A role for ARF6 in dendritic cell podosome formation and migration

ARF6 promotes Streptococcus suis suilysin induced apoptosis in HBMECs

Streptococcus suis (S. suis) is a significant zoonotic microorganism that causes a severe illness in both pigs and humans and is characterized by severe meningitis and septicemia. Suilysin (SLY), which is secreted by S. suis, plays a crucial role as a virulence factor in the disease. To date, the interaction between SLY and host cells is not fully understood. In this study, we identified the interacting proteins between SLY and human brain microvascular endothelial cells (HBMECs) using the TurboID-mediated proximity labeling method. 251 unique proteins were identified in TurboID-SLY treated group, of which six plasma membrane proteins including ARF6, GRK6, EPB41L5, DSC1, TJP2, and PNN were identified. We found that the proteins capable of interacting with SLY are ARF6 and PNN. Subsequent investigations revealed that ARF6 substantially increased the invasive ability of S. suis in HBMECs. Furthermore, ARF6 promoted SLY-induced the activation of p38 MAPK signaling pathway in HBMECs. Moreover, ARF6 promoted the apoptosis in HBMECs through the activation of p38 MAPK signaling pathway induced by SLY. Finally, we confirmed that ARF6 could increase the virulence of SLY in C57BL/6 mice. These findings offer valuable insights that contribute to a deeper understanding of the pathogenic mechanism of SLY.

Arf6 exacerbates allergic asthma through cell-to-cell transmission of ASC inflammasomes

Asthma is a chronic inflammatory disease of the airways associated with excess production of Th2 cytokines and lung eosinophil accumulation. This inflammatory response persists in spite of steroid administration that blocks autocrine/paracrine loops of inflammatory cytokines, and the detailed mechanisms underlying asthma exacerbation remain unclear. Here, we show that asthma exacerbation is triggered by airway macrophages through a prion-like cell-to-cell transmission of extracellular particulates, including ASC protein, that assemble inflammasomes and mediate IL-1β production. OVA-induced allergic asthma and associated IL-1β production were alleviated in mice with small GTPase Arf6-deficient macrophages. The extracellular ASC specks were slightly engulfed by Arf6^–/– macrophages, and the IL-1β production was reduced in Arf6^–/– macrophages compared with that in WT macrophages. Furthermore, pharmacological inhibition of the Arf6 guanine nucleotide exchange factor suppressed asthma-like allergic inflammation in OVA-challenged WT mice. Collectively, the Arf6-dependent intercellular transmission of extracellular ASC specks contributes to the amplification of allergic inflammation and subsequent asthma exacerbation.

Chlortetracycline, a Novel Arf Inhibitor That Decreases the Arf6-Dependent Invasive Properties of Breast Cancer Cells

Breast cancer is a major disease for women worldwide, where mortality is associated with tumour cell dissemination to distant organs. While the number of efficient anticancer therapies increased in the past 20 years, treatments targeting the invasive properties of metastatic tumour cells are still awaited. Various studies analysing invasive breast cancer cell lines have demonstrated that Arf6 is an important player of the migratory and invasive processes. These observations make Arf6 and its regulators potential therapeutic targets. As of today, no drug effective against Arf6 has been identified, with one explanation being that the activation of Arf6 is dependent on the presence of lipid membranes that are rarely included in drug screening. To overcome this issue we have set up a fluorescence-based high throughput screening that follows overtime the activation of Arf6 at the surface of lipid membranes. Using this unique screening assay, we isolated several compounds that affect Arf6 activation, among which the antibiotic chlortetracycline (CTC) appeared to be the most promising. In this report, we describe CTC in vitro biochemical characterization and show that it blocks both the Arf6-stimulated collective migration and cell invasion in a 3D collagen I gel of the invasive breast cancer cell line MDA-MB-231. Thus, CTC appears as a promising hit to target deadly metastatic dissemination and a powerful tool to unravel the molecular mechanisms of Arf6-mediated invasive processes.

