The retinal pigment epithelium: something more than a constituent of the blood-retinal barrier--implications for the pathogenesis of diabetic retinopathy

Abstract

The retinal pigment epithelium (RPE) is an specialized epithelium lying in the interface between the neural retina and the choriocapillaris where it forms the outer blood-retinal barrier (BRB). The main functions of the RPE are the following: (1) transport of nutrients, ions, and water, (2) absorption of light and protection against photooxidation, (3) reisomerization of all-trans-retinal into 11-cis-retinal, which is crucial for the visual cycle, (4) phagocytosis of shed photoreceptor membranes, and (5) secretion of essential factors for the structural integrity of the retina. An overview of these functions will be given. Most of the research on the physiopathology of diabetic retinopathy has been focused on the impairment of the neuroretina and the breakdown of the inner BRB. By contrast, the effects of diabetes on the RPE and in particular on its secretory activity have received less attention. In this regard, new therapeutic strategies addressed to modulating RPE impairment are warranted.

Figure 1

Retinal section of the retina showing the location of the retinal pigment epithelium (RPE). In the box are listed the main functions of RPE.

Figure 2

Confocal microscopy showing the expression of somatostatin (SST) in the human retina. As can be appreciated SST expression (in red) is higher in the RPE than in the neuroretina.

Figure 3

Confocal microscopy of human RPE showing the expression of both erythropoietin (Epo) in green and Epo receptor (Epo-R) in red. At the bottom the merged image shows partial colocalization of Epo and Epo-R.

Figure 4

(a) Immunoblot showing higher protein content of apolipoprotein A1 (apoA1) in RPEs from diabetic donors in comparison with RPEs from nondiabetic donors. (b) Inmmunofluorescent image of apoA1 (red) in ARPE cells (spontaneously immortalized cell line of human RPE).