RKIKK motif in the intracellular domain is critical for spatial and dynamic organization of ICAM-1: functional implication for the leukocyte adhesion and transmigration

Exploring the evolving function of soluble intercellular adhesion molecule-1 in junction dynamics during spermatogenesis

Intercellular adhesion molecule-1 (ICAM-1) is a transmembrane glycoprotein expressed on immune, endothelial, and epithelial cells. Its ectodomain can be proteolytically cleaved to release a circulating soluble form called sICAM-1. Clinical studies demonstrate sICAM-1 is upregulated in various diseases and associated with disease severity. Research has identified sICAM-1 as a regulator of the blood-testis barrier (BTB) and spermatogenesis. Overexpression of sICAM-1 weakened the BTB in vitro and in vivo, downregulated junction proteins including N-cadherin, γ-catenin, and connexin 43, and caused germ cell loss. This contrasts with barrier-strengthening effects of membrane-bound ICAM-1. sICAM-1 may act as a molecular switch enabling germ cells to open BTB and Sertoli-germ cell adhesion for transport across the seminiferous epithelium. While the mechanism remains unclear, reduced SRC family kinase (SFK) signaling was observed following sICAM-1 overexpression. SRC promotes BTB protein endocytosis and degradation, influences cytoskeletal dynamics, and affects cell polarity. As sICAM-1 overexpression phenocopies SRC inhibition, SRC may operate downstream of sICAM-1 in regulating BTB dynamics and spermatogenesis. Investigating sICAM-1's structure-function regions and downstream targets will elucidate the molecular mechanisms of junction disruption. This knowledge could enable strategies targeting sICAM-1/SRC to modulate BTB permeability and treat male infertility or diseases involving endothelial/epithelial barrier dysfunction.

Primary adhered neutrophils increase JNK1-MARCKSL1-mediated filopodia to promote secondary neutrophil transmigration

During inflammation, leukocytes extravasate the vasculature to areas of inflammation in a process termed transendothelial migration. Previous research has shown that transendothelial migration hotspots exist, areas in the vasculature that are preferred by leukocytes to cross. Several factors that contribute to hotspot-mediated transmigration have been proposed already, but whether one leukocyte transmigration hotspot can be used subsequently by a second wave of leukocytes and thereby can increase the efficiency of leukocyte transmigration is not well understood. Here, we show that primary neutrophil adhesion to the endothelium triggers endothelial transmigration hotspots, allowing secondary neutrophils to cross the endothelium more efficiently. Mechanistically, we show that primary neutrophil adhesion increases the number of endothelial apical filopodia, resulting in an increase in the number of adherent secondary neutrophils. Using fluorescence resonance energy transfer (FRET)-based biosensors, we found that neutrophil adhesion did not trigger the activity of the small GTPase Cdc42. We used kinase translocation reporters to study the activity of mitogen-activated protein (MAP) kinases and Akt in endothelial cells on a single-cell level with a high temporal resolution during the process of leukocyte transmigration and found that c-Jun N-terminal kinase (JNK) is rapidly activated upon neutrophil adhesion, whereas extracellular regulated kinase (ERK), p38, and Akt are not. Additionally, we show that short-term chemical inhibition of endothelial JNK successfully prevents the adhesion of neutrophils to the endothelium. Furthermore, we show that neutrophil-induced endothelial JNK1 but not JNK2 increases the formation of filopodia and thereby the adhesion of secondary neutrophils. JNK1 needs its downstream substrate MARCKSL1 to trigger additional apical filopodia and consequently neutrophil adhesion. Overall, our data show that primary neutrophils can trigger the endothelial transmigration hotspot by activating JNK1 and MARCKSL1 to induce filopodia that trigger more neutrophils to transmigrate at the endothelial hotspot area.

Endothelial transmigration hotspots limit vascular leakage through heterogeneous expression of ICAM-1

Upon inflammation, leukocytes leave the circulation by crossing the endothelial monolayer at specific transmigration "hotspot" regions. Although these regions support leukocyte transmigration, their functionality is not clear. We found that endothelial hotspots function to limit vascular leakage during transmigration events. Using the photoconvertible probe mEos4b, we traced back and identified original endothelial transmigration hotspots. Using this method, we show that the heterogeneous distribution of ICAM-1 determines the location of the transmigration hotspot. Interestingly, the loss of ICAM-1 heterogeneity either by CRISPR/Cas9-induced knockout of ICAM-1 or equalizing the distribution of ICAM-1 in all endothelial cells results in the loss of TEM hotspots but not necessarily in reduced TEM events. Functionally, the loss of endothelial hotspots results in increased vascular leakage during TEM. Mechanistically, we demonstrate that the 3 extracellular Ig-like domains of ICAM-1 are crucial for hotspot recognition. However, the intracellular tail of ICAM-1 and the 4th Ig-like dimerization domain are not involved, indicating that intracellular signaling or ICAM-1 dimerization is not required for hotspot recognition. Together, we discovered that hotspots function to limit vascular leakage during inflammation-induced extravasation.

Expanding TOR Complex 2 Signaling: Emerging Regulators and New Connections

Almost three decades after its seminal discovery, our understanding of the remarkable TOR pathway continues to expand. As a TOR complex, TORC2 lies at the nexus of many signaling pathways and directs a diverse array of fundamental processes such as cell survival, proliferation, and metabolism by integrating environmental and intracellular cues. The dysregulation of TORC2 activity disrupts cellular homeostasis and leads to many pathophysiological conditions. With continued efforts at mapping the signaling landscape, the pace of discovery in TORC2 regulation has been accelerated in recent years. Consequently, emerging evidence has expanded the repertoire of upstream regulators and has revealed unexpected diversity in the modes of TORC2 regulation. Multiple environmental cues and plasma membrane proteins that fine-tune TORC2 activity are unfolding. Furthermore, TORC2 signaling is intricately intertwined with other major signaling pathways. Therefore, feedback and crosstalk regulation also extensively modulate TORC2. In this context, we provide a comprehensive overview of revolutionary concepts regarding emerging regulators of TORC2 and discuss evidence of feedback and crosstalk regulation that shed new light on TORC2 biology.

Neutrophil transendothelial migration hotspots - mechanisms and implications

During inflammation, leukocytes circulating in the blood stream exit the vasculature in a process called leukocyte transendothelial migration (TEM). The current paradigm of this process comprises several well-established steps, including rolling, adhesion, crawling, diapedesis and sub-endothelial crawling. Nowadays, the role of the endothelium in transmigration is increasingly appreciated. It has been established that leukocyte exit sites on the endothelium and in the pericyte layer are in fact not random but instead may be specifically recognized by migrating leukocytes. Here, we review the concept of transmigration hotspots, specific sites in the endothelial and pericyte layer where most transmigration events take place. Chemokine cues, adhesion molecules and membrane protrusions as well as physical factors, such as endothelial junction stability, substrate stiffness, the presence of pericytes and basement membrane composition, may all contribute to local hotspot formation to facilitate leukocytes exiting the vasculature. In this Review, we discuss the biological relevance of such hotspots and put forward multiple mechanisms and factors that determine a functional TEM hotspot.