The Small GTPase Arf6: An Overview of Its Mechanisms of Action and of Its Role in Host⁻Pathogen Interactions and Innate Immunity

The small GTase Arf6 has several important functions in intracellular vesicular trafficking and regulates the recycling of different types of cargo internalized via clathrin-dependent or -independent endocytosis. It activates the lipid modifying enzymes PIP 5-kinase and phospholipase D, promotes actin polymerization, and affects several functionally distinct processes in the cell. Arf6 is used for the phagocytosis of pathogens and can be directly or indirectly targeted by various pathogens to block phagocytosis or induce the uptake of intracellular pathogens. Arf6 is also used in the signaling of Toll-like receptors and in the activation of NADPH oxidases. In this review, we first give an overview of the different roles and mechanisms of action of Arf6 and then focus on its role in innate immunity and host–pathogen interactions.

PLD-dependent phosphatidic acid microdomains are signaling platforms for podosome formation

Local membrane phospholipid enrichment serves as docking platform for signaling proteins involved in many processes including cell adhesion and migration. Tissue-resident dendritic cells (DCs) assemble actomyosin-based structures called podosomes, which mediate adhesion and degradation of extracellular matrix for migration and antigen sampling. Recent evidence suggested the involvement of phospholipase D (PLD) and its product phosphatidic acid (PA) in podosome formation, but the spatiotemporal control of this process is poorly characterized. Here we determined the role of PLD1 and PLD2 isoforms in regulating podosome formation and dynamics in human primary DCs by combining PLD pharmacological inhibition with a fluorescent PA sensor and fluorescence microscopy. We found that ongoing PLD2 activity is required for the maintenance of podosomes, whereas both PLD1 and PLD2 control the early stages of podosome assembly. Furthermore, we captured the formation of PA microdomains accumulating at the membrane cytoplasmic leaflet of living DCs, in dynamic coordination with nascent podosome actin cores. Finally, we show that both PLD1 and PLD2 activity are important for podosome-mediated matrix degradation. Our results provide novel insight into the isoform-specific spatiotemporal regulation of PLD activity and further our understanding of the role of cell membrane phospholipids in controlling localized actin polymerization and cell protrusion.

A key role for PTP1B in dendritic cell maturation, migration, and T cell activation

Dendritic cells (DC) are the major antigen-presenting cells bridging innate and adaptive immunity, a function they perform by converting quiescent DC to active, mature DC with the capacity to activate naïve T cells. They do this by migrating from the tissues to the T cell area of the secondary lymphoid tissues. Here, we demonstrate that myeloid cell-specific genetic deletion of PTP1B (LysM PTP1B) leads to defects in lipopolysaccharide-driven bone marrow-derived DC (BMDC) activation associated with increased levels of phosphorylated Stat3. We show that myeloid cell-specific PTP1B deletion also causes decreased migratory capacity of epidermal DC, as well as reduced CCR7 expression and chemotaxis to CCL19 by BMDC. PTP1B deficiency in BMDC also impairs their migration in vivo. Further, immature LysM PTP1B BMDC display fewer podosomes, increased levels of phosphorylated Src at tyrosine 527, and loss of Src localization to podosome puncta. In co-culture with T cells, LysM PTP1B BMDC establish fewer and shorter contacts than control BMDC. Finally, LysM PTP1B BMDC fail to present antigen to T cells as efficiently as control BMDC. These data provide first evidence for a key regulatory role for PTP1B in mediating a central DC function of initiating adaptive immune responses in response to innate immune cell activation.

