Neutrophil polarization: spatiotemporal dynamics of RhoA activity support a self-organizing mechanism

Competing signaling pathways controls electrotaxis

Understanding how cells follow exogenous cues is a key question for biology, medicine, and bioengineering. Growing evidence shows that electric fields represent a precise and programmable method to control cell migration. Most data suggest that the polarization of membrane proteins and the following downstream signaling are central to electrotaxis. Unfortunately, how these multiple mechanisms coordinate with the motile machinery of the cell is still poorly understood. Here, we develop a mechanistic model that explains electrotaxis across different cell types. Using the zebrafish proteome, we identify membrane proteins directly related to migration signaling pathways that polarize anodally and cathodally. Further, we show that the simultaneous and asymmetric distribution of these membrane receptors establish multiple cooperative and competing stimuli for directing the anodal and cathodal migration of the cell. Using electric fields, we enhance, cancel, or switch directed cell migration, with clear implications in promoting tissue regeneration or arresting tumor progression.

The Protein Kinase aPKC as Well as the Small GTPases RhoA and Cdc42 Regulates Neutrophil Chemotaxis Partly by Recruiting the ROCK Kinase to the Leading Edge

The small GTPases RhoA and Cdc42 and their effector proteins play crucial roles in neutrophil chemotaxis. However, endogenous localization and regulation of these proteins have remained largely unknown. Here, we show, using a trichloroacetic acid fixation method, that endogenous RhoA and Cdc42 are preferentially accumulated at the F-actin-rich leading edge (pseudopod) during chemotaxis of human neutrophil-like PLB-985 cells in response to the chemoattractant C5a. Interestingly, the enrichment of RhoA is impaired by knockdown of Cdc42, indicating a positive regulation by Cdc42. Depletion of Cdc42 or RhoA each induces the formation of multiple pseudopods, confirming their significance in cell polarization with an organized actin network at the front. The Rho-associated kinase ROCK is also recruited to the leading edge during chemotaxis in a manner dependent on not only RhoA and Cdc42 but also aPKC, a Cdc42-interacting kinase that can also bind to ROCK. ROCK promotes phosphorylation of the myosin light chain at the front, possibly regulating pseudopod contractility. Knockdown of aPKC suppresses neutrophil chemotaxis by disturbing pseudopod orientation without forming multiple protrusions. An incorrectly oriented pseudopod is also observed in ROCK-depleted cells. Thus, aPKC, as well as RhoA and Cdc42, likely regulates neutrophil chemotaxis partly by recruiting ROCK to the leading edge for correct directionality.

Model supports asymmetric regulation across the intercellular junction for collective cell polarization

Symmetry breaking, which is ubiquitous in biological cells, functionally enables directed cell movement and organized embryogenesis. Prior to movement, cells break symmetry to form a well-defined cell front and rear in a process called polarization. In developing and regenerating tissues, collective cell movement requires the coordination of the polarity of the migration machineries of neighboring cells. Though several works shed light on the molecular basis of polarity, fewer studies have focused on the regulation across the cell-cell junction required for collective polarization, thus limiting our ability to connect tissue-level dynamics to subcellular interactions. Here, we investigated how polarity signals are communicated from one cell to its neighbor to ensure coordinated front-to-rear symmetry breaking with the same orientation across the group. In a theoretical setting, we systematically searched a variety of intercellular interactions and identified that co-alignment arrangement of the polarity axes in groups of two and four cells can only be achieved with strong asymmetric regulation of Rho GTPases or enhanced assembly of complementary F-actin structures across the junction. Our results held if we further assumed the presence of an external stimulus, intrinsic cell-to-cell variability, or larger groups. The results underline the potential of using quantitative models to probe the molecular interactions required for macroscopic biological phenomena. Lastly, we posit that asymmetric regulation is achieved through junction proteins and predict that in the absence of cytoplasmic tails of such linker proteins, the likeliness of doublet co-polarity is greatly diminished.

Galectin-9 regulates dendritic cell polarity and uropod contraction by modulating RhoA activity

Adaptive immunity relies on dendritic cell (DC) migration to transport antigens from tissues to lymph nodes. Galectins, a family of β-galactoside-binding proteins, control cell membrane organisation, exerting crucial roles in multiple physiological processes. Here, we report a novel mechanism underlying cell polarity and uropod retraction. We demonstrate that galectin-9 regulates chemokine-driven and basal DC migration both in humans and mice, indicating a conserved function for this lectin. We identified the underlying mechanism, namely a deficiency in cell rear contractility mediated by galectin-9 interaction with CD44 that in turn regulates RhoA activity. Analysis of DC motility in the 3D tumour-microenvironment revealed galectin-9 is also required for DC infiltration. Moreover, exogenous galectin-9 rescued the motility of tumour-immunocompromised human blood DCs, validating the physiological relevance of galectin-9 in DC migration and underscoring its implications for DC-based immunotherapies. Our results identify galectin-9 as a necessary mechanistic component for DC motility and highlight a novel role for the lectin in regulating cell polarity and contractility.

Long range mutual activation establishes Rho and Rac polarity during cell migration

In migrating cells, the GTPase Rac organizes a protrusive front, whereas Rho organizes a contractile back. How these GTPases are appropriately positioned at the opposite poles of a migrating cell is unknown. Here we leverage optogenetics, manipulation of cell mechanics, and mathematical modeling to reveal a surprising long-range mutual activation of the front and back polarity programs that complements their well-known local mutual inhibition. This long-range activation is rooted in two distinct modes of mechanochemical crosstalk. Local Rac-based protrusion stimulates Rho activation at the opposite side of the cell via membrane tension-based activation of mTORC2. Conversely, local Rho-based contraction induces cortical-flow-based remodeling of membrane-to-cortex interactions leading to PIP2 release, PIP3 generation, and Rac activation at the opposite side of the cell. We develop a minimal unifying mechanochemical model of the cell to explain how this long-range mechanical facilitation complements local biochemical inhibition to enable robust global Rho and Rac partitioning. Finally, we validate the importance of this long-range facilitation in the context of chemoattractant-based cell polarization and migration in primary human lymphocytes. Our findings demonstrate that the actin cortex and plasma membrane function as an integrated mechanochemical system for long-range partitioning of Rac and Rho during cell migration and likely other cellular contexts.

Excitable Rho dynamics control cell shape and motility by sequentially activating ERM proteins and actomyosin contractility

Migration of endothelial and many other cells requires spatiotemporal regulation of protrusive and contractile cytoskeletal rearrangements that drive local cell shape changes. Unexpectedly, the small GTPase Rho, a crucial regulator of cell movement, has been reported to be active in both local cell protrusions and retractions, raising the question of how Rho activity can coordinate cell migration. Here, we show that Rho activity is absent in local protrusions and active during retractions. During retractions, Rho rapidly activated ezrin-radixin-moesin proteins (ERMs) to increase actin-membrane attachment, and, with a delay, nonmuscle myosin 2 (NM2). Rho activity was excitable, with NM2 acting as a slow negative feedback regulator. Strikingly, inhibition of SLK/LOK kinases, through which Rho activates ERMs, caused elongated cell morphologies, impaired Rho-induced cell contractions, and reverted Rho-induced blebbing. Together, our study demonstrates that Rho activity drives retractions by sequentially enhancing ERM-mediated actin-membrane attachment for force transmission and NM2-dependent contractility.

