Under the hood: an investigation into the molecular mechanisms of metastasis

Cells have some sort of mailing system. They release small molecular balls called extracellular vesicles (EVs) containing biological substances or messages, and can attach to other cells to share what is contained.

In cancer, EVs often start from tumor cells and disseminate the cancer elsewhere in the body, leading to metastasis. However, how it is connected to the recipient cell to provide the payload remains a mystery until now. A team of researchers based in Japan has uncovered the molecular mechanisms that underpin the process of small EVs (SEVs).

The team published their findings in the Journal of Cell Biology.

“In recent years, EVS has gained a lot of attention as a mediator of cell-to-cell communication,” said author Kenichi Gn Suzuki, professor at GIFU’s Glicocoa Institute and chief of the Advanced Bioimaging Department at the National Cancer Center Institute in Tokyo.

He explained that EVs could function as biomarkers. Because they carry certain proteins and genetic material that can show disease progression. Researchers have also begun exploring the possibilities of treating cancer by inhibiting binding to host cells or promoting EV binding via therapeutic payloads.

“However, the mechanisms underlying selective binding to recipient cell membranes remain elusive,” Suzuki said. “This study sought to elucidate these mechanisms.”

Researchers focused on understanding the role of integrin heterodimers, a molecule that helps SEVs attach to host cells. The team previously discovered that SEVs can be sorted into subtypes with different properties, depending on which tetraspanin protein. This type of protein is small, but is important for EV formation and regulation, Suzuki said.

Using this understanding, researchers sorted and tracked SEVs by single molecule resolution.

They examined subtypes sorted by super-resolution microscopy and found that all subtypes bind to laminin using integrin heterodimers associated with a specific tetraspanin protein known primarily as CD151, and a carbohydrate- and fat-containing molecule called GM1 to bind to laminin and to cell membrane-resistant proteins.

Laminin is a particularly glycoprotein. That is, it is a carbohydrate, or a protein with sugar, or molecules. It resides in extracellular matrices, or in molecular networks surrounding cells, supporting signaling and structure.

“Due to quantitative analysis using single molecule imaging and super-resolution microscopy, it was important that all EV subtypes derived from four different tumor cell lines, primarily CD151-related integrin heterodimers and laminin via GM1, are bound to recipient-bound laMinin. Fibronectin is another protein responsible for cell adhesion in the extracellular matrix.

He also pointed out that two other proteins associated with EV adhesion, Talin and Kindlin, did not activate integrin heterodimers. Putting it all together, the researchers concluded that GM1 and integrin heterodimers associated with CD151 are keys to EV binding. This understanding helps researchers to suppress or encourage binding as needed in the name of disease treatment, Suzuki said.

“Although EVs are widely explored as biomarkers, attempts have begun to use EVs as therapeutic agents,” Suzuki said. “Given the molecular mechanisms underlying EV binding to recipient cells, our findings are expected to advance the development of EV-based therapies.”