Characterization of cells with different mitochondrial membrane potential during apoptosis

Background: Until now, the simultaneous analysis of several parameters during apoptosis, including DNA content and mitochondrial membrane potential (Delta Psi), has not been possible because of the spectral characteristics of the commonly used dyes. Using polychromatic flow cytometry based upon multiple laser and UV lamp excitation, we have characterized cells with different Delta Psi during apoptosis. Methods: U937 cells were treated with the flavonoid quercetin (Qu) and stained with JC-1 to detect AT, propidium iodide (PI) for cell viability, Hoechst 33342 for DNA content, Annexin V conjugated with Alexa Fluor-647 for detection of phosphatidilserine (PS) exposure, marker of early apoptosis, or Mitotracker Deep Red for the determination of mitochondrial mass. Results: Treatment with Qu provoked the onset of three cell populations with different Delta Psi: (1) healthy cells, with normal Delta Psi, DNA content and physical parameters, high mitochondrial mass, PI- and Annexin V-negative; (2) cells with intermediate Delta Psi and normal DNA content, but with physical parameters typical of apoptotic cells and low mitochondrial mass; most of them were PI+ and Annexin V+; (3) cells with collapsed Delta Psi that had low mitochondrial mass and were Annexin-V+, PI+; half of them showed diminished DNA content. Similar results, i.e. the presence of cells with intermediate Delta Psi, were observed in other models of apoptosis. Conclusions: During Qu-induced apoptosis, loss of Delta Psi, PS exposure, and decrease of mitochondrial mass are early events that precede permeability to PI and loss of DNA. Populations of cells with different Delta Psi, as revealed by flow cytometry after JC-1 staining, differed also for other parameters associated to apoptosis. Thus, the simultaneous analysis of several parameters by polychromatic flow cytometry permits a better identification of many stages of cell death, and, more in general, allows to evaluate the eventual heterogenic sensibility of the population under study to a given compound.