Mitochondria and Reactive Oxygen Species in Aging and Age-Related Diseases

Abstract

Aging has been linked to several degenerative processes that, through the accumulation of molecular and cellular damage, can progressively lead to cell dysfunction and organ failure. Human aging is linked with a higher risk for individuals to develop cancer, neurodegenerative, cardiovascular, and metabolic disorders. The understanding of the molecular basis of aging and associated diseases has been one major challenge of scientific research over the last decades. Mitochondria, the center of oxidative metabolism and principal site of reactive oxygen species (ROS) production, are crucial both in health and in pathogenesis of many diseases. Redox signaling is important for the modulation of cell functions and several studies indicate a dual role for ROS in cell physiology. In fact, high concentrations of ROS are pathogenic and can cause severe damage to cell and organelle membranes, DNA, and proteins. On the other hand, moderate amounts of ROS are essential for the maintenance of several biological processes, including gene expression. In this review, we provide an update regarding the key roles of ROS-mitochondria cross talk in different fundamental physiological or pathological situations accompanying aging and highlighting that mitochondrial ROS may be a decisive target in clinical practice.

Keywords: Age-related neurodegenerative disorders; Aging; Anti-ROS intervention; Antioxidant defense; Mitochondria; Mitochondrial dysfunction–related pathologies; ROS.

Figure 1. Potential role of mitochondrial ROS increase in aging.

During aging, mitochondrial ROS production steadily increases, leading to mitochondrial damage and decreased life span. Here we report the major events contributing to aging (upper panel), or most important chemicals and experimental interventions, which may promote longevity (bottom panel).

Figure 2. Schematic representations of two pivotal apoptotic molecular routes, involved during oxidative stress and oncogenic stress, respectively.

(A) Oxidative stress-induced activation of PKCβ leads to phosphorylation of p66shc at Serine 36 residue, allowing translocation of protein to mitochondria by PIN1-dependent mechanism. The mitochondrial pool of p66shc oxidizes cytochrome c and catalyzes the reduction of O₂ to H₂O₂, inducing ROS production and successively apoptotic induction. (B) Oncogenic stress promotes the binding between FHIT and mitochondrial-import complex Hsp60/Hsp10. In mitochondria FHIT interacts with FDXR promoting ROS production and cytochrome c release, respectively, leading to apoptotic response.

Figure 3. Mitochondrial network displays huge heterogeneity and shape rearrangements.

Examples of mitochondrial network are shown in both immortalized [(A) HEK, (B) Cos7, (C) IB3] and primary cultured cell lines (D) human fibroblast, (E) rat myotube, (F) rat adipocyte, (G) rat oligodendrocyte progenitor). Rearrangement of mitochondrial network is also a typical adaptation to stress stimulus such as oxidative stress as shown for mouse embryonic fibroblast before (Hi) and after (Hii) exposure to H₂O₂.

Figure 4. The number of contact sites between mitochondria and the ER in young and senescent human fibroblasts.

(A) Image of the endoplasmic reticulum (ER) (green) and mitochondria (red) in young and senescent human fibroblasts. Maximum intensity projections of confocal micrographs from young and old fibroblasts expressing mitochondria-targeted Cherry (magenta) and ER-targeted GFP (green) contacts sites are represented by colocalization areas (white). Colocalization extents were quantified using Pearson’s and Mander’s coefficients. (B) Activity of senescence-associated β-galactosidase in young and senescent human fibroblasts. Cells referred as “young” fibroblasts were at 4th passage, and the “old” ones (senescent) at 16th passage.

Figure 5. Fenton and Haber–Weiss reaction.

In the presence of catalytic amounts of trace metals (such as iron and copper), highly reactive hydroxyl radicals (^•OH) are generated from hydrogen peroxide via the Fenton and Haber–Weiss reaction.

Figure 6. Putative pathways linking oxidative stress, mitochondrial dysfunction, and hyperglycemia.

GSH, glutathione; ROS, reactive oxygen species; UCP2, uncoupling protein-2.

Figure 7. ROS as causative agents in cardiovascular diseases (CVDs) and therapeutic approaches targeting mitochondrial oxidative damage.

