Exosomes are emerging as effective therapeutic tools for various pathologies. These extracellular vesicles can bypass biological barriers, including the blood-brain barrier, and can serve as powerful drug and gene therapy transporters. However, the progress of therapy development is impeded by several challenges, including insufficient data on exosome trafficking and bio-distribution, and the difficulty to image deep brain structures in vivo.
Here, Bar-Ilan University researchers established a method for non-invasive in vivo neuroimaging and tracking of exosomes, based on glucose-coated gold nanoparticle (GNP) labeling and computed tomography (CT) imaging. Labeling of exosomes with the GNPs was achieved directly, as opposed to the typical, and less efficient, indirect labeling mode through parent cells. On the mechanistic level, the researchers found that the glucose-coated GNPs were uptaken into MSC-derived exosomes via an active, energy-dependent mechanism that is mediated by the glucose transporter GLUT-1 and involves endocytic proteins. Next, they determined optimal parameters of size and administration route; they demonstrated that 5 nm GNPs enabled improved exosome labeling, and that intranasal, compared to intravenous, administration led to superior brain accumulation, and thus enhanced in vivo neuroimaging. Furthermore, using a mouse model of focal brain ischemia, the researchers non-invasively tracked intranasally administered GNP-labeled exosomes, which showed increased accumulation at the lesion site over 24 hrs, as compared to non-specific migration and clearance from control brains over the same period. Thus, this exosome labeling technique can serve as a powerful diagnostic tool for various brain disorders, and could potentially enhance exosome-based treatments for neuronal recovery.
Imaging of double-labeled exosomes
(a) Spectral unmixing image (Maestro) of brain induced with striatal stroke, 24 hrs post IN administration of exosomes (red). (b) ImageJ analysis of Spectral imaging; Color density, translated into intensity was analyzed in three regions: the cerebellum, the left hemisphere and the right hemisphere (where the stroke was induced). (c) ImageJ analysis of CT imaging; Color density, translated into intensity was examined in the same three regions as above. (d) Bright field microscopy of the ischemic striatum (representative section; magnification X40). (e) Dark field microscopy image of the same section. GNPs are seen in yellow (magnification X40). (f) Histological analysis of the same section, stained with PKH26 (red) and DAPI (blue, for all cells) (magnification X60).