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1.  Tetraspanin CD9 links junctional adhesion molecule-A to αvβ3 integrin to mediate basic fibroblast growth factor–specific angiogenic signaling 
Molecular Biology of the Cell  2013;24(7):933-944.
This paper describes a ternary protein complex consisting of junctional adhesion molecule-A (JAM-A), tetraspanin CD9, and αvβ3 integrin in endothelial cells. In this complex, CD9 links JAM-A to αvβ3 integrin to regulate basic fibroblast growth factor–specific mitogen-activated protein kinase activation, endothelial cell migration, and tube formation. Our findings contribute to a better understanding of the signaling events during angiogenesis.
Junctional adhesion molecule-A (JAM-A) is a member of the immunoglobulin family with diverse functions in epithelial cells, including cell migration, cell contact maturation, and tight junction formation. In endothelial cells, JAM-A has been implicated in basic fibroblast growth factor (bFGF)-regulated angiogenesis through incompletely understood mechanisms. In this paper, we identify tetraspanin CD9 as novel binding partner for JAM-A in endothelial cells. CD9 acts as scaffold and assembles a ternary JAM-A-CD9-αvβ3 integrin complex from which JAM-A is released upon bFGF stimulation. CD9 interacts predominantly with monomeric JAM-A, which suggests that bFGF induces signaling by triggering JAM-A dimerization. Among the two vitronectin receptors, αvβ3 and αvβ5 integrin, which have been shown to cooperate during angiogenic signaling with bFGF and vascular endothelial growth factor (VEGF), respectively, CD9 links JAM-A specifically to αvβ3 integrin. In line with this, knockdown of CD9 blocks bFGF- but not VEGF-induced ERK1/2 activation. JAM-A or CD9 knockdown impairs endothelial cell migration and tube formation. Our findings indicate that CD9 incorporates monomeric JAM-A into a complex with αvβ3 integrin, which responds to bFGF stimulation by JAM-A release to regulate mitogen-activated protein kinase (MAPK) activation, endothelial cell migration, and angiogenesis. The data also provide new mechanistic insights into the cooperativity between bFGF and αvβ3 integrin during angiogenic signaling.
PMCID: PMC3608503  PMID: 23389628
2.  JAM-A associates with ZO-2, afadin, and PDZ-GEF1 to activate Rap2c and regulate epithelial barrier function 
Molecular Biology of the Cell  2013;24(18):2849-2860.
Intestinal barrier function is regulated by epithelial tight junctions, structures that control paracellular permeability. JAM-A regulates epithelial permeability through association with ZO-2, afadin, and PDZ-GEF1 to activate Rap2c and control contraction of the apical cytoskeleton.
Intestinal barrier function is regulated by epithelial tight junctions (TJs), structures that control paracellular permeability. Junctional adhesion molecule-A (JAM-A) is a TJ-associated protein that regulates barrier; however, mechanisms linking JAM-A to epithelial permeability are poorly understood. Here we report that JAM-A associates directly with ZO-2 and indirectly with afadin, and this complex, along with PDZ-GEF1, activates the small GTPase Rap2c. Supporting a functional link, small interfering RNA–mediated down-regulation of the foregoing regulatory proteins results in enhanced permeability similar to that observed after JAM-A loss. JAM-A–deficient mice and cultured epithelial cells demonstrate enhanced paracellular permeability to large molecules, revealing a potential role of JAM-A in controlling perijunctional actin cytoskeleton in addition to its previously reported role in regulating claudin proteins and small-molecule permeability. Further experiments suggest that JAM-A does not regulate actin turnover but modulates activity of RhoA and phosphorylation of nonmuscle myosin, both implicated in actomyosin contraction. These results suggest that JAM-A regulates epithelial permeability via association with ZO-2, afadin, and PDZ-GEF1 to activate Rap2c and control contraction of the apical cytoskeleton.
PMCID: PMC3771947  PMID: 23885123
3.  Junctional adhesion molecule-C is a soluble mediator of angiogenesis 
Junctional adhesion molecule-C (JAM-C) is an adhesion molecule expressed by endothelial cells that plays a role in tight junction formation, leukocyte adhesion, and trans-endothelial migration. In the present study, we investigated whether JAM-C is found in soluble form and if soluble JAM-C (sJAM-C) mediates angiogenesis. We found that JAM-C is present in soluble form in normal serum and elevated in rheumatoid arthritis (RA) serum. The concentration of sJAM-C is also elevated locally in RA synovial fluid compared to RA serum or osteoarthritis synovial fluid. sJAM-C was also present in the culture supernatant of human microvascular endothelial cells (HMVECs) and immortalized human dermal microvascular endothelial cells (HMEC-1s), and its concentration was increased following cytokine stimulation. In addition, sJAM-C cleavage from the cell surface was mediated in part by a disintegrin and metalloproteinase 10 (ADAM10) and ADAM17. In functional assays, sJAM-C was both chemotatic and chemokinetic for HMVECs, and induced HMVEC tube formation on Matrigel in vitro. Neutralizing anti-JAM-C antibodies inhibited RA synovial fluid induced HMVEC chemotaxis and sJAM-C induced HMVEC tube formation on Matrigel. sJAM-C also induced angiogenesis in vivo in the Matrigel plug and sponge granuloma models. Moreover, sJAM-C mediated HMVEC chemotaxis was dependent on Src, p38, and PI3K. Our results show that JAM-C exists in soluble form, and suggest that modulation of sJAM-C may provide a novel route for controling pathological angiogenesis.