Follow the flow: Actin and membrane act as an integrated system to globally coordinate cell shape and movement

Migratory cells are polarized with protrusive fronts and contractile rears. This spatial organization necessitates long-range coordination of the signals that organize protrusions and contractions. Cells leverage reciprocal interactions of short-range biochemical signals and long-range mechanical forces for this integration. The interface between the plasma membrane and actin cortex is where this communication occurs. Here, we review how the membrane and cortex form an integrated system for long-range coordination of cell polarity. We highlight the role of membrane-to-cortex-attachment proteins as regulators of tension transmission across the cell and discuss the interplay between actin-membrane and polarity signaling complexes. Rather than presenting an exhaustive list of recent findings, we focus on important gaps in our current understanding.

Cdc42 activity in the trailing edge is required for persistent directional migration of keratinocytes

Fibroblasts migrate discontinuously by generating transient leading-edge protrusions and irregular, abrupt retractions of a narrow trailing edge. In contrast, keratinocytes migrate persistently and directionally via a single, stable, broad protrusion paired with a stable trailing-edge. The Rho GTPases Rac1, Cdc42 and RhoA are key regulators of cell protrusions and retractions. However, how these molecules mediate cell-type specific migration modes is still poorly understood. In fibroblasts, all three Rho proteins are active at the leading edge, suggesting short-range coordination of protrusive Rac1 and Cdc42 signals with RhoA retraction signals. Here, we show that Cdc42 was surprisingly active in the trailing-edge of migrating keratinocytes. Elevated Cdc42 activity colocalized with the effectors MRCK and N-WASP suggesting that Cdc42 controls both myosin activation and actin polymerization in the back. Indeed, Cdc42 was required to maintain the highly dynamic contractile acto-myosin retrograde flow at the trailing edge of keratinocytes, and its depletion induced ectopic protrusions in the back, leading to decreased migration directionality. These findings suggest that Cdc42 is required to stabilize the dynamic cytoskeletal polarization in keratinocytes, to enable persistent, directional migration.

Rho GTPase activity crosstalk mediated by Arhgef11 and Arhgef12 coordinates cell protrusion-retraction cycles

Rho GTPases play a key role in the spatio-temporal coordination of cytoskeletal dynamics during cell migration. Here, we directly investigate crosstalk between the major Rho GTPases Rho, Rac and Cdc42 by combining rapid activity perturbation with activity measurements in mammalian cells. These studies reveal that Rac stimulates Rho activity. Direct measurement of spatio-temporal activity patterns show that Rac activity is tightly and precisely coupled to local cell protrusions, followed by Rho activation during retraction. Furthermore, we find that the Rho-activating Lbc-type GEFs Arhgef11 and Arhgef12 are enriched at transient cell protrusions and retractions and recruited to the plasma membrane by active Rac. In addition, their depletion reduces activity crosstalk, cell protrusion-retraction dynamics and migration distance and increases migration directionality. Thus, our study shows that Arhgef11 and Arhgef12 facilitate exploratory cell migration by coordinating cell protrusion and retraction by coupling the activity of the associated regulators Rac and Rho.

Proteomic insight into human directed selection of the domesticated chicken Gallus gallus

Chicken domestication began at least 3,500 years ago for purposes of divination, cockfighting, and food. Prior to industrial scale chicken production, domestication selected larger birds with increased egg production. In the mid-20th century companies began intensive selection with the broiler (meat) industry focusing on improved feed conversion, rapid growth, and breast muscle yield. Here we present proteomic analysis comparing the modern broiler line, Ross 708, with the UIUC legacy line which is not selected for growth traits. Breast muscle proteome analysis identifies cellular processes that have responded to human directed artificial selection. Mass spectrometry was used to identify protein level differences in the breast muscle of 6-day old chicks from Modern and Legacy lines. Our results indicate elevated levels of stress proteins, ribosomal proteins and proteins that participate in the innate immune pathway in the Modern chickens. Furthermore, the comparative analyses indicated expression differences for proteins involved in multiple biochemical pathways. In particular, the Modern line had elevated levels of proteins affecting the pentose phosphate pathway, TCA cycle and fatty acid oxidation while proteins involved in the first phase of glycolysis were reduced compared to the Legacy line. These analyses provide hypotheses linking the morphometric changes driven by human directed selection to biochemical pathways. These results also have implications for the poultry industry, specifically Wooden Breast disease which is linked to rapid breast muscle growth.

Optical regulation of endogenous RhoA reveals selection of cellular responses by signal amplitude

How protein signaling networks respond to different input strengths is an important but poorly understood problem in cell biology. For example, RhoA can promote focal adhesion (FA) growth or disassembly, but how RhoA activity mediates these opposite outcomes is not clear. Here, we develop a photoswitchable RhoA guanine nucleotide exchange factor (GEF), psRhoGEF, to precisely control endogenous RhoA activity. Using this optical tool, we discover that peak FA disassembly selectively occurs upon activation of RhoA to submaximal levels. We also find that Src activation at FAs selectively occurs upon submaximal RhoA activation, identifying Src as an amplitude-dependent RhoA effector. Finally, a pharmacological Src inhibitor reverses the direction of the FA response to RhoA activation from disassembly to growth, demonstrating that Src functions to suppress FA growth upon RhoA activation. Thus, rheostatic control of RhoA activation by psRhoGEF reveals that cells can use signal amplitude to produce multiple responses to a single biochemical signal.

Two Complementary Signaling Pathways Depict Eukaryotic Chemotaxis: A Mechanochemical Coupling Model

Many eukaryotic cells, including neutrophils and Dictyostelium cells, are able to undergo correlated random migration in the absence of directional cues while reacting to shallow gradients of chemoattractants with exquisite precision. Although progress has been made with regard to molecular identities, it remains elusive how molecular mechanics are integrated with cell mechanics to initiate and manipulate cell motility. Here, we propose a two dimensional (2D) cell migration model wherein a multilayered dynamic seesaw mechanism is accompanied by a mechanical strain-based inhibition mechanism. In biology, these two mechanisms can be mapped onto the biochemical feedback between phosphoinositides (PIs) and Rho GTPase and the mechanical interplay between filamin A (FLNa) and FilGAP. Cell migration and the accompanying morphological changes are demonstrated in numerical simulations using a particle-spring model, and the diffusion in the cell membrane are simulations using a one dimensional (1D) finite differences method (FDM). The fine balance established between endogenous signaling and a mechanically governed inactivation scheme ensures the endogenous cycle of self-organizing pseudopods, accounting for the correlated random migration. Furthermore, this model cell manifests directional and adaptable responses to shallow graded signaling, depending on the overwhelming effect of the graded stimuli guidance on strain-based inhibition. Finally, the model cell becomes trapped within an obstacle-ridden spatial region, manifesting a shuttle run for local explorations and can chemotactically “escape”, illustrating again the balance required in the complementary signaling pathways.

