Change in the characteristics of ferritin induces iron imbalance in prion disease affected brains

Abstract

Prion disease associated neurotoxicity is mainly attributed to PrP-scrapie (PrPSc), the disease associated isoform of a normal protein, the prion protein (PrPC). Participation of other proteins and processes is suspected, but their identity and contribution to the pathogenic process is unclear. Emerging evidence implicates imbalance of brain iron homeostasis as a significant cause of prion disease-associated neurotoxicity. The underlying cause of this change, however, remains unclear. We demonstrate that iron is sequestered in heat and SDS-stable protein complexes in sporadic-Creutzfeldt-Jakob- disease (sCJD) brains, creating a phenotype of iron deficiency. The underlying cause is change in the characteristics of ferritin, an iron storage protein that becomes aggregated, detergent-insoluble, and partitions with denatured ferritin using conventional methods of ferritin purification. A similar phenotype of iron deficiency is noted in the lumbar spinal cord (SC) tissue of scrapie infected hamsters, a site unlikely to be affected by massive neuronal death and non-specific iron deposition. As a result, the iron uptake protein transferrin (Tf) is upregulated in scrapie infected SC tissue, and increases with disease progression. A direct correlation between Tf and PrPSc suggests sequestration of iron in dysfunctional ferritin that either co-aggregates with PrPSc or is rendered dysfunctional by PrPSc through an indirect process. Surprisingly, amplification of PrPSc in vitro by the protein-misfolding-cyclic-amplification (PMCA) reaction using normal brain homogenate as substrate does not increase the heat and SDS-stable pool of iron even though both PrPSc and ferritin aggregate by this procedure. These observations highlight important differences between PrPSc-protein complexes generated in vivo during disease progression and in vitro by the PMCA reaction, and the significance of these complexes in PrPSc-associated neurotoxicity.

Disrupted iron homeostasis causes dopaminergic neurodegeneration in mice

“…Decreased activity of TFR1 may contribute to neurodegeneration in Huntington's disease (78). Creuztfeldt-Jakob disease was recently reported to involve neuronal iron deficiency, apparently due to sequestration of the iron storage protein ferritin in prion aggregates (79). Additional work will be needed to better understand the possible role of iron deficiency in neurodegenerative disorders.…”

Brain Iron Homeostasis: From Molecular Mechanisms To Clinical Significance and Therapeutic Opportunities

“…It is likely that co-aggregation of wild-type Ft chains is initiated by the free radicals generated by mutant Ft-L, creating iron imbalance in the affected brains (25). Accumulation of iron-rich, aggregated ferritin has also been observed in sCJD brains, suggesting the presence of common pathogenic events initiated by brain iron imbalance and free radicals (407,410).…”

“…Likewise, ferritin isolated from sCJD brain homogenates shows significant alterations in its biochemical characteristics. Unlike normal brain ferritin, ferritin isolated from prion disease-affected brains is insoluble, partitions with denatured ferritin when purified using conventional methods, and retains associated iron even after boiling in the presence of SDS (410). The PrP Sc -ferritin aggregates are rich in iron and induce the aggregation of additional PrP C to the PrP Sc form due to their pro-oxidant characteristics (36).…”

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.…”