Novel Therapeutic Approaches Targeting Oxidative Stress

By Pawan Kumar Maurya

Novel Therapeutic Approaches Targeting Oxidative Stress investigates the role of oxidative stress in disease and explores the latest methods and approaches to targeting oxidative stress for treatment and diagnosis. The book begins with an introduction to oxidative stress and its significance. Subsequent sections cover biochemical methods for detecting free radicals and novel therapeutic approaches for targeting oxidative stress in a number of different diseases. This includes age-related illnesses, neuropsychiatric disorders such as schizophrenia and bipolar disorder, and neurodegenerative diseases like Alzheimer’s and Parkinson’s disease. Novel approaches for targeting oxidative stress in cancer and cardiovascular diseases are also explored.

The book then moves on to discuss advances in drug delivery systems and detecting oxidative stress biomarkers using biosensors. It concludes with case studies that illustrate the targeting of oxidative stress and future perspectives.

• Explores oxidative stress in a variety of diseases, including neurological disorders, cardiovascular diseases, age-related diseases, and cancer
• Covers a range of therapeutic approaches to target oxidative stress
• Includes chapters on the application of novel drug delivery systems and diagnostic biosensors to oxidative stress
• Features case studies illustrating the targeting of oxidative stress

• Language: English
• Publisher: Academic Press
• Release date: Feb 18, 2022
• ISBN: 9780323909068

Book preview

Chapter 1: Novel therapeutic approaches targeting oxidative stress in mood disorders

Alexander Bambokiana; Fabiano A. Gomesa,b; Calvin Sjaardaa,b; Claudio N. Soaresa,b; Roumen Mileva,b; Elisa Brietzkea,b    a Centre for Neuroscience Studies (CNS), Queen’s University, Kingston, ON, Canada

b Department of Psychiatry, Queen’s University School of Medicine, Kingston, ON, Canada

Abstract

Mood disorders, such as major depressive disorder (MDD) and bipolar disorder (BD), have been treated with a wide range of pharmacological therapies, including but not restricted to antidepressants, mood stabilizers, and second-generation antipsychotics. Current gold standard interventions primarily target numerous monoaminergic brain systems, to prompt relief of depressive symptoms. However, these interventions provide symptomatic relief in only ~   50% of patients. To address these limitations, there has been a greater emphasis on understanding the role of neuronal oxidative stress and its subsequent impact on the pathology of various mood disorders. Numerous sources have been identified within neurons of the central nervous system (CNS) that contribute significantly to both reactive oxygen species (ROS) and reactive nitrogen species (RNS) accumulation within the brain. The aerobic mitochondrial process of oxidative phosphorylation and various neuronal enzymes are indirectly associated with the production of common ROS and RNS. Numerous anatomical and physiological properties promote a greater susceptibility to neuronal ROS-inflicted subcellular damage, interrupting the functional and structural integrity of cells across the CNS. The association of mood disorder pathology with increased oxidative stress markers has prompted interventional strategies that reduce the cellular REDOX state and ensures that an adequate balance of ROS is maintained within the neuron. Whether the ROS will have a detrimental or beneficial effect on the neuron is heavily reliant upon the activity of several key enzymes associated with ROS homeostasis. Though the individual sources of neuronal ROS can be targeted, antioxidant systems are critical in moderating ROS accumulation. Thus, the exploration of antioxidants as adjunctive therapy for mood disorders provides a promising mechanism for targeting neuronal oxidative stress and increasing symptomatic relief across patients.

Keywords

Mood disorders; Depression; Bipolar disorder; Antidepressants; Neuronal oxidative stress; Reactive oxygen species; Reactive nitrogen species; Mitochondria; Nitric oxide synthase; Adjunctive therapy

1: Introduction

Mood disorders, including major depressive disorder (MDD) and bipolar disorder (BD), have been treated with a wide range of pharmacological therapies, including but not restricted to antidepressants, mood stabilizers, and second-generation antipsychotics [1]. However, these interventions are not always effective and may be associated with side effects, reducing the therapeutic options for a large group of individuals. For example, current gold standard antidepressant monotherapies only improve the severity of depressive symptoms in approximately 50% of patients, with only 30% achieving remission [1]. A possible explanation for this limitation is that the main target for all these medications is the monoaminergic brain systems.

