Cell surface accumulation of a truncated transmembrane prion protein in Gerstmann-Straussler-Scheinker disease P102L

Prion protein promotes kidney iron uptake via its ferrireductase activity

Brain iron-dyshomeostasis is an important cause of neurotoxicity in prion disorders, a group of neurodegenerative conditions associated with the conversion of prion protein (PrP(C)) from its normal conformation to an aggregated, PrP-scrapie (PrP(Sc)) isoform. Alteration of iron homeostasis is believed to result from impaired function of PrP(C) in neuronal iron uptake via its ferrireductase activity. However, unequivocal evidence supporting the ferrireductase activity of PrP(C) is lacking. Kidney provides a relevant model for this evaluation because PrP(C) is expressed in the kidney, and ∼370 μg of iron are reabsorbed daily from the glomerular filtrate by kidney proximal tubule cells (PT), requiring ferrireductase activity. Here, we report that PrP(C) promotes the uptake of transferrin (Tf) and non-Tf-bound iron (NTBI) by the kidney in vivo and mainly NTBI by PT cells in vitro. Thus, uptake of (59)Fe administered by gastric gavage, intravenously, or intraperitoneally was significantly lower in PrP-knock-out (PrP(-/-)) mouse kidney relative to PrP(+/+) controls. Selective in vivo radiolabeling of plasma NTBI with (59)Fe revealed similar results. Expression of exogenous PrP(C) in immortalized PT cells showed localization on the plasma membrane and intracellular vesicles and increased transepithelial transport of (59)Fe-NTBI and to a smaller extent (59)Fe-Tf from the apical to the basolateral domain. Notably, the ferrireductase-deficient mutant of PrP (PrP(Δ51-89)) lacked this activity. Furthermore, excess NTBI and hemin caused aggregation of PrP(C) to a detergent-insoluble form, limiting iron uptake. Together, these observations suggest that PrP(C) promotes retrieval of iron from the glomerular filtrate via its ferrireductase activity and modulates kidney iron metabolism.

Prion protein regulates iron transport by functioning as a ferrireductase

Prion protein (PrPC) is implicated in the pathogenesis of prion disorders, but its normal function is unclear. We demonstrate that PrPC is a ferrireductase (FR), and its absence causes systemic iron deficiency in PrP knock-out mice (PrP-/-). When exposed to non-transferrin-bound (NTB) radioactive-iron (59FeCl3) by gastric-gavage, PrP-/- mice absorb significantly more 59Fe from the intestinal lumen relative to controls, indicating appropriate systemic response to the iron deficiency. Chronic exposure to excess dietary iron corrects this deficiency, but unlike wild-type (PrP+/+) controls that remain iron over-loaded, PrP-/- mice revert back to the iron deficient phenotype after 5 months of chase on normal diet. Bone marrow (BM) preparations of PrP-/- mice on normal diet show relatively less stainable iron, and this phenotype is only partially corrected by intraperitoneal administration of excess iron-dextran. Cultured PrP-/- BM-macrophages incorporate significantly less NTB-59Fe in the absence or presence of excess extracellular iron, indicating reduced uptake and/or storage of available iron in the absence of PrPC. When expressed in neuroblastoma cells, PrPC exhibits NAD(P)H-dependent cell-surface and intracellular FR activity that requires the copper-binding octa-peptide-repeat region and linkage to the plasma membrane for optimal function. Incorporation of NTB-59Fe by neuroblastoma cells correlates with FR activity of PrPC, implicating PrPC in cellular iron uptake and metabolism. These observations explain the correlation between PrPC expression and cellular iron levels, and the cause of iron imbalance in sporadic-Creutzfeldt-Jakob-disease brains where PrPC accumulates as insoluble aggregates.

