The PrPC C1 fragment derived from the ovine A136R154R171 PRNP allele is highly abundant in sheep brain and inhibits fibrillisation of full-length PrPC protein in vitro

Abstract

Expression of the cellular prion protein (PrP(C)) is crucial for the development of prion diseases. Resistance to prion diseases can result from reduced availability of the prion protein or from amino acid changes in the prion protein sequence. We propose here that increased production of a natural PrP α-cleavage fragment, C1, is also associated with resistance to disease. We show, in brain tissue, that ARR homozygous sheep, associated with resistance to disease, produced PrP(C) comprised of 25% more C1 fragment than PrP(C) from the disease-susceptible ARQ homozygous and highly susceptible VRQ homozygous animals. Only the C1 fragment derived from the ARR allele inhibits in-vitro fibrillisation of other allelic PrP(C) variants. We propose that the increased α-cleavage of ovine ARR PrP(C) contributes to a dominant negative effect of this polymorphism on disease susceptibility. Furthermore, the significant reduction in PrP(C) β-cleavage product C2 in sheep of the ARR/ARR genotype compared to ARQ/ARQ and VRQ/VRQ genotypes, may add to the complexity of genetic determinants of prion disease susceptibility.

Physiological Functions of the Cellular Prion Protein

“…α-Cleavage is thought to occur either in an acidic endosomal compartment (Shyng et al, 1993) or within the Golgi apparatus (Walmsley et al, 2009) and results in the production of the N-terminal fragment N1, which is released from the cell, and the C-terminal fragment C1, which seems to be trafficked to the cell membrane as per the full length protein (Harris et al, 1993; Vincent et al, 2000; Laffont-Proust et al, 2006). A large proportion of the cellular pool of PrP molecules may be in the form of C1—around 50% on average in sheep cerebral cortex, for example (Campbell et al, 2013). In addition to the α form of processing, PrP C can be subject to cleavage within its octapeptide repeat region (McMahon et al, 2001).…”

“…β-Cleavage seems to be dependent upon the combined presence of Cu 2+ and reactive oxygen species (ROS) (McMahon et al, 2001), although enzymatic processing by ADAM8 is also a possibility (McDonald et al, 2014). The apparent role for ROS in β-cleavage suggests that it might be a response to oxidative stress (Watt et al, 2005), although β-cleavage also seems to occur physiologically, since the resulting C-terminal fragment, C2, is found in healthy brain tissues from various species, albeit in small amounts (Mange et al, 2004; Campbell et al, 2013). β-Cleavage is thought to take place at the cell surface, leading to retainment of C2 on the cell membrane and release of the N-terminal fragment N2 (Mange et al, 2004; Watt et al, 2005).…”

The sheddase ADAM10 is a potent modulator of prion disease

“…These cleavages likely influence physiological functions of PrP C ( Aguzzi et al, 2008 ; Linden et al, 2008 ) and, importantly, its role in neurodegenerative diseases (reviewed in Altmeppen et al, 2013 ). While α-cleavage, occurring in the middle of the PrP C sequence and producing a soluble N1 and a membrane-attached C1 fragment, confers neuroprotection with regard to prion diseases ( Lewis et al, 2009 ; Westergard et al, 2011 ; Turnbaugh et al, 2012 ; Campbell et al, 2013 ) and Aβ-associated neurotoxicity ( Guillot-Sestier et al, 2009 ; Resenberger et al, 2011 ; Beland et al, 2012 ; Guillot-Sestier et al, 2012 ; Fluharty et al, 2013 ), the role of an extreme C-terminal cleavage in close proximity to the plasma membrane, termed shedding, remains enigmatic. On the one hand, by reducing the substrate for prion conversion and the receptor for neurotoxicity, this proteolytic release of almost full length PrP C from the plasma membrane could be protective with regard to prion diseases ( Marella et al, 2002 ; Heiseke et al, 2008 ; Altmeppen et al, 2012 ).…”

A New Paradigm for Enzymatic Control of α-Cleavage and β-Cleavage of the Prion Protein

“…Conversely, the C1 fragment promotes apoptosis through p53-dependent caspase-3 activity, although it appears as though the protective effects of N1 significantly outweigh the pro-apoptotic effects of C1 (14). Perhaps more importantly, substoichiometric levels of C1 protect against PrP Sc propagation (15,16).…”