Phosphoinositide 3-Kinase δ Inhibition Improves Neutrophil Bacterial Killing in Critically Ill Patients at High Risk of Infection

Acquired neutrophil dysfunction frequently develops during critical illness, independently increasing the risk for intensive care unit-acquired infection. PI3Kδ is implicated in driving neutrophil dysfunction and can potentially be targeted pharmacologically. The aims of this study were to determine whether PI3Kδ inhibition reverses dysfunction in neutrophils from critically ill patients and to describe potential mechanisms. Neutrophils were isolated from blood taken from critically ill patients requiring intubation and mechanical ventilation, renal support, or blood pressure support. In separate validation experiments, neutrophil dysfunction was induced pharmacologically in neutrophils from healthy volunteers. Phagocytosis and bacterial killing assays were performed, and activity of RhoA and protein kinase A (PKA) was assessed. Inhibitors of PI3Kδ, 3-phosphoinositide-dependent protein kinase-1 (PDK1), and PKA were used to determine mechanisms of neutrophil dysfunction. Sixty-six patients were recruited. In the 27 patients (40.9%) with impaired neutrophil function, PI3Kδ inhibition consistently improved function and significantly increased bacterial killing. These findings were validated in neutrophils from healthy volunteers with salbutamol-induced dysfunction and extended to demonstrate that PI3Kδ inhibition restored killing of clinical isolates of nine pathogens commonly associated with intensive care unit-acquired infection. PI3Kδ activation was associated with PDK1 activation, which in turn phosphorylated PKA, which drove phosphorylation and inhibition of the key regulator of neutrophil phagocytosis, RhoA. These data indicate that, in a significant proportion of critically ill patients, PI3Kδ inhibition can improve neutrophil function through PDK1- and PKA-dependent processes, suggesting that therapeutic use of PI3Kδ inhibitors warrants investigation in this setting.

Single-Component Optogenetic Tools for Inducible RhoA GTPase Signaling

Optogenetic tools are created to control RhoA GTPase, a central regulator of actin organization and actomyosin contractility. RhoA GTPase, or its upstream activator ARHGEF11, is fused to BcLOV4, a photoreceptor that can be dynamically recruited to the plasma membrane by a light-regulated protein-lipid electrostatic interaction with the inner leaflet. Direct membrane recruitment of these proteins induces potent contractile signaling sufficient to separate adherens junctions with as little as one pulse of blue light. Induced cytoskeletal morphology changes are dependent on the alignment of the spatially patterned stimulation with the underlying cell polarization. RhoA-mediated cytoskeletal activation drives yes-associated protein (YAP) nuclear localization within minutes and consequent mechanotransduction verified by YAP-transcriptional enhanced associate domain transcriptional activity. These single-transgene tools do not require protein binding partners for dynamic membrane localization and permit spatiotemporally precise control over RhoA signaling to advance the study of its diverse regulatory roles in cell migration, morphogenesis, and cell cycle maintenance.

RhoA Signaling in Immune Cell Response and Cardiac Disease

Chronic inflammation, the activation of immune cells and their cross-talk with cardiomyocytes in the pathogenesis and progression of heart diseases has long been overlooked. However, with the latest research developments, it is increasingly accepted that a vicious cycle exists where cardiomyocytes release cardiocrine signaling molecules that spiral down to immune cell activation and chronic state of low-level inflammation. For example, cardiocrine molecules released from injured or stressed cardiomyocytes can stimulate macrophages, dendritic cells, neutrophils and even T-cells, which then subsequently increase cardiac inflammation by co-stimulation and positive feedback loops. One of the key proteins involved in stress-mediated cardiomyocyte signal transduction is a small GTPase RhoA. Importantly, the regulation of RhoA activation is critical for effective immune cell response and is being considered as one of the potential therapeutic targets in many immune-cell-mediated inflammatory diseases. In this review we provide an update on the role of RhoA at the juncture of immune cell activation, inflammation and cardiac disease.

HIF2α is a direct regulator of neutrophil motility

Orchestrated recruitment of neutrophils to inflamed tissue is essential during the initiation of inflammation. Inflamed areas are usually hypoxic, and adaptation to reduced oxygen pressure is typically mediated by hypoxia pathway proteins. However, it remains unclear how these factors influence the migration of neutrophils to and at the site of inflammation during their transmigration through the blood-endothelial cell barrier, as well as their motility in the interstitial space. Here, we reveal that activation of hypoxia-inducible factor 2 (HIF2α) as a result of a deficiency in HIF prolyl hydroxylase domain protein 2 (PHD2) boosts neutrophil migration specifically through highly confined microenvironments. In vivo, the increased migratory capacity of PHD2-deficient neutrophils resulted in massive tissue accumulation in models of acute local inflammation. Using systematic RNA sequencing analyses and mechanistic approaches, we identified RhoA, a cytoskeleton organizer, as the central downstream factor that mediates HIF2α-dependent neutrophil motility. Thus, we propose that the novel PHD2-HIF2α-RhoA axis is vital to the initial stages of inflammation because it promotes neutrophil movement through highly confined tissue landscapes.

Physics of compartmentalization: How phase separation and signaling shape membrane and organelle identity

Compartmentalization of cellular functions is at the core of the physiology of eukaryotic cells. Recent evidences indicate that a universal organizing process - phase separation - supports the partitioning of biomolecules in distinct phases from a single homogeneous mixture, a landmark event in both the biogenesis and the maintenance of membrane and non-membrane-bound organelles. In the cell, 'passive' (non energy-consuming) mechanisms are flanked by 'active' mechanisms of separation into phases of distinct density and stoichiometry, that allow for increased partitioning flexibility and programmability. A convergence of physical and biological approaches is leading to new insights into the inner functioning of this driver of intracellular order, holding promises for future advances in both biological research and biotechnological applications.

