Prion protein facilitates retinal iron uptake and is cleaved at the β-site: Implications for retinal iron homeostasis in prion disorders

Abstract

Prion disease-associated retinal degeneration is attributed to PrP-scrapie (PrPSc), a misfolded isoform of prion protein (PrPC) that accumulates in the neuroretina. However, a lack of temporal and spatial correlation between PrPSc and cytotoxicity suggests the contribution of host factors. We report retinal iron dyshomeostasis as one such factor. PrPC is expressed on the basolateral membrane of retinal-pigment-epithelial (RPE) cells, where it mediates uptake of iron by the neuroretina. Accordingly, the neuroretina of PrP-knock-out mice is iron-deficient. In RPE19 cells, silencing of PrPC decreases ferritin while over-expression upregulates ferritin and divalent-metal-transporter-1 (DMT-1), indicating PrPC-mediated iron uptake through DMT-1. Polarization of RPE19 cells results in upregulation of ferritin by ~10-fold and β-cleavage of PrPC, the latter likely to block further uptake of iron due to cleavage of the ferrireductase domain. A similar β-cleavage of PrPC is observed in mouse retinal lysates. Scrapie infection causes PrPSc accumulation and microglial activation, and surprisingly, upregulation of transferrin despite increased levels of ferritin. Notably, detergent-insoluble ferritin accumulates in RPE cells and correlates temporally with microglial activation, not PrPSc accumulation, suggesting that impaired uptake of iron by PrPSc combined with inflammation results in retinal iron-dyshomeostasis, a potentially toxic host response contributing to prion disease-associated pathology.

Prion protein modulates iron transport in the anterior segment: Implications for ocular iron homeostasis and prion transmission

“…Since the ciliary epithelium is a continuation of the retina, further studies were focused on whether PrP C facilitates iron transport across these cells as reported for RPE cells in the outer retina (Asthana et al, 2017). …”

“…To explore this possibility, subsequent experiments were conducted on mouse models lacking PrP C (PrP −/− ) or expressing human PrP C (PrP +/+ ) to determine whether PrP C modulates iron transport as reported in RPE cells and other cell types (Asthana et al, 2017) (Haldar et al, 2015; Tripathi et al, 2015). …”

“…Fixed eyes were paraffin embedded, and thin sections were immunostained as described (Asthana et al, 2017). In brief, sections were rehydrated with xylene followed by decreasing concentrations of ethanol (100%, 90% and 70%), and finally, three washes of 5 min each with 1× PBS.…”

“…The eye is protected further from fluctuations in systemic iron by the outer and inner blood-retinal barriers comprised of retinal pigment epithelial cells and capillary endothelial cells respectively, and the blood aqueous barrier formed by epithelial cells of the ciliary body (CB). Although the regulation of iron transport across blood-retinal barriers has been the subject of several recent reports (Asthana et al, 2017; Chaudhary et al, 2018; Sterling et al, 2017), relatively little is known about the exchange of iron, if any, across the blood-aqueous barrier.…”

“…Recently, we reported that the prion protein (PrP C ), the principal protein implicated in the pathogenesis of human and animal prion disorders, is expressed on retinal pigment epithelial cells and facilitates iron uptake by functioning as a ferrireductase partner for divalent metal transporters (Asthana et al, 2017). Since this layer is continuous with pigmented epithelium of the CB, it is plausible that PrP C plays a similar role at this site.…”