ADGRB1 contributes to astrocyte-mediated phagocytosis of excitatory synapses

Synapse refinement through the elimination of excess synapses is crucial for proper neuronal circuitry during development and adulthood, and the phagocytic activity of astrocytes plays an important role in this process. Failure to remove excess synapses can lead to neurological and neurodevelopmental disorders like epilepsy and autism spectrum disorder (ASD). 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. Our 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 (Adgrb1^exon2-/-) and wildtype (WT) littermates, we found that Adgrb1^exon2-/- astrocytes exhibit significantly reduced phagocytic capacity when compared to similarly prepared astrocytes from WT littermates. Immunostaining of astrocytes and pre-synaptic sites showed less engulfed pre-synaptic elements in astrocytes from Adgrb1^exon2-/- mutants. Finally, immunostaining of pre- and post-synaptic sites revealed a significantly higher density of excitatory synapses in Adgrb1^exon2-/- mutants. Previous studies have shown that ADGRB1 variants are associated with ASD, and mice lacking ADGRB1 exhibit impaired social behavior and increased seizure susceptibility. The findings of this study suggest that the lack of ADGRB1 in astrocytes may result in deficits in astrocyte-mediated phagocytosis, which could potentially contribute to the behavioral abnormalities reported in prior studies.