Cells on the run
In the human body, the cells that make up our skin, bones, blood vessels and organs sit tightly together – 100 trillion of them in each of us. Some cells have to be able to wriggle out of this jam-packed environment – especially immune cells, which patrol through tissue to hunt down pathogens and defective cells. They possess certain abilities that help them on their mission. It was recently discovered that immune cells can recognise and avoid nearby bottlenecks. What the cells in our bodies are measuring, however, is not just their surroundings but also themselves: they immediately recognise when they are being too tightly compressed in a bottleneck and activate an escape mechanism.
Daniel Müller’s group from the Department of Biosystems and Systems Engineering at ETH Zurich in Basel has joined forces with an international team of scientists – including from Paris Sciences et Lettres University and the Ludwig Boltzmann Institute for Rare and Undiagnosed Diseases in Vienna – to examine this mechanism more closely and their findings have been published in the journal Science. In future, these findings could help improve immunotherapy for cancers.
Getting out of a tight spot
Crucial observations relied on a special atomic force microscope at ETH. This instrument was equipped with a glass microcantilever developed by biophysicist Cédric Cattin, who at the time was a postdoc in Müller’s group. Using this sensitive measuring device, the researchers were able to gradually and very precisely compress individual cells and observe when and how the cells reacted to being deformed in this way. The device registered not only the force that the microcantilever exerted on the cells, but also whether or not the cells pushed back.
It turns out that the cells can tolerate significant force without resistance. “All the cells examined let themselves be compressed from their normal round shape with a diameter of approximately 25 micrometres, to a flatter one measuring 10 micrometres across,” Müller says. But compressing the cells any further provoked a reaction: at the latest when they were flattened to 5 micrometres, the cells pushed back while mobilising to escape the confined space.
The nucleus as gauge
During further examinations, the team discerned that a cell’s nucleus is responsible for this escape response – or more specifically the nuclear membrane, which generally features wrinkles similar to the skin over our knuckles. As soon as a cell is compressed far enough for the nucleus to deform, its membrane unfolds and stretches. “This stretching of the membrane signals the start of the escape response,” Müller explains. The stretched membrane releases calcium ions that activate a specific enzyme, which in turn starts a reaction that activates the cell’s actomyosin system. This is responsible for the cell’s movements and initiates contractions in the cell structure, which allows the cell to generate counter pressure and escape.
“In other words, the nucleus acts as a gauge that measures when the cell becomes too confined,” Müller explains. This mechanism becomes particularly visible under an atomic force microscope, provided the two proteins actin and myosin are exposed to fluorescent dyes. As soon as the nuclear membrane is stretched, whole bubbles of these two proteins mushroom within the cell.
The researchers subsequently confirmed their findings from these experiments with the microcantilever through further tests. These included sending cells through bottlenecks in minute glass capillaries as well as observing how cells move through cell cultures of varying densities. Here, too, the cells escaped bottlenecks measuring between 5 and 10 micrometres by activating their actomyosin system. The researchers also observed this behaviour in every type of cell they tested, including tumour cells and immune cells from mice. “We infer from this that most types of cell have this capability,” Müller says.
The new findings have implications for a variety of applications, such as research into artificial tissues. To give such tissues the form desired to produce, say, artificial skin or organs, human cells are grown on a synthetic matrix. The new observations of how cells move should help enhance the design of such matrices. They may also prove useful in advancing immunotherapy, which for many years has been the great hope in beating cancer. Treatment involves encouraging the body’s own immune cells to attack tumour cells in a more targeted way. But immune cells sometimes find it hard to reach cancer cells because tumours grow more densely than healthy tissue. Müller says that to improve this, researchers could now harness the discovery of the escape mechanism.