Prion Protein Regulates Iron Transport by Functioning as a Ferrireductase

Abstract

Prion protein (PrPC) is implicated in the pathogenesis of prion disorders, but its normal function is unclear. We demonstrate that PrPC is a ferrireductase (FR), and its absence causes systemic iron deficiency in PrP knock-out mice (PrP−/−). When exposed to non-transferrin-bound (NTB) radioactive-iron (59FeCl3) by gastric-gavage, PrP−/− mice absorb significantly more 59Fe from the intestinal lumen relative to controls, indicating appropriate systemic response to the iron deficiency. Chronic exposure to excess dietary iron corrects this deficiency, but unlike wild-type (PrP+/+) controls that remain iron over-loaded, PrP−/− mice revert back to the iron deficient phenotype after 5 months of chase on normal diet. Bone marrow (BM) preparations of PrP−/− mice on normal diet show relatively less stainable iron, and this phenotype is only partially corrected by intraperitoneal administration of excess iron-dextran. Cultured PrP−/− BM-macrophages incorporate significantly less NTB-59Fe in the absence or presence of excess extracellular iron, indicating reduced uptake and/or storage of available iron in the absence of PrPC. When expressed in neuroblastoma cells, PrPC exhibits NAD(P)H-dependent cell-surface and intracellular FR activity that requires the copper-binding octa-peptide-repeat region and linkage to the plasma membrane for optimal function. Incorporation of NTB-59Fe by neuroblastoma cells correlates with FR activity of PrPC, implicating PrPC in cellular iron uptake and metabolism. These observations explain the correlation between PrPC expression and cellular iron levels, and the cause of iron imbalance in sporadic-Creutzfeldt-Jakob-disease brains where PrPC accumulates as insoluble aggregates.

Biology of ferritin in mammals: an update on iron storage, oxidative damage and neurodegeneration

“…Interestingly mice lacking the gene showed signs of systemic iron deficiency (Singh et al 2009b) and a progressive decrease of the metal is described in brains from affected patients or animals, despite an increase in total iron load (Singh et al 2009a) together with loss of function of PrPC. There is evidence that PrPc could have ferroxidase activity and its absence could reduce iron uptake in diseased brain (Singh et al 2013), but an important role could be attributed to the formation of aggregates between the PrPsc and iron-rich ferritin (Singh et al 2012). The ferritin isolated form affected brains is insoluble and retains iron even when exposed to harsh denaturing conditions.…”

Prion Protein Promotes Kidney Iron Uptake via Its Ferrireductase Activity

“…It is remarkable that the majority of PrP C in the urine is similar to the anchorless, N-terminally processed form observed in human and animal brains and neuronal cell models, suggesting similar processing of PrP C by kidney epithelial and cortical neuronal cells (20 -22). More importantly, the presence of PrP C in the urine of healthy individuals suggests that it is expressed on PT cells where it may provide FR activity necessary for the uptake of Tf-iron and NTBI (4,5,17).…”

“…However, excess NTBI and hemin aggregate PrP C to a detergent-insoluble form, preventing further iron uptake. 59 Fe-citrate diluted in normal saline as described in previous reports (5,25). For the specific radiolabeling of NTBI, serum Tf was saturated with excess unlabeled iron by injecting ferric-ammonium citrate (FAC) (70 g/22 g of mouse) intraperitoneally followed by 59 Fe-citrate after an interval of 30 min (26).…”

“…Cumulative evidence indicates brain iron dyshomeostasis as a prominent feature of human and animal prion disorders, implicating redox-active iron in disease-associated neurotoxicity (3,4). The underlying cause of iron dyshomeostasis is not clear, although recent reports suggest loss or altered function of PrP C in cellular iron uptake via its ferrireductase (FR) activity as a possible cause (5). Although demonstrated in neuroblastoma cells expressing exogenous PrP C in vitro (5), unequivocal evidence supporting PrP C -associated FR activity in vivo is lacking mainly because of the complex nature of regulation of the iron redox state as well as transport into different compartments in the brain.…”

Iron, Aging, and Neurodegeneration

“…Despite the changes in uptake and storage of iron suggested by these studies, it has also been suggested that PrP c itself is a ferrireductase (Figure 1) , which is then more likely to be transported into the cell, thus resulting in increased cellular iron. However, unlike alpha-synuclein, for which kinetic studies on purified protein suggested iron reduction is a result of true enzymatic catalysis109], studies on the ferrireductase activity of PrP c have only been performed on cell extracts [140]. As PrP c has already been suggested to be associated with redox balance in cells [123], this observation may only be reporting a secondary effect rather than a direct function of PrP c in iron reduction.…”