Merger of laser capture microdissection and mass spectrometry: a window into the amyloid plaque proteome

The occurrence of protein accumulation and aggregation in the brain is one of the pathological hallmarks of neurodegenerative diseases such as Alzheimer's disease (AD). Although it is instructive to analyze the aggregated proteins in the brain, biochemical purification and identification of these proteins have been challenging. Recent developments in laser capture microdissection (LCM) and mass spectrometry (MS) enable large-scale protein profiling of captured tissue samples. We present here the method of analyzing senile plaques from postmortem AD brains by coupling LCM and highly sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS). First, the senile plaques were stained with thioflavin-S and precisely isolated by adjusted laser beams under a microscope. Total proteins in the isolated tissues were extracted and resolved on an SDS gel. To identify all proteins in the samples, the gel was excised into multiple pieces followed by trypsin digestion. The resulting peptides were further separated by reverse-phase chromatography and analyzed by tandem mass spectrometry. A database search of acquired MS/MS spectra allowed the identification of hundreds to thousands of peptides/proteins in the original samples. Moreover, quantitative comparison of protein composites in different LCM samples could be achieved by MS strategies. For instance, the comparison between plaques and surrounding nonplaque tissues from the same specimen revealed tens of proteins specifically enriched in the plaques. Finally, the data were corroborated by independent experiments using the approach of immunohistochemistry. Taken together, the merger of LCM and MS is a powerful tool to probe the proteome of any given pathological lesions.