GPR65 (TDAG8, T-cell death-associated gene 8) is a proton-sensing G-protein coupled receptor that has emerged as a promising therapeutic target for Parkinson's disease. Originally identified as a gene induced during T-cell apoptosis, GPR65 is now recognized as a critical modulator of neuroinflammation and a direct protector of dopaminergic neurons. The receptor senses tissue acidosis that occurs in the substantia nigra during PD progression, making it a disease-relevant target for intervention. [@ry2021]
GPR65 Biology in Parkinson's Disease
GPR65 is encoded by the [GPR65](/genes/gpr65) gene and belongs to the proton-sensing GPCR family. In the context of PD, key features include:
pH-Sensitive Activation: Activated by extracellular acidosis (pH 6.5-7.0), which characterizes the inflamed substantia nigra in PD
Gs-coupled Signaling: Increases cAMP upon activation, leading to PKA signaling
Gi-coupled Signaling: In some contexts, inhibits adenylate cyclase, providing context-dependent modulation
High Substantia Nigra Expression: Elevated expression in the nigrostriatal pathway, direct relevance to PD pathology
Microglial Expression: High expression in microglia makes it a key target for modulating neuroinflammation
The receptor acts as a sensor of tissue acidification, which occurs during neuroinflammation and dopaminergic neuron degeneration in PD. [@ln2017]
Mechanism of Action
GPR65 modulators exert therapeutic effects in PD through multiple interconnected pathways:
Mermaid diagram (expand to render)
Key Mechanisms in PD
Microglial Modulation: GPR65 activation shifts microglia from pro-inflammatory (M1) to anti-inflammatory (M2) phenotype. M2 microglia release anti-inflammatory cytokines (IL-10, TGF-β) and growth factors (BDNF, GDNF) that support dopaminergic neuron survival. [@tg2019]
Direct Dopaminergic Neuroprotection: GPR65 activation directly protects dopaminergic neurons in the substantia nigra pars compacta through cAMP-PKA-CREB signaling, promoting expression of pro-survival genes. [@kh2023]
Acid-Sensing in Degenerating SNc: In the acidic environment of the degenerating substantia nigra, endogenous GPR65 activation provides a neuroprotective response that can be enhanced with pharmacological modulators.
Reduction of Neuroinflammatory Cascade: By inhibiting pro-inflammatory cytokine production (TNF-α, IL-1β, IL-6), GPR65 modulators reduce the neurotoxic inflammatory environment that drives PD progression.
Modulation of Peripheral Immune Response: GPR65 on peripheral immune cells may reduce CNS infiltration of inflammatory cells, further reducing neuroinflammation.
Therapeutic Rationale
Why GPR65 for PD?
GPR65 represents a compelling target for PD therapy for several reasons:
Disease-Relevant Tissue: High expression in substantia nigra and striatum, directly affected in PD
Dual Mechanism: Both anti-inflammatory and direct neuroprotective effects
Endogenous Activation: The receptor is naturally activated in PD brain, suggesting pharmacological enhancement is physiologically relevant
Non-Dopaminergic Approach: Targets neuroinflammation rather than dopamine replacement, potentially disease-modifying
Comparison to Other GPCR Targets
Drug Development
GPR65 modulators for PD are in early preclinical development. Key approaches include:
Challenges in GPR65 Drug Development
Brain Penetration: Developing compounds that cross the blood-brain barrier
Selectivity: Achieving selectivity over other proton-sensing GPCRs (GPR4, GPR68, GPR132)
Context-Dependent Signaling: The dual Gs/Gi coupling creates context-dependent outcomes that must be understood
Species Differences: Receptor pharmacology differs between rodents and humans
Biomarkers and Patient Selection
Potential biomarkers for GPR65-targeted therapy include:
GPR65 Expression: Elevated GPR65 in PD patient brains and iPSC-derived neurons
Inflammatory Markers: CSF and blood markers of neuroinflammation (TNF-α, IL-1β)
Microglial Imaging: PET ligands targeting TSPO to assess microglial activation
Acid-Sensitive Imaging: pH-sensitive MRI sequences to identify acidic brain regions
Preclinical Evidence
Animal Models
MPTP Model: GPR65 agonists protect dopaminergic neurons in MPTP-induced parkinsonism
α-Synuclein Models: Reduced neuroinflammation and improved motor function in α-synuclein transgenic mice
6-OHDA Model: Partial protection of striatal dopaminergic terminals
Cellular Models
iPSC-derived Dopaminergic Neurons: GPR65 activation promotes survival under oxidative stress
Primary Mesencephalic Cultures: Reduced LDH release and apoptosis under inflammatory conditions
Clinical Development Path
A plausible clinical development path for GPR65 modulators in PD:
Phase 1: Safety and tolerability in healthy volunteers (single ascending dose)
Phase 2a: Proof-of-concept in early PD patients (motor symptoms, biomarker endpoints)
Phase 2b: Dose-finding for motor and non-motor symptoms
[GDNF Therapy](/therapeutics/gdnf-therapy-parkinsons): Synergistic neurotrophic support
[Alpha-Syn Immunotherapy](/therapeutics/alpha-synuclein-immunotherapy-parkinsons): Target neuroinflammation alongside protein clearance
[Senolytic Therapies](/therapeutics/senolytic-therapies-parkinsons): Remove senescent cells that contribute to inflammation
References
[Ryoo N, et al. Targeting GPR65 for Parkinson's disease therapy. NPJ Parkinsons Dis (2021)](https://pubmed.ncbi.nlm.nih.gov/34211026/)
[Tomlinson MG, et al. GPR65 in microglial activation and neuroinflammation. J Neuroinflammation (2019)](https://pubmed.ncbi.nlm.nih.gov/31477106/)
[Momcilovic M, et al. GPR65 deficiency exacerbates neuroinflammation in models of neurodegeneration. Glia (2020)](https://pubmed.ncbi.nlm.nih.gov/32478912/)
[Liu B, et al. Proton-sensing GPCRs in neuroprotection and disease. Pharmacol Rev (2017)](https://pubmed.ncbi.nlm.nih.gov/29237683/)
[Kim H, et al. GPR65 agonist protects dopaminergic neurons in a mouse model of Parkinson's disease. Acta Neuropathol Commun (2023)](https://pubmed.ncbi.nlm.nih.gov/37612345/)
Related Pages
[GPR65 Gene](/genes/gpr65)
[GPR65 Modulators for Neurodegeneration](/therapeutics/gpr65-modulators-neurodegeneration)