Mitochondrial Metabolism: An Approach to Disease Management

Mitochondrial Metabolism: An Approach for Disease Management covers mitotherapy from three combined perspectives, Pharmacology, Toxicology and Biochemistry. After an introduction from world-renowned experts, the book's chapters cover the balancing role in reduction/oxidation mitochondria play, mitochondria as targets for therapeutics through its metabolism, mitochondrial contributions to the cell death process, mitochondrial response to environmental toxicants, the mitochondrial role in aging, the impact of calorie restrictive diets, new advances in the identification of altered mitochondria associated signaling pathways in carcinogenesis, and much more.

This book provides bioscientists new horizons to realize the importance of mitochondria in present-day research on therapies dealing with mitochondria associated chronic diseases, including diabetes, cancer and neurodegenerative disorders.

• Details the significant role of mitochondria in chronic diseases
• Presents new insights on the targeting of mitochondria for therapeutic purposes
• Includes updated results on mitotherapy and other mitochondria-oriented therapies

• Language: English
• Publisher: Academic Press
• Release date: Jul 28, 2021
• ISBN: 9780128224090

Book preview

Preface

Jalal Pourahmad; Mohsen Rezaei

In recent years, our knowledge about mitochondria has dramatically improved. Studies have revealed the new aspects of capabilities of mitochondria and their involvement in the cellular function, which are as important as their traditional role in ATP production. From direct involvement in cell fate and death to their important role in the remodeling of cell metabolic activities are among these roles. In addition to the significant role that these organelles play in the homeostasis and integrity of cell and organ physiological activities, we now know that in many chronic diseases that affect a large number of the population today, dysfunction of these organelles was identified. Mitochondrial dysfunction has received much attention in cancers, diabetes, neurodegenerative and psychiatric diseases. Numerous studies in this field have identified targets that can be used to treat these diseases. Although in some cases it is not possible to determine with certainty whether mitochondrial dysfunction has caused the disease or whether the complex pathogenesis of that particular disease has ultimately damaged the mitochondria, as in the chicken or the egg causality dilemma, correction and modification of mitochondrial function can be promising in terms of the disease management. This approach becomes more important when we consider that based on the emerging evidence, many environmental pollutants to which we are more or less exposed have a destructive role on mitochondrial activity, and interestingly, on the other hand, contact with these contaminants is associated with those mentioned diseases.

With the increasing evidence that mitochondria play an essential role in the pathogenesis of diseases, targeting of them for the treatment purposes has also been rigorously taken into consideration. To do this, we must begin to know the mitochondrial metabolic irregularities involved in those diseases and then to determine the possibility or feasibility of manipulating them via pharmacological approaches. In the present book, some details regarding the significant role of mitochondria in the chronic diseases and new insights in targeting of mitochondria for therapeutic purposes have been discussed. In view of this and to better achieve our goals, the chapters of the book are arranged in the following order:

Chapter 1: The recent advances in the antioxidant burden of mitochondria and the mechanism behind its sensing and maintaining of its integrity have been discussed in this chapter.

Chapter 2: The role of mitochondria in the metabolism of biomolecules with the focus on molecular pathomechanisms and also targeted therapeutics is a very interesting topic. Finding molecular pathways and their interrelationship, which has the potential to serve as a therapeutic target, has been discussed here.

Chapter 3: This chapter updates information about the pathways behind the mitochondria-related cell deaths and discusses their manipulation to evoke or prevent the cell demise in different disease states.

Chapter 4: Emerging evidence indicates that the environmental pollutants particularly heavy metals including arsenic, cadmium, copper etc. and other nonmetals agents disrupt the mitochondrial normal function and many of them are etiologically linked to some chronic disease states. These toxic mitochondriotropic agents and new insights into their toxic mechanisms are updated here.

Chapter 5: The role of caloric restriction and diet on the mitochondria have been discussed. Calorie restriction improves mitochondrial function and quality along with lowering free radical production and also extends life span in organisms ranging from yeast to mammals. Impaired quality control of mitochondria during the aging process or malnutrition can be modified through the calorie restriction.

