Prion protein expression in different species: analysis with a panel of new mAbs

The Humanization and Maturation of an Anti-PrPc Antibody

The cellular prion protein (PrPc) is a cell surface glycoprotein that is highly expressed in a variety of cancer tissues in addition to the nervous system, and its elevated expression is correlated to poor prognosis in many cancer patients. Our team previously found that patients with colorectal cancer (CRC) with high-level PrPc expression had significantly poorer survival than those with no or low-level PrPc expression. Mouse antibodies for PrPc inhibited tumor initiation and liver metastasis of PrPc-positive human CRC cells in mouse model experiments. PrPc is a candidate target for CRC therapy. In this study, we newly cloned a mouse anti-PrPc antibody (Clone 6) and humanized it, then affinity-matured this antibody using a CHO cell display with a peptide antigen and full-length PrPc, respectively. We obtained two humanized antibody clones with affinities toward a full-length PrPc of about 10- and 100-fold of that of the original antibody. The two humanized antibodies bound to the PrPc displayed significantly better on the cell surface than Clone 6. Used for Western blotting and immunohistochemistry, the humanized antibody with the highest affinity is superior to the two most frequently used commercial antibodies (8H4 and 3F4). The two new antibodies have the potential to be developed as useful reagents for PrPc detection and even therapeutic antibodies targeting PrPc-positive cancers.

Expression of the cellular prion protein by mast cells in the human carotid body

Prion diseases are fatal neurologic disorders that can be transmitted by blood transfusion. The route for neuroinvasion following exposure to infected blood is not known. Carotid bodies (CBs) are specialized chemosensitive structures that detect the concentration of blood gasses and provide feedback for the neural control of respiration. Sensory cells of the CB are highly perfused and densely innervated by nerves that are synaptically connected to the brainstem and thoracic spinal cord, known to be areas of early prion deposition following oral infection. Given their direct exposure to blood and neural connections to central nervous system (CNS) areas involved in prion neuroinvasion, we sought to determine if there were cells in the human CB that express the cellular prion protein (PrPC), a characteristic that would support CBs serving as a route for prion neuroinvasion. We collected CBs from cadaver donor bodies and determined that mast cells located in the carotid bodies express PrPC and that these cells are in close proximity to blood vessels, nerves, and nerve terminals that are synaptically connected to the brainstem and spinal cord.

Emerging roles of the cellular prion protein (PrPC) and 37/67 kDa laminin receptor (RPSA) interaction in cancer biology

The cellular prion protein (PrPC) is well-known for its involvement, under its pathogenic protease-resistant form (PrPSc), in a group of neurodegenerative diseases, known as prion diseases. PrPC is expressed in nervous system, as well as in other peripheral organs, and has been found overexpressed in several types of solid tumors. Notwithstanding, studies in recent years have disclosed an emerging role for PrPC in various cancer associated processes. PrPC has high binding affinity for 37/67 kDa laminin receptor (RPSA), a molecule that acts as a key player in tumorigenesis, affecting cell growth, adhesion, migration, invasion and cell death processes. Recently, we have characterized at cellular level, small molecules able to antagonize the direct PrPC binding to RPSA and their intracellular trafficking. These findings are very crucial considering that the main function of RPSA is to modulate key events in the metastasis cascade. Elucidation of the role played by PrPC/RPSA interaction in regulating tumor development, progression and response to treatment, represents a very promising challenge to gain pathogenetic information and discover novel specific biomarkers and/or therapeutic targets to be exploited in clinical settings. This review attempts to convey a detailed description of the complexity surrounding these multifaceted proteins from the perspective of cancer hallmarks, but with a specific focus on the role of their interaction in the control of proliferation, migration and invasion, genome instability and mutation, as well as resistance to cell death controlled by autophagic pathway.

An optimized Western blot assay provides a comprehensive assessment of the physiological endoproteolytic processing of the prion protein

The prion protein (PrP^C) is subjected to several conserved endoproteolytic events producing bioactive fragments that are of increasing interest for their physiological functions and their implication in the pathogenesis of prion diseases and other neurodegenerative diseases. However, systematic and comprehensive investigations on the full spectrum of PrP^C proteoforms have been hampered by the lack of methods able to identify all PrP^C-derived proteoforms. Building on previous knowledge of PrP^C endoproteolytic processing, we thus developed an optimized Western blot assay able to obtain the maximum information about PrP^C constitutive processing and the relative abundance of PrP^C proteoforms in a complex biological sample. This approach led to the concurrent identification of the whole spectrum of known endoproteolytic-derived PrP^C proteoforms in brain homogenates, including C-terminal, N-terminal and, most importantly, shed PrP^C-derived fragments. Endoproteolytic processing of PrP^C was remarkably similar in the brain of widely used wild type and transgenic rodent models, with α-cleavage-derived C1 representing the most abundant proteoform and ADAM10-mediated shedding being an unexpectedly prominent proteolytic event. Interestingly, the relative amount of shed PrP^C was higher in WT mice than in most other models. Our results indicate that constitutive endoproteolytic processing of PrP^C is not affected by PrP^C overexpression or host factors other than PrP^C but can be impacted by PrP^C primary structure. Finally, this method represents a crucial step in gaining insight into pathophysiological roles, biomarker suitability, and therapeutic potential of shed PrP^C and for a comprehensive appraisal of PrP^C proteoforms in therapies, drug screening, or in the progression of neurodegenerative diseases.

PrP concentration in the central nervous system: regional variability, genotypic effects, and pharmacodynamic impact

Prion protein (PrP) concentration controls the kinetics of prion replication and is a genetically and pharmacologically validated therapeutic target for prion disease. In order to evaluate PrP concentration as a pharmacodynamic biomarker and assess its contribution to known prion disease risk factors, we developed and validated a plate-based immunoassay reactive for PrP across six species of interest and applicable to brain and cerebrospinal fluid (CSF). PrP concentration varies dramatically between different brain regions in mice, cynomolgus macaques, and humans. PrP expression does not appear to contribute to the known risk factors of age, sex, or common PRNP genetic variants. CSF PrP is lowered in the presence of rare pathogenic PRNP variants, with heterozygous carriers of P102L displaying 55% and of D178N just 31% the CSF PrP concentration of mutation-negative controls. In rodents, pharmacologic reduction of brain Prnp RNA is reflected in brain parenchyma PrP, and in turn in CSF PrP, validating CSF as a sampling compartment for the effect of PrP-lowering therapy. Our findings support the use of CSF PrP as a pharmacodynamic biomarker for PrP-lowering drugs, and suggest that relative reduction from individual baseline CSF PrP concentration may be an appropriate marker for target engagement.

Membrane Domain Localization and Interaction of the Prion-Family Proteins, Prion and Shadoo with Calnexin

The cellular prion protein (PrP^C) is renowned for its infectious conformational isoform PrP^Sc, capable of templating subsequent conversions of healthy PrP^Cs and thus triggering the group of incurable diseases known as transmissible spongiform encephalopathies. Besides this mechanism not being fully uncovered, the protein’s physiological role is also elusive. PrP^C and its newest, less understood paralog Shadoo are glycosylphosphatidylinositol-anchored proteins highly expressed in the central nervous system. While they share some attributes and neuroprotective actions, opposing roles have also been reported for the two; however, the amount of data about their exact functions is lacking. Protein–protein interactions and membrane microdomain localizations are key determinants of protein function. Accurate identification of these functions for a membrane protein, however, can become biased due to interactions occurring during sample processing. To avoid such artifacts, we apply a non-detergent-based membrane-fractionation approach to study the prion protein and Shadoo. We show that the two proteins occupy similarly raft and non-raft membrane fractions when expressed in N2a cells and that both proteins pull down the chaperone calnexin in both rafts and non-rafts. These indicate their possible binding to calnexin in both types of membrane domains, which might be a necessary requisite to aid the inherently unstable native conformation during their lifetime.

Structurally distinct external solvent-exposed domains drive replication of major human prions

There is a limited understanding of structural attributes that encode the iatrogenic transmissibility and various phenotypes of prions causing the most common human prion disease, sporadic Creutzfeldt-Jakob disease (sCJD). Here we report the detailed structural differences between major sCJD MM1, MM2, and VV2 prions determined with two complementary synchrotron hydroxyl radical footprinting techniques—mass spectrometry (MS) and conformation dependent immunoassay (CDI) with a panel of Europium-labeled antibodies. Both approaches clearly demonstrate that the phenotypically distant prions differ in a major way with regard to their structural organization, and synchrotron-generated hydroxyl radicals progressively inhibit their seeding potency in a strain and structure-specific manner. Moreover, the seeding rate of sCJD prions is primarily determined by strain-specific structural organization of solvent-exposed external domains of human prion particles that control the seeding activity. Structural characteristics of human prion strains suggest that subtle changes in the organization of surface domains play a critical role as a determinant of human prion infectivity, propagation rate, and targeting of specific brain structures.

