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As the interface between the cell and its environment, the cell cortex must be able to respond to a variety of external stimuli. This is made possible in part by cortical excitability, a behavior driven by coupled positive and negative feedback loops that generate propagating waves of actin assembly in the cell cortex. Cortical excitability is best known for promoting cell protrusion and allowing the interpretation of and response to chemoattractant gradients in migrating cells. It has recently become apparent, however, that cortical excitability is involved in the response of the cortex to internal signals from the cell-cycle regulatory machinery and the spindle during cell division. Two overlapping functions have been ascribed to cortical excitability in cell division: control of cell division plane placement, and amplification of the activity of the small GTPase Rho at the equatorial cortex during cytokinesis. Here, we propose that cortical excitability explains several important yet poorly understood features of signaling during cell division. We also consider the potential advantages that arise from the use of cortical excitability as a signaling mechanism to regulate cortical dynamics in cell division., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Emergence of the cytokinetic Rho zone that orchestrates formation and ingression of the cleavage furrow had been explained previously via microtubule-dependent cortical concentration of Ect2, a guanine nucleotide exchange factor for Rho. The results of a recent publication now demonstrate that, en route from resting cortex to fully established furrow, there lies a regime of cortical excitability in which Rho activity and F-actin play the roles of the prototypical activator and inhibitor, respectively. This cortical excitability is manifest as dramatic traveling waves on the cortex of oocytes and embryos of frogs and starfish. These waves are initiated by autocatalytic activation of Rho at the wave front and extinguished by F-actin-dependent inhibition at their back. It is still unclear how propagating excitable Rho-actin waves give rise to the stable co-existence of Rho activity and F-actin density in the static cleavage furrow during cytokinesis. It is possible that some central spindle-associated signaling molecule simply turns off the inhibition of Rho activity by F-actin. However, mathematical modeling suggests a distinct scenario in which local "re-wiring" of the Rho-actin coupling in the furrow is no longer necessary. Instead, the model predicts that the continuously rising level of Ect2 produces in the furrow a qualitatively new stable steady state that replaces excitability and brings about the stable co-existence of high Rho activity and dense F-actin despite the continuing inhibition of Rho by F-actin.

Animal cell cytokinesis results from patterned activation of the small GTPase Rho, which directs assembly of actomyosin in the equatorial cortex. Cytokinesis is restricted to a portion of the cell cycle following anaphase onset in which the cortex is responsive to signals from the spindle. We show that shortly after anaphase onset oocytes and embryonic cells of frogs and echinoderms exhibit cortical waves of Rho activity and F-actin polymerization. The waves are modulated by cyclin-dependent kinase 1 (Cdk1) activity and require the Rho GEF (guanine nucleotide exchange factor), Ect2. Surprisingly, during wave propagation, although Rho activity elicits F-actin assembly, F-actin subsequently inactivates Rho. Experimental and modelling results show that waves represent excitable dynamics of a reaction-diffusion system with Rho as the activator and F-actin the inhibitor. We propose that cortical excitability explains fundamental features of cytokinesis including its cell cycle regulation.

Cytokinesis in animal cells depends on spindle-derived spatial cues that culminate in Rho activation, and thereby actomyosin assembly, in a narrow equatorial band. Although the nature, origin, and variety of such cues have long been obscure, one component is certainly the Rho activator Ect2. Here we describe the behavior and function of Ect2 in echinoderm embryos, showing that Ect2 migrates from spindle midzone to astral microtubules in anaphase and that Ect2 shapes the pattern of Rho activation in incipient furrows. Our key finding is that Ect2 and its binding partner Cyk4 accumulate not only at normal furrows, but also at furrows that form in the absence of associated spindle, midzone, or chromosomes. In all these cases, the cell assembles essentially the same cytokinetic signaling ensemble—opposed astral microtubules decorated with Ect2 and Cyk4. We conclude that if multiple signals contribute to furrow induction in echinoderm embryos, they likely converge on the same signaling ensemble on an analogous cytoskeletal scaffold., (© 2014 Su et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).)

Animal cells decide where to build the cytokinetic apparatus by sensing the position of the mitotic spindle. Reflecting a long-standing presumption that a furrow-inducing stimulus travels from spindle to cortex via microtubules, debate continues about which microtubules, and in what geometry, are essential for accurate cytokinesis. We used live imaging in urchin and frog embryos to evaluate the relationship between microtubule organization and cytokinetic furrow position. In normal cells, the cytokinetic apparatus forms in a region of lower cortical microtubule density. Remarkably, cells depleted of astral microtubules conduct accurate, complete cytokinesis. Conversely, in anucleate cells, asters alone can support furrow induction without a spindle, but only when sufficiently separated. Ablation of a single centrosome displaces furrows away from the remaining centrosome; ablation of both centrosomes causes broad, inefficient furrowing. We conclude that the asters confer accuracy and precision to a primary furrow-inducing signal that can reach the cell surface from the spindle without transport on microtubules.

