Imaging with isotopes: high resolution and quantitation
© BioMed Central Ltd 2006
Published: 5 October 2006
Mass spectrometry technology provides a clear image of the future of quantitative microscopy
Hooke was fascinated by the new vision of the world and the planets afforded by the lenses of the early light microscopes and telescopes of the 17th century. Ever since these discoveries, researchers have been gazing at the microscopic world and developing better and better instruments to do so. Over the centuries, the demanding needs of biologists have fuelled countless improvements in imaging technologies. For example, electron microscopy has become a standard instrument for high-resolution imaging (in the nanometer range) in biology, and scanning probe microscopy techniques provide three-dimensional images of atomic surfaces.
Quantitative imaging with mass spectrometry
Lechene's demanding requirements were important because he was keen to do experiments using the 15N isotope. 15N had been used for the pioneering experiments of Schoenheimer , to demonstrate protein turnover, and by Meselson and Stahl , to confirm the semiconservative nature of DNA replication. The problem is that nitrogen atoms hardly ionize and must therefore be examined as cyanide (CN-) ions. Lechene needed a system that could distinguish between the different isobars, such as 12C15N- (mass 27) and 13C14N- (also mass 27) and other similar atomic clusters. Slodzian's instruments enabled both high spatial resolution and the high mass resolution necessary for separating isobars at high secondary-ion transmissions.
A plethora of applications
Lechene teamed up with biologists from different disciplines to demonstrate how MIMS could be applied to quantitative imaging of biological samples. The Lechene study  is full of examples looking at turnover of proteins, DNA and fatty acids and at subcellular localization. Although these are spectacular examples of the MIMS technique, many researchers agree that this is just the tip of the iceberg. "The labelling of the lymph node cells by 15N is really convincing and suggests that MIMS may be highly useful in immunology and cancer research," says Brad Amos of the MRC Laboratory of Molecular Biology in Cambridge, UK. "The paper shows that a remarkable amount of fine detail can be seen. This may turn out to be a key paper in the development of a really important imaging method."
"The most significant feature of this technique is that it opens up a whole new world of imaging; we haven't yet imagined all that we can do with it," says Peter Gillespie from the Oregon Health & Science University in Portland, USA. He agrees with Amos that the technology represents an imaging revolution. "The novelty of the technique means it will take some time for the details to be absorbed, [but it] sets a spectacular new standard for spatial resolution and detection of stable and radioactive compounds in cells." Vickerman is also enthusiastic about the applications: "This study is important in that it demonstrates across a range of demanding applications that SIMS can deliver unique information, inaccessible by other means."
But Vickerman adds a cautionary note about how widely MIMS technology will be applied in the future. "It is clear that the technique has great potential in medicine and biology, but there are two issues that have to be overcome: the conservative approach of much of the potential user community and the cost of the equipment." Gillespie agrees "Once commercial instruments are available, will they be affordable and easy enough to use that we will do many experiments with them? Or will they be like electron microscopes, where the expense of the instruments and the difficulty in operation means that relatively few people use them well?"
"It may take a long time (EM took a long time), but I am convinced that in 10–15 years this will be an easily accessible technique, with routine instruments in many departments of biological research," responds Lechene. A machine based on Slodzian's prototype is already sold by a French company called Cameca Inc., and Lechene notes that there are over a dozen around the world. At a cost of two million dollars they are beyond the budget of most laboratories. But Lechene is keen to point out that more and more mass spectrometry machines are being purchased. "After all, it's just the price of five electron microscopes, but it does so much more!"
Perhaps we should leave the last word to Hooke , whose prophecies echo through three centuries of improvements in microscopy: "Tis not unlikely, but that there may be yet invented several other helps for the eye, at much exceeding those already found, as those do the bare eye, such as by which we may perhaps be able to discover ... the figures of the compounding Particles of matter and the particular Schematisms and Textures of Bodies."
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