S-nitrosylation is a reversible post-translational modification (PTM) in which a nitric oxide (NO) group is covalently attached to the thiol side chain of cysteine residues, forming an S-nitrosothiol (SNO). This modification serves as a critical signaling mechanism in the nervous system, regulating protein function, neuronal communication, and cellular homeostasis. However, dysregulated S-nitrosylation has emerged as a key pathological mechanism in neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD) [1]. [@nitric]
S-nitrosylation is a reversible post-translational modification (PTM) in which a nitric oxide (NO) group is covalently attached to the thiol side chain of cysteine residues, forming an S-nitrosothiol (SNO). This modification serves as a critical signaling mechanism in the nervous system, regulating protein function, neuronal communication, and cellular homeostasis. However, dysregulated S-nitrosylation has emerged as a key pathological mechanism in neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD) [1]. [@nitric]
Unlike chronic inflammation and oxidative stress, which represent broad cellular stress responses, S-nitrosylation represents a precise molecular pathway through which nitric oxide orchestrates both physiological signaling and pathological cascades. The modification is mediated by nitric oxide synthases (NOS) — neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS) — and is reversed by denitrosylases including thioredoxin (Trx) and S-nitrosoglutathione reductase (GSNOR). [@snitrosylation]
S-nitrosylation occurs through multiple biochemical pathways: [@snitrosylationa]
Enzymatic S-nitrosylation: The three NOS isoforms catalyze the production of NO, which can directly react with cysteine thiols to form S-nitrosocysteine: [@snitrosylated]
Protein-SNO + Cysteine → Protein + Protein-Cys-SNO
This reaction is catalyzed by proteins including thioredoxin and protein disulfide isomerase (PDI). [@snitrosylationc]
Denitrosylation: The reverse reaction is equally important physiologically: [@role]
Not all cysteines are equally susceptible to S-nitrosylation. Susceptibility depends on: [@thioredoxin]
S-nitrosylation of Parkin represents one of the best-characterized pathological S-nitrosylation events. Parkin is an E3 ubiquitin ligase critical for mitophagy — the selective [autophagy](/entities/autophagy) of damaged mitochondria [2].
Normal function: Parkin, activated by PINK1 phosphorylation, ubiquitinates mitochondrial outer membrane proteins, targeting damaged mitochondria for degradation.
S-nitrosylation pathology: In PD and related disorders, excessive nNOS-derived NO S-nitrosylates Parkin at Cys³⁶⁵, inhibiting its E3 ligase activity [2]. This impairs mitophagy, leading to accumulation of dysfunctional mitochondria, increased oxidative stress, and dopaminergic neuron death.
The S-nitrosylated Parkin is functionally inactive, unable to ubiquitinate mitochondrial proteins, creating a vicious cycle where impaired mitophagy leads to further oxidative/nitrosative stress.
Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key glycolytic enzyme that is also subject to pathological S-nitrosylation.
S-nitrosylation at Cys¹⁵⁰: In [neurons](/entities/neurons) exposed to NO donors or under oxidative stress, GAPDH is S-nitrosylated at Cys¹⁵⁰ [3]. This modification:
PDI is an ER chaperone that catalyzes disulfide bond formation and reduction. S-nitrosylation of PDI in neurodegenerative diseases impairs its protective function [4].
ER stress amplification: When PDI is S-nitrosylated, it loses its chaperone and isomerase activity, leading to accumulation of misfolded proteins in the ER lumen [4]. This triggers the [unfolded protein response](/entities/unfolded-protein-response) (UPR), and when chronic, leads to ER stress-induced [apoptosis](/entities/apoptosis).
In ALS, PDI S-nitrosylation contributes to the accumulation of misfolded SOD1 and [TDP-43](/mechanisms/tdp-43-proteinopathy) aggregates [4].
Caspase-3, the executioner caspase of apoptosis, can be regulated by S-nitrosylation.
