Differential Targeting of Neurons by Prion Strains Part 1

Introduction

A basic principle of microbiology that applies to all conventional infectious pathogens is that the disease phenotype is a function of both the infecting agent and the host’s response to it. All evidence indicates that this principle is also true for diseases acquired by infection with prions, given that inoculation of different scrapie prion strains into inbred mouse strains shows that reproducible differences in the disease phenotype are determined by both the strain of scrapie prion and a host gene or genes (1-6). The disease parameters used to characterize and define each prion strain, then and now, include: the relative or complete failure of transmission of a prion strain from one animal species to another, designated the "host species barrier"; incubation time, defined as the time from inoculation of a prion strain to the onset of clinical signs; the neuroanatomic distribution of spongiform degeneration, also designated the "lesion profile"; and whether or not PrP amyloid plaques are formed in the brain. Of these parameters, least is known about how each prion strain targets a different population of neurons for degeneration to create the strain-specific lesion profile. The goal of this report is to review the evidence, which argues that PrPSc is the main and perhaps sole prion factor determining the disease phenotype, and that PrPC expressed by the host animal is the predominant host factor determining the disease phenotype, including differential targeting of neurons.


Basic Prion Biology

Prions

Prusiner (7) named the infectious agent that transmits scrapie a "prion" when he discovered that it is composed solely of a single, protease-resistant protein, designated the scrapie prion protein, or PrPSc. Subsequently, it was found that PrPSc is derived from a constitutively expressed and protease-sensitive mammalian protein designated PrPC (8-10). Today, a large mass of reproducible data supports the "protein only" hypothesis, which states that a prion is a pro-teinaceous infectious particle that lacks nucleic acid (11). The properties of the prion protein that give it the behavioral characteristics of a conventional infectious agent, such as a virus, are: its ability to exist in two conformations, one that is largely a-helical (PrPC) and one that is largely P-sheet (PrPSc), and the ability of PrPSc to induce an identical P-sheeted conformation in PrPC and, in doing so, begin a self-perpetuating process that results in increasing prion infectivity titers.

Nascent PrPSc Is Derived from the Host’s PrPC

Diverse, reproducible data indicate that the PrPSc, which comprises an infecting prion, catalyzes the conversion of the host’s PrPC to nascent PrPSc for reviews, (see refs. 12 and 13). The factors and steps involved in propagation of prions can be summarized as follows:

1. PrPC and PrPSc have different conformations: PrPSc is 43% p-sheet and 30% a-helix, whereas PrPC is 3% P-sheet and 42% a-helix (14,15);

2. Formation of nascent PrPSc requires synthesis of PrPC by the host and transport of PrPC to the cell surface, since blocking its export from the endoplasmic reticu-lum-Golgi complex to the plasma membrane inhibits formation of PrPSc (16), since exposure of scrapie-infected cells to phosphatidylinositol-specific phos-pholipase C (PIPLC), which releases PrPC from the cell surface, also inhibits formation of PrPSc (17), and since PrPSc is not formed in PrP knockout mice exposed acutely or chronically to scrapie prions (18-21);

3. An infecting PrPSc binds to PrPC (22-24), a step which appears to require binding of PrPC to a host factor, provisionally designated "protein X" (24,25);

4. The conformation of PrPSc is replicated precisely in PrPC during the latter’s conversion to nascent PrPSc (26,27);

5. PrPSc accumulates in the brain, because it is protease-resistant; PrPC maintains a steady-state concentration in the brain, because it is degradable and its rate of synthesis is equal to its rate of degradation.

In the central nervous system, it is currently assumed that these steps occur in caveolae-like domains (CLD) of neuronal plasma membranes, because 90% of PrPC and PrPSc are localized to caveolae-like domains (28).

The Conversion of PrP to Nascent PrPSc, not Only the Presence of PrPSc, Is a Requirement for Neurodegeneration

Both acute and chronic exposure of the brain in PrP knockout mice to prions have failed to result in propagation of prions or to cause neuropathological changes (18-21). This indicates that exogenously derived PrPSc by itself is not pathogenic; rather, in order for PrPSc to cause neuronal degeneration, it must be derived from PrPC. It is likely that nascent PrPSc derived from glycolipid anchored PrPC enters a cellular compartment, where it can disrupt functions that PrPSc in the extracellular space cannot. It is possible that PrPSc itself must be glycosylphosphatidylinositol (GPI)-anchored to membranes, to be pathogenic, and that it can only do so if it is derived from GPI anchored PrPC. A corollary to these findings is that the conversion of PrPC to PrPSc must occur in neurons, in order for PrPSc to cause neuronal dysfunction and degeneration.

