I got an early start this morning. P icked up a tea (compliments of the student hospitality room) and a croissant and made my way to the Nanosymposium on Presynaptic Mechanisms
A. Denker’s talk: Synaptic vesicle recycling in vivo
She started by stating that although it is reported that ~500,000 vesicles can occupy the terminals in neuromuscular junctions (NMJ) her data suggests that only ~400 are used per actionpotential. Her data also supports the consensus of the 60s recycling time (fairly slow) for the synaptic vesicle. Using an endocytic dye FM1-43, she concluded that in the NMJ in many animals only about 2-5% of the vesicles were used over the course of the experiment (a few hours). She also looked at nerve terminals in C. Elegans, insects and mice and observed the same thing. Even under ‘high stress’ life AND death situations (in insects) 95% of the vesicles remained.
I am curious to know how this relates to the ‘kiss and run’ theory and if the contents (neurotransmitters) in the vesicles are constantly being secreted recycled without so much of the membrane being recycled. The resulting ‘diluted’ content may not be as readily detected by their methods (ie, density or quantification of the ROS measured inside the vesicle). Technique concerns aside, the question remains: what are the other unused vesicles doing? Acting as reservoir? Or storage? For a published review, click here.
R. Sinha’s Talk: Stochiometry of SNARE complexes sufficient for fast calcium-triggered exocytosis in central nervous system synapses
Speaking of vesicles, Sinha talked about the number of SNARE proteins needed for vesicle fusion. The current theory is anywhere from 1-15. The authors observed single vesicle fusion events using a genetically encoded probe, synaptopHluorin (spH). It is a pH-sensitive GFP fused to the luminal domain of synaptobrevin. Their single molecule experiments revealed that only 2-3 spH molecules are incorporated per synaptic vesicle. Thus, two SNARE complexes are necessary and sufficient for SV fusion during fast synaptic transmission.