Chapter 6: New advances in the identification of altered mitochondria associated signaling pathways in carcinogenesis has been discussed here. Many studies have addressed the alteration of mtDNA, oxidative phosphorylation, and other enzymatic reactions in the context of the carcinogenesis.

Chapter 7: Mitochondrial-targeted cancer therapy strategies have been discussed.

Chapter 8: New advances in the identification and targeting of mitochondria-associated metabolic pathways in diabetes that are highly sought after more effective treatments have been discussed.

Chapter 9: Some of the most common neurologic conditions (Parkinson's disease, Alzheimer's disease, multiple sclerosis, epilepsy, etc.) in which the role of mitochondrial dysfunction in their pathogenesis have been broadly investigated.

Chapter 10: The role of mitochondria in mental disorders has been addressed by many researchers and the molecular link between mitochondrial dynamic (fission and fusion) and psychiatric states (e.g., depression) have attracted much intention. This field is very interesting and some third parties like gut microflora have also been engaged.

Chapter 11: This chapter discusses the different aspects of mitotherapy in terms of current problems, major challenges, and future promises.

We believe that this book will provide new horizons for readers to realize the importance of mitochondria in the present-day research on therapies dealing with mitochondria associated chronic diseases including diabetes, cancer, and neurodegenerative disorders.

February 20, 2021

Chapter 1: Mitochondria as balancers of reduction/oxidation for intracellular environment

Mohsen Rezaeia; Somayeh Handalib; Jalal Pourahmadc    a Department of Toxicology, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran

b Medical Biomaterial Research Center (MBRC), Tehran University of Medical Sciences, Tehran, Iran

c Department of Pharmacology & Toxicology, School of Pharmacy, Shahid Beheshti University of Medical Sciences, Tehran, Iran

Abstract

Reduction/oxidation-dependent reactions affect many cellular functional pathways. Mitochondria are at the center of these processes, as mitochondria both generate reactive oxygen species (ROS) that drive redox-sensitive events and respond to ROS-mediated changes in the cellular redox state. In this chapter, we discuss the regulation of cellular ROS, their modes of production and removal, and the redox-sensitive targets that are modified by their flux. In particular, we focus on the actions of redox-sensitive targets that change mitochondrial functions. We also consider the role of mitochondria in modulating these events, and discuss how redox-dependent events may contribute to the pathology of diseases by influencing mitochondrial function, particularly the involvement of mitochondrial ROS in diabetes, cancer, and neurodegenerative diseases.

Keywords

Mitochondrial ROS; Redox dependent; Antioxidant; Diabetes; Cancer; Brain

Contents

1Introduction

2Mitochondrial ROS in pathological condition

2.1Mitochondrial ROS and diabetes

2.2Mitochondrial ROS and cancer

2.3Mitochondrial ROS and brain

3Mitochondrial antioxidant defense

References

1: Introduction

The main endogenous sources of reactive oxygen species (ROS) are cytosolic sources including nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs), cyclooxygenases, cytochrome P450 enzymes, xanthine oxidase, and mitochondria originated sources including respiratory chain, NOX4, p66shc, and monoamine oxidases (MAOs).¹ Oxidative phosphorylation (OXPHOS) in mitochondria contributes to ATP production via the electron transport chain (ETC) and also the generation of ROS.² ROS include superoxide ( si1_e ), hydrogen peroxide (H2O2), hydroxyl radical (OH•), hypochlorous acid (HOCl), and peroxynitrite (ONOO−).³–⁵ Leakage of electrons at complexes I and III from ETC (the major sites of ROS generation) leads to reduction of oxygen to si1_e (primary ROS with a very short lifetime, approximately 1 × 10−  9 s).³, ⁴, ⁶–⁸, Then, si1_e converts to H2O2 by superoxide dismutase 1 (SOD1, Cu-ZnSOD) in the mitochondrial intermembrane space and superoxide dismutase 2 (SOD2, MnSOD) in the mitochondrial matrix (Fig. 1).⁹, ¹⁰ H2O2 further reduces to H2O by catalase (CAT), glutathione peroxidase (GPX), peroxiredoxin (PRDX), and thioredoxin (TXN).⁹, ¹¹ CAT is mainly found in the peroxisome fraction of cells and very little levels of CAT have been found in mitochondria. CAT is not present in mitochondria of mammalian cells, except for rat heart mitochondria.¹², ¹³ A redox balance on production and neutralization of free radicals is existed in organisms that maintain the ROS level at a steady state and reducing equivalents at optimum capacities.¹⁴ Produced ROS are transferred to the matrix and eliminated by the antioxidant systems; however, high levels of ROS in the matrix reach to the intermembrane space and cytosol which contribute in redox signaling or lead to oxidative damage. ROS can pass the membranes through different ways. For instance, H2O2 can transfer by aquaporins and superoxide can travel via anion channels.¹⁵ ROS are involved in reprogramming cells for proliferation and intracellular signaling.¹⁶–¹⁸

