Mobilization of lysosomal calcium regulates the externalization of phosphatidylserine during Apoptosis

A hallmark of apoptotic cells is the Ca²⁺-dependent appearance of phosphatidylserine (PS) at the cell surface as a result of its redistribution from the inner-to-outer plasma membrane leaflet. Although endoplasmic reticulum and mitochondrial Ca²⁺ are known to participate in apoptosis, their role in PS externalization has not been established. In this study, several organelle-specific fluorescent markers and Ca²⁺-sensitive probes were used to identify the source of Ca²⁺ critical to PS externalization. By employing Rhod-2AM, fluorescein-labeled high molecular weight dextran, and Calcium Green 1, we provide evidence that lysosomes respond to apoptotic stimuli by releasing their luminal Ca²⁺ to the cytosol. Cells treated with the cytosolic phospholipase A₂ inhibitor, cPLA2α, had no effect on caspase activation but exhibited a significant decrease in lysosomal Ca²⁺ release and externalization of PS in response to apoptotic stimuli. Similarly, cells depleted of lysosomal Ca²⁺ underwent programmed cell death yet failed to externalize PS. These data indicate that although Ca²⁺ release from other intracellular organelles to the cytosol is adequate for apoptosis, the release of Ca²⁺ from lysosomes is critical for PS externalization.