Lon protease preferentially degrades oxidized mitochondrial aconitase by an ATP-stimulated mechanism

Lon protease inactivation in Drosophila causes unfolded protein stress and inhibition of mitochondrial translation

Mitochondrial dysfunction is a frequent participant in common diseases and a principal suspect in aging. To combat mitochondrial dysfunction, eukaryotes have evolved a large repertoire of quality control mechanisms. One such mechanism involves the selective degradation of damaged or misfolded mitochondrial proteins by mitochondrial resident proteases, including proteases of the ATPase Associated with diverse cellular Activities (AAA⁺) family. The importance of the AAA⁺ family of mitochondrial proteases is exemplified by the fact that mutations that impair their functions cause a variety of human diseases, yet our knowledge of the cellular responses to their inactivation is limited. To address this matter, we created and characterized flies with complete or partial inactivation of the Drosophila matrix-localized AAA⁺ protease Lon. We found that a Lon null allele confers early larval lethality and that severely reducing Lon expression using RNAi results in shortened lifespan, locomotor impairment, and respiratory defects specific to respiratory chain complexes that contain mitochondrially encoded subunits. The respiratory chain defects of Lon knockdown (Lon^KD) flies appeared to result from severely reduced translation of mitochondrially encoded genes. This translational defect was not a consequence of reduced mitochondrial transcription, as evidenced by the fact that mitochondrial transcripts were elevated in abundance in Lon^KD flies. Rather, the translational defect of Lon^KD flies appeared to be derived from sequestration of mitochondrially encoded transcripts in highly dense ribonucleoparticles. The translational defect of Lon^KD flies was also accompanied by a substantial increase in unfolded mitochondrial proteins. Together, our findings suggest that the accumulation of unfolded mitochondrial proteins triggers a stress response that culminates in the inhibition of mitochondrial translation. Our work provides a foundation to explore the underlying molecular mechanisms.

Study on the oxidation of fibrinogen using Fe3O4 magnetic nanoparticles and its influence to the formation of fibrin

Oxidative stress accompanies various diseases associated with chronic inflammation. In this work, H₂O₂ and H₂O₂-Fe₃O₄ magnetic nanoparticles were used as two reactive oxygen species to study the oxidative stress for the structure and polymerization behaviour of fibrinogen molecules. The alterations of secondary structure and component of fibrinogen molecule were characterized by circular dichroism spectra, ultraviolet-visible spectra and fluorescence spectra, the viscoelasticity of fibrinogen solution was studied by dynamic light scattering microrheology. Based on the molecular dynamics simulations and fluorescence properties, the possible oxidative stress sites were analyzed and confirmed by Tb³⁺ probe. The hydrophobicity/philicity and electrostatic net charges present on the exterior part of the fibrinogen molecules were measured with zeta potential. The height and image analysis obtained from atomic force microscope indicated that oxidative stress of fibrinogen molecules could influence the equilateral junctions of protofibrils and the different cross-linking patterns between the α- and γ-chains, result in the decrease of the fibre size, form a higher proportion of branching and a denser aggregation state. This study will provide insights into the misfolding and fibril formation of disease-associated fibrinogen, facilitate an increased understanding of how oxidative stress in vivo affects the formation and polymerization of fibrin, and support efforts for the improved treatment of patients suffering from the thrombotic disease.

Aerobic Exercise Preconception and During Pregnancy Enhances Oxidative Capacity in the Hindlimb Muscles of Mice Offspring

Liu, J, Lee, I, Feng, H-Z, Galen, SS, Hüttemann, PP, Perkins, GA, Jin, J-P, Hüttemann, M, and Malek, MH. Aerobic exercise preconception and during pregnancy enhances oxidative capacity in the hindlimb muscles of mice offspring. J Strength Cond Res 32(5): 1391-1403, 2018-Little is known about the effect of maternal exercise on offspring skeletal muscle health. The purpose of this study, therefore, was to determine whether maternal exercise (preconception and during pregnancy) alters offspring skeletal muscle capillarity and mitochondrial biogenesis. We hypothesized that offspring from exercised dams would have higher capillarity and mitochondrial density in the hindlimb muscles compared with offspring from sedentary dams. Female mice in the exercise condition had access to a running wheel in their individual cage 30 days before mating and throughout pregnancy, whereas the sedentary group did not have access to the running wheel before mating and during pregnancy. Male offspring from both groups were killed when they were 2 months old, and their tissues were analyzed. The results indicated no significant (p > 0.05) mean differences for capillarity density, capillarity-to-fiber ratio, or regulators of angiogenesis such as VEGF-A and TSP-1. Compared with offspring from sedentary dams, however, offspring from exercised dams had an increase in protein expression of myosin heavy chain type I (MHC I) (∼134%; p = 0.009), but no change in MHC II. For mitochondrial morphology, we found significant (all p-values ≤ 0.0124) increases in mitochondrial volume density (∼55%) and length (∼18%) as well as mitochondria per unit area (∼19%). For mitochondrial enzymes, there were also significant (all p-values ≤ 0.0058) increases in basal citrate synthase (∼79%) and cytochrome c oxidase activity (∼67%) in the nonoxidative muscle fibers as well as increases in basal (ATP) (∼52%). Last, there were also significant mean differences in protein expression for regulators (FIS1, Lon protease, and TFAM) of mitochondrial biogenesis. These findings suggest that maternal exercise before and during pregnancy enhances offspring skeletal muscle mitochondria functionality, but not capillarity.

Adaptive homeostasis and the free radical theory of ageing

The Free Radical Theory of Ageing, was first proposed by Denham Harman in the mid-1950's, based largely on work conducted by Rebeca Gerschman and Daniel Gilbert. At its core, the Free Radical Theory of Ageing posits that free radical and related oxidants, from the environment and internal metabolism, cause damage to cellular constituents that, over time, result in an accumulation of structural and functional problems. Several variations on the original concept have been advanced over the past six decades, including the suggestion of a central role for mitochondria-derived reactive species, and the proposal of an age-related decline in the effectiveness of protein, lipid, and DNA repair systems. Such innovations have helped the Free Radical Theory of Aging to achieve widespread popularity. Nevertheless, an ever-growing number of apparent 'exceptions' to the Theory have seriously undermined its acceptance. In part, we suggest, this has resulted from a rather simplistic experimental approach of knocking-out, knocking-down, knocking-in, or overexpressing antioxidant-related genes to determine effects on lifespan. In some cases such experiments have yielded results that appear to support the Free Radical Theory of Aging, but there are just as many published papers that appear to contradict the Theory. We suggest that free radicals and related oxidants are but one subset of stressors with which all life forms must cope over their lifespans. Adaptive Homeostasis is the mechanism by which organisms dynamically expand or contract the homeostatic range of stress defense and repair systems, employing a veritable armory of signal transduction pathways (such as the Keap1-Nrf2 system) to generate a complex profile of inducible and enzymatic protection that best fits the particular need. Viewed as a component of Adaptive Homeostasis, the Free Radical Theory of Aging appears both viable and robust.

Skeletal muscle mitochondrial remodeling in exercise and diseases

Skeletal muscle fitness and plasticity is an important determinant of human health and disease. Mitochondria are essential for maintaining skeletal muscle energy homeostasis by adaptive re-programming to meet the demands imposed by a myriad of physiologic or pathophysiological stresses. Skeletal muscle mitochondrial dysfunction has been implicated in the pathogenesis of many diseases, including muscular dystrophy, atrophy, type 2 diabetes, and aging-related sarcopenia. Notably, exercise counteracts the effects of many chronic diseases on skeletal muscle mitochondrial function. Recent studies have revealed a finely tuned regulatory network that orchestrates skeletal muscle mitochondrial biogenesis and function in response to exercise and in disease states. In addition, increasing evidence suggests that mitochondria also serve to "communicate" with the nucleus and mediate adaptive genomic re-programming. Here we review the current state of knowledge relevant to the dynamic remodeling of skeletal muscle mitochondria in response to exercise and in disease states.

