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Abstract
Unique domains of the retinal interphotoreceptor matrix (IPM), termed cone matrix sheaths, are composed largely of chondroitin 6-sulfate proteoglycan in most higher mammalian species. Recent investigations suggest that cone matrix sheaths participate in the maintenance of normal retinal attachment. To investigate the potential functional roles of IPM proteoglycans further, the synthesis of cone matrix sheath chondroitin 6-sulfate proteoglycan was perturbed in vivo. Intravitreal injections of p-nitrophenyl-beta-D-xylopyranoside (xyloside), a sugar that inhibits chondroitin sulfate proteoglycan synthesis, were administered to Yucatan micropigs. Their eyes were examined funduscopically and electroretinographically. At selected times, the eyes were enucleated and examined histochemically and immunohistochemically with various probes directed against cone photoreceptor cells and cone matrix sheaths. The IPM was affected selectively after xyloside administration; no inner retinal pathology or dysfunction was detected morphologically or electroretinographically. The degree of xyloside-induced perturbation was dependent on the duration of xyloside exposure and dose. It was classified into three stages, based on morphologic and histochemical criteria. Although all three stages could be observed in a given retina, a single stage typically predominated, depending on the particular dosage regimen. The early stage was characterized by IPM disruption, as evidenced by disorganization of chondroitin 6-sulfate and peanut agglutinin (PNA)-binding glycoconjugates. Cone photoreceptor cell outer segment degeneration and markedly decreased chondroitin 6-sulfate immunoreactivity distinguished the middle stage. During the late stage, there was a near complete absence of both immunoreactive chondroitin 6-sulfate and PNA-binding glycoconjugates in the IPM. Shallow retinal detachments that appeared funduscopically as patches of retinal whitening frequently were observed after moderate durations of xyloside exposure; these progressed peripherally with continued xyloside exposure. Histologically, the areas of retinal whitening corresponded to regions in which cone matrix sheaths were split transversely (ie, in a plane perpendicular to the longitudinal axes of the photoreceptor cell outer segments) or were separated completely from cone outer segments. Similar effects were not observed in control eyes. These results suggest that adhesion between the neural retina and retinal pigmented epithelium may be dependent, in part, on continuous synthesis of cone matrix sheath-associated proteoglycans and, potentially, other IPM proteoglycans. In addition, these proteoglycans appear to be necessary for the maintenance of cone photoreceptor cell outer segment integrity.