Imaging mass spectrometry enables molecular profiling of mouse and human pancreatic tissue
The molecular response and function of pancreatic islet cells during metabolic stress is a complex process. The anatomical location and small size of pancreatic islets coupled with current methodological limitations have prevented the achievement of a complete, coherent picture of the role that lipids and proteins play in cellular processes under normal conditions and in diseased states. Herein, we describe the development of untargeted tissue imaging mass spectrometry (IMS) technologies for the study of in situ protein and, more specifically, lipid distributions in murine and human pancreases.
We developed matrix-assisted laser desorption/ionisation (MALDI) IMS protocols to study metabolite, lipid and protein distributions in mouse (wild-type and ob/ob mouse models) and human pancreases. IMS allows for the facile discrimination of chemically similar lipid and metabolite isoforms that cannot be distinguished using standard immunohistochemical techniques. Co-registration of MS images with immunofluorescence images acquired from serial tissue sections allowed accurate cross-registration of cell types. By acquiring immunofluorescence images first, this serial section approach guides targeted high spatial resolution IMS analyses (down to 15 μm) of regions of interest and leads to reduced time requirements for data acquisition.
MALDI IMS enabled the molecular identification of specific phospholipid and glycolipid isoforms in pancreatic islets with intra-islet spatial resolution. This technology shows that subtle differences in the chemical structure of phospholipids can dramatically affect their distribution patterns and, presumably, cellular function within the islet and exocrine compartments of the pancreas (e.g. 18:1 vs 18:2 fatty acyl groups in phosphatidylcholine lipids). We also observed the localisation of specific GM3 ganglioside lipids [GM3(d34:1), GM3(d36:1), GM3(d38:1) and GM3(d40:1)] within murine islet cells that were correlated with a higher level of GM3 synthase as verified by immunostaining. However, in human pancreas, GM3 gangliosides were equally distributed in both the endocrine and exocrine tissue, with only one GM3 isoform showing islet-specific localisation.
The development of more complete molecular profiles of pancreatic tissue will provide important insight into the molecular state of the pancreas during islet development, normal function, and diseased states. For example, this study demonstrates that these results can provide novel insight into the potential signalling mechanisms involving phospholipids and glycolipids that would be difficult to detect by targeted methods, and can help raise new hypotheses about the types of physiological control exerted on endocrine hormone-producing cells in islets. Importantly, the in situ measurements afforded by IMS do not require a priori knowledge of molecules of interest and are not susceptible to the limitations of immunohistochemistry, providing the opportunity for novel biomarker discovery. Notably, the presence of multiple GM3 isoforms in mouse islets and the differential localisation of lipids in human tissue underscore the important role these molecules play in regulating insulin modulation and suggest species, organ, and cell specificity. This approach demonstrates the importance of both high spatial resolution and high molecular specificity to accurately survey the molecular composition of complex, multi-functional tissues such as the pancreas.
KeywordsDiabetes Ganglioside Imaging mass spectrometry MALDI Pancreas Phospholipid
ion cyclotron resonance
Imaging mass spectrometry
Matrix-assisted laser desorption/ionisation
Tandem mass spectrometry
Nanospray desorption electrospray ionisation
Total ion current
BMP, NJH, NP, RH, MB, JLN, ACP and RMC conceived and designed the research. NJH and NP performed glucose tolerance tests and animal euthanasia. BMP and AJ prepared tissue sections. BMP performed MALDI IMS and MS/MS analyses. NJH, NP, RH, AJ and RA conceived experiments and performed hormone IHC. AJ, CLL and KLB performed GM3 synthase IHC. BMP analysed MS data. BMP, NJH, NP, RH, JMS, RWS, CVW, MB, JLN, ACP and RMC interpreted results of experiments. BMP prepared figures and drafted the manuscript prior to revision and approval by the other authors. RMC is the guarantor of this work.
This work was supported by the National Institutes of Health (NIH) under award P41 GM103391-07 (National Institute of General Medical Sciences [NIGMS]) and by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)-supported Human Islet Research Network (HIRN, RRID:SCR_014393; https://hirnetwork.org; UC4DK104211, DK108120, DK104218 and DK112232), DK106755, DK72473, DK89572, DK97829, DK94199, the Vanderbilt Diabetes Research and Training Center (DK20593) and grants from the JDRF, the Leona M. and Harry B. Helmsley Charitable Trust, and the Department of Veterans Affairs. TPSR is supported by the NIH under awards 5P30 CA68485-19 (National Cancer Institute [NCI]) and 2U24 DK059637-16 (Vanderbilt Mouse Metabolic Phenotyping Center). BMP was supported by the NIH/NIDDK under award F32 FDK105841A.
Duality of interest
The authors declare that there is no duality of interest associated with this manuscript.
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