Reduction of SNAP-25 expression impacts on neuronal calcium currents and network activity

SNAP25 is a member of the SNARE protein complex that participates in synaptic vesicle exocytosis. We previously demonstrated that differential SNAP25 expression in hippocampal neurons regulates intracellular calcium dynamics (Verderio et al., Neuron 2004) and that negative modulation of neuronal voltage-gated calcium channels requires SNAP25 activity-dependent phosphorylation on Ser187 (Pozzi et al., PNAS 2008). A dysregulation of calcium dynamics due to alterations of SNAP25 expression may thus lead to unbalances of neuronal network activity. In line with this possibility, alterations of SNAP25 expression have been associated to several neurological diseases, such as schizophrenia, ADHD and epilepsy (reviewed in Corradini et al., 2009). In this study we used SNAP25+/- mice to investigate whether reduction of SNAP25 expression induce altered network activity in vitro and in vivo, possibly leading to neurological phenotypes. A significant increase in calcium responsiveness to depolarization was observed in SNAP25+/- neurons as well as in synaptosomes from heterozygous mice relative to wild-type. No structural changes or alterations in protein expression were observed either in developing (P2) or adult (P40) brains, analyzed by immunohistochemistry and western blotting. Behavioural analysis of mice revealed lack of schizophrenic phenotype, as indicated by the absence of differences in prepulse inhibition of the acoustic startle response (PPI) test. Similarly, the elevated plus-maze and 0-maze tests revealed the absence of anxious behaviour. However, the occurrence of partial defects in learning were revealed in heterozygous mice by passive avoidance, object recognition and T maze tests . Heterozygous mice also presented an increased spontaneous motor activity by activity cage in comparison with wild-type mice; furthermore, in vivo electroencephalographic recordings from both cortex and hippocampus showed a significant increase in the number of spikes in SNAP25 +/- mice. Treatment of SNAP25+/- mice with antiepileptic drugs, such as ethosuximide and valproic acid, caused a significant reduction in the number of cortical spikes recorded by EEG. Finally, heterozygous mice showed a high susceptibility to kainate-induced seizures and a strong sprouting of mossy fibers, which is a marker of spontaneous epileptogenesis in mice after kainate injection. These data suggest that alterations of SNAP25 expression may contribute to epilepsy, possibly by dysregulating the normal calcium dynamics and thus altering network activity.