Integrin leukocyte function-associated antigen-1-mediated cell binding can be activated by clustering of membrane rafts

RAS GTPases and Interleaflet Coupling in the Plasma Membrane

RAS genes are frequently mutated in cancer. The primary signaling compartment of wild-type and constitutively active oncogenic mutant RAS proteins is the inner leaflet of the plasma membrane (PM). Thus, a better understanding of the unique environment of the PM inner leaflet is important to shed further light on RAS function. Over the past few decades, an integrated approach of superresolution imaging, molecular dynamic simulations, and biophysical assays has yielded new insights into the capacity of RAS proteins to sort lipids with specific headgroups and acyl chains, to assemble signaling nanoclusters on the inner PM. RAS proteins also sense and respond to changes in components of the outer PM leaflet, including glycophosphatidylinositol-anchored proteins, sphingophospholipids, glycosphingolipids, and galectins, as well as cholesterol that translocates between the two leaflets. Such communication between the inner and outer leaflets of the PM, called interleaflet coupling, allows RAS to potentially integrate extracellular mechanical and electrostatic information with intracellular biochemical signaling events, and reciprocally allows mutant RAS-transformed tumor cells to modify tumor microenvironments. Here, we review RAS-lipid interactions and speculate on potential mechanisms that allow communication between the opposing leaflets of the PM.

Prospects of antibodies targeting CD47 or CD24 in the treatment of glioblastoma

Glioma is a malignant tumor with the highest incidence among all brain tumors (about 46% of intracranial tumors) and is the most common primary intracranial tumor. Among them, glioblastoma (GBM) is highly malignant and is one of the three refractory tumors with the highest mortality rate in the world. The survival time from glioblastoma diagnosis to death is only 14–16 months for patients with standard treatment such as surgery plus radiotherapy and chemotherapy. Due to its high malignancy and poor prognosis, in‐depth studies have been conducted to explore effective therapeutic strategies for glioblastoma. In addition to the conventional surgery, radiotherapy, and chemotherapy, the glioblastoma treatments also include targeted therapy, immunotherapy, and electric field treatment. However, current treatment methods provide limited benefits because of the heterogeneity of glioblastoma and the complexity of the immune microenvironment within a tumor. Therefore, seeking an effective treatment plan is imperative. In particular, developing an active immunotherapy for glioblastoma has become an essential objective in the field. This article reviews the feasibility of CD47/CD24 antibody treatment, either individually or in combination, to target the tumor stem cells and the antitumor immunity in glioblastoma. The potential mechanisms underlying the antitumor effects of CD47/CD24 antibodies are also discussed.

Novel insights into the function of CD24: A driving force in cancer

CD24 is a highly glycosylated protein with a small protein core that is linked to the plasma membrane via a glycosyl-phosphatidylinositol anchor. CD24 is primarily expressed by immune cells but is often overexpressed in human tumors. In cancer, CD24 is a regulator of cell migration, invasion and proliferation. Its expression is associated with poor prognosis and it is used as cancer stemness marker. Recently, CD24 on tumor cells was identified as a phagocytic inhibitor ("do not eat me" signal) having a suppressive role in tumor immunity via binding to Siglec-10 on macrophages. This finding is reminiscent of the demonstration that soluble CD24-Fc can dampen the immune system in autoimmune disease. In the present review, we summarize recent progress on the role of the CD24-Siglec-10 binding axis at the interface between tumor cells and the immune system, and the role of CD24 genetic polymorphisms in cancer. We describe the specific function of cytoplasmic CD24 and discuss the presence of CD24 on tumor-released extracellular vesicles. Finally, we evaluate the potential of CD24-based immunotherapy.

Roles of Membrane Domains in Integrin-Mediated Cell Adhesion

The composition and organization of the plasma membrane play important functional and regulatory roles in integrin signaling, which direct many physiological and pathological processes, such as development, wound healing, immunity, thrombosis, and cancer metastasis. Membranes are comprised of regions that are thick or thin owing to spontaneous partitioning of long-chain saturated lipids from short-chain polyunsaturated lipids into domains defined as ordered and liquid-disorder domains, respectively. Liquid-ordered domains are typically 100 nm in diameter and sometimes referred to as lipid rafts. We posit that integrin β senses membrane thickness and that mechanical force on the membrane regulates integrin activation through membrane thinning. This review examines what we know about the nature and mechanism of the interaction of integrins with the plasma membrane and its effects on regulating integrins and its binding partners.

Loss of Thy-1 may reduce lung regeneration after pneumonectomy in mice

BACKGROUND

Lung regeneration plays an important role in lung repair after injury. It is reliant upon proliferation of multiple cell types in the lung, including endothelium, epithelium, and fibroblasts, as well as remodeling of the extracellular matrix.

METHODS

Lung regeneration following injury progresses via an initial infammatory response during which macrophages clear the tissue of cellular debris. This process continues through cellular proliferation when existing cells and progenitors act to repopulate cells lost during injury, followed by tissue maturation in which newly formed cells achieve a diferentiated phenotype.

RESULTS

Signaling pathways critical for lung regeneration include FGF, EGF, WNT, and NOTCH. In addition, HDACs, miRNAs, ELASTIN, and MMP14 have been shown to regulate lung regeneration. Partial pneumonectomy (PNX) has been used as a therapeutic and investigational tool for several decades. Following PNX the remaining lung increases in size to compensate for loss of volume and respiratory capacity.

CONCLUSIONS

Much has been learned about the triggers and mechanisms regulating pulmonary regeneration. However, the role of thymocyte differentiation antigen-1(thy-1) in post-PNX lung growth remains incompletely characterized. Thy-1 is a phosphatidylinositol glycoprotein with a relative molecular weight of 25000~37000 Da, which is expressed in almost all types of fibroblasts and regulates many biological functions. It not only supports the structure of fibroblasts, but also can balance cell proliferation, migration and regulate the synthesis of immune inflammatory mediators.

Role of Lipid Rafts in Hematopoietic Stem Cells Homing, Mobilization, Hibernation, and Differentiation

Hematopoietic stem cells (HSCs) are multipotent, self-renewing cells that can differentiate into myeloid or lymphoid cells. The mobilization and differentiation processes are affected by the external environment, such as extracellular matrix and soluble molecules in the niche, where the lipid rafts (LRs) of the HSCs act as the receptors and control platforms for these effectors. LRs are membrane microdomains that are enriched in cholesterol, sphingolipid, and proteins. They are involved in diverse cellular processes including morphogenesis, cytokinesis, signaling, endocytic events, and response to the environment. They are also involved in different types of diseases, such as cancer, Alzheimer's, and prion disease. LR clustering and disruption contribute directly to the differentiation, homing, hibernation, or mobilization of HSCs. Thus, characterization of LR integrity may provide a promising approach to controlling the fate of stem cells for clinical applications. In this review, we show the critical role of LR modification (clustering, disruption, protein incorporation, and signal responding) in deciding the fate of HSCs, under the effect of soluble cytokines such as stem cell factor (SCF), transforming growth factor- β (TGF-β), hematopoietic-specific phospholipase Cβ2 (PLC-β2), and granulocyte colony-stimulating factor (G-CSF).

