Bidirectional signaling by cell-adhesion molecules is thought to mediate synapse formation, but the mechanisms involved remain elusive. Here we found that the adhesion-G-protein-coupled receptors latrophilin-2 and latrophilin-3 selectively directed formation of perforant-path and Schaffer-collateral synapses, respectively, to hippocampal CA1-region neurons. Latrophilin-3 binds to two trans-cellular ligands, fibronectin leucine-rich-repeat transmembrane proteins (FLRTs) and teneurins. In vivo, both binding activities were required for input-specific synapse formation, suggesting that coincident binding of both ligands is necessary for synapse formation. In vitro, teneurin or FLRT alone did not induce excitatory synapse formation, whereas together they potently did so. Thus, postsynaptic latrophilins promote excitatory synapse formation by simultaneous binding of two unrelated presynaptic ligands, which is required for formation of synaptic inputs at specific dendritic localizations. INTRODUCTION: In brain, synaptic connections form neuronal communication networks, thereby constructing neural circuits. Synaptic connections are exquisitely specific and dynamic, but the underlying molecular mechanisms remain largely unexplored. In the hippocampus, Schaffer-collateral axons from the CA3 region form synapses on CA1 region pyramidal neurons exclusively on dendritic domains in the S. oriens and S. radiatum of these neurons. In contrast, perforant-path axons from the entorhinal cortex form synapses on CA1 region pyramidal neurons exclusively on dendritic domains in the S. lacunosum-moleculare. How this synaptic input specificity is achieved, however, and what signaling mechanisms maintain the two classes of synapses, is unknown. RATIONALE: Synapse formation is thought to involve bidirectional signaling by trans-synaptic cell-adhesion molecules. Building on recent observations that the adhesion G-protein coupled receptor (GPCR) latrophilin-2 is essential for synapses in the S. lacunosum-moleculare of the CA1 region, we asked whether distinct latrophilins are localized to different dendritic domains of CA1 region neurons. Moreover, latrophilins are known to form trans-cellular interactions with two classes of cell-adhesion molecules, teneurins and fibronectin leucine-rich-repeat transmembrane proteins (FLRTs). Thus we hypothesized that latrophilins may act in synapse formation via trans-synaptic interactions with these adhesion molecules as ligands, and that such interactions may contribute to the specificity of synapse formation. RESULTS: We produced genetic manipulations that allow monitoring the localization of endogenous latrophilin-2 and latrophilin-3 in vivo and that enable their conditional deletion. Using these manipulations, we found that latrophilin-2 and latrophilin-3 were specifically localized to postsynaptic spines in non-overlapping dendritic domains of CA1 region pyramidal neurons. Latrophilin-2 was targeted only to excitatory synapses in the S. lacunosum-moleculare, whereas latrophilin-3 was targeted o...

SUMMARY Neuronal activity influences genes involved in circuit development and information processing. However, the molecular basis of this process remains poorly understood. We found that HDAC4, a histone deacetylase that shuttles between the nucleus and cytoplasm, controls a transcriptional program essential for synaptic plasticity and memory. The nuclear import of HDAC4 and its association with chromatin is negatively regulated by NMDA receptors. In the nucleus, HDAC4 represses genes encoding constituents of central synapses, thereby affecting synaptic architecture and strength. Furthermore, we show that a truncated form of HDAC4 encoded by an allele associated with mental retardation is a gain-of-function nuclear repressor that abolishes transcription and synaptic transmission despite the loss of the deacetylase domain. Accordingly, mice carrying a mutant that mimics this allele exhibit deficits in neurotransmission and spatial memory. These studies elucidate a mechanism of experience-dependent plasticity and define the biological role of HDAC4 in the brain.

Teneurins (TENs) are cell-surface adhesion proteins with critical roles in tissue development and axon guidance. Here, we report the 3.1-Å cryoelectron microscopy structure of the human TEN2 extracellular region (ECR), revealing a striking similarity to bacterial Tc-toxins. The ECR includes a large β barrel that partially encapsulates a C-terminal domain, which emerges to the solvent through an opening in the mid-barrel region. An immunoglobulin (Ig)-like domain seals the bottom of the barrel while a β propeller is attached in a perpendicular orientation. We further show that an alternatively spliced region within the β propeller acts as a switch to regulate trans-cellular adhesion of TEN2 to latrophilin (LPHN), a transmembrane receptor known to mediate critical functions in the central nervous system. One splice variant activates trans-cellular signaling in a LPHN-dependent manner, whereas the other induces inhibitory postsynaptic differentiation. These results highlight the unusual structural organization of TENs giving rise to their multifarious functions.

