Allosteric stabilization of calcium and phosphoinositide dual binding engages several synaptotagmins in fast exocytosis

Abstract

Synaptic communication relies on the fusion of synaptic vesicles with the plasma membrane, which leads to neurotransmitter release. This exocytosis is triggered by brief and local elevations of intracellular Ca2+ with remarkably high sensitivity. How this is molecularly achieved is unknown. While synaptotagmins confer the Ca2+ sensitivity of neurotransmitter exocytosis, biochemical measurements reported Ca2+ affinities too low to account for synaptic function. However, synaptotagmin's Ca2+ affinity increases upon binding the plasma membrane phospholipid PI(4,5)P2 and, vice versa, Ca2+-binding increases synaptotagmin's PI(4,5)P2 affinity, indicating a stabilization of the Ca2+/PI(4,5)P2 dual-bound syt. Here we devise a molecular exocytosis model based on this positive allosteric stabilization and the assumptions that (1.) synaptotagmin Ca2+/PI(4,5)P2 dual binding lowers the energy barrier for vesicle fusion and that (2.) the effect of multiple synaptotagmins on the energy barrier is additive. The model, which relies on biochemically measured Ca2+/PI(4,5)P2 affinities and protein copy numbers, reproduced the steep Ca2+ dependency of neurotransmitter release. Our results indicate that each synaptotagmin dual binding Ca2+/PI(4,5)P2 lowers the energy barrier for vesicle fusion by ~5 kBT and that allosteric stabilization of this state enables the synchronized engagement of several (typically three) synaptotagmins for fast exocytosis. Furthermore, we show that mutations altering synaptotagmin’s allosteric properties may show dominant-negative effects, even though synaptotagmins act independently on the energy barrier, and that dynamic changes of local PI(4,5)P2 (e.g. upon vesicle movement) dramatically impact synaptic responses. We conclude that allosterically stabilized Ca2+/PI(4,5)P2 dual binding enables synaptotagmins to exert their coordinated function in neurotransmission.

Data availability

All data and software codes generated and used during this study are included in the manuscript and supporting files. Source data is included for all figures.

Article and author information

Author details

  1. Janus RL Kobbersmed

    Department of Mathematical Sciences, University of Copenhagen, Copenhagen, Denmark
    Competing interests
    The authors declare that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0003-0313-6205
  2. Manon MM Berns

    Department of Neuroscience, University of Copenhagen, Copenhagen, Denmark
    Competing interests
    The authors declare that no competing interests exist.
  3. Susanne Ditlevsen

    Department of Mathematical Sciences, University of Copenhagen, Copenhagen, Denmark
    Competing interests
    The authors declare that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0002-1998-2783
  4. Jakob Balslev Sørensen

    Department of Neuroscience, University of Copenhagen, Copenhagen, Denmark
    Competing interests
    The authors declare that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0001-5465-3769
  5. Alexander M Walter

    Department of Mathematical Sciences, University of Copenhagen, Copenhagen, Denmark
    For correspondence
    awalter@sund.ku.dk
    Competing interests
    The authors declare that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0001-5646-4750

Funding

Novo Nordisk Fonden (NNF19OC0056047)

  • Alexander M Walter

Novo Nordisk Fonden (NNF20OC0062958)

  • Susanne Ditlevsen

Novo Nordisk Fonden (NNF19OC0058298)

  • Jakob Balslev Sørensen

Lundbeckfonden (R277-2018-802)

  • Jakob Balslev Sørensen

Independent research fund Denmark (8020-00228A)

  • Jakob Balslev Sørensen

Deutsche Forschungsgemeinschaft , Transregio SFB 186 (278001972)

  • Alexander M Walter

Deutsche Forschungsgemeinschaft , Emmy Noether Programme (261020751)

  • Alexander M Walter

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Reviewing Editor

  1. Henrique von Gersdorff, Oregon Health and Science University, United States

Publication history

  1. Received: October 18, 2021
  2. Accepted: August 4, 2022
  3. Accepted Manuscript published: August 5, 2022 (version 1)

Copyright

© 2022, Kobbersmed et al.

This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.

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  1. Janus RL Kobbersmed
  2. Manon MM Berns
  3. Susanne Ditlevsen
  4. Jakob Balslev Sørensen
  5. Alexander M Walter
(2022)
Allosteric stabilization of calcium and phosphoinositide dual binding engages several synaptotagmins in fast exocytosis
eLife 11:e74810.
https://doi.org/10.7554/eLife.74810

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