Local Myo9b RhoGAP activity regulates cell motility

To migrate, cells assume a polarized morphology, extending forward with a leading edge with their trailing edge retracting back toward the cell body. Both cell extension and retraction critically depend on the organization and dynamics of the actin cytoskeleton, and the small, monomeric GTPases Rac and Rho are important regulators of actin. Activation of Rac induces actin polymerization and cell extension, whereas activation of Rho enhances acto-myosin II contractility and cell retraction. To coordinate migration, these processes must be carefully regulated. The myosin Myo9b, a Rho GTPase-activating protein (GAP), negatively regulates Rho activity and deletion of Myo9b in leukocytes impairs cell migration through increased Rho activity. However, it is not known whether cell motility is regulated by global or local inhibition of Rho activity by Myo9b. Here, we addressed this question by using Myo9b-deficient macrophage-like cells that expressed different recombinant Myo9b constructs. We found that Myo9b accumulates in lamellipodial extensions generated by Rac-induced actin polymerization as a function of its motor activity. Deletion of Myo9b in HL-60-derived macrophages altered cell morphology and impaired cell migration. Reintroduction of Myo9b or Myo9b motor and GAP mutants revealed that local GAP activity rescues cell morphology and migration. In summary, Rac activation leads to actin polymerization and recruitment of Myo9b, which locally inhibits Rho activity to enhance directional cell migration.

Cellular self-organization: generating order from the abyss

While the organization of inanimate systems such as gases or liquids is predominantly thermodynamically driven—a mixture of two gases will tend to mix until they reach equilibrium—biological systems frequently exhibit organization that is far from a well-mixed equilibrium. The anisotropies displayed by cells are evident in some of the dynamic processes that constitute life including cell development, movement, and division. These anisotropies operate at different length-scales, from the meso- to the nanoscale, and are proposed to reflect self-organization, a characteristic of living systems that is becoming accessible to reconstitution from purified components, and thus a more thorough understanding. Here, some examples of self-organization underlying cellular anisotropies at the cellular level are reviewed, with an emphasis on Rho-family GTPases operating at the plasma membrane. Given the technical challenges of studying these dynamic proteins, some of the successful approaches that are being employed to study their self-organization will also be considered.

A hybrid stochastic-deterministic mechanochemical model of cell polarization

Polarization is a crucial component in cell differentiation, development, and motility, but its details are not yet well understood. At the onset of cell locomotion, cells break symmetry to form well-defined cell fronts and rears. This polarity establishment varies across cell types: in Dictyostelium discoideum cells, it is mediated by biochemical signaling pathways and can function in the absence of a cytoskeleton, while in keratocytes, it is tightly connected to cytoskeletal dynamics and mechanics. Theoretical models that have been developed to understand the onset of polarization have explored either signaling or mechanical pathways, yet few have explored mechanochemical mechanisms. However, many motile cells rely on both signaling modules and actin cytoskeleton to break symmetry and achieve a stable polarized state. We propose a general mechanochemical polarization model based on coupling between a stochastic model for the segregation of signaling molecules and a simplified mechanical model for actin cytoskeleton network competition. We find that local linear coupling between minimally nonlinear signaling and cytoskeletal systems, separately not supporting stable polarization, yields a robustly polarized cell state. The model captures the essence of spontaneous polarization of neutrophils, which has been proposed to emerge due to the competition between frontness and backness pathways.

Semaphorin 3F signaling actively retains neutrophils at sites of inflammation

Neutrophilic inflammation is central to disease pathogenesis, for example, in chronic obstructive pulmonary disease, yet the mechanisms that retain neutrophils within tissues remain poorly understood. With emerging evidence that axon guidance factors can regulate myeloid recruitment and that neutrophils can regulate expression of a class 3 semaphorin, SEMA3F, we investigated the role of SEMA3F in inflammatory cell retention within inflamed tissues. We observed that neutrophils upregulate SEMA3F in response to proinflammatory mediators and following neutrophil recruitment to the inflamed lung. In both zebrafish tail injury and murine acute lung injury models of neutrophilic inflammation, overexpression of SEMA3F delayed inflammation resolution with slower neutrophil migratory speeds and retention of neutrophils within the tissues. Conversely, constitutive loss of sema3f accelerated egress of neutrophils from the tail injury site in fish, whereas neutrophil-specific deletion of Sema3f in mice resulted in more rapid neutrophil transit through the airways, and significantly reduced time to resolution of the neutrophilic response. Study of filamentous-actin (F-actin) subsequently showed that SEMA3F-mediated retention is associated with F-actin disassembly. In conclusion, SEMA3F signaling actively regulates neutrophil retention within the injured tissues with consequences for neutrophil clearance and inflammation resolution.

A neutrophil-centric view of chemotaxis

Neutrophils are key players of the innate immune system, that are involved in coordinating the initiation, propagation and resolution of inflammation. Accurate neutrophil migration (chemotaxis) to sites of inflammation in response to gradients of chemoattractants is pivotal to these roles. Binding of chemoattractants to dedicated G-protein-coupled receptors (GPCRs) initiates downstream signalling events that promote neutrophil polarisation, a prerequisite for directional migration. We provide a brief summary of some of the recent insights into signalling events and feedback loops that serve to initiate and maintain neutrophil polarisation. This is followed by a discussion of recent developments in the understanding of in vivo neutrophil chemotaxis, a process that is frequently referred to as 'recruitment' or 'trafficking'. Here, we summarise neutrophil mobilisation from and homing to the bone marrow, and briefly discuss the role of glucosaminoglycan-immobilised chemoattractants and their corresponding receptors in the regulation of neutrophil extravasation and neutrophil swarming. We furthermore touch on some of the most recent insights into the roles of atypical chemokine receptors (ACKRs) in neutrophil recruitment, and discuss neutrophil reverse (transendothelial) migration together with potential function(s) in the dissemination and/or resolution of inflammation.

Simple Rho GTPase Dynamics Generate a Complex Regulatory Landscape Associated with Cell Shape

Migratory cells exhibit a variety of morphologically distinct responses to their environments that manifest in their cell shape. Some protrude uniformly to increase substrate contacts, others are broadly contractile, some polarize to facilitate migration, and yet others exhibit mixtures of these responses. Prior studies have identified a discrete collection of shapes that the majority of cells display and demonstrated that activity levels of the cytoskeletal regulators Rac1 and RhoA GTPase regulate those shapes. Here, we use computational modeling to assess whether known GTPase dynamics can give rise to a sufficient diversity of spatial signaling states to explain the observed shapes. Results show that the combination of autoactivation and mutually antagonistic cross talk between GTPases, along with the conservative membrane binding, generates a wide array of distinct homogeneous and polarized regulatory phenotypes that arise for fixed model parameters. From a theoretical perspective, these results demonstrate that simple GTPase dynamics can generate complex multistability in which six distinct stable steady states (three homogeneous and three polarized) coexist for a fixed set of parameters, each of which naturally maps to an observed morphology. From a biological perspective, although we do not explicitly model the cytoskeleton or resulting cell morphologies, these results, along with prior literature linking GTPase activity to cell morphology, support the hypothesis that GTPase signaling dynamics can generate the broad morphological characteristics observed in many migratory cell populations. Further, the observed diversity may be the result of cells populating a complex morphological landscape generated by GTPase regulation rather than being the result of intrinsic cell-cell variation. These results demonstrate that Rho GTPases may have a central role in regulating the broad characteristics of cell shape (e.g., expansive, contractile, polarized, etc.) and that shape heterogeneity may be (at least partly) a reflection of the rich signaling dynamics regulating the cytoskeleton rather than intrinsic cell heterogeneity.

