Extracellular vesicles (EVs) are important mediators of intercellular communication. Their role in disease processes, uncovered mostly over the last two decades, makes them potential biomarkers, leading to a need to fundamentally understand EV biology. Direct visualization of EVs can provide insights into EV behavior, but current labeling techniques are often restricted by false-positive signals and rapid photobleaching. Hence, we developed a method of labeling EVs through conjugation with quantum dots (QDs)-high photoluminescent nanosized semi-conductors-using click chemistry. We showed that QD-EV conjugation could be tailored by altering QD to EV ratio or by using a catalyst. This conjugation chemistry was stable in a biological environment and upon storage for up to a week. Using size-exclusion chromatography, QD-EV conjugates could be separated from unconjugated QDs, enabling EV-specific signal detection. We demonstrate that these QD-EV conjugates can be live- and fixed-imaged in high resolution on cells and in tissue sheets, and the conjugates have better photostability compared with the commonly used EV dye DiI. We labeled two distinct EV populations: human semen EVs (sEVs) from fresh semen samples donated by healthy volunteers and brain EVs (bEVs) from excised rat brain tissues. We visualized QD-sEVs in epithelial sheets isolated from human vaginal mucosa and time-lapse imaged QD-bEV interactions with microglial BV-2 cells. The development of the QD-EV conjugate will benefit the study of EV localization, movement, and function and accelerate their potential use as biomarkers, therapeutic agents, or drug-delivery vehicles.
Keywords: click chemistry; extracellular vesicles; labeling; live imaging; quantum dots.