Secretion correlation between matrix metalloproteinases and extracellular vesicles revealed by synchronized dynamic monitoring of single cells on a microfluidic chip

Despite growing interest in matrix metalloproteinases (MMPs) as promising therapeutic targets for cancer treatments, cellular trafficking and release of MMPs remain underexplored. To evaluate to what content the MMP secretion is associated with extracellular vesicles (EVs), in this study, we report a droplet microfluidic assay for simultaneous dynamic monitoring of the MMP and EV secretion at the single-cell level. Microdroplets containing single cells were generated in a flow-focusing microchannel and then guided to a microcolumn array for on-chip patterning, incubation and time-lapse microscopy. Utilizing a green fluorescent substrate for MMP activity and a red fluorescent protein reporter for EVs, temporal secretion dynamics of MMPs and EVs from individual cells were recorded respectively. With this dual-reporter system, a strong correlation between MMP and EV secretion was revealed. We further demonstrated that promotion or inhibition of EV release by altering intracellular calcium levels would induce a rise or fall in single-cell MMP secretion, which was masked by population-based experiments. More importantly, the activated release of this EV subpopulation (exosomes) also resulted in a higher correlation coefficient between MMP and EV secretion, and vice versa. Therefore, compared to other EV subpopulations, exosomes should be more closely connected with MMP secretion, and thus may represent a potential target for combination therapy. When in contrast with bulk experiments, single-cell analysis of the secretion correlation between MMPs and EVs not only reveals extensive cellular heterogeneity, but also exhibits a higher resolution.