Sterol metabolism in the filasterean Capsaspora owczarzaki has features that resemble both fungi and animals

Sterols are essential for several physiological processes in most eukaryotes. Sterols regulate membrane homeostasis and participate in different signalling pathways not only as precursors of steroid hormones and vitamins, but also through its role in the formation of lipid rafts. Two major types of sterols, cholesterol and ergosterol, have been described so far in the opisthokonts, the clade that comprise animals, fungi and their unicellular relatives. Cholesterol predominates in derived bilaterians, whereas ergosterol is what generally defines fungi. We here characterize, by a combination of bioinformatic and biochemical analyses, the sterol metabolism in the filasterean Capsaspora owczarzaki, a close unicellular relative of animals that is becoming a model organism. We found that C. owczarzaki sterol metabolism combines enzymatic activities that are usually considered either characteristic of fungi or exclusive to metazoans. Moreover, we observe a differential transcriptional regulation of this metabolism across its life cycle. Thus, C. owczarzaki alternates between synthesizing 7-dehydrocholesterol de novo, which happens at the cystic stage, and the partial conversion—via a novel pathway—of incorporated cholesterol into ergosterol, the characteristic fungal sterol, in the filopodial and aggregative stages.

Subcellular domain-dependent molecular hierarchy of SFK and FAK in mechanotransduction and cytokine signaling

Focal adhesion kinase (FAK) and Src family kinases (SFK) are known to play critical roles in mechanotransduction and other crucial cell functions. Recent reports indicate that they reside in different microdomains of the plasma membrane. However, little is known about their subcellular domain-dependent roles and responses to extracellular stimuli. Here, we employed fluorescence resonance energy transfer (FRET)-based biosensors in conjunction with collagen-coupled agarose gels to detect subcellular activities of SFK and FAK in three-dimensional (3D) settings. We observed that SFK and FAK in the lipid rafts and nonrafts are differently regulated by fluid flow and pro-inflammatory cytokines. Inhibition of FAK in the lipid rafts blocked SFK response to fluid flow, while inhibition of SFK in the non-rafts blocked FAK activation by the cytokines. Ex-vivo FRET imaging of mouse cartilage explants showed that intermediate level of interstitial fluid flow selectively decreased cytokine-induced SFK/FAK activation. These findings suggest that SFK and FAK exert distinctive molecular hierarchy depending on their subcellular location and extracellular stimuli.

CDCP1 drives triple-negative breast cancer metastasis through reduction of lipid-droplet abundance and stimulation of fatty acid oxidation

Triple-negative breast cancer (TNBC) is notoriously aggressive with high metastatic potential, which has recently been linked to high rates of fatty acid oxidation (FAO). Here we report the mechanism of lipid metabolism dysregulation in TNBC through the prometastatic protein, CUB-domain containing protein 1 (CDCP1). We show that a "low-lipid" phenotype is characteristic of breast cancer cells compared with normal breast epithelial cells and negatively correlates with invasiveness in 3D culture. Using coherent anti-Stokes Raman scattering and two-photon excited fluorescence microscopy, we show that CDCP1 depletes lipids from cytoplasmic lipid droplets (LDs) through reduced acyl-CoA production and increased lipid utilization in the mitochondria through FAO, fueling oxidative phosphorylation. These findings are supported by CDCP1's interaction with and inhibition of acyl CoA-synthetase ligase (ACSL) activity. Importantly, CDCP1 knockdown increases LD abundance and reduces TNBC 2D migration in vitro, which can be partially rescued by the ACSL inhibitor, Triacsin C. Furthermore, CDCP1 knockdown reduced 3D invasion, which can be rescued by ACSL3 co-knockdown. In vivo, inhibiting CDCP1 activity with an engineered blocking fragment (extracellular portion of cleaved CDCP1) lead to increased LD abundance in primary tumors, decreased metastasis, and increased ACSL activity in two animal models of TNBC. Finally, TNBC lung metastases have lower LD abundance than their corresponding primary tumors, indicating that LD abundance in primary tumor might serve as a prognostic marker for metastatic potential. Our studies have important implications for the development of TNBC therapeutics to specifically block CDCP1-driven FAO and oxidative phosphorylation, which contribute to TNBC migration and metastasis.

GPI-Anchored Proteins and Glycosphingolipid-Rich Rafts: Platforms for Adhesion and Signaling

Glycosylphosphatidylinositol (GPI)-anchored proteins in mammalian cells play a role in adhesion and signaling. They are sorted in the trans-Golgi network into glycosphingolipid- and cholesterol-rich microdomains termed rafts. Such rafts can be isolated from many cell types including epithelial cells, neural cells, and lymphocytes. In polarized cells, the rafts segregate in distinct regions of the cell. The rafts constitute platforms for signal transduction via raft-associated srcfamily tyrosine kinases. This review compares the sorting, distribution, and signaling of GPI-anchored proteins and rafts in epithelial cells, lymphocytes, and neural cells. A possible involvement of rafts in distinct diseases is also addressed.

Membrane dynamics in mammalian embryogenesis: Implication in signal regulation

Eukaryotes have evolved an array of membrane compartments constituting secretory and endocytic pathways that allow the flow of materials. Both pathways perform important regulatory roles. The secretory pathway is essential for the production of extracellular, secreted signal molecules, but its function is not restricted to a mere route connecting intra- and extracellular compartments. Post-translational modifications also play an integral function in the secretory pathway and are implicated in developmental regulation. The endocytic pathway serves as a platform for relaying signals from the extracellular stimuli to intracellular mediators, and then ultimately inducing signal termination. Here, we discuss recent studies showing that dysfunction in membrane dynamics causes patterning defects in embryogenesis and tissue morphogenesis in mammals.

Proteome Based Construction of the Lymphocyte Function-Associated Antigen 1 (LFA-1) Interactome in Human Dendritic Cells

The β2-integrin lymphocyte function-associated antigen 1 (LFA-1) plays an important role in the migration, adhesion and intercellular communication of dendritic cells (DCs). During the differentiation of human DCs from monocyte precursors, LFA-1 ligand binding capacity is completely lost, even though its expression levels were remained constant. Yet LFA-1-mediated adhesive capacity on DCs can be regained by exposing DCs to the chemokine CCL21, suggesting a high degree of regulation of LFA-1 activity during the course of DC differentiation. The molecular mechanisms underlying this regulation of LFA-1 function in DCs, however, remain elusive. To get more insight we attempted to identify specific LFA-1 binding partners that may play a role in regulating LFA-1 activity in DCs. We used highly sensitive label free quantitative mass-spectrometry to identify proteins co-immunoprecipitated (co-IP) with LFA-1 from ex vivo generated DCs. Among the potential binding partners we identified not only established components of integrin signalling pathways and cytoskeletal proteins, but also several novel LFA-1 binding partners including CD13, galectin-3, thrombospondin-1 and CD44. Further comparison to the LFA-1 interaction partners in monocytes indicated that DC differentiation was accompanied by an overall increase in LFA-1 associated proteins, in particular cytoskeletal, signalling and plasma membrane (PM) proteins. The here presented LFA-1 interactome composed of 78 proteins thus represents a valuable resource of potential regulators of LFA-1 function during the DC lifecycle.

The extracellular matrix modulates H2O2 degradation and redox signaling in endothelial cells

The molecular processes that are crucial for cell function, such as proliferation, migration and survival, are regulated by hydrogen peroxide (H₂O₂). Although environmental cues, such as growth factors, regulate redox signaling, it was still unknown whether the ECM, a component of the cell microenvironment, had a function in this process.

