Differential Expression of IRBP, Albumin, BIGH3 and PNA Binding in the Interphotoreceptor Matrix - RPE/Choroid Complex in Diabetes, AMD and Glaucoma
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Purpose: Interphotoreceptor retinoid-binding protein (IRBP) is a 145 kDa glycolipoprotein originally described in the vitreous and interphotoreceptor matrix (IPM). Interestingly, its concentration in the vitreous is reduced in early stages of diabetic retinopathy (DR).  In the retina, IRBP is restricted to the subretinal space. Our long-term goal is to understand the mechanism and significance for the reduced vitreous IRBP in diabetes. Specifically, we look at (1) National Development and Research Institutes (NDRI) human eye cases pathologic diagnosis and staging, using H&E, silver stain and Periodic Acid Schiff (PAS) stain, (2) disease pattern reflected by immunohistochemistry of IPM and choroid markers, (3) comparison of disease pattern study between human tissue and rodents'. Methods: We used immunohistochemistry to compare the distribution of IRBP and peanut agglutinin (PNA), with that of the extracellular choroidal proteins, albumin and BIGH3 (transforming β induced gene-human clone 3). Globes were retrieved from 34 donors (30 to 98 yo) with diabetes, age-related macular degeneration (AMD), glaucoma, and normal controls. The interval between death and placement of the eye in formalin was under 7.5 hours except one control. For all cases, a detailed gross and histopathological study were performed, and correlated with clinical history. Cases: Diabetes [4 no apparent diabetic retinopathy, 5 non-proliferative retinopathy (NPDR), 2 proliferative retinopathy (PDR)]; AMD (15 cases); glaucoma (13 cases), and normal controls (6 cases). Eleven had more than one diagnosis. We further extended this immunohistochemistry study on rodent tissues including streptozotocin (STZ)- induced 1-month diabetic rat (1 responding case with 2 controls), and STZ-induced 3-months diabetic mouse (4 DR cases with 2 controls). Results: Except when the postmortem interval was beyond about 12 hours, and the photoreceptors showed autolysis, IRBP was restricted to the IPM and albumin to the choroid. However, in diabetes, IRBP was present in the choroid, and was reduced in the IPM in proportion to disease severity, and albumin was present in the IPM. In contrast, no significant IRBP was found in the choroid of patients with glaucoma. In AMD, decreased IPM IRBP was noted whenever there was drusen formation, or when there was photoreceptor degeneration. BIGH3 and PNA labeling did not show evidence of diffusion out of the choroid or IPM, respectively. However, in severe DR, PNA showed decreased staining. In wet AMD, BIGH3 showed some staining in the IPM. In rodents, IRBP, albumin, PNA or BIGH3 redistribution were not found. PNA and IRBP staining in the photoreceptor/IPM appeared to be decreased in rodents. Conclusions: In human, reduced expression of IPM IRBP, and its appearance in the choroid was characteristic of diabetes compared to AMD, glaucoma or normal controls. The concomitant movement of albumin into the IPM is consistent with reduced RPE integrity. The lack of redistribution of BIGH3 and PNA binding glyconjugates may be due to local binding to choroidal integrins, or the IPM hyaluronan scaffold such as SPACA (sialoprotein associated with rods and cones), respectively. Our data suggests that increased clearance of IRBP may contribute to decreased vitreous IRBP in diabetes. In contrast, in rodent diabetic models, no obvious IRBP or albumin redistribution was shown, which indicate outer blood-retinal barrier (BRB) breakdown may not be present at this stage. However, the possible decreased staining of IRBP and PNA may suggest that the degeneration of photoreceptors could be a sign in the early stage of rodent diabetic retinopathy.