Two subpopulations of mitochondria in the aging rat heart display heterogenous levels of oxidative stress

Hyperbaric oxygen treatment reveals spatiotemporal OXPHOS plasticity in the porcine heart

Cardiomyocytes meet their high ATP demand almost exclusively by oxidative phosphorylation (OXPHOS). Adequate oxygen supply is an essential prerequisite to keep OXPHOS operational. At least two spatially distinct mitochondrial subpopulations facilitate OXPHOS in cardiomyocytes, i.e. subsarcolemmal (SSM) and interfibrillar mitochondria (IFM). Their intracellular localization below the sarcolemma or buried deep between the sarcomeres suggests different oxygen availability. Here, we studied SSM and IFM isolated from piglet hearts and found significantly lower activities of electron transport chain enzymes and F1FO-ATP synthase in IFM, indicative for compromised energy metabolism. To test the contribution of oxygen availability to this outcome, we ventilated piglets under hyperbaric hyperoxic (HBO) conditions for 240 min. HBO treatment raised OXPHOS enzyme activities in IFM to the level of SSM. Complexome profiling analysis revealed that a high proportion of the F1FO-ATP synthase in the IFM was in a disassembled state prior to the HBO treatment. Upon increased oxygen availability, the enzyme was found to be largely assembled, which may account for the observed increase in OXPHOS complex activities. Although HBO also induced transcription of genes involved in mitochondrial biogenesis, a full proteome analysis revealed only minimal alterations, meaning that HBO-mediated tissue remodeling is an unlikely cause for the observed differences in OXPHOS. We conclude that a previously unrecognized oxygen-regulated mechanism endows cardiac OXPHOS with spatiotemporal plasticity that may underlie the enormous metabolic and contractile adaptability of the heart.

The emerging importance of the α-keto acid dehydrogenase complexes in serving as intracellular and intercellular signaling platforms for the regulation of metabolism

The α-keto acid dehydrogenase complex (KDHc) class of mitochondrial enzymes is composed of four members: pyruvate dehydrogenase (PDHc), α-ketoglutarate dehydrogenase (KGDHc), branched-chain keto acid dehydrogenase (BCKDHc), and 2-oxoadipate dehydrogenase (OADHc). These enzyme complexes occupy critical metabolic intersections that connect monosaccharide, amino acid, and fatty acid metabolism to Krebs cycle flux and oxidative phosphorylation (OxPhos). This feature also imbues KDHc enzymes with the heightened capacity to serve as platforms for propagation of intracellular and intercellular signaling. KDHc enzymes serve as a source and sink for mitochondrial hydrogen peroxide (mtH2O2), a vital second messenger used to trigger oxidative eustress pathways. Notably, deactivation of KDHc enzymes through reversible oxidation by mtH2O2 and other electrophiles modulates the availability of several Krebs cycle intermediates and related metabolites which serve as powerful intracellular and intercellular messengers. The KDHc enzymes also play important roles in the modulation of mitochondrial metabolism and epigenetic programming in the nucleus through the provision of various acyl-CoAs, which are used to acylate proteinaceous lysine residues. Intriguingly, nucleosomal control by acylation is also achieved through PDHc and KGDHc localization to the nuclear lumen. In this review, I discuss emerging concepts in the signaling roles fulfilled by the KDHc complexes. I highlight their vital function in serving as mitochondrial redox sensors and how this function can be used by cells to regulate the availability of critical metabolites required in cell signaling. Coupled with this, I describe in detail how defects in KDHc function can cause disease states through the disruption of cell redox homeodynamics and the deregulation of metabolic signaling. Finally, I propose that the intracellular and intercellular signaling functions of the KDHc enzymes are controlled through the reversible redox modification of the vicinal lipoic acid thiols in the E2 subunit of the complexes.

Aging-induced mitochondrial dysfunction: two distinct populations of mitochondria versus a combined population

Mitochondrial function in aged hearts is impaired, and studies of isolated mitochondria are commonly used to assess their function. The two populations of cardiac mitochondria, subsarcolemmal mitochondria (SSM) and interfibrillar mitochondria (IFM), are affected by aging. However, the yield of these mitochondria, particularly SSM, is limited in the mouse heart because of the smaller heart size. To address this issue, the authors developed a method to isolate a mixed population (MIX) of SSM and IFM mitochondria from a single mouse heart. The aim of the study was to compare the mitochondrial function between SSM, IFM, and the MIX population from young and aged mouse hearts. The MIX population had a higher yield of total protein and citrate synthase activity from both young and aged hearts compared with the individual yields of SSM or IFM. Oxidative phosphorylation (OXPHOS) decreased in aged SSM and IFM compared with young SSM and IFM, as well as in the MIX population isolated from aged hearts compared with young hearts, when using complex I or IV substrates. Furthermore, aging barely affected the sensitivity to mitochondrial permeability transition pore (MPTP) opening in SSM, whereas the sensitivity was increased in IFM isolated from aged hearts and in the MIX population from aged hearts compared with the corresponding populations isolated from young hearts. These results suggest that mitochondrial dysfunction exists in aged hearts and the isolation of a MIX population of mitochondria from the mouse heart is a potential approach to studying mitochondrial function in the mouse heart.NEW & NOTEWORTHY We developed two methods to isolate mitochondria from a single mouse heart. We compared mitochondrial function in young and aged mice using mitochondria isolated with different methods. Both methods can be successfully used to isolate cardiac mitochondria from single mouse hearts. Our results provide the flexibility to isolate mitochondria from a single mouse heart based on the purpose of the study.

Metabolic landscape in cardiac aging: insights into molecular biology and therapeutic implications

Cardiac aging is evident by a reduction in function which subsequently contributes to heart failure. The metabolic microenvironment has been identified as a hallmark of malignancy, but recent studies have shed light on its role in cardiovascular diseases (CVDs). Various metabolic pathways in cardiomyocytes and noncardiomyocytes determine cellular senescence in the aging heart. Metabolic alteration is a common process throughout cardiac degeneration. Importantly, the involvement of cellular senescence in cardiac injuries, including heart failure and myocardial ischemia and infarction, has been reported. However, metabolic complexity among human aging hearts hinders the development of strategies that targets metabolic susceptibility. Advances over the past decade have linked cellular senescence and function with their metabolic reprogramming pathway in cardiac aging, including autophagy, oxidative stress, epigenetic modifications, chronic inflammation, and myocyte systolic phenotype regulation. In addition, metabolic status is involved in crucial aspects of myocardial biology, from fibrosis to hypertrophy and chronic inflammation. However, further elucidation of the metabolism involvement in cardiac degeneration is still needed. Thus, deciphering the mechanisms underlying how metabolic reprogramming impacts cardiac aging is thought to contribute to the novel interventions to protect or even restore cardiac function in aging hearts. Here, we summarize emerging concepts about metabolic landscapes of cardiac aging, with specific focuses on why metabolic profile alters during cardiac degeneration and how we could utilize the current knowledge to improve the management of cardiac aging.

Age-Related Changes in Skeletal Muscle Iron Homeostasis

Sarcopenia is an age-related condition of slow, progressive loss of muscle mass and strength, which contributes to frailty, increased risk of hospitalization and mortality, and increased health care costs. The incidence of sarcopenia is predicted to increase to >200 million affected older adults worldwide over the next 40 years, highlighting the urgency for understanding biological mechanisms and developing effective interventions. An understanding of the mechanisms underlying sarcopenia remains incomplete. Iron in the muscle is important for various metabolic functions, including oxygen supply and electron transfer during energy production, yet these same chemical properties of iron may be deleterious to the muscle when either in excess or when biochemically unshackled (eg, in ferroptosis), it can promote oxidative stress and induce inflammation. This review outlines the mechanisms leading to iron overload in muscle with aging and evaluates the evidence for the iron overload hypothesis of sarcopenia. Based on current evidence, studies are needed to (a) determine the mechanisms leading to iron overload in skeletal muscle during aging; and (b) investigate whether skeletal muscles are functionally deficient in iron during aging leading to impairments in oxidative metabolism.