PMCID: PMC3003428  PMID: 20592283
4.  Dual Interaction of JAM-C with JAM-B and αMβ2 Integrin: Function in Junctional Complexes and Leukocyte AdhesionD⃞ 
Molecular Biology of the Cell  2005;16(10):4992-5003.
The junctional adhesion molecules (JAMs) have been recently described as interendothelial junctional molecules and as integrin ligands. Here we show that JAM-B and JAM-C undergo heterophilic interaction in cell-cell contacts and that JAM-C is recruited and stabilized in junctional complexes by JAM-B. In addition, soluble JAM-B dissociates soluble JAM-C homodimers to form JAM-B/JAM-C heterodimers. This suggests that the affinity of JAM-C monomers to form dimers is higher for JAM-B than for JAM-C. Using antibodies against JAM-C, the formation of JAM-B/JAM-C heterodimers can be abolished. This liberates JAM-C from its vascular binding partner JAM-B and makes it available on the apical side of vessels for interaction with its leukocyte counterreceptor αMβ2 integrin. We demonstrate that the modulation of JAM-C localization in junctional complexes is a new regulatory mechanism for αMβ2-dependent adhesion of leukocytes.
PMCID: PMC1237098  PMID: 16093349
5.  Junctional Adhesion Molecule-A Regulates Vascular Endothelial Growth Factor Receptor-2 Signaling-Dependent Mouse Corneal Wound Healing 
PLoS ONE  2013;8(5):e63674.
Inflammation and angiogenesis are integral parts of wound healing. However, excessive and persistent wound-induced inflammation and angiogenesis in an avascular tissue such as the cornea may be associated with scarring and visual impairment. Junctional adhesion molecule A (Jam-A) is a tight junction protein that regulates leukocyte transmigration as well as fibroblast growth factor-2 (FGF-2)-induced angiogenesis. However its function in wound-induced inflammation and angiogenesis is still unknown. In this study, we report spontaneous corneal opacity in Jam-A deficient mice associated with inflammation, angiogenesis and the presence of myofibroblasts. Since wounds and/or corneal infections cause corneal opacities, we tested the role of Jam-A in wound-induced inflammation, angiogenesis and scarring by subjecting Jam-A deficient mice to full thickness corneal wounding. Analysis of these wounds demonstrated increased inflammation, angiogenesis, and increased number of myofibroblasts thereby indicating that Jam-A regulates the wound-healing response by controlling wound-induced inflammation, angiogenesis and scarring in the cornea. These effects were not due to inflammation alone since the inflammation-induced wound-healing response in Jam-A deficient mice was similar to wild type mice. In order to determine the molecular mechanism associated with the observed aberrant corneal wound healing in Jam-A deficient mice, we assessed the expression of the components of vascular endothelial growth factor A (VEGF-A)/vascular endothelial growth factor receptor- 2(VEGFR-2) signaling pathway. Interestingly, we observed increased levels of VEGF-A mRNA in Jam-A deficient eyes. We also observed nuclear localization of phosphorylated SMAD3 (pSMAD3) indicative of TGFβ pathway activation in the Jam-A deficient eyes. Furthermore the increased wound-induced corneal inflammation, angiogenesis, and scarring in Jam-A deficient mice was attenuated by treatment with DC101, an anti-vascular endothelial growth factor receptor-2 (VEGFR-2) antibody. Our results suggest that in the absence of Jam-A, the VEGF-A/VEGFR-2 pathway is upregulated, thereby augmenting wound induced corneal inflammation, angiogenesis, and myofibroblast accumulation leading to scarring.
PMCID: PMC3648504  PMID: 23667656
6.  Evaluation of Soluble Junctional Adhesion Molecule-A as a Biomarker of Human Brain Endothelial Barrier Breakdown 
PLoS ONE  2010;5(10):e13568.
An inducible release of soluble junctional adhesion molecule-A (sJAM-A) under pro-inflammatory conditions was described in cultured non-CNS endothelial cells (EC) and increased sJAM-A serum levels were found to indicate inflammation in non-CNS vascular beds. Here we studied the regulation of JAM-A expression in cultured brain EC and evaluated sJAM-A as a serum biomarker of blood-brain barrier (BBB) function.
Methodology/Principal Findings
As previously reported in non-CNS EC types, pro-inflammatory stimulation of primary or immortalized (hCMEC/D3) human brain microvascular EC (HBMEC) induced a redistribution of cell-bound JAM-A on the cell surface away from tight junctions, along with a dissociation from the cytoskeleton. This was paralleled by reduced immunocytochemical staining of occludin and zonula occludens-1 as well as by increased paracellular permeability for dextran 3000. Both a self-developed ELISA test and Western blot analysis detected a constitutive sJAM-A release by HBMEC into culture supernatants, which importantly was unaffected by pro-inflammatory or hypoxia/reoxygenation challenge. Accordingly, serum levels of sJAM-A were unaltered in 14 patients with clinically active multiple sclerosis compared to 45 stable patients and remained unchanged in 13 patients with acute ischemic non-small vessel stroke over time.
