LFA-1 binding destabilizes the JAM-A homophilic interaction during leukocyte transmigration

Thy-1 (CD90)-regulated cell adhesion and migration of mesenchymal cells: insights into adhesomes, mechanical forces, and signaling pathways

Cell adhesion and migration depend on the assembly and disassembly of adhesive structures known as focal adhesions. Cells adhere to the extracellular matrix (ECM) and form these structures via receptors, such as integrins and syndecans, which initiate signal transduction pathways that bridge the ECM to the cytoskeleton, thus governing adhesion and migration processes. Integrins bind to the ECM and soluble or cell surface ligands to form integrin adhesion complexes (IAC), whose composition depends on the cellular context and cell type. Proteomic analyses of these IACs led to the curation of the term adhesome, which is a complex molecular network containing hundreds of proteins involved in signaling, adhesion, and cell movement. One of the hallmarks of these IACs is to sense mechanical cues that arise due to ECM rigidity, as well as the tension exerted by cell-cell interactions, and transduce this force by modifying the actin cytoskeleton to regulate cell migration. Among the integrin/syndecan cell surface ligands, we have described Thy-1 (CD90), a GPI-anchored protein that possesses binding domains for each of these receptors and, upon engaging them, stimulates cell adhesion and migration. In this review, we examine what is currently known about adhesomes, revise how mechanical forces have changed our view on the regulation of cell migration, and, in this context, discuss how we have contributed to the understanding of signaling mechanisms that control cell adhesion and migration.

Dissecting the molecular control of immune cell accumulation in the inflamed joint

Mechanisms governing entry and exit of immune cells into and out of inflamed joints remain poorly understood. We sought herein to identify the key molecular pathways regulating such migration. Using murine models of inflammation in conjunction with mice expressing a photoconvertible fluorescent protein, we characterized the migration of cells from joints to draining lymph nodes and performed RNA-Seq analysis on isolated cells, identifying genes associated with migration and retention. We further refined the gene list to those specific for joint inflammation. RNA-Seq data revealed pathways and genes previously highlighted as characteristic of rheumatoid arthritis in patient studies, validating the methodology. Focusing on pathways associated with cell migration, adhesion, and movement, we identified genes involved in the retention of immune cells in the inflamed joint, namely junctional adhesion molecule A (JAM-A), and identified a role for such molecules in T cell differentiation in vivo. Thus, using a combination of cell-tracking approaches and murine models of inflammatory arthritis, we identified genes, pathways, and anatomically specific tissue signatures regulating cell migration in a variety of inflamed sites. This skin- and joint-specific data set will be an invaluable resource for the identification of therapeutic targets for arthritis and other inflammatory disorders.

Understanding the Role of LFA-1 in Leukocyte Adhesion Deficiency Type I (LAD I): Moving towards Inflammation?

LFA-1 (Lymphocyte function-associated antigen-1) is a heterodimeric integrin (CD11a/CD18) present on the surface of all leukocytes; it is essential for leukocyte recruitment to the site of tissue inflammation, but also for other immunological processes such as T cell activation and formation of the immunological synapse. Absent or dysfunctional expression of LFA-1, caused by mutations in the ITGB2 (integrin subunit beta 2) gene, results in a rare immunodeficiency syndrome known as Leukocyte adhesion deficiency type I (LAD I). Patients suffering from severe LAD I present with recurrent infections of the skin and mucosa, as well as inflammatory symptoms complicating the clinical course of the disease before and after allogeneic hematopoietic stem cell transplantation (alloHSCT); alloHSCT is currently the only established curative treatment option. With this review, we aim to provide an overview of the intrinsic role of inflammation in LAD I.

The Roles of Junctional Adhesion Molecules (JAMs) in Cell Migration

The review briefly summarizes the role of the family of adhesion molecules, JAMs (junctional adhesion molecules), in various cell migration, covering germ cells, epithelial cells, endothelial cells, several leukocytes, and different cancer cells. These functions affect multiple diseases, including reproductive diseases, inflammation-related diseases, cardiovascular diseases, and cancers. JAMs bind to both similar and dissimilar proteins and take both similar and dissimilar effects on different cells. Concluding relevant results provides a reference to further research.

Astrocytic junctional adhesion molecule-A regulates T cell entry past the glia limitans to promote central nervous system autoimmune attack

Contact-mediated interactions between the astrocytic endfeet and infiltrating immune cells within the perivascular space are underexplored, yet represent potential regulatory check-points against CNS autoimmune disease and disability. Reactive astrocytes upregulate junctional adhesion molecule-A, an immunoglobulin-like cell surface receptor that binds to T cells via its ligand, the integrin, lymphocyte function-associated antigen-1. Here, we tested the role of astrocytic junctional adhesion molecule-A in regulating CNS autoinflammatory disease. In cell co-cultures, we found that junctional adhesion molecule-A-mediated signalling between astrocytes and T cells increases levels of matrix metalloproteinase-2, C–C motif chemokine ligand 2 and granulocyte-macrophage colony-stimulating factor, pro-inflammatory factors driving lymphocyte entry and pathogenicity in multiple sclerosis and experimental autoimmune encephalomyelitis, an animal model of CNS autoimmune disease. In experimental autoimmune encephalomyelitis, mice with astrocyte-specific JAM-A deletion (mGFAP:CreJAM-A^fl/fl) exhibit decreased levels of matrix metalloproteinase-2, reduced ability of T cells to infiltrate the CNS parenchyma from the perivascular spaces and a milder histopathological and clinical course of disease compared with wild-type controls (JAM-A^fl/fl). Treatment of wild-type mice with intraperitoneal injection of soluble junctional adhesion molecule-A blocking peptide decreases the severity of experimental autoimmune encephalomyelitis, highlighting the potential of contact-mediated astrocyte–immune cell signalling as a novel translational target against neuroinflammatory disease.

The F11 Receptor (F11R)/Junctional Adhesion Molecule-A (JAM-A) (F11R/JAM-A) in cancer progression

The F11 Receptor (F11R), also called Junctional Adhesion Molecule-A (JAM-A) (F11R/JAM-A), is a transmembrane glycoprotein of the immunoglobulin superfamily, which is mainly located in epithelial and endothelial cell tight junctions and also expressed on circulating platelets and leukocytes. It participates in the regulation of various biological processes, as diverse as paracellular permeability, tight junction formation and maintenance, leukocyte transendothelial migration, epithelial-to-mesenchymal transition, angiogenesis, reovirus binding, and platelet activation. Dysregulation of F11R/JAM-A may result in pathological consequences and disorders in normal cell function. A growing body of evidence points to its role in carcinogenesis and invasiveness, but its tissue-specific pro- or anti-tumorigenic role remains a debated issue. The following review focuses on the F11R/JAM-A tissue-dependent manner in tumorigenesis and metastasis and also discusses the correlation between poor patient clinical outcomes and its aberrant expression. In the future, it will be required to clarify the signaling pathways that are activated or suppressed via the F11R/JAM-A protein in various cancer types to understand its multiple roles in cancer progression and further use it as a novel direct target for cancer treatment.

