Lipoprotein Association Fluorometry (LAF) as a Semi-Quantitative Characterization Tool to Assess Extracellular Vesicle-Lipoprotein Binding

Extracellular vesicles (EVs) are biological nanoparticles that play important roles in (patho)physiological processes and are promising new therapeutic and diagnostic tools. Recent evidence suggests that other circulating biological nanoparticles, primarily lipoproteins, bind to EVs, changing their biological identity. Such binding has been demonstrated with complex qualitative techniques, such as cryogenic transmission electron microscopy. There is a need to rapidly and simply quantify EV-lipoprotein binding, as such complexes could have major implications for EV biology and medical applications. This study developed lipoprotein association fluorometry (LAF; based on fluorescent lipophilic indocarbocyanine dyes), as a first-of-its-kind, simple and quick assay to assess EV binding to lipoproteins. The LAF assay was validated with synthetic nanoparticles, small molecules, polymers and proteins that display known interactions with lipoproteins. The LAF assay demonstrates that EVs from various human and non-human (nematode and bacteria) sources bind to very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL). Notably, EVs derived from cancerous cells displayed substantially increased binding to VLDL, LDL and plasma compared to EVs from normal cells. Additionally, the LAF assay revealed that EVs from metastatic cancer cells bound to VLDL to a greater extent than those from corresponding patient-matched non-metastatic cancer cells. On the contrary, EVs displayed minimal binding to high-density lipoprotein (HDL). Taken together, the LAF assay is capable of measuring EV-lipoprotein binding in a simple, rapid and semi-quantitative manner, leading to new opportunities to probe EV biology and develop novel therapeutics, and diagnostics.