SPACRCAN, a novel human interphotoreceptor matrix hyaluronan-binding proteoglycan synthesized by photoreceptors and pinealocytes

Abstract

The interphotoreceptor matrix is a unique extracellular complex occupying the interface between photoreceptors and the retinal pigment epithelium in the fundus of the eye. Because of the putative supportive role in photoreceptor maintenance, it is likely that constituent molecules play key roles in photoreceptor function and may be targets for inherited retinal disease. In this study we identify and characterize SPACRCAN, a novel chondroitin proteoglycan in this matrix. SPACRCAN was cloned from a human retinal cDNA library and the gene localized to chromosome 3q11.2. Analysis of SPACRCAN mRNA and protein revealed that SPACRCAN is expressed exclusively by photoreceptors and pinealocytes. SPACRCAN synthesized by photoreceptors is localized to the interphotoreceptor matrix where it surrounds both rods and cones. The functional protein contains 1160 amino acids with a large central mucin domain, three consensus sites for glycosaminoglycan attachment, two epidermal growth factor-like repeats, a putative hyaluronan-binding motif, and a potential transmembrane domain near the C-terminal. Lectin and Western blotting indicate an M(r) around 400,000 before and 230,000 after chondroitinase ABC digestion. Removal of N- and O-linked oligosaccharides reduces the Mr to approximately 160,000, suggesting that approximately 60% of the mass of SPACRCAN is carbohydrate. Finally, we demonstrate that SPACRCAN binds hyaluronan and propose that associations between SPACRCAN and hyaluronan may be involved in organization of the insoluble interphotoreceptor matrix, particularly as SPACRCAN is the major proteoglycan present in this matrix.