October 9, 2006
Imaging Nanoscale Membrane Organization
Understanding the chemical composition and organization of cell membranes – what components reside next to each other, how many of each there are and how they respond to their environment – may reveal the secret lives of cells in both health and disease. Now, thanks to a novel application of mass spectroscopy, researchers at Stanford University have developed a way to image cell membranes with unprecedented resolution – on the order of 100 nanometers.
Reporting its work in the journal Science, a multi-institutional team of investigators led by Steven Boxer, Ph.D., at Stanford University, describes its use of a highly specialized mass spectrometer that analyzes the mass of small molecular ions formed when a focused ion beam runs across the surface of a sample. “You take everything in the beam's focal area, which is about 100 nanometers in diameter and about 10 nanometers deep for our experiment, and you obliterate it,” Boxer said, explaining how the machine works. “Then you sample the fragments by mass spectrometry. Then you move over and you go another 100 nanometers and you obliterate everything. And now you see if what's in each 100 nanometer region is the same or different from the next region. And so you just raster this beam across the surface, and by rastering over and over and over again, you build an image.”
Called NanoSIMS 50, the mass spectrometer allows researchers to probe the composition of cell membranes with a higher resolution than light microscopy. By providing information about chemical composition of a sample, it fills a gap left by atomic force microscopy, which provides high-resolution information about topography, but not chemistry, as its microscope tip “feels” its way through samples. Plus it handles samples less ordered than those addressed by x-ray crystallography, which requires that samples be turned into crystals before analysis.
Boxer's group used atomic force microscopy to locate interesting features in a cell membrane and then employed the NanoSIMS 50 to determine what was there chemically. “Either technique by itself would be, I think, insufficient, but combined, they're really powerful,” Boxer said. The combination of techniques allowed the researchers to distinguish debris from features of interest.
“The real point is that you can do quantitative analysis,” Boxer said, emphasizing that this research allowed the first high-resolution mapping of chemical features in a region of interest. “We can analyze a few percent of one component in the presence of other components. It's exquisitely sensitive.” Sensitivity is important because cell membranes are not pure materials. “We're looking at mixtures of things, and we want to be able to say that we've got one molecule in 20 of type A mixed in with type B, or something like that,” Boxer said.
This work is detailed in a paper titled, “Phase separation of lipid membranes analyzed with high-resolution secondary ion mass spectrometry.” Investigators from the Lawrence Livermore National Laboratory and the University of California, Davis, also participated in this study. An abstract of this paper is available through PubMed.