Thermodynamic stabilization of the folded domain of prion protein inhibits prion infection in vivo

Prion diseases, or transmissible spongiform encephalopathies (TSEs), are associated with the conformational conversion of the cellular prion protein, PrP(C), into a protease-resistant form, PrP(Sc). Here, we show that mutation-induced thermodynamic stabilization of the folded, α-helical domain of PrP(C) has a dramatic inhibitory effect on the conformational conversion of prion protein in vitro, as well as on the propagation of TSE disease in vivo. Transgenic mice expressing a human prion protein variant with increased thermodynamic stability were found to be much more resistant to infection with the TSE agent than those expressing wild-type human prion protein, in both the primary passage and three subsequent subpassages. These findings not only provide a line of evidence in support of the protein-only model of TSEs but also yield insight into the molecular nature of the PrP(C)→PrP(Sc) conformational transition, and they suggest an approach to the treatment of prion diseases.

A Drosophila model of GSS syndrome suggests defects in active zones are responsible for pathogenesis of GSS syndrome

We have established a Drosophila model of Gerstmann-Sträussler-Scheinker (GSS) syndrome by expressing mouse prion protein (PrP) having leucine substitution at residue 101 (MoPrP(P101L)). Flies expressing MoPrP(P101L), but not wild-type MoPrP (MoPrP(3F4)), showed severe defects in climbing ability and early death. Expressed MoPrP(P101L) in Drosophila was differentially glycosylated, localized at the synaptic terminals and mainly present as deposits in adult brains. We found that behavioral defects and early death of MoPrP(P101L) flies were not due to Caspase 3-dependent programmed cell death signaling. In addition, we found that Type 1 glutamatergic synaptic boutons in larval neuromuscular junctions of MoPrP(P101L) flies showed significantly increased numbers of satellite synaptic boutons. Furthermore, the amount of Bruchpilot and Discs large in MoPrP(P101L) flies was significantly reduced. Brains from scrapie-infected mice showed significantly decreased ELKS, an active zone matrix marker compared with those of age-matched control mice. Thus, altered active zone structures at the molecular level may be involved in the pathogenesis of GSS syndrome in Drosophila and scrapie-infected mice.

Paradoxical role of prion protein aggregates in redox-iron induced toxicity

BACKGROUND

Imbalance of iron homeostasis has been reported in sporadic Creutzfeldt-Jakob-disease (sCJD) affected human and scrapie infected animal brains, but the contribution of this phenotype to disease associated neurotoxicity is unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Using cell models of familial prion disorders, we demonstrate that exposure of cells expressing normal prion protein (PrP(C)) or mutant PrP forms to a source of redox-iron induces aggregation of PrP(C) and specific mutant PrP forms. Initially this response is cytoprotective, but becomes increasingly toxic with time due to accumulation of PrP-ferritin aggregates. Mutant PrP forms that do not aggregate are not cytoprotective, and cells show signs of acute toxicity. Intracellular PrP-ferritin aggregates induce the expression of LC3-II, indicating stimulation of autophagy in these cells. Similar observations are noted in sCJD and scrapie infected hamster brains, lending credence to these results. Furthermore, phagocytosis of PrP-ferritin aggregates by astrocytes is cytoprotective, while culture in astrocyte conditioned medium (CM) shows no measurable effect. Exposure to H(2)O(2), on the other hand, does not cause aggregation of PrP, and cells show acute toxicity that is alleviated by CM.

CONCLUSIONS/SIGNIFICANCE

These observations suggest that aggregation of PrP in response to redox-iron is cytoprotective. However, subsequent co-aggregation of PrP with ferritin induces intracellular toxicity unless the aggregates are degraded by autophagosomes or phagocytosed by adjacent scavenger cells. H(2)O(2), on the other hand, does not cause aggregation of PrP, and induces toxicity through extra-cellular free radicals. Together with previous observations demonstrating imbalance of iron homeostasis in prion disease affected brains, these observations provide insight into the mechanism of neurotoxicity by redox-iron, and the role of PrP in this process.

