During the synaptic vesicle fusion at the active zone a fusion pore is formed that results in a formation of an Ω-shape intermediate structure (Ω-profile) at the plasma membrane for releasing contents, followed by closure (called kiss-and-run) or merging of the Ω-profile into the plasma membrane (called full fusion). Ω-profile closure limits vesicular content release and cargo delivery, but recycles vesicles economically. In contrast, Ω-profile merging allows for rapid, complete content release and cargo delivery, but couples exocytosis to classical endocytosis, involving membrane invagination, Ω-profile formation and fission, for retrieving merged vesicles. In other words, Ω-profile merging defines the mode of fusion (full fusion instead of kiss-and-run) and the mode of endocytosis (classical endocytosis instead of kiss-and-run) [1-3]. Despite these fundamental roles, the mechanism underlying Ω-profile merging is unclear in neurons, in which vesicles are less then 50 nm in diameter and fusion takes place rapidly after calcium influx.
Giant presynaptic nerve terminals in lamprey that allow intracellular microinjections of active compounds were used in our experiments to investigate the role of actin dynamics during the synaptic vesicle fusion in synapses. Compounds perturbing the actin dynamics, such as latrunculin A and cytochalasin D, phalloidin and non-dissociable profilin-actin directly tagged with fluorescence or along with a fluorescent carrier to monitor microinjections, were introduced into giant synapses stimulated at 5 Hz and 20 Hz and studied by confocal and electron microscopy. Phalloidin labeling was observed at the synaptic active zone and a large number of omega-shaped membrane invaginations with dimensions corresponded to the size of synaptic vesicles and larger were observed at active zones in synapses microinjected with compounds disrupting actin polymerization as compared to control synapses, injected with dextran or GST and stimulated at the same rate (Fig. 1). In synapses simulated at 20 Hz a larger number of vesicles was observed at active zones at sites of microinjection supporting that SV release was inhibited.
Our results indicate that actin dynamics is involved at stages when synaptic vesicle merges with the active zone and when its membrane relocates to the periactive zone for endocytosis.
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 Wu,L.G., Hamid,E., Shin,W., & Chiang,H.C. Exocytosis and endocytosis: modes, functions, and coupling mechanisms. Annu. Rev. Physiol 76, 301-331 (2014).
 Jahn,R. & Fasshauer,D. Molecular machines governing exocytosis of synaptic vesicles. Nature 490, 201-207 (2012).
To cite this abstract:Gianvito Arpino, Tuomas Näreoja, Elena Sopova, Oleg Shupliakov; Actin-dependent mechanisms during vesicle fusion link exo- and endocytosis in synapses. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/actin-dependent-mechanisms-during-vesicle-fusion-link-exo-and-endocytosis-in-synapses/. Accessed: October 21, 2021
EMC Abstracts - https://emc-proceedings.com/abstract/actin-dependent-mechanisms-during-vesicle-fusion-link-exo-and-endocytosis-in-synapses/