In all synapses, Ca2+ triggers neurotransmitter release to initiate signal transmission. Ca2+ presumably acts by activating synaptic Ca2+ sensors, but the nature of these sensors--which are the gatekeepers to neurotransmission--remains unclear. One of the candidate Ca2+ sensors in release is the synaptic Ca2+-binding protein synaptotagmin I. Here we have studied a point mutation in synaptotagmin I that causes a twofold decrease in overall Ca2+ affinity without inducing structural or conformational changes. When introduced by homologous recombination into the endogenous synaptotagmin I gene in mice, this point mutation decreases the Ca2+ sensitivity of neurotransmitter release twofold, but does not alter spontaneous release or the size of the readily releasable pool of neurotransmitters. Therefore, Ca2+ binding to synaptotagmin I participates in triggering neurotransmitter release at the synapse.

Synaptogenesis, the generation and maturation of functional synapses between nerve cells, is an essential step in the development of neuronal networks in the brain. It is thought to be triggered by members of the neuroligin family of postsynaptic cell adhesion proteins, which may form transsynaptic contacts with presynaptic alpha- and beta-neurexins and have been implicated in the etiology of autism. We show that deletion mutant mice lacking neuroligin expression die shortly after birth due to respiratory failure. This respiratory failure is a consequence of reduced GABAergic/glycinergic and glutamatergic synaptic transmission and network activity in brainstem centers that control respiration. However, the density of synaptic contacts is not altered in neuroligin-deficient brains and cultured neurons. Our data show that neuroligins are required for proper synapse maturation and brain function, but not for the initial formation of synaptic contacts.

Neuroligins enhance synapse formation in vitro, but surprisingly are not required for the generation of synapses in vivo. We now show that in cultured neurons, neuroligin-1 overexpression increases excitatory, but not inhibitory, synaptic responses, and potentiates synaptic NMDAR/AMPAR ratios. In contrast, neuroligin-2 overexpression increases inhibitory, but not excitatory, synaptic responses. Accordingly, deletion of neuroligin-1 in knockout mice selectively decreases the NMDAR/AMPAR ratio, whereas deletion of neuroligin-2 selectively decreases inhibitory synaptic responses. Strikingly, chronic inhibition of NMDARs or CaM-Kinase II, which signals downstream of NMDARs, suppresses the synapse-boosting activity of neuroligin-1, whereas chronic inhibition of general synaptic activity suppresses the synapse-boosting activity of neuroligin-2. Taken together, these data indicate that neuroligins do not establish, but specify and validate, synapses via an activity-dependent mechanism, with different neuroligins acting on distinct types of synapses. This hypothesis reconciles the overexpression and knockout phenotypes and suggests that neuroligins contribute to the use-dependent formation of neural circuits.

Neurotransmitter release at synapses between nerve cells is mediated by calcium-triggered exocytotic fusion of synaptic vesicles. Before fusion, vesicles dock at the presynaptic release site where they mature to a fusion-competent state. Here we identify Munc13-1, a brain-specific presynaptic phorbol ester receptor, as an essential protein for synaptic vesicle maturation. We show that glutamatergic hippocampal neurons from mice lacking Munc13-1 form ultrastructurally normal synapses whose synaptic-vesicle cycle is arrested at the maturation step. Transmitter release from mutant synapses cannot be triggered by action potentials, calcium-ionophores or hypertonic sucrose solution. In contrast, release evoked by alpha-latrotoxin is indistinguishable from wild-type controls, indicating that the toxin can bypass Munc13-1-mediated vesicle maturation. A small subpopulation of synapses of any given glutamatergic neuron as well as all synapses of GABA (gamma-aminobutyric acid)-containing neurons are unaffected by Munc13-1 loss, demonstrating the existence of multiple and transmitter-specific synaptic vesicle maturation processes in synapses.

Neurons release neurotransmitters by calcium-dependent exocytosis of synaptic vesicles. However, the molecular steps transducing the calcium signal into membrane fusion are still an enigma. It is reported here that synaptotagmin, a highly conserved synaptic vesicle protein, binds calcium at physiological concentrations in a complex with negatively charged phospholipids. This binding is specific for calcium and involves the cytoplasmic domain of synaptotagmin. Calcium binding is dependent on the intact oligomeric structure of synaptotagmin (it is abolished by proteolytic cleavage at a single site). These results suggest that synaptotagmin acts as a cooperative calcium receptor in exocytosis.

In the mammalian CNS, each neuron typically receives thousands of synaptic inputs from diverse classes of neurons. Synaptic transmission to the postsynaptic neuron relies on localized and transmitter-specific differentiation of the plasma membrane with postsynaptic receptor, scaffolding, and adhesion proteins accumulating in precise apposition to presynaptic sites of transmitter release. We identified protein interactions of the synaptic adhesion molecule neuroligin 2 that drive postsynaptic differentiation at inhibitory synapses. Neuroligin 2 binds the scaffolding protein gephyrin through a conserved cytoplasmic motif and functions as a specific activator of collybistin, thus guiding membrane tethering of the inhibitory postsynaptic scaffold. Complexes of neuroligin 2, gephyrin and collybistin are sufficient for cell-autonomous clustering of inhibitory neurotransmitter receptors. Deletion of neuroligin 2 in mice perturbs GABAergic and glycinergic synaptic transmission and leads to a loss of postsynaptic specializations specifically at perisomatic inhibitory synapses.

Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Heritability and polygenic predictionIn the EUR sample, the SNP-based heritability (h 2 SNP ) (that is, the proportion of variance in liability attributable to all measured SNPs)

Graphical Abstract Highlights d SynGO is a public knowledge base and online analysis platform for synapse research d SynGO has annotated 1,112 genes with synaptic localization and/or function d SynGO genes are exceptionally large, well conserved, and intolerant to mutations d SynGO genes are strongly enriched among genes associated with brain disorders Correspondence guus.smit@cncr.vu.nl (A.B.S.), matthijs@cncr.vu.nl (M.V.) In BriefThe SynGO consortium presents a framework to annotate synaptic protein locations and functions and annotations for 1,112 synaptic genes based on published experimental evidence. SynGO reports exceptional features and disease associations for synaptic genes and provides an online data analysis platform. SUMMARYSynapses are fundamental information-processing units of the brain, and synaptic dysregulation is central to many brain disorders (''synaptopathies''). However, systematic annotation of synaptic genes and ontology of synaptic processes are currently lacking. We established SynGO, an interactive knowledge base that accumulates available research about synapse biology using Gene Ontology (GO) annotations to novel ontology terms: 87 synaptic locations and 179 synaptic processes. SynGO annotations are exclusively based on published, expert-curated evidence. Using 2,922 annotations for 1,112 genes, we show that synaptic genes are exceptionally well conserved and less tolerant to mutations than other genes. Many SynGO terms are significantly overrepresented among gene variations associated with intelligence, educational attainment, ADHD, autism, and bipolar disorder and among de novo variants associated with neurodevelopmental disorders, including schizophrenia. SynGO is a public, universal reference for synapse research and an online analysis platform for interpretation of large-scale -omics data (https://syngoportal.org and