Soluble JAM-A was not suited as a biomarker of BBB breakdown in our hands. The unexpected non-inducibility of sJAM-A release at the human BBB might contribute to a particular resistance of brain EC to inflammatory stimuli, protecting the CNS compartment.
PMCID: PMC2958838  PMID: 21060661
7.  JAM4, a Junctional Cell Adhesion Molecule Interacting with a Tight Junction Protein, MAGI-1 
Molecular and Cellular Biology  2003;23(12):4267-4282.
MAGI-1 is a membrane-associated guanylate kinase protein at tight junctions in epithelial cells. It interacts with various molecules and functions as a scaffold protein at cell junctions. We report here a novel MAGI-1-binding protein that we named junctional adhesion molecule 4 (JAM4). JAM4 belongs to an immunoglobulin protein family. JAM4 was colocalized with ZO-1 in kidney glomeruli and in intestinal epithelial cells. Biochemical in vitro studies revealed that JAM4 bound to MAGI-1 but not to ZO-1, whereas JAM1 did not bind to MAGI-1. JAM4 and MAGI-1 interacted with each other and formed clusters in COS-7 cells when coexpressed. JAM4 mediated calcium-independent homophilic adhesion and was accumulated at cell-cell contacts when expressed in L cells. MAGI-1, ZO-1, and occludin were recruited to JAM4-based cell contacts. JAM4 also reduced the permeability of CHO cell monolayers. MAGI-1 strengthened JAM4-mediated cell adhesion in L cells and sealing effects in CHO cells. These findings suggest that JAM4 together with MAGI-1 provides an adhesion machinery at tight junctions, which may regulate the permeability of kidney glomerulus and small intestinal epithelial cells.
PMCID: PMC156145  PMID: 12773569
8.  Expression of JAM-A in the Human Corneal Endothelium and Retinal Pigment Epithelium: Localization and Evidence for Role in Barrier Function 
Junctional adhesion molecules (JAMs) are a family of adhesion proteins found in intercellular junctions. Evidence suggests that JAM-A is important for the regulation of tight junction assembly and epithelial barrier function. The authors recently reported that JAM-A is expressed in rabbit corneal endothelium and that antibody to JAM-A produces corneal swelling. In the present study, they investigate JAM-A expression in the human corneal endothelium and retinal pigment epithelium (RPE) and examine the effect of a function-blocking antibody to JAM-A on the permeability of cultured RPE cell monolayers.
Expression of JAM-A in human corneal endothelium, human RPE tissue, and cultured ARPE-19 monolayers was assessed by immunofluorescence confocal microscopy. Localization of JAM-A was compared with the tight junction-associated protein zonula occludens-1 (ZO-1). To investigate JAM-A function in ARPE-19 cells, ARPE-19 monolayers were subjected to a calcium switch protocol to disrupt cell junctions and treated with a function-blocking antibody to JAM-A or an isotype-matched control. Dextran flux assays were performed to assess the effect of JAM-A antibody on ARPE-19 monolayer permeability.
Expression of JAM-A was observed in human corneal endothelium, and its distribution correlated with the tight junction-associated protein ZO-1. In addition, expression of JAM-A was observed in human RPE and in intercellular junctions of ARPE-19 monolayers. The localization pattern of JAM-A in the RPE and ARPE-19 monolayers was similar to that of ZO-1. ARPE-19 monolayers treated with antibody to JAM-A demonstrated a 33% increase in permeability to 10,000 MWt dextran compared with monolayers treated with control antibody.
Results of this study provide new information about JAM-A expression in tight junctions of the human corneal endothelium and human RPE. The observation that antibodies to JAM-A increase ARPE-19 monolayer permeability is consistent with previous findings of JAM-A function in epithelial tight junctions and suggests JAM-A may have a role in the regulation of RPE barrier function.
PMCID: PMC2074894  PMID: 17724169
9.  Junctional Adhesion Molecule 2 Mediates the Interaction between Hatched Blastocyst and Luminal Epithelium: Induction by Progesterone and LIF 
PLoS ONE  2012;7(4):e34325.
Junctional adhesion molecule 2 (Jam2) is a member of the JAM superfamily. JAMs are localized at intercellular contacts and participated in the assembly and maintenance of junctions, and control of cell permeability. Because Jam2 is highly expressed in the luminal epithelium on day 4 of pregnancy, this study was to determine whether Jam2 plays a role in uterine receptivity and blastocyst attachment in mouse uterus.
Methodology/Principal Findings
Jam2 is highly expressed in the uterine luminal epithelium on days 3 and 4 of pregnancy. Progesterone induces Jam2 expression in ovariectomized mice, which is blocked by progesterone antagonist RU486. Jam2 expression on day 4 of pregnancy is also inhibited by RU486 treatment. Leukemia inhibitory factor (LIF) up-regulates Jam2 protein in isolated luminal epithelium from day 4 uterus, which is blocked by S3I-201, a cell-permeable inhibitor for Stat3 phosphorylation. Under adhesion assay, recombinant Jam2 protein increases the rate of blastocyst adhesion. Both soluble recombinant Jam2 and Jam3 can reverse this process.
