Live tracking of endogenous exosomes in vivo

• Published in: Journal of extracellular vesicles 2018-01, Vol.7, p.187-187
• Main Authors: Verweij, Frederik, Revenu, Celine, Arras, Guillaume, Loew, Damarys, Herbomel, Philippe, Allio, Guillaume, Goetz, Jacky, del Bene, Filippo, Raposo, Graca, van Niel, Guillaume
• Format: Article
• Language: English
• Subjects: Biosynthesis; Blood flow; CD63 antigen; Danio rerio; Dynamin; Electron microscopy; Embryos; Endocytosis; Endothelial cells; Exosomes; Fluid flow; Macrophages; Nutrient transport; Scavenger receptors; Secretion; Vascular system; Yolk; Zebrafish
• ISSN: 2001-3078

Background: Extracellular vesicles (EVs) such as exosomes are released by a wide variety of cell types and found in all organism tested so far. Even though EVs are implicated in many important physio- and pathological processes, our understanding of their relevance in vivo remains poorly understood, mainly due to the lack of model organisms that allow the accurate spatiotemporal assessment of EV biogenesis, transfer and fate at single-vesicle level. Methods: We developed an animal model to study endogenous exosomes in vivo by expressing CD63-pHluorin, a fluorescent reporter for exosome secretion, in zebrafish embryos. Using a combination of lightand electron microscopy techniques and proteomic ex vivo analysis, we explored the physiology of exosomes, including their biogenesis, transfer, uptake and fate. Results: We observed exosome release in vivo and tracked a massive pool of endogenous EVs in the blood-flow and interstitial fluid of zebrafish embryos. We identified the yolk syncytial layer (YSL), a cell layer with essential nutrient transport functions, as a major contributor to this pool of EVs by cell layer-specific expression. We further identified YSL-derived EVs as exosomes generated in a syntenin-dependent manner and enriched, among others, in solute carriers. These exosomes were directly released into the vascular system, and, by following the blood flow, went through the whole organism to finally reach and accumulate in the caudal vein plexus. Here, exosomes were specifically captured and endocytosed by scavenging early macrophages and endothelial cells. While endocytosis of exosomes was dynamin-dependent in both cell types, inhibition of scavenger receptors interfered mostly with uptake in endothelial cells. Interestingly, we could not observe functional transfer of Cre-protein. In fact, selective inhibition of vATPases showed massive accumulation of YSL-derived exosomes in late endo-/lysosomal compartments destined for degradation. Functionally, the origin, distribution and fate of these EVs are compatible with a role in trophic support for the developing embryo. Summary/Conclusion: Altogether, these data reveal for the first time the release, journey, targets and fate of endogenous exosomes in vivo and support a role in nutrient delivery.