Impaired removal of dying brain cells by microglia in GPR34 deficient mice

GPR34, a G protein coupled receptor present selectively in microglia and other myeloid cells, is highly expressed in homeostatic microglia but is downregulated in disease associated microglia such as are found in Alzheimer’s disease brain. However, little is known about GPR34’s role in microglia function or brain development. Here, we studied Gpr34 knockout (KO) mice at postnatal 18-day (18 d) and 3 months (3 mo) age. In the brains of 18 d GPR34 KO mice, there were elevated numbers of neurons, oligodendrocytes and microglia, many of which were IHC-positive for cell death markers cleaved-caspase 3, phospho-RIP3, or annexin V. There was no increase in cell death markers or in steady state numbers of neurons, oligos and microglia at 3 months, indicating that GPR34-independent mechanisms are able to compensate during brain maturation. Based on RNA sequencing analysis and phagocytosis functional assays, as well as computational modeling, we provide evidence that Gpr34 KO microglia have deficiencies in chemotaxis, but not phagocytic efficiency, which leads to a slower clearance of dead cells from the developing brain. Collectively, these results indicate that Gpr34 is required for microglia to clear dead or dying cells.