Deep Lipidomics and Molecular Imaging of Unsaturated Lipid Isomers: A Universal Strategy Initiated by mCPBA Epoxidation

Cellular
lipidome is highly regulated through lipogenesis, rendering
diverse double-bond positional isomers (CC isomer) of a given
unsaturated lipid species. In recent years, there are increasing reports
indicating the physiological roles of CC isomer compositions
associated with diseases, while the biochemistry has not been broadly
investigated due to the challenge in characterizing lipid isomers
inherent to conventional mass spectrometry-based lipidomics. To address
this challenge, we reported a universal, user-friendly, derivatization-based
strategy, MELDI (mCPBA Epoxidation
for Lipid Double-bond Identification),
which enables both large-scale identification and spatial mapping
of biological CC isomers using commercial mass spectrometers
without any instrument modification. With the developed liquid-chromatography
mass spectrometry (LC-MS) lipidomics workflow, we elucidated more
than 100 isomers among monounsaturated and polyunsaturated fatty acids
and glycerophospholipids in human serum, where uncommon isomers of
low abundance were quantified for the first time. The capability of
MELDI-LC-MS in lipidome analysis was further demonstrated using the
differentiated 3T3-L1 adipocytes, providing an insight into the cellular
lipid reprogramming upon stearoyl-coenzyme A desaturase 1 (SCD1) inhibition.
Finally, we highlighted the versatility of MELDI coupled with ambient
mass spectrometry imaging to spatially resolve cancer-associated alteration
of lipid isomers in a metastatic mouse tissue section. Our results
suggested that MELDI will contribute to current lipidomics pipelines
with a deeper level of structural information, allowing us to investigate
the underlying lipid biochemistry.