A RAB5/RAB4 recycling circuitry induces a proteolytic invasive program and promotes tumor dissemination

The mechanisms by which tumor cells metastasize and the role of endocytic proteins in this process are not well understood. We report that overexpression of the GTPase RAB5A, a master regulator of endocytosis, is predictive of aggressive behavior and metastatic ability in human breast cancers. RAB5A is necessary and sufficient to promote local invasion and distant dissemination of various mammary and nonmammary tumor cell lines, and this prometastatic behavior is associated with increased intratumoral cell motility. Specifically, RAB5A is necessary for the formation of invadosomes, membrane protrusions specialized in extracellular matrix (ECM) degradation. RAB5A promotes RAB4- and RABENOSYN-5-dependent endo/exocytic cycles (EECs) of critical cargos (membrane-type 1 matrix metalloprotease [MT1-MMP] and β3 integrin) required for invadosome formation in response to motogenic stimuli. This trafficking circuitry is necessary for spatially localized hepatocyte growth factor (HGF)/MET signaling that drives invasive, proteolysis-dependent chemotaxis in vitro and for conversion of ductal carcinoma in situ to invasive ductal carcinoma in vivo. Thus, RAB5A/RAB4 EECs promote tumor dissemination by controlling a proteolytic, mesenchymal invasive program.

Arf1 and Arf6 promote ventral actin structures formed by acute activation of protein kinase C and Src

Arf proteins regulate membrane traffic and organelle structure. Although Arf6 is known to initiate actin-based changes in cell surface architecture, Arf1 may also function at the plasma membrane. Here we show that acute activation of protein kinase C (PKC) induced by the phorbol ester PMA led to the formation of motile actin structures on the ventral surface of Beas-2b cells, a lung bronchial epithelial cell line. Ventral actin structures also formed in PMA-treated HeLa cells that had elevated levels of Arf activation. For both cell types, formation of the ventral actin structures was enhanced by expression of active forms of either Arf1 or Arf6 and by the expression of guanine nucleotide exchange factors that activate these Arfs. By contrast, formation of these structures was blocked by inhibitors of PKC and Src and required phosphatidylinositol 4, 5-bisphosphate, Rac, Arf6, and Arf1. Furthermore, expression of ASAP1, an Arf1 GTPase activating protein (GAP) was more effective at inhibiting the ventral actin structures than was ACAP1, an Arf6 GAP. This study adds to the expanding role for Arf1 in the periphery and identifies a requirement for Arf1, a "Golgi Arf," in the reorganization of the cortical actin cytoskeleton on ventral surfaces, against the substratum.

ARF6 inhibition stabilizes the vasculature and enhances survival during endotoxic shock

The vascular endothelium responds to infection by destabilizing endothelial cell-cell junctions to allow fluid and cells to pass into peripheral tissues, facilitating clearance of infection and tissue repair. During sepsis, endotoxin and other proinflammatory molecules induce excessive vascular leak, which can cause organ dysfunction, shock, and death. Current therapies for sepsis are limited to antibiotics and supportive care, which are often insufficient to reduce morbidity and prevent mortality. Previous attempts at blocking inflammatory cytokine responses in humans proved ineffective at reducing the pathologies associated with sepsis, highlighting the need for a new therapeutic strategy. The small GTPase ARF6 is activated by a MyD88-ARNO interaction to induce vascular leak through disruption of endothelial adherens junctions. In this study, we show that the MyD88-ARNO-ARF6-signaling axis is responsible for LPS-induced endothelial permeability and is a destabilizing convergence point used by multiple inflammatory cues. We also show that blocking ARF6 with a peptide construct of its N terminus is sufficient to reduce vascular leak and enhance survival during endotoxic shock, without inhibiting the host cytokine response. Our data highlight the therapeutic potential of blocking ARF6 and reducing vascular leak for the treatment of inflammatory conditions, such as endotoxemia.

Migration of dendritic cells: physical principles, molecular mechanisms, and functional implications

Dendritic cells (DCs) constitute a complex cell population that resides in both peripheral tissues and lymphoid organs. Their major function in tissues is to patrol their environment in search of danger-associated antigens to transport to lymph nodes and present to T lymphocytes. This process constitutes the first step of the adaptive immune response and relies on specific DC properties, including a high endocytic capacity as well as efficient motility in confined three-dimensional environments. Although cell motility has been widely studied, little is known on how the geometric characteristics of the environment influence DC migration and function. In this review, we give an overview of the basic physical principles and molecular mechanisms that control DC migration under confinement and discuss how such mechanisms impact the environment-patrolling capacity of DCs.