Sporadic human prion diseases are conceivably the most heterogenous neurodegenerative disorders and a growing body of research indicates that they are caused by distinct strains of prions. By parallel monitoring their replication potency and progressive hydroxyl radical modification of amino acid side chains during synchrotron irradiation, we identified major differences in the structural organization that correlate with distinct inactivation susceptibility of a given human prion strain. Furthermore, our data demonstrated, for the first time, that seeding activity of different strains of infectious brain-derived human prions is primarily function of distinct solvent-exposed structural domains, and implicate them in the initial binding of cellular isoform of prion protein (PrPC) as a critical step in human prion replication and infectivity.

Harnessing the Physiological Functions of Cellular Prion Protein in the Kidneys: Applications for Treating Renal Diseases

A cellular prion protein (PrP^C) is a ubiquitous cell surface glycoprotein, and its physiological functions have been receiving increased attention. Endogenous PrP^C is present in various kidney tissues and undergoes glomerular filtration. In prion diseases, abnormal prion proteins are found to accumulate in renal tissues and filtered into urine. Urinary prion protein could serve as a diagnostic biomarker. PrP^C plays a role in cellular signaling pathways, reno-protective effects, and kidney iron uptake. PrP^C signaling affects mitochondrial function via the ERK pathway and is affected by the regulatory influence of microRNAs, small molecules, and signaling proteins. Targeting PrP^C in acute and chronic kidney disease could help improve iron homeostasis, ameliorate damage from ischemia/reperfusion injury, and enhance the efficacy of mesenchymal stem/stromal cell or extracellular vesicle-based therapeutic strategies. PrP^C may also be under the influence of BMP/Smad signaling and affect the progression of TGF-β-related renal fibrosis. PrP^C conveys TNF-α resistance in some renal cancers, and therefore, the coadministration of anti-PrP^C antibodies improves chemotherapy. PrP^C can be used to design antibody-drug conjugates, aptamer-drug conjugates, and customized tissue inhibitors of metalloproteinases to suppress cancer. With preclinical studies demonstrating promising results, further research on PrP^C in the kidney may lead to innovative PrP^C-based therapeutic strategies for renal disease.

A New Cell Model for Investigating Prion Strain Selection and Adaptation

Prion diseases are fatal neurodegenerative diseases that affect humans and animals. Prion strains, conformational variants of misfolded prion proteins, are associated with distinct clinical and pathological phenotypes. Host-strain interactions result in the selective damage of distinct brain areas and they are responsible for strain selection and/or adaptation, but the underlying molecular mechanisms are unknown. Prion strains can be distinguished by their cell tropism in vivo and in vitro, which suggests that susceptibility to distinct prion strains is determined by cellular factors. The neuroblastoma cell line PK1 is refractory to the prion strain Me7, but highly susceptible to RML. We challenged a large number of clonal PK1 lines with Me7 and successfully selected highly Me7-susceptible subclones (PME) to investigate whether the prion strain repertoire of PK1 can be expanded. Notably, the Me7-infected PME clones were more protease-resistant when compared to RML-infected PME clones, which suggested that cell-adapted Me7 and RML are distinct prion strains. Strikingly, Me7-refractory cells, including PK1 and astrocytes in cortico-hippocampal cultures, are highly susceptible to prions, being derived from homogenates of Me7-infected PME cells, suggesting that the passage of Me7 in PME cells leads to an extended host range. Thus, PME clones represent a compelling cell model for strain selection and adaptation.

Epitope mapping of the protease resistant products of RT-QuIC does not allow the discrimination of sCJD subtypes

Sporadic Creutzfeldt-Jakob disease (sCJD) is a transmissible, rapidly progressive and fatal neurodegenerative disease. The transmissible agent linked to sCJD is composed of the misfolded form of the host-encoded prion protein. The combination of histopathological and biochemical analyses has allowed the identification and sub-classification of six sCJD subtypes. This classification depends on the polymorphic variability of codon 129 of the prion protein gene and the PrPres isotype, and appears to be associated with neuropathological and clinical features. Currently, sCJD subtyping is only fully achievable post mortem. However, a rapid and non-invasive method for discriminating sCJD subtypes in vita would be invaluable for the clinical management of affected individuals, and for the selection of participants for clinical trials. The CSF analysis by Real Time Quaking Induced Conversion (RT-QuIC) reaction is the most sensitive and specific ante mortem sCJD diagnostic test available to date, and it is used by a number of laboratories internationally. RT-QuIC takes advantage of the natural replication mechanisms of prions by template-induced misfolding, employing recombinant prion protein as reaction substrate. We asked whether epitope mapping, of the RT-QuIC reaction products obtained from seeding RT-QuIC with brain and CSF samples from each of the six molecular subtypes of sCJD could be employed to distinguish them and therefore achieve in vita sCJD molecular subtyping. We found that it is possible to distinguish the RT-QuIC products generated by sCJD biological samples from the ones generated by spontaneous conversion in the negative controls, but that different sCJD subtypes generate very similar, if not identical RT-QuIC reaction products. We concluded that whilst RT-QuIC has demonstrable diagnostic value it has limited prognostic value at this point in time.

Artificial strain of human prions created in vitro

The molecular mechanism that determines under physiological conditions transmissibility of the most common human prion disease, sporadic Creutzfeldt-Jakob disease (sCJD) is unknown. We report the synthesis of new human prion from the recombinant human prion protein expressed in bacteria in reaction seeded with sCJD MM1 prions and cofactor, ganglioside GM1. These synthetic human prions were infectious to transgenic mice expressing non-glycosylated human prion protein, causing neurologic dysfunction after 459 and 224 days in the first and second passage, respectively. The neuropathology, replication potency, and biophysical profiling suggest that a novel, particularly neurotoxic human prion strain was created. Distinct biological and structural characteristics of our synthetic human prions suggest that subtle changes in the structural organization of critical domains, some linked to posttranslational modifications of the pathogenic prion protein (PrP^Sc), play a crucial role as a determinant of human prion infectivity, host range, and targetting of specific brain structures in mice models.

Synthetic prions have previously been generated from recombinant rodent PrP. Here the authors generate synthetic human prions, by seeding human PrP with CJD prions, and characterize its infectivity in mice.

Hepatitis C virus-induced prion protein expression facilitates hepatitis C virus replication

Hepatitis C virus (HCV) infects approximately 180 million people worldwide. Significant progress has been made since the establishment of in vitro HCV infection models in cells. However, the replication of HCV is complex and not completely understood. Here, we found that the expression of host prion protein (PrP) was induced in an HCV replication cell model. We then showed that increased PrP expression facilitated HCV genomic replication. Finally, we demonstrated that the KKRPK motif on the N-terminus of PrP bound nucleic acids and facilitated HCV genomic replication. Our results provided important insights into how viruses may harness cellular protein to achieve propagation.

Prion protein facilitates retinal iron uptake and is cleaved at the β-site: Implications for retinal iron homeostasis in prion disorders

Prion disease-associated retinal degeneration is attributed to PrP-scrapie (PrP^Sc), a misfolded isoform of prion protein (PrP^C) that accumulates in the neuroretina. However, a lack of temporal and spatial correlation between PrP^Sc and cytotoxicity suggests the contribution of host factors. We report retinal iron dyshomeostasis as one such factor. PrP^C is expressed on the basolateral membrane of retinal-pigment-epithelial (RPE) cells, where it mediates uptake of iron by the neuroretina. Accordingly, the neuroretina of PrP-knock-out mice is iron-deficient. In RPE19 cells, silencing of PrP^C decreases ferritin while over-expression upregulates ferritin and divalent-metal-transporter-1 (DMT-1), indicating PrP^C-mediated iron uptake through DMT-1. Polarization of RPE19 cells results in upregulation of ferritin by ~10-fold and β-cleavage of PrP^C, the latter likely to block further uptake of iron due to cleavage of the ferrireductase domain. A similar β-cleavage of PrP^C is observed in mouse retinal lysates. Scrapie infection causes PrP^Sc accumulation and microglial activation, and surprisingly, upregulation of transferrin despite increased levels of ferritin. Notably, detergent-insoluble ferritin accumulates in RPE cells and correlates temporally with microglial activation, not PrP^Sc accumulation, suggesting that impaired uptake of iron by PrP^Sc combined with inflammation results in retinal iron-dyshomeostasis, a potentially toxic host response contributing to prion disease-associated pathology.

