Abstract
Different models for animal cell cytokinesis posit that the stiffness of the equatorial cortex is either increased or decreased relative to the stiffness of the polar cortex. A recent work has suggested that the critical cytokinesis signaling complex centralspindlin may reduce the stiffness of the equatorial cortex by inactivating the small GTPase Rac. To determine if such a reduction occurs and if it depends on centralspindlin, we devised a method to estimate cortical bending stiffness with high spatio-temporal resolution from in vivo cell shapes. Using the early Caenorhabditis elegans embryo as a model, we show that the stiffness of the equatorial cell surface is reduced during cytokinesis, whereas the stiffness of the polar cell surface remains stiff. The equatorial reduction of stiffness was compromised in cells with a mutation in the gene encoding the ZEN-4/kinesin-6 subunit of centralspindlin. Theoretical modeling showed that the absence of the equatorial reduction of stiffness could explain the arrest of furrow ingression in the mutant. By contrast, the equatorial reduction of stiffness was sufficient to generate a cleavage furrow even without the constriction force of the contractile ring. In this regime, the contractile ring had a supportive contribution to furrow ingression. We conclude that stiffness is reduced around the equator in a centralspindlin-dependent manner. In addition, computational modeling suggests that proper regulation of stiffness could be sufficient for cleavage furrow ingression.
Highlights
Cytokinesis is the final step of cell division that mechanically separates a mother cell into two daughter cells
Our analysis indicates that the stiffness of the equatorial cell surface is reduced during cytokinesis and that this reduction depends on the centralspindlin component ZEN-4
Quantification of cell shape To examine whether cell surface stiffness is reduced around the cleavage furrow, we estimated spatio-temporal changes in surface stiffness by fitting in vivo cell shapes to a mathematical model
Summary
Cytokinesis is the final step of cell division that mechanically separates a mother cell into two daughter cells. Cytokinesis is accomplished via constriction of a cortical contractile ring. The constriction force generated by the actomyosinbased contractile ring is typically considered to be the principal mechanical component for cleavage furrow ingression [1], the mechanical properties of the cell surface contribute to cleavage furrow ingression [2]. One example that illustrates the importance of cortical mechanics is the fact that furrow ingression is completely inhibited by the disruption of cell surface actin filaments around the polar regions [3]. Some gene products required for cytokinesis are involved in cell surface stiffness, e.g., the actin regulator racE of Dictyostelium discoideum [4]. For a fundamental understanding of the mechanics of cytokinesis, a consideration of cell surface mechanics is essential
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