AP-MALDI imaging of neuropeptides in mouse pituitary gland with 5 μm spatial resolution and high mass accuracy

Abstract

MALDI MS imaging is a powerful tool to visualize the spatial distribution of endogenous biomolecules such as lipids or neuropeptides. Direct identification of analytes is often difficult due to the complexity of biological tissue samples. Today reliable analyte identification is routinely done with mass spectrometers featuring high mass resolving power, high mass accuracy and MS/MS capability. These mass spectrometers, however, typically have a rather poor spatial resolution when used in MALDI MS imaging experiments. In this work a linear ion trap orbital trapping mass spectrometer combined with an in-house developed atmospheric pressure MALDI imaging ion source was used to image neuropeptides in mouse pituitary gland with a spatial resolution of 5 μm. Ten neuropeptides were identified by their accurate mass in the mass range up to 2500 u via targeted database search. The ion images of the peptides show down to the cellular level that their appearances within the pituitary gland are restricted to accurately defined tissue types. This is in excellent agreement with the gland's structure and biological function. The identity of the neuropeptides was confirmed by additional MS/MS measurements from single 10 μm sample spots, obtained directly from tissue. Furthermore, MS/MS imaging of two different peptides at 10 μm spatial resolution resulted in product ion images, which were in good accordance with the distributions of their peptide precursor ions, confirming their identity and excluding possible analyte interferences. Since the method offers high performance mass spectrometry in combination with high spatial resolution, it appears to be ideally suited for imaging peptide signatures on a cellular level with high confidence in identification and with high sensitivity.