The Role of Prion Protein Expression in Predicting Gastric Cancer Prognosis

Previous reports indicated that prion protein (PrP) is involved in gastric cancer (GC) development and progression, but its role in GC prognosis has been poorly characterized. A total of 480 GC patients were recruited in this retrospective study. PrP expression in cancerous and non-cancerous gastric tissues was detected by using the tissue microarray and immunohistochemical staining techniques. Our results showed that the PrP expression in GC was significantly less frequent than that in the non-cancerous gastric tissue (44.4% vs 66.4%, P < 0.001). Cox regression analysis revealed that PrP expression was associated with TNM stage, survival status and survival time. GC patients with higher TNM stages (stages II, III and IV) had significantly lower PrP expression levels in tumors than those with lower TNM stages (stages 0 and I). Kaplan-Meier survival curves revealed that negative PrP expression was associated with poor overall survival (log-rank test: P < 0.001). The mean survival time for patients with negative PrP expression was significant lower than those with positive PrP expression (43.0±28.5m vs. 53.9±31.1m, P<0.001). In multivariate Cox hazard regression, PrP expression was an independent prognostic factor for GC survival, with a HR (hazard ratio) of 0.687 (95%CI:0.520-0.907, P=0.008). Our results revealed that negative PrP expression could independently predict worse outcome in GC and thereby could be used to guide the clinical practice.

The 37/67 kDa laminin receptor (LR) inhibitor, NSC47924, affects 37/67 kDa LR cell surface localization and interaction with the cellular prion protein

The 37/67 kDa laminin receptor (LR) is a non-integrin protein, which binds both laminin-1 of the extracellular matrix and prion proteins, that hold a central role in prion diseases. The 37/67 kDa LR has been identified as interactor for the prion protein (PrP(C)) and to be required for pathological PrP (PrP(Sc)) propagation in scrapie-infected neuronal cells, leading to the possibility that 37/67 kDa LR-PrP(C) interaction is related to the pathogenesis of prion diseases. A relationship between 37/67 kDa LR and PrP(C) in the presence of specific LR inhibitor compounds has not been investigated yet. We have characterized the trafficking of 37/67 kDa LR in both neuronal and non-neuronal cells, finding the receptor on the cell surface and nuclei, and identified the 67 kDa LR as the almost exclusive isoform interacting with PrP(C). Here, we show that the treatment with the 37/67 kDa LR inhibitor, NSC47924, affects both the direct 37/67 kDa LR-PrP(C) interaction in vitro and the formation of the immunocomplex in live cells, inducing a progressive internalization of 37/67 kDa LR and stabilization of PrP(C) on the cell surface. These data reveal NSC47924 as a useful tool to regulate PrP(C) and 37/67 kDa LR trafficking and degradation, representing a novel small molecule to be tested against prion diseases.

Glycan-deficient PrP stimulates VEGFR2 signaling via glycosaminoglycan

Whether the two N-linked glycans are important in prion, PrP, biology is unresolved. In Chinese hamster ovary (CHO) cells, the two glycans are clearly not important in the cell surface expression of transfected human PrP. Compared to fully-glycosylated PrP, glycan-deficient PrP preferentially partitions to lipid raft. In CHO cells glycan-deficient PrP also interacts with glycosaminoglycan (GAG) and vascular endothelial growth factor receptor 2 (VEGFR2), resulting in VEGFR2 activation and enhanced Akt phosphorylation. Accordingly, CHO cells expressing glycan-deficient PrP lacking the GAG binding motif or cells treated with heparinase to remove GAG show diminished Akt signaling. Being in lipid raft is critical, chimeric glycan-deficient PrP with CD4 transmembrane and cytoplasmic domains is absent in lipid raft and does not activate Akt signaling. CHO cells bearing glycan-deficient PrP also exhibit enhanced cellular adhesion and migration. Based on these findings, we propose a model in which glycan-deficient PrP, GAG, and VEGFR2 interact, activating VEGFR2 and resulting in changes in cellular behavior.

Glycosylphosphatidylinositol Anchor Modification Machinery Deficiency Is Responsible for the Formation of Pro-Prion Protein (PrP) in BxPC-3 Protein and Increases Cancer Cell Motility

The normal cellular prion protein (PrP) is a glycosylphosphatidylinositol (GPI)-anchored cell surface glycoprotein. However, in pancreatic ductal adenocarcinoma cell lines, such as BxPC-3, PrP exists as a pro-PrP retaining its glycosylphosphatidylinositol (GPI) peptide signaling sequence. Here, we report the identification of another pancreatic ductal adenocarcinoma cell line, AsPC-1, which expresses a mature GPI-anchored PrP. Comparison of the 24 genes involved in the GPI anchor modification pathway between AsPC-1 and BxPC-3 revealed 15 of the 24 genes, including PGAP1 and PIG-F, were down-regulated in the latter cells. We also identified six missense mutations in DPM2, PIG-C, PIG-N, and PIG-P alongside eight silent mutations. When BxPC-3 cells were fused with Chinese hamster ovary (CHO) cells, which lack endogenous PrP, pro-PrP was successfully converted into mature GPI-anchored PrP. Expression of the individual gene, such as PGAP1, PIG-F, or PIG-C, into BxPC-3 cells does not result in phosphoinositide-specific phospholipase C sensitivity of PrP. However, when PIG-F but not PIG-P is expressed in PGAP1-expressing BxPC-3 cells, PrP on the surface of the cells becomes phosphoinositide-specific phospholipase C-sensitive. Thus, low expression of PIG-F and PGAP1 is the major factor contributing to the accumulation of pro-PrP. More importantly, BxPC-3 cells expressing GPI-anchored PrP migrate much slower than BxPC-3 cells bearing pro-PrP. In addition, GPI-anchored PrP-bearing AsPC-1 cells also migrate slower than pro-PrP bearing BxPC-3 cells, although both cells express filamin A. "Knocking out" PRNP in BxPC-3 cell drastically reduces its migration. Collectively, these results show that multiple gene irregularity in BxPC-3 cells is responsible for the formation of pro-PrP, and binding of pro-PrP to filamin A contributes to enhanced tumor cell motility.

Prion Infectivity Plateaus and Conversion to Symptomatic Disease Originate from Falling Precursor Levels and Increased Levels of Oligomeric PrPSc Species

In lethal prion neurodegenerative diseases, misfolded prion proteins (PrP(Sc)) replicate by redirecting the folding of the cellular prion glycoprotein (PrP(C)). Infections of different durations can have a subclinical phase with constant levels of infectious particles, but the mechanisms underlying this plateau and a subsequent exit to overt clinical disease are unknown. Using tandem biophysical techniques, we show that attenuated accumulation of infectious particles in presymptomatic disease is preceded by a progressive fall in PrP(C) level, which constricts replication rate and thereby causes the plateau effect. Furthermore, disease symptoms occurred at the threshold associated with increasing levels of small, relatively less protease-resistant oligomeric prion particles (oPrP(Sc)). Although a hypothetical lethal isoform of PrP cannot be excluded, our data argue that diminishing residual PrP(C) levels and continuously increasing levels of oPrP(Sc) are crucial determinants in the transition from presymptomatic to symptomatic prion disease.

IMPORTANCE

Prions are infectious agents that cause lethal brain diseases; they arise from misfolding of a cell surface protein, PrP(C) to a form called PrP(Sc). Prion infections can have long latencies even though there is no protective immune response. Accumulation of infectious prion particles has been suggested to always reach the same plateau in the brain during latent periods, with clinical disease only occurring when hypothetical toxic forms (called PrP(L) or TPrP) begin to accumulate. We show here that infectivity plateaus arise because PrP(C) precursor levels become downregulated and that the duration of latent periods can be accounted for by the level of residual PrP(C), which transduces a toxic effect, along with the amount of oligomeric forms of PrP(Sc).

Structural determinants of phenotypic diversity and replication rate of human prions

The infectious pathogen responsible for prion diseases is the misfolded, aggregated form of the prion protein, PrP^Sc. In contrast to recent progress in studies of laboratory rodent-adapted prions, current understanding of the molecular basis of human prion diseases and, especially, their vast phenotypic diversity is very limited. Here, we have purified proteinase resistant PrP^Sc aggregates from two major phenotypes of sporadic Creutzfeldt-Jakob disease (sCJD), determined their conformational stability and replication tempo in vitro, as well as characterized structural organization using recently emerged approaches based on hydrogen/deuterium (H/D) exchange coupled with mass spectrometry. Our data clearly demonstrate that these phenotypically distant prions differ in a major way with regard to their structural organization, both at the level of the polypeptide backbone (as indicated by backbone amide H/D exchange data) as well as the quaternary packing arrangements (as indicated by H/D exchange kinetics for histidine side chains). Furthermore, these data indicate that, in contrast to previous observations on yeast and some murine prion strains, the replication rate of sCJD prions is primarily determined not by conformational stability but by specific structural features that control the growth rate of prion protein aggregates.