Protection against apoptosis: At low NO concentrations, S-nitrosylation of caspase-3 at Cys¹⁷⁸ protects the enzyme from activation, providing a cytoprotective effect.
Pathological implications: However, in chronic neurodegenerative conditions, the balance shifts — prolonged nitrosative stress can ultimately lead to caspase activation and apoptosis through mechanisms including mitochondrial permeabilization and cytochrome c release.
In ALS and frontotemporal dementia (FTD), TDP-43 forms characteristic cytoplasmic inclusions. S-nitrosylation may influence TDP-43 aggregation propensity [5].
Modulation of aggregation: S-nitrosylation of specific cysteine residues in TDP-43 may alter its conformational stability and aggregation kinetics [5]. The relationship is complex, with both protective and pathological effects reported depending on the specific cysteine and cellular context.
In AD, multiple pathways converge to produce excessive nitrosative stress:
Amyloid-β interaction: [Aβ](/proteins/amyloid-beta) peptides stimulate nNOS and iNOS activity, increasing NO production in the vicinity of synapses [1]. This leads to:
PD is particularly associated with S-nitrosylation due to the vulnerability of dopaminergic neurons to nitrosative stress [6]:
Dopamine metabolism: Dopamine oxidation produces [reactive oxygen species](/entities/reactive-oxygen-species) and can stimulate NOS activity. Combined with mitochondrial complex I deficiency, this creates a permissive environment for S-nitrosylation [6].
Parkin S-nitrosylation: As described above, this represents a critical pathogenic mechanism, impairing mitophagy [2].
[α-Synuclein](/proteins/alpha-synuclein): S-nitrosylation of α-synuclein at Cys⁵⁰ may influence its aggregation propensity, though this remains an area of active investigation [5].
DJ-1: The Parkin co-substrate DJ-1 (PARK7), which loses its protective function in early-onset PD, can be S-nitrosylated. This modification may regulate its oxidative stress-sensing function.
ALS presents a particularly compelling case for S-nitrosylation involvement [7]:
SOD1 mutations: While most ALS-causing SOD1 mutations are not directly related to nitrosylation, the presence of mutant SOD1 leads to increased oxidative and nitrosative stress [7].
TDP-43 pathology: S-nitrosylation may influence TDP-43 mislocalization and aggregation [5].
Motor neuron vulnerability: Motor neurons appear particularly susceptible to nitrosative stress, possibly due to their high metabolic demands and excitability [7].
iNOS upregulation: In ALS spinal cord, iNOS is upregulated in activated [microglia](/cell-types/microglia-neuroinflammation) and [astrocytes](/entities/astrocytes), producing high local NO concentrations [7].
In HD, mutant [huntingtin](/proteins/huntingtin) (mHtt) protein promotes nitrosative stress through multiple mechanisms:
nNOS activation: mHtt can bind to and activate nNOS, increasing NO production.
Mitochondrial dysfunction: S-nitrosylation of mitochondrial proteins contributes to the bioenergetic deficit characteristic of HD.
Transcription regulation: S-nitrosylation of transcription factors including [NF-κB](/entities/nf-kb) and HIF-1α alters gene expression patterns relevant to neuronal survival.
Non-selective NOS inhibitors have been tested in neurodegenerative models but face challenges:
Enhancing endogenous denitrosylation pathways represents a promising strategy:
Thioredoxin (Trx) system: Overexpression of Trx or Trx reductase protects against nitrosative stress in models [8].
GSNOR inhibition: Paradoxically, GSNOR inhibition may increase S-nitrosylation in the short term but can trigger adaptive antioxidant responses.
S-nitrosoglutathione (GSNO) and related compounds can:
Given the close relationship between oxidative and nitrosative stress:
S-nitrosylation does not occur in isolation — it interacts with other PTMs:
Detecting S-nitrosylated proteins in tissue and biological samples remains methodologically challenging:
Key questions remain: