A water window on the world of cells
There are two keys to the success of the x-ray microscopy live-cell imaging techniques being developed by cell biologist Carolyn Larabell -- the microscope itself, XM-1, and the way her cells are prepared for imaging.
XM-1 is a direct-imaging transmission x-ray microscope at beamline 6.1.2 of the Advanced Light Source, a third-generation synchrotron light source. The photon-energy range of XM-1 extends from 250 to 950 electron volts, a range that covers the so-called "water window," which is the energy span over which water is transparent to x-rays but carbon-containing materials are not. What this means is that high-contrast images of proteins and other interior cell structures can be obtained at XM-1 without the need for staining, a common microscopy procedure that can alter the integrity of the sample being imaged.
To prepare for x-ray imaging, Larabell and her group treat their cells with a fixing agent to lock proteins into position, then wash the cells in a special detergent that removes lipids from their outer membrane. This yields a cell surface that is perforated with holes through which antibodies coated with a cluster of fluorescent and nanosized gold particles can be introduced.
Inside the cell, the fluorogold-labeled antibodies attach themselves to specific proteins. Because the nanosized particles of gold are too small to be seen even with x-rays, prior to imaging at XM-1 the treated cells are enhanced with a coating of silver, similar to the use of silver emulsions to bring out images on photographic film.
The combination of XM-1 and fluorogold labeling has already been used
by Larabell and her colleagues to obtain detailed information on the distribution
within cell nuclei of a protein that plays an important role in the expression
of certain genes. This proof-of-principle experiment should be considered
merely a preview of coming attractions.
A slide show on soft x-ray microscopy