Developing a clearer molecular picture

Vanderbilt University Medical Centre investigators have developed a new way to take pictures of molecules in a slice of tissue.

The technique, called ‘Imaging Mass Spectrometry,’ is said to offer scientists a new tool for visualising where proteins are located in cells and tissues. This kind of information is important to understanding how proteins work and how they change in disease states.

‘One of our goals is to look at tumour tissues and attempt to find changes in expressed proteins that are the result of, or contribute to, tumour development,’ said Richard Caprioli, Ph.D., director of the Mass Spectrometry Research Centre. ‘We know from this and other work, for example, that the pattern of proteins expressed in the outer edge of a growing tumour is different from that of the interior, and that both of these are different from the normal tissue right next to the tumour.’

Caprioli hopes that someday the technology can be used to assess tumour margins during surgery and to detect molecular changes in a biopsy sample before a tumour has started any significant development.

The new Imaging Mass Spectrometry technology uses a standard MALDI (Matrix-Assisted Laser Desorption Ionisation) mass spectrometer, an instrument that analyses molecules based on their mass-to-charge.

To take the new molecular photographs, a chunk of tissue is first frozen so that it can be cut into super-thin slices. A tissue slice to be analysed is then coated with a matrix material and introduced into the mass spectrometer, where a laser beam blasts successive spots on the tissue to release molecules for analysis. Each spot becomes a ‘pixel’ in the final image, with each ‘pixel’ containing a record of the molecules located in that tiny spot.

Computer processing can then be used to display the locations of selected proteins, based on their size. Caprioli’s team successfully used the new imaging technology to take molecular pictures of two types of tissues-normal mouse brain and human brain tumour grown in a mouse. In the brain tumour slices, the researchers identified proteins uniquely located in the tumour’s interior and at its invasive outer edge.

‘The presumption is that molecules which are distinct for this invasive tumour region are related to the proliferation process and might serve as good diagnostic markers or molecular drug targets,’ Caprioli said.

Compared to other imaging techniques, Imaging Mass Spectrometry offers the advantage that it does not require a directed search for a specific protein. Using other techniques, investigators generally must add chemicals or antibodies that will ‘mark’ a certain protein in order to determine its location. These strategies require that researchers know what molecule to look for, in order to select or produce the right antibodies, said Caprioli.

Imaging Mass Spectrometry, however, is said to perform an unbiased scan of the tissue, cataloguing all of the proteins it finds.

‘You’re closest to finding out what’s going on in vivo if you don’t have to manipulate the tissue a great deal,’ said Caprioli. ‘With Imaging Mass Spectrometry, there’s really minimal preparation of the tissue. It’s an important tool in the discovery process when you don’t necessarily know what the target molecule is.’