Targeting Opposing Immunological Roles of the Junctional Adhesion Molecule-A in Autoimmunity and Cancer

The junctional adhesion molecule-A (JAM-A) is a cell surface adhesion molecule expressed on platelets, epithelial cells, endothelial cells and leukocytes (e. g. monocytes and dendritic cells). JAM-A plays a relevant role in leukocyte trafficking and its therapeutic potential has been studied in several pathological conditions due to its capacity to induce leukocyte migration out of inflamed sites or infiltration into tumor sites. However, disruption of JAM-A pathways may worsen clinical pathology in some cases. As such, the effects of JAM-A manipulation on modulating immune responses in the context of different diseases must be better understood. In this mini-review, we discuss the potential of JAM-A as a therapeutic target, summarizing findings from studies manipulating JAM-A in the context of inflammatory diseases (e.g. autoimmune diseases) and cancer and highlighting described mechanisms.

Analysis of leukocyte transepithelial migration using an in vivo murine colonic loop model

Molecular mechanisms that control leukocyte migration across the vascular endothelium (transendothelial migration; TEndoM) have been extensively characterized in vivo, but details of leukocyte transepithelial migration (TEpM) and its dysregulation (a pathologic feature of many mucosal diseases) are missing due to the lack of suitable animal models. Here, we describe a murine model that utilizes a vascularized proximal colonic segment (pcLoop) and enables quantitative studies of leukocyte trafficking across colonic epithelium. Consistent with previous in vitro studies, intraluminal injection of antibodies against integrin CD11b/CD18 reduced recruitment of polymorphonuclear neutrophils (PMN) into the lumen of pcLoops, and it increased subepithelial accumulation of PMN. We extended studies using the pcLoop to determine contributions of Junctional Adhesion Molecule-A (JAM-A, or F11R) in PMN TEpM and confirmed that mice with total loss of JAM-A or mice with intestinal epithelial selective loss of JAM-A had increased colonic permeability. Furthermore, there was reduced PMN migration into the colonic lumen that paralleled subepithelial accumulation of PMN in global-KO mice, as well as in intestinal epithelial-targeted JAM-A-deficient mice. These findings highlight a potentially novel role for JAM-A in regulating PMN TEpM in vivo and demonstrate utility of this model for identifying receptors that may be targeted in vivo to reduce pathologic intestinal inflammation.

Junctional adhesion molecule-A: functional diversity through molecular promiscuity

Cell adhesion molecules (CAMs) of the immunoglobulin superfamily (IgSF) regulate important processes such as cell proliferation, differentiation and morphogenesis. This activity is primarily due to their ability to initiate intracellular signaling cascades at cell-cell contact sites. Junctional adhesion molecule-A (JAM-A) is an IgSF-CAM with a short cytoplasmic tail that has no catalytic activity. Nevertheless, JAM-A is involved in a variety of biological processes. The functional diversity of JAM-A resides to a large part in a C-terminal PDZ domain binding motif which directly interacts with nine different PDZ domain-containing proteins. The molecular promiscuity of its PDZ domain motif allows JAM-A to recruit protein scaffolds to specific sites of cell-cell adhesion and to assemble signaling complexes at those sites. Here, we review the molecular characteristics of JAM-A, including its dimerization, its interaction with scaffolding proteins, and the phosphorylation of its cytoplasmic domain, and we describe how these characteristics translate into diverse biological activities.

Junctional Adhesion Molecules (JAMs): The JAM-Integrin Connection

Junctional adhesion molecules (JAMs) are cell surface adhesion receptors of the immunoglobulin superfamily. JAMs are involved in a variety of biological processes both in the adult organism but also during development. These include processes such as inflammation, angiogenesis, hemostasis, or epithelial barrier formation, but also developmental processes such as hematopoiesis, germ cell development, and development of the nervous system. Several of these functions of JAMs depend on a physical and functional interaction with integrins. The JAM – integrin interactions in trans regulate cell-cell adhesion, their interactions in cis regulate signaling processes originating at the cell surface. The JAM – integrin interaction can regulate the function of the JAM as well as the function of the integrin. Beyond the physical interaction with integrins, JAMs can regulate integrin function through intracellular signaling indicating an additional level of JAM – integrin cross-talk. In this review, we describe the various levels of the functional interplay between JAMs and integrins and the role of this interplay during different physiological processes.

Junctional Adhesion Molecules (JAMs): Cell Adhesion Receptors With Pleiotropic Functions in Cell Physiology and Development

Junctional adhesion molecules (JAM)-A, -B and -C are cell-cell adhesion molecules of the immunoglobulin superfamily which are expressed by a variety of tissues, both during development and in the adult organism. Through their extracellular domains, they interact with other adhesion receptors on opposing cells. Through their cytoplasmic domains, they interact with PDZ domain-containing scaffolding and signaling proteins. In combination, these two properties regulate the assembly of signaling complexes at specific sites of cell-cell adhesion. The multitude of molecular interactions has enabled JAMs to adopt distinct cellular functions such as the regulation of cell-cell contact formation, cell migration, or mitotic spindle orientation. Not surprisingly, JAMs regulate diverse processes such as epithelial and endothelial barrier formation, hemostasis, angiogenesis, hematopoiesis, germ cell development, and the development of the central and peripheral nervous system. This review summarizes the recent progress in the understanding of JAMs, including their characteristic structural features, their molecular interactions, their cellular functions, and their contribution to a multitude of processes during vertebrate development and homeostasis.