Here, we showed that the extracellular matrix (ECM) differently regulated H₂O₂ consumption by endothelial cells and that this effect was not general for all types of cells. The analysis of biophysical properties of the endothelial cell membrane suggested that this modification in H₂O₂ consumption rates was not due to altered membrane permeability. Instead, we found that the ECM regulated GPx activity, a known H₂O₂ scavenger. Finally, we showed that the extent of PTEN oxidation was dependent on the ECM, indicating that the ECM was able to modulate H₂O₂-dependent protein oxidation.

Thus, our results unraveled a new mechanism by which the ECM regulates endothelial cell function by altering redox balance. These results pinpoint the ECM as an important component of redox-signaling.

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The extracellular matrix (ECM) regulates H₂O₂ consumption by endothelial cells.

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Membrane biophysical properties are not affected by the ECM.

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The ECM regulates GPx activity and the extent of PTEN oxidation.

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We propose that the ECM modulates redox-signaling by controlling H₂O₂ degradation.

Elastic Membrane Deformations Govern Interleaflet Coupling of Lipid-Ordered Domains

The mechanism responsible for domain registration in two membrane leaflets has thus far remained enigmatic. Using continuum elasticity theory, we show that minimum line tension is achieved along the rim between thicker (ordered) and thinner (disordered) domains by shifting the rims in opposing leaflets by a few nanometers relative to each other. Increasing surface tension yields an increase in line tension, resulting in larger domains. Because domain registration is driven by lipid deformation energy, it does not require special lipid components or interactions at the membrane midplane.

The nanoscale organization of signaling domains at the plasma membrane

In this chapter, we present an overview of the role of the nanoscale organization of signaling domains in regulating key cellular processes. In particular, we illustrate the importance of protein and lipid nanodomains as triggers and mediators of cell signaling. As particular examples, we summarize the state of the art of understanding the role of nanodomains in the mounting of an immune response, cellular adhesion, intercellular communication, and cell proliferation. Thus, this chapter underlines the essential role the nanoscale organization of key signaling proteins and lipid domains. We will also see how nanodomains play an important role in the lifecycle of many pathogens relevant to human disease and therefore illustrate how these structures may become future therapeutic targets.

Cell clustering and delay/arrest in T-cell division implicate a novel mechanism of immune modulation by E. coli heat-labile enterotoxin B-subunits

The B-subunits of heat-labile enterotoxins LT-I (LT-IB) and LT-IIa (LT-IIaB) are strong adjuvants that bind to cell-surface receptors, including gangliosides G(M1) and GD1b, respectively. LT-IIaB also binds TLR-2. We demonstrate for the first time that co-incubation with the B-subunits induces significant clustering of B cells after only 4h, and B and T cells in 24h. Clustering was dependent on intact B-subunits, but not on the TLR-2 binding activity of LT-IIaB, indicating it was ganglioside-mediated. Treatment of B cells with LT-IB, a mixture of LT-IB+LT-IIaB, but not LT-IIaB alone, caused a delay in T cell division following ovalbumin endocytosis. B cell receptor-mediated uptake in presence of each treatment caused an arrest, but with increased production of IL-2. Further, treatments differentially increased the proportion of macrophages expressing MHC class-II. These results highlight the outcomes of interplay between signals involving different receptors and implicate a novel mechanism of adjuvanticity.

Adaptor regulation of LFA-1 signaling in T lymphocyte migration: Potential druggable targets for immunotherapies?

The integrin lymphocyte function associated antigen-1 (LFA-1) plays a key role in leukocyte trafficking and in adaptive immune responses through interactions with adhesive ligands, such as ICAM-1. Specific blockade of these interactions has validated LFA-1 as a therapeutic target in many chronic inflammatory diseases, however LFA-1 antagonists have not been clinically successful due to the development of a general immunosuppression, causing fatal side effects. Growing evidence has now established that LFA-1 mediates an array of intracellular signaling pathways by triggering a number of downstream molecules. In this context, a class of multimodular domain-containing proteins capable of recruiting two or more effector molecules, collectively known as "adaptor proteins," has emerged as important mediators in LFA-1 signal transduction. Here, we provide an overview of the adaptor proteins involved in the intracellular signaling cascades by which LFA-1 regulates T-cell motility and immune responses. The complexity of the LFA-1-associated signaling delineated in this review suggests that it may be an important and challenging focus for future research, enabling the identification of "tunable" targets for the development of immunotherapies.

Single particle tracking with sterol modulation reveals the cholesterol-mediated diffusion properties of integrin receptors

A combination of sterol modulation with cyclodextrins plus fluorescence microscopy revealed a biophysical mechanism behind cholesterol's influence on the diffusion of a ubiquitous class of receptors called integrins. The heterogeneous diffusion of integrins bound to ligand-coated quantum dots was measured using single particle tracking (SPT), and the ensemble changes in integrin diffusion were measured by fluorescence recovery after photobleaching (FRAP). A 25 ± 1% reduction of membrane cholesterol resulted in three significant changes to the diffusion of ligand-bound αPS2CβPS integrins as measured by SPT. There was a 23% increase in ligand-bound mobile integrins; there was a statistically significant increase in the average diffusion coefficient inside zones of confined diffusion, and histograms of confined integrin trajectories showed an increased frequency in the range of 0.1-1 μm(2) s(-1) and a decreased frequency in the 0.001-0.1 μm(2) s(-1) range. No statistical change was measured in the duration of confinement nor the size of confined zones. Restoring the cholesterol-depleted cells with exogenous cholesterol or exogenous epicholesterol resulted in similar diffusion properties. Epicholesterol differs from cholesterol in the orientation of a single hydroxyl group. The ability of epicholesterol to substitute for cholesterol suggests a biophysical mechanism for cholesterol's effect on integrin diffusion. Influences of bilayer thickness, viscosity and organization are discussed as possible explanations for the measured changes in integrin diffusion when the membrane cholesterol concentration is reduced.

Prohibitin, an essential protein for Colorado potato beetle larval viability, is relevant to Bacillus thuringiensis Cry3Aa toxicity

Bacillus thuringienesis (Bt) Cry toxins constitute the most extensively used environmentally safe biopesticide and their mode of action relies on the interaction of the toxins with membrane proteins in the midgut of susceptible insects that mediate toxicity and insect specificity. Therefore, identification of Bt Cry toxin interacting proteins in the midgut of target insects and understanding their role in toxicity is of great interest to exploit their insecticidal action. Using ligand blot, we demonstrated that Bt Cry3Aa toxin bound to a 30kDa protein in Colorado potato beetle (CPB) larval midgut membrane, identified by sequence homology as prohibitin-1 protein. Prohibitins comprise a highly conserved family of proteins implicated in important cellular processes. We obtained the complete CPB prohibitin-1 DNA coding sequence of 828pb, in silico translated into a 276-amino acid protein. The analysis at the amino acid level showed that the protein contains a prohibitin-homology domain (Band7_prohibitin, cd03401) conserved among prohibitin proteins. A striking feature of the CPB identified prohibitin-1 is the predicted presence of cadherin elements, potential binding sites for Cry toxins described in other Bt susceptible insects. We also showed that CPB prohibitin-1 protein partitioned into both, detergent soluble and insoluble membrane fractions, as well as a prohibitin-2 homologous protein, previously reported to form functional complexes with prohibitin-1 in other organisms. Prohibitin complexes act as membrane scaffolds ensuring the recruitment of membrane proteases to facilitate substrate processing. Accordingly, sequestration of prohibitin-1 by an anti-prohibitin-1 antibody impaired the Cry3Aa toxin inhibition of the proteolytic cleavage of a fluorogenic synthetic substrate of an ADAM-like metalloprotease previously reported to proteolize this toxin. In this work, we also demonstrated that prohibitin-1 RNAi silencing in CPB larvae produced deleterious effects and together with a LD50 Cry3Aa toxin treatment resulted in a highly efficient short term response since 100% larval mortality was achieved just 5days after toxin challenge. Therefore, the combination of prohibitin RNAi and Cry toxin reveals as an effective strategy to improve crop protection.

