Semi-automated, quantitative immunofluorescence analysis of single cell cytochrome c release during apoptotic cell...

Apoptosis, a tightly regulated form of programmed cell death, eliminates damaged or malignant cells and is triggered by internal or external stress signals. A critical decision point is mitochondrial outer membrane permeabilization (MOMP), governed by BCL-2 family proteins. Pro-apoptotic members such as BAX and BAK form pores in the mitochondrial outer membrane, releasing intermembrane space proteins like cytochrome c into the cytoplasm. Once cytosolic, cytochrome c binds APAF-1 to form the apoptosome, which activates caspase-9 and subsequently caspase-3, driving apoptosis through cleavage of key cellular substrates. Cytochrome c release serves as a hallmark and point of no return in the apoptotic cascade. However, cytochrome c release can be variable, occurring at submaximal levels or from only a subset of mitochondria, which complicates detection in heterogeneous cell populations. To address this, we developed a semi-automated imaging-based method to quantify cytochrome c release at the single-cell level using immunofluorescence microscopy. Our approach uses CellProfiler, an open-source image analysis platform, to implement a pipeline that segments adherent cells into nuclear, mitochondrial, and cytoplasmic compartments based on compartment-specific reference stains. The pipeline quantifies cytochrome c distribution across these compartments, calculating the ratio of mitochondrially retained to cytoplasmic cytochrome c for each cell. Automation of segmentation and measurement ensures rapid, robust, and reproducible analysis, with only image acquisition and data interpretation performed manually. This method provides a quantitative readout of MOMP and can be readily adapted to any immunofluorescence-detectable protein given an appropriate compartmental marker, expanding its utility for broader cellular studies.