Keywords

5.1 Introduction

The cellular prion protein (PrPC) is a universally expressed membrane protein present predominantly in the central nervous system (CNS). Deposition in the CNS of its pathologic isoform (PrPSc), derived from PrPC via a conformational transition, is a molecular hallmark of prion diseases, a group of fatal transmissible neurodegenerative disorders in humans and animals. Although the physiologic functions of PrPC are unclear, it has nevertheless been proposed that PrPC has beneficial and deleterious effects on cognition (Collinge et al. 1994; Laurén et al. 2009; Linden et al. 2008; Westaway et al. 2011). Moreover, it has been well demonstrated that the coexistence of PrPC and PrPSc is the prerequisite for the emergence of prion diseases (PrDs). The two PrP conformers mainly studied so far are believed to be implicated in these diseases. PrPC and PrPSc share the same primary sequence but have distinct secondary structures (Meyer et al. 1986; Caughey et al. 1991; Pan et al. 1993). PrPC is monomeric, rich in α-helical structure, sensitive to proteinase K (PK) digestion, soluble in non-denaturing detergents, non-precipitable by anti-DNA antibodies or DNA-binding proteins, noninfectious, and present in both uninfected and scrapie-infected brains. PrPSc, on the other hand, is oligomeric or aggregate, rich in β-sheet structure, partially resistant to PK digestion, insoluble in detergents, precipitable by anti-DNA antibodies or DNA-binding proteins, infectious, and present only in infected brains. Soluble PrPC has been the only conformer detected in the uninfected mammalian brain. In contrast, insoluble PrPSc exhibits chameleon-like conformations, which may underlie the distinct prion strains and phenotypes of PrDs identified in animals and humans (Bessen and Marsh 1992; Parchi et al. 1996; Caughey et al. 1998; Safar et al. 1998; Zou and Gambetti 2007; Collinge and Clarke 2007). Recent identification of insoluble cellular PrP (iPrPC) in the uninfected human and animal brain raises two possibilities: that the PrPC species in the brain may also exhibit chameleon-like conformations that are implicated in the beneficial or deleterious effects of PrPC, and that these species may play a role in the pathogenesis of PrDs and other neurodegenerative disorders (Yuan et al. 2006; Zou 2010; Zou et al. 2011b).

5.2 Prion Protein Is Characterized by the Presence of an Intrinsically Chameleon-Like Conformation

Studies using recombinant PrP (rPrP) in vitro indicated that PrP possesses a highly variable conformation. In aqueous solutions, rPrP could be folded into pH-dependent α-helical conformations, a thermodynamically more stable β-sheet, and various stable or transient intermediates (Zhang et al. 1997). A stopped-flow kinetic study demonstrated that PrP folded by a three-state mechanism involving a monomeric intermediate (Apetri and Surewicz 2002). It was found that the population of this partially structured PrP intermediate increased in the presence of relatively low concentrations of urea and was more stable at acidic pH 4.8, compared to neutral pH 7.0. Moreover, this approach revealed that PrP mutations, linked with naturally occurring familial prion diseases, showed a pronounced stabilization of the folding intermediate (Apetri et al. 2004). This characteristic strongly suggested that these intermediates play a crucial role in PrP conversion and serve as direct precursors of the pathologic PrPSc isoform. The existence of a PrP folding intermediate was also indicated by hydrogen exchange experiments (Nicholson et al. 2002), and by studies using high pressure NMR and fluorescence spectroscopy (Kuwata et al. 2002; Martins et al. 2003). In addition to a β-oligomer and an amyloid fibril (Baskakov et al. 2001; Morillas et al. 2001; Lu and Chang 2002; Sokolowski et al. 2003; Baskakov et al. 2004), two additional polymeric transient intermediates were also identified during fibrillogenesis of rPrP in vitro (Baskakov et al. 2002).

PrPC in vivo is anchored to the cell membrane. Several experiments have indicated that the PrP conformation is affected by its local conditions. For example, the interaction of the anchorless recombinant PrP with lipids in a membrane-like environment resulted in a conformational transition (Wang et al. 2007; Re et al. 2008). Increasing the local concentration of membrane-anchored PrPC seems to induce a conformational transition accompanied by oligomerization of PrPC (Elfrink et al. 2008). Therefore, the tendency of PrP to form multiple nonnative β-sheet-rich isoforms in vitro, as demonstrated in biophysical studies on rPrP, may represent a unique intrinsic feature of this protein.

