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Vascular Biology |
From the Department of Molecular Cardiovascular Research (A.Z., E.A.L., L.F., P.v.H., R.R.K., C.W.), RWTH University Hospital, Aachen, Germany; Mario Negri Institute for Pharmacological Research and FIRC Institute of Molecular Oncology (M.C., E.D.); and the Department of Biomolecular and Biotechnological Sciences School of Sciences (E.D.), University of Milan, Italy.
Correspondence to Dr Christian Weber, Kardiovaskuläre Molekularbiologie, Universitätsklinikum Aachen, Pauwelsstrasse 30, D-52074 Aachen, Germany. E-mail cweber{at}ukaachen.de
| Abstract |
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Methods and Results Here we show that the genetic deletion of JAM-A in apolipoprotein Edeficient (apoE/) mice significantly reduced neointimal hyperplasia after wire injury of carotid arteries without altering medial area. This was associated with a significant decrease in neointimal macrophage content, whereas the relative content of smooth muscle cells and endothelial recovery was unaltered in JAM-A/apoE/ compared with JAM-A+/+apoE/ lesions. In carotid arteries perfused ex vivo, deficiency in JAM-A significantly impaired the recruitment of monocytes 1 week, but not 1 day, after injury. These effects were paralleled by an attenuation of monocyte arrest and transmigration on activated JAM-A/apoE/ versus JAM-A+/+apoE/ endothelial cells under flow conditions in vitro. A mechanism underlying reduced recruitment was implied by findings that the luminal expression of the arrest chemokine RANTES in injured arteries and its endothelial deposition by activated platelets in vitro were diminished by JAM-A deficiency.
Conclusions Our data provide the first evidence to our knowledge for a crucial role of JAM-A in accelerated lesion formation and monocyte infiltration in atherosclerosis-prone mice.
Key Words: adhesion molecule atherosclerosis chemokine monocytes
| Introduction |
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Inflammatory monocyte recruitment has emerged as the crucial force driving the initiation and progression of atherosclerotic lesion formation and is controlled by the expression of adhesion molecules and pro-inflammatory cytokines.9,10 Recently, JAM-A has been found to be upregulated on atherosclerotic endothelium in carotid arteries of apolipoprotein Edeficient (apoE/) mice on high-cholesterol diet and to participate in the recruitment of monocytes and T cells to carotid arteries perfused ex vivo.11 In addition, platelet activation and adhesion to activated endothelium can be mediated by homophilic engagement of JAM-A.12 Neointimal hyperplasia after arterial injury is characterized by endothelial denudation, exposure of extracellular matrix, and adhesion of activated platelets, which contribute to the inflammatory recruitment and accumulation of smooth muscle cells. Given the pro-atherogenic role of mononuclear cell infiltration and activated platelets in exacerbating lesion formation,1315 we were prompted to study a functional contribution of JAM-A to accelerated neointima formation after arterial injury by genetic deletion in atherosclerosis-prone apoE/ mice.
| Methods |
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Immunofluorescence and Morphometry
Neointimal and medial areas were quantified in serial pentachrome stained sections (5 µm) within 500 µm from the bifurcation (11 per mouse) by planimetry (Diskus software; Hilgers).15 Areas within external elastic lamina, internal elastic lamina, or lumen were determined by computer-assisted planimetry (Diskus software). Neointimal area was defined as internal elastic lamina-lumen, and medial area was defined as external elastic lamina-internal elastic lamina. Staining for Mac-2 (CL8942AP; Cedarlane), smooth muscle cell
-actin (clone 1A4; DAKO), CD3 (48-2B), CD31 (clone M20), vascular endothelial (VE)cadherin (goat polyclonal), or regulated on activation, normal T cell expressed and secreted (RANTES, C-19; all Santa Cruz) was visualized with a fluorescein isothiocyanate-conjugated mAb (Sigma) in 3 sections from each carotid artery, recorded using a Leica DMLB fluorescence microscope, and quantified using Diskus software.15
Endothelial Cell Isolation and Cell Culture
Mouse aortal endothelial cells (ECs) were cultured as described.16 Rat tail collagen I (1.5 mg/mL) in DMEM supplemented with 10% fetal calf serum was aliquoted into 24-well plates, allowed to gel at 37°C for 60 minutes, and equilibrated with complete EC medium (RPMI-1640, 20% fetal calf serum, 4 µL/L 2-mercaptoethanol, 10 µg/mL gentamicin, and 50 µg/mL EC growth supplement; BD Biosciences) overnight. Aortas from JAM-A+/+apoE/ or JAM-A/apoE/ mice were isolated and cleaned from peri-adventitial fat, cut into rings, opened, and placed endothelial side-down onto the collagen gel. After 2 days, complete medium was added and changed every third day. After 10 days, aortic segments were removed and the gel was digested with 0.3% collagenase H solution. Cells were used between passages 1 and 8. Mono Mac 6 cells were maintained as described.15
Ex Vivo Perfusion of Carotid Arteries
Carotid arteries from JAM-A+/+apoE/ or JAM-A/apoE/ mice were isolated for ex vivo perfusion,11 transferred onto a microscope stage, and perfused with Mono Mac 6 cells (106/mL) labeled with calcein-AM (Molecular Probes) in MOPS-buffered physiological salt solution at 4 µL/min. Adhesive interactions were recorded using stroboscopic epifluorescence illumination (Drelloscop 250; Drello, Mönchengladbach, Germany) and determined after 10 minutes of perfusion.
Monocyte Adhesion Assay, Platelet Perfusion, and Immunofluorescence on Endothelium
For laminar flow assays,14,15,17 confluent aortal JAM-A+/+apoE/ or JAM-A/apoE/ ECs in Petri dishes were activated (100 U/mL tumor necrosis factor-
, 200 U/mL IFN-
) for 12 hours. Platelets were isolated from platelet-rich plasma by centrifugation of blood from JAM-A+/+apoE/ or JAM-A/apoE/ mice and bound to 3-aminopropyl-triethoxysilane-treated glass slides (30 minutes, 37°C)18 and adherent platelets were activated with thrombin (1 U/mL, 30 minutes, 37°C). Mono Mac 6 cells (0.5x106/mL) were perfused at 1.5 dyn/cm2. After 4 minutes, firmly adherent and transmigrated cells were counted in multiple fields recorded by video microscopy. Surface-bound RANTES was detected on interleukin (IL)-1ß (10 ng/mL, 12 hours) activated JAM-A+/+apoE/ aortal ECs on glass cover slides after perfusion with JAM-A+/+apoE/ or JAM-A/apoE/ platelets (1.5 dyn/cm2, 15 minutes)17 and quantified using AnalySIS software (Soft-Imaging Systems).14
Statistical Analysis
Data represent mean±SEM and were analyzed by 2-tailed Student t test or 1-way ANOVA followed by Newman-Keuls post-test when appropriate (InStat software; GraphPad).
