Mfrp Gene Function is Necessary to Maintain Normal Structure of Rod Photoreceptors and the Interphotoreceptor Matrix

Abstract

Purpose: <br /> Nanophthalmos with retinopathy (NNO2) is a genetic disorder associated with extreme hyperopia, insufficient growth of the eye, angle closure glaucoma, macular edema, progressive degeneration of rod photoreceptors and retinal detachment. It is caused by recessive mutations in the MFRP gene, which encodes a Frizzled-related transmembrane protein expressed on the apical surface of the retinal pigment epithelium (RPE). In contrast with the human disease, mutations of Mfrp in the mouse rd6 mutant have little effect on ocular shape or growth. However, they do exhibit a progressive photoreceptor dysfunction and degeneration that is very similar to the human disorder. To better understand the mechanism of this retinopathy, we have examined the structure and biochemisty of the rd6 mutant retina.

Methods: Eyes of homozygous mutant Mfrp^rd6 mice (2 to 24 weeks), were perfusion-fixed and cryosectioned. Immunocytochemistry with confocal microscopy was used to image proteins and carbohydrates. Eyes were also embedded in LR white plastic and sectioned for light or electron microscopy. Critical point-dried retina was fractured and examined by scanning electron microscopy.

Results: We have found that mice lacking Mfrp selectively accumulate SPACR, a major protein component of the extracellular matrix that surrounds photoreceptor inner and outer segments. Structural anomalies of the interphotoreceptor matrix are already present on eye opening at 2 weeks, and matrix accumulation is apparent by 5 weeks. Photoreceptor cell death becomes evident at 12 weeks. Scanning electron microscopy of the Mfrp^rd6 homozygous mutant retina reveals distinctive structural abnormalities of the inner and outer segments, as well as the outer neural epithelium. Progressive accumulation of the interphotoreceptor matrix in the mutant suggests that Mfrp is normally required for its turnover. Whatever its role, Mfrp does not appear to function as an endocytic receptor for the matrix, as it is not found in SPACR-loaded endocytic vesicles.

Conclusions: We have identified novel structural defects in photoreceptor inner and outer segments that are caused by loss of function of the Mfrp gene. The normal function of Mfrp protein in the RPE has remained elusive. Our results suggest that it participates in the turnover and structural maintenance of the interphotoreceptor matrix.