Imaging of Lipids in Atheroma by Desorption Electrospray Ionization Mass Spectrometry

One of the hallmarks of atherosclerosis is the accumulation of lipoproteins within the wall of blood vessels. The lipid composition can vary among atheroma, even within a single individual, and is also dynamic, changing as the lesion progresses. One desirable characteristic of atheroma is their stability, as the rupture of unstable plaques can interfere with normal blood flow to the brain or heart, leading to stroke or heart attack. Desorption electrospray ionization mass spectrometry (DESI-MS) was used in this study for the profiling and imaging of arterial plaques. DESI-MS is an ambient ionization method in which a charged, nebulized solvent spray is directed a surface. In the positive and negative ion modes, sodium and chloride adducts, respectively, of diacyl glycerophosphocholines (GPChos), sphingomyelins (SMs), and hydrolyzed GPChos were detected. Also, cholesteryl esters were detected via adduct formation with ammonium cations. Finally, cholesterol was imaged in the atheroma by doping the charge labeling reagent betaine aldehyde directly into the DESI solvent spray, leading to in situ chemical derivatization of the otherwise nonionic cholesterol. DESI imaging experiments, in which the spatial distribution of the various chemical species is determined by scanning the DESI probe across an entire sample surface, revealed that there are lipid rich regions within the arterial walls, and the lipid rich regions seem to have one of two different lipid profiles. These lipid rich regions likely correspond to the areas of the tissue where lipoprotein particles have accumulated. It is also possible that the different lipid distributions may correlate with the stability or vulnerability of that particular region of the plaque.