Sialoglycoconjugate profiling of human choroid, retinal pigment epithelium, and basal laminar deposits

Age-related macular degeneration is a leading cause of central vision loss in the elderly. Early hallmarks of the disease include basal laminar deposit beneath the retinal pigment epithelium (RPE) and choriocapillaris degeneration. We utilized sialic acid binding lectins Sambucus nigra/Elderberry Bark Lectin (EBL) and Maackia amurensis lectin II (MAL-II), to assess the localization of ɑ-2,6 and ɑ-2,3 sialic acids, respectively, in human macular retina, RPE, basal laminar deposits, and choroid. Photoreceptor carbohydrate epitopes differ based on retinal topography, with MAL-II recognizing foveal (but not extrafoveal) cones. Both MAL-II and EBL react with apical RPE, and both bind basal laminar deposits. In the choroid, MAL-II predominantly labels the choriocapillaris endothelium, while EBL also shows robust labeling of Bruch's membrane and extracellular domains surrounding the microvasculature (intercapillary pillars). EBL labeling overlaps with the distribution of complement factor H to a greater extent than MAL-II. After treatment with neuraminidase to remove terminal sialic acids, a battery of lectins was applied to sections of choroids. Lectins that recognize β-galactose, N-acetyllactosamine, galactose (β-1,3) N-acetylgalactosamine, and ɑ- or β-N-acetylgalactosamine showed increased reactivity, indicating the presence of abundant sialoglycans in basal laminar deposits. This study provides insight into the location and partial identities of sialoglycoconjugates in the human choroid, with possible implications for understanding the pathogenesis of macular degeneration.