Mitochondrial heterogeneity during staurosporine-induced apoptosis in HL60 cells: Analysis at the single cell and single organelle level

Background: Apoptosis is a complex phenomenon during which several events occur. A growing interest exists on the role and functionality of mitochondria during this type of cell death. The responsibility of modifications in mitochondrial membrane potential (Delta Psi) in triggering apoptosis is under investigation. Methods: We evaluated Delta Psi changes in HL60 cells treated with staurosporine (STS). Flow cytometry and confocal microscopy have been used to analyze samples stained with two Delta Psi-sensitive probes, JC1 and MitoTracker(TM) Red CMXRos. Results: At the cellular level, we found heterogeneic behavior. Indeed, after STS treatment, some cells displayed typical markers of apoptosis and a collapse in Delta Psi. Others were apoptotic with no changes in Delta Psi, others changed Delta Psi without being apoptotic, and others were healthy. The same heterogeneic response to STS was found at the single organelle level. In a given cell, some mitochondria were depolarized whereas others were not. Conclusion: In this model of apoptosis, changes in Delta Psi can be different among cells of the same type and among different organelles of the same cell. The collapse in Delta Psi is thus a heterogeneic phenomenon that seems to be an ancillary event following the irreversible phase of the apoptotic process.