This video is from the high numerical aperture TIRF-SIM microscope showing 181 frames at 5-second intervals in a COS-7 cell at 37°C expressing mEmerald-CLTA (green) and mCherry-Lifeact (orange-red). [Betzig Lab, HHMI/Janelia Research Campus]
Technological advances in the field of microscopy and imaging have seen a flurry of activity over the past several years, with the Nobel Prize in Chemistry going to the development of super-resolved fluorescence microscopy in 2014. One of the three scientists awarded the Noble for this advancement has been actively pursuing new methods to visualize living cells with far superior clarity. He believes the new methods dramatically improve the spatial resolution provided by structured illumination microscopy, which is currently one of the best imaging methods for seeing inside living cells.
“These methods set a new standard for how far you can push the speed and non-invasiveness of super-resolution imaging,” explained senior author and Nobel Laureate Eric Betzig, Ph.D., group leader at the Howard Hughes Medical Institute's Janelia Research Campus. “This will bring super-resolution to live-cell imaging for real.”
The findings from this study were published recently in Science through an article entitled “Extended-resolution structured illumination imaging of endocytic and cytoskeletal dynamics.”
The images and videos produced with this new technology, a variation of structured illumination microscopy (SIM), can now show the movement and interactions of proteins as cells remodel their structural supports or reorganize their membranes to take up molecules from outside the cell. This adds to the tools available for super-resolution optical microscopy since it had been impractical for use in imaging living cells.
In traditional SIM, the sample under the lens is observed while it is illuminated by a pattern of light (similar to scanning a barcode). Several different light patterns are applied, and the resulting patterns, called moiré, are captured from various angles each time by a digital camera. Computer software then extracts the information from the moiré images and translates it into a three-dimensional, high-resolution reconstruction. The final reconstructed image has twice the spatial resolution that can be obtained with traditional light microscopy.
“I fell in love with SIM because of its speed and the fact that it took so much less light than the other methods,” Dr. Betzig noted.
Dr. Betzig and his team were convinced that SIM had the potential to generate significant insights into cellular mechanics, and he suspected that improving the technique's spatial resolution would go a long way toward increasing its use by biologists.
However, traditional SIM was not suitable for living cells, as the method was designed to generate an image by switching on all of the fluorescent labels in the cell and then deactivate them with a different wavelength of light—repeating the process 25 times or more to construct a high-resolution image.
“The problem with this approach is that you first turn on all the molecules, then you immediately turn off almost all the molecules,” stated Dr. Betzig. “The molecules you've turned off don't contribute anything to the image, but you've just fried them twice. You're stressing the molecules, and it takes a lot of time, which you don't have because the cell is moving.”
To Dr. Betzig and his team, the solution seemed almost too simple: “Don't turn on all of the molecules. There's no need to do that.” Alternatively, the new method, called patterned photoactivation non-linear SIM, begins by switching on just a subset of fluorescent labels in a sample with a pattern of light. “The patterning of that gives you some high-resolution information already,” Dr. Betzig explained.
The scientists created a new pattern of light to deactivate molecules and extract information from their deactivation. The combined effect of those patterns led to final images with 62-nanometer resolution—much better than standard SIM and a threefold improvement over the limits imposed by the wavelength of light.
“We can do it and we can do it fast,” said Dr. Betzig. “If something in the cell is moving at a micron a second and I have a one-micron resolution, I can take that image in a second. But if I have a 1/10-micron resolution, I have to take the data in a tenth of a second, or else it will smear out.”
Dr. Betzig and his team were excited by their results and hope to make their methods available to more researchers in the near future. However, he went on to note that “most of the magic is in the software, not the hardware.”