Misfolded PrP impairs the UPS by interaction with the 20S proteasome and inhibition of substrate entry

Abstract

Prion diseases are associated with the conversion of cellular prion protein (PrP C ) to toxic b-sheet isoforms (PrP Sc ), which are reported to inhibit the ubiquitin-proteasome system (UPS). Accordingly, UPS substrates accumulate in prion-infected mouse brains, suggesting impairment of the 26S proteasome. A direct interaction between its 20S core particle and PrP isoforms was demonstrated by immunoprecipitation. b-PrP aggregates associated with the 20S particle, but did not impede binding of the PA26 complex, suggesting that the aggregates do not bind to its ends. Aggregated b-PrP reduced the 20S proteasome's basal peptidase activity, and the enhanced activity induced by C-terminal peptides from the 19S ATPases or by the 19S regulator itself, including when stimulated by polyubiquitin conjugates. However, the 20S proteasome was not inhibited when the gate in the a-ring was open due to a truncation mutation or by association with PA26/PA28. These PrP aggregates inhibit by stabilising the closed conformation of the substrate entry channel. A similar inhibition of substrate entry into the proteasome may occur in other neurodegenerative diseases where misfolded b-sheet-rich proteins accumulate.

Dendritic spinopathy in transgenic mice expressing ALS/dementia-linked mutant UBQLN2

“…Thus, it is conceivable that if this positive-feedback cycle continues, the aggregates would increase in number and size over time and be observed as a pathological hallmark in diseased tissues. Consistent with this hypothesis, proteasome subunits and ubiquitin have been previously observed in protein aggregates associated with various neurodegenerative diseases (23); and inhibition of the UPS by the disease-associated oligomers of amyloid-β, α-synuclein, prion, or polyQ repeats have also been suggested (24)(25)(26)(27)(28). These data support the notion that the failure of clearance of ubiquitinated proteins in specific cellular compartments, such as dendritic spines, may represent a common defect shared by a wide spectrum of neurodegenerative disorders, although the causes and upstream pathways are different.…”

Misfolded PrP and a novel mechanism of proteasome inhibition

“…This is exactly what was observed when β-sheetrich PrP was incubated with either α3ΔN open-gated 20S or 26S particles, in contrast to the inhibitory actions seen in wild-type proteasomes. 11 These data suggest that disease-associated misfolded PrP isoforms inhibit gate opening in the 20S proteasome, which accounts for the inhibition of 26S proteasomal function.…”

“…Misfolded PrP aggregates are able to reduce substrate hydrolysis by CtRpt5-activated 20S proteasomes as they do the 26S proteasome complex. 11 However, in the presence of β-sheet-rich PrP, the CtRpt5 peptide is still able to induce some increase in 20S proteasome gate opening. Thus, rather than directly blocking the actions of CtRpt5, it appears that disease-associated misfolded PrP conformers enhance the closed state of the 20S proteasome gate, which in the presence of the gate-opening properties of CtRpt5 results in a partial gate opening response.…”

“…It is intriguing, therefore, that disease-associated PrP conformers are not able to reduce the activity of 11S-activated 20S proteasomes, in stark contrast to their effect on 20S proteasome gate opening by the 19S ATPases (or their mimetic, the CtRpt5 peptide). 11 The 11S activators Each of these proteolytic activities is reduced when proteasomes are exposed to β-sheet-rich PrP. 9 The two outer rings of the 20S particle contain seven α-subunits, the N-termini of which interact to form a gate that regulates substrate entry.…”

FK506 reduces abnormal prion protein through the activation of autolysosomal degradation and prolongs survival in prion-infected mice

“…Although inhibition of the UPS causes cytosolic PRNP C accumulation, it is unlikely that the degradation of PRNP Sc occurs mainly in the UPS, 35,36 as PRNP Sc aggregates are unable to enter the narrow channel into the proteasome. On the other hand, the UPS has been reported to be impaired in TSEs, 37,38 and it has been demonstrated that both PRNP Sc and aggregated β-sheet-rich recPRNP inhibit the activity of proteasomes. Thus, it is reasonable to postulate that autophagy is one of the major degradation pathways of PRNP Sc .…”