In vitro expression in eukaryotic cells of a prion protein gene cloned from scrapie-infected mouse brain

Persistent ER stress causes GPI anchor deficit to convert a GPI-anchored prion protein into pro-PrP via the ATF6-miR449c-5p-PIGV axis

Endoplasmic reticulum (ER) stress and unfolded protein response are cells' survival strategies to thwart disruption of proteostasis. Tumor cells are continuously being challenged by ER stress. The prion protein, PrP, normally a glycosylphosphatidylinositol (GPI)-anchored protein exists as a pro-PrP retaining its GPI-peptide signal sequence in human pancreatic ductal cell adenocarcinoma (PDAC). Higher abundance of pro-PrP indicates poorer prognosis in PDAC patients. The reason why PDAC cells express pro-PrP is unknown. Here, we report that persistent ER stress causes conversion of GPI-anchored PrP to pro-PrP via a conserved ATF6-miRNA449c-5p-PIGV axis. Mouse neurons and AsPC-1, a PDAC cell line, express GPI-anchored PrP. However, continuous culture of these cells with the ER stress inducers thapsigargin or brefeldin A results in the conversion of a GPI-anchored PrP to pro-PrP. Such a conversion is reversible; removal of the inducers allows the cells to re-express a GPI-anchored PrP. Mechanistically, persistent ER stress increases the abundance of an active ATF6, which increases the level of miRNA449c-5p (miR449c-5p). By binding the mRNA of PIGV at its 3'-UTRs, miR449c-5p suppresses the level of PIGV, a mannosyltransferase pivotal in the synthesis of the GPI anchor. Reduction of PIGV leads to disruption of the GPI anchor assembly, causing pro-PrP accumulation and enhancing cancer cell migration and invasion. The importance of ATF6-miR449c-5p-PIGV axis is recapitulated in PDAC biopsies as the higher levels of ATF6 and miR449c-5p and lower levels of PIGV are markers of poorer outcome for patients with PDAC. Drugs targeting this axis may prevent PDAC progression.

PrP P102L and Nearby Lysine Mutations Promote Spontaneous In Vitro Formation of Transmissible Prions

Accumulation of fibrillar protein aggregates is a hallmark of many diseases. While numerous proteins form fibrils by prion-like seeded polymerization in vitro, only some are transmissible and pathogenic in vivo To probe the structural features that confer transmissibility to prion protein (PrP) fibrils, we have analyzed synthetic PrP amyloids with or without the human prion disease-associated P102L mutation. The formation of infectious prions from PrP molecules in vitro has required cofactors and/or unphysiological denaturing conditions. Here, we demonstrate that, under physiologically compatible conditions without cofactors, the P102L mutation in recombinant hamster PrP promoted prion formation when seeded by minute amounts of scrapie prions in vitro Surprisingly, combination of the P102L mutation with charge-neutralizing substitutions of four nearby lysines promoted spontaneous prion formation. When inoculated into hamsters, both of these types of synthetic prions initiated substantial accumulation of prion seeding activity and protease-resistant PrP without transmissible spongiform encephalopathy (TSE) clinical signs or notable glial activation. Our evidence suggests that PrP's centrally located proline and lysine residues act as conformational switches in the in vitro formation of transmissible PrP amyloids.IMPORTANCE Many diseases involve the damaging accumulation of specific misfolded proteins in thread-like aggregates. These threads (fibrils) are capable of growing on the ends by seeding the refolding and incorporation of the normal form of the given protein. In many cases such aggregates can be infectious and propagate like prions when transmitted from one individual host to another. Some transmitted aggregates can cause fatal disease, as with human iatrogenic prion diseases, while other aggregates appear to be relatively innocuous. The factors that distinguish infectious and pathogenic protein aggregates from more innocuous ones are poorly understood. Here we have compared the combined effects of prion seeding and mutations of prion protein (PrP) on the structure and transmission properties of synthetic PrP aggregates. Our results highlight the influence of specific sequence features in the normally unstructured region of PrP that influence the infectious and neuropathogenic properties of PrP-derived aggregates.

