Synaptotagmin-7 is an asynchronous calcium sensor for synaptic transmission in neurons expressing SNAP-23

PLoS One. 2014 Nov 25;9(11):e114033. doi: 10.1371/journal.pone.0114033. eCollection 2014.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Line
  • Mice
  • Mice, Knockout
  • Neurons / metabolism*
  • Qb-SNARE Proteins / genetics
  • Qb-SNARE Proteins / metabolism*
  • Qc-SNARE Proteins / genetics
  • Qc-SNARE Proteins / metabolism*
  • Synaptic Transmission / physiology*
  • Synaptotagmins / physiology*

Substances

  • Qb-SNARE Proteins
  • Qc-SNARE Proteins
  • Snap23 protein, mouse
  • Syt7 protein, mouse
  • Synaptotagmins

Grant support

This work was supported by the Lundbeck Foundation, the Novo Nordisk Foundation, the Danish Medical Research Council (J.B.S.), the European Union Seventh Framework Program under grant agreement no. HEALTH-F2-2009-242167 (‘SynSys’ project; J.B.S.), and Deutsche Forschungsgemeinschaft (SFB 1027; R.M.). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.