Redox control of prion and disease pathogenesis

Imbalance of brain metal homeostasis and associated oxidative stress by redox-active metals like iron and copper is an important trigger of neurotoxicity in several neurodegenerative conditions, including prion disorders. Whereas some reports attribute this to end-stage disease, others provide evidence for specific mechanisms leading to brain metal dyshomeostasis during disease progression. In prion disorders, imbalance of brain-iron homeostasis is observed before end-stage disease and worsens with disease progression, implicating iron-induced oxidative stress in disease pathogenesis. This is an unexpected observation, because the underlying cause of brain pathology in all prion disorders is PrP-scrapie (PrP(Sc)), a beta-sheet-rich conformation of a normal glycoprotein, the prion protein (PrP(C)). Whether brain-iron dyshomeostasis occurs because of gain of toxic function by PrP(Sc) or loss of normal function of PrP(C) remains unclear. In this review, we summarize available evidence suggesting the involvement of oxidative stress in prion-disease pathogenesis. Subsequently, we review the biology of PrP(C) to highlight its possible role in maintaining brain metal homeostasis during health and the contribution of PrP(Sc) in inducing brain metal imbalance with disease progression. Finally, we discuss possible therapeutic avenues directed at restoring brain metal homeostasis and alleviating metal-induced oxidative stress in prion disorders.

Prion protein-detergent micelle interactions studied by NMR in solution

Cellular prion proteins, PrP(C), carrying the amino acid substitutions P102L, P105L, or A117V, which confer increased susceptibility to human transmissible spongiform encephalopathies, are known to form structures that include transmembrane polypeptide segments. Herein, we investigated the interactions between dodecylphosphocholine micelles and the polypeptide fragments 90-231 of the recombinant mouse PrP variants carrying the amino acid replacements P102L, P105L, A117V, A113V/A115V/A118V, K110I/H111I, M129V, P105L/M129V, and A117V/M129V. Wild-type mPrP-(90-231) and mPrP[M129V]-(91-231) showed only weak interactions with dodecylphosphocholine micelles in aqueous solution at pH 7.0, whereas discrete interaction sites within the polypeptide segment 102-127 were identified for all other aforementioned mPrP variants by NMR chemical shift mapping. These model studies thus provide evidence that amino acid substitutions within the polypeptide segment 102-127 affect the interactions of PrP(C) with membranous structures, which might in turn modulate the physiological function of the protein in health and disease.

Prion protein modulates cellular iron uptake: a novel function with implications for prion disease pathogenesis

Converging evidence leaves little doubt that a change in the conformation of prion protein (PrP^C) from a mainly α-helical to a β-sheet rich PrP-scrapie (PrP^Sc) form is the main event responsible for prion disease associated neurotoxicity. However, neither the mechanism of toxicity by PrP^Sc, nor the normal function of PrP^C is entirely clear. Recent reports suggest that imbalance of iron homeostasis is a common feature of prion infected cells and mouse models, implicating redox-iron in prion disease pathogenesis. In this report, we provide evidence that PrP^C mediates cellular iron uptake and transport, and mutant PrP forms alter cellular iron levels differentially. Using human neuroblastoma cells as models, we demonstrate that over-expression of PrP^C increases intra-cellular iron relative to non-transfected controls as indicated by an increase in total cellular iron, the cellular labile iron pool (LIP), and iron content of ferritin. As a result, the levels of iron uptake proteins transferrin (Tf) and transferrin receptor (TfR) are decreased, and expression of iron storage protein ferritin is increased. The positive effect of PrP^C on ferritin iron content is enhanced by stimulating PrP^C endocytosis, and reversed by cross-linking PrP^C on the plasma membrane. Expression of mutant PrP forms lacking the octapeptide-repeats, the membrane anchor, or carrying the pathogenic mutation PrP^102L decreases ferritin iron content significantly relative to PrP^C expressing cells, but the effect on cellular LIP and levels of Tf, TfR, and ferritin is complex, varying with the mutation. Neither PrP^C nor the mutant PrP forms influence the rate or amount of iron released into the medium, suggesting a functional role for PrP^C in cellular iron uptake and transport to ferritin, and dysfunction of PrP^C as a significant contributing factor of brain iron imbalance in prion disorders.

