Microglia in the Mouse Retina Alter the Structure and Function of Retinal Pigmented Epithelial Cells: A Potential Cellular Interaction Relevant to AMD

Wong WT (2009) Microglia in the Mouse Retina Alter the Structure and Function of Retinal Pigmented Epithelial Cells: A Potential Cellular Interaction Relevant to AMD. PLoS ONE 4(11): e7945. doi:10.1371/journal.pone.0007945 Microglia in the Mouse Retina Alter the Structure and Function of Retinal Pigmented Epithelial Cells: A Potential Cellular Interaction Relevant to AMD Wenxin Ma 0 Lian Zhao 0 Aurora M. Fontainhas 0 Robert N. Fariss 0 Wai T. Wong 0 Karl-Wilhelm Koch, University of Oldenburg, Germany 0 1 Unit on Neuron-Glia Interactions in Retinal Disease, National Eye Institute, National Institutes of Health , Bethesda , Maryland, United States of America , 2 Biological Imaging Core , Office of the Scientific Director, National Eye Institute, National Institutes of Health , Bethesda, Maryland , United States of America Background: Age-related macular degeneration (AMD) is a leading cause of legal blindness in the elderly in the industrialized word. While the immune system in the retina is likely to be important in AMD pathogenesis, the cell biology underlying the disease is incompletely understood. Clinical and basic science studies have implicated alterations in the retinal pigment epithelium (RPE) layer as a locus of early change. Also, retinal microglia, the resident immune cells of the retina, have been observed to translocate from their normal position in the inner retina to accumulate in the subretinal space close to the RPE layer in AMD eyes and in animal models of AMD. Methodology/Principal Findings: In this study, we examined the effects of retinal microglia on RPE cells using 1) an in vitro model where activated retinal microglia are co-cultured with primary RPE cells, and 2) an in vivo mouse model where retinal microglia are transplanted into the subretinal space. We found that retinal microglia induced in RPE cells 1) changes in RPE structure and distribution, 2) increased expression and secretion of pro-inflammatory, chemotactic, and pro-angiogenic molecules, and 3) increased extent of in vivo choroidal neovascularization in the subretinal space. Conclusions/Significance: These findings share similarities with important pathological features found in AMD and suggest the relevance of microglia-RPE interactions in AMD pathogenesis. We speculate that the migration of retinal microglia into the subretinal space in early stages of the disease induces significant changes in RPE cells that perpetuate further microglial accumulation, increase inflammation in the outer retina, and fosters an environment conducive for the formation of neovascular changes responsible for much of vision loss in advanced AMD. - Funding: This work has been supported by the National Eye Institute Intramural Research Program and a grant from the Prevention of Blindness Society of Metropolitan Washington (www.youreyes.org). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Age-related macular degeneration (AMD) is the leading cause of legal blindness and visual disability for individuals aged 60 and over in the Western hemisphere[14]. While the pathogenesis of AMD is still unknown, multiple early disease alterations have been noted to involve the retinal pigment epithelium (RPE) monolayer, a highly specialized layer of epithelial cells essential for supporting photoreceptors and maintaining the outer blood retina barrier [5]. In early and intermediate stages of AMD, lipoproteinaceous deposits called drusen collect beneath the RPE layer [6]. In addition, RPE cells develop changes in structure and distribution that are evident as patchy hyper- and hypopigmentary changes in the fundus (so-called pigment mottling)on clinical examination [7], and also as RPE depigmentation, hypertrophy, hyperplasia, and atrophy, as seen on histopathological examination [8]. As AMD progresses, choroidal neovascularization (CNV) (or wet AMD) can arise when new blood vessels from the choroid invade the basement membrane of the RPE layer, the Bruchs membrane, and extend into the outer retina, with ensuing photoreceptor cell atrophy and vision loss [9]. While early structural changes in the RPE layer are often considered a harbinger of AMD disease progression [10], the initiating cellular and molecular factors inducing pathogenetic alterations in the RPE are incompletely understood. The role of inflammation has been prominently implicated in the pathogenesis of AMD [11,12]. The subretinal space, the interface between the apical surface of the RPE and the outer segments of photoreceptors, is a locus of particular interest in the relationship between inflammation and AMD. Under normal conditions, the subretinal space is a zone of special immune privilege[13], maintained by the activity of RPE cells which secrete immunosuppressive factors into this space[14], and also by the notable absence of retinal microglia, which perform dynamic immune surveillance in the inner retina, but are largely excluded from the outer retina [15]. However, under conditions of advanced age and photoreceptor injury, retinal microglia translocate into the outer retina [16], accumulate in the subretinal space [17], and acquire morphological features of activation [18]. Significantly, activated microglia have also been found in the subretinal space of patients with AMD [19], and are juxtaposed in close proximity with RPE cells overlying drusen (Wong and Fariss, unpublished data). Additionally, in a number of mouse models for AMD, involving the absence of chemokine ligands/receptors, CCL2 and/or CX3CR1 [20,21], microglia accumulation in the subretinal space was accelerated and more pronounced. This accumulation of subretinal microglia was in turn associated with multiple features reminiscent of AMD histopathology including the accumulation of drusen-like deposits, local RPE structural changes, and CNV formation [20,21]. The anatomical separation between retinal microglia and RPE cells under normal conditions and their juxtaposition into direct and intimate contact in the subretinal space under pathological situations suggest that cellular interactions between these two retinal cell types may be of particular pathogenetic significance. Microglia, serving as resident immune cells of the retina, can have multiple functional states and carry out diverse functions [22]. Capable of rapid dynamism and motility, they also synthesize and release multiple cytokines, chemokines, neurotrophic factors, and neurotransmitters that allow them to interact with multiple CNS cell types and exert cytotoxic or cytoprotective effects depending on the tissue context [23]. While there has been evidence that retinal microglia may interact with signals from retinal neurons [24], photoreceptors [25], and retinal vessels [26,27], the direct influences of retinal microglia on RPE structure, physiology, an (...truncated)