Astrocytic tight junctions control inflammatory CNS lesion pathogenesis

Lesions and neurologic disability in inflammatory CNS diseases such as multiple sclerosis (MS) result from the translocation of leukocytes and humoral factors from the vasculature, first across the endothelial blood-brain barrier (BBB) and then across the astrocytic glia limitans (GL). Factors secreted by reactive astrocytes open the BBB by disrupting endothelial tight junctions (TJs), but the mechanisms that control access across the GL are unknown. Here, we report that in inflammatory lesions, a second barrier composed of reactive astrocyte TJs of claudin 1 (CLDN1), CLDN4, and junctional adhesion molecule A (JAM-A) subunits is induced at the GL. In a human coculture model, CLDN4-deficient astrocytes were unable to control lymphocyte segregation. In models of CNS inflammation and MS, mice with astrocyte-specific Cldn4 deletion displayed exacerbated leukocyte and humoral infiltration, neuropathology, motor disability, and mortality. These findings identify a second inducible barrier to CNS entry at the GL. This barrier may be therapeutically targetable in inflammatory CNS disease.

Tension on JAM-A activates RhoA via GEF-H1 and p115 RhoGEF

Forces on JAM-A activate RhoA to increase cell stiffness. Activation of RhoA requires GEF-H1 and p115 RhoGEF activation downstream of FAK/ERK and Src family kinases, respectively.

Junctional adhesion molecule A (JAM-A) is a broadly expressed adhesion molecule that regulates cell–cell contacts and facilitates leukocyte transendothelial migration. The latter occurs through interactions with the integrin LFA-1. Although we understand much about JAM-A, little is known regarding the protein’s role in mechanotransduction or as a modulator of RhoA signaling. We found that tension imposed on JAM-A activates RhoA, which leads to increased cell stiffness. Activation of RhoA in this system depends on PI3K-mediated activation of GEF-H1 and p115 RhoGEF. These two GEFs are further regulated by FAK/ERK and Src family kinases, respectively. Finally, we show that phosphorylation of JAM-A at Ser-284 is required for RhoA activation in response to tension. These data demonstrate a direct role of JAM-A in mechanosignaling and control of RhoA and implicate Src family kinases in the regulation of p115 RhoGEF.

Junctional proteins of the blood-brain barrier: New insights into function and dysfunction

The blood-brain barrier (BBB) is a highly complex and dynamic barrier. It is formed by an interdependent network of brain capillary endothelial cells, endowed with barrier properties, and perivascular cells (astrocytes and pericytes) responsible for inducing and maintaining those properties. One of the primary properties of the BBB is a strict regulation of paracellular permeability due to the presence of junctional complexes (tight, adherens and gap junctions) between the endothelial cells. Alterations in junction assembly and function significantly affect BBB properties, particularly barrier permeability. However, such alterations are also involved in remodeling the brain endothelial cell surface and regulating brain endothelial cell phenotype. This review summarizes the characteristics of brain endothelial tight, adherens and gap junctions and highlights structural and functional alterations in junctional proteins that may contribute to BBB dysfunction.

F11R mRNA expression and promoter polymorphisms in patients with rheumatoid arthritis

AIM

Although F11 receptor (F11R), also named junctional adhesion molecular A (JAM-A), participates in leukocyte migration, its role in autoimmune diseases has not been specifically disclosed. In this study, we examined the association of F11R expression with the development and clinical manifestations of rheumatoid arthritis (RA).

METHOD

RNA from peripheral blood mononuclear cells (PBMCs) and DNA from the peripheral blood in RA patients and a healthy control group were extracted. F11R messenger RNA (mRNA) expression was determined by quantitative real-time polymerase chain reaction. The F11R polymorphisms were determined by the TaqMan genotyping assay.

RESULTS

There was more F11R mRNA expression in the PBMCs of RA patients than those of the control group (P = 0.018). In F11R promoter -688 A > C, C carriers have lower titers of the anticyclic citrullinated peptide (anti-CCP) antibodies (P = 0.002) and fewer positive rates of Schirmer's tests (P = 0.009). The effect is independent of the existence of HLA-DR4. Different genotypes in F11R promoter -688 A > C and -436 A > G do not lead to changes of the gene expression in RA patients.

CONCLUSION

RA patients have higher mRNA expression of F11R. In RA patients, F11R -688 C may be a protective factor for the development of anti-CCP antibodies and positive rates of Schirmer's tests.

N-glycosylation controls the function of junctional adhesion molecule-A

Junctional adhesion molecule-A (JAM-A) is an adherens and tight junction protein expressed by endothelial and epithelial cells. JAM-A serves many roles and contributes to barrier function and cell migration and motility, and it also acts as a ligand for the leukocyte receptor LFA-1. JAM-A is reported to contain N-glycans, but the extent of this modification and its contribution to the protein's functions are unknown. We show that human JAM-A contains a single N-glycan at N185 and that this residue is conserved across multiple mammalian species. A glycomutant lacking all N-glycans, N185Q, is able to reach the cell surface but exhibits decreased protein half-life compared with the wild- type protein. N-glycosylation of JAM-A is required for the protein's ability to reinforce barrier function and contributes to Rap1 activity. We further show that glycosylation of N185 is required for JAM-A-mediated reduction of cell migration. Finally, we show that N-glycosylation of JAM-A regulates leukocyte adhesion and LFA-1 binding. These findings identify N-glycosylation as critical for JAM-A's many functions.

IL-18 Is Involved in Eosinophil-Mediated Tumoricidal Activity against a Colon Carcinoma Cell Line by Upregulating LFA-1 and ICAM-1

Eosinophils are multifunctional leukocytes that are involved in innate and adaptive immune responses through the expression of various receptors and mediators. Previously, we showed that human eosinophils and T cells shared cytotoxic activities against tumor cells that involved the γ-δ TCR and cell-cell contact. In this study, we investigated the molecules involved in eosinophil-tumor cell interactions. Given the role of IL-18 in cell adhesion and in protecting against colon cancer, we evaluated its role in eosinophil-mediated cytotoxicity against Colo-205, a human colon carcinoma cell line. We found that human eosinophils exerted dose- and time-dependent tumoricidal activity against Colo-205 cells. Neutralization of IL-18 significantly reduced eosinophil-mediated Colo-205 apoptosis and inhibited cell-cell adhesion. Moreover, addition of rIL-18 led to upregulation of CD11a and ICAM-1 adhesion molecules, which were involved in the contact between eosinophils and Colo-205 cells. Our results indicated that IL-18 was involved in the eosinophil-mediated death of Colo-205 by facilitating contact between effector and target cells. These data underscored the involvement of an additional mediator in eosinophil-mediated antitumor cytotoxicity. Our findings support existing evidence that eosinophils could play a beneficial role in the context of colon cancer.

