Formation of poly(ADP-ribose) (PAR) marks intracellular stress signaling and is notably induced upon DNA damage. PAR polymerases (PARPs) catalyze PAR synthesis upon genotoxic stress and thereby recruit multiple proteins to damaged chromatin. PAR induction is transient and antagonized by the action of PAR glycohydrolase (PARG). Given that poly(ADP-ribosyl)ation (PARylation) is involved in genome integrity maintenance and other vital cellular functions, but also in light of the recent approval of PARP inhibitors for cancer treatments, reliable measurements of intracellular PAR formation have gained importance. Here we provide a detailed protocol for PAR measurements by quantitative image-based cytometry. This technique combines the high spatial resolution of single-cell microscopy with the advantages of cell population measurements through automated high-content imaging. Such upscaling of immunofluorescence-based PAR detection not only increases the robustness of the measurements through averaging across large cell populations but also allows for the discrimination of subpopulations and thus enables multivariate measurements of PAR levels and DNA damage signaling. We illustrate how this technique can be used to assess the dynamics of the cellular response to oxidative damage as well as to PARP inhibitor-induced genotoxicity in a cell cycle resolved manner. Due to the possibility to use any automated microscope for quantitative image-based cytometry, the presented method has widespread applicability in the area of PARP biology and beyond.
Keywords: ARTD1; Cell cycle; Cytometry; DNA damage; High-content microscopy; Olaparib; PARP inhibitors; PARP1; Poly(ADP-ribose) (PAR); Quantitative single-cell analyses.