Jam2 is highly expressed in the luminal epithelium of receptive uterus and up-regulated by progesterone and LIF via tyrosine phosphorylation of Stat3. Jam2 may play a role in the interaction between hatched blastocyst and receptive uterus.
PMCID: PMC3325240  PMID: 22511936
10.  Junctional Adhesion Molecule (JAM)-C Deficient C57BL/6 Mice Develop a Severe Hydrocephalus 
PLoS ONE  2012;7(9):e45619.
The junctional adhesion molecule (JAM)-C is a widely expressed adhesion molecule regulating cell adhesion, cell polarity and inflammation. JAM-C expression and function in the central nervous system (CNS) has been poorly characterized to date. Here we show that JAM-C−/− mice backcrossed onto the C57BL/6 genetic background developed a severe hydrocephalus. An in depth immunohistochemical study revealed specific immunostaining for JAM-C in vascular endothelial cells in the CNS parenchyma, the meninges and in the choroid plexus of healthy C57BL/6 mice. Additional JAM-C immunostaining was detected on ependymal cells lining the ventricles and on choroid plexus epithelial cells. Despite the presence of hemorrhages in the brains of JAM-C−/− mice, our study demonstrates that development of the hydrocephalus was not due to a vascular function of JAM-C as endothelial re-expression of JAM-C failed to rescue the hydrocephalus phenotype of JAM-C−/− C57BL/6 mice. Evaluation of cerebrospinal fluid (CSF) circulation within the ventricular system of JAM-C−/− mice excluded occlusion of the cerebral aqueduct as the cause of hydrocephalus development but showed the acquisition of a block or reduction of CSF drainage from the lateral to the 3rd ventricle in JAM-C−/− C57BL/6 mice. Taken together, our study suggests that JAM-C−/− C57BL/6 mice model the important role for JAM-C in brain development and CSF homeostasis as recently observed in humans with a loss-of-function mutation in JAM-C.
PMCID: PMC3445510  PMID: 23029139
11.  Junctional adhesion molecule (JAM)-C mediates leukocyte infiltration in response to ischemia reperfusion injury 
JAM-C is an adhesion molecule that has multiple roles in inflammation and vascular biology but many aspects of its functions under pathological conditions are unknown. Here we investigated the role of JAM-C in leukocyte migration in response to ischemia reperfusion (I/R) injury.
Methods and Results
Pre-treatment of mice with soluble JAM-C (sJAM-C), used as a pharmacological blocker of JAM-C-mediated reactions, significantly suppressed leukocyte migration in models of kidney and cremaster muscle I/R injury (39 and 51% inhibition, respectively). Furthermore, in the cremaster muscle model (studied by intravital microscopy), both leukocyte adhesion and transmigration were suppressed in JAM-C deficient mice (JAM-C−/−) and enhanced in mice over-expressing JAM-C in their endothelial cells (ECs). Analysis of JAM-C subcellular expression by immunoelectron microscopy indicated that in I/R-injured tissues, EC JAM-C was redistributed from cytoplasmic vesicles and EC junctional sites to non-junctional plasma membranes, a response that may account for the role of JAM-C in both leukocyte adhesion and transmigration under conditions of I/R injury.
The findings demonstrate a role for EC JAM-C in mediating leukocyte adhesion and transmigration in response to I/R injury and indicate the existence of a novel regulatory mechanism for redistribution and hence function of EC JAM-C in vivo.
PMCID: PMC2746810  PMID: 19574560
JAM-C; Ischemia reperfusion injury; Leukocyte transmigration; Inflammation; Adhesion molecules
12.  Schwann cell-specific JAM-C-deficient mice reveal novel expression and functions for JAM-C in peripheral nerves 
The FASEB Journal  2012;26(3):1064-1076.
Junctional adhesion molecule-C (JAM-C) is an adhesion molecule expressed at junctions between adjacent endothelial and epithelial cells and implicated in multiple inflammatory and vascular responses. In addition, we recently reported on the expression of JAM-C in Schwann cells (SCs) and its importance for the integrity and function of peripheral nerves. To investigate the role of JAM-C in neuronal functions further, mice with a specific deletion of JAM-C in SCs (JAM-C SC KO) were generated. Compared to wild-type (WT) controls, JAM-C SC KO mice showed electrophysiological defects, muscular weakness, and hypersensitivity to mechanical stimuli. In addressing the underlying cause of these defects, nerves from JAM-C SC KO mice were found to have morphological defects in the paranodal region, exhibiting increased nodal length as compared to WTs. The study also reports on previously undetected expressions of JAM-C, namely on perineural cells, and in line with nociception defects of the JAM-C SC KO animals, on finely myelinated sensory nerve fibers. Collectively, the generation and characterization of JAM-C SC KO mice has provided unequivocal evidence for the involvement of SC JAM-C in the fine organization of peripheral nerves and in modulating multiple neuronal responses.—Colom, B., Poitelon, Y., Huang, W., Woodfin, A., Averill, S., Del Carro, U., Zambroni, D., Brain, S. D., Perretti, M., Ahluwalia, A., Priestley, J. V., Chavakis, T., Imhof, B. A., Feltri, M. L., Nourshargh, S. Schwann cell-specific JAM-C-deficient mice reveal novel expression and functions for JAM-C in peripheral nerves.