Sporadic Creutzfeldt-Jakob disease (sCJD) represents ~90% of all human prion diseases worldwide. This neurodegenerative disease, which is transmissible and invariably fatal, is characterized by variable progression rates and remarkable diversity of clinical and pathological traits. The infectious sCJD prions propagating the pathology mainly in the brain are assemblies of abnormally folded isoform (PrP^Sc) of a host-encoded prion protein (PrP^C). The structure and replication mechanisms of human prions are unknown, and whether the PrP^Sc subtypes identified by proteolytic fragmentation represent distinct strains of sCJD prions has been debated. Here, we isolated sCJD prions from patients with two very distant phenotypes of the disease, compared their structural organization using recently developed biophysical techniques, and investigated their replication in vitro. Our data indicate that these sCJD prions are characterized by different secondary structure organization and quaternary packing arrangements, and that these structural differences are responsible for distinct prion replication rates and unique phenotypic characteristics of the disease. Furthermore, our analysis reveals that, contrary to previous observations for yeast prions, the replication tempo of sCJD prions is determined not so much by their conformational stability but by specific structural features that control the growth speed of prion particles.

The glycosylation status of PrPC is a key factor in determining transmissible spongiform encephalopathy transmission between species

The risk of transmission of transmissible spongiform encephalopathies (TSE) between different species has been notoriously unpredictable because the mechanisms of transmission are not fully understood. A transmission barrier between species often prevents infection of a new host with a TSE agent. Nonetheless, some TSE agents are able to cross this barrier and infect new species, with devastating consequences. The host PrP(C) misfolds during disease pathogenesis and has a major role in controlling the transmission of agents between species, but sequence compatibility between host and agent PrP(C) does not fully explain host susceptibility. PrP(C) is posttranslationally modified by the addition of glycan moieties which have an important role in the infectious process. Here, we show in vivo that glycosylation of the host PrP(C) has a significant impact on the transmission of TSE between different host species. We infected mice carrying different glycosylated forms of PrP(C) with two human agents (sCJDMM2 and vCJD) and one hamster strain (263K). The absence of glycosylation at both or the first PrP(C) glycosylation site in the host results in almost complete resistance to disease. The absence of the second site of N-glycan has a dramatic effect on the barrier to transmission between host species, facilitating the transmission of sCJDMM2 to a host normally resistant to this agent. These results highlight glycosylation of PrP(C) as a key factor in determining the transmission efficiency of TSEs between different species.

IMPORTANCE

The risks of transmission of TSE between different species are difficult to predict due to a lack of knowledge over the mechanisms of disease transmission; some strains of TSE are able to cross a species barrier, while others do not. The host protein, PrP(C), plays a major role in disease transmission. PrP(C) undergoes posttranslational glycosylation, and the addition of these glycans may play a role in disease transmission. We infected mice that express different forms of glycosylated PrP(C) with three different TSE agents. We demonstrate that changing the glycosylation status of the host can have profound effects on disease transmission, changing host susceptibility and incubation times. Our results show that PrP(C) glycosylation is a key factor in determining risks of TSE transmission between species.

Prion protein (PrP) gene-knockout cell lines: insight into functions of the PrP

Elucidation of prion protein (PrP) functions is crucial to fully understand prion diseases. A major approach to studying PrP functions is the use of PrP gene-knockout (Prnp (-/-)) mice. So far, six types of Prnp (-/-) mice have been generated, demonstrating the promiscuous functions of PrP. Recently, other PrP family members, such as Doppel and Shadoo, have been found. However, information obtained from comparative studies of structural and functional analyses of these PrP family proteins do not fully reveal PrP functions. Recently, varieties of Prnp (-/-) cell lines established from Prnp (-/-) mice have contributed to the analysis of PrP functions. In this mini-review, we focus on Prnp (-/-) cell lines and summarize currently available Prnp (-/-) cell lines and their characterizations. In addition, we introduce the recent advances in the methodology of cell line generation with knockout or knockdown of the PrP gene. We also discuss how these cell lines have provided valuable insights into PrP functions and show future perspectives.

Prion protein-specific antibodies that detect multiple TSE agents with high sensitivity

This paper describes the generation, characterisation and potential applications of a panel of novel anti-prion protein monoclonal antibodies (mAbs). The mAbs were generated by immunising PRNP null mice, using a variety of regimes, with a truncated form of recombinant ovine prion protein spanning residues 94-233. Epitopes of specific antibodies were mapped using solid-phase Pepscan analysis and clustered to four distinct regions within the PrP molecule. We have demonstrated the utility of these antibodies by use of Western blotting and immunohistochemistry in tissues from a range of different species affected by transmissible spongiform encephalopathy (TSE). In comparative tests against extensively-used and widely-published, commercially available antibodies, similar or improved results can be obtained using these new mAbs, specifically in terms of sensitivity of detection. Since many of these antibodies recognise native PrPC, they could also be applied to a broad range of immunoassays such as flow cytometry, DELFIA analysis or immunoprecipitation. We are using these reagents to increase our understanding of TSE pathogenesis and for use in potential diagnostic screening assays.

Prion disease tempo determined by host-dependent substrate reduction

The symptoms of prion infection can take years or decades to manifest following the initial exposure. Molecular markers of prion disease include accumulation of the misfolded prion protein (PrPSc), which is derived from its cellular precursor (PrPC), as well as downregulation of the PrP-like Shadoo (Sho) glycoprotein. Given the overlapping cellular environments for PrPC and Sho, we inferred that PrPC levels might also be altered as part of a host response during prion infection. Using rodent models, we found that, in addition to changes in PrPC glycosylation and proteolytic processing, net reductions in PrPC occur in a wide range of prion diseases, including sheep scrapie, human Creutzfeldt-Jakob disease, and cervid chronic wasting disease. The reduction in PrPC results in decreased prion replication, as measured by the protein misfolding cyclic amplification technique for generating PrPSc in vitro. While PrPC downregulation is not discernible in animals with unusually short incubation periods and high PrPC expression, slowly evolving prion infections exhibit downregulation of the PrPC substrate required for new PrPSc synthesis and as a receptor for pathogenic signaling. Our data reveal PrPC downregulation as a previously unappreciated element of disease pathogenesis that defines the extensive, presymptomatic period for many prion strains.

Human tonsil-derived follicular dendritic-like cells are refractory to human prion infection in vitro and traffic disease-associated prion protein to lysosomes

The molecular mechanisms involved in human cellular susceptibility to prion infection remain poorly defined. This is due, in part, to the absence of any well characterized and relevant cultured human cells susceptible to infection with human prions, such as those involved in Creutzfeldt-Jakob disease. In variant Creutzfeldt-Jakob disease, prion replication is thought to occur first in the lymphoreticular system and then spread into the brain. We have, therefore, examined the susceptibility of a human tonsil-derived follicular dendritic cell-like cell line (HK) to prion infection. HK cells were found to display a readily detectable, time-dependent increase in cell-associated abnormal prion protein (PrP(TSE)) when exposed to medium spiked with Creutzfeldt-Jakob disease brain homogenate, resulting in a coarse granular perinuclear PrP(TSE) staining pattern. Despite their high level of cellular prion protein expression, HK cells failed to support infection, as judged by longer term maintenance of PrP(TSE) accumulation. Colocalization studies revealed that exposure of HK cells to brain homogenate resulted in increased numbers of detectable lysosomes and that these structures immunostained intensely for PrP(TSE) after exposure to Creutzfeldt-Jakob disease brain homogenate. Our data suggest that human follicular dendritic-like cells and perhaps other human cell types are able to avoid prion infection by efficient lysosomal degradation of PrP(TSE).

Association of prion protein genotype and scrapie prion protein type with cellular prion protein charge isoform profiles in cerebrospinal fluid of humans with sporadic or familial prion diseases

The present study investigates whether posttranslational modifications of cellular prion protein (PrP(C)) in the cerebrospinal fluid (CSF) of humans with prion diseases are associated with methionine (M) and/or valine (V) polymorphism at codon 129 of the prion protein gene (PRNP), scrapie prion protein (PrP(Sc)) type in sporadic Creutzfeldt-Jakob disease (sCJD), or PRNP mutations in familial Creutzfeldt-Jakob disease (fCJD/E200K), and fatal familial insomnia (FFI). We performed comparative 2-dimensional immunoblotting of PrP(C) charge isoforms in CSF samples from cohorts of diseased and control donors. Mean levels of total PrP(C) were significantly lower in the CSF from fCJD patients than from those with sCJD or FFI. Of the 12 most abundant PrP(C) isoforms in the examined CSF, one (IF12) was relatively decreased in (1) sCJD with VV (vs. MM or MV) at PRNP codon 129; (2) in sCJD with PrP(Sc) type 2 (vs. PrP(Sc) type 1); and (3) in FFI versus sCJD or fCJD. Furthermore, truncated PrP(C) species were detected in sCJD and control samples without discernible differences. Finally, serine 43 of PrP(C) in the CSF and brain tissue from CJD patients showed more pronounced phosphorylation than in control donors.

