Why stroke accelerates glioma growth in mice

In several mouse models, stroke accelerates glioma growth and changes where tumor cells spread. Rather than expanding uniformly, glioma cells infiltrated brain regions damaged by the interruption of blood supply. A study published in Nature Cancer traces this effect to changes in the cells surrounding the tumor, particularly astrocytes and immune cells.

Gliomas arise from cells of the nervous system or their precursors. High-grade gliomas, including glioblastoma, are especially difficult to treat because malignant cells can infiltrate apparently healthy brain tissue well beyond the visible tumor. This invasive behavior makes complete surgical removal extremely challenging.

Epidemiological studies have previously suggested an association between a history of stroke and an increased risk of brain tumors. Whether brain injury can directly change the behavior of glioma cells, however, has remained unclear.

Researchers at Baylor College of Medicine and collaborating institutions investigated this question using several experimental models.

Tumor cells move toward injured brain tissue

In one experiment, the researchers first induced a localized ischemic stroke in mice. Seven days later, they implanted glioblastoma cells originally obtained from human patients. Control animals underwent a comparable procedure without the stroke injury.

Tumors in mice that had experienced stroke were larger and contained more proliferating cells. They also invaded surrounding tissue more extensively, with tumor infiltration preferentially directed toward the injured region.

The researchers observed similar effects in other glioma models, including high-grade tumors that developed within the mouse brain. Across these experiments, stroke promoted tumor progression and was associated with shorter survival.

This suggested that the injured brain was providing conditions that favored tumor expansion.

To investigate those conditions, the team analyzed cells within and around the tumors using single-cell RNA sequencing and spatial transcriptomics. These techniques make it possible to examine the molecular state of individual cell populations while preserving information about their location within the tumor environment.

One cell population stood out: astrocytes.

Astrocytes lose part of their calcium signaling

Astrocytes are abundant cells that support neurons, help regulate the chemical environment of the brain and participate in responses to injury.

Changes in intracellular calcium are among the signals astrocytes use to regulate their own activity and respond to surrounding cells.

Following stroke, the researchers identified a distinct population of tumor-associated astrocytes at the invasive edge of gliomas. These cells exhibited reduced calcium activity.

The change was accompanied by an accumulation of tumor-associated microglia and macrophages. Microglia are the brain's resident immune cells, whereas macrophages can also enter from the circulation. Under certain conditions, these immune populations can adopt states that support tumor progression.

The researchers then identified a molecular connection between altered astrocyte signaling and immune-cell recruitment.

A protein called SLC4A4, which transports sodium and bicarbonate across the cell membrane, emerged as an important regulator of astrocyte calcium activity.

Disrupting SLC4A4 function in astrocytes reduced calcium signaling and increased the production of CCL2, a signaling molecule that attracts certain immune cells.

This was accompanied by an increased accumulation of tumor-associated immune cells and more aggressive glioma growth.

The results suggested that a change in astrocyte physiology could help reorganize the surrounding immune environment into one that favors tumor progression.

Interrupting the process

Observing these changes did not establish whether they were responsible for accelerated tumor growth. The researchers therefore tested interventions targeting different parts of the pathway.

Experimentally enhancing calcium signaling in tumor-associated astrocytes suppressed stroke-induced glioma progression.

Depleting tumor-associated microglia and macrophages also reduced the effect of stroke on tumor growth.

Further experiments provided additional evidence for the proposed mechanism. Increasing astrocytic SLC4A4 activity restricted glioma growth, while genetically inhibiting astrocyte-derived CCL2 counteracted the tumor-promoting effects of SLC4A4 loss.

Together, these findings indicate that astrocyte calcium signaling and immune-cell recruitment contribute functionally to the effect of brain injury on glioma progression in the experimental models.

Several limitations are important when interpreting the results. The causal experiments were primarily conducted in mice, including models implanted with human glioblastoma cells. Some experimental groups were small. For example, one comparison of tumor volume involved three control mice and four mice exposed to stroke. Although the investigators reproduced key effects across multiple models, clinical relevance has yet to be established.

The study does not demonstrate that stroke directly causes glioma to develop in humans. Its central experimental finding concerns how injury affects the growth and infiltration of tumor cells that are already present or have been experimentally implanted.

Nor have astrocyte calcium signaling, SLC4A4 or CCL2 been established as therapeutic targets in patients with glioma. The interventions reported here remain experimental.

What the study reveals is a specific way in which brain injury can alter the environment surrounding a malignant tumor. In these models, the region affected by stroke became a preferred direction for glioma invasion, with altered astrocyte signaling playing a central role in the process.