Opposite effects of tumor necrosis factor and soluble fibronectin on junctional adhesion molecule-A in endothelial cells.

Junctional adhesion molecule-A (JAM-A) regulates key inflammatory responses, such as edema formation and leukocyte transmigration. Although it has been reported that the inflammatory cytokine tumor necrosis factor (TNF) causes the disassembly of JAM-A from the intercellular junctions, the mechanism has not been elucidated fully. Here, we report that TNF enhances the solubility of JAM-A in Triton X-100 and increases the amount of Triton-soluble JAM-A dimers at the cell surface but does not change the total levels of cellular JAM-A. Thus we hypothesized that TNF causes the redistribution of JAM-A from the junctions to the cell surface and that junction disassembly is sufficient to account for JAM-A redistribution. Intriguingly, however, even after complete disassembly of the junctions (with EDTA and trypsin), higher levels of JAM-A are detectable at the cell surface (by FACS analysis) in cells that had been previously incubated in the presence of TNF than in its absence. Thus we propose that TNF causes not only the disassembly of JAM-A from the junctions and its subsequent redistribution to the cell surface but also its dispersal in such a way that JAM-A becomes more easily accessible to the antibodies used for FACS analysis. Finally, we evaluated whether soluble fibronectin might attenuate the effects of TNF on JAM-A, as some inflammatory conditions are associated with the depletion of plasma fibronectin. We found that fibronectin reduces the effect of TNF on the disassembly of JAM-A, but not on its dispersal, thus further stressing that disassembly and dispersal can be functionally dissociated.

Expression of junctional adhesion molecule-A prevents spontaneous and random motility.

Junctional adhesion molecule-A (JAM-A) is a cell-surface glycoprotein that localizes to intercellular junctions and associates with intracellular proteins via PSD95-Dlg-ZO1-binding residues. To define the functional consequences of JAM-A expression, we have produced endothelial cells from JAM-A-deficient mice. We report here that the absence of JAM-A enhanced spontaneous and random motility. In turn, the enhanced motility of JAM-A-negative cells was abrogated either on transfection of exogenous JAM-A or on treatment with inhibitors of glycogen synthase kinase-3beta (GSK-3beta). In addition, in JAM-A-positive cells, motility was enhanced on inactivation of protein kinase Czeta (PKCzeta), which is an inhibitor of GSK-3beta. Although these findings suggested that JAM-A might inhibit GSK-3beta, we found that expression per se of JAM-A did not change the levels of inactive GSK-3beta. Thus, JAM-A expression may regulate effectors of motility that are also downstream of the PKCzeta/GSK-3beta axis. In support of this view, we found that JAM-A absence increased the number of actin-containing protrusions, reduced the stability of microtubules and impaired the formation of focal adhesions. Notably, all the functional consequences of JAM-A absence were reversed either on treatment with GSK-3beta inhibitors or on transfection of full-length JAM-A, but not on transfection of a JAM-A deletion mutant devoid of the PSD95-Dlg-ZO1-binding residues. Thus, by regulating cytoskeletal and adhesive structures, JAM-A expression prevents cell motility, probably in a PSD95-Dlg-ZO1-dependent manner.

Increased DC trafficking to lymph nodes and contact hypersensitivity in junctional adhesion molecule-A-deficient mice.

Junctional adhesion molecule-A (JAM-A) is a transmembrane adhesive protein expressed at endothelial junctions and in leukocytes. In the present work, we found that DCs also express JAM-A. To evaluate the biological relevance of this observation, Jam-A(-/-) mice were generated and the functional behavior of DCs in vitro and in vivo was studied. In vitro, Jam-A(-/-) DCs showed a selective increase in random motility and in the capacity to transmigrate across lymphatic endothelial cells. In vivo, Jam-A(-/-) mice showed enhanced DC migration to lymph nodes, which was not observed in mice with endothelium-restricted deficiency of the protein. Furthermore, increased DC migration to lymph nodes was associated with enhanced contact hypersensitivity (CHS). Adoptive transfer experiments showed that JAM-A-deficient DCs elicited increased CHS in Jam-A(+/+) mice, further supporting the concept of a DC-specific effect. Thus, we identified here a novel, non-redundant role of JAM-A in controlling DC motility, trafficking to lymph nodes, and activation of specific immunity.