An Electrostatic Energy Barrier for SNARE-Dependent Spontaneous and Evoked Synaptic Transmission

Abstract

Graphical Abstract Highlights d The energy barrier for vesicle fusion depends on SNAREcomplex surface charge d Positive charges decrease and negative charges increase the energy barrier for fusion d Addition of 35 positive charges per SNARE-complex fuses vesicles with evoked rates d Synaptotagmin-1 acts as an electrostatic switch, adding 18 charges by binding to Ca 2+ SUMMARYInformation transfer across CNS synapses depends on the very low basal vesicle fusion rate and the ability to rapidly upregulate that rate upon Ca 2+ influx. We show that local electrostatic repulsion participates in creating an energy barrier, which limits spontaneous synaptic transmission. The barrier amplitude is increased by negative charges and decreased by positive charges on the SNAREcomplex surface. Strikingly, the effect of charges on the barrier is additive and this extends to evoked transmission, but with a shallower charge dependence. Action potential-driven synaptic release is equivalent to the abrupt addition of $35 positive charges to the fusion machine. Within an electrostatic model for triggering, the Ca 2+ sensor synaptotagmin-1 contributes $18 charges by binding Ca 2+ , while also modulating the fusion barrier at rest. Thus, the energy barrier for synaptic vesicle fusion has a large electrostatic component, allowing synaptotagmin-1 to act as an electrostatic switch and modulator to trigger vesicle fusion.

Post-tetanic potentiation lowers the energy barrier for synaptic vesicle fusion independently of Synaptotagmin-1

“…1) and Syt1 9Pro expressing synapses (Fig. 2) that mEPSC frequencies were increased, in line with previous studies in autapses [5,20,55] and networks [28,29,36,56]. It remains enigmatic why others found a similar increase in spontaneous release in networks but not in autapses after Syt1 deletion [57,58], but differences in culture protocol [59], or genetic background could play a role.…”

“…However, Syt1 is also know to act as an inhibitor of spontaneous and asynchronous release [15][16][17][18][19][20]28,29]. This inhibition may act directly on the fusion machinery [5,30,31], suggesting an increase in the fusion energy barrier [5]. Alternatively, Syt1 may inhibit a second high-affinity Ca 2+sensor [15,28], reducing sensitivity to local Ca 2+ -fluctuations [32][33][34], but likely not affecting the energy barrier.…”

“…We explored how the Syt1 D232N mutation could affect Ca 2+ sensitivity in this model. Based on proposed electrostatic effects on the fusion barrier [5], the effect of adding one charge to the C2A domain can be modelled by increasing the basal fusion rate…”

“…is under the assumption that one charge is half as effective as two charges during Ca 2+ binding [5], with ∆ the energy barrier reduction from one bound Ca 2+ (Fig. 7.…”

SNARE Complex Alters the Interactions of the Ca2+sensor Synaptotagmin 1 with Lipid Bilayers

“…Syt1 can interact with t-SNARE proteins, and multiple studies suggest an important role for Syt1-SNARE interactions during fusion [18][19][20][21][22][23]. However, other studies have argued against this possibility [24,25], and it is still debated how the Syt1-SNARE complex is formed in vivo and what is the role of the Syt1-SNARE interactions in the fusion process [26][27][28][29][30][31][32].…”