Mutant prion protein D202N associated with familial prion disease is retained in the endoplasmic reticulum and forms 'curly' intracellular aggregates

Transmissible Spongiform Encephalopathies are fatal neurodegenerative disorders of humans and animals that are familial, sporadic, and infectious in nature. Familial disorders of humans include Gerstmann-Straussler-Scheinker disease (GSS), familial Creutzfeldt-Jakob disease (CJD), and fatal familial insomnia, and result from point mutations in the prion protein gene. Although neurotoxicity in familial cases is believed to result from a spontaneous change in conformation of mutant prion protein (PrP) to the pathogenic PrP-scrapie (PrPSc) form, emerging evidence indicates otherwise. We have investigated the processing and metabolism of mutant PrP D202N (PrP202N) in cell models to elucidate possible mechanisms of cytotoxicity. In this report, we demonstrate that PrP202N expressed in human neuroblastoma cells fails to achieve a mature conformation following synthesis and accumulates in the endoplasmic reticulum as 'curly' aggregates. In addition, PrP202N cells show increased sensitivity to free radicals, indicating that neuronal susceptibility to oxidative damage may account for the neurotoxicity observed in cases of GSS resulting from PrP D202N mutation.

Ovine plasma prion protein levels show genotypic variation detected by C-terminal epitopes not exposed in cell-surface PrPC

Ovine PBMCs (peripheral blood mononuclear cells) express PrP(C) [cellular PrP (prion-related protein)] and have the potential to harbour and release disease-associated forms of PrP during scrapie in sheep. Cell-surface PrP(C) expression by PBMCs, together with plasma PrP(C) levels, may contribute to the regulatory mechanisms that determine susceptibility and resistance to natural scrapie in sheep. Here, we have correlated cell-surface PrP(C) expression on normal ovine PBMCs by FACS with the presence of PrP(C) in plasma measured by capture-detector immunoassay. FACS showed similar levels of cell-surface PrP(C) on homozygous ARR (Ala136-Arg154-Arg171), ARQ (Ala136-Arg154-Gln171) and VRQ (Val136-Arg154-Gln171) PBMCs. Cell-surface ovine PrP(C) showed modulation of N-terminal epitopes, which was more evident on homozygous ARR cells. Ovine plasma PrP(C) levels showed genotypic variation and the protein displayed C-terminal epitopes not available in cell-surface PrP(C). Homozygous VRQ sheep showed the highest plasma PrP(C) level and homozygous ARR animals the lowest. For comparison, similar analyses were performed on normal bovine PBMCs and plasma. PrP(C) levels in bovine plasma were approx. 4-fold higher than ovine homozygous ARQ plasma despite similar levels of PBMC cell-surface PrP(C) expression. Immunoassays using C-terminal-specific anti-PrP monoclonal antibodies as capture and detector reagents revealed the highest level of PrP(C) in both ovine and bovine plasma, whilst lower levels were detected using N-terminal-specific monoclonal antibody FH11 as the capture reagent. This suggested that a proportion of plasma PrP(C) was N-terminally truncated. Our results indicate that the increased susceptibility to natural scrapie displayed by homozygous VRQ sheep correlates with a higher level of plasma PrP(C).

Role of N-terminal Familial Mutations in Prion Protein Fibrillization and Prion Amyloid Propagation in Vitro*

A self-perpetuating conformational conversion of the prion protein (PrP) is believed to underlie pathology and transmission of prion diseases. Here we explore the effects of N-terminal pathogenic mutations (P102L, P105L, A117V) and the residue 129 polymorphism on amyloid fibril formation by the human PrP fragment 23-144, an in vitro conversion model that can reproduce certain characteristics of prion replication such as strains and species barriers. We find that these amino acid substitutions neither affect PrP23-144 amyloidogenicity nor introduce barriers to cross-seeding of soluble protein. However, the polymorphism strongly influences the conformation of the amyloid fibrils, as determined by infrared spectroscopy. Intriguingly, unlike conformational features governed by the critical amyloidogenic region of PrP23-144 (residues 138-139), the structural features distinguishing Met-129 and Val-129 PrP23-144 amyloid fibrils are not transmissible by cross-seeding. While based only on in vitro data, these findings provide fundamental insight into the mechanism of prion-based conformational transmission, indicating that only conformational features controlling seeding specificity (e.g. those in critical intermolecular contact sites of amyloid fibrils) are necessarily transmissible by cross-seeding; conformational traits in other parts of the PrP molecule may not be "heritable" from the amyloid template.

