BAI adhesion-GPCRs perform distinct functions in neural development differentially controlled by RTN4R and C1ql ligands

BAI1, BAI2, and BAI3 (for 'Brain-specific Angiogenesis Inhibitor-1, -2, and -3') are adhesion-GPCRs implicated in neuronal development. The precise roles of individual BAIs remain unclear. BAIs interact with two sets of ligands, secreted C1ql proteins and membrane-bound RTN4R proteins (a.k.a. NoGo receptors), but which of these ligands regulate specific functions of BAIs is incompletely understood. To address these key questions, we here systematically examine the functions of the three BAIs in neuronal development using hippocampal neuron-glia cultures, genetic knockouts, and rescue experiments. In a direct comparison, we demonstrate that deletions of BAI1 or BAI3, but not of BAI2, increase axonal and dendritic arborizations but decrease excitatory synapse formation, while inhibitory synapse formation remains unaffected. Since biochemical and cellular assays reveal that only BAI3 binds to both RTN4Rs and C1qls, we analyzed the role of these two ligands in controlling BAI3 functions using rescue experiments. We find that RTN4R-binding to BAI3 is essential for restricting axonal and dendritic arborizations and for enabling excitatory synapse formation, whereas C1ql-binding to BAI3 is only required for synapse organization as monitored in hippocampal neuron-glia cultures. Thus, BAI1 and BAI3 perform diverse functions that shape multiple facets of neuronal development and that require their interaction with RTN4Rs.