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Plasma membrane integrity is essential for cellular homeostasis. In vivo, cells experience plasma membrane damage from a multitude of stressors in the extra- and intra-cellular environment. To avoid lethal consequences, cells are equipped with repair pathways to restore membrane integrity. Here, we assess plasma membrane damage and repair from a whole-body perspective. We highlight the role of tissue-specific stressors in health and disease and examine membrane repair pathways across diverse cell types. Furthermore, we outline the impact of genetic and environmental factors on plasma membrane integrity and how these contribute to disease pathogenesis in different tissues.

The Rho GTPases — RHOA, RAC1 and CDC42 — are small GTP binding proteins that regulate basic biological processes such as cell locomotion, cell division and morphogenesis by promoting cytoskeleton-based changes in the cell cortex. This regulation results from active (GTP-bound) Rho GTPases stimulating target proteins that, in turn, promote actin assembly and myosin 2-based contraction to organize the cortex. This basic regulatory scheme, well supported by in vitro studies, led to the natural assumption that Rho GTPases function in vivo in an essentially linear matter, with a given process being initiated by GTPase activation and terminated by GTPase inactivation. However, a growing body of evidence based on live cell imaging, modelling and experimental manipulation indicates that Rho GTPase activation and inactivation are often tightly coupled in space and time via signalling circuits and networks based on positive and negative feedback. In this Review, we present and discuss this evidence, and we address one of the fundamental consequences of coupled activation and inactivation: the ability of the Rho GTPases to self-organize, that is, direct their own transition from states of low order to states of high order. We discuss how Rho GTPase self-organization results in the formation of diverse spatiotemporal cortical patterns such as static clusters, oscillatory pulses, travelling wave trains and ring-like waves. Finally, we discuss the advantages of Rho GTPase self-organization and pattern formation for cell function.The Rho GTPases are small GTP binding proteins that regulate basic biological processes such as cell locomotion, cell division and morphogenesis by promoting cytoskeleton-based changes in the cell cortex. These different functions are driven by the ability of Rho GTPases to self-organize, forming diverse spatiotemporal cortical patterns.

The RhoGTPases are characterized as membrane-associated molecular switches that cycle between active, GTP-bound and inactive, GDP-bound states. However, 90–95% of RhoGTPases are maintained in a soluble form by RhoGDI, which is generally viewed as a passive shuttle for inactive RhoGTPases. Our current understanding of RhoGTPase:RhoGDI dynamics has been limited by two experimental challenges: direct visualization of the RhoGTPases in vivo and reconstitution of the cycle in vitro. We developed methods to directly image vertebrate RhoGTPases in vivo or on lipid bilayers in vitro. Using these methods, we identified pools of active and inactive RhoGTPase associated with the membrane, found that RhoGDI can extract both inactive and active RhoGTPases, and found that extraction of active RhoGTPase contributes to their spatial regulation around cell wounds. These results indicate that RhoGDI directly contributes to the spatiotemporal patterning of RhoGTPases by removing active RhoGTPases from the plasma membrane. Organisms rely on many signaling molecules to control how their cells grow, divide and heal. For example, when the cell membrane is damaged, two signaling proteins, Rho and Cdc42, are recruited to wounds and activated to promote repair. Active Rho and active Cdc42 form two concentric rings at the membrane to direct the closure of the wound. Rho and Cdc42 belong to the RhoGTPase family, a group of proteins that act as molecular switches and alternate between active and inactive forms. At the level of the cell, RhoGTPases are only active in the tiny patches of the membrane where they bind. However, individual proteins hop on and off membranes in a matter of seconds, only staying bound for short periods. This mechanism is controlled by a regulatory protein known as RhoGDI, and it allows RhoGTPases to form precise patterns of activity at membranes – such as the rings that surround a wound site. However, it was not known exactly how RhoGDI regulates the activity of RhoGTPases over space and time, partly because it is difficult to study these proteins in the laboratory. To fill this knowledge gap, Golding, Visco et al. developed new fluorescent probes to track Rho and Cdc42 in wounded cells from frogs and on artificial membranes. The experiments showed that pools of inactive Cdc42 accumulated on membranes, alongside the active form of the protein. RhoGDI removed both active and inactive RhoGTPases from artificial and frog cell membranes. In fact, removing active Rho and Cdc42 proteins from the cell membrane was necessary to form the spatial patterns of RhoGTPase activity observed in wounded frog cells. The findings of Golding, Visco et al. help to understand how RhoGDI proteins regulate RhoGTPases and provide new tools to further study these proteins. In humans, mutations in either RhoGDI or Cdc42 are responsible for severe conditions such as Nephrotic Syndrome Type 8 or Takenouchi-Kosaki syndrome. In the future, this work may aid the development of treatments and cures for these conditions.

