonC. Protomers of all Lons are single-chain polypeptides and contain two functional domains, ATPase and protease. The LonA enzymes additionally include a large N-terminal region, and different Lons may also include non-conserved inserts in the principal domains. These ATP-dependent proteases function as homohexamers, in which unfolded substrates are translocated to a large central chamber where they undergo proteolysis by a processive mechanism. X-ray crystal structures provided high-resolution models which verified that Lons are hydrolases with the rare Ser-Lys catalytic dyad. Full-length LonA enzymes have been investigated by cryo-electron microscopy (cryo-EM), providing description of the functional enzyme at different stages of the catalytic cycle, indicating extensive flexibility of their N-terminal domains, and revealing insights into the substrate translocation mechanism. Structural studies of Lon proteases provide an interesting case for symbiosis of X-ray crystallography and cryo-EM, currently the two principal techniques for determination of macromolecular structures.

Proteomic analysis demonstrates the role of the quality control protease LONP1 in mitochondrial protein aggregation

The mitochondrial matrix protease LONP1 is an essential part of the organellar protein quality control system. LONP1 has been shown to be involved in respiration control and apoptosis. Furthermore, a reduction in LONP1 level correlates with aging. Up to now, the effects of a LONP1 defect were mostly studied by utilizing transient, siRNA-mediated knockdown approaches. We generated a new cellular model system for studying the impact of LONP1 on mitochondrial protein homeostasis by a CRISPR/Cas-mediated genetic knockdown (gKD). These cells showed a stable reduction of LONP1 along with a mild phenotype characterized by absent morphological differences and only small negative effects on mitochondrial functions under normal culture conditions. To assess the consequences of a permanent LONP1 depletion on the mitochondrial proteome, we analyzed the alterations of protein levels by quantitative mass spectrometry, demonstrating small adaptive changes, in particular with respect to mitochondrial protein biogenesis. In an additional proteomic analysis, we determined the temperature-dependent aggregation behavior of mitochondrial proteins and its dependence on a reduction of LONP1 activity, demonstrating the important role of the protease for mitochondrial protein homeostasis in mammalian cells. We identified a significant number of mitochondrial proteins that are affected by a reduced LONP1 activity especially with respect to their stress-induced solubility. Taken together, our results suggest a very good applicability of the LONP1 gKD cell line as a model system for human aging processes.

CHIP control degradation of mutant ETF:QO through ubiquitylation in late-onset multiple acyl-CoA dehydrogenase deficiency

Late-onset multiple acyl-CoA dehydrogenase deficiency (MADD) is the most common form of lipid storage myopathy. The disease is mainly caused by mutations in electron-transfer flavoprotein dehydrogenase gene (ETFDH), which leads to decreased levels of ETF:QO in skeletal muscle. However, the specific underlying mechanisms triggering such degradation remain unknown. We constructed expression plasmids containing wild type ETF:QO and mutants ETF:QO-A84T, R175H, A215T, Y333C, and cultured patient-derived fibroblasts containing the following mutations in ETFDH: c.250G>A (p.A84T), c.998A>G (p.Y333C), c.770A>G (p.Y257C), c.1254_1257delAACT (p. L418TfsX10), c.524G>A (p.R175H), c.380T>A (p.L127P), and c.892C>T (p.P298S). We used in vitro expression systems and patient-derived fibroblasts to detect stability of ETF:QO mutants then evaluated their interaction with Hsp70 interacting protein CHIP with active/inactive ubiquitin E3 ligase carboxyl terminus using western blot and immunofluorescence staining. This interaction was confirmed in vitro and in vivo by co-immunoprecipitation and immunofluorescence staining. We confirmed the existence two ubiquitination sites in mutant ETF:QO using mass spectrometry (MS) analysis. We found that mutant ETF:QO proteins were unstable and easily degraded in patient fibroblasts and in vitro expression systems by ubiquitin-proteasome pathway, and identified the specific ubiquitin E3 ligase as CHIP, which forms complex to control mutant ETF:QO degradation through poly-ubiquitination. CHIP-dependent degradation of mutant ETF:QO proteins was confirmed by MS and site-directed mutagenesis of ubiquitination sites. Hsp70 is directly involved in this process as molecular chaperone of CHIP. CHIP plays an important role in ubiquitin-proteasome pathway dependent degradation of mutant ETF:QO by working as a chaperone-assisted E3 ligase, which reveals CHIP's potential role in pathological mechanisms of late-onset MADD.

Targeting cellular senescence based on interorganelle communication, multilevel proteostasis, and metabolic control

Cellular senescence, a stable cell division arrest caused by severe damage and stress, is a hallmark of aging in vertebrates including humans. With progressing age, senescent cells accumulate in a variety of mammalian tissues, where they contribute to tissue aging, identifying cellular senescence as a major target to delay or prevent aging. There is an increasing demand for the discovery of new classes of small molecules that would either avoid or postpone cellular senescence by selectively eliminating senescent cells from the body (i.e., ‘senolytics’) or inactivating/switching damage‐inducing properties of senescent cells (i.e., ‘senostatics/senomorphics’), such as the senescence‐associated secretory phenotype. Whereas compounds with senolytic or senostatic activity have already been described, their efficacy and specificity has not been fully established for clinical use yet. Here, we review mechanisms of senescence that are related to mitochondria and their interorganelle communication, and the involvement of proteostasis networks and metabolic control in the senescent phenotype. These cellular functions are associated with cellular senescence in in vitro and in vivo models but have not been fully exploited for the search of new compounds to counteract senescence yet. Therefore, we explore possibilities to target these mechanisms as new opportunities to selectively eliminate and/or disable senescent cells with the aim of tissue rejuvenation. We assume that this research will provide new compounds from the chemical space which act as mimetics of caloric restriction, modulators of calcium signaling and mitochondrial physiology, or as proteostasis optimizers, bearing the potential to counteract cellular senescence, thereby allowing healthy aging.