Inactivation of Prions and Amyloid Seeds with Hypochlorous Acid

Hypochlorous acid (HOCl) is produced naturally by neutrophils and other cells to kill conventional microbes in vivo. Synthetic preparations containing HOCl can also be effective as microbial disinfectants. Here we have tested whether HOCl can also inactivate prions and other self-propagating protein amyloid seeds. Prions are deadly pathogens that are notoriously difficult to inactivate, and standard microbial disinfection protocols are often inadequate. Recommended treatments for prion decontamination include strongly basic (pH ≥~12) sodium hypochlorite bleach, ≥1 N sodium hydroxide, and/or prolonged autoclaving. These treatments are damaging and/or unsuitable for many clinical, agricultural and environmental applications. We have tested the anti-prion activity of a weakly acidic aqueous formulation of HOCl (BrioHOCl) that poses no apparent hazard to either users or many surfaces. For example, BrioHOCl can be applied directly to skin and mucous membranes and has been aerosolized to treat entire rooms without apparent deleterious effects. Here, we demonstrate that immersion in BrioHOCl can inactivate not only a range of target microbes, including spores of Bacillus subtilis, but also prions in tissue suspensions and on stainless steel. Real-time quaking-induced conversion (RT-QuIC) assays showed that BrioHOCl treatments eliminated all detectable prion seeding activity of human Creutzfeldt-Jakob disease, bovine spongiform encephalopathy, cervine chronic wasting disease, sheep scrapie and hamster scrapie; these findings indicated reductions of ≥10³- to 10⁶-fold. Transgenic mouse bioassays showed that all detectable hamster-adapted scrapie infectivity in brain homogenates or on steel wires was eliminated, representing reductions of ≥~10^5.75-fold and >10⁴-fold, respectively. Inactivation of RT-QuIC seeding activity correlated with free chlorine concentration and higher order aggregation or destruction of proteins generally, including prion protein. BrioHOCl treatments had similar effects on amyloids composed of human α-synuclein and a fragment of human tau. These results indicate that HOCl can block the self-propagating activity of prions and other amyloids.

Many serious diseases have been linked to pathogenic states of various proteins. These naturally occurring proteins can be corrupted to form aggregates such as prions and amyloids that propagate in and between tissues by acting as seeds that convert the normal form of the protein into more of the pathological form. For example, corrupted prion protein can cause fatal transmissible neurodegenerative diseases such as Creutzfeldt-Jakob disease in humans, chronic wasting disease in cervids and bovine spongiform encephalopathy. Other amyloid-forming protein aggregates are pathogenic in Parkinson’s, Alzheimer’s, and other diseases. The fact that prions and amyloids are composed predominantly of tough, tightly packed proteins makes them unusually resistant to conventional microbial disinfection procedures. Infectious prions can persist indefinitely in, or on, a variety of materials such as tissues, fluids, tools, instruments, and environmental surfaces, making it important to identify decontaminants that are effective without being dangerous or damaging. Here we show that hypochlorous acid, a disinfectant that is produced naturally by certain cells within the body, has strong anti-prion and anti-amyloid activity. We find that a non-irritating and broadly applicable hypochlorous acid preparation can disinfect prions in tissue homogenates and on stainless steel wires serving as surrogates for surgical instruments.

Prion Protein Prolines 102 and 105 and the Surrounding Lysine Cluster Impede Amyloid Formation

Human prion diseases can have acquired, sporadic, or genetic origins, each of which results in the conversion of prion protein (PrP) to transmissible, pathological forms. The genetic prion disease Gerstmann-Straussler-Scheinker syndrome can arise from point mutations of prolines 102 or 105. However, the structural effects of these two prolines, and mutations thereof, on PrP misfolding are not well understood. Here, we provide evidence that individual mutations of Pro-102 or Pro-105 to noncyclic aliphatic residues such as the Gerstmann-Straussler-Scheinker-linked leucines can promote the in vitro formation of PrP amyloid with extended protease-resistant cores reminiscent of infectious prions. This effect was enhanced by additional charge-neutralizing mutations of four nearby lysine residues comprising the so-called central lysine cluster. Substitution of these proline and lysine residues accelerated PrP conversion such that spontaneous amyloid formation was no longer slower than scrapie-seeded amyloid formation. Thus, Pro-102 and Pro-105, as well as the lysines in the central lysine cluster, impede amyloid formation by PrP, implicating these residues as key structural modulators in the conversion of PrP to disease-associated types of amyloid.

Charge neutralization of the central lysine cluster in prion protein (PrP) promotes PrP(Sc)-like folding of recombinant PrP amyloids

The structure of the infectious form of prion protein, PrP(Sc), remains unclear. Most pure recombinant prion protein (PrP) amyloids generated in vitro are not infectious and lack the extent of the protease-resistant core and solvent exclusion of infectious PrP(Sc), especially within residues ∼90-160. Polyanionic cofactors can enhance infectivity and PrP(Sc)-like characteristics of such fibrils, but the mechanism of this enhancement is unknown. In considering structural models of PrP(Sc) multimers, we identified an obstacle to tight packing that might be overcome with polyanionic cofactors, namely, electrostatic repulsion between four closely spaced cationic lysines within a central lysine cluster of residues 101-110. For example, in our parallel in-register intermolecular β-sheet model of PrP(Sc), not only would these lysines be clustered within the 101-110 region of the primary sequence, but they would have intermolecular spacings of only ∼4.8 Å between stacked β-strands. We have now performed molecular dynamics simulations predicting that neutralization of the charges on these lysine residues would allow more stable parallel in-register packing in this region. We also show empirically that substitution of these clustered lysine residues with alanines or asparagines results in recombinant PrP amyloid fibrils with extended proteinase-K resistant β-sheet cores and infrared spectra that are more reminiscent of bona fide PrP(Sc). These findings indicate that charge neutralization at the central lysine cluster is critical for the folding and tight packing of N-proximal residues within PrP amyloid fibrils. This charge neutralization may be a key aspect of the mechanism by which anionic cofactors promote PrP(Sc) formation.

