Chemotaxis in a lymphocyte cell line transfected with C-C chemokine receptor 2B: evidence that directed migration is mediated by betagamma dimers released by activation of Galphai-coupled receptors

CC Chemokine Family Members' Modulation as a Novel Approach for Treating Central Nervous System and Peripheral Nervous System Injury-A Review of Clinical and Experimental Findings

Despite significant progress in modern medicine and pharmacology, damage to the nervous system with various etiologies still poses a challenge to doctors and scientists. Injuries lead to neuroimmunological changes in the central nervous system (CNS), which may result in both secondary damage and the development of tactile and thermal hypersensitivity. In our review, based on the analysis of many experimental and clinical studies, we indicate that the mechanisms occurring both at the level of the brain after direct damage and at the level of the spinal cord after peripheral nerve damage have a common immunological basis. This suggests that there are opportunities for similar pharmacological therapeutic interventions in the damage of various etiologies. Experimental data indicate that after CNS/PNS damage, the levels of 16 among the 28 CC-family chemokines, i.e., CCL1, CCL2, CCL3, CCL4, CCL5, CCL6, CCL7, CCL8, CCL9, CCL11, CCL12, CCL17, CCL19, CCL20, CCL21, and CCL22, increase in the brain and/or spinal cord and have strong proinflammatory and/or pronociceptive effects. According to the available literature data, further investigation is still needed for understanding the role of the remaining chemokines, especially six of them which were found in humans but not in mice/rats, i.e., CCL13, CCL14, CCL15, CCL16, CCL18, and CCL23. Over the past several years, the results of studies in which available pharmacological tools were used indicated that blocking individual receptors, e.g., CCR1 (J113863 and BX513), CCR2 (RS504393, CCX872, INCB3344, and AZ889), CCR3 (SB328437), CCR4 (C021 and AZD-2098), and CCR5 (maraviroc, AZD-5672, and TAK-220), has beneficial effects after damage to both the CNS and PNS. Recently, experimental data have proved that blockades exerted by double antagonists CCR1/3 (UCB 35625) and CCR2/5 (cenicriviroc) have very good anti-inflammatory and antinociceptive effects. In addition, both single (J113863, RS504393, SB328437, C021, and maraviroc) and dual (cenicriviroc) chemokine receptor antagonists enhanced the analgesic effect of opioid drugs. This review will display the evidence that a multidirectional strategy based on the modulation of neuronal-glial-immune interactions can significantly improve the health of patients after CNS and PNS damage by changing the activity of chemokines belonging to the CC family. Moreover, in the case of pain, the combined administration of such antagonists with opioid drugs could reduce therapeutic doses and minimize the risk of complications.

Computational design of dynamic receptor-peptide signaling complexes applied to chemotaxis

Engineering protein biosensors that sensitively respond to specific biomolecules by triggering precise cellular responses is a major goal of diagnostics and synthetic cell biology. Previous biosensor designs have largely relied on binding structurally well-defined molecules. In contrast, approaches that couple the sensing of flexible compounds to intended cellular responses would greatly expand potential biosensor applications. Here, to address these challenges, we develop a computational strategy for designing signaling complexes between conformationally dynamic proteins and peptides. To demonstrate the power of the approach, we create ultrasensitive chemotactic receptor-peptide pairs capable of eliciting potent signaling responses and strong chemotaxis in primary human T cells. Unlike traditional approaches that engineer static binding complexes, our dynamic structure design strategy optimizes contacts with multiple binding and allosteric sites accessible through dynamic conformational ensembles to achieve strongly enhanced signaling efficacy and potency. Our study suggests that a conformationally adaptable binding interface coupled to a robust allosteric transmission region is a key evolutionary determinant of peptidergic GPCR signaling systems. The approach lays a foundation for designing peptide-sensing receptors and signaling peptide ligands for basic and therapeutic applications.

A meta-analysis indicates that the regulation of cell motility is a non-intrinsic function of chemoattractant receptors that is governed independently of directional sensing

Chemoattraction, defined as the migration of a cell toward a source of a chemical gradient, is controlled by chemoattractant receptors. Chemoattraction involves two basic activities, namely, directional sensing, a molecular mechanism that detects the direction of a source of chemoattractant, and actin-based motility, which allows the migration of a cell towards it. Current models assume first, that chemoattractant receptors govern both directional sensing and motility (most commonly inducing an increase in the migratory speed of the cells, i.e. chemokinesis), and, second, that the signaling pathways controlling both activities are intertwined. We performed a meta-analysis to reassess these two points. From this study emerge two main findings. First, although many chemoattractant receptors govern directional sensing, there are also receptors that do not regulate cell motility, suggesting that is the ability to control directional sensing, not motility, that best defines a chemoattractant receptor. Second, multiple experimental data suggest that receptor-controlled directional sensing and motility can be controlled independently. We hypothesize that this independence may be based on the existence of separated signalling modules that selectively govern directional sensing and motility in chemotactic cells. Together, the information gathered can be useful to update current models representing the signalling from chemoattractant receptors. The new models may facilitate the development of strategies for a more effective pharmacological modulation of chemoattractant receptor-controlled chemoattraction in health and disease.

Gαi protein subunit: A step toward understanding its non-canonical mechanisms

The heterotrimeric G protein family plays essential roles during a varied array of cellular events; thus, its deregulation can seriously alter signaling events and the overall state of the cell. Heterotrimeric G-proteins have three subunits (α, β, γ) and are subdivided into four families, Gαi, Gα12/13, Gαq, and Gαs. These proteins cycle between an inactive Gα-GDP state and active Gα-GTP state, triggered canonically by the G-protein coupled receptor (GPCR) and by other accessory proteins receptors independent also known as AGS (Activators of G-protein Signaling). In this review, we summarize research data specific for the Gαi family. This family has the largest number of individual members, including Gαi1, Gαi2, Gαi3, Gαo, Gαt, Gαg, and Gαz, and constitutes the majority of G proteins α subunits expressed in a tissue or cell. Gαi was initially described by its inhibitory function on adenylyl cyclase activity, decreasing cAMP levels. Interestingly, today Gi family G-protein have been reported to be importantly involved in the immune system function. Here, we discuss the impact of Gαi on non-canonical effector proteins, such as c-Src, ERK1/2, phospholipase-C (PLC), and proteins from the Rho GTPase family members, all of them essential signaling pathways regulating a wide range of physiological processes.

Chemokinesis-driven accumulation of active colloids in low-mobility regions of fuel gradients

Many motile cells exhibit migratory behaviors, such as chemotaxis (motion up or down a chemical gradient) or chemokinesis (dependence of speed on chemical concentration), which enable them to carry out vital functions including immune response, egg fertilization, and predator evasion. These have inspired researchers to develop self-propelled colloidal analogues to biological microswimmers, known as active colloids, that perform similar feats. Here, we study the behavior of half-platinum half-gold (Pt/Au) self-propelled rods in antiparallel gradients of hydrogen peroxide fuel and salt, which tend to increase and decrease the rods' speed, respectively. Brownian Dynamics simulations, a Fokker-Planck theoretical model, and experiments demonstrate that, at steady state, the rods accumulate in low-speed (salt-rich, peroxide-poor) regions not because of chemotaxis, but because of chemokinesis. Chemokinesis is distinct from chemotaxis in that no directional sensing or reorientation capabilities are required. The agreement between simulations, model, and experiments bolsters the role of chemokinesis in this system. This work suggests a novel strategy of exploiting chemokinesis to effect accumulation of motile colloids in desired areas.