Functional wiring of proteostatic and mitostatic modules ensures transient organismal survival during imbalanced mitochondrial dynamics

Being an assembly of protein machines, cells depend on adequate supply of energetic molecules for retaining their homeodynamics. Consequently, mitochondria functionality is ensured by quality control systems and mitochondrial dynamics (fusion/fission). Similarly, proteome stability is maintained by the machineries of the proteostasis network. We report here that reduced mitochondrial fusion rates in Drosophila caused developmental lethality or if induced in the adult accelerated aging. Imbalanced mitochondrial dynamics were tolerable for various periods in young flies, where they caused oxidative stress and proteome instability that mobilized Nrf2 and foxo to upregulate cytoprotective antioxidant/proteostatic modules. Consistently, proteasome inhibition or Nrf2, foxo knock down in young flies exaggerated perturbed mitochondrial dynamics toxicity. Neither Nrf2 overexpression (with concomitant proteasome activation) nor Atg8a upregulation suppressed the deregulated mitochondrial dynamics toxicity, which was mildly mitigated by antioxidants. Thus, despite extensive functional wiring of mitostatic and antioxidant/proteostatic modules, sustained loss-of mitostasis exhausts adaptation responses triggering premature aging.

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Reduced mitochondrial fusion rates cause severe organismal toxicity and progeria.

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Perturbed mitostasis activates cytoprotective antioxidant and proteostatic modules.

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Nrf2 or Foxo KD exaggerates the imbalanced mitochondrial dynamics induced toxicity.

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Antioxidants mildly alleviate loss-of mitochondrial dynamics-mediated progeria.

Modulation of age related protein expression changes by gelam honey in cardiac mitochondrial rats

Aging is characterized by progressive decline in biochemical and physiological functions. According to the free radical theory of aging, aging results from oxidative damage due to the accumulation of excess reactive oxygen species (ROS). Mitochondria are the main source of ROS production and are also the main target for ROS. Therefore, a diet high in antioxidant such as honey is potentially able to protect the body from ROS and oxidative damage. Gelam honey is higher in flavonoid content and phenolic compounds compared to other local honey. This study was conducted to determine the effects of gelam honey on age related protein expression changes in cardiac mitochondrial rat. A total of 24 Sprague-Dawley male rats were divided into two groups: the young group (2 months old), and aged group (19 months old). Each group were then subdivided into two groups: control group (force-fed with distilled water), and treatment group (force-fed with gelam honey, 2.5 g/kg), and were treated for 8 months. Comparative proteomic analysis of mitochondria from cardiac tissue was then performed by high performance mass spectrometry (Q-TOF LCMS/MS) followed by validation of selected proteins by Western blotting. Proteins were identified using Spectrum Mill software and were subjected to stringent statistical analysis. A total of 286 proteins were identified in the young control group (YC) and 241 proteins were identified in the young gelam group (YG). In the aged group, a total of 243 proteins were identified in control group (OC), and 271 proteins in gelam group (OG). Comparative proteome profiling identified 69 proteins with different abundance (p < 0.05) in OC when compared to YC, and also in YG when compared to YC. On the other hand, 55 proteins were found to be different in abundance when comparing OG with OC. In the aged group, gelam honey supplementation affected the relative abundance of 52 proteins with most of these proteins showing a decrease in the control group. Bioinformatics analysis showed that the majority of the affected proteins were involved in the respiratory chain (OXPHOS) which play an important role in maintaining mitochondrial function.

LonP1 Differently Modulates Mitochondrial Function and Bioenergetics of Primary Versus Metastatic Colon Cancer Cells

Mitochondrial Lon protease (LonP1) is a multi-function enzyme that regulates mitochondrial functions in several human malignancies, including colorectal cancer (CRC). The mechanism(s) by which LonP1 contributes to colorectal carcinogenesis is not fully understood. We found that silencing LonP1 leads to severe mitochondrial impairment and apoptosis in colon cancer cells. Here, we investigate the role of LonP1 in mitochondrial functions, metabolism, and epithelial-mesenchymal transition (EMT) in colon tumor cells and in metastasis. LonP1 was almost absent in normal mucosa, gradually increased from aberrant crypt foci to adenoma, and was most abundant in CRC. Moreover, LonP1 was preferentially upregulated in colorectal samples with mutated p53 or nuclear β-catenin, and its overexpression led to increased levels of β-catenin and decreased levels of E-cadherin, key proteins in EMT, in vitro. LonP1 upregulation also induced opposite changes in oxidative phosphorylation, glycolysis, and pentose pathway in SW480 primary colon tumor cells when compared to SW620 metastatic colon cancer cells. In conclusion, basal LonP1 expression is essential for normal mitochondrial function, and increased LonP1 levels in SW480 and SW620 cells induce a metabolic shift toward glycolysis, leading to EMT.

Oxidative damage and impairment of protein quality control systems in keratinocytes exposed to a volatile organic compounds cocktail

Compelling evidence suggests that volatile organic compounds (VOCs) have potentially harmful effects to the skin. However, knowledge about cellular signaling events and toxicity subsequent to VOC exposure to human skin cells is still poorly documented. The aim of this study was to focus on the interaction between 5 different VOCs (hexane, toluene, acetaldehyde, formaldehyde and acetone) at doses mimicking chronic low level environmental exposure and the effect on human keratinocytes to get better insight into VOC-cell interactions. We provide evidence that the proteasome, a major intracellular proteolytic system which is involved in a broad array of processes such as cell cycle, apoptosis, transcription, DNA repair, protein quality control and antigen presentation, is a VOC target. Proteasome inactivation after VOC exposure is accompanied by apoptosis, DNA damage and protein oxidation. Lon protease, which degrades oxidized, dysfunctional, and misfolded proteins in the mitochondria is also a VOC target. Using human skin explants we found that VOCs prevent cell proliferation and also inhibit proteasome activity in vivo. Taken together, our findings provide insight into potential mechanisms of VOC-induced proteasome inactivation and the cellular consequences of these events.

Global Protein Oxidation Profiling Suggests Efficient Mitochondrial Proteome Homeostasis During Aging

The free radical theory of aging is based on the idea that reactive oxygen species (ROS) may lead to the accumulation of age-related protein oxidation. Because themajority of cellular ROS is generated at the respiratory electron transport chain, this study focuses on the mitochondrial proteome of the aging model Podospora anserina as target for ROS-induced damage. To ensure the detection of even low abundant modified peptides, separation by long gradient nLC-ESI-MS/MS and an appropriate statistical workflow for iTRAQ quantification was developed. Artificial protein oxidation was minimized by establishing gel-free sample preparation in the presence of reducing and iron-chelating agents. This first large scale, oxidative modification-centric study for P. anserina allowed the comprehensive quantification of 22 different oxidative amino acid modifications, and notably the quantitative comparison of oxidized and nonoxidized protein species. In total 2341 proteins were quantified. For 746 both protein species (unmodified and oxidatively modified) were detected and the modification sites determined. The data revealed that methionine residues are preferably oxidized. Further prominent identified modifications in decreasing order of occurrence were carbonylation as well as formation of N-formylkynurenine and pyrrolidinone. Interestingly, for the majority of proteins a positive correlation of changes in protein amount and oxidative damage were noticed, and a general decrease in protein amounts at late age. However, it was discovered that few proteins changed in oxidative damage in accordance with former reports. Our data suggest that P. anserina is efficiently capable to counteract ROS-induced protein damage during aging as long as protein de novo synthesis is functioning, ultimately leading to an overall constant relationship between damaged and undamaged protein species. These findings contradict a massive increase in protein oxidation during aging and rather suggest a protein damage homeostasis mechanism even at late age.