Epitope scanning indicates structural differences in brain-derived monomeric and aggregated mutant prion proteins related to genetic prion diseases

Genetic Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia and prion protein cerebral amyloid angiopathy are clinically and neuropathologically distinct neurodegenerative diseases linked to mutations in the PRNP gene encoding the cellular prion protein (PrPC). How sequence variants of PRNP encode the information to specify these disease phenotypes is not known. It is suggested that each mutation produces a misfolded variant of PrPC with specific neurotoxic properties. However, structural studies of recombinant PrP did not detect major differences between wild-type and mutant molecules, pointing to the importance of investigating mutant PrPs from mammalian brains. We used surface plasmon resonance and a slot-blot immunoassay to analyse the antibody-binding profiles of soluble and insoluble PrP molecules extracted from the brains of transgenic mice modelling different prion diseases. By measuring the reactivity of monoclonal antibodies against different PrP epitopes, we obtained evidence of conformational differences between wild-type and mutant PrPs, and among different mutants. We detected structural heterogeneity in both monomeric and aggregated PrP, supporting the hypothesis that the phenotype of genetic prion diseases is encoded by mutant PrP conformation and assembly state.

Co-existence of distinct prion types enables conformational evolution of human PrPSc by competitive selection

The unique phenotypic characteristics of mammalian prions are thought to be encoded in the conformation of pathogenic prion proteins (PrP(Sc)). The molecular mechanism responsible for the adaptation, mutation, and evolution of prions observed in cloned cells and upon crossing the species barrier remains unsolved. Using biophysical techniques and conformation-dependent immunoassays in tandem, we isolated two distinct populations of PrP(Sc) particles with different conformational stabilities and aggregate sizes, which frequently co-exist in the most common human prion disease, sporadic Creutzfeldt-Jakob disease. The protein misfolding cyclic amplification replicates each of the PrP(Sc) particle types independently and leads to the competitive selection of those with lower initial conformational stability. In serial propagation with a nonglycosylated mutant PrP(C) substrate, the dominant PrP(Sc) conformers are subject to further evolution by natural selection of the subpopulation with the highest replication rate due to its lowest stability. Cumulatively, the data show that sporadic Creutzfeldt-Jakob disease PrP(Sc) is not a single conformational entity but a dynamic collection of two distinct populations of particles. This implies the co-existence of different prions, whose adaptation and evolution are governed by the selection of progressively less stable, faster replicating PrP(Sc) conformers.

Post-translational changes to PrP alter transmissible spongiform encephalopathy strain properties

The agents responsible for transmissible spongiform encephalopathies (TSEs), or prion diseases, contain as a major component PrP(Sc), an abnormal conformer of the host glycoprotein PrP(C). TSE agents are distinguished by differences in phenotypic properties in the host, which nevertheless can contain PrP(Sc) with the same amino-acid sequence. If PrP alone carries information defining strain properties, these must be encoded by post-translational events. Here we investigated whether the glycosylation status of host PrP affects TSE strain characteristics. We inoculated wild-type mice with three TSE strains passaged through transgenic mice with PrP devoid of glycans at the first, second or both N-glycosylation sites. We compared the infectious properties of the emerging isolates with TSE strains passaged in wild-type mice by in vivo strain typing and by the standard scrapie cell assay in vitro. Strain-specific characteristics of the 79A TSE strain changed when PrP(Sc) was devoid of one or both glycans. Thus infectious properties of a TSE strain can be altered by post-translational changes to PrP which we propose result in the selection of mutant TSE strains.

Association of prion protein expression with pancreatic adenocarcinoma survival in the SEER residual tissue repository

Pancreatic ductal adenocarcinoma (PDAC) is an important cause of cancer death with no clear prognostic biomarker. Expression of prion (PrP) has been reported to be a marker of poor prognosis in a series of Caucasian PDAC cases. We determined the prognostic value of PrP in a racially and geographically diverse population-based series of PDAC cases. PrP expression was examined in 142 PDAC cases from three cancer registries. Cases included 71 Caucasian, 54 Asian/Pacific Islanders and 17 Blacks diagnosed from 1983-2000, and followed through 2008. Hazard ratios (HR) and 95% confidence intervals (CIs) for the association of PrP expression with survival were computed after adjustment for case attributes. The risk of death was about four times higher (HR=3.8; 95% PDAC cases with PrP(+) tumors (median survival 5 months) compared to the 34 cases with PrP(-) tumors (median survival 20 months). Of 51 cases with resected, localized PDAC median survival was 74 months for 17 cases with PrP(-) tumors versus 14 months for 34 cases with PrP(+) tumors (HR=6.7; 95% CI: 2.6, 17.4). All 6 surviving cases had PrP(-) negative tumors (median survival, > 10 years). PrP may have potential as a prognostic biomarker in PDAC patient management.

Small protease sensitive oligomers of PrPSc in distinct human prions determine conversion rate of PrP(C)

The mammalian prions replicate by converting cellular prion protein (PrP^C) into pathogenic conformational isoform (PrP^Sc). Variations in prions, which cause different disease phenotypes, are referred to as strains. The mechanism of high-fidelity replication of prion strains in the absence of nucleic acid remains unsolved. We investigated the impact of different conformational characteristics of PrP^Sc on conversion of PrP^C in vitro using PrP^Sc seeds from the most frequent human prion disease worldwide, the Creutzfeldt-Jakob disease (sCJD). The conversion potency of a broad spectrum of distinct sCJD prions was governed by the level, conformation, and stability of small oligomers of the protease-sensitive (s) PrP^Sc. The smallest most potent prions present in sCJD brains were composed only of∼20 monomers of PrP^Sc. The tight correlation between conversion potency of small oligomers of human sPrP^Sc observed in vitro and duration of the disease suggests that sPrP^Sc conformers are an important determinant of prion strain characteristics that control the progression rate of the disease.

Mammalian prion diseases were originally characterized by accumulation of protease-resistant prion protein (PrP^Sc), often forming large amyloid deposits and fibrils. However, the apparent absence of protease-resistant PrP^Sc or amyloid fibrils in growing number of prion diseases raised several fundamental questions; specifically, whether presumably protease-sensitive forms of PrP^Sc exist as distinct conformers; and whether they comprise the initial steps in prion replication or are related to the alternative misfolding pathway generating noninfectious aggregates. We investigated the conformational characteristics of protease sensitive conformers of PrP^Sc and their role in the pathogenesis of sporadic Creutzfeldt-Jakob disease (sCJD). Using two different in vitro prion protein (PrP^C) conversion techniques in tandem with biophysical methods, we identified small oligomers of protease sensitive PrP^Sc present in sCJD brains as the most potent initiators of PrP^C conversion. Their concentration and conformational stability determine the distinctly different replication potency of PrP^Sc in individual isolates of sCJD and each of these characteristics correlates tightly with duration of the disease. These features argue for a broad range of distinct prion strains causing the sCJD and imply that small oligomers of protease sensitive conformers of pathogenic prion protein are encoding incubation time and progression rate of the disease.

Molecular pathogenesis of sporadic prion diseases in man

The yeast, fungal and mammalian prions determine heritable and infectious traits that are encoded in alternative conformations of proteins. They cause lethal sporadic, familial and infectious neurodegenerative conditions in man, including Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome (GSS), kuru, sporadic fatal insomnia (SFI) and likely variable protease-sensitive prionopathy (VPSPr). The most prevalent of human prion diseases is sporadic (s)CJD. Recent advances in amplification and detection of prions led to considerable optimism that early and possibly preclinical diagnosis and therapy might become a reality. Although several drugs have already been tested in small numbers of sCJD patients, there is no clear evidence of any agent’s efficacy. Therefore, it remains crucial to determine the full spectrum of sCJD prion strains and the conformational features in the pathogenic human prion protein governing replication of sCJD prions. Research in this direction is essential for the rational development of diagnostic as well as therapeutic strategies. Moreover, there is growing recognition that fundamental processes involved in human prion propagation – intercellular induction of protein misfolding and seeded aggregation of misfolded host proteins – are of far wider significance. This insight leads to new avenues of research in the ever-widening spectrum of age-related human neurodegenerative diseases that are caused by protein misfolding and that pose a major challenge for healthcare.

Determining the role of mononuclear phagocytes in prion neuroinvasion from the skin

Many prion diseases are acquired by peripheral exposure, and skin lesions are an effective route of transmission. Following exposure, early prion replication, upon FDCs in the draining LN is obligatory for the spread of disease to the brain. However, the mechanism by which prions are conveyed to the draining LN is uncertain. Here, transgenic mice were used, in which langerin(+) cells, including epidermal LCs and langerin(+) classical DCs, were specifically depleted. These were used in parallel with transgenic mice, in which nonepidermal CD11c(+) cells were specifically depleted. Our data show that prion pathogenesis, following exposure via skin scarification, occurred independently of LC and other langerin(+) cells. However, the depletion of nonepidermal CD11c(+) cells impaired the early accumulation of prions in the draining LN, implying a role for these cells in the propagation of prions from the skin. Therefore, together, these data suggest that the propagation of prions from the skin to the draining LN occurs via dermal classical DCs, independently of langerin(+) cells.

