Localizing and quantifying metabolites in situ with luminometry: Induced metabolic bioluminescence imaging (imBI)

Abstract

The technique of induced metabolic bioluminescence imaging (imBI) has been developed to obtain a “snapshot” of the momentary metabolic status of biological tissues. Using cryo-sections of snap-frozen tissue specimens, imBI combines highly specifi c and sensitive in situ detection of metabolites with spatial resolution on a microscopic level and with metabolic imaging in relation to tissue histology. Establishing appropriate enzyme mixtures, various metabolites, such as ATP, glucose, lactate, pyruvate, and many others can be biochemically linked to the emission of light by luciferases in a stoichiometrical and calibratable manner. Spatial resolution is achieved with a particular sandwich technique inducing local bioluminescence in the tissue sections. Two-dimensional light intensity profi les can be registered with a high performance microscope and a cooled back-illuminated EM-CCD camera. These profi les, which can be calibrated in micromole of metabolite per gram of tissue (μmol/g; equivalent to mmol/L or mM in solution), are routinely displayed in a colorcoded way. In the standard confi guration, the minimal detectable metabolite concentration is in the range of 100–200 μM, the maximal linear spatial resolution is 20–25 μm. Serial sectioning and a specifi cally designed technique for precise overlay allow for signal acquisition in selected histological areas, e.g., distinct zones of organs, such as renal medulla versus renal cortex. The method has been applied by different investigators in various fi elds of biology and medicine including quantitative metabolic imaging in plant seedlings, 2D or 3D cell cultures, blood vessel walls, tissue wounds, various animal organs, experimental tumors, and cancers in patients. While not used extensively in neuroscience yet, this technology has a great potential to unravel the local distribution of metabolites in the context of brain metabolism. In summary, biologically and clinically relevant and signifi cant results were obtained with imBI most likely due to the “natural way” of detecting metabolites, i.e., using them as substrates of specifi c enzymes.