Modulation of Reactive Oxygen Species Homeostasis as a Pleiotropic Effect of Commonly Used Drugs

Age-associated diseases represent a growing burden for global health systems in our aging society. Consequently, we urgently need innovative strategies to counteract these pathological disturbances. Overwhelming generation of reactive oxygen species (ROS) is associated with age-related damage, leading to cellular dysfunction and, ultimately, diseases. However, low-dose ROS act as crucial signaling molecules and inducers of a vaccination-like response to boost antioxidant defense mechanisms, known as mitohormesis. Consequently, modulation of ROS homeostasis by nutrition, exercise, or pharmacological interventions is critical in aging. Numerous nutrients and approved drugs exhibit pleiotropic effects on ROS homeostasis. In the current review, we provide an overview of drugs affecting ROS generation and ROS detoxification and evaluate the potential of these effects to counteract the development and progression of age-related diseases. In case of inflammation-related dysfunctions, cardiovascular- and neurodegenerative diseases, it might be essential to strengthen antioxidant defense mechanisms in advance by low ROS level rises to boost the individual ROS defense mechanisms. In contrast, induction of overwhelming ROS production might be helpful to fight pathogens and kill cancer cells. While we outline the potential of ROS manipulation to counteract age-related dysfunction and diseases, we also raise the question about the proper intervention time and dosage.

Thiol-based redox-active proteins as cardioprotective therapeutic agents in cardiovascular diseases

Thiol-based redox compounds, namely thioredoxins (Trxs), glutaredoxins (Grxs) and peroxiredoxins (Prxs), stand as a pivotal group of proteins involved in antioxidant processes and redox signaling. Glutaredoxins (Grxs) are considered as one of the major families of proteins involved in redox regulation by removal of S-glutathionylation and thereby reactivation of other enzymes with thiol-dependent activity. Grxs are also coupled to Trxs and Prxs recycling and thereby indirectly contribute to reactive oxygen species (ROS) detoxification. Peroxiredoxins (Prxs) are a ubiquitous family of peroxidases, which play an essential role in the detoxification of hydrogen peroxide, aliphatic and aromatic hydroperoxides, and peroxynitrite. The Trxs, Grxs and Prxs systems, which reversibly induce thiol modifications, regulate redox signaling involved in various biological events in the cardiovascular system. This review focuses on the current knowledge of the role of Trxs, Grxs and Prxs on cardiovascular pathologies and especially in cardiac hypertrophy, ischemia/reperfusion (I/R) injury and heart failure as well as in the presence of cardiovascular risk factors, such as hypertension, hyperlipidemia, hyperglycemia and metabolic syndrome. Further studies on the roles of thiol-dependent redox systems in the cardiovascular system will support the development of novel protective and therapeutic strategies against cardiovascular diseases.

The Role of Nrf2 in D-Galactose-Induced Cardiac Aging in Mice: Involvement of Oxidative Stress

INTRODUCTION

Cardiac aging is the major risk factor for advanced heart disease, which is the leading cause of death in developed countries, accounting for >30% of deaths worldwide.

OBJECTIVE

To discover the detailed mechanism of cardiac aging and develop an effective therapeutic candidate drug to treat or delay cardiac aging.

METHODS

We used D-galactose to induce cardiac aging in Nrf2+/+ and Nrf2-/- mice, and then treated these mice with vehicle or the Nrf2 activator, CDDO-imidazolide (CDDO-Im).

RESULTS AND CONCLUSIONS

D-galactose injection significantly induced cardiac aging, cell apoptosis, and oxidative stress in Nrf2+/+ mice, all of which were further exacerbated in Nrf2-/- mice. CDDO-Im treatment can effectively weaken oxidative stress and enhance the activities of antioxidant enzymes, but CDDO-Im lost its antioxidative effect in the Nrf2-/- mice. Nrf2 activator CDDO-Im could therefore effectively protect against D-galactose-induced cardiac aging by inhibiting oxidative stress, suggesting that CDDO-Im might be a potential and promising therapeutic candidate drug to treat cardiac aging.

Mitochondria and Calcium Homeostasis: Cisd2 as a Big Player in Cardiac Ageing

The ageing of human populations has become a problem throughout the world. In this context, increasing the healthy lifespan of individuals has become an important target for medical research and governments. Cardiac disease remains the leading cause of morbidity and mortality in ageing populations and results in significant increases in healthcare costs. Although clinical and basic research have revealed many novel insights into the pathways that drive heart failure, the molecular mechanisms underlying cardiac ageing and age-related cardiac dysfunction are still not fully understood. In this review we summarize the most updated publications and discuss the central components that drive cardiac ageing. The following characters of mitochondria-related dysfunction have been identified during cardiac ageing: (a) disruption of the integrity of mitochondria-associated membrane (MAM) contact sites; (b) dysregulation of energy metabolism and dynamic flexibility; (c) dyshomeostasis of Ca²⁺ control; (d) disturbance to mitochondria–lysosomal crosstalk. Furthermore, Cisd2, a pro-longevity gene, is known to be mainly located in the endoplasmic reticulum (ER), mitochondria, and MAM. The expression level of Cisd2 decreases during cardiac ageing. Remarkably, a high level of Cisd2 delays cardiac ageing and ameliorates age-related cardiac dysfunction; this occurs by maintaining correct regulation of energy metabolism and allowing dynamic control of metabolic flexibility. Together, our previous studies and new evidence provided here highlight Cisd2 as a novel target for developing therapies to promote healthy ageing

Mitochondrial Glutathione: Recent Insights and Role in Disease

Mitochondria are the main source of reactive oxygen species (ROS), most of them deriving from the mitochondrial respiratory chain. Among the numerous enzymatic and non-enzymatic antioxidant systems present in mitochondria, mitochondrial glutathione (mGSH) emerges as the main line of defense for maintaining the appropriate mitochondrial redox environment. mGSH’s ability to act directly or as a co-factor in reactions catalyzed by other mitochondrial enzymes makes its presence essential to avoid or to repair oxidative modifications that can lead to mitochondrial dysfunction and subsequently to cell death. Since mitochondrial redox disorders play a central part in many diseases, harboring optimal levels of mGSH is vitally important. In this review, we will highlight the participation of mGSH as a contributor to disease progression in pathologies as diverse as Alzheimer’s disease, alcoholic and non-alcoholic steatohepatitis, or diabetic nephropathy. Furthermore, the involvement of mitochondrial ROS in the signaling of new prescribed drugs and in other pathologies (or in other unmet medical needs, such as gender differences or coronavirus disease of 2019 (COVID-19) treatment) is still being revealed; guaranteeing that research on mGSH will be an interesting topic for years to come.

Age-Associated Changes in Antioxidants and Redox Proteins of Rat Heart

Oxidative stress and decline in cellular redox regulation have been hypothesized to play a key role in cardiovascular aging; however, data on antioxidant and redox regulating systems in the aging heart are controversial. The aim of the present study was to examine the effect of aging on critical antioxidant enzymes and two major redox-regulatory systems glutathione (GSH) and thioredoxin (Trx) system in hearts from adult (6-month-old), old (15-month-old), and senescent (26-month-old) rats. Aging was associated with a non-uniform array of changes, including decline in contents of reduced GSH and total mercaptans in the senescent heart. The activities of Mn-superoxide dismutase (SOD2), glutathione peroxidase (GPx), glutathione reductase (GR), and thioredoxin reductase (TrxR) exhibited an age-related decline, whereas catalase was unchanged and Cu,Zn-superoxide dismutase (SOD1) displayed only slight decrease in old heart and was unchanged in the senescent heart. GR, Trx, and peroxiredoxin levels were significantly reduced in old and/or senescent hearts, indicating a diminished expression of these proteins. In contrast, SOD2 level was unchanged in the old heart and was slightly elevated in the senescent heart. Decline in GPx activity was accompanied by a loss of GPx level only in old rats, the level in senescent heart was unchanged. These results indicate age-related posttranslational protein modification of SOD2 and GPx. In summary, our data suggest that changes are more pronounced in senescent than in old rat hearts and support the view that aging is associated with disturbed redox balance that could alter cellular signaling and regulation.