5.3 Insoluble Cellular Prion Protein Is Present in Normal Mammalian Brains

If the tendency of PrP to form multiple conformations in vitro represents a unique intrinsic feature of this protein, it is conceivable that other PrP conformers would be present in the normal brain in addition to the well-characterized PrPC. To test for this possibility, we examined uninfected human and animal brains using a combination of biophysical and biochemical approaches to determine whether there are additional PrP conformers (Yuan et al. 2006). We identified a novel conformer which forms insoluble cellular PrP aggregates and protease-resistant PrP species in uninfected human brains (Yuan et al. 2006). Using gel filtration, we revealed that PrP in uninfected human brains is present not only in monomers but also in oligomers and large aggregates (Fig. 5.1). The new PrP conformer, which we termed insoluble cellular PrP (iPrPC), accounts for approximately 5–25% of total PrP including full-length and N terminally truncated forms, and a portion of iPrPC is resistant to PK digestion even at 50 μg/ml (Yuan et al. 2006). Notably, the PK-resistant iPrPC has an immunoreactive behavior different from that of classic PrPSc detected in prion-infected brains; its affinity is much lower for 3F4 while higher for 1E4, compared to the affinity of those antibodies for classic PrPSc (Yuan et al. 2006, 2008; Zou et al. 2010a, 2011a) (Fig. 5.2). The epitopes of the two antibodies 3F4 and 1E4 are adjacent and the C terminus of the 1E4 epitope between PrP97-105 is connected to the N terminus of the 3F4 epitope between PrP 106-112 (Yuan et al. 2008; Zou et al. 2010a). Antibody 3F4 is the most widely used antibody in the detection of human PrPC and PrPSc, including PrPSc types 1 and 2 seen in sCJD and inherited CJD, and the internal PrPSc fragment PrP7-8 seen in GSS. In addition, the new conformer reveals high affinity for the gene 5 protein (g5p, a single-stranded DNA-binding protein) and sodium phosphotungstate (NaPTA), both of which also specifically bind to PrPSc but not to soluble PrPC (Zou et al. 2004; Yuan et al. 2006; Safar et al. 1998; Wadsworth et al. 2001). To rule out the possibility that PrP aggregates detected in the uninfected human brain result from postmortem autolysis of autopsy tissues or from other neurodegenerative disorders, we also examined frozen uninfected human biopsy brain tissues or normal animal brain tissues from hamsters and cows. We discovered that the insoluble PrPC was also detectable in these tissues, a finding which confirmed that iPrPC is a de novo generated PrP conformer (Yuan et al. 2006). Using gel filtration, we recently further demonstrated that not only soluble PrPC monomers but also soluble PrPC oligomers are present in the uninfected human brain (Xiao et al. 2012).

Fig. 5.1
figure 00051

Gel filtration of PrP from uninfected human brains. Gel filtration fractions of uninfected brain homogenates were subjected to SDS-PAGE and Western blotting with 3F4. Molecular mass (kDa) of various PrP species recovered in different fractions is indicated by an arrow and molecular mass markers used include dextran blue (2,000 kDa), thyroglobulin (669 kDa), apoferritin (443 kDa), β-amylase (200 kDa), and albumin (66 kDa). PrP was detected not only in fractions with molecular mass less than 66 kDa after fraction 59 but also in fractions with molecular mass greater than 66 kDa before fraction 59 including fraction 33 containing large PrP aggregates (2,000 kDa)

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Fig. 5.2
figure 00052

PK-resistance of PrP in uninfected human brains. Brain homogenates from two ­uninfected human brains received at autopsy were treated with PK at 0, 5, 10, 25, 50, 100 μg/ml (upper two panels A and B) or PK plus PNGase F (lower three panels C, D, and E). The samples were subjected to SDS-PAGE and Western blotting with 3F4, 1E4, and Anti-C antibodies. No PK-resistant PrP was detectable with 3F4 antibody. In contrast, PK-resistant PrP was detected with 1E4 and Anti-C up to 100 μg/ml. With PK alone, three PrP bands migrating at 30-29 kDa, 27-26 kDa, and 21-20 kDa were detected, in which the upper band (∼30-29 kDa, dark asterisk) was predominant while the intensity of the middle band was lowest, which is apparently different from those of PrPSc type 1 (T1) and type 2 (T2). After PNGase F treatment, only one band was detected with 1E4 and Anti-C migrating at ∼20 kDa and ∼18 kDa, respectively (PrP*20 and PrP*18, gray asterisk). Interestingly, a band migrating at ∼12–13 kDa was also detected with Anti-C at low PK concentration (5–10 μg/ml, white asterisk)