Role of inflammatory chemokines in hypertension

Hypertension is associated with immune cells activation and their migration into the kidney, vasculature, heart and brain. These inflammatory mechanisms are critical for blood pressure regulation and mediate target organ damage, creating unique novel targets for pharmacological modulation. In response to angiotensin II and other pro-hypertensive stimuli, the expression of several inflammatory chemokines and their receptors is increased in the target organs, mediating homing of immune cells. In this review, we summarize the contribution of key inflammatory chemokines and their receptors to increased accumulation of immune cells in target organs and effects on vascular dysfunction, remodeling, oxidative stress and fibrosis, all of which contribute to blood pressure elevation. In particular, the role of CCL2, CCL5, CXCL8, CXCL9, CXCL10, CXCL11, CXCL16, CXCL1, CX3CL1, XCL1 and their receptors in the context of hypertension is discussed. Recent studies have tested the efficacy of pharmacological or genetic targeting of chemokines and their receptors on the development of hypertension. Promising results indicate that some of these pathways may serve as future therapeutic targets to improve blood pressure control and prevent target organ consequences including kidney failure, heart failure, atherosclerosis or cognitive impairment.

Biased agonism at chemokine receptors

In the human chemokine system, interactions between the approximately 50 known endogenous chemokine ligands and 20 known chemokine receptors (CKRs) regulate a wide range of cellular functions and biological processes including immune cell activation and homeostasis, development, angiogenesis, and neuromodulation. CKRs are a family of G protein-coupled receptors (GPCR), which represent the most common and versatile class of receptors in the human genome and the targets of approximately one third of all Food and Drug Administration-approved drugs. Chemokines and CKRs bind with significant promiscuity, as most CKRs can be activated by multiple chemokines and most chemokines can activate multiple CKRs. While these ligand-receptor interactions were previously regarded as redundant, it is now appreciated that many chemokine:CKR interactions display biased agonism, the phenomenon in which different ligands binding to the same receptor signal through different pathways with different efficacies, leading to distinct biological effects. Notably, these biased responses can be modulated through changes in ligand, receptor, and or the specific cellular context (system). In this review, we explore the biochemical mechanisms, functional consequences, and therapeutic potential of biased agonism in the chemokine system. An enhanced understanding of biased agonism in the chemokine system may prove transformative in the understanding of the mechanisms and consequences of biased signaling across all GPCR subtypes and aid in the development of biased pharmaceuticals with increased therapeutic efficacy and safer side effect profiles.

Concepts of GPCR-controlled navigation in the immune system

G‐protein–coupled receptor (GPCR) signaling is essential for the spatiotemporal control of leukocyte dynamics during immune responses. For efficient navigation through mammalian tissues, most leukocyte types express more than one GPCR on their surface and sense a wide range of chemokines and chemoattractants, leading to basic forms of leukocyte movement (chemokinesis, haptokinesis, chemotaxis, haptotaxis, and chemorepulsion). How leukocytes integrate multiple GPCR signals and make directional decisions in lymphoid and inflamed tissues is still subject of intense research. Many of our concepts on GPCR‐controlled leukocyte navigation in the presence of multiple GPCR signals derive from in vitro chemotaxis studies and lower vertebrates. In this review, we refer to these concepts and critically contemplate their relevance for the directional movement of several leukocyte subsets (neutrophils, T cells, and dendritic cells) in the complexity of mouse tissues. We discuss how leukocyte navigation can be regulated at the level of only a single GPCR (surface expression, competitive antagonism, oligomerization, homologous desensitization, and receptor internalization) or multiple GPCRs (synergy, hierarchical and non‐hierarchical competition, sequential signaling, heterologous desensitization, and agonist scavenging). In particular, we will highlight recent advances in understanding GPCR‐controlled leukocyte navigation by intravital microscopy of immune cells in mice.

Gβγ signaling to the chemotactic effector P-REX1 and mammalian cell migration is directly regulated by Gαq and Gα13 proteins

G protein-coupled receptors stimulate Rho guanine nucleotide exchange factors that promote mammalian cell migration. Rac and Rho GTPases exert opposing effects on cell morphology and are stimulated downstream of Gβγ and Gα_12/13 or Gα_q, respectively. These Gα subunits might in turn favor Rho pathways by preventing Gβγ signaling to Rac. Here, we investigated whether Gβγ signaling to phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchange factor 1 (P-REX1), a key Gβγ chemotactic effector, is directly controlled by Rho-activating Gα subunits. We show that pharmacological inhibition of Gα_q makes P-REX1 activation by G_q/G_i-coupled lysophosphatidic acid receptors more effective. Moreover, chemogenetic control of G_i and G_q by designer receptors exclusively activated by designer drugs (DREADDs) confirmed that G_i differentially activates P-REX1. GTPase-deficient Gα_qQL and Gα₁₃QL variants formed stable complexes with Gβγ, impairing its interaction with P-REX1. The N-terminal regions of these variants were essential for stable interaction with Gβγ. Pulldown assays revealed that chimeric Gα_13-i2QL interacts with Gβγ unlike to Gα_i2-13QL, the reciprocal chimera, which similarly to Gα_i2QL could not interact with Gβγ. Moreover, Gβγ was part of tetrameric Gβγ-Gα_qQL-RGS2 and Gβγ-Gα_13-i2QL-RGS4 complexes, whereas Gα₁₃QL dissociated from Gβγ to interact with the PDZ-RhoGEF-RGS domain. Consistent with an integrated response, Gβγ and AKT kinase were associated with active SDF-1/CXCL12-stimulated P-REX1. This pathway was inhibited by Gα_qQL and Gα₁₃QL, which also prevented CXCR4-dependent cell migration. We conclude that a coordinated mechanism prioritizes Gα_q- and Gα₁₃-mediated signaling to Rho over a Gβγ-dependent Rac pathway, attributed to heterotrimeric G_i proteins.

Arginine 313 of the putative 8th helix mediates Gαq/14 coupling of human CC chemokine receptors CCR2a and CCR2b

In man, two CC chemokine receptor isoforms, CCR2a and CCR2b, are present that belong to the rhodopsin-like G protein-coupled receptor family, and couple to G_i and G_q family members. The CCR2 receptors are known to regulate canonical functions of chemokines such as directed migration of leukocytes, and to potentially control non-canonical functions such as differentiation, proliferation, and gene transcription of immune and non-immune cells. We recently reported on the activation of phospholipase C isoenzymes and RhoA GTPases by coupling of the two CCR2 receptors to members of the G_q family, in particular Gα_q and Gα₁₄. So far little is known about the structural requirements for the CCR2/G_q/14 interaction. Interestingly, the CCR2 receptor isoforms are identical up to arginine 313 (R³¹³) that is part of the putative 8th helix in CCR2 receptors, and the 8th helix has been implicated in the interaction of rhodopsin-like G protein-coupled receptors with Gα_q. In the present work we describe that the 8th helix of both CCR2a and CCR2b is critically involved in selectively activating Gα_q/14-regulated signaling. Refined analysis using various CCR2a and CCR2b mutants and analyzing their cellular signaling, e.g. ligand-dependent (i) activation of phospholipase C isoenzymes, (ii) stimulation of serum response factor-mediated gene transcription, (iii) activation of mitogen-activated protein kinases, (iv) internalization, and (v) changes in intracellular calcium concentrations, identified arginine 313 within the amino terminal portion of helix 8 to play a role for the agonist-mediated conformational changes and the formation of a Gα_q/14 binding surface. We show that R³¹³ determines Gα_q/14 protein-dependent but not G_i protein-dependent cellular signaling, and plays no role in G_q/G_i-independent receptor internalization, indicating a role of R³¹³ in biased signaling of CCR2 receptors.

miR‑1273g‑3p promotes proliferation, migration and invasion of LoVo cells via cannabinoid receptor 1 through activation of ERBB4/PIK3R3/mTOR/S6K2 signaling pathway

