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

Date Published: November 20, 2009

Publisher: Public Library of Science

Author(s): Wenxin Ma, Lian Zhao, Aurora M. Fontainhas, Robert N. Fariss, Wai T. Wong, Karl-Wilhelm Koch.

Abstract: 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.

Partial Text: 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[1]–[4]. 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 Bruch’s 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.

In the present study, we found that as a consequence of coming into close proximity with activated retinal microglia, primary RPE cells developed multiple structural and functional alterations. These involved decreased expression levels of visual cycle protein, RPE65, and proteins in the tight junctions (ZO-1 and claudin-1), a disruption of ZO-1 localization in cell junctions, and a loss of regular RPE cell shape and distribution. Interestingly, RPE cells co-cultured with retinal microglia did not undergo higher rates of apoptosis but instead exhibited a higher proliferative capacity. These changes indicate that RPE cells, under the influence of activated microglia, may lose integrity in their cellular morphology and intercellular contacts, proliferate in a less regulated manner, and thus lose its original configuration as a uniformly-spaced monolayer and form irregular cellular aggregates as seen in our in vitro and in vivo experiments. Indeed, in eyes with early and intermediate AMD, prior to the onset of CNV, analogous changes, the form of RPE hypertrophy and clumping in the subretinal space and outer retina seen may be seen on histopathological and clinical examination [7], [8]. Photoreceptor loss [8] and synaptic pathology [37] have also been observed in AMD eyes in areas of drusen and pigmentary alteration, which may potentially be related to decreases in expression of RPE65 in RPE cells, inducing dysfunctional changes in visual pigment cycling and photoreceptor physiology [38]. Taken together, the changes induced by retinal microglia on RPE cells in our in vitro and in vivo models bear resemblances to aspects of RPE alterations in AMD, suggesting that the in vivo accumulation of retinal microglia in the subretinal space seen in AMD[19] may indeed drive relevant pathogenic mechanisms. In the late atrophic form of AMD, RPE cells undergo eventual atrophy in a contiguous manner in the form of geographic atrophy [39]; while we did not observe an increase in RPE apoptosis over the time scale of our in vitro co-culture systems, the possibility that prolonged co-culture with retinal microglia may result in pro-apoptotic effects cannot be ruled out.



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