Stable Isotopes for Tracing Mammalian-Cell Metabolism In Vivo

Stable-isotope measurements are increasingly used to probe mammalian-cell metabolism in vivo. The selection of stable-isotope tracer(s) and tracer administration approach is key to maximize the information extracted from in vivo measurements. Metabolic models integrating stable-isotope tracer measurements in tissues and plasma allow quantitative readouts of in vivo metabolism at the whole-organ/whole-body level to be obtained. Tissue heterogeneity and metabolic compartmentalization need to be considered during data interpretation. The development of single-cell/single-organelle metabolomic approaches will advance our understanding of in vivo metabolism. Metabolism is at the cornerstone of all cellular functions and mounting evidence of its deregulation in different diseases emphasizes the importance of a comprehensive understanding of metabolic regulation at the whole-organism level. Stable-isotope measurements are a powerful tool for probing cellular metabolism and, as a result, are increasingly used to study metabolism in in vivo settings. The additional complexity of in vivo metabolic measurements requires paying special attention to experimental design and data interpretation. Here, we review recent work where in vivo stable-isotope measurements have been used to address relevant biological questions within an in vivo context, summarize different experimental and data interpretation approaches and their limitations, and discuss future opportunities in the field. Metabolism is at the cornerstone of all cellular functions and mounting evidence of its deregulation in different diseases emphasizes the importance of a comprehensive understanding of metabolic regulation at the whole-organism level. Stable-isotope measurements are a powerful tool for probing cellular metabolism and, as a result, are increasingly used to study metabolism in in vivo settings. The additional complexity of in vivo metabolic measurements requires paying special attention to experimental design and data interpretation. Here, we review recent work where in vivo stable-isotope measurements have been used to address relevant biological questions within an in vivo context, summarize different experimental and data interpretation approaches and their limitations, and discuss future opportunities in the field. the set of metabolic pathways responsible for transforming carbon from nutrients into biomass and energy inside the cell, including glycolysis, gluconeogenesis, the pentose phosphate pathway, the TCA cycle, the glyoxylate shunt, and the methyl-citrate cycle. gas chromatography; analytical chromatographic technique used to separate volatile substances in the gas phase, based on their different interactions with a stationary phase (or column). The mobile phase is not involved in the interactions per se, but is rather a chemically inert gas that serves to carry the molecules through the stationary phase. collection of MS techniques capable of high resolution in (m/z), typically characterized by an accuracy of four or more decimal places, allowing the detection of differences in mass between compounds with the same nominal mass but different chemical formulas. a condition during an isotope-labeling experiment in which the isotopic enrichment in a given metabolite is stable over time. This should not be confused with the term metabolic steady state, representing a condition in which all layers of metabolism (i.e., metabolite concentrations and metabolic fluxes) remain constant over time in a biological system, irrespective of isotopic labeling. instances of the same molecule that differ in their isotope composition (and consequently in mass). Isotopologues are often referred to in the literature as mass isotopomers. The use of the latter term is, however, discouraged, as it may lead to incorrect identification with the term isotopomer itself (see below). instances of the same isotopologue that differ in the position of their isotopes (and thus not in mass). Resolving the different isotopomers of a given isotopologue requires analytical techniques capable of distinguishing positional isotopic enrichment, such as nuclear magnetic resonance. liquid chromatography; analytical chromatographic technique used to separate ions or molecules based on their different interactions with a liquid mobile phase (where sample ions or molecules are dissolved) and a solid stationary phase (or column). in an isotope labeling experiment, a vector representing the fractional abundances of different isotopologues of a given metabolite in a sample, originating from label incorporation, relative to the total pool of that metabolite in the sample. For a metabolite with N atoms susceptible of label incorporation, the corresponding MDV will have N + 1 components, ranging from the unlabeled isotopologue (no label incorporation) to the fully labeled isotopologue (maximum label incorporation). analytical technique to measure the mass-to-charge ratio (m/z) of one or more ionized molecular species present in a sample. MS-based technique used to determine the spatial abundance profiles of different molecular species within a two-dimensional tissue or sample. the rate at which a whole organism jointly consumes (or produces) a given nutrient to maintain whole-body metabolic homeostasis. a molecule in which one or more atoms are substituted by heavy stable (i.e., nonradioactive) isotopes of the same chemical element. The labeled atoms will generally present the same biological behavior as their unlabeled counterparts, but their increased mass enables their distinction from the latter by appropriate analytical techniques.