Integrins modulate the infection efficiency of West Nile virus into cells

The underlying mechanisms allowing West Nile virus (WNV) to replicate in a large variety of different arthropod, bird and mammal species are largely unknown but are believed to rely on highly conserved proteins relevant for viral entry and replication. Consistent with this, the integrin αvβ3 has been proposed lately to function as the cellular receptor for WNV. More recently published data, however, are not in line with this concept. Integrins are highly conserved among diverse taxa and are expressed by almost every cell type at high numbers. Our study was designed to clarify the involvement of integrins in WNV infection of cells. A cell culture model, based on wild-type and specific integrin knockout cell lines lacking the integrin subunits αv, β1 or β3, was used to investigate the susceptibility to WNV, and to evaluate binding and replication efficiencies of four distinct strains (New York 1999, Uganda 1937, Sarafend and Dakar). Though all cell lines were permissive, clear differences in replication efficiencies were observed. Rescue of the β3-integrin subunit resulted in enhanced WNV yields of up to 90 %, regardless of the virus strain used. Similar results were obtained for β1-expressing and non-expressing cells. Binding, however, was not affected by the expression of the integrins in question, and integrin blocking antibodies failed to have any effect. We conclude that integrins are involved in WNV infection but not at the level of binding to target cells.

Synthetic immunomodulatory peptide IDR-1002 enhances monocyte migration and adhesion on fibronectin

Regulation of the immune system by immunomodulatory agents, such as the synthetic innate defense regulator (IDR) peptides, has been proposed as a potential strategy to strengthen host immune responses against infection. IDR peptides confer protection in vivo against a range of bacterial infections and have been developed as components of single-dose vaccine adjuvants due to their ability to modulate innate immunity, correlating with an increased recruitment of monocytes to sites of infection or immunization. However, the mechanisms by which IDR peptides augment monocyte recruitment remain poorly defined. Anti-infective peptide IDR-1002 was demonstrated here to lack direct monocyte chemoattractive activity yet enhance, by up to 5-fold, the ability of human monocytes to migrate on fibronectin towards chemokines. This effect correlated with an increased adhesion of monocytes and THP-1 cells to fibronectin by IDR-1002 and other IDR peptides and the adhesion of THP-1 cells to fibronectin occurred in a β(1)-integrin-dependent manner, corresponding with an increased activation of β(1)-integrins and the phosphoinositide 3-kinase (PI3K)-Akt pathway. PI3K- and Akt-specific inhibitors abrogated IDR-1002-induced adhesion and activation of β(1)-integrins, whereas p38 and MEK1 inhibitors did not affect, or moderately inhibited, adhesion, respectively. Furthermore, IDR-1002 enhancement of monocyte migration towards chemokines and activation of β(1)-integrins was abrogated in the presence of PI3K- and Akt-specific inhibitors. In summary, IDR-1002 enhanced monocyte migration on fibronectin through promotion of β(1)-integrin-mediated interactions regulated by the PI3K-Akt pathway, revealing a mechanism by which IDR-1002 promotes monocyte recruitment.

Overview: imaging in the study of integrins

Integrins play critical adhesion and signaling roles during development, wound healing, immunity, and cancer. Central to their function is a unique ability to dynamically modulate their adhesiveness and signaling properties through changes in conformation, both homo- and heterotypic protein-protein interactions and cellular distribution. Genetic, biochemical and structural studies have been instrumental in uncovering overall functions, describing ligand and regulatory protein interactions and elucidating the molecular architecture of integrins. However, such approaches alone are inadequate to describe how dynamic integrin behaviors are orchestrated in intact cells. To fill this void, a wide array of distinct light microscopy (largely fluorescence-based) imaging approaches have been developed and employed. Various microscopy technologies, including wide-field, optical sectioning (laser-scanning confocal, spinning-disk confocal, and multiphoton), TIRF and range of novel "Super-Resolution" techniques have been used in combination with diverse imaging modalities (such as IRM, FRET, FRAP, CALI, and fluorescence speckle imaging) to address distinct aspects of integrin function and regulation. This chapter provides an overview of these imaging approaches and how they have advanced our understanding of integrins.

Intracellular lipid flux and membrane microdomains as organizing principles in inflammatory cell signaling

Lipid rafts and caveolae play a pivotal role in organization of signaling by TLR4 and several other immune receptors. Beyond the simple cataloguing of signaling events compartmentalized by these membrane microdomains, recent studies have revealed the surprisingly central importance of dynamic remodeling of membrane lipid domains to immune signaling. Simple interventions upon membrane lipid, such as changes in cholesterol loading or crosslinking of raft lipids, are sufficient to induce micrometer-scale reordering of membranes and their protein cargo with consequent signal transduction. In this review, using TLR signaling in the macrophage as a central focus, we discuss emerging evidence that environmental and genetic perturbations of membrane lipid regulate protein signaling, illustrate how homeostatic flow of cholesterol and other lipids through rafts regulates the innate immune response, and highlight recent attempts to harness these insights toward therapeutic development.

Streptolysin O inhibits clathrin-dependent internalization of group A Streptococcus

Group A Streptococcus (GAS) can be internalized by epithelial cells, including keratinocytes from human skin or pharyngeal epithelium. Internalization of GAS by epithelial cells has been postulated both to play a role in host defense and to provide a sanctuary site for GAS survival. The cholesterol-binding cytolysin streptolysin O (SLO) appears to enhance virulence in part by inhibiting GAS internalization by human keratinocytes and by disrupting the lysosomal degradation of internalized GAS. We now report that low-level production of SLO by an inducible expression system reduced GAS internalization by keratinocytes. Induced SLO expression also prevented lysosomal colocalization with intracellular bacteria and acidification of GAS-containing vacuoles. Exogenous recombinant SLO mimicked the inhibitory effect of SLO secretion on GAS entry but not that on colocalization with the lysosomal marker LAMP-1, implying that disruption of lysosomal degradation requires intracellular secretion of SLO. The internalization of SLO-negative GAS was blocked by the depletion of host cell cholesterol and by the inhibition or knocking down of the expression of clathrin or dynamin. SLO also inhibited the cellular uptake of other cargos that are internalized by clathrin-mediated uptake or by macropinocytosis. We conclude that SLO interferes with the internalization of GAS through local perturbation of the keratinocyte cell membrane and disruption of a clathrin-dependent uptake pathway.