Mitochondrial Lon protease at the crossroads of oxidative stress, ageing and cancer

Lon protease is a nuclear DNA-encoded mitochondrial enzyme highly conserved throughout evolution, involved in the degradation of damaged and oxidized proteins of the mitochondrial matrix, in the correct folding of proteins imported in mitochondria, and in the maintenance of mitochondrial DNA. Lon expression is induced by various stimuli, including hypoxia and reactive oxygen species, and provides protection against cell stress. Lon down-regulation is associated with ageing and with cell senescence, while up-regulation is observed in tumour cells, and is correlated with a more aggressive phenotype of cancer. Lon up-regulation contributes to metabolic reprogramming observed in cancer, favours the switch from a respiratory to a glycolytic metabolism, helping cancer cell survival in the tumour microenvironment, and contributes to epithelial to mesenchymal transition. Silencing of Lon, or pharmacological inhibition of its activity, causes cell death in various cancer cells. Thus, Lon can be included in the growing class of proteins that are not responsible for oncogenic transformation, but that are essential for survival and proliferation of cancer cells, and that can be considered as a new target for development of anticancer drugs.

Mitochondrial proteases and protein quality control in ageing and longevity

Mitochondria have been implicated in the ageing process and the lifespan modulation of model organisms. Mitochondria are the main providers of energy in eukaryotic cells but also represent both a major source of reactive oxygen species and targets for protein oxidative damage. Since protein damage can impair mitochondrial function, mitochondrial proteases are critically important for protein maintenance and elimination of oxidized protein. In the mitochondrial matrix, protein quality control is mainly achieved by the Lon and Clp proteases which are also key players in damaged mitochondrial proteins degradation. Accumulation of damaged macromolecules resulting from oxidative stress and failure of protein maintenance constitutes a hallmark of cellular and organismal ageing and is believed to participate to the age-related decline of cellular function. Hence, age-related impairment of mitochondrial protein quality control may therefore contribute to the age-associated build-up of oxidized protein and alterations of mitochondrial redox and protein homeostasis.

Control of mitochondrial integrity in ageing and disease

Various molecular and cellular pathways are active in eukaryotes to control the quality and integrity of mitochondria. These pathways are involved in keeping a 'healthy' population of this essential organelle during the lifetime of the organism. Quality control (QC) systems counteract processes that lead to organellar dysfunction manifesting as degenerative diseases and ageing. We discuss disease- and ageing-related pathways involved in mitochondrial QC: mtDNA repair and reorganization, regeneration of oxidized amino acids, refolding and degradation of severely damaged proteins, degradation of whole mitochondria by mitophagy and finally programmed cell death. The control of the integrity of mtDNA and regulation of its expression is essential to remodel single proteins as well as mitochondrial complexes that determine mitochondrial functions. The redundancy of components, such as proteases, and the hierarchies of the QC raise questions about crosstalk between systems and their precise regulation. The understanding of the underlying mechanisms on the genomic, proteomic, organellar and cellular levels holds the key for the development of interventions for mitochondrial dysfunctions, degenerative processes, ageing and age-related diseases resulting from impairments of mitochondria.

Krebs cycle intermediates regulate DNA and histone methylation: epigenetic impact on the aging process

Many aging theories have proposed that mitochondria and energy metabolism have a major role in the aging process. There are recent studies indicating that Krebs cycle intermediates can shape the epigenetic landscape of chromatin by regulating DNA and histone methylation. A growing evidence indicates that epigenetics plays an important role in the regulation of healthspan but also is involved in the aging process. 2-Oxoglutarate (α-ketoglutarate) is a key metabolite in the Krebs cycle but it is also an obligatory substrate for 2-oxoglutarate-dependent dioxygenases (2-OGDO). The 2-OGDO enzyme family includes the major enzymes of DNA and histone demethylation, i.e. Ten-Eleven Translocation (TETs) and Jumonji C domain containing (JmjC) demethylases. In addition, 2-OGDO members can regulate collagen synthesis and hypoxic responses in a non-epigenetical manner. Interestingly, succinate and fumarate, also Krebs cycle intermediates, are potent inhibitors of 2-OGDO enzymes, i.e. the balance of Krebs cycle reactions can affect the level of DNA and histone methylation and thus control gene expression. We will review the epigenetic mechanisms through which Krebs cycle intermediates control the DNA and histone methylation. We propose that age-related disturbances in the Krebs cycle function induce stochastic epigenetic changes in chromatin structures which in turn promote the aging process.

Mitochondrial reactive oxygen species (ROS) in animal cells: relevance to aging and normal physiology

In animal mitochondria, the four electron reduction of molecular oxygen to produce water at respiratory complex IV is the terminal step in substrate oxidation. However, respiratory complexes I, II, and III can participate in the single electron reduction of oxygen to produce the radical species superoxide. This progenitor reactive oxygen species (ROS) participates in a number of reactions that generate other ROS. These molecules may react with, and damage, intracellular macromolecules, leading to cellular dysfunction. Mitochondrial ROS production is often considered from this perspective of macromolecular damage and is central to the “oxidative damage theory of aging”, which suggests the accumulation of oxidative damage in animal cells underlies the aging phenotype and limits lifespan. In this review, we discuss some experimental results accumulated over the past decade that are inconsistent with this theory. A limitation of the theory is that it presupposes mitochondrial ROS are inherently harmful. However, it is increasingly apparent that some basic cellular functions are physiologically regulated by normal levels of mitochondrial ROS. For example, cell growth and division, the apoptotic pathway, and mitochondrial fusion–fission dynamics all appear to be redox-regulated by mitochondrial ROS and perhaps the matrix manganese superoxide dismutase (MnSOD). Therefore, it is less clear how the balance between ROS regulation of normal cellular activities and ROS-mediated macromolecular damage is maintained and how this relates to aging and longevity in animals.

