The formation of Ca^(2+) gradients at the cleavage furrows during cytokinesis of Zebrafish embryos

In dividing embryos,a localized elevation in intracellular Ca^(2+)([Ca^(2+)]i)at the cleavage furrow has been shown to be essential for cytokinesis.However,the underlying mechanisms for generating and maintaining these[Ca^(2+)]_(i) gradients throughout cytokinesis are not fully understood.In the present study,we analyzed the role of inositol 1,4,5-trisphosphate receptors(IP3Rs)and endoplasmic reticulum(ER)distribution in determining the intracellular Ca^(2+) gradients in early zebrafish blastomeres.Application of the injected Ca^(2+) indicator,Indo-1,showed that during the first cell division a standing Ca^(2+) gradient was formed~35 min after fertilization,with the[Ca^(2+)]_(i) spatially decaying from 500–600 nmol/L at the cleavage furrow to 100–200 nmol/L around the nucleus.While the IP3R immunohistochemical fluorescence was relatively concentrated in the peri-furrow region,ER labeling was relatively enriched in both peri-furrow and peri-nuclear regions.Numeric simulation suggested that a divergence in the spatial distribution of IP3R and the locations of Ca^(2+) uptake within the ER was essential for the formation of a standing Ca^(2+) gradient,and the Ca^(2+) gradient could only be well-established under an optimal stoichiometry of Ca^(2+) uptake and release.Indeed,while inhibition of IP3R Ca^(2+) release blocked the generation of the Ca^(2+)gradient at a lower[Ca^(2+)]_(i) level,both Ca^(2+) release stimulation by inositol 1,4,5-trisphosphate(IP3)injection and ER Ca^(2+) pump inhibition by cyclopiazonic acid also eliminated the Ca^(2+) gradients at higher[Ca^(2+)]_(i) levels.Our results suggest a dynamic relationship between ER-mediated Ca^(2+) release and uptake that underlies the maintenance of the perifurrow Ca^(2+) gradient and is essential for cytokinesis of zebrafish embryos.