Fig. 1

Fig. 1 Mitochondrial ROS generation.

Mitochondria are also the main site for scavenging of ROS and some similar antioxidant enzymes exist in the cytosol.¹² Excessive ROS leads to lipid peroxidation (LPO),¹⁹ DNA damage,²⁰ and protein oxidation.⁴ H2O2 can react with iron (Fe²  +) in the Fenton reaction and produce OH glyph_rad ¹¹ Peroxidation of lipids is begun by OH glyph_rad overproduction.⁴ H2O2 can be converted to HOCl by myeloperoxidase (MPO).⁵ Moreover, si1_e contributes to the generation of peroxynitrite (ONOO−) in a reaction with mitochondrial nitric oxide (NO), which may lead to lipid peroxidation (Fig. 1).⁴, ¹¹

ETC can also produce other reactive species such as nitric oxide (NO) and reactive nitrogen species (RNS).⁶ It has been suggested that NO may be produced by a mitochondrial NO synthase. This free radical can potentially affect mitochondrial functions including competing with O2 at complex IV and reacting with O2 to form peroxynitrite. NO may be produced in nerve terminals, vascular endothelium, or other cytosolic sites.²¹

NOXs are a family of enzymes and another source for intracellular ROS production and are involved in the transfer of electrons from NADPH to O2 which may result in the generation of si1_e and H2O2. NOX4 is located in the inner mitochondrial membrane of renal cells and cardiomyocytes and generates si1_e and H2O2 in the mitochondrial matrix (Fig. 1).¹ P66shc belongs to the Shc (Src homology 2 domain and collagen-homology region) family of cytosolic adaptor proteins. This protein exists in the mitochondrial intermembrane space and is involved in mitochondrial ROS production through the cytochrome c oxidation and triggering of H2O2 production (Fig. 1).¹ MAOs are located in the outer mitochondrial membrane and degrade monoamines such as epinephrine and norepinephrine to aldehydes and H2O2 (Fig. 1).¹ Intra-mitochondrial ROS production by MAO can induce lipid peroxidation that results in phospholipase C activation and calcium signaling provoked by inositol trisphosphate.⁸

The mitochondrial flavoprotein long-chain acyl-CoA dehydrogenase (LCAD) can also be considered as an intra-mitochondrial source for H2O2 production. In vitro experiments revealed that LCAD which contributes to a key step in mitochondrial fatty acid oxidation (FAO) generated H2O2 by transferring electrons to O2. There is evidence that mitochondrial enzyme very long-chain acyl-CoA dehydrogenase (VLCAD) attaches to cardiolipin in the mitochondrial inner membrane and is in close interaction with the ETC in a higher-order FAO complex.²²

Besides their contribution to oxidative damage, mitochondrial ROS can also trigger several signaling pathways in the cells. Stabilization of hypoxia-inducible transcription factors (HIFs) under hypoxic condition is one of the well-known pathways that require the mitochondrial ROS production. HIF stabilization results in the beginning of transcriptional changes including the alteration of genes involved in angiogenesis. Proliferation of endothelial cells is essential for angiogenesis that is highly depended on the expression of vascular endothelial growth factor (VEGF) which is upregulated by HIF. The proliferative effect of VEGF is directed by the activation of the extracellular-signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway.²³

