Retinal Pigment Epithelial Cells Synthesize Laminins, Including Laminin 5, and Adhere to Them through α3- and α6-Containing Integrins

The interface between the neural retina and retinal pigment epithelium (RPE) is formed during the unusual juxtaposition of two epithelial apical surfaces as the optic cup folds in from the neural tube. The inner limit of the retina is formed by the epithelial basal surface of the neural retina, the structured basement membrane known as the inner limiting membrane. Because the apical surface of the neural retina is juxtaposed to the apical surface of the RPE, the outer limit of the retina is also formed by an epithelial basal surface: that of the RPE, Bruch's membrane (BrM). BrM serves functions analogous to basement membranes in other tissues, including anchoring subjacent cells, acting as a barrier and a filter, and stabilizing the structure of the tissue.1 Epithelial basement membranes are minimally composed of a structural framework built from a combination of members of several families of glycoproteins.2,3 These families include fibronectins, the polymer-forming collagens (collagens type IV), and the laminins. Together, this complex forms the electrondense structure visible in electron micrographs apposed to the basal surface of epithelial cells. BrM is a five-layered structure that consists of two basement membranes bordering an inner core of two collagenous layers (composed largely of collagen type I) and an elastic layer (composed largely of elastin). The choroidal border of BrM is formed by the basement membrane of the choriocapillaris. Subjacent to the RPE itself is another classic basement membrane, the RPE basement membrane, which, like other basement membranes, contains fibronectin, collagen type IV, and laminins.4–6 One of the many changes associated with the RPE basement membrane during disease is potentially critical: a splitting of Bruch's membrane between the basement membrane of RPE cells and the inner collagenous layer of BrM. This splitting occurs in retinas of patients with protein and lipid deposits known as drusen, during the development of choroidal neovascularization and, most obviously, in pigment epithelium detachment (either serous or fibrovascular). Changes in the composition and distribution of BrM extracellular matrix (ECM) components, including proteins promoting adhesion such as collagen IV, fibronectin, and laminins, can promote this splitting of BrM. Changes in these BrM ECM components also lead to changes in BrM hydroconductivity, thereby promoting pigment epithelium detachment.7,8 However, it is not known which of these ECM components—in particular, which laminins—are present and functional in the RPE basement membrane. Laminins are large heterotrimeric glycoproteins consisting of an α, a β, and a γ chain.2,9 Vertebrates produce five α chains, three β chains, and three γ chains; these chains combine to form at least 15 different laminins.10,11 The distinct biological activity of each of these laminins is the result of combined properties of the individual chains. Of the 15 reported laminins, several have quite restricted tissue distribution and elicit distinct biological responses in cells with which they interact. One of the most distinctive is laminin 5, a critical component of skin stability.12 Although it is possible to purify heterotrimeric laminins from some tissues, the presence of a given laminin heterotrimer is usually inferred by the presence of its component chains. Most chains are components of several heterotrimers; however, the notable exceptions are the β3 and γ2 chains, which are thought only to exist as components of laminin 5. Thus, the presence of the β3 or γ2 chains infers the presence of the laminin 5 heterotrimer. Epithelial cells adhere to their basement membranes via myriad membrane-associated receptors including dystroglycan13 and, critically for many laminins, a large family of transmembrane receptors, the integrins. Individual integrin heterodimers show some ligand specificity but also can be somewhat promiscuous in their binding.14 Integrins are composed of one each of the 18 α and 8 β subunits.14 Integrin subunits that are present on the basal side of the RPE include α3, α5, α6, and β1.15–17 Of these, potential heterodimeric laminin receptors are α3β1 and α6β1. Antibodies against the β1 subunit block attachment of RPE cells on isolated BrM18 and inhibit migration of RPE cells on isolated BrM,19 suggesting that β1-containing integrins can function in RPE interactions with laminins of Bruch's membrane. Genetic disruptions demonstrate that interactions between integrins on epithelial cells and laminins in their subjacent basement membranes are critical for tissue function (see the references in the Discussion section). We propose that similar interactions between integrins in RPE and specific laminins in the RPE-basement membrane are vital for RPE stability and RPE-BrM interface integrity.