Tracking Exosomes in Vitro and in Vivo To Elucidate Their Physiological Functions: Implications for Diagnostic and Therapeutic Nanocarriers

Exosomes are nanosized extracellular vesicles (EVs) secreted by a variety of cell types and widely distributed in body fluids. With a lipid-bilayer membrane encapsulating genetic and proteomic information, exosomes play an important role in cell-to-cell communication. As newly emerged EVs, exosomes have gathered wide scientific and clinical research interest owing to their important functions under both physiological and pathological conditions. Furthermore, because some bioactive cargo, e.g., mRNA, miRNA, and proteins, is specifically sorted into exosomes from parent cells, noninvasive analysis of the molecular profiles of circulating exosomes may reveal potential biomarkers for disease diagnosis and prognosis. Moreover, inherent features of stability, low immunogenicity, targeted delivery, and the ability to overcome natural barriers facilitate the potential for exosomes to serve as drug-delivery vehicles for cancer, neurodegenerative diseases, and regenerative medicine. In practice, the further investigation and application of exosomes requires accurate knowledge of their specific metabolic pathways in vitro and in vivo, such as their release from parent cells and uptake by recipient cells, tissue distribution, and body liquid levels. The establishment of exosomes tracking protocols can effectively solve this problem. Using cells or other enclosed vesicles as reference, exosomes labeling and tracking strategies first adopted fluorescence microscopy, following which bioluminescence imaging, magnetic resonance imaging, and computed tomography were also successively applied to investigate the exosomes function in realistic pathophysiological contexts. In this review, we first introduce the biogenesis and composition of exosomes, together with two other kinds of EVs, microvesicles and apoptotic bodies, and then discuss their applications in the field of diagnosis and therapeutics. Subsequently, the latest developments of exosomes tracking techniques are reviewed. Finally, we present the remaining challenges and further perspectives in this exciting and promising field.