Protease-sensitive conformers in broad spectrum of distinct PrPSc structures in sporadic Creutzfeldt-Jakob disease are indicator of progression rate

The origin, range, and structure of prions causing the most common human prion disease, sporadic Creutzfeldt-Jakob disease (sCJD), are largely unknown. To investigate the molecular mechanism responsible for the broad phenotypic variability of sCJD, we analyzed the conformational characteristics of protease-sensitive and protease-resistant fractions of the pathogenic prion protein (PrP^Sc) using novel conformational methods derived from a conformation-dependent immunoassay (CDI). In 46 brains of patients homozygous for polymorphisms in the PRNP gene and exhibiting either Type 1 or Type 2 western blot pattern of the PrP^Sc, we identified an extensive array of PrP^Sc structures that differ in protease sensitivity, display of critical domains, and conformational stability. Surprisingly, in sCJD cases homozygous for methionine or valine at codon 129 of the PRNP gene, the concentration and stability of protease-sensitive conformers of PrP^Sc correlated with progression rate of the disease. These data indicate that sCJD brains exhibit a wide spectrum of PrP^Sc structural states, and accordingly argue for a broad spectrum of prion strains coding for different phenotypes. The link between disease duration, levels, and stability of protease-sensitive conformers of PrP^Sc suggests that these conformers play an important role in the pathogenesis of sCJD.

Sporadic Creutzfeldt-Jakob disease (sCJD) is the most common human prion disease worldwide. This neurodegenerative disease, which is transmissible and invariably fatal, is characterized by the accumulation of an abnormally folded isoform (PrP^Sc) of a host-encoded protein (PrP^C), predominantly in the brain. Most researchers believe that PrP^Sc is the infectious agent and five or six subtypes of sCJD have been identified. Whether or not these subtypes represent distinct strains of sCJD prions is debated in the context of the extraordinary variability of sCJD phenotypes, frequent co-occurrence of different PrP^Sc fragments in the same brain, and the fact that up to 90% of protease-sensitive PrP^Sc eludes the conventional analysis because it is destroyed by protease treatment. Using novel conformational methods, we identified within each clinical and pathological category an array of PrP^Sc structures that differ in protease-sensitivity, display of critical domains, and conformational stability. Each of these features offers evidence of a distinct conformation. The link between the rate at which the disease progresses, on the one hand, and the concentration and stability of protease-sensitive conformers of PrP^Sc on the other, suggests that these conformers play an important role in how the disease originates and progresses.

PrP(Sc)-specific antibodies with the ability to immunodetect prion oligomers

The development of antibodies with binding capacity towards soluble oligomeric forms of PrPSc recognised in the aggregation process in early stage of the disease would be of paramount importance in diagnosing prion diseases before extensive neuropathology has ensued. As blood transfusion appears to be efficient in the transmission of the infectious prion agent, there is an urgent need to develop reagents that would specifically recognize oligomeric forms of the abnormally folded prion protein, PrPSc.

To that end, we show that anti-PrP monoclonal antibodies (called PRIOC mAbs) derived from mice immunised with native PrP-coated microbeads are able to immunodetect oligomers/multimers of PrPSc. Oligomer-specific immunoreactivity displayed by these PRIOC mAbs was demonstrated as large aggregates of immunoreactive deposits in prion-permissive neuroblastoma cell lines but not in equivalent non-infected or prn-p^0/0 cell lines. In contrast, an anti-monomer PrP antibody displayed diffuse immunoreactivity restricted to the cell membrane. Furthermore, our PRIOC mAbs did not display any binding with monomeric recombinant and cellular prion proteins but strongly detected PrPSc oligomers as shown by a newly developed sensitive and specific ELISA. Finally, PrioC antibodies were also able to bind soluble oligomers formed of Aβ and α-synuclein. These findings demonstrate the potential use of anti-prion antibodies that bind PrPSc oligomers, recognised in early stage of the disease, for the diagnosis of prion diseases in blood and other body fluids.

Non-infectious aggregates of the prion protein react with several PrPSc-directed antibodies

The key event in the pathogenesis of prion diseases is the conformational conversion of the normal prion protein (PrP) (PrP(C)) into an infectious, aggregated isoform (PrP(Sc)) that has a high content of beta-sheet. Historically, a great deal of effort has been devoted to developing antibodies that specifically recognize PrP(Sc) but not PrP(C), as such antibodies would have enormous diagnostic and experimental value. A mouse monoclonal IgM antibody (designated 15B3) and three PrP motif-grafted monoclonal antibodies (referred to as IgG 19-33, 89-112, and 136-158) have been previously reported to react specifically with infectious PrP(Sc) but not PrP(C). In this study, we extend the characterization of these four antibodies by testing their ability to immunoprecipitate and immunostain infectious and non-infectious aggregates of wild-type, mutant, and recombinant PrP. We find that 15B3 as well as the motif-grafted antibodies recognize multiple types of aggregated PrP, both infectious and non-infectious, including forms found in brain, in transfected cells, and induced in vitro from purified recombinant protein. These antibodies are exquisitely selective for aggregated PrP, and do not react with soluble PrP even when present in vast excess. Our results suggest that 15B3 and the motif-grafted antibodies recognize structural features common to both infectious and non-infectious aggregates of PrP. Our study extends the utility of these antibodies for diagnostic and experimental purposes, and it provides new insight into the structural changes that accompany PrP oligomerization and prion propagation.

Binding of pro-prion to filamin A: by design or an unfortunate blunder

Over the last decades, cancer research has focused on tumor suppressor genes and oncogenes. Genes in other cellular pathways has received less attention. Between 0.5% to 1% of the mammalian genome encodes for proteins that are tethered on the cell membrane via a glycosylphosphatidylinositol (GPI)-anchor. The GPI modification pathway is complex and not completely understood. Prion (PrP), a GPI-anchored protein, is infamous for being the only normal protein that when misfolded can cause and transmit a deadly disease. Though widely expressed and highly conserved, little is known about the functions of PrP. Pancreatic cancer and melanoma cell lines express PrP. However, in these cell lines the PrP exists as a pro-PrP as defined by retaining its GPI anchor peptide signal sequence (GPI-PSS). Unexpectedly, the GPI-PSS of PrP has a filamin A (FLNA) binding motif and binds FLNA. FLNA is a cytolinker protein, and an integrator of cell mechanics and signaling. Binding of pro-PrP to FLNA disrupts the normal FLNA functions. Although normal pancreatic ductal cells lack PrP, about 40% of patients with pancreatic ductal cell adenocarcinoma express PrP in their cancers. These patients have significantly shorter survival time compared with patients whose cancers lack PrP. Pro-PrP is also detected in melanoma in situ but is undetectable in normal melanocyte, and invasive melanoma expresses more pro-PrP. In this review, we will discuss the underlying mechanisms by which binding of pro-PrP to FLNA disrupts normal cellular physiology and contributes to tumorigenesis, and the potential mechanisms that cause the accumulation of pro-PrP in cancer cells.

Characterization of the prion protein in human urine

The presence of the prion protein (PrP) in normal human urine is controversial and currently inconclusive. This issue has taken a special relevance because prion infectivity has been demonstrated in urine of animals carrying experimental or naturally occurring prion diseases, but the actual presence and tissue origin of the infectious prion have not been determined. We used immunoprecipitation, one- and two-dimensional electrophoresis, and mass spectrometry to prove definitely the presence of PrP in human urine and its post-translational modifications. We show that urinary PrP (uPrP) is truncated mainly at residue 112 but also at other residues up to 122. This truncation makes uPrP undetectable with some commonly used antibodies to PrP. uPrP is glycosylated and carries an anchor which, at variance with that of cellular PrP, lacks the inositol-associated phospholipid moiety, indicating that uPrP is probably shed from the cell surface. The detailed characterization of uPrP reported here definitely proves the presence of PrP in human urine and will help determine the origin of prion infectivity in urine.

Codon 129 polymorphism and the E200K mutation do not affect the cellular prion protein isoform composition in the cerebrospinal fluid from patients with Creutzfeldt-Jakob disease

The cellular prion protein (PrP(c)) is a multifunctional, highly conserved and ubiquitously expressed protein. It undergoes a number of modifications during its post-translational processing, resulting in different PrP(c) glycoforms and truncated PrP(c) fragments. Limited data are available in humans on the expression and cleavage of PrP(c). In this study we investigated the PrP(c) isoform composition in the cerebrospinal fluid from patients with different human prion diseases. The first group of patients was affected by sporadic Creutzfeldt-Jakob disease exhibiting different PrP codon 129 genotypes. The second group contained patients with a genetic form of Creutzfeldt-Jakob disease (E200K). The third group consisted of patients with fatal familial insomnia and the last group comprised cases with the Gerstmann-Sträussler-Scheinker syndrome. We examined whether the PrP codon 129 polymorphism in sporadic Creutzfeldt-Jakob disease as well as the type of prion disease in human patients has an impact on the glycosylation and processing of PrP(c). Immunoblotting analyses using different monoclonal PrP(c) antibodies directed against various epitopes of PrP(c) revealed, for all examined groups of patients, a consistent predominance of the glycosylated PrP(c) isoforms as compared with the unglycosylated form. In addition, the antibody SAF70 recognized a variety of PrP(c) fragments with sizes of 21, 18, 13 and 12 kDa. Our findings indicate that the polymorphisms at PrP codon 129, the E200K mutation at codon 200 or the examined types of human transmissible spongiform encephalopathies do not exert a measurable effect on the glycosylation and processing of PrP(c) in human prion diseases.