Cell migration: an overview

Cell migration is a fundamental process that controls morphogenesis and inflammation. Its deregulation causes or is part of many diseases, including autoimmune syndromes, chronic inflammation, mental retardation, and cancer. Cell migration is an integral part of the cell biology, embryology, immunology, and neuroscience fields; as such, it has benefited from quantum leaps in molecular biology, biochemistry, and imaging techniques, and the emergence of the genomic and proteomic era. Combinations of these techniques have revealed new and exciting insights that explain how cells adhere and move, how the migration of multiple cells are coordinated and regulated, and how the cells interact with neighboring cells and/or react to changes in their microenvironment. This introduction provides a primer of the molecular and cellular insights, particularly the signaling networks, which control the migration of individual cells as well as collective migrations. The rest of the chapters are devoted to describe in detail some of the most salient technical advances that have illuminated the field of cell migration in recent years.

VEGF₁₆₄ differentially regulates neutrophil and T cell adhesion through ItgaL- and ItgaM-dependent mechanisms

Leukocyte recruitment to inflamed tissues is the cornerstone of inflammatory responses and the driving force behind the establishment of inflammatory bowel disease, consisting of Crohn's disease and ulcerative colitis. It has been reported that angiogenic cytokines contribute to this inflammatory response that facilitates the chronic nature of disease. We have previously reported (Goebel S, Huang M, Davis WC, Jennings M, Siahaan TJ, Alexander JS, Kevil CG. Am J Physiol Gastrointest Liver Physiol 290: G648-G654, 2006) that vascular endothelial growth factor (VEGF)-A can stimulate neutrophil adhesion to colon microvascular endothelial cells in a β₂-integrin (Itgb2)-dependent manner. However, it is not known which of the specific leukocyte integrins are critical for VEGF-A-dependent neutrophil and T cell recruitment. Here we examine the differential importance of either α-integrin (Itga)L or ItgaM in governing neutrophil and T cell adhesion to VEGF-A-activated colonic endothelium. Using an in vitro parallel-plate flow chamber model, we found that genetic deficiency of ItgaM completely blunted neutrophil adhesion to VEGF-A-stimulated endothelium, whereas ItgaL deficiency only partly blocked neutrophil adhesion. Deficiency of ItgaM did significantly decrease neutrophil rolling, whereas deficiency of ItgaL did not. We found that genetic deficiency of either ItgaL or ItgaM did significantly blunt T cell adhesion to VEGF-A-stimulated colon endothelium. We also found that genetic deficiency of these Itgas significantly attenuated T cell rolling behavior. Lastly, we examined whether VEGF-A-mediated leukocyte recruitment occurred through different VEGF receptor (VEGFR) pathways and found that VEGFR2 activation regulates neutrophil recruitment, whereas both VEGFR1 and VEGFR2 modulate T cell recruitment. Together, these data identify differential molecular mechanisms of VEGF-A-mediated leukocyte recruitment.

Noninvasive measurements of integrin microclustering under altered membrane cholesterol levels

Reported herein is a method that can be used to study the role of cholesterol in the microclustering of a ubiquitous class of membrane receptors, termed integrins. Integrin microclustering was measured using a fluorescence resonance energy transfer assay that does not require direct attachment of fluorescent donors or acceptors onto the integrins, and thus minimizes unwanted perturbations to integrin clustering. Membrane cholesterol levels were reduced using methyl-beta-cyclodextrin (mbetaCD), as confirmed by Amplex Red assays of total cellular lipid or plasma membrane lipid extract. Subsequent changes in integrin microclustering were measured in cells expressing wild-type (WT) or mutant integrins. Although less integrin microclustering was measured after 27% membrane cholesterol depletion in a cell line expressing WT integrins, there was no statistically significant change for cells expressing alpha-cytoplasmic integrin mutants after a 45% reduction in plasma membrane cholesterol, and a significant increase in clustering for cells expressing ligand-binding domain integrin mutants after a 57% decrease in membrane cholesterol. These results are explained by differences in WT and mutant integrin partitioning into lipid nanodomains. Restoration of original cholesterol levels was used to confirm that the measured changes in membrane properties were cholesterol-dependent. No correlations between lipid diffusion and integrin microclustering were measured by means of fluorescence recovery after photobleaching using a fluorescent lipid mimetic. Similar lipid diffusion coefficients were measured after cholesterol depletion, irrespective of the integrins being expressed.

Cell physician: reading cell motion: a mathematical diagnostic technique through analysis of single cell motion

Cell motility is an essential phenomenon in almost all living organisms. It is natural to think that behavioral or shape changes of a cell bear information about the underlying mechanisms that generate these changes. Reading cell motion, namely, understanding the underlying biophysical and mechanochemical processes, is of paramount importance. The mathematical model developed in this paper determines some physical features and material properties of the cells locally through analysis of live cell image sequences and uses this information to make further inferences about the molecular structures, dynamics, and processes within the cells, such as the actin network, microdomains, chemotaxis, adhesion, and retrograde flow. The generality of the principals used in formation of the model ensures its wide applicability to different phenomena at various levels. Based on the model outcomes, we hypothesize a novel biological model for collective biomechanical and molecular mechanism of cell motion.

Regulation of adhesion behavior of murine macrophage using supported lipid membranes displaying tunable mannose domains

Highly uniform, strongly correlated domains of synthetically designed lipids can be incorporated into supported lipid membranes. The systematic characterization of membranes displaying a variety of domains revealed that the equilibrium size of domains significantly depends on the length of fluorocarbon chains, which can be quantitatively interpreted within the framework of an equivalent dipole model. A mono-dispersive, narrow size distribution of the domains enables us to treat the inter-domain correlations as two-dimensional colloidal crystallization and calculate the potentials of mean force. The obtained results demonstrated that both size and inter-domain correlation can precisely be controlled by the molecular structures. By coupling α-D-mannose to lipid head groups, we studied the adhesion behavior of the murine macrophage (J774A.1) on supported membranes. Specific adhesion and spreading of macrophages showed a clear dependence on the density of functional lipids. The obtained results suggest that such synthetic lipid domains can be used as a defined platform to study how cells sense the size and distribution of functional molecules during adhesion and spreading.

Regulation of Integrin β 1 recycling to lipid rafts by Rab1a to promote cell migration

Rab1a is a member of the Rab family of small GTPases with a well characterized function in the regulation of vesicle trafficking from the endoplasmic reticulum to the Golgi apparatus and within Golgi compartments. The integrin family heterodimeric transmembrane proteins serve as major receptors for extracellular matrix proteins, which play essential roles in cell adhesion and migration. Although effects on intracellular trafficking of integrins or other key cargos by Rab1a could influence cell migration, the regulatory mechanisms linking Rab1a to cell migration are not well understood. Here, we report identification of Rab1a as a novel regulator of cell migration using an unbiased RNAi screen targeting GTPases. Inhibition of Rab1a reduced integrin-mediated cell adhesion and spreading on fibronectins, reduced integrin β1 localization to lipid rafts, and decreased recycling of integrin β1 to the plasma membrane. Analysis of Rab1a effector molecules showed that p115 mediated Rab1a regulation of integrin recycling and lipid raft localization in cell migration. Taken together, these results suggest a novel function for Rab1a in the regulation of cell migration through controlling integrin β1 recycling and localization to lipid rafts via a specific downstream effector pathway.

