Mitochondrial transfer in glioblastoma through cell nanotubes

Mitochondrial transfer in glioblastoma appears to be a two-way process, at least in the laboratory models examined in this study. Tumor cells could send damaged mitochondria to neighboring astrocytes while receiving astrocyte-derived mitochondria that were able to become incorporated into the tumor cell’s own mitochondrial network.

The route between the cells consisted of extremely thin membrane bridges known as tunneling nanotubes, or TNTs. Live imaging allowed the researchers to watch entire mitochondria move through these connections. Their stop-and-go trajectories were consistent with active transport rather than simple passive movement.

Many of the nanotubes extended for tens of micrometers. Some survived for less than an hour, while others persisted for several hours. Cryo-electron microscopy also showed that what looked like a single tube under conventional microscopy could actually contain a bundle of much narrower nanotubes only tens of nanometers across.

Damaged mitochondria went one way

The most revealing part of the study came when the researchers followed what happened to the transferred organelles after they reached another cell.

Tumor-derived mitochondria found inside astrocytes lacked normal signs of mitochondrial membrane potential and strongly colocalized with lysosomes, the cellular compartments that break down unwanted material. The pattern was consistent with mitophagy, the selective disposal of damaged mitochondria.

Transfer in the opposite direction produced a different outcome. Astrocyte-derived mitochondria entered glioblastoma cells, and when a recipient cell contained a larger number of them, some became integrated into its existing mitochondrial network and retained markers of mitochondrial activity.

The traffic was not symmetrical and varied considerably among experimental models. In co-cultures using the widely studied U251 glioblastoma cell line, roughly half of the tumor cells showed evidence of receiving astrocytic mitochondria, whereas fewer than one in ten astrocytes received mitochondria from the tumor cells. Two patient-derived glioblastoma stem-like cell populations showed lower transfer rates, but the same overall bias toward astrocyte-to-tumor transfer.

The researchers then compared glioblastoma stem-like cells that had acquired astrocytic mitochondria with cells in which no transferred mitochondria were detected. The two populations had different transcriptional profiles, with enrichment of genes involved in mitochondrial metabolism, electron transport and membrane-potential regulation.

That association fits the idea that acquiring mitochondria may alter the energetic state of recipient cancer cells. It does not, however, prove that mitochondrial transfer itself caused every transcriptional difference. The authors note that pre-existing differences among tumor cells could make some cells both more likely to acquire mitochondria and more likely to display a particular gene-expression program.

Watching the bridges inside a living tumor

A persistent problem in this field has been that tunneling nanotubes are relatively easy to investigate in cultured cells but extremely difficult to resolve inside living tissue. They are thin, fragile and dynamic, and there is no unique molecular marker that identifies them.

The imaging approach used in this study could not provide the necessary resolution deep inside the brain. For the in vivo experiments, the team therefore switched from glioblastoma to mouse models of head and neck squamous cell carcinoma growing in the tongue.

High-resolution intravital imaging revealed thin tumor-cell connections with dimensions, persistence and formation mechanisms resembling the TNTs observed in culture. Mitochondria could be seen inside some of these structures. In a separate mouse model in which host mitochondria were fluorescently labeled, host-derived mitochondria were detected inside tumor cells.

This provides evidence that whole-mitochondrion transfer can occur within a living tumor rather than being solely a cell-culture phenomenon. It does not directly demonstrate astrocyte-to-glioblastoma mitochondrial exchange in the living brain or in human tumors. The authors also cannot completely exclude the presence of a different class of larger cancer-cell projections known as tumor microtubes.

The study therefore establishes two connected but distinct findings. In glioblastoma models in vitro, damaged tumor mitochondria can be passed toward astrocytes while astrocytic mitochondria move into tumor cells. In living mouse tumors of another cancer type, TNT-like connections and mitochondrial transfer can be observed within the tumor environment itself.

The broader picture is of a cancer cell that is not metabolically isolated. In these experimental systems, maintaining its mitochondrial population can involve physical exchange with neighboring cells, extending mitochondrial quality control beyond the boundary of a single cell.