The small GTPase Arf6 is essential for the Tram/Trif pathway in TLR4 signaling

Recognition of lipopolysaccharides (LPS) by Toll-like receptor 4 (TLR4) at the plasma membrane triggers NF-κB activation through recruitment of the adaptor proteins Mal and MyD88. Endocytosis of the activated TLR4 allows recruitment of the adaptors Tram and Trif, leading to activation of the transcription factor IRF3 and interferon production. The small GTPase ADP-ribosylation factor 6 (Arf6) was shown to regulate the plasma membrane association of Mal. Here we demonstrate that inhibition of Arf6 also markedly reduced LPS-induced cytokine production in Mal(-/-) mouse macrophages. In this article, we focus on a novel role for Arf6 in the MyD88-independent TLR4 pathway. MyD88-independent IRF3 activation and IRF3-dependent gene transcription were strictly dependent on Arf6. Arf6 was involved in transport of Tram to the endocytic recycling compartment and internalization of LPS, possibly explaining its requirement for LPS-induced IRF3 activation. Together, these results show a critical role for Arf6 in regulating Tram/Trif-dependent TLR4 signaling.

Methodology of fibroblast and mesenchymal stem cell coating of surgical meshes: a pilot analysis

Coating of various synthetic, absorbable, and biologic meshes with mesenchymal stem cells (MSCs) and fibroblasts was analyzed qualitatively and quantitatively. Five hernia meshes-light weight monofilament polypropylene (Soft Mesh), polyester (Parietex-TET), polylactide composite (TIGR), heavy weight monofilament polypropylene (Marlex), and porcine dermal collagen (Strattice)-were coated with three cell lines: human dermal fibroblasts (HFs), rat kidney fibroblasts (NRKs), and rat MSCs. Cell densities were determined at different time points. Samples also underwent histology and transmission electron microscopic (TEM) analyses. It required HFs 3 weeks to cover the entire mesh, while only 2 weeks for NRKs and MSCs to do so. MSCs had no preference for any of the meshes and produced the highest cell densities on Parietex and TIGR. Substrate-preference accounted for the significantly lower fibroblast densities on TIGR than Parietex. Fibroblasts failed to coat Marlex. Strattice, which had the least surface area, generated comparable cell densities to Parietex. Both histology and TEM confirmed cell coating of mesh surface. Various prosthetics can be coated by certain cell strains. Both mesh composition and cell preference dramatically influence the coating process. This methodology provides foundation for novel avenues of modulation of host response to various modern synthetic and biologic meshes.

Differential regulation of Rab GTPase expression in monocyte-derived dendritic cells upon lipopolysaccharide activation: a correlation to maturation-dependent functional properties

The regulation of Rab expression to modulate cellular function has recently been proposed. Dendritic cells are a prototypic example of cells that drastically alter their function in response to environmental cues by reducing endocytosis, secreting cytokines, changing surface protein repertoires and altering morphology and migration. This is not a binary event, but is subject to fluctuations through the activation process, termed maturation. Consequently, DCs transiently increase endocytosis and production of major histocompatibility complex class II molecules, and secrete inflammatory cytokines in infected tissues before migrating to secondary lymph nodes and releasing T cell polarizing factors. All these cellular processes rely on intracellular membrane transport, which is regulated by Rab family GTPases and their diverse effectors. Here we examine how the Rabs likely to be involved in these functions are regulated throughout DC maturation. We find that Rab expression is altered upon lipopolysaccharide-induced activation, and discuss how this correlates to the reported functions of these cells during maturation.

