Metabolic flux analysis

Citric acid cycle

Organic chemistry

Glycolysis

Gluconeogenesis

Glutamine

Biology

Endocrinology

Medulla

Chemistry

Metabolic pathway

Tricarboxylic acid

Metabolism

Amino acid

Biochemistry

Flux (metallurgy)

Nature Methods

Isotope tracing has helped to determine the metabolic activities of organs. Methods to probe metabolic heterogeneity within organs are less developed. We couple stable-isotope-labeled nutrient infusion to matrix-assisted laser desorption ionization imaging mass spectrometry (iso-imaging) to quantitate metabolic activity in mammalian tissues in a spatially resolved manner. In the kidney, we visualize gluconeogenic flux and glycolytic flux in the cortex and medulla, respectively. Tricarboxylic acid cycle substrate usage differs across kidney regions; glutamine and citrate are used preferentially in the cortex and fatty acids are used in the medulla. In the brain, we observe spatial gradations in carbon inputs to the tricarboxylic acid cycle and glutamate under a ketogenic diet. In a carbohydrate-rich diet, glucose predominates throughout but in a ketogenic diet, 3-hydroxybutyrate contributes most strongly in the hippocampus and least in the midbrain. Brain nitrogen sources also vary spatially; branched-chain amino acids contribute most in the midbrain, whereas ammonia contributes in the thalamus. Thus, iso-imaging can reveal the spatial organization of metabolic activity. Iso-imaging integrates stable-isotope infusions with imaging mass spectrometry to enable quantitative analysis of metabolic activity in mammalian tissues with spatial resolution. IsoScope software facilitates analysis of iso-imaging data.

Spatially resolved isotope tracing reveals tissue metabolic activity