The role of chaperones in Parkinson's disease and prion diseases

The etiologies of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, polyglutamine diseases, or prion diseases may be diverse; however, aberrations in protein folding, processing, and/or degradation are common features of these entities, implying a role of quality control systems, such as molecular chaperones and the ubiquitin-proteasome pathway. There is substantial evidence for a causal role of protein misfolding in the pathogenic process coming from neuropathology, genetics, animal modeling, and biophysics. The presence of protein aggregates in all neurodegenerative diseases gave rise to the hypothesis that protein aggregates, be it intracellular or extracellular deposits, may perturb the cellular homeostasis and disintegrate neuronal function (Table 1). More recently, however, an increasing number of studies have indicated that protein aggregates are not toxic per se and might even serve a protective role by sequestering misfolded proteins. Specifically, experimental models of polyglutamine diseases, Alzheimer's disease, and Parkinson's disease revealed that the appearance of aggregates can be dissociated from neuronal toxicity, while misfolded monomers or oligomeric intermediates seem to be the toxic species. The unique features of molecular chaperones to assist in the folding of nascent proteins and to prevent stress-induced misfolding was the rationale to exploit their effects in different models of neurodegenerative diseases. This chapter concentrates on two neurodegenerative diseases, Parkinson's disease and prion diseases, with a special focus on protein misfolding and a possible role of molecular chaperones.

Copper binding is the governing determinant of prion protein turnover

The cellular isoform of the prion protein (PrP(c)) is located at the cell membrane, anchored externally by a glycosylphosphatidylinositol (GPI) anchor. It is a copper (Cu) binding glycoprotein with a rapid basal turnover. Previous studies have shown that exposure of cells to Cu causes internalisation of PrP(c) in vitro. In this study, we show that physiological levels of Cu promote internalisation of PrP(c). Interaction between PrP(c) and Cu was found to be the overriding factor in stimulating the internalisation response with other metals showing no effect. Deletion mutation studies have shown that two domains are essential for copper-induced internalisation to occur. These two domains are the octameric repeat region, encompassing amino acids 51-89, and the palindromic region, amino acids 112-119 with the sequence AGAAAAGA. The decrease in detectable levels of PrP(c) at the cell surface following Cu treatment was found to be the result of rapid internalisation rather than loss into the surrounding environment. These results have implications for both normal metabolism of PrP(c) and the possible mechanism of conversion of PrP(c) to PrP(sc).

Protease-resistant human prion protein and ferritin are cotransported across Caco-2 epithelial cells: implications for species barrier in prion uptake from the intestine

Foodborne transmission of bovine spongiform encephalopathy (BSE) to humans as variant Creutzfeldt-Jakob disease (CJD) has affected over 100 individuals, and probably millions of others have been exposed to BSE-contaminated food substances. Despite these obvious public health concerns, surprisingly little is known about the mechanism by which PrP-scrapie (PrP(Sc)), the most reliable surrogate marker of infection in BSE-contaminated food, crosses the human intestinal epithelial cell barrier. Here we show that digestive enzyme (DE) treatment of sporadic CJD brain homogenate generates a C-terminal fragment similar to the proteinase K-resistant PrP(Sc) core of 27-30 kDa implicated in prion disease transmission and pathogenesis. Notably, DE treatment results in a PrP(Sc)-protein complex that is avidly transcytosed in vesicular structures across an in vitro model of the human intestinal epithelial cell barrier, regardless of the amount of endogenous PrP(C) expression. Unexpectedly, PrP(Sc) is cotransported with ferritin, a prominent component of the DE-treated PrP(Sc)-protein complex. The transport of PrP(Sc)-ferritin is sensitive to low temperature, brefeldin-A, and nocodazole treatment and is inhibited by excess free ferritin, implicating a receptor- or transporter-mediated pathway. Because ferritin shares considerable homology across species, these data suggest that PrP(Sc)-associated proteins, in particular ferritin, may facilitate PrP(Sc) uptake in the intestine from distant species, leading to a carrier state in humans.