Evolution and significance of the Lon gene family in Arabidopsis organelle biogenesis and energy metabolism

Lon is the first identified ATP-dependent protease highly conserved across all kingdoms. Model plant species Arabidopsis thaliana has a small Lon gene family of four members. Although these genes share common structural features, they have distinct properties in terms of gene expression profile, subcellular targeting and substrate recognition motifs. This supports the notion that their functions under different environmental conditions are not necessarily redundant. This article intends to unravel the biological role of Lon proteases in energy metabolism and plant growth through an evolutionary perspective. Given that plants are sessile organisms exposed to diverse environmental conditions and plant organelles are semi-autonomous, it is tempting to suggest that Lon genes in Arabidopsis are paralogs. Adaptive evolution through repetitive gene duplication events of a single archaic gene led to Lon genes with complementing sets of subfunctions providing to the organism rapid adaptability for canonical development under different environmental conditions. Lon1 function is adequately characterized being involved in mitochondrial biogenesis, modulating carbon metabolism, oxidative phosphorylation and energy supply, all prerequisites for seed germination and seedling establishment. Lon is not a stand-alone proteolytic machine in plant organelles. Lon in association with other nuclear-encoded ATP-dependent proteases builds up an elegant nevertheless, tight interconnected circuit. This circuitry channels properly and accurately, proteostasis and protein quality control among the distinct subcellular compartments namely mitochondria, chloroplasts, and peroxisomes.

Effect of Lon protease knockdown on mitochondrial function in HeLa cells

ATP-dependent proteases are currently emerging as key regulators of mitochondrial functions. Among these proteolytic systems, Lon protease is involved in the control of selective protein turnover in the mitochondrial matrix. In the absence of Lon, yeast cells have been shown to accumulate electron-dense inclusion bodies in the matrix space, to loose integrity of mitochondrial genome and to be respiratory deficient. In order to address the role of Lon in mitochondrial functionality in human cells, we have set up a HeLa cell line stably transfected with a vector expressing a shRNA under the control of a promoter which is inducible with doxycycline. We have demonstrated that reduction of Lon protease results in a mild phenotype in this cell line in contrast with what have been observed in other cell types such as WI-38 fibroblasts. Nevertheless, deficiency in Lon protease led to an increase in ROS production and to an accumulation of carbonylated protein in the mitochondria. Our study suggests that Lon protease has a wide variety of targets and is likely to play different roles depending of the cell type.

Oxidative stress, neurodegeneration, and the balance of protein degradation and protein synthesis

Oxidative stress occurs in a variety of disease settings and is strongly linked to the development of neuron death and neuronal dysfunction. Cells are equipped with numerous pathways to prevent the genesis, as well as the consequences, of oxidative stress in the brain. In this review we discuss the various forms and sources of oxidative stress in the brain and briefly discuss some of the complexities in detecting the presence of oxidative stress. We then focus the review on the interplay between the diverse cellular proteolytic pathways and their roles in regulating oxidative stress in the brain. Additionally, we discuss the involvement of protein synthesis in regulating the downstream effects of oxidative stress. Together, these components of the review demonstrate that the removal of damaged proteins by effective proteolysis and the synthesis of new and protective proteins are vital in the preservation of brain homeostasis during periods of increased levels of reactive oxygen species. Last, studies from our laboratory and others have demonstrated that protein synthesis is intricately linked to the rates of protein degradation, with impairment of protein degradation sufficient to decrease the rates of protein synthesis, which has important implications for successfully responding to periods of oxidative stress. Specific neurodegenerative diseases, including Alzheimer disease, Parkinson disease, amyotrophic lateral sclerosis, and stroke, are discussed in this context. Taken together, these findings add to our understanding of how oxidative stress is effectively managed in the healthy brain and help elucidate how impairments in proteolysis and/or protein synthesis contribute to the development of neurodegeneration and neuronal dysfunction in a variety of clinical settings.

Cerebral ischemia-reperfusion-induced autophagy protects against neuronal injury by mitochondrial clearance

Cerebral ischemia-reperfusion (I-R) is a complex pathological process. Although autophagy can be evoked by ischemia, its involvement in the reperfusion phase after ischemia and its contribution to the fate of neurons remains largely unknown. In the present investigation, we found that autophagy was activated in the reperfusion phase, as revealed in both mice with middle cerebral artery occlusion and oxygen-glucose deprived cortical neurons in culture. Interestingly, in contrast to that in permanent ischemia, inhibition of autophagy (by 3-methyladenine, bafilomycin A 1, Atg7 knockdown or in atg5(-/-) MEF cells) in the reperfusion phase reinforced, rather than reduced, the brain and cell injury induced by I-R. Inhibition of autophagy either with 3-methyladenine or Atg7 knockdown enhanced the I-R-induced release of cytochrome c and the downstream activation of apoptosis. Moreover, MitoTracker Red-labeled neuronal mitochondria increasingly overlapped with GFP-LC3-labeled autophagosomes during reperfusion, suggesting the presence of mitophagy. The mitochondrial clearance in I-R was reversed by 3-methyladenine and Atg7 silencing, further suggesting that mitophagy underlies the neuroprotection by autophagy. In support, administration of the mitophagy inhibitor mdivi-1 in the reperfusion phase aggravated the ischemia-induced neuronal injury both in vivo and in vitro. PARK2 translocated to mitochondria during reperfusion and Park2 knockdown aggravated ischemia-induced neuronal cell death. In conclusion, the results indicated that autophagy plays different roles in cerebral ischemia and subsequent reperfusion. The protective role of autophagy during reperfusion may be attributable to mitophagy-related mitochondrial clearance and inhibition of downstream apoptosis. PARK2 may be involved in the mitophagy process.

Stress proteins in aging and life span

Heat shock proteins (HSP) are molecular chaperones and have been implicated in longevity and aging in many species. Their major functions include chaperoning misfolded or newly synthesised polypeptides, protecting cells from proteotoxic stress, and processing of immunogenic agents. These proteins are expressed constitutively and can be induced by stresses such as heat, oxidative stress and many more. The induction of HSP in aging could potentially maintain protein homeostasis and longevity by refolding the damaged proteins which accumulate during aging and are toxic to cells. HSP are shown to increase life span in model organisms such as Caenorhabditis elegans and decrease aging-related proteotoxicity. Thus, decrease in HSP in aging is associated with disruption of cellular homeostasis which causes diseases such as cancer, cell senescence and neurodegeneration. HSP levels are decreased with aging in most organs including neurons. Aging also causes attenuation or alteration of many signalling pathways as well as the expression of transcription factors such as heat shock factor (HSF). The alteration in regulation and synthesis of Forkhead box O3a (FoxO3a) family of transcription factors as well as major antioxidant enzymes (manganese superoxide dismutase, catalase) are also seen in aging. Among many signalling mechanisms involved in altering longevity and aging, the insulin/IGF-1 pathway and the Sir2 deacetylase are highly significant. This review enquires into the role of some of these pathways in longevity/aging along with HSP.