Age-related loss of mitochondrial glutathione exacerbates menadione-induced inhibition of Complex I

The role of mitochondrial GSH (mGSH) in the enhanced age-related susceptibility to xenobiotic toxicity is not well defined. We determined mGSH status and indices of mitochondrial bioenergetics in hepatocytes from young and old F344 rats treated with 300 μM menadione, a concentration that causes 50% cell death in old. At this concentration, mGSH was significantly lost only in hepatocytes from old rats, and with near total depletion due to lower basal mGSH in aged cells. In old hepatocytes, menadione caused mitochondrial membrane potential to collapse, as well as significant deficits in maximal O₂ consumption and respiratory reserve capacity, indicators of cellular bioenergetic resiliency. Further examination revealed that the menadione-mediated loss of respiratory reserve capacity in aged hepatocytes was from significant inhibition of Complex I activity and increased proton leak, for which an increase in Complex II activity was not able to compensate. These data demonstrate an age-related increase in mitochondrial susceptibility to a redox-cycling challenge, particularly in regards to Complex I activity, and provide a plausible mechanism to link this vulnerability to mGSH perturbations.

Administration of Enalapril Started Late in Life Attenuates Hypertrophy and Oxidative Stress Burden, Increases Mitochondrial Mass, and Modulates Mitochondrial Quality Control Signaling in the Rat Heart

Mitochondrial dysfunction is a relevant mechanism in cardiac aging. Here, we investigated the effects of late-life enalapril administration at a non-antihypertensive dose on mitochondrial genomic stability, oxidative damage, and mitochondrial quality control (MQC) signaling in the hearts of aged rats. The protein expression of selected mediators (i.e., mitochondrial antioxidant enzymes, energy metabolism, mitochondrial biogenesis, dynamics, and autophagy) was measured in old rats randomly assigned to receive enalapril (n = 8) or placebo (n = 8) from 24 to 27 months of age. We also assessed mitochondrial DNA (mtDNA) content, citrate synthase activity, oxidative lesions to protein and mtDNA (i.e., carbonyls and the abundance of mtDNA⁴⁸³⁴ deletion), and the mitochondrial transcription factor A (TFAM) binding to specific mtDNA regions. Enalapril attenuated cardiac hypertrophy and oxidative stress-derived damage (mtDNA oxidation, mtDNA⁴⁸³⁴ deletion, and protein carbonylation), while increasing mitochondrial antioxidant defenses. The binding of mitochondrial transcription factor A to mtDNA regions involved in replication and deletion generation was enhanced following enalapril administration. Increased mitochondrial mass as well as mitochondriogenesis and autophagy signaling were found in enalapril-treated rats. Late-life enalapril administration mitigates age-dependent cardiac hypertrophy and oxidative damage, while increasing mitochondrial mass and modulating MQC signaling. Further analyses are needed to conclusively establish whether enalapril may offer cardioprotection during aging.

The Aging Mitochondria

Mitochondrial dysfunction is a central event in many pathologies and contributes as well to age-related processes. However, distinguishing between primary mitochondrial dysfunction driving aging and a secondary mitochondrial impairment resulting from other cell alterations remains challenging. Indeed, even though mitochondria undeniably play a crucial role in aging pathways at the cellular and organismal level, the original hypothesis in which mitochondrial dysfunction and production of free radicals represent the main driving force of cell degeneration has been strongly challenged. In this review, we will first describe mitochondrial dysfunctions observed in aged tissue, and how these features have been linked to mitochondrial reactive oxygen species (ROS)–mediated cell damage and mitochondrial DNA (mtDNA) mutations. We will also discuss the clues that led to consider mitochondria as the starting point in the aging process, and how recent research has showed that the mitochondria aging axis represents instead a more complex and multifactorial signaling pathway. New working hypothesis will be also presented in which mitochondria are considered at the center of a complex web of cell dysfunctions that eventually leads to cell senescence and death.

Exploring the mitochondrial microRNA import pathway through Polynucleotide Phosphorylase (PNPase)

Cardiovascular disease is the primary cause of mortality for individuals with type 2 diabetes mellitus. During the diabetic condition, cardiovascular dysfunction can be partially attributed to molecular changes in the tissue, including alterations in microRNA (miRNA) interactions. MiRNAs have been reported in the mitochondrion and their presence may influence cellular bioenergetics, creating decrements in functional capacity. In this study, we examined the roles of Argonaute 2 (Ago2), a protein associated with cytosolic and mitochondrial miRNAs, and Polynucleotide Phosphorylase (PNPase), a protein found in the inner membrane space of the mitochondrion, to determine their role in mitochondrial miRNA import. In cardiac tissue from human and mouse models of type 2 diabetes mellitus, Ago2 protein levels were unchanged while PNPase protein expression levels were increased; also, there was an increase in the association between both proteins in the diabetic state. MiRNA-378 was found to be significantly increased in db/db mice, leading to decrements in ATP6 levels and ATP synthase activity, which was also exhibited when overexpressing PNPase in HL-1 cardiomyocytes and in HL-1 cells with stable miRNA-378 overexpression (HL-1-378). To assess potential therapeutic interventions, flow cytometry evaluated the capacity for targeting miRNA-378 species in mitochondria through antimiR treatment, revealing miRNA-378 level-dependent inhibition. Our study establishes PNPase as a contributor to mitochondrial miRNA import through the transport of miRNA-378, which may regulate bioenergetics during type 2 diabetes mellitus. Further, our data provide evidence that manipulation of PNPase levels may enhance the delivery of antimiR therapeutics to mitochondria in physiological and pathological conditions.

Age and ischemia differentially impact mitochondrial ultrastructure and function in a novel model of age-associated estrogen deficiency in the female rat heart

Altered mitochondrial respiration, morphology, and quality control collectively contribute to mitochondrial dysfunction in the aged heart. Because myocardial infarction remains the leading cause of death in aged women, the present study utilized a novel rodent model to recapitulate human menopause to interrogate the combination of age and estrogen deficiency on mitochondrial ultrastructure and function with cardiac ischemia/reperfusion (I/R) injury. Female F344 rats were ovariectomized (OVX) at 15 months and studied at 24 months (MO OVX; n = 40) vs adult ovary intact (6 months; n = 41). Temporal declines in estrogen concomitant with increased visceral adipose tissue were observed in MO OVX vs adult. Following in vivo coronary artery ligation or sham surgery, state 3 mitochondrial respiration was selectively reduced by age in subsarcolemmal mitochondria (SSM) and by I/R in interfibrillar mitochondria (IFM); left ventricular maximum dP/dt was reduced in MO OVX (p < 0.05). Elevated cyclophilin D and exacerbated I/R-induced mitochondrial acetylation in MO OVX suggest permeability transition pore involvement and reduced protection vs adult (p < 0.05). Mitochondrial morphology by TEM revealed an altered time course of autophagy coordinate with attenuated Drp1 and LC3BII protein levels with age-associated estrogen loss (p < 0.05). Here, reductions in both SSM and IFM function may play an additive role in enhanced susceptibility to regional I/R injury in aged estrogen-deficient female hearts. Moreover, novel insight into altered cardiac mitochondrial quality control garnered here begins to unravel the potentially important regulatory role of mitochondrial dynamics on sustaining respiratory function in the aged female heart.

