Selected Protein Monitoring
in Histological Sections
by Targeted MALDI-FTICR In-Source Decay Imaging
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Abstract
Matrix-assisted laser desorption/ionization mass spectrometry
imaging
(MALDI MSI) is a rapidly growing method in biomedical research allowing
molecular mapping of proteins on histological sections. The images
can be analyzed in terms of spectral pattern to define regions of
interest. However, the identification and the differential quantitative
analysis of proteins require off line or in situ proteomic methods
using enzymatic digestion. The rapid identification of biomarkers
holds great promise for diagnostic research, but the major obstacle
is the absence of a rapid and direct method to detect and identify
with a sufficient dynamic range a set of specific biomarkers. In the
current work, we present a proof of concept for a method allowing
one to identify simultaneously a set of selected biomarkers on histological
slices with minimal sample treatment using in-source decay (ISD) MSI
and MALDI-Fourier transform ion cyclotron resonance (FTICR). In the
proposed method, known biomarkers are spotted next to the tissue of
interest, the whole MALDI plate being coated with 1,5-diaminonaphthalene
(1,5-DAN) matrix. The latter enhances MALDI radical-induced ISD, providing
large tags of the amino acid sequences. Comparative analysis of ISD
fragments between the reference spots and the specimen in imaging
mode allows for unambiguous identification of the selected biomarker
while preserving full spatial resolution. Moreover, the high resolution/high
mass accuracy provided by FTICR mass spectrometry allows the identification
of proteins. Well-resolved peaks and precise measurements of masses
and mass differences allow the construction of reliable sequence tags
for protein identification. The method will allow the use of MALDI-FTICR
MSI as a method for rapid targeted biomarker detection in complement
to classical histology