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The presence of PrP conformers besides the typical PrPC in uninfected brains was also implied in observations reported by other groups. Consistent with our findings, small amounts of PrP (less than 5% of total PrPC) were also reported to be precipitated by NaPTA from uninfected human brains (Wadsworth et al. 2001). Moreover, by a differential SDS solubility assay, PrPC species with either lower or higher solubility were differentiated in brain homogenates of noninfected humans, sheep, and cattle (Kuczius et al. 2009, 2011). Notably, a purified hamster brain PrPC displayed an unexpectedly high β-sheet component under native conditions; this finding provided the first evidence that the full-length native PrPC isolated from animal brains exhibited intrinsic conformational plasticity (Pergami et al. 1999). Moreover, mammalian brain PrPC from six species was observed to be initially degraded to an intermediate fragment prior to complete proteolysis, suggesting an intrinsic partial PK-resistance (Buschmann et al. 1998). Interestingly, PrP aggregates have also been reported in pancreatic beta-cells of uninfected rats in response to hyperglycemia (Strom et al. 2007). In sum, the cumulative evidence shows that insoluble PrPC is present in tissues and organs from uninfected animals and humans.

5.4 Spontaneous Formation of the Insoluble Cellular Prion Protein Has Been Modeled with Cultured Cells and May Result from PrP Cytosolic Accumulation

Lehmann and Harris (1996) modeled spontaneous formation of PrPSc-like insoluble PrP in cultured Chinese hamster ovary (CHO) cells expressing wild-type or mutant mouse PrP. Significant amounts of mutant PrP with point mutation at residue 199 (E199K) (∼60%) or six octapeptide repeat insertion mutation between residues 51 and 90 (∼90%) linked to inherited human prion disease were detergent; notably approximately 15% wild-type PrPC was also detergent insoluble (Lehmann and Harris 1996). While approximately 5% mutant PrP was resistant to the digestion by PK at 3.3 μg/ml for 20 min, wild-type PrP was completely degraded. Because the two mutant PrP molecules but not wild-type PrP were tightly associated with the plasma membrane, it was hypothesized that acquisition of PrPSc-like properties results from an alternation in membrane topology or affinity (Lehmann and Harris 1996). Using the same models, the same group further identified a three-step endocytic pathway by which mutant PrP forms a PrPSc-like conformer: initially hydrophobic, then detergent insoluble, and finally partially PK resistant (Daude et al. 1997). Using human neuroblastoma cells, Singh et al. also revealed that PrP with Q217R mutation linked to GSS formed a PrPSc-like form (Singh et al. 1997).

In addition to above PrP mutations, the two N-linked glycosylation sites located at residue 181, Asn–Ile–Thr residues 181–183, and at residue 197, Asn–Phe–Thr residues 197–199 (Puckett et al. 1991) are believed to play a crucial role in the stabilization of prion protein conformation. The naturally occurring mutations at residue 183, Thr to Ala (PrPT183A), or at residue 198, Phe to Ser (PrPF198S), falling in the two consensus sites, are linked to two distinct familial prion diseases (Nitrini et al. 1997; Tagliavini et al. 1991). Elimination of either site, or of both by mutagenesis of hamster PrP in CV1 cells, induced intracellular accumulation of mutant proteins (Rogers et al. 1990). Lehmann and Harris observed that mouse PrP mutated at T182 alone, or at both T182 and T198 in CHO cells, failed to reach the cell surface but the PrP with T198 mutation did. Moreover, all three mutant PrP’s acquired PrPSc-like physicochemical properties reminiscent of PrPSc; PrPWt did so only when synthesized in the presence of N-linked glycosylation inhibitor tunicamycin (Lehmann and Harris 1997). Using M17 cells expressing human PrPN181G or PrPT183A, Capellari et al. observed that PrPN181G, but not PrPT183A, reached the cell surface even though both mutations eliminated glycosylation at the first site (Capellari et al. 2000). This observation indicates that the Thr to Ala mutation itself, rather than the elimination of the first glycosylation site, altered the physical properties of the mutant protein (Capellari et al. 2000). Although the F198S mutation falls within the second glycosylation site, Asn–Phe–Thr residues 197–199, PrPF198S slightly increased the efficiency of glycosylation at the first glycosylation site (N181), and greatly increased the efficiency of glycosylation at the second site (N197) in cultured cells (Zaidi et al. 2005).

