Orphan G protein-coupled receptor 3 as a potential modulator of microglial inflammatory responses: implications for tau-associated neuroinflammation in Alzheimer's disease.

Abstract

INTRODUCTION: Neuroinflammation, driven largely by microglial activation, is a core pathological feature of Alzheimer's disease (AD). Pathological tau species represent one of the key drivers of inflammatory activation in AD; however, the upstream regulators governing microglial inflammatory responses remain incompletely understood. Orphan G protein-coupled receptors (GPCRs) are emerging neuroimmune modulators, yet their specific roles in tau-mediated neuroinflammation are not fully characterized. This study investigated selected orphan GPCRs as modulators of this response. METHODS: Transcriptomic analyses of postmortem AD hippocampal tissue were performed to identify dysregulated orphan GPCRs, followed by functional network analysis. Key findings were validated in a cis P-tau-induced tauopathy mouse model and in cis P-tau-stimulated BV-2 microglial cultures using immunofluorescence, qPCR, Western blotting, and ELISA. Functional relevance was assessed using DNAzyme-mediated Gpr3 knockdown. RESULTS: Transcriptomic profiling identified Gpr3, Gpr6, and Gpr12 as significantly upregulated orphan GPCRs in AD hippocampal tissue, with network analyses implicating them in inflammatory signaling pathways. In the cis P-tau model, hippocampal neuroinflammation was accompanied by microglial activation, increased inflammatory markers, and receptor upregulation. Immunofluorescence revealed a spatial association between increased GPR3 expression and microglial activation. In vitro, cis P-tau stimulation of BV-2 microglia induced Gpr3 expression, and its downregulation attenuated inflammatory outputs at both transcriptional and signaling levels, including reduced NF-κB protein level, modulation of AKT phosphorylation, and decreased TNF-α and IL-6 secretion. CONCLUSION: These findings support a previously underexplored role for GPR3 in modulating microglial inflammatory responses under tau-associated pathological conditions in AD, highlighting its potential relevance as a therapeutic target for maladaptive neuroinflammation.