Measurement of Contractile Ring Tension Using Two-photon Laser Ablation during Drosophila Cellularization.

Cytokinesis occurs at the final step of cell division and leads to the separation of daughter cells. It requires assembly and constriction of the actomyosin contractile ring. The phases of assembly and constriction of the contractile ring show an increase in tension in the actomyosin complex. The measurement of tension in the contractile ring is of interest to probe the mechanics of contractile ring formation. Drosophila cellularization is a powerful genetic model system to study the mechanisms regulating actomyosin contractility during contractile ring constriction. Cellularization occurs in the interphase of syncytial cycle 14, where the plasma membrane extends around individual nuclei and forms complete cells with the help of a contractile ring at the bottom. The contractile ring forms at the furrow tip during the extension around individual nuclei and its assembly requires the coordinated action of cytoskeletal and plasma membrane-associated proteins. Laser ablation of the contractile ring enables the measurement of the contractility of the actomyosin network during cytokinesis. This protocol outlines the method used for estimating the contractility at the actomyosin ring during cellularization by laser ablation, in both control and mutant embryos for a Rho guanosine triphosphatase activating protein (RhoGAP) containing protein called GRAF (GTPase regulator associated with focal adhesion kinase-1). Physical cutting of the contractile ring by a two-photon laser at 800 nm leads to the displacement of the actomyosin ring edges, at a rate dependent upon the tension. This can be carried out at distinct steps of the contractile ring assembly during furrow extension in cellularization. Quantification of the extent of displacement and recoil velocity of movement of the edges at different stages of cellularization provides a quantitative measure of contractility in the system. This protocol describes the experimental procedure containing the preparation of live embryos, optimization of laser power, acquisition settings, and post-acquisition analysis of actomyosin contractility during Drosophila cellularization.