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

Abstract

SNAP-25 is a key component of the synaptic-vesicle fusion machinery, involved in several psychiatric diseases including schizophrenia and ADHD. SNAP-25 protein expression is lower in different brain areas of schizophrenic patients and in ADHD mouse models. How the reduced expression of SNAP-25 alters the properties of synaptic transmission, leading to a pathological phenotype, is unknown. We show that, unexpectedly, halved SNAP-25 levels at 13-14 DIV not only fail to impair synaptic transmission but instead enhance evoked glutamatergic neurotransmission. This effect is possibly dependent on presynaptic voltage-gated calcium channel activity and is not accompanied by changes in spontaneous quantal events or in the pool of readily releasable synaptic vesicles. Notably, synapses of 13-14 DIV neurons with reduced SNAP-25 expression show paired-pulse depression as opposed to paired-pulse facilitation occurring in their wild-type counterparts. This phenotype disappears with synapse maturation. As alterations in short-term plasticity represent a new mechanism contributing to cognitive impairments in intellectual disabilities, our data provide mechanistic clues for neuronal circuit alterations in psychiatric diseases characterized by reduced expression of SNAP-25.

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

“…Finally, miniature excitatory postsynaptic current (mEPSC) analysis in SNAP-25 Het networks revealed a significant reduction in both the frequency and the amplitude relative to controls (Figure 2i). Notably, no difference in mEPSC frequency and amplitude occurs in primary hippocampal cultures at earlier SNAP-25 in spine formation and function G Fossati et al developmental stages, 18,20 thus confirming that morphological and functional postsynaptic defects become evident only at later stages during neuronal development.…”

“…Immunocytochemical staining for the synaptic vesicle protein SV2A, for the active zone component Bassoon, and for the postsynaptic scaffold protein PSD-95 was performed at 14 and 21 days in vitro (DIV) and percentages of colocalization among these proteins were quantified. Since the levels of SV2A do not differ between wt and Het cultures, 20 we used this protein as a reference marker. The percentage of juxtaposed pre-and postsynaptic terminals relative to the total presynaptic sites (SV2A&PSD-95/SV2A), the percentage of synapses showing immunoreactivity for the all three markers (SV2A&PSD-95&Bsn/SV2A) and the percentage of mature presynaptic terminals (SV2A&Bsn/SV2A) were quantified (Figures 2a-c).…”

“…18 The mechanisms by which reduced SNAP-25 expression may result in a psychiatric disease are still undefined, although alterations in neurotransmitter release have been indicated as potential causative processes. 19,20 Recently an unexpected postsynaptic role of SNAP-25 has been described, with the protein controlling NMDA and kainate receptor trafficking. 21 Furthermore, it has been lately found that acute SNAP-25 downregulation results in LTP impairment 21,22 and that acute reduction of the protein affects spine morphogenesis in vitro, through a process involving the protein p140Cap.…”

VGLUT1/VGAT co-expression sustains glutamate-gaba co-release and is regulated by activity

“…; Antonucci et al, 2013) and paired-pulse depression (PPD) (PPR50.5245760.08087; Debanne et al, 1996), respectively, all mixed synapses exhibited PPD (PPR50.5859760.05726) (Fig. 1F).…”

“…1E by normalizing the 'mixed' evoked potentials to 'pure' glutamatergic (left) or 'pure' GABAergic (right) responses. 'Mixed' synapses displayed different functional properties, as revealed by the analysis of short term plasticity, where two synaptic responses are evoked by closely spaced presynaptic stimuli (Antonucci et al, 2013).…”