Slow Retinal Degeneration in the Retinal Pigment Epithelium (RPE)-Ezrin Knockout Model

Abstract

Purpose: : In the eye, ezrin (an actin-binding protein) has been localized to apical microvilli of Müller cells and RPE apical microvilli and basal infoldings. We have previously shown that ezrin is expressed and plays an important role in establishing and maintaining apical microvilli of RPE and Müller cells, and RPE basal infoldings. In ezrin-/- mice, we noted that photoreceptor development was delayed in comparison to wild type. To better understand the role of ezrin in retinal development and function, we generated an RPE-specific ezrin knockout mouse model.

Methods: : RPE-ezrin-/- mice were generated by breeding an ezrin flox/flox line with a transgenic line carrying an RPE-specific promoter driving the expression of cre recombinase. At 5 months and 1 year, eyecups were fixed and analyzed by TEM, cryosections of RPE-ezrin-/- mice and control littermates were processed for immunofluorescence with antibodies specific to RPE and photoreceptors, and photoreceptor and RPE function was assessed by ERG.

Results: : TEM analysis indicates that while the RPE microvilli and basal infoldings are formed, they appear abnormal ultrastructurally. The membranes of apical microvilli and cytoplasm were very electrondense, as were the membranes of the basal infoldings membranes. In the RPE-ezrin-/- retina, cone opsins were distributed to the outer segments and to the photoreceptor terminal. Although an increased content of rhodopsin was observed it remained confined to the outer segments . RPE-ezrin-/- expressed low levels of microvilli transporters such as the glucose transporter Glut-1 and displayed decreased levels of CRALBP and RPE65. ERG studies of RPE-ezrin-/- mice noted only minor changes under dark- and light-adapted conditions; all RPE-driven components were present, although at reduced levels.

Conclusions: : RPE-ezrin-/- mice demonstrate that ezrin is required for the formation and maintenance of the RPE polarized phenotype and that ezrin plays a crucial role in the morphogenesis of both apical microvilli and basal infoldings. These mice will prove useful to understand the role of RPE function in the retina.

Keywords: retinal pigment epithelium • transgenics/knock-outs • retinal degenerations: cell biology