Hydrogen Peroxide and Amyotrophic Lateral Sclerosis: From Biochemistry to Pathophysiology

Abstract

Free radicals are unstable chemical reactive species produced during Redox dyshomeostasis (RDH) inside living cells and are implicated in the pathogenesis of various neurodegenerative diseases. One of the most complicated and life-threatening motor neurodegenerative diseases (MND) is amyotrophic lateral sclerosis (ALS) because of the poor understanding of its pathophysiology and absence of an effective treatment for its cure. During the last 25 years, researchers around the globe have focused their interest on copper/zinc superoxide dismutase (Cu/Zn SOD, SOD1) protein after the landmark discovery of mutant SOD1 (mSOD1) gene as a risk factor for ALS. Substantial evidence suggests that toxic gain of function due to redox disturbance caused by reactive oxygen species (ROS) changes the biophysical properties of native SOD1 protein thus, instigating its fibrillization and misfolding. These abnormal misfolding aggregates or inclusions of SOD1 play a role in the pathogenesis of both forms of ALS, i.e., Sporadic ALS (sALS) and familial ALS (fALS). However, what leads to a decrease in the stability and misfolding of SOD1 is still in question and our scientific knowledge is scarce. A large number of studies have been conducted in this area to explore the biochemical mechanistic pathway of SOD1 aggregation. Several studies, over the past two decades, have shown that the SOD1-catalyzed biochemical reaction product hydrogen peroxide (H₂O₂) at a pathological concentration act as a substrate to trigger the misfolding trajectories and toxicity of SOD1 in the pathogenesis of ALS. These toxic aggregates of SOD1 also cause aberrant localization of TAR-DNA binding protein 43 (TDP-43), which is characteristic of neuronal cytoplasmic inclusions (NCI) found in ALS. Here in this review, we present the evidence implicating the pivotal role of H₂O₂ in modulating the toxicity of SOD1 in the pathophysiology of the incurable and highly complex disease ALS. Also, highlighting the role of H₂O₂ in ALS, we believe will encourage scientists to target pathological concentrations of H₂O₂ thereby halting the misfolding of SOD1.

Keywords: TAR-DNA binding protein 43; aggregation; amyotrophic lateral sclerosis; fibrilization; hydrogen peroxide; misfolding; motor neurodegenerative disease; mutant superoxide dismutase 1; neuronal cytoplasmic inclusions; reactive oxygen species; redox dyshomeostasis; superoxide dismutase 1.

Figure 1

Successive four-electron reduction of molecular oxygen to generate water and different reactive oxygen species during cellular metabolism.

Figure 2

Role of H₂O₂ controlling the oxidative redox balance of the cell at different concentrations. Hydrogen peroxide (H₂O₂) controls the cell signaling process at physiological concentration called oxidative Eustress. Whereas, at pathological concentration, H₂O₂ causes cell death due to oxidative distress.

Figure 3

Antioxidant actions of various natural defense systems present in our body to scavenge hydrogen peroxide (H₂O₂). (A) Catalase system. (B) Glutathione peroxidase (GPxs) system. (C) Peroxiredoxin system. Abbreviations: (SH), thiol; (-SOH), sulphenic acid; (-S-S-), disulfide bond; NADP+, nicotinamide adenine dinucleotide phosphate; NADPH(+)H+, reduced nicotinamide adenine dinucleotide phosphate; GSH, reduced glutathione; GSSH, oxidized glutathione.

Figure 4

Generation of highly reactive oxidant hydroxyl radicals (HO^•). Free ferrous ion (Fe⁺²) initiates Fenton reaction with hydrogen peroxide (H₂O₂), leading to the generation of a highly reactive hydroxyl free radical (HO^•). Ascorbic acid (AscH) also, take part in recycling the ferric ion (Fe⁺³), through the generation of (Fe⁺²) and ascorbyl radical (AscH^•). Superoxide radical anion (O₂^•−) can also react with (Fe⁺³) in the Haber-Weiss reaction leading to the production of (Fe⁺²)^, which then again starts redox cycling to generate HO^•. The HO^• leads to oxidative stress-induced lipid peroxidation, mitochondrial dysfunction, and an increase in intracellular free-calcium concentration, and finally causing redox imbalance within the cell and ultimately leading to cell death.

Figure 5

Structure of superoxide dismutase1 (SOD1) dimer (pdb code: 1SPD). The catalytic role of copper (Cu) in the dismutation reaction of superoxide radical anion (O₂^•−).

Figure 6

The X-ray crystallographic structure of wild-type SOD1 (wSOD1) (Pdb#2C9V) [211] is shown, modeled in Chimera. Cysteine 111(Cys¹¹¹) highlighted in the yellow act as a “HOT SPOT” for oxidative modification by hydrogen peroxide (H₂O₂) and labeled red in the cartoon. The Zinc (Zn) and Copper (Cu) atoms are shown in cyan and orange, respectively [210].

Figure 7

A hypothetical model implicating how pathologic concentrations of hydrogen peroxide (H₂O₂) prompt the change in conformation and biophysical properties of superoxide dismutase (SOD1) via change in thiol (SH) status of Cys¹¹¹, and thus instigate SOD1 and TAR DNA-binding protein (TDP-43) toxicity in motor neuronal cells leading to a degeneration of motor neurons in amyotrophic lateral sclerosis (ALS). Nascent SOD1 (unfolded, apoSOD1) released from the ribosomes, undergo post-translational modifications (PTMs) to form homodimerized matured SOD1 (folded, holoSOD1) through the addition of Copper (Cu) and Zinc (Zn) on the correct binding sites of apoSOD1 and formation of disulfide bond (-S-S-). Mature SOD1 catalyzes the dismutation of superoxide radical anion (O₂^•−) to H₂O₂. H₂O₂ acts as a double edge sword molecule and in low concentration i.e., the physiological concentration of (1–10nM) acts as a signal molecule to create eustress in the cell. However, at higher concentrations i.e., the pathological concentration of (>100 nM) acts to create an oxidative environment for the cells to create cellular distress. The pathological concentration of H₂O₂ acts as a reactive oxygen species (ROS) and could cause misfolding of SOD1 via two molecular pathways (Path a and path b).