ERM Proteins at the Crossroad of Leukocyte Polarization, Migration and Intercellular Adhesion

Ezrin, radixin and moesin proteins (ERMs) are plasma membrane (PM) organizers that link the actin cytoskeleton to the cytoplasmic tail of transmembrane proteins, many of which are adhesion receptors, in order to regulate the formation of F-actin-based structures (e.g., microspikes and microvilli). ERMs also effect transmission of signals from the PM into the cell, an action mainly exerted through the compartmentalized activation of the small Rho GTPases Rho, Rac and Cdc42. Ezrin and moesin are the ERMs more highly expressed in leukocytes, and although they do not always share functions, both are mainly regulated through phosphatidylinositol 4,5-bisphosphate (PIP₂) binding to the N-terminal band 4.1 protein-ERM (FERM) domain and phosphorylation of a conserved Thr in the C-terminal ERM association domain (C-ERMAD), exerting their functions through a wide assortment of mechanisms. In this review we will discuss some of these mechanisms, focusing on how they regulate polarization and migration in leukocytes, and formation of actin-based cellular structures like the phagocytic cup-endosome and the immune synapse in macrophages/neutrophils and lymphocytes, respectively, which represent essential aspects of the effector immune response.

Passing the Vascular Barrier: Endothelial Signaling Processes Controlling Extravasation

A central function of the vascular endothelium is to serve as a barrier between the blood and the surrounding tissue of the body. At the same time, solutes and cells have to pass the endothelium to leave or to enter the bloodstream to maintain homeostasis. Under pathological conditions, for example, inflammation, permeability for fluid and cells is largely increased in the affected area, thereby facilitating host defense. To appropriately function as a regulated permeability filter, the endothelium uses various mechanisms to allow solutes and cells to pass the endothelial layer. These include transcellular and paracellular pathways of which the latter requires remodeling of intercellular junctions for its regulation. This review provides an overview on endothelial barrier regulation and focuses on the endothelial signaling mechanisms controlling the opening and closing of paracellular pathways for solutes and cells such as leukocytes and metastasizing tumor cells.

Outside-in HLA class I signaling regulates ICAM-1 clustering and endothelial cell-monocyte interactions via mTOR in transplant antibody-mediated rejection

Antibody-mediated rejection (AMR) resulting in transplant allograft vasculopathy (TAV) is the major obstacle for long-term survival of solid organ transplants. AMR is caused by donor-specific antibodies to HLA, which contribute to TAV by initiating outside-in signaling transduction pathways that elicit monocyte recruitment to activated endothelium. Mechanistic target of rapamycin (mTOR) inhibitors can attenuate TAV; therefore, we sought to understand the mechanistic underpinnings of mTOR signaling in HLA class I Ab-mediated endothelial cell activation and monocyte recruitment. We used an in vitro model to assess monocyte binding to HLA I Ab-activated endothelial cells and found mTOR inhibition reduced ezrin/radixin/moesin (ERM) phosphorylation, intercellular adhesion molecule 1 (ICAM-1) clustering, and monocyte firm adhesion to HLA I Ab-activated endothelium. Further, in a mouse model of AMR, in which C57BL/6. RAG1^-/- recipients of BALB/c cardiac allografts were passively transferred with donor-specific MHC I antibodies, mTOR inhibition significantly reduced vascular injury, ERM phosphorylation, and macrophage infiltration of the allograft. Taken together, these studies indicate mTOR inhibition suppresses ERM phosphorylation in endothelial cells, which impedes ICAM-1 clustering in response to HLA class I Ab and prevents macrophage infiltration into cardiac allografts. These findings indicate a novel therapeutic application for mTOR inhibitors to disrupt endothelial cell-monocyte interactions during AMR.

Action and Traction: Cytoskeletal Control of Receptor Triggering at the Immunological Synapse

It is well known that F-actin dynamics drive the micron-scale cell shape changes required for migration and immunological synapse (IS) formation. In addition, recent evidence points to a more intimate role for the actin cytoskeleton in promoting T cell activation. Mechanotransduction, the conversion of mechanical input into intracellular biochemical changes, is thought to play a critical role in several aspects of immunoreceptor triggering and downstream signal transduction. Multiple molecules associated with signaling events at the IS have been shown to respond to physical force, including the TCR, costimulatory molecules, adhesion molecules, and several downstream adapters. In at least some cases, it is clear that the relevant forces are exerted by dynamics of the T cell actomyosin cytoskeleton. Interestingly, there is evidence that the cytoskeleton of the antigen-presenting cell also plays an active role in T cell activation, by countering the molecular forces exerted by the T cell at the IS. Since actin polymerization is itself driven by TCR and costimulatory signaling pathways, a complex relationship exists between actin dynamics and receptor activation. This review will focus on recent advances in our understanding of the mechanosensitive aspects of T cell activation, paying specific attention to how F-actin-directed forces applied from both sides of the IS fit into current models of receptor triggering and activation.

Leukocytes Crossing the Endothelium: A Matter of Communication

Leukocytes cross the endothelial vessel wall in a process called transendothelial migration (TEM). The purpose of leukocyte TEM is to clear the causing agents of inflammation in underlying tissues, for example, bacteria and viruses. During TEM, endothelial cells initiate signals that attract and guide leukocytes to sites of tissue damage. Leukocytes react by attaching to these sites and signal their readiness to move back to endothelial cells. Endothelial cells in turn respond by facilitating the passage of leukocytes while retaining overall integrity. In this review, we present recent findings in the field and we have endeavored to synthesize a coherent picture of the intricate interplay between endothelial cells and leukocytes during TEM.

Nanoscale Imaging Reveals a Tetraspanin-CD9 Coordinated Elevation of Endothelial ICAM-1 Clusters

Endothelial barriers have a central role in inflammation as they allow or deny the passage of leukocytes from the vasculature into the tissue. To bind leukocytes, endothelial cells form adhesive clusters containing tetraspanins and ICAM-1, so-called endothelial adhesive platforms (EAPs). Upon leukocyte binding, EAPs evolve into docking structures that emanate from the endothelial surface while engulfing the leukocyte. Here, we show that TNF-α is sufficient to induce apical protrusions in the absence of leukocytes. Using advanced quantitation of atomic force microscopy (AFM) recordings, we found these structures to protrude by 160 ± 80 nm above endothelial surface level. Confocal immunofluorescence microscopy proved them positive for ICAM-1, JAM-A, tetraspanin CD9 and f-actin. Microvilli formation was inhibited in the absence of CD9. Our findings indicate that stimulation with TNF-α induces nanoscale changes in endothelial surface architecture and that—via a tetraspanin CD9 depending mechanism—the EAPs rise above the surface to facilitate leukocyte capture.