Glycosylation of PrPC determines timing of neuroinvasion and targeting in the brain following transmissible spongiform encephalopathy infection by a peripheral route

Transmissible spongiform encephalopathy (TSE) infectivity naturally spreads from site of entry in the periphery to the central nervous system where pathological lesions are formed. Several routes and cells within the host have been identified as important for facilitating the infectious process. Expression of the glycoprotein cellular PrP (PrP(C)) is considered a key factor for replication of infectivity in the central nervous system (CNS) and its transport to the brain, and it has been suggested that the infectious agent propagates from cell to cell via a domino-like effect. However, precisely how this is achieved and what involvement the different glycoforms of PrP have in these processes remain to be determined. To address this issue, we have used our unique models of gene-targeted transgenic mice expressing different glycosylated forms of PrP. Two TSE strains were inoculated intraperitoneally into these mice to assess the contribution of diglycosylated, monoglycosylated, and unglycosylated PrP in spreading of infectivity to the brain. This study demonstrates that glycosylation of host PrP has a profound effect in determining the outcome of disease. Lack of diglycosylated PrP slowed or prevented disease onset after peripheral challenge, suggesting an important role for fully glycosylated PrP in either the replication of the infectious agent in the periphery or its transport to the CNS. Moreover, mice expressing unglycosylated PrP did not develop clinical disease, and mice expressing monoglycosylated PrP showed strikingly different neuropathologic features compared to those expressing diglycosylated PrP. This demonstrates that targeting in the brain following peripheral inoculation is profoundly influenced by the glycosylation status of host PrP.

Dramatic reduction of PrP C level and glycosylation in peripheral nerves following PrP knock-out from Schwann cells does not prevent transmissible spongiform encephalopathy neuroinvasion

Expression of the prion protein (PrP(C)) is a requirement for host susceptibility to the transmissible spongiform encephalopathies (TSEs) and thought to be necessary for the replication and transport of the infectious agent. The mechanism of TSE neuroinvasion is not fully understood, although the routing of infection has been mapped through the peripheral nervous system (PNS) and Schwann cells have been implicated as a potential conduit for transport of the TSE infectious agent. To address whether Schwann cells are a requirement for spread of the TSE agent from the site of infection to the CNS, PrP(C) expression was selectively removed from Schwann cells in vivo. This dramatically reduced total PrP(C) within peripheral nerves by 90%, resulting in the selective loss of glycosylated PrP(C) species. Despite this, 139A and ME7 mouse-passaged scrapie agent strains were efficiently replicated and transported to the CNS following oral and intraperitoneal exposure. Thus, the myelinating glial cells within the PNS do not appear to play a significant role in TSE neuroinvasion.

A novel, drug-based, cellular assay for the activity of neurotoxic mutants of the prion protein

In prion diseases, the infectious isoform of the prion protein (PrP(Sc)) may subvert a normal, physiological activity of the cellular isoform (PrP(C)). A deletion mutant of the prion protein (Delta105-125) that produces a neonatal lethal phenotype when expressed in transgenic mice provides a window into the normal function of PrP(C) and how it can be corrupted to produce neurotoxic effects. We report here the surprising and unexpected observation that cells expressing Delta105-125 PrP and related mutants are hypersensitive to the toxic effects of two classes of antibiotics (aminoglycosides and bleomycin analogues) that are commonly used for selection of stably transfected cell lines. This unusual phenomenon mimics several essential features of Delta105-125 PrP toxicity seen in transgenic mice, including rescue by co-expression of wild type PrP. Cells expressing Delta105-125 PrP are susceptible to drug toxicity within minutes, suggesting that the mutant protein enhances cellular accumulation of these cationic compounds. Our results establish a screenable cellular phenotype for the activity of neurotoxic forms of PrP, and they suggest possible mechanisms by which these molecules could produce their pathological effects in vivo.

Immunopurification of pathological prion protein aggregates

Background

Prion diseases are fatal neurodegenerative disorders that can arise sporadically, be genetically inherited or acquired through infection. The key event in these diseases is misfolding of the cellular prion protein (PrP^C) into a pathogenic isoform that is rich in β-sheet structure. This conformational change may result in the formation of PrP^Sc, the prion isoform of PrP, which propagates itself by imprinting its aberrant conformation onto PrP^C molecules. A great deal of effort has been devoted to developing protocols for purifying PrP^Sc for structural studies, and testing its biological properties. Most procedures rely on protease digestion, allowing efficient purification of PrP27-30, the protease-resistant core of PrP^Sc. However, protease treatment cannot be used to isolate abnormal forms of PrP lacking conventional protease resistance, such as those found in several genetic and atypical sporadic cases.

Principal Findings

We developed a method for purifying pathological PrP molecules based on sequential centrifugation and immunoprecipitation with a monoclonal antibody selective for aggregated PrP. With this procedure we purified full-length PrP^Sc and mutant PrP aggregates at electrophoretic homogeneity. PrP^Sc purified from prion-infected mice was able to seed misfolding of PrP^C in a protein misfolding cyclic amplification reaction, and mutant PrP aggregates from transgenic mice were toxic to cultured neurons.

Significance

The immunopurification protocol described here isolates biologically active forms of aggregated PrP. These preparations may be useful for investigating the structural and chemico-physical properties of infectious and neurotoxic PrP aggregates.

Co-existence of scrapie prion protein types 1 and 2 in sporadic Creutzfeldt-Jakob disease: its effect on the phenotype and prion-type characteristics

Five phenotypically distinct subtypes have been identified in sporadic Creutzfeldt-Jakob disease (sCJD), based on the methionine/valine polymorphic genotype of codon 129 of the prion protein (PrP) gene and the presence of either one of the two protease K-resistant scrapie prion protein (PrP(Sc)) types identified as 1 and 2. The infrequent co-existence of both PrP(Sc) types in the same case has been known for a long time. Recently, it has been reported, using type-specific antibodies, that the PrP(Sc) type 1 is present in all cases of sCJD carrying PrP(Sc) type 2. The consistent co-occurrence of both PrP(Sc) types complicates the diagnosis and the current classification of sCJD, and has implications for the pathogenesis of naturally occurring prion diseases. In the present study, we investigated the prevalence of PrP(Sc) types 1 and 2 co-occurrence, along with its effects on the disease phenotype and PrP(Sc) strain characteristics, comparatively analysing 34 cases of sCJD, all methionine homozygous at codon 129 of the PrP gene (sCJDMM). To minimize overestimating the prevalence of the sCJDMM cases carrying PrP(Sc) types 1 and 2 (sCJDMM1-2), we used proteinase K concentrations designed to hydrolyse all fragments resulting from an incomplete digestion, while preserving the protease-resistant PrP(Sc) core. Furthermore, we used several antibodies to maximize the detection of both PrP(Sc) types. Our data show that sCJDMM cases associated exclusively with either PrP(Sc) type 1 (sCJDMM1) or PrP(Sc) type 2 (sCJDMM2) do exist; we estimate that they account for approximately 56% and 5% of all the sCJDMM cases, respectively; while in 39% of the cases, both PrP(Sc) types 1 and 2 are present together (sCJDMM1-2) either mixed in the same anatomical region or separate in different regions. Clinically, sCJDMM1-2 had an average disease duration intermediate between the other two sCJDMM subtypes. The histopathology was also intermediate, except for the cerebellum where it resembled that of sCJDMM1. These features, along with the PrP immunostaining pattern, offer a diagnostic clue. We also observed a correlation between the disease duration and the prevalence of PrP(Sc) type 2 and sCJDMM2 phenotypes. The use of different antibodies and of the conformational stability immunoassay indicated that the co-existence of types 1 and 2 in the same anatomical region may confer special conformational characteristics to PrP(Sc) types 1 and 2. All of these findings indicate that sCJDMM1-2 should be considered as a separate entity at this time.

