From Reactive Oxygen and Nitrogen Species to Therapy


Excessive free radicals, including reactive oxygen species (ROS) and nitric oxide (NO), contribute to pathological production of misfolded proteins, synaptic damage and apoptosis. Misfolded protein aggregates occur in several chronic neurodegenerative disorders, including Parkinson and Alzheimer diseases. In rare cases, the cause is a genetic mutation; the majority is sporadic and may be a response to environmental factors that generate free radicals. Overactivity of the N‐methyl‐d‐aspartate (NMDA)‐subtype of glutamate receptor can generate ROS and NO species that mediate protein misfolding, synaptic damage and apoptosis, and thus, neurodegenerative disease. We review current evidence that excessive ROS and NO contribute to protein misfolding by S‐nitrosylation of the E3 ubiquitin ligase parkin and protein‐disulfide isomerase. We also discuss NMDA receptor antagonists memantine and NitroMemantine, drugs that block excessive glutamate excitation, thereby limiting production of ROS and NO, and therapeutic electrophiles, drugs that protect cells from oxidative stress by activating the Nrf2 transcriptional pathway.

Key Concepts:

  • S‐nitrosylation or transfer of the NO group to critical cysteine thiol can regulate protein function.

  • Under pathological circumstances, aberrant S‐nitrosylation reactions can contribute to protein misfolding, neuronal injury and neurodegeneration.

Keywords: reactive oxygen species; S‐nitrosylation; molecular chaperone; ubiquitin‐proteasome system; protein misfolding; electrophiles

Figure 1.

Excess activation of the NMDA receptor (NMDAR) by glutamate (Glu) and glycine (Gly) induces Ca2+ influx and activates excitotoxic pathways. NMDAR hyperactivation triggers (1) generation of NO by neuronal NO synthase (nNOS) and (2) production and release of ROS from mitochondria. Additionally, generation of mitochondrial ROS occurs on environmental toxin stimuli or during the normal ageing process. Excessively produced NO and ROS contribute to protein misfolding, neuronal cell injury and death.

Figure 2.

Chemical pathways for nitrogen and oxygen free radical species. Production of ROS by the mitochondrial respiratory chain or certain enzymatic reactions (e.g. NADPH oxidase) can lead to the formation of superoxide anion (O2•−) or other toxic substances such as hydrogen peroxide (H2O2) and hydroxyl radical (OH). A series of antioxidative enzymes, including superoxide dismutase (SOD), catalase and glutathione peroxidase (GPx), decomposes such toxic oxygen radicals to water. When the rate of free radical production exceeds the antioxidative defence mechanisms, oxidative stress occurs. NOS produces NO from l‐arginine, and NO reacts with sulfhydryl groups to form S‐nitrosothiols (R‐SNO). NO activates soluble guanylate cyclase (sGC) to produce cGMP, and cGMP can activate cGMP‐dependent protein kinase. Peroxynitrite (ONOO), derived from a reaction of NO and superoxide anion, can oxidize vicinal sulfhydryl groups to disulfide bonds and can also nitrate tyrosine residues to form 3‐nitrotyrosine (3‐NT). Accumulation of R‐SNO and 3‐NT contributes to nitrosative stress.

Figure 3.

S‐nitrosylation of neuronal proteins. Physiological levels of NO mediate neuroprotective effects, at least in part, by S‐nitrosylating the NMDAR and caspases, thus inhibiting their activity. In contrast, overproduction of NO can be neurotoxic via S‐nitrosylation of Parkin, PDI, GAPDH, MMP‐2/9, PrxII and COX‐2. S‐nitrosylated parkin and PDI contribute to neuronal cell injury by triggering accumulation of misfolded proteins. S‐nitrosylation of Drp1 causes excessive mitochondrial fragmentation in neurodegenerative conditions.

Figure 4.

Possible mechanism whereby S‐nitrosylated species contribute to the accumulation of aberrant proteins and neuronal damage. S‐nitrosylation of parkin (forming SNO‐PARK) and PDI (forming SNO‐PDI) can contribute to neuronal cell injury in part by triggering accumulation of misfolded proteins. Specifically, S‐nitrosylation of parkin disturbs its E3 ligase activity, thus inducing dysfunction in the ubiquitin proteasome system (UPS) (left). S‐nitrosylated PDI decreases its molecular chaperone and disulfide isomerase activity (right).

Figure 5.

S‐nitrosylation and Memantine/NitroMemantine regulate NMDA receptor activity. (a) NO‐induced inhibition of NMDA receptor. Relatively hypoxic conditions (brain oxygen levels are normally low under physiological conditions) render the NMDA receptor exquisitely sensitive to S‐nitrosylation of Cys744 and Cys798 on the NR1 subunit. NO modification of these two thiols in NR1 further enhances S‐nitrosylation of the Cys399 site on the NR2A subunit, resulting in the inhibition of receptor activity by very low levels of NO. Under certain physiological conditions, NR2A can also be nitrosylated at Cys87 and Cys320 (not shown for clarity). (b) Memantine and NitroMemantine preferentially block excessive extrasynaptic NMDA receptor activity. (Left) Normal (physiological/synaptic) activity of the NMDAR is required for synaptic function and neuronal survival. (Middle) Excessive activation of the NMDAR, predominantly at extrasynaptic sites, is thought to induce neuronal cell injury and death, and is associated with the accumulation of misfolded proteins. (Right) Memantine (Mem) and the newer NitroMemantine drugs (NitroMem) preferentially block excessive (pathological) extrasynaptic NMDA receptor activity, whereas relatively sparing normal (physiological) synaptic activity.



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McKercher, Scott R, Nakamura, Tomohiro, and Lipton, Stuart A(Dec 2009) From Reactive Oxygen and Nitrogen Species to Therapy. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0021989]