(A) Principal mechanisms of ROS production and subsequent mitochondrial dysfunction leading to CVDs. Mitochondria participate in the pathogenesis of hypertension: increased blood pressure has been associated with an excessive endothelial cells (EC) production of superoxide (${\mathrm{O}}_2^{•\text{-}}$) and H₂O₂; UCP1 expression also increases ${\mathrm{O}}_2^{•\text{-}}$ production and decreases the availability of NO. ROS production by endothelial mitochondria contribute to heart disease: angiotensin-II, hyperglycemia, or hypoxia increases mitochondrial ROS production in EC, which then stimulates the NADPH oxidase; moreover ROS, produced by the NADPH oxidase, activate mitochondrial KATP channels, suggesting a possible feedback amplification system. Exposure of endothelial cells to oxidized lipids (oxLDL) induces ROS formation, which has a pivotal role in atherogenesis. During ischemia O₂ is lacking and mitochondria hydrolyze ATP to maintain the mitochondrial membrane potential (ΔΨ); the cardiac cell strives to maintain ATP production, this eventually results in mitochondrial Ca²⁺ overload, mitochondrial depolarization, and increases the generation of ROS. Restoration of blood flow will help to restore ATP levels, but the damaged mitochondria generate enormous amounts of ROS during reperfusion and promote mitochondrial permeability transition pore (mPTP) opening and activation of apoptosis, triggering ischemia/reperfusion (I/R) injury. (B) Approaches to deliver drugs to the mitochondria and prevent ROS-induced mitochondrial oxidative damage. Compounds conjugated to the lipophilic triphenylphosphonium cation (TPP⁺) can be delivered selectively into the mitochondrial matrix in a potential-driven manner; a series of mitochondria-targeted antioxidants have been designed to decrease superoxide (MitoSOD), hydrogen peroxide (MitoPeroxidase), ferrous iron (MitoTEMPOL), lipid peroxidation (MitoQ, MitoE), and preventing mPTP opening (TPP⁺-CsA). Recently, Szeto and Schiller (SS) peptides targeting the inner mitochondrial membrane have been developed; they concentrate in a potential independent manner and possess intrinsic mitoprotective activities.

Figure 8. Mitochondrial calcium uptake as a function of cell culture passage number.

Representative quantitation of mitochondrial Ca²⁺ uptake elicited by ATP 100 μM n mouse embryonic fibroblasts (MEFs) from low and high passage rates. Mitochondrial calcium concentration ([Ca²⁺]_m).

Figure 9. Mitochondrial-related alterations in principal neurodegenerative disorders.

In Alzheimer’s disease amyloid peptides (Ab) can aggregate to mitochondria and interact with Ab alcohol dehydrogenase (ABAD) to induce cytochrome C release and ROS production. Contemporary altered activity of Krebs cycle enzymes (KC), complexes of respiratory chain, and proteins involved in mitochondria fusion and fission leads to impaired mitochondrial remodeling and ROS production. Similar readouts on mitochondrial physiology are observed in Parkinson’s disease. Aggregation of mutant synuclein (syn) impair activity of respiratory complex I similarly as many pro-parkinsonian compounds. In this scenario should be added the activity of mutant proteins such as PINK1, DJ-1, Parkin, LRRK2 that impair mitochondrial modeling and recycling as promoting induction of cytochrome C release. In Huntington disease mutant huntingtin (carrying poliQ expansion) can impair activity of II respiratory complex (with consequent ROS production) and mitochondria transport along filaments in axons. Also superoxide dismutase 1 (SOD1) mutations causing ALS can induce appearance of toxic mitochondrial aggregates that lead to improved toxic ROS production. Altered protein or OXPHOS complexes activities are shown with a plus or minus symbol in a circle, mutant proteins instead are marked by an M in a circle.

Figure 10. Schematic view of various compounds that reduce oxidative stress in diseases mentioned in the text and their correlation with neurodegenerative disorders.

Mitochondrial selective compounds can act in directly buffering ROS production, such as Szeto–Shiller peptides (SS), vitamine E or coenzyme Q (CoQ), and derivatives. Buffering of ROS production could by obtained by inhibiting interaction between Keap1 and the nuclear respiratory factor 2 (NRF2). Restoration of mitochondrial functions has been also obtained by the use of drugs targeting Kreb’s cycle, such as lipoic acid, acetyl-l-carnitine (ALCAR), or creatine.