Acid Sphingomyelinase-Derived Ceramide Regulates ICAM-1 Function during T Cell Transmigration across Brain Endothelial Cells

Multiple sclerosis (MS) is a chronic demyelinating disorder of the CNS characterized by immune cell infiltration across the brain vasculature into the brain, a process not yet fully understood. We previously demonstrated that the sphingolipid metabolism is altered in MS lesions. In particular, acid sphingomyelinase (ASM), a critical enzyme in the production of the bioactive lipid ceramide, is involved in the pathogenesis of MS; however, its role in the brain vasculature remains unknown. Transmigration of T lymphocytes is highly dependent on adhesion molecules in the vasculature such as intercellular adhesion molecule-1 (ICAM-1). In this article, we hypothesize that ASM controls T cell migration by regulating ICAM-1 function. To study the role of endothelial ASM in transmigration, we generated brain endothelial cells lacking ASM activity using a lentiviral shRNA approach. Interestingly, although ICAM-1 expression was increased in cells lacking ASM activity, we measured a significant decrease in T lymphocyte adhesion and consequently transmigration both in static and under flow conditions. As an underlying mechanism, we revealed that upon lack of endothelial ASM activity, the phosphorylation of ezrin was perturbed as well as the interaction between filamin and ICAM-1 upon ICAM-1 clustering. Functionally this resulted in reduced microvilli formation and impaired transendothelial migration of T cells. In conclusion, in this article, we show that ASM coordinates ICAM-1 function in brain endothelial cells by regulating its interaction with filamin and phosphorylation of ezrin. The understanding of these underlying mechanisms of T lymphocyte transmigration is of great value to develop new strategies against MS lesion formation.

Crossing the Vascular Wall: Common and Unique Mechanisms Exploited by Different Leukocyte Subsets during Extravasation

Leukocyte extravasation is one of the essential and first steps during the initiation of inflammation. Therefore, a better understanding of the key molecules that regulate this process may help to develop novel therapeutics for treatment of inflammation-based diseases such as atherosclerosis or rheumatoid arthritis. The endothelial adhesion molecules ICAM-1 and VCAM-1 are known as the central mediators of leukocyte adhesion to and transmigration across the endothelium. Engagement of these molecules by their leukocyte integrin receptors initiates the activation of several signaling pathways within both leukocytes and endothelium. Several of such events have been described to occur during transendothelial migration of all leukocyte subsets, whereas other mechanisms are known only for a single leukocyte subset. Here, we summarize current knowledge on regulatory mechanisms of leukocyte extravasation from a leukocyte and endothelial point of view, respectively. Specifically, we will focus on highlighting common and unique mechanisms that specific leukocyte subsets exploit to succeed in crossing endothelial monolayers.

Na(+) /H(+) exchanger regulatory factor 1 knockout mice have an attenuated hepatic inflammatory response and are protected from cholestatic liver injury

The intercellular adhesion molecule 1 (ICAM-1) is induced in mouse liver after bile duct ligation (BDL) and plays a key role in neutrophil-mediated liver injury in BDL mice. ICAM-1 has been shown to interact with cytoskeletal ezrin-radixin-moesin (ERM) proteins that also interact with the PDZ protein, Na(+) /H(+) exchanger regulatory factor 1 (NHERF-1/EBP50). In NHERF-1(-/-) mice, ERM proteins are significantly reduced in brush-border membranes from kidney and small intestine. ERM knockdown reduces ICAM-1 expression in response to tumor necrosis factor alpha. Here we show that NHERF-1 assembles ERM proteins, ICAM-1 and F-actin into a macromolecule complex that is increased in mouse liver after BDL. Compared to wild-type (WT) mice, both sham-operated and BDL NHERF-1(-/-) mice have lower levels of activated ERM and ICAM-1 protein in the liver accompanied by significantly reduced hepatic neutrophil accumulation, serum alanine aminotransferase, and attenuated liver injury after BDL. However, total bile acid concentrations in serum and liver of sham and BDL NHERF-1(-/-) mice were not significantly different from WT controls, although hepatic tetrahydroxylated bile acids and Cyp3a11 messenger RNA levels were higher in NHERF-1(-/-) BDL mice.

CONCLUSION

NHERF-1 participates in the inflammatory response that is associated with BDL-induced liver injury. Deletion of NHERF-1 in mice leads to disruption of the formation of ICAM-1/ERM/NHERF-1 complex and reduction of hepatic ERM proteins and ICAM-1, molecules that are up-regulated and are essential for neutrophil-mediated liver injury in cholestasis. Further study of the role of NHERF-1 in the inflammatory response in cholestasis and other forms of liver injury should lead to discovery of new therapeutic targets in hepatic inflammatory diseases.

Cell-stiffness-induced mechanosignaling - a key driver of leukocyte transendothelial migration

The breaching of cellular and structural barriers by migrating cells is a driving factor in development, inflammation and tumor cell metastasis. One of the most extensively studied examples is the extravasation of activated leukocytes across the vascular endothelium, the inner lining of blood vessels. Each step of this leukocyte transendothelial migration (TEM) process is regulated by distinct endothelial adhesion receptors such as the intercellular adhesion molecule 1 (ICAM1). Adherent leukocytes exert force on these receptors, which sense mechanical cues and transform them into localized mechanosignaling in endothelial cells. In turn, the function of the mechanoreceptors is controlled by the stiffness of the endothelial cells and of the underlying substrate representing a positive-feedback loop. In this Commentary, we focus on the mechanotransduction in leukocytes and endothelial cells, which is induced in response to variations in substrate stiffness. Recent studies have described the first key proteins involved in these mechanosensitive events, allowing us to identify common regulatory mechanisms in both cell types. Finally, we discuss how endothelial cell stiffness controls the individual steps in the leukocyte TEM process. We identify endothelial cell stiffness as an important component, in addition to locally presented chemokines and adhesion receptors, which guides leukocytes to sites that permit TEM.

Rho-GTPase signaling in leukocyte extravasation: an endothelial point of view

Leukocyte transendothelial migration (TEM) is one of the crucial steps during inflammation. A better understanding of the key molecules that regulate leukocyte extravasation aids to the development of novel therapeutics for treatment of inflammation-based diseases, such as atherosclerosis and rheumatoid arthritis. The adhesion molecules ICAM-1 and VCAM-1 are known as central mediators of TEM. Clustering of these molecules by their leukocytic integrins initiates the activation of several signaling pathways within the endothelium, including a rise in intracellular Ca (2+), activation of several kinase cascades, and the activation of Rho-GTPases. Activation of Rho-GTPases has been shown to control adhesion molecule clustering and the formation of apical membrane protrusions that embrace adherent leukocytes during TEM. Here, we discuss the potential regulatory mechanisms of leukocyte extravasation from an endothelial point of view, with specific focus on the role of the Rho-GTPases.

MUC1-mediated motility in breast cancer: a review highlighting the role of the MUC1/ICAM-1/Src signaling triad

Breast cancer is the most common cancer in women with the leading cause of death being metastasis, the spread of cancer to distant organs. For those patients with high-risk estrogen receptor positive (ER+) breast cancer, an increased expression of the glycoprotein MUC1 is associated with resistance to anti-hormonal therapy, metastasis and death. Tumor cells may use MUC1 to metastasize by exploiting the vascular adhesion pathways used by leukocytes during the inflammatory response. MUC1 is a type 1 transmembrane protein whose cytoplasmic tail acts as a scaffold for several signaling pathways including the non-receptor kinase Src, a signaling molecule involved in cell differentiation, proliferation, adhesion and motility. This review will highlight our current knowledge of how MUC1/ICAM-1 binding can lead to the recruitment and activation of Src and propose a novel role for lipid raft microdomains in this promigratory signaling. Improved understanding of the mechanism of metastases and the underlying signaling cascade is a prerequisite to the discovery of therapeutic targets to prevent metastasis and death in ER+ breast cancer patients.