Regulation of RhoA activity by adhesion molecules and mechanotransduction

The low molecular weight GTP-binding protein RhoA regulates many cellular events, including cell migration, organization of the cytoskeleton, cell adhesion, progress through the cell cycle and gene expression. Physical forces influence these cellular processes in part by regulating RhoA activity through mechanotransduction of cell adhesion molecules (e.g. integrins, cadherins, Ig superfamily molecules). RhoA activity is regulated by guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs) that are themselves regulated by many different signaling pathways. Significantly, the engagement of many cell adhesion molecules can affect RhoA activity in both positive and negative ways. In this brief review, we consider how RhoA activity is regulated downstream from cell adhesion molecules and mechanical force. Finally, we highlight the importance of mechanotransduction signaling to RhoA in normal cell biology as well as in certain pathological states.

JAM-related proteins in mucosal homeostasis and inflammation

Mucosal surfaces are lined by epithelial cells that form a physical barrier protecting the body against external noxious substances and pathogens. At a molecular level, the mucosal barrier is regulated by tight junctions (TJs) that seal the paracellular space between adjacent epithelial cells. Transmembrane proteins within TJs include junctional adhesion molecules (JAMs) that belong to the cortical thymocyte marker for Xenopus family of proteins. JAM family encompasses three classical members (JAM-A, JAM-B, and JAM-C) and related molecules including JAM4, JAM-like protein, Coxsackie and adenovirus receptor (CAR), CAR-like membrane protein and endothelial cell-selective adhesion molecule. JAMs have multiple functions that include regulation of endothelial and epithelial paracellular permeability, leukocyte recruitment during inflammation, angiogenesis, cell migration, and proliferation. In this review, we summarize the current knowledge regarding the roles of the JAM family members in the regulation of mucosal homeostasis and leukocyte trafficking with a particular emphasis on barrier function and its perturbation during pathological inflammation.

Atherogenic mononuclear cell recruitment is facilitated by oxidized lipoprotein-induced endothelial junctional adhesion molecule-A redistribution

BACKGROUND

Junctional adhesion molecule (JAM-) A is a transmembrane protein expressed in many cell types and maintains junctional integrity in endothelial cells. Upon inflammatory stimulation, JAM-A relocates to the apical surface and might thereby facilitate the recruitment of leukocytes.

OBJECTIVE

Although inflammatory JAM-A redistribution is an established process, further effort is required to understand its exact role in the transmigration of mononuclear cells, particularly under atherogenic conditions.

METHODS

By the use of RNA interference and genetic deletion, the role of JAM-A in the transmigration of T cells and monocytes through aortic endothelial cells was investigated. JAM-A-localization and subsequent mononuclear cell rolling, adhesion and transmigration were explored during endothelial inflammation, induced by oxidized LDL or cytokines.

RESULTS

RNA interference or genetic deletion of JAM-A in aortic endothelial cells resulted in a decreased transmigration of mononuclear cells. Treatment of the endothelial cells with oxLDL resulted in an increase of both permeability and apical JAM-A presentation, as shown by bead adhesion and confocal microscopy experiments. Redistribution of JAM-A resulted in an increased leukocyte adhesion and transmigration, which could be inhibited with antibodies against JAM-A or by lovastatin-treatment, but not with the peroxisome proliferator activated receptor gamma-agonist pioglitazone.

CONCLUSIONS

This study demonstrates that redistribution of JAM-A in endothelial cells after stimulation with pro-atherogenic oxidized lipoproteins results in increased transmigration of mononuclear cells. This inflammatory dispersal of JAM-A could be counteracted with statins, revealing a novel aspect of their mechanism of action.

Inhibition of junctional adhesion molecule-A/LFA interaction attenuates leukocyte trafficking and inflammation in brain ischemia/reperfusion injury

Proinflammatory mediators trigger intensive postischemic inflammatory remodeling of the blood-brain barrier (BBB) including extensive brain endothelial cell surface and junctional complex changes. Junctional adhesion molecule-A (JAM-A) is a component of the brain endothelial junctional complex with dual roles: paracellular route occlusion and regulating leukocyte docking and migration. The current study examined the contribution of JAM-A to the regulation of leukocyte (neutrophils and monocytes/macrophages) infiltration and the postischemic inflammatory response in brain ischemia/reperfusion (I/R injury). Brain I/R injury was induced by transient middle cerebral artery occlusion (MCAO) for 30min in mice followed by reperfusion for 0-5days, during which time JAM-A antagonist peptide (JAM-Ap) was administered. The peptide, which inhibits JAM-A/leukocyte interaction by blocking the interaction of the C2 domain of JAM-A with LFA on neutrophils and monocytes/macrophages, attenuated I/R-induced neutrophil and monocyte infiltration into brain parenchyma. Consequently, mice treated with JAM-A peptide during reperfusion had reduced expression (~3-fold) of inflammatory mediators in the ischemic penumbra, reduced infarct size (94±39 vs 211±38mm3) and significantly improved neurological score. BBB hyperpermeability was also reduced. Collectively, these results indicate that JAM-A has a prominent role in regulating leukocyte infiltration after brain I/R injury and could be a new target in limiting post-ischemic inflammation.

Physical biology in cancer. 4. Physical cues guide tumor cell adhesion and migration

As tumor cells metastasize from the primary tumor location to a distant secondary site, they encounter an array of biologically and physically heterogeneous microenvironments. While it is well established that biochemical signals guide all stages of the metastatic cascade, mounting evidence indicates that physical cues also direct tumor cell behavior, including adhesion and migration phenotypes. Physical cues acting on tumor cells in vivo include extracellular matrix mechanical properties, dimensionality, and topography, as well as interstitial flow, hydrodynamic shear stresses, and local forces due to neighboring cells. State-of-the-art technologies have recently enabled us and other researchers to engineer cell microenvironments that mimic specific physical properties of the cellular milieu. Through integration of these engineering strategies, along with physics, molecular biology, and imaging techniques, we have acquired new insights into tumor cell adhesion and migration mechanisms. In this review, we focus on the extravasation and invasion stages of the metastatic cascade. We first discuss the physical role of the endothelium during tumor cell extravasation and invasion and how contractility of endothelial and tumor cells contributes to the ability of tumor cells to exit the vasculature. Next, we examine how matrix dimensionality and stiffness coregulate tumor cell adhesion and migration beyond the vasculature. Finally, we summarize how tumor cells translate and respond to physical cues through mechanotransduction. Because of the critical role of tumor cell mechanotransduction at various stages of the metastatic cascade, targeting signaling pathways involved in tumor cell mechanosensing of physical stimuli may prove to be an effective therapeutic strategy for cancer patients.