To further investigate the formation of iPrPC and the effect of mutations on the formation of iPrPC, we examined iPrPC in cultured M17 cells expressing human wild-type (PrPWt) and mutant PrP (Yuan et al. 2008; Zou et al. 2011a). We confirmed that the de novo generated iPrP was detectable not only in cells expressing mutant PrP (PrPT183A or PrPF198S) linked to naturally occurring genetic Creutzfeldt–Jakob disease and Gerstmann–Sträussler–Scheinker disease, respectively, but also in cells expressing wild-type PrP. Compared to cells expressing wild-type PrP, significantly increased amounts of iPrP forming PrP aggregates and PK-resistant PrP were found in cells expressing mutant PrP. Most of PrPT183A was composed of oligomers and large aggregates; virtually no monomeric form was present. In PrPF198S, however, monomeric species were still dominant despite an increase in the amounts of aggregates. The enhanced tendency of PrPT183A to form aggregates may result from the intracellular accumulation of the mutant protein. The F198S mutation did not significantly diminish the ability of PrPF198S to reach the cell surface (Zaidi et al. 2005), although the mutation may change the structure around the V14 epitope previously found to be localized between human PrP185-196 (Zou et al. 2011a; Moudjou et al. 2004; Rezaei et al. 2005). Therefore, the majority of the iPrPC associated with the T183A mutation may result from PrP intracellular accumulation, raising the possibility that iPrPC is derived predominantly from intracellular PrP species. Immunofluorescence microscopy of tagged PrP also indicated that PrPT183A accumulates within the cell, whereas PrPF198S was distributed both inside the cell and on the cell surface, consistent with previous observations (Zou et al. 2011a; Capellari et al. 2000; Zaidi et al. 2005).

In uninfected cultured cells, we also confirmed that the PK-resistant PrP exhibited higher affinity for 1E4 than for 3F4 that was initially observed in tissue samples (Zou et al. 2011a; Yuan et al. 2006, 2008). In Western blotting with cell lysates, virtually no PrP was detected by 1E4, and PrP was detectable only after PK treatment. However, PrP was stainable by 1E4 in fixed cultured cells treated with or without PK although the PrP signal was weaker in treated than in untreated cells (Zou et al. 2011a). It is worth noting that an antibody against human PrP95-110 (termed 8G8), that actually extends merely two more amino acids toward the N and C terminuses of the 1E4 epitope, respectively, stained PrP-expressing cells with a brilliant cytoplamic fluorescence (Krasemann et al. 1999). However, the number of positive cells was smaller than that of cells strained with antibodies against other PrP regions. Moreover, despite sharing a similar amino acid sequence within the corresponding region, only cattle, but not mouse and hamster PrP, was observed to react with 8G8 (Krasemann et al. 1999). In contrast to 3F4, 1E4 indeed seems to detect intracellular PrP in cultured cells (Zou et al. 2011a). Therefore, like 8G8, 1E4 may recognize a PrP species with a unique conformation in its epitope region.

In the absence of scrapie infection, aggregation of the cellular wild-type prion protein in cultured cells was first observed only when proteasome inhibitors were used (Yedidia et al. 2001). It was later reported that PrPWt accumulated in the cytoplasm of cultured cells under other conditions as well, such as in a reducing environment, or when expressing PrP without both N and C terminal signal peptides (Ma and Lindquist 2001, 2002; Drisaldi et al. 2003; Grenier et al. 2006). Cytosolic PrP forms aggregates that are insoluble in non-ionic detergents and partially resistant to PK (Ma and Lindquist 2001). Accumulated cytosolic PrP aggregates induced by ER stress and inhibition of proteasomal activity were recently observed to travel through the secretory pathway and reach the plasma membrane (Nunziante et al. 2011). Cytosolic PrP was observed not only in cultured cells but also in subpopulations of neurons in the hippocampus, neocortex, and thalamus in uninfected wild-type mice (Mironov et al. 2003). In addition, soluble PrPC was observed to switch to insoluble PrPC by treatment with acidic buffers in vitro (Zou and Cashman 2002).