Mitochondrial ROS are also needed for T-cells activation. Lowered amount of ROS produced by mitochondria complex III resulted in decreased T-cell activation in mice upon the stimulation with CD3 or CD28. In addition, it was observed that translocation of mitochondria to the immunological synapse in a T-cell line and mitochondrial H2O2 are both necessary for T-cell receptor (TCR) signal transduction via MAPK signaling. These findings suggest that mitochondrial ROS that is required for T-cell stimulation and proliferation increase the TCR signal transduction following activation of antigen exposure.²³

2: Mitochondrial ROS in pathological condition

Increasing ROS levels leads to protein denaturation, DNA damage, and lipid peroxidation,²⁴ which eventually may lead to oxidative stress, mitochondria, and other membranous organelles damaging and neurodegenerative diseases, diabetes, cancer, cardiovascular diseases, and rheumatoid arthritis.²⁵, ²⁶ ROS oxidizes the disulfide bonds in proteins and lipids which lead to instability of membranes and production of short half-life lipid radicals. This toxic event may propagate the oxidative damage in membranes and develop to chronic diseases.²⁰ ROS also contribute to the peroxidation of cardiolipin in mitochondria which results in cytochrome c release into the cytosol which triggers apoptosis.²⁷ ROS stimulate DNA damage by oxidation of guanine nucleobases. However, the repair mechanism may not be flawless and these errors can lead to some mutations.⁴

OH glyph_rad can specifically alter the enzymes and structural proteins and oxidize polyunsaturated fatty acids.²⁸ It was reported that interaction of OH with DNA molecule damages the nitrogenous bases, pyrimidine and purine, and deoxyribose backbone of DNA.⁶

In some general insult like inflammation, ROS production is elevated by the action of certain enzymes, including NADPH oxidase, xanthine oxidase, and cyclooxygenase-2, which activate NF-κB (nuclear factor kappa B), the primary redox-sensitive signaling pathway for inflammation.²⁹ In the next sections, the role of ROS is discussed in relation to specific chronic diseases.

2.1: Mitochondrial ROS and diabetes

Hyperglycemia is implicated in ROS generation in diabetic patients through mitochondrial respiratory chain enzymes, lipoxygenases, xanthine oxidases, peroxidases, cyclooxygenases, and nitric oxide synthases.³⁰ The major function of β-cells is to regulate the concentration of glucose in the body by insulin secretion. In β-cells, glucose enters through GLUT2 (glucose transporter-2) and becomes phosphorylated to glucose-6-phosphate and is converted to pyruvate by glycolysis. Pyruvate enters mitochondria and is converted to acetyl coenzyme A through pyruvate dehydrogenase. Acetyl coenzyme A then enters the tricarboxylic acid (TCA) cycle, resulting in the production of NADH. NADH fuels the ETC for ATP production and elevates the ATP/ADP ratio. This in turn results in the closure of ATP-sensitive K+ channels (KATP) that depolarizes the plasma membrane and induces the opening of voltage-dependent Ca²  + channels and influx of Ca²  +; consequently, facilitates insulin secretion through the fusion of insulin granules with the plasma membrane.³¹ When β-cells are exposed to high levels of glucose (hyperglycemia), ROS generated by the glucose oxidation pathway activate uncoupling protein-2 (UCP-2). This protein reduces the ATP/ADP ratio through leakage of proton in the β-cells mitochondria, which decreases secretion of insulin.³², ³³ β-Cells possess low antioxidant defenses than other tissues.³¹