Bordetella adenylate cyclase toxin mobilizes its beta2 integrin receptor into lipid rafts to accomplish translocation across target cell membrane in two steps

Bordetella adenylate cyclase toxin (CyaA) binds the α_Mβ₂ integrin (CD11b/CD18, Mac-1, or CR3) of myeloid phagocytes and delivers into their cytosol an adenylate cyclase (AC) enzyme that converts ATP into the key signaling molecule cAMP. We show that penetration of the AC domain across cell membrane proceeds in two steps. It starts by membrane insertion of a toxin ‘translocation intermediate’, which can be ‘locked’ in the membrane by the 3D1 antibody blocking AC domain translocation. Insertion of the ‘intermediate’ permeabilizes cells for influx of extracellular calcium ions and thus activates calpain-mediated cleavage of the talin tether. Recruitment of the integrin-CyaA complex into lipid rafts follows and the cholesterol-rich lipid environment promotes translocation of the AC domain across cell membrane. AC translocation into cells was inhibited upon raft disruption by cholesterol depletion, or when CyaA mobilization into rafts was blocked by inhibition of talin processing. Furthermore, CyaA mutants unable to mobilize calcium into cells failed to relocate into lipid rafts, and failed to translocate the AC domain across cell membrane, unless rescued by Ca²⁺ influx promoted in trans by ionomycin or another CyaA protein. Hence, by mobilizing calcium ions into phagocytes, the ‘translocation intermediate’ promotes toxin piggybacking on integrin into lipid rafts and enables AC enzyme delivery into host cytosol.

The adenylate cyclase toxin (CyaA) of pathogenic Bordetellae eliminates the first line of host innate immune defense. It penetrates myeloid phagocytes, such as neutrophils, macrophage or dendritic cells, and subverts their signaling by catalyzing an extremely rapid conversion of intracellular ATP to the key signaling molecule cAMP. This efficiently inhibits the oxidative burst and complement-mediated opsonophagocytic killing of bacteria, thus enabling the pathogen to colonize host airways. We show that translocation of CyaA into phagocyte cytosol occurs in two steps. The toxin first binds the integrin CD11b/CD18 and inserts into phagocyte membrane to mediate influx of calcium ions into cells. This promotes relocation of the toxin-receptor complex into specific lipid microdomains within cell membrane called rafts. The increased concentrations of cholesterol within rafts and their particular lipid organization then support translocation of the adenylate cyclase enzyme directly into the cytoplasmic compartment of cells. The mechanism of CyaA penetration into cells sets a new paradigm for membrane translocation of toxins of the RTX family.

T cell receptor "inside-out" pathway via signaling module SKAP1-RapL regulates T cell motility and interactions in lymph nodes

Although essential for T cell function, the identity of the T cell receptor "inside-out" pathway for lymphocyte function-associated antigen 1 (LFA-1) adhesion has proved elusive. Here, we define the "inside-out" pathway mediated by N-terminal SKAP1 (SKAP-55) domain binding to the C-terminal SARAH domain of RapL. TcR induced Rap1-RapL complex formation and LFA-1 binding failed to occur in Skap1(-/-) primary T cells. SKAP1 generated a SKAP1-RapL-Rap1 complex that bound to LFA-1, whereas a RapL mutation (L224A) that abrogated SKAP1 binding without affecting MST1 disrupted component colocalization in vesicles as well as T cell-dendritic cell (DC) conjugation. RapL expression also "slowed" T cell motility in D011.10 transgenic T cells in lymph nodes (LNs), an effect reversed by the L224A mutation with reduced dwell times between T cells and DCs. Overall, our findings define a TCR "inside-out" pathway via N-SKAP1-C-RapL that regulates T cell adhesion, motility, and arrest times with DCs in LNs.

CD24: from A to Z

As a testament to the importance of CD24, researchers with diverse interests, including adaptive immunity, inflammation, autoimmune diseases and cancer, have encountered CD24. CD24 is overexpressed in many cancers and appears oncogenic. In the adaptive immune response, CD24 is a redundant costimulatory molecule in costimulation-rich lymphoid organs but is essential in selected target organs tested, such as brain and skin. More recent studies suggest it may have a role in discriminating danger and pathogen-associated molecular patterns by dendritic cells. The biology of CD24 is intriguing but poorly understood. Here we summarize the major findings associated with CD24 to stimulate new ideas for further research that may reveal the underlying link among the diverse processes mediated by CD24.

PI 3-Kinase p110β regulation of platelet integrin α(IIb)β3

Hemopoietic cells express relatively high levels of the type I phosphoinositide (PI) 3-kinase isoforms, with p110δ and γ exhibiting specialized signaling functions in neutrophils, monocytes, mast cells, and lymphocytes. In platelets, p110β appears to be the dominant PI 3-kinase isoform regulating platelet activation, irrespective of the nature of the primary platelet activating stimulus. Based on findings with isoform-selective p110β pharmacological inhibitors and more recently with p110β-deficient platelets, p110β appears to primarily signal downstream of G(i)- and tyrosine kinase-coupled receptors. Functionally, inhibition of p110β kinase function leads to a marked defect in integrin α(IIb)β₃ adhesion and reduced platelet thrombus formation in vivo. This defect in platelet adhesive function is not associated with increased bleeding, suggesting that therapeutic targeting of p110β may represent a safe approach to reduce thrombotic complications in patients with cardiovascular disease.

Phosphoinositide 3-kinase p110 beta regulates integrin alpha IIb beta 3 avidity and the cellular transmission of contractile forces

Phosphoinositide (PI) 3-kinase (PI3K) signaling processes play an important role in regulating the adhesive function of integrin alpha(IIb)beta(3), necessary for platelet spreading and sustained platelet aggregation. PI3K inhibitors are effective at reducing platelet aggregation and thrombus formation in vivo and as a consequence are currently being evaluated as novel antithrombotic agents. PI3K regulation of integrin alpha(IIb)beta(3) activation (affinity modulation) primarily occurs downstream of G(i)-coupled and tyrosine kinase-linked receptors linked to the activation of Rap1b, AKT, and phospholipase C. In the present study, we demonstrate an important role for PI3Ks in regulating the avidity (strength of adhesion) of high affinity integrin alpha(IIb)beta(3) bonds, necessary for the cellular transmission of contractile forces. Using knock-out mouse models and isoform-selective PI3K inhibitors, we demonstrate that the Type Ia p110 beta isoform plays a major role in regulating thrombin-stimulated fibrin clot retraction in vitro. Reduced clot retraction induced by PI3K inhibitors was not associated with defects in integrin alpha(IIb)beta(3) activation, actin polymerization, or actomyosin contractility but was associated with a defect in integrin alpha(IIb)beta(3) association with the contractile cytoskeleton. Analysis of integrin alpha(IIb)beta(3) adhesion contacts using total internal reflection fluorescence microscopy revealed an important role for PI3Ks in regulating the stability of high affinity integrin alpha(IIb)beta(3) bonds. These studies demonstrate an important role for PI3K p110 beta in regulating the avidity of high affinity integrin alpha(IIb)beta(3) receptors, necessary for the cellular transmission of contractile forces. These findings may provide new insight into the potential antithrombotic properties of PI3K p110 beta inhibitors.

