Epileptiform Activity and Cognitive Deficits in SNAP-25+/− Mice are Normalized by Antiepileptic Drugs

Abstract

Synaptosomal-associated protein of 25 kDa (SNAP-25) is a protein that participates in the regulation of synaptic vesicle exocytosis through the formation of the soluble NSF attachment protein receptor complex and modulates voltage-gated calcium channels activity. The Snap25 gene has been associated with schizophrenia, attention deficit hyperactivity disorder, and bipolar disorder, and lower levels of SNAP-25 have been described in patients with schizophrenia. We used SNAP-25 heterozygous (SNAP-25(+/-)) mice to investigate at which extent the reduction of the protein levels affects neuronal network function and mouse behavior. As interactions of genotype with the specific laboratory conditions may impact behavioral results, the study was performed through a multilaboratory study in which behavioral tests were replicated in at least 2 of 3 distinct European laboratories. Reductions of SNAP-25 levels were associated with a moderate hyperactivity, which disappeared in the adult animals, and with impaired associative learning and memory. Electroencephalographic recordings revealed the occurrence of frequent spikes, suggesting a diffuse network hyperexcitability. Consistently, SNAP-25(+/-) mice displayed higher susceptibility to kainate-induced seizures, paralleled by degeneration of hilar neurons. Notably, both EEG profile and cognitive defects were improved by antiepileptic drugs. These results indicate that reduction of SNAP-25 expression is associated to generation of epileptiform discharges and cognitive dysfunctions, which can be effectively treated by antiepileptic drugs.

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

“…Such a decrease was accompanied by a reduction in the number of mushroom spines and an increase in the number of thin spines ( Figure 1c). Adult Het mice show similar but minor defects in spine density and morphology (Supplementary Figure S1D and E), probably due to the higher protein level 25 in the adult Het animals with respect to the SNAP-25 shRNA injected hippocampi.…”

“…We validated SNAP-25 knockdown in mouse primary neuronal cultures by real-time PCR analysis (Supplementary Figure S1B). To functionally confirm in vivo the SNAP-25 shRNA efficacy, we took advantage of the previous demonstration that SNAP-25 heterozygous mice show defective conditioned test aversion (CTA) memory 25 and that SNAP-25 immunoreactivity increases in the gustatory cortices in response to food and chemical stimulation. 26 We then used CTA as a paradigm test to provide the functional demonstration that acute downregulation of SNAP-25 is effective and induces the same behavioral defects already described in transgenic mice.…”

“…26 We then used CTA as a paradigm test to provide the functional demonstration that acute downregulation of SNAP-25 is effective and induces the same behavioral defects already described in transgenic mice. 25 We therefore injected the indicated vectors into the gustatory cortex, which resides within the insular cortex, and CTA memory was tested 7 days after. The scrambled-injected rats displayed normal CTA 27 while SNAP-25 shRNA-injected rats were impaired (Supplementary Figure S1C), thus indicating that acute reduction of the protein recapitulates the functional defects already shown in SNAP-25 heterozygous mice.…”

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

“…Notably, at 21 DIV, when SNAP-25 expression was significantly increased, differences in evoked responses (eEPSC and eIPSC) and in shortterm plasticity disappeared (eEPSC (nA): wt ¼ 0.360 ± 0.023, Fig 2A), indicating that neurotransmission and short-term plasticity defects can be fully recovered in parallel with increases in protein expression. The milder phenotype observed in het hippocampal slices, as compared with cultured neurons, might be owing therefore to upregulation of protein expression, which occurs also in vivo during postnatal development [32]. Notably, the recovery of the presynaptic altered phenotype in het neurons at 21 DIV is associated with the development of postsynaptic defects as indicated by the significantly lower amplitude of eEPSCs at 21 DIV (Fig 3I), which is accompanied by reduced PSD-95 density and altered spine maturation (Fossati et al, unpublished work).…”

Mutant SNAP25B causes myasthenia, cortical hyperexcitability, ataxia, and intellectual disability

“…Whereas Snap25 2/2 mice die at birth of respiratory failure, heterozygous (Snap25 1/2 ) mice develop cognitive deficits and cortical hyperexcitability. 33 Still other studies indicate that prenatal replacement of the mature Snap25b isoform with Snap25a isoform results in developmental defects, seizures, and impaired short-term plasticity, while the introduction of the isoform change in adult mice results in severe learning impairment, morphologic changes in hippocampus, and altered neuropeptide expression. 34 Finally, in Drosophila melanogaster with a wild-type background, a heterozygous Arg206Ala (198 in vertebrates) mutation in Snap25b at a botulinum toxin A cleavage site curtails the MEPP frequency at the neuromuscular junction and is predicted to hinder contacts within rosettes of SNARE complexes that assemble to effect fusion of the synaptic vesicle with the presynaptic membrane.…”