Table 2 |.
Tagging strategies for EVs
| Strategy | Biogenesis | Secretion | Transfer | Biodistribution | uptake | Functional transfer | Subcellular resolution | Body-wide resolution | Microscopy techniques | Modality |
|---|---|---|---|---|---|---|---|---|---|---|
| Lipid dyes | ||||||||||
| PKH, MemBright, DiI, DiO, DiR | − | −/+ | + | + | + | − | + | −/+ | CM, SDM, BFM | Live, fixed |
| Radiolabels and metabolic labels | ||||||||||
| Radioisotopes (that is, 99mTc) | − | − | − | ++ | − | − | − | ++ | SPECT, PET | Live |
| Metabolic labeling (for example, glycan) | − | − | − | ++ | + | − | −/+ | ++ | CM, SDM, BFM | Live |
| Genetic labeling strategies | ||||||||||
| Protein fused to fluorescent protein (for example, TSPAN–XFP) | + | ++ | + | + | + | − | + | −/+ | IEM, CM, SDM, TIRFM, BFM | Live, fixed |
| Degron tagging | − | + | + | + | + | − | + | −/+ | CM, SDM, BFM | Live, fixed |
| Cre/loxP | − | − | − | −/+ | −/+ | ++ | − | + | CM, SDM, BFM | Live, fixed |
| APEX | + | − | − | − | + | + | + | − | EM | Fixed |
| Nanoluciferase | − | + | − | + | + | − | + | ++ | BLIM, IEM | Live, fixed |
Different labeling strategies are suitable for visualizing EV (subtype) biogenesis, secretion, transfer, biodistribution, uptake, and functional (cargo) transfer, as well as for live or fixed imaging at subcellular or body-wide resolution. −, unsuitable; −/+, low suitability; +, suitable; ++, highly suitable. BFM, bright-field microscopy; BLIM, bioluminescence imaging microscopy; CM, confocal microscopy; IEM, immuno-electron microscopy; PET, positron emission tomography; SDM, spinning disk microscopy; SPECT, single-photon emission computed tomography; TIRFM, total internal reflection fluorescence microscopy.