Spatial Systems Lipidomics Reveals Nonalcoholic Fatty Liver Disease Heterogeneity

Hepatocellular
lipid accumulation characterizes nonalcoholic fatty
liver disease (NAFLD). However, the types of lipids associated with
disease progression are debated, as is the impact of their localization.
Traditional lipidomics analysis using liver homogenates or plasma
dilutes and averages lipid concentrations, and does not provide spatial
information about lipid distribution. We aimed to characterize the
distribution of specific lipid species related to NAFLD severity by
performing label-free molecular analysis by mass spectrometry imaging
(MSI). Fresh frozen liver biopsies from obese subjects undergoing
bariatric surgery (n = 23) with various degrees of
NAFLD were cryosectioned and analyzed by matrix-assisted laser desorption/ionization
(MALDI)-MSI. Molecular identification was verified by tandem MS. Tissue
sections were histopathologically stained, annotated according to
the Kleiner classification, and coregistered with the MSI data set.
Lipid pathway analysis was performed and linked to local proteome
networks. Spatially resolved lipid profiles showed pronounced differences
between nonsteatotic and steatotic tissues. Lipid identification and
network analyses revealed phosphatidylinositols and arachidonic acid
metabolism in nonsteatotic regions, whereas low–density lipoprotein
(LDL) and very low–density lipoprotein (VLDL) metabolism was
associated with steatotic tissue. Supervised and unsupervised discriminant
analysis using lipid based classifiers outperformed simulated analysis
of liver tissue homogenates in predicting steatosis severity. We conclude
that lipid composition of steatotic and nonsteatotic tissue is highly
distinct, implying that spatial context is important for understanding
the mechanisms of lipid accumulation in NAFLD. MSI combined with principal
component–linear discriminant analysis linking lipid and protein
pathways represents a novel tool enabling detailed, comprehensive
studies of the heterogeneity of NAFLD.