Microtubule asymmetry during neutrophil polarization and migration

Helicobacter pylori-infected human neutrophils exhibit impaired chemotaxis and a uropod retraction defect

Helicobacter pylori is a major human pathogen that colonizes the gastric mucosa and plays a causative role in development of peptic ulcers and gastric cancer. Neutrophils are heavily infected with this organism in vivo and play a prominent role in tissue destruction and disease. Recently, we demonstrated that H. pylori exploits neutrophil plasticity as part of its virulence strategy eliciting N1-like subtype differentiation that is notable for profound nuclear hypersegmentation. We undertook this study to test the hypothesis that hypersegmentation may enhance neutrophil migratory capacity. However, EZ-TAXIScan™ video imaging revealed a previously unappreciated and progressive chemotaxis defect that was apparent prior to hypersegmentation onset. Cell speed and directionality were significantly impaired to fMLF as well as C5a and IL-8. Infected cells oriented normally in chemotactic gradients, but speed and direction were impaired because of a uropod retraction defect that led to cell elongation, nuclear lobe trapping in the contracted rear and progressive narrowing of the leading edge. In contrast, chemotactic receptor abundance, adhesion, phagocytosis and other aspects of cell function were unchanged. At the molecular level, H. pylori phenocopied the effects of Blebbistatin as indicated by aberrant accumulation of F-actin and actin spikes at the uropod together with enhanced ROCKII-mediated phosphorylation of myosin IIA regulatory light chains at S19. At the same time, RhoA and ROCKII disappeared from the cell rear and accumulated at the leading edge whereas myosin IIA was enriched at both cell poles. These data suggest that H. pylori inhibits the dynamic changes in myosin IIA contractility and front-to-back polarity that are essential for chemotaxis. Taken together, our data advance understanding of PMN plasticity and H. pylori pathogenesis.

Multiple centrosomes enhance migration and immune cell effector functions of mature dendritic cells

Centrosomes play a crucial role during immune cell interactions and initiation of the immune response. In proliferating cells, centrosome numbers are tightly controlled and generally limited to one in G1 and two prior to mitosis. Defects in regulating centrosome numbers have been associated with cell transformation and tumorigenesis. Here, we report the emergence of extra centrosomes in leukocytes during immune activation. Upon antigen encounter, dendritic cells pass through incomplete mitosis and arrest in the subsequent G1 phase leading to tetraploid cells with accumulated centrosomes. In addition, cell stimulation increases expression of polo-like kinase 2, resulting in diploid cells with two centrosomes in G1-arrested cells. During cell migration, centrosomes tightly cluster and act as functional microtubule-organizing centers allowing for increased persistent locomotion along gradients of chemotactic cues. Moreover, dendritic cells with extra centrosomes display enhanced secretion of inflammatory cytokines and optimized T cell responses. Together, these results demonstrate a previously unappreciated role of extra centrosomes for regular cell and tissue homeostasis.

Roles of the nucleus in leukocyte migration

Leukocytes patrol our bodies in search of pathogens and migrate to sites of injury in response to various stimuli. Rapid and directed leukocyte motility is therefore crucial to our immunity. The nucleus is the largest and stiffest cellular organelle and a mechanical obstacle for migration through constrictions. However, the nucleus is also essential for 3D cell migration. Here, we review the roles of the nucleus in leukocyte migration, focusing on how cells deform their nuclei to aid cell motility and the contributions of the nucleus to cell migration. We discuss the regulation of the nuclear biomechanics by the nuclear lamina and how it, together with the cytoskeleton, modulates the shapes of leukocyte nuclei. We then summarize the functions of nesprins and SUN proteins in leukocytes and discuss how forces are exerted on the nucleus. Finally, we examine the mechanical roles of the nucleus in cell migration, including its roles in regulating the direction of migration and path selection.

Centrosome Positioning in Migrating Dictyostelium Cells

Directional cell migration and the establishment of polarity play an important role in development, wound healing, and host cell defense. While actin polymerization provides the driving force at the cell front, the microtubule network assumes a regulatory function, in coordinating front protrusion and rear retraction. By using Dictyostelium discoideum cells as a model for amoeboid movement in different 2D and 3D environments, the position of the centrosome relative to the nucleus was analyzed using live-cell microscopy. Our results showed that the centrosome was preferentially located rearward of the nucleus under all conditions tested for directed migration, while the nucleus was oriented toward the expanding front. When cells are hindered from straight movement by obstacles, the centrosome is displaced temporarily from its rearward location to the side of the nucleus, but is reoriented within seconds. This relocalization is supported by the presence of intact microtubules and their contact with the cortex. The data suggest that the centrosome is responsible for coordinating microtubules with respect to the nucleus. In summary, we have analyzed the orientation of the centrosome during different modes of migration in an amoeboid model and present evidence that the basic principles of centrosome positioning and movement are conserved between Dictyostelium and human leukocytes.

Neutrophils-From Bone Marrow to First-Line Defense of the Innate Immune System

Neutrophils (polymorphonuclear cells; PMNs) form a first line of defense against pathogens and are therefore an important component of the innate immune response. As a result of poorly controlled activation, however, PMNs can also mediate tissue damage in numerous diseases, often by increasing tissue inflammation and injury. According to current knowledge, PMNs are not only part of the pathogenesis of infectious and autoimmune diseases but also of conditions with disturbed tissue homeostasis such as trauma and shock. Scientific advances in the past two decades have changed the role of neutrophils from that of solely immune defense cells to cells that are responsible for the general integrity of the body, even in the absence of pathogens. To better understand PMN function in the human organism, our review outlines the role of PMNs within the innate immune system. This review provides an overview of the migration of PMNs from the vascular compartment to the target tissue as well as their chemotactic processes and illuminates crucial neutrophil immune properties at the site of the lesion. The review is focused on the formation of chemotactic gradients in interaction with the extracellular matrix (ECM) and the influence of the ECM on PMN function. In addition, our review summarizes current knowledge about the phenomenon of bidirectional and reverse PMN migration, neutrophil microtubules, and the microtubule organizing center in PMN migration. As a conclusive feature, we review and discuss new findings about neutrophil behavior in cancer environment and tumor tissue.

