Expression, Localization, and Function of Junctional Adhesion Molecule-C (JAM-C) in Human Retinal Pigment Epithelium

The blood–retinal barrier (BRB) has two components. The inner BRB at the vitreous surface of the retina is formed by tightly opposed retinal endothelial cells and the pericytes surrounding them, whereas the outer BRB consists of a uniform monolayer of retinal pigment epithelial (RPE) cells. The outer component of the BRB is particularly important for maintaining the health and integrity of the retina–RPE complex. In the distal retina, the RPE helps maintain the volume and chemical composition of the extracellular spaces on both the retinal and choroidal sides of the tissue. The RPE apical processes are in close anatomic association with photoreceptor outer segments, and consequently this interface mediates a wide range of metabolic, electrical, and functional interactions.1,2 In particular, the RPE avidly participates in the phagocytosis of the photoreceptor outer segments and the recycling of visual pigments during the light– dark cycle,3 whereas pathophysiological processes in the RPE–photoreceptor complex can lead to widespread photoreceptor degeneration and vision loss.4,5 The integrity of the RPE monolayer depends on the interepithelial junctions that are subdivided to include the tight and adherens junctions and the desmosomes. Adherens junctions are formed by cadherins linked to the actin cytoskeleton via intracellular catenins and the main constituents of tight junctions are three families of transmembrane proteins: occludins, claudins, and junctional adhesion molecules (JAMs).6 JAMs are part of the immunoglobulin superfamily and have two extra-cellular Ig domains, a single transmembrane region and one short cytoplasmic tail. The PDZ domain binding motif located in the cytoplasmic tail of JAMs mediates interactions with intracellular scaffolding proteins such as ZO-1, thereby providing a link to the cytoskeleton.7 JAM-A is found in several types of cells, including endothelial and epithelial cells, and it functions as a gatekeeper by regulating permeability through both endothelial and epithelial monolayers and leukocyte transmigration through endothelial cells.8–10 Furthermore, JAM-A promotes the apicobasal polarization of epithelial cells, including RPE cells.11–13 In contrast, JAM-B is principally found in endothelial cells and also has a role in endothelial cell permeability and leukocyte transmigration through these cells.14 The third member of the JAM family, JAM-C15 has been identified in various cell types including endothelial and gut epithelial cells, platelets, smooth muscle cells and B cells and recently also RPE.8,16 Its localization within intercellular contacts may be divergent among different cell types, as both association with tight junctions and desmosomes have been reported.17,18 Previous studies have shown that JAM-C may interact with factors that regulate cell polarization, tight junction assembly, and paracellular permeability in different cell types.17,19,20 In addition, JAM-C has been implicated in inflammatory processes and shown to participate in the transmigration of leukocytes through endothelial and gut epithelial cells, which may be attributed to the propensity of JAM-C to interact with JAM-B21 as well as with the leukocyte β2-integrin Mac-1.18,22,23 There is growing recognition of the RPE’s involvement in immune mediated processes and diseases24–28 like age-related macular degeneration (AMD) and other inflammatory processes in the posterior pole of the eye. Given that the integrity of the RPE junctions is essential to its function as a barrier between the blood and the retina, that JAM-A and JAM-C have been identified in the RPE junctions, and that JAM-C is known to mediate leukocyte transepithelial migration in other systems, we sought to examine the expression, localization, and function of JAM-C in human RPE. We found that JAM-C is localized at the tight junctions of intact monolayers of human RPE, that it is found at the initial cell– cell contacts of newly forming junctions, and that it helps initiate hfRPE junction formation and polarization. JAM-C also promotes the transepithelial migration of granulocytes through intact monolayers of cultured hfRPE. Thus, in the intact eye, JAM-C may be an important determinant of RPE initial junction formation, cell polarization, and immune system–mediated pathophysiology at the retina-RPE interface.