Chemotaxis by polymorphonuclear leukocytes

High levels of short-chain fatty acids secreted by Candida albicans hyphae induce neutrophil chemotaxis via free fatty acid receptor 2

Candida albicans belongs to our commensal mucosal flora and in immune-competent individuals in the absence of epithelial damage, this fungus is well tolerated and controlled by our immune defense. However, C. albicans is an opportunistic microorganism that can cause different forms of infections, ranging from superficial to life-threatening systemic infections. C. albicans is polymorphic and switches between different phenotypes (e.g. from yeast form to hyphal form). C. albicans hyphae are invasive and can grow into tissues to eventually reach circulation. During fungal infections, neutrophils in particular play a critical role for the defense, but how neutrophils are directed toward the invasive forms of fungi is less well understood. We set out to investigate possible neutrophil chemoattractants released by C. albicans into culture supernatants. We found that cell-free culture supernatants from the hyphal form of C. albicans induced both neutrophil chemotaxis and concomitant intracellular calcium transients. Size separation and hydrophobic sorting of supernatants indicated small hydrophilic factors as responsible for the activity. Further analysis showed that the culture supernatants contained high levels of short-chain fatty acids with higher levels from hyphae as compared to yeast. Short-chain fatty acids are known neutrophil chemoattractants acting via the neutrophil free fatty acid receptor 2. In line with this, the calcium signaling in neutrophils induced by hyphae culture supernatants was blocked by a free fatty acid receptor 2 antagonist and potently increased in the presence of a positive allosteric modulator. Our data imply that short-chain fatty acids may act as a recruitment signal whereby neutrophils can detect C. albicans hyphae.

Quantitative Monitoring of GPCR-Mediated Spatiotemporal IP3 Dynamics Using Confocal Fluorescence Microscopy

Activation of G protein-coupled receptors upon chemoattractant stimulation induces activation of multiple signaling pathways. To fully understand how these signaling pathway coordinates to achieve directional migration of neutrophils, it is essential to determine the dynamics of the spatiotemporal activation profile of signaling components at the level of single living cells. Here, we describe a detailed methodology for monitoring and quantitatively analyzing the spatiotemporal dynamics of 1,4,5-inositol trisphosphate (IP3) in neutrophil-like HL60 cells in response to various chemoattractant fields by applying Förster resonance energy transfer (FRET) fluorescence microscopy.

Understanding How Cells Probe the World: A Preliminary Step towards Modeling Cell Behavior?

Cell biologists have long aimed at quantitatively modeling cell function. Recently, the outstanding progress of high-throughput measurement methods and data processing tools has made this a realistic goal. The aim of this paper is twofold: First, to suggest that, while much progress has been done in modeling cell states and transitions, current accounts of environmental cues driving these transitions remain insufficient. There is a need to provide an integrated view of the biochemical, topographical and mechanical information processed by cells to take decisions. It might be rewarding in the near future to try to connect cell environmental cues to physiologically relevant outcomes rather than modeling relationships between these cues and internal signaling networks. The second aim of this paper is to review exogenous signals that are sensed by living cells and significantly influence fate decisions. Indeed, in addition to the composition of the surrounding medium, cells are highly sensitive to the properties of neighboring surfaces, including the spatial organization of anchored molecules and substrate mechanical and topographical properties. These properties should thus be included in models of cell behavior. It is also suggested that attempts at cell modeling could strongly benefit from two research lines: (i) trying to decipher the way cells encode the information they retrieve from environment analysis, and (ii) developing more standardized means of assessing the quality of proposed models, as was done in other research domains such as protein structure prediction.

pH variation in medical implant biofilms: Causes, measurements, and its implications for antibiotic resistance

The advent of implanted medical devices has greatly improved the quality of life and increased longevity. However, infection remains a significant risk because bacteria can colonize device surfaces and form biofilms that are resistant to antibiotics and the host's immune system. Several factors contribute to this resistance, including heterogeneous biochemical and pH microenvironments that can affect bacterial growth and interfere with antibiotic biochemistry; dormant regions in the biofilm with low oxygen, pH, and metabolites; slow bacterial growth and division; and poor antibody penetration through the biofilm, which may also be regions with poor acid product clearance. Measuring pH in biofilms is thus key to understanding their biochemistry and offers potential routes to detect and treat latent infections. This review covers the causes of biofilm pH changes and simulations, general findings of metabolite-dependent pH gradients, methods for measuring pH in biofilms, effects of pH on biofilms, and pH-targeted antimicrobial-based approaches.

Ras inhibitors gate chemoattractant concentration range for chemotaxis through controlling GPCR-mediated adaptation and cell sensitivity

Chemotaxis plays an essential role in recruitment of leukocytes to sites of inflammation. Eukaryotic cells sense chemoattractant with G protein-coupled receptors (GPCRs) and chemotax toward gradients with an enormous concentration range through adaptation. Cells in adaptation no longer respond to the present stimulus but remain sensitive to stronger stimuli. Thus, adaptation provides a fundamental strategy for eukaryotic cells to chemotax through a gradient. Ras activation is the first step in the chemosensing GPCR signaling pathways that displays a transient activation behavior in both model organism Dictyostelium discoideum and mammalian neutrophils. Recently, it has been revealed that C2GAP1 and CAPRI control the GPCR-mediated adaptation in D. discoideum and human neutrophils, respectively. More importantly, both Ras inhibitors regulate the sensitivity of the cells. These findings suggest an evolutionarily conserved molecular mechanism by which eukaryotic cells gate concentration range of chemoattractants for chemotaxis.

A Stable Chemokine Gradient Controls Directional Persistence of Migrating Dendritic Cells

Navigation of dendritic cells (DCs) from the site of infection to lymphoid organs is guided by concentration gradients of CCR7 ligands. How cells interpret chemokine gradients and how they couple directional sensing to polarization and persistent chemotaxis has remained largely elusive. Previous experimental systems were limited in the ability to control fast de novo formation of the final gradient slope, long-lasting stability of the gradient and to expose cells to dynamic stimulation. Here, we used a combination of microfluidics and quantitative in vitro live cell imaging to elucidate the chemotactic sensing strategy of DCs. The microfluidic approach allows us to generate soluble gradients with high spatio-temporal precision and to analyze actin dynamics, cell polarization, and persistent directional migration in both static and dynamic environments. We demonstrate that directional persistence of DC migration requires steady-state characteristics of the soluble gradient instead of temporally rising CCL19 concentration, implying that spatial sensing mechanisms control chemotaxis of DCs. Kymograph analysis of actin dynamics revealed that the presence of the CCL19 gradient is essential to stabilize leading edge protrusions in DCs and to determine directionality, since both cytoskeletal polarization and persistent chemotaxis are abrogated in the range of seconds when steady-state gradients are perturbed. In contrast to Dictyostelium amoeba, DCs are unable to decode oscillatory stimulation of soluble chemokine traveling waves into a directional response toward the wave source. These findings are consistent with the notion that DCs do not employ adaptive temporal sensing strategies that discriminate temporally increasing and decreasing chemoattractant concentrations in our setting. Taken together, in our experimental system DCs do not depend on increasing absolute chemokine concentration over time to induce persistent migration and do not integrate oscillatory stimulation. The observed capability of DCs to migrate with high directional persistence in stable gradients but not when subjected to periodic temporal cues, identifies spatial sensing as a key requirement for persistent chemotaxis of DCs.

Mechanotransduction as a major driver of cell behaviour: mechanisms, and relevance to cell organization and future research

How do cells process environmental cues to make decisions? This simple question is still generating much experimental and theoretical work, at the border of physics, chemistry and biology, with strong implications in medicine. The purpose of mechanobiology is to understand how biochemical and physical cues are turned into signals through mechanotransduction. Here, we review recent evidence showing that (i) mechanotransduction plays a major role in triggering signalling cascades following cell-neighbourhood interaction; (ii) the cell capacity to continually generate forces, and biomolecule properties to undergo conformational changes in response to piconewton forces, provide a molecular basis for understanding mechanotransduction; and (iii) mechanotransduction shapes the guidance cues retrieved by living cells and the information flow they generate. This includes the temporal and spatial properties of intracellular signalling cascades. In conclusion, it is suggested that the described concepts may provide guidelines to define experimentally accessible parameters to describe cell structure and dynamics, as a prerequisite to take advantage of recent progress in high-throughput data gathering, computer simulation and artificial intelligence, in order to build a workable, hopefully predictive, account of cell signalling networks.

How Phagocytes Acquired the Capability of Hunting and Removing Pathogens From a Human Body: Lessons Learned From Chemotaxis and Phagocytosis of Dictyostelium discoideum (Review)

How phagocytes find invading microorganisms and eliminate pathogenic ones from human bodies is a fundamental question in the study of infectious diseases. About 2.5 billion years ago, eukaryotic unicellular organisms–protozoans–appeared and started to interact with various bacteria. Less than 1 billion years ago, multicellular animals–metazoans–appeared and acquired the ability to distinguish self from non-self and to remove harmful organisms from their bodies. Since then, animals have developed innate immunity in which specialized white-blood cells phagocytes- patrol the body to kill pathogenic bacteria. The social amoebae Dictyostelium discoideum are prototypical phagocytes that chase various bacteria via chemotaxis and consume them as food via phagocytosis. Studies of this genetically amendable organism have revealed evolutionarily conserved mechanisms underlying chemotaxis and phagocytosis and shed light on studies of phagocytes in mammals. In this review, we briefly summarize important studies that contribute to our current understanding of how phagocytes effectively find and kill pathogens via chemotaxis and phagocytosis.

The Role of Neutrophils in the Immune System: An Overview

Neutrophils, also known as polymorphonuclear neutrophils (PMNs), have long been considered as the short-lived, nonspecific white cells that form pus-and also happen to kill invading microbes. Indeed, neutrophils were often neglected (and largely not considered) as immune cells. This historic view of neutrophils has changed considerably over the past several decades, and we now know that in addition to playing the predominant role in the clearance of bacteria and fungi, they have a major role in shaping the host response to infection and immune system homeostasis. The change in our view of the role of neutrophils in the immune system has been due in large part to the study of these cells in vitro. Such work has been made possible by new and/or improved methods and approaches used to investigate neutrophils. These methods are the focus of this volume.

Staphylococcus aureus and Host Immunity in Recurrent Furunculosis

Staphylococcus aureus is one of the severest and most persistent bacterial pathogens. The most frequent S. aureus infections include impetigo, folliculitis, furuncles, furunculosis, abscesses, hidradenitis suppurativa, and mastitis. S. aureus produces a great variety of cellular and extracellular factors responsible for its invasiveness and ability to cause pathological lesions. Their expression depends on the growth phase, environmental factors, and location of the infection. Susceptibility to staphylococcal infections is rooted in multiple mechanisms of host immune responses and reactions to bacterial colonization. Immunological and inflammatory processes of chronic furunculosis are based on the pathogenicity of S. aureus as well as innate and acquired immunity. In-depth knowledge about them may help to discover the whole pathomechanism of the disease and to develop effective therapeutic options. In this review, we focus on the S. aureus-host immune interactions in the pathogenesis of recurrent furunculosis according to the most recent experimental and clinical findings.

Filling GAPs in G protein- coupled receptor (GPCR)-mediated Ras adaptation and chemotaxis

Eukaryotic cells sense and migrate toward chemoattractant gradients using G protein-coupled receptor (GPCR) signaling pathways. The fascinating feature of chemotaxis is that cells migrate through chemoattractant gradients with huge concentration ranges by "adaptation." Adaptive cells no longer respond to the present stimulus but remain sensitive to stronger stimuli, providing the fundamental strategy for chemotaxis through gradients with a broad range of concentrations. Ras activation is the first step in the GPCR-mediated chemosensing signaling pathways that displays adaptation. However, the molecular mechanism of Ras adaptation is not fully understood. Here, we highlight C2GAP1, a GPCR-activated Ras negative regulator, that locally inhibits Ras signaling for adaptation and long-range chemotaxis in D. discoideum.

Neutrophil chemotaxis

Neutrophils are the primary cells recruited to inflamed sites during an innate immune response to tissue damage and/or infection. They are finely sensitive to inciting stimuli to reach in great numbers and within minutes areas of inflammation and tissue insult. For this effective response, they can detect extracellular chemical gradients and move towards higher concentrations, the so-called chemotaxis process or guided cell migration. This directed neutrophil recruitment is orchestrated by chemoattractants, a chemically diverse group of molecular guidance cues (e.g., lipids, N-formylated peptides, complement, anaphylotoxins and chemokines). Neutrophils respond to these guidance signals in a hierarchical manner and, based on this concept, they can be further subdivided into two groups: "end target" and "intermediary" chemoattractants, the signals of the former dominant over the latter. Neutrophil chemoattractants exert their effects through interaction with heptahelical G protein-coupled receptors (GPCRs) expressed on cell surfaces and the chemotactic response is mainly regulated by the Rho family of GTPases. Additionally, neutrophil behavior might differ and be affected in different complex scenarios such as disease conditions and type of vascular bed in specific organs. Finally, there are different mechanisms to disrupt neutrophil chemotaxis either associated to the resolution of inflammation or to bacterial escape and systemic infection. Therefore, in the present review, we will discuss the different molecular players involved in neutrophil chemotaxis, paying special attention to the different chemoattractants described and the way that they interact intra- and extravascularly for neutrophils to properly reach the target tissue.

GPCR-controlled membrane recruitment of negative regulator C2GAP1 locally inhibits Ras signaling for adaptation and long-range chemotaxis

Eukaryotic cells chemotax in a wide range of chemoattractant concentration gradients, and thus need inhibitory processes that terminate cell responses to reach adaptation while maintaining sensitivity to higher-concentration stimuli. However, the molecular mechanisms underlying inhibitory processes are still poorly understood. Here, we reveal a locally controlled inhibitory process in a GPCR-mediated signaling network for chemotaxis in Dictyostelium discoideum We identified a negative regulator of Ras signaling, C2GAP1, which localizes at the leading edge of chemotaxing cells and is activated by and essential for GPCR-mediated Ras signaling. We show that both C2 and GAP domains are required for the membrane targeting of C2GAP1, and that GPCR-triggered Ras activation is necessary to recruit C2GAP1 from the cytosol and retains it on the membrane to locally inhibit Ras signaling. C2GAP1-deficient c2gapA^- cells have altered Ras activation that results in impaired gradient sensing, excessive polymerization of F actin, and subsequent defective chemotaxis. Remarkably, these cellular defects of c2gapA^- cells are chemoattractant concentration dependent. Thus, we have uncovered an inhibitory mechanism required for adaptation and long-range chemotaxis.

Imaging G Protein-coupled Receptor-mediated Chemotaxis and its Signaling Events in Neutrophil-like HL60 Cells

Eukaryotic cells sense and move towards a chemoattractant gradient, a cellular process referred as chemotaxis. Chemotaxis plays critical roles in many physiological processes, such as embryogenesis, neuron patterning, metastasis of cancer cells, recruitment of neutrophils to sites of inflammation, and the development of the model organism Dictyostelium discoideum. Eukaryotic cells sense chemo-attractants using G protein-coupled receptors. Visual chemotaxis assays are essential for a better understanding of how eukaryotic cells control chemoattractant-mediated directional cell migration. Here, we describe detailed methods for: 1) real-time, high-resolution monitoring of multiple chemotaxis assays, and 2) simultaneously visualizing the chemoattractant gradient and the spatiotemporal dynamics of signaling events in neutrophil-like HL60 cells.

The Novel Functions of the PLC/PKC/PKD Signaling Axis in G Protein-Coupled Receptor-Mediated Chemotaxis of Neutrophils

Chemotaxis, a directional cell migration guided by extracellular chemoattractant gradients, plays an essential role in the recruitment of neutrophils to sites of inflammation. Chemotaxis is mediated by the G protein-coupled receptor (GPCR) signaling pathway. Extracellular stimuli trigger activation of the PLC/PKC/PKD signaling axis, which controls several signaling pathways. Here, we concentrate on the novel functions of PLC/PKC/PKD signaling in GPCR-mediated chemotaxis of neutrophils.

Motile Human Neutrophils Sense Ligand Density Over Their Entire Contact Area

Neutrophils are key components of the immune system and motility is central their function during the inflammatory response. We have previously demonstrated that neutrophils are capable of switching their motile phenotype between amoeboid-like and keratocyte-like in response to the ligand density of adhesion molecules (Henry et al. in Int Biol 6:348-356, 2014). In this study, we engineered planar micropatterned surfaces that presented adhesion molecules in local islands of high density, separated by regions largely devoid of ligands. By controlling the geometry of islands we made arrays in which the local (on island) adhesion density was high but the global (multi-island) adhesion density over the entire cell-substrate interface was low. Neutrophils in contact with these island arrays assumed a well-spread and directionally-persistent motile phenotype (keratocyte-like) in contrast to the classical amoeboid morphology they display on uniform fields of high adhesion density. By virtue of our rationally designed substrates, we were able to conclude that neutrophils were integrating the stimulation received across their entire contact interface; furthermore, they were able to mount this whole cell response on the timescale of seconds. This work demonstrates the capacity of adhesive microenvironments to direct the phenotype of cell motility, which has broader implications in physiologic processes such as inflammation and cancer metastasis.

Ligand density elicits a phenotypic switch in human neutrophils

Neutrophils are mediators of innate immunity and motility is critical to their function. We used microcontact printing to investigate the relationship between density of adhesive ligands and the dynamics of neutrophil motility. We show that neutrophils adopt a well-spread morphology without a uropod on moderate densities of adhesion ligand. As density is increased, the morphology switches to a classic amoeboid shape. In addition to the morphological differences, the dynamics of motility were quantitatively distinct. Well-spread cells without uropods glide slowly with high persistence, while amoeboid cells made frequent directional changes migrating quickly with low persistence. Using an antibody panel against various integrin chains, we show that adhesion and motility on fibronectin are mediated by MAC-1 (αMβ2). The phenotypic switch could be generalized to other surface ligands, such as bovine serum albumin, to which the promiscuous MAC-1 also binds. These results suggest that neutrophils are capable of displaying multiple modes of motility as dictated by their adhesive environment.

The role of neutrophils in the immune system: an overview

Neutrophils, also known as polymorphonuclear leukocytes (PMNs), have long been considered as the short-lived, nonspecific white cells that form pus-and also happen to kill invading microbes. Indeed, neutrophils were often neglected (and largely not considered) as immune cells. This historic view of neutrophils has changed considerably over the past several decades, and we know now that, in addition to playing the predominant role in the clearance of bacteria and fungi, they play a major role in shaping the host response to infection and immune system homeostasis. The change in our view of the role of neutrophils in the immune system has been due in large part to the study of these cells in vitro. Such work has been made possible by new and/or improved methods and approaches used to investigate neutrophils. These methods are the focus of this volume.

Chemotaxis in densely populated tissue determines germinal center anatomy and cell motility: a new paradigm for the development of complex tissues

Germinal centers (GCs) are complex dynamic structures that form within lymph nodes as an essential process in the humoral immune response. They represent a paradigm for studying the regulation of cell movement in the development of complex anatomical structures. We have developed a simulation of a modified cyclic re-entry model of GC dynamics which successfully employs chemotaxis to recapitulate the anatomy of the primary follicle and the development of a mature GC, including correctly structured mantle, dark and light zones. We then show that correct single cell movement dynamics (including persistent random walk and inter-zonal crossing) arise from this simulation as purely emergent properties. The major insight of our study is that chemotaxis can only achieve this when constrained by the known biological properties that cells are incompressible, exist in a densely packed environment, and must therefore compete for space. It is this interplay of chemotaxis and competition for limited space that generates all the complex and biologically accurate behaviors described here. Thus, from a single simple mechanism that is well documented in the biological literature, we can explain both higher level structure and single cell movement behaviors. To our knowledge this is the first GC model that is able to recapitulate both correctly detailed anatomy and single cell movement. This mechanism may have wide application for modeling other biological systems where cells undergo complex patterns of movement to produce defined anatomical structures with sharp tissue boundaries.

Neutrophils in innate host defense against Staphylococcus aureus infections

Staphylococcus aureus has been an important human pathogen throughout history and is currently a leading cause of bacterial infections worldwide. S. aureus has the unique ability to cause a continuum of diseases, ranging from minor skin infections to fatal necrotizing pneumonia. Moreover, the emergence of highly virulent, drug-resistant strains such as methicillin-resistant S. aureus in both healthcare and community settings is a major therapeutic concern. Neutrophils are the most prominent cellular component of the innate immune system and provide an essential primary defense against bacterial pathogens such as S. aureus. Neutrophils are rapidly recruited to sites of infection where they bind and ingest invading S. aureus, and this process triggers potent oxidative and non-oxidative antimicrobial killing mechanisms that serve to limit pathogen survival and dissemination. S. aureus has evolved numerous mechanisms to evade host defense strategies employed by neutrophils, including the ability to modulate normal neutrophil turnover, a process critical to the resolution of acute inflammation. Here we provide an overview of the role of neutrophils in host defense against bacterial pathogens and discuss strategies employed by S. aureus to circumvent neutrophil function.

In vitro effect of the reproductive hormones on the oxidative burst activity of polymorphonuclear leucocytes from cows: a flow cytometric study

In this study, the effect of reproductive hormones and substances with hormonal activity on the oxidative burst activity of blood polymorphonuclear leucocytes (PMN) high yielding dairy cows was evaluated. Different concentrations of: progesterone, oestradiol 17β, FSH, LH, GnRH, cortisol and PGF2α were incubated in vitro for 4 h with PMN of seven high milk yielding cows, during the period of anoestrous postpartum. Controls were run in parallel in which each hormone was replaced by its solvent. After incubation with hormones the competence of PMN to generate H(2) O(2) was monitored by flow cytometry. A down-regulation on the oxidative burst activity of PMA-stimulated PMN was observed when cells were incubated with progesterone. Significant (p ≤ 0.001) differences between control and progesterone incubated cells were observed from 6.56 μg/ml. The same predisposition was observed when PMNs were incubated with cortisol. Besides for all concentrations employed, a decrease in the burst activity was observed, only beyond 0.19 mg/ml, statistical differences between the results obtained by the control and the cortisol incubated cells were obtained. Concerning oestradiol 17β, an increase on H(2) O(2) -production was observed when PMN were incubated with 15 pg/ml and 45 pg/ml of this steroid (p ≤ 0.05), followed by a depression of the cell's activity when unphysiological concentrations were employed. Significant (p ≤ 0.05) differences between the obtained with the control and oestradiol 17β incubated cells were observed only in the highest concentration of oestradiol. No statistical differences were observed in the metabolic burst activity of PMN incubated with FSH, GnRH and LH when compared with the results obtained by the control.

Evaluation of the function of polymorphonuclear neutrophilic leukocytes in healthy dogs given a high dose of methylprednisolone sodium succinate

OBJECTIVE

To evaluate effects of a high dose of methylprednisolone sodium succinate (MPSS) on function of polymorphonuclear neutrophilic leukocytes (PMNs) in dogs.

ANIMALS

7 healthy male Beagles (body weight, 10.5 to 15 kg; age, 2 to 4 years).

PROCEDURES

All dogs were treated by IV administration of a high dose of MPSS (30 mg/kg). Additional doses of MPSS (15 mg/kg) were administered IV at 2 and 6 hours and then at 6-hour intervals until 48 hours after the initial dose. Blood samples were collected before and 1, 2, 4, 7, and 14 days after completion of the MPSS administrations and used for evaluation of PMN functions. Isolated PMNs were used for assessment of functions, such as adhesion, migration, phagocytosis, and oxidative burst.

RESULTS

On days 1, 2, and 4 after completion of MPSS administration, there was a decrease in PMN expression of adhesion markers such as CD11b and CD18. There was a decrease in the phagocytotic ability of PMNs on days 1, 2, and 7 after completion of MPSS administration, with a reduction in the oxidative burst of PMNs detected on day 7. No significant changes were identified for migration. All functional changes returned to their pretreatment values by 14 days after completion of MPSS treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

Treatment with a high dose of MPSS suppressed PMN functions in dogs. Analysis of these results suggested that treatment with a high dose of MPSS can suppress some of the major functions of PMNs for at least 7 days.

Locally controlled inhibitory mechanisms are involved in eukaryotic GPCR-mediated chemosensing

Gprotein-coupled receptor (GPCR) signaling mediates a balance of excitatory and inhibitory activities that regulate Dictyostelium chemosensing to cAMP. The molecular nature and kinetics of these inhibitors are unknown. We report that transient cAMP stimulations induce PIP3 responses without a refractory period, suggesting that GPCR-mediated inhibition accumulates and decays slowly. Moreover, exposure to cAMP gradients leads to asymmetric distribution of the inhibitory components. The gradients induce a stable accumulation of the PIP3 reporter PHCrac-GFP in the front of cells near the cAMP source. Rapid withdrawal of the gradient led to the reassociation of G protein subunits, and the return of the PIP3 phosphatase PTEN and PHCrac-GFP to their pre-stimulus distribution. Reapplication of cAMP stimulation produces a clear PHCrac-GFP translocation to the back but not to the front, indicating that a stronger inhibition is maintained in the front of a polarized cell. Our study demonstrates a novel spatiotemporal feature of currently unknown inhibitory mechanisms acting locally on the PI3K activation pathway.

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.

Quantitative imaging of single live cells reveals spatiotemporal dynamics of multistep signaling events of chemoattractant gradient sensing in Dictyostelium

Activation of G-protein-coupled chemoattractant receptors triggers dissociation of Galpha and Gbetagamma subunits. These subunits induce intracellular responses that can be highly polarized when a cell experiences a gradient of chemoattractant. Exactly how a cell achieves this amplified signal polarization is still not well understood. Here, we quantitatively measure temporal and spatial changes of receptor occupancy, G-protein activation by FRET imaging, and PIP3 levels by monitoring the dynamics of PH(Crac)-GFP translocation in single living cells in response to different chemoattractant fields. Our results provided the first direct evidence that G-proteins are activated to different extents on the cell surface in response to asymmetrical stimulations. A stronger, uniformly applied stimulation triggers not only a stronger G-protein activation but also a faster adaptation of downstream responses. When naive cells (which have not experienced chemoattractant) were abruptly exposed to stable cAMP gradients, G-proteins were persistently activated throughout the entire cell surface, whereas the response of PH(Crac)-GFP translocation surprisingly consisted of two phases, an initial transient and asymmetrical translocation around the cell membrane, followed by a second phase producing a highly polarized distribution of PH(Crac)-GFP. We propose a revised model of gradient sensing, suggesting an important role for locally controlled components that inhibit PI3Kinase activity.

Cytoskeletal organization of limulus amebocytes pre- and post-activation: comparative aspects

One of the major functions of circulating Limulus amebocytes is to effect blood coagulation upon receipt of appropriate signals. However, the hypothesis that Limulus amebocytes are fundamentally similar to vertebrate thrombocytes and platelets has not been tested sufficiently in previous studies of their cytoskeletal organization. Whereas the earlier data were derived from transmission electron microscopy (TEM) of thin sections of a limited number of cells, improved fluorescence labeling methods that retain cell morphology have now enabled us to survey F-actin and microtubule organization in intact individual amebocytes and in large amebocyte populations pre- and post-activation. Anti-tubulin immunofluorescence showed the marginal band (MB) of microtubules to be ellipsoidal in most unactivated cells, with essentially no other microtubules present. However, minor subpopulations of cells with discoidal or pointed shape, containing corresponding arrangements of microtubules suggestive of morphogenetic intermediates, were also observed. Texas-red phalloidin labeled an F-actin-rich cortex in unactivated amebocytes, accounting for MB and granule separation from the plasma membrane as visualized in TEM thin sections, and supporting earlier models for MB maintenance of flattened amebocyte morphology by pressure against a cortical layer. Shape transformation after activation by bacterial lipopolysaccharide was attributable principally to spiky and spreading F-actin in outer cell regions, with the MB changing to twisted, nuclei-associated forms and eventually becoming unrecognizable. These major pre- and post-activation cytoskeletal features resemble those of platelets and non-mammalian vertebrate thrombocytes, supporting recognition of the Limulus amebocyte as a representative evolutionary precursor of more specialized clotting cell types.

Release of a potent polymorphonuclear leukocyte chemoattractant is regulated by white-opaque switching in Candida albicans

Previous studies employing transmembrane assays suggested that Candida albicans and related species, as well as Saccharomyces cerevisiae, release chemoattractants for human polymorphonuclear leukocytes (PMNs). Because transmembrane assays do not definitively distinguish between chemokinesis and chemotaxis, single-cell chemotaxis assays were used to confirm these findings and test whether mating-type or white-opaque switching affects the release of attractant. Our results demonstrate that C. albicans, C. dubliniensis, C. tropicalis, C. parapsilosis, and C. glabrata release bona fide chemoattractants for PMNs. S. cerevisiae, however, releases a chemokinetic factor but not a chemoattractant. Characterization of the C. albicans chemoattractant revealed that it is a peptide of approximately 1 kDa. Whereas the mating type of C. albicans did not affect the release of chemoattractant, switching did. White-phase cells released chemoattractant, but opaque-phase cells did not. Since the opaque phase of C. albicans represents the mating-competent phenotype, it may be that opaque-phase cells selectively suppress the release of chemoattractant to facilitate mating.

A stochastic view of lymphocyte motility and trafficking within the lymph node

Two-photon microscopy is providing literal insight into the cellular dynamics of lymphoid organs and, guided by analysis of three-dimensional images, into mechanisms that underlie cell migration and antigen recognition in vivo. This review describes lymphocyte motility and antigen recognition in the native tissue environment and compares these results with a much more extensive literature on lymphocyte motility, signaling, and chemotaxis in vitro. We discuss the in vitro literature on dynamic aspects of lymphocyte motility, chemotaxis, and the response to antigen and present the view that random migration of lymphocytes may drive a stochastic mechanism of antigen recognition in lymphoid organs, rather than being guided by chemotaxis.

Human polymorphonuclear leukocytes respond to waves of chemoattractant, like Dictyostelium

It has been assumed that the natural chemotactic signal that attracts human polymorphonuclear leukocytes (PMNs) over long distances to sites of infection is in the form of a standing spatial gradient of chemoattractant. We have questioned this assumption on the grounds, first, that standing spatial gradients may not be stable over long distances for long periods of time and, second, that in the one animal cell chemotaxis system in which the natural chemotactic signal has been described in space and time, aggregation of Dicytostelium discoideum, the signal is in the form of an outwardly relayed, nondissipating wave of attractant. Here, it is demonstrated that PMNs alter their behavior in each of the four phases of a wave of PMN chemoattractant, fashioned after the Dictyostelium wave, in a manner similar to Dictyostelium. These results demonstrate that PMNs have all of the machinery to respond to a natural wave of attractant, providing support to the hypothesis that the natural signal that attracts PMNs over large distances to sites of infection in the human body may also be in the form of a wave.

Eukaryotic chemotaxis: distinctions between directional sensing and polarization

Directional sensing and polarization are fundamental cellular responses that play a central role in health and disease. In this review we define each process and evaluate a series of models previously proposed to explain these phenomena. New findings show that directional sensing by G protein-coupled receptors is localized at a discrete step in the signaling pathway downstream of G protein activation but upstream of the accumulation of PIP3. Local levels of PIP3, whether triggered by chemoattractants, particle binding, or spontaneous events, determine the sites of new actin-filled projections. Robust control of the temporal and spatial levels of PIP3 is achieved by reciprocal regulation of PI3K and PTEN. These observations suggest that a local excitation-global inhibition model can account for the localization of PI3K and PTEN and thereby explain directional sensing. However, elements of other models, including positive feedback and the reaction of the cytoskeleton, must be invoked to account for polarization.

Chemotaxis of capacitated rabbit spermatozoa to follicular fluid revealed by a novel directionality-based assay

Precontact communication between gametes is established by chemotaxis. Sperm chemotaxis toward factor(s) in follicular fluid (FF) has been demonstrated in humans and mice. In humans, the chemotactic responsiveness is restricted to capacitated spermatozoa. Here, we investigated whether sperm chemotaxis to factor(s) present in FF also occurs in rabbits and, if so, whether only capacitated spermatozoa are chemotactically responsive. Chemotaxis assays were performed by videomicroscopy in a Zigmond chamber. We measured chemotactic responsiveness as a function of FF dilution by means of a novel directionality-based method that considers the ratio between the distances traveled by the spermatozoa both parallel to the chemoattractant gradient and perpendicular to it. A peak of maximal response was observed at 10(-4) dilution of FF, resulting in a typical chemotactic concentration-dependent curve in which 23% of the spermatozoa were chemotactically responsive. In contrast, the percentage of cells exhibiting FF-dependent enhanced speed of swimming increased with the FF concentration, whereas the percentage of cells maintaining linear motility decreased with the FF concentration. The percentages of chemotactically responsive cells were very similar to those of capacitated spermatozoa. Depletion of the latter by stimulation of the acrosome reaction resulted in a total loss of the chemotactic response, whereas the reappearance of capacitated cells resulted in a recovery of chemotactic responsiveness. We conclude that rabbit spermatozoa, like human spermatozoa, are chemotactically responsive to FF factor(s) and acquire this responsiveness as part of the capacitation process.

Temporal and spatial regulation of chemotaxis

The ability to sense and respond to shallow gradients of extracellular signals is remarkably similar in Dictyostelium discoideum amoebae and mammalian leukocytes. Chemoattractant receptors and G proteins are fairly evenly distributed along the cell surface. Receptor occupancy generates local excitatory and global inhibitory processes that balance to control the chemotactic response. Uniform stimuli transiently recruit PI3Ks to, and release PTEN from, the plasma membrane, while gradients of chemoattractant cause the two enzymes to bind to the membrane at the front and back of the cell, respectively. Interference with PI3Ks alters chemotaxis, and disruption of PTEN broadens PI localization and actin polymerization in parallel. Thus, counteracting signals from the upstream elements of the pathway converge to regulate the key enzymes of PI metabolism, localize these lipids, and direct pseudopod formation.

Neutrophils lacking phosphoinositide 3-kinase gamma show loss of directionality during N-formyl-Met-Leu-Phe-induced chemotaxis

Confocal imaging and time-lapsed videomicroscopy were used to study the directionality, motility, rate of cell movement, and morphologies of phosphoinositide 3-kinase gamma (PI3K)gamma(-/-) neutrophils undergoing chemotaxis in Zigmond chambers containing N-formyl-Met-Leu-Phe gradients. Most of the PI3Kgamma(-/-) neutrophils failed to translocate up the chemotactic gradient. A partial reduction in cell motility and abnormal morphologies were also observed. In the wild-type neutrophils, the pleckstrin homology domain-containing protein kinase B (AKT) and F-actin colocalize to the leading edge of polarized neutrophils oriented toward the gradient, which was not observed in PI3Kgamma(-/-) neutrophils. In PI3Kgamma(-/-) neutrophils, AKT staining consistently failed to perfectly overlap with the F-actin. This failure was observed as an F-actin-filled region of 2.3 +/- 0.5 microm between AKT and the cell membrane. These data suggest that PI3Kgamma regulates neutrophil chemotaxis primarily by controlling the direction of cell migration and the intracellular colocalization of AKT and F-actin to the leading edge.

Characterization of the priming effect by pituitary canine growth hormone on canine polymorphonuclear neutrophil granulocyte function

In this report, we demonstrate that canine growth hormone (cGH) is capable of priming canine polymorphonuclear neutrophil granulocytes (PMN) in a manner resembling that of human PMN. The cGH influences important functions that are involved in the process of recruitment of PMN, i.e., shape change, chemotaxis, CD11b/CD18 expression, adhesion, and subsequent transendothelial migration. Also, intracellular O(2)(-) production was evaluated. We investigated the priming effect by incubating PMN with purified pituitary cGH at various concentrations (10 to 800 microg/liter). The capacity for shape change was significantly (P < 0.05) enhanced, whereas the chemotactic response under agarose was significantly (P < 0.05) reduced. The chemotactic migration in Boyden chambers (10-microm-thick polycarbonate filter; lower surface count technique) was significantly (P < 0.05) enhanced, presumably due to cGH-induced hyperadhesiveness to the lower surface of the filters. The adhesion in albumin-coated microtiter plates and adherence to canine pulmonary fibroblasts were significantly (P < 0.05) increased, and the increased adhesion resulted in a significant (P < 0.01) increase in transendothelial migration using canine jugular vein endothelial cells. The increase in adhesion was associated with a significant increase in CD11b/CD18 expression. Furthermore, intracellular O(2)(-) production was significantly enhanced in response to both phorbol myristate acetate (P < 0.01) and opsonized zymosan (P < 0.05). In the absence of a PMN-stimulating agent, cGH did not influence the effector functions investigated except for an increased expression of CD11b/CD18.

Dynamic expression of L-selectin in cell-to-cell interactions between neutrophils and endothelial cells in vitro

Neutrophil-endothelial cell interactions are regulated by cell adhesion molecules and their cognate ligands. It has been proposed that L-selectin and Mac-1 (CD11b/CD18), two neutrophil adhesion receptors, have sequential roles in neutrophil extravasation during inflammation. In this model, L-selectin mediates rolling and initial adherence of neutrophils to endothelial cells, while Mac-1 strengthens this initial adherence and also facilitates migration of neutrophils through endothelial cells. L-selectin and Mac-1 expression are known to be inversely regulated. Here an in vitro culture system has been developed to investigate in situ expression of L-selectin during cell-to-cell interactions between neutrophils and endothelial cell monolayers by confocal immunofluorescence analysis. Neutrophils underwent profound cell shape change from round to polarized cell morphology with pseudopod formation after 5 to 15 min coculture with IL-1-stimulated human endothelial cells. L-selectin was redistributed to the pseudopod of the polarized neutrophils in correlation with such cellular changes. During initial cell attachment, neutrophils bound to IL-1-stimulated endothelial cells expressed a high level of L-selectin in a polarized pattern. L-selectin expression decreased over time during neutrophil-endothelial cell interactions.

Microtubules are required in amoeba chemotaxis for preferential stabilization of appropriate pseudopods

Amoebae of Physarum polycephalum exhibit chemotactic responses to glucose and to cAMP. The chemotaxing amoebae exhibit alternating locomotive movements: relatively linear locomotion and movements that change the direction of the locomotion. Such locomotive activity is tightly coupled with the changes in the number and the positions of the pseudopods; cells have one pseudopod at the leading edge during their linear locomotion, while they have multiple pseudopods when they are changing the direction of locomotion. Treatment of cells with microtubule-disrupting reagents inhibited the chemotaxis of the cells. To characterize the role of the microtubule system in chemotaxis, we quantitatively analyzed the relationship between the positions of multiple pseudopods of the amoebae and the relative stability of the pseudopods during reorientation. No significant differences were observed in the pseudopod dynamics between the untreated and the treated amoebae. In both cases, one pseudopod at the leading edge continued to expand during linear locomotion. It then split into two to three pseudopods in the reorientation phase, and the positions of the multiple pseudopods were random. Among multiple pseudopods, however, the pseudopods closer to the microneedle tip were selectively stabilized more often than those distant from the tip in the presence of the microtubule system. By contrast, such preferential stabilization of the appropriate pseudopods was completely abolished by microtubule inhibitors. The microtubule-dependent selection of appropriately located pseudopods enables amoebae to turn correctly at the reorientation step.

The regulation of chemotaxis and chemokinesis in Dictyostelium amoebae by temporal signals and spatial gradients of cyclic AMP

The tactic and kinetic locomotion of Dictyostelium discoideum amoebae were examined in cyclic AMP (cAMP) spatial gradient and temporal signal fields. The distributions of migrating cells were examined within 150 microns-thick micropore filters after incubation with different cAMP concentrations, [cAMP], applied in three ways across the fields: as positively or negatively developing gradients, generated either by increasing or decreasing the [cAMP] on one side of the filter, respectively, or as static, linear gradients after negative development. Chemotaxis was only induced by oriented, temporally increasing [cAMP]. Pulses propagated by molecular diffusion or mechanical flow were equally effective. Negatively developing cAMP gradients had no initial effect on cell accumulation. However, if the subsequent static spatial gradient was maintained by an infusion system, some gradients also induced cell accumulation, whose degree and direction depended on the gradient [cAMP]. The basis of this new effect was examined by tracking individual cells by computer-assisted videomicroscopy during locomotion in different [cAMP]. Cells produced a triphasic [cAMP]-dependent response, with optimal cell motility induced by 10–30 nM. The results demonstrate that cell accumulation either up-field or down-field in spatial gradients is governed by the field locations of the attractant concentrations that induce the relative locomotory maxima and minima in the gradient field. Cells perceive the ambient [cAMP], but cannot read the spatial gradient orientation in static or yet steeper regions of developing gradients. Accumulation in static spatial gradients is a function of klino- and orthokinesis, but chemotaxis requires an oriented cAMP pulse or impulse.(ABSTRACT TRUNCATED AT 250 WORDS)

Recruitment of CD11b/CD18 to the neutrophil surface and adherence-dependent cell locomotion

Chemotactic stimulation of neutrophils results in translocation of CD11b/CD18 (Mac-1) from intracellular storage pools to the cell surface. Though results from several laboratories indicate that the newly arrived surface Mac-1 is not involved in the adherence induced by the initial stimulus, the present study addresses the hypothesis that this Mac-1 plays a role in subsequent adherence-dependent functions. The response of human neutrophils to changing concentrations of a chemotactic stimulus was evaluated by determining the amount of newly arrived surface Mac-1, and Mac-1-dependent adhesion and locomotion. Small step-wise increases in the concentration of f-Met-Leu-Phe (FMLP) resulted in proportional stepwise increases in surface Mac-1 that plateaued within 2-4 min. This newly arrived Mac-1 supported adhesion to protein-coated surfaces only when the cells were exposed to an additional increase in the FMLP stimulus level. Adherence-dependent cellular locomotion was evaluated in chambers that allowed rapid changes in the stimulus concentration. Repeated small increments in the stimulus level at 200-s intervals resulted in significantly longer migration paths than a single-step increase in the stimulus. The results support the hypothesis that small increments in the chemotactic stimulus bring Mac-1 to the cell surface, and this newly mobilized Mac-1 is available for adherence-dependent locomotion with subsequent increases in the concentration of the stimulus.

Using proteinase trapping to detect revertants of inactive rhinoviral 2A proteinase mutants

The 2A proteinase of human rhinovirus 2 cleaves itself off the growing polyprotein at its own N terminus during translation; this property was used to develop an in vivo screening system with the lacZ gene fragment of M13mp18. The fusion of an active 2A proteinase to the C-terminus of the alpha-fragment did not affect alpha-complementation, as the proteinase cleaved itself off the alpha-fragment. However, an inactive 2A proteinase remained fused to the alpha-fragment hindering alpha-complementation. Random mutations were then introduced into the 2A gene site by PCR amplification. Mutants defective in alpha-complementation (thus containing an inactive 2A proteinase) were obtained at an efficiency of 5%, mutants showing reduced 2A activity at an efficiency of 1%. Mutants showing reduced or no 2A activity were then subjected to PCR mutagenesis. Three mutants reactivating an inactive 2A proteinase were examined and the compensatory changes determined.

Adherence of neutrophils to hemodialysis membranes: role of complement receptors

Complement activation occurs during hemodialysis using cellulosic dialysis membranes with the consequent deposition of C3 activation and degradation products on the membrane surface. To determine if these complement fragments are functionally active, we examined their capacity to mediate leukocyte adherence to cuprophan membranes. Immunoblotting of proteins eluted from plasma-treated cuprophan membranes confirmed the presence of both C3b and iC3b. Incubation of cuprophan membranes with heparinized whole blood resulted in adherence of leukocytes but not erythrocytes. Neutrophils were the primary cell type bound, with monocytes comprising less than 5% of the adherent cells. Studies using indium-labeled neutrophils demonstrated that the binding was plasma dependent and increased with time up to two hours. Neutrophil binding was inhibited by preincubation of the plasma-treated cuprophan membrane with anti-C3 or preincubation of neutrophils with an antibody directed against the alpha chain of complement receptor type 3 (CR3). These observations indicate that iC3b deposited on cuprophan membrane surface as a result of complement activation mediates neutrophil adherence via interaction with CR3. They also support the hypothesis that, in addition to the anaphylatoxins released into the fluid phase, complement activation products that remained membrane bound during hemodialysis also stimulate pathophysiological responses.

Calcium gradients underlying polarization and chemotaxis of eosinophils

The concentration of intracellular free calcium ([Ca2+]i) in polarized eosinophils was imaged during chemotaxis by monitoring fluorescence of the calcium-sensitive dye Fura-2 with a modified digital imaging microscope. Chemotactic stimuli caused [Ca2+]i to increase in a nonuniform manner that was related to cell activity. In cells moving persistently in one direction, [Ca2+]i was highest at the rear and lowest at the front of the cell. Before cells turned, [Ca2+]i transiently increased. The region of the cell that became the new leading edge had the lowest [Ca2+]i. These changes in [Ca2+]i provide a basis for understanding the organization and local activity of cytoskeletal proteins thought to underlie the directed migration of many cells.

A head signal influences apical migration of interstitial cells in Hydra vulgaris

Although interstitial cells of hydra can migrate either apically or basally along the body column, there is a distinct bias toward apical cell accumulation. This apical bias could be produced by a local vectorial property of the tissue or it may be controlled by a more global property, such as a signal from the apical head region. The migration behavior of BrdU-labeled interstitial cells was examined in several types of grafts to distinguish between these two general types of migration control. Grafting BrdU-labeled midgastric region tissue into a host in either the normal or the reverse orientation had no effect on the apical bias, indicating that a local vectorial cue was probably not guiding cells apically. In grafts with heads or with feet at both ends of the body column, there was no directional bias in migration if the labeled tissue was equidistant from both ends. In the two-headed grafts, if the labeled tissue was closer to one end, there was a bias in the direction of the closer head. The results suggest that a graded signal emanating from the head creates the apical bias and may attract cells via chemotaxis. The apical bias is enhanced in decapitated animals regenerating a head, indicating that the attracting signal is present and is possibly stronger in regenerating heads. The signal for cell migration may be involved in a patterning process underlying head regeneration.