PMCID: PMC3370675  PMID: 22090315
adhesion molecules; tight junctions; peripheral nerves
13.  Endothelial cell junctional adhesion molecule C plays a key role in the development of tumors in a murine model of ovarian cancer 
The FASEB Journal  2013;27(10):4244-4253.
Junctional adhesion molecule C (JAM-C) is a transmembrane protein with significant roles in regulation of endothelial cell (EC) functions, including immune cell recruitment and angiogenesis. As these responses are important in promoting tumor growth, the role of EC JAM-C in tumor development was investigated using the ID8 syngeneic model of ovarian cancer. Within 10–15 wk, intraperitoneally injected ID8 cells form multiple tumor deposits and ascites that resemble human high-grade serous ovarian cancer. Compared to wild-type mice, survival in this model was increased in EC JAM-C knockouts (KOs; 88 vs. 96 d, P=0.04) and reduced in EC JAM-C transgenics (88 vs. 78.5 d, P=0.03), mice deficient in or overexpressing EC JAM-C, respectively. While tumor growth was significantly reduced in EC JAM-C KOs (87% inhibition at 10 wk, P<0.0005), this was not associated with alterations in tumor vessel density or immune cell infiltration. However, tumor microvessels from EC JAM-C-deficient mice exhibited reduced pericyte coverage and increased vascular leakage, suggesting a role for EC JAM-C in the development of functional tumor vessels. These findings provide evidence for a role for EC JAM-C in tumor growth and aggressiveness as well as recruitment of pericytes to newly formed blood vessels in a model of ovarian cancer.—Leinster, D. A., Colom, B., Whiteford, J. R., Ennis, D. P., Lockley, M., McNeish, I. A., Aurrand-Lions, M., Chavakis, T., Imhof, B. A., Balkwill, F. R., Nourshargh, S. Endothelial cell junctional adhesion molecule C plays a key role in the development of tumors in a murine model of ovarian cancer.
PMCID: PMC3819510  PMID: 23825230
pericytes; angiogenesis; vascular development; immune cell infiltrate
14.  Evidence for cross-reactivity of JAM-C antibodies: implications for cellular localization studies 
Background information
JAM-C (junctional adhesion molecule C) has been implicated in the regulation of leukocyte migration, cell polarity, spermatogenesis, angiogenesis and nerve conduction. JAM-C has been also reported to concentrate at TJs (tight junctions) and desmosomes, although detailed localization studies remain incomplete.
Monoclonal (LUCA14, MAB1189, Gi11, and PACA4) and polyclonal (40–9000) antibodies were employed to evaluate JAM-C expression/localization in various epithelial cell lines. However, RT–PCR (reverse transcription– PCR) assays revealed no JAM-C mRNA in SK-CO15, HeLa and HPAF-II cells, whereas abundant mRNA was detected in platelets, Caco-2 and ARPE cells. Interestingly, immunofluorescence localization in all cells revealed strong intercellular junctional staining with all of the above antibodies, except PACA4. Given the positive staining results in cells lacking JAM-C mRNA, immunoblot analyses were performed. Western blots revealed a prominent protein band at 52 kDa in all cells tested with all antibodies except PACA4. However, the correct size of JAM-C (37 kDa) was only detected in cells containing JAM-C mRNA. Immunofluorescence staining of JAM-C mRNA-expressing Caco-2 cells using mAb PACA4 revealed co-localization with occludin and ZO-1 (zonula occludens 1) at TJs. Analyses by MS identified the cross-reactive 52 kDa protein band as K8 (keratin 8). Furthermore, siRNA (small interfering RNA)-mediated downregulation of K8 in JAM-C mRNA-negative cells resulted in diminished junctional staining along with a reduction in the intensity of the 52 kDa protein band. Using an antibody specific for K8 phosphorylated at Ser73, the 52 kDa protein was identified as this phosphorylated form of K8.
The results from the present study demonstrate that a majority of available anti-human JAM-C antibodies cross-react with phosphorylated K8 and suggest that cellular localization studies using these reagents should be interpreted with caution. Of the JAM-C antibodies tested, only mAb PACA4 is monospecific for human JAM-C. Analyses using PACA4 reveal that JAM-C expression is variable in different epithelial cell lines with co-localization at TJs.
PMCID: PMC2877264  PMID: 19143587
antibody specificity; epithelial cell; junctional adhesion molecule C (JAM-C); keratin; tight junction
15.  Trans-dimerization of JAM-A regulates Rap2 and is mediated by a domain that is distinct from the cis-dimerization interface 
Molecular Biology of the Cell  2014;25(10):1574-1585.
Junctional adhesion molecule-A (JAM-A) is a tight junction–associated signaling protein that homodimerizes across cells at a unique motif to activate the small GTPase Rap2, previously implicated in the regulation of barrier function. JAM-A may therefore act as a barrier-inducing molecular switch that is activated when cells become confluent.