Rejuvenation of Cardiac Tissue Developed from Reprogrammed Aged Somatic Cells

Induced pluripotent stem cells (iPSCs) derived via somatic cell reprogramming have been reported to reset aged somatic cells to a more youthful state, characterized by elongated telomeres, a rearranged mitochondrial network, reduced oxidative stress, and restored pluripotency. However, it is still unclear whether the reprogrammed aged somatic cells can function normally as embryonic stem cells (ESCs) during development and be rejuvenated. In the current study, we applied the aggregation technique to investigate whether iPSCs derived from aged somatic cells could develop normally and be rejuvenated. iPSCs derived from bone marrow myeloid cells of 2-month-old (2 M) and 18-month-old (18 M) C57BL/6-Tg (CAG-EGFP)1Osb/J mice were aggregated with embryos derived from wild-type ICR mice to produce chimeras (referred to as 2 M CA and 18 M CA, respectively). Our observations focused on comparing the ability of the iPSCs derived from 18 M and 2 M bone marrow cells to develop rejuvenated cardiac tissue (the heart is the most vital organ during aging). The results showed an absence of p16 and p53 upregulation, telomere length shortening, and mitochondrial gene expression and deletion in 18 M CA, whereas slight changes in mitochondrial ultrastructure, cytochrome C oxidase activity, ATP production, and reactive oxygen species production were observed in CA cardiac tissues. The data implied that all of the aging characteristics observed in the newborn cardiac tissue of 18 M CA were comparable with those of 2 M CA newborn cardiac tissue. This study provides the first direct evidence of the aging-related characteristics of cardiac tissue developed from aged iPSCs, and our observations demonstrate that partial rejuvenation can be achieved by reprogramming aged somatic cells to a pluripotent state.

Mitochondria and ageing: role in heart, skeletal muscle and adipose tissue

Age is the most important risk factor for most diseases. Mitochondria play a central role in bioenergetics and metabolism. In addition, several lines of evidence indicate the impact of mitochondria in lifespan determination and ageing. The best-known hypothesis to explain ageing is the free radical theory, which proposes that cells, organs, and organisms age because they accumulate reactive oxygen species (ROS) damage over time. Mitochondria play a central role as the principle source of intracellular ROS, which are mainly formed at the level of complex I and III of the respiratory chain. Dysfunctional mitochondria generating less ATP have been observed in various aged organs. Mitochondrial dysfunction comprises different features including reduced mitochondrial content, altered mitochondrial morphology, reduced activity of the complexes of the electron transport chain, opening of the mitochondrial permeability transition pore, and increased ROS formation. Furthermore, abnormalities in mitochondrial quality control or defects in mitochondrial dynamics have also been linked to senescence. Among the tissues affected by mitochondrial dysfunction are those with a high-energy demand and thus high mitochondrial content. Therefore, the present review focuses on the impact of mitochondria in the ageing process of heart and skeletal muscle. In this article, we review different aspects of mitochondrial dysfunction and discuss potential therapeutic strategies to improve mitochondrial function. Finally, novel aspects of adipose tissue biology and their involvement in the ageing process are discussed.

Mitochondrial Metabolism in Aging Heart

Altered mitochondrial metabolism is the underlying basis for the increased sensitivity in the aged heart to stress. The aged heart exhibits impaired metabolic flexibility, with a decreased capacity to oxidize fatty acids and enhanced dependence on glucose metabolism. Aging impairs mitochondrial oxidative phosphorylation, with a greater role played by the mitochondria located between the myofibrils, the interfibrillar mitochondria. With aging, there is a decrease in activity of complexes III and IV, which account for the decrease in respiration. Furthermore, aging decreases mitochondrial content among the myofibrils. The end result is that in the interfibrillar area, there is ≈50% decrease in mitochondrial function, affecting all substrates. The defective mitochondria persist in the aged heart, leading to enhanced oxidant production and oxidative injury and the activation of oxidant signaling for cell death. Aging defects in mitochondria represent new therapeutic targets, whether by manipulation of the mitochondrial proteome, modulation of electron transport, activation of biogenesis or mitophagy, or the regulation of mitochondrial fission and fusion. These mechanisms provide new ways to attenuate cardiac disease in elders by preemptive treatment of age-related defects, in contrast to the treatment of disease-induced dysfunction.

Unique morphological characteristics of mitochondrial subtypes in the heart: the effect of ischemia and ischemic preconditioning

RATIONALE

Three subsets of mitochondria have been described in adult cardiomyocytes - intermyofibrillar (IMF), subsarcolemmal (SSM), and perinuclear (PN). They have been shown to differ in physiology, but whether they also vary in morphological characteristics is unknown. Ischemic preconditioning (IPC) is known to prevent mitochondrial dysfunction induced by acute myocardial ischemia/reperfusion injury (IRI), but whether IPC can also modulate mitochondrial morphology is not known.

AIMS

Morphological characteristics of three different subsets of adult cardiac mitochondria along with the effect of ischemia and IPC on mitochondrial morphology will be investigated.

METHODS

Mouse hearts were subjected to the following treatments (N=6 for each group): stabilization only, IPC (3x5 min cycles of global ischemia and reperfusion), ischemia only (20 min global ischemia); and IPC and ischemia. Hearts were then processed for electron microscopy and mitochondrial morphology was assessed subsequently.

RESULTS

In adult cardiomyocytes, IMF mitochondria were found to be more elongated and less spherical than PN and SSM mitochondria. PN mitochondria were smaller in size when compared to the other two subsets. SSM mitochondria had similar area to IMF mitochondria but their sphericity measures were similar to PN mitochondria. Ischemia was shown to increase the sphericity parameters of all 3 subsets of mitochondria; reduce the length of IMF mitochondria, and increase the size of PN mitochondria. IPC had no effect on mitochondrial morphology either at baseline or after ischemia.

CONCLUSION

The three subsets of mitochondria in the adult heart are morphologically different. IPC does not appear to modulate mitochondrial morphology in adult cardiomyocytes.

In female rat heart mitochondria, oophorectomy results in loss of oxidative phosphorylation

Oophorectomy in adult rats affected cardiac mitochondrial function. Progression of mitochondrial alterations was assessed at one, two and three months after surgery: at one month, very slight changes were observed, which increased at two and three months. Gradual effects included decrease in the rates of oxygen consumption and in respiratory uncoupling in the presence of complex I substrates, as well as compromised Ca²⁺ buffering ability. Malondialdehyde concentration increased, whereas the ROS-detoxifying enzyme Mn²⁺ superoxide dismutase (MnSOD) and aconitase lost activity. In the mitochondrial respiratory chain, the concentration and activity of complex I and complex IV decreased. Among other mitochondrial enzymes and transporters, adenine nucleotide carrier and glutaminase decreased. 2-Oxoglutarate dehydrogenase and pyruvate dehydrogenase also decreased. Data strongly suggest that in the female rat heart, estrogen depletion leads to progressive, severe mitochondrial dysfunction.

Vascular calcification abrogates the nicorandil mediated cardio-protection in ischemia reperfusion injury of rat heart

The present study was aimed to determine the efficacy of nicorandil in treating cardiac reperfusion injury with an underlying co-morbidity of vascular calcification (VC). Adenine diet was used to induce VC in Wistar rat and the heart was isolated to induce global ischemia reperfusion (IR) by Langendorff method, with and without the nicorandil (7.5mg/kg) pre-treatment and compared with those fed on normal diet. The adenine-treated rats displayed abnormal ECG changes and altered mitochondrial integrity compared to a normal rat heart. These hearts, when subjected to IR increased the infarct size, cardiac injury (measured by lactate dehydrogenase and creatine kinase activity in the coronary perfusate) and significantly altered the hemodynamics compared to the normal perfused heart. Nicorandil pretreatment in rat fed on normal diet enhanced the hemodynamics significantly (P<0.05) along with a substantial reduction in the mitochondrial dysfunction (measured by high ADP to oxygen consumption ratio, respiratory control ratio, enzyme activities and less swelling behavior) when subjected to IR. However, this cardio-protective effect of nicorandil was absent in rat heart with underlying calcification. Our results suggest that, the protective effect of nicorandil, a known mitochondrial ATP linked K⁺ channel opener, against myocardial reperfusion injury was confined to normal rat heart.