In animal cells, cytokinesis is powered by a contractile ring of actin filaments (F-actin) and myosin-2. Formation of the contractile ring is dependent on the small GTPase RhoA, which is activated in a precise zone at the cell equator. It has long been assumed that cytokinesis and other Rho-dependent processes are controlled in a sequential manner, whereby Rho activation by guanine nucleotide exchange factors (GEFs) initiates a particular event, and Rho inactivation by GTPase activating proteins (GAPs) terminates that event. MgcRacGAP is a conserved cytokinesis regulator thought to be required only at the end of cytokinesis. Here we show that GAP activity of MgcRacGAP is necessary early during cytokinesis for the formation and maintenance of the Rho activity zone. Disruption of GAP activity by point mutation results in poorly focused Rho activity zones, whereas complete removal of the GAP domain results in unfocused zones that show lateral instability and/or rapid side-to-side oscillations. We propose that the GAP domain of MgcRacGAP has two unexpected roles throughout cytokinesis: first, it transiently anchors active Rho, and second, it promotes local Rho inactivation, resulting in the constant flux of Rho through the GTPase cycle.

Vertebrate oocyte maturation is an extreme form of asymmetric cell division, producing a mature egg alongside a diminutive polar body. Critical to this process is the attachment of one spindle pole to the oocyte cortex prior to anaphase. We report here that asymmetric spindle pole attachment and anaphase initiation are required for localized cortical activation of Cdc42, which in turn defines the surface of the impending polar body. The Cdc42 activity zone overlaps with dynamic F-actin and is circumscribed by a RhoA-based actomyosin contractile ring. During cytokinesis, constriction of the RhoA contractile ring is accompanied by Cdc42-mediated membrane outpocketing such that one spindle pole and one set of chromosomes are pulled into the Cdc42 enclosure. Unexpectedly, the guanine nucleotide exchange factor Ect2, which is necessary for contractile ring formation, does not colocalize with active RhoA. Polar body emission thus requires a classical RhoA contractile ring and Cdc42-mediated membrane protrusion.

Cytokinesis in animal cells is powered by the cytokinetic apparatus, a ring of filamentous actin and myosin-2 that underlies the plasma membrane and closes between the separating chromosomes. Formation of the cytokinetic apparatus is at least partially dependent on the small GTPase, Rho. Similar to other small GTPases, Rho cycles between the active (GTP-bound) and inactive (GDP-bound) states. Because of this switch-like behaviour, Rho and other members of the Rho GTPase family, such as Rac and Cdc42, have long been thought to work in a manner such that their activation and inactivation are not tightly coupled. That is, a given Rho-dependent event, such as cytokinesis, has been thought to be initiated by activation of Rho, and then, many minutes later, terminated by inactivation of Rho. Here we discuss evidence suggesting that in fact Rho undergoes rapid movement through the GTPase cycle throughout the entire process of cytokinesis, and that this cycling is necessary for proper cytokinetic apparatus function.

Cytokinesis, the final step in cell division, is dependent on formation and closure of a ring of actin filaments (F-actin) and myosin-2 which is, in turn, dependent on activation of the small GTPase, RhoA, at the cell equator. Four new papers, including two in this issue of Developmental Cell (Petronczki et al., 2007; Birkenfeld et al., 2007), provide new insights into how RhoA activation at the equator is initiated and maintained.

During vertebrate egg maturation, cytokinesis initiates after one pole of the bipolar metaphase I spindle attaches to the oocyte cortex, resulting in the formation of a polar body and the mature egg. It is not known what signal couples the spindle pole positioning to polar body formation. We approached this question by drawing an analogy to mitotic exit in budding yeast, as asymmetric spindle attachment to the appropriate cortical region is the common regulatory cue. In budding yeast, the small G protein Cdc42 plays an important role in mitotic exit following the spindle pole attachment . We show here that inhibition of Cdc42 activation blocks polar body formation. The oocytes initiate anaphase but fail to properly form and direct a contractile ring. Endogenous Cdc42 is activated at the spindle pole-cortical contact site immediately prior to polar body formation. The cortical Cdc42 activity zone, which directly overlays the spindle pole, is circumscribed by a cortical RhoA activity zone; the latter defines the cytokinetic contractile furrow . As the RhoA ring contracts during cytokinesis, the Cdc42 zone expands, maintaining its complementary relationship with the RhoA ring. Cdc42 signaling may thus be an evolutionarily conserved mechanism that couples spindle positioning to asymmetric cytokinesis.

Cytokinesis in animal cells results from the assembly and constriction of a circumferential array of actin filaments and myosin-2. Microtubules of the mitotic apparatus determine the position at which the cytokinetic actomyosin array forms, but the molecular mechanisms by which they do so remain unknown. The small GTPase RhoA has previously been implicated in cytokinesis. Using four-dimensional microscopy and a probe for active RhoA, we show that active RhoA concentrates in a precisely bounded zone before cytokinesis and is independent of actin assembly. Cytokinetic RhoA activity zones are common to four echinoderm species, the vertebrate Xenopus laevis, and the highly asymmetric cytokinesis accompanying meiosis. Microtubules direct the formation and placement of the RhoA activity zone, and the zone is repositioned after physical spindle displacement. We conclude that microtubules specify the cytokinetic apparatus via a dynamic zone of local RhoA activity.