Establishment of bovine prion peptide-based monoclonal antibodies for identifying bovine prion

To obtain high titer monoclonal antibodies (McAbs) which can react with mammalian prion protein (PrP), Balb/C mice were immunized with bovine (Bo) PrP peptide (BoPrP 209-228 aa) coupled to keyhole limpet hemocyanin (KLH). The hybridoma cell lines secreting monoclonal antibodies against the peptide were established by cell fusion and cloning. The obtained McAbs were applied to detect recombinant human, bovine and hamster PrP, cellular prion protein (PrP(c)) in normal bovine brain and pathogenic scrapie prion protein (PrP(Sc)) accumulated in the medulla oblongata of bovine spongiform encephalopathy(BSE)specimen with Western blot and immunohistochemical detection, respectively. The current procedure might offer a simple, feasible method to raise high titer antibodies for studying biological features of PrP in mammals, as well as detection of transmissible spongiform encephalopathy (TSE) and diagnosis of BSE, in particular.

Misfolding of the prion protein: linking biophysical and biological approaches

Prion diseases are a group of neurodegenerative diseases that can arise spontaneously, be inherited, or acquired by infection in mammals. The propensity of the prion protein to adopt different structures is a clue to its pathological and perhaps biological role too. While the normal monomeric PrP is well characterized, the misfolded conformations responsible for neurodegeneration remain elusive despite progress in this field. Both structural dynamics and physico-chemical approaches are thus fundamental for a better knowledge of the molecular basis of this pathology. Indeed, multiple misfolding pathways combined with extensive posttranslational modifications of PrP and probable interaction(s) with cofactors call for a combination of approaches. In this review, we outline the current physico-chemical knowledge explaining the conformational diversities of PrP in relation with postulated or putative cellular partners such as proteic or non-proteic ligands.

The role of rafts in the fibrillization and aggregation of prions

A key molecular event in prion diseases is the conversion of the prion protein (PrP) from its normal cellular form (PrP(C)) to the disease-specific form (PrP(Sc)). The transition from PrP(C) to PrP(Sc) involves a major conformational change, resulting in amorphous aggregates and/or fibrillar amyloid deposits. Here several lines of evidence implicating membranes in the conversion of PrP are reviewed with a particular emphasis on the role of lipid rafts in the conformational transition of prion proteins. New correlations between in vitro biophysical studies and findings from cell biology work on the role of rafts in prion conversion are highlighted and a mechanism for the role of rafts in prion conversion is proposed.

On-column purification and refolding of recombinant bovine prion protein: using its octarepeat sequences as a natural affinity tag

Prion protein has a key role in the occurrence of transmissible spongiform encephalopathy (TSE) and development of these diseases. Here, we provide a convenient procedure for on-column purification and refolding of the full-length mature bovine prion protein (bPrP) from Escherichia coli using immobilized metal (Ni) affinity chromatography, based on the metal-binding property of its unusual octarepeat sequences containing six tandem copies. Following extensive washing, the bPrP pellet was solubilized by guanidine hydrochloride and subjected to Ni-NTA agarose column. Purification and refolding were achieved by stepwise gradient washing with reduced guanidine hydrochloride concentrations. Triton X-100 and beta-mercaptoethanol were required in this rapid refolding process. The isolated prion protein was identified by monoclonal antibodies and its integrity was monitored by mass spectroscopy. Its correct folding was confirmed from circular dichroism (CD) experiments. Moreover, thioflavin T-binding assay showed that the recombinant bPrP could be transformed into amyloid fiber structures like that of the infectious prion isoform PrP(sc).