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.

Protein and cell wall polysaccharide carbonyl determination by a neutral pH 2,4-dinitrophenylhydrazine-based photometric assay

A new 2,4-dinitrophenylhydrazine (DNPH)-based photometric assay is developed for the quantification of carbonyls in protein samples from any biological source by protein carbonyl-DNPH hydrazone formation at acidic pH in the presence of denaturing urea, and subsequent hydrazone solubilization in the presence of SDS and stabilization from acid hydrolysis at pH 7.0. At this neutral (ntr) pH, interfering unreacted DNPH is uncharged and its thus increased hydrophobicity permits its 100% effective removal from the solubilizate with ethyl acetate/hexane wash. The ntrDNPH assay is more reliable and sensitive than the standard (std) DNPH photometric assay because it eliminates its main limitations: (i) interfering unreacted DNPH (pKa 1.55) that is nonspecifically bound to the TCA (pKa 0.7)-protein pellet is not effectively removed after wash with EtOH: ethyl acetate because it is positively charged, (ii) acid (TCA-induced) hydrolysis of the protein carbonyl-DNPH hydrazone, (iii) sample protein concentration re-determination, (iv) loss of sample acid (TCA)-soluble proteins, (v) DNA interference, and (vi) requires high protein quantity samples (≥ 1 mg). Considering ntrDNPH assay's very low protein limit (1 µg), its cumulative and functional sensitivities are 2600- and 2000-fold higher than those of the stdDNPH assay, respectively. The present study elucidates the DNA interference mechanism on the stdDNPH assay, and also develops a standardized protocol for sample protein treatment and fractionation (into cytoplasmic/aqueous, membrane/lipid-bound, and histone/DNA-bound proteins; see Supplement section V) in order to ensure reproducible carbonyl determination on defined cell protein fractions, and to eliminate assay interference from protein samples containing (i) Cys sulfenic acid groups (via their neutralization with dithiothreitol), and (ii) DNA (via its removal by streptomycin sulfate precipitation). Lastly, the ntrDNPH assay determines carbonyl groups on cell wall polysaccharides, thus paving the way on studies to investigate cell walls acting as antioxidant defense in plants, fungi, bacteria and lichens.

The role of oxidative stress in anxiety disorder: cause or consequence?

Anxiety disorders are the most common mental illness in the USA affecting 18% of the population. The cause(s) of anxiety disorders is/are not completely clear, and research in the neurobiology of anxiety at the molecular level is still rather limited. Although mounting clinical and preclinical evidence now indicates that oxidative stress may be a major component of anxiety pathology, whether oxidative stress is the cause or consequence remains elusive. Studies conducted over the past few years suggest that anxiety disorders may be characterised by lowered antioxidant defences and increased oxidative damage to proteins, lipids, and nucleic acids. In particular, oxidative modifications to proteins have actually been proposed as a potential factor in the onset and progression of several psychiatric disorders, including anxiety and depressive disorders. Oxidised proteins are normally degraded by the proteasome proteolytic complex in the cell cytoplasm, nucleus, and endoplasmic reticulum. The Lon protease performs a similar protective function inside mitochondria. Impairment of the proteasome and/or the Lon protease results in the accumulation of toxic oxidised proteins in the brain, which can cause severe neuronal trauma. Recent evidence points to possible proteolytic dysfunction and accumulation of damaged, oxidised proteins as factors that may determine the appearance and severity of psychotic symptoms in mood disorders. Thus, critical interactions between oxidative stress, proteasome, and the Lon protease may provide keys to the molecular mechanisms involved in emotional regulation, and may also be of great help in designing and screening novel anxiolytics and antidepressants.

Coordinating Mitochondrial Biology Through the Stress-Responsive Regulation of Mitochondrial Proteases

Proteases are localized throughout mitochondria and function as critical regulators of all aspects of mitochondrial biology. As such, the activities of these proteases are sensitively regulated through transcriptional and post-translational mechanisms to adapt mitochondrial function to specific cellular demands. Here, we discuss the stress-responsive mechanisms responsible for regulating mitochondrial protease activity and the implications of this regulation on mitochondrial function. Furthermore, we describe how imbalances in the activity or regulation of mitochondrial proteases induced by genetic, environmental, or aging-related factors influence mitochondria in the context of disease. Understanding the molecular mechanisms by which cells regulate mitochondrial function through alterations in protease activity provide insights into the contributions of these proteases in pathologic mitochondrial dysfunction and reveals new therapeutic opportunities to ameliorate this dysfunction in the context of diverse classes of human disease.

Novel Insights Into the Anti-aging Role of Mitophagy

Aging is a complex biological process affecting almost all living organisms. Although its detrimental effects on animals' physiology have been extensively documented, several aspects of the biology of aging are insufficiently understood. Mitochondria, the central energy producers of the cell, play vital roles in a wide range of cellular processes, including regulation of bioenergetics, calcium signaling, metabolic responses, and cell death, among others. Thus, proper mitochondrial function is a prerequisite for the maintenance of cellular and organismal homeostasis. Several mitochondrial quality control mechanisms have evolved to allow adaptation to different metabolic conditions, thereby preserving cellular homeostasis and survival. A tight coordination between mitochondrial biogenesis and mitochondrial selective autophagy, known as mitophagy, is a common characteristic of healthy biological systems. The balanced interplay between these two opposing cellular processes dictates stress resistance, healthspan, and lifespan extension. Mitochondrial biogenesis and mitophagy efficiency decline with age, leading to progressive accumulation of damaged and/or unwanted mitochondria, deterioration of cellular function, and ultimately death. Several regulatory factors that contribute to energy homeostasis have been implicated in the development and progression of many pathological conditions, such as neurodegenerative, metabolic, and cardiovascular disorders, among others. Therefore, mitophagy modulation may serve as a novel potential therapeutic approach to tackle age-associated pathologies. Here, we review the molecular signaling pathways that regulate and coordinate mitophagy with mitochondrial biogenesis, highlighting critical factors that hold promise for the development of pharmacological interventions toward enhancing human health and quality of life throughout aging.

Mitochondrial dysfunction reduces yeast replicative lifespan by elevating RAS-dependent ROS production by the ER-localized NADPH oxidase Yno1

Mitochondrial dysfunction leads to the accumulation of reactive oxygen species (ROS) which is associated with cellular dysfunction, disease etiology, and senescence. Here, we used the eukaryotic model Saccharomyces cerevisiae, commonly studied for cellular aging, to demonstrate how defective mitochondrial function affects yeast replicative lifespan (RLS). We show that RLS of respiratory-deficient cells decreases significantly, indicating that the maintenance of RLS requires active respiration. The shortening of RLS due to mitochondrial dysfunction was not related to the accumulation of extrachromosomal ribosomal DNA circles, a well-known cause of aging in yeast. Instead, intracellular ROS and oxidatively damaged proteins increased in respiratory-deficient mutants. We show that, while the protein kinase A activity is not elevated, ROS generation in respiratory-deficient cells depends on RAS signaling pathway. The ER-localized NADPH oxidase Yno1 also played a role in producing ROS. Our data suggest that a severe defect in mitochondrial respiration accelerates cellular aging by disturbing protein homeostasis in yeast.

