Cytokinesis Triggers Two Separate Spikes of Intracellular Calcium

Abstract

Cytokinesis is the final stage of cell division during which two daughter cells separate. In some animal embryos including those of sea urchin and zebrafish, cytoplasmic calcium concentration increases sharply (spike) at the cell division plane during cytokinesis. However, it remained unclear how evolutionally conserved this process is and whether the intracellular calcium is important to cytokinesis. Here we employed GCaMP6s, a genetically encoded calcium reporter, to measure intracellular calcium concentration during cytokinesis of the model organism Schizosacchromyces pombe. First, we validated this novel fission yeast calcium reporter by quantifying the calcium concentration of single cells with live fluorescence microscopy. As expected, although the calcium concentration remained constant in most of the unstimulated cells, external stimuli triggered frequent spikes of intracellular calcium concentration. In the presence of excessive extracellular calcium, the intracellular calcium concentration spiked frequently. Hypo-osmotic shocks, applied through microfluidics, triggered a transient but sharp increase in the frequency of calcium spikes. We then measured the intracellular calcium concentration throughout cell division and found cytokinesis triggered two calcium spikes. The first followed the initiation of the cleavage furrow ingression. The second spike accompanied the initiation of cell separation, exhibiting asymmetrically in two daughter cells. Both spikes are global, not restricted to the cell division plane. Lastly, we examined whether these calcium spikes are essential to cytokinesis. Inhibition of these spikes reduced the rate of ring closure and triggered frequent post-cytokinesis cell lysis. Our study demonstrated that cytokinesis triggered calcium spikes are evolutionally conserved. We propose that this transient increase of intracellular calcium concentration is essential for turgor regulation to support the furrow ingression and the cell separation.