Novel SCAMPs Lacking NPF Repeats: Ubiquitous and Synaptic Vesicle-Specific Forms Implicate SCAMPs in Multiple Membrane- Trafficking Functions

In vertebrates, secretory carrier membrane proteins (SCAMPs) 1–3 constitute a family of putative membrane-trafficking proteins composed of cytoplasmic N-terminal sequences with NPF repeats, four central transmembrane regions (TMRs), and a cytoplasmic tail. SCAMPs probably function in endocytosis by recruiting EH-domain proteins to the N-terminal NPF repeats but may have additional functions mediated by their other sequences. We now demonstrate that SCAMPs form a much larger and more heterogeneous protein family than envisioned previously, with an evolutionary conservation extending to invertebrates and plants. Two novel vertebrate SCAMPs (SCAMPs 4 and 5), single SCAMP genes in Caenorhabditis elegans and Drosophila melanogaster, and multiple SCAMPs in Arabidopsis thaliana were identified. Interestingly, the novel SCAMPs 4 and 5 lack the N-terminal NPF repeats that are highly conserved in all other SCAMPs. RNA and Western blotting experiments showed that SCAMPs 1–4 are ubiquitously coexpressed, whereas SCAMP 5 is only detectable in brain where it is expressed late in development coincident with the elaboration of mature synapses. Immunocytochemistry revealed that SCAMP 5 exhibits a synaptic localization, and subcellular fractionations demonstrated that SCAMP 5 is highly enriched in synaptic vesicles. Our studies characterize SCAMPs as a heterogeneous family of putative trafficking proteins composed of three isoforms that are primarily synthesized outside of neurons (SCAMPs 2–4), one isoform that is ubiquitously expressed but highly concentrated on synaptic vesicles (SCAMP 1), and one brain-specific isoform primarily localized to synaptic vesicles (SCAMP 5). The conservation of the TMRs in all SCAMPs with the variable presence of N-terminal NPF repeats suggests that in addition to the role of some SCAMPs in endocytosis mediated by their NPF repeats, all SCAMPs perform a “core” function in membrane traffic mediated by their TMRs

Three-Dimensional Structure of the Complexin/SNARE Complex

During neurotransmitter release, the neuronal SNARE proteins synaptobrevin/VAMP, syntaxin, and SNAP-25 form a four-helix bundle, the SNARE complex, that pulls the synaptic vesicle and plasma membranes together possibly causing membrane fusion. Complexin binds tightly to the SNARE complex and is essential for efficient Ca2+-evoked neurotransmitter release. A combined X-ray and TROSY-based NMR study now reveals the atomic structure of the complexin/SNARE complex. Complexin binds in an antiparallel α-helical conformation to the groove between the synaptobrevin and syntaxin helices. This interaction stabilizes the interface between these two helices, which bears the repulsive forces between the apposed membranes. These results suggest that complexin stabilizes the fully assembled SNARE complex as a key step that enables the exquisitely high speed of Ca2+-evoked neurotransmitter release

Examining Synaptotagmin 1 Function in Dense Core Vesicle Exocytosis under Direct Control of Ca2+

We tested the long-standing hypothesis that synaptotagmin 1 is the Ca2+ sensor for fast neurosecretion by analyzing the intracellular Ca2+ dependence of large dense-core vesicle exocytosis in a mouse strain carrying a mutated synaptotagmin C2A domain. The mutation (R233Q) causes a twofold increase in the KD of Ca2+-dependent phospholipid binding to the double C2A-C2B domain of synaptotagmin. Using photolysis of caged calcium and capacitance measurements we found that secretion from mutant cells had lower secretory rates, longer secretory delays, and a higher intracellular Ca2+-threshold for secretion due to a twofold increase in the apparent KD of the Ca2+ sensor for fast exocytosis. Single amperometric fusion events were unchanged. We conclude that Ca2+-dependent phospholipid binding to synaptotagmin 1 mirrors the intracellular Ca2+ dependence of exocytosis

A Role for cAMP in Long-Term Depression at Hippocampal Mossy Fiber Synapses

AbstractMossy fiber synapses on hippocampal CA3 pyramidal cells, in addition to expressing an NMDA receptor–independent form of long-term potentiation (LTP), have recently been shown to express a novel presynaptic form of long-term depression (LTD). We have studied the mechanisms underlying mossy fiber LTD and present evidence that it is triggered, at least in part, by a metabotropic glutamate receptor–mediated decrease in adenylyl cyclase activity, which leads to a decrease in the activity of the cAMP-dependent protein kinase (PKA) and a reversal of the presynaptic processes responsible for mossy fiber LTP. The bidirectional control of synaptic strength at mossy fiber synapses by activity therefore appears to be due to modulation of the cAMP-PKA signaling pathway in mossy fiber boutons

Synaptotagmin-12, a synaptic vesicle phosphoprotein that modulates spontaneous neurotransmitter release

Central synapses exhibit spontaneous neurotransmitter release that is selectively regulated by cAMP-dependent protein kinase A (PKA). We now show that synaptic vesicles contain synaptotagmin-12, a synaptotagmin isoform that differs from classical synaptotagmins in that it does not bind Ca2+. In synaptic vesicles, synaptotagmin-12 forms a complex with synaptotagmin-1 that prevents synaptotagmin-1 from interacting with SNARE complexes. We demonstrate that synaptotagmin-12 is phosphorylated by cAMP-dependent PKA on serine97, and show that expression of synaptotagmin-12 in neurons increases spontaneous neurotransmitter release by approximately threefold, but has no effect on evoked release. Replacing serine97 by alanine abolishes synaptotagmin-12 phosphorylation and blocks its effect on spontaneous release. Our data suggest that spontaneous synaptic-vesicle exocytosis is selectively modulated by a Ca2+-independent synaptotagmin isoform, synaptotagmin-12, which is controlled by cAMP-dependent phosphorylation

Lipid-Anchored SNAREs Lacking Transmembrane Regions Fully Support Membrane Fusion during Neurotransmitter Release

SummarySynaptic vesicle fusion during neurotransmitter release is mediated by assembly of SNARE- and SM-protein complexes composed of syntaxin-1, SNAP-25, synaptobrevin-2/VAMP2, and Munc18-1. Current models suggest that SNARE-complex assembly catalyzes membrane fusion by pulling the transmembrane regions (TMRs) of SNARE proteins together, thus allowing their TMRs to form a fusion pore. These models are consistent with the requirement for TMRs in viral fusion proteins. However, the role of the SNARE TMRs in synaptic vesicle fusion has not yet been tested physiologically. Here, we examined whether synaptic SNAREs require TMRs for catalysis of synaptic vesicle fusion, which was monitored electrophysiologically at millisecond time resolution. Surprisingly, we find that both lipid-anchored syntaxin-1 and lipid-anchored synaptobrevin-2 lacking TMRs efficiently promoted spontaneous and Ca2+-triggered membrane fusion. Our data suggest that SNARE proteins function during fusion primarily as force generators, consistent with the notion that forcing lipid membranes close together suffices to induce membrane fusion

Proteolytic regulation of calcium channels - avoiding controversy

The publication of papers containing data obtained with suboptimal rigor in the experimental design and choice of key reagents, such as antibodies, can result in a lack of reproducibility and generate controversy that can both needlessly divert resources and, in some cases, damage public perception of the scientific enterprise. This exemplary paper by Buonarati et al. (2018)1 shows how a previously published, potentially important paper on calcium channel regulation falls short of the necessary mark, and aims to resolve the resulting controversy

LTP Requires a Unique Postsynaptic SNARE Fusion Machinery

SummaryMembrane fusion during exocytosis is mediated by assemblies of SNARE (soluble NSF-attachment protein receptor) and SM (Sec1/Munc18-like) proteins. The SNARE/SM proteins involved in vesicle fusion during neurotransmitter release are well understood, whereas little is known about the protein machinery that mediates activity-dependent AMPA receptor (AMPAR) exocytosis during long-term potentiation (LTP). Using direct measurements of LTP in acute hippocampal slices and an in vitro LTP model of stimulated AMPAR exocytosis, we demonstrate that the Q-SNARE proteins syntaxin-3 and SNAP-47 are required for regulated AMPAR exocytosis during LTP but not for constitutive basal AMPAR exocytosis. In contrast, the R-SNARE protein synaptobrevin-2/VAMP2 contributes to both regulated and constitutive AMPAR exocytosis. Both the central complexin-binding and the N-terminal Munc18-binding sites of syntaxin-3 are essential for its postsynaptic role in LTP. Thus, postsynaptic exocytosis of AMPARs during LTP is mediated by a unique fusion machinery that is distinct from that used during presynaptic neurotransmitter release

A stoichiometric complex of neurexins and dystroglycan in brain

In nonneuronal cells, the cell surface protein dystroglycan links the intracellular cytoskeleton (via dystrophin or utrophin) to the extracellular matrix (via laminin, agrin, or perlecan). Impairment of this linkage is instrumental in the pathogenesis of muscular dystrophies. In brain, dystroglycan and dystrophin are expressed on neurons and astrocytes, and some muscular dystrophies cause cognitive dysfunction; however, no extracellular binding partner for neuronal dystroglycan is known. Regular components of the extracellular matrix, such as laminin, agrin, and perlecan, are not abundant in brain except in the perivascular space that is contacted by astrocytes but not by neurons, suggesting that other ligands for neuronal dystroglycan must exist. We have now identified α- and β-neurexins, polymorphic neuron-specific cell surface proteins, as neuronal dystroglycan receptors. The extracellular sequences of α- and β-neurexins are largely composed of laminin-neurexin–sex hormone–binding globulin (LNS)/laminin G domains, which are also found in laminin, agrin, and perlecan, that are dystroglycan ligands. Dystroglycan binds specifically to a subset of the LNS domains of neurexins in a tight interaction that requires glycosylation of dystroglycan and is regulated by alternative splicing of neurexins. Neurexins are receptors for the excitatory neurotoxin α-latrotoxin; this toxin competes with dystroglycan for binding, suggesting overlapping binding sites on neurexins for dystroglycan and α-latrotoxin. Our data indicate that dystroglycan is a physiological ligand for neurexins and that neurexins' tightly regulated interaction could mediate cell adhesion between brain cells

Proteolysis of SNAP-25 by types E and A botulinal neurotoxins

Clostridial neurotoxins, tetanus toxin (TeTx) and the seven related but serologically distinct botulinal neurotoxins (BoNT/A to BoNT/G), are potent inhibitors of synaptic vesicle exocytosis in nerve endings. Recently it was reported that the light chains of clostridial neurotoxins act as zinc-dependent metalloproteases which specifically cleave synaptic target proteins such as synaptobrevin/VAMPs, HPC-1/syntaxin (BoNT/C1), and SNAP-25 (BoNT/A). We show here that BoNT/E, like BoNT/A, cleaves SNAP-25, as generated by in vitro translation or by expression in Escherichia coli. BoNT/E cleaves the Arg180-Ile181 bond. This site is different from that of BoNT/A, which cleaves SNAP-25 between the amino acid residues Gln197 and Arg198. These findings further support the view that clostridial neurotoxins have evolved from an ancestral protease recognizing the exocytotic fusion machinery of synaptic vesicles whereby individual toxins target different members of the membrane fusion complex