The dendritic cell cytoskeleton promotes T cell adhesion and activation by constraining ICAM-1 mobility

Integrity of the dendritic cell (DC) actin cytoskeleton is essential for T cell priming, but the underlying mechanisms are poorly understood. We show that the DC F-actin network regulates the lateral mobility of intracellular cell adhesion molecule 1 (ICAM-1), but not MHCII. ICAM-1 mobility and clustering are regulated by maturation-induced changes in the expression and activation of moesin and α-actinin-1, which associate with actin filaments and the ICAM-1 cytoplasmic domain. Constrained ICAM-1 mobility is important for DC function, as DCs expressing a high-mobility ICAM-1 mutant lacking the cytoplasmic domain exhibit diminished antigen-dependent conjugate formation and T cell priming. These defects are associated with inefficient induction of leukocyte functional antigen 1 (LFA-1) affinity maturation, which is consistent with a model in which constrained ICAM-1 mobility opposes forces on LFA-1 exerted by the T cell cytoskeleton, whereas ICAM-1 clustering enhances valency and further promotes ligand-dependent LFA-1 activation. Our results reveal an important new mechanism through which the DC cytoskeleton regulates receptor activation at the immunological synapse.

Role of Hic-5 in the formation of microvilli-like structures and the monocyte-endothelial interaction that accelerates atherosclerosis

AIMS

The adhesion of circulating monocytes to endothelial cells (ECs) is an early and critical event in the formation of atherosclerotic plaques. Hydrogen peroxide-inducible clone 5 (Hic-5) serves as an adaptor molecule in cell adhesion complexes. However, the role of endothelial Hic-5 in monocyte-EC interaction and atherogenesis remains unclear. We examined the roles of endothelial Hic-5 in monocyte-EC interaction and atherogenesis using mouse models of atherosclerosis and cultured human umbilical vein endothelial cells (HUVECs).

METHODS AND RESULTS

Hic-5 was expressed in ECs, but not in monocytes/macrophages. An ex vivo monocyte adhesion assay revealed that adhesion of THP-1 monocytes to aortas isolated from Apoe(-/-) and LDLR(-/-) mice stimulated by TNF-α or oxidized LDL was suppressed by Hic-5 deficiency. Scanning electron microscopic observations of aortas harvested from Apoe(-/-) mice revealed that TNF-α- or oxidized LDL-induced microvilli-like structures were markedly suppressed by Hic-5 deficiency. Relative Hic-5 deficiency suppressed 60% of the atherosclerotic lesions in aortas from Apoe(-/-) and LDLR(-/-) mice. In contrast, overexpression of Hic-5 in HUVECs promoted induction of microvilli-like structures and adherence of THP-1 cells in an adhesion receptor such as intercellular adhesion molecule-1- and vascular cell adhesion molecule-1-dependent manner.

CONCLUSION

Hic-5 in ECs plays an important role in the formation of microvilli-like structures and in the interaction between ECs and monocytes, leading to monocyte recruitment and subsequent development of atherosclerosis.

Selective acquisition of host-derived ICAM-1 by HIV-1 is a matrix-dependent process

HIV-1 acquires an impressive number of foreign components during its formation. Despite all previous efforts spent studying the nature and functionality of virus-anchored host molecules, the exact mechanism(s) through which such constituents are acquired by HIV-1 is still unknown. However, in the case of ICAM-1, one of the most extensively studied transmembrane proteins found associated with mature virions, the Pr55(Gag) precursor polyprotein appears to be a potential interaction partner. We investigated and characterized at the molecular level the process of ICAM-1 incorporation using initially a Pr55(Gag)-based virus-like particle (VLP) model. Substitution of various domains of Pr55(Gag), such as the nucleocapsid, SP2, or p6, had no effect on the acquisition of ICAM-1. We found that the structural matrix protein (MA) is mandatory for ICAM-1 incorporation within VLPs, and we confirmed this novel observation with the replication-competent HIV-1 molecular clone NL4.3. Additional studies suggest that the C-terminal two-thirds of MA, and especially 13 amino acids positioned inside the fifth α-helix, are important. Moreover, based on three-dimensional (3D) modeling of protein-protein interactions (i.e., protein-protein docking) and further validation by a virus capture assay, we found that a series of acidic residues in the MA domain interact with basic amino acids located in the ICAM-1 cytoplasmic tail. Our findings provide new insight into the molecular mechanism governing the acquisition of ICAM-1, a host molecule known to enhance HIV-1 infectivity in a significant manner. Altogether, these observations offer a new avenue for the development of antiviral therapeutics that are directed at a target of host origin.

IMPORTANCE

Intercellular adhesion molecule 1 (ICAM-1) is a cell surface host component known to be efficiently inserted within emerging HIV-1 particles. It has been demonstrated that host-derived ICAM-1 molecules act as a strong attachment factor and increase HIV-1 infectivity substantially. Despite previous efforts spent studying virus-associated host molecules, the precise mechanism(s) through which such constituents are inserted within emerging HIV-1 particles still remains obscure. Previous data suggest that the Pr55(Gag) precursor polyprotein appears as a potential interaction partner with ICAM-1. In the present study, we demonstrate that the HIV-1 matrix domain plays a key role in the ICAM-1 incorporation process. Some observations were confirmed with whole-virus preparations amplified in primary human cells, thereby providing physiological significance to our data.

Intercellular adhesion molecule-1 expression by skeletal muscle cells augments myogenesis

We previously demonstrated that the expression of intercellular adhesion molecule-1 (ICAM-1) by skeletal muscle cells after muscle overload contributes to ensuing regenerative and hypertrophic processes in skeletal muscle. The objective of the present study is to reveal mechanisms through which skeletal muscle cell expression of ICAM-1 augments regenerative and hypertrophic processes of myogenesis. This was accomplished by genetically engineering C2C12 myoblasts to stably express ICAM-1, and by inhibiting the adhesive and signaling functions of ICAM-1 through the use of a neutralizing antibody or cell penetrating peptide, respectively. Expression of ICAM-1 by cultured skeletal muscle cells augmented myoblast-myoblast adhesion, myotube formation, myonuclear number, myotube alignment, myotube-myotube fusion, and myotube size without influencing the ability of myoblasts to proliferate or differentiate. ICAM-1 augmented myotube formation, myonuclear accretion, and myotube alignment through a mechanism involving adhesion-induced activation of ICAM-1 signaling, as these dependent measures were reduced via antibody and peptide inhibition of ICAM-1. The adhesive and signaling functions of ICAM-1 also facilitated myotube hypertrophy through a mechanism involving myotube-myotube fusion, protein synthesis, and Akt/p70s6k signaling. Our findings demonstrate that ICAM-1 expression by skeletal muscle cells augments myogenesis, and establish a novel mechanism through which the inflammatory response facilitates growth processes in skeletal muscle.

