A new twist on a technique called cryo-electron tomography offers a closer-than-ever look inside a human skin cell: it generates a 3-D image with resolution fine enough to distinguish the structures of proteins. The new method, which involves freezing a cell and slicing it into thin sections, will allow scientists to probe how proteins organize and interact deep within a cell without disturbing them from their native states.
“At this resolution, the cell is essentially an uncharted territory,” says Achilleas Frangakis, a biologist at the European Molecular Biology Laboratory, in Heidelberg, Germany, who led the work. The images have a resolution of three to four nanometers, allowing scientists to discern the structures of individual proteins. Because the proteins have not been disrupted from their native positions, the scientists can glean clues about how they function and interact with one another in a living cell. “When you see the proteins, you immediately also see their interaction partners–how they interact in an undisturbed environment,” says Frangakis.
Traditional electron tomography can generate 3-D extreme close-ups of cells, but the procedure comes at a cost. Samples to be studied typically undergo elaborate chemical treatment that allows them to withstand the vacuum within the microscope and the powerful beam of electrons used to generate the image. However, that chemical processing also disturbs proteins and organelles from their natural configurations, destroying valuable information about how they function.
Scientists can circumvent this problem by freezing a sample so quickly that ice crystals–which would ravage the cell’s delicate internal structures–don’t have time to form. But since samples must be extremely thin for cryo-electron tomography to work, most cell types were ineligible. Only tiny bacterial cells and the thin fringes of eukaryotic cells made the cut.
Now Frangakis and his team have developed a way to cut frozen cells into miniscule slices, revealing the previously unavailable innards of much thicker cells. This includes eukaryotic cells–cells with nuclei–like those that make up human tissues. The scientists then use a lower-power electron beam to image the sample, so that it holds up longer in the microscope. They have also refined the software needed to build a 3-D representation of the slice.
Frozen cells: A thin, frozen slice of a human skin cell was bombarded with electrons and then reconstructed with specialized software to create this 3-D color-coded image. Each cellular structure has its own color: blue for the nucleus and its envelope, red for nuclear pores, purple for mitochondria, and brown for the cadherin proteins that allow the cell to adhere to its neighbors.