Photoreceptor degeneration and altered distribution of interphotoreceptor matrix proteoglycans in the mucopolysaccharidosis VII mouse

Abstract

Recent studies suggest that chondroitin sulfate proteoglycans in the retinal interphotoreceptor matrix (IPM) play a role in maintaining photoreceptor viability. We report herein studies on the retinas from mice with mucopolysaccharidosis type VII (MPS VII), a storage disorder resulting from virtual absence of beta-glucuronidase, an enzyme that is involved in the lysosomal degradation of chondroitin sulfate and other beta-glucuronide-containing proteoglycans. The distribution of IPM chondroitin sulfate proteoglycans was examined by immunohistochemistry and lectin-based histochemistry, and compared with the morphology of photoreceptor and retinal pigmented epithelial (RPE) cells at various ages in MPS VII-affected mice. A number of lectins and antibodies were employed that react with epitopes of IPM chondroitin sulfate proteoglycans, including Triticum vulgaris agglutinin (WGA), Arachis hypogaea agglutinin (PNA), Phaseolus vulgaris agglutinin (PHA-L), and an antibody directed against chondroitin 6-sulfate (AC6S). In MPS VII-affected animals, slight shortening of photoreceptor outer segments occurs between postnatal months 1 and 8. This is associated with changes in the distribution of some of the IPM chondroitin sulfate proteoglycans and hypertrophy of the retinal pigmented epithelium due to the accumulation of cytoplasmic membrane-bounded vesicles. WGA- and PHA-L-, but not AC6S-binding glycoconjugates accumulate within the RPE of affected mice during this time. Immunoreactive chondroitin 6-sulfate is not observed within the RPE of affected animals, probably since the antibody employed does not label free chondroitin sulfate glycosaminoglycan fragments. A loss of the normal apical-basal distribution of chondroitin 6-sulfate and PHA-L-binding IPM proteoglycans is apparent by 4 postnatal months. In contrast, WGA-binding proteoglycan remains uniformly distributed through 8 months. Pyknotic photoreceptor nuclei are observed in months 2-5 and photoreceptor loss is observed by 6 months. Cone photoreceptor loss appears to occur prior to that of rod photoreceptors. These observations suggest that the absence of beta-glucuronidase in the RPE of MPS VII mice may lead to an altered distribution of at least some IPM chondroitin sulfate proteoglycans. The resultant changes in the biochemical composition and/or physical structure of the IPM may affect subsequently its photoreceptor cell-supportive function leading to photoreceptor degeneration.