Abstract
Purpose: :
Ezrin, a member of the ezrin/moesin/radixin (ERM) family, localizes to microvilli of epithelia in vivo, where it functions as a bridge between actin filaments and plasma membrane proteins. In the eye, ezrin has been localized to both apical microvilli of Müller cells and retinal pigment epithelium (RPE) apical microvilli and basal infoldings. Two specific morphogenetic roles for ezrin in RPE cells have been demonstrated, namely the generation of apical microvilli and basal infoldings. However, all the available data regarding ezrin function in RPE cells was generated in studies carried out in cell culture. Our hypothesis is that ezrin expression is important to establish and maintain the RPE apical microvilli and basal infoldings and the apical Müller cells microvilli in vivo. To test this hypothesis, in the present study we followed the development of these structures in the eyes of early postnatal ezrin knockout mice.
Methods: :
The ezrin–mutant (Ez–/–) mice have normal appearance at birth. However, neonates failed to thrive postnatally and did not survive past weaning. Accordingly, our observations were carried out on animals at postnatal day 7 and 10. Eyecups were fixed in 2.5% glutaraldehyde in 0.1M cacodylate buffer and analyzed by TEM. Also, cryosections of Ez–/– mice and control littermates were processed for immunofluorescence with antibodies specific to RPE, photoreceptor and IPM markers.
Results: :
TEM analysis indicates that the loss of ezrin leads to substantial reductions in number and length of apical microvilli and basal infoldings in RPE cells and in the Müller cell apical microvilli. The absence of apical microvilli in the RPE is accompanied by the presence of microvilli–like inclusions (MIs) in the RPE cytoplasm. Photoreceptors in the ezrin knockout animals showed substantial retardation in development as compared to their wild type littermates.
Conclusions: :
In Ez–/– mice we observed morphological defects in the two cell types previously described to express ezrin, the RPE and Müller cells. Specifically, we showed that ezrin is not required for the formation and maintenance of the RPE polarized phenotype per se, but it plays a crucial role in the morphogenesis of both apical microvilli and basal infoldings. Moreover, ezrin is also involved in the morphogenesis of Müller apical microvilli. Finally, the photoreceptors of the Ez–/– mice are significantly affected by the lack of ezrin in RPE and Müller cells.
Keywords: retinal pigment epithelium • retinal degenerations: cell biology • cytoskeleton