Prion Protein Modulates Cellular Iron Uptake: A Novel Function with Implications for Prion Disease Pathogenesis

Abstract

Converging evidence leaves little doubt that a change in the conformation of prion protein (PrPC) from a mainly α-helical to a β-sheet rich PrP-scrapie (PrPSc) form is the main event responsible for prion disease associated neurotoxicity. However, neither the mechanism of toxicity by PrPSc, nor the normal function of PrPC is entirely clear. Recent reports suggest that imbalance of iron homeostasis is a common feature of prion infected cells and mouse models, implicating redox-iron in prion disease pathogenesis. In this report, we provide evidence that PrPC mediates cellular iron uptake and transport, and mutant PrP forms alter cellular iron levels differentially. Using human neuroblastoma cells as models, we demonstrate that over-expression of PrPC increases intra-cellular iron relative to non-transfected controls as indicated by an increase in total cellular iron, the cellular labile iron pool (LIP), and iron content of ferritin. As a result, the levels of iron uptake proteins transferrin (Tf) and transferrin receptor (TfR) are decreased, and expression of iron storage protein ferritin is increased. The positive effect of PrPC on ferritin iron content is enhanced by stimulating PrPC endocytosis, and reversed by cross-linking PrPC on the plasma membrane. Expression of mutant PrP forms lacking the octapeptide-repeats, the membrane anchor, or carrying the pathogenic mutation PrP102L decreases ferritin iron content significantly relative to PrPC expressing cells, but the effect on cellular LIP and levels of Tf, TfR, and ferritin is complex, varying with the mutation. Neither PrPC nor the mutant PrP forms influence the rate or amount of iron released into the medium, suggesting a functional role for PrPC in cellular iron uptake and transport to ferritin, and dysfunction of PrPC as a significant contributing factor of brain iron imbalance in prion disorders.

Molecular Dynamics as an Approach to Study Prion Protein Misfolding and the Effect of Pathogenic Mutations

“…Determining the function of PrP C has proved to be a major challenge, complicated by the fact that PrP knock-out mice lack an obvious phenotype [12]. Many different putative functions have been proposed, indicating that PrP C is a multifunctional protein that plays a role in cell signaling [13,14] and metal metabolism [15][16][17]. When PrP aggregates and forms fibrils, however, it has significantly changed conformation and becomes largely insoluble and proteinase K resistant.…”

Redox Control of Prion and Disease Pathogenesis

“…The protective function of PrP C is further exemplified by the fact that expression of a single copy of PrP C can rescue the ataxic phenotype of transgenic mice expressing a deletion construct of PrP (PrP~32-121=134) (199), slow neurodegeneration in transgenic mice expressing a pathogenic mutation of PrP (217), rescue neurons from Doppel-induced toxicity (149), and protect brain tissue from ischemia-and traumainduced insult (210,238). Moreover, PrP C is believed to function as a copper-and iron-uptake protein, suggesting a role in the maintenance of cellular copper and iron homeostasis (33,126,205,226). Collectively, these reports suggest that loss of normal function of PrP C can be as significant as accumulation of PrP Sc in inducing prion disease-associated neurotoxicity (Fig.…”

“…Although the underlying cause of metal imbalance is not clear, recent reports indicating a facilitative role of PrP C in iron and copper uptake and interference with manganese uptake suggest that loss of this function as a result of aggregation to the PrP Sc form may contribute to brain metal dyshomeostasis, resulting in metalinduced oxidative damage (147,166,204,205). Furthermore, iron and copper may remain associated with PrP Sc , thus rendering the complex redox active and neurotoxic (12,203).…”

“…Collectively, the available evidence points to a functional role for PrP C in the uptake and metabolism of copper and iron (117,142,143,204,205), leading to the hypothesis that dysfunction of PrP C due to conversion to the PrP Sc form may contribute to the imbalance of brain metal homeostasis observed in prion disorders (203). Other mechanisms leading to altered metal metabolism and disease pathogenesis have been proposed, and consensus on this issue is still lacking (40,44).…”

“…Diverse functions have been attributed to PrP C based on the model and the experimental design used for evaluation (126,129,239). However, because PrP C is involved in copper and iron uptake, it is possible that loss of normal function of PrP C in copper and iron metabolism, combined with gain of toxic function by redox-active PrP Sc -ferritin aggregates, induce a state of brain metal imbalance, resulting in metal-induced oxidative stress and neurotoxicity (12,166,(203)(204)(205)239). The accumulated redoxactive iron and copper would further react with dioxygen species abundant in the metabolically active environment of the brain, further aggravating the oxidative damage (169).…”