The Role of the Octarepeat Region in Neuroprotective Function of the Cellular Prion Protein

Abstract

Structural alterations of the cellular prion protein (PrP C ) seem to be the core of the pathogenesis of prion diseases. However, the physiological function of PrP C remains an enigma. Cell culture experiments have indicated that PrP C and in particular its Nterminal octarepeat region together with the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathways have a fundamental involvement in neuroprotection and oxidative stress reactions. We used wild-type mice, PrP knockout (Prnp -/-) animals and transgenic mice that lack the octarepeat region (C4/-) and subjected them to controlled ischemia. We identified an increased cleavage and synthesis of PrP C in ischemic brain areas of wild-type mice compared with sham controls. The infarct size in Prnp -/-animals was increased threefold when compared with wild-type mice. The infarct size in C4/-animals was identical to Prnp -/-mice, that is, around three times larger than in wild-type mice. We showed that the PrP in C4/-mice does not functionally rescue the Prnp -/-phenotype; furthermore it is unable to undergo b cleavage, although an increased amount of C1 fragments was found in ischemic brain areas compared with sham controls. We demonstrated that the N-terminal octarepeat region has a lead function in PrP C physiology and neuroprotection against oxidative stress in vivo. Pathol 2007;17:174-183. Brain INTRODUCTIONThe cellular prion protein (PrP C ) is a copper-binding protein (3, 17) located at the synapse (13) and has a primary role in the pathogenesis of prion diseases. A templated conformational transition of PrP C seems to be the cause of transmissibility and pathogenesis (25). PrP C has been shown to play a role in cellular antioxidative defense mechanisms (15,27) and to protect human neurons in primary culture against Bax-mediated cell death (2). Experiments in cerebellar granule cells (5) have shown a loss of neuroprotective activity of PrP C by a deletion of all five N-terminal octarepeats (PrP∆51-90). We have recently identified a functional link between PrP C expression and phosphatidylinositol 3-kinase (PI3K) activation, a regulator of Akt phosphorylation and a protein kinase that plays a pivotal role in cell survival (33). Our findings are in agreement with published results that showed reduced Phospho-Akt expression levels in Prnp −/− mouse brains following ischemia (38). We could also demonstrate that hippocampal cells that lack the octarepeat region showed reduced PI3K levels and decreased survival under stress conditions similar to PrP C knockout cells. In neuroblastoma cells it could be demonstrated that the octapeptide repeats are required for the β cleavage and their absence (PrP∆oct) correlates with increased sensitivity of cells to oxidative stress (36). The cleavage of the molecule seems to play a major role in the biological properties of PrP C but succession of these proteolysis procedures as well as relations between the two cleavages is controversial and still remains to be elucidated and in particular, the relevance of the octapeptide re...

Hemin Interactions and Alterations of the Subcellular Localization of Prion Protein

“…These findings suggest that PrP C participates in the modulation of the activity of these enzymes and/or PrP C itself functions as an antioxidant molecule. Moreover, it has been reported that the flexible N-terminal domain of PrP C , which we have shown contains heminbinding sites, is influential in cellular responses to oxidative stress (50,51). This suggests that the peroxidase activity of hemin-PrP C complexes may play a protective role against oxidative stress.…”

Trafficking of PrPcto mitochondrial raft-like microdomains during cell apoptosis

“…38 A neuroprotective activity of PrP C was also reported by Mitteregger et al who revealed that both the C-terminal GPI anchor and the unstructured N-terminal domain are required for this physiological activity. 39 Thus, PrP C might act as a signaling molecule at the cell surface to promote stress-protective signaling under physiological conditions, which can be switched to toxic signaling through an interaction with β-sheet-rich conformers. Similarly to other GPI-anchored proteins involved in signal transduction, PrP C may act as a co-receptor in concert with a transmembrane protein to transduce the signal into the cell.…”

Conserved Stress-protective Activity between Prion Protein and Shadoo

“…The design of the Sho mutants was based on PrP mutants that have been characterized previously. PrP⌬N, a mutant lacking large parts of the intrinsically disordered N terminus (⌬aa 27-89), can promote propagation of infectious prions (14) but shows an impaired neuroprotective activity (9,15). PrP⌬HD lacks the highly conserved internal hydrophobic domain (⌬aa 113-133) and is characterized by a neurotoxic potential that can be blocked by the co-expression of wild type (WT) PrP C (9,(17)(18)(19).…”

“…13). Interestingly, the octarepeat region in the N-terminal domain as well as the C-terminal GPI anchor are dispensable for the generation of infectious prions (8,14) but are required for the stress-protective activity of PrP C (9,15). Although in the majority of prion diseases there is a correlation between the accumulation of PrP Sc , formation of infectious prions, and neurodegeneration, there are some interesting exceptions.…”