Purinergic signaling refers to the signaling pathways mediated by purine nucleotides and nucleosides (ATP, ADP, adenosine) through activation of purinergic receptors (P1, P2X, P2Y). This ancient signaling system plays crucial roles in neural development, synaptic transmission, glial function, and neuroinflammation. Dysregulation of purinergic signaling contributes to neurodegenerative processes through multiple mechanisms [1].
Key aspects include:
P2X receptors are ligand-gated ion channels activated by extracellular ATP. Seven subtypes (P2X1-P2X7) form trimeric assemblies that permit cation influx (Na+, Ca2+, K+). In the CNS, the most relevant are [2]:
| Receptor | Primary CNS Expression | Key Functions |
|---|---|---|
| P2X4 | Microglia, neurons | Microglial activation, BDNF release, neuropathic pain |
| P2X7 | Microglia, neurons | Nociception, neurotransmission, inflammasome activation |
| P2X1 | Smooth muscle, platelets | Vasoconstriction, platelet aggregation |
P2Y receptors are G-protein-coupled receptors activated by ATP, ADP, UTP, or UDP. Eight subtypes signal through Gq/11, Gi/o, or Gs pathways [3]:
| Receptor | Primary CNS Expression | Key Functions |
|---|---|---|
| P2Y1 | Astrocytes, neurons | Calcium waves, glutamate release |
| P2Y6 | Microglia | UDP-activated phagocytosis |
| P2Y12 | Homeostatic microglia | Chemotaxis, surveillance, synaptic monitoring |
| P2Y13 | Microglia, oligodendrocytes | Process extension, ADP sensing |
Adenosine receptors are G-protein-coupled receptors activated by adenosine, the breakdown product of ATP via ectonucleotidases (CD39 and CD73):
| Receptor | Coupling | Key Functions |
|---|---|---|
| A1 | Gi/o (inhibitory) | Neuroprotection, presynaptic inhibition, anti-inflammatory |
| A2A | Gs (excitatory) | Pro-inflammatory microglial activation, synaptic modulation |
| A2B | Gs | Astrocyte activation, vascular regulation |
| A3 | Gi/o | Microglial process motility, mixed effects |
In the healthy brain, ATP is released in a controlled manner from synaptic vesicles and through pannexin channels during normal neurotransmission. In neurodegenerative diseases, pathological ATP release occurs through multiple mechanisms [4]:
Extracellular ATP is rapidly metabolized by ectonucleotidases:
ATP - (CD39/NTPDase1) -> ADP - (CD39) -> AMP - (CD73/ecto-5'-nucleotidase) -> Adenosine - (adenosine deaminase) -> Inosine
This cascade determines the balance between ATP-driven inflammation (via P2 receptors) and adenosine-mediated neuroprotection (via A1 receptors). In neurodegeneration, changes in ectonucleotidase expression alter this balance, often favoring sustained ATP signaling.
The P2X7 receptor (P2X7R) has several properties that distinguish it from other P2X family members and make it particularly important in neurodegeneration [5]:
The P2X7R-NLRP3 inflammasome axis represents a critical link between purinergic signaling and neuroinflammation [6]:
In AD, amyloid-beta aggregates activate P2X7R on perivascular microglia, driving chronic IL-1beta release and creating a self-amplifying neuroinflammatory cycle [4:1]. Key findings:
The adenosine A1 receptor provides constitutive neuroprotection through:
A2A receptors have a complex role in neurodegeneration:
Astrocytes are major participants in purinergic signaling:
Multiple P2X7R antagonists are being developed for neurological indications [2:1]:
| Compound | Company | Development Status | Notes |
|---|---|---|---|
| JNJ-54175446 | Janssen | Phase 1 completed | Brain-penetrant, showed target engagement in hippocampus |
| CE-224,535 | Pfizer | Phase 2 RA (ceased) | Anti-inflammatory |
| AZD9056 | AstraZeneca | Phase 2 (ceased) | Inflammatory conditions |
| GSK1482160 | GSK | Preclinical | PET ligand and therapeutic candidate |
| Compound | Status | Indication |
|---|---|---|
| Istradefylline (Nourianz) | FDA-approved 2020 | PD OFF episodes |
| Preladenant | Phase 3 (ceased) | PD |
| Tozadenant | Phase 3 (ceased) | PD |
| Caffeine | Epidemiologic | Neuroprotective (epidemiological data) |
Targeting ectonucleotidases to shift the ATP/adenosine balance represents a newer therapeutic approach. Enhancing CD39 activity could reduce pro-inflammatory ATP levels while increasing neuroprotective adenosine.
Strategies to maintain or restore microglial P2Y12 expression in disease states could help preserve homeostatic surveillance and neuroprotection.
Several purinergic components are being investigated as biomarkers:
| Dimension | Score |
|---|---|
| Supporting Studies | 10 primary references |
| Replication | Multiple studies replicated across AD, PD, ALS |
| Effect Sizes | Moderate |
| Contradicting Evidence | Some conflicting data on A2A antagonism timing |
| Mechanistic Completeness | 50% |
Overall Confidence: 45%
Calovi S, Mut-Arbona P, Sperlágh B. "Glial Purinergic Signaling in Neurodegeneration". Front Neurol. 2021. ↩︎
Recourt K, et al. "Characterization of the central nervous system penetration of the P2X7 antagonist JNJ-54175446 in healthy volunteers". J Psychopharmacol. 2020. ↩︎ ↩︎
Ribeiro DE, et al. "Mechanisms of astrocytic and microglial purinergic signaling in homeostatic regulation and implications for neurological disease". Neuroscience. 2021. ↩︎
Illes P, et al. "P2X7 receptor and purinergic signaling in neurodegenerative diseases". Front Pharmacol. 2024. ↩︎ ↩︎
Deussing JM, Bhatt DP. "P2X7 receptor: an emerging target in central nervous system diseases". Trends Pharmacol Sci. 2022. ↩︎
Bhatt DP, et al. "Targeting neuroinflammation with brain penetrant P2X7 antagonists as novel therapeutics for neuropsychiatric disorders". Neuropsychopharmacology. 2019. ↩︎