Efficient Front-Rear Coupling in Neutrophil Chemotaxis by Dynamic Myosin II Localization

Efficient chemotaxis requires rapid coordination between different parts of the cell in response to changing directional cues. Here, we investigate the mechanism of front-rear coordination in chemotactic neutrophils. We find that changes in the protrusion rate at the cell front are instantaneously coupled to changes in retraction at the cell rear, while myosin II accumulation at the rear exhibits a reproducible 9-15-s lag. In turning cells, myosin II exhibits dynamic side-to-side relocalization at the cell rear in response to turning of the leading edge and facilitates efficient turning by rapidly re-orienting the rear. These manifestations of front-rear coupling can be explained by a simple quantitative model incorporating reversible actin-myosin interactions with a rearward-flowing actin network. Finally, the system can be tuned by the degree of myosin regulatory light chain (MRLC) phosphorylation, which appears to be set in an optimal range to balance persistence of movement and turning ability.

The role of polarisation of circulating tumour cells in cancer metastasis

Metastasis is the spread of cancer cells from a primary tumour to a distant site of the body. Metastasising tumour cells have to survive and readjust to different environments, such as heterogeneous solid tissues and liquid phase in lymph- or blood circulation, which they achieve through a high degree of plasticity that renders them adaptable to varying conditions. One defining characteristic of the metastatic process is the transition of tumour cells between different polarised phenotypes, ranging from differentiated epithelial polarity to migratory front-rear polarity. Here, we review the polarisation types adopted by tumour cells during the metastatic process and describe the recently discovered single-cell polarity in liquid phase observed in circulating tumour cells. We propose that single-cell polarity constitutes a mode of polarisation of the cell cortex that is uncoupled from the intracellular polarisation machinery, which distinguishes single-cell polarity from other types of polarity identified so far. We discuss how single-cell polarity can contribute to tumour metastasis and the therapeutic potential of this new discovery.

The trafficking protein JFC1 regulates Rac1-GTP localization at the uropod controlling neutrophil chemotaxis and in vivo migration

Neutrophil chemotaxis is essential in responses to infection and underlies inflammation. In neutrophils, the small GTPase Rac1 has discrete functions at both the leading edge and in the retraction of the trailing structure at the cell's rear (uropod), but how Rac1 is regulated at the uropod is unknown. Here, we identified a mechanism mediated by the trafficking protein synaptotagmin-like 1 (SYTL1 or JFC1) that controls Rac1-GTP recycling from the uropod and promotes directional migration of neutrophils. JFC1-null neutrophils displayed defective polarization and impaired directional migration to N-formyl-methionine-leucyl-phenylalanine in vitro, but chemoattractant-induced actin remodeling, calcium signaling and Erk activation were normal in these cells. Defective chemotaxis was not explained by impaired azurophilic granule exocytosis associated with JFC1 deficiency. Mechanistically, we show that active Rac1 localizes at dynamic vesicles where endogenous JFC1 colocalizes with Rac1-GTP. Super-resolution microscopy (STORM) analysis shows adjacent distribution of JFC1 and Rac1-GTP, which increases upon activation. JFC1 interacts with Rac1-GTP in a Rab27a-independent manner to regulate Rac1-GTP trafficking. JFC1-null cells exhibited Rac1-GTP accumulation at the uropod and increased tail length, and Rac1-GTP uropod accumulation was recapitulated by inhibition of ROCK or by interference with microtubule remodeling. In vivo, neutrophil dynamic studies in mixed bone marrow chimeric mice show that JFC1^-/- neutrophils are unable to move directionally toward the source of the chemoattractant, supporting the notion that JFC1 deficiency results in defective neutrophil migration. Our results suggest that defective Rac1-GTP recycling from the uropod affects directionality and highlight JFC1-mediated Rac1 trafficking as a potential target to regulate chemotaxis in inflammation and immunity.

Genetically Encoded Fluorescent Biosensors Illuminate the Spatiotemporal Regulation of Signaling Networks

Cellular signaling networks are the foundation which determines the fate and function of cells as they respond to various cues and stimuli. The discovery of fluorescent proteins over 25 years ago enabled the development of a diverse array of genetically encodable fluorescent biosensors that are capable of measuring the spatiotemporal dynamics of signal transduction pathways in live cells. In an effort to encapsulate the breadth over which fluorescent biosensors have expanded, we endeavored to assemble a comprehensive list of published engineered biosensors, and we discuss many of the molecular designs utilized in their development. Then, we review how the high temporal and spatial resolution afforded by fluorescent biosensors has aided our understanding of the spatiotemporal regulation of signaling networks at the cellular and subcellular level. Finally, we highlight some emerging areas of research in both biosensor design and applications that are on the forefront of biosensor development.

Time course of chemotaxis and chemokinesis of neutrophils following stimulation with IL-8 or FMLP

Polymorphonuclear cells (PMNs) attend to inflammatory sites by chemotactic movement, caused by chemoattractants (CAs) like n-formyl-l-methionyl-l-leucyl-l-phenylalanine (FMLP) and interleukin-8 (IL-8). However, distinct but applicable assays for investigations of PMNs’ migration limit in vitro examination. We integrated CD15-bead-based isolation of PMNs with analysing their chemotaxis in a novel 3D-µ-Slide migration chamber. The PMNs were exposed to different concentrations of FMLP and IL-8 (1, 10 and 100 nM) and observed for 180 min in cell-physiological environment conditions. Moving PMNs’ percentage (median and interquartile range) decreased from 62% (27%) to 36% (31%) without CA, from 88% (30%) to 22% (26%) for 1 nM IL-8, from 70% (22%) to 28% (13%) for 100 nM IL-8, from 30% (23%) to 18% (46%) for 1 nM FMLP and from 76% (20%) to 28% (13%) for 100 nM FMLP. Centres of cell movement turned towards the CAs (negative values) within a single 30-min observation period: 5.37 µm (16.82 µm) without CA, −181.37 µm (132.18 µm) with 10 nM and −239.34 µm (152.19 µm) with 100 nM IL-8; −116.2 µm (69.07 µm) with 10 nM and −71.59 µm (98.58 µm) with 100 nM FMLP. FMLP and IL-8 ensure chemotaxis without increase of chemokinesis. 3D-µ-Slide chemotaxis chambers facilitate time course analyses of PMNs’ migration in stable conditions over a long time with concise distinction of chemotaxis and chemokinesis.