A Transmembrane Form of the Prion Protein Is Localized in the Golgi Apparatus of Neurons

(Ctm)PrP is a transmembrane version of the prion protein that has been proposed to be a neurotoxic intermediate underlying prion-induced pathogenesis. In previous studies, we found that PrP molecules carrying mutations in the N-terminal signal peptide (L9R) and the transmembrane domain (3AV) were synthesized exclusively in the (Ctm)PrP form in transfected cell lines. To characterize the properties of (Ctm)PrP in a neuronal setting, we have utilized cerebellar granule neurons cultured from Tg(L9R-3AV) mice that developed a fatal neurodegenerative illness. We found that about half of the L9R-3AV PrP synthesized in these neurons represents (Ctm)PrP, with the rest being (Sec)PrP, the glycolipid anchored form that does not span the membrane. Both forms contained an uncleaved signal peptide, and they are differentially glycosylated. (Sec)PrP was localized on the surface of neuronal processes. Most surprisingly, (Ctm)PrP was concentrated in the Golgi apparatus, rather in the endoplasmic reticulum as it is in transfected cell lines. Our study is the first to analyze the properties of (Ctm)PrP in a neuronal context, and our results suggest new hypotheses about how this form may exert its neurotoxic effects.

Conformational variation between allelic variants of cell-surface ovine prion protein

The distribution of prion infectivity and PrPSc between peripheral lymphoid tissues suggests their possible haematogenic spread during the progression of natural scrapie in susceptible sheep. Since ovine PBMCs (peripheral blood mononuclear cells) express PrPC, they have the potential to carry or harbour disease-associated forms of PrP. To detect the possible presence of disease-associated PrP on the surface of blood cells, an understanding is required of the conformations that normal ovine cell-surface PrPC may adopt. In the present study, we have used monoclonal antibodies that recognize epitopes in either the N- or C-terminal portions of PrP to probe the conformations of PrPC on ovine PBMCs by flow cytometry. Although PBMCs from scrapie-susceptible and -resistant genotypes of sheep expressed similar levels of cell-surface PrPC, as judged by their reactivity with N-terminal-specific anti-PrP monoclonal antibodies, there was considerable genotypic heterogeneity in the region between helix-1 and residue 171. Cells from PrP-VRQ (V136R154Q171) sheep showed uniform reactivity with monoclonal antibodies that bound to epitopes around helix-1, whereas cells from PrP-ARQ (A136R154Q171) and PrP-ARR (A136R154R171) sheep showed variable binding. The region between b-strand-2 and residue 171, which includes a YYR motif, was buried or obscured in cell-surface PrPC on PBMCs from scrapie-susceptible and -resistant sheep. However, an epitope of PrPC that is influenced by residue 171 was more exposed on PBMCs from PrP-VRQ sheep than on PBMCs from the PrP-ARQ genotype. Our results highlight conformational variation between scrapie-susceptible and -resistant forms of cell-surface PrPC and also between allelic variants of susceptible genotypes.