Individual synapses vary significantly in their neurotransmitter release properties, which underlie complex information processing in neural circuits. Presynaptic Ca2+ homeostasis plays a critical role in specifying neurotransmitter release properties, but the mechanisms regulating synapse-specific Ca2+ homeostasis in the mammalian brain are still poorly understood. Using electrophysiology and genetically encoded Ca2+ sensors targeted to the mitochondrial matrix or to presynaptic boutons of cortical pyramidal neurons, we demonstrate that the presence or absence of mitochondria at presynaptic boutons dictates neurotransmitter release properties through Mitochondrial Calcium Uniporter (MCU)-dependent Ca2+ clearance. We demonstrate that the serine/threonine kinase LKB1 regulates MCU expression, mitochondria-dependent Ca2+ clearance, and thereby, presynaptic release properties. Re-establishment of MCU-dependent mitochondrial Ca2+ uptake at glutamatergic synapses rescues the altered neurotransmitter release properties characterizing LKB1-null cortical axons. Our results provide novel insights into the cellular and molecular mechanisms whereby mitochondria control neurotransmitter release properties in a bouton-specific way through presynaptic Ca2+ clearance.

Hippocampal CA1 region neurons specifically target latrophilin-2 (Lphn2), an adhesion-type GPCR, to dendritic spines in the stratum lacunosum-moleculare. In this study, Lphn2 controls assembly of excitatory synapses formed by presynaptic entorhinal cortex afferents but not by Schaffer-collateral afferents, suggesting a synaptic recognition function.

SUMMARY Synaptic excitation mediates a broad spectrum of structural changes in neural circuits across the brain. Here, we examine the morphologies, wiring, and architectures of single synapses of projection neurons in the murine hippocampus that developed in virtually complete absence of vesicular glutamate release. While these neurons had smaller dendritic trees and/or formed fewer contacts in specific hippocampal subfields, their stereotyped connectivity was largely preserved. Furthermore, loss of release did not disrupt the morphogenesis of presynaptic terminals and dendritic spines, suggesting that glutamatergic neurotransmission is unnecessary for synapse assembly and maintenance. These results underscore the instructive role of intrinsic mechanisms in synapse formation.

SUMMARY At presynaptic active zones, exocytosis of neurotransmitter vesicles (SVs) is driven by SNARE complexes that recruit Syb2 and SNAP25. However, it remains unknown which SNAREs promote the secretion of neuronal proteins, including those essential for circuit development and experience-dependent plasticity. Here, we demonstrate that Syb2 and SNAP25 mediate the vesicular release of BDNF in axons and dendrites of cortical neurons, suggesting these SNAREs act in multiple spatially-segregated secretory pathways. Remarkably, axonal secretion of BDNF is also strongly regulated by SNAP47 which interacts with SNAP25 but appears to be dispensable for exocytosis of SVs. Cell-autonomous ablation of SNAP47 disrupts the layer-specific branching of callosal axons of projection cortical neurons in vivo, and this phenotype is recapitulated by ablation of BDNF or its receptor, TrkB. Our results provide insights into the molecular mechanisms of protein secretion and define the functions of SNAREs in BDNF signaling and regulation of neuronal connectivity.

Summary FLRTs are cell-adhesion molecules with emerging functions in cortical development and synapse formation. Their extracellular regions interact with LPHNs to mediate synapse development, and with UNC5/Netrin receptors to control the migration of neurons in the developing cortex. Here, we present the crystal structures of FLRT3 in isolation and in complex with LPHN3. The FLRT3/LPHN3 structure reveals that LPHN3 binds to FLRT3 at a distinct site from UNC5. Structure-based mutations specifically disrupt FLRT3/LPHN3 binding, but do not disturb their interactions with other proteins or their cell-membrane localization. Thus, they can be used as molecular tools to dissect the functions of FLRTs and LPHNs in vivo. Our results suggest that UNC5 and LPHN3 can simultaneously bind to FLRT3 forming a trimeric complex and that FLRT3 may form trans-synaptic complexes with both LPHN3 and UNC5. These findings provide molecular insights for understanding the role of cell-adhesion proteins in synapse function.