Excitatory synapses are polarized structures that primarily reside on dendritic spines in the brain. The small GTPase Rac1 regulates the development and plasticity of synapses and spines by modulating actin dynamics. By restricting the Rac1-guanine nucleotide exchange factor Tiam1 to spines, the polarity protein Par3 promotes synapse development by spatially controlling Rac1 activation. However, the mechanism for recruiting Par3 to spines is unknown. Here, we identify brain-specific angiogenesis inhibitor 1 (BAI1) as a synaptic adhesion GPCR that is required for spinogenesis and synaptogenesis in mice and rats. We show that BAI1 interacts with Par3/Tiam1 and recruits these proteins to synaptic sites. BAI1 knockdown results in Par3/Tiam1 mislocalization and loss of activated Rac1 and filamentous actin from spines. Interestingly, BAI1 also mediates Rac-dependent engulfment in professional phagocytes through its interaction with a different Rac1-guanine nucleotide exchange factor module, ELMO/DOCK180. However, this interaction is dispensable for BAI1’s role in synapse development because a BAI1 mutant that cannot interact with ELMO/DOCK180 rescues spine defects in BAI1-knockdown neurons, whereas a mutant that cannot interact with Par3/Tiam1 rescues neither spine defects nor Par3 localization. Further, overexpression of Tiam1 rescues BAI1 knockdown spine phenotypes. These results indicate that BAI1 plays an important role in synaptogenesis that is mechanistically distinct from its role in phagocytosis. Furthermore, our results provide the first example of a cell surface receptor that targets members of the PAR polarity complex to synapses.

Excitatory synapses are specialized cell-cell contacts located on actin-rich dendritic spines that mediate information flow and storage in the brain. The postsynaptic adhesion-G protein-coupled receptor (A-GPCR) BAI1 is a critical regulator of excitatory synaptogenesis, which functions in part by recruiting the Par3-Tiam1 polarity complex to spines, inducing local Rac1 GTPase activation and actin cytoskeletal remodeling. However, a detailed mechanistic understanding of how BAI1 controls synapse and spine development remains elusive. Here, we confirm that BAI1 is required for hippocampal spine development, and we identify three distinct signaling mechanisms mediating BAI1's prosynaptogenic functions. Using electroporation to sparsely knock down BAI1 expression in hippocampal pyramidal neurons, we show that BAI1 cell-autonomously promotes spinogenesis in the developing mouse brain. BAI1 appears to function as a receptor at synapses, as its extracellular N-terminal segment is required for both its prospinogenic and prosynaptogenic functions. Moreover, BAI1 activation with a -derived peptide, which mimics a tethered agonist motif found in A-GPCRs, drives synaptic Rac1 activation and subsequent spine and synapse development. We also reveal, for the first time, a trans-synaptic function for BAI1, demonstrating in a mixed-culture assay that BAI1 induces the clustering of presynaptic vesicular glutamate transporter 1 (vGluT1) in contacting axons, indicative of presynaptic differentiation. Finally, we show that BAI1 forms a receptor complex with the synaptogenic cell-adhesion molecule Neuroligin-1 (NRLN1) and mediates NRLN1-dependent spine growth and synapse development. Together, these findings establish BAI1 as an essential postsynaptic A-GPCR that regulates excitatory synaptogenesis by coordinating bidirectional trans-synaptic signaling in cooperation with NRLN1. Adhesion-G protein-coupled receptors are cell-adhesion receptors with important roles in nervous system development, function, and neuropsychiatric disorders. The postsynaptic adhesion-G protein-coupled receptor BAI1 is a critical regulator of dendritic spine and excitatory synapse development. However, the mechanism by which BAI1 controls these functions remains unclear. Our study identifies three distinct signaling paradigms for BAI1, demonstrating that it mediates forward, reverse, and lateral signaling in spines. Activation of BAI1 by a -dependent mechanism induces local Rac1 activation and subsequent spinogenesis/synaptogenesis. BAI1 also signals trans-synaptically to promote presynaptic differentiation. Furthermore, BAI1 interacts with the postsynaptic cell-adhesion molecule Neuroligin-1 (NRLN1) and facilitates NRLN1-dependent spine growth and excitatory synaptogenesis. Thus, our findings establish BAI1 as a functional synaptogenic receptor that promotes presynaptic and postsynaptic development in cooperation with synaptic organizer NRLN1.

At the Drosophila melanogaster bithorax complex (BX-C) over 330kb of intergenic DNA is responsible for directing the transcription of just three homeotic (Hox) genes during embryonic development. A number of distinct enhancer cis-regulatory modules (CRMs) are responsible for controlling the specific expression patterns of the Hox genes in the BX-C. While it is has proven possible to identify orthologs of known BX-C CRMs in different Drosophila species using overall sequence conservation, this approach has not proven sufficiently effective for identifying novel CRMs or defining the key functional sequences within enhancer CRMs. Here we demonstrate that the specific spatial clustering of transcription factor (TF) binding sites is important for BX-C enhancer activity. A bioinformatic search for combinations of putative TF binding sites in the BX-C suggests that simple clustering of binding sites is frequently not indicative of enhancer activity. However, through molecular dissection and evolutionary comparison across the Drosophila genus we discovered that specific TF binding site clustering patterns are an important feature of three known BX-C enhancers. Sub-regions of the defined IAB5 and IAB7b enhancers were both found to contain an evolutionarily conserved signature motif of clustered TF binding sites which is critical for the functional activity of the enhancers. Together, these results indicate that the spatial organization of specific activator and repressor binding sites within BX-C enhancers is of greater importance than overall sequence conservation and is indicative of enhancer functional activity.

Synapses mediate communication between neurons and enable the brain to change in response to experience, which is essential for learning and memory. The sites of most excitatory synapses in the brain, dendritic spines, undergo rapid remodeling that is important for neural circuit formation and synaptic plasticity. Abnormalities in synapse and spine formation and plasticity are associated with a broad range of brain disorders, including intellectual disabilities, autism spectrum disorders (ASD), and schizophrenia. Thus, elucidating the mechanisms that regulate these neuronal processes is critical for understanding brain function and disease. The brain-specific angiogenesis inhibitor (BAI) subfamily of adhesion G-protein-coupled receptors (adhesion-GPCRs) has recently emerged as central regulators of synapse development and plasticity. In this review, we will summarize the current knowledge regarding the roles of BAIs at synapses, highlighting their regulation, downstream signaling, and physiological functions, while noting the roles of other adhesion-GPCRs at synapses. We will also discuss the relevance of BAIs in various neurological and psychiatric disorders and consider their potential importance as pharmacological targets in the treatment of these diseases.

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