Abstract
The retinal pigment epithelium (RPE) is a pigmented monolayer of cells lying between the photoreceptors and a layer of fenestrated capillaries, the choriocapillaris. Choroideremia (CHM) is an X-linked progressive degeneration of these three layers caused by the loss of function of Rab Escort protein-1 (REP1). REP1 is involved in the prenylation of Rab proteins, key regulators of membrane trafficking. To study the pathological consequences of chronic disruption of membrane traffic in the RPE we used a cell type-specific knock-out mouse model of the disease, where the Chm/Rep1 gene is deleted only in pigmented cells (ChmFlox, Tyr-Cre+). Transmission electron microscopy (TEM) was used to quantitate the melanosome distribution in the RPE and immunofluorescent staining of rhodopsin was used to quantitate phagocytosed rod outer segments in retinal sections. The ultrastructure of the RPE and Bruch’s membrane at different ages was characterised by TEM to analyse age-related changes occurring as a result of defects in membrane traffic pathways. Chm/Rep1 gene knockout in RPE cells resulted in reduced numbers of melanosomes in the apical processes and delayed phagosome degradation. In addition, the RPE accumulated pathological changes at 5–6 months of age similar to those observed in 2-year old controls. These included the intracellular accumulation of lipofuscin-containing deposits, disorganised basal infoldings and the extracellular accumulation of basal laminar and basal linear deposits. The phenotype of the ChmFlox, Tyr-Cre+ mice suggests that loss of the Chm/Rep1 gene causes premature accumulation of features of aging in the RPE. Furthermore, the striking similarities between the present observations and some of the phenotypes reported in age-related macular degeneration (AMD) suggest that membrane traffic defects may contribute to the pathogenesis of AMD.
Highlights
The retinal pigment epithelium (RPE) provides nutrients, growth factors and ions to the photoreceptors, removes waste products of retinal metabolism and is essential for photoreceptor survival and, for vision
The percentage of melanosomes found in the apical processes of the RPE in 7 to 12-month old ChmFlox, Tyr-Cre+ mice 6 to 8 hours after light onset was reduced compared to littermate ChmFlox controls (Fig. 1A–D, 18.2% vs 6.2%, respectively), but did not show the complete exclusion from the apical processes that was exhibited by the ashen RPE (Fig. 1C)
In this study we examine the in vivo consequences of chronic defects in multiple membrane traffic pathways induced in the RPE
Summary
The retinal pigment epithelium (RPE) provides nutrients, growth factors and ions to the photoreceptors, removes waste products of retinal metabolism and is essential for photoreceptor survival and, for vision. RPE dysfunction is associated with aging and multiple inherited retinal degenerative diseases. One such disease, choroideremia (CHM), is an X-linked chorioretinal degeneration caused by functional defects in CHM/REP1, a chaperone protein for Rab GTPases [1], which are critical regulators of membrane trafficking [2]. Loss of function of REP1 in CHM is functionally compensated by a related protein, REP2 [3] This compensation is only partial as a subset of Rabs are underprenylated in peripheral lymphoblasts of CHM patients and in mouse models of CHM [4,5]. Given that Rab GTPases regulate multiple steps in membrane traffic pathways including vesicle budding, movement and fusion with the destination compartment, the partial loss of function of multiple Rabs is predicted to affect multiple intracellular trafficking pathways. The pathology of CHM cannot be explained solely by compromised Rab27a function as the ashen mouse, which lacks functional Rab27a, does not reproduce the retinal degeneration observed in CHM patients or in CHM mouse models [4,7]
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