Oxidative stress
Reactive oxygen species, or ROS, are highly reactive free radical molecules derived from oxygen. Reactive nitrogen species also exist but ROS are more important. They are often described as damaging molecules, but that is only part of the story. In healthy cells, ROS are continuously produced at low levels and serve important roles in signaling, helping cells respond to changes in metabolism, stress, and the environment. Free radicals play a critical role in the chemical reactions that underpin biological processes like metabolism. However, when there are too many, and the cell’s natural neutralization processes are not enough, oxidative stress occurs. When ROS levels rise too high, they can damage proteins, lipids, and DNA, disrupting normal cellular function. This imbalance is a common feature across many neurological diseases. ROS and oxidative stress are tightly linked to mitochondrial function, which is why this page builds directly on the mitochondrial section. Across diseases such as Parkinson’s, Alzheimer’s, ALS, Huntington’s, MS, CIPN, stroke, and TBI, oxidative stress is not just a byproduct of damage. It often becomes an amplifier, pushing cells further toward dysfunction and degeneration.
Normal biology
ROS are generated as a natural byproduct of cellular metabolism, especially during oxidative phosphorylation in mitochondria. As electrons move through the electron transport chain, a small fraction can prematurely react with oxygen to form superoxide, one of the primary ROS species. Importantly, ROS are not inherently harmful. At controlled levels, they function as signaling molecules. They can regulate gene expression for pathways involved in cell growth, immune responses, and adaptation to stress. This concept is referred to as redox signaling. Cells maintain tight control over ROS levels using antioxidant systems. These include enzymes such as superoxide dismutase (SOD), catalase, and glutathione peroxidase, as well as small molecules like glutathione. Together, these systems convert reactive molecules into less harmful forms and repair oxidative damage when it occurs. The goal is not to eliminate ROS entirely, but to maintain a balance where ROS can perform signaling functions without causing excessive damage.
Dysfunction
Oxidative stress arises when ROS production exceeds the capacity of antioxidant systems. This can happen through increased ROS generation, impaired antioxidant defenses, or a combination. One common trigger is mitochondrial dysfunction, in which impaired electron transport leads to increased electron leakage and ROS production. Environmental toxins like paraquat, inflammation, and metabolic disturbances can also increase ROS levels. At the same time, antioxidant systems may become overwhelmed or impaired. For example, depletion of glutathione, one of the cell’s most important antioxidants, can leave cells more vulnerable to oxidative damage. Mutations in genes that encode antioxidant enzymes, most notably SOD1, also impair the cell’s ability to respond to oxidative stress. Once oxidative stress begins, it can become self-perpetuating. ROS damage mitochondrial components, which in turn leads to more ROS production. They also damage proteins involved in antioxidant defense, further weakening the cell’s ability to cope. This creates an environment that amplifies cellular stress.
Disease Connections
Oxidative stress is a consistent feature across many neurodegenerative and neurological conditions. In Parkinson’s disease, dopaminergic neurons are especially vulnerable to oxidative stress due to dopamine metabolism and high mitochondrial activity. Elevated ROS contributes to protein aggregation and neuronal death. Paraquat and manganese are two environmental toxins known to both induce ROS and cause Parkinson’s. In ALS oxidative stress contributes to motor neuron injury and disease progression. One of the most common familial causes of ALS is a mutation in the gene SOD1, which encodes an antioxidant enzyme responsible for quenching mitochondrial superoxide. In Huntington’s disease, mutant huntingtin disrupts mitochondrial function which increases oxidative stress, contributing to neuronal vulnerability. In CIPN, chemotherapy-induced mitochondrial damage leads to increased ROS, which contributes directly to axonal degeneration and sensory symptoms. In ischemic stroke and TBI, ROS are generated both during the initial injury and during reperfusion and inflammation, amplifying tissue damage. Across these diseases, ROS serve as a common downstream mediator that links diverse upstream insults to cellular injury.
Molecular Consequences
At the molecular level, ROS can modify a wide range of cellular components. One major target is proteins. ROS can oxidize amino acid side chains, alter protein structure, and impair enzyme function. Given the vast number of functions proteins can play in cells, ROS’s impacts on proteins can have wide ranging effects. Importantly these include impaired metabolism, DNA repair, and redox. ROS also damage lipids, particularly in cellular membranes. Lipid peroxidation can compromise membrane integrity, disrupt ion gradients, and generate secondary reactive molecules that further damage the cell. In a process called ferroptosis, lipid peroxidation can even directly cause cell death. Damage to DNA, including mitochondrial DNA, is another key consequence. This can impair gene expression, create mutations, and further disrupt mitochondrial function, contributing to long-term dysfunction. ROS also interact with metal ions, such as iron, to generate highly reactive species through reactions like the Fenton reaction. This is especially relevant in the context of iron-sulfur cluster biology and mitochondrial metabolism. Importantly, ROS are not purely destructive. They can also modify signaling pathways, activating stress responses such as the unfolded protein response, inflammatory signaling, and adaptive antioxidant pathways. The problem arises when these responses become chronic or dysregulated.
Therapeutic Targeting
Given the central role of oxidative stress, antioxidant therapies have long been explored as treatments for neurodegenerative diseases. However, clinical success has been limited. One reason is that broad antioxidants do not address the underlying sources of ROS. Simply scavenging reactive molecules may not be sufficient if mitochondrial dysfunction or inflammation continues to generate them. More generally, the effects of antioxidant supplementation do not have an established effect on lifespan in people. Another issue is that ROS are also important for normal signaling. Completely suppressing ROS can interfere with physiological processes, which may explain why some antioxidant strategies have failed to show benefit in clinical trials. In fact, an interesting study showed that treating the model system C. elegans with low doses of paraquat to increase their ROS actually improved lifespan (do not try this, paraquat is extremely toxic and you’re not a worm). More recent approaches focus on targeted interventions, such as mitochondria-specific antioxidants, enhancing the cell’s antioxidant systems, or restoring metabolic balance. There is also interest in modulating redox-sensitive signaling pathways rather than eliminating ROS entirely. Overall, the field is shifting away from the idea of ROS as purely harmful and toward a more nuanced view of redox balance and signaling.
Research Directions
Current research is focused on better understanding how ROS function as both signaling molecules and sources of damage. One major direction is identifying thresholds: when does ROS signaling shift from helpful to harmful? This is likely context-dependent and may vary between cell types and disease states. Another area of interest is the interaction between ROS and other pathways, including mitochondrial dysfunction, protein aggregation, inflammation, and autophagy. ROS often sit at the intersection of these processes, acting as both a cause and a consequence of cellular stress. There is also growing interest in spatial and temporal dynamics of ROS, meaning when and where they are produced and scavenged. ROS produced in different parts of the cell can have very different effects, and short bursts of ROS may have different consequences than chronic elevation. Finally, new tools are allowing researchers to measure ROS more precisely in living cells and tissues. This may help resolve longstanding questions about when and where oxidative stress becomes truly pathogenic, and how best to intervene.
Sources
- Sies, H. (1997). Oxidative stress: oxidants and antioxidants.
- Finkel, T., & Holbrook, N. J. (2000). Oxidants, oxidative stress and the biology of ageing.
- Murphy, M. P. (2009). How mitochondria produce reactive oxygen species.
- Schieber, M., & Chandel, N. S. (2014). ROS function in redox signaling and oxidative stress.
- Cobley, J. N., Fiorello, M. L., & Bailey, D. M. (2018). 13 reasons why the brain is susceptible to oxidative stress.
- Bora, S., et al. (2021). Paraquat exposure over generation affects lifespan and reproduction through mitochondrial disruption in C. elegans.