Frangakis’s group tried out its new procedure on human skin cells, which are thick enough that their inner workings are invisible using conventional cryo-electron tomography. The team first froze cells from the arm of a healthy donor to a frigid -200 ºC by plunging them into liquid nitrogen. Keeping the cells’ temperature below -140 ºC, the scientists sliced them into 50-nanometer-thick sections with a diamond-bladed knife and then bombarded them with a relatively low-power beam of electrons in the microscope. Special software pared down the resulting image into even thinner virtual slices–as tiny as half a nanometer thick–to construct a 3-D representation called a tomogram. The results were published last week in the journal Nature.
These tomograms allowed the research team to resolve, with unprecedented detail, the Velcro-like mechanism that skin cells use to attach to one another. Proteins called cadherins protrude from the cells’ surfaces and hook together in elaborate structures that had previously remained obscure. Thanks to the new imaging technique, Frangakis was able to determine precisely how the cadherin molecules interact. Each protein linked up with other cadherins in two different ways–one for proteins from the same cell, and one for proteins from a neighboring cell–to form an interlocking lattice.
Frangakis’s accomplishment is “truly a first,” says Grant Jensen, a biologist at the California Institute of Technology who specializes in cryo-electron tomography. While using electron tomography on thinly sliced frozen cells had been attempted before, he says, “it never worked well enough to make any significant conclusions from it.”
Visualizing cells in as close to their natural state as possible is the key benefit of cryo-electron tomography, says Jensen. Frangakis’s slicing technique, known as cryo-sectioning, opens the door to reaping that benefit on virtually any cell–not just on skinny ones. “Cryo-sectioning is going to allow us to look at any cell we want,” Jensen says.
For his next project, Frangakis plans to use his new technique to plumb the innermost realms of rat kidney cells. He hopes to tease apart the structure of nuclear pore complexes–sophisticated protein assemblies that act as gatekeepers to the cell’s nucleus, allowing only certain molecules to pass through.