Hotspots of GPI-anchored proteins and integrin nanoclusters function as nucleation sites for cell adhesion

Recruitment of receptor proteins to lipid rafts has been proposed as an important mechanism to regulate their cellular function. In particular, rafts have been implicated in regulation of integrin-mediated cell adhesion, although the underlying mechanism remains elusive. We used single-molecule near-field optical microscopy (NSOM) with localization accuracy of approximately 3 nm, to capture the spatio-functional relationship between the integrin LFA-1 and raft components (GPI-APs) on immune cells. Dual color nanoscale imaging revealed the existence of a nanodomain GPI-AP subpopulation that further concentrated in regions smaller than 250 nm, suggesting a hierarchical prearrangement of GPI-APs on resting monocytes. We previously demonstrated that in quiescent monocytes, LFA-1 preorganizes in nanoclusters. We now show that integrin nanoclusters are spatially different but reside proximal to GPI-AP nanodomains, forming hotspots on the cell surface. Ligand-mediated integrin activation resulted in an interconversion from monomers to nanodomains of GPI-APs and the generation of nascent adhesion sites where integrin and GPI-APs colocalized at the nanoscale. Cholesterol depletion significantly affected the reciprocal distribution pattern of LFA-1 and GPI-APs in the resting state, and LFA-1 adhesion to its ligand. As such, our data demonstrate the existence of nanoplatforms as essential intermediates in nascent cell adhesion. Since raft association with a variety of membrane proteins other than LFA-1 has been documented, we propose that hotspots regions enriched with raft components and functional receptors may constitute a prototype of nanoscale inter-receptor assembly and correspond to a generic mechanism to offer cells with privileged areas for rapid cellular function and responses to the outside world.

Epac increases melanoma cell migration by a heparan sulfate-related mechanism

Melanoma, the most malignant form of human skin cancer, has a poor prognosis due to its strong metastatic ability. It was recently demonstrated that Epac, an effector molecule of cAMP, is involved in regulating cell migration; however, the role of Epac in melanoma cell migration remains unclear. We thus examined whether Epac regulates cell migration and metastasis of melanoma. Epac activation, by either specific agonist or overexpression of Epac, increased melanoma cell migration. Deletion of endogenous Epac with small interfering RNA decreased basal melanoma cell migration. These data suggested a major role of Epac in melanoma cell migration. Epac-induced cell migration was mediated by translocation of syndecan-2, a cell-surface heparan sulfate proteoglycan, to lipid rafts. This syndecan-2 translocation was regulated by tubulin polymerization via the Epac/phosphoinositol-3 kinase pathway. Epac-induced cell migration was also regulated by the production of heparan sulfate, a major extracellular matrix. Epac-induced heparan sulfate production was attributable to the increased expression of N-deacetylase/N-sulfotransferase-1 (NDST-1) accompanied by an increased NDST-1 translation rate. Finally, Epac overexpression enhanced lung colonization of melanoma cells in mice. Taken together, these data indicate that Epac regulates melanoma cell migration/metastasis mostly via syndecan-2 translocation and heparan sulfate production.

Cytoskeleton-membrane interactions in membrane raft structure

Cell membranes are structurally heterogeneous, composed of discrete domains with unique physical and biological properties. Membrane domains can form through a number of mechanisms involving lipid-lipid and protein-lipid interactions. One type of membrane domain is the cholesterol-dependent membrane raft. How rafts form remains a current topic in membrane biology. We review here evidence of structuring of rafts by the cortical actin cytoskeleton. This includes evidence that the actin cytoskeleton associates with rafts, and that many of the structural and functional properties of rafts require an intact actin cytoskeleton. We discuss the mechanisms of the actin-dependent raft organization, and the properties of the actin cytoskeleton in regulating raft-associated signaling events. We end with a discussion of membrane rafts and the actin cytoskeleton in T cell activation, which function synergistically to initiate the adaptive immune response.

Nuclear translocation and signalling of L1-CAM in human carcinoma cells requires ADAM10 and presenilin/gamma-secretase activity

L1-CAM (L1 cell-adhesion molecule), or more simply L1, plays an important role in the progression of human carcinoma. Overexpression promotes tumour-cell invasion and motility, growth in nude mice and tumour metastasis. It is feasible that L1-dependent signalling contributes to these effects. However, little is known about its mechanism in tumour cells. We reported previously that L1 is cleaved by ADAM (a disintegrin and metalloprotease) and that the cytoplasmic part is essential for L1 function. Here we analysed more closely the role of proteolytic cleavage in L1-mediated nuclear signalling. Using OVMz carcinoma cells and L1-transfected cells as a model, we found that ADAM10-mediated cleavage of L1 proceeds in lipid raft and non-raft domains. The cleavage product, L1-32, is further processed by PS (presenilin)/gamma-secretase to release L1-ICD, an L1 intracellular domain of 28 kDa. Overexpression of dominant-negative PS1 or use of a specific gamma-secretase inhibitor leads to an accumulation of L1-32. Fluorescence and biochemical analysis revealed a nuclear localization for L1-ICD. Moreover, inhibition of ADAM10 and/or gamma-secretase blocks nuclear translocation of L1-ICD and L1-dependent gene regulation. Overexpression of recombinant L1-ICD mediates gene regulation in a similar manner to full-length L1. Our results establish for the first time that regulated proteolytic processing by ADAM10 and PS/gamma-secretase is essential for the nuclear signalling of L1 in human carcinoma cell lines.

The ins and outs of leukocyte integrin signaling

Integrins are the principal cell adhesion receptors that mediate leukocyte migration and activation in the immune system. These receptors signal bidirectionally through the plasma membrane in pathways referred to as inside-out and outside-in signaling. Each of these pathways is mediated by conformational changes in the integrin structure. Such changes allow high-affinity binding of the receptor with counter-adhesion molecules on the vascular endothelium or extracellular matrix and lead to association of the cytoplasmic tails of the integrins with intracellular signaling molecules. Leukocyte functional responses resulting from outside-in signaling include migration, proliferation, cytokine secretion, and degranulation. Here, we review the key signaling events that occur in the inside-out versus outside-in pathways, highlighting recent advances in our understanding of how integrins are activated by a variety of stimuli and how they mediate a diverse array of cellular responses.

Clustering T-cell GM1 lipid rafts increases cellular resistance to shear on fibronectin through changes in integrin affinity and cytoskeletal dynamics

Lipid rafts are small laterally mobile microdomains that are highly enriched in lymphocyte signaling molecules. GM1 gangliosides are a common lipid raft component and have been shown to be important in many T cell functions. The aggregation of specific GM1 lipid rafts can control many T cell activation events, including their novel association with T cell integrins. We found that clustering GM1 lipid rafts can regulate β1 integrin function. This was apparent through increased resistance to shear flow dependent detachment of T cells adherent to the α4β1 and α5β1 integrin ligand fibronectin (FN). Adhesion strengthening as a result of clustering GM1 enriched lipid rafts correlated with increased cellular rigidity and morphology through the localization of cortical F-actin, the resistance to shear induced cell stretching, and an increase in the surface area and symmetry of the contact area between the cell surface and adhesive substrate. Furthermore, clustering GM1 lipid rafts could initiate integrin “inside-out” signaling mechanisms. This was seen through increased integrin-cytoskeleton associations and enhanced soluble binding of FN and VCAM-1 suggesting the induction of high affinity integrin conformations. The activation of these adhesion strengthening characteristics appear to be specific for the aggregation of GM1 lipid rafts as the aggregation of the heterogeneous raft associated molecule CD59 failed to activate these functions. These findings indicate a novel mechanism to signal to β1 integrins and to activate adhesion strengthening processes.