Binding of prion proteins to lipid membranes

A key molecular event in prion diseases is the conversion of the normal cellular form of the prion protein (PrPC) to an aberrant form known as the scrapie isoform, PrPSc. Under normal physiological conditions PrPC is attached to the outer leaflet of the plasma membrane via a GPI-anchor. It has been proposed that a direct interaction between PrP and lipid membranes could be involved in the conversion of PrPC to its disease-associated corrupted conformation, PrPSc. Recombinant PrP can be refolded into an alpha-helical structure, designated alpha-PrP isoform, or into beta-sheet-rich states, designated beta-PrP isoform. The current study investigates the binding of recombinant PrP isoforms to model lipid membranes using surface plasmon resonance spectroscopy. The binding of alpha- and beta-PrP to negatively charged lipid membranes of POPG, zwitterionic membranes of DPPC, and model raft membranes composed of DPPC, cholesterol, and sphingomyelin is compared at pH 7 and 5, to simulate the environment at the plasma membrane and within endosomes, respectively. It is found that PrP binds strongly to lipid membranes. The strength of the association of PrP with lipid membranes depends on the protein conformation and pH, and involves both hydrophobic and electrostatic lipid-protein interactions. Competition binding measurements established that the binding of alpha-PrP to lipid membranes follows a decreasing order of affinity to POPG>DPPC>rafts.

Prion protein and the molecular features of transmissible spongiform encephalopathy agents

Transmissible spongiform encephalopathy (TSE) diseases, or prion diseases, are neurodegenerative diseases found in a number of mammals, including man. Although they are generally rare, TSEs are always fatal, and as of yet there are no practical therapeutic avenues to slow the course of disease. The epidemic of bovine spongiform encephalopathy (BSE) in the UK greatly increased the awareness of TSE diseases. Although it appears that BSE has not spread to North America, chronic wasting disease (CWD), a TSE found in cervids, is causing significant concern. Despite decades of investigation, the exact nature of the infectious agent of the TSEs is still controversial. Although many questions remain, substantial efforts have been made to understand the molecular features of TSE agents, with the hope of enhancing diagnosis and treatment of disease, as well as understanding the fundamental nature of the infectious agent itself. This review summarizes the current understanding of these molecular features, focusing on the role of the prion protein (PrP(c)) and its relationship to the disease-associated isoform (PrP(Sc)).

Effect of glycosylphosphatidylinositol anchor-dependent and -independent prion protein association with model raft membranes on conversion to the protease-resistant isoform

Prion protein (PrP) is usually bound to membranes by a glycosylphosphatidylinositol (GPI) anchor that associates with detergent-resistant membranes, or rafts. To examine the effect of membrane association on the interaction between the normal protease-sensitive PrP isoform (PrP-sen) and the protease-resistant isoform (PrP-res), a model system was employed using PrP-sen reconstituted into sphingolipid-cholesterol-rich raft-like liposomes (SCRLs). Both full-length (GPI(+)) and GPI anchor-deficient (GPI(-)) PrP-sen produced in fibroblasts stably associated with SCRLs. The latter, alternative mode of membrane association was not detectably altered by glycosylation and was markedly reduced by deletion of residues 34-94. The SCRL-associated PrP molecules were not removed by treatments with either high salt or carbonate buffer. However, only GPI(+) PrP-sen resisted extraction with cold Triton X-100. PrP-sen association with SCRLs was pH-independent. PrP-sen was also one of a small subset of phosphatidylinositol-specific phospholipase C (PI-PLC)-released proteins from fibroblast cells found to bind SCRLs. A cell-free conversion assay was used to measure the interaction of SCRL-bound PrP-sen with exogenous PrP-res as contained in microsomes. SCRL-bound GPI(+) PrP-sen was not converted to PrP-res until PI-PLC was added to the reaction or the combined membrane fractions were treated with the membrane-fusing agent polyethylene glycol (PEG). In contrast, SCRL-bound GPI(-) PrP-sen was converted to PrP-res without PI-PLC or PEG treatment. Thus, of the two forms of raft membrane association by PrP-sen, only the GPI anchor-directed form resists conversion induced by exogenous PrP-res.

In vitro conversion of normal prion protein into pathologic isoforms

The in vitro conversion techniques in cell-free and cell culture systems have provided tools to adequately study the underlying mechanism of TSEs, namely PrP conversion. These systems also have provided tools that make it easier to study the interspecies and intraspecies transmissibilities of TSEs. Finally, these systems also may assist in the discovery of TSE therapeutic strategies and in the development of extremely sensitive TSE detection techniques. In vivo TSE transmission studies are limited to (transgenic) animals (mostly mice). Although the cell culture systems also are restricted in their species-range (mostly mouse), the currently used cell-free systems. Allow studying almost all possible species barriers (including the potential transmission of various TSEs to humans). One advantage of the cell culture systems, however, is that they generate do novo TSE infectivity. Studies using cell cultures also take into account several cofactors in addition to PrP that might be involved in replication the TSE agent. Although the in vitro systems provide accurate tools to study TSE agent parameters, they mainly or only focus on the molecular processes of PrP conversion. Other factors (i.e., host genetic factors [99]) that, for example, determine the differential uptake of the TSE agent from the environment, might play an additional role in determining the susceptibility of hosts for TSEs and on the transmission of the disease among individuals.