Apicobasal polarity controls lymphocyte adhesion to hepatic epithelial cells

Loss of apicobasal polarity is a hallmark of epithelial pathologies. Leukocyte infiltration and crosstalk with dysfunctional epithelial barriers are crucial for the inflammatory response. Here, we show that apicobasal architecture regulates the adhesion between hepatic epithelial cells and lymphocytes. Polarized hepatocytes and epithelium from bile ducts segregate the intercellular adhesion molecule 1 (ICAM-1) adhesion receptor onto their apical, microvilli-rich membranes, which are less accessible by circulating immune cells. Upon cell depolarization, hepatic ICAM-1 becomes exposed and increases lymphocyte binding. Polarized hepatic cells prevent ICAM-1 exposure to lymphocytes by redirecting basolateral ICAM-1 to apical domains. Loss of ICAM-1 polarity occurs in human inflammatory liver diseases and can be induced by the inflammatory cytokine tumor necrosis factor alpha (TNF-α). We propose that adhesion receptor polarization is a parenchymal immune checkpoint that allows functional epithelium to hamper leukocyte binding. This contributes to the haptotactic guidance of leukocytes toward neighboring damaged or chronically inflamed epithelial cells that expose their adhesion machinery.

Actin-binding proteins differentially regulate endothelial cell stiffness, ICAM-1 function and neutrophil transmigration

Chronic vascular inflammation is driven by interactions between activated leukocytes and the endothelium. Leukocyte β2-integrins bind to endothelial intercellular adhesion molecule 1 (ICAM-1), which allows leukocyte spreading, crawling and transendothelial migration. Leukocytes scan the vascular endothelium for permissive sites to transmigrate, which suggests that there is apical membrane heterogeneity within the endothelium. However, the molecular basis for this heterogeneity is unknown. Leukocyte adhesion induces ICAM-1 clustering, which promotes its association to the actin-binding proteins filamin B, α-actinin-4 and cortactin. We show that these endothelial proteins differentially control adhesion, spreading and transmigration of neutrophils. Loss of filamin B, α-actinin-4 and cortactin revealed adaptor-specific effects on a nuclear-to-peripheral gradient of endothelial cell stiffness. By contrast, increasing endothelial cell stiffness stimulates ICAM-1 function. We identify endothelial α-actinin-4 as a key regulator of endothelial cell stiffness and of ICAM-1-mediated neutrophil transmigration. Finally, we found that the endothelial lining of human and murine atherosclerotic plaques shows elevated levels of α-actinin-4. These results identify endothelial cell stiffness as an important regulator of endothelial surface heterogeneity and of ICAM-1 function, which in turn controls the adhesion and transmigration of neutrophils.

The RhoA guanine nucleotide exchange factor, LARG, mediates ICAM-1-dependent mechanotransduction in endothelial cells to stimulate transendothelial migration

RhoA-mediated cytoskeletal rearrangements in endothelial cells (ECs) play an active role in leukocyte transendothelial cell migration (TEM), a normal physiological process in which leukocytes cross the endothelium to enter the underlying tissue. Although much has been learned about RhoA signaling pathways downstream from ICAM-1 in ECs, little is known about the consequences of the tractional forces that leukocytes generate on ECs as they migrate over the surface before TEM. We have found that after applying mechanical forces to ICAM-1 clusters, there is an increase in cellular stiffening and enhanced RhoA signaling compared with ICAM-1 clustering alone. We have identified that leukemia-associated Rho guanine nucleotide exchange factor (LARG), also known as Rho GEF 12 (ARHGEF12) acts downstream of clustered ICAM-1 to increase RhoA activity, and that this pathway is further enhanced by mechanical force on ICAM-1. Depletion of LARG decreases leukocyte crawling and inhibits TEM. To our knowledge, this is the first report of endothelial LARG regulating leukocyte behavior and EC stiffening in response to tractional forces generated by leukocytes.

Interactome analysis reveals ezrin can adopt multiple conformational states

Ezrin, a member of the ezrin-radixin-moesin family (ERM), is an essential regulator of the structure of microvilli on the apical aspect of epithelial cells. Ezrin provides a linkage between membrane-associated proteins and F-actin, oscillating between active/open and inactive/closed states, and is regulated in part by phosphorylation of a C-terminal threonine. In the open state, ezrin can bind a number of ligands, but in the closed state the ligand-binding sites are inaccessible. In vitro analysis has proposed that there may be a third hyperactivated form of ezrin. To gain a better understanding of ezrin, we conducted an unbiased proteomic analysis of ezrin-binding proteins in an epithelial cell line, Jeg-3. We refined our list of interactors by comparing the interactomes using quantitative mass spectrometry between wild-type ezrin, closed ezrin, open ezrin, and hyperactivated ezrin. The analysis reveals several novel interactors confirmed by their localization to microvilli, as well as a significant class of proteins that bind closed ezrin. Taken together, the data indicate that ezrin can exist in three different conformational states, and different ligands "perceive" ezrin conformational states differently.

Combined effects of icam-1 single-nucleotide polymorphisms and environmental carcinogens on oral cancer susceptibility and clinicopathologic development

BACKGROUND

In Taiwan, oral cancer has causally been associated with environmental carcinogens. Intercellular adhesion molecule (ICAM)-1, a cell adhesion molecule with a key role in inflammation and immunosurveillance, was implicated in carcinogenesis by facilitating instability in the tumor environment. The current study explored the combined effect of ICAM-1 gene polymorphisms and exposure to environmental carcinogens on the susceptibility of developing oral squamous cell carcinoma (OSCC) and the clinicopathological characteristics of the tumors.

METHODOLOGY AND PRINCIPAL FINDINGS

Four single-nucleotide polymorphisms (SNPs) of the ICAM-1 gene from 595 patients with oral cancer and 561 non-cancer controls were analyzed by a real-time PCR. We found that the ICAM-1 rs5498 polymorphism and the TAGG or TACG haplotype of 4 ICAM-1 SNPs (rs3093030, rs5491, rs281432, and rs5498) combined were associated with oral-cancer susceptibility. Among 727 smokers, ICAM-1 polymorphisms carriers with the betel-nut chewing habit had a 27.49-36.23-fold greater risk of having oral cancer compared to ICAM-1 wild-type (WT) carriers without the betel-nut chewing habit. Among 549 betel-nut chewers, ICAM-1 polymorphisms carriers who smoked had a 9.93-14.27-fold greater risk of having oral cancer compared to those who carried the WT but did not smoke. Finally, patients with oral cancer who had at least 1 T allele of ICAM-1 rs5491 or 1 G allele of rs281432 were at lower risk of developing an advanced clinical stage (III/IV) (p<0.05), compared to those patients with AA or CC homozygotes.

CONCLUSIONS

Our results suggest that the ICAM-1 rs5498 SNP and either of 2 haplotypes of 4 SNPs combined have potential predictive significance in oral carcinogenesis. Gene-environment interactions of ICAM-1 polymorphisms, smoking, and betel-nut chewing might alter oral-cancer susceptibility. ICAM-1 rs5491 and rs281432 may be applied as factors to predict the clinical stage in OSCC patients.

