SNAP-25 negatively modulates voltage-gated calcium channels and network activity

SNAP25 is a member of the SNARE proteins that participates in synaptic vesicle exocytosis. We previously demonstrated that differential SNAP25 expression in hippocampal neurons regulates intracellular calcium dynamics (Verderio C. et al., Neuron 2004) and that negative modulation of neuronal voltage-gated calcium channels requires SNAP25 activity-dependent phosphorylation on Ser187 (Pozzi D. et al., PNAS 2007). A dysregulation of calcium dynamics due to alterations of SNAP25 expression may thus lead to unbalances of neuronal network activity. In line with these data, alterations of SNAP25 expression have been associated to several neurological diseases, such as schizophrenia (Young et al., 1998; Thompson et al., 1998 and 2003; Lewis et al., 2003; Jeans et al., 2007) and ADHD (Faraone S.V. et al., 2006; Kim J.W. et al., 2007; Choi T.K. et al., 2007; Renner T.J. et al., 2008). In this study we aim to investigate whether reduction or lack of SNAP25 may lead to altered network activity in vitro and in vivo. This possibility, previously raised in the coloboma mice model (Hess E.J. et al., 1992 and 1996; Zhang Y. et al., J Neurosci 2004), has now been tested using SNAP+/- mice, which, differently from coloboma, display selective reduction of this protein. A significant increase in calcium responsiveness to depolarization was observed in SNAP25 +/- and -/- neurons in comparison with wild-type neurons; this result was confirmed in synaptosomes from adult wild-type and heterozygous mice, while no effect was observed on Glutamate and GABA release. Behavioural tests on wild-type and SNAP25-heterozygous mice revealed an increased spontaneous motor activity and a deficit in long term memory in SNAP25+/- mice in comparison with wild-type mice. In vivo electroencephalographic recordings showed a significant increase in the number of spikes in SNAP25 +/- mice. Moreover, these 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 the pathology epilepsy and other neuropsychiatric disorders, possibly by dysregulating the normal calcium dynamics and thus altering network activity.