The above observations with cell models may suggest that the formation of iPrPC or the aggregation of PrPC is associated not only with mutations of the protein but also with altered cellular conditions that cause abnormal traffic and distribution of PrP in cells including reductive/oxidative stress.

5.5 Physiology and Pathophysiology of Insoluble PrPC

5.5.1 Long-Term Memory Storage

The iPrPC with a conformation potentially different from soluble PrPC may have a physiologic function. It has been hypothesized that prion-like conformational changes are indispensable for the maintenance of structural synaptic changes required for long-term memory (Si et al. 2003, 2010; Papassotiropoulos et al. 2005; Shorter and Lindquist 2005). Conceivably, the conversion of soluble PrPC monomers into insoluble PrP oligomers or aggregates could be associated with long-term memory storage in the normal human brain (Zou et al. 2011c). The iPrPC molecule is able to bind to g5p, the single-stranded DNA-binding protein (Yuan et al. 2006, 2008). The possible binding of iPrPC to mRNA in vivo cannot be ruled out. Based on the observation that 24 h after a word-list learning task, carriers of either the polymorphism methionine/methionine (M/M) at residue 129 (129MM) or M/valine (V) (129 MV) genotype recalled 17% more information than did 129VV carriers (Papassotiropoulos et al. 2005). The PrP gene is believed to be genetically associated with human long-term memory performance. Therefore, the polymorphism at the residue 129 of PrP may participate in mediating human memory, in which the 129 M allele may have a beneficial effect on long-term memory. Interestingly, the impact of a putative PrP conformation rather than pathologic PrPSc on long-term memory in healthy humans was proposed to be related to physiologically occurring conformational changes (Tompa and Friedrich 1998; Papassotiropoulos et al. 2005).

5.5.2 Prion Disease

The in vivo pathway by which PrPC forms PrPSc remains poorly understood. Two non-exclusive conversion models were proposed: refolding (Griffith 1967; Prusiner 1991) and seeding (Jarrett and Lansbury 1993). In the former, the exogenous PrPSc binds to the PrPC species that has been partially unfolded and the PrPSc-bound PrPC molecule undergoes a refolding process during which the nascent PrPSc is derived from this PrPC species via a conformational transition. The latter proposes that a small amount of abnormal PrPSc or PrPSc-like form (PrP*) is present in the normal brain and is in reversible equilibrium with PrPC. When several monomeric PrP* molecules form a highly ordered nucleus, PrPC is converted to PrPSc polymers. Obviously, two key elements are required by the seeding model. One is the presence in the uninfected brain of a small amount of endogenous PrPSc or PrP* and the second is the formation of PrPSc-derived oligomers. The seeding model, with the two elements, has been recapitulated in vitro using PrP from various fungal and mammalian sources (Ross et al. 2005; Castilla et al. 2005; Tanaka et al. 2005).

Because iPrPC possesses PrPSc-like physicochemical properties, it is possible that iPrPC represents endogenous PrPSc, an intermediate form (PrP*) between PrPC and PrPSc, or silent prion (Hall and Edskes 2004; Weissmann 2004; Yuan et al. 2006; Zou et al. 2011a). Based on the observation that the brains of bigenic mice are capable of clearing prions, it has been proposed that the normal brain contains low levels of PrPSc (Safar et al. 2005). Under normal circumstances, despite the presence of a small amount of PrPSc, the brain may maintain equilibrium between the formation and clearance of PrPSc. The amount of PrPSc may be too small to induce a neurodegenerative disorder, which presumably, remain in a silent state. However, prion diseases may be triggered when the levels of the silent prions are significantly increased due to infection, PrP mutation, or unknown causes. Using protein misfolding cyclic amplification (PMCA), Barria and coworkers generated a new infectious prion without adding exogenous PrPSc seeds (Barria et al. 2009). This study may raise two possibilities (1) PMCA replicated an intermediate PrPSc which was present in the brain homogenate; or (2) the silent prion was activated by the sonication–incubation cycles during PMCA.