Junctional adhesion molecule-A (JAM-A) is a tight junction–associated signaling protein that regulates epithelial cell proliferation, migration, and barrier function. JAM-A dimerization on a common cell surface (in cis) has been shown to regulate cell migration, and evidence suggests that JAM-A may form homodimers between cells (in trans). Indeed, transfection experiments revealed accumulation of JAM-A at sites between transfected cells, which was lost in cells expressing cis- or predicted trans-dimerization null mutants. Of importance, microspheres coated with JAM-A containing alanine substitutions to residues 43NNP45 (NNP-JAM-A) within the predicted trans-dimerization site did not aggregate. In contrast, beads coated with cis-null JAM-A demonstrated enhanced clustering similar to that observed with wild-type (WT) JAM-A. In addition, atomic force microscopy revealed decreased association forces in NNP-JAM-A compared with WT and cis-null JAM-A. Assessment of effects of JAM-A dimerization on cell signaling revealed that expression of trans- but not cis-null JAM-A mutants decreased Rap2 activity. Furthermore, confluent cells, which enable trans-dimerization, had enhanced Rap2 activity. Taken together, these results suggest that trans-dimerization of JAM-A occurs at a unique site and with different affinity compared with dimerization in cis. Trans-dimerization of JAM-A may thus act as a barrier-inducing molecular switch that is activated when cells become confluent.
PMCID: PMC4019489  PMID: 24672055
16.  Breast cancer cell migration is regulated through junctional adhesion molecule-A-mediated activation of Rap1 GTPase 
The adhesion protein junctional adhesion molecule-A (JAM-A) regulates epithelial cell morphology and migration, and its over-expression has recently been linked with increased risk of metastasis in breast cancer patients. As cell migration is an early requirement for tumor metastasis, we sought to identify the JAM-A signalling events regulating migration in breast cancer cells.
MCF7 breast cancer cells (which express high endogenous levels of JAM-A) and primary cultures from breast cancer patients were used for this study. JAM-A was knocked down in MCF7 cells using siRNA to determine the consequences for cell adhesion, cell migration and the protein expression of various integrin subunits. As we had previously demonstrated a link between the expression of JAM-A and β1-integrin, we examined activation of the β1-integrin regulator Rap1 GTPase in response to JAM-A knockdown or functional antagonism. To test whether JAM-A, Rap1 and β1-integrin lie in a linear pathway, we tested functional inhibitors of all three proteins separately or together in migration assays. Finally we performed immunoprecipitations in MCF7 cells and primary breast cells to determine the binding partners connecting JAM-A to Rap1 activation.
JAM-A knockdown in MCF7 breast cancer cells reduced adhesion to, and migration through, the β1-integrin substrate fibronectin. This was accompanied by reduced protein expression of β1-integrin and its binding partners αV- and α5-integrin. Rap1 activity was reduced in response to JAM-A knockdown or inhibition, and pharmacological inhibition of Rap1 reduced MCF7 cell migration. No additive anti-migratory effect was observed in response to simultaneous inhibition of JAM-A, Rap1 and β1-integrin, suggesting that they lie in a linear migratory pathway. Finally, in an attempt to elucidate the binding partners putatively linking JAM-A to Rap1 activation, we have demonstrated the formation of a complex between JAM-A, AF-6 and the Rap1 activator PDZ-GEF2 in MCF7 cells and in primary cultures from breast cancer patients.
Our findings provide compelling evidence of a novel role for JAM-A in driving breast cancer cell migration via activation of Rap1 GTPase and β1-integrin. We speculate that JAM-A over-expression in some breast cancer patients may represent a novel therapeutic target to reduce the likelihood of metastasis.
PMCID: PMC3219194  PMID: 21429211
17.  Junctional Adhesion Molecule C Mediates Leukocyte Adhesion to Rheumatoid Arthritis Synovium 
Arthritis and rheumatism  2008;58(10):3020-3029.
Leukocyte infiltration into the rheumatoid arthritis (RA) synovium is a multistep process in which leukocytes leave the bloodstream and invade the synovial tissue (ST). Leukocyte transendothelial migration and adhesion to RA ST requires adhesion molecules on the surface of endothelial cells and RA ST fibroblasts. This study was undertaken to investigate the role of junctional adhesion molecule C (JAM-C) in mediating leukocyte recruitment and retention in the RA joint.
Immunohistologic analysis was performed on RA, osteoarthritis (OA), and normal ST samples to quantify JAM-C expression. Fibroblast JAM-C expression was also analyzed using Western blotting, cell surface enzyme-linked immunosorbent assay, and immunofluorescence. To determine the role of JAM-C in leukocyte retention in the RA synovium, in vitro and in situ adhesion assays and RA ST fibroblast transmigration assays were performed.
JAM-C was highly expressed by RA ST lining cells, and its expression was increased in OA ST and RA ST endothelial cells compared with normal ST endothelial cells. JAM-C was also expressed on the surface of OA ST and RA ST fibroblasts. Furthermore, we demonstrated that myeloid U937 cell adhesion to both OA ST and RA ST fibroblasts and to RA ST was dependent on JAM-C. U937 cell migration through an RA ST fibroblast monolayer was enhanced in the presence of neutralizing antibodies against JAM-C.
Our results highlight the novel role of JAM-C in recruiting and retaining leukocytes in the RA synovium and suggest that targeting JAM-C may be important in combating inflammatory diseases such as RA.
PMCID: PMC2911024  PMID: 18821692
18.  Targeting of Junctional Adhesion Molecule-C Inhibits Experimental Choroidal Neovascularization 
JAM-C blockade may be useful for CNV suppression by inhibiting macrophage transmigration, RPE cell migration, and monolayer RPE barrier malfunction. These data reveal a novel function of JAM-C and demonstrate that JAM-C may be a compelling target for CNV therapy.