Cell-surface retention of PrPC by anti-PrP antibody prevents protease-resistant PrP formation

The C-terminal portion of the prion protein (PrP), corresponding to a protease-resistant core fragment of the abnormal isoform of the prion protein (PrPSc), is essential for prion propagation. Antibodies to the C-terminal portion of PrP are known to inhibit PrPScaccumulation in cells persistently infected with prions. Here it was shown that, in addition to monoclonal antibodies (mAbs) to the C-terminal portion of PrP, a mAb recognizing the octapeptide repeat region in the N-terminal part of PrP that is dispensable for PrPScformation reduced PrPScaccumulation in cells persistently infected with prions. The 50 % effective dose was as low as ∼1 nM, and, regardless of their epitope specificity, the inhibitory mAbs shared the ability to bind cellular prion protein (PrPC) expressed on the cell surface. Flow cytometric analysis revealed that mAbs that bound to the cell surface during cell culture were not internalized even after their withdrawal from the growth medium. Retention of the mAb–PrPCcomplex on the cell surface was also confirmed by the fact that internalization was enhanced by treatment of cells with dextran sulfate. These results suggested that anti-PrP mAb antagonizes PrPScformation by interfering with the regular PrPCdegradation pathway.

Doxycycline and protein folding agents rescue the abnormal phenotype of familial CJD H187R in a cell model

Familial Creutzfeldt-Jakob disease (CJD) comprises a group of neurodegenerative disorders for which currently there is no treatment. In this study, we evaluated the efficacy of drugs approved for human use, and protein folding agents in reversing the mutant phenotype of familial CJD H187R in a cell model. For an efficient experimental readout, green fluorescent protein (GFP)-tagged mutant prion protein (PrP(187R-GFP)) and wild-type PrP (PrP(C-GFP)) were expressed in human neuroblastoma cells. We report that unlike PrP(C-GFP) that is expressed on the cell surface, PrP(187R-GFP) accumulates in the lysosomes of transfected cells. Treatment of PrP(187R-GFP) cells with quinacrine or doxycycline, agents known to inhibit the replication of PrP-scrapie (PrP(Sc)) in experimental models, gave conflicting results; doxycycline reverted the mutant phenotype of PrP(187R-GFP) cells, whereas quinacrine had no effect. The concentration of doxycycline used in these studies is well within the plasma concentration of patients receiving a 250-600 mg dose two to three times daily. Interestingly, exposure of PrP(187R-GFP) cells to low temperature (28 degrees C) or to the chemical chaperones dimethyl sulphoxide (DMSO) and glycerol also reversed the mutant phenotype. These data suggest that doxycycline and protein folding agents may hold promise as therapeutic agents for familial CJD H187R and other familial disorders that share similar pathogenic mechanisms.

Humanin rescues cortical neurons from prion-peptide-induced apoptosis

We recently demonstrated that a soluble oligomeric prion peptide, the putative 118-135 transmembrane domain of prion protein (PrP), exhibited membrane fusogenic properties and induced apoptotic cell death both in vitro and in vivo. A recently discovered rescue factor humanin (HN) was shown to protect neuronal cells from various insults involved in human neurodegenerative diseases. We thus addressed the question of whether HN might modulate the apoptosis induced by the soluble PrP(118-135) fragment. We found that the incubation of rat cortical neurons with 10 microM HN prevented soluble PrP(118-135) fragment-induced cell death concomitantly with inhibition of apoptotic events. An HN variant, termed HNG, exhibited a 500-fold increase in the protective activity in cortical neurons, whereas the HNA variant displayed no protective effect. The effects of HN and HNG peptides did not require a preincubation with the PrP(118-135) fragment, strongly suggesting that these peptides rescue cells independently of a direct interaction with the prion peptide. By contrast, and in agreement with a previous study, HN had no effect on the fibrillar PrP(106-126) peptide-induced cell death. This protective effect for neurons from PrP(118-135)-induced cell death strongly suggests that PrP(118-135) and PrP(106-126) peptides may trigger different pathways leading to neuronal apoptosis.

