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

Abstract

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.

Reduced SNAP‐25 alters short‐term plasticity at developing glutamatergic synapses

“…We recorded miniature excitatory (mEPSCs) or inhibitory (mIPSCs) currents (Fig 1A-B), holding neurons at the reversal potential for GABA (g-aminobutyric acid)-and glutamate-mediated responses (-70 and þ 5 mV, respectively) in the presence of 1 mM tetrodotoxin. Frequency and amplitudes of both mEPSCs and mIPSCs were not significantly different in het neurons relative to wt, in line with previous reports [19,3,18] Fig 1C,E). Conversely, a slight, although significant, reduction of evoked IPSC amplitude was recorded in het neurons relative to wt (0.52±0.03 versus 0.36±0.03 nA, wt versus het, Po0.05; Fig 1D,E).…”

SNAP-25 regulates spine formation through postsynaptic binding to p140Cap

“…SNAP-25 phosphorylation by Protein Kinase C (PKC) is of fundamental importance for the insertion of NMDA receptors in neuronal plasma membrane 7 . In line with the SNARE role in postsynaptic function, amplitude of mEPSCs was reported to be reduced in the absence of SNAP-25 46,47 , while exogenous expression of either SNAP-25a or SNAP-25b rescued the amplitude value 47 . Finally, a recent report showed that downregulation of the protein impairs potentiation phenomena 48 .…”

Reduced SNAP-25 increases PSD-95 mobility and impairs spine morphogenesis

“…55 Given that SNAP-25 has a critical role at the presynaptic terminal during synaptic vesicle fusion and considering the possible role of presynaptic function in the maturation of the postsynaptic compartment, the postsynaptic effects observed here might have been the result of SNAP-25-related alterations in presynaptic release, 7 even if electron microscopy studies showed that reduced levels of SNAP-25 do not alter the presynaptic terminal structure and vesicles content. 56 By taking advantage of mixed cultures of wt and Het neurons, we demonstrated that postsynaptic defects in In recent years SNARE/SM proteins have been shown to be involved in activity-dependent AMPA receptor exocytosis during LTP 22 and SNAP-25, in particular, has been reported to have a role in the removal of kainate receptors from the postsynaptic membrane, 57 in the insertion of NMDA receptors in neuronal plasma membrane 21 and of P/Q and L typevoltage gated calcium channels. 10,58 Also, acute regulation of SNAP-25 expression has been found to impair synaptic plasticity probably affecting NMDAR trafficking.…”