“We have developed a unique technique to map, on a scale of milliseconds, the elasticity of a cell’s internal components. This has opened up a whole new area of research in the field of mechanobiology research to study the dynamics of displacements inside cells and understand the impact of these forces on disease and treatments,” stated Guy Cloutier, a researcher at the University of Montreal Hospital Research Centre (CRCHUM) and professor at the Université de Montréal.
This technology, called “cell quake elastography”, is presented in an article published today in the Proceedings of the National Academy of Sciences (PNAS).
Elasticity is a basic property of cells related to the anatomy, function and pathological state of cells and tissues. A cancerous tumour becomes rigid. Atherosclerosis and vascular aneurysms start with a loss of elasticity in the cells and arteries. Endothelial cells release transmitters that cause the vasoconstriction or vasodilation of blood vessels, depending on the mechanical shear conditions associated with vessel flow and geometry.
“Until now, we had a hard time measuring the mechanical changes continuously occurring in cells. Current techniques, based on cell deformation by atomic force microscopy or Brillouin scattering, can require several dozen minutes to measure elasticity. For some applications, that’s too slow, because thousands of events can occur in a cell, such as ion transfers, neuron stimulations and cell death. These events could be easier to track and the new imaging technology being developed here can very quickly measure the mechanical properties of cells,” explained Pol Grasland-Mongrain, a postdoctoral trainee in Guy Cloutier’s laboratory and the study’s first author.
With simple equipment consisting of a standard microscope, micropipettes and a high-speed camera, the researchers developed a revolutionary method for observing, in real time, the displacements and forces present in mice oocytes. According to CRCHUM researcher and Université de Montréal professor Greg FitzHarris, “with this new cell imaging modality, we will be able to study new mechanisms involved in cell division during embryo formation.”
“In the beginning, we had no idea of the level of spatial resolution that we could hope for, which is why we thought of using mice oocytes. These cells are large, about 80 microns in diameter, so they are easier to view. In our experiments, we were able to observe mechanical vibrations in the cell lasting less than a thousandth of a second, thanks to the camera used, which can capture 200,000 frames per second,” explained Cloutier. By comparison a movie usually has 25 frames per second.
The images captured make it possible to measure these vibrations using displacement-tracking algorithms. But how can a map of intracellular elasticity be reconstructed based on these vibrations? The solution is to use a noise-correlation technique, an approach developed by seismologists when they measure the vibrations produced by earthquakes to determine the composition of subterranean rocks.
“We used this same principle to explore the interior of a cell from a mechanical perspective, by applying shear wave elastography at the micrometer scale. A wave is sent into the cell at a speed proportional to the elasticity of the components that it encounters, such as the cytoplasm, nucleus or other cell components. Elasticity is measured by evaluating the speed of vibrations in time and space,” explained Stefan Catheline, a physicist at the Institut national de la santé et de la recherche médicale (INSERM) and a researcher at the Université de Lyon in France.
Pol Grasland-Mongrain explained the process he used to make his observations. “Using a micropipette, we induced high-frequency vibrations (15,000 cycles per second) in live mouse oocytes. With the images captured by the camera, we measured the vibrations inside the cell, using an optical flow algorithm developed initially for ultrasound applications. Then, the noise correlation algorithm allowed us to map the elasticity of the entire cell. In this study, we were able to show, among other things, that there was a significant reduction in the elasticity of mouse oocytes treated with cytochalasin, a substance known to disrupt their cytoskeleton,” he said.
This innovative technology for mapping cell elasticity opens the door to many practical applications in biology and medicine, whether for cancer, pathogenic infection or tissue scarring and engineering. “Now that we have shown the feasibility of the technology, we want to fine-tune the method and develop collaborations with experts in cellular and molecular biology, physiology and mechanobiology to explore the incredible potential of this area of research,” said Cloutier.