The dynamics of SNARE complex assembly and disassembly in response to Ca ²⁺ current activation in live chromaffin...

A SNAP25 based FRET construct named SCORE (SNARE COmplex REporter) has revealed a transient FRET increase that specifically occurred at fusion sites preceding fusion events by tens of milliseconds and presumably reflects vesicle priming. The FRET increase lasts for a few seconds until it is reversed. In those experiments, the FRET increase was found to be localized to areas <0.5 µm ² at sites of transmitter release as detected amperometrically using electrochemical detector arrays. Due to the localization to such small areas, it was unknown if the reversal of the FRET increase is due to local dispersion of high-FRET SCORE copies leaving the site after fusion and exchange with surrounding low-FRET copies, or if it reflects disassembly of the high-FRET complexes. To resolve this question, we performed whole-cell patch clamp pulse stimulation experiments, imaging the entire footprint of the cells in Total Internal Reflection Fluorescence (TIRF) excitation mode such that diffusional exchange between high-FRET and low-FRET copies does not produce a net FRET change. We show here that pulse stimulation of calcium currents results in FRET ratio transients with a time course very similar that related to fusion events. By comparing the kinetics of the FRET ratio decay with analytical and numerical diffusion simulation results, we show that the experimentally observed kinetics cannot be explained by diffusional exchange and conclude that the SCORE FRET ratio transients reflect incorporation of SCORE in SNARE complexes followed by SNARE complex disassembly. Experiments using Synaptobrevin 2/Cellubrevin double knock-out mouse embryonal chromaffin cells showed no pulse induced FRET change, indicating that the vSNARE is required for the incorporation of SCORE (or SNAP25 in wild type cells) in the SNARE complex during priming.

In chromaffin cells, SNAP25-based FRET constructs (SCORE) revealed transient FRET increases within <0.5 µm ² areas, preceding individual fusion events that reversed within seconds. It remained unknown whether this reversal stems from high-FRET complex disassembly or diffusion-mediated exchange with low-FRET complexes. Here, we performed whole-cell patch-clamp pulse stimulation with TIRF microscopy, imaging large ∼30 µm ² areas of the cell footprint. Calcium currents induced FRET transients with the same decay time constant of ∼1.5 s, significantly shorter than the time scale of diffusion. The SCORE FRET ratio thus reports in real time the dynamics of SNRE complex assembly and disassembly in live cells. Using Synaptobrevin 2/Cellubrevin double knock-out mouse chromaffin cells we show that vSNAREs are required for SNAP25 incorporation into SNARE complexes during priming.