PrPSc Conformation Encodes Prion Strain Behavior

Persuasive evidence that the prion factor that gives it strain-like properties is the three-dimensional conformation of its PrPSc has come from transmissions of familial Creutzfeldt-Jakob disease (CJD) cases into transgenic (Tg) mice expressing a chimeric mouse-human-mouse (MHu2M) PrP transgene (27). In familial fatal insomnia while that is genetically linked to a mutation of PRNP gene at codon 178, FFI(D178N), the protease-resistant fragment of PrPSc after deglycosylation, migrates as a single 19-kDa fragment, which from familial CJD linked to a mutation at codon 200, fCJD(E200K), is 21 kDa. Subsequently, the difference in molecular size was found to result from different degrees of proteolytic cleavage of the N-termini of these human PrPSc molecules (29). The reproducibility of the size differences argues for different stable molecular conformations that protect different lengths of the N-terminus from proteolysis. It is not surprising that mutated (mu) PrPScs, with different amino acid substitutions, should have different molecular conformations, because their amino acid sequences are different. The question remained whether those different conformations could be transferred to any PrPC.

Chimeric mouse-human-mouse PrPC in which the amino acid sequence between residues 80 and 150 is identical to that in humans and the sequences on either side are identical to those in mice, PrPC(MHu2M), is readily converted to PrPSc(MHu2M) by human prions (24). On the first passage of FFI(D178N) prions to Tg(MHu2M) mice, incubation times were about 206 d, and the most intense vacuolation was confined to the thalamus; in contrast, incubation times in Tg(MHu2M) mice inoculated with fCJD(E200K) prions were about 170 d, and the most intense vacuolation and PrPSc deposition were widespread throughout the cerebral hemispheres and brainstem (Fig. 1; see ref. 27). Moreover, Western analysis of the deglycosylated, protease-resistant portions of the PrPScs in the mouse brain homogenates migrated to 19 kDa for FFI and 21 kDa for fCJD(E200K), like the respective human PrPScs.

On the second sequential passage of FFI prions in Tg(MHu2M) mice, incubation time was reduced to about 130 d, vacuolation and PrPSc accumulation patterns remained the same, and the same 19-kDa PrPSc digestion product was found. Similarly, the second passage of fCJD(E200K) remained about 170 d, vacuolation and PrPSc accumulation patterns remained the same, and the same 21-kDa PrPSc digestion product was found. The persistence of the 19-kDa and 21 kDa PrP fragments from the human brain, and through two sequential transmissions in Tg(MHu2M), supported the idea that the different tertiary conformations of the respective FFI and fCJD prions remained unchanged, even with a change in amino acid sequence from mutated HuPrPCs to the chimeric PrPC(MHu2M) in the transgenic mice.

These results argue that PrPSc conformation differences are a prion strain-specific property and are independent of PrPC’s amino acid sequence. The results also imply that PrPSc acts as a template for the conversion of PrPC to nascent PrPSc and, in so doing, imparts a conformation to the nascent PrPSc that determines the size of the protease resistance fragment of PrPSc. Finally, the different reproducible vacuolation and PrPSc deposition profiles and incubation times indicate that two different human prion strains are formed in the human brain as the result of the D178N and E200K PRNP mutations.

More recently, we identified two cases of sporadic fatal insomnia (SFI), in which the clinical and neuropathological features were identical to those in FFI, except there was no family history of such a neurodegenerative disorder, and no PRNP gene mutation was present in the patient’s PRNP gene (30). Deglycosylated, protease-resistant PrPSc in these cases, also migrated at 19 kDa, as in FFI, and differently than sporadic and familial CJD cases, which migrated at 21 kDa. The same PrP migration patterns were identified in the brains of Tg(MHu2M) mice inoculated with SFI, and the vacuolation histograms and PrPSc deposition pattern were virtually identical to those found for FFI (Fig. 1). These results support the view that, from the perspective of the prion, the clinical and neuropathological disease phenotypes are determined more by the conformation of PrPSc and less by its amino acid sequence. Furthermore, the PrPSc conformation, which causes the fatal insomnia can arise as the result of an inherited D178N PRNP mutation, or can arise spontaneously. The SFI and FFI phenotypes occur rarely in humans, compared to the number of sporadic and familial CJD phenotypes.