Mitochondrial quality control: impact on aging and life span - a mini-review

A fundamental impact of mitochondria on biological aging has been suggested decades ago. One prominent theory explains aging as the result of the age-related accumulation of random molecular damage of biomolecules resulting from the reaction of reactive oxygen species, the majority of which are generated in mitochondria. Although this concept appeared to be very attractive and strongly influenced aging research, in recent years more and more data accumulated which seem to contradict this theory. However, since these data are derived from reductionist approaches and do not integrate the various components and pathways which are affected as a result of a primary experimental intervention, they are prone to misinterpretation and have to be taken with some caution. Here, after a general introduction of mitochondrial function, we discuss the relevance of various pathways which are involved in keeping mitochondria functional over time. Moreover, we provide examples which emphasize the importance of a critical interpretation of experimental data and the necessity for a holistic analysis of the aging process. The success of such a systems biology approach is strongly dependent on the development of methods for data mining and an efficient analysis and modeling of the huge data sets that are raised.

S1 kills MCF-7/ADR cells more than MCF-7 cells: A protective mechanism of endoplasmic reticulum stress

Drug resistance in chemotherapy for breast cancer is associated with high levels of P-glycoprotein (P-gp) as well as endoplasmic reticulum (ER) stress. In this paper, we aimed to evaluate the efficacy of a pan-BH3 mimetic S1 on drug-resistant MCF-7/ADR cells, and the roles of S1-induced ER stress in cell death. S1 exhibited greater and faster mitochondrial apoptosis in MCF-7/ADR cells than in MCF-7 cells. We demonstrated by Bax/Bak activation and cyrochrome c release that the p-glycprotein had little influence on S1 entering cells and hitting its targets in MCF-7/ADR cells. An IRE1-mediated ER stress response followed by c-Jun N-terminal kinase (JNK) and extracellular signal-regulated protein kinase (ERK) activation was specifically induced by S1 in MCF-7 cells, but not in MCF-7/ADR cells. Coimmunoprecipitation and western blotting analysis determined that ER stress played a protective role in S1-induced apoptosis by triggering Bcl-2 phosphorylation, which grabbed more pro-apoptotic proteins. The synergism effect of ERK inhibitor PD98059 with S1 confirmed the protective role of ER stress. Defective ER stress in MCF-7/ADR cells confers the more sensitivity toward S1.

Quality control of mitochondria during aging: is there a good and a bad side of mitochondrial dynamics?

Maintenance of functional mitochondria is essential in order to prevent degenerative processes leading to disease and aging. Mitochondrial dynamics plays a crucial role in ensuring mitochondrial quality but may also generate and spread molecular damage through a population of mitochondria. Computational simulations suggest that this dynamics is advantageous when mitochondria are not or only marginally damaged. In contrast, at a higher degree of damage, mitochondrial dynamics may be disadvantageous. Deceleration of fusion-fission cycles could be one way to adapt to this situation and to delay a further decline in mitochondrial quality. However, this adaptive response makes the mitochondrial network more vulnerable to additional molecular damage. The "mitochondrial infectious damage adaptation" (MIDA) model explains a number of inconsistent and counterintuitive data such as the "clonal expansion" of mutant mitochondrial DNA. We propose that mitochondrial dynamics is a double-edged sword and suggest ways to test this experimentally.

Aging of the dopaminergic system and motor behavior in mice intoxicated with the parkinsonian toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication of mice is a standard model of Parkinson's disease (PD). However, it does not reproduce functionally PD. Given the occurrence of PD during aging, symptoms might only be detected in MPTP-intoxicated mice after aging. To address this, mice injected with MPTP at 2.5 months were followed up to a maximum age of 21 months. There was no loss of dopamine cells with aging in control mice; moreover, the initial post-MPTP intoxication decrease in dopamine cell was no longer significant at 21 months. With aging, striatal dopamine level remained constant, but concentrations of the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were markedly reduced in both groups. There was also a late impairment of fine motor skills. After MPTP intoxication, hyperactivity was immediately detected and it became greater than in control mice from 14 months of age; fine motor skills were also more impaired; both these symptoms were correlated with striatal dopamine, DOPAC and HVA concentrations. In bothgroups, neither motor symptoms nor dopamine changes worsened with age. These findings do not support the notion that PD develops with age in mice after MPTP intoxication and that the motor deficits seen are because of an aging process.

The multifaceted role of Lon proteolysis in seedling establishment and maintenance of plant organelle function: living from protein destruction

Intracellular selective proteolysis is an important post-translational regulatory mechanism maintaining protein quality control by removing defective, damaged or even deleterious protein aggregates. The ATP-dependent Lon protease is a key component of protein quality control that is highly conserved across the kingdoms of living organisms. Major advancements have been made in bacteria and in non-plant organisms to understand the role of Lon in protection against protein oxidation, ageing and neurodegenerative diseases. This review presents the progress currently made in plants. The Lon gene family in Arabidopsis consists of four members that produce distinct protein isoforms localized in several organelles. Lon1 and Lon4 that potentially originate from a recent gene duplication event are dual-targeted to mitochondria and chloroplasts through distinct mechanisms revealing divergent evolution. Arabidopsis mutant analysis showed that mitochondria and peroxisomes biogenesis or maintenance of function is modulated by Lon1 and Lon2, respectively. Consequently, the lack of Lon selective proteolysis leading to growth retardation and impaired seedling establishment can be attributed to defects in the oil reserve mobilization pathway. The current progress in Arabidopsis research uncovers the role of Lon in the proteome homeostasis of plant organelles and stimulates biotechnology scenarios of plant tolerance against harsh abiotic conditions because of climate instability.

Activation of the cAMP cascade in human fibroblast cultures rescues the activity of oxidatively damaged complex I

A study of the relationship between cAMP/PKA-dependent phosphorylation and oxidative damage of subunits of complex I of the mitochondrial respiratory chain is presented. It is shown that, in fibroblast cultures, PKA-mediated phosphorylation of the NDUFS4 subunit of complex I rescues the activity of the oxidatively damaged complex. Evidence is presented showing that this effect is mediated by phosphorylation-dependent exchange of carbonylated NDUFS4 subunit in the assembled complex with the de novo synthesized subunit. These results indicate a potential use for β-adrenoceptor agonists in preventing/reversing the detrimental effects of oxidative stress in the mitochondrial respiratory system.

