Abstract
A common clinical phenotype of several neurodegenerative and systemic disorders including Alzheimer's disease and atherosclerosis is the abnormal accumulation of extracellular material, which interferes with routine cellular functions. Similarly, patients with age-related macular degeneration (AMD), the leading cause of vision loss among the aged population, present with extracellular lipid- and protein-filled basal deposits in the back of the eye. While the exact mechanism of growth and formation of these deposits is poorly understood, much has been learned from investigating their composition, providing critical insights into AMD pathogenesis, prevention, and therapeutics. We identified human osteopontin (OPN), a phosphoprotein expressed in a variety of tissues in the body, as a newly discovered component of basal deposits in AMD patients, with a distinctive punctate staining pattern. OPN expression within these lesions, which are associated with AMD disease progression, were found to co-localize with abnormal calcium deposition. Additionally, OPN puncta colocalized with an AMD risk-associated complement pathway protein, but not with apolipoprotein E or vitronectin, two other well-established basal deposit components. Mechanistically, we found that retinal pigment epithelial cells, cells vulnerable in AMD, will secrete OPN into the extracellular space, under oxidative stress conditions, supporting OPN biosynthesis locally within the outer retina. Finally, we report that OPN levels in plasma of aged (non-AMD) human donors were significantly higher than levels in young (non-AMD) donors, but were not significantly different from donors with the different clinical subtypes of AMD. Collectively, our study defines the expression pattern of OPN in the posterior pole as a function of disease, and its local expression as a potential histopathologic biomarker of AMD.
Highlights
Age-related macular degeneration (AMD), is an ocular neurodegenerative disease of aging, and the most common cause of irreversible blindness in the elderly in developed countries[1,2,3], with consequent detrimental effects on physical and mental health[4,5]
Chronic inflammation has emerged as a pathogenic mechanism that may contribute to formation and accumulation of basal deposits, a thought given further credence by the observed localization of several factors associated with immune regulation, including members of the complement pathway, within these lipid- and protein-rich foci[25,26,27,28]
OPN is expressed in the posterior human retina To identify ocular cells and tissue compartments that express OPN, gene expression was examined in cells isolated from the posterior retina, including cultured human primary RPE (hRPE) cells, the human derived ARPE19
Summary
Age-related macular degeneration (AMD), is an ocular neurodegenerative disease of aging, and the most common cause of irreversible blindness in the elderly in developed countries[1,2,3], with consequent detrimental effects on physical and mental health[4,5]. The initial stage of AMD development, known as early nonexudative, is characterized by accumulation of lipid- and protein-filled extracellular deposits, basal to the retinal pigment epithelium (RPE)[6,7,8], a cuboidal epithelial cell layer that serves as support for everyday functions of the overlying neurosensory retina[9,10]. OPN has been investigated in neurodegenerative diseases that share common pathogenic pathways with AMD, including Alzheimer’s disease, atherosclerosis, and multiple sclerosis, and found to be differentially regulated in response to either inflammation or injury[45]. The impact of AMD-relevant stressors on extracellular OPN secretion from human RPE cells, Immunohistochemistry Protein localization in paraffin sections probed with antibodies (Table 2), was performed as previously described[55,59,60]. Peptide competition immunofluorescence assay To further demonstrate specificity of the OPN antibody, immunofluores-
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