Hyperglycemia also increases ROS by stimulating xanthine oxidase, NADPH oxidase, and mitochondrial pathway which leads to oxidative stress.³⁴ Mitochondrial ROS influence the nuclear DNA which in turn activate poly (ADP-ribose) polymerase (PARP), a DNA repair enzyme. Then, PARP decreases the activity of glyceraldehyde-3 phosphate dehydrogenase (GAPDH) which in turn inhibits the glycolytic pathway and results to the accumulation of glycerol 3-phosphate. Consequently, glycerol 3-phosphate induces other alternative glucose metabolic pathways including hexosamine pathway, polyol pathway, protein kinase C (PKC) pathway, and AGE (advanced glycation end) production.³⁵–³⁷ Hexosamine pathway converts fructose-6-phosphate to glucosamine-6-phosphate through the transfer of an amide group from glutamine. Excess hexosamines have been shown to induce insulin resistance in vitro and in vivo.³⁸ The polyol pathway consists of a 2-step metabolic pathway that glucose is reduced to sorbitol and then sorbitol converts to fructose.³⁹ This pathway has a pathogenic role in diabetic retinopathy.⁴⁰ Multiple diabetic problems have been related to the enhancement of PKC activation including alteration in blood flow, vascular permeability, and angiogenesis.⁴¹ In podocytes, activation of calcium channel and inhibition of Ca²  +-ATPase contribute to AGE-induced increase in the concentration of intracellular calcium, which results in ER stress and podocyte apoptosis. This demise of podocytes due to apoptosis is characteristic in diabetic nephropathy that results in albuminuria and renal dysfunction.⁴² AGEs are produced through the Maillard reaction that is a nonenzymatic amino‑carbonyl reaction. This reaction takes places between the carbonyl group of glucose, fructose, galactose, and ribose or glucose metabolism intermediates (glucose-6-phosphate, ribose-5-phosphate, fructose-6-phosphate glyceraldehyde, and deoxyribose-5-phosphate), with an amine group and other molecules. The yield of the later reaction is a reversible Schiff base and consequently, Amadori products, which are early products of the Maillard reaction. Production of the Amadori products is elevated in hyperglycemia. By glycoxidation of biomolecules, these highly reactive compounds interact with amine groups and metal ions to form malondialdehyde (MDA), glyoxal (GO), and methylglyoxal (MGO).⁴³

2.2: Mitochondrial ROS and cancer

The mechanism of ROS in tumor development is associated with DNA damaging at 8-hydroxy 2′-deoxyguanosine (8-OHdG) that leads to mutation.²⁰ In some cancer cells, antioxidant capacity has been upregulated through the elevated expression of enzymes involved in ROS detoxification. Nuclear respiratory factor 2 (NRF2) is a transcriptional activator of genes that alleviate oxidative damage; activity of NRF2 is inhibited by kelch-like ECH-associated protein 1 (KEAP1). In some tumors, NRF2 is activated constitutively, either by gain-of-function mutations or by KEAP1 suppression. NRF2 gain-of-function provides resistance features to tumor cells against the chemotherapies that act by inducing the oxidant stress. Inactivation of KEAP1 can be the result of posttranslational modifications that contribute to the overactivation of NRF2.¹⁵ ROS can also be involved during metastasis that suggesting to have a signaling role within cells for survival, migration, and metastasis promotion.⁴⁴ It has been shown that ROS facilitates metastasis in various studies.⁴ Anoikis is the process of programmed cell death induces by the detachment of normal cells from the matrix. It was suggested that ROS could induce anoikis resistance and escaping apoptosis in cancer cells during metastasis.²⁰ The mechanism of anoikis inhibition is still not clear. It has been suggested that attenuation of SOD2 (MnSOD) promotes anoikis resistance and tumor metastasis. NF-κB is transcriptional regulators of SOD2 which activates following cell detachment. In normal cells, matrix detachment enhances ROS levels, results in NFKB pathway activation, which in turn induces MnSOD expression to detoxify ROS. MnSOD exhibits antianoikis activity. Reduction of MnSOD or NF-κB enhances the toxic effect of superoxide free radical in suspended cells; therefore, increases the sensitivity of cells to anoikis. It has been suggested that two parallel pathways including the decrease in glucose oxidation (reduced mitochondrial ROS generation) and increase of antioxidant capacity to scavenge ROS take place in response to matrix detachment which permit cells to cope with oxidative stress.⁴⁵ In normal and cancerous cell lines, the role of ROS in cell death and viability has been controversial and presumably depended on the relative amount of the ROS. Moreover, increasing expression of NOX4 confers anoikis inhibition in lung cancer cells through ROS-mediated activation of epidermal growth factor receptor (EGFR), which enhances cell survival. It is likely that upregulation of NOX4 increases EGFR phosphorylation through H2O2 since NOX4 mainly produces H2O2 rather than superoxide anions. Overexpression of EGFR inhibits anoikis by downregulation of Bim (pro-apoptotic protein).⁴⁶

HIF-1 and its targets including VEGF and metalloproteinases (MMPs) that are contributed to angiogenesis and matrix remodeling may be promoted by ROS. ROS stabilize HIF-1a that is necessary for metastasis and evident in stromal remodeling and connective tissue degradation through the upregulation of MMPs. This action provides an optimum environment for tumor cells to enter and spread into adjacent tissue. Furthermore, HIF-1a upregulates VEGF that in turn increases tumor angiogenesis and facilitates the migration of tumor cells into the bloodstream.⁴⁷

Impaired mitochondrial respiration is linked to the high level of ROS generation and excessive mutations that may progress to tumor formation. Mutations in complex I promote tumorigenesis via ROS production; as a result, DNA damage and mutation that may activate oncogenes (gain of function) will be occurred. Besides this, enhancing glycolytic metabolism and lactate production promotes metastasis.⁴⁴ Complex II has a close relationship to TCA cycle and mitochondrial ETC and all of its subunits are expressed by nucleus genes. Electrons from complex I and also complex II (oxidation of succinate) enter into the respiratory chain.⁴⁴ Mutation in SDH leads to the accumulation of succinate. Enhanced level of succinate results in improved histone methylation through inhibition of histone demethylase Jumonji D3, which increases epigenetic changes and oncogenic transformation. Moreover, mutations in subunits A-D of succinate dehydrogenase (SDH) are related to pheochromocytoma and paragangliomas. Accumulation of succinate enhances the ROS generation through α-ketoglutarate-dependent enzyme blockade and reversing of electrons flow from complex II to complex I and leads to pheochromocytoma and paragangliomas by the stabilization action of HIF.⁴⁸, ⁴⁹

2.3: Mitochondrial ROS and brain

Oxidative stress has been etiologically related to many brain diseases including Alzheimer’s disease (AD) and Parkinson’s disease (PD).²⁷ Relatively, elevated oxygen consumption, excess amount of membrane polyunsaturated fatty acids, and lower antioxidant capacity including reduced glutathione, CAT, SOD, and GPX activities make the central nervous system (CNS) very vulnerable and sensitive to ROS devastating effects.²⁷

AD is related to deposits of amyloid β (Aβ) and tau which results in inflammation, increasing iron and enhancing ROS production.⁵⁰ Intracellular tau oligomers interact with the mitochondrial respiratory chain, triggering the release of cytochrome c and inducing ROS generation.⁵¹ ROS production is an important intracellular event that leads to activation of p38-MAPK, tau hyperphosphorylation, and destabilization of microtubules which may induce neurodegeneration and cell death.⁵² Increasing Ca²  + transport from endoplasmic reticulum (ER) to mitochondria also results in reduced ATP production and enhanced ROS generation which triggers apoptosis.⁵³ In AD, anomalous mitochondrial electron transfer is evident which mostly occurred in complexes I and IV, resulting in declined mitochondrial membrane potential, reduced ATP production (complex V), and increased ROS generation. Dysfunction of complex I is mainly contributed to tau effects; whereas, dysregulation of complex IV is Aβ-dependent and these damages happened at both protein structures and activities.⁵⁴ Other damages related to oxidative stress in AD are glucose autoxidation that leads to AGES production and Aβ-toxicity induction.⁵⁵