Effect of the E200K mutation on prion protein metabolism. Comparative study of a cell model and human brain

The hallmark of prion diseases is the cerebral accumulation of a conformationally altered isoform (PrP(Sc)) of a normal cellular protein, the prion protein (PrP(C)). In the inherited form, mutations in the prion protein gene are thought to cause the disease by altering the metabolism of the mutant PrP (PrP(M)) engendering its conversion into PrP(Sc). We used a cell model to study biosynthesis and processing of PrP(M) carrying the glutamic acid to lysine substitution at residue 200 (E200K), which is linked to the most common inherited human prion disease. PrP(M) contained an aberrant glycan at residue 197 and generated an increased quantity of truncated fragments. In addition, PrP(M) showed impaired transport of the unglycosylated isoform to the cell surface. Similar changes were found in the PrP isolated from brains of patients affected by the E200K variant of Creutzfeldt-Jakob disease. Although the cellular PrP(M) displayed some characteristics of PrP(Sc), the PrP(Sc) found in the E200K brains was quantitatively and qualitatively different. We propose that the E200K mutation cause the same metabolic changes of PrP(M) in the cell model and in the brain. However, in the brain, PrP(M) undergoes additional modifications, by an age-dependent mechanism that leads to the formation of PrP(Sc) and the development of the disease.

Structural dependence of the cellular isoform of prion protein on solvent: spectroscopic characterization of an intermediate conformation

Using circular dichroism, fluorescence, and infrared spectroscopies, we studied the secondary structure of purified hamster PrP(C) in the presence of the mild, nonionic detergent octylglucoside. Under these native conditions, PrP(C) displayed an unexpectedly high beta-sheet component, intermediate between the values previously reported for PrP(Sc) and an isoform of PrP(C) isolated in a zwitterionic detergent. The structure of PrP(C) appeared to depend strongly on the detergent and/or phase. Switching from octylglucoside to zwitterion 3-14 drastically modified PrP secondary structure by increasing the alpha-helix while abolishing the beta-sheet component. In contrast, the conformation of PrP(C) in zwitterion was highly stable, since reverting to octylglucoside did not restore the original native structure. These and other results show that native PrP(C) in octylglucoside has some of the conformational characteristics that make the protein susceptible to conversion into PrP(Sc). Most importantly, this is the first study to demonstrate the intrinsic plasticity of the full-length native PrP(C) isolated from animal brains.

Methods for studying prion protein (PrP) metabolism and the formation of protease-resistant PrP in cell culture and cell-free systems. An update

Transmissible spongiform encephalopathies (TSE) or prion diseases result in aberrant metabolism of prion protein (PrP) and the accumulation of a protease-resistant, insoluble, and possibly infectious form of PrP, PrP-res. Studies of PrP biosynthesis, intracellular trafficking, and degradation has been studied in a variety of tissue culture cells. Pulse-chase metabolic labeling studies in scrapie-infected cells indicated that PrP-res is made posttranslationally from an apparently normal protease-sensitive precursor, PrP-sen, after the latter reaches the cell surface. Cell-free reactions have provided evidence that PrP-res itself can induce the conversion of PrP-sen to PrP-res in a highly species- and strain-specific manner. These studies have shed light on the mechanism of PrP-res formation and suggest molecular bases for TSE species barrier effects and agent strain propagation.

Early destruction of the extracellular matrix around parvalbumin-immunoreactive interneurons in Creutzfeldt-Jakob disease

GABA-interneurons immunoreactive (IR) for the calcium-binding protein parvalbumin are lost during the early stages of Creutzfeldt-Jakob disease (CJD) and diminution in their number may partially account for the neurological disturbances manifested in patients suffering from this condition. The disease is characterized by a transformation of the prion protein, PrP(c)-a host-coded sialoglycoprotein-to its protease-resistant and putatively pathological form, PrP(CJD). And since this conversion is likely to take place at the cell surface, we were curious to know whether the "perineuronal net"-a characteristic accumulation of extracellular matrix in intimate contact with the surface of parvalbumin-IR neurons-is implicated in the early disappearance of the mantled cells. Using various lectins and antibodies as markers for the perineuronal net in brains of 21 CJD victims, we observed that this meshwork of extracellular matrix molecules is lost before the embraced parvalbumin-IR neurons themselves disappear. Hence, an interaction of PrP(c) and/or PrP(CJD) with components of the extracellular matrix around this subpopulation of nerve cells precipitates a sequence of as yet unknown events which culminates in the replacement of perineuronal nets by deposits of insoluble PrP(CJD). This change in the environment of the GABA-interneurons IR for parvalbumin may ultimately provoke their death.