Isoforms of Vitamin E Differentially Regulate PKC α and Inflammation: A Review

Vitamin E regulation of disease has been extensively studied but most studies focus on the α-tocopherol isoform of vitamin E. These reports indicate contradictory outcomes for anti-inflammatory functions of the α-tocopherol isoform of vitamin E with regards to animal and clinical studies. These seemingly disparate results are consistent with our recent studies demonstrating that purified natural forms of vitamin E have opposing regulatory functions during inflammation. In this review, we discuss that α-tocopherol inhibits whereas γ-tocopherol elevates allergic inflammation, airway hyperresponsiveness, leukocyte transendothelial migration, and endothelial cell adhesion molecule signaling through protein kinase Cα. Moreover, we have demonstrated that α-tocopherol is an antagonist and γ-tocopherol is an agonist of PKCα through direct binding to a regulatory domain of PKCα. In summary, we have determined mechanisms for opposing regulatory functions of α-tocopherol and γ-tocopherol on inflammation. Information from our studies will have significant impact on the design of clinical studies and on vitamin E consumption.

Distinct sites within the vascular cell adhesion molecule-1 (VCAM-1) cytoplasmic domain regulate VCAM-1 activation of calcium fluxes versus Rac1 during leukocyte transendothelial migration

Vascular adhesion molecules regulate the migration of leukocytes from the blood into tissue during inflammation. Binding of leukocytes to vascular cell adhesion molecule-1 (VCAM-1) activates signals in endothelial cells, including Rac1 and calcium fluxes. These VCAM-1 signals are required for leukocyte transendothelial migration on VCAM-1. However, it has not been reported whether the cytoplasmic domain of VCAM-1 is necessary for these signals. Interestingly, the 19-amino acid sequence of the VCAM-1 cytoplasmic domain is 100% conserved among many mammalian species, suggesting an important functional role for the domain. To examine the function of the VCAM-1 cytoplasmic domain, we deleted the VCAM-1 cytoplasmic domain or mutated the cytoplasmic domain at amino acid N724, S728, Y729, S730, or S737. The cytoplasmic domain and S728, Y729, S730, or S737 were necessary for leukocyte transendothelial migration. S728 and Y729, but not S730 or S737, were necessary for VCAM-1 activation of calcium fluxes. In contrast, S730 and S737, but not S728 or Y729, were necessary for VCAM-1 activation of Rac1. These functional data are consistent with our computational model of the structure of the VCAM-1 cytoplasmic domain as an α-helix with S728 and Y729, and S730 and S737, on opposite sides of the α-helix. Together, these data indicate that S728 and Y729, and S730 and S737, are distinct functional sites that coordinate VCAM-1 activation of calcium fluxes and Rac1 during leukocyte transendothelial migration.

Endothelial membrane reorganization during leukocyte extravasation

Leukocyte trafficking from the bloodstream to inflamed tissues across the endothelial barrier is an essential response in innate immunity. Leukocyte adhesion, locomotion, and diapedesis induce signaling in endothelial cells and this is accompanied by a profound reorganization of the endothelial cell surfaces that is only starting to be unveiled. Here we review the current knowledge on the leukocyte-mediated alterations of endothelial membrane dynamics and their role in promoting leukocyte extravasation. The formation of protein- and lipid-mediated cell adhesion nanodomains at the endothelial apical surface, the extension of micrometric apical membrane docking structures, which are derived from microvilli and embrace adhered leukocytes, as well as the vesicle-trafficking pathways that are required for efficient leukocyte diapedesis, are discussed. The coordination between these different endothelial membrane-remodeling events probably provides the road map for transmigrating leukocytes to find exit points in the vessel wall, in a context of severe mechanical and inflammatory stress. A better understanding of how vascular endothelial cells respond to immune cell adhesion should enable new therapeutic strategies to be developed that can abrogate uncontrolled leukocyte extravasation in inflammatory diseases.

Vitamin E isoforms differentially regulate intercellular adhesion molecule-1 activation of PKCα in human microvascular endothelial cells

Aims

ICAM-1-dependent leukocyte recruitment in vivo is inhibited by the vitamin E isoform d-α-tocopherol and elevated by d-γ-tocopherol. ICAM-1 is reported to activate endothelial cell signals including protein kinase C (PKC), but the PKC isoform and the mechanism for ICAM-1 activation of PKC are not known. It is also not known whether ICAM-1 signaling in endothelial cells is regulated by tocopherol isoforms. We hypothesized that d-α-tocopherol and d-γ-tocopherol differentially regulate ICAM-1 activation of endothelial cell PKC.

Results

ICAM-1 crosslinking activated the PKC isoform PKCα but not PKCβ in TNFα-pretreated human microvascular endothelial cells. ICAM-1 activation of PKCα was blocked by the PLC inhibitor U73122, ERK1/2 inhibitor PD98059, and xanthine oxidase inhibitor allopurinol. ERK1/2 activation was blocked by inhibition of XO and PLC but not by inhibition of PKCα, indicating that ERK1/2 is downstream of XO and upstream of PKCα during ICAM-1 signaling. During ICAM-1 activation of PKCα, the XO-generated ROS did not oxidize PKCα. Interestingly, d-α-tocopherol inhibited ICAM-1 activation of PKCα but not the upstream signal ERK1/2. The d-α-tocopherol inhibition of PKCα was ablated by the addition of d-γ-tocopherol.

Conclusions

Crosslinking ICAM-1 stimulated XO/ROS which activated ERK1/2 that then activated PKCα. ICAM-1 activation of PKCα was inhibited by d-α-tocopherol and this inhibition was ablated by the addition of d-γ-tocopherol. These tocopherols regulated ICAM-1 activation of PKCα without altering the upstream signal ERK1/2. Thus, we identified a mechanism for ICAM-1 activation of PKC and determined that d-α-tocopherol and d-γ-tocopherol have opposing regulatory functions for ICAM-1-activated PKCα in endothelial cells.

The Rho-guanine nucleotide exchange factor Trio controls leukocyte transendothelial migration by promoting docking structure formation

Neutrophils induce endothelial docking structures prior to crossing the blood vessel wall. The Rho guanine nucleotide exchange factor Trio regulates the formation of these structures through ICAM-1 clustering in a filamin-dependent fashion. We show that Trio is a crucial mediator of the signaling pathway that controls leukocyte extravasation through docking structure formation.

Leukocyte transendothelial migration involves the active participation of the endothelium through the formation of apical membrane protrusions that embrace adherent leukocytes, termed docking structures. Using live-cell imaging, we find that prior to transmigration, endothelial docking structures form around 80% of all neutrophils. Previously we showed that endothelial RhoG and SGEF control leukocyte transmigration. In this study, our data reveal that both full-length Trio and the first DH-PH (TrioD1) domain of Trio, which can activate Rac1 and RhoG, interact with ICAM-1 and are recruited to leukocyte adhesion sites. Moreover, upon clustering of ICAM-1, the Rho-guanine nucleotide exchange factor Trio activates Rac1, prior to activating RhoG, in a filamin-dependent manner. We further show that docking structure formation is initiated by ICAM-1 clustering into ring-like structures, which is followed by apical membrane protrusion. Interestingly, we find that Rac1 is required for ICAM-1 clustering, whereas RhoG controls membrane protrusion formation. Finally, silencing endothelial Trio expression or reducing TrioD1 activity without affecting SGEF impairs both docking structure formation and leukocyte transmigration. We conclude that Trio promotes leukocyte transendothelial migration by inducing endothelial docking structure formation in a filamin-dependent manner through the activation of Rac1 and RhoG.

