Apoptosis induction-related cytosolic calcium responses revealed by the dual FRET imaging of calcium signals and caspase-3 activation in a single cell

Stimulus-induced changes in the intracellular Ca2+ concentration control cell fate decision, including apoptosis. However, the precise patterns of the cytosolic Ca2+ signals that are associated with apoptotic induction remain unknown. We have developed a novel genetically encoded sensor of activated caspase-3 that can be applied in combination with a genetically encoded sensor of the Ca2+ concentration and have established a dual imaging system that enables the imaging of both cytosolic Ca2+ signals and caspase-3 activation, which is an indicator of apoptosis, in the same cell. Using this system, we identified differences in the cytosolic Ca2+ signals of apoptotic and surviving DT40 B lymphocytes after B cell receptor (BCR) stimulation. In surviving cells, BCR stimulation evoked larger initial Ca2+ spikes followed by a larger sustained elevation of the Ca2+ concentration than those in apoptotic cells; BCR stimulation also resulted in repetitive transient Ca2+ spikes, which were mediated by the influx of Ca2+ from the extracellular space. Our results indicate that the observation of both Ca2+ signals and cells fate in same cell is crucial to gain an accurate understanding of the function of intracellular Ca2+ signals in apoptotic induction.