Synapsin II Directly Suppresses Epileptic Seizures In Vivo

Ryan Schwark,Rodrigo Andrade,Maria Bykhovskaia

doi:10.3390/brainsci12030325

Abstract

The synapsin family offers a strong linkage between synaptic mechanisms and the epileptic phenotype. Synapsins are phosphoproteins reversibly associated with synaptic vesicles. Synapsin deficiency can cause epilepsy in humans, and synapsin II (SynII) in knockout (KO) mice causes generalized epileptic seizures. To differentiate between the direct effect of SynII versus its secondary adaptations, we used neonatal intracerebroventricular injections of the adeno-associated virus (AAV) expressing SynII. We found that SynII reintroduction diminished the enhanced synaptic activity in Syn2 KO hippocampal slices. Next, we employed the epileptogenic agent 4-aminopyridine (4-AP) and found that SynII reintroduction completely rescued the epileptiform activity observed in Syn2 KO slices upon 4-AP application. Finally, we developed a protocol to provoke behavioral seizures in young Syn2 KO animals and found that SynII reintroduction balances the behavioral seizures. To elucidate the mechanisms through which SynII suppresses hyperexcitability, we injected the phospho-incompetent version of Syn2 that had the mutated protein kinase A (PKA) phosphorylation site. The introduction of the phospho-incompetent SynII mutant suppressed the epileptiform and seizure activity in Syn2 KO mice, but not to the extent observed upon the reintroduction of native SynII. These findings show that SynII can directly suppress seizure activity and that PKA phosphorylation contributes to this function.

Highlights

Epilepsy is a complex and multifaceted disorder, and it has been established that changes in the presynaptic function can contribute to the etiology of epilepsy [1]
These findings show that synapsin II (SynII) can directly suppress seizure activity and that protein kinase A (PKA) phosphorylation contributes to this function
This hypothesis was further supported by the discovery that Syn II maintains the asynchronous component of inhibitory transmission [38], which contributes to balancing seizure activity [39]

Summary

Introduction

Epilepsy is a complex and multifaceted disorder, and it has been established that changes in the presynaptic function can contribute to the etiology of epilepsy [1]. Some forms of epilepsy have been linked to deficiencies in synapsin function [2]. The synapsins are a family of neuronal phosphoproteins abundantly expressed in the brain [3]. They are encoded by three genes (Syn, Syn, and Syn3) [4] and are concentrated in synaptic terminals. Within synaptic terminals, they reversibly associate with synaptic vesicles and play key roles in vesicle clustering and mobilization for the release process [5–8]. The synapsin function depends on its phosphorylated state, which is regulated by multiple kinases and phosphatases [6]. Synapsin binding to and dissociation from vesicles are mediated through phosphorylation at a highly conserved site P1 [6], a target for protein kinase A (PKA) and calcium/calmodulin kinase I (CaMKI) [9]

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