Mitochondrial Dysfunction in Cardiovascular Aging

Mitochondria are the prime source of ATP in cardiomyocytes. Impairment of mitochondrial metabolism results in damage to existing proteins and DNA. Such deleterious effects are part and parcel of the aging process, reducing the ability of cardiomyocytes to counter stress, such as myocardial infarction and consequent reperfusion. In such conditions, mitochondria in the heart of aged individuals exhibit decreased oxidative phosphorylation, decreased ATP production, and increased net reactive oxygen species production; all of these effects are independent of the decrease in number of mitochondria that occurs in these situations. Rather than being associated with the mitochondrial population in toto, these defects are almost exclusively confined to those organelles positioned between myofibrils (interfibrillar mitochondria). It is in complex III and IV where these dysfunctional aspects are manifested. In an apparent effort to correct mitochondrial metabolic defects, affected organelles are to some extent eliminated by mitophagy; at the same time, new, unaffected organelles are generated by fission of mitochondria. Because these cardiac health issues are localized to specific mitochondria, these organelles offer potential targets for therapeutic approaches that could favorably affect the aging process in heart.

Respiratory supercomplexes and the functional segmentation of the CoQ pool

The evidence accumulated during the last fifteen years on the existence of respiratory supercomplexes and their proposed functional implications has changed our understanding of the OXPHOS system complexity and regulation. The plasticity model is a point of encounter accounting for the apparently contradictory experimental observations claimed to support either the solid or the fluid models. It allows the explanation of previous observations such as the dependence between respiratory complexes, supercomplex assembly dynamics or the existence of different functional ubiquinone pools. With the general acceptation of respiratory supercomplexes as true entities, this review evaluates the supporting evidences in favor or against the existence of different ubiquinone pools and the relationship between supercomplexes, ROS production and pathology.

Sensitivity of Interfibrillar and Subsarcolemmal Mitochondria to Cobalt Chloride-induced Oxidative Stress and Hydrogen Sulfide Treatment

Oxidative stress plays a significant role not only in cardiovascular disease but also in non-communicable diseases, where it plays a significant role the mortality rate. Hydrogen sulfide, the biological gaseous signaling molecule that preserves mitochondria in its mode of action, is an effective cardioprotective drug. However, cardiac mitochondria comprise of two distinct populations, namely interfibrillar and subsarcolemmal mitochondria, which respond distinctly in cardiovascular disease. This study was designed to determine the direct impact of cobalt chloride-induced oxidative stress in isolated mitochondrial subpopulations with an intention to examine the efficacy of hydrogen sulfide in preserving interfibrillar and subsarcolemmal mitochondria functional activities when they were incubated as pretreated, co-treated and post-treated agent. Mitochondrial subpopulations were isolated from the heart of male Wistar rats and subjected to cobalt chloride treatment (500 μM) for 20 min, followed by incubation with 10 μM sodium hydrosulfide in three different ways (Pre, Co, and Post-cobalt chloride treatment). Mitochondrial oxidative stress was measured by the concentration of thiobarbituric acid reactive species, reduced glutathione and the activities of enzymes like superoxide dismutase, catalase and glutathione peroxidase. Mitochondrial membrane potential, swelling behavior and enzyme activities were measured to assess its function. The increased level of lipid peroxidation and the decreased level of reduced glutathione in cobalt chloride-induced group confirm the induction of oxidative stress and were more predominant in the subsarcolemmal mitochondria. Hydrogen sulfide treatment to interfibrillar and subsarcolemmal mitochondria preserved their functional activities, but the effect was prominent only with co-treated group. In conclusion, the present study demonstrated that subsarcolemmal mitochondria are more prone to oxidative stress and the co-treatment of the mitochondria with hydrogen sulfide preserved the enzyme activity in the in vitro conditions.

Physiological and structural differences in spatially distinct subpopulations of cardiac mitochondria: influence of cardiac pathologies

Cardiac tissue contains discrete pools of mitochondria that are characterized by their subcellular spatial arrangement. Subsarcolemmal mitochondria (SSM) exist below the cell membrane, interfibrillar mitochondria (IFM) reside in rows between the myofibrils, and perinuclear mitochondria are situated at the nuclear poles. Microstructural imaging of heart tissue coupled with the development of differential isolation techniques designed to sequentially separate spatially distinct mitochondrial subpopulations have revealed differences in morphological features including shape, absolute size, and internal cristae arrangement. These findings have been complemented by functional studies indicating differences in biochemical parameters and, potentially, functional roles for the ATP generated, based upon subcellular location. Consequently, mitochondrial subpopulations appear to be influenced differently during cardiac pathologies including ischemia/reperfusion, heart failure, aging, exercise, and diabetes mellitus. These influences may be the result of specific structural and functional disparities between mitochondrial subpopulations such that the stress elicited by a given cardiac insult differentially impacts subcellular locales and the mitochondria contained within. The goal of this review is to highlight some of the inherent structural and functional differences that exist between spatially distinct cardiac mitochondrial subpopulations as well as provide an overview of the differential impact of various cardiac pathologies on spatially distinct mitochondrial subpopulations. As an outcome, we will instill a basis for incorporating subcellular spatial location when evaluating the impact of cardiac pathologies on the mitochondrion. Incorporation of subcellular spatial location may offer the greatest potential for delineating the influence of cardiac pathology on this critical organelle.

Mitochondrial and cellular mechanisms for managing lipid excess

Current scientific debates center on the impact of lipids and mitochondrial function on diverse aspects of human health, nutrition and disease, among them the association of lipotoxicity with the onset of insulin resistance in skeletal muscle, and with heart dysfunction in obesity and diabetes. Mitochondria play a fundamental role in aging and in prevalent acute or chronic diseases. Lipids are main mitochondrial fuels however these molecules can also behave as uncouplers and inhibitors of oxidative phosphorylation. Knowledge about the functional composition of these contradictory effects and their impact on mitochondrial-cellular energetics/redox status is incomplete. Cells store fatty acids (FAs) as triacylglycerol and package them into cytoplasmic lipid droplets (LDs). New emerging data shows the LD as a highly dynamic storage pool of FAs that can be used for energy reserve. Lipid excess packaging into LDs can be seen as an adaptive response to fulfilling energy supply without hindering mitochondrial or cellular redox status and keeping low concentration of lipotoxic intermediates. Herein we review the mechanisms of action and utilization of lipids by mitochondria reported in liver, heart and skeletal muscle under relevant physiological situations, e.g., exercise. We report on perilipins, a family of proteins that associate with LDs in response to loading of cells with lipids. Evidence showing that in addition to physical contact, mitochondria and LDs exhibit metabolic interactions is presented and discussed. A hypothetical model of channeled lipid utilization by mitochondria is proposed. Direct delivery and channeled processing of lipids in mitochondria could represent a reliable and efficient way to maintain reactive oxygen species (ROS) within levels compatible with signaling while ensuring robust and reliable energy supply.

Complex oscillatory redox dynamics with signaling potential at the edge between normal and pathological mitochondrial function

The time-keeping properties bestowed by oscillatory behavior on functional rhythms represent an evolutionarily conserved trait in living systems. Mitochondrial networks function as timekeepers maximizing energetic output while tuning reactive oxygen species (ROS) within physiological levels compatible with signaling. In this work, we explore the potential for timekeeping functions dependent on mitochondrial dynamics with the validated two-compartment mitochondrial energetic-redox (ME-R) computational model, that takes into account (a) four main redox couples [NADH, NADPH, GSH, Trx(SH)₂], (b) scavenging systems (glutathione, thioredoxin, SOD, catalase) distributed in matrix and extra-matrix compartments, and (c) transport of ROS species between them. Herein, we describe that the ME-R model can exhibit highly complex oscillatory dynamics in energetic/redox variables and ROS species, consisting of at least five frequencies with modulated amplitudes and period according to power spectral analysis. By stability analysis we describe that the extent of steady state—as against complex oscillatory behavior—was dependent upon the abundance of Mn and Cu, Zn SODs, and their interplay with ROS production in the respiratory chain. Large parametric regions corresponding to oscillatory dynamics of increasingly complex waveforms were obtained at low Cu, Zn SOD concentration as a function of Mn SOD. This oscillatory domain was greatly reduced at higher levels of Cu, Zn SOD. Interestingly, the realm of complex oscillations was located at the edge between normal and pathological mitochondrial energetic behavior, and was characterized by oxidative stress. We conclude that complex oscillatory dynamics could represent a frequency- and amplitude-modulated H₂O₂ signaling mechanism that arises under intense oxidative stress. By modulating SOD, cells could have evolved an adaptive compromise between relative constancy and the flexibility required under stressful redox/energetic conditions.

