Question

Explain in great detail how to determine if MALDI-MS can resolve protein ions from a section of brain tissue and within a cell. To make these determinations, the only other information to work with is the dimensions of the tissue and the diameter of the cell.

Answer 1

Matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) is a powerful tool for investigating the distribution of proteins and small molecules within biological systems through the in situ analysis of tissue sections. MALDI-IMS can determine the distribution of hundreds of unknown compounds in a single measurement and enables the acquisition of cellular expression profiles while maintaining the cellular and molecular integrity. In recent years, a great many advances in the practice of imaging mass spectrometry have taken place, making the technique more sensitive, robust, and ultimately useful. In this review, we focus on the current state of the art of MALDI-IMS, describe basic technological developments for MALDI-IMS of animal and human tissues, and discuss some recent applications in basic research and in clinical settings.

To use a mass spectrometer as an imaging instrument, it is essential for the spectrometer to be equipped with an automatic rastering function, automatic data acquisition system, and visualization software. Recently, several manufacturers have released novel instruments having these features. Almost all manufacturers have developed in-house software for their instruments and have included a driver for instrument and image reconstruction. Instruments used for imaging mass spectrometry can be classified according to how ions are generated from the sample: either by irradiation by pulsed laser or bombardment by energetic particles. Laser based systems include MALDI and laser desorption ionization instruments. Secondary ion mass spectrometry (SIMS) systems use particle bombardment with a continuous beam of highly-focused, energetic ions. In general, these imaging instruments were based mostly on MALDI-TOF, MALDI-TOF/TOF, or TOF-SIMS. MALDI-TOF imaging instruments are high throughput, and TOF-SIMS imaging instruments can provide submicrometer spatial resolution of ~500 nm (Altelaar et al. 2006). Both instruments have powerful advantages in imaging mass spectrometry. There are several published articles discussing and comparing MALDI and SIMS imaging (Heeren et al. 2005; McDonnell and Heeren 2007; Shimma et al. 2008). This review focuses on MALDI imaging.

For the interpretation of MALDI-IMS results it is an absolute necessity to correlate the MALDI image with the histological information. Advanced MALDI-IMS software allows superimposing the MALDI images over a macroscopic or microscopic optical image of the sample taken before the MALDI measurement. While that primary macroscopic optical image is sufficient to recognize the outline of the tissue and to define the measurement area, it is usually not possible to see histological features in that image in contrast to microscopic images. For a histomorphological interpretation it is necessary to use stained tissue sections. Two approaches have been used to correlate histology with the MALDI-IMS result so far.