Real-time imaging of multivesicular body–plasma membrane fusion to quantify exosome release from single cells

Exosomes are small extracellular vesicles with a diameter of 40–150 nm, and are implicated in cellular homeostasis and cell–cell 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 MVB–PM 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 (30–60 min per condition), and semiautomatic image analysis by using a newly developed ImageJ macro (±30 min per condition).