Interactions between microtubules and actin are a basic phenomenon that underlies many fundamental processes in which dynamic cellular asymmetries need to be established and maintained. These are processes as diverse as cell motility, neuronal pathfinding, cellular wound healing, cell division and cortical flow. Microtubules and actin exhibit two mechanistic classes of interactions — regulatory and structural. These interactions comprise at least three conserved 'mechanochemical activity modules' that perform similar roles in these diverse cell functions.

The cell cortex plays many critical roles, including interpreting and responding to internal and external signals. One behavior which supports a cell’s ability to respond to both internal and externally-derived signaling is cortical excitability, wherein coupled positive and negative feedback loops generate waves of actin polymerization and depolymerization at the cortex. Cortical excitability is a highly conserved behavior, having been demonstrated in many cell types and organisms. One system well-suited to studying cortical excitability is Xenopus laevis , in which cortical excitability is easily monitored for many hours after fertilization. Indeed, recent investigations using X. laevis have furthered our understanding of the circuitry underlying cortical excitability and how it contributes to cytokinesis. Here, we describe the impact of wounding, which represents both a chemical and a physical signal, on cortical excitability. In early embryos (zygotes to early blastulae), we find that wounding results in a transient cessation (“freezing”) of wave propagation followed by transport of frozen waves toward the wound site. We also find that wounding near cell-cell junctions results in the formation of an F-actin (actin filament)-based structure that pulls the junction toward the wound; at least part of this structure is based on frozen waves. In later embryos (late blastulae to gastrulae), we find that cortical excitability diminishes and is progressively replaced by epithelial excitability, a process in which wounded cells communicate with other cells via wave-like increases of calcium and apical F-actin. While the F-actin waves closely follow the calcium waves in space and time, under some conditions the actin wave can be uncoupled from the calcium wave, suggesting that they may be independently regulated by a common upstream signal. We conclude that as cortical excitability disappears from the level of the individual cell within the embryo, it is replaced by excitability at the level of the embryonic epithelium itself.

Proper spindle positioning and orientation are essential for asymmetric cell division and require microtubule–actin filament (F-actin) interactions in many systems 1 , 2 . Such interactions are particularly important in meiosis 3 , where they mediate nuclear anchoring 4 , 5 , 6 , as well as meiotic spindle assembly and rotation 7 , 8 , two processes required for asymmetric cell division. Myosin-10 proteins are phosphoinositide-binding 9 , actin-based motors that contain carboxy-terminal MyTH4 and FERM domains of unknown function 10 . Here we show that Xenopus laevis myosin-10 (Myo10) associates with microtubules in vitro and in vivo , and is concentrated at the point where the meiotic spindle contacts the F-actin-rich cortex. Microtubule association is mediated by the MyTH4-FERM domains, which bind directly to purified microtubules. Disruption of Myo10 function disrupts nuclear anchoring, spindle assembly and spindle–F-actin association. Thus, this myosin has a novel and critically important role during meiosis in integrating the F-actin and microtubule cytoskeletons.

Living systems are driven far from thermodynamic equilibrium through the continuous consumption of ambient energy. In the cell cortex, this energy is invested in the formation of diverse patterns in chemical and mechanical activities, whose spatial and temporal dynamics determine the cell phenotypes and behaviours. How cells partition internal energy between these activities is unknown. Here we measured the entropy production rate of both chemical and mechanical subsystems of the cell cortex across a variety of patterns as the system is driven further from equilibrium. We do this by manipulating the Rho GTPase pathway, which controls the cortical actin filaments and myosin-II. At lower levels of GTPase-activating protein expression, which produce pulses or choppy Rho and actin filament waves, energy is proportionally partitioned between the two subsystems and is subject to the constraint of Onsager reciprocity. Within the range of reciprocity, the entropy production rate is maximized in choppy waves. As the cortex is driven into labyrinthine or spiral travelling waves, reciprocity is broken, marking an increasingly differential partitioning of energy and an uncoupling of chemical and mechanical activities. We further demonstrate that energy partitioning and reciprocity are determined by the competing timescales between chemical reaction and mechanical relaxation. How cells manage the internal energetic budget to drive mechanical and chemical dynamics is still an open question. Now it is shown that the allocation of energy depends on the distance from thermodynamic equilibrium.

In cytokinesis, formation of the contractile ring depends on localized activation of RhoA at the cell equator. This study demonstrates that GAP activity of MgcRacGAP is necessary throughout cytokinesis to maintain a focused zone of Rho activity. 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 1 , 2 , which is activated in a precise zone at the cell equator 3 . 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 4 , 5 . 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.

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. Using live imaging of Xenopus and starfish oocytes and embryos undergoing cytokinesis, Bement and colleagues show that anaphase onset promotes cortical waves of Rho and F-actin, which can be modelled by reaction–diffusion dynamics.