Mechanochemical modeling of neutrophil migration based on four signaling layers, integrin dynamics, and substrate stiffness

Directional neutrophil migration during human immune responses is a highly coordinated process regulated by both biochemical and biomechanical environments. In this paper, we developed an integrative mathematical model of neutrophil migration using a lattice Boltzmann-particle method built in-house to solve the moving boundary problem with spatiotemporal regulation of biochemical components. The mechanical features of the cell cortex are modeled by a series of spring-connected nodes representing discrete cell-substrate adhesive sites. The intracellular signaling cascades responsible for cytoskeletal remodeling [e.g., small GTPases, phosphoinositide-3-kinase (PI3K), and phosphatase and tensin homolog] are built based on our previous four-layered signaling model centered on the bidirectional molecular transport mechanism and implemented as reaction-diffusion equations. Focal adhesion dynamics are determined by force-dependent integrin-ligand binding kinetics and integrin recycling and are thus integrated with cell motion. Using numerical simulations, the model reproduces the major features of cell migration in response to uniform and gradient biochemical stimuli based on the quantitative spatiotemporal regulation of signaling molecules, which agree with experimental observations. The existence of multiple types of integrins with different binding kinetics could act as an adaptation mechanism for substrate stiffness. Moreover, cells can perform reversal, U-turn, or lock-on behaviors depending on the steepness of the reversal biochemical signals received. Finally, this model is also applied to predict the responses of mutants in which PTEN is overexpressed or disrupted.

Tctex-1, a novel interaction partner of Kidney Injury Molecule-1, is required for efferocytosis

Kidney injury molecule-1 (KIM-1) is a phosphatidylserine receptor that is specifically upregulated on proximal tubular epithelial cells (PTECs) during acute kidney injury and mitigates tissue damage by mediating efferocytosis (the phagocytic clearance of apoptotic cells). The signaling molecules that regulate efferocytosis in TECs are not well understood. Using a yeast two-hybrid screen, we identified the dynein light chain protein, Tctex-1, as a novel KIM-1-interacting protein. Immunoprecipitation and confocal imaging studies suggested that Tctex-1 associates with KIM-1 in cells at baseline, but, dissociates from KIM-1 within 90 min of initiation of efferocytosis. Interfering with actin or microtubule polymerization interestingly prevented the dissociation of KIM-1 from Tctex-1. Moreover, the subcellular localization of Tctex-1 changed from being microtubule-associated to mainly cytosolic upon expression of KIM-1. Short hairpin RNA-mediated silencing of endogenous Tctex-1 in cells significantly inhibited efferocytosis to levels comparable to that of knock down of KIM-1 in the same cells. Importantly, Tctex-1 was not involved in the delivery of KIM-1 to the cell-surface. On the other hand, KIM-1 expression significantly inhibited the phosphorylation of Tctex-1 at threonine 94 (T94), a post-translational modification which is known to disrupt the binding of Tctex-1 to dynein on microtubules. In keeping with this, we found that KIM-1 bound less efficiently to the phosphomimic (T94E) mutant of Tctex-1 compared to wild type Tctex-1. Surprisingly, expression of Tctex-1 T94E did not influence KIM-1-mediated efferocytosis. Our studies uncover a previously unknown role for Tctex-1 in KIM-1-dependent efferocytosis in epithelial cells.

Studying Chemoattractant Signal Transduction Dynamics in Dictyostelium by BRET

Understanding the dynamics of chemoattractant signaling is key to our understanding of the mechanisms underlying the directed migration of cells, including that of neutrophils to sites of infections and of cancer cells during metastasis. A model frequently used for deciphering chemoattractant signal transduction is the social amoeba Dictyostelium discoideum. However, the methods available to quantitatively measure chemotactic signaling are limited. Here, we describe a protocol to quantitatively study chemoattractant signal transduction in Dictyostelium by monitoring protein-protein interactions and conformational changes using Bioluminescence Resonance Energy Transfer (BRET).

Catenins Steer Cell Migration via Stabilization of Front-Rear Polarity

Cell migration plays a pivotal role in morphogenetic and pathogenetic processes. To achieve directional migration, cells must establish a front-to-rear axis of polarity. Here we show that components of the cadherin-catenin complex function to stabilize this front-rear polarity. Neural crest and glioblastoma cells undergo directional migration in vivo or in vitro. During this process, αE-catenin accumulated at lamellipodial membranes and then moved toward the rear with the support of a tyrosine-phosphorylated β-catenin. This relocating αE-catenin bound to p115RhoGEF, leading to gathering of active RhoA in front of the nucleus where myosin-IIB arcs assemble. When catenins or p115RhoGEF were removed, cells lost the polarized myosin-IIB assembly, as well as the capability for directional movement. These results suggest that, apart from its well-known function in cell adhesion, the β-catenin/αE-catenin complex regulates directional cell migration by restricting active RhoA to perinuclear regions and controlling myosin-IIB dynamics at these sites.

Role of the Cytoskeleton in Myeloid Cell Function

During an innate immune response, myeloid cells undergo complex morphological adaptations in response to inflammatory cues, which allow them to exit the vasculature, enter the tissues, and destroy invading pathogens. The actin and microtubule cytoskeletons are central to many of the most essential cellular functions including cell division, cell morphology, migration, intracellular trafficking, and signaling. Cytoskeletal structure and regulation are crucial for many myeloid cell functions, which require rapid and dynamic responses to extracellular signals. In this chapter, we review the roles of the actin and microtubule cytoskeletons in myeloid cells, focusing primarily on their roles in chemotaxis and phagocytosis. The role of myeloid cell cytoskeletal defects in hematological disorders is highlighted throughout.

Cross-talk between Rho GTPases and PI3K in the neutrophil

Neutrophils are short-lived, abundant peripheral blood leukocytes that provide a first line of defense against bacterial and fungal infections while also being a key part of the inflammatory response. Chemokines induce neutrophil recruitment to inflammatory sites, where neutrophils perform several diverse functions that are aimed at fighting infections. Neutrophil effector functions are tightly regulated processes that are governed by an array of intracellular signaling pathways and initiated by receptor-ligand binding events. Dysregulated neutrophil activation can result in excessive inflammation and host damage, as is evident in several autoimmune diseases. Rho family small GTPases and agonist-activated phosphoinositide 3-kinases (PI3Ks) represent 2 classes of key regulators of the highly specialized neutrophil. Here we review cross-talk between these important signaling intermediates in the context of neutrophil functions. We include PI3K-dependent activation of Rho family small GTPases and of their guanine nucleotide exchange factors and GTPase activating proteins, as well as Rho GTPase-dependent regulation of PI3K.

