DUAL ROLE OF SNAP-25 AT PRE- AND POST-SYNAPTIC LEVEL DURING DEVELOPMENT

Impairment of synaptic function can lead to neurological and psychiatric disorders collectively referred to as synaptopathies. SNAP-25, a SNARE protein controlling synaptic vesicle exocytosis and fundamental presynaptic functions, is implicated in several brain pathologies and, indeed, brain areas of psychiatric patients often display reduced SNAP-25 expression. We observed that halved SNAP-25 levels at 13–14DIV not only fail to impair synaptic transmission, but instead enhance evoked glutamatergic neurotransmission. This effect is probably dependent on presynaptic voltage-gated calcium channel activity and it is not followed by changes in spontaneous quantal events or in the pool of readily releasable synaptic vesicles. Notably, synapses of neurons with reduced SNAP-25 levels show paired-pulse depression as opposed to paired-pulse facilitation occurring in their wild-type counterpart. These phenotypes disappear with synapse maturation, where instead a reduction of evoked glutamatergic transmission and mEPSC amplitude emerge in heterozygous neurons thus suggesting the onset of a postsynaptic defect. In fact, it has been recently reported that a peculiar postsynaptic SNARE complex is required for long-term potentiation; however, the role of SNAP-25 in this process is not completely understood. We recently demonstrated that acute down-regulation of SNAP-25 in vitro affects spine morphogenesis through binding to p140Cap, thus suggesting that the protein may exert a structural role at the postsynaptic level. Here we demonstrate that in vivo acute down-regulation of SNAP-25 in CA1 hippocampal neurons affects spine number and morphology and causes a specific reduction of the postsynaptic protein PSD-95. Consistently, hippocampal neurons from SNAP-25 het mice show a flawed maturation of postsynaptic specializations, reduced densities of dendritic spines and defective PSD-95 clustering. These effects do not stem from impaired presynaptic function, but as a direct consequence of reduced SNAP-25 levels in the postsynaptic compartment. By co-immunoprecipitation and LUMIER Assay, we show that SNAP-25, PSD-95 and p140Cap are part of the same molecular complex in the brain, with p140Cap being intrinsically capable to bind either to SNAP-25 and PSD-95. These data provide new mechanistic insights as to SNAP-25 involvement in synaptopathies that go beyond the protein’s known roles in presynaptic function.