Interplay between Rab35 and Arf6 controls cargo recycling to coordinate cell adhesion and migration

Cells inversely adjust the plasma membrane levels of integrins and cadherins during cell migration and cell-cell adhesion but the regulatory mechanisms that coordinate these trafficking events remain unknown. Here, we demonstrate that the small GTPase Rab35 maintains cadherins at the cell surface to promote cell-cell adhesion. Simultaneously, Rab35 supresses the activity of the GTPase Arf6 to downregulate an Arf6-dependent recycling pathway for β1-integrin and EGF receptors, resulting in inhibition of cell migration and attenuation of signaling downstream of these receptors. Importantly, the phenotypes of decreased cell adhesion and increased cell migration observed following Rab35 knock down are consistent with the epithelial-mesenchymal transition, a feature of invasive cancer cells, and we show that Rab35 expression is suppressed in a subset of cancers characterized by Arf6 hyperactivity. Our data thus identify a key molecular mechanism that efficiently coordinates the inverse intracellular sorting and cell surface levels of cadherin and integrin receptors for cell migration and differentiation.

ARF6 activated by the LHCG receptor through the cytohesin family of guanine nucleotide exchange factors mediates the receptor internalization and signaling

The luteinizing hormone chorionic gonadotropin receptor (LHCGR) is a G(s)-coupled GPCR that is essential for the maturation and function of the ovary and testis. LHCGR is internalized following its activation, which regulates the biological responsiveness of the receptor. Previous studies indicated that ADP-ribosylation factor (ARF)6 and its GTP-exchange factor (GEF) cytohesin 2 regulate LHCGR internalization in follicular membranes. However, the mechanisms by which ARF6 and cytohesin 2 regulate LHCGR internalization remain incompletely understood. Here we investigated the role of the ARF6 signaling pathway in the internalization of heterologously expressed human LHCGR (HLHCGR) in intact cells using a combination of pharmacological inhibitors, siRNA and the expression of mutant proteins. We found that human CG (HCG)-induced HLHCGR internalization, cAMP accumulation and ARF6 activation were inhibited by Gallein (βγ inhibitor), Wortmannin (PI 3-kinase inhibitor), SecinH3 (cytohesin ARF GEF inhibitor), QS11 (an ARF GAP inhibitor), an ARF6 inhibitory peptide and ARF6 siRNA. However, Dynasore (dynamin inhibitor), the dominant negative mutants of NM23-H1 (dynamin activator) and clathrin, and PBP10 (PtdIns 4,5-P2-binding peptide) inhibited agonist-induced HLHCGR and cAMP accumulation but not ARF6 activation. These results indicate that heterotrimeric G-protein, phosphatidylinositol (PI) 3-kinase (PI3K), cytohesin ARF GEF and ARF GAP function upstream of ARF6 whereas dynamin and clathrin act downstream of ARF6 in the regulation of HCG-induced HLHCGR internalization and signaling. In conclusion, we have identified the components and molecular details of the ARF6 signaling pathway required for agonist-induced HLHCGR internalization.

CD14 and NFAT mediate lipopolysaccharide-induced skin edema formation in mice

Inflammation is a multistep process triggered when innate immune cells - for example, DCs - sense a pathogen or injured cell or tissue. Edema formation is one of the first steps in the inflammatory response; it is fundamental for the local accumulation of inflammatory mediators. Injection of LPS into the skin provides a model for studying the mechanisms of inflammation and edema formation. While it is known that innate immune recognition of LPS leads to activation of numerous transcriptional activators, including nuclear factor of activated T cells (NFAT) isoforms, the molecular pathways that lead to edema formation have not been determined. As PGE2 regulates many proinflammatory processes, including swelling and pain, and it is induced by LPS, we hypothesized that PGE2 mediates the local generation of edema following LPS exposure. Here, we show that tissue-resident DCs are the main source of PGE2 and the main controllers of tissue edema formation in a mouse model of LPS-induced inflammation. LPS exposure induced expression of microsomal PGE synthase-1 (mPGES-1), a key enzyme in PGE2 biosynthesis. mPGES-1 activation, PGE2 production, and edema formation required CD14 (a component of the LPS receptor) and NFAT. Therefore, tissue edema formation induced by LPS is DC and CD14/NFAT dependent. Moreover, DCs can regulate free antigen arrival at the draining lymph nodes by controlling edema formation and interstitial fluid pressure in the presence of LPS. We therefore suggest that the CD14/NFAT/mPGES-1 pathway represents a possible target for antiinflammatory therapies.