Protein quality control in neurodegenerative disease

The accumulation of misfolded proteins is a common feature of many neurodegenerative diseases. These observations suggest a potential link between these disorders and protein quality control, a collection of cellular pathways that sense damage to proteins and facilitate their turnover. Consistent with this idea, activation of quality control components, such as molecular chaperones, has been shown to be protective in multiple neurodegenerative disease models. In addition, key studies have suggested that quality control deteriorates with age, further supporting a relationship between these processes. In this chapter, we discuss the evidence linking neurodegeneration to quality control and present the emerging models. We also speculate on why proper quality control is so difficult for certain proteins.

Mitochondrial calpain 10 is degraded by Lon protease after oxidant injury

Calpain 10 is ubiquitously expressed and is one of four mitochondrial matrix proteases. We determined that over-expression or knock-down of mitochondrial calpain 10 results in cell death, demonstrating that mitochondrial calpain 10 is required for viability. Thus, we studied calpain 10 degradation in isolated mitochondrial matrix, mitochondria and in renal proximal tubular cells (RPTC) under control and toxic conditions. Using isolated renal cortical mitochondria and mitochondrial matrix, calpain 10 underwent rapid degradation at 37°C that was blocked with Lon inhibitors but not by calpain or proteasome inhibitors. While exogenous Ca(2+) addition, Ca(2+) chelation or exogenous ATP addition had no effect on calpain 10 degradation, the oxidants tert-butyl hydroperoxide (TBHP) or H(2)O(2) increased the rate of degradation. Using RPTC, mitochondrial and cytosolic calpain 10 increased in the presence of MG132 (Lon/proteasome inhibitor) but only cytosolic calpain 10 increased in the presence of epoxomicin (proteasome inhibitor). Furthermore, TBHP and H(2)O(2) oxidized mitochondrial calpain 10, decreased mitochondrial, but not cytosolic calpain 10, and pretreatment with MG132 blocked TBHP-induced degradation of calpain 10. In summary, mitochondrial calpain 10 is selectively degraded by Lon protease under basal conditions and is enhanced under and oxidizing conditions, while cytosolic calpain 10 is degraded by the proteasome.

Two proteases, trypsin domain-containing 1 (Tysnd1) and peroxisomal lon protease (PsLon), cooperatively regulate fatty acid β-oxidation in peroxisomal matrix

The molecular mechanisms underlying protein turnover and enzyme regulation in the peroxisomal matrix remain largely unknown. Trypsin domain-containing 1 (Tysnd1) and peroxisomal Lon protease (PsLon) are newly identified peroxisomal matrix proteins that harbor both a serine protease-like domain and a peroxisome-targeting signal 1 (PTS1) sequence. Tysnd1 processes several PTS1-containing proteins and cleaves N-terminal presequences from PTS2-containing protein precursors. Here we report that knockdown of Tysnd1, but not PsLon, resulted in accumulation of endogenous β-oxidation enzymes in their premature form. The protease activity of Tysnd1 was inactivated by intermolecular self-conversion of the 60-kDa form to 15- and 45-kDa chains, which were preferentially degraded by PsLon. Peroxisomal β-oxidation of a very long fatty acid was significantly decreased by knockdown of Tysnd1 and partially lowered by PsLon knockdown. Taken together, these data suggest that Tysnd1 is a key regulator of the peroxisomal β-oxidation pathway via proteolytic processing of β-oxidation enzymes. The proteolytic activity of oligomeric Tysnd1 is in turn controlled by self-cleavage of Tysnd1 and degradation of Tysnd1 cleavage products by PsLon.

Phosphorylation of Serine 114 on Atg32 mediates mitophagy

Mitophagy, which selectively degrades mitochondria via autophagy, has a significant role in mitochondrial quality control. When mitophagy is induced in yeast, mitochondrial residential protein Atg32 binds Atg11, an adaptor protein for selective types of autophagy, and it is recruited into the vacuole along with mitochondria. The Atg11-Atg32 interaction is believed to be the initial molecular step in which the autophagic machinery recognizes mitochondria as a cargo, although how this interaction is mediated is poorly understood. Therefore, we studied the Atg11-Atg32 interaction in detail. We found that the C-terminus region of Atg11, which included the fourth coiled-coil domain, interacted with the N-terminus region of Atg32 (residues 100-120). When mitophagy was induced, Ser-114 and Ser-119 on Atg32 were phosphorylated, and then the phosphorylation of Atg32, especially phosphorylation of Ser-114 on Atg32, mediated the Atg11-Atg32 interaction and mitophagy. These findings suggest that cells can regulate the amount of mitochondria, or select specific mitochondria (damaged or aged) that are degraded by mitophagy, by controlling the activity and/or localization of the kinase that phosphorylates Atg32. We also found that Hog1 and Pbs2, which are involved in the osmoregulatory signal transduction cascade, are related to Atg32 phosphorylation and mitophagy.

Rat liver mitochondrial proteome: changes associated with aging and acetyl-L-carnitine treatment

Oxidative stress has a central role in aging and in several age-linked diseases such as neurodegenerative diseases, diabetes and cancer. Mitochondria, as the main cellular source and target of reactive oxygen species (ROS) in aging, are recognized as very important players in the above reported diseases. Impaired mitochondrial oxidative phosphorylation has been reported in several aging tissues. Defective mitochondria are not only responsible of bioenergetically less efficient cells but also increase ROS production further contributing to tissues oxidative stress. Acetyl-L-carnitine (ALCAR) is a biomolecule able to limit age-linked mitochondrial decay in brain, liver, heart and skeletal muscles by increasing mitochondrial efficiency. Here the global changes induced by aging and by ALCAR supplementation to old rat on the mitochondrial proteome of rat liver has been analyzed by means of the two-dimensional polyacrylamide gel electrophoresis. Mass spectrometry has been used to identify the differentially expressed proteins. A significant age-related change occurred in 31 proteins involved in several metabolisms. ALCAR supplementation altered the levels of 26 proteins. In particular, ALCAR reversed the age-related alterations of 10 mitochondrial proteins relative to mitochondrial cristae morphology, to the oxidative phosphorylation and antioxidant systems, to urea cycle, to purine biosynthesis.

Downregulation of mitochondrial lon protease impairs mitochondrial function and causes hepatic insulin resistance in human liver SK-HEP-1 cells

AIMS/HYPOTHESIS

Lon protease degrades oxidatively damaged proteins in mitochondrial matrix. To examine the relationships between mitochondrial quality control, mitochondrial functions and diabetes, we investigated whether lon protease deficiency influences insulin resistance by affecting mitochondrial function.

METHODS

Lon protease-specific small interfering RNA (siRNA) was transfected into human liver SK-HEP-1 cells and changes in molecules related to insulin resistance were analysed.

RESULTS

Reduction in lon protease was achieved using specific siRNA-induced mitochondrial dysfunction in human liver SK-HEP-1 cells. Concurrently, insulin signalling and subsequent insulin action were impaired and levels of gluconeogenic enzymes were increased by lon protein deficiency. Moreover, the activity of mitogen-activated protein kinases and transcription factors related to hepatic gluconeogenesis were elevated in LON (also known as LONP1) siRNA-transfected cells via increased intracellular reactive oxygen species production. Overproduction of lon protease restored mitochondrial function and also diminished the insulin resistance induced by treatment with cholesterol and palmitate. In addition, levels of lon protease decreased dramatically in livers of diabetic db/db mice compared with their lean mice counterparts.

CONCLUSIONS/INTERPRETATION

Here we have demonstrated that reduction of lon protease induced hepatic insulin resistance by lowering mitochondrial function. This is the first study to report that defects in mitochondrial protein quality control could cause insulin resistance and diabetes.

Proteomic Profiling of Mitochondrial Enzymes during Skeletal Muscle Aging

Mitochondria are of central importance for energy generation in skeletal muscles. Expression changes or functional alterations in mitochondrial enzymes play a key role during myogenesis, fibre maturation, and various neuromuscular pathologies, as well as natural fibre aging. Mass spectrometry-based proteomics suggests itself as a convenient large-scale and high-throughput approach to catalogue the mitochondrial protein complement and determine global changes during health and disease. This paper gives a brief overview of the relatively new field of mitochondrial proteomics and discusses the findings from recent proteomic surveys of mitochondrial elements in aged skeletal muscles. Changes in the abundance, biochemical activity, subcellular localization, and/or posttranslational modifications in key mitochondrial enzymes might be useful as novel biomarkers of aging. In the long term, this may advance diagnostic procedures, improve the monitoring of disease progression, help in the testing of side effects due to new drug regimes, and enhance our molecular understanding of age-related muscle degeneration.

Possible existence of lysosome-like organella within mitochondria and its role in mitochondrial quality control

The accumulation of unhealthy mitochondria results in mitochondrial dysfunction, which has been implicated in aging, cancer, and a variety of degenerative diseases. However, the mechanism by which mitochondrial quality is regulated remains unclear. Here, we show that Mieap, a novel p53-inducible protein, induces intramitochondrial lysosome-like organella that plays a critical role in mitochondrial quality control. Mieap expression is directly regulated by p53 and is frequently lost in human cancer as result of DNA methylation. Mieap dramatically induces the accumulation of lysosomal proteins within mitochondria and mitochondrial acidic condition without destroying the mitochondrial structure (designated MALM, for Mieap-induced accumulation of lysosome-like organelles within mitochondria) in response to mitochondrial damage. MALM was not related to canonical autophagy. MALM is involved in the degradation of oxidized mitochondrial proteins, leading to increased ATP synthesis and decreased reactive oxygen species generation. These results suggest that Mieap induces intramitochondrial lysosome-like organella that plays a critical role in mitochondrial quality control by eliminating oxidized mitochondrial proteins. Cancer cells might accumulate unhealthy mitochondria due to p53 mutations and/or Mieap methylation, representing a potential cause of the Warburg effect.

Mieap, a p53-inducible protein, controls mitochondrial quality by repairing or eliminating unhealthy mitochondria

Maintenance of healthy mitochondria prevents aging, cancer, and a variety of degenerative diseases that are due to the result of defective mitochondrial quality control (MQC). Recently, we discovered a novel mechanism for MQC, in which Mieap induces intramitochondrial lysosome-like organella that plays a critical role in the elimination of oxidized mitochondrial proteins (designated MALM for Mieap-induced accumulation of lysosome-like organelles within mitochondria). However, a large part of the mechanisms for MQC remains unknown. Here, we report additional mechanisms for Mieap-regulated MQC. Reactive oxygen species (ROS) scavengers completely inhibited MALM. A mitochondrial outer membrane protein NIX interacted with Mieap in a ROS-dependent manner via the BH3 domain of NIX and the coiled-coil domain of Mieap. Deficiency of NIX also completely impaired MALM. When MALM was inhibited, Mieap induced vacuole-like structures (designated as MIV for Mieap-induced vacuole), which engulfed and degraded the unhealthy mitochondria by accumulating lysosomes. The inactivation of p53 severely impaired both MALM and MIV generation, leading to accumulation of unhealthy mitochondria. These results suggest that (1) mitochondrial ROS and NIX are essential factors for MALM, (2) MIV is a novel mechanism for lysosomal degradation of mitochondria, and (3) the p53-Mieap pathway plays a pivotal role in MQC by repairing or eliminating unhealthy mitochondria via MALM or MIV generation, respectively.

Mitochondrial Lon protease regulates mitochondrial DNA copy number and transcription by selective degradation of mitochondrial transcription factor A (TFAM)

Lon is the major protease in the mitochondrial matrix in eukaryotes, and is well conserved among species. Although a role for Lon in mitochondrial biogenesis has been proposed, the mechanistic basis is unclear. Here, we demonstrate a role for Lon in mtDNA metabolism. An RNA interference (RNAi) construct was designed that reduces Lon to less than 10% of its normal level in Drosophila Schneider cells. RNAi knockdown of Lon results in increased abundance of mitochondrial transcription factor A (TFAM) and mtDNA copy number. In a corollary manner, overexpression of Lon reduces TFAM levels and mtDNA copy number. Notably, induction of mtDNA depletion in Lon knockdown cells does not result in degradation of TFAM, thereby causing a dramatic increase in the TFAMmtDNA ratio. The increased TFAMmtDNA ratio in turn causes inhibition of mitochondrial transcription. We conclude that Lon regulates mitochondrial transcription by stabilizing the mitochondrial TFAMmtDNA ratio via selective degradation of TFAM.

Oxidized mitochondrial protein degradation and repair in aging and oxidative stress

Proteins are main targets for oxidative damage that occurs during aging and in oxidative stress situations. Since the mitochondria is a major source of reactive oxygen species, mitochondrial proteins are especially exposed to oxidative modification, and elimination of oxidized proteins is crucial for maintaining the integrity of this organelle. Hence, enzymatic reversal of protein oxidation and protein degradation is critical for protein homeostasis while protein maintenance failure has been implicated in the age-related accumulation of oxidized proteins. Within the mitochondrial matrix, the ATP-stimulated mitochondrial Lon protease is believed to play an important role in the degradation of oxidized protein, and age-associated impairment of Lon-like protease activity has been suggested to contribute to oxidized protein buildup in the mitochondria. Oxidized protein repair is limited to certain oxidation products of the sulfur-containing amino acids cysteine and methionine. Oxidized protein repair systems, thioredoxin/thioredoxin reductase or glutaredoxin/glutathione/glutathione reductase that catalytically reduce disulfide bridges or sulfenic acids, and methionine sulfoxide reductase that reverses methionine sulfoxide back to methionine within proteins, are present in the mitochondrial matrix. Thus, the role of the mitochondrial Lon protease and the oxidized protein repair system methionine sulfoxide reductase is further addressed in the context of oxidative stress and aging.

Regulation of mitochondrial processes: a target for heart failure

Cardiac mitochondria, the main source of energy as well as free radicals, are vital organelles for normal functioning of the heart. Mitochondrial number, structure, turnover and function are regulated by processes such as mitochondrial protein quality control, mitochondrial fusion and fission and mitophagy. Recent studies suggest that abnormal changes in these mitochondrial regulatory processes may contribute to the pathology of heart failure (HF). Here we discuss these processes and their potential as therapeutic targets.

Salsolinol, a catechol neurotoxin, induces modification of ferritin: Protection by histidine dipeptide

1-Methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol), an endogenous neurotoxin present in the mammalian brain, is known to perform a role in the pathogenesis of Parkinson's disease. In this study, we evaluated oxidative modifications of ferritin occurring after incubation with salsolinol. When ferritin was incubated with salsolinol, protein aggregation increased in a time-dependent manner. Free radical scavengers inhibited this salsolinol-mediated ferritin modification. The exposure of ferritin to salsolinol also results in the generation of protein carbonyl compounds and the formation of dityrosine. The results of this study show that free radicals may perform a pivotal role in salsolinol-mediated ferritin modification. Histidine dipeptides, such as carnosine, have been proposed to function as antioxidant agents in vivo. In this study, we also attempted to determine whether the histidine dipeptides, carnosine and N-acetyl-carnosine, could prevent salsolinol-mediated oxidative modification of ferritin. Our results showed that both carnosine and N-acetyl-carnosine significantly reduced ferritin aggregation. Both compounds effectively inhibited the formation of both carbonyl compounds and dityrosine. These results suggest that carnosine derivatives can, indeed, protect against salsolinol-mediated ferritin modification, as the consequence of free radical-scavenging activity.

Methionine-oxidized amyloid fibrils are poor substrates for human methionine sulfoxide reductases A and B2

A common feature of many amyloid diseases is the appearance of oxidized, aggregated proteins. Methionine is one of the most readily oxidized amino acids, and its oxidative state is regulated in vivo by the methionine sulfoxide reductases (Msr). Here, we have explored the basis by which methionine oxidation is linked to amyloid disease by comparing the reduction of oxidized amyloid fibrils and monomer. We show that oxidized amyloid fibrils are not as effectively reduced by the Msr enzymes as the monomer. This work suggests a mechanism by which oxidized proteins and aggregates can accumulate as a part of degenerative disease.

Quantitation of (a)symmetric inheritance of functional and of oxidatively damaged mitochondrial aconitase in the cell division of old yeast mother cells

Asymmetric segregation of oxidatively damaged proteins is discussed in the literature as a mechanism in cell division cycles which at the same time causes rejuvenation of the daughter cell and aging of the mother cell. This process must be viewed as cooperating with the cellular degradation processes like autophagy, proteasomal degradation and others. Together, these two mechanisms guarantee survival of the species and prevent clonal senescence of unicellular organisms, like yeast. It is widely believed that oxidative damage to proteins is primarily caused by oxygen radicals and their follow-up products produced in the mitochondria. As we have shown previously, old yeast mother cells in contrast to young cells contain reactive oxygen species and undergo programmed cell death. Here we show that aconitase of the mitochondrial matrix is readily inactivated by oxidative stress, but even in its inactive form is relatively long-lived and retains fluorescence in the Aco1p-eGFP form. The fluorescent protein is distributed between old mothers and their daughters approximately corresponding to the different sizes of mother and daughter cells. However, the remaining active enzyme is primarily inherited by the daughter cells. This indicates that asymmetric distribution of the still active enzyme takes place and a mechanism for discrimination between active and inactive enzyme must exist. As the aconitase remains mitochondrial during aging and cell division, our findings could indicate discrimination between active and no longer active mitochondria during the process.

Proteomic DIGE analysis of the mitochondria-enriched fraction from aged rat skeletal muscle

Skeletal muscle aging is associated with a loss in tissue mass and contractile strength, as well as fiber type shifting and bioenergetic adaptation processes. Since mitochondria represent the primary site for energy generation via oxidative phosphorylation, we investigated potential changes in the expression pattern of the mitochondrial proteome using the highly sensitive DIGE approach. The comparative analysis of the mitochondria-enriched fraction from young adult versus aged muscle revealed an age-related change in abundance for 39 protein species. MS technology identified the majority of altered proteins as constituents of muscle mitochondria. An age-dependent increase was observed for NADH dehydrogenase, the mitochondrial inner membrane protein mitofilin, peroxiredoxin isoform PRX-III, ATPase synthase, succinate dehydrogenase, mitochondrial fission protein Fis1, succinate-coenzyme A ligase, acyl-coenzyme A dehydrogenase, porin isoform VDAC2, ubiquinol-cytochrome c reductase core I protein and prohibitin. Immunoblotting, enzyme testing and confocal microscopy were used to validate proteomic findings. The DIGE-identified increase in key mitochondrial elements during aging agrees with the concept that sarcopenia is associated with a shift to a slower contractile phenotype and more pronounced aerobic-oxidative metabolism. This suggests that mitochondrial markers are reliable candidates that should be included in the future establishment of a biomarker signature of skeletal muscle aging.