MicroRNAs (miR) are important in various crucial cell processes including proliferation, migration and invasion. Dysregulation of miRNAs have been increasingly reported to contribute to colorectal cancer. However, the detailed biological function and potential mechanisms of miR‑1273g‑3p in colorectal cancer remain poorly understood. The expression levels of miR‑1273g‑3p in human colorectal cancer LoVo cell lines were detected via reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR). The target genes of miR‑1273g‑3p were predicted by bioinformatics and verified by a luciferase reporter assay, RT‑qPCR and western blotting. The MTT, wound‑healing and Transwell assays were used to examine the biological functions of miR‑1273g‑3p in LoVo cells. The potential molecular mechanisms of miR‑1273g‑3p on LoVo cell proliferation, migration and invasion was detected by western blotting. The results of the present study demonstrated that miR‑1273g‑3p expression was extensively upregulated in LoVo cells compared with the normal colon epithelial NCM460 cell line. Further studies indicated that miR‑1273g‑3p inhibitor significantly suppressed LoVo cell proliferation, migration and invasion compared with inhibitor control. Following this, the cannabinoid receptor 1 (CNR1) was identified as a direct target gene of miR‑1273g‑3p. Knockdown of CNR1 restored the phenotypes of LoVo cells transfected with miR‑1273g‑3p inhibitor. Furthermore, the potential molecular mechanism of miR‑1273g‑3p on LoVo cell proliferation, migration and invasion may be mediated by activating the Erb‑B2 receptor tyrosine kinase 4 (ERBB4)/phosphoinositide‑3‑kinase regulatory subunit 3 (PIK3R3)/mechanistic target of rapamycin (mTOR)/S6 kinase 2 (S6K2) signaling pathway. These observations indicated that miR‑1273g‑3p promoted the proliferation, migration and invasion of LoVo cells via CNR1, and this may have occurred through activation of the ERBB4/PIK3R3/mTOR/S6K2 signaling pathway, suggesting that miR‑1273g‑3p may serve as a novel therapeutic target for the effective treatment of colorectal cancer.

Regulation of Chemokine Signal Integration by Activator of G-Protein Signaling 4 (AGS4)

Activator of G-protein signaling 4 (AGS4)/G-protein signaling modulator 3 (Gpsm3) contains three G-protein regulatory (GPR) motifs, each of which can bind Gαi-GDP free of Gβγ We previously demonstrated that the AGS4-Gαi interaction is regulated by seven transmembrane-spanning receptors (7-TMR), which may reflect direct coupling of the GPR-Gαi module to the receptor analogous to canonical Gαβγ heterotrimer. We have demonstrated that the AGS4-Gαi complex is regulated by chemokine receptors in an agonist-dependent manner that is receptor-proximal. As an initial approach to investigate the functional role(s) of this regulated interaction in vivo, we analyzed leukocytes, in which AGS4/Gpsm3 is predominantly expressed, from AGS4/Gpsm3-null mice. Loss of AGS4/Gpsm3 resulted in mild but significant neutropenia and leukocytosis. Dendritic cells, T lymphocytes, and neutrophils from AGS4/Gpsm3-null mice also exhibited significant defects in chemoattractant-directed chemotaxis and extracellular signal-regulated kinase activation. An in vivo peritonitis model revealed a dramatic reduction in the ability of AGS4/Gpsm3-null neutrophils to migrate to primary sites of inflammation. Taken together, these data suggest that AGS4/Gpsm3 is required for proper chemokine signal processing in leukocytes and provide further evidence for the importance of the GPR-Gαi module in the regulation of leukocyte function.

Chemokine Receptor Signaling and the Hallmarks of Cancer

The chemokines are a family of chemotactic cytokines that mediate their activity by acting on seven-transmembrane-spanning G protein-coupled receptors. Both the ability of the chemokines and their receptors to form homo- and heterodimers and the promiscuity of the chemokine-chemokine receptor interaction endow this protein family with enormous signaling plasticity and complexity that are not fully understood at present. Chemokines were initially identified as essential regulators of homeostatic and inflammatory trafficking of innate and adaptive leucocytes from lymphoid organs to tissues. Chemokines also mediate the host response to cancer. Nevertheless, chemokine function in this response is not limited to regulating leucocyte infiltration into the tumor microenvironment. It is now known that chemokines and their receptors influence most-if not all-hallmark processes of cancer; they act on both neoplastic and untransformed cells in the tumor microenvironment, including fibroblasts, endothelial cells (blood and lymphatic), bone marrow-derived stem cells, and, obviously, infiltrating leucocytes. This review begins with an overview of chemokine and chemokine receptor structure, to better define how chemokines affect the proliferation, survival, stemness, and metastatic potential of neoplastic cells. We also examine the main mechanisms by which chemokines regulate tumor angiogenesis and immune cell infiltration, emphasizing the pro- and antitumorigenic activity of this protein superfamily in these interrelated processes.

Biased and g protein-independent signaling of chemokine receptors

Biased signaling or functional selectivity occurs when a 7TM-receptor preferentially activates one of several available pathways. It can be divided into three distinct forms: ligand bias, receptor bias, and tissue or cell bias, where it is mediated by different ligands (on the same receptor), different receptors (with the same ligand), or different tissues or cells (for the same ligand–receptor pair). Most often biased signaling is differentiated into G protein-dependent and β-arrestin-dependent signaling. Yet, it may also cover signaling differences within these groups. Moreover, it may not be absolute, i.e., full versus no activation. Here we discuss biased signaling in the chemokine system, including the structural basis for biased signaling in chemokine receptors, as well as in class A 7TM receptors in general. This includes overall helical movements and the contributions of micro-switches based on recently published 7TM crystals and molecular dynamics studies. All three forms of biased signaling are abundant in the chemokine system. This challenges our understanding of “classic” redundancy inevitably ascribed to this system, where multiple chemokines bind to the same receptor and where a single chemokine may bind to several receptors – in both cases with the same functional outcome. The ubiquitous biased signaling confers a hitherto unknown specificity to the chemokine system with a complex interaction pattern that is better described as promiscuous with context-defined roles and different functional outcomes in a ligand-, receptor-, or cell/tissue-defined manner. As the low number of successful drug development plans implies, there are great difficulties in targeting chemokine receptors; in particular with regard to receptor antagonists as anti-inflammatory drugs. Un-defined and putative non-selective targeting of the complete cellular signaling system could be the underlying cause of lack of success. Therefore, biased ligands could be the solution.

Subcellular optogenetic inhibition of G proteins generates signaling gradients and cell migration

Cells sense gradients of extracellular cues and generate polarized responses such as cell migration and neurite initiation. There is static information on the intracellular signaling molecules involved in these responses, but how they dynamically orchestrate polarized cell behaviors is not well understood. A limitation has been the lack of methods to exert spatial and temporal control over specific signaling molecules inside a living cell. Here we introduce optogenetic tools that act downstream of native G protein-coupled receptor (GPCRs) and provide direct control over the activity of endogenous heterotrimeric G protein subunits. Light-triggered recruitment of a truncated regulator of G protein signaling (RGS) protein or a Gβγ-sequestering domain to a selected region on the plasma membrane results in localized inhibition of G protein signaling. In immune cells exposed to spatially uniform chemoattractants, these optogenetic tools allow us to create reversible gradients of signaling activity. Migratory responses generated by this approach show that a gradient of active G protein αi and βγ subunits is sufficient to generate directed cell migration. They also provide the most direct evidence so for a global inhibition pathway triggered by Gi signaling in directional sensing and adaptation. These optogenetic tools can be applied to interrogate the mechanistic basis of other GPCR-modulated cellular functions.

Defective chemokine signal integration in leukocytes lacking activator of G protein signaling 3 (AGS3)

Activator of G-protein signaling 3 (AGS3, gene name G-protein signaling modulator-1, Gpsm1), an accessory protein for G-protein signaling, has functional roles in the kidney and CNS. Here we show that AGS3 is expressed in spleen, thymus, and bone marrow-derived dendritic cells, and is up-regulated upon leukocyte activation. We explored the role of AGS3 in immune cell function by characterizing chemokine receptor signaling in leukocytes from mice lacking AGS3. No obvious differences in lymphocyte subsets were observed. Interestingly, however, AGS3-null B and T lymphocytes and bone marrow-derived dendritic cells exhibited significant chemotactic defects as well as reductions in chemokine-stimulated calcium mobilization and altered ERK and Akt activation. These studies indicate a role for AGS3 in the regulation of G-protein signaling in the immune system, providing unexpected venues for the potential development of therapeutic agents that modulate immune function by targeting these regulatory mechanisms.

Clusterin stimulates the chemotactic migration of macrophages through a pertussis toxin sensitive G-protein-coupled receptor and Gβγ-dependent pathways

Clusterin induces the expression of various chemotactic cytokines including tumor necrosis factor-α (TNF-α) in macrophages and is involved in the cell migration. According to the results of this study, clusterin induced the directional migration (chemotaxis) of macrophages based on a checkerboard analysis. The chemotactic activity of clusterin was prevented by pretreatment with pertussis toxin (PTX), indicating that the Gαi/o-protein coupled receptor (GPCR) was involved in the chemotactic response of clusterin. Clusterin-stimulated chemotaxis was abrogated in a dose-dependent manner by pretreatment with gallein (a Gβγ inhibitor), indicating the involvement of Gβγ released from the GPCR. In addition, inhibitors of phospholipase C (PLC, U73122) and phosphoinositide 3-kinase (PI3K, LY294002), the key targets of Gβγ binding and activation, suppressed chemotactic migration by clusterin. The phosphorylation of Akt induced by clusterin was blocked by pretreatment with gallein or LY294002 but not with U73122, indicating that Gβγ released from the PTX-sensitive Gi protein complex activated PLC and PI3K/Akt signaling pathways separately. The activation of cellular MAP kinases was essential in that their inhibitors blocked clusterin-induced chemotaxis, and Gβγ was required for the activation of MAP kinases because gallein reduced their phosphorylations induced by clusterin. In addition, the inflammation-induced migration of macrophages was greatly reduced in clusterin-deficient mice based on a thioglycollate-induced peritonitis model system. These results suggest that clusterin stimulates the chemotactic migration of macrophages through a PTX-sensitive GPCR and Gβγ-dependent pathways and describe a novel role of clusterin as a chemoattractant of monocytes/macrophages, suggesting that clusterin may serve as a molecular bridge between inflammation and its remodeling of related tissue by recruiting immune cells.

PC3-secreted microprotein is a novel chemoattractant protein and functions as a high-affinity ligand for CC chemokine receptor 2

PC3-secreted microprotein (PSMP) or microseminoprotein is a newly discovered secreted protein whose function is currently unknown. In this study, PSMP was found to possess chemotactic ability toward monocytes and lymphocytes, and its functional receptor was identified as CCR2B. PSMP was identified as a chemoattractant protein from a PBMC chemoattractant platform screen that we established. The mature secreted PSMP was able to chemoattract human peripheral blood monocytes, PBLs, and CCR2B-expressing THP-1 cells, but not peripheral blood neutrophils, even though it does not contain the classical structure of chemokines. CCR2B was identified as one receptor for PSMP-mediated chemotaxis by screening HEK293 cells that transiently expressed classical chemokine receptors; results obtained from the chemotaxis, calcium flux, receptor internalization, and radioligand-binding assays all confirmed this finding. To further identify the major function of PSMP, we analyzed its expression profile in tissues. PSMP is highly expressed in benign prostatic hyperplasia and in some prostate cancers, and can also be detected in breast tumor tissue. In response to PSMP stimulation, phosphorylated ERK levels downstream of CCR2B signaling were upregulated in the PC3 cell line. Taken together, our data collectively suggest that PSMP is a chemoattractant protein acting as a novel CCR2 ligand that may influence inflammation and cancer development.

Emerging non-canonical functions for heterotrimeric G proteins in cellular signaling

Classically heterotrimeric G proteins have been described as the principal signal transducing machinery for G-protein-coupled receptors. Receptor activation catalyzes nucleotide exchange on the Gα protein, enabling Gα-GTP and Gβγ-subunits to engage intracellular effectors to generate various cellular effects such as second messenger production or regulation of ion channel conductivity. Recent genetic and proteomic screens have identified novel heterotrimeric G-protein-interacting proteins and expanded their functional roles. This review highlights some examples of recently identified interacting proteins and summarizes how they functionally connect heterotrimeric G proteins to previously underappreciated cellular roles.

The chemokine receptor CCR1 is constitutively active, which leads to G protein-independent, β-arrestin-mediated internalization

Activation of G protein-coupled receptors by their associated ligands has been extensively studied, and increasing structural information about the molecular mechanisms underlying ligand-dependent receptor activation is beginning to emerge with the recent expansion in GPCR crystal structures. However, some GPCRs are also able to adopt active conformations in the absence of agonist binding that result in the initiation of signal transduction and receptor down-modulation. In this report, we show that the CC-type chemokine receptor 1 (CCR1) exhibits significant constitutive activity leading to a variety of cellular responses. CCR1 expression is sufficient to induce inhibition of cAMP formation, increased F-actin content, and basal migration of human and murine leukocytes. The constitutive activity leads to basal phosphorylation of the receptor, recruitment of β-arrestin-2, and subsequent receptor internalization. CCR1 concurrently engages Gαi and β-arrestin-2 in a multiprotein complex, which may be accommodated by homo-oligomerization or receptor clustering. The data suggest the presence of two functional states for CCR1; whereas receptor coupled to Gαi functions as a canonical GPCR, albeit with high constitutive activity, the CCR1·β-arrestin-2 complex is required for G protein-independent constitutive receptor internalization. The pertussis toxin-insensitive uptake of chemokine by the receptor suggests that the CCR1·β-arrestin-2 complex may be related to a potential scavenging function of the receptor, which may be important for maintenance of chemokine gradients and receptor responsiveness in complex fields of chemokines during inflammation.

PLEKHG2 promotes heterotrimeric G protein βγ-stimulated lymphocyte migration via Rac and Cdc42 activation and actin polymerization

PLEKHG2 is a Dbl family Rho guanine nucleotide exchange factor (RhoGEF) whose gene was originally identified as being upregulated in a leukemia mouse model and was later shown to be activated by heterotrimeric G protein βγ (Gβγ) subunits. However, its function and activation mechanisms remain elusive. Here we show that, compared to its expression in primary human T cells, its expression is upregulated in several leukemia cell lines, including Jurkat T cells. Downregulation of PLEKHG2 in Jurkat T cells by small interfering RNAs (siRNAs) specifically inhibited Gβγ-stimulated Rac and Cdc42, but not RhoA, activation. Consequently, suppressing PLEKHG2 expression blocked actin polymerization and SDF1α-stimulated lymphocyte migration. Additional studies indicate that Gβγ likely activates PLEKHG2, in part by binding the N terminus of PLEKHG2 to release an autoinhibition imposed by its C terminus, which interacts with a region encompassing the catalytic Dbl homology (DH) domain. As a result, overexpressing either the N terminus or the C terminus of PLEKHG2 blocked Gβγ-stimulated Rac and Cdc42 activation and prevented Jurkat T cells from forming membrane protrusions and migrating. Together, our studies have provided the first evidence for the endogenous function of PLEKHG2, which may serve as a key Gβγ-stimulated RhoGEF that regulates lymphocyte chemotaxis via Rac and Cdc42 activation and actin polymerization.

CC chemokine receptors and chronic inflammation--therapeutic opportunities and pharmacological challenges

Chemokines are a family of low molecular weight proteins with an essential role in leukocyte trafficking during both homeostasis and inflammation. The CC class of chemokines consists of at least 28 members (CCL1-28) that signal through 10 known chemokine receptors (CCR1-10). CC chemokine receptors are expressed predominantly by T cells and monocyte-macrophages, cell types associated predominantly with chronic inflammation occurring over weeks or years. Chronic inflammatory diseases including rheumatoid arthritis, atherosclerosis, and metabolic syndrome are characterized by continued leukocyte infiltration into the inflammatory site, driven in large part by excessive chemokine production. Over years or decades, persistent inflammation may lead to loss of tissue architecture and function, causing severe disability or, in the case of atherosclerosis, fatal outcomes such as myocardial infarction or stroke. Despite the existence of several clinical strategies for targeting chronic inflammation, these diseases remain significant causes of morbidity and mortality globally, with a concomitant economic impact. Thus, the development of novel therapeutic agents for the treatment of chronic inflammatory disease continues to be a priority. In this review we introduce CC chemokine receptors as critical mediators of chronic inflammatory responses and explore their potential role as pharmacological targets. We discuss functions of individual CC chemokine receptors based on in vitro pharmacological data as well as transgenic animal studies. Focusing on three key forms of chronic inflammation--rheumatoid arthritis, atherosclerosis, and metabolic syndrome--we describe the pathologic function of CC chemokine receptors and their possible relevance as therapeutic targets.

CCR2 acts as scavenger for CCL2 during monocyte chemotaxis

BACKGROUND

Leukocyte migration is essential for effective host defense against invading pathogens and during immune homeostasis. A hallmark of the regulation of this process is the presentation of chemokines in gradients stimulating leukocyte chemotaxis via cognate chemokine receptors. For efficient migration, receptor responsiveness must be maintained whilst the cells crawl on cell surfaces or on matrices along the attracting gradient towards increasing concentrations of agonist. On the other hand agonist-induced desensitization and internalization is a general paradigm for chemokine receptors which is inconsistent with the prolonged migratory capacity.

METHODOLOGY/PRINCIPAL FINDINGS

Chemotaxis of monocytes was monitored in response to fluorescent CCL2-mCherry by time-lapse video microscopy. Uptake of the fluorescent agonist was used as indirect measure to follow the endogenous receptor CCR2 expressed on primary human monocytes. During chemotaxis CCL2-mCherry becomes endocytosed as cargo of CCR2, however, the internalization of CCR2 is not accompanied by reduced responsiveness of the cells due to desensitization.

CONCLUSIONS/SIGNIFICANCE

During chemotaxis CCR2 expressed on monocytes internalizes with the bound chemoattractant, but cycles rapidly back to the plasma membrane to maintain high responsiveness. Moreover, following relocation of the source of attractant, monocytes can rapidly reverse their polarization axis organizing a new leading edge along the newly formed gradient, suggesting a uniform distribution of highly receptive CCR2 on the plasma membrane. The present observations further indicate that during chemotaxis CCR2 acts as scavenger consuming the chemokine forming the attracting cue.

The WD40 repeat protein WDR26 binds Gβγ and promotes Gβγ-dependent signal transduction and leukocyte migration

The Gβγ subunits of heterotrimeric G proteins transmit signals to control many cellular processes, including leukocyte migration. Gβγ signaling may regulate and be regulated by numerous signaling partners. Here, we reveal that WDR26, a member of the WD40 repeat protein family, directly bound free Gβγ in vitro, and formed a complex with endogenous Gβγ in Jurkat T cells stimulated by the chemokine SDF1α. Suppression of WDR26 by siRNAs selectively inhibited Gβγ-dependent phospholipase Cβ and PI3K activation, and attenuated chemotaxis in Jurkat T cells and differentiated HL60 cells in vitro and Jurkat T cell homing to lymphoid tissues in scid mice. Similarly, disruption of the WDR26/Gβγ interaction via expression of a WDR26 deletion mutant impaired Gβγ signaling and Jurkat T cell migration, indicating that the function of WDR26 depends on its binding to Gβγ. Additional data show that WDR26 also controlled RACK1, a negative regulator, in binding Gβγ and inhibiting leukocyte migration. Collectively, these experiments identify WDR26 as a novel Gβγ-binding protein that is required for the efficacy of Gβγ signaling and leukocyte migration.

Image-based analysis of primary human neutrophil chemotaxis in an automated direct-viewing assay

Multi-well assays based on the Boyden chamber have enabled highly parallel studies of chemotaxis-the directional migration of cells in response to molecular gradients-while direct-viewing approaches have allowed more detailed questions to be asked at low throughput. Boyden-based plates provide a count of cells that pass through a membrane, but no information about cell appearance. In contrast, direct-viewing devices enable the observation of cells during chemotaxis, which allows measurement of many parameters including area, shape, and location. Here we show automated chemotaxis and cell morphology assays in a 96-unit direct-viewing plate. Using only 12000 primary human neutrophils per datum, we measured dose-dependent stimulation and inhibition of chemotaxis and quantified the effects of inhibitors on cell area and elongation. With 60 parallel conditions we demonstrated 5-fold increase in throughput compared to previously reported direct-viewing approaches.

Oxytocin induces the migration of prostate cancer cells: involvement of the Gi-coupled signaling pathway

Expression of genes that encode oxytocin (OXT) and vasopressin (AVP) and their cognate receptors in normal and diseased prostates are only partially characterized. Reverse transcription and PCR were used to examine the expression of these genes in normal prostate epithelial and stromal cell lines, k-ras-transformed prostate epithelial cell lines, and in four prostate cancer cell lines. Secreted and cell-associated OXT peptide was measured by an enzyme immunoassay. OXT and its receptor (OXTR) were expressed in all eight prostate cell lines. Cell-associated OXT peptide was also found in all prostate epithelial cell lines except in DU145 cells. Neither AVP nor its cognate receptors (V1a receptor and V2 receptor) were expressed in any prostate cell line examined. These data point to the OXTR as the primary target of OXT and AVP, and suggest that OXT might be an autocrine/paracrine regulator in human prostate. We found that OXT induces the migration of PC3 and PC3M, but not DU145 prostate cancer cells. The effect of OXT is distinct from the epidermal growth factor (EGF)-induced migration of prostate cancer cells, in which ERK1/2 and EGF receptor kinase activities were required. When cells were pretreated with pertussis toxin, the effect of OXT, but not EGF, on cell migration was abolished. Pretreatment with the cyclic AMP analogue, 8-Br-cAMP, did not affect OXT-induced cell migration, which eliminated the nonspecific effect of pertussis toxin. We conclude that a Gi-dependent mechanism is involved in OXTR-mediated migration of prostate cancer cells, and indicates a role for OXTR in prostate cancer metastasis.

Variations in Gnai2 and Rgs1 expression affect chemokine receptor signaling and the organization of secondary lymphoid organs

Ligand bound chemoattractant receptors activate the heterotrimeric G protein Gi to stimulate downstream signaling pathways to properly position lymphocytes in lymphoid organs. Here we show how variations the expression of a chemokine receptor and in two components in the signaling pathway, Gα_i2 and RGS1, affects the output fidelity of the signaling pathway. Examination of B cells from mice with varying numbers of intact alleles of Ccr7, Rgs1, Gnai2, and Gnai3 provided the basis for these results. Loss of a single allele of either Gnai2 or Rgs1 affected CCL19 triggered chemotaxis, while loss of a single allele of Ccr7, which encodes the cognate CCL19 receptor, had little effect. Emphasizing the importance of Gnai2, B cells lacking Gnai3 expression responded to chemokines better than did wild type B cells. At an organismal level, variations in Rgs1 and Gnai2 expression affected marginal zone B cell development, splenic architecture, lymphoid follicle size, and germinal center morphology. Gnai2 expression was also needed for the proper alignment of MOMA-1⁺ macrophages and MAdCAM-1⁺ endothelial cells along marginal zone sinuses in the spleen. These data indicate that chemoattractant receptors, heterotrimeric G-proteins, and RGS protein expression levels have a complex inter-relationship that affects the responses to chemoattractant exposure.

The obesity and inflammatory marker haptoglobin attracts monocytes via interaction with chemokine (C-C motif) receptor 2 (CCR2)

Background

Obesity is a chronic low inflammatory state. In the obesity condition the white adipose tissue (WAT) is massively infiltrated with monocytes/macrophages, and the nature of the signals recruiting these inflammatory cells has yet to be fully elucidated. Haptoglobin (Hp) is an inflammatory marker and its expression is induced in the WAT of obese subjects. In an effort to elucidate the biological significance of Hp presence in the WAT and of its upregulation in obesity we formulated the hypothesis that Hp may serve as a macrophage chemoattractant.

Results

We demonstrated by chemotaxis assay that Hp is able to attract chemokine (C-C motif) receptor 2 (CCR2)-transfected pre-B lymphocytes and monocytes in a dose-dependent manner. Moreover, Hp-mediated migration of monocytes is impaired by CCR2-specific inhibition or previous cell exposure to monocyte chemoattractant protein 1 (MCP1) (also known as CCR2 ligand or chemokine (C-C motif) ligand 2 (CCL2)). Downstream effects of Hp/CCR2 interaction were also investigated: flow cytometry proved that monocytes treated with Hp show reduced CCR2 expression on their surface; Hp interaction induces calcium release that is reduced upon pretreatment with CCR2 antagonist; extracellular signal-regulated kinase (ERK)1/2, a signal transducer activated by CCR2, is phosphorylated following Hp treatment and this phosphorylation is reduced when cells are pretreated with a specific CCR2 inhibitor. Consistently, blocking the ERK1/2 pathway with U0126, the selective inhibitor of the ERK upstream mitogen-activated protein (MAP)-ERK kinase (MEK), results in a dramatic reduction (by almost 100%) of the capability of Hp to induce monocyte migration.

Conclusions

Our data show that Hp is a novel monocyte chemoattractant and that its chemotactic potential is mediated, at least in part. by its interaction with CCR2.

Chemokine receptor dimerization and chemotaxis

A broad array of biological responses ranging from cell polarization, movement, immune and inflammatory responses, as well as prevention of HIV-1 infection, are triggered by the chemokines, a family of structurally related chemoattractant proteins that bind to specific seven-transmembrane receptors linked to G proteins. Although it was initially believed that chemokine receptors act as monomeric entities, it has now been shown that they function as oligomers. Chemokine receptor homo- and heterodimers are found on the cell membrane; binding to their ligands stabilizes specific receptor conformations and activates distinct signaling cascades. Thorough analysis of the conformations adopted by the receptors at the membrane is therefore a prerequisite for understanding the function of these inflammatory mediators. For study of the chemokine receptor conformations at the cell surface, we focus here on conventional biochemical and genetic methods, as well as on new imaging techniques such as those based on resonance energy transfer; we also evaluate in vitro and in vivo methods to determine certain chemokine receptor functions.

Chemoattract receptor signaling and its role in lymphocyte motility and trafficking

Intravital microscopy has provided extraordinary glimpses of lymphocytes crossing high endothelial venules, detailed the movements and interactions of lymphocytes within lymph organs, and recorded lymphocytes crossing the lymphatic endothelium into the efferent lymph. Helping to orchestrate these movements are signals generated by the engagement of chemoattractants with their cognate receptors. Chemokines present on high endothelial venules and within lymph organs, and the high levels of sphingosine l-phosphate in the lymph, provide signposts to help guide lymphocytes and provide intracellular signals that affect lymphocyte polarity and motility. Within lymph nodes, T and B lymphocytes migrate along networks of fibroblastic reticular cells and follicular dendritic, respectively, which provide an adhesive platform and solid phased chemokines. Illustrating the importance of chemoattractant receptors in these processes, lymphocytes that lack CXCR4, CXCR5, CCR7 or S1PR1, or which lack crucial signaling molecules activated by these receptors, exhibit defects in lymph node entrance, positioning, polarity, motility, and/or lymph node egress. This review will focus on the contributions of in vivo imaging of lymphocytes from various mouse mutants to our understanding of the roles chemoattractants play in lymphocyte entrance into and exit from lymph nodes, and in coordinating and facilitating the movements of lymphocytes within lymph nodes.

Imaging techniques: new insights into chemokine/chemokine receptor biology at the immune system

Our current knowledge of molecular and cellular responses in vivo is based mainly on event reconstruction from time-freeze observations. Conventional biochemical and genetic methods consider the cell as an individual entity and ligand/receptor pairs as isolated systems. In addition, the data refer to the average behavior of a pool of cells and/or receptors removed from their real-life context. The use of new technologies, particularly real-time imaging approaches, is showing us that biological responses are highly dynamic and extremely dependent on the context in which they take place, and therefore much more diverse than initially envisaged. This review focuses on the mechanistic insights that new imaging techniques, such as those based on resonance energy transfer and two-photon microscopy, contribute to our understanding of how receptors work within a single cell, and how cells work within a tissue. Cell movement is a complex and regulated process; it has a key role in embryogenesis, organogenesis, wound-healing and tumor invasion. Nonetheless, it is in immune system homeostasis and response that cell movement becomes essential. For this reason, immunology is being radically transformed and enriched by these new approaches. We will discuss the use of these techniques for studying chemokine/chemokine receptors and their role in the immune system function, and comment on the potential contribution to the design of therapeutic strategies.

Phospholipase cbeta is critical for T cell chemotaxis

Chemokines acting through G protein-coupled receptors play an essential role in the immune response. PI3K and phospholipase C (PLC) are distinct signaling molecules that have been proposed in the regulation of chemokine-mediated cell migration. Studies with knockout mice have demonstrated a critical role for PI3K in G(alphai) protein-coupled receptor-mediated neutrophil and lymphocyte chemotaxis. Although PLCbeta is not essential for the chemotactic response of neutrophils, its role in lymphocyte migration has not been clearly defined. We compared the chemotactic response of peripheral T cells derived from wild-type mice with mice containing loss-of-function mutations in both of the two predominant lymphocyte PLCbeta isoforms (PLCbeta2 and PLCbeta3), and demonstrate that loss of PLCbeta2 and PLCbeta3 significantly impaired T cell migration. Because second messengers generated by PLCbeta lead to a rise in intracellular calcium and activation of PKC, we analyzed which of these responses was critical for the PLCbeta-mediated chemotaxis. Intracellular calcium chelation decreased the chemotactic response of wild-type lymphocytes, but pharmacologic inhibition of several PKC isoforms had no effect. Furthermore, calcium efflux induced by stromal cell-derived factor-1alpha was undetectable in PLCbeta2beta3-null lymphocytes, suggesting that the migration defect is due to the impaired ability to increase intracellular calcium. This study demonstrates that, in contrast to neutrophils, phospholipid second messengers generated by PLCbeta play a critical role in T lymphocyte chemotaxis.

Impaired trafficking of Gnai2+/- and Gnai2-/- T lymphocytes: implications for T cell movement within lymph nodes

Signals generated by the engagement of chemoattractants with their cognate receptors orchestrate lymphocyte movements into and out of lymphoid organs and sites of inflammation. Yet, the role of chemokines in organizing lymphocyte movements in lymphoid organs is controversial. Recent evidence suggests that the extensive network of fibroblastic reticular cells within the T cell areas helps guide T cells. The expression of adhesion molecules and chemokines by fibroblastic reticular cells most likely facilitates their influence on T cell movements. Consistent with this hypothesis, CD4 T cells with defective chemokine receptor signaling move very differently within lymph nodes than do normal cells. For the imaging studies, we used CD4 T cells prepared from Gnai2(-/-) mice, which lack G(alphai2) expression. We first demonstrate that CD4 as well as CD8 T cells from these mice are markedly defective in chemokine receptor signaling. Gnai2(-/-) T cells have profound defects in chemokine-induced intracellular calcium mobilization, chemotaxis, and homing, whereas Gnai2(+/-) T cells exhibit modest defects. Intravital imaging revealed that within the inguinal lymph nodes Gnai2(-/-) CD4 T accumulate at the cortical ridge, poorly accessing the lymph node paracortex. They also lack the customary amoeboid-like cell movements and active membrane projections observed with normal CD4 T cells. These results demonstrate the importance of G(alphai2) for T lymphocyte chemokine receptor signaling and argue that local chemoattractants regulate the movement of CD4 T cells in lymph nodes.

The adaptor protein Lad associates with the G protein beta subunit and mediates chemokine-dependent T-cell migration

Lck-interacting adaptor protein/Rlk/Itk-binding protein (Lad/RIBP) was previously identified as an adaptor protein involved in TCR-mediated T-cell activation. To elucidate the functions of Lad further, we here performed yeast 2-hybrid screening using Lad as bait and discovered that the G protein beta subunit (G beta) is a Lad-binding partner. Since the most well-known G protein-coupled receptor in T cells is the chemokine receptor, we investigated whether Lad is involved in chemokine signaling. We found that, upon chemokine treatment, Lad associated with G beta in Jurkat T cells. Furthermore, ectopic expression of dominant-negative Lad or the reduction of endogenous Lad expression by siRNA impaired the chemokine-induced migration of T cells, indicating that Lad is required for chemokine-induced T-cell migration. Subsequent investigation of the signaling pathways revealed that, in response to chemokine, Lad associated with the tyrosine kinases Lck and Zap-70 and that Lad was essential for the activation of Zap-70. Moreover, Lad was required for the chemokine-dependent tyrosine phosphorylation of focal adhesion molecules that included Pyk2 and paxillin. Taken together, these data show that, upon chemokine stimulation, Lad acts as an adaptor protein that links the G protein beta subunit to the tyrosine kinases Lck and Zap-70, thereby mediating T-cell migration.

Identification of the Intracellular Region of the Leukotriene B4 Receptor Type 1 That Is Specifically Involved in Gi Activation

Many G-protein-coupled receptors can activate more than one G-protein subfamily member. Leukotriene B4 receptor type 1 (BLT1) is a high affinity G-protein-coupled receptors for leukotriene B4 functioning in host defense, inflammation, and immunity. Previous studies have shown that BLT1 utilizes different G-proteins (the Gi family and G16 G-proteins) in mediating diverse cellular events and that truncation of the cytoplasmic tail of BLT1 does not impair activation of Gi and G16 proteins. To determine responsive regions of BLT1 for G-protein coupling, we performed an extensive mutagenesis study of its intracellular loops. Three intracellular loops (i1, i2, and i3) of BLT1 were found to be important for both Gi and G16 coupling, as judged by Gi-dependent guanosine 5'-(gamma-thio) triphosphate (GTPgammaS) binding and G16-dependent inositol phosphate accumulation assays. The i3-1 mutant, with a mutation at the i3 amino terminus, exhibited greatly reduced GTPgammaS binding but intact inositol phosphate accumulation triggered by leukotriene B4 stimulation. These results suggest that the i3-1 region is required only for Gi activation. Moreover, in the i3-1 mutant, the deficiency in Gi activation was accompanied by a loss of the high affinity leukotriene B4 binding state seen with the wild type receptor. A three-dimensional model of BLT1 constructed based on the structure of bovine rhodopsin suggests that the i3-1 region may consist of the cytoplasmic end of the transmembrane helix V, which protrudes the helix into the cytoplasm. From mutational studies and three-dimensional modeling, we propose that the extended cytoplasmic helix connected to the transmembrane helix V of BLT1 might be a key region for selective activation of Gi proteins.

Chemoattractant receptor signaling and the control of lymphocyte migration

This review focuses on mechanisms by which chemoattractant receptors activate downstream signaling pathways in lymphocytes. An emphasis is placed on heterotrimeric G protein signaling with a discussion of the specific heterotrimeric G-proteins involved in lymphocyte chemotaxis and motility and the role of regulator of G protein signaling (RGS) proteins in controlling the activation of downstream effectors. Also considered are those direct downstream effectors known to function in lymphocyte chemotaxis and/or motility. The consequences of targeting genes suspected, known, or serendipitously found to be involved in chemokine receptor signaling pathways form much of a basis for the review. When needed for clarification, reference to studies of chemoattractant signaling in model organisms and in neutrophils will be compared and contrasted to studies in lymphocytes. Finally, the emergence of tools to image lymphocyte in vitro and in vivo will be mentioned as they are increasing helpful for the analysis of lymphocyte trafficking and amendable to the study of chemokine receptor signaling.

Silent chemoattractant receptors: D6 as a decoy and scavenger receptor for inflammatory CC chemokines

The chemokine system includes at least three "silent" receptors, DARC, D6 and CCX CKR, with distinct specificity and tissue distribution. D6 binds most inflammatory, but not homeostatic, CC chemokines and shuttles in a ligand-independent way from the plasma membrane to endocytic compartments where chemokines are targeted to degradation. In vitro and in vivo evidence, including results with gene-targeted mice, is consistent with the view that D6 acts as a decoy and scavenger for inflammatory CC chemokines. Thus, D6 has unique functional and structural features, which make it ideally adapted to act as a chemokine decoy and scavenger receptor, strategically located on lymphatic endothelium to dampen inflammation in tissues and draining lymph nodes.

CCR3 expression and function in asthmatic airway smooth muscle cells

Asthma is characterized by an increase in airway smooth muscle mass and a decreased distance between the smooth muscle layer and the epithelium. Furthermore, there is evidence to indicate that airway smooth muscle cells (ASMC) express a wide variety of receptors involved in the immune response. The aims of this study were to examine the expression of CCR3 on ASMC, to compare this expression between asthmatic and nonasthmatic subjects, and to determine the implications of CCR3 expression in the migration of ASMC. We first demonstrated that ASMC constitutively express CCR3 at both mRNA and protein levels. Interestingly, TNF-alpha increases ASMC surface expression of CCR3 from 33 to 74%. Furthermore, using FACS analysis, we found that ASMC CCR3 is expressed to a greater degree in asthmatic vs control subjects (95 vs 75%). Functionality of the receptor was demonstrated by calcium assay; the addition of CCR3 ligand eotaxin to ASMC resulted in an increase in intracellular calcium production. Interestingly, ASMC was seen to demonstrate a positive chemotactic response to eotaxin. Indeed, ASMC significantly migrated toward 100 ng/ml eotaxin (2.2-fold increase, compared with control). In conclusion, the expression of CCR3 by ASMC is increased in asthmatics, and our data show that a CCR3 ligand such as eotaxin induces migration of ASMC in vitro. These results may suggest that eotaxin could be involved in the increased smooth muscle mass observed in asthmatics through the activation of CCR3.

Biochemistry and biomechanics of cell motility

Cell motility is an essential cellular process for a variety of biological events. The process of cell migration requires the integration and coordination of complex biochemical and biomechanical signals. The protrusion force at the leading edge of a cell is generated by the cytoskeleton, and this force generation is controlled by multiple signaling cascades. The formation of new adhesions at the front and the release of adhesions at the rear involve the outside-in and inside-out signaling mediated by integrins and other adhesion receptors. The traction force generated by the cell on the extracellular matrix (ECM) regulates cell-ECM adhesions, and the counter force exerted by ECM on the cell drives the migration. The polarity of cell migration can be amplified and maintained by the feedback loop between the cytoskeleton and cell-ECM adhesions. Cell migration in three-dimensional ECM has characteristics distinct from that on two-dimensional ECM. The migration of cells is initiated and modulated by external chemical and mechanical factors, such as chemoattractants and the mechanical forces acting on the cells and ECM, as well as the surface density, distribution, topography, and rigidity of the ECM.

Proline 326 in the C terminus of murine CX3CR1 prevents G-protein and phosphatidylinositol 3-kinase-dependent stimulation of Akt and extracellular signal-regulated kinase in Chinese hamster ovary cells

Naturally occurring single nucleotide polymorphisms have been identified in human CX3CR1, the chemokine receptor for fractalkine (FKN/CX3CL1). Individuals carrying the I249/M280 variant of CX3CR1 have a lower risk of cardiovascular disease compared with those homozygous for the common variant (V249/T280). The precise molecular basis for this phenotype is unclear, although differences in FKN binding, adhesive properties, and signaling efficiency between the CX3CR1 variants have been reported. FKN binding to CX3CR1 leads to an increase in intracellular calcium, actin rearrangement, and activation of the mitogen-activated protein kinase and phosphoinositide 3-kinase (PI3K) pathways. Regulation of these signaling pathways underlies the known roles for FKN in cell survival, proliferation, and migration. In the present study, we demonstrate that FKN stimulates phosphorylation of protein kinase B (Akt/PKB) in Chinese hamster ovary cells individually expressing the naturally occurring variants of human CX3CR1-, as well as rat CX3CR1-, but not in murine CX3CR1-expressing cells. Substitution of Pro326 in the C terminus of murine CX3CR1 with Ser (residue found in the analogous position of human CX3CR1) produced a mutant receptor that mimicked the human receptor in its ability to stimulate the phosphorylation of both Akt and extracellular signal-regulated kinase in a time-, PI3K-, and pertussis toxin-sensitive G-protein-dependent manner. These results identify a critical structural determinant of CX3CR1 important for activation of downstream signaling pathways.

CCL17 and CCL22 attenuate CCL5-induced mast cell migration

BACKGROUND

Mast cells (MCs) accumulate at sites of allergic mucosal inflammation where they act as central effectors and regulatory cells. Chemokines are believed to be crucial for the recruitment of MCs to sites of inflammation. We recently reported that human umbilical cord blood MCs (CBMCs) expresses the CC chemokine receptors, CCR1 and CCR4. We found a unique response profile to ligands of the respective receptors in which, of all tested ligands, only CCL5/RANTES-induced migration.

OBJECTIVE

To further investigate the function of CCR4 in MCs.

METHODS

CBMCs were used for competition binding experiments, migration, and intracellular calcium mobilization and release response studies.

RESULTS

The natural ligands for CCR4, CCL17/TARC and CCL22/MDC could both compete for binding with radiolabelled CCL5. Further, both CCL17 and CCL22 act as CCR4 antagonists by inhibiting CCL5-induced migration. Although both CCL17 and CCL22 caused mobilization of intracellular calcium, none of them induced migration or histamine release.

CONCLUSIONS

These results suggest that CCL5-induced migration of MCs via CCR4 can be regulated by the natural agonists CCL17 and CCL22, which are up-regulated at sites of allergic inflammation.

Rgs1 and Gnai2 regulate the entrance of B lymphocytes into lymph nodes and B cell motility within lymph node follicles

Signaling by G protein-coupled receptors coupled to Galpha(i) assists in triggering lymphocyte movement into and out of lymph nodes. Here, we show that modulating the signaling output from these receptors dramatically alters B cell trafficking. Intravital microscopy of adoptively transferred B cells from wild-type and Rgs1-/- mice revealed that Rgs1-/- B cells stick better to lymph node high endothelial venules, home better to lymph nodes, and move more rapidly within lymph node follicles than do wild-type B cells. In contrast, B cells from Gnai2-/- mice enter lymph nodes poorly and move more slowly than do wild-type B cells. The Gnai2-/- mice often lack multiple peripheral lymph nodes, and their B cells respond poorly to chemokines, indicating that Galpha(i1) and Galpha(i3) poorly compensate for the loss of Galpha(i2). These results demonstrate opposing roles for Rgs1 and Gnai2 in B cell trafficking into and within lymph nodes.

Virus-encoded chemokine receptors--putative novel antiviral drug targets

Large DNA viruses, in particular herpes- and poxviruses, have evolved proteins that serve as mimics or decoys for endogenous proteins in the host. The chemokines and their receptors serve key functions in both innate and adaptive immunity through control of leukocyte trafficking, and have as such a paramount role in the antiviral immune responses. It is therefore not surprising that viruses have found ways to exploit and subvert the chemokine system by means of molecular mimicry. By ancient acts of molecular piracy and by induction and suppression of endogenous genes, viruses have utilized chemokines and their receptors to serve a variety of roles in viral life-cycle. This review focuses on the pharmacology of virus-encoded chemokine receptors, yet also the family of virus-encoded chemokines and chemokine-binding proteins will be touched upon. Key properties of the virus-encoded receptors, compared to their closest endogenous homologs, are interactions with a wider range of chemokines, which can act as agonists, antagonists and inverse agonists, and the exploitation of many signal transduction pathways. High constitutive activity is another key property of some--but not all--of these receptors. The chemokine receptors belong to the superfamily of G-protein coupled 7TM receptors that per se are excellent drug targets. At present, non-peptide antagonists have been developed against many chemokine receptors. The potentials of the virus-encoded chemokine receptors as drug targets--ie. as novel antiviral strategies--will be highlighted here together with the potentials of the virus-encoded chemokines and chemokine-binding proteins as novel anti-inflammatory biopharmaceutical strategies.

Neutrophil microtubules suppress polarity and enhance directional migration

How do microtubules, which maintain and direct polarity of many eukaryotic cells, regulate polarity of blood neutrophils? In sharp contrast to most cells, disrupting a neutrophil's microtubule network with nocodazole causes it to polarize and migrate [Niggli, V. (2003) J. Cell Sci. 116, 813-822]. Nocodazole induces the same responses in differentiated HL-60 cells, a model neutrophil cell line, and reduces their chemotactic prowess by causing them to pursue abnormally circuitous paths in migrating toward a stationary point source of an attractant, f-Met-Leu-Phe (fMLP). The chemotactic defect stems from dramatic nocodazole-induced imbalance between the divergent, opposed fMLP-induced "backness" and "frontness" signals responsible for neutrophil polarity. Nocodazole (i) stimulates backness by increasing Rho- and actomyosin-dependent contractility, as reported by Niggli, and also (ii) impairs fMLP-dependent frontness: pseudopods are flatter, contain less F-actin, and show decreased membrane translocation of PH-Akt-GFP, a fluorescent marker for 3'-phosphoinositide lipids. Inhibiting backness with a pharmacologic inhibitor of a Rho-dependent kinase substantially reverses nocodazole's effects on chemotaxis, straightness of migration paths, morphology, and PH-Akt-GFP translocation. Thus, microtubules normally balance backness vs. frontness signals, preventing backness from reducing the strength of pseudopods and from impairing directional migration.

Cyclic AMP mediates keratinocyte directional migration in an electric field

Re-epithelialization of wounded skin is necessary for wound closure and restoration of barrier function and requires directional keratinocyte migration towards the center of the wound. The electric field (EF) generated immediately upon wounding could be the earliest signal keratinocytes receive to initiate directional migration and healing. Keratinocytes express many beta2-adrenergic receptors (beta2-ARs), but their role in the epidermis is unknown. We have previously shown that beta-AR agonists decrease keratinocyte migration in a cyclic AMP (cAMP) independent mechanism involving the activation of protein phosphatase 2A (PP2A). Here, we ask whether beta2-ARs play a role in keratinocyte galvanotaxis. We report a bimodal response. When keratinocytes were exposed to higher concentrations of beta-AR agonist (0.1 microM), their tracked migratory speed was inhibited, in both the presence (directional migration) and the absence (random migration) of a 100 mV mm(-1) EF, as expected. At lower agonist concentrations (0.1 pM to 0.1 nM), there was no effect on migratory speed; however, all directionality was lost - essentially, cells were 'blinded' to the directional cue. Preincubating the cells with beta-antagonist restored directional migration, demonstrating that the 'blindness' was beta2-AR mediated. Incubation of keratinocytes with agents known to increase intracellular cAMP levels, such as sp-cAMP, pertussis toxin and forskolin, resulted in similar 'blinding' to the EF, whereas random migration was unaffected. The inactive cAMP analog rp-cAMP had no effect on keratinocyte migration, whether directional or random. However, rp-cAMP pretreatment before beta-agonist addition fully restored galvanotaxis, demonstrating the complete cAMP dependence of the attenuation of keratinocyte directional migration. This is the first report that cAMP is capable of mediating keratinocyte galvanotaxis. beta-AR agonists and antagonists could be valuable tools for modulating re-epithelialization, an essential step in the wound-healing process. Thus, beta-ARs regulate the two distinct components of keratinocyte directional migration differently: migration speed via a cAMP-independent mechanism and galvanotaxis by a cAMP-dependent one.