The oncogenic TBC domain protein USP6/TRE17 regulates cell migration and cytokinesis

BACKGROUND INFORMATION

Cancer cells are characterized by their intrinsic ability to rapidly divide and migrate and to invade other tissues. How these processes are regulated at a molecular level is largely unknown.

RESULTS

Here, we identify the oncogenic TBC (Tre-2/Bub2/Cdc16) domain protein USP6 (also termed TRE17) as a regulator of both cell migration and division. We show that manipulating USP6 expression levels alters the ability of cells to migrate and to divide. Furthermore, we observe that cell proliferation and progression through cytokinesis depend on USP6 expression via a pathway that involves the small GTPase Arf6 and its GTPase-activating protein ACAP1.

CONCLUSIONS

Our data suggest a model whereby the oncogenic potential of USP6 is linked to its ability to integrate cell migration and cytokinesis by regulating Arf6/ACAP1.

Antigen stored in dendritic cells after macropinocytosis is released unprocessed from late endosomes to target B cells

B lymphocytes can be triggered in lymph nodes by nonopsonized antigens (Ag), potentially in their native form. However, the mechanisms that promote encounter of B lymphocytes with unprocessed antigens in lymph nodes are still elusive. We show here that antigens are detected in B cells in the draining lymph nodes of mice injected with live, but not fixed, dendritic cells (DCs) loaded with antigens. This highlights active processes in DCs to promote Ag transfer to B lymphocytes. In addition, antigen-loaded DCs found in the draining lymph node were CD103+. Using 3 different model Ag, we then show that immature DCs efficiently take up Ag by macropinocytosis and store the internalized material in late endocytic compartments. We find that DCs have a unique ability to release antigens from these compartments in the extracellular medium, which is controlled by Rab27. B cells take up the regurgitated Ag and the chemokine CXCL13, essential to attract B cells in lymph nodes, enhances this transfer. Our results reveal a unique property of DCs to regurgitate unprocessed Ag that could play an important role in B-cell activation.

The human cytokine TSLP triggers a cell-autonomous dendritic cell migration in confined environments

Dendritic cells (DCs) need to migrate in the interstitial environment of peripheral tissues to reach secondary lymphoid organs and initiate a suitable immune response. Whether and how inflamed tissues instruct DCs to emigrate is not fully understood. In this study, we report the unexpected finding that the epithelial-derived cytokine TSLP triggers chemokinesis of resting primary human DCs in a cell-autonomous manner. TSLP induced the polarization of both microtubule and actin cytoskeletons and promoted DC 3-dimensional migration in transwell as well as in microfabricated channels that mimic the confined environment of peripheral tissues. TSLP-induced migration relied on the actin-based motor myosin II and was inhibited by blebbistatin. Accordingly, TSLP triggered the redistribution of phosphorylated myosin II regulatory light chain to the actin cortex, indicating that TSLP induces DC migration by promoting actomyosin contractility. Thus, TSLP produced by epithelial cells in inflamed tissue has a critical function in licensing DCs for cell-autonomous migration. This indicates that cytokines can directly trigger cell migration, which has important implications in immune physiopathology and vaccine design.