Over the past decade, the main efforts of the scientific community for developing new treatments for mood disorders have focused on going beyond the monoaminergic system, concentrating mainly on three main pathways: modulation of neuroplasticity, mitochondrial dysfunction, and oxidative stress. Modulation of neuroplasticity includes changes in synaptic plasticity, dendritic arborization, and cell resilience [2]. Indeed, neuroplasticity impairments became the most prevalent theoretical paradigm to reconcile findings from different lines of evidence. For example, it is well known that neuroplasticity is compromised by hypothalamus-pituitary-adrenal-axis (HPA) activation due to stress, changes in gonadal hormones (e.g., testosterone, estrogen, progesterone), thyroid hormones, persistent long-term inflammatory activation, exposure to alcohol and drugs, and sleep deprivation [3]. In addition, neuroimaging studies have demonstrated reduction of hippocampus volume in MDD and some reduction of cortical thickness and hippocampus volume in BD [4–6].

More recently, treatments have focused on abnormalities in the process by which neurons and glia use energy [7]. For example, there is replicated evidence of mitochondrial dysfunction in both MDD [8] and BD [9,10]. From a more clinical point of view, several studies report a positive association between MDD and BD and systemic energy regulation dysfunction, clinically expressed by a disproportionally high prevalence of obesity and metabolic syndrome in individuals with mood disorders, which happens even in drug-naïve individuals [11,12]. Mansur et al. [13] proposed a model that centered on energy expenditure regulation as a disease model for BD. According to the authors, disruption of energy expenditure is a primary and sufficient cause of BD, with changes in neuroplasticity and neurotransmission being secondary to energy imbalances and general medical comorbidities as compensatory mechanisms [13].

Finally, reducing oxidative stress markers is promising to be an effective therapy in individuals with MDD and BD. Oxidative stress results from an imbalance between reactive oxygen species (ROS) production and their degradation by antioxidant defenses [14]. Physiological brain activity generates ROS, which antioxidant defenses should counterbalance. Over the past 15 years, mounting evidence on the role of oxidative stress has affirmed that both manic and depressive episodes are associated with oxidative stress [15,16]. Most of these studies quantified changes in oxidative stress markers throughout the disease, and during posttherapy periods, in individuals with MDD and BD when compared to healthy controls [17].

Increasing our understanding of the role that oxidative stress plays in the onset of mood disorders like MDD and BD, and development of novel therapeutic approaches targeting these mechanisms, may facilitate (and has already begun to do so) alternative therapies for treatment-resistant patients. However, to understand the therapeutic implications of targeting oxidative stress, it is critical first to recognize the essential role and sources of ROS across the central nervous system (CNS), as well as the physiological properties and cellular structures of the brain that make neurons highly susceptible to oxidative damage.

2: The production of ROS in the brain

The processes by which ROS are produced within neurons are similar to those recorded in other cell types. Oxidative phosphorylation, the mitochondrial metabolic process that provides aerobic organisms with most of their cellular adenosine trisphosphate (ATP), is a significant source of cellular ROS production within neurons. Occasionally during this process, oxygen (O2) is subjected to a single-electron reduction, forming the superoxide anion radical (O2 glyph_rad −), which most commonly occurs at Complex III of the electron transport chain (ETC)². Due to its extremely high reactivity, the superoxide anion can immediately react with, and subsequently damage, nearby subcellular structures. Typically, this superoxide anion is rapidly converted into hydrogen peroxide (H2O2) via a superoxide dismutase-2 (SOD2) catalyzed dismutation. Despite its greater stability, hydrogen peroxide is also a ROS capable of initiating widespread neuronal damage due to its propensity to diffuse freely across both cellular and subcellular membranes [18].

Though oxidative phosphorylation generates the greatest concentration of intracellular ROS, various other enzymes also contribute to ROS production within the brain, though to a lesser degree. Neuronal nitric oxide synthase (nNOS) is an isoform of the NOS enzyme family that catalyzes L-arginine oxidation, which produces nitric oxide (NO) in both the central and peripheral nervous systems. Though NO production functions to mediate smooth muscle relaxation, regulate blood pressure, and provide synaptic plasticity to the CNS, NO and nNOS are responsible for contributing minor concentrations of ROS³. During periods where L-arginine concentrations are reduced, hydrogen peroxide and superoxide are produced in place of NO [18]. Additionally, any preexisting intracellular superoxide anions can interact with NO, creating the highly reactive peroxynitrite (NO3−), a reactive nitrogen species that can modify and damage intracellular proteins and disrupt the structural and functional integrity of the neuron. nNOS activity is indirectly regulated via intracellular calcium (Ca2+) concentrations, specifically through interactions with the Ca2+-binding protein calmodulin. In the brain, nNOS is found primarily anchored to the postsynaptic membrane of neurons, located in close proximity to Ca2+-transporting N-methyl-d-aspartate (NMDA) receptors [19]. This distribution enables efficient regulation of NO and ROS production based on postsynaptic neuronal depolarization.

The monoamine oxidase (MAO) enzyme family, specifically the MAO-A and MAO-B isoforms, is an additional ROS production source within neurons. MAO utilizes O2 to catalyze the oxidative deamination of biogenic monoamines, producing H2O2 as a by-product. These isoforms function to inactivate monoamine neurotransmitters across the CNS and are found naturally distributed across different neuronal cell types. Though each catalyzes the same reaction, MAO-A is preferentially localized within catecholaminergic neurons, while MOA-B is predominantly situated in serotonergic and histaminergic neurons and glial cells [19,20]. Each form of MAO is bound to the outer mitochondrial membrane and contributes to oxidative stress in congruence with ROS produced through oxidative phosphorylation [21,22].

A third source of neuronal ROS may evolve from nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX), a family of plasma membrane-bound enzyme complexes whose expression across the CNS has only recently been described. Whereas the previous sources that have been discussed produce ROS as a by-product, NOX functions to specifically produce O2 glyph_rad − via a single-electron transfer from NADPH to molecular oxygen [19]. Superoxide plays critical physiological roles within the brain, such as aiding immune protection against pathogens. However, if not regulated effectively, the overproduction of NOX-generated superoxide can damage nearby subcellular structures. Although the association of NOX with the pathogenesis of several neurodegenerative diseases, including Alzheimer’s, Huntington’s, and Parkinson’s disease is not clearly understood, each disease displays increased NOX activity and heightened levels of oxidative stress markers [23]. The identification of other sources of neuronal ROS, and their role in neurodegenerative diseases and mood disorders, is a highly emphasized field in neuroscience research.

3: The elimination of ROS in the brain

Whether the ROS will have a detrimental or beneficial effect on the neuron is heavily reliant upon the activity of several key enzymes associated with ROS homeostasis. Numerous antioxidant systems ensure that any excess ROS, produced from the sources mentioned previously, is rapidly removed prior to cellular damage. This includes superoxide dismutase (SOD), a group of metalloenzymes that function to provide the first line of defense against ROS-inflicted cellular damage. SOD catalyzes the dismutation of superoxide radical anions into hydrogen peroxide and molecular oxygen, thus comprising an antioxidant mechanism that provides effective regulation over superoxide production [24]. Three SOD isotypes have been identified in humans, each distinguished based on their subcellular or extracellular location. These include SOD-1, SOD-2, and SOD-3, localized specifically in the cytoplasm, mitochondria, and extracellular matrix, respectively [24]. The distribution of the SOD isoforms throughout the cellular interior, and exterior, ensures rapid removal of superoxide, regardless of where it is produced within the cell. Given the significant production of superoxide originating from mitochondrial oxidative phosphorylation, SOD-2 operates as a critical component associated with regulating and thus preventing ROS overproduction. The rise in oxidative stress markers recorded in neurons often implicates SOD-2 dysfunction as a critical constituent associated with mood disorder pathology, though further investigation is required. Though total neuronal superoxide concentrations are heavily reliant upon the activity of mitochondrial SOD-2, SOD-1 and SOD-3 are also critical to maintaining neuronal integrity and preventing additional ROS production outside the mitochondria. SOD-1 maintains appropriate levels of superoxide within the cytoplasm of neurons and SOD-3 regulates extracellular superoxide in the extracellular space surrounding neurons across the CNS. Though not directly associated with neuronal protection, SOD-3 aids the metabolic regulation of neurons throughout the CNS, specifically by altering vascular tone and blood flow to the brain [24]. In unity, all three SOD isoforms provide neurons with an effective antioxidant mechanism that ensures tight regulation over the highly reactive superoxide anion radical.

Glutathione, a cysteine, glycine, and glutamic acid tripeptide, also protects neurons against oxidative stress through its antioxidant properties. Glutathione exists within cells in one of two redox states: a single molecule of reduced (GSH), or as an oxidized glutathione dimer (GSSG), consisting of the two molecules of glutathione bound together via a disulfide bond between cysteine residues. GSH provides antioxidant protection by reducing/neutralizing ROS, such as free radicals and peroxides, into oxygen species that pose a lower threat to cellular damage. This has established glutathione as a widely recognized ROS scavenger. During this process, the ROS act as an oxidizing agent, converting two GSH molecules into GSSG. Though glutathione comprises a wide variety of physiological functions, including xenobiotic metabolism through conjugation, its antioxidant properties play a significant role in neuroprotection against oxidative stress. Cellular ratios of GSH:GSSG have been effectively used to predict the redox state of an individual cell where a low GSH:GSSG ratio indicates low oxidative stress and a high GSH:GSSG ratio indicates high oxidative stress, thus providing a metric for determining intracellular levels of oxidative stress [25]. Evidence suggests that healthy cells in their resting state have a GSH:GSSG ratio greater than 100:1, compared to 1:10 during periods of oxidative stress [25]. Once depleted, intraneuronal sources of GSH can become restored through one of three methods: the regeneration of oxidized glutathione (GSSG) back to its reduced form (GSH) by glutathione reductase, de novo synthesis from its constituent amino acids via a two-step catalyzed reaction, and the recycling of cysteine residues from conjugated glutathione [25]. The cystine residue in GSH regulates de novo synthesis of glutathione based on its intracellular availability, and thus encompasses a rate-limiting step that can be manipulated when targeting oxidative stress with respect to treating mood disorders.

4: Oxidative stress in the brain

Ultimately, oxidative stress will occur when the damaging effects of the ROS exceed the biological system’s ability to neutralize the ROS and repair cellular damage. As seen across many cell types, oxidative conditions that overwhelm repair mechanisms across the CNS can cause potentially irreversible damage to neuronal proteins, lipids, and DNA. Amino acid side chains and protein backbones can become oxidized by ROS, eliciting detrimental effects on structural cytoskeletal and functional neuronal proteins. The loss of cellular integrity induced by protein modification can result in neuronal dysfunction, an outcome that has a significant implication for many neurodegenerative diseases and mood disorders. Base modification/damage and strand breaks by ROS, in both DNA and mitochondrial DNA (mtDNA), are responsible for a majority of ROS-inflicted neuronal dysfunction. More than 20 variations of oxidized bases have been discovered, though guanine oxidation to 8-oxoguanine has been recorded most frequently [26]. Polyunsaturated fatty acids are also common oxidative substrates, often subject to free radical or singlet oxygen oxidation.

Oxidative damage to mtDNA, proteins, and lipids can impair mitochondrial ATP production, as metabolic processes such as oxidative phosphorylation, fatty acid oxidation, and the tricarboxylic acid (TCA) cycle, among others, become disrupted. If sufficient damage has occurred such that the structural integrity of the mitochondria has been compromised, apoptotic factors, including cytochrome c and apoptosis-inducing factor (AIF), may leak from the mitochondrial intermembrane space into the cytosol, prompting apoptosis [27]. Alternatively, if significant concentrations of ROS are present, necrosis may result. Regardless of the macromolecular damage sustained by ROS, the loss of structural and functional neuronal integrity can result in cell death.

Considering the anatomical features of cells across the CNS, neurons are very vulnerable to oxidative damage compared to other cell types that compose different systems. The brain consumes 20% of the body’s oxygen supply through aerobic processes such as oxidative phosphorylation. The significant utilization of oxygen throughout the CNS promotes ROS formation through metabolic processes at a rate that exceeds other cell types. Thus, if antioxidant mechanisms become overwhelmed by the large concentration of ROS produced throughout the CNS, the neurons can become subject to widespread macromolecular damage. Furthermore, the brain is particularly enriched with polyunsaturated fatty acids (PUFAs), such as arachidonic and docosahexaenoic acids, which can act as a suitable oxidative substrate [28]. PUFAs have several essential functions across the nervous system, including participation in axonal growth, neuronal development, memory, inflammatory response, neuronal membranes, and myelin formation [28]. These critical functions can become impaired following oxidative modification/damage, promoting neurodegeneration, cell death, and possibly the onset of affective disorders.

However, current research suggests that neurodegeneration may not only occur as a result of ROS-inflicted damage, but also have an effect on the activation of inflammatory pathways [29]. Oxidative damage of neuronal proteins and lipids can result in significant modification of cellular structures, generating novel, highly immunogenic epitopes [29]. Lipid membrane components that are modified by oxidative stress can be recognized by numerous immune cells, which can elicit a robust inflammatory response against the neuron. The interactions between oxidative stress and inflammatory pathway-mediated neurodegeneration are currently under review, particularly to determine their role in the pathogenesis of numerous mood disorders.

5: Markers of oxidative stress and mood disorders

Several converging bodies of evidence support the role of oxidative stress in mood disorders [30]. To date, more than 400 studies support the association of oxidative stress and MDD or BD [30]. This research includes data from both animal and human studies, especially those evaluating the levels of different oxidative mediators in peripheral blood. It is thought that oxidative mediators are secondary messengers for the transcriptional regulation of genes involved in the regulation of neuroplasticity [31]. Here, we summarize the results of the human studies supporting the association between products of oxidative stress and mood