As mentioned above, iPrPC possesses a unique immunoreactive behavior showing poor affinity for 3F4 and higher affinity for 1E4, different from other types of human PrPSc identified so far (Yuan et al. 2006, 2008; Zou et al. 2011a). The two antibodies have adjacent epitopes on PrP (Yuan et al. 2008; Zou et al. 2010b). Thus, the possibility cannot be ruled out that iPrP is a distinct PrP species with an altered conformation and that iPrPC may be a conformer which, when it increases, induces an atypical form of prion disease. Some previous observations with experimental animals may favor this hypothesis. A novel neurologic syndrome was reported in Tg mice overexpressing wild-type PrP and these mice exhibited degeneration of skeletal muscle, peripheral nerves, and the central nervous system (Westaway et al. 1994). The increased amounts of wild-type PrPC might form aggregates that induce degeneration in those mice. Chiesa et al. indeed observed that homozygous Tg mice overexpressing wild-type PrP at approximately tenfold but not hemizygous mice overexpressing wild-type PrP at approximately fivefold developed a spontaneous neurodegenerative disorder manifesting tremor and paresis (Chiesa et al. 2008). Nevertheless, abnormal PrP deposits and enlarged synaptic terminals with a dramatic proliferation of membranous structures were found in both types of mice. It was also observed that the overexpressed PrP assembled into insoluble aggregates with mild PK resistance but acquired no infectivity (Chiesa et al. 2008). Misfolding and neurotoxicity of wild-type PrP in transgenic flies were observed to be sequence dependent: Hamster PrP formed large amounts of PrP aggregates with spongiform degeneration, whereas rabbit PrP formed only small amounts of PrP aggregates without spongiform degeneration (Fernandez-Funez et al. 2010). Moreover, the same study also found that although small amounts of PrP aggregates were similarly detected in young flies (day 1) expressing hamster PrP, spongiform degeneration was not evident. Therefore, the small amounts of PrP aggregates were unable to induce spongiform degeneration. Interestingly, spongiform degeneration was observed to occur in older flies (day 30) only when the concentrations of PrP aggregates increased.

The same unique immunoreactivity behavior with 1E4 has also been observed in a new PrPSc species we recently identified from variably protease-sensitive prionopathy (VPSPr), a novel human prion disease (Gambetti et al. 2008; Zou et al. 2010b). VPSPr exhibits an abnormal PrP species with peculiar glycosylation and enzymatic proteolysis (Zou et al. 2010b, 2011c). The 1E4-detected pathogenetic PK-resistant PrPSc with a ladder-like electrophoretic profile is the molecular hallmark of VPSPr. PrPSc from VPSPr exhibits not only the peculiar immunoreactivity behavior but also three PK-resistant core fragments, which is similar to iPrPC (Zou et al. 2010b, 2011c). These similarities may suggest that they share a common molecular metabolic pathway. Similar to sCJD, VPSPr affects patients regardless of their PrP genotypes defined by 129 MV polymorphism; however, the allelic prevalence is distinct in the two diseases (Zou et al. 2010b; Gambetti et al. 2011a). Notably, the amounts of PK-resistant PrPSc in VPSPr seem to be dependent on the polymorphism, a characteristic which has not been observed in sCJD. Preliminary data revealed no clinical phenotype during the normal life span of the transgenic mice expressing human PrP-129V at sixfold inoculated with brain homogenates from cases of VPSPr-129VV (Gambetti et al. 2011a), suggesting that the infectivity of PrPSc from VPSPr may be much lower than that of PrPSc from sCJD. Only 30% of the mice exhibited peculiar PrP plaques with a distinctive topography and minimal or no spongiform degeneration, compared to the typical neuropathological changes found in 100% mice inoculated with the classical sCJD control. Some of these mice inoculated with VPSPr also had the PK-resistant PrPSc whose profile exhibited the ladder-like electrophoresis detected by 1E4. Therefore, it is possible that VPSPr characterized by the deposition in the brain of iPrPC-like PrPSc represents a prion disease, distinct from classical prion diseases and bearing more resemblance to other neurodegenerative diseases such as Alzheimer disease and tauopathies (Gambetti et al. 2011b). Because of the similarities between iPrPC and PrPSc from VPSPr, the possibility that VPSPr results from an increase in the amount of iPrPC cannot be excluded.

