GPCRomics of Homeostatic and Disease-Associated Human Microglia

Microglia are critical immune cells of the central nervous system that maintain homeostasis and respond to pathology. A significant portion of their functional regulation is governed by G-protein-coupled receptors (GPCRs), the largest family of membrane sensors in eukaryotic cells. Despite their importance, a systematic understanding of the microglial GPCR landscape, or "GPCRome," has been limited.

This study utilized publicly available bulk and single-cell RNA sequencing (RNAseq) data from human and mouse brain tissue to map the GPCRs present in microglia. The authors identified a set of 17 core GPCR genes reliably expressed across homeostatic microglia. To test for functional significance, the team performed flow cytometry to quantify protein expression and transwell migration assays to evaluate how human microglia respond to ligands like SDF-1.

The study identified core microglial signature genes including CX3CR1, GPR34, P2RY12, and ADGRG1. A critical discovery was that the expression of these homeostatic markers is rapidly lost when cells are cultured ex vivo, highlighting the difficulty of studying these cells in standard laboratory conditions. However, the study showed that induced pluripotent stem cell (iPSC)-derived microglial precursors, when transplanted into mouse brains, recover this gene expression profile.

The researchers also observed significant regional and disease-related differences. For instance, CXCR4 and PTGER4 expression is higher in subcortical white matter microglia than in grey matter microglia. In the context of multiple sclerosis (MS), active lesions contain a subset of microglia that downregulate homeostatic markers while upregulating CXCR4. This CXCR4 expression is functionally active, as evidenced by microglial migration toward the ligand SDF-1, suggesting a potential pathway for therapeutic intervention in neuroinflammatory diseases.