Signaling networks regulating leukocyte podosome dynamics and function

Podosomes are ventral adhesion structures prominent in cells of the myeloid lineage. A common aspect of these cells is that they are highly motile and must to traverse multiple tissue barriers in order to perform their functions. Recently podosomes have gathered attention from researchers as important cellular structures that can influence cell adhesion, motility and matrix remodeling. Adhesive and soluble ligands act via transmembrane receptors and propagate signals to the leukocyte cytoskeleton via small G proteins of the Rho family, tyrosine kinases and scaffold proteins and are able to induce podosome formation and rearrangements. Manipulation of the signals that regulate podosome formation and dynamics can therefore be a strategy to interfere with leukocyte functions in a multitude of pathological settings, such as infections, atherosclerosis and arthritis. Here, we review the major signaling molecules that act in the formation and regulation of podosomes.

Imaging podosome dynamics and matrix degradation

Invasive cell migration is critical for leukocyte trafficking into tissues. Podosomes are matrix-degrading adhesive structures that are formed by macrophages and are necessary for macrophage migration and invasion. Here, we describe methods for imaging and quantifying podosomes in primary human macrophages and in THP-1 cells, a monocyte cell line that can be differentiated to a macrophage-like state. Moreover, we outline detailed methods for live imaging of podosomes, which are highly dynamic, and for the quantification of rates of podosome turnover. Finally, we discuss methods for the quantitative analysis of matrix degradation on fluorescent-gelatin-coated cover slips.

Cdc42 localization and cell polarity depend on membrane traffic

Cell polarity is essential for cell division, cell differentiation, and most differentiated cell functions including cell migration. The small G protein Cdc42 controls cell polarity in a wide variety of cellular contexts. Although restricted localization of active Cdc42 seems to be important for its distinct functions, mechanisms responsible for the concentration of active Cdc42 at precise cortical sites are not fully understood. In this study, we show that during directed cell migration, Cdc42 accumulation at the cell leading edge relies on membrane traffic. Cdc42 and its exchange factor βPIX localize to intracytosplasmic vesicles. Inhibition of Arf6-dependent membrane trafficking alters the dynamics of Cdc42-positive vesicles and abolishes the polarized recruitment of Cdc42 and βPIX to the leading edge. Furthermore, we show that Arf6-dependent membrane dynamics is also required for polarized recruitment of Rac and the Par6-aPKC polarity complex and for cell polarization. Our results demonstrate influence of membrane dynamics on the localization and activation of Cdc42 and consequently on directed cell migration.

Lysophosphatidylcholine-mediated functional inactivation of syndecan-4 results in decreased adhesion and motility of dendritic cells

Following antigen contact, maturation and migration of DCs into lymphatic tissues are crucial to the developing immune response or maintenance of tolerance. Lysophosphatidylcholine (LysoPC) is generated during apoptosis of cells and acts as a "find-and-eat-me" signal thought to prevent autoimmunity. Moreover, LysoPC can activate PKCδ and initiates a signaling cascade that leads to phosphorylation and inactivation of syndecan-4 (SDC4), a heparansulfate proteoglycan integrin co-receptor. In human monocyte-derived DCs, we recently demonstrated that SDC4 is upregulated during maturation thereby stimulating DC motility. Here, we investigate the effects of LysoPC on DC motility as well as on the involvement of PKCδ phosphorylation-dependent regulation of DC motility by SDC4 and PKCα. Employing a static adhesion assay and videomicroscopy, we show that LysoPC inhibits adhesion of DCs to fibronectin and motility of DCs by decreasing podosome formation. Moreover, DC podosome formation and motility, which both are regulated by SDC4 and subject to control by PKCδ-dependent phosphorylation of SDC4, were inhibited in LysoPC-matured DCs. Thus, these DC are defective in adhesion and migration. Based on our results, we hypothesize that LysoPC released during apoptosis might delay DC migration to lymphoid organs and thus prevent autoimmunity.

ARF6-mediated endocytic recycling impacts cell movement, cell division and lipid homeostasis

A wide range of cellular activities depends upon endocytic recycling. ARF6, a small molecular weight GTPase, regulates the processes of endocytosis and endocytic recycling in concert with various effector molecules and other small GTPases. This review highlights three critical processes that involve ARF6-mediated endosomal membrane trafficking-cell motility, cytokinesis, and cholesterol homeostasis. In each case, the function of ARF6-mediated trafficking varies-including localization of specific protein and lipid cargo, regulation of bulk membrane movement, and modulation of intracellular signaling. As described in this review, mis-regulation of endocytic traffic can result in human disease when it compromises the cell's ability to regulate cell movement and invasion, cell division, and lipid homeostasis.