Analysis of heat-induced protein aggregation in human mitochondria

Proteins in mammalian cells exhibit optimal stability at physiological temperatures, and even small temperature variations may cause unfolding and nonspecific aggregation. Because this process leads to a loss of function of the affected polypeptides and to cytotoxic stress, formation of protein aggregates has been recognized as a major pathogenic factor in human diseases. In this study, we determined the impact of physiological heat stress on mitochondria isolated from HeLa cells. We found that the heat-stressed mitochondria had lower membrane potential and ATP level and exhibited a decreased production of reactive oxygen species. An analysis of the mitochondrial proteome by 2D PAGE showed that the overall solubility of endogenous proteins was only marginally affected by elevated temperatures. However, a small subset of polypeptides exhibited an high sensitivity to heat stress. The mitochondrial translation elongation factor Tu (Tufm), a protein essential for organellar protein biosynthesis, was highly aggregation-prone and lost its solubility already under mild heat-stress conditions. Moreover, mitochondrial translation and the import of cytosolic proteins were defective in the heat-stressed mitochondria. Both types of nascent polypeptides, produced by translation or imported into the mitochondria, exhibited a strong tendency to aggregate in the heat-exposed mitochondria. We propose that a fast and specific inactivation of elongation factors may prevent the accumulation of misfolded nascent polypeptides and may thereby attenuate proteotoxicity under heat stress.

An Integrated Proteomic Approach Uncovers Novel Substrates and Functions of the Lon Protease in Escherichia coli

Controlling the cellular abundance and proper function of proteins by proteolysis is a universal process in all living organisms. In Escherichia coli, the ATP-dependent Lon protease is crucial for protein quality control and regulatory processes. To understand how diverse substrates are selected and degraded, unbiased global approaches are needed. We employed a quantitative Super-SILAC (stable isotope labeling with amino acids in cell culture) mass spectrometry approach and compared the proteomes of a lon mutant and a strain producing the protease to discover Lon-dependent physiological functions. To identify Lon substrates, we took advantage of a Lon trapping variant, which is able to translocate substrates but unable to degrade them. Lon-associated proteins were identified by label-free LC-MS/MS. The combination of both approaches revealed a total of 14 novel Lon substrates. Besides the identification of known pathways affected by Lon, for example, the superoxide stress response, our cumulative data suggests previously unrecognized fundamental functions of Lon in sulfur assimilation, nucleotide biosynthesis, amino acid and central energy metabolism.

To adapt or not to adapt: Consequences of declining Adaptive Homeostasis and Proteostasis with age

Many consequences of ageing can be broadly attributed to the inability to maintain homeostasis. Multiple markers of ageing have been identified, including loss of protein homeostasis, increased inflammation, and declining metabolism. Although much effort has been focused on characterization of the ageing phenotype, much less is understood about the underlying causes of ageing. To address this gap, we outline the age-associated consequences of dysregulation of 'Adaptive Homeostasis' and its proposed contributing role as an accelerator of the ageing phenotype. Adaptive Homeostasis is a phenomenon, shared across cells and tissues of both simple and complex organisms, that enables the transient plastic expansion or contraction of the homeostatic range to modulate stress-protective systems (such as the Proteasome, the Immunoproteasome, and the Lon protease) in response to varying internal and external environments. The age-related rise in the baseline of stress-protective systems and the inability to increase beyond a physiological ceiling is likely a contributor to the reduction and loss of Adaptive Homeostasis. We propose that dysregulation of Adaptive Homeostasis in the final third of lifespan is a significant factor in the ageing process, while successful maintenance of Adaptive Homeostasis below a physiological ceiling results in extended longevity.

Cardiovascular Adaptive Homeostasis in Exercise

Adaptive Homeostasis has been defined as, "The transient expansion or contraction of the homeostatic range in response to exposure to sub-toxic, non-damaging, signaling molecules or events, or the removal or cessation of such molecules or events." (Davies, 2016). I propose that one of the most significant examples of adaptive homeostasis may be the adaptation of the cardiovascular system to exercise training. In particular, endurance type training involves the generation of increased levels of free radicals such as ubisemiquinone, superoxide, nitric oxide, and other (non-radical) reactive oxygen species such as hydrogen peroxide (H2O2), in a repetitive manner, typically several times per week. As long as the training intensity and duration are sub-maximal and not exhaustive these reactive species do not cause damage, but rather activate signal transduction pathways to induce mitochondrial biogenesis-the foundation of increased exercise endurance. Particularly important are the NFκB and Nrf2 signal transduction pathways which respond to reactive oxygen and nitrogen species generated during exercise. As with other examples of adaptive homeostasis the effects are transient, lasting only as long as the training is maintained. Unfortunately, the ability to adapt to exercise training declines with age, perhaps as a result of impaired Nrf2 and NFκB signaling, as does adaptive homeostasis capacity in general. Since this is an Hypothesis/Theory Paper and not a review, I have not tried to provide a comprehensive discussion of all the literature relating to exercise adaptation and the cardiovascular system. Rather, I have attempted to develop the Hypothesis or Theory that adaptive homeostasis is the foundation for adaptation of the cardiovascular system to exercise training, largely based on work from my own laboratory, that of close collaborators, and that of key contributors over a period of almost 40 years.

Redox crosstalk at endoplasmic reticulum (ER) membrane contact sites (MCS) uses toxic waste to deliver messages

Many cellular redox reactions housed within mitochondria, peroxisomes and the endoplasmic reticulum (ER) generate hydrogen peroxide (H₂O₂) and other reactive oxygen species (ROS). The contribution of each organelle to the total cellular ROS production is considerable, but varies between cell types and also over time. Redox-regulatory enzymes are thought to assemble at a “redox triangle” formed by mitochondria, peroxisomes and the ER, assembling “redoxosomes” that sense ROS accumulations and redox imbalances. The redoxosome enzymes use ROS, potentially toxic by-products made by some redoxosome members themselves, to transmit inter-compartmental signals via chemical modifications of downstream proteins and lipids. Interestingly, important components of the redoxosome are ER chaperones and oxidoreductases, identifying ER oxidative protein folding as a key ROS producer and controller of the tri-organellar membrane contact sites (MCS) formed at the redox triangle. At these MCS, ROS accumulations could directly facilitate inter-organellar signal transmission, using ROS transporters. In addition, ROS influence the flux of Ca²⁺ ions, since many Ca²⁺ handling proteins, including inositol 1,4,5 trisphosphate receptors (IP₃Rs), SERCA pumps or regulators of the mitochondrial Ca²⁺ uniporter (MCU) are redox-sensitive. Fine-tuning of these redox and ion signaling pathways might be difficult in older organisms, suggesting a dysfunctional redox triangle may accompany the aging process.

Mitochondrial Lon is over-expressed in high-grade gliomas, and mediates hypoxic adaptation: potential role of Lon as a therapeutic target in glioma

Mitochondrial dysfunction is a hallmark of cancer biology. Tumor mitochondrial metabolism is characterized by an abnormal ability to function in scarce oxygen conditions through glycolysis (the Warburg effect), and accumulation of mitochondrial DNA defects are present in both hereditary neoplasia and sporadic cancers. Mitochondrial Lon is a major regulator of mitochondrial metabolism and the mitochondrial response to free radical damage, and plays an essential role in the maintenance and repair of mitochondrial DNA. Despite these critical cellular functions of Lon, very little has been reported regarding its role in glioma. Lon expression in gliomas and its relevance with patient survival was examined using published databases and human tissue sections. The effect of Lon in glioma biology was investigated through siRNA targeting Lon. We also tested the in vitro antitumor activity of Lon inhibitor, CC4, in the glioma cell lines D-54 and U-251. High Lon expression was associated with high glioma tumor grade and poor patient survival. While Lon expression was elevated in response to a variety of stimuli, Lon knockdown in glioma cell lines decreased cell viability under normal conditions, and dramatically impaired glioma cell survival under hypoxic conditions. Furthermore, the Lon inhibitor, CC4, efficiently prohibited glioma cell proliferation and synergistically enhanced the therapeutic efficacy of the chemotherapeutic agents, temozolomide (TMZ) and cisplatin. We demonstrate that Lon plays a key role in glioma cell hypoxic survival and mitochondrial respiration, and propose Lon as a promising therapeutic target in the treatment of malignant gliomas.

