Hydrogen sulfide (H₂S) is a gaseous signaling molecule that serves as an important neuromodulator in the brain. Alongside nitric oxide (NO) and carbon monoxide (CO), H₂S belongs to the family of gasotransmitters that regulate neuronal function, synaptic plasticity, and cellular stress responses. In the brain, H₂S is produced endogenously by three main enzymes—cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3-MST)—and plays crucial roles in neuroprotection, anti-oxidation, anti-inflammation, and mitochondrial function 1. [1]
The biological significance of H₂S in the nervous system has become increasingly apparent over the past two decades. Unlike its reputation as a toxic gas, H₂S at physiological concentrations (nanomolar to low micromolar) serves as a critical signaling molecule with diverse effects on neuronal survival, synaptic transmission, and cellular homeostasis. The dysregulation of H₂S signaling has been implicated in the pathogenesis of multiple neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD) 1.
The brain produces H₂S through three distinct enzymatic pathways, each with unique cellular distribution and physiological functions:
| Enzyme | Gene | Cellular Location | Substrate | Primary Role | [2]
|--------|------|-------------------|-----------|--------------| [3]
| CBS | CBS | Neurons, astrocytes | L-cysteine | Major CNS H₂S production | [4]
| CSE | CTH | Endothelial cells, peripheral tissues | L-cysteine | Peripheral H₂S production, vascular signaling | [5]
| 3-MST | MPST | Brain mitochondria, neurons | 3-mercaptopyruvate | Mitochondrial H₂S, redox regulation | [2:1]
The three enzymes exhibit distinct subcellular localization: CBS is primarily cytosolic, CSE is partly membrane-associated, and 3-MST localizes to mitochondria where it participates in cellular energy metabolism and redox homeostasis 2. This differential distribution allows for spatially regulated H₂S production in different cellular compartments.
CBS Regulation:
CSE Regulation:
3-MST Regulation:
H₂S exerts its biological effects through multiple signaling pathways:
Ion Channel Modulation
Receptor Interaction
Kinase Pathways
Transcription Factor Activation
Alzheimer's disease presents a complex relationship with H₂S signaling, characterized by compensatory changes in H₂S production machinery and altered H₂S homeostasis.
CBS Dysregulation in AD:
CBS activity is significantly elevated in AD brain tissue, particularly in the hippocampus and frontal cortex, representing a compensatory neuroprotective response to accumulating oxidative stress and beta-amyloid pathology 4. This upregulation appears to be driven by increased oxidative stress and inflammatory cytokines, which stimulate CBS expression as a endogenous protective mechanism 5. Paradoxically, while CBS activity increases, overall H₂S levels are decreased in AD patients, suggesting either increased consumption or impaired release of H₂S at synaptic terminals.
Amyloid Pathology:
H₂S reduces amyloid-beta production through multiple mechanisms:
Tau Pathology:
H₂S inhibits tau phosphorylation and aggregation through:
Synaptic Protection:
H₂S preserves synaptic plasticity and memory formation through multiple mechanisms 7:
Oxidative Stress:
H₂S upregulates the Nrf2 pathway and antioxidant enzymes including:
In Parkinson's disease, H₂S signaling exhibits distinct patterns of dysregulation compared to AD, with particular vulnerability in dopaminergic neurons of the substantia nigra.
CBS Deficiency:
CBS expression is significantly decreased in the substantia nigra of PD patients, contributing to dopaminergic neuron vulnerability 8. This deficiency results in:
Mitochondrial Protection:
H₂S preserves mitochondrial function through:
α-Synuclein Pathology:
H₂S modulates α-synuclein aggregation and clearance:
Neuroinflammation:
H₂S inhibits microglial activation and neuroinflammatory responses through:
Dopaminergic Protection:
H₂S protects against MPTP and 6-OHDA toxicity through multiple mechanisms 11:
CSE-derived H₂S provides specific protection against 6-OHDA toxicity, demonstrating the importance of the CSE pathway in dopaminergic neuron survival 12.
ALS shows distinctive patterns of H₂S dysregulation, particularly in motor neurons.
CBS Dysregulation:
CBS expression is dysregulated in motor neurons of ALS patients, with both increased and decreased expression reported depending on disease stage and cellular context 13. This dysregulation contributes to:
H₂S Donors in ALS:
H₂S donors protect motor neurons from:
Mitochondrial Dysfunction:
H₂S improves mitochondrial bioenergetics through:
Excitotoxicity:
H₂S modulates NMDA receptor activity, providing protection against glutamate-induced excitotoxicity.
H₂S homeostasis is disrupted in HD models, with implications for disease progression.