High-throughput flow cytometry screening reveals a role for junctional adhesion molecule a as a cancer stem cell maintenance factor

Stem cells reside in niches that regulate the balance between self-renewal and differentiation. The identity of a stem cell is linked with the ability to interact with its niche through adhesion mechanisms. To identify targets that disrupt cancer stem cell (CSC) adhesion, we performed a flow cytometry screen on patient-derived glioblastoma (GBM) cells and identified junctional adhesion molecule A (JAM-A) as a CSC adhesion mechanism essential for self-renewal and tumor growth. JAM-A was dispensable for normal neural stem/progenitor cell (NPC) function, and JAM-A expression was reduced in normal brain versus GBM. Targeting JAM-A compromised the self-renewal of CSCs. JAM-A expression negatively correlated to GBM patient prognosis. Our results demonstrate that GBM-targeting strategies can be identified through screening adhesion receptors and JAM-A represents a mechanism for niche-driven CSC maintenance.

Investigation of the monocyte diapedesis-related LFA-1 and JAM-A gene variants in Turkish coronary heart disease patients

Background

LFA-1/JAM-A interaction plays a significant role in early steps of leukocyte transendothelial migration (diapedesis) which takes part in atherosclerosis pathogenesis. In this population-based case–control study, the frequencies of JAM-A rs790056 and LFA-1 rs8058823 gene polymorphisms in patients with coronary heart disease (CHD) and healthy subjects were investigated and the correlations between the different genotypes and cardiovascular risk factors were analyzed.

Methods

The JAM-A and LFA-1 genotypes were determined in 153 patients with CHD and 124 controls by PCR–RFLP assay.

Results

In CHD patient group, the frequency of JAM-A rs790056 TT genotype and the frequency of T allele were higher when compared with the control group (p = 0.03 and p = 0.007,respectively). In patient groups, the frequency of LFA-1 rs8058823 AA genotype was higher (p = 0.000), and the frequency of AG genotype was lower when compared with the control group (p = 0.031). In the control group, LFA-1 rs8058823 G allele carriers had higher SBP than subjects with AA genotype (p = 0.038), whereas in the CHD patient group, G allele carriers had lower DBP than subjects with AA genotype (p = 0.007). The multivariate logistic regression analysis confirmed that the JAM-A rs790056 TT genotype (OR = 2.472, p = 0.045) and LFA-1 rs8058823 AA genotype (OR = 6.751, p = 0.000) were risk factors for CHD development.

Conclusion

These results suggest that the wild type genotypes and alleles of JAM-A rs790056 (TT genotype and T allele) and LFA-1 rs8058823 (AA genotype and A allele) were found to be risk factors for CHD, whereas rare genotypes and alleles were found to be higher in healthy controls thus being protective.

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JAM-A common genotype and allele were more frequent in CHD group than controls.

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LFA-1 common genotype was more frequent in CHD group than controls.

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LFA-1 rare allele had lower DBP than subjects with common genotype.

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JAM-A and LFA-1 common genotypes were risk factors for CHD development.

TNF-α and IFN-γ promote lymphocyte adhesion to endothelial junctional regions facilitating transendothelial migration

Inflammatory conditions induce redistribution of junctional adhesion receptors toward the apical regions of endothelial cells promoting lymphocyte TEM. Much of the molecular structures of TEM have been revealed; however, the biophysical mechanisms underlying this process remain to be fully elucidated. Here, we used immunofluorescence microscopy and AFM to study endothelial distribution of adhesion molecules upon lymphocyte activation and transmigration. Our immunofluorescence results revealed redistribution of JAM-A and PECAM-1 but not ICAM-1 or VCAM-1 toward the apical junctional regions of HUVECs following a 6-h stimulation with TNF-α and IFN-γ. Consistently, our SCFS studies revealed that Jurkat cell adhesion to stimulated HUVEC monolayers was significantly greater in junctional regions. Enhanced adhesion was mediated mostly by JAM-A receptors. Further AFM adhesion mapping of the homophilic JAM-A/JAM-A interaction on the surfaces of HUVECs revealed a greater number of JAM-A receptors available for binding along junctional regions after TNF-α and IFN-γ stimulation. Our data reveal for the first time that adhesion "hot spots" of JAM-A receptors are involved in initiating lymphocyte TEM under inflammatory conditions.

Endothelial junctional adhesion molecule-a guides monocytes into flow-dependent predilection sites of atherosclerosis

BACKGROUND

Junctional adhesion molecule (JAM)-A expressed in endothelial, epithelial, and blood cells can regulate permeability and leukocyte extravasation. Atherosclerosis develops at sites of disturbed flow in large arteries, but the mechanisms guiding inflammatory cells into these predilection sites remain unknown.

METHODS AND RESULTS

To characterize cell-specific functions of JAM-A in atherosclerosis, we used apolipoprotein E-deficient mice with a somatic or endothelium-specific deficiency in JAM-A and bone marrow chimeras with JAM-A-deficient leukocytes. We show that impaired JAM-A expression in endothelial cells reduced mononuclear cell recruitment into the arterial wall and limited atherosclerotic lesion formation in hyperlipidemic mice. In contrast, JAM-A deficiency in bone marrow cells impeded monocyte de-adhesion, thereby increasing vascular permeability and lesion formation, whereas somatic JAM-A deletion revealed no significant effects. Regions with disturbed flow displayed a focal enrichment and luminal redistribution of endothelial JAM-A and were preferentially protected by its deficiency. The functional expression and redistribution of endothelial JAM-A was increased by oxidized low-density lipoprotein, but confined by atheroprotective laminar flow through an upregulation of microRNA (miR)-145, which repressed JAM-A.

CONCLUSIONS

Our data identify endothelial JAM-A as an important effector molecule integrating atherogenic conditions to direct inflammatory cell entry at predilection sites of atherosclerosis.

Human neutrophil cytoskeletal dynamics and contractility actively contribute to trans-endothelial migration

Transmigration through the endothelium is a key step in the immune response. In our recent work, the mechanical properties of the subendothelial matrix and biophysical state of the endothelium have been identified as key modulators of leukocyte trans-endothelial migration. Here, we demonstrated that neutrophil contractile forces and cytoskeletal dynamics also play an active biophysical role during transmigration through endothelial cell-cell junctions. Using our previously-established model for leukocyte transmigration, we first discovered that >93% of human neutrophils preferentially exploit the paracellular mode of transmigration in our in vitro model, and that is independent of subendothelial matrix stiffness. We demonstrated that inhibition of actin polymerization or depolymerization completely blocks transmigration, thus establishing a critical role for neutrophil actin dynamics in transmigration. Next, inhibition of neutrophil myosin II-mediated contractile forces renders 44% of neutrophils incapable of retracting their trailing edge under the endothelium for several minutes after the majority of the neutrophil transmigrates. Meanwhile, inhibition of neutrophil contractile forces or stabilization of microtubules doubles the time to complete transmigration for the first neutrophils to cross the endothelium. Notably, the time to complete transmigration is significantly reduced for subsequent neutrophils that cross through the same path as a previous neutrophil and is less dependent on neutrophil contractile forces and microtubule dynamics. These results suggest that the first neutrophil induces a gap in endothelial cell-cell adhesions, which “opens the door” in the endothelium and facilitates transmigration of subsequent neutrophils through the same hole. Collectively, this work demonstrates that neutrophils play an active biophysical role during the transmigration step of the immune response.

The receptor for activated complement factor 5 (C5aR) conveys myocardial ischemic damage by mediating neutrophil transmigration

Tissue loss after myocardial ischemia with reperfusion (MI/R) is in part conveyed by neutrophil recruitment to post-ischemic myocardium. Strategies to prevent reperfusion injury would help to limit myocardial damage. The receptor for activated complement factor 5 (C5aR) plays a prominent role in inflammation. We examine the effects of C5aR-deficiency on reperfusion injury after MI/R. C5aR(-/-)-mice and their C57BL/6- (WT) littermates underwent transient myocardial ischemia followed by different time points of reperfusion. Infarct size and leukocyte infiltration were determined. Expression of C5aR, inflammatory cytokines and adhesion molecules were analyzed by real-time RT-PCR. Leukocyte-endothelial interactions were assessed by low-shear adhesion- and transmigration-assays in vitro. Myocardial C5aR mRNA expression was 2.8-fold increased by ischemia. Infarct size per area-at-risk and leukocyte recruitment into infarctions were reduced in C5aR(-/-)-compared to WT-mice as well as in WT mice treated with the C5aR-antagonist JPE1375. IL-6, IL-1β, ICAM-1 and VCAM-1 expression were not different, while TNFα expression was reduced in C5aR(-/-)-mice after MI/R. In vitro, C5aR on leukocytes is required for effective transendothelial migration but not adhesion. Expression of MMP9 and JAM-A, molecules that are involved in leukocyte transmigration, were reduced in C5aR(-/-) mice in vivo. Genetic C5aR deficiency blunts the inflammatory response in murine MI/R resulting in reduced inflammatory cell recruitment, which is due to a C5aR-dependent effect on leukocyte transmigration across inflamed endothelium into the ischemic myocardium. This effect could be related to MMP9- and JAM-A expression in response to ischemia and reperfusion.

A novel role for junctional adhesion molecule-A in tumor proliferation: modulation by an anti-JAM-A monoclonal antibody

To identify new potential targets in oncology, functional approaches were developed using tumor cells as immunogens to select monoclonal antibodies targeting membrane receptors involved in cell proliferation. For that purpose cancer cells were injected into mice and resulting hybridomas were screened for their ability to inhibit cell proliferation in vitro. Based on this functional approach coupled to proteomic analysis, a monoclonal antibody specifically recognizing the human junctional adhesion molecule-A (JAM-A) was defined. Interestingly, compared to both normal and tumor tissues, we observed that JAM-A was mainly overexpressed on breast, lung and kidney tumor tissues. In vivo experiments demonstrated that injections of anti-JAM-A antibody resulted in a significant tumor growth inhibition of xenograft human tumors. Treatment with monoclonal antibody induced a decrease of the Ki67 expression and downregulated JAM-A levels. All together, our results show for the first time that JAM-A can interfere with tumor proliferation and suggest that JAM-A is a potential novel target in oncology. The results also demonstrate that a functional approach coupled to a robust proteomic analysis can be successful to identify new antibody target molecules that lead to promising new antibody-based therapies against cancers.

LFA-1 and ICAM-1 expression induced during melanoma-endothelial cell co-culture favors the transendothelial migration of melanoma cell lines in vitro

Background

Patients with metastatic melanoma have a poor median rate of survival. It is therefore necessary to increase our knowledge about melanoma cell dissemination which includes extravasation, where cancer cells cross the endothelial barrier. Extravasation is well understood during travelling of white blood cells, and involves integrins such as LFA-1 (composed of two chains, CD11a and CD18) expressed by T cells, while ICAM-1 is induced during inflammation by endothelial cells. Although melanoma cell lines cross endothelial cell barriers, they do not express LFA-1. We therefore hypothesized that melanoma-endothelial cell co-culture might induce the LFA-1/ICAM ligand/receptor couple during melanoma transmigration.

Methods

A transwell approach has been used as well as blocking antibodies against CD11a, CD18 and ICAM-1. Data were analyzed with an epifluorescence microscope. Fluorescence intensity was quantified with the ImageJ software.

Results

We show here that HUVEC-conditioned medium induce cell-surface expression of LFA-1 on melanoma cell lines. Similarly melanoma-conditioned medium activates ICAM-1 expression in endothelial cells. Accordingly blocking antibodies of ICAM-1, CD11a or CD18 strongly decrease melanoma transmigration. We therefore demonstrate that melanoma cells can cross endothelial monolayers in vitro due to the induction of ICAM-1 and LFA-1 occurring during the co-culture of melanoma and endothelial cells. Our data further suggest a role of LFA-1 and ICAM-1 in the formation of melanoma cell clumps enhancing tumor cell transmigration.

Conclusion

Melanoma-endothelial cell co-culture induces LFA-1 and ICAM-1 expression, thereby favoring in vitro melanoma trans-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.

Function of junctional adhesion molecules (JAMs) in leukocyte migration and homeostasis

Homeostasis is a word widely used in the scientific community to refer to the property of a system to maintain its uniformity and functionality. In living organisms, the word refers to the concept enunciated 150 years ago by C. Bernard by which external variations must be compensated for in order to maintain internal conditions compatible with life. This is especially true in the case of highly dynamic system such as the hematopoietic system that requires the coordinated control of cell proliferation and death within specialized microenvironments that are anatomically distinct. As a consequence, hematopoietic cell adhesion and migration must be tightly controlled in order for hematopoietic cells to reach and to be maintained in appropriate microenvironments. The junctional adhesion molecules (JAMs) are adhesion molecules that belong to the immunoglobulin superfamily (IgSf) and that have been initially identified as important players controlling vascular permeability and leukocyte transendothelial migration. This involves the regulated localization of the JAMs at lateral endothelial cell/cell borders and their interaction with leukocyte integrins. More recently, some of the JAM family members have also been found to be expressed by stromal cells and to regulate chemokine secretion within lymphoid organs, acting not only on leukocyte transendothelial migration, but also on hematopoietic cell retention within specialized microenvironments. This review summarizes recent progress in understanding the role of the JAMs in leukocyte adhesion and migration to tentatively draw an integrated view of the homeostatic function of the JAMs within the hematopoietic system.