Regulation of antigen uptake, migration, and lifespan of dendritic cell by Toll-like receptors

Dendritic cells (DCs) sense the presence of pathogens through germline-encoded pattern recognition receptors (PRRs), which recognize molecular patterns expressed by various microorganisms and endogenous stimuli. Toll-like receptors (TLRs) are the best characterized PRRs. TLR activation has a profound effect on a number of DC activities, including endocytosis, cytoskeleton rearrangement, migration, antigen processing and presentation, survival, and death. The goal of TLR-induced DC reprogramming is to promote the appropriate activation and differentiation of lymphocytes bearing clonally distributed antigen-specific receptors. In this review, we will focus on the functional consequences of TLR engagement for conventional DCs.

Virus entry by macropinocytosis

As obligatory intracellular parasites, viruses rely on host-cell functions for most aspects of their replication cycle. This is born out during entry, when most viruses that infect vertebrate and insect cells exploit the endocytic activities of the host cell to move into the cytoplasm. Viruses belonging to vaccinia, adeno, picorna and other virus families have been reported to take advantage of macropinocytosis, an endocytic mechanism normally involved in fluid uptake. The virus particles first activate signalling pathways that trigger actin-mediated membrane ruffling and blebbing. Usually, this is followed by the formation of large vacuoles (macropinosomes) at the plasma membrane, internalization of virus particles and penetration by the viruses or their capsids into the cytosol through the limiting membrane of the macropinosomes. We review the molecular machinery involved in macropinocytosis and describe what is known about its role in virus entry.

Mechanisms of endocytosis

Endocytic mechanisms control the lipid and protein composition of the plasma membrane, thereby regulating how cells interact with their environments. Here, we review what is known about mammalian endocytic mechanisms, with focus on the cellular proteins that control these events. We discuss the well-studied clathrin-mediated endocytic mechanisms and dissect endocytic pathways that proceed independently of clathrin. These clathrin-independent pathways include the CLIC/GEEC endocytic pathway, arf6-dependent endocytosis, flotillin-dependent endocytosis, macropinocytosis, circular doral ruffles, phagocytosis, and trans-endocytosis. We also critically review the role of caveolae and caveolin1 in endocytosis. We highlight the roles of lipids, membrane curvature-modulating proteins, small G proteins, actin, and dynamin in endocytic pathways. We discuss the functional relevance of distinct endocytic pathways and emphasize the importance of studying these pathways to understand human disease processes.

TLR ligand-induced podosome disassembly in dendritic cells is ADAM17 dependent

Toll-like receptor (TLR) signaling induces a rapid reorganization of the actin cytoskeleton in cultured mouse dendritic cells (DC), leading to enhanced antigen endocytosis and a concomitant loss of filamentous actin-rich podosomes. We show that as podosomes are lost, TLR signaling induces prominent focal contacts and a transient reduction in DC migratory capacity in vitro. We further show that podosomes in mouse DC are foci of pronounced gelatinase activity, dependent on the enzyme membrane type I matrix metalloprotease (MT1-MMP), and that DC transiently lose the ability to degrade the extracellular matrix after TLR signaling. Surprisingly, MMP inhibitors block TLR signaling-induced podosome disassembly, although stimulated endocytosis is unaffected, which demonstrates that the two phenomena are not obligatorily coupled. Podosome disassembly caused by TLR signaling occurs normally in DC lacking MT1-MMP, and instead requires the tumor necrosis factor alpha-converting enzyme ADAM17 (a disintegrin and metalloprotease 17), which demonstrates a novel role for this "sheddase" in regulating an actin-based structure.