Radioluminescence Microscopy: Measuring the Heterogeneous Uptake of Radiotracers in Single Living Cells

Abstract

Radiotracers play an important role in interrogating molecular processes both in vitro and in vivo. However, current methods, such as PET, autoradiography, and scintillation counting are limited to measuring average radiotracer uptake in large cell populations and, as a result, lack the ability to quantify cell-to-cell variations. To overcome this limitation, we apply a new technique, termed radioluminescence microscopy, to visualize radiotracer uptake in single living cells, in a standard fluorescence microscopy environment [Pratx et al. PLoS One 2013].Live cells are cultured sparsely on a thin scintillator plate and incubated with a radiotracer. Radioluminescence microscopy utilizes the scintillator plate, which is in contact with the cells of interest, to convert ionizing radiation from emitted beta particles into visible-range photons detectable in a sensitive microscope. The optical scintillation signal can be measured with an EM-CCD with a high resolution and enables the detection minute concentrations of radiotracers. The transparency of the plate in the visible range allows for conventional fluorescence and brightfield microscopy, which provides a rich environment to characterize the biological status of a population of living cells. Radioluminescence microscopy revealed strong heterogeneity in the uptake of [18F]fluoro-deoxyglucose (FDG) in single cells, which was found consistent with fluorescence imaging of a glucose analog. We also verified that dynamic uptake of FDG in single cells followed the standard two-tissue compartmental model. A difference in FDG uptake rates using single-cell analysis versus bulk cell analysis revealed that radioluminescence microscopy is an important tool to investigate single-cell biology. Biological studies at the single-cell level aim at elucidating the effect of the cell cycle on the FDG uptake in breast cancer cell populations. These experiments can also find metabolic differences to determine subpopulations in clonal cell populations.