Cellular biology of prion diseases

Prion diseases are fatal neurodegenerative disorders of humans and animals that are important because of their impact on public health and because they exemplify a novel mechanism of infectivity and biological information transfer. These diseases are caused by conformational conversion of a normal host glycoprotein (PrPC) into an infectious isoform (PrPSc) that is devoid of nucleic acid. This review focuses on the current understanding of prion diseases at the cell biological level. The characteristics of the diseases are introduced, and a brief history and description of the prion hypothesis are given. Information is then presented about the structure, expression, biosynthesis, and possible function of PrPC, as well as its posttranslational processing, cellular localization, and trafficking. The latest findings concerning PrPSc are then discussed, including cell culture systems used to generate this pathogenic isoform, the subcellular distribution of the protein, its membrane attachment, proteolytic processing, and its kinetics and sites of synthesis. Information is also provided on molecular models of the PrPC-->PrPSc conversion reaction and the possible role of cellular chaperones. The review concludes with suggestions of several important avenues for future investigation.

Overexpression of nonconvertible PrPc delta114-121 in scrapie-infected mouse neuroblastoma cells leads to trans-dominant inhibition of wild-type PrP(Sc) accumulation

One hallmark of prion diseases is the accumulation of the abnormal isoform PrP(Sc) of a normal cellular glycoprotein, PrPc, which is characterized by a high content of beta-sheet structures and by its partial resistance to proteinase K. It was hypothesized that the PrP region comprising amino acid residues 109 to 122 [PrP(109-122)], which spontaneously forms amyloid when it is synthesized as a peptide but which does not display significant secondary structure in the context of the full-length PrPc molecule, should play a role in promoting the conversion into PrP(Sc). By using persistently scrapie-infected mouse neuroblastoma (Sc+-MNB) cells as a model system for prion replication, we set out to design dominant-negative mutants of PrPc that are capable of blocking the conversion of endogenous, wild-type PrPc into PrP(Sc). We constructed a deletion mutant (PrPc delta114-121) lacking eight codons that span most of the highly amyloidogenic part, AGAAAAGA, of PrP(109-122). Transient transfections of mammalian expression vectors encoding either wild-type PrPc or PrPc delta114-121 into uninfected mouse neuroblastoma cells (Neuro2a) led to overexpression of the respective PrPc versions, which proved to be correctly localized on the extracellular face of the plasma membrane. Transfection of Sc+-MNB cells revealed that PrPc delta114-121 was not a substrate for conversion into a proteinase K-resistant isoform. Furthermore, its presence led to a significant reduction in the steady-state levels of PrP(Sc) derived from endogenous PrPc. Thus, we showed that the presence of amino acids 114 to 121 of mouse PrPc plays an important role in the conversion process of PrPc into PrP(Sc) and that a deletion mutant lacking these codons indeed behaves as a dominant-negative mutant with respect to PrP(Sc) accumulation. This mechanism could form a basis for a new gene therapy and/or a prevention concept for prion diseases.

The complete mature bovine prion protein highly expressed in Escherichia coli: biochemical and structural studies

According to the 'protein only' hypothesis, modification of the 3-dimensional fold of the constituent cellular protein, PrP(C), into the disease-associated isoform, PrP(Sc), is the cause of neurodegenerative diseases in animals and humans. Here we describe the high-level synthesis in Escherichia coli, and purification in the monomeric form, of a histidine-tagged full-length mature PrP (25-249) of bovine brain, termed His-PrP. Based on biochemical and spectroscopic data, His-PrP displays characteristics expected for the PrP(C) isoform. The reported expression system should allow the production of quantities of bovine PrP(C) sufficient to permit 3-dimensional structure determinations.

Effect of the D178N mutation and the codon 129 polymorphism on the metabolism of the prion protein

Prion diseases are thought to be caused by the conversion of the normal, or cellular, prion protein (PrPC)(PrPres). There are three familial forms of human prion disease, Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler-Scheinker syndrome, and fatal familial insomnia (FFI) which are all expressed at advanced age despite the congenital presence of the mutant prion protein (PrPM). The cellular mechanisms that result in the age-dependent conversion of PrPM into PrPres and the unique phenotypes associated with each PrPM are unknown. FFI and a familial type of Creutzfeldt-Jakob disease (CJD178), share the D178N mutation in the PrP gene but have distinct phenotypes linked to codon 129, the site of a methionine/valine polymorphism (129M/V). We analyzed PrP processing in cells transfected with constructs reproducing the FFI and CJD178 genotypes. The D178N mutation results in instability of the mutant PrP which is partially corrected by N-glycosylation. Hence, only the glycosylated forms of PrPM reach the cell surface whereas the unglycosylated PrPM is also under-represented in the brain of FFI patients validating the cell model. These results offer new insight into the effect of the D178N mutation on the metabolism of the prion protein.

