C3aR expression in microglial cells after brain injury in mice

Complement 3 alpha receptor (C3aR) is expressed in microglia during embryonic development with a role in pruning and debris clearance. We observed increased complement 3 (C3) immunoreactivity in mouse injured brain after cold injury, likely from blood origin. Recent findings suggest a role of C3-C3aR in brain damage progression, making it a promising target. We aimed to study C3aR expression using qPCR and fluorescence microscopy (epifluorescence and confocal) in a cold injury model in young adult female and male mice (C57/4-5 months). We observed increased C3aR mRNA expression in the injured hemisphere in both males and females. Co-labeling of C3aR with astrocyte marker GFAP (glial fibrillary acidic protein) or microglia marker IBA1 (Ionized calcium-binding adaptor molecule 1) strongly suggests microglial (but not astroglial) receptor expression at 1, 3 and 7 days post lesion. In vitro, using primary cultures of astrocytes and microglia, we observed C3aR expression in microglial cells but not astrocytes under control conditions. After LPS (Lipopolysaccharide) stimulation, microglia showed increased C3aR immunoreactivity, while astrocytes remained C3aR-negative, suggesting strong repression in these cells. Co-treatment with histone deacetylase inhibitor, trichostatin A, did not promote C3aR expression in astrocytes. Given that C3 is known to modulates gliosis, our results position microglia as a first responder and a potential target for C3aR pharmacological inhibition.