Osteopontin (OPN) is present in drusen and basal deposits in human eyes with age-related macular degeneration

Purpose: AMD is the most common cause of irreversible vision loss. Chronic inflammation has been proposed to contribute to formation of diffuse basal deposits and lipid-protein rich focal deposits called drusen, characteristic phenotypes of the early dry form of AMD. OPN is a secreted phosphoprotein, which has been shown to play a regulatory role in inflammation in Alzheimer’s disease, atherosclerosis and fatty liver disease. Given that these diseases share common pathogenic pathways with AMD, we sought to investigate the localization of OPN in eyes from normal and AMD donors. Further, to identify potential inducers of OPN, we tested the effect of constituents of cigarette smoke and lipids, on OPN expression in human retinal pigment epithelial (RPE) cells in vitro. Methods: OPN was localized via indirect immunofluorescence in ten micron thick paraffin sections cut from normal and diagnosed AMD human donor eyes (N=5). AMD eyes contained drusen, basal deposits and RPE changes. ARPE19 cells grown to post-confluence were exposed to cigarette smoke components (dioxin, hydroquinone) and lipids (OXLDL) for 24 hours prior to RNA and protein extraction. mRNA and protein expression of OPN relative to a housekeeping gene or protein were measured with qPCR and Western blot analysis respectively (n=3). Results: In normal eyes OPN immunolocalized to the retinal ganglion cell layer. In AMD eyes staining was also seen throughout diffuse deposits and as vesicles or spherical particulates within drusen. In vitro experiments confirmed RPE cells as a potential source of OPN as they express both RNA and protein. A significant increase in OPN mRNA in ARPE19 cells was seen following treatment with dioxin (2 fold), hydroquinone (15 fold) and oxLDL (3 fold). OPN protein expression also increased following treatment with these drugs. Conclusions: These findings demonstrate the novel observation that OPN accumulates in drusen and basal deposits in human dry AMD. RPE cells express OPN and may be a local source. Drugs associated with initiation and progression of AMD were shown to stimulate and induce OPN expression in RPE cells. It is plausible that OPN serves as a signal for promoting AMD progression through its proposed functions in recruitment and retention of macrophages and inflammation. This potential aspect of OPN biology in AMD is currently under investigation

Apolipoprotein E allele-dependent pathogenesis: A model for age-related retinal degeneration

Age-related macular degeneration (AMD) is a late-onset, multifactorial, neurodegenerative disease of the retina and the leading cause of irreversible vision loss in the elderly in the Western world. We describe here a murine model that combines three known AMD risk factors: advanced age, high fat cholesterol-rich (HF-C) diet, and apolipoprotein E (apoE) genotype. Eyes of aged, targeted replacement mice expressing human apoE2, apoE3, or apoE4 and maintained on a HF-C diet show apoE isoform-dependent pathologies of differential severity. ApoE4 mice are the most severely affected. They develop a constellation of changes that mimic the pathology associated with human AMD. These alterations include diffuse sub-retinal pigment epithelial deposits, drusenoid deposits, thickened Bruch's membrane, and atrophy, hypopigmentation, and hyperpigmentation of the retinal pigment epithelium. In extreme cases, apoE4 mice also develop marked choroidal neovascularization, a hallmark of exudative AMD. Neither age nor HF-C diet alone is sufficient to elicit these changes. We document choroidal neovascularization and other AMD-like ocular pathologies in an animal model that exploits known AMD risk factors. The model is additionally attractive because it is not complicated by invasive experimental intervention. Our findings in this model implicate the human apoE E4 allele as a susceptibility gene for AMD and support the hypothesis that common pathogenic mechanisms may underlie AMD and Alzheimer's disease