title: Synaptotagmin-7 is an asynchronous calcium sensor for synaptic transmission in neurons expressing SNAP-23.
abstract: Synchronization of neurotransmitter release with the presynaptic action potential is essential for maintaining fidelity of information transfer in the central nervous system. However, synchronous release is frequently accompanied by an asynchronous release component that builds up during repetitive stimulation, and can even play a dominant role in some synapses. Here, we show that substitution of SNAP-23 for SNAP-25 in mouse autaptic glutamatergic hippocampal neurons results in asynchronous release and a higher frequency of spontaneous release events (mEPSCs). Use of neurons from double-knock-out (SNAP-25, synaptotagmin-7) mice in combination with viral transduction showed that SNAP-23-driven release is triggered by endogenous synaptotagmin-7. In the absence of synaptotagmin-7 release became even more asynchronous, and the spontaneous release rate increased even more, indicating that synaptotagmin-7 acts to synchronize release and suppress spontaneous release. However, compared to synaptotagmin-1, synaptotagmin-7 is a both leaky and asynchronous calcium sensor. In the presence of SNAP-25, consequences of the elimination of synaptotagmin-7 were small or absent, indicating that the protein pairs SNAP-25/synaptotagmin-1 and SNAP-23/synaptotagmin-7 might act as mutually exclusive calcium sensors. Expression of fusion proteins between pHluorin (pH-sensitive GFP) and synaptotagmin-1 or -7 showed that vesicles that fuse using the SNAP-23/synaptotagmin-7 combination contained synaptotagmin-1, while synaptotagmin-7 barely displayed activity-dependent trafficking between vesicle and plasma membrane, implying that it acts as a plasma membrane calcium sensor. Overall, these findings support the idea of alternative syt∶SNARE combinations driving release with different kinetics and fidelity.
authors.affiliation: Department of Neuroscience and Pharmacology, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark; Department of Functional Genomics, Center for Neurogenomics and Cognitive Research, Vrije Universiteit, Amsterdam, The Netherlands.
authors.forename: Jens P
authors.initials: JP
authors.lastname: Weber
authors.affiliation: Department of Neuroscience and Pharmacology, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Trine L
authors.initials: TL
authors.lastname: Toft-Bertelsen
authors.affiliation: Department of Physiology, University of Saarland, Homburg, Germany.
authors.forename: Ralf
authors.initials: R
authors.lastname: Mohrmann
authors.affiliation: Singapore Institute of Neurotechnology, National University of Singapore, Singapore.
authors.forename: Ignacio
authors.initials: I
authors.lastname: Delgado-Martinez
authors.affiliation: Department of Neuroscience and Pharmacology, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark; Lundbeck Foundation Center for Biomembranes in Nanomedicine, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Jakob B
authors.initials: JB
authors.lastname: Sørensen
mesh_terms: D000818:Animals; D002460:Cell Line; D051379:Mice; D018345:Mice, Knockout; D009474:Neurons / Q000378:metabolism*; D050766:Qb-SNARE Proteins / Q000235:genetics / Q000378:metabolism*; D050767:Qc-SNARE Proteins / Q000235:genetics / Q000378:metabolism*; D009435:Synaptic Transmission / Q000502:physiology*; D050857:Synaptotagmins / Q000502:physiology*
chemical_list: D050766:Qb-SNARE Proteins; D050767:Qc-SNARE Proteins; C107017:Snap23 protein, mouse; C494303:Syt7 protein, mouse; D050857:Synaptotagmins