The regional distribution of PrPSc in Tg(MHu2M)/Prnpo/o mice inoculated with brain extracts from fCJD(E200K), is different than that caused by inocula from FFI(D178N) and SFI. Histoblots of coronal brain sections at the level of the thalamus and hippocampus (A,B,C) and transverse sections of the upper pons-lower midbrain (D,E,F) were immunostained specifically for protease-resistant PrPSc. (A,D), fCJD(E200K); (B,E), FFI(D178N); and (C,F) SFI.

Fig. 1. The regional distribution of PrPSc in Tg(MHu2M)/Prnpo/o mice inoculated with brain extracts from fCJD(E200K), is different than that caused by inocula from FFI(D178N) and SFI. Histoblots of coronal brain sections at the level of the thalamus and hippocampus (A,B,C) and transverse sections of the upper pons-lower midbrain (D,E,F) were immunostained specifically for protease-resistant PrPSc. (A,D), fCJD(E200K); (B,E), FFI(D178N); and (C,F) SFI.

It is possible that SFI is caused by an acquired D178N mutation of the PRNP gene in a single neuron from which sufficient PrPSc is formed with the fatal insomnia conformation to trigger the self-perpetuating conversion of wild-type PrPC to nascent PrPSc.

PrPSc Is the Sole Functionally Relevant Component of the Prion

Because of the difficulty eliminating contaminants from purified prions, biochemical enrichment of prions alone cannot completely exclude the possibility that prions contain another functionally important factor (31,32). The strongest argument in favor of the hypothesis that PrPSc is the sole component of the prion is whether or not all characteristics of prion diseases and prion propagation can be explained solely by the PrPSc component. The evidence that this is true comes mostly from studies of prion propagation in Tg mice expressing different PrP constructs. Tg mice serve the dual function of a biological system in which PrPSc and PrPC interactions can be examined by manipulating their amino acid sequences, and as a bioassay of the resulting disease phenotype from which prion strain identification can be inferred.

In Tg mice, the host species barrier was found to result from a relatively large difference in the amino acid sequence between PrPSc comprising the infecting prion, which is determined by the animal from which it was derived, and PrPC expressed by the host animal (22,33). Further support for this concept is the failure of transmission of any prion strain in Prnp gene knockout mice (Prnpo/o mice) (18-21). Scrapie incubation times in mice appear to be determined, at least in part, by two polymorphisms of the Prnp gene at codons 108 and 189 in inbred mouse strains which result in synthesis of either PrPC-A or PrPC-B (34,35). These small amino acid sequence differences in the host’s PrPC appear to be sufficient to affect its interaction with PrPSc, the rate of its conversion to nascent PrPSc, and/or the rate of PrPSc’s accumulation in the brain, and to account for long and short incubation times. Furthermore, transgenic and congenic mice expressing different proportions and levels of PrPC-A and PrPC-B have shown that scrapie incubation time is inversely proportional to the level of allotype expression (36).

The homotypic interaction of PrPSc with PrPC, which initiates the conversion of PrPC to nascent PrPSc, appears to involve sequence homology of a relatively small domain of the PrP molecule. This was first discovered in Tg mice expressing chimeric SHa and mouse (Mo or M) PrPC in the absence of wild-type MoPrPC (23). Two chimeric PrP constructs were made on the MoPrP background, one containing two SHaPrP-specific amino acid substitutions at residues 108 and 111, designated MHM2, and the other containing three additional SHaPrP substitutions at residues 138, 154, and 169, designated MH2M. Three Tg mouse lines expressing the former construct, Tg(MHM2)Prnpo/o mice, were resistant to SHa(Sc237) prions similar to non-Tg mice; however, all Tg mice expressing the transgene with five SHa amino acid substitutions, Tg(MH2M)Prnpo/o mice, became clinically ill with SHa(Sc237) prions. It was concluded that 100% homology is not necessary to overcome the host species barrier, and that the PrP domain between residues 90 and 160 is particularly important for PrPSc and PrPC dimerization (Fig. 2).