Binding of pro-prion to filamin A disrupts cytoskeleton and correlates with poor prognosis in pancreatic cancer

The cellular prion protein (PrP) is a highly conserved, widely expressed, glycosylphosphatidylinositol-anchored (GPI-anchored) cell surface glycoprotein. Since its discovery, most studies on PrP have focused on its role in neurodegenerative prion diseases, whereas its function outside the nervous system remains unclear. Here, we report that human pancreatic ductal adenocarcinoma (PDAC) cell lines expressed PrP. However, the PrP was neither glycosylated nor GPI-anchored, existing as pro-PrP and retaining its GPI anchor peptide signal sequence (GPI-PSS). We also showed that the PrP GPI-PSS has a filamin A-binding (FLNa-binding) motif and interacted with FLNa, an actin-associated protein that integrates cell mechanics and signaling. Binding of pro-PrP to FLNa disrupted cytoskeletal organization. Inhibition of PrP expression by shRNA in the PDAC cell lines altered the cytoskeleton and expression of multiple signaling proteins; it also reduced cellular proliferation and invasiveness in vitro as well as tumor growth in vivo. A subgroup of human patients with pancreatic cancer was found to have tumors that expressed pro-PrP. Most importantly, PrP expression in tumors correlated with a marked decrease in patient survival. We propose that binding of pro-PrP to FLNa perturbs FLNa function, thus contributing to the aggressiveness of PDAC. Prevention of this interaction could provide an attractive target for therapeutic intervention in human PDAC.

Anti-PrP Mab 6D11 suppresses PrP(Sc) replication in prion infected myeloid precursor line FDC-P1/22L and in the lymphoreticular system in vivo

The pathogenesis of prion diseases is related to conformational transformation of cellular prion protein (PrP(C)) into a toxic, infectious, and self-replicating conformer termed PrP(Sc). Following extracerebral inoculation, the replication of PrP(Sc) is confined for months to years to the lymporeticular system (LRS) before the secondary CNS involvement results in occurrence of neurological symptoms. Therefore, replication of PrP(Sc), in the early stage of infection can be targeted by therapeutic approaches, which like passive immunization have limited blood-brain-barrier penetration. In this study, we show that 6D11 anti-PrP monoclonal antibody (Mab) prevents infection on a FDC-P1 myeloid precursor cell line stably infected with 22L mouse adapted scrapie strain. Passive immunization of extracerebrally infected CD-1 mice with Mab 6D11 resulted in effective suppression of PrP(Sc) replication in the LRS. Although, a rebound of PrP(Sc) presence occurred when the Mab 6D11 treatment was stopped, passively immunized mice showed a prolongation of the incubation period by 36.9% (pb0.0001) and a significant decrease in CNS pathology compared to control groups receiving vehicle or murine IgG. Our results indicate that antibody-based therapeutic strategies can be used, even on a short-term basis, to delay or prevent disease in subjects accidentally exposed to prions.

Evaluation of the possible transmission of BSE and scrapie to gilthead sea bream (Sparus aurata)

In transmissible spongiform encephalopathies (TSEs), a group of fatal neurodegenerative disorders affecting many species, the key event in disease pathogenesis is the accumulation of an abnormal conformational isoform (PrP(Sc)) of the host-encoded cellular prion protein (PrP(C)). While the precise mechanism of the PrP(C) to PrP(Sc) conversion is not understood, it is clear that host PrP(C) expression is a prerequisite for effective infectious prion propagation. Although there have been many studies on TSEs in mammalian species, little is known about TSE pathogenesis in fish. Here we show that while gilthead sea bream (Sparus aurata) orally challenged with brain homogenates prepared either from a BSE infected cow or from scrapie infected sheep developed no clinical prion disease, the brains of TSE-fed fish sampled two years after challenge did show signs of neurodegeneration and accumulation of deposits that reacted positively with antibodies raised against sea bream PrP. The control groups, fed with brains from uninfected animals, showed no such signs. Remarkably, the deposits developed much more rapidly and extensively in fish inoculated with BSE-infected material than in the ones challenged with the scrapie-infected brain homogenate, with numerous deposits being proteinase K-resistant. These plaque-like aggregates exhibited congophilia and birefringence in polarized light, consistent with an amyloid-like component. The neurodegeneration and abnormal deposition in the brains of fish challenged with prion, especially BSE, raises concerns about the potential risk to public health. As fish aquaculture is an economically important industry providing high protein nutrition for humans and other mammalian species, the prospect of farmed fish being contaminated with infectious mammalian PrP(Sc), or of a prion disease developing in farmed fish is alarming and requires further evaluation.

The cellular prion protein and its role in Alzheimer disease

The cellular prion protein (PrP(C)) is a membrane-bound glycoprotein especially abundant in the central nervous system (CNS). The scrapie prion protein (PrP(Sc,) also termed prions) is responsible of transmissible spongiform encephalopathies (TSE), a group of neurodegenerative diseases which affect humans and other mammal species, although the presence of PrP(C) is needed for the establishment and further evolution of prions. The present work compares the expression and localization of PrP(C) between healthy human brains and those suffering from Alzheimer disease (AD). In both situations we have observed a rostrocaudal decrease in the amount of PrP(C) within the CNS, both by immunoblotting and immunohistochemistry techniques. PrP(C) is higher expressed in our control brains than in AD cases. There was a neuronal loss and astogliosis in our AD cases. There was a tendency of a lesser expression of PrP(C) in AD cases than in healthy ones. And in AD cases, the intensity of the expression of the unglycosylated band is higher than the di- and monoglycosylated bands. With regards to amyloid plaques, those present in AD cases were positively labeled for PrP(C), a result which is further supported by the presence of PrP(C) in the amyloid plaques of a transgenic line of mice mimicking AD. The work was done according to Helsinki Declaration of 1975, and approved by the Ethics Committee of the Faculty of Medicine of the University of Navarre.

Aggregated, wild-type prion protein causes neurological dysfunction and synaptic abnormalities

The neurotoxic forms of the prion protein (PrP) that cause neurodegeneration in prion diseases remain to be conclusively identified. Considerable evidence points to the importance of noninfectious oligomers of PrP in the pathogenic process. In this study, we describe lines of Tg(WT) transgenic mice that over-express wild-type PrP by either approximately 5-fold or approximately 10-fold (depending on whether the transgene array is, respectively, hemizygous or homozygous). Homozygous but not hemizygous Tg(WT) mice develop a spontaneous neurodegenerative illness characterized clinically by tremor and paresis. Both kinds of mice accumulate large numbers of punctate PrP deposits in the molecular layer of the cerebellum as well as in several other brain regions, and they display abnormally enlarged synaptic terminals accompanied by a dramatic proliferation of membranous structures. The over-expressed PrP in Tg(WT) mice assembles into an insoluble form that is mildly protease-resistant and is recognizable by aggregation-specific antibodies, but that is not infectious in transmission experiments. Together, our results demonstrate that noninfectious aggregates of wild-type PrP are neurotoxic, particularly to synapses, and they suggest common pathogenic mechanisms shared by prion diseases and nontransmissible neurodegenerative disorders associated with protein misfolding.

[Production of monoclonal antibodies to the prion protein and their characterization]

Antibodies to the prion protein (PrP), particularly, monoclonal antibodies, are necessary tools in the diagnostics and study of prion diseases and potential means of their immunotherapy. For the production of monoclonal antibodies, BALB/c mice were immunized by a recombinant bovine PrP. Three stable hybridomas producing antibodies of IgM class were prepared. The antibodies were bound to PrP in a solid-phase enzyme immunoassay and immunoblotting. The epitope mapping accomplished with the use of synthetic peptides showed that an epitope located in region 25-36 of PrP corresponds to one antibody, and epitopes located in region 222-229, to the other two. The antibodies to fragment 222-229 purified by affinity chromatography recognized with a high specificity conglomerates of a pathogenic prion in the brain tissue of cows suffering from spongiform encephalopathy. Thus, in nontransgenic mice, PrP-specific monoclonal antibodies were produced, useful in studies and diagnostics of prion diseases.

Ligand binding promotes prion protein aggregation--role of the octapeptide repeats

Aggregation of the normal cellular prion protein, PrP, is important in the pathogenesis of prion disease. PrP binds glycosaminoglycan (GAG) and divalent cations, such as Cu(2+) and Zn(2+). Here, we report our findings that GAG and Cu(2+) promote the aggregation of recombinant human PrP (rPrP). The normal cellular prion protein has five octapeptide repeats. In the presence of either GAG or Cu(2+), mutant rPrPs with eight or ten octapeptide repeats are more aggregation prone, exhibit faster kinetics and form larger aggregates than wild-type PrP. When the GAG-binding motif, KKRPK, is deleted the effect of GAG but not that of Cu(2+) is abolished. By contrast, when the Cu(2+)-binding motif, the octapeptide-repeat region, is deleted, neither GAG nor Cu(2+) is able to promote aggregation. Therefore, the octapeptide-repeat region is critical in the aggregation of rPrP, irrespective of the promoting ligand. Furthermore, aggregation of rPrP in the presence of GAG is blocked with anti-PrP mAbs, whereas none of the tested anti-PrP mAbs block Cu(2+)-promoted aggregation. However, a mAb that is specific for an epitope at the N-terminus enhances aggregation in the presence of either GAG or Cu(2+). Therefore, although binding of either GAG or Cu(2+) promotes the aggregation of rPrP, their aggregation processes are different, suggesting multiple pathways of rPrP aggregation.