Conformation of PrP(C) on the cell surface as probed by antibodies

We have investigated the conformation of Syrian hamster PrP(C) on the surface of transfected CHO cells by performing cross-competition experiments between a set of nine monoclonal antibody fragments (Fab) directed to defined epitopes throughout the protein. No competition was observed between antibodies recognizing epitopes located within the unstructured N-terminal portion of PrP(C) and those recognizing epitopes located within the ordered C-terminal half of the molecule. However, competition was observed between antibodies recognizing overlapping epitopes and between antibodies recognizing epitopes lying adjacent to one another in the PrP sequence. Titrating the reactivity of each Fab against cell-surface PrP(C) revealed a clear heterogeneity in the accessibility of different specific epitopes. Fab D18, recognizing sequence incorporating the first alpha-helix of PrP(C), bound the largest fraction of the cell-surface PrP population. In contrast, Fab E123, binding an epitope at the extreme N terminus of PrP, and Fab 13A5, binding an epitope in the central region of PrP, were able to recognize fewer than half the number of PrP(C) molecules bound by Fab D18. The pattern of antibody reactivity we observed may, in part, result from N-terminal truncation of a proportion of PrP(C) molecules found at the cell surface. However, truncation cannot account for the marked disparity between exposure of the Fab D18 and 13A5 epitopes, which lie adjacent in the PrP sequence. The relative inaccessibility of the 13A5 epitope likely reflects either PrP(C)-PrP(C) interaction, interaction between PrP(C) and other constituents on the cell membrane, or the existence of PrP(C) subspecies with distinct conformations.

Mutant prion protein-mediated aggregation of normal prion protein in the endoplasmic reticulum: implications for prion propagation and neurotoxicity

Familial prion disorders are believed to result from spontaneous conversion of mutant prion protein (PrPM) to the pathogenic isoform (PrPSc). While most familial cases are heterozygous and thus express the normal (PrPC) and mutant alleles of PrP, the role of PrPC in the pathogenic process is unclear. Plaques from affected cases reveal a heterogeneous picture; in some cases only PrPM is detected, whereas in others both PrPC and PrPM are transformed to PrPSc. To understand if the coaggregation of PrPC is governed by PrP mutations or is a consequence of the cellular compartment of PrPM aggregation, we coexpressed PrPM and PrPC in neuroblastoma cells, the latter tagged with green fluorescent protein (PrPC-GFP) for differentiation. Two PrPM forms (PrP231T, PrP217R/231T) that aggregate spontaneously in the endoplasmic reticulum (ER) were generated for this analysis. We report that PrPC-GFP aggregates when coexpressed with PrP231T or PrP217R/231T, regardless of sequence homology between the interacting forms. Furthermore, intracellular aggregates of PrP231T induce the accumulation of a C-terminal fragment of PrP, most likely derived from a potentially neurotoxic transmembrane form of PrP (CtmPrP) in the ER. These findings have implications for prion pathogenesis in familial prion disorders, especially in cases where transport of PrPM from the ER is blocked by the cellular quality control.

Cotranslational partitioning of nascent prion protein into multiple populations at the translocation channel

The decisive events that direct a single polypeptide such as the prion protein (PrP) to be synthesized at the endoplasmic reticulum in both fully translocated and transmembrane forms are poorly understood. In this study, we demonstrate that the topological heterogeneity of PrP is determined cotranslationally, while at the translocation channel. By evaluating sequential intermediates during PrP topogenesis, we find that signal sequence-mediated initiation of translocation results in an interaction between nascent PrP and endoplasmic reticulum chaperones, committing the N terminus to the lumen. Synthesis of the transmembrane domain before completion of this step allows it to direct the generation of (Ctm)PrP, a transmembrane form with its N terminus in the cytosol. Thus, segregation of nascent PrP into different topological configurations is critically dependent on the precise timing of signal-mediated initiation of N-terminus translocation. Consequently, this step could be experimentally tuned to modify PrP topogenesis, including complete reversal of the elevated (Ctm)PrP caused by disease-associated mutations in the transmembrane domain. These results delineate the sequence of events involved in PrP biogenesis, explain the mechanism of action of (Ctm)PrP-favoring mutations associated with neurodegenerative disease, and more generally, reveal that translocation substrates can be cotranslationally partitioned into multiple populations at the translocon.