Integrating Ex Vivo Isotope Labeling with Mass Spectrometry Imaging to Spatially Trace Heterogeneous Metabolic Activity in Rat Kidney Tissue

Spatially resolved isotope tracing provides a powerful
means for
achieving deeper and more accurate characterization of metabolic activities
in biological tissues. However, its application in mammals remains
limited by several key technical challenges, including complex surgical
procedures, the need for high tracer doses, and poor suitability by
clinical tissue samples. In this study, we developed a novel spatially
resolved isotope tracing method by integrating ex vivo tissue labeling
with airflow-assisted desorption electrospray ionization mass spectrometry
imaging (AFADESI-MSI). Using liquid chromatography–mass spectrometry,
we identified 263 labeled metabolites in ex vivo U–13C glucose-labeled rat kidney tissue, including amino acids, nucleotides,
organic acids, and lipids. Moreover, AFADESI-MSI enabled the in situ
characterization of 27 labeled metabolites. This method allowed the
simultaneous visualization of complex metabolic networks and their
spatially dynamic activities without the need for in vivo tracer injection.
Our results revealed that the kidney cortex exhibited significantly
higher glucose uptake efficiency and gluconeogenic activity compared
with the medulla, whereas the medulla demonstrated greater activity
in reducing pyruvate to lactate. We anticipate that this method will
provide new strategies and analytical tools for investigating metabolic
heterogeneity in clinical tumors and other complex tissues.