Human decay-accelerating factor and CEACAM receptor-mediated internalization and intracellular lifestyle of Afa/Dr diffusely adhering Escherichia coli in epithelial cells

We used transfected epithelial CHO-B2 cells as a model to identify the mechanism mediating internalization of Afa/Dr diffusely adhering Escherichia coli. We provide evidence that neither the alpha5 or beta1 integrin subunits nor alpha5beta1 integrin functioned as a receptor mediating the adhesion and/or internalization of Dr or Afa-III fimbria-positive bacteria. We also demonstrated that (i) whether or not the AfaD or DraD invasin subunits were present, there was no difference in the cell association and entry of bacteria and that (ii) DraE or AfaE-III adhesin subunits are necessary and sufficient to promote the receptor-mediated bacterial internalization into epithelial cells expressing human decay-accelerating factor (DAF), CEACAM1, CEA, or CEACAM6. Internalization of Dr fimbria-positive E. coli within CHO-DAF, CHO-CEACAM1, CHO-CEA, or CHO-CEACAM6 cells occurs through a microfilament-independent, microtubule-dependent, and lipid raft-dependent mechanism. Wild-type Dr fimbria-positive bacteria survived better within cells expressing DAF than bacteria internalized within CHO-CEACAM1, CHO-CEA, or CHO-CEACAM6 cells. In DAF-positive cells, internalized Dr fimbria-positive bacteria were located in vacuoles that contained more than one bacterium, displaying some of the features of late endosomes, including the presence of Lamp-1 and Lamp-2, and some of the features of CD63 proteins, but not of cathepsin D, and were acidic. No interaction between Dr fimbria-positive-bacterium-containing vacuoles and the autophagic pathway was observed.

CD24 induces localization of beta1 integrin to lipid raft domains

The expression of the glycosyl phosphatidylinositol (GPI)-anchored protein CD24 correlates with poor prognosis in a variety of carcinomas. However, little is known about the cellular mechanisms of the CD24-mediated effects. In this study, we present evidence that CD24 affects the lateral localization of beta1 integrin. Using stably CD24-transfected A125 and MDA-MB-435S carcinoma cells we show that CD24 augments beta1-dependent cell motility and stimulates transmigration and invasion across a monolayer of endothelial cells. Furthermore, as demonstrated by sucrose density gradient centrifugation and Western Blot analysis, CD24 recruits beta1 integrin into lipid raft domains. We suggest that CD24 acts as a gate-keeper for lipid rafts, thereby regulating the activity of integrins and other proteins.

Membranal cholesterol is not required for L-selectin adhesiveness in primary lymphocytes but controls a chemokine-induced destabilization of L-selectin rolling adhesions

Cholesterol-enriched lipid microdomains regulate L-selectin signaling, but the role of membrane cholesterol in L-selectin adhesion is unclear. Arrest chemokines are a subset of endothelial chemokines that rapidly activate leukocyte integrin adhesiveness under shear flow. In the absence of integrin ligands, these chemokines destabilize L-selectin-mediated leukocyte rolling. In the present study, we investigated how cholesterol extraction from the plasma membrane of peripheral blood T or B cells affects L-selectin adhesions and their destabilization by arrest chemokines. Unlike the Jurkat T cell line, whose L-selectin-mediated adhesion is cholesterol dependent, in primary human PBLs and in murine B cells and B cell lines, cholesterol depletion did not impair any intrinsic adhesiveness of L-selectin, consistent with low selectin partitioning into lipid rafts in these cells. However, cholesterol raft disruption impaired the ability of two arrest chemokines, CXCL12 and CXCL13, but not of a third arrest chemokine, CCL21, to destabilize L-selectin-mediated rolling of T lymphocytes. Actin capping by brief incubation with cytochalasin D impaired the ability of all three chemokines to destabilize L-selectin rolling. Blocking of the actin regulatory phosphatidylinositol lipid, phosphatidylinositol 4,5-bisphosphate, did not affect chemokine-mediated destabilization of L-selectin adhesions. Collectively, our results suggest that L-selectin adhesions are inhibited by actin-associated, cholesterol-stabilized assemblies of CXCL12- and CXCL13-binding receptors on both T and B lymphocytes. Thus, the regulation of L-selectin by cholesterol-enriched microdomains varies with the cell type as well as with the identity of the destabilizing chemokine.

Silencing of insulin-like growth factor-binding protein-2 in human glioblastoma cells reduces both invasiveness and expression of progression-associated gene CD24

Glioblastoma multiforme (GBM) is a malignant brain tumor characterized by rapid growth and extensive invasiveness. Overexpression of insulin-like growth factor-binding protein-2 (IGFBP-2) has been reported in GBM. However, it remains to be determined how IGFBP-2 is involved in the progression of GBM. We utilized short hairpin-RNA (shRNA) expression retroviral vectors to inactivate the IGFBP-2 gene permanently in two human GBM cell lines, U251 and YKG-1. The stable knockdown of IGFBP-2 resulted in decreased invasiveness, decreased saturation density of the cells in vitro, and decreased tumorigenicity in nude mice. Transcriptional profiling of both lines revealed several genes that were significantly down-regulated by inactivation of IGFBP-2. One such gene was CD24, which has been implicated in progression of various cancers. Indeed, CD24 was expressed in most GBM cases and the inactivation of CD24 in GBM cells suppressed cellular invasiveness, as was the case for IGFBP-2. Forced overexpression of CD24 led to increased invasiveness of both IGFBP-2-inactivated GBM cell lines and also A172, a human GBM cell line with low endogenous CD24. Further supporting the inter-relationship between IGFBP-2 and CD24, knockdown of IGFBP-2 suppressed the CD24 promoter activity. Moreover, both CD24 promoter activity and in vitro invasiveness were restored in knockdown cells by transfection with an IGFBP-2 expression plasmid. These results indicate that CD24 is modulated by IGFBP-2 and contributes to IGFBP-2-enhanced invasiveness of GBM cells.

On the role of CD26 in CD4 memory T cells

We studied an in vivo mouse model to evaluate the relationships between CD26--a glycoprotein with dipeptidyl peptidase IV (DPP-IV) activity implicated in the regulation of immune functions--and T cells expressing the effector/memory phenotype CD45RB. We report that CD26 does not define a differentiation stage of CD4 T cells because the density and frequency of CD26 on CD4 T cells from the spleen, inguinal and mesenteric lymph node was similar within the CD45RB+ (naïve) and CD45RB- (antigen primed) subsets. This observation was confirmed using CD4 T cells from a T-cell receptor transgenic (tg) model. CD4 tg T cells specific for ovalbumin (OVA) were adoptively transferred and challenged in vivo with antigen. CD26 expression was the same on naive and antigen-stimulated CD4 T cells. Depleting CD4 T cells with an anti-CD4 antibody preferentially depleted the CD45RB+ subset. In CD4 depleted animals CD26 expression was not altered on the CD45RB- subset but the density of CD26 was marginally increased on the remaining CD45RB+ CD4 T cells. The results suggest that, unlike the human, CD26 in the mouse was not directly linked with T cell activation.