Munc18-1 expression levels control synapse recovery by regulating readily releasable pool size

Abstract

Prompt recovery after intense activity is an essential feature of most mammalian synapses. Here we show that synapses with reduced expression of the presynaptic gene munc18-1 suffer from increased depression during intense stimulation at glutamatergic, GABAergic, and neuromuscular synapses. Conversely, munc18-1 overexpression makes these synapses recover faster. Concomitant changes in the readily releasable vesicle pool and its refill kinetics were found. The number of vesicles docked at the active zone and the total number of vesicles per terminal correlated with both munc18-1 expression levels and the size of the releasable vesicle pool. These data show that varying expression of a single gene controls synaptic recovery by modulating the number of docked, release-ready vesicles and thereby replenishment of the secretion capacity.autapse ͉ docking ͉ exocytosis ͉ secretion ͉ synaptic transmission R eliable and sustainable neurotransmitter release is essential for effective neuronal communication. However, neurons only have a limited number of fusion-ready vesicles that reside in a vesicle pool at the membrane of the presynaptic terminal (1). During periods of increased activity, this vesicle pool is depleted, resulting in a decreased reliability of neurotransmission. To ensure efficient neurotransmission, neurons need to be able to increase the initial number of fusion-ready vesicles [the so-called readily releasable pool (RRP)] and͞or the rate at which this pool is replenished during activity. However, surprisingly little is known about the molecular mechanisms that control the size of the RRP and the way vesicles are recruited to this pool.The Sec1͞Munc18-like (SM) protein Munc18-1 has emerged as a key component for calcium-dependent neurotransmitter release (2). SM proteins function in all intracellular membrane trafficking pathways across species. Genetic deletion of Munc18-1 and most other SM genes involved in synaptic-vesicle release across species results in a complete block of neurotransmitter release (3-5), which shows that Munc18-1 and probably all SM proteins are indispensable factors that promote vesicle secretion (2, 6, 7). However, identifying where SM proteins act in the cascade of events leading to the release of neurotransmitter has proven to be difficult and has generated apparently conflicting data (8-10).Here, we analyzed the effect of different Munc18-1 expression levels on synaptic function in autaptic synapses of GABAergic and glutamatergic central neurons, as well as in the peripheral neuromuscular junction (NMJ). We combined electrophysiological and optical measurements to show that Munc18-1 controls synapse efficacy in a bidirectional way via the control of the size and replenishment rate of the RRP. ResultsIn homozygous munc18-1-null mutant mice, synapses are silent (3), identifying munc18-1 as an essential gene but providing little information on its molecular function. Heterozygous mice (munc18-1 ϩ/Ϫ ) had a 50% reduction of Munc18-1 protein expression but no reduction in the levels ...

Analysis of conditional heterozygous STXBP1 mutations in human neurons

“…Although hundreds of patients with STXBP1 mutations have been described, the pathogenetic mechanism of these mutations is unclear. In mice, it has been well established that homozygous deletion of STXBP1 produces a total block in synaptic transmission and neurodegeneration (31), but that the heterozygous deletion of STXBP1 induces only a mild phenotype (37,38). Thus, the question arises of how a heterozygous STXBP1 mutation that in mice has only modest effects can cause severe clinical impairment in human patients.…”

Short-Term Presynaptic Plasticity

“…The use of knockout animals established that calcium-dependent PKC accounts for 80% of PTP at the calyx of Held . The involvement of calcium-sensitive isoforms suggests that PKC responds to Ca res to produce PTP, possibly by phosphorylating Munc18-1 to alter the effective pool size and the probability of release (Nili et al 2006;Toonen et al 2006;Toonen and Verhage 2007). Munc13, a calciumsensitive presynaptic protein that regulates the priming and probability of release of vesicles, has been shown to influence short-term synaptic plasticity during and following tetanic activation (Brose and Rosenmund 2002;Junge et al 2004;Beierlein et al 2007;Shin et al 2010).…”

Munc18-1 redistributes in nerve terminals in an activity- and PKC-dependent manner

“…The SM protein Munc18-1 is a soluble protein initially found as an interacting partner of syntaxin-1 (Hata et al, 1993;Garcia et al, 1994;Pevsner et al, 1994). Deletion of munc18-1 completely arrests synaptic transmission , whereas increased Munc18-1 levels result in a larger readily releasable vesicle pool (RRP) and increased synaptic efficacy (Toonen et al, 2006b). Mutations in munc18-1 are found in patients with epilepsy and intellectual disability, and Munc18-1 dysregulation is implicated in Alzheimer's disease and schizophrenia (Jacobs et al, 2006;Saitsu et al, 2008;Hamdan et al, 2009;Milh et al, 2011;Vatta et al, 2012;Mastrangelo et al, 2013;Urigüen et al, 2013).…”