Prion protein facilitates retinal iron uptake and is cleaved at the \u03b2-site: Implications for retinal iron homeostasis in prion disorders

Abhishek Asthana,Shilpita Karmakar,Ajay Ashok,Robyn Kokemuller,Najiba Mammadova,Qingzhong Kong,Neena Singh,Mary Heather Greenlee,Shounak Baksi

doi:10.1038/s41598-017-08821-1

Abhishek Asthana, Shilpita Karmakar + Show 7 more

Open Access

https://doi.org/10.1038/s41598-017-08821-1

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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.

Highlights

Brain iron dyshomeostasis is believed to result from loss of function of PrPC in iron uptake combined with sequestration of iron in dysfunctional ferritin, creating functional iron deficiency[12, 13]
A phenotype of relative iron deficiency despite sufficient or increased total iron has been demonstrated in biopsy and autopsy brain samples from sporadic Creutzfeldt-Jakob disease cases, the most common human prion disorder, and brain tissue harvested from scrapie-infected hamsters and mice during prion disease progression[12, 13]
To differentiate between these processes, it is important to understand the functional role of PrPC in iron uptake, and the effect of PrPSc and inflammation on iron metabolism at the cellular and organ level in vivo in a relatively simple model where temporal and spatial correlation between the appearance of PrPSc, microglial activation, and accumulation of ferritin can be mapped during disease progression

Summary

Introduction

Involves conversion of PrPC to a dysfunctional, aggregated form that induces microglial activation[4, 15, 16], it is likely that loss of function of PrPC in iron uptake, aggregation of ferritin, and PrPSc-induced inflammation evolve simultaneously, making it difficult to identify the contribution of each to iron dyshomeostasis To differentiate between these processes, it is important to understand the functional role of PrPC in iron uptake, and the effect of PrPSc and inflammation on iron metabolism at the cellular and organ level in vivo in a relatively simple model where temporal and spatial correlation between the appearance of PrPSc, microglial activation, and accumulation of ferritin can be mapped during disease progression. The neuroretina accumulates PrPSc and undergoes degeneration in sCJD and scrapie-infected animal models[3, 16, 18,19,20], providing an experimentally manipulable model to understand the role of PrPC in iron transport and the role of PrPSc and inflammation in retinal iron homeostasis and neuroretinal degeneration

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