To identify the expression of junctional adhesion molecule-C (JAM-C) in choroidal neovascularization (CNV) and evaluate the effect of JAM-C targeting on CNV formation and on cellular functions relevant to CNV in vitro, such as macrophage transmigration, human retinal pigment epithelial (hRPE) cell migration, and monolayer RPE permeability.
JAM-C expression in CNV was analyzed by real-time PCR, immunoblot analysis, and immunofluorescence staining. CNV area and blood vessel leakage were quantified using isolectin B4 staining and fluorescein angiography, respectively, 1 week after laser treatment. Macrophage infiltration within the CNV area was measured by immunofluorescence, and transmigration through monolayer RPE was analyzed using a transepithelial migration assay. After JAM-C shRNA transfection, human RPE cell migration was quantified using a transwell assay, and monolayer RPE permeability was determined by measuring the apical-to-basolateral movements of sodium fluorescein.
JAM-C expression was upregulated during CNV formation after laser treatment in a time-dependent manner. However, no change in JAM-C expression was found in the retina up to 14 days after laser treatment. JAM-C targeting by intravitreal injection of JAM-C Fc chimera inhibited CNV, blood vessel leakage, and macrophage infiltration. JAM-C Fc chimera inhibited basolateral-to-apical transmigration in vitro through a monolayer of hRPE of macrophages from patients with wet AMD. In addition, shRNA-mediated JAM-C knockdown inhibited hRPE cell migration and hRPE permeability.
JAM-C blockade may prove useful for CNV suppression by inhibiting macrophage transmigration, RPE cell migration, and monolayer RPE barrier malfunction.
PMCID: PMC3339919  PMID: 22323465
19.  Relocalization of Junctional Adhesion Molecule A during Inflammatory Stimulation of Brain Endothelial Cells 
Molecular and Cellular Biology  2012;32(17):3414-3427.
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.
PMCID: PMC3422009  PMID: 22733993
20.  Blockade but Not Overexpression of the Junctional Adhesion Molecule C Influences Virus-Induced Type 1 Diabetes in Mice 
PLoS ONE  2013;8(1):e54675.
Type 1 diabetes (T1D) results from the autoimmune destruction of insulin-producing beta-cells in the pancreas. Recruitment of inflammatory cells is prerequisite to beta-cell-injury. The junctional adhesion molecule (JAM) family proteins JAM-B and JAM–C are involved in polarized leukocyte transendothelial migration and are expressed by vascular endothelial cells of peripheral tissue and high endothelial venules in lympoid organs. Blocking of JAM-C efficiently attenuated cerulean-induced pancreatitis, rheumatoid arthritis or inflammation induced by ischemia and reperfusion in mice. In order to investigate the influence of JAM-C on trafficking and transmigration of antigen-specific, autoaggressive T-cells, we used transgenic mice that express a protein of the lymphocytic choriomeningitis virus (LCMV) as a target autoantigen in the β-cells of the islets of Langerhans under the rat insulin promoter (RIP). Such RIP-LCMV mice turn diabetic after infection with LCMV. We found that upon LCMV-infection JAM-C protein was upregulated around the islets in RIP-LCMV mice. JAM-C expression correlated with islet infiltration and functional beta-cell impairment. Blockade with a neutralizing anti-JAM-C antibody reduced the T1D incidence. However, JAM-C overexpression on endothelial cells did not accelerate diabetes in the RIP-LCMV model. In summary, our data suggest that JAM-C might be involved in the final steps of trafficking and transmigration of antigen-specific autoaggressive T-cells to the islets of Langerhans.
PMCID: PMC3556033  PMID: 23372751
21.  Jam1a – Jam2a interactions regulate haematopoietic stem cell fate through Notch signalling 
Nature  2014;512(7514):319-323.
Notch signalling plays a key role in the generation of haematopoietic stem cells (HSCs) during vertebrate development1-3 and requires intimate contact between signal emitting and receiving cells, although little is known regarding when, where, and how these intercellular events occur. We previously reported that the somitic Notch ligands, Dlc and Dld, are essential for HSC specification4. It has remained unclear, however, how these somitic requirements are connected to the later emergence of HSCs from the dorsal aorta (DA). Here we show that Notch signalling establishes HSC fate as their shared vascular precursors migrate across the ventral face of the somite and that Junctional adhesion molecules (JAMs) mediate this required Notch signal transduction. HSC precursors express jam1a and migrate axially across the ventral somite, where Jam2a and Notch ligands Dlc and Dld are expressed. Despite no alteration in the expression of Notch ligand or receptor genes, loss of function of jam1a led to loss of Notch signalling and loss of HSCs. Enforced activation of Notch in shared vascular precursors rescued HSCs in jam1a or jam2a deficient embryos. Together, these results indicate that Jam1a – Jam2a interactions facilitate the transduction of requisite Notch signals from the somite to the precursors of HSCs, and that these events occur well before formation of the DA.
PMCID: PMC4237229  PMID: 25119047
22.  Structural determinants of Junctional Adhesion Molecule A (JAM-A) function and mechanisms of intracellular signaling 
Current opinion in cell biology  2009;21(5):701-707.
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 β1 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 likely involved. Recent insights into JAM-A outside-in signaling events that regulate these cellular functions are discussed.
PMCID: PMC2776699  PMID: 19608396
23.  JAM-A regulates permeability and inflammation in the intestine in vivo 
The Journal of Experimental Medicine  2007;204(13):3067-3076.
Recent evidence has linked intestinal permeability to mucosal inflammation, but molecular studies are lacking. Candidate regulatory molecules localized within the tight junction (TJ) include Junctional Adhesion Molecule (JAM-A), which has been implicated in the regulation of barrier function and leukocyte migration. Thus, we analyzed the intestinal mucosa of JAM-A–deficient (JAM-A−/−) mice for evidence of enhanced permeability and inflammation. Colonic mucosa from JAM-A−/− mice had normal epithelial architecture but increased polymorphonuclear leukocyte infiltration and large lymphoid aggregates not seen in wild-type controls. Barrier function experiments revealed increased mucosal permeability, as indicated by enhanced dextran flux, and decreased transepithelial electrical resistance in JAM-A−/− mice. The in vivo observations were epithelial specific, because monolayers of JAM-A−/− epithelial cells also demonstrated increased permeability. Analyses of other TJ components revealed increased expression of claudin-10 and -15 in the colonic mucosa of JAM-A−/− mice and in JAM-A small interfering RNA–treated epithelial cells. Given the observed increase in colonic inflammation and permeability, we assessed the susceptibility of JAM-A−/− mice to the induction of colitis with dextran sulfate sodium (DSS). Although DSS-treated JAM-A−/− animals had increased clinical disease compared with controls, colonic mucosa showed less injury and increased epithelial proliferation. These findings demonstrate a complex role of JAM-A in intestinal homeostasis by regulating epithelial permeability, inflammation, and proliferation.
PMCID: PMC2150975  PMID: 18039951
24.  The proadhesive phenotype of systemic sclerosis skin promotes myeloid cell adhesion via ICAM-1 and VCAM-1 
Rheumatology (Oxford, England)  2009;48(7):734-740.
Objective. SSc is characterized by microvascular abnormalities and leucocyte infiltration. Previous studies have suggested a proadhesive phenotype in SSc skin, but the functional consequences of this phenotype are not fully understood. Molecules known to mediate leucocyte adhesion include those present at intracellular junctions, such as junctional adhesion molecule-B (JAM-B), JAM-C and CD99, as well as intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1). The aim of this study was to examine adhesive interactions in SSc skin.
Methods. The expression of JAM-B, JAM-C, CD99, ICAM-1 and VCAM-1 in SSc skin was determined by immunohistology and cell surface ELISA. Myeloid U937 cell–SSc dermal fibroblast adhesion assays or in situ adhesion assays to SSc skin were performed.
Results. JAM-C and CD99 expression on endothelial cells (ECs) in SSc skin was decreased compared with expression on normal ECs. CD99 was overexpressed on mononuclear cells in SSc skin and on SSc dermal fibroblasts. Neutralizing ICAM-1 inhibited the binding of U937 cells to SSc dermal fibroblasts. In addition, blocking both ICAM-1 and VCAM-1 inhibited U937 cell adhesion to either proximal (less involved) or distal (more involved) SSc skin.
Conclusions. These studies show that JAM-C and CD99 are aberrantly expressed in SSc skin. However, these adhesion molecules do not mediate myeloid cell–SSc skin adhesion. In contrast, we demonstrate an important role for ICAM-1 and VCAM-1 in the retention of myeloid cells in SSc skin, suggesting that targeting these molecules may be useful SSc therapies.
PMCID: PMC2696027  PMID: 19439502
Systemic sclerosis; Adhesion molecules; Junctional adhesion molecules; VCAM-1; ICAM-1
25.  JAM-A is present in Mammalian Spermatozoa where it is Essential for Normal Motility 
Developmental biology  2007;313(1):246-255.
Junctional Adhesion Molecules (JAMs) that are expressed in endothelial and epithelial cells and function in tight junction assembly, also perform important roles in testis where the closely-related JAM-A, JAM-B, and JAM-C are found. Disruption of murine Jam-B and Jam-C has varying effects on sperm development and function, however deletion of Jam-A has not yet been studied. Here we show for the first time that in addition to expression in the Sertoli-Sertoli tight junctions in the seminiferous tubules, the ∼32 kDa murine JAM-A is present in elongated spermatids and in the plasma membrane of the head and flagellum of sperm. Deletion of Jam-A, using the gene trap technology, results in flagellar defects at the ultrastructural level. In Jam-A-deficient mice, which have reduced litter size, both progressive and hyperactived motility are significantly affected (P<0.0001) before and, more severely, after capacitation. The findings show that JAM-A is involved in sperm tail formation and is essential for normal motility, which may occur via its signal transduction and protein phosphorylation properties. Detection of JAM-A in human sperm protein indicates that its role may be conserved in sperm motility and that JAM-A may be a candidate gene for the analysis of idiopathic sperm motility defects resulting in male subfertility in the human population.
PMCID: PMC2254371  PMID: 18022613
spermiogenesis; elongated spermatid; progressive and hyperactivated motility; sperm flagellar defects; sperm membrane protein

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