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Membrane frizzled-related protein is necessary for the normal development and maintenance of photoreceptor outer segments

Published online by Cambridge University Press:  01 July 2008

The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine
The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine
Research Service, Cleveland VA Medical Center, Cleveland, Ohio Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio
Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio
The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine
The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine
The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine
*Address correspondence and reprint requests to: Patsy M. Nishina, The Jackson Laboratory, 600 Main Street, Bar Harbor, ME 04609. E-mail:


A 4 base pair deletion in a splice donor site of the Mfrp (membrane-type frizzled-related protein) gene, herein referred to as Mfrprd6/rd6, is predicted to lead to the skipping of exon 4 and photoreceptor degeneration in retinal degeneration 6 (rd6) mutant mice. Little, however, is known about the function of the protein or how the mutation causes the degenerative retinal phenotype. Here we examine ultrastructural changes in the retina of Mfrprd6/rd6 mice to determine the earliest effects of the mutation. We also extend the reported observations of the expression pattern of the dicistronic Mfrp/C1qtnf5 message and the localization of these and other retinal pigment epithelium (RPE) and retinal proteins during development and assess the ability of RPE cells to phagocytize outer segments (OSs) in mutant and wild-type (WT) mice. At the ultrastructural level, OSs do not develop normally in Mfrprd6/rd6 mutants. They are disorganized and become progressively shorter as mutant mice age. Additionally, there are focal areas in which there is a reduction of apical RPE microvilli. At P25, the rod electroretinogram (ERG) a-wave of Mfrprd6/rd6 mice is reduced in amplitude by ~50% as are ERG components generated by the RPE. Examination of β-catenin localization and Fos and Tcf-1 expression, intermediates of the canonical Wnt pathway, showed that they were not different between mutant and WT mice, suggesting that MFRP may operate through an alternative pathway. Finally, impaired OS phagocytosis was observed in Mfrprd6/rd6 mice both in standard ambient lighting conditions and with bright light exposure when compared to WT controls.

Research Articles
Copyright © Cambridge University Press 2008

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