5.5.3 Alzheimer Disease

The insoluble PrPC has been recently demonstrated to be the main species that interacts with Aβ in AD (Zou et al. 2011b). Moreover, using a peptide membrane array involving 13-mer human PrP peptides, and two Aβ peptides (Aβ42 and Aβ40), we identified 17 Aβ binding regions distributed on N terminal, internal, and C terminal PrP domains. Two distinct types of Aβ-binding sites were differentiated: one specifically binds to Aβ42 and the other binds to both Aβ42 and Aβ40. Notably, Aβ42-specific binding sites are localized predominantly in the octapeptide repeat region, whereas sites that bind both Aβ40 and Aβ42 are mainly in the extreme N terminal and C terminal domains of PrP (Zou et al. 2011b). Our study is consistent with other observations. PrP deposits often histologically accompany Aβ-positive plaques in AD brains (Esiri et al. 2000; Ferrer et al. 2001; Kovacs et al. 2002). In addition, Freir et al. also observed that interaction between PrP and toxic Aβ assemblies can be therapeutically targeted at multiple sites (Freir et al. 2011), indicating that their binding sites are not limited only to the internal domain. Remarkably, Kudo et al. showed more recently that not only anti-PrP antibodies but also PrPC peptides identified in our previous study (Zou et al. 2011b) rescued Aβ oligomer-induced neurotoxiciy (Kudo et al. 2012).

Although the exact biological relevance of the interaction between iPrPC and Aβ remains unclear, aggregation of one protein was observed to facilitate aggregation of the other (Morales et al. 2010). Moreover, synergistic interactions between other amyloidogenic proteins associated with neurodegeneration have also been reported to promote each other’s fibrillization, amyloid deposition, and formation of filamentous inclusions in transgenic mice (Schwarze-Eicker et al. 2005). An increase in the efficiency of Aβ42 aggregation in vitro was dependent on PrPSc dosage (Morales et al. 2010). Moreover, insoluble PrPSc aggregates also seemed to facilitate Aβ42 aggregation in vivo; AD mice developed a strikingly higher load of cerebral amyloid plaques that appeared much faster in prion infected than in uninfected mice (Morales et al. 2010). Our finding that Aβ42 binds to iPrP may suggest that iPrP (the PrPSc-like forms in uninfected human brains) facilitates the fibrillization of Aβ42 in AD. Similarly, the possibility should be considered that a significant increase in the total number of Aβ plaques observed in bigenic mice overexpressing PrP (Schwarze-Eicker et al. 2005) might result from an increase in the formation of iPrP. Since the less toxic insoluble Aβ42 aggregates constitute the end products of highly toxic soluble Aβ42 oligomers, it is conceivable that formation of the large aggregates facilitated by iPrPC may reduce the amount of Aβ42 oligomers. The decrease in the levels of toxic Aβ42 oligomers would then attenuate the cognitive impairment induced by Aβ42 oligomers in AD. If this is the case, iPrPC may play a protective role in AD. Given that iPrPC interacts with insoluble Aβ42, whereas soluble PrPC binds soluble Aβ42 in vivo (Zou et al. 2011b), it is possible that distinct PrP conformers binding to different Aβ42 species thereby function either as receptors for soluble Aβ42 oligomers or as modulators of insoluble Aβ42 deposition. It would be interesting to test this hypothesis by intracerebrally injecting anti-PrP antibodies against either soluble or insoluble PrP species in AD animal models. This experiment would establish that the multiple conformers of PrPC are coupled with its beneficial and deleterious effects.

5.6 Conclusions

Like PrPSc whose chameleon-like conformations are believed to link to transmissible and non-transmissible prion diseases with highly heterogeneous phenotypes (Zou 2007; Zou and Gambetti 2007), the chameleon-like conformations of PrPC may be linked to its beneficial and deleterious effects (Zou et al. 2011c). Demonstration of the presence of insoluble PrP in normal mammalian brains and its potential association with AD and atypical prion disease may open a new avenue in the exploration of prion formation and in the physiology and pathophysiology of prion protein.