Regulation of hippocampal excitatory synapse development by the adhesion G-protein coupled receptor Brain-specific angiogenesis inhibitor 2 (BAI2/ADGRB2).

Glutamatergic synapses and their associated dendritic spines are critical information processing sites within the brain. Proper development of these specialized cellular junctions is important for normal brain functionality. Synaptic adhesion G protein-coupled receptors (aGPCRs) have been identified as regulators of synapse development and function. While two members of the Brain-specific angiogenesis inhibitor (BAI/ADGRB) subfamily of synaptic aGPCRs, BAI1/ADGRB1 and BAI3/ADGRB3, have been found to mediate synapse and spine formation, BAI2/ADGRB2 function remains uncharacterized at the synapse. Here, we show that endogenous BAI2 is expressed throughout the nervous system with prominent expression in synapse dense regions of the hippocampus. In dissociated hippocampal cultures, BAI2 is highly enriched at large postsynaptic sites, defined by the size of the postsynaptic scaffold PSD95. Loss of BAI2 negatively impacts glutamatergic synapses across development in dissociated hippocampal cultures. In contrast, GABAergic synapse density is unchanged. Furthermore, BAI2 deficient neurons have significant alterations in spine morphology with decreased density of mature PSD95-containing mushroom-shaped spines compared with wild-type neurons. Interestingly, no major alterations in dendritic complexity were observed in BAI2 deficient neurons, in contrast to previous results for the other BAIs. The reduction in mature mushroom-shaped spine is commensurate with a reduction in spine volume and head diameter. Altogether, these results demonstrate that the aGPCR BAI2 is an important regulator of glutamatergic synapse and PSD95-associated spine development in cultured hippocampal neurons. These results expand the knowledge of the BAI subfamily of aGPCRs in mediating excitatory synapse and spine development and highlight differences unique to BAI2.