A novel method based on click chemistry, which overcomes limitations of cell cycle analysis by classical determination of BrdU incorporation, allowing multiplex antibody staining

Abstract

Quantification of BrdU incorporation into DNA is a widely used technique to assess the cell cycle status of cells. DNA denaturation is required for BrdU detection with the drawback that most protein epitopes are destroyed and classical antibody staining techniques for multiplex analysis are not possible. To address this issue we have developed a novel method that overcomes the DNA denaturation step but still allows detection of BrdU. Cells were pulsed for a short time by 5-ethynyl-2'-deoxyuridine, which is incorporated into DNA. The exposed nucleotide alkyne group of DNA was then derivatized in physiologic conditions by the copper (I)-catalyzed azide-alkyne cycloaddition (CuAAC) using BrdU azides. The resulting DNA-bound bromouracil moiety was subsequently detected by commercial anti-BrdU mAb without the need for a denaturation step. Continuous labeling with EdU showed a slightly increased anti-proliferative activity compared to BrdU. However, using a lower concentration of EdU for labeling can compensate for this. Alkynyl tags could be detected quickly by a highly specific reaction using BrdU azides. Fluorescence quenching by the DNA dye PI using both BrdU azides was negligible. Our labeling method is suitable for FCM and HCA and shows a higher signal to noise ratio than other methods. This method also allowed multiplex analysis by simultaneous detection of EdU-BrdU, caspase-3, and phospho-histone 3 mAbs, proving sensitivity and feasibility of this new technique. In addition, it has the potential for use in vivo, as exemplified for bone marrow studies. We have established a new method to determine the position of cells in the cell cycle. This is superior when compared to traditional BrdU detection since it allows multiplex analysis, is more sensitive and shows less quenching with PI. The method provides new opportunities to investigate changes in protein expression at different cell cycle stages using pulse labeling experiments.