Intercellular adhesion molecule 1 engagement modulates sphingomyelinase and ceramide, supporting uptake of drug carriers by the vascular endothelium

OBJECTIVE

Engagement of intercellular adhesion molecule 1 (ICAM-1) on endothelial cells by ICAM-1-targeted carriers induces cell adhesion molecule-mediated endocytosis, providing intraendothelial delivery of therapeutics. This pathway differs from classical endocytic mechanisms and invokes aspects of endothelial signaling during inflammation. ICAM-1 interacts with Na(+)/H(+) exchanger NHE1 during endocytosis, but it is unclear how this regulates plasmalemma and cytoskeletal changes. We studied such aspects in this work.

METHODS AND RESULTS

We used fluorescence and electron microscopy, inhibitors and knockout tools, cell culture, and mouse models. ICAM-1 engagement by anti-ICAM carriers induced sphingomyelin-enriched engulfment structures. Acid sphingomyelinase (ASM), an acidic enzyme that hydrolyzes sphingomyelin into ceramide (involved in plasmalemma deformability and cytoskeletal reorganization), redistributed to ICAM-1-engagement sites at ceramide-enriched areas. This induced actin stress fibers and carrier endocytosis. Inhibiting ASM impaired ceramide enrichment, engulfment structures, cytoskeletal reorganization, and carrier uptake, which was rescued by supplying this enzyme activity exogenously. Interfering with NHE1 rendered similar outcomes, suggesting that Na(+)/H(+) exchange might provide an acidic microenvironment for ASM at the plasmalemma.

CONCLUSIONS

These findings are consistent with the ability of endothelial cells to internalize relatively large ICAM- 1--targeted drug carriers and expand our knowledge on the regulation of the sphingomyelin/ceramide pathway by the vascular endothelium.

Recycling and LFA-1-dependent trafficking of ICAM-1 to the immunological synapse

Little is known about how adhesion molecules on APCs accumulate at immunological synapses. We show here that ICAM-1 on APCs is continuously internalized and rapidly recycled back to the interface after antigen-priming T-cell contact. The internalization rate is high in APCs, including Raji B cells and dendritic cells, but low in endothelial cells. Internalization is significantly reduced by inhibitors of Na(+)/H(+) exchangers (NHEs), suggesting that members of the NHE-family regulate this process. Once internalized, ICAM-1 is co-localized with MHC class II in the polarized recycling compartment. Surprisingly, not only ICAM-1, but also MHC class II, is targeted to the immunological synapse through LFA-1-dependent adhesion. Cytosolic ICAM-1 is highly mobile and forms a tubular structure. Inhibitors of microtubule or actin polymerization can reduce ICAM-1 mobility, and thereby block accumulation at immunological synapses. Membrane ICAM-1 also moves to the T-cell contact zone, presumably through an active, cytoskeleton-dependent mechanism. Collectively, these results demonstrate that ICAM-1 can be transported to the immunological synapse through the recycling compartment. Furthermore, the high-affinity state of LFA-1 on T cells is critical to induce targeted movements of both ICAM-1 and MHC class II to the immunological synapse on APCs.

The latest developments in synthetic peptides with immunoregulatory activities

In the past few years, many researches have provided us with much data demonstrating the abilities of synthetic peptides to impact immune response in vitro and in vivo. These peptides were designed according to the structure of some important protein molecules which play a key role in immune response, so they act with specific targets. The class I and II MHC-derived peptides inhibit the TCR recognition of antigen peptide-MHC complex. Rationally designed CD80 and CD154-binding peptides block the interaction between cell surface costimulatory molecules on antigen-presenting cells (APCs) and T cells. Some peptides were designed to inhibit the activities of cell signal proteins, including JNK, NF-κB and NFAT. Some peptide antagonists competitively bind to important cytokines and inhibit their activities, such as TNF-α, TGF-β and IL-1β inhibitory peptides. Adhesion molecule ICAM-1 derived peptides block the T cell adhesion and activation. These immunoregulatory peptides showed therapeutic effect in several animal models, including collagen-induced arthritis (CIA), autoimmune cystitis model, murine skin transplant model and cardiac allograft model. These results give us important implications for the development of a novel therapy for immune mediated diseases.

Neutrophils, from marrow to microbes

Neutrophils are produced in the bone marrow from stem cells that proliferate and differentiate to mature neutrophils fully equipped with an armory of granules. These contain proteins that enable the neutrophil to deliver lethal hits against microorganisms, but also to cause great tissue damage. Neutrophils circulate in the blood as dormant cells. At sites of infection, endothelial cells capture bypassing neutrophils and guide them through the endothelial cell lining whereby the neutrophils are activated and tuned for the subsequent interaction with microbes. Once in tissues, neutrophils kill microorganisms by microbicidal agents liberated from granules or generated by metabolic activation. As a final act, neutrophils can extrude stands of DNA with bactericidal proteins attached that act as extracellular traps for microorganisms.

Intermediate monomer-dimer equilibrium structure of native ICAM-1: implication for enhanced cell adhesion

Dimeric intercellular adhesion molecule-1 (ICAM-1) has been known to more efficiently mediate cell adhesion than monomeric ICAM-1. Here, we found that truncation of the intracellular domain of ICAM-1 significantly enhances surface dimerization based on the two criteria: 1) the binding degree of monomer-specific antibody CA-7 and 2) the ratio of dimer/monomer when a mutation (L42→C42) was introduced in the interface of domain 1. Mutation analysis revealed that the positively charged amino acids, including very membrane-proximal ⁵⁰⁵R, are essential for maintaining the structural transition between the monomer and dimer. Despite a strong dimer presentation, the ICAM-1 mutants lacking an intracellular domain (IC1ΔCTD) or containing R to A substitution in position 505 (⁵⁰⁵R/A) supported a lower degree of cell adhesion than did wild-type ICAM-1. Collectively, these results demonstrate that the native structure of surface ICAM-1 is not a dimer, but is an intermediate monomer-dimer equilibrium structure by which the effectiveness of ICAM-1 can be fully achieved.

Inside-out regulation of ICAM-1 dynamics in TNF-alpha-activated endothelium

BACKGROUND

During transendothelial migration, leukocytes use adhesion molecules, such as ICAM-1, to adhere to the endothelium. ICAM-1 is a dynamic molecule that is localized in the apical membrane of the endothelium and clusters upon binding to leukocytes. However, not much is known about the regulation of ICAM-1 clustering and whether membrane dynamics are linked to the ability of ICAM-1 to cluster and bind leukocyte integrins. Therefore, we studied the dynamics of endothelial ICAM-1 under non-clustered and clustered conditions.