The relevance of the 90-160 domain for successful interaction of human prions with the mouse’s PrPC was also demonstrated in Tg mice expressing the chimeric PrPC(MHu2M) construct, in which residues 90-160 contain the human sequence, but is flanked on both sides by mouse sequences (24). The PrP 90-160 peptide is highly amyloidogenic, because it has the propensity to form p-sheet bonds with other PrP90-160 peptides and, in doing so, to polymerize into the massive deposits of PrP amyloid characteristic of Gerstmam-Straussler-Scheinker (GSS) syndromes (37). This tendency to form intermolecular p-sheet bonds is facilitated by some amino acid substitutions within and outside the 90-160 region. Thus, in GSS pedigrees, the N-terminus and C-terminus of mutated PrPs is highly truncated: The amyloidogenic PrP peptide in GSS(P102L) consists of residues from about 90 to 170; in GSS(A117V), residues 81-146; in GSS(F198S), residues 58 to 150; in GSS(F198S), residues 81-150; and GSS(Q217R), residues 81-146. As with all amyloids, these peptides polymerize into straight filaments, with p-sheet molecular interactions. The latter finding is one of the arguments that the amyloid phenotype in prion diseases is a function of the amino acid sequence of PrPC expressed by the host gene.

Selective Targeting of Neurons by Prion Strains in Transgenic Mice Expressing Glycosylation Site Mutants

The host factors involved in selective targeting of neurons for neurodegeneration by prion strains are poorly understood. Several lines of evidence indicate that, like the propagation of prions, selective neuronal degeneration is also related to the conversion of PrPC to PrPSc. Thus, vacuolar degeneration of neurons, and reactive astrocytic gliosis in a brain region follow the local increase in PrPSc concentration, and colocalize precisely with sites of PrPSc deposition (38-41). A correlation between mutated PrP and neuropathological changes has not yet been established for familial prion diseases of the GSS-type, because nonamyloid plaque PrP in the neuropil is relatively protease-sensitive (42) and because it may have a transmembrane topography that requires specialized techniques to quantify (43). Consistent with the neuroanatomic correlation between sites of vacuolar degeneration and PrPSc deposition in prion diseases acquired by infection, we and others have found that the neuroanatomic pattern of PrPSc accumulation in the brain is characteristic of each prion strain, and is itself a strain-defining pheno-typic parameter (Fig. 1) (40,44,45). These and other observations outlined above have led to the unifying hypothesis that the propagation of prions and neurodegeneration in prion diseases are both linked to the conversion of PrPC to PrPSc.

In the context of the steps involved in the conversion of PrPC to nascent PrPSc, we hypothesized that selective targeting of neuronal populations in the CNS may be determined by cell-specific differences in the affinity of PrPSc for PrPC, which in turn determines brain region differences in the rate of nascent PrPSc formation. Since PrPC is glycosylated at Asn residues 181 and 197 (Fig. 2) (46,47), and since Asn-linked oligosaccharide side chains are known to modify the conformation and interaction of glycoproteins (48), it seemed reasonable to postulate that variations in PrPC’s CHOs may alter the size of the energy barrier that must be traversed during formation of PrPSc.

Structural model of the SHaPrPC molecule. The purpose of the model is to depict the relative sizes of and locations of the Asn-linked oligosaccharides relative to the published structure of SHaPrP fragments inferred from NMR spectroscopy (56,57). SHaPrPC is shown attached to the plasma membrane by its GPI anchor, to indicate how the range of movement of the N-terminal half of the molecule might be constrained in vivo. A growing body of evidence suggests that the normal physiological role of PrPC is to store and present copper to cells (78). Recombinant SHaPrP(29-231) binds two Cu++ molecules to His residues in the octarepeat region near the N-termi-nus, and, in doing so, brings about a conformational change (79). The model was generated by V. Daggett and D. Alanso from the Department of Medicinal Chemistry, University of Washington, Seattle, and is adapted with permission from ref. (52). The more ordered portion of the molecule, residues 125-231, contains helices B (residues 172-193) and C (200-227). A disulfide bridge links Cys179 of helix B with Cys214 of helix C (space-filling group). The Asn-linked CHO at residue 181, which is attached to helix B, and the CHO at Asn 197, which is on the bridging peptide between helix B and C, represent the predominant defucosylated moieties present in PrPSc reported by Endo et al. (46). The putative protein X-binding sites are indicated with an "X," with lines pointing to the discontinuous epitope on helices C and B, with which it interacts (25). The disordered portion of the molecule, residues 23-160, was modeled in a random conformation. Whether or not helix A (residues 144-157) exists is debatable. Constraints were applied to the putative Cu++ binding histidine residues (ringed side chains) near the N-terminus to bring them into proximity.