Mitochondrial network responses in oxidative physiology and disease

Mitochondrial activities are linked directly or indirectly to all cellular functions in aerobic eukaryotes. Omics methods enable new approaches to study functional organization of mitochondria and their adaptive and maladaptive network responses to bioenergetic fuels, physiologic demands, environmental challenges and aging. In this review, we consider mitochondria collectively within a multicellular organism as a macroscale "mitochondriome", functioning to organize bioenergetics and metabolism as an organism utilizes environmental resources and protects against environmental threats. We address complexities of knowledgebase-driven functional mapping of mitochondrial systems and then consider data-driven network mapping using omics methods. Transcriptome-metabolome-wide association study (TMWAS) shows connectivity and organization of nuclear transcription with mitochondrial transport systems in cellular responses to mitochondria-mediated toxicity. Integration of redox and respiratory measures with TMWAS shows central redox hubs separating systems linked to oxygen consumption rate and H₂O₂ production. Combined redox proteomics, metabolomics and transcriptomics further shows that physiologic network structures can be visualized separately from toxicologic networks. These data-driven integrated omics methods create new opportunities for mitochondrial systems biology.

Redox Regulation of Homeostasis and Proteostasis in Peroxisomes

Peroxisomes are highly dynamic intracellular organelles involved in a variety of metabolic functions essential for the metabolism of long-chain fatty acids, d-amino acids, and many polyamines. A byproduct of peroxisomal metabolism is the generation, and subsequent detoxification, of reactive oxygen and nitrogen species, particularly hydrogen peroxide (H2O2). Because of its relatively low reactivity (as a mild oxidant), H2O2 has a comparatively long intracellular half-life and a high diffusion rate, all of which makes H2O2 an efficient signaling molecule. Peroxisomes also have intricate connections to mitochondria, and both organelles appear to play important roles in regulating redox signaling pathways. Peroxisomal proteins are also subject to oxidative modification and inactivation by the reactive oxygen and nitrogen species they generate, but the peroxisomal LonP2 protease can selectively remove such oxidatively damaged proteins, thus prolonging the useful lifespan of the organelle. Peroxisomal homeostasis must adapt to the metabolic state of the cell, by a combination of peroxisome proliferation, the removal of excess or badly damaged organelles by autophagy (pexophagy), as well as by processes of peroxisome inheritance and motility. More recently the tumor suppressors ataxia telangiectasia mutate (ATM) and tuberous sclerosis complex (TSC), which regulate mTORC1 signaling, have been found to regulate pexophagy in response to variable levels of certain reactive oxygen and nitrogen species. It is now clear that any significant loss of peroxisome homeostasis can have devastating physiological consequences. Peroxisome dysregulation has been implicated in several metabolic diseases, and increasing evidence highlights the important role of diminished peroxisomal functions in aging processes.

Determination of Aconitase Activity: A Substrate of the Mitochondrial Lon Protease

Mitochondrial aconitase is a reversible enzyme that catalyzes the conversion of citrate to isocitrate in the tricarboxylic acid cycle. Mitochondrial aconitase is very sensitive to oxidative inactivation and can aggregate and accumulate in the mitochondrial matrix causing mitochondrial dysfunction. Lon protease, one of the major quality control proteases in mitochondria, degrades oxidized aconitase maintaining mitochondrial homeostasis. This chapter describes a step-by-step protocol for a simple and reliable measurement of mitochondrial aconitase, as well as citrate synthase activity, using isolated mitochondria from cells. The protocol is simple and fast, and it is optimized for a 96-well plate using a microplate reader.

Lon protease: a novel mitochondrial matrix protein in the interconnection between drug-induced mitochondrial dysfunction and endoplasmic reticulum stress

BACKGROUND AND PURPOSE

Mitochondria-associated membranes (MAMs) are specific endoplasmic reticulum (ER) domains that enable it to interact directly with mitochondria and mediate metabolic flow and Ca2+ transfer. A growing list of proteins have been identified as MAMs components, but how they are recruited and function during complex cell stress situations is still not understood, while the participation of mitochondrial matrix proteins is largely unrecognized.

EXPERIMENTAL APPROACH

This work compares mitochondrial/ER contact during combined ER stress/mitochondrial dysfunction using a model of human hepatoma cells (Hep3B cell line) treated for 24 h with classic pharmacological inducers of ER stress (thapsigargin), mitochondrial dysfunction (carbonyl cyanide m-chlorophenyl hydrazone or rotenone) or both (the antiretroviral drug efavirenz used at clinically relevant concentrations).

KEY RESULTS

Markers of mitochondrial dynamics (dynamin-related protein 1, optic atrophy 1 and mitofusin 2) were expressed differently with these stimuli, pointing to a specificity of combined ER/mitochondrial stress. Lon, a matrix protease involved in protein and mtDNA quality control, was up-regulated at mRNA and protein levels under all conditions. However, only efavirenz decreased the mitochondrial content of Lon while increasing its extramitochondrial presence and its localization to MAMs. This latter effect resulted in an enhanced mitochondria/ER interaction, as shown by co-immunoprecipitation experiments of MAMs protein partners and confocal microscopy imaging.

CONCLUSION AND IMPLICATIONS

A specific dual drug-induced mitochondria-ER effect enhances the MAMs content of Lon and its extramitochondrial expression. This is the first report of this phenomenon and suggests a novel MAMs-linked function of Lon protease.

A LON-ClpP Proteolytic Axis Degrades Complex I to Extinguish ROS Production in Depolarized Mitochondria

Mitochondrial dysfunction is implicated in numerous neurodegenerative disorders and in Parkinson's disease (PD) in particular. PINK1 and Parkin gene mutations are causes of autosomal recessive PD, and these respective proteins function cooperatively to degrade depolarized mitochondria (mitophagy). It is widely assumed that impaired mitophagy causes PD, as toxic reactive oxygen species (ROS)-producing mitochondria accumulate and progressively drive neurodegeneration. Instead, we report that a LON-ClpP proteolytic quality control axis extinguishes ROS in depolarized mitochondria by degrading the complex I ROS-generating domain. Complex I deficiency has also been identified in PD brain, and our study provides a compelling non-genetic mechanistic rationale to explain this observation: intact complex I depletes if mitochondrial bioenergetic capacity is robustly attenuated.

Zinc enhances the cellular energy supply to improve cell motility and restore impaired energetic metabolism in a toxic environment induced by OTA

Exogenous nutrient elements modulate the energetic metabolism responses that are prerequisites for cellular homeostasis and metabolic physiology. Although zinc is important in oxidative stress and cytoprotection processes, its role in the regulation of energetic metabolism remains largely unknown. In this study, we found that zinc stimulated aspect in cell motility and was essential in restoring the Ochratoxin A (OTA)-induced energetic metabolism damage in HEK293 cells. Moreover, using zinc supplementation and zinc deficiency models, we observed that zinc is conducive to mitochondrial pyruvate transport, oxidative phosphorylation, carbohydrate metabolism, lipid metabolism and ultimate energy metabolism in both normal and toxic-induced oxidative stress conditions in vitro, and it plays an important role in restoring impaired energetic metabolism. This zinc-mediated energetic metabolism regulation could also be helpful for DNA maintenance, cytoprotection and hereditary cancer traceability. Therefore, zinc can widely adjust energetic metabolism and is essential in restoring the impaired energetic metabolism of cellular physiology.

Diminished stress resistance and defective adaptive homeostasis in age-related diseases

Adaptive homeostasis is defined as the transient expansion or contraction of the homeostatic range following exposure to subtoxic, non-damaging, signaling molecules or events, or the removal or cessation of such molecules or events (Mol. Aspects Med. (2016) 49, 1-7). Adaptive homeostasis allows us to transiently adapt (and then de-adapt) to fluctuating levels of internal and external stressors. The ability to cope with transient changes in internal and external environmental stress, however, diminishes with age. Declining adaptive homeostasis may make older people more susceptible to many diseases. Chronic oxidative stress and defective protein homeostasis (proteostasis) are two major factors associated with the etiology of age-related disorders. In the present paper, we review the contribution of impaired responses to oxidative stress and defective adaptive homeostasis in the development of age-associated diseases.

Intermittent hypoxia leads to functional reorganization of mitochondria and affects cellular bioenergetics in marine molluscs

Fluctuations in oxygen (O2) concentrations represent a major challenge to aerobic organisms and can be extremely damaging to their mitochondria. Marine intertidal molluscs are well-adapted to frequent O2 fluctuations, yet it remains unknown how their mitochondrial functions are regulated to sustain energy metabolism and prevent cellular damage during hypoxia and reoxygenation (H/R). We used metabolic control analysis to investigate the mechanisms of mitochondrial responses to H/R stress (18 h at <0.1% O2 followed by 1 h of reoxygenation) using hypoxia-tolerant intertidal clams Mercenaria mercenaria and hypoxia-sensitive subtidal scallops Argopecten irradians as models. We also assessed H/R-induced changes in cellular energy balance, oxidative damage and unfolded protein response to determine the potential links between mitochondrial dysfunction and cellular injury. Mitochondrial responses to H/R in scallops strongly resembled those in other hypoxia-sensitive organisms. Exposure to hypoxia followed by reoxygenation led to a strong decrease in the substrate oxidation (SOX) and phosphorylation (PHOS) capacities as well as partial depolarization of mitochondria of scallops. Elevated mRNA expression of a reactive oxygen species-sensitive enzyme aconitase and Lon protease (responsible for degradation of oxidized mitochondrial proteins) during H/R stress was consistent with elevated levels of oxidative stress in mitochondria of scallops. In hypoxia-tolerant clams, mitochondrial SOX capacity was enhanced during hypoxia and continued rising during the first hour of reoxygenation. In both species, the mitochondrial PHOS capacity was suppressed during hypoxia, likely to prevent ATP wastage by the reverse action of FO,F1-ATPase. The PHOS capacity recovered after 1 h of reoxygenation in clams but not in scallops. Compared with scallops, clams showed a greater suppression of energy-consuming processes (such as protein turnover and ion transport) during hypoxia, indicated by inactivation of the translation initiation factor EIF-2α, suppression of 26S proteasome activity and a dramatic decrease in the activity of Na(+)/K(+)-ATPase. The steady-state levels of adenylates were preserved during H/R exposure and AMP-dependent protein kinase was not activated in either species, indicating that the H/R exposure did not lead to severe energy deficiency. Taken together, our findings suggest that mitochondrial reorganizations sustaining high oxidative phosphorylation flux during recovery, combined with the ability to suppress ATP-demanding cellular functions during hypoxia, may contribute to high resilience of clams to H/R stress and help maintain energy homeostasis during frequent H/R cycles in the intertidal zone.

Utilization of Mechanistic Enzymology to Evaluate the Significance of ADP Binding to Human Lon Protease

Lon, also known as Protease La, is one of the simplest ATP-dependent proteases. It is a homooligomeric enzyme comprised of an ATPase domain and a proteolytic domain in each enzyme subunit. Despite sharing about 40% sequence identity, human and Escherichia coli Lon proteases utilize a highly conserved ATPase domain found in the AAA+ family to catalyze ATP hydrolysis, which is needed to activate protein degradation. In this study, we utilized mechanistic enzymology techniques to show that despite comparable k_cat and K_m parameters found in the ATPase activity, human and E. coli Lon exhibit significantly different susceptibility to ADP inhibition. Due to the low affinity of human Lon for ADP, the conformational changes in human Lon generated from the ATPase cycle are also different. The relatively low affinity of human Lon for ADP cannot be accounted for by reversibility in ATP hydrolysis, as a positional isotope exchange experiment demonstrated both E. coli Lon and human Lon catalyzed ATP hydrolysis irreversibly. A limited tryptic digestion study however indicated that human and E. coli Lon bind to ADP differently. Taken together, the findings reported in this research article suggest that human Lon is not regulated by a substrate-promoted ADP/ATP exchange mechanism as found in the bacterial enzyme homolog. The drastic difference in structural changes associated with ADP interaction with the two protease homologs offer potential for selective inhibitor design and development through targeting the ATPase sites. In addition to revealing unique mechanistic differences that distinguish human vs. bacterial Lon, this article underscores the benefit of mechanistic enzymology in deciphering the physiological mechanism of action of Lon proteases and perhaps other closely related ATP-dependent proteases in the future.

Mitochondrial quality control in amyotrophic lateral sclerosis: towards a common pathway?

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder characterized by loss of upper and lower motor neurons. Different mechanisms contribute to the disease initiation and progression, including mitochondrial dysfunction which has been proposed to be a central determinant in ALS pathogenesis. Indeed, while mitochondrial defects have been mainly described in ALS-linked SOD1 mutants, it is now well established that mitochondria become also dysfunctional in other ALS conditions. In such context, the mitochondrial quality control system allows to restore normal functioning of mitochondria and to prevent cell death, by both eliminating and replacing damaged mitochondrial components or by degrading the entire organelle through mitophagy. Recent evidence shows that ALS-related genes interfere with the mitochondrial quality control system. This review highlights how ineffective mitochondrial quality control may render motor neurons defenseless towards the accumulating mitochondrial damage in ALS.

Mitochondrial LON protease-dependent degradation of cytochrome c oxidase subunits under hypoxia and myocardial ischemia

The mitochondrial ATP dependent matrix protease, Lon, is involved in the maintenance of mitochondrial DNA nucleoids and degradation of abnormal or misfolded proteins. The Lon protease regulates mitochondrial Tfam (mitochondrial transcription factor A) level and thus modulates mitochondrial DNA (mtDNA) content. We have previously shown that hypoxic stress induces the PKA-dependent phosphorylation of cytochrome c oxidase (CcO) subunits I, IVi1, and Vb and a time-dependent reduction of these subunits in RAW 264.7 murine macrophages subjected to hypoxia and rabbit hearts subjected to ischemia/reperfusion. Here, we show that Lon is involved in the preferential turnover of phosphorylated CcO subunits under hypoxic/ischemic stress. Induction of Lon protease occurs at 6 to 12 h of hypoxia and this increase coincides with lower CcO subunit contents. Over-expression of flag-tagged wild type and phosphorylation site mutant Vb and IVi1 subunits (S40A and T52A, respectively) caused marked degradation of wild type protein under hypoxia while the mutant proteins were relatively resistant. Furthermore, the recombinant purified Lon protease degraded the phosphorylated IVi1 and Vb subunits, while the phosphorylation-site mutant proteins were resistant to degradation. 3D structural modeling shows that the phosphorylation sites are exposed to the matrix compartment, accessible to matrix PKA and Lon protease. Hypoxic stress did not alter CcO subunit levels in Lon depleted cells, confirming its role in CcO turnover. Our results therefore suggest that Lon preferentially degrades the phosphorylated subunits of CcO and plays a role in the regulation of CcO activity in hypoxia and ischemia/reperfusion injury.

Dysfunction of mitochondrial Lon protease and identification of oxidized protein in mouse brain following exposure to MPTP: Implications for Parkinson disease

Compelling evidence suggests that mitochondrial dysfunction leading to reactive oxygen species (ROS) production and protein oxidation could represent a critical event in the pathogenesis of Parkinson's disease (PD). Pioneering studies have shown that the mitochondrial matrix contains the Lon protease, which degrades oxidized, dysfunctional, and misfolded protein. Using the PD animal model of 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) intoxication in mice, we showed that Lon protease expression increased in the ventral mesencephalon of intoxicated animals, concomitantly with the appearance of oxidized proteins and dopaminergic cell loss. In addition, we report that Lon is inactivated by ROS. Moreover, proteomic experiments provide evidence of carbonylation in α-ketoglutarate dehydrogenase (KGDH), aconitase or subunits of respiratory chain complexes. Lon protease inactivation upon MPTP treatment in mice raises the possibility that Lon protease dysfunction is an early event in the pathogenesis of PD.

TLR7/8 activation in neutrophils impairs immune complex phagocytosis through shedding of FcgRIIA

Lood et al. find that neutrophil TLR7/8 activation shifts neutrophils from phagocytosis of immune complexes to NETosis. Reduced phagocytosis of immune complexes is associated with partial proteolytic cleavage of FcgRIIA. Cleaved FcgRIIA is found in SLE neutrophils ex vivo.

Neutrophils play a crucial role in host defense. However, neutrophil activation is also linked to autoimmune diseases such as systemic lupus erythematosus (SLE), where nucleic acid–containing immune complexes (IC) drive inflammation. The role of Toll-like receptor (TLR) signaling in processing of SLE ICs and downstream inflammatory neutrophil effector functions is not known. We observed that TLR7/8 activation leads to a furin-dependent proteolytic cleavage of the N-terminal part of FcgRIIA, shifting neutrophils away from phagocytosis of ICs toward the programmed form of necrosis, NETosis. TLR7/8-activated neutrophils promoted cleavage of FcgRIIA on plasmacytoid dendritic cells and monocytes, resulting in impaired overall clearance of ICs and increased complement C5a generation. Importantly, ex vivo derived activated neutrophils from SLE patients demonstrated a similar cleavage of FcgRIIA that was correlated with markers of disease activity, as well as complement activation. Therapeutic approaches aimed at blocking TLR7/8 activation would be predicted to increase phagocytosis of circulating ICs, while disarming their inflammatory potential.

Oxidative DNA damage & repair: An introduction

This introductory article should be viewed as a prologue to the Free Radical Biology & Medicine Special Issue devoted to the important topic of Oxidatively Damaged DNA and its Repair. This special issue is dedicated to Professor Tomas Lindahl, co-winner of the 2015 Nobel Prize in Chemistry for his seminal discoveries in the area repair of oxidatively damaged DNA. In the past several years it has become abundantly clear that DNA oxidation is a major consequence of life in an oxygen-rich environment. Concomitantly, survival in the presence of oxygen, with the constant threat of deleterious DNA mutations and deletions, has largely been made possible through the evolution of a vast array of DNA repair enzymes. The articles in this Oxidatively Damaged DNA & Repair special issue detail the reactions by which intracellular DNA is oxidatively damaged, and the enzymatic reactions and pathways by which living organisms survive such assaults by repair processes.

Tetramethylpyrazine blocks TFAM degradation and up-regulates mitochondrial DNA copy number by interacting with TFAM

The natural small molecule compound: 2,3,5,6-tetramethylpyrazine (TMP), is a major component of the Chinese medicine Chuanxiong, which has wide clinical applications in dilating blood vessels, inhibiting platelet aggregation and treating thrombosis. Recent work suggests that TMP is also an antitumour agent. Despite its chemotherapeutic potential, the mechanism(s) underlying TMP action are unknown. Herein, we demonstrate that TMP binds to mitochondrial transcription factor A (TFAM) and blocks its degradation by the mitochondrial Lon protease. TFAM is a key regulator of mtDNA replication, transcription and transmission. Our previous work showed that when TFAM is not bound to DNA, it is rapidly degraded by the ATP-dependent Lon protease, which is essential for mitochondrial proteostasis. In cultured cells, TMP specifically blocks Lon-mediated degradation of TFAM, leading to TFAM accumulation and subsequent up-regulation of mtDNA content in cells with substantially low levels of mtDNA. In vitro protease assays show that TMP does not directly inhibit mitochondrial Lon, rather interacts with TFAM and blocks degradation. Pull-down assays show that biotinylated TMP interacts with TFAM. These findings suggest a novel mechanism whereby TMP stabilizes TFAM and confers resistance to Lon-mediated degradation, thereby promoting mtDNA up-regulation in cells with low mtDNA content.

Mitochondrial Chaperonin HSP60 Is the Apoptosis-Related Target for Myrtucommulone

The acylphloroglucinol myrtucommulone A (MC) causes mitochondrial dysfunctions by direct interference leading to apoptosis in cancer cells, but the molecular targets involved are unknown. Here, we reveal the chaperonin heat-shock protein 60 (HSP60) as a molecular target of MC that seemingly modulates HSP60-mediated mitochondrial functions. Exploiting an unbiased, discriminative protein fishing approach using MC as bait and mitochondrial lysates from leukemic HL-60 cells as target source identified HSP60 as an MC-binding protein. MC prevented HSP60-mediated reactivation of denatured malate dehydrogenase in a protein refolding assay. Interference of MC with HSP60 was accompanied by aggregation of two proteins in isolated mitochondria under heat shock that were identified as Lon protease-like protein (LONP) and leucine-rich PPR motif-containing protein (LRP130). Together, our results reveal HSP60 as a direct target of MC, proposing MC as a valuable tool for studying HSP60 biology and for evaluating its value as a target in related diseases, such as cancer.

Eat, breathe, ROS: controlling stem cell fate through metabolism

INTRODUCTION

Research reveals cardiac regeneration exists at levels previously deemed unattainable. Clinical trials using stem cells demonstrate promising cardiomyogenic and regenerative potential but insufficient contractile recovery. Incomplete understanding of the biology of administered cells likely contributes to inconsistent patient outcomes. Metabolism is a core component of many well-characterized stem cell types, and metabolic changes fundamentally alter stem cell fate from self-renewal to lineage commitment, and vice versa. However, the metabolism of stem cells currently studied for cardiac regeneration remains incompletely understood. Areas covered: Key metabolic features of stem cells are reviewed and unique stem cell metabolic characteristics are discussed. Metabolic changes altering stem cell fate are considered from quiescence and self-renewal to lineage commitment. Key metabolic concepts are applied toward examining cardiac regeneration through stem cell-based approaches, and clinical implications of current cell therapies are evaluated to identify potential areas of improvement. Expert commentary: The metabolism and biology of stem cells used for cardiac therapy remain poorly characterized. A growing appreciation for the fundamental relationship between stem cell functionality and metabolic phenotype is developing. Future studies unraveling links between cardiac stem cell metabolism and regenerative potential may considerably improve treatment strategies and therapeutic outcomes.

The role of Lon-mediated proteolysis in the dynamics of mitochondrial nucleic acid-protein complexes

Mitochondrial nucleoids consist of several different groups of proteins, many of which are involved in essential cellular processes such as the replication, repair and transcription of the mitochondrial genome. The eukaryotic, ATP-dependent protease Lon is found within the central nucleoid region, though little is presently known about its role there. Aside from its association with mitochondrial nucleoids, human Lon also specifically interacts with RNA. Recently, Lon was shown to regulate TFAM, the most abundant mtDNA structural factor in human mitochondria. To determine whether Lon also regulates other mitochondrial nucleoid- or ribosome-associated proteins, we examined the in vitro digestion profiles of the Saccharomyces cerevisiae TFAM functional homologue Abf2, the yeast mtDNA maintenance protein Mgm101, and two human mitochondrial proteins, Twinkle helicase and the large ribosomal subunit protein MrpL32. Degradation of Mgm101 was also verified in vivo in yeast mitochondria. These experiments revealed that all four proteins are actively degraded by Lon, but that three of them are protected from it when bound to a nucleic acid; the Twinkle helicase is not. Such a regulatory mechanism might facilitate dynamic changes to the mitochondrial nucleoid, which are crucial for conducting mitochondrial functions and maintaining mitochondrial homeostasis.

The age- and sex-specific decline of the 20s proteasome and the Nrf2/CncC signal transduction pathway in adaption and resistance to oxidative stress in Drosophila melanogaster

Hallmarks of aging include loss of protein homeostasis and dysregulation of stress-adaptive pathways. Loss of adaptive homeostasis, increases accumulation of DNA, protein, and lipid damage. During acute stress, the Cnc-C (Drosophila Nrf2 orthologue) transcriptionally-regulated 20S proteasome degrades damaged proteins in an ATP-independent manner. Exposure to very low, non-toxic, signaling concentrations of the redox-signaling agent hydrogen peroxide (H2O2) cause adaptive increases in the de novo expression and proteolytic activity/capacity of the 20S proteasome in female D. melanogaster (fruit-flies). Female 20S proteasome induction was accompanied by increased tolerance to a subsequent normally toxic but sub-lethal amount of H2O2, and blocking adaptive increases in proteasome expression also prevented full adaptation. We find, however, that this adaptive response is both sex- and age-dependent. Both increased proteasome expression and activity, and increased oxidative-stress resistance, in female flies, were lost with age. In contrast, male flies exhibited no H2O2 adaptation, irrespective of age. Furthermore, aging caused a generalized increase in basal 20S proteasome expression, but proteolytic activity and adaptation were both compromised. Finally, continual knockdown of Keep1 (the cytosolic inhibitor of Cnc-C) in adults resulted in older flies with greater stress resistance than their age-matched controls, but who still exhibited an age-associated loss of adaptive homeostasis.

Mitochondrial contribution to lipofuscin formation

Mitochondria have been in the focus of oxidative stress and aging research for decades due to their permanent production of ROS during the oxidative phosphorylation. The hypothesis exists that mitochondria are involved in the formation of lipofuscin, an autofluorescent protein aggregate that accumulates progressively over time in lysosomes of post-mitotic and senescent cells. To investigate the influence and involvement of mitochondria in lipofuscinogenesis, we analyzed lipofuscin amounts as well as the mitochondrial function in young and senescent cells. In addition we used an aging model and Lon protease deficient HeLa cells to investigate the influence of mitochondrial degradation processes on lipofuscin formation.

We were able to show that mitophagy is impaired in senescent cells resulting in an increased mitochondrial mass and superoxide formation. In addition, the inhibition of mitochondrial fission leads to increased lipofuscin formation.

Moreover, we observed that Lon protease downregulation is linked to a higher lipofuscinogenesis whereas the application of the mitochondrial-targeted antioxidant mitoTEMPO is able to prevent the accumulation of this protein aggregate.

FABP4/aP2 Regulates Macrophage Redox Signaling and Inflammasome Activation via Control of UCP2

Obesity-linked metabolic disease is mechanistically associated with the accumulation of proinflammatory macrophages in adipose tissue, leading to increased reactive oxygen species (ROS) production and chronic low-grade inflammation. Previous work has demonstrated that deletion of the adipocyte fatty acid-binding protein (FABP4/aP2) uncouples obesity from inflammation via upregulation of the uncoupling protein 2 (UCP2). Here, we demonstrate that ablation of FABP4/aP2 regulates systemic redox capacity and reduces cellular protein sulfhydryl oxidation and, in particular, oxidation of mitochondrial protein cysteine residues. Coincident with the loss of FABP4/aP2 is the upregulation of the antioxidants superoxide dismutase (SOD2), catalase, methionine sulfoxide reductase A, and the 20S proteasome subunits PSMB5 and αβ. Reduced mitochondrial protein oxidation in FABP4/aP2^-/- macrophages attenuates the mitochondrial unfolded-protein response (mtUPR) as measured by expression of heat shock protein 60, Clp protease, and Lon peptidase 1. Consistent with a diminished mtUPR, FABP4/aP2^-/- macrophages exhibit reduced expression of cleaved caspase-1 and NLRP3. Secretion of interleukin 1β (IL-1β), in response to inflammasome activation, is ablated in FABP4/aP2^-/- macrophages, as well as in FABP4/aP2 inhibitor-treated cells, but partially rescued in FABP4/aP2-null macrophages when UCP2 is silenced. Collectively, these data offer a novel pathway whereby FABP4/aP2 regulates macrophage redox signaling and inflammasome activation via control of UCP2 expression.

Mitochondrial Lon Protease and Cancer

ATP-dependent Lon protease of mitochondrial matrix is encoded by nuclear DNA and highly evolutionarily conserved throughout all organisms, which is involved in the quality control of proteins by selective degradation of misfolded, oxidized, and short-lived regulatory proteins within mitochondrial matrix, maintenance of mitochondrial genome (mtDNA), and folding of mitochondria proteins. Various stimuli such as hypoxia and oxidative and ER stress lead to upregulation of Lon expression. Inhibition of protease activity or downregulation of Lon promotes cancer cell death and enhances sensitivity of cancer cells to anticancer drugs through metabolic reprogramming, thus reducing the viability of cancer cell in tumor microenvironment and epithelial to mesenchymal transition (EMT). Moreover, mitochondrial ATP-dependent Lon protease may serve as a potential biomarker for cancer diagnosis and novel target for the development of anticancer drugs and for predicting of the efficiency and effectiveness of chemotherapy of a variety of cancers.

The Mitochondrial Lon Protease Is Required for Age-Specific and Sex-Specific Adaptation to Oxidative Stress

Multiple human diseases involving chronic oxidative stress show a significant sex bias, including neurodegenerative diseases, cancer, immune dysfunction, diabetes, and cardiovascular disease. However, a possible molecular mechanism for the sex bias in physiological adaptation to oxidative stress remains unclear. Here, we report that Drosophila melanogaster females but not males adapt to hydrogen peroxide stress, whereas males but not females adapt to paraquat (superoxide) stress. Stress adaptation in each sex requires the conserved mitochondrial Lon protease and is associated with sex-specific expression of Lon protein isoforms and proteolytic activity. Adaptation to oxidative stress is lost with age in both sexes. Transgenic expression of transformer gene during development transforms chromosomal males into pseudo-females and confers the female-specific pattern of Lon isoform expression, Lon proteolytic activity induction, and H₂O₂ stress adaptation; these effects were also observed using adult-specific transformation. Conversely, knockdown of transformer in chromosomal females eliminates the female-specific Lon isoform expression, Lon proteolytic activity induction, and H₂O₂ stress adaptation and produces the male-specific paraquat (superoxide) stress adaptation. Sex-specific expression of alternative Lon isoforms was also observed in mouse tissues. The results develop Drosophila melanogaster as a model for sex-specific stress adaptation regulated by the Lon protease, with potential implications for understanding sexual dimorphism in human disease.

The Proteasome and Oxidative Stress in Alzheimer's Disease

SIGNIFICANCE

Alzheimer's disease is a neurodegenerative disorder that is projected to exceed more than 100 million cases worldwide by 2050. Aging is considered the primary risk factor for some 90% of Alzheimer's cases but a significant 10% of patients suffer from aggressive, early-onset forms of the disease. There is currently no effective Alzheimer's treatment and this, coupled with a growing aging population, highlights the necessity to understand the mechanism(s) of disease initiation and propagation. A major hallmark of Alzheimer's disease pathology is the accumulation of amyloid-β (Aβ) aggregates (an early marker of Alzheimer's disease), and neurofibrillary tangles, comprising the hyper-phosphorylated microtubule-associated protein Tau. Recent Advances: Protein oxidation is frequently invoked as a potential factor in the progression of Alzheimer's disease; however, whether it is a cause or a consequence of the pathology is still being debated. The Proteasome complex is a major regulator of intracellular protein quality control and an essential proteolytic enzyme for the processing of both Aβ and Tau. Recent studies have indicated that both protein oxidation and excessive phosphorylation may limit Proteasomal processing of Aβ and Tau in Alzheimer's disease.

CRITICAL ISSUES

Thus, the Proteasome may be a key factor in understanding the development of Alzheimer's disease pathology; however, its significance is still very much under investigation.

FUTURE DIRECTIONS

Discovering how the proteasome is affected, regulated, or dysregulated in Alzheimer's disease could be a valuable tool in the efforts to understand and, ultimately, eradicate the disease. Antioxid. Redox Signal. 25, 886-901.

Mitochondrial Lon protease in human disease and aging: Including an etiologic classification of Lon-related diseases and disorders

The Mitochondrial Lon protease, also called LonP1 is a product of the nuclear gene LONP1. Lon is a major regulator of mitochondrial metabolism and response to free radical damage, as well as an essential factor for the maintenance and repair of mitochondrial DNA. Lon is an ATP-stimulated protease that cycles between being bound (at the inner surface of the inner mitochondrial membrane) to the mitochondrial genome, and being released into the mitochondrial matrix where it can degrade matrix proteins. At least three different roles or functions have been ascribed to Lon: 1) Proteolytic digestion of oxidized proteins and the turnover of specific essential mitochondrial enzymes such as aconitase, TFAM, and StAR; 2) Mitochondrial (mt)DNA-binding protein, involved in mtDNA replication and mitogenesis; and 3) Protein chaperone, interacting with the Hsp60-mtHsp70 complex. LONP1 orthologs have been studied in bacteria, yeast, flies, worms, and mammals, evincing the widespread importance of the gene, as well as its remarkable evolutionary conservation. In recent years, we have witnessed a significant increase in knowledge regarding Lon's involvement in physiological functions, as well as in an expanding array of human disorders, including cancer, neurodegeneration, heart disease, and stroke. In addition, Lon appears to have a significant role in the aging process. A number of mitochondrial diseases have now been identified whose mechanisms involve various degrees of Lon dysfunction. In this paper we review current knowledge of Lon's function, under normal conditions, and we propose a new classification of human diseases characterized by a either over-expression or decline or loss of function of Lon. Lon has also been implicated in human aging, and we review the data currently available as well as speculating about possible interactions of aging and disease. Finally, we also discuss Lon as potential therapeutic target in human disease.

Mitochondrial H2O2 signaling is controlled by the concerted action of peroxiredoxin III and sulfiredoxin: Linking mitochondrial function to circadian rhythm

Mitochondria produce hydrogen peroxide (H₂O₂) during energy metabolism in most mammalian cells as well as during the oxidation of cholesterol associated with the synthesis of steroid hormones in steroidogenic cells. Some of the H₂O₂ produced in mitochondria is released into the cytosol, where it serves as a key regulator of various signaling pathways. Given that mitochondria are equipped with several H₂O₂-eliminating enzymes, however, it had not been clear how mitochondrial H₂O₂ can escape destruction by these enzymes for such release. Peroxiredoxin III (PrxIII) is the most abundant and efficient H₂O₂-eliminating enzyme in mitochondria of most cell types. We found that PrxIII undergoes reversible inactivation through hyperoxidation of its catalytic cysteine residue to cysteine sulfinic acid, and that release of mitochondrial H₂O₂ likely occurs as a result of such PrxIII inactivation. The hyperoxidized form of PrxIII (PrxIII-SO₂H) is reduced and reactivated by sulfiredoxin (Srx). We also found that the amounts of PrxIII-SO₂H and Srx undergo antiphasic circadian oscillation in mitochondria of the adrenal gland, heart, and brown adipose tissue of mice maintained under normal conditions. Cytosolic Srx was found to be imported into mitochondria via a mechanism that requires formation of a disulfide-linked complex with heat shock protein 90, which is likely promoted by H₂O₂ released from mitochondria. The imported Srx was found to be degraded by Lon protease in a manner dependent on PrxIII hyperoxidation state. The coordinated import and degradation of Srx underlie Srx oscillation and consequent PrxIII-SO₂H oscillation in mitochondria. The rhythmic change in the amount of PrxIII-SO₂H suggests that mitochondrial release of H₂O₂ is also likely a circadian event that conveys temporal information on steroidogenesis in the adrenal gland and on energy metabolism in heart and brown adipose tissue to cytosolic signaling pathways.

Mitochondrial H2O2 signaling is controlled by the concerted action of peroxiredoxin III and sulfiredoxin: Linking mitochondrial function to circadian rhythm

Mitochondria produce hydrogen peroxide (H₂O₂) during energy metabolism in most mammalian cells as well as during the oxidation of cholesterol associated with the synthesis of steroid hormones in steroidogenic cells. Some of the H₂O₂ produced in mitochondria is released into the cytosol, where it serves as a key regulator of various signaling pathways. Given that mitochondria are equipped with several H₂O₂-eliminating enzymes, however, it had not been clear how mitochondrial H₂O₂ can escape destruction by these enzymes for such release. Peroxiredoxin III (PrxIII) is the most abundant and efficient H₂O₂-eliminating enzyme in mitochondria of most cell types. We found that PrxIII undergoes reversible inactivation through hyperoxidation of its catalytic cysteine residue to cysteine sulfinic acid, and that release of mitochondrial H₂O₂ likely occurs as a result of such PrxIII inactivation. The hyperoxidized form of PrxIII (PrxIII-SO₂H) is reduced and reactivated by sulfiredoxin (Srx). We also found that the amounts of PrxIII-SO₂H and Srx undergo antiphasic circadian oscillation in mitochondria of the adrenal gland, heart, and brown adipose tissue of mice maintained under normal conditions. Cytosolic Srx was found to be imported into mitochondria via a mechanism that requires formation of a disulfide-linked complex with heat shock protein 90, which is likely promoted by H₂O₂ released from mitochondria. The imported Srx was found to be degraded by Lon protease in a manner dependent on PrxIII hyperoxidation state. The coordinated import and degradation of Srx underlie Srx oscillation and consequent PrxIII-SO₂H oscillation in mitochondria. The rhythmic change in the amount of PrxIII-SO₂H suggests that mitochondrial release of H₂O₂ is also likely a circadian event that conveys temporal information on steroidogenesis in the adrenal gland and on energy metabolism in heart and brown adipose tissue to cytosolic signaling pathways.

GRIM-19: A master regulator of cytokine induced tumor suppression, metastasis and energy metabolism

Cytokines induce cell proliferation or growth suppression depending on the context. It is increasingly becoming clear that success of standard radiotherapy and/or chemotherapeutics to eradicate solid tumors is dependent on IFN signaling. In this review we discuss the molecular mechanisms of tumor growth suppression by a gene product isolated in our laboratory using a genome-wide expression knock-down strategy. Gene associated with retinoid-IFN-induced mortality -19 (GRIM-19) functions as non-canonical tumor suppressor by antagonizing oncoproteins. As a component of mitochondrial respiratory chain, GRIM-19 influences the degree of "Warburg effect" in cancer cells as many advanced and/or aggressive tumors show severely down-regulated GRIM-19 levels. In addition, GRIM-19 appears to regulate innate and acquired immune responses in mouse models. Thus, GRIM-19 is positioned at nodes that favor cell protection and/or prevent aberrant cell growth.