Researchers from the laboratory of Gijs Wuite at Vrije Universiteit in Amsterdam, The Netherlands, collaborating with scientists from Weizmann Institute of Science in Israel, University of Groningen in The Netherlands, and the University of Barcelona in Spain, have developed a method that uses sound to measure cell mechanics across samples of red blood cells (RBCs). Lead investigator Raya Sorkin explained that the method, called Acoustic Force Spectroscopy (AFS), can be used to detect the softness or stiffness of cells as altered by the cellular uptake of secreted extracellular vesicles or as a result of chemical treatments. Their report, “Probing cellular mechanics with acoustic force spectroscopy,” was published in the August 8, 2018, special issue of Molecular Biology of the Cell, which was entitled Forces on and within Cells (Mol. Biol. Cell 29, 2005–2011).
In previous work of the collaborators at the Weizmann Institute and other researchers, elevated vesicle levels during disease were observed. “We realized that in many disease states, the concentration of secreted vesicles increases drastically (10 times higher during malaria infection, for example), and we wondered what effects these vesicles have on cells that uptake them,” Sorkin said. “We hypothesized that vesicle uptake should increase cell deformability, and we were looking for a way to test this hypothesis. While having several atomic force microscopy and optical tweezer instruments at hand, we realized that these methods, while proven to be suitable for cell mechanics measurements, are very time-consuming. We wanted to be able to apply a big range of forces and to have large statistics to be sure of our results, and this led us to develop the AFS-based method.”
Much like how your body might feel the vibrations of very loud music, AFS uses sound to apply a force to cells. Combined with microfluidics technology, researchers can make quick changes to the conditions the cells are exposed to and measure cell mechanics on samples of tens or even hundreds of cells all at once. Because there is no time delay, as there would be with a single cell analysis method, AFS eliminates the chance for variances in cell mechanics that might occur over time. This is especially important because even in cell samples taken from the same donor, there can be a great deal of heterogeneity, Sorkin said.
Sorkin explained the method like this: “The acoustic pressure field essentially consists of acoustic standing waves. If you imagine a water pool with standing waves, these trapped waves don’t even look as if they are traveling back and forth, but instead seem to be appearing and disappearing regularly at the same spots, while other spots stand still. These are the nodes of the waves. If you had a little rubber duck floating on top of the wave, it would be pushed towards the nodes. Similarly, the (microspheres used in this study) are pushed towards the nodes of the acoustic pressure field, towards the place where they would feel zero force.”
“There are many methods that can be used to measure cell mechanics,” Sorkin added. “Ours is quick and easy, offers force stability over time and multiple simultaneous measurements, but the important thing is that we demonstrated something that is biologically important and interesting: that vesicle uptake increases cell deformability.”
Furthermore, AFS has possible clinical applications. “In diabetes, RBCs are softer, and in malaria infection and sickle cell disease, RBCs are stiffer. Therefore our method can potentially be used in the clinic for diagnostic purposes.” Sorkin added that changes in cell mechanics following uptake of vesicles may be pathologically relevant in conditions where concentrations of extracellular vesicles are “significantly elevated,” such as in trauma patients and in women with breast cancer.