Abstract
Exosomes are small extracellular vesicles with a diameter of 40â150 nm, and are implicated in cellular homeostasis and cellâcell communication. They can be secreted in bulk in response to cell-extrinsic and cell-intrinsic signals that cause multivesicular body (MVB) fusion with the plasma membrane (PM). However, research on the regulation of exosome release is hampered by the failure of current methods to capture the dynamics of exosome release. Here we describe how live imaging with tetraspanin-based pH-sensitive fluorescent reporters can quantify the MVBâPM fusion rate of single cells. Our approach enables identification of exogenous stimuli, signaling pathways, and fusion complexes, and can map subcellular sites of fusion events. In addition, dual-color imaging can be used to assess simultaneous release of different cargo by MVB exocytosis. This protocol describes the complete imaging experiment, consisting of transient expression of tetraspanin reporters (2 d), live-cell (dual-color) total internal reflection fluorescence microscopy (30â60 min per condition), and semiautomatic image analysis by using a newly developed ImageJ macro (±30 min per condition).
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Data availability
The data that support this study are available from the corresponding author upon reasonable request.
Code availability
The AMvBE macro, a Readme file, and example data can be found in Supplementary Software 1. The software in this protocol has been peer reviewed.
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Acknowledgements
We acknowledge the NeurImag facility of the Institute of Psychiatry and Neuroscience of Paris where the AMvBE macro has been developed. This Protocol was funded by a Dutch Organizations for Scientific ResearchâAmsterdam Institute for Molecules, Medicines, and Systems STAR Graduate Program grant (022.005.031) to M.P.B., a Dutch Cancer Fund (KWF-5510) and a Cancer Center AmsterdamâVU University Medical Center grant to D.M.P., and a European Molecular Biology Organization grant (EMBO ALTF 1383-2014) and a Fondation ARC pour la Recherché sur le Cancer fellowship (PJA 20161204808) to F.J.V.
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M.P.B. and F.J.V. performed the experiments, prepared the figures, and wrote the paper. G.v.N., P.B., S.H., and D.M.P. provided critical feedback and helped to shape the paper. P.B. developed the AMvBE macro. S.H. helped to set up the dual-color TIRF experiments. D.M.P. conceived the study. F.J.V. and D.M.P. supervised the study.
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Peer review information Nature Protocols thanks David Perrais and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Key references using this protocol
Verweij, F. J. et al. J Cell Biol, 217 (3) 1129â1142 (2018): http://jcb.rupress.org/content/217/3/1129/
Supplementary information
Supplementary Software 1
AMvBE macro, Readme file and example data
Supplementary Video 1
TIRF microscopy of a CD63-pHluorin expressing HeLa cell at 8à speed. Scale bar, 20 μm
Supplementary Video 2
Dual-color TIRF microscopy of a CD63-pHluorin (green) and CD81-pHuji (red) expressing HeLa cell at 8à speed. Scale bar, 20 μm
Supplementary Video 3
Dual-color TIRF microscopy of a CD63-pHluorin (green) and CD63-mRFP (red) expressing HeLa cell at 8à speed. Scale bar, 20 μm. White arrows highlight MVBs containing CD63-pHluorin and CD63-mRFP that are visible in red before fusion
Supplementary Video 4
Dual-color TIRF microscopy of a CD63-C-term-pHluorin (green) and CD63- pHuji (red) expressing HeLa cell at 8à speed. Scale bar, 20 μm. Original source: ref. 17
Supplementary Video 5
TIRF microscopy of HeLa cells stably expressing CD63-pHluorin 6 weeks post-transduction at 8à speed. Scale bar, 20 μm
Supplementary Video 6
TIRF microscopy of a CD63-pHluorin expressing HUVEC cell at 8à speed. Scale bar, 20 μm
Supplementary Video 7
TIRF microscopy of a CD63-pHluorin expressing HEK293T cell at 8à speed. Scale bar, 20 μm
Supplementary Video 8
Example of the summary TIFF file as created by the AMvBE macro upon analysis of an MVB-PM fusion event (highlighted by a green circle) or a neutral CD63-pHluorin-positive vesicle moving in the TIRF plane (highlighted by a red circle). Scale bar, 5 μm
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Bebelman, M.P., Bun, P., Huveneers, S. et al. Real-time imaging of multivesicular bodyâplasma membrane fusion to quantify exosome release from single cells. Nat Protoc 15, 102â121 (2020). https://doi.org/10.1038/s41596-019-0245-4
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DOI: https://doi.org/10.1038/s41596-019-0245-4