PRINCIPAL FINDINGS

Detailed scanning electron and fluorescent microscopy showed that the apical surface of endothelial cells constitutively forms small filopodia-like protrusions that are positive for ICAM-1 and freely move within the lateral plane of the membrane. Clustering of ICAM-1, using anti-ICAM-1 antibody-coated beads, efficiently and rapidly recruits ICAM-1. Using fluorescence recovery after photo-bleaching (FRAP), we found that clustering increased the immobile fraction of ICAM-1, compared to non-clustered ICAM-1. This shift required the intracellular portion of ICAM-1. Moreover, biochemical assays showed that ICAM-1 clustering recruited beta-actin and filamin. Cytochalasin B, which interferes with actin polymerization, delayed the clustering of ICAM-1. In addition, we could show that cytochalasin B decreased the immobile fraction of clustered ICAM-1-GFP, but had no effect on non-clustered ICAM-1. Also, the motor protein myosin-II is recruited to ICAM-1 adhesion sites and its inhibition increased the immobile fraction of both non-clustered and clustered ICAM-1. Finally, blocking Rac1 activation, the formation of lipid rafts, myosin-II activity or actin polymerization, but not Src, reduced the adhesive function of ICAM-1, tested under physiological flow conditions.

CONCLUSIONS

Together, these findings indicate that ICAM-1 clustering is regulated in an inside-out fashion through the actin cytoskeleton. Overall, these data indicate that signaling events within the endothelium are required for efficient ICAM-1-mediated leukocyte adhesion.

ICAM-1/LFA-1 interaction contributes to the induction of endothelial cell-cell separation: implication for enhanced leukocyte diapedesis

The basic route and mechanism for diapedesis has not yet to be fully defined. Here we present evidence that "cell-cell separation" between endothelial cells (ECs) may provide a route for leukocyte diapedesis. We unexpectedly found that extensive interaction between peripheral blood leukocytes and ECs that were activated by TNF-alpha induced the opening of EC contacts and, surprisingly, resulted in cell-cell separation. This event was specific to the intercellular adhesion molecules-1 (ICAM-1)/leukocyte function- associated antigen-1 interaction, as demonstrated by the following: (1) ICAM-1 expression correlated with increased EC contraction; and (2) the blocking of ICAM-1 selectively inhibited EC separation. Thus, we suggest that "cell-cell separation" could be a mechanism for diapedesis in situations that may require massive leukocyte infiltration.

Hug tightly and say goodbye: role of endothelial ICAM-1 in leukocyte transmigration

Stable adhesion of leukocytes to endothelium is crucial for transendothelial migration (TEM) of leukocytes evoked during inflammatory responses, immune surveillance, and homing and mobilization of hematopoietic progenitor cells. The basis of stable adhesion involves expression of intercellular adhesion molecule-1 (ICAM-1), an inducible endothelial adhesive protein that serves as a counter-receptor for beta(2)-integrins on leukocytes. Interaction of ICAM-1 with beta(2)-integrins enables leukocytes to adhere firmly to the vascular endothelium and subsequently, to migrate across the endothelial barrier. The emerging paradigm is that ICAM-1, in addition to firmly capturing leukocytes, triggers intracellular signaling events that may contribute to active participation of the endothelium in facilitating the TEM of adherent leukocytes. The nature, duration, and intensity of ICAM-1-dependent signaling events may contribute to the determination of the route (paracellular vs. transcellular) of leukocyte passage; these aspects of ICAM-1 signaling may in turn be influenced by density and distribution of ICAM-1 on the endothelial cell surface, the source of endothelial cells it is present on, and the type of leukocytes with which it is engaged. This review summarizes our current understanding of the "ICAM-1 paradigm" of TEM with an emphasis on the signaling events mediating ICAM-1 expression and activated by ICAM-1 engagement in endothelial cells.

Neutrophil recruitment under shear flow: it's all about endothelial cell rings and gaps

Leukocyte recruitment to tissues and organs is an essential component of host defense. The molecular mechanisms controlling this process are complex and remain under active investigation. The combination of biochemical techniques and live cell imaging using in vivo and in vitro flow-model approaches have shed light on several aspects of neutrophil transmigration through the vascular endothelial lining of blood vessels. Here, we focus on the role of adhesion molecule signaling in endothelial cells and their downstream targets during the process of transendothelial migration at cell-cell borders (paracellular transmigration). An emerging model involves the leukocyte beta2 integrin engagement of endothelial cell ICAM-1, which triggers integrin-ICAM-1 clustering (rings) and stabilizes leukocyte adhesion at cell-cell junctions. This step recruits nonreceptor tyrosine kinases that phosphorylate key tyrosine residues in the cytoplasmic tail of VE-cadherin, which destabilizes its linkage to catenins and the actin cytoskeleton, triggering the transient opening of VE-cadherin homodimers to form a gap in the cell junction, through which the neutrophil transmigrates. Interestingly, the signaling events that lead to neutrophil transmigration occur independently of shear flow in vitro.

Thermal facilitation of lymphocyte trafficking involves temporal induction of intravascular ICAM-1

OBJECTIVE

Fever is associated with improved survival, although its beneficial mechanisms are poorly understood. Previous studies indicate that the thermal element of fever augments lymphocyte migration across high endothelial venules (HEVs) of lymphoid organs by increasing the intravascular display of a gatekeeper trafficking molecule, intercellular adhesion molecule-1 (ICAM-1). Here, we evaluated the spatio-temporal relationship between the thermal induction of intravascular ICAM-1 and lymphocyte trafficking.

METHODS

Intravascular ICAM-1 density was quantified by immunofluorescence staining in mice exposed to fever-range whole-body hyperthermia (39.5+/-0.5 degrees C). ICAM-1-dependent lymphocyte trafficking was measured in short-term homing assays.

RESULTS

A linear relationship was observed between the duration of heat treatment and intravascular ICAM-1 density in HEVs with maximal responses requiring sustained (i.e., five hours) thermal stress. Circulating lymphocytes were found to sense incremental changes in ICAM-1 on HEVs, such that trafficking is proportional to the intravascular density of ICAM-1. We further identified a hydroxamate-sensitive shedding mechanism that restores ICAM-1 expression to homeostatic levels following the cessation of thermal stress.

CONCLUSIONS

The time-dependent response to thermal stress indicates that ICAM-1 density governs the efficiency of lymphocyte interactions with HEVs in vivo. These studies highlight the dynamic role of the microcirculation in promoting immune surveillance during febrile inflammatory responses.

Filamin B mediates ICAM-1-driven leukocyte transendothelial migration

During inflammation, the endothelium mediates rolling and firm adhesion of activated leukocytes. Integrin-mediated adhesion to endothelial ligands of the Ig-superfamily induces intracellular signaling in endothelial cells, which promotes leukocyte transendothelial migration. We identified the actin cross-linking molecule filamin B as a novel binding partner for intracellular adhesion molecule-1 (ICAM-1). Immune precipitation as well as laser scanning confocal microscopy confirmed the specific interaction and co-localization of endogenous filamin B with ICAM-1. Importantly, clustering of ICAM-1 promotes the ICAM-1-filamin B interaction. To investigate the functional consequences of filamin B binding to ICAM-1, we used small interfering RNA to reduce filamin B expression in ICAM-1-GFP expressing HeLa cells. We found that filamin B is required for the lateral mobility of ICAM-1 and for ICAM-1-induced transmigration of leukocytes. Reducing filamin B expression in primary human endothelial cells resulted in reduced recruitment of ICAM-1 to endothelial docking structures, reduced firm adhesion of the leukocytes to the endothelium, and inhibition of transendothelial migration. In conclusion, this study identifies filamin B as a molecular linker that mediates ICAM-1-driven transendothelial migration.