Neuroprotective Effects:
H₂S reduces mutant huntingtin aggregation through:
Transcriptional Regulation:
H₂S modulates BDNF expression, promoting neuronal survival in HD models 15.
Metabolic Dysfunction:
H₂S improves energy metabolism in HD through:
H₂S exerts potent antioxidant effects through multiple mechanisms 3:
H₂S promotes neuronal survival through kinase-mediated anti-apoptotic pathways:
H₂S exerts anti-inflammatory effects through multiple mechanisms 10:
H₂S is particularly important for mitochondrial health 9:
H₂S plays critical roles in synaptic function 16:
Multiple H₂S-releasing compounds have been developed for therapeutic applications:
| Compound | Mechanism | Specificity | Evidence | Status |
|---|---|---|---|---|
| NaHS | Fast H₂S release | Non-selective | Preclinical | Research |
| GYY4137 | Slow H₂S release | Non-selective | Preclinical | Investigational |
| AP39 | Mitochondria-targeted | Complex IV | Preclinical | Research |
| JK-1 | Caged H₂S donor | Controlled release | Preclinical | Research |
| A-419259 | CBS activator | CBS-specific | Preclinical | Research |
| S-propargyl-cysteine (SPC) | H₂S donor | S-sulfhydration | Preclinical | Research |
| Danioquinone-CBD | H₂S donor | Mitochondria | Preclinical | Research |
AP39 has shown particular promise in PD models, demonstrating neuroprotection through mitochondria-targeted H₂S delivery 17.
H₂S-Releasing NSAIDs:
Novel H₂S-releasing non-steroidal anti-inflammatory drugs combine anti-inflammatory and neuroprotective properties 18:
While no large-scale clinical trials of H₂S therapy in neurodegenerative diseases have completed as of 2024, several early-phase studies are underway:
| Marker | Tissue | Change in Neurodegeneration | Disease Specificity |
|---|---|---|---|
| CBS expression | Brain | ↑ in AD, ↓ in PD | Disease-specific |
| CSE expression | Brain | ↓ in ALS | ALS-specific |
| H₂S levels | Brain, CSF | ↓ in AD, PD | Non-specific |
| 3-MST activity | Brain | ↓ in aging | Age-related |
| Cystathionine | Plasma | ↑ in AD | AD marker |
Hydrogen sulfide is a versatile gasotransmitter with significant neuroprotective potential in neurodegenerative diseases. Through its antioxidant, anti-apoptotic, anti-inflammatory, and mitochondrial-protective effects, H₂S addresses multiple pathological mechanisms common to AD, PD, ALS, and HD. While preclinical evidence is compelling, successful translation to clinical practice requires advances in drug delivery, dosing strategies, and patient selection. The ongoing development of mitochondria-targeted H₂S donors represents a particularly promising approach for future therapeutic intervention.
The reciprocal relationship between H₂S signaling and neurodegeneration suggests that restoring H₂S homeostasis may provide broad neuroprotective benefits. CBS upregulation in AD and deficiency in PD demonstrate disease-specific patterns that could inform personalized therapeutic approaches. The convergence of H₂S effects on multiple pathological processes—including oxidative stress, neuroinflammation, mitochondrial dysfunction, and protein aggregation—makes it an attractive target for neurodegenerative disease modification.
Recent advances in hydrogen sulfide signaling research have revealed additional neuroprotective mechanisms:
Mitochondrial Function: H₂S donors preserve mitochondrial bioenergetics in neuronal cells through sulfhydration of complex IV subunits 9
Neuroinflammation: Studies demonstrate H₂S suppresses NLRP3 inflammasome activation in microglia, reducing neuroinflammatory responses 10
AD Therapeutic Potential: Novel H₂S-releasing NSAIDs show dual anti-amyloid and anti-inflammatory effects in AD models 18
PD Protection: H₂S donors protect dopaminergic neurons from oxidative stress and mitochondrial dysfunction in PD models 11
Synaptic Function: Research reveals H₂S modulates synaptic plasticity through presynaptic calcium channel regulation 16
Clinical Translation: First-in-human safety data supports advancement of H₂S donors to neurological disease trials 20
'Hydrogen sulfide in neurodegeneration: Biological functions and therapeutic potential (2021)'. 2021. ↩︎
3-Mercaptopyruvate sulfurtransferase in neuronal stress response (2022). 2022. ↩︎ ↩︎
H2S sulfhydrates mitochondrial complex IV to preserve bioenergetics (2023). 2023. ↩︎
CBS deficiency in Parkinson's disease substantia nigra (2019). 2019. ↩︎
CSE-derived H2S protects against 6-OHDA toxicity (2021). 2021. ↩︎