Dynamic changes in Rap1 activity are required for cell retraction and spreading during mitosis

Abstract

At the onset of mitosis, most adherent cells undergo cell retraction characterised by the disassembly of focal adhesions and actin stress fibres. Mitosis takes place in rounded cells, and the two daughter cells spread again after cytokinesis. Because of the well-documented ability of the small GTPase Rap1 to stimulate integrin-dependent adhesion and spreading, we assessed its role during mitosis. We show that Rap1 activity is regulated during this process. Changes in Rap1 activity play an essential role in regulating cell retraction and spreading, respectively, before and after mitosis of HeLa cells. Indeed, endogenous Rap1 is inhibited at the onset of mitosis; conversely, constitutive activation of Rap1 inhibits the disassembly of premitotic focal adhesions and of the actin cytoskeleton, leading to delayed mitosis and to cytokinesis defects. Rap1 activity slowly increases after mitosis ends; inhibition of Rap1 activation by the ectopic expression of the dominant-negative Rap1[S17A] mutant prevents the rounded cells from spreading after mitosis. For the first time, we provide evidence for the direct regulation of adhesion processes during mitosis via the activity of the Rap1 GTPase.

Conserved role for autophagy in Rho1-mediated cortical remodeling and blood cell recruitment

“…Interestingly, a growing list of Rho pathway components with roles relevant to control of cell protrusion and spreading have been found to undergo ubiquitination and changes in protein stability (34)(35)(36), and recent connections suggest a potential reciprocal regulatory relationship between the Rho and autophagic pathways (37,38). Components that are involved in changing levels of focal adhesion activity and that correspond to a rounding-up of cells also have been identified as being under ubiquitin regulation (39,40). Connections also have been made linking focal adhesion components directly to the process of autophagy (20,41,42), as well as to cell spreading.…”

Rho GTPases as regulators of mitosis and cytokinesis in mammalian cells

“…Cell rounding involves extensive rearrangement of the actin cytoskeleton, de-adhesion, and an increase in cortical rigidity. 10,11 Recent time-lapse microscopy analysis of HeLa cells reveals that cell rounding is one of the earliest mitotic events in prophase occurring prior to centrosome separation and visible chromatin condensation followed by nuclear envelope breakdown.…”

“…Instead this involves inactivation of the small GTPase Rap1. 10 RhoA mediates cell rounding and cortical stiffening through activation of its downstream effectors, Rho kinase (ROCK) and subsequent phosphorylation of myosin II 12,13 in an analogous pathway for contractile ring assembly and function during the ingression phase of cytokinesis.…”

The metastatic cancer cell cortex: An adaptation to enhance robust cell division in novel environments?

“…A number of pathways are involved, likely acting in parallel, downstream of the mitotic kinase, Cdk1 [16]. The focal adhesion complexes that act as points of attachment to the ECM are disassembled, in a process dependent on inactivation of the small GTPase Rap1 [23]. At the same time, the F-actin cytoskeleton is assembled to form the rigid mitotic cortex [17].…”