High Resolution Mass Spectrometry of Cystine-Containing Neuropeptides in Histological Sections of Human FFPE Tissue Banks

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Abstract
Using our earlier developed protocol, mass spectrometry imaging of small endogenous peptides (and a selection of small metabolites) can be successfully performed directly in tissue sections of formaldehyde-fixed paraffin-embedded (FFPE) samples, such as those available in Homo sapiens biobanks. In analogy with immunohistochemistry (IHC) which employs antibodies as detection probes, this method was designated mass spectrometry histochemistry (MSHC) as it solely relies on (top-down) mass spectrometry for analyte detection. We demonstrate that MSHC enables the localization of cystine-containing neuropeptides in histological sections of human FFPE biobanked tissue and illustrate this on pituitary adenomas and non-diseased pituitary tissues archived for several years in an academic hospital pathology biobank. The instrumental setup consists of high-resolution mass spectrometers (several orbitrap systems and one dedicated hybrid TOF instrument) fitted with atmospheric pressure (AP) scanning matrix-assisted laser desorption/ionization MALDI. Currently, the best spatial resolution routinely achievable with such (MALDI) apparatus is 5m. The high mass spectrometric resolution obtained allows revealing the full isotope envelopes of the peptides. As such both reduced and oxidized cysteine-containing proteoforms of e.g., the neurosecretory nonapeptides vasopressin and oxytocin can be visualized in biobanked FFPE tissue, demonstrating yet a novel application of MSHC.

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