The shift in GH3 cell shape and cell motility is dependent on MLCK and ROCK

Cytoskeletal organization, actin-myosin contractility and the cell membrane together regulate cell morphology in response to the cell environment, wherein the extracellular matrix (ECM) is an indispensable component. Plasticity in cell shape enables cells to adapt their migration mode to their surroundings. GH3 endocrine cells respond to different ECM proteins, acquiring different morphologies: a rounded on collagen I-III (C I-III) and an elongated on collagen IV (C IV). However, the identities of the molecules that participate in these responses remain unknown. Considering that actin-myosin contractility is crucial to maintaining cell shape, we analyzed the participation of MLCK and ROCK in the acquisition of cell shape, the generation of cellular tension and the cell motility mode. We found that a rounded shape with high cortical tension depends on MLCK and ROCK, whereas in cells with an elongated shape, MLCK is the primary protein responsible for cell spreading. Further, in cells with a slow and directionally persistent motility, MLCK predominates, while rapid and erratic movement is ROCK-dependent. This behavior also correlates with GTPase activation. Cells on C I-III exhibited higher Rho-GTPase activity than cells on C IV and vice versa with Rac-GTPase activity, showing a plastic response of GH3 cells to their environment, leading to the generation of different cytoskeleton and membrane organizations and resulting in two movement strategies, rounded and fibroblastoid-like.

Neutrophil Microparticles Deliver Active Myeloperoxidase to Injured Mucosa To Inhibit Epithelial Wound Healing

Neutrophil (PMN) infiltration of the intestinal mucosa often leads to severe epithelial injury; however, how this process occurs is unclear. This article describes a novel mechanism whereby membrane-derived microparticles released by tissue infiltrating PMNs (PMN-MPs) serve as shuttles to protect and deliver active mediators to locally modulate cellular function during inflammation. Specifically, myeloperoxidase (MPO), which is abundantly expressed in PMN azurophilic granules and is used for microbial killing, was found to be mobilized to the PMN surface and subsequently released in association with PMN-MPs upon PMN activation and binding to intestinal epithelial cells (IECs). The enzymatic activity of PMN-MP-associated MPO was enhanced compared with soluble protein, leading to potent inhibition of wound closure following PMN-MP binding to IECs. Importantly, localized microinjection of PMN-MPs into wounded colonic mucosa was sufficient to impair epithelial wound healing in vivo. PMN-MP/MPO-dependent inhibition of IEC wound healing was due to impaired IEC migration and proliferation, resulting from impeded actin dynamics, cell spreading, and cell cycle arrest. Thus, our findings provide new insight into mechanisms governing PMN-induced tissue injury and implicate PMN-MPs and MPO as important regulators of cellular function.

Modeling the Mechanosensitivity of Neutrophils Passing through a Narrow Channel

Recent experiments have found that neutrophils may be activated after passing through microfluidic channels and filters. Mechanical deformation causes disassembly of the cytoskeleton and a sudden drop of the elastic modulus of the neutrophil. This fluidization is followed by either activation of the neutrophil with protrusion of pseudopods or a uniform recovery of the cytoskeleton network with no pseudopod. The former occurs if the neutrophil traverses the narrow channel at a slower rate. We propose a chemo-mechanical model for the fluidization and activation processes. Fluidization is treated as mechanical destruction of the cytoskeleton by sufficiently rapid bending. Loss of the cytoskeleton removes a pathway by which cortical tension inhibits the Rac protein. As a result, Rac rises and polarizes through a wave-pinning mechanism if the chemical reaction rate is fast enough. This leads to recovery and reinforcement of the cytoskeleton at the front of the neutrophil, and hence protrusion and activation. Otherwise the Rac signal returns to a uniform pre-deformation state and no activation occurs. Thus, mechanically induced neutrophil activation is understood as the competition between two timescales: that of chemical reaction and that of mechanical deformation. The model captures the main features of the experimental observation.

Stochastic modelling suggests that an elevated superoxide anion - hydrogen peroxide ratio can drive extravascular phagocyte transmigration by lamellipodium formation

Chemotaxis, integrates diverse intra- and inter-cellular molecular processes into a purposeful patho-physiological response; the operatic rules of which, remain speculative. Here, I surmise, that superoxide anion induced directional motility, in a responding cell, results from a quasi pathway between the stimulus, surrounding interstitium, and its biochemical repertoire. The epochal event in the mounting of an inflammatory response, is the extravascular transmigration of a phagocyte competent cell towards the site of injury, secondary to the development of a lamellipodium. This stochastic-to-markovian process conversion, is initiated by the cytosolic-ROS of the damaged cell, but is maintained by the inverse association of a de novo generated pool of self-sustaining superoxide anions and sub-critical hydrogen peroxide levels. Whilst, the exponential rise of O2(.-) is secondary to the focal accumulation of higher order lipid raft-Rac1/2-actin oligomers; O2(.-) mediated inactivation and redistribution of ECSOD, accounts for the minimal concentration of H2O2 that the phagocyte experiences. The net result of this reciprocal association between ROS/ RNS members, is the prolonged perturbation and remodeling of the cytoskeleton and plasma membrane, a prelude to chemotactic migration. The manuscript also describes the significance of stochastic modeling, in the testing of plausible molecular hypotheses of observable phenomena in complex biological systems.

Symmetry breaking in spreading RAT2 fibroblasts requires the MAPK/ERK pathway scaffold RACK1 that integrates FAK, p190A-RhoGAP and ERK2 signaling

The spreading of adhering cells is a morphogenetic process during which cells break spherical or radial symmetry and adopt migratory polarity with spatially segregated protruding cell front and non-protruding cell rear. The organization and regulation of these symmetry-breaking events, which are both complex and stochastic, are not fully understood. Here we show that in radially spreading cells, symmetry breaking commences with the development of discrete non-protruding regions characterized by large but sparse focal adhesions and long peripheral actin bundles. Establishment of this non-protruding static region specifies the distally oriented protruding cell front and thus determines the polarity axis and the direction of cell migration. The development of non-protruding regions requires ERK2 and the ERK pathway scaffold protein RACK1. RACK1 promotes adhesion-mediated activation of ERK2 that in turn inhibits p190A-RhoGAP signaling by reducing the peripheral localization of p190A-RhoGAP. We propose that sustained ERK signaling at the prospective cell rear induces p190A-RhoGAP depletion from the cell periphery resulting in peripheral actin bundles and cell rear formation. Since cell adhesion activates both ERK and p190A-RhoGAP signaling this constitutes a spatially confined incoherent feed-forward signaling circuit.

Feedback Amplification of Neutrophil Function

As the first line of innate immune defense, neutrophils need to mount a rapid and robust antimicrobial response. Recent studies implicate various positive feedback amplification processes in achieving that goal. Feedback amplification ensures effective migration of neutrophils in shallow chemotactic gradients, multiple waves of neutrophil recruitment to the site of inflammation, and the augmentation of various effector functions of the cells. We review here such positive feedback loops including intracellular and autocrine processes, paracrine effects mediated by lipid (LTB4), chemokine, and cytokine mediators, and bidirectional interactions with the complement system and with other immune and non-immune cells. These amplification mechanisms are not only involved in antimicrobial immunity but also contribute to neutrophil-mediated tissue damage under pathological conditions.

