The role of calcium signaling during cytokinesis has long remained ambiguous. Past studies of embryonic cell division discovered that calcium concentration increases transiently at the division plane just before the cleavage furrow ingression, suggesting that these calcium transients could trigger the contractile ring constriction. However, such calcium transients have only been found in animal embryos and their function remains controversial. Here we explored cytokinetic calcium transients in the fission yeast Schizosaccharomyces pombe by adopting GCaMP, a genetically encoded calcium indicator, to determine the intracellular calcium level of this model organism. We validated GCaMP as a highly sensitive calcium reporter in fission yeast, allowing us to capture calcium transients triggered by osmotic shocks. We identified a correlation between the intracellular calcium level and cell division, consistent with the existence of calcium transients during cytokinesis. Using time-lapse microscopy and quantitative image analysis, we discovered calcium spikes both at the start of the cleavage furrow ingression and the end of cell separation. Inhibition of these calcium spikes slowed the furrow ingression and led to frequent lysis of daughter cells. We conclude that like the larger animal embryos fission yeast triggers calcium transients that may play an important role in cytokinesis (197). [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text]

The role of calcium during cell division has long remained ambiguous. The intracellular calcium concentration of many animal embryos increases transiently during cytokinesis, leading to the long-standing proposal that calcium transients may trigger the contraction of actomyosin ring.However, it remains unknown whether these calcium transients can be found in cells beyond those large embryos and whether they have any role in cytokinesis. Here we addressed these questions in the unicellular model organism fission yeast by adopting GCaMP, a genetically encoded indicator, to determine its intracellular calcium level. With confocal microscopy, we captured both the calcium homeostasis and the calcium transients in live cells using this calcium reporter. We searched for the cytokinetic calcium transients using two independent approaches.Both analyses of the intracellular calcium in a population and time-lapse microscopy of dividing cell revealed two cytokinetic calcium spikes. The first initiated at the beginning of the cleavage furrow ingression and the second one at the end of the cell separation. The temporal regulation of these spikes bears a strong similarity to the two calcium waves discovered in the cytokinesis of fish embryos. These slow calcium spikes propagated intracellularly, not restricted to the cell division plane. Although depletion of these spikes did not prevent the contractile ring from contracting as predicted, it reduced the rate of contraction and led to lysis of many daughter cells.We conclude that the fission yeast cytokinetic calcium spikes promote the contractile ring closure and the integrity of separating cells, but they are not required for triggering the ring contraction. Our discovery suggests that transient increase of intracellular calcium may be a conserved regulatory mechanism of cytokinesis among eukaryotes.