Exosome-Mediated Bidirectional Signaling Regulates Glioma Stem-Cell Homeostasis and Radiation-Induced Plasticity

Glioblastoma (GBM) is a highly aggressive brain cancer marked by poor treatment response and a tendency to recur. Within GBM, glioma-initiating cells (GICs), or glioma stem cells (GSCs), play a role in these characteristics through their ability to self-renew and resist damage from chemotherapy. Radiation therapy can paradoxically boost the number of GICs by giving non-stem glioma cells stem-like qualities.

Researchers looked into whether exosomes, tiny particles produced by cells, facilitate a two-way communication that keeps the balance between GICs and non-stem cells in steady state and after radiation.

Exosomes were extracted from enriched GIC gliomaspheres and from cultures of patient-derived GBM lines, HK-374 and HK-390. These exosomes were confirmed to have the right characteristics using various tests. The recipient cells were then treated with exosomes, either before or after being exposed to radiation. The study looked at the number of GICs and their function using a specific reporter, sphere formation tests, and a method called extreme limiting dilution analysis.

The proteins carried by the exosomes were identified using a technique called liquid chromatography-tandem mass spectrometry, and an analysis of the genes they are connected to was performed. Single-cell RNA sequencing was used to see how the mix of cell types changed when treated with exosomes.

The exosomes taken from the gliomaspheres were shaped like typical exosomes, were 41.6 +/- 14.7 nanometers in diameter, and had a protein called CD63 on their surface. After radiation, the number of GICs that showed the reporter marker increased by a factor of 12 compared to those that did not. Exosomes from gliomaspheres reduced this radiation-induced change, reducing the number of GICs and their ability to self-renew in both GBM lines.

Conversely, exosomes from cultures of differentiated cells increased the number of reporter-positive cells by two to three times and made existing GICs more numerous and self-renewing. Proteomic analysis found 1,796 exosomal proteins, with 336 of them differentially abundant. Exosomes taken from gliomaspheres had more proteins related to making proteins, carrying genetic material, organizing and moving the actin cytoskeleton, regulating the cell's internal structure, and moving materials in and out of the cell.

Exosomes from cultures of differentiated cells had more proteins from the extracellular matrix and the niche where stem cells live, including NID2, LAMA5, LAMB1, LAMB2, LAMC1, THBS1, TGFBI, IQGAP3, and APOE.

Analyzing the RNA of the cells showed that exosomes from gliomaspheres stopped the radiation-induced increase in neural progenitor-like cells, suggesting they suppressed an induced stem-like state. Treating gliomaspheres with exosomes from cultures of differentiated cells left the overall mix of cell types largely unchanged but increased the activity of programs linked to forming new spheres, E2F proteins, and oxidative phosphorylation, which may partly explain the increase in sphere formation.

The study concludes that GBM cells use exosomes to create a two-way communication between the stem-like and differentiated tumor-cell groups. GIC-derived exosomes prevent radiation from turning non-stem glioma cells into GICs, while exosomes from differentiated cells promote a niche that keeps GICs alive and able to self-renew, possibly by increasing a mixed population of cells that act like blood vessels and neurovascular progenitors.

Whether the protein IQGAP3, found in exosomes, plays a role in expanding this GIC population is still unclear. The research shows that exosome-mediated communication might be a key factor in regulating how GBM cells maintain their balance, resist treatment, and recur.