Single-molecule force spectroscopy of the Aplysia cell adhesion molecule reveals two homophilic bonds

Aplysia californica neurons comprise a powerful model system for quantitative analysis of cellular and biophysical properties that are essential for neuronal development and function. The Aplysia cell adhesion molecule (apCAM), a member of the immunoglobulin superfamily of cell adhesion molecules, is present in the growth cone plasma membrane and involved in neurite growth, synapse formation, and synaptic plasticity. apCAM has been considered to be the Aplysia homolog of the vertebrate neural cell adhesion molecule (NCAM); however, whether apCAM exhibits similar binding properties and neuronal functions has not been fully established because of the lack of detailed binding data for the extracellular portion of apCAM. In this work, we used the atomic force microscope to perform single-molecule force spectroscopy of the extracellular region of apCAM and show for the first time (to our knowledge) that apCAM, like NCAM, is indeed a homophilic cell adhesion molecule. Furthermore, like NCAM, apCAM exhibits two distinct bonds in the trans configuration, although the kinetic and structural parameters of the apCAM bonds are quite different from those of NCAM. In summary, these single-molecule analyses further indicate that apCAM and NCAM are species homologs likely performing similar functions.

Relocalization of junctional adhesion molecule A during inflammatory stimulation of brain endothelial cells

Junctional adhesion molecule A (JAM-A) is a unique tight junction (TJ) transmembrane protein that under basal conditions maintains endothelial cell-cell interactions but under inflammatory conditions acts as a leukocyte adhesion molecule. This study investigates the fate of JAM-A during inflammatory TJ complex remodeling and paracellular route formation in brain endothelial cells. The chemokine (C-C motif) ligand 2 (CCL2) induced JAM-A redistribution from the interendothelial cell area to the apical surface, where JAM-A played a role as a leukocyte adhesion molecule participating in transendothelial cell migration of neutrophils and monocytes. JAM-A redistribution was associated with internalization via macropinocytosis during paracellular route opening. A tracer study with dextran-Texas Red indicated that internalization occurred within a short time period (~10 min) by dextran-positive vesicles and then became sorted to dextran-positive/Rab34-positive/Rab5-positive vesicles and then Rab4-positive endosomes. By ~20 min, most internalized JAM-A moved to the brain endothelial cell apical membrane. Treatment with a macropinocytosis inhibitor, 5-(N-ethyl-N-isopropyl)amiloride, or Rab5/Rab4 depletion with small interfering RNA oligonucleotides prevented JAM-A relocalization, suggesting that macropinocytosis and recycling to the membrane surface occur during JAM-A redistribution. Analysis of the signaling pathways indicated involvement of RhoA and Rho kinase in JAM-A relocalization. These data provide new insights into the molecular and cellular mechanisms involved in blood-brain barrier remodeling during inflammation.

Mechanisms of dendritic cell trafficking across the blood-brain barrier

Although the central nervous system (CNS) is considered to be an immunoprivileged site, it is susceptible to a host of autoimmune as well as neuroinflammatory disorders owing to recruitment of immune cells across the blood-brain barrier into perivascular and parenchymal spaces. Dendritic cells (DCs), which are involved in both primary and secondary immune responses, are the most potent immune cells in terms of antigen uptake and processing as well as presentation to T cells. In light of the emerging importance of DC traficking into the CNS, these cells represent good candidates for targeted immunotherapy against various neuroinflammatory diseases. This review focuses on potential physiological events and receptor interactions between DCs and the microvascular endothelial cells of the brain as they transmigrate into the CNS during degeneration and injury. A clear understanding of the underlying mechanisms involved in DC migration may advance the development of new therapies that manipulate these mechanistic properties via pharmacologic intervention. Furthermore, therapeutic validation should be in concurrence with the molecular imaging techniques that can detect migration of these cells in vivo. Since the use of noninvasive methods to image migration of DCs into CNS has barely been explored, we highlighted potential molecular imaging techniques to achieve this goal. Overall, information provided will bring this important leukocyte population to the forefront as key players in the immune cascade in the light of the emerging contribution of DCs to CNS health and disease.

Pathobiology of junctional adhesion molecules

Junctional adhesion molecules are transmembrane proteins that belong to the immunoglobulin superfamily. In addition to their localization in close proximity to the tight junctions in endothelial and epithelial cells, junctional adhesion molecules are also expressed in circulating cells that do not form junctions, such as leukocytes and platelets. As a consequence, these proteins are associated not only with the permeability-regulating barrier function of the tight junctions, but also with other biologic processes, such as inflammatory reactions, responses to vascular injury, and tumor angiogenesis. Furthermore, because of their transmembrane topology, junctional adhesion molecules are poised both for receiving inputs from the cell interior (their expression, localization, and function being regulated in response to inflammatory cytokines and growth factors) and for translating extracellular adhesive events into functional responses. This review focuses on the different roles of junctional adhesion molecules in normal and pathologic conditions, with emphasis on inflammatory reactions and vascular responses to injury.

Atomic force microscopy: a nanoscopic window on the cell surface

Atomic force microscopy (AFM) techniques provide a versatile platform for imaging and manipulating living cells to single-molecule resolution, thereby enabling us to address pertinent questions in key areas of cell biology, including cell adhesion and signalling, embryonic and tissue development, cell division and shape, and microbial pathogenesis. In this review, we describe the principles of AFM, and survey recent breakthroughs made in AFM-based cell nanoscopy, showing how the technology has increased our molecular understanding of the organization, mechanics, interactions and processes of the cell surface. We also discuss the advantages and limitations of AFM techniques, and the challenges remaining to be addressed in future research.

