In-situ observation of mitochondrial biogenesis as the early event of apoptosis

SUMMARY Mitochondrial biogenesis is a cell response to external stimuli which is generally believed to suppress apoptosis. However, during the process of apoptosis, whether mitochondrial biogenesis occurs in the early stage of the apoptotic cells remains unclear. To address this question, we constructed the COX8-EGFP-ACTIN-mCherry HeLa cells with recombinant fluorescent proteins respectively tagged on the nucleus and mitochondria, and monitored the mitochondrial changes in living cells exposed to gamma-ray radiation. Besides in situ detection of mitochondrial fluorescence changes, we also examined the cell viability, nuclear DNA damage, reactive oxygen species (ROS), Mitochondrial superoxide, citrate synthase activity, ATP, cytoplasmic and mitochondrial calcium, mitochondrial DNA copy number and expression of transcription genes related to mitochondrial biogenesis as well as the apoptosis biomarkers. As a result, we confirmed that significant mitochondrial biogenesis took place preceding the radiation-induced apoptosis, and the change of mitochondrial biogenesis at early time was closely correlated with the apoptotic cells at late stage. The involved mechanism was also discussed. Highlights A dual fluorescence reporter system was successfully constructed for in-situ observation of mitochondrial biogenesis in living cells. The whole process of radiation-induced mitochondrial biogenesis and apoptosis was scrutinized. The conception of the relationship between mitochondrial biogenesis and apoptosis was revised. Assessment of the early event of mitochondrial biogenesis is critical for prediction of the late fate of cells. Graphical abstract