INTRODUCTION

Fig. 1. Retinal pigment epithelium (RPE) and photoreceptor contacts in the subretinal space. The subretinal space is separated from the vascularized inner retina and choroidal tissue by tight junctions (t. j.). Tight junctions between photoreceptor inner segments and Müller cells form the external limiting membrane (ELM). Tight junctions of RPE cells separate apical and basolateral domains of the RPE and form the outer blood retinal barrier. Cadherin-based adherens junctions (a. j.) between RPE cells further stabilize the RPE monolayer. Basally, RPE cells adhere to Bruch’s membrane via cell–substrate interactions. The subretinal space contains the interphotoreceptor matrix (IPM) with distinct composition around cones and rods (X and ▲ as indicated), photoreceptor inner segments (IS) and outer segments (OS), and RPE apical membrane with microvilli that extend to ensheathe the photoreceptor OS.

In addition, these multiprotein complexes seal off the subretinal space from the underlying connective tissue to prevent free exchange of extracellular molecules by transepithelial diffusion. Together with the external limiting membrane (ELM) generated by photoreceptorMüller cell junctions, the tight junctions of the RPE form the outer blood-retinal barrier isolating the avascular area of the subretinal space, which contains the apical plasma membrane domain of the RPE, the IPM, and photo-receptor rod and cone outer segments (Fig. 1). Regardless of size and charge, molecules trafficking between the subretinal space and the underlying choroidal circulation must cross the RPE via paracellular or transcellular routes that are largely controlled by RPE cells themselves.

Concentrations of soluble compounds such as ions, metabolites, and macromolecules in the subretinal space are highly dynamic, varying both spatially and temporally. Changes that are best understood occur as a direct consequence of photoisomerization and phototransduction (recently reviewed in [1]): (1) All-trans retinal released from rhodopsin is reduced to all-trans retinol in the outer segment, which moves through the subretinal space to the RPE for reisomerization. Trafficking of 11-cis retinal from the RPE to the outer segment completes the visual cycle of rod photopigment regeneration. Several specialized retinoid-binding proteins sequester retinoids en route. Indeed, interphotoreceptor retinoid-binding protein (IRBP) is one of the most abundant proteins in the subretinal space [2]. (2) Rhodopsin isomerization in POSs closes cyclic guanosine monophosphate (cGMP)-gated cation channels in the plasma membrane of the outer segment, reducing the dark current that involves flux of Na+ into and K+ out of the outer segment. Thus, photoreceptor illumination directly decreases K+ (from ~5 to 2 mM) and increases Na+ in the subretinal space. These changes in ion concentrations are rapid and transient as ion transporters and channels of the apical surface of RPE cells and of photoreceptor inner segments respond to these changes with activity changes that compensate for them.

The distribution and mobility of macromolecules of the IPM in the subretinal space are only poorly understood. Both RPE and photoreceptor cells contribute components to the IPM, generating a regionalized network of numerous glycoproteins and proteoglycans that sequesters small metabolites such as fatty acids and other nutrients [3–7]. Distinct sheaths consisting of high molecular weight proteoglycans and glycoproteins surround cones and rods [8, 9]. These matrix sheaths likely provide important mechanical support for outer segments and RPE apical microvilli that stabilizes their alignment. Moreover, they may serve to bind and sequester nutrients, growth factors, and other proteins for optimal access by either rods or cones [10].

There is much evidence that macromolecule distribution in the subretinal space is tightly controlled. Uehara in the LaVail group first demonstrated in 1990 that the binding pattern of wheat germ agglutinin lectin (WGA) to adult rat retina cross sections depends on illumination. WGA (as well as colloidal iron and antibodies to chondroi- tin-6-sulfate or IRBP) exhibited staining that was diffusely distributed and restricted to apical and basal zones of the outer segment layer in darkand light-adapted retina, respectively [11]. The same investigators later showed that recognition sites for peanut agglutinin, which specifically labels components of the cone matrix sheath, do not redistribute in response to differences in illumination [12]. These are very intriguing observations, suggesting that the organization of extracellular macromolecules in the subretinal space may change to facilitate functions specifically associated with rod outer segments, such as the classical visual cycle that employs reisomerization within the RPE. However, the precise molecular nature of these changes and their physiological purpose remain unknown.

Taken together, photoreceptor and RPE cells interact in the highly dynamic environment of the subretinal space. In the following, we discuss in detail interactions that take place between POSs and the apical surface of the RPE in the context of this complex extracellular milieu during retinal adhesion and during POS renewal.