Comparative advantages of mechanical biosensors

Mechanical interactions are fundamental to biology. Mechanical forces of chemical origin determine motility and adhesion on the cellular scale, and govern transport and affinity on the molecular scale. Biological sensing in the mechanical domain provides unique opportunities to measure forces, displacements and mass changes from cellular and subcellular processes. Nanomechanical systems are particularly well matched in size with molecular interactions, and provide a basis for biological probes with single-molecule sensitivity. Here we review micro- and nanoscale biosensors, with a particular focus on fast mechanical biosensing in fluid by mass- and force-based methods, and the challenges presented by non-specific interactions. We explain the general issues that will be critical to the success of any type of next-generation mechanical biosensor, such as the need to improve intrinsic device performance, fabrication reproducibility and system integration. We also discuss the need for a greater understanding of analyte-sensor interactions on the nanoscale and of stochastic processes in the sensing environment.

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.

Stimulated single-cell force spectroscopy to quantify cell adhesion receptor crosstalk

To control their attachment to substrates and other cells, cells regulate their adhesion receptors. One regulatory process is receptor crosstalk, where the binding of one type of cell adhesion molecule influences the activity of another type. To identify such crosstalk and gain insight into their mechanisms, we developed the stimulated single-cell force spectroscopy assay. In this assay, the influence of a cells adhesion to one substrate on the strength of its adhesion to a second substrate is examined. The assay quantifies the adhesion of the cell and the contributions of specific adhesion receptors. This allows mechanisms by which the adhesion is regulated to be determined. Using the assay we identified crosstalk between collagen-binding integrin alpha(1)beta(1) and fibronectin-binding integrin alpha(5)beta(1) in HeLa cells. This crosstalk was unidirectional, from integrin alpha(1)beta(1) to integrin alpha(5)beta(1), and functioned by regulating the endocytosis of integrin alpha(5)beta(1). The single-cell assay should be expandable for the screening and quantification of crosstalk between various cell adhesion molecules and other cell surface receptors.

A biophysical view of the interplay between mechanical forces and signaling pathways during transendothelial cell migration

The vascular endothelium is exposed to an array of physical forces, including shear stress via blood flow, contact with other cells such as neighboring endothelial cells and leukocytes, and contact with the basement membrane. Endothelial cell morphology, protein expression, stiffness and cytoskeletal arrangement are all influenced by these mechanochemical forces. There are many biophysical tools that are useful in studying how forces are transmitted in endothelial cells, and these tools are also beginning to be used to investigate biophysical aspects of leukocyte transmigration, which is a ubiquitous mechanosensitive process. In particular, the stiffness of the substrate has been shown to have a significant impact on cellular behavior, and this is true for both endothelial cells and leukocytes. Thus, the stiffness of the basement membrane as an endothelial substrate, as well as the stiffness of the endothelium as a leukocyte substrate, is relevant to the process of transmigration. In this review, we discuss recent work that has related the biophysical aspects of endothelial cell interactions and leukocyte transmigration to the biochemical pathways and molecular interactions that take place during this process. Further use of biophysical tools to investigate the biological process of leukocyte transmigration will have implications for tissue engineering, as well as atherosclerosis, stroke and immune system disease research.

Recent developments and complexities in neutrophil transmigration

PURPOSE OF REVIEW

As the migration of neutrophils from blood to inflamed tissues is an essential component of innate immunity and a key contributing factor to the pathogenesis of inflammatory disorders, this aspect of leukocyte biology continues to be a highly dynamic field of research. This review summarizes recent findings in this area, focusing on the mechanisms that mediate neutrophil transmigration, an area where significant progress has been made.

RECENT FINDINGS

The topics to be covered will include responses that are prerequisite to neutrophil migration through venular walls, such as leukocyte luminal crawling and cellular and molecular changes in leukocytes and endothelial cells (e.g. formation of protrusions) that collectively support leukocyte transendothelial cell migration. Advances in both paracellular and transcellular neutrophil migration through endothelial cells will be discussed, addressing the associated roles and regulation of expression of endothelial cell luminal and junctional adhesion molecules. Beyond the endothelium, migration through the vascular pericyte coverage and basement membrane will be reviewed.

SUMMARY

The unquestionable role of neutrophils in the development and progression of inflammatory conditions suggests that a better understanding of the tissue-specific and stimulus-specific mechanisms that mediate this response may identify novel pathways that could be exploited for the development of more specific anti-inflammatory interventions.

Structural determinants of Junctional Adhesion Molecule A (JAM-A) function and mechanisms of intracellular signaling

Junctional Adhesion Molecule A (JAM-A) is a multifunctional cell surface protein that has multiple evolutionarily conserved structural features. There is now conclusive evidence that discrete structural elements on JAM-A mediate intracellular signaling events that alter cell migration and paracellular permeability. Specifically, self-dimerization between extracellular Ig-like loops and close apposition of PDZ-dependent, JAM-A-associated intracellular scaffold proteins such as Afadin and guanine-nucleotide exchange factors mediate activation of Rap1 and modulation of epithelial cell migration by effects on beta1 integrin. While the same JAM-A structural features also modulate migration of other cell types and paracellular permeability in epithelia/endothelia, additional signaling proteins/mechanisms are probably involved. Recent insights into JAM-A outside-in signaling events that regulate these cellular functions are discussed.

Regulated release and functional modulation of junctional adhesion molecule A by disintegrin metalloproteinases

Junctional adhesion molecule A (JAM-A) is a transmembrane adhesive glycoprotein that participates in the organization of endothelial tight junctions and contributes to leukocyte transendothelial migration. We demonstrate here that cultured endothelial cells not only express a cellular 43-kDa variant of JAM-A but also release considerable amounts of a 33-kDa soluble JAM-A variant. This release is enhanced by treatment with proinflammatory cytokines and is associated with the down-regulation of surface JAM-A. Inhibition experiments, loss/gain-of-function experiments, and cleavage experiments with recombinant proteases indicated that cleavage of JAM-A is mediated predominantly by the disintegrin and metalloproteinase (ADAM) 17 and, to a lesser extent, by ADAM10. Cytokine treatment of mice increased JAM-A serum level and in excised murine aortas increased ADAM10/17 activity correlated with enhanced JAM-A release. Functionally, soluble JAM-A blocked migration of cultured endothelial cells, reduced transendothelial migration of isolated neutrophils in vitro, and decreased neutrophil infiltration in a murine air pouch model by LFA-1– and JAM-A–dependent mechanisms. Therefore, shedding of JAM-A by inflamed vascular endothelium via ADAM17 and ADAM10 may not only generate a biomarker for vascular inflammation but could also be instrumental in controlling JAM-A functions in the molecular zipper guiding transendothelial diapedesis of leukocytes.