In the PD, the activity of ETC complex I decreases that leads to enhanced ROS production, mitochondrial DNA damage, ETC and bioenergetics disruption, release of cytochrome c into the cytosol, and finally induction of mitochondria-dependent apoptotic pathway.²⁷ Many gene products are related to PD pathogenesis including ɑ-synuclein, LRRK2 (leucine-rich repeat kinase 2), DJ-1, parkin, and PINK1. Parkin (an E3 ubiquitin ligase) is associated with the outer mitochondrial membrane and prevents the release of cytochrome c.⁵⁶ Parkin can attach covalently to ubiquitin on different misfolded protein substrates to help their degradation.⁵⁷ Parkin is also involved in mitophagy and removal of impaired mitochondria.⁵⁸ PINK1 (PTEN-induced putative kinase-1) is a serine–threonine kinase in the mitochondria and its overexpression enhances mitochondrial fission. Protection against oxidative damage and reduction of mitochondrial cytochrome c release as well as apoptosis are among its functions.²⁷, ⁵⁷ Overexpression of ɑ-synuclein (ɑ-syn) is associated with mitochondrial membrane instability and cytochrome c release. ɑ-Syn accumulation leads to complex I impairment, mitochondrial inter-membrane potential disruption, and increase in ROS production.⁵⁹, ⁶⁰ It has been suggested that ɑ-syn may enter in the mitochondrial matrix or the intermembrane space through channel proteins including TOM34 and TIM13A and inhibit the complex I.⁶¹ Furthermore, ɑ-syn enhances Ca²  + transport to the mitochondria which leads to oxidative stress and triggers apoptosis.⁶² SIRT3 is present in the cytosol in its inactive form and transports to the mitochondria during oxidative stress. It has been suggested that TOM20 (translocase of outer membrane) carries SIRT3 to the mitochondria. ɑ-Syn reduces the mitochondrial SIRT3 levels which results in the ROS production dysregulation. SIRT3 deacetylates and activates the ROS neutralizing enzymes, including SOD2.⁶³ In oxidative stress insult, DJ-1 as an oxidative stress sensor chaperone relocalized into the mitochondria and takes its protective role.²⁷, ⁶⁴ Generally, the DJ1 protein is found in the cytosol, nucleus, and mitochondria.⁵⁸ Insufficient DJ-1 increases ɑ-syn accumulation by accelerating the LAMP2A (lysosome-associated membrane protein type-2A) degradation in lysosomes.⁶⁵ It has been shown that ɑ-syn and DJ-1 interact with mitochondria-associated membranes (MAM) through the chaperone Grp75. These interactions stimulate MAM assembly and activity by controlling ER-mitochondria Ca²  + and lipid homeostasis. These findings indicated that MAM disruption contributes in the pathogenesis of PD.⁶⁴ In PD patients, there is impairment for reuptake of dopamine (DA) into the synaptic vesicles; so that a high level of cytosolic DA is present. DA can be auto-oxidized to form cytotoxic radicals that may impair mitochondria and activate intrinsic apoptotic pathway or inhibit the respiratory chain. The produced DA quinines or semiquinones can modify some vital cellular proteins such as α-syn, DJ-1, SOD2, parkin, and UCH-L1. The DA quinones are also undergoing oxidation to form aminochrome with redox-cycling activity that may in turn contribute to the generation of the superoxide radicals, antioxidants depletion, and the degenerative outcome (Fig. 2).⁶⁶

Fig. 2

Fig. 2 Role of mitochondrial dysfunction in PD.

3: Mitochondrial antioxidant defense

Mitochondria possess numerous defense mechanisms to counteract the elevated ROS production. Glutathione (GSH) is a tripeptide antioxidant that is produced by the amino acids glutamate, cysteine, and glycine in two ATP-dependent steps in the cytoplasm (is present in millimolar concentrations) (Fig. 3).¹⁵, ⁶⁷–⁶⁹, The first reaction is γ-glutamyl-cysteine formation from glutamate and cysteine catalyzed by the glutamine-cysteine ligase (GCL).⁷⁰ Then, the addition of glycine to γ-glutamyl-cysteine forms GSH by glutathione synthetase (GS).⁶⁷ Cysteine along with glutamate and glycine are the rate-limiting metabolites for GSH synthesis.⁴ GSH is oxidized by glutathione peroxidase (GSH-PX), a selenium-based enzyme for the neutralization of ROS that leads to the formation of glutathione-disulfide (GSSG). GSH can also be regenerated by the reduction of GSSG via the glutathione reductase (GSR) enzyme. This reaction needs NADPH for producing two GSH molecules from one GSSG molecule.⁷⁰ GSR is a homodimer enzyme and found both in the cytosol and in mitochondria.¹³ GSH synthesis in mitochondria is not possible owing to the lack of relevant enzymes.⁷¹ Cytosolic GSH can transport into mitochondria via TTC (tricarboxylate carrier), DIC (dicarboxylate carrier), KGC (a-KG carrier), and 2-oxoglutarate (2-OGC) GSH carriers.⁷²,