Fig. 2. Structural model of the SHaPrPC molecule. The purpose of the model is to depict the relative sizes of and locations of the Asn-linked oligosaccharides relative to the published structure of SHaPrP fragments inferred from NMR spectroscopy (56,57). SHaPrPC is shown attached to the plasma membrane by its GPI anchor, to indicate how the range of movement of the N-terminal half of the molecule might be constrained in vivo. A growing body of evidence suggests that the normal physiological role of PrPC is to store and present copper to cells (78). Recombinant SHaPrP(29-231) binds two Cu++ molecules to His residues in the octarepeat region near the N-termi-nus, and, in doing so, brings about a conformational change (79). The model was generated by V. Daggett and D. Alanso from the Department of Medicinal Chemistry, University of Washington, Seattle, and is adapted with permission from ref. (52). The more ordered portion of the molecule, residues 125-231, contains helices B (residues 172-193) and C (200-227). A disulfide bridge links Cys179 of helix B with Cys214 of helix C (space-filling group). The Asn-linked CHO at residue 181, which is attached to helix B, and the CHO at Asn 197, which is on the bridging peptide between helix B and C, represent the predominant defucosylated moieties present in PrPSc reported by Endo et al. (46). The putative protein X-binding sites are indicated with an "X," with lines pointing to the discontinuous epitope on helices C and B, with which it interacts (25). The disordered portion of the molecule, residues 23-160, was modeled in a random conformation. Whether or not helix A (residues 144-157) exists is debatable. Constraints were applied to the putative Cu++ binding histidine residues (ringed side chains) near the N-terminus to bring them into proximity.

If this is the case, then regional variations in CHO structure could account for formation of PrPSc in particular areas of the brain. To test this hypothesis, we constructed Tg mice that express SHaPrPs mutated at either or both of the glycosylation consensus sites (49).

Expression of Glycosylation-Site Mutant SHaPrPC in Transgnic Mice

Asn-linked oligosaccharides are attached to Asn residues 181 and 197 in SHaPrPC. To delete one or both of these CHOs, the threonine residues were mutated to Ala within the NXT consensus sequence sites (50). Single and double glycosylation site mutations were expressed in Tg mice deficient for MoPrP (Prnpo/o). The distribution of mutant SHaPrPC was analyzed by the histoblot technique (51). Here, analysis is confined to the hippocampus.

Wild-type SHaPrPC was confined almost exclusively to the dendritic tree region of the CA1-CA4 regions of Ammon’s horn and of the dentate gyrus (Fig. 3A). It was absent from the nerve cell bodies of the pyramidal and granule cell layers in the respective regions; additionally, it was mostly absent from white matter tracts, such as the corpus callosum that overlies the hippocampus. In contrast, mutation of either one or both glycosylation consensus sites had a profound effect on the anatomical distribution of SHaPrPC. Mutation of the first glycosylation site alone, or in combination with mutation of the second site, resulted in low brain levels of mutated SHaPrPC(T183A) and SHaPrPC(T183A, T199A), accumulation of the respective mutated SHaPrPCs in nerve cell bodies, and little or none in the dendritic trees (Fig. 3B, C). When the second glycosylation site was mutated and the first left intact, the brain levels of SHaPrPC(T199A) were about the same as wild-type SHaPrPC. SHaPrPC(T199A) was distributed to all neuronal compartments including nerve cell bodies, the dendritic tree, and axons of the white matter (Fig. 3D). These results suggest that the CHO at residue 181 is required for trafficking of PrPC out of the nerve cell body and into nerve cell processes.

Transmission of Scrapie to Transgenic Mice Expressing Mutant SHaPrPs

To examine the effects of mutations of the consensus sites for Asn-linked glycosylation on the scrapie phenotype, Tg mice expressing the mutant SHaPrPCs were inoculated with either the Sc237 or 139H hamster-adapted prion strains. Of the Tg mice expressing the three different mutant PrPs, only Tg(SHaPrP-T199A) mice were capable of forming PrPSc. Two lines of Tg(SHaPrP-T199A)Prnpo/o mice developed signs typical of scrapie, about 550 d postinoculation with Sc237, but not with 139H.

Mutations of the Asn-linked glycosylation consensus sites alter the distribution of SHaPrPC in the brains of Tg mice: (A) Wild-type SHaPrPC; (B) SHaPrPC(T183A); (C) SHaPrPC(T183A, T199A); and (D) SHaPrPC(T199A). Histoblots of coronal sections of the hippocampal regions, from normal, uninfected animals, were stained specifically for protease-sensitive PrPC by the histoblot technique.

