Jam-A Unleashed Incites Thromboinflammatory Coronary Artery Disease

Corresponding Author

Junctional adhesion molecule A (JAM-A, JAM-1, F11 receptor) is a single transmembrane member of the immunoglobulin superfamily, expressed on the surface of many cell types, including vascular cells, leukocytes, and platelets.^1,2 Through homophilic interactions mediated by its extracellular domain 1, JAM-A is enriched in intercellular junctions and maintains barrier function in epithelial and endothelial cell layers. Under inflammatory conditions, this barrier is lost as JAM-A moves out of the endothelial junctions toward the apical side of the cells where it undergoes adhesive interactions with platelets and leukocytes through homophilic and heterophilic binding to JAM-A and lymphocyte function-associated antigen 1, respectively.^1,2 Inflammation also causes proteolytic release of JAM-A from the surface of endothelial cells and platelets through the action of a disintegrin and metalloproteases-10 and -17.³ Accordingly, increased circulating levels of soluble JAM-A (sJAM-A) were found to be associated with (vascular) pathologic conditions such as atherosclerosis, hypertension, end-stage kidney disease, and particular forms of cancer. However, it has been unclear whether the release of JAM-A is an epiphenomenon of cell surface protease activation or whether sJAM-A plays rather an active role in regulating cellular actions during disease.

Another open question concerns the function of JAM-A on platelets. It was initially identified as the molecular target for a platelet-activating monoclonal antibody (F11) and termed F11 receptor in 1990.⁴ However, its exact function in platelets long remained obscure and JAM-A was later rediscovered as JAM-1, and henceforth mainly known as an endothelial and epithelial junction molecule.⁵ Although JAM-A does not appear to transduce signals by itself, later studies showed that JAM-A down-regulates platelet activation by association with integrin α_IIbβ₃ and that genetic deletion of JAM-A led to platelet hyperreactivity. Studies in mice demonstrated that JAM-A–deficient platelets had a lower activation threshold, increased aggregation, more robust adhesion to fibrinogen and collagen, and increased thrombosis. In addition, the development of atherosclerosis was accelerated in mice carrying a platelet-specific deletion of JAM-A, which may be attributed to increased platelet reactivity. Although a functional role of JAM-A in vascular inflammation and thrombosis has been clearly demonstrated in mice, evidence in humans is still sparse.

This knowledge gap has been addressed in the study by Rath et al⁶ in this issue of JACC: Basic to Translational Science. Here, a possible proinflammatory and prothrombotic role of JAM-A was investigated in patients with acute and chronic coronary syndromes. Rath et al have performed a single nucleotide variation (SNV, formerly SNP) analysis and measured sJAM-A levels in cohorts of patients with acute and chronic coronary syndromes and found that homozygosity of 2 previously identified minor alleles of the SNVs rs2774276 and rs790056 was associated with a worse event-free survival during long-term follow-up. Consistent with previous findings, homozygous carriers of the rs2774276 and rs790056 minor alleles had elevated plasma levels of sJAM-A. When the patients were stratified in 2 groups of below- and above-median sJAM-A concentrations, elevated sJAM-A levels by themselves were found to be an independent predictor of recurrent myocardial infarction. Given the pivotal role of platelets in acute coronary syndrome, the researchers focused on JAM-A on platelets. On activation, platelets were found to up-regulate surface expression of JAM-A and to shed JAM-A, partly associated with extracellular vesicles, into solution. In patients with coronary artery disease, surface JAM-A expression correlated with that of platelet activation markers. These findings in patients suggest that both plasma levels of sJAM-A and surface expression of JAM-A on platelets can provide information on the presence and future outcome of coronary syndromes.

To investigate possible functional consequences of the release of sJAM-A into circulation, Rath et al⁶ performed studies with isolated platelets and recombinant isolated sJAM-A extracellular domains. When sJAM-A was added alone to platelets, this did not lead to a response. However, when platelets were stimulated with a soluble agonist in combination with sJAM-A, this resulted in a notable up-regulation of platelet aggregation, spreading, and degranulation. On a structural level, the platelet costimulating function of JAM-A could be attributed to extracellular domain 1, suggesting that the level of JAM-A multimerization at the cell surface governs its functions. These findings in model experiments were supported by studies in mice, because injection of sJAM-A resulted in quicker carotid artery occlusion and enhanced thrombus formation in an arterial injury model. An antibody against JAM-A blocked the observed prothrombotic effects by sJAM-A. sJAM-A also enhanced thrombus formation in an ex vivo perfusion model, and this effect was not observed when isolated platelets from mice lacking JAM-A were used. On the level of intracellular signal transduction, addition of sJAM resulted in the phosphorylation of several key kinases involved in platelet signaling (eg, proto-oncogene c-Src, protein kinase B and C, and phosphatidylinositol 3-kinase). In addition, the integrin β₃ chain was phosphorylated on addition of sJAM-A, further highlighting the putative role of JAM-A in platelet spreading and clot retraction.

To investigate a role for sJAM-A in inflammation associated with acute thrombotic events, the effects of sJAM-A on platelet–monocyte interactions were studied. The addition of sJAM-A was found to enhance platelet-monocyte aggregate formation and platelet internalization by monocytes, and this might explain the enhanced foam cell formation induced by sJAM-A–treated platelets and the subsequently enhanced release of proinflammatory cytokines by the monocytes that have taken up sJAM-A–treated platelets. The overall conclusion of the study is that soluble JAM-A acts as a thromboinflammatory factor, and that it can serve as a biomarker for the risk assessment of patients with coronary artery disease.

The study by Rath et al⁶ links clinical findings with observations from experimental model systems both in vitro and in vivo. Beyond revealing a role of sJAM-A as a disease biomarker, it also links the proteolytically released protein to key platelet-driven processes in the pathophysiology of coronary artery disease. This work thereby provides compelling evidence of a functional role of (s)JAM-A in human disease and provides a perspective for future clinical use of JAM-A–related biomarkers.

Nevertheless, some questions remain to be addressed. First, because the release of JAM-A may occur from cell-types other than platelets, notably endothelial cells, information on the origin of sJAM-A would provide further knowledge on the processes both preceding and accompanying acute coronary disease. Perhaps such information can be obtained by having a closer look at the JAM-A associated with extracellular vesicles, because these also contain several remnant molecules derived from the parent cell. In addition, it would be interesting to elaborate on possible functions of extracellular vesicle-bound JAM-A in the future. Another open question concerns the proteases involved in the release of JAM-A, which might be others than the already known a disintegrin and metalloproteases, and their activation might be specific for the (patho)physiologic context. Although platelets, leukocytes, and endothelial cells are numerous, whether the sJAM-A (locally) released by these cells is sufficient to achieve effective concentrations to induce the effects seen in the in vitro experiments might be a subject for debate. Taken together, surface-bound JAM-A is proteolytically unleashed to propagate thromboinflammatory events during acute coronary artery disease.

Funding Support and Author Disclosures

Dr Weber is a van der Laar Professor of Atherosclerosis. Dr Koenen has reported that he has no relationships relevant to the contents of this paper to disclose.