Reduced level of interphotoreceptor retinoid-binding protein (IRBP), a possible cause for retinal degeneration in the Abyssinian cat

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Abstract
Retinae of Abyssinian cats homozygous for a retinal degeneration gene, and normal controls, have been investigated using antibodies directed against opsin, transducin α (TD-α), S-antigen (48K protein), interphotoreceptor retinoid-binding protein (IRBP), and cone outer segments. IRBP-immunoreactivity (IR) is much reduced at stage 2 of the disease in affected retinae; later massive photoreceptor cell death occurs. In cats, at a late stage of the disease, the retina exhibits few S-antigen-IR cells in the peripheral part of the retina whereas, in the central part, some patches of cells exhibiting opsin-IR, TD-α-IR, and S-antigen-IR are present in remnants of the outer nuclear layer (ONL). No IRBP-IR is detectable at this stage. The form and size of the majority of these remaining cells, however, does not resemble that of normal photoreceptors. No, or only rudimentary, inner and outer segments are present; long bifurcating basal protrusions often occur. These cells, which could be remains of cone elements, are S-antigen immunoreactive. Double labelling for different retina-specific proteins reveals a co-localization of opsin, TD-α and S-antigen in some, but not all, remaining photoreceptor elements. Cells exhibiting opsin-IR also show TD-α-IR and S-antigen-IR located within the entire cell and its protrusions. In control retinae and retinae at early stages of the disease, immunoreactions are comparable with all antibodies used. However, TD-α-IR is less intensive in the photoreceptor terminals. S-antigen-IR cones are most frequently present in the peripheral retina. Reduction of IRBP at an early stage of the disease could be one of the factors leading to photoreceptor cell death at later stages.