First-in-class cardiolipin-protective compound as a therapeutic agent to restore mitochondrial bioenergetics

A decline in energy is common in aging, and the restoration of mitochondrial bioenergetics may offer a common approach for the treatment of numerous age-associated diseases. Cardiolipin is a unique phospholipid that is exclusively expressed on the inner mitochondrial membrane where it plays an important structural role in cristae formation and the organization of the respiratory complexes into supercomplexes for optimal oxidative phosphorylation. The interaction between cardiolipin and cytochrome c determines whether cytochrome c acts as an electron carrier or peroxidase. Cardiolipin peroxidation and depletion have been reported in a variety of pathological conditions associated with energy deficiency, and cardiolipin has been identified as a target for drug development. This review focuses on the discovery and development of the first cardiolipin-protective compound as a therapeutic agent. SS-31 is a member of the Szeto-Schiller (SS) peptides known to selectively target the inner mitochondrial membrane. SS-31 binds selectively to cardiolipin via electrostatic and hydrophobic interactions. By interacting with cardiolipin, SS-31 prevents cardiolipin from converting cytochrome c into a peroxidase while protecting its electron carrying function. As a result, SS-31 protects the structure of mitochondrial cristae and promotes oxidative phosphorylation. SS-31 represents a new class of compounds that can recharge the cellular powerhouse and restore bioenergetics. Extensive animal studies have shown that targeting such a fundamental mechanism can benefit highly complex diseases that share a common pathogenesis of bioenergetics failure. This review summarizes the mechanisms of action and therapeutic potential of SS-31 and provides an update of its clinical development programme.

The interplay between autophagy and mitochondrial dysfunction in oxidative stress-induced cardiac aging and pathology

Aging is accompanied by a progressive increase in the incidence and prevalence of cardiovascular disease (CVD). Prolonged exposure to cardiovascular risk factors, together with intrinsic age-dependent declines in cardiac functionality, increases the vulnerability of the heart to both endogenous and exogenous stressors, ultimately enhancing the susceptibility to developing CVD in late life. Both increased levels of oxidative damage and the accumulation of dysfunctional mitochondria have been observed in a wide range of cardiac diseases, which may therefore represent a common ground upon which many aspects of CVD develop. In this review, we summarize the current knowledge on the mechanisms whereby oxidative stress arising from mitochondrial dysfunction is involved in the process of cardiac aging and in the pathogenesis of CVD highly prevalent in late life (e.g., heart failure and ischemic heart disease). Special emphasis is placed on recent evidence about the role played by alterations in cellular quality control systems, in particular autophagy/mitophagy and mitochondrial dynamics (fusion and fission), and their interconnections in the context of age-related CVD. Cardioprotective interventions acting through the modulation of mitochondrial autophagy (calorie restriction, calorie restriction mimetics, and the gasotransmitter hydrogen sulfide) are also presented. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

Neuroprotective effects of Hu-Yi-Neng, a diet supplement, on SH-SY5Y human neuroblastoma cells

OBJECTIVES

Oxidative stress is considered the potential risk to the development of dementia. Some medicines, vitamins, and diet supplements have been suggested to have possible benefits via the antioxidative effects to slow the decline of cognitive function in demented and non-demented individuals. However, few studies were conducted to examine their functions, especially in composite diet supplements. Hu-Yi-Neng is a composite diet supplement, including ginkgo biloba, extract of pine bark, phosphatidyl serine, docosahexaenoic acid, and folic acid, used extensively in Taiwan. Therefore, our aim is to investigate the potential protective effects of Hu-Yi-Neng on human neuron cells. MATERALS AND METHODS: H2O2-induced neuronal toxicity was characterized in SH-SY5Y human neuroblastoma cells by the decrease of cell viability using PrestoBlue™ assay and by the increase of intracellular reactive oxygen species (ROS) level using DCFH-DA (2', 7'-dichlorodihydrofluorescin diacetate) assays. HO-1 mRNA expression was detected by real-time PCR. Akt and Erk 1/2 proteins were detected by western blotting.

RESULTS

Pretreatment with Hu-Yi-Neng significantly reversed the decrease in cell viability induced by H2O2 in SH-SY5Y cells. Furthermore, Hu-Yi-Neng dose-dependently suppressed the elevation of intracellular reactive oxygen species (ROS) level. Hu-Yi-Neng protected SH-SY5Y cells from oxidative stress may via the increase in mRNA expression of heme oxygenase-1 (HO-1), an antioxidant enzyme. In addition, Hu-Yi-Neng inhibited H2O2-induced phosphorylation of Akt kinase but further increased the phosphorylation of Erk 1/2.

CONCLUSION

Our results suggest that Hu-Yi-Neng has protective effect against oxidative stress-induced neuron cell loss and it could be an ideal composite diet supplement for preventing neurodegenerative diseases.

Aging-dependent changes in rat heart mitochondrial glutaredoxins--Implications for redox regulation

Clinical and animal studies have documented that hearts of the elderly are more susceptible to ischemia/reperfusion damage compared to young adults. Recently we found that aging-dependent increase in susceptibility of cardiomyocytes to apoptosis was attributable to decrease in cytosolic glutaredoxin 1 (Grx1) and concomitant decrease in NF-κB-mediated expression of anti-apoptotic proteins. Besides primary localization in the cytosol, Grx1 also exists in the mitochondrial intermembrane space (IMS). In contrast, Grx2 is confined to the mitochondrial matrix. Here we report that Grx1 is decreased by 50–60% in the IMS, but Grx2 is increased by 1.4–2.6 fold in the matrix of heart mitochondria from elderly rats. Determination of in situ activities of the Grx isozymes from both subsarcolemmal (SSM) and interfibrillar (IFM) mitochondria revealed that Grx1 was fully active in the IMS. However, Grx2 was mostly in an inactive form in the matrix, consistent with reversible sequestration of the active-site cysteines of two Grx2 molecules in complex with an iron–sulfur cluster. Our quantitative evaluations of the active/inactive ratio for Grx2 suggest that levels of dimeric Grx2 complex with iron–sulfur clusters are increased in SSM and IFM in the hearts of elderly rats. We found that the inactive Grx2 can be fully reactivated by sodium dithionite or exogenous superoxide production mediated by xanthine oxidase. However, treatment with rotenone, which generates intramitochondrial superoxide through inhibition of mitochondrial respiratory chain Complex I, did not lead to Grx2 activation. These findings suggest that insufficient ROS accumulates in the vicinity of dimeric Grx2 to activate it in situ.

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Glutaredoxins play key roles in cellular redox regulation, which is sensitive to aging-dependent dysregulation.

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Grx1 is diminished in the intermembrane space of mitochondria from aged heart; matrix Grx2 is increased but mostly in an inactive form.

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The inactive Grx2 is selectively activated by superoxide.

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Mitochondrial glutaredoxin changes may contribute to dysregulation of redox homeostasis during aging.

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Changes in in situ activities of heart mitochondrial Grx1 and Grx2 with aging provide mechanistic insights for future studies.

Differential expression and glycative damage affect specific mitochondrial proteins with aging in rat liver

Aging is accompanied by the gradual deterioration of cell functions. Particularly, mitochondrial dysfunction, associated with an accumulation of damaged proteins, is of key importance due to the central role of these organelles in cellular metabolism. However, the detailed molecular mechanisms involved in such impairment have not been completely elucidated. In the present study, proteomic analyses looking at both changes at the expression level as well as to glycative modifications of the mitochondrial proteome were performed. Two-dimensional difference gel electrophoresis analysis revealed 16 differentially expressed proteins with aging. Thirteen exhibited a decreased expression and are crucial enzymes related to OXPHOS chain complex I/V components, TCA cycle or fatty acid β-oxidation reaction. On the other hand, 2 enzymes involved in fatty acid β-oxidation cycle were increased in aged mitochondria. Immunodetection and further identification of glycated proteins disclosed a set of advanced glycation end product-modified proteins, including 6 enzymes involved in the fatty acid β-oxidation process, and 2 enzymes of the TCA/urea cycles. A crucial antioxidant enzyme, catalase, was among the most strongly glycated proteins. In addition, several AGE-damaged enzymes (aldehyde dehydrogenase 2, medium chain acyl-CoA dehydrogenase and 3-ketoacyl-CoA dehydrogenase) exhibited a decreased activity with age. Taken together, these data suggest that liver mitochondria in old rats suffer from a decline in their capacity for energy production, due to (i) decreased expression of OXPHOS complex I/V components and (ii) glycative damage to key fatty acid β-oxidation and TCA/urea cycle enzymes.

