ADGRB1 contributes to astrocyte-mediated phagocytosis of excitatory synapses

The phagocytic activity of astrocytes plays an important role in synapse refinement through the elimination of excess synapses during brain development. The adhesion G protein-coupled receptor BAI1/ADGRB1 contributes to phagocytosis in various tissues, including the clearance of apoptotic myoblasts in skeletal muscle and epithelial cells in the intestine. However, the phagocytic function of ADGRB1 in the brain has not been thoroughly investigated. Given that Adgrb1 is highly expressed in astrocytes but not in microglia, we examined changes in astrocyte gene expression resulting from the loss of ADGRB1. RNA-seq analysis revealed that astrocytes lacking ADGRB1 exhibit altered expression of genes associated with cytoskeleton organization and chemotaxis, processes that are required for phagocytosis. Using cultured astrocytes from mice lacking full-length ADGRB1 (Adgrb1exon2-/-) and wildtype (WT) littermates, we found that Adgrb1exon2-/- astrocytes exhibit significantly reduced phagocytic capacity when compared to similarly prepared astrocytes from WT littermates. Immunostaining of astrocytes and pre-synaptic markers showed less engulfed pre-synaptic elements in astrocytes from Adgrb1exon2-/- mutants. Finally, immunostaining of pre-synaptic and post synaptic markers revealed a significantly higher density of excitatory synapses in Adgrb1exon2-/- mutants. These findings highlight the importance of ADGRB1 in synaptic remodeling and raise the possibility that reduced astrocyte-mediated phagocytosis might underlie the behavioral alterations previously observed in Adgrb1exon2-/- mutants, including increased seizure susceptibility and deficits in social memory.