GPCRs and actin-cytoskeleton dynamics

A multitude of physiological processes regulated by G protein-coupled receptors (GPCRs) signaling are accomplished by the participation of active rearrangements of the cytoskeleton. In general, it is common that a cross talk occurs among networks of microfilaments, microtubules, and intermediate filaments in order to reach specific cell responses. In particular, actin-cytoskeleton dynamics regulate processes such as cell shape, cell division, cell motility, and cell polarization, among others. This chapter describes the current knowledge about the regulation of actin-cytoskeleton dynamic by diverse GPCR signaling pathways, and also includes some protocols combining immunofluorescence and confocal microscopy for the visualization of the different rearrangements of the actin-cytoskeleton. We report how both the S1P-GPCR/G12/13/Rho/ROCK and glucagon-GPCR/Gs/cAMP axes induce differential actin-cytoskeleton rearrangements in epithelial cells. We also show that specific actin-binding molecules, like phalloidin and LifeAct, are very useful to analyze F-actin reorganization by confocal microscopy, and also that both molecules show similar results in fixed cells, whereas the anti-actin antibody is useful to detect both the G- and F-actin, as well as their compartmentalization. Thus, it is highly recommended to utilize different approaches to investigate the regulation of actin dynamics by GPCR signaling, with the aim to get a better picture of the phenomenon under study.

Directional memory arises from long-lived cytoskeletal asymmetries in polarized chemotactic cells

Chemotaxis, the directional migration of cells in a chemical gradient, is robust to fluctuations associated with low chemical concentrations and dynamically changing gradients as well as high saturating chemical concentrations. Although a number of reports have identified cellular behavior consistent with a directional memory that could account for behavior in these complex environments, the quantitative and molecular details of such a memory process remain unknown. Using microfluidics to confine cellular motion to a 1D channel and control chemoattractant exposure, we observed directional memory in chemotactic neutrophil-like cells. We modeled this directional memory as a long-lived intracellular asymmetry that decays slower than observed membrane phospholipid signaling. Measurements of intracellular dynamics revealed that moesin at the cell rear is a long-lived element that when inhibited, results in a reduction of memory. Inhibition of ROCK (Rho-associated protein kinase), downstream of RhoA (Ras homolog gene family, member A), stabilized moesin and directional memory while depolymerization of microtubules (MTs) disoriented moesin deposition and also reduced directional memory. Our study reveals that long-lived polarized cytoskeletal structures, specifically moesin, actomyosin, and MTs, provide a directional memory in neutrophil-like cells even as they respond on short time scales to external chemical cues.

Locally excitable Cdc42 signals steer cells during chemotaxis

Neutrophils and other amoeboid cells chemotax by steering their front towards chemoattractant. While Ras, Rac, Cdc42, and RhoA small GTPases all regulate chemotaxis, it has been unclear how they spatiotemporally control polarization and steering. Using fluorescence biosensors in neutrophil-like PLB-985 cells and photorelease of chemoattractant, we show that local Cdc42 signals, but not those of Rac, RhoA or Ras, precede cell turning during chemotaxis. Furthermore, preexisting local Cdc42 signals in morphologically unpolarized cells predict the future direction of movement upon uniform stimulation. Moreover, inhibition of actin polymerization uncovers recurring local Cdc42 activity pulses, suggesting that Cdc42 has the excitable characteristic of the compass activity proposed in models of chemotaxis. Globally, Cdc42 antagonizes RhoA, and maintains a steep spatial activity gradient during migration, while Ras and Rac form shallow gradients. Thus, chemotactic steering and de novo polarization are both directed by locally excitable Cdc42 signals.

The mechanisms of spatial and temporal patterning of cell-edge dynamics

Adherent cells migrate and change their shape by means of protrusion and retraction at their edges. When and where these activities occur defines the shape of the cell and the way it moves. Despite a great deal of knowledge about the structural organization, components, and biochemical reactions involved in protrusion and retraction, the origins of their spatial and temporal patterns are still poorly understood. Chemical signaling circuitry is believed to be an important source of patterning, but recent studies highlighted mechanisms based on physical forces, motion, and mechanical feedback.

Myosin light chain kinase regulates cell polarization independently of membrane tension or Rho kinase

Use of embryonic zebrafish keratocytes as a model system shows that increased myosin light chain kinase (MLCK) activity promotes the formation of multiple protrusions independently of ROCK by increasing myosin accumulation in lamellipodia.

Cells polarize to a single front and rear to achieve rapid actin-based motility, but the mechanisms preventing the formation of multiple fronts are unclear. We developed embryonic zebrafish keratocytes as a model system for investigating establishment of a single axis. We observed that, although keratocytes from 2 d postfertilization (dpf) embryos resembled canonical fan-shaped keratocytes, keratocytes from 4 dpf embryos often formed multiple protrusions despite unchanged membrane tension. Using genomic, genetic, and pharmacological approaches, we determined that the multiple-protrusion phenotype was primarily due to increased myosin light chain kinase (MLCK) expression. MLCK activity influences cell polarity by increasing myosin accumulation in lamellipodia, which locally decreases protrusion lifetime, limiting lamellipodial size and allowing for multiple protrusions to coexist within the context of membrane tension limiting protrusion globally. In contrast, Rho kinase (ROCK) regulates myosin accumulation at the cell rear and does not determine protrusion size. These results suggest a novel MLCK-specific mechanism for controlling cell polarity via regulation of myosin activity in protrusions.

Traction stress analysis and modeling reveal that amoeboid migration in confined spaces is accompanied by expansive forces and requires the structural integrity of the membrane-cortex interactions

Leukocytes and tumor cells migrate via rapid shape changes in an amoeboid-like manner, distinct from mesenchymal cells such as fibroblasts. However, the mechanisms of how rapid shape changes are caused and how they lead to migration in the amoeboid mode are still unclear. In this study, we confined differentiated human promyelocytic leukemia cells between opposing surfaces of two pieces of polyacrylamide gels and characterized the mechanics of fibronectin-dependent mesenchymal versus fibronectin-independent amoeboid migration. On fibronectin-coated gels, the cells form lamellipodia and migrate mesenchymally. Whereas in the absence of cell-substrate adhesions through fibronectin, the same cells migrate by producing blebs and "chimneying" between the gel sheets. To identify the orientation and to quantify the magnitude of the traction forces, we found by traction force microscopy that expanding blebs push into the gels and generate anchoring stresses whose magnitude increases with decreasing gap size while the resulting migration speed is highest at an intermediate gap size. To understand why there exists such an optimal gap size for migration, we developed a computational model and showed that the chimneying speed depends on both the magnitude of intracellular pressure as well as the distribution of blebs around the cell periphery. The model also predicts that the optimal gap size increases with weakening cell membrane to actin cortex adhesion strength. We verified this prediction experimentally, by weakening the membrane-cortex adhesion strength using the ezrin inhibitor, baicalein. Thus, the chimneying mode of amoeboid migration requires a balance between intracellular pressure and membrane-cortex adhesion strength.