Role of mitochondrial dysfunction and altered autophagy in cardiovascular aging and disease: from mechanisms to therapeutics

Advanced age is associated with a disproportionate prevalence of cardiovascular disease (CVD). Intrinsic alterations in the heart and the vasculature occurring over the life course render the cardiovascular system more vulnerable to various stressors in late life, ultimately favoring the development of CVD. Several lines of evidence indicate mitochondrial dysfunction as a major contributor to cardiovascular senescence. Besides being less bioenergetically efficient, damaged mitochondria also produce increased amounts of reactive oxygen species, with detrimental structural and functional consequences for the cardiovascular system. The age-related accumulation of dysfunctional mitochondrial likely results from the combination of impaired clearance of damaged organelles by autophagy and inadequate replenishment of the cellular mitochondrial pool by mitochondriogenesis. In this review, we summarize the current knowledge about relevant mechanisms and consequences of age-related mitochondrial decay and alterations in mitochondrial quality control in the cardiovascular system. The involvement of mitochondrial dysfunction in the pathogenesis of cardiovascular conditions especially prevalent in late life and the emerging connections with neurodegeneration are also illustrated. Special emphasis is placed on recent discoveries on the role played by alterations in mitochondrial dynamics (fusion and fission), mitophagy, and their interconnections in the context of age-related CVD and endothelial dysfunction. Finally, we discuss pharmacological interventions targeting mitochondrial dysfunction to delay cardiovascular aging and manage CVD.

HEART AND AGE (PART I): AGEING THEORIES AND MORPHOLOGICAL CHANGES

Heart ageing is a complex process including multiple cellular and molecular-level changes and resulting in different ageing phenotypes within the same biological species. According to the existing ageing theories, the multiple interacting mechanisms of ageing include somatic mutations, telomere shortening, oxidative stress, and mitochondrial defects. The review presents the most accepted ageing theories and discusses morphological characteristics of heart ageing on subcellular, cellular, and organ levels.

Mitochondria as a source and target of lipid peroxidation products in healthy and diseased heart

The heart is a highly oxidative organ in which cardiomyocyte turnover is virtually absent, making it particularly vulnerable to accumulation of lipid peroxidation products (LPP) formed as a result of oxidative damage. Reactive oxygen and nitrogen species are the most common electrophiles formed during lipid peroxidation and lead to the formation of both stable and unstable LPP. Of the LPP formed, highly reactive aldehydes are a well-recognized causative factor in ageing and age-associated diseases, including cardiovascular disease and diabetes. Recent studies have identified that the mitochondria are both a primary source and target of LPP, with specific emphasis on aldehydes in cardiomyocytes and how these affect the electron transport system and Ca(2+) balance. Numerous studies have found that there are functional consequences in the heart following exposure to specific aldehydes (acrolein, trans-2-hexanal, 4-hydroxynonenal and acetaldehyde). Because these LPP are known to form in heart failure, cardiac ischaemia-reperfusion injury and diabetes, they may have an underappreciated role in the pathophysiology of these disease processes. Lipid peroxidation products are involved in the transcriptional regulation of endogenous anti-oxidant systems. Recent evidence demonstrates that transient increases in LPP may be beneficial in cardioprotection by contributing to mitohormesis (i.e. induction of anti-oxidant systems) in cardiomyocytes. Thus, exploitation of the cardioprotective actions of the LPP may represent a novel therapeutic strategy for future treatment of heart disease.

Cardiac subsarcolemmal and interfibrillar mitochondria display distinct responsiveness to protection by diazoxide

Objective

Cardiac subsarcolemmal (SSM) and interfibrillar (IFM) mitochondrial subpopulations possess distinct biochemical properties and differ with respect to their protein and lipid compositions, capacities for respiration and protein synthesis, and sensitivity to metabolic challenge, yet their responsiveness to mitochondrially active cardioprotective therapeutics has not been characterized. This study assessed the differential responsiveness of the two mitochondrial subpopulations to diazoxide, a cardioprotective agent targeting mitochondria.

Methods

Mitochondrial subpopulations were freshly isolated from rat ventricles and their morphologies assessed by electron microscopy and enzymatic activities determined using standard biochemical protocols with a plate reader. Oxidative phosphorylation was assessed from State 3 respiration using succinate as a substrate. Calcium dynamics and the status of Ca²⁺-dependent mitochondrial permeability transition (MPT) pore and mitochondrial membrane potential were assessed using standard Ca²⁺ and TPP⁺ ion-selective electrodes.

Results

Compared to IFM, isolated SSM exhibited a higher sensitivity to Ca²⁺ overload-mediated inhibition of adenosine triphosphate (ATP) synthesis with decreased ATP production (from 375±25 to 83±15 nmol ATP/min/mg protein in SSM, and from 875±39 to 583±45 nmol ATP/min/mg protein in IFM). In addition, SSM exhibited reduced Ca²⁺-accumulating capacity as compared to IFM (230±13 vs. 450±46 nmol Ca²⁺/mg protein in SSM and IFM, respectively), suggestive of increased Ca²⁺ sensitivity of MPT pore opening. Despite enhanced susceptibility to stress, SSM were more responsive to the protective effect of diazoxide (100 μM) against Ca²⁺ overload-mediated inhibition of ATP synthesis (67% vs. 2% in SSM and IFM, respectively).

Conclusion

These results provide evidence for the distinct sensitivity of cardiac SSM and IFM toward Ca²⁺-dependent metabolic stress and the protective effect of diazoxide on mitochondrial energetics.

A comparative 'bottom up' proteomics strategy for the site-specific identification and quantification of protein modifications by electrophilic lipids

We report a mass spectrometry-based comparative "bottom up" proteomics approach that combines d(0)/d(4)-succinic anhydride labeling with commercially available hydrazine (Hz)-functionalized beads (Affi-gel Hz beads) for detection, identification and relative quantification of site-specific oxylipid modifications in biological matrices. We evaluated and applied this robust and simple method for the quantitative analysis of oxylipid protein conjugates in cardiac mitochondrial proteome samples isolated from 3- and 24-month-old rat hearts. The use of d(0)/d(4)-succinic anhydride labeling, Hz-bead based affinity enrichment, nanoLC fractionation and MALDI-ToF/ToF tandem mass spectrometry yielded relative quantification of oxylipid conjugates with residue-specific modification information. Conjugation of acrolein (ACR), 4-hydroxy-2-hexenal (HHE), 4-hydroxy-2-nonenal (HNE) and 4-oxo-2-noneal (ONE) to cysteine, histidine and lysine residues were identified. HHE conjugates were the predominant subset of Michael-type adducts detected in this study. The HHE conjugates showed higher levels in mitochondrial preparations from young heart congruent with previous findings by others that the n-3/n-6 PUFA ratio is higher in young heart mitochondrial membranes. Although this study focuses on protein adducts of reactive oxylipids, the method might be equally applicable to protein carbonyl modifications caused by metal catalyzed oxidation reactions.