Overexpression of active Syrian golden hamster prion protein PrPc as a glutathione S-transferase fusion in heterologous systems

This article describes a procedure which permits for the first time the isolation of the prion protein PrPc from the Syrian golden hamster in heterologous systems. Using a glutathione S-transferase (GST) fusion approach, milligram amounts of stable, soluble, and homogeneous GST::PrPc protein were obtained in Escherichia coli and with baculovirus-infected insect cells. Authentic PrPc was released from the immobilized fusion protein by direct cleavage with thrombin. GST::PrPc expressed in these two expression systems and also authentic PrPc released by thrombin cleavage were recognized by a polyclonal antibody directed against amino acid 95 to 110 of the golden hamster PrPc protein. GST::PrPc was not detected by a monoclonal antibody recognizing the region encompassing amino acids 138 to 152 of the human prion protein. The fusion protein was sensitive to proteinase K digestion, demonstrating that the cellular rather than the proteinase K-resistant scrapie isoform was produced.

A 60-kDa prion protein (PrP) with properties of both the normal and scrapie-associated forms of PrP

Scrapie is a transmissible spongiform encephalopathy of sheep and other mammals in which disease appears to be caused by the accumulation of an abnormal form of a host protein, prion protein (PrP), in the brain and other tissues. The process by which the normal protease-sensitive form of PrP is converted into the abnormal protease-resistant form is unknown. Several hypotheses predict that oligomeric forms of either the normal or abnormal PrP may act as intermediates in the conversion process. We have now identified a 60-kDa PrP derived from hamster PrP expressed in murine neuroblastoma cells. Peptide mapping studies provided evidence that the 60-kDa PrP was composed solely of PrP and, based on its molecular mass, appeared to be a PrP dimer. The 60-kDa PrP was not dissociated under several harsh denaturing conditions, which indicated that it was covalently linked. It was similar to the disease-associated form of PrP in that it formed large aggregates. However, it resembled the normal form of PrP in that it was sensitive to proteinase K and had a short metabolic half-life. The 60-kDa PrP, therefore, had characteristics of both the normal and disease-associated forms of PrP. Formation and aggregation of the 60-kDa hamster PrP occurs in uninfected mouse neuroblastoma cells, which suggests that hamster PrP has a predisposition to aggregate even in the absence of scrapie infectivity. Similar 60-kDa PrP bands were identified in scrapie-infected hamster brain but not in uninfected brain. Therefore, a 60-kDa molecule might participate in the scrapie-associated conversion of protease-sensitive PrP to protease-resistant PrP.

Scrapie-associated PrP accumulation and its inhibition: revisiting the amyloid-glycosaminoglycan connection

An abnormal protease-resistant isoform of the protein PrP accumulates in the brain of hosts with transmissible spongiform encephalopathies (TSEs) and appears to be centrally involved in TSE pathogenesis. Studies with scrapie-infected tissue culture cells have indicated that this abnormal PrP is formed from an apparently normal precursor on the plasma membrane or along an endocytic pathway to the lysosomes. Inhibitors of protease-resistant PrP accumulation might serve as tools for studying the basic mechanism of protease-resistant PrP formation and as potential drugs for TSE therapy. Using scrapie-infected neuroblastoma cells to screen for such compounds in vitro, we found that the amyloid binding dye Congo red and certain sulfated glycans potently inhibited the accumulation of protease-resistant PrP in scrapie-infected cells without apparent effects on the metabolism of the normal isoform. The relative potencies of the sulfated glycans corresponded with their previously determined anti-scrapie activities in vivo, suggesting that the prophylactic effects of sulfated polyanions may be due to inhibition of protease-resistant PrP accumulation. Since protease-resistant PrP amyloid is known to contain sulfated glycosaminoglycans, as do other naturally derived amyloids, we hypothesize that these sulfated inhibitors competitively block binding between PrP and endogenous glycosaminoglycans that is important for its accumulation in a protease-resistant, potentially amyloidogenic state. Drugs which interfere with this (pre)amyloid-glycosaminoglycan interaction may be useful for treating a variety of amyloidoses.

Inhibition of scrapie-associated PrP accumulation. Probing the role of glycosaminoglycans in amyloidogenesis

Accumulation of an abnormal, protease-resistant form of an endogenous protein, PrP, is a characteristic feature of scrapie and related transmissible spongiform encephalopathies. This abnormal isoform is also present in the amyloid plaques that are often observed in these diseases. In mouse neuroblastoma cells persistently infected with scrapie, the abnormal protease-resistant isoform of PrP is derived from an operationally normal protease-sensitive precursor. Conversion of PrP to the protease-resistant state occurs either on the plasma membrane or along an endocytic pathway by an unknown mechanism. Inhibitors of protease-resistant PrP accumulation have been identified, and these include the amyloid-binding dye Congo red and certain sulfated glycans. The similarity of these compounds to sulfated glycosaminoglycans, which are components of all natural amyloids, has led to the hypothesis that the inhibitors act by competitively blocking an interaction between endogenous glycosaminoglycan(s) and PrP that is critical for amyloidogenic PrP accumulation. The proven prophylactic effect of these sulfated glycans in animal models of scrapie suggests that they represent a group of compounds that might interfere with the pathogenic formation of amyloid in a variety of diseases, such as Alzheimer's disease.

