Individual proteins play a key role in the development of a host of diseases, including Alzheimer’s, Parkinson’s, and Huntington’s. A number of new imaging techniques can reveal the behavior of single biomolecules, but these approaches are tricky and expensive. Now a new technique, developed at Harvard University, could provide a cheaper and simpler way to measure and track molecules as they move freely through a solution.
Proteins are small–around two nanometers on average–and they flit around quickly, making them difficult to track under a microscope. A popular way to observe interactions between two proteins is to tether one to a surface and wait until another molecule comes by and interacts. The problem with this approach, explains Adam Cohen, assistant professor of chemistry at Harvard University and a TR35 Award winner in 2007, is that proteins behave differently when they are attached to a surface, since they have less freedom to move.
Cohen’s lab has adapted a regular fluorescence microscope to make single-molecule imaging simpler. The new technique, called Convex Lens-Induced Confinement (CLIC), squeezes molecules between a flat sheet of glass and a curved sheet, so they are confined but not tethered. While there are other ways to immobilize single molecules, they generally require specialized devices, which can be complex or expensive.
One of Cohen’s postdoctoral researchers, Sabrina Leslie, modified a regular fluorescence microscope using a mechanical setup. A convex lens touches the center of a flat piece of glass covered with a protein-containing solution. The curved surface of the lens is placed facedown. Proteins diffuse through the solution, but their size limits how far they can travel toward the center, where the space between the flat glass and curving glass gets smaller. How far the proteins can travel lets researchers figure out the size of each protein.
The lens also controls the depth of the solution. This prevents proteins from becoming layered on top of each other, as normally occurs. The setup also makes it easier to observe individual proteins for longer, because they are confined between the flat glass and the lens.
“This is a beautifully simple, novel approach,” says Julio Fernández, a professor of biological studies at Columbia University whose lab studies protein dynamics. He says that watching molecules for a very long time at high resolution will give researchers enough time to see how single proteins behave. “It’s much better to observe something with its dynamics unchanged,” he says.
The new technique might help researchers understand, for example, how a single protein contributes to the formation of amyloid plaques–tangles of proteins found between nerve cells in Alzheimer’s. “This should broaden the scope of experiments that could be possible,” says Cohen.
Another advantage of the CLIC setup is that it’s cheap. Microscopes designed for imaging single proteins cost around $100,000. “You can do better with CLIC, and it would cost you a couple hundred dollars,” says Fernández. “It doesn’t require any particular software or expensive equipment,” he says, adding that he plans to try the technique in his own lab.
Fernández emphasizes that it will take time and experiments to confirm how useful the technique will be, but says, “I think it looks more than promising.”