Fig. 3. Mutations of the Asn-linked glycosylation consensus sites alter the distribution of SHaPrPC in the brains of Tg mice: (A) Wild-type SHaPrPC; (B) SHaPrPC(T183A); (C) SHaPrPC(T183A, T199A); and (D) SHaPrPC(T199A). Histoblots of coronal sections of the hippocampal regions, from normal, uninfected animals, were stained specifically for protease-sensitive PrPC by the histoblot technique.

In contrast, incubation times were about 55 d in Tg(SHaPrP)/Prnpo/o mice expressing levels of wild-type SHaPrPC, comparable to those expressing mutant SHaPrPC in Tg(SHaPrP-T199A)Prnpo/o mice. These findings suggest that deletion of the of the CHO at Asn187 created a host species barrier to 139H prions, and also resulted in a 10-fold increase in Sc237 incubation time.

The neuroanatomic distribution of protease-resistant mutant SHaPrPSc(T199A) accumulation in two Tg(SHaPrP-T199A)Prnpo/o mouse lines, and the distribution of protease-resistant wild-type SHaPrPSc in two Tg(SHaPrP)/Prnpo/o mouse lines, were compared (22,33). Distributions of wild-type SHaPrPSc in the two Tg(SHaPrP)/Prnpo/o lines were similar, and the distributions of mutant SHaPrPSc in the two Tg(SHaPrP-T199A)Prnpo/o were similar; however, the distributions of wild-type SHaPrPSc and mutant SHaPrPSc were markedly different (Fig. 4). One of the chief differences was little or no mutant SHaPrPSc in the thalamus and habenula.

These results reveal; that deletion of the CHO at Asn197 has a profound effect on the PrPSc distribution phenotype, and therefore, on the lesion profile.

The regional distribution of wild-type SHaPrPSc (A,B) is markedly different than mutant SHaPrPSc(T199A) (C,D) in Tg mice inoculated with Sc237 prions. Histoblots of coronal sections through the thalamus and hippocampus were immunostained for pro-tease-resistant PrPSc. (A and B) Two different Tg mouse lines expressing wild-type SHaPrPC (C and D) Two different Tg mouse lines expressing SHaPrPC(T199A). (E) Tg(SHaPrP -T199A)/Prnpo/o mice inoculated with the 139H scrapie prion strain did not develop signs of scrapie nor form protease-resistant PrPSc; only weak, nonspecific staining of white matter is seen. Arrows in (A) and (B) point to sites of PrP amyloid plaque formation. (F) Diagram: Hb, habenula; Hp, hippocampus; Hy, hypothalamus; NC, neocortex; ZI, zona incerta.

Fig. 4. The regional distribution of wild-type SHaPrPSc (A,B) is markedly different than mutant SHaPrPSc(T199A) (C,D) in Tg mice inoculated with Sc237 prions. Histoblots of coronal sections through the thalamus and hippocampus were immunostained for pro-tease-resistant PrPSc. (A and B) Two different Tg mouse lines expressing wild-type SHaPrPC (C and D) Two different Tg mouse lines expressing SHaPrPC(T199A). (E) Tg(SHaPrP -T199A)/Prnpo/o mice inoculated with the 139H scrapie prion strain did not develop signs of scrapie nor form protease-resistant PrPSc; only weak, nonspecific staining of white matter is seen. Arrows in (A) and (B) point to sites of PrP amyloid plaque formation. (F) Diagram: Hb, habenula; Hp, hippocampus; Hy, hypothalamus; NC, neocortex; ZI, zona incerta.

Moreover, the PrP amyloid plaque phenotype was also altered. Amyloid plaques composed of wild-type SHaPrPSc were easily identified in both Tg(SHaPrP)/Prnpo/o lines by their characteristic subcallosal location and size (Fig. 4), as reported earlier (22); however, no PrP amyloid-like deposits were identified in Tg(SHaPrP-T199A)Prnpo/o mouse lines. This implies that deletion of CHO at Asn197 results in a poorly amyloidogenic SHaPrPSc. In summary, deletion of the Asn-lined CHO at residue 197 affected all of the prion strain related phenotypic parameters: the host species barrier, scrapie incubation time, the PrPSc distribution pattern in the brain (and, therefore, the lesion profile), and whether or not PrP amyloid forms.

Next post:

Previous post: