The tension of cell membranes (which are made of lipid bilayers) plays an important role in a host of biological processes, such as cell motility, endocytosis and cell division, but measuring this tension is no easy task. Researchers at the University of Geneva in Switzerland have now invented a new fluorescent push-pull probe to do just this. The device, which can accurately determine the membrane tension of live cells, might help in the development of many new biomedical applications, including the detection of cancer cells, which characteristically have very high membrane tension compared to healthy cells.
Cell membranes are fluid surfaces around 4 nm thick that surround a cell and prevent its contents from “spilling” out. Since the volume of a cell changes dramatically during everyday biological processes, cells have evolved to continuously monitor the tension of their membranes. For example, when the tension becomes too high, they increase the amount of lipid in the membrane. And when it becomes too low, they decrease it, which has the effect of “tightening” the membrane. Cell membranes are pretty resistant to stretching though and can withstand tensions of up to 10-2 N/m before breaking apart.
Important though it is, membrane tension is notoriously difficult to measure in cells. The only technique available today involves making measurements on small membrane tubes that have been extracted from the outer membrane of the cell (its plasma membrane). Although this approach has provided much valuable information in the past, it is complicated – both to perform and obtain results from.
A push-pull system: FliptR
The new probe, created by a team led by Aurélien Roux and Stefan Matile, works in a completely different way. Dubbed FliptR (for fluorescent lipid tension reporter), it consists of two large fluorescent flipper groups made of dithienothiophene molecules connected by a single carbon bond.
“This chemistry was designed by Matile’s group so that the two flippers are in a twisted configuration at rest,” explains Roux. “The molecule also has an electron donor group at one end and an electron group on the other end – which is why we call it a push-pull system.”
When the molecule is inserted into a cell, it partially untwists (“planarizes”) because of the pressure exerted by the lipid tails on the cell membrane. This untwisted structure increases the time it takes for the molecule to fluoresce (that is, the time it takes for electrons to transfer through the molecule). Since this fluorescent lifetime increases as the membrane tension of the cell increases, the researchers are easily able to quantify it using fluorescence lifetime imaging microscopy. Indeed, they have already produced florescent lifetime calibration curves for the membrane tension of two of the most commonly used cell lines in biology, MDCK and HeLa cells.
Access to internal membranes too
“The technique is an improvement on existing methods to measure tension that apply local force (using pipettes or optical tweezers, for example) to pull on the membrane and monitor the reaction,” says Roux. “These approaches are thus limited solely to the outer, plasma membrane. Our technique, on the other hand, allows us to image the membrane tension all over the cell and follow tension gradients and inhomogeneity,” he tells Physics World. “And last but not least, it allows us to access the internal membranes of organelles for the first time, so we could now start measuring their tension too.”
Indeed, researchers in Matile’s team say they are busy designing such internal membrane probes.
The research is detailed in Nature Chemistry 10.1038/s41557-018-0127-3.