Yee Seir Kee

Mitosis-Specific Mechanosensing and Contractile-Protein Redistribution Control Cell Shape

Because cell-division failure is deleterious, promoting tumorigenesis in mammals, cells utilize numerous mechanisms to control their cell-cycle progression. Though cell division is considered a well-ordered sequence of biochemical events, cytokinesis, an inherently mechanical process, must also be mechanically controlled to ensure that two equivalent daughter cells are produced with high fidelity. Given that cells respond to their mechanical environment, we hypothesized that cells utilize mechanosensing and mechanical feedback to sense and correct shape asymmetries during cytokinesis. Because the mitotic spindle and myosin II are vital to cell division, we explored their roles in responding to shape perturbations during cell division. We demonstrate that the contractile proteins myosin II and cortexillin I redistribute in response to intrinsic and externally induced shape asymmetries. In early cytokinesis, mechanical load overrides spindle cues and slows cytokinesis progression while contractile proteins accumulate and correct shape asymmetries. In late cytokinesis, mechanical perturbation also directs contractile proteins but without apparently disrupting cytokinesis. Significantly, this response only occurs during anaphase through cytokinesis, does not require microtubules, and is independent of spindle orientation, but is dependent on myosin II. Our data provide evidence for a mechanosensory system that directs contractile proteins to regulate cell shape during mitosis.

14-3-3 Coordinates Microtubules, Rac, and Myosin II to Control Cell Mechanics and Cytokinesis

BACKGROUND During cytokinesis, regulatory signals are presumed to emanate from the mitotic spindle. However, what these signals are and how they lead to the spatiotemporal changes in the cortex structure, mechanics, and regional contractility are not well understood in any system. RESULTS To investigate pathways that link the microtubule network to the cortical changes that promote cytokinesis, we used chemical genetics in Dictyostelium to identify genetic suppressors of nocodazole, a microtubule depolymerizer. We identified 14-3-3 and found that it is enriched in the cortex, helps maintain steady-state microtubule length, contributes to normal cortical tension, modulates actin wave formation, and controls the symmetry and kinetics of cleavage furrow contractility during cytokinesis. Furthermore, 14-3-3 acts downstream of a Rac small GTPase (RacE), associates with myosin II heavy chain, and is needed to promote myosin II bipolar thick filament remodeling. CONCLUSIONS 14-3-3 connects microtubules, Rac, and myosin II to control several aspects of cortical dynamics, mechanics, and cytokinesis cell shape change. Furthermore, 14-3-3 interacts directly with myosin II heavy chain to promote bipolar thick filament remodeling and distribution. Overall, 14-3-3 appears to integrate several critical cytoskeletal elements that drive two important processes-cytokinesis cell shape change and cell mechanics.

A mechanosensory system governs myosin II accumulation in dividing cells.

A mechanosensory system is characterized that fine-tunes the level of myosin II at the cleavage furrow. This mechanosensory system consists of the mechanosensory module composed of myosin II and cortexillin I and a mechanotransduction loop that includes IQGAP2, kinesin-6, and INCENP.

Cytokinesis through biochemical-mechanical feedback loops.

Cytokinesis is emerging as a control system defined by interacting biochemical and mechanical modules, which form a system of feedback loops. This integrated system accounts for the regulation and kinetics of cytokinesis furrowing and demonstrates that cytokinesis is a whole-cell process in which the global and equatorial cortices and cytoplasm are active players in the system. Though originally defined in Dictyostelium, features of the control system are recognizable in other organisms, suggesting a universal mechanism for cytokinesis regulation and contractility.

Motor Proteins: Myosin Mechanosensors

Mechanosensation is emerging as a general principle of myosin motors. As demonstrated in a recent study, the single-headed myosin I molecule is an exquisite mechanosensor, able to sense strain over a very small force range.

Micropipette Aspiration for Studying Cellular Mechanosensory Responses and Mechanics

Micropipette aspiration (MPA) is a widely applied method for studying cortical tension and deformability. Based on simple hydrostatic principles, this assay allows the application of a specific magnitude of mechanical stress on cells. This powerful method has revealed insights about cell mechanics and mechanosensing, not only in Dictyostelium discoideum but also in other cell types. In this chapter, we present how to set up a micropipette aspiration system and the experimental procedures for determining cortical tension and mechanosensory responses.

7.5 Understanding How Dividing Cells Change Shape

Cytokinesis is an essential cellular process with significant developmental and medical implications. Fundamentally mechanical, this geometrically simple cell shape change encompasses nearly all cellular processes. Particularly featured are cytoskeletal mechanics, molecular motor mechanochemistry, fluid dynamics, and cellular physiology, all of which are carried out by genetically encoded biomolecules. This chapter presents the current understanding of how these processes and features contribute to the physical aspects of cytokinesis. The chapter is rounded out with a synthesis of the processes into what is emerging as an integrated control system characterized by mechanical and biochemical feedback loops.