| Results and Discussion |
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The recruitment of monocytes to lesions in situ was analyzed in carotid arteries perfused ex vivo. Although no difference in monocyte accumulation to denuded segments was observed 1 day after wire injury, arrest was significantly diminished in JAM-A/apoE/ compared with JAM-A+/+apoE/ arteries 1 week after injury (Figure 2A and 2B). After endothelial denudation, activated platelets adhere to the exposed subendothelial matrix and provide a substrate for the initial recruitment of leukocytes.10,18 The engagement of JAM-A on platelets has been shown to mediate degranulation and aggregation21 and to participate in the interactions of platelets or platelet microparticles, with endothelium subsequently supporting leukocyte recruitment.12,17 To further dissect a platelet-dependent role of JAM-A after injury, monocyte adhesion to activated platelets under flow conditions was analyzed in vitro. As compared with JAM-A+/+apoE/ platelets, shear-resistant arrest of monocytes to activated platelets from JAM-A/apoE/ mice was not attenuated (Figure 2C), particularly when adjusting for the number of adherent platelets, which was slightly lower for JAM-A/apoE/ platelets (43±7x103 versus 55±6x103/mm2). Monocyteplatelet interactions may thus be primarily mediated by Mac-1 and its ligands, consistent with the results in ex vivo perfused arteries, whereas homophilic interactions of JAM-A, as for platelet arrest on activated endothelium, or heterophilic binding of platelet JAM-A by monocyte LFA-1 may serve as adjuvant signals triggering outside-in integrin activation or platelet degranulation.4,18,22
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To explore the role of endothelial JAM-A, monocytes were perfused on aortic ECs isolated from JAM-A/apoE/ or JAM-A+/+apoE/ mice activated with tumor necrosis factor-
/IFN-
. Compared with JAM-A+/+, JAM-A/ ECs supported significantly less monocyte adhesion and showed substantially reduced (by 80%) monocytes transmigration under flow conditions (Figure 2D). Previously, soluble JAM-A or JAM-A antibody have been used to establish a contribution of JAM-A to monocyte and T cell arrest on activated or atherosclerotic endothelium in concert with interactions by VLA-4.11 In contrast to blocking antibodies or soluble proteins, the genetic deletion of JAM-A more directly unveils its cooperative role in monocyte adhesion and its importance in transmigration.
The infiltration with mononuclear cells is crucially involved in atherogenesis but also determines the degree of neointimal hyperplasia.9,10 Accordingly, our data confirm that attenuated neointima formation in JAM-A/apoE/ carotid arteries may at least in part be caused by a reduction in inflammatory infiltration with monocytes and T cells after injury.9,10 These data extend previous findings that JAM-A is upregulated on atherosclerotic endothelium in carotid arteries of apoE/ mice, where it cooperates with VLA-4 in the atherogenic recruitment of mononuclear cells.11 The finding that JAM-A mediated monocyte recruitment to carotid arteries 1 week after injury, whereas being less important immediately after denudation may reflect a preferential involvement of JAM-A in emigration on newly reconstituted endothelium versus arrest on adherent platelets, paralleling a differential role in neutrophil transmigration but not adhesion after hepatic reperfusion injury.7
Activated platelets can deposit the CC chemokine CCL5/RANTES on the surface of monocytes or atherosclerotic endothelium in a P-selectindependent process.13 Moreover, microparticles shed from activated platelets can mediate the deposition of RANTES via transient interactions on activated endothelium involving JAM-Adependent signals.17 Given the well-established role of RANTES deposited by activated platelets in subsequent monocyte recruitment and plaque formation,1315 we analyzed the deposition of RANTES on activated endothelium under flow conditions. Perfusion of JAM-A+/+apoE/ platelets over activated aortal ECs led to a marked deposition of RANTES, whereas endothelial staining was markedly reduced after preperfusion with JAM-A/apoE/ platelets (Figure 3A and 3B). This was paralleled by immunofluorescence analysis of RANTES deposition in vivo demonstrating robust staining for RANTES preferentially on the luminal surface but also in neointimal cells in JAM-A+/+apoE/ but not JAM-A/apoE/ carotid arteries 1 week but not 1 day after wire injury (Figure 3C and 3D; data not shown). Our findings imply that interactions of platelets with the vessel wall involving engagement of JAM-A are required for the effective deposition of RANTES under flow conditions. In line with data that platelet accumulation after hepatic ischemia reperfusion was unaffected by JAM-A deficiency,7 this suggests that transient interactions are sufficient for the role of platelets. With regard to the importance of RANTES in accelerated lesion formation,13,15 attenuated RANTES delivery may constitute another pivotal mechanisms underlying the reduction in neointima formation in JAM-A/apoE/ mice.
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Taken together, our data indicate that JAM-A plays a crucial role in neointimal lesion formation and infiltration after arterial injury in an atherosclerotic context, which can be ascribed to a direct contribution to monocyte recruitment and deposition of platelet RANTES on activated or recovering endothelium and may be targeted to limit lesion development after injury.
| Acknowledgments |
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Received August 21, 2005; accepted November 2, 2005.
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