Cooperative regulation of neurotransmitter release by Rab3a and synapsin II

Abstract

To understand how the presynaptic proteins synapsin and Rab3a may interact in the regulation of the synaptic vesicle cycle and the release process, we derived a double knockout (DKO) mouse lacking both synapsin II and Rab3a. We found that Rab3a deletion rescued epileptic-like seizures typical for synapsin II gene deleted animals (SynII(−)). Furthermore, action potential evoked release was drastically reduced in DKO synapses, although spontaneous release remained normal. At low Ca2+ conditions, quantal content was equally reduced in Rab3a(−) and DKO synapses, but as Ca2+ concentration increased, the increase in quantal content was more prominent in Rab3a(−). Electron microscopy analysis revealed that DKO synapses have a combined phenotype, with docked vesicles being reduced similar to Rab3a(−), and intraterminal vesicles being depleted similar to SynII(−). Consistently, both SynII(−) and DKO terminals had increased synaptic depression and incomplete recovery. Taken together, our results suggest that synapsin II and Rab3a have separate roles in maintaining the total store of synaptic vesicles and cooperate in promoting the latest steps of neuronal secretion.

Synapsin II desynchronizes neurotransmitter release at inhibitory synapses by interacting with presynaptic calcium channels

“…We cannot exclude that the interaction between Syn II and Cav-2.1 type Ca 2+ -channels is indirect and mediated by modulatory proteins of the active zone, such as via the Rab3-RIM-Ca 2+ -channel pathway. This hypothesis is supported by the fact that Syns interact with Rab35152 and may thereby interfere with RIM binding to N and P/Q-type Ca 2+ channels at the active zone53, decreasing the efficiency, speed and synchrony of release. As Syn II appears to bind more tightly to SVs than Syn I (F. Benfenati, unpublished observations), it is possible that Syn II remains to a large extent attached to SVs during the post-docking steps of release, possibly acting as a brake for the synchronous release of SVs docked in the vicinity of a specific Ca 2+ channel.…”

Inhibition of exocytosis or endocytosis blocks activity‐dependent redistribution of synapsin

“…1995; Baldelli et al. 2007; Coleman and Bykhovskaia 2010); and (v) finally, perturbing synapsin redistribution by pharmacological means (Menegon et al. 2006) or by mutagenesis of its phosphorylation sites (Chi et al.…”

Synapsin II Directly Suppresses Epileptic Seizures In Vivo

“…These studies have shown that the seizure progression typically involves forelimb myoclonus, followed by hindlimb myoclonus/clonus and truncus tonic-clonic activity (epistotonus, emprostotonus, and emprostoclonus), which is sometimes followed by a running fit. Since it was shown that seizures in Syn2 KO adult mice can be provoked by gently lifting a mouse by the tail [20,21,27], we employed the tail suspension (TS) method [28]. Each TS mouse was individually videotaped over the course of 3 min.…”

“…Seizures were provoked by lifting mice by the tail, since earlier studies showed that this protocol robustly induces generalized tonic-clonic seizures in Syn2 KO mice [19][20][21]27], resulting in seizures typically involving forelimb myoclonus followed by hindlimb myoclonus/clonus and truncus tonic-clonic activity. To quantify seizure activity, we videotaped the mice over the course of 3 min tail suspensions (TS) [28] and determined the percentage of time displaying seizures.…”