Extracellular vesicles as circulating biomarkers and therapeutic targets in brain tumors

Extracellular vesicles (EVs) are small lipid-bound particles that play major roles in cell-cell interactions. EVs contain a diverse cargo of proteins, lipids and nucleic acids that reflect the pathophysiological state of the secreting cells. Recent studies demonstrated that tumor EVs play pivotal roles in the crosstalk of tumors and their microenvironment which promotes tumor progression. We employed cancer stem cells isolated from primary and metastatic brain tumor specimens to study the role of EVs in the bidirectional interaction of tumor and glial cells. Using orthotopic xenografts and co-cultures of CSCs and human neural cells, we demonstrated that EVs secreted by CSCs induced a switch in microglia polarization to a predominantly M1 phenotype, and a mixed pro/anti-inflammatory phenotype in co-cultured astrocytes. Microglia and astrocyte EVs promoted CSC stemness, mesenchymal transit and increased PD-L1 expression in these cells. RNA sequencing analysis of secreted EVs identified specific miRNAs and lncRNAs that were associated with the CSC-glia cell interactions. Using specific reporters, antagomiRs and CRISPR/Cas9 we demonstrated that EV-associated miR-21, miR-1246 and the lncRNAs TALNEC2, PARROT and TUG1 played major roles in the activation of glial cells in vitro and in vivo. Significantly higher levels of these non-coding RNAs and PD-L1 were also identified in serum EVs of patients with brain tumors compared with healthy controls and in serum of mice carrying CSC-derived xenografts. In conclusion, we identified EV-associated novel non-coding RNAs that mediate the crosstalk of CSCs with glial cells and represent potential circulating biomarkers and therapeutic targets in these tumors.