Synaptic dysfunction is an early pathological feature of neurodegenerative diseases associated with Tau, including Alzheimer's disease. Interfering with early synaptic dysfunction may be therapeutically beneficial to prevent cognitive decline and disease progression, but the mechanisms underlying synaptic defects associated with Tau are unclear. In disease conditions, Tau mislocalizes into pre- and postsynaptic compartments; here we show that, under pathological conditions, Tau binds to presynaptic vesicles in Alzheimer's disease patient brain. We define that the binding of Tau to synaptic vesicles is mediated by the transmembrane vesicle protein Synaptogyrin-3. In fly and mouse models of Tauopathy, reduction of Synaptogyrin-3 prevents the association of presynaptic Tau with vesicles, alleviates Tau-induced defects in vesicle mobility, and restores neurotransmitter release. This work therefore identifies Synaptogyrin-3 as the binding partner of Tau on synaptic vesicles, revealing a new presynapse-specific Tau interactor, which may contribute to early synaptic dysfunction in neurodegenerative diseases associated with Tau.

Lowering Synaptogyrin-3 expression rescues Tauinduced memory defects and synaptic loss in the presence of microglial activation Highlights d Tau accrues at Synaptogyrin-3-positive synapses in a tauopathy mouse model and AD patient brains d Lowering Synaptogyrin-3 levels is benign in mice d Lowering Synaptogyrin-3 expression rescues Tau-induced synaptic plasticity and working memory defects d Tau-induced synaptic loss can be rescued in the presence of neuroinflammation

Excitatory and inhibitory neurons are connected into microcircuits that generate circuit output. Central in the hippocampal CA3 microcircuit is the mossy fiber (MF) synapse, which provides powerful direct excitatory input and indirect feedforward inhibition to CA3 pyramidal neurons. Here, we dissect its cell-surface protein (CSP) composition to discover novel regulators of MF synaptic connectivity. Proteomic profiling of isolated MF synaptosomes uncovers a rich CSP composition, including many CSPs without synaptic function and several that are uncharacterized. Cell-surface interactome screening identifies IgSF8 as a neuronal receptor enriched in the MF pathway. Presynaptic Igsf8 deletion impairs MF synaptic architecture and robustly decreases the density of bouton filopodia that provide feedforward inhibition. Consequently, IgSF8 loss impairs excitation/inhibition balance and increases excitability of CA3 pyramidal neurons. Our results provide insight into the CSP landscape and interactome of a specific excitatory synapse and reveal IgSF8 as a critical regulator of CA3 microcircuit connectivity and function.

Synaptic diversity is a key feature of neural circuits. Its underlying molecular basis is largely unknown, due to the challenge of analyzing the protein composition of specific synapse types.Here, we isolate the hippocampal mossy fiber (MF) synapse, taking advantage of its unique size and architecture, and dissect its proteome. We identify a rich cell-surface repertoire that includes 5 adhesion proteins, guidance cue receptors, extracellular matrix (ECM) proteins, and proteins of unknown function. Among the latter, we find IgSF8, a previously uncharacterized neuronal receptor, and uncover its role in regulating MF synapse architecture and feedforward inhibition on CA3 pyramidal neurons. Our findings reveal a diverse MF synapse surface proteome and highlight the role of neuronal surface-ECM interactions in the specification of synapse identity and circuit 10 formation. One Sentence Summary:Proteomic dissection of a specific synapse 15 3 Main Text:Neural circuits are composed of distinct neuronal cell types connected in highly specific patterns.Establishing these patterns of connectivity critically relies on cell-surface proteins (CSPs) expressed in cell type-specific combinations. CSPs, including transmembrane, membraneanchored, and secreted proteins, engage in networks of interactions that control neurite guidance, 5 target selection, and synapse development required for the formation of functional circuits (1).Single-cell RNA sequencing has enabled the characterization of cell type-specific CSP repertoires (2-6), but determining how these dictate complex patterns of connectivity (7, 8) poses a major challenge.This challenge is exemplified by pyramidal neurons, which receive different types of 10 synapses on their dendritic arbor, each with a distinct architecture, subcellular location, and functional properties. This synaptic diversity is essential for information processing in pyramidal neurons (9). Recent studies reveal a synapse type-specific localization and function of postsynaptic adhesion molecules in hippocampal pyramidal neuron dendrites (10-12), suggesting that compartmentalized distributions of CSPs contribute to the specification of synaptic structure and 15 function. Analogous to single-cell sequencing, probing the mechanisms underlying synaptic diversity requires dissecting the molecular composition of specific synapse types. This has remained challenging, as microdissection or chemical labeling strategies combined with mass spectrometry (MS) (13-15) average different synapse types, and affinity purification of synapse type-specific protein complexes (16) requires genetically engineered mice. Here, we isolate the 20 hippocampal mossy fiber (MF) synapse, a large and morphologically complex excitatory synapse (17) connecting dentate granule cell axons (mossy fibers) and CA3 pyramidal neuron dendrites in stratum lucidum (SL) (Fig. 1A, B), from wild-type (WT) tissue and map its CSP landscape. 4To isolate a specific synapse type from the hippocampus, we started with a previously published approach (18) ...