Optical detection of three modes of endocytosis at hippocampal synapses
Abstract
Coupling of synaptic vesicle fusion and retrieval constitutes a core mechanism ensuring maintenance of presynaptic function. Recent studies using fast-freeze electron microscopy and capacitance measurements reported an ultrafast mode of endocytosis operating at physiological temperatures. Here, using rat hippocampal neurons, we optically monitored single synaptic vesicle endocytosis with high time resolution using the vesicular glutamate transporter, synaptophysin and the V0a1 subunit of the vacuolar ATPase as probes. In this setting, we could distinguish three components of retrieval operating at ultrafast (~150-250 ms, ~20% of events), fast (~5-12s, ~40% of events) and ultraslow speeds (>20s, ~40% of events). While increasing Ca2+ slowed the fast events, increasing temperature accelerated their time course. In contrast, the kinetics of ultrafast events were only mildly affected by these manipulations. These results suggest that synaptic vesicle proteins can be retrieved with ultrafast kinetics, although a majority of evoked fusion events are coupled to slower retrieval mechanisms.
Data availability
All data generated or analysed during this study are included in the manuscript and supporting files as histograms or box plots.
Article and author information
Author details
Funding
National Institute of Mental Health (MH066198)
- Ege T Kavalali
The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
Reviewing Editor
- Axel T Brunger, Stanford University Medical Center, United States
Ethics
Animal experimentation: All animal protocols were approved by the Institutional Care and Use Committee at UT Southwestern Medical Center. The work presented in this study is covered by the Animal Protocol Numbers APN 2016-101416.
Version history
- Received: February 21, 2018
- Accepted: April 20, 2018
- Accepted Manuscript published: April 23, 2018 (version 1)
- Version of Record published: May 18, 2018 (version 2)
Copyright
© 2018, Chanaday & Kavalali
This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.
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Further reading
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- Neuroscience
Vesicles within presynaptic terminals are thought to be segregated into a variety of readily releasable and reserve pools. The nature of the pools and trafficking between them is not well understood, but pools that are slow to mobilize when synapses are active are often assumed to feed pools that are mobilized more quickly, in a series. However, electrophysiological studies of synaptic transmission have suggested instead a parallel organization where vesicles within slowly and quickly mobilized reserve pools would separately feed independent reluctant- and fast-releasing subdivisions of the readily releasable pool. Here, we use FM-dyes to confirm the existence of multiple reserve pools at hippocampal synapses and a parallel organization that prevents intermixing between the pools, even when stimulation is intense enough to drive exocytosis at the maximum rate. The experiments additionally demonstrate extensive heterogeneity among synapses in the relative sizes of the slowly and quickly mobilized reserve pools, which suggests equivalent heterogeneity in the numbers of reluctant and fast-releasing readily releasable vesicles that may be relevant for understanding information processing and storage.
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