Mitochondria in the pathogenesis of diabetes: a proteomic view

Diabetes mellitus is a complex metabolic disorder characterized by chronic hyperglycemia due to absolute or relative lack of insulin. Though great efforts have been made to investigate the pathogenesis of diabetes, the underlying mechanism behind the development of diabetes and its complications remains unexplored. Cumulative evidence has linked mitochondrial modification to the pathogenesis of diabetes and its complications and they are also observed in various tissues affected by diabetes. Proteomics is an attractive tool for the study of diabetes since it allows researchers to compare normal and diabetic samples by identifying and quantifying the differentially expressed proteins in tissues, cells or organelles. Great progress has already been made in mitochondrial proteomics to elucidate the role of mitochondria in the pathogenesis of diabetes and its complications. Further studies on the changes of mitochondrial protein specifically post-translational modifications during the diabetic state using proteomic tools, would provide more information to better understand diabetes.

Age-related decline in mitochondrial bioenergetics: does supercomplex destabilization determine lower oxidative capacity and higher superoxide production?

Mitochondrial decay plays a central role in the aging process. Although certainly multifactorial in nature, defective operation of the electron transport chain (ETC) constitutes a key mechanism involved in the age-associated loss of mitochondrial energy metabolism. Primarily, mitochondrial dysfunction affects the aging animal by limiting bioenergetic reserve capacity and/or increasing oxidative stress via enhanced electron leakage from the ETC. Even though the important aging characteristics of mitochondrial decay are known, the molecular events underlying inefficient electron flux that ultimately leads to higher superoxide appearance and impaired respiration are not completely understood. This review focuses on the potential role(s) that age-associated destabilization of the macromolecular organization of the ETC (i.e. supercomplexes) may be important for development of the mitochondrial aging phenotype, particularly in post-mitotic tissues.

Acetyl-L-carnitine supplementation reverses the age-related decline in carnitine palmitoyltransferase 1 (CPT1) activity in interfibrillar mitochondria without changing the L-carnitine content in the rat heart

The aging heart displays a loss of bioenergetic reserve capacity partially mediated through lower fatty acid utilization. We investigated whether the age-related impairment of cardiac fatty acid catabolism occurs, at least partially, through diminished levels of L-carnitine, which would adversely affect carnitine palmitoyltransferase 1 (CPT1), the rate-limiting enzyme for fatty acyl-CoA uptake into mitochondria for β-oxidation. Old (24-28 mos) Fischer 344 rats were fed±acetyl-L-carnitine (ALCAR; 1.5% [w/v]) for up to four weeks prior to sacrifice and isolation of cardiac interfibrillar (IFM) and subsarcolemmal (SSM) mitochondria. IFM displayed a 28% (p<0.05) age-related loss of CPT1 activity, which correlated with a decline (41%, p<0.05) in palmitoyl-CoA-driven state 3 respiration. Interestingly, SSM had preserved enzyme function and efficiently utilized palmitate. Analysis of IFM CPT1 kinetics showed both diminished V(max) and K(m) (60% and 49% respectively, p<0.05) when palmitoyl-CoA was the substrate. However, no age-related changes in enzyme kinetics were evident with respect to L-carnitine. ALCAR supplementation restored CPT1 activity in heart IFM, but not apparently through remediation of L-carnitine levels. Rather, ALCAR influenced enzyme activity over time, potentially by modulating conditions in the aging heart that ultimately affect palmitoyl-CoA binding and CPT1 kinetics.

Decreased cytochrome c oxidase subunit VIIa in aged rat heart mitochondria: immunocytochemistry

Aging decreases oxidative phosphorylation through cytochrome oxidase (COX) in cardiac interfibrillar mitochondria (IFM) in 24-month old (aged) rats compared to 6-month old adult Fischer 344 rats, whereas subsarcolemmal mitochondria (SSM) located beneath the plasma membrane remain unaffected. Immunoelectron microscopy (IEM) reveals in aged rats a 25% reduction in cardiac COX subunit VIIa in cardiac IFM, but not in SSM. In contrast, the content of subunit IV remains unchanged in both SSM and IFM, irrespective of age. These subunits are localized mainly on cristae membranes. In contrast, semi-quantitative immunoblotting, which detects denatured protein, indicates that the content of COX VIIa is similar in IFM and SSM from both aged and adult hearts. IEM provides a sensitive method for precise localizing and quantifying specific mitochondrial proteins. The lack of immunoreaction of COX VIIa subunit by IEM in aged IFM is not explained by a reduction in protein, but rather by a masking phenomenon or by an in situ change in protein structure affecting COX activity.

Tandem mass spectrometric characterization of thiol peptides modified by the chemoselective cationic sulfhydryl reagent (4-iodobutyl)triphenylphosphonium--effects of a cationic thiol derivatization on peptide fragmentation

Fixed charge chemical modifications on peptides and proteins can impact fragmentation behaviors in tandem mass spectrometry (MS/MS). In this study, we employed a thiol-specific cationic alkylation reagent, (4-iodobutyl)triphenylphosphonium (IBTP), to selectively modify cysteine thiol groups in mitochondrial proteome samples. Tandem mass spectrometric characteristics of butyltriphenylphosphonium (BTP)-modified peptides were evaluated by comparison to their carbamidomethylated (CAM) analogues using a quadrupole time-of-flight (Q-TOF) instrument under low energy collision-induced dissociation (CID) conditions. Introduction of the fixed charge modification resulted in the observation of peptide and fragment (b(n) and y(n)) ions with higher charge states than those observed for CAM-modified analogues. The charged BTP moiety had a significant effect on the neighboring amide bond fragmentation products. A decrease in relative abundances of the product ions at the corresponding cleavage sites was observed compared with those from the CAM-modified derivatives. This effect was particularly noticeable when an Xxx-Pro bond was in the vicinity of a BTP group. We hypothesized that the presence of a phosphonium moiety will reduce the tendency for protonation of the proximal amide bonds in the peptide backbone. Indeed, calculations indicated that proton affinities of backbone amide bonds close to the modified cysteine residues were generally 20-50 kcal/mol lower for BTP-modified peptides than for the unmodified or CAM-modified analogues with the sequence motif -Ala-Cys-Ala(n)-Ala(2)-, -Ala-Cys-Ala(n)-Pro-Ala-, and -Ala-Pro-Ala(n)-Cys-Ala-, n=0-3.

Site-specific proteomic analysis of lipoxidation adducts in cardiac mitochondria reveals chemical diversity of 2-alkenal adduction

The modification of proteins by lipid peroxidation products has been linked to numerous diseases and age-related disorders. Here we report on the identification of endogenous protein targets of electrophilic 2-alkenals in cardiac mitochondria. An aldehyde/keto-specific chemical labeling and affinity strategy in combination with LC-MS/MS resulted in 39 unique lipoxidation sites on 27 proteins. Several of the target sites were modified by a variety of 2-alkenal products including acrolein, β-hydroxyacrolein, crotonaldehyde, 4-hydroxy-2-hexenal, 4-hydroxy-2-nonenal and 4-oxo-2-nonenal. Many of the adduction sites are implicated in the catalytic function of key mitochondrial enzymes suggesting potential impact on pathways and overall mitochondrial function.

Effects of aging on activities of mitochondrial electron transport chain complexes and oxidative damage in rat heart

Mitochondrial dysfunction and accumulation of oxidative damage have been implicated to be the major factors of aging. However, data on age-related changes in activities of mitochondrial electron transport chain (ETC) complexes remain controversial and molecular mechanisms responsible for ETC dysfunction are still largely unknown. In this study, we examined the effect of aging on activities of ETC complexes and oxidative damage to proteins and lipids in cardiac mitochondria from adult (6-month-old), old (15-month-old) and senescent (26-month-old) rats. ETC complexes I-IV displayed different extent of inhibition with age. The most significant decline occurred in complex IV activity, whereas complex II activity was unchanged in old rats and was only slightly reduced in senescent rats. Compared to adult, old and senescent rat hearts had significantly higher levels of malondialdehyde, 4-hydroxynonenal (HNE) and dityrosine, while thiol group content was reduced. Despite marked increase in HNE content with age (25 and 76 % for 15- and 26-month-old rats, respectively) Western blot analysis revealed only few HNE-protein adducts. The present study suggests that non-uniform decline in activities of ETC complexes is due, at least in part, to mitochondrial oxidative damage; however, lipid peroxidation products appear to have a limited impact on enzyme functions.