Differential Effects of SNAP-25 Deletion on Ca2+-Dependent and Ca2+-Independent Neurotransmission

Department of Neuroscience, The University of Texas Southwestern Medical Center, Dallas, Texas 75390-9111, USA.
Journal of Neurophysiology (Impact Factor: 2.89). 09/2007; 98(2):794-806. DOI: 10.1152/jn.00226.2007
Source: PubMed


At the synapse, SNAP-25, along with syntaxin/HPC-1 and synaptobrevin/VAMP, forms SNARE N-ethylmaleimide-sensitive factor [soluble (NSF) attachment protein receptor] complexes that are thought to catalyze membrane fusion. Results from neuronal cultures of synaptobrevin-2 knockout (KO) mice showed that loss of synaptobrevin has a more severe effect on calcium-evoked release than on spontaneous release or on release evoked by hypertonicity. In this study, we recorded neurotransmitter release from neuronal cultures of SNAP-25 KO mice to determine whether they share this property. In neurons lacking SNAP-25, as those deficient in synaptobrevin-2, we found that approximately 10-12% of calcium-independent excitatory and inhibitory neurotransmitter release persisted. However, in contrast to synaptobrevin-2 knockouts, this remaining readily releasable pool in SNAP-25-deficient synapses was virtually insensitive to calcium-dependent-evoked stimulation. Although field stimulation reliably evoked neurotransmitter release in synaptobrevin-2 KO neurons, responses were rare in neurons lacking SNAP-25, and unlike synaptobrevin-2-deficient synapses, SNAP-25-deficient synapses did not exhibit facilitation of release during high-frequency stimulation. This severe loss of evoked exocytosis was matched by a reduction, but not a complete loss, of endocytosis during evoked stimulation. Moreover, synaptic vesicle turnover probed by FM-dye uptake and release during hypertonic stimulation was relatively unaffected by the absence of SNAP-25. This last difference indicates that in contrast to synaptobrevin, SNAP-25 does not directly function in endocytosis. Together, these results suggest that SNAP-25 has a more significant role in calcium-secretion coupling than synaptobrevin-2.

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    • "Neurons were used for experiments between 14 to 18 days in vitro. Dissociated hippocampal cultures from SNAP25 knockout mice and their wild-type littermates were generated from E17-20 embryos and were plated on poly-d-lysine coated coverslips as described previously (Bronk et al., 2007). Neurons were used for experiments 14–18 days in vitro. "
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    ABSTRACT: Spontaneous glutamate release-driven NMDA receptor activity exerts a strong influence on synaptic homeostasis. However, the properties of Ca(2+) signals that mediate this effect remain unclear. Here, using hippocampal neurons labeled with the fluorescent Ca(2+) probes Fluo-4 or GCAMP5, we visualized action potential-independent Ca(2+) transients in dendritic regions adjacent to fluorescently labeled presynaptic boutons in physiological levels of extracellular Mg(2+). These Ca(2+) transients required NMDA receptor activity, and their propensity correlated with acute or genetically induced changes in spontaneous neurotransmitter release. In contrast, they were insensitive to blockers of AMPA receptors, L-type voltage-gated Ca(2+) channels, or group I mGluRs. However, inhibition of Ca(2+)-induced Ca(2+) release suppressed these transients and elicited synaptic scaling, a process which required protein translation and eukaryotic elongation factor-2 kinase activity. These results support a critical role for Ca(2+)-induced Ca(2+) release in amplifying NMDA receptor-driven Ca(2+) signals at rest for the maintenance of synaptic homeostasis.
    eLife Sciences 07/2015; 4. DOI:10.7554/eLife.09262 · 9.32 Impact Factor
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    • "However, we found via DKO mice the roles of STX1 in Ca2+-triggered synchronous release and in neuronal viability. The phenotypes of the DKO neurons were similar to SNAP-25−/− neurons in several apparent aspects such as embryonic lethality, reduced neuron survival in culture and smaller amplitude of spontaneous events [10], [35], [36]. Cleavage of STX1 or SNAP-25 by botulinum neurotoxin induces degeneration of neurons, suggesting that STX1 and SNAP-25 cooperate to support neuron survival [37]. "
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    ABSTRACT: Two syntaxin 1 (STX1) isoforms, HPC-1/STX1A and STX1B, are coexpressed in neurons and function as neuronal target membrane (t)-SNAREs. However, little is known about their functional differences in synaptic transmission. STX1A null mutant mice develop normally and do not show abnormalities in fast synaptic transmission, but monoaminergic transmissions are impaired. In the present study, we found that STX1B null mutant mice died within 2 weeks of birth. To examine functional differences between STX1A and 1B, we analyzed the presynaptic properties of glutamatergic and GABAergic synapses in STX1B null mutant and STX1A/1B double null mutant mice. We found that the frequency of spontaneous quantal release was lower and the paired-pulse ratio of evoked postsynaptic currents was significantly greater in glutamatergic and GABAergic synapses of STX1B null neurons. Deletion of STX1B also accelerated synaptic vesicle turnover in glutamatergic synapses and decreased the size of the readily releasable pool in glutamatergic and GABAergic synapses. Moreover, STX1A/1B double null neurons showed reduced and asynchronous evoked synaptic vesicle release in glutamatergic and GABAergic synapses. Our results suggest that although STX1A and 1B share a basic function as neuronal t-SNAREs, STX1B but not STX1A is necessary for the regulation of spontaneous and evoked synaptic vesicle exocytosis in fast transmission.
    PLoS ONE 02/2014; 9(2):e90004. DOI:10.1371/journal.pone.0090004 · 3.23 Impact Factor
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    • "Although neurons from their wild-type littermates showed swift responses to Reelin application (Figure 4B), neurons lacking SNAP-25 (SNAP25 À/À ) showed no response to Reelin and had an overall lower mEPSC frequency (Figure 4C) (Bronk et al., 2007). Here, it is important to note that SNAP-25 deficient synapses respond to other secretagogues such as hypertonic sucrose, ionomycin, or a-latrotoxin (Bronk et al., 2007; Deá k et al., 2009). These data indicate that Reelin causes an increase in SV fusion frequency that requires the function of the plasma membrane-associated SNARE, SNAP-25, in agreement with an earlier study suggesting a SNAP-25-dependent role for Reelin in presynaptic function (Hellwig et al., 2011). "
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    Neuron 11/2013; 80(4). DOI:10.1016/j.neuron.2013.08.024 · 15.05 Impact Factor
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