Presynaptic Short-Term Plasticity Persists in the Absence of PKC Phosphorylation of Munc18-1

J Neurosci. 2021 Sep 1;41(35):7329-7339. doi: 10.1523/JNEUROSCI.0347-21.2021. Epub 2021 Jul 21.

Abstract

Post-tetanic potentiation (PTP) is a form of short-term plasticity that lasts for tens of seconds following a burst of presynaptic activity. It has been proposed that PTP arises from protein kinase C (PKC) phosphorylation of Munc18-1, an SM (Sec1/Munc-18 like) family protein that is essential for release. To test this model, we made a knock-in mouse in which all Munc18-1 PKC phosphorylation sites were eliminated through serine-to-alanine point mutations (Munc18-1SA mice), and we studied mice of either sex. The expression of Munc18-1 was not altered in Munc18-1SA mice, and there were no obvious behavioral phenotypes. At the hippocampal CA3-to-CA1 synapse and the granule cell parallel fiber (PF)-to-Purkinje cell (PC) synapse, basal transmission was largely normal except for small decreases in paired-pulse facilitation that are consistent with a slight elevation in release probability. Phorbol esters that mimic the activation of PKC by diacylglycerol still increased synaptic transmission in Munc18-1SA mice. In Munc18-1SA mice, 70% of PTP remained at CA3-to-CA1 synapses, and the amplitude of PTP was not reduced at PF-to-PC synapses. These findings indicate that at both CA3-to-CA1 and PF-to-PC synapses, phorbol esters and PTP enhance synaptic transmission primarily by mechanisms that are independent of PKC phosphorylation of Munc18-1.SIGNIFICANCE STATEMENT A leading mechanism for a prevalent form of short-term plasticity, post-tetanic potentiation (PTP), involves protein kinase C (PKC) phosphorylation of Munc18-1. This study tests this mechanism by creating a knock-in mouse in which Munc18-1 is replaced by a mutated form of Munc18-1 that cannot be phosphorylated. The main finding is that most PTP at hippocampal CA3-to-CA1 synapses or at cerebellar granule cell-to-Purkinje cell synapses does not rely on PKC phosphorylation of Munc18-1. Thus, mechanisms independent of PKC phosphorylation of Munc18-1 are important mediators of PTP.

Keywords: Munc18-1; PKC; plasticity; post-tetanic potentiation; synapse.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Substitution
  • Animals
  • Female
  • Gene Knock-In Techniques
  • Hippocampus / physiology
  • Male
  • Mice
  • Mice, Knockout
  • Miniature Postsynaptic Potentials / drug effects
  • Miniature Postsynaptic Potentials / physiology
  • Munc18 Proteins / deficiency
  • Munc18 Proteins / metabolism*
  • Mutation, Missense
  • Neuronal Plasticity / physiology*
  • Phorbol Esters / pharmacology
  • Phosphorylation
  • Point Mutation
  • Protein Kinase C / deficiency
  • Protein Kinase C / metabolism*
  • Protein Processing, Post-Translational*
  • Purkinje Cells / physiology
  • Recombinant Proteins / metabolism
  • Synaptic Transmission / drug effects

Substances

  • Munc18 Proteins
  • Phorbol Esters
  • Recombinant Proteins
  • Stxbp1 protein, mouse
  • Protein Kinase C