Microtubule destabilization is a critical checkpoint of chemotaxis and transendothelial migration in melanoma cells but not in T cells

Microtubules (MTs) control cell shape and intracellular cargo transport. The role of MT turnover in the migration of slow-moving cells through endothelial barriers remains unclear. To irreversibly interfere with MT disassembly, we have used the MT-stabilizing agent zampanolide (ZMP) in Β16F10 melanoma as amodel of slow-moving cells. ZMP-treated B16 cells failed to follow chemotactic gradients across rigid confinements and could not generate stable sub-endothelial pseudopodia under endothelial monolayers. In vivo, ZMP-treated Β16 cells failed to extravasate though lung capillaries. In contrast to melanoma cells, the chemotaxis and transendothelial migration of ZMP-treated Tcells were largely conserved. This is afirst demonstration that MT disassembly is akey checkpoint in the directional migration of cancer cells but not of lymphocytes.

Dynamic Microtubule Arrays in Leukocytes and Their Role in Cell Migration and Immune Synapse Formation

The organization of microtubule arrays in immune cells is critically important for a properly operating immune system. Leukocytes are white blood cells of hematopoietic origin, which exert effector functions of innate and adaptive immune responses. During these processes the microtubule cytoskeleton plays a crucial role for establishing cell polarization and directed migration, targeted secretion of vesicles for T cell activation and cellular cytotoxicity as well as the maintenance of cell integrity. Considering this large spectrum of distinct effector functions, leukocytes require flexible microtubule arrays, which timely and spatially reorganize allowing the cells to accommodate their specific tasks. In contrast to other specialized cell types, which typically nucleate microtubule filaments from non-centrosomal microtubule organizing centers (MTOCs), leukocytes mainly utilize centrosomes for sites of microtubule nucleation. Yet, MTOC localization as well as microtubule organization and dynamics are highly plastic in leukocytes thus allowing the cells to adapt to different environmental constraints. Here we summarize our current knowledge on microtubule organization and dynamics during immune processes and how these microtubule arrays affect immune cell effector functions. We particularly highlight emerging concepts of microtubule involvement during maintenance of cell shape and physical coherence.

Targeting tubulin polymerization and DNA binding of 4-thiazolidinone–umbelliferone hybrids: synthesis and cytotoxicity evaluation

The discovery of a series of combretastatin A-4 inspired novel molecular hybrids of 4-thiazolidinone–umbelliferone as prominent cytotoxic agents was reported.

Microtubules in cell migration

Directed cell migration is critical for embryogenesis and organ development, wound healing and the immune response. Microtubules are dynamic polymers that control directional migration through a number of coordinated processes: microtubules are the tracks for long-distance intracellular transport, crucial for delivery of new membrane components and signalling molecules to the leading edge of a migrating cell and the recycling of adhesion receptors. Microtubules act as force generators and compressive elements to support sustained cell protrusions. The assembly and disassembly of microtubules is coupled to Rho GTPase signalling, thereby controlling actin polymerisation, myosin-driven contractility and the turnover of cellular adhesions locally. Cross-talk of actin and microtubule dynamics is mediated through a number of common binding proteins and regulators. Furthermore, cortical microtubule capture sites are physically linked to focal adhesions, facilitating the delivery of secretory vesicles and efficient cross-talk. Here we summarise the diverse functions of microtubules during cell migration, aiming to show how they contribute to the spatially and temporally coordinated sequence of events that permit efficient, directional and persistent migration.

Extracellular vesicles as mediators of in vitro neutrophil swarming on a large-scale microparticle array

Neutrophils combat infections and promote healing of damaged tissues while protecting the surrounding healthy tissue through a process called swarming. Swarming neutrophils release soluble factors that recruit additional neutrophils and shape the inflammation response. Additionally, neutrophils release extracellular vesicles (EVs), which are gaining attention as important intercellular mediators. We developed a large-scale array of bioparticles on a glass substrate that triggers neutrophil swarming in vitro in a spatially and temporally controlled manner that facilitates the analysis of neutrophil migration. Our platform can generate 30 000 neutrophil swarms on a glass slide in a highly reproducible manner (98% patterning efficiency), which produces an EV-rich supernatant that enables quantitative characterization of inflammation-specific EVs. Healthy neutrophils were able to form uniform swarms across the bioparticle array, which demonstrates a high degree of intercellular coordination. However, neutrophils swarming on the bioparticle array tended to have a lower radial velocity than neutrophils swarming toward a single target. After collecting and isolating EVs released by swarming and non-swarming neutrophils, we found that neutrophils constitutively release exosomes and microvesicles. Furthermore, EVs released by swarming neutrophils cause neutrophil activation and contain the proinflammatory mediator galectin-3, suggesting that EVs have an active role during neutrophil swarming. Ultimately, understanding EVs' role in intercellular communication during swarming will improve understanding of the complex signaling pathways involved in the regulation of inflammation.

Nuclear positioning facilitates amoeboid migration along the path of least resistance

During metazoan development, immune surveillance and cancer dissemination, cells migrate in complex three-dimensional microenvironments^1-3. These spaces are crowded by cells and extracellular matrix, generating mazes with differently sized gaps that are typically smaller than the diameter of the migrating cell^4,5. Most mesenchymal and epithelial cells and some-but not all-cancer cells actively generate their migratory path using pericellular tissue proteolysis⁶. By contrast, amoeboid cells such as leukocytes use non-destructive strategies of locomotion⁷, raising the question how these extremely fast cells navigate through dense tissues. Here we reveal that leukocytes sample their immediate vicinity for large pore sizes, and are thereby able to choose the path of least resistance. This allows them to circumnavigate local obstacles while effectively following global directional cues such as chemotactic gradients. Pore-size discrimination is facilitated by frontward positioning of the nucleus, which enables the cells to use their bulkiest compartment as a mechanical gauge. Once the nucleus and the closely associated microtubule organizing centre pass the largest pore, cytoplasmic protrusions still lingering in smaller pores are retracted. These retractions are coordinated by dynamic microtubules; when microtubules are disrupted, migrating cells lose coherence and frequently fragment into migratory cytoplasmic pieces. As nuclear positioning in front of the microtubule organizing centre is a typical feature of amoeboid migration, our findings link the fundamental organization of cellular polarity to the strategy of locomotion.

Chemokine-triggered microtubule polymerization promotes neutrophil chemotaxis and invasion but not transendothelial migration

Microtubules (MTs) are critically involved in the transport of material within cells, but their roles in chemotactic leukocyte motility and effector functions are still obscure. Resting neutrophils contain few MTs assembled in an MT organizing center (MTOC) behind their multilobular nuclei. Using a probe of real-time tubulin polymerization, SiR-tubulin, we found that neutrophils elongated their MTs within minutes in response to signals from the two prototypic chemotactic peptides, CXCL1 and fMLP. Taxol, a beta-tubulin binding and MT stabilizing drug, was found to abolish this CXCL1- and fMLP-stimulated MT polymerization. Nevertheless, taxol treatment as well as disruption of existing and de novo generated MTs did not impair neutrophil protrusion and squeezing through IL-1β-stimulated endothelial monolayers mediated by endothelial deposited CXCL1 and neutrophil CXCR2. Notably, CXCL1-dependent neutrophil TEM was not associated with neutrophil MT polymerization. Chemokinetic neutrophil motility on immobilized CXCL1 was also not associated with MT polymerization, and taxol treatment did not interfere with this motility. Nevertheless, and consistent with its ability to suppress MT polymerization induced by soluble CXCL1 and fMLP, taxol treatment inhibited neutrophil chemotaxis toward both chemotactic peptides. Taxol treatment also suppressed CXCL1- and fMLP-triggered elastase-dependent neutrophil invasion through collagen I barriers. Collectively, our results highlight de novo chemoattractant-triggered MT polymerization as key for neutrophil chemotaxis and elastase-dependent invasion but not for chemotactic neutrophil crossing of inflamed endothelial barriers.

The Neutrophil Nucleus: An Important Influence on Neutrophil Migration and Function

Neutrophil nuclear morphology has historically been used in haematology for neutrophil identification and characterisation, but its exact role in neutrophil function has remained enigmatic. During maturation, segmentation of the neutrophil nucleus into its mature, multi-lobulated shape is accompanied by distinct changes in nuclear envelope composition, resulting in a unique nucleus that is believed to be imbued with extraordinary nuclear flexibility. As a rate-limiting factor for cell migration, nuclear morphology and biomechanics are particularly important in the context of neutrophil migration during immune responses. Being an extremely plastic and fast migrating cell type, it is to be expected that neutrophils have an especially deformable nucleus. However, many questions still surround the dynamic capacities of the neutrophil nucleus, and which nuclear and cytoskeletal elements determine these dynamics. The biomechanics of the neutrophil nucleus should also be considered for their influences on the production of neutrophil extracellular traps (NETs), given this process sees the release of chromatin "nets" from nucleoplasm to extracellular space. Although past studies have investigated neutrophil nuclear composition and shape, in a new era of more sophisticated biomechanical and genetic techniques, 3D migration studies, and higher resolution microscopy we now have the ability to further investigate and understand neutrophil nuclear plasticity at an unprecedented level. This review addresses what is currently understood about neutrophil nuclear structure and its role in migration and the release of NETs, whilst highlighting open questions surrounding neutrophil nuclear dynamics.

ROS and glutathionylation balance cytoskeletal dynamics in neutrophil extracellular trap formation

Neutrophils can release their genomic DNA as extracellular traps (NETs), which ensnare bacteria and limit their replication. Stojkov et al. find that modulation of cytoskeletal dynamics by reactive oxygen species and glutathionylation controls the degranulation and release of mitochondrial DNA required for NET formation.

The antimicrobial defense activity of neutrophils partly depends on their ability to form neutrophil extracellular traps (NETs), but the underlying mechanism controlling NET formation remains unclear. We demonstrate that inhibiting cytoskeletal dynamics with pharmacological agents or by genetic manipulation prevents the degranulation of neutrophils and mitochondrial DNA release required for NET formation. Wiskott-Aldrich syndrome protein–deficient neutrophils are unable to polymerize actin and exhibit a block in both degranulation and DNA release. Similarly, neutrophils with a genetic defect in NADPH oxidase fail to induce either actin and tubulin polymerization or NET formation on activation. Moreover, neutrophils deficient in glutaredoxin 1 (Grx1), an enzyme required for deglutathionylation of actin and tubulin, are unable to polymerize either cytoskeletal network and fail to degranulate or release DNA. Collectively, cytoskeletal dynamics are achieved as a balance between reactive oxygen species–regulated effects on polymerization and glutathionylation on the one hand and the Grx1-mediated deglutathionylation that is required for NET formation on the other.

Live imaging reveals distinct modes of neutrophil and macrophage migration within interstitial tissues

Cell motility is required for diverse processes during immunity and inflammation. Classically, leukocyte motility is defined as an amoeboid type of migration, however some leukocytes, like macrophages, also employ a more mesenchymal mode of migration. Here, we sought to characterize the mechanisms that regulate neutrophil and macrophage migration in vivo by using real-time imaging of leukocyte motility within interstitial tissues in zebrafish larvae. Neutrophils displayed a rounded morphology and rapid protease-independent motility, lacked defined paxillin puncta, and had persistent rearward polarization of stable F-actin and the microtubule network. By contrast, macrophages displayed an elongated morphology with reduced speed and increased directional persistence and formed paxillin-containing puncta but had a less-defined polarization of the microtubule and actin networks. We also observed differential effects of protease inhibition, microtubule disruption and ROCK inhibition on the efficiency of neutrophil and macrophage motility. Taken together, our findings suggest that larval zebrafish neutrophils and macrophage display distinct modes of migration within interstitial tissues in vivo.

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.

Leading from the Back: The Role of the Uropod in Neutrophil Polarization and Migration

Cell motility is required for diverse biological processes including development, homing of immune cells, wound healing, and cancer cell invasion. Motile neutrophils exhibit a polarized morphology characterized by the formation of leading-edge pseudopods and a highly contractile cell rear known as the uropod. Although it is known that perturbing uropod formation impairs neutrophil migration, the role of the uropod in cell polarization and motility remains incompletely understood. Here we discuss cell intrinsic mechanisms that regulate neutrophil polarization and motility, with a focus on the uropod, and examine how relationships among regulatory mechanisms change when cells change their direction of migration.

Aquaporin-9-expressing neutrophils are required for the establishment of contact hypersensitivity

Aquaporin-9 (AQP9), a water/glycerol channel protein, is expressed in several immune cells including neutrophils; however, its role in immune response remains unknown. Here we show the involvement of AQP9 in hapten-induced contact hypersensitivity (CHS), as a murine model of skin allergic contact dermatitis, using AQP9 knockout (AQP9^−/−) mice. First, the CHS response to hapten dinitrofluorobenzene (DNFB) was impaired in AQP9^−/− mice compared with wild-type (WT) mice. Adoptive transfer of sensitized AQP9^−/− draining lymph node (dLN) cells into WT recipients resulted in a reduced CHS response, indicating impaired sensitization in AQP9^−/− mice. Second, administration of WT neutrophils into AQP9^−/− mice during sensitization rescued the impaired CHS response. Neutrophil recruitment to dLNs upon hapten application was attenuated by AQP9 deficiency. Coincidentally, AQP9^−/− neutrophils showed a reduced CC-chemokine receptor 7 (CCR7) ligand-induced migration efficacy, which was attributed to the attenuated recruitment of neutrophils to dLNs. Furthermore, we found that neutrophil deficiency, observed in AQP9^−/− or neutrophil-depleted mice, decreased IL-17A production by dLN cells, which might be responsible for T cell activation during a subsequent CHS response. Taken together, these findings suggest that AQP9 is required for the development of sensitization during cutaneous acquired immune responses via regulating neutrophil function.

Critical behavior of subcellular density organization during neutrophil activation and migration

Physical theories of active matter continue to provide a quantitative understanding of dynamic cellular phenomena, including cell locomotion. Although various investigations of the rheology of cells have identified important viscoelastic and traction force parameters for use in these theoretical approaches, a key variable has remained elusive both in theoretical and experimental approaches: the spatiotemporal behavior of the subcellular density. The evolution of the subcellular density has been qualitatively observed for decades as it provides the source of image contrast in label-free imaging modalities (e.g., differential interference contrast, phase contrast) used to investigate cellular specimens. While these modalities directly visualize cell structure, they do not provide quantitative access to the structures being visualized. We present an established quantitative imaging approach, non-interferometric quantitative phase microscopy, to elucidate the subcellular density dynamics in neutrophils undergoing chemokinesis following uniform bacterial peptide stimulation. Through this approach, we identify a power law dependence of the neutrophil mean density on time with a critical point, suggesting a critical density is required for motility on 2D substrates. Next we elucidate a continuum law relating mean cell density, area, and total mass that is conserved during neutrophil polarization and migration. Together, our approach and quantitative findings will enable investigators to define the physics coupling cytoskeletal dynamics with subcellular density dynamics during cell migration.

Midline 1 controls polarization and migration of murine cytotoxic T cells

Midline 1 (MID1) is a microtubule-associated ubiquitin ligase that regulates protein phosphatase 2 A levels. Loss-of-function mutations in MID1 lead to the human X-linked Opitz G/BBB (OS) syndrome characterized by defective midline development during embryogenesis. We have recently shown that MID1 is strongly up-regulated in murine cytotoxic T lymphocytes (CTLs), and that it has a significant impact on exocytosis of lytic granules and the killing capacity of CTLs. The aims of the present study were to determine the localization of MID1 in migrating CTLs, and to investigate whether MID1 affects CTL polarization and migration. We found that MID1 mainly localizes to the uropod of migrating CTLs and that it has a substantial impact on CTL polarization and migration in vitro. Furthermore, analysis of contact hypersensitivity responses supported that MID1 controls effector functions of CTLs in hapten-challenged skin in vivo. These results provide significant new knowledge on the role of MID1 in CTL biology.

Microtubule dynamics regulation contributes to endothelial morphogenesis

Because little is known how microtubules contribute to cell migration in a physiological three-dimensional environment, we analyzed microtubule function and dynamics during in vitro angiogenesis in which endothelial cells form networks on a reconstituted basement membrane. Endothelial network formation resulted from distinct cell behaviors: matrix reorganization by myosin-mediated contractile forces, and active cell migration along reorganized, bundled matrix fibers. Inhibition of microtubule dynamics inhibited persistent cell migration, but not matrix reorganization. In addition, microtubule polymerization dynamics and CLASP2-binding to microtubules were spatially regulated to promote microtubule growth into endothelial cell protrusions along matrix tension tracks. We propose that microtubules counter-act contractile forces of the cortical actin cytoskeleton and are required to stabilize endothelial cell protrusions in a soft three-dimensional environment.

Wnt5a directs polarized calcium gradients by recruiting cortical endoplasmic reticulum to the cell trailing edge

Wnt5a directs the assembly of the Wnt-receptor-actin-myosin-polarity (WRAMP) structure, which integrates cell-adhesion receptors with F-actin and myosin to form a microfilament array associated with multivesicular bodies (MVBs). The WRAMP structure is polarized to the cell posterior, where it directs tail-end membrane retraction, driving forward translocation of the cell body. Here we define constituents of the WRAMP proteome, including regulators of microfilament and microtubule dynamics, protein interactions, and enzymatic activity. IQGAP1, a scaffold for F-actin nucleation and crosslinking, is necessary for WRAMP structure formation, potentially bridging microfilaments and MVBs. Vesicle coat proteins, including coatomer-I subunits, localize to and are required for the WRAMP structure. Electron microscopy and live imaging demonstrate movement of the ER to the WRAMP structure and plasma membrane, followed by elevation of intracellular Ca2+. Thus, Wnt5a controls directional movement by recruiting cortical ER to mobilize a rear-directed, localized Ca2+ signal, activating actomyosin contraction and adhesion disassembly for membrane retraction.

Angiostatin inhibits activation and migration of neutrophils

There is a critical need to identify molecules that modulate the biology of neutrophils because activated neutrophils, though necessary for host defense, cause exuberant tissue damage through production of reactive oxygen species and increased lifespan. Angiostatin, an endogenous anti-angiogenic cleavage product of plasminogen, binds to integrin αvβ3, ATP synthase and angiomotin and its expression is increased in inflammatory conditions. We test the hypothesis that angiostatin inhibits neutrophil activation, induces apoptosis and blocks recruitment in vivo and in vitro. The data show immuno-reactivity for plasminogen/angiostatin in resting neutrophils. Angiostatin conjugated to FITC revealed that angiostatin was endocytozed by activated mouse and human neutrophils in a lipid raft-dependent fashion. Co-immunoprecipitation of human neutrophil lysates, confocal microscopy of isolated mouse and human neutrophils and functional blocking experiments showed that angiostatin complexes with flotillin-1 along with integrin αvβ3 and ATP synthase. Angiostatin inhibited fMLP-induced neutrophil polarization, as well as caused inhibition of hsp-27 phosphorylation and stabilization of microtubules. Angiostatin treatment, before or after LPS-induced neutrophil activation, inhibited phosphorylation of p38 and p44/42 MAPKs, abolished reactive oxygen species production and released the neutrophils from suppressed apoptosis, as indicated by expression of activated caspase-3 and morphological evidence of apoptosis. Finally, intravital microscopy and myeloperoxidase assay showed inhibition of neutrophil recruitment in post-capillary venules of TNFα-treated cremaster muscle in mouse. These in vitro and in vivo data demonstrate angiostatin as a broad deactivator and silencer of neutrophils and an inhibitor of their migration. These data potentially open new avenues for the development of anti-inflammatory drugs.

Identifying network motifs that buffer front-to-back signaling in polarized neutrophils

Neutrophil polarity relies on local, mutual inhibition to segregate incompatible signaling circuits to the leading and trailing edges. Mutual inhibition alone should lead to cells having strong fronts and weak backs or vice versa. However, analysis of cell-to-cell variation in human neutrophils revealed that back polarity remains consistent despite changes in front strength. How is this buffering achieved? Pharmacological perturbations and mathematical modeling revealed a functional role for microtubules in buffering back polarity by mediating positive, long-range crosstalk from front to back; loss of microtubules inhibits buffering and results in anticorrelation between front and back signaling. Furthermore, a systematic, computational search of network topologies found that a long-range, positive front-to-back link is necessary for back buffering. Our studies suggest a design principle that can be employed by polarity networks: short-range mutual inhibition establishes distinct signaling regions, after which directed long-range activation insulates one region from variations in the other.

Cdc42 regulates neutrophil migration via crosstalk between WASp, CD11b, and microtubules

Chemotaxis promotes neutrophil participation in cellular defense by enabling neutrophil migration to infected tissue and is controlled by persistent cell polarization. One long-standing question of neutrophil polarity has been how the pseudopod and the uropod are coordinated. In our previous report, we suggested that Rho GTPase Cdc42 controls neutrophil polarity through CD11b signaling at the uropod, albeit through an unknown mechanism. Here, we show that Cdc42 controls polarity, unexpectedly, via its effector WASp. Cdc42 controls WASp activation and its distant localization to the uropod. At the uropod, WASp regulates the reorganization of CD11b integrin into detergent resistant membrane domains; in turn, CD11b recruits the microtubule end binding protein EB1 to capture and stabilize microtubules at the uropod. This organization is necessary to maintain neutrophil polarity during migration and is critical for neutrophil emigration into inflamed lungs. These results suggest unrecognized mechanism of neutrophil polarity in which WASp mediates long-distance control of the uropod by Cdc42 to maintain a proper balance between the pseudopod and the uropod. Our study reveals a new function for WASp in the control of neutrophil polarity via crosstalk between CD11b and microtubules.

The role of microtubules in neutrophil polarity and migration in live zebrafish

Microtubules control cell motility by positively regulating polarization in many cell types. However, how microtubules regulate leukocyte migration is not well understood, particularly in living organisms. Here we exploited the zebrafish system to study the role of microtubules in neutrophil migration in vivo. The localization of microtubules was visualized in motile neutrophils using various bioprobes, revealing that, in contrast to what has been seen in studies in vitro, the microtubule organizing center is positioned in front of the nucleus (relative to the direction of migration) in motile neutrophils. Microtubule disassembly impaired attraction of neutrophils to wounds but enhanced the polarity of F-actin dynamics as measured by the distribution of stable and dynamic F-actin. Microtubule depolymerization inhibited polarized phosphoinositol 3-kinase (PI(3)K) activation at the leading edge and induced rapid PI(3)K independent motility. Finally, we show that microtubules exert their effects on neutrophil polarity and motility at least in part by the negative regulation of both Rho and Rac activity. These results provide new insight into the role of microtubules in neutrophil migration in a living vertebrate and show that the motility of these professional migratory cells are subject to distinctly different rules from those established for other cell types.

Network crosstalk dynamically changes during neutrophil polarization

How complex signaling networks shape highly coordinated, multistep cellular responses is poorly understood. Here, we made use of a network-perturbation approach to investigate causal influences, or "crosstalk," among signaling modules involved in the cytoskeletal response of neutrophils to chemoattractant. We quantified the intensity and polarity of cytoskeletal marker proteins over time to characterize stereotyped cellular responses. Analyzing the effects of network disruptions revealed that, not only does crosstalk evolve rapidly during polarization, but also that intensity and polarity responses are influenced by different patterns of crosstalk. Interestingly, persistent crosstalk is arranged in a surprisingly simple circuit: a linear cascade from front to back to microtubules influences intensities, and a feed-forward network in the reverse direction influences polarity. Our approach provided a rational strategy for decomposing a complex, dynamically evolving signaling system and revealed evolving paths of causal influence that shape the neutrophil polarization response.

Disassembly of actin filaments by botulinum C2 toxin and actin-filament-disrupting agents induces assembly of microtubules in human leukaemia cell lines

BACKGROUND INFORMATION

C(2) toxin produced by Clostridium botulinum types C and D ADP-ribosylates actin monomers and inactivates their polymerization activities. The disassembly of actin filaments by C(2) toxin induces a polarization of cultured human leukaemia cell lines.

RESULTS

The polarization induced by C(2) toxin was temperature dependent and was prevented by nocodazole, a microtubule-disrupting agent, whereas it was promoted by paclitaxel, a microtubule-stabilizing agent. The fluorescence staining of polarized cells indicated an increase in microtubule assembly accompanying disassembly of actin filaments. Furthermore, several actin-filament-disrupting agents, other than C(2) toxin, also induced microtubule assembly and cell polarization, irrespective of their different mechanisms of action. The effects induced by some of the agents, which have lower binding affinities for actin, were reversible in response to the re-assembly of actin filaments.

CONCLUSIONS

Thus the disassembly of actin filaments by C(2) toxin and actin-filament-disrupting agents induces assembly of microtubules followed by polarization of human leukaemia cell lines, indicating that the assembly/disassembly equilibrium of actin filaments influences the dynamics of microtubules, which control cell morphology and, in turn, diverse cellular processes.

Review series: TOR kinase complexes and cell migration

Cell migration is a fundamental process in a wide array of biological and pathological responses. It is regulated by complex signal transduction pathways in response to external cues that couple to growth factor and chemokine receptors. In recent years, the target of rapamycin (TOR) kinase, as part of either TOR complex 1 (TORC1) or TOR complex 2 (TORC2), has been shown to be an important signaling component linking external signals to the cytoskeletal machinery in a variety of cell types and organisms. Thus, these complexes have emerged as key regulators of cell migration and chemotaxis.

An angiogenic inhibitor, cyclic RGDfV, attenuates MPTP-induced dopamine neuron toxicity

We previously demonstrated that several dopamine (DA) neurotoxins produced punctate areas of FITC-labeled albumin (FITC-LA) leakage in the substantia nigra and striatum suggesting blood brain barrier (BBB) dysfunction. Further, this leakage was co-localized with αvβ3 integrin up-regulation, a marker for angiogenesis. This suggested that the FITC-LA leakage might have been a result of angiogenesis. To assess the possible role of angiogenesis in DA neuron loss, we treated mice with 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) and on the following day treated with cyRGDfV, a cyclic peptide that binds to integrin αvβ3 and prevents angiogenesis. Post-treatment for 3 days (b.i.d.) with cyRGDfV blocked the MPTP-induced upregulation of integrin β3 immunoreactivity (a marker for angiogenesis), leakage of FITC-LA into brain parenchyma (a marker for BBB disruption) as well as the down regulation of Zona Occludin-1 (ZO-1; a marker for tight junction integrity). In addition, cyRGDfV also completely prevented tyrosine hydroxylase immunoreactive cell loss (a marker for DA neurons) and markedly attenuated the up-regulation of activated microglia (Iba1 cell counts and morphology). These data suggest that cyRGDfV, and perhaps other anti-angiogenic drugs, are neuroprotective following acute MPTP treatment and may suggest that compensatory angiogenesis and BBB dysfunction may contribute to inflammation and DA neuron loss.

Rear polarization of the microtubule-organizing center in neointimal smooth muscle cells depends on PKCα, ARPC5, and RHAMM

Directed migration of smooth muscle cells (SMCs) from the media to the intima in arteries occurs during atherosclerotic plaque formation and during restenosis after angioplasty or stent application. The polarized orientation of the microtubule-organizing center (MTOC) is a key determinant of this process, and we therefore investigated factors that regulate MTOC polarity in vascular SMCs. SMCs migrating in vivo from the medial to the intimal layer of the rat carotid artery following balloon catheter injury were rear polarized, with the MTOC located posterior of the nucleus. In tissue culture, migrating neointimal cells maintained rear polarization, whereas medial cells were front polarized. Using phosphoproteomic screening and mass spectrometry, we identified ARPC5 and RHAMM as protein kinase C (PKC)-phosphorylated proteins associated with rear polarization of the MTOC in neointimal SMCs. RNA silencing of ARPC5 and RHAMM, PKC inhibition, and transfection with a mutated nonphosphorylatable ARPC5 showed that these proteins regulate rear polarization by organizing the actin and microtubule cytoskeletons in neointimal SMCs. Both ARPC5 and RHAMM, in addition to PKC, were required for migration of neointimal SMCs.

Microtubules regulate migratory polarity through Rho/ROCK signaling in T cells

Background

Migrating leukocytes normally have a polarized morphology with an actin-rich lamellipodium at the front and a uropod at the rear. Microtubules (MTs) are required for persistent migration and chemotaxis, but how they affect cell polarity is not known.

Methodology/Principal Findings

Here we report that T cells treated with nocodazole to disrupt MTs are unable to form a stable uropod or lamellipodium, and instead often move by membrane blebbing with reduced migratory persistence. However, uropod-localized receptors and ezrin/radixin/moesin proteins still cluster in nocodazole-treated cells, indicating that MTs are required specifically for uropod stability. Nocodazole stimulates RhoA activity, and inhibition of the RhoA target ROCK allows nocodazole-treated cells to re-establish lamellipodia and uropods and persistent migratory polarity. ROCK inhibition decreases nocodazole-induced membrane blebbing and stabilizes MTs. The myosin inhibitor blebbistatin also stabilizes MTs, indicating that RhoA/ROCK act through myosin II to destabilize MTs.

Conclusions/Significance

Our results indicate that RhoA/ROCK signaling normally contributes to migration by affecting both actomyosin contractility and MT stability. We propose that regulation of MT stability and RhoA/ROCK activity is a mechanism to alter T-cell migratory behavior from lamellipodium-based persistent migration to bleb-based migration with frequent turning.

Glycogen synthase kinase 3 in the world of cell migration

Glycogen synthase kinase 3 (GSK3) is one of the few master switch kinases that regulate many aspects of cell functions. Recent studies on cell polarization and migration have shown that GSK3 is also essential for proper regulation of these processes. GSK3 influences cell migration as one of the regulators of the spatiotemporally controlled dynamics of the actin cytoskeleton, microtubules, and cell-to-matrix adhesions. In this mini-review, the effects of GSK3 on these three aspects of cell migration will be discussed.

Human neutrophils coordinate chemotaxis by differential activation of Rac1 and Rac2

Rac1 and Rac2, members of the small Rho GTPase family, play essential roles in coordinating directional migration and superoxide production during neutrophil responses to chemoattractants. Although earlier studies in Rac1 and Rac2 knockout mice have demonstrated unique roles for each Rac isoform in chemotaxis and NADPH oxidase activation, it is still unclear how human neutrophils use Rac1 and Rac2 to achieve their immunological responses to foreign agent stimulation. In the current study, we used TAT dominant-negative Rac1-T17N and Rac2-T17N fusion proteins to acutely alter the activity of Rac1 and Rac2 individually in human neutrophils. We demonstrate distinct activation kinetics and different roles for Rac1 and Rac2 in response to low vs high concentrations of fMLP. These observations were verified using neutrophils from mice in which Rac1 or Rac2 was genetically absent. Based on these results, we propose a model to explain how human neutrophils kill invading microbes while limiting oxidative damage to the adjacent surrounding healthy tissue through the differential activation of Rac1 and Rac2 in response to different concentrations of chemoattractant.

Adaptive-control model for neutrophil orientation in the direction of chemical gradients

Neutrophils have a remarkable ability to detect the direction of chemoattractant gradients and move directionally in response to bacterial infections and tissue injuries. For their role in health and disease, neutrophils have been extensively studied, and many of the molecules involved in the signaling mechanisms of gradient detection and chemotaxis have been identified. However, the cellular-scale mechanisms of gradient sensing and directional neutrophil migration have been more elusive, and existent models provide only limited insight into these processes. Here, we propose a what we believe is a novel adaptive-control model for the initiation of cell polarization in response to gradients. In this model, the neutrophils first sample the environment by extending protrusions in random directions and subsequently adapt their sensitivity depending on localized, temporal changes in stimulation levels. Our results suggest that microtubules may play a critical role in integrating all the sensing events from the cellular periphery through their redistribution inside the neutrophils, and may also be involved in modulating local signaling. An unexpected finding was that model neutrophils exhibit significant randomness in timing and directionality of activation, comparable to our experimental observations in microfluidic devices. Moreover, their responses are robust against alterations of the rate and amplitude of the signaling reactions, and for a broad range in chemoattractant concentrations and spatial gradients.

Microtubule network asymmetry in motile cells: role of Golgi-derived array

Cell migration requires polarization of the cell into the leading edge and the trailing edge. Microtubules (MTs) are indispensable for polarized cell migration in the majority of cell types. To support cell polarity, MT network has to be functionally and structurally asymmetric. How is this asymmetry achieved? In interphase cells, MTs form a dynamic system radiating from a centrosome-based MT-organizing center (MTOC) to the cell edges. Symmetry of this radial array can be broken according to four general principles. Asymmetry occurs due to differential modulation of MT dynamics, relocation of existing MTs within a cell, adding an asymmetric nucleation site, and/or repositioning of a symmetric nucleation site to one side of a cell. Combinations of these asymmetry regulation principles result in a variety of asymmetric MT networks typical for diverse motile cell types. Importantly, an asymmetric MT array is formed at a non-conventional MT nucleation site, the Golgi. Here, we emphasize the contribution of this array to the asymmetry of MT network.

Temperature pretreatment alters the polarization response of human neutrophils to the chemoattractant N-formyl-Met-Leu-Phe

Neutrophils present a polarized morphology upon stimulation of chemoattractants, which play a vital role in host-defense mechanisms. Many studies have been published on neutrophil polarization, in which three different temperatures pretreatment (4 degrees C, 25 degrees C and 37 degrees C) have been used. However, no study has investigated whether different temperature pretreatments affect neutrophil polarization. In the current study, we examined the effects of 4 degrees C, 25 degrees C and 37 degrees C pretreatment temperatures on short-term (1 or 3 min) chemoattractant-induced polarization. Human neutrophils were polarized upon the stimulation of N-formyl-Met-Leu-Phe (fMLP) after pretreated by different temperature. The morphological changes of the neutrophils were investigated under the microscopy. The F-actin polymerization was determined by immunological histological chemistry. There were more head-tail polarized cells (>50% of the cells) in the 25 degrees C and 37 degrees C pretreatment groups than in the 4 degrees C group (32.4%). The average lengths of the pseudopod were 3.2 +/- 1.1 microm (n = 17), 5.3 +/- 2.1 microm (n = 12) and 7.4 +/- 2.7 microm (n = 21) in the 4 degrees C, 25 degrees C and 37 degrees C pretreatment groups, respectively; the 4 degrees C and 37 degrees C pretreatment groups were statistically different (P < 0.05). Additionally, there was a statistically significant difference in the pseudopod extension rate among the three groups, as well as the Lamellipod percentage between the 4 degrees C group and the other two groups within 1 min of stimulation with fMLP. This study demonstrates that different temperature pretreatments affect neutrophil polarization upon short-term stimulation with fMLP.

Collective cell migration requires vesicular trafficking for chemoattractant delivery at the trailing edge

Chemoattractant signaling induces the polarization and directed movement of cells secondary to the activation of multiple effector pathways. In addition, chemotactic signals can be amplified and relayed to proximal cells via the synthesis and secretion of additional chemoattractant. The mechanisms underlying such remarkable features remain ill defined. We show that the asymmetrical distribution of adenylyl cyclase (ACA) at the back of Dictyostelium discoideum cells, an essential determinant of their ability to migrate in a head-to-tail fashion, requires vesicular trafficking. This trafficking results in a local accumulation of ACA-containing intracellular vesicles and involves intact actin, microtubule networks, and de novo protein synthesis. We also show that migrating cells leave behind ACA-containing vesicles, likely secreted as multivesicular bodies and presumably involved in the formation of head-to-tail arrays of migrating cells. We propose that similar compartmentalization and shedding mechanisms exist in mammalian cells during embryogenesis, wound healing, neuron growth, and metastasis.

An experimental and computational study of effects of microtubule stabilization on T-cell polarity

T-killer cells eliminate infected and cancerous cells with precision by positioning their centrosome near the interface (immunological synapse) with the target cell. The mechanism of centrosome positioning has remained controversial, in particular the role of microtubule dynamics in it. We re-examined the issue in the experimental model of Jurkat cells presented with a T cell receptor-binding artificial substrate, which permits controlled stimulation and reproducible measurements. Neither 1-µM taxol nor 100-nM nocodazole inhibited the centrosome positioning at the “synapse” with the biomimetic substrate. At the same time, in micromolar taxol but not in nanomolar nocodazole the centrosome adopted a distinct peripheral rather than the normally central position within the synapse. This effect was reproduced in a computational energy-minimization model that assumed no microtubule dynamics, but only a taxol-induced increase in the length of the microtubules. Together, the experimental and computational results indicate that microtubule dynamics are not essential for the centrosome positioning, but that the fit of the microtubule array in the deformed body of the conjugated T cell is a major factor. The possibility of modulating the T-cell centrosome position with well-studied drugs and of predicting their effects in silico appears attractive for designing anti-cancer and antiviral therapies.

Analysis of neutrophil polarization and chemotaxis

Neutrophil polarization and directed migration (chemotaxis) are critical for the inflammatory response. Neutrophil chemotaxis is achieved by the sensing of narrow gradients of chemoattractant and the subsequent polarization and directed migration toward the chemotactic source. Despite recent progress, the signaling mechanisms that regulate neutrophil polarization during chemotaxis have not been clearly defined. Here, we describe methods to analyze neutrophil polarization and asymmetric redistribution of signaling components induced by chemoattractant using immunofluorescence. Further, methods are described to dissect the role of specific signaling pathways during chemotaxis by the use of murine neutrophils from transgenic mouse models. Finally, methods for time-lapse microscopy and transwell assay for the analysis of neutrophil chemotaxis will also be discussed.

Control of cell flattening and junctional remodeling during squamous epithelial morphogenesis in Drosophila

Diverse types of epithelial morphogenesis drive development. Similar cytoskeletal and cell adhesion machinery orchestrate these changes, but it is unclear how distinct tissue types are produced. Thus, it is important to define and compare different types of morphogenesis. We investigated cell flattening and elongation in the amnioserosa, a squamous epithelium formed at Drosophila gastrulation. Amnioserosa cells are initially columnar. Remarkably, they flatten and elongate autonomously by perpendicularly rotating the microtubule cytoskeleton--we call this 'rotary cell elongation'. Apical microtubule protrusion appears to initiate the rotation and microtubule inhibition perturbs the process. F-actin restrains and helps orient the microtubule protrusions. As amnioserosa cells elongate, they maintain their original cell-cell contacts and develop planar polarity. Myosin II localizes to anterior-posterior contacts, while the polarity protein Bazooka (PAR-3) localizes to dorsoventral contacts. Genetic analysis revealed that Myosin II and Bazooka cooperate to properly position adherens junctions. These results identify a specific cellular mechanism of squamous tissue morphogenesis and molecular interactions involved.

The PCH family member proline-serine-threonine phosphatase-interacting protein 1 targets to the leukocyte uropod and regulates directed cell migration

Pombe Cdc15 homology (PCH) family members have emerged as important regulators of membrane-cytoskeletal interactions. Here we show that PSTPIP1, a PCH family member expressed in hematopoietic cells, regulates the motility of neutrophil-like cells and is a novel component of the leukocyte uropod where it colocalizes with other uropod components, such as type I PIPKIgamma. Furthermore, we show that PSTPIP1 association with the regulator of endocytosis, dynamin 2, and PSTPIP1 expression impairs transferrin uptake and endocytosis. We also show that PSTPIP1 localizes at the rear of neutrophils with a subpopulation of F-actin that is specifically detected by the binding of an F-actin probe that detects a more stable population of actin. Finally, we show that actin polymerization, but not the microtubule network, is necessary for the polarized distribution of PSTPIP1 toward the rear of the cell. Together, our findings demonstrate that PSTPIP1 is a novel component of the leukocyte uropod that regulates endocytosis and cell migration.

Elevated cholesterol levels in the plasma membranes of macrophages inhibit migration by disrupting RhoA regulation

OBJECTIVE

Atherogenesis begins as small subendothelial accumulations of foam cells that develop through unregulated uptake of modified and aggregated low-density lipoprotein (LDL). The reason why foam cells remain in the atherosclerotic plaque rather than migrating out of the area is unclear. We tested the hypothesis that elevated membrane cholesterol levels, which may result from interactions with aggregated LDL, affect macrophage migration.

METHODS AND RESULTS

Cholesterol loading by incubation with cholesterol-chelated methyl-beta-cyclodextrin decreased migration of J774A.1 macrophages toward complement 5a (C5a) in transwell migration assays, even though cholesterol-loaded macrophages responded to a bath application of C5a. In a micropipette polarization assay, cholesterol-loaded cells polarized toward a C5a gradient. In a transwell migration assay, cholesterol-loaded cells extended lamellae through the filter pores but were unable to translocate their cell bodies. Cholesterol loading decreased both the cellular levels of GTP-bound active RhoA and the phosphorylation of myosin light chain. Expression of constitutively active RhoA largely prevented the inhibition of cell migration by cholesterol loading.

CONCLUSIONS

These results suggest that increases in plasma membrane cholesterol content alter RhoA activation, resulting in inhibition of cell migration. These findings provide one possible explanation for the retention of foam cells in atherosclerotic lesions.

Establishment and maintenance of cell polarity during leukocyte chemotaxis

The term polarity refers to the differential distribution of the macromolecular elements of a cell, resulting in its asymmetry in function, shape and/or content. Polarity is a fundamental property of all metazoan cells in at least some stages, and is pivotal to processes such as epithelial differentiation (apical/basal polarity), coordinated cell activity within the plane of a tissue (planar cell polarity), asymmetric cell division, and cell migration. In the last case, an apparently symmetric cell responds to directional cues provided by chemoattractants, creating a polarity axis that runs from the cell anterior, or leading edge, in which actin polymerization takes place, to the cell posterior (termed uropod in leukocytes), in which acto-myosin contraction occurs. Here we will review some of the molecular mechanisms through which chemoattractants break cell symmetry to trigger directed migration, focusing on cells of the immune system. We briefly highlight some common or apparently contradictory pathways reported as important for polarity in other cells, as this suggests conserved or cell type-specific mechanisms in eukaryotic cell chemotaxis.

Bias in the gradient-sensing response of chemotactic cells

We apply linear stability theory and perform perturbation studies to better characterize, and to generate new experimental predictions from, a model of chemotactic gradient sensing in eukaryotic cells. The model uses reaction-diffusion equations to describe 3(') phosphoinositide signaling and its regulation at the plasma membrane. It demonstrates a range of possible gradient-sensing mechanisms and captures such characteristic behaviors as strong polarization in response to static gradients, adaptation to differing mean levels of stimulus, and plasticity in response to changing gradients. An analysis of the stability of polarized steady-state solutions indicates that the model is most sensitive to off-axis perturbations. This biased sensitivity is also reflected in responses to localized external stimuli, and leads to a clear experimental prediction, namely, that a cell which is polarized in a background gradient will be most sensitive to transient point-source stimuli lying within a range of angles that are oblique with respect to the polarization axis. Stimuli at angles below this range will elicit responses whose directions overshoot the stimulus angle, while responses to stimuli applied at larger angles will undershoot the stimulus angle. We argue that such a bias is likely to be a general feature of gradient sensing in highly motile cells, particularly if they are optimized to respond to small gradients. Finally, an angular bias in gradient sensing might lead to preferred turn angles and zigzag movements of cells moving up chemotactic gradients, as has been noted under certain experimental conditions.