Scrapie-associated PrP accumulation and agent replication: effects of sulphated glycosaminoglycan analogues

An abnormally protease-resistant and apparently neuropathogenic form of PrP accumulates in the brains of hosts with scrapie and related transmissible spongiform encephalopathies. Studies with scrapie-infected neuroblastoma cells have highlighted dramatic differences in the metabolism of the normal (protease-sensitive) and scrapie-associated (protease-resistant) isoforms of PrP. Furthermore, this model has been useful in identifying inhibitors of protease-resistant PrP accumulation and scrapie agent replication which are valuable as potential therapeutic agents and as probes of the mechanism of protease-resistant PrP formation. These inhibitors include the amyloid stain Congo red and certain sulphated glycans which are glycosaminoglycans themselves or glycosaminoglycan analogues. The relative potencies of various sulphated glycans correlate with their previously determined anti-scrapie activities in vivo, suggesting that the prophylactic effects of sulphated polyanions is due to inhibition of protease-resistant PrP accumulation. These and other observations suggest that an interaction of PrP with endogenous sulphated glycosaminoglycans or proteoglycans is important in protease-resistant PrP accumulation, and raise the possibility that therapies for transmissible spongiform encephalopathies and other amyloidoses could be based on blocking (pre)amyloid-glycosaminoglycan interactions.

PrP protein is associated with follicular dendritic cells of spleens and lymph nodes in uninfected and scrapie-infected mice

Abnormal forms of a host protein, PrP, accumulate in the central nervous system in scrapie-affected animals. Here, PrP protein was detected immunocytochemically in tissue sections of spleen, lymph node, Peyer's patches, thymus, and pancreas from uninfected mice and from mice infected with a range of mouse-passaged scrapie strains and bovine spongiform encephalopathy (BSE). In the spleen, lymph node and Peyer's patches, PrP-positive cells were identified as follicular dendritic cells (FDC) by their location, appearance, and immune complex trapping function, whereas in the thymus they appeared to be two types of stromal cells: interdigitating cells (IDC) and cortical epithelial cells. In pancreas, PrP-containing cells were confined to the islets of Langerhans. Although the distribution of PrP immunolabelling was the same in tissues from scrapie-affected and uninfected mice, there was evidence that PrP accumulated in abnormal forms in FDC of infected mice. If, as is likely, PrP is essential for agent replication, our results suggest that FDC are the site of scrapie and BSE replication in the spleen and lymph node.

Comparative sequence analysis and expression of bovine PrP gene in mouse L-929 cells

A cDNA clone encoding bovine scrapie-associated fibril protein, PrP, from a bovine brain cDNA library and six amplified genomic DNA clones of bovine PrP were characterized. These clones possessed specific characteristics observed in other animal PrP genes. However, the bovine PrP was divided into two types by the number of repeats. One possessed four octapeptide repetitive sequences, like other animal PrP genes, and consisted of 256 amino acids; the other had five such repetitive sequences and 264 amino acids. The amino acid sequence of the former bovine PrP agreed with that of sheep PrP up to the 165th amino acid from the N-terminus. Bovine PrP cDNA introduced into mouse L-929 cells were stably expressed. The expression level of recombinant bovine PrP in the cells judged by immunofluorescence was higher than that of authentic mouse PrP. The recombinant PrP comigrated with authentic bovine PrP in SDS-polyacrylamide gel electrophoresis, suggesting that the recombinant product was fully glycosylated in L-929 cells. Distinct bundles of the intermediate filaments were frequently seen at the perinuclear region of the cells.

N-terminal truncation of the scrapie-associated form of PrP by lysosomal protease(s): implications regarding the site of conversion of PrP to the protease-resistant state

Scrapie and related transmissible spongiform encephalopathies result in the accumulation of a protease-resistant form of an endogenous brain protein called PrP. As an approach to understanding the scrapie-associated modification of PrP, we have studied the processing and sedimentation properties of protease-resistant PrP (PrP-res) in scrapie-infected mouse neuroblastoma cells. Like brain-derived PrP-res, the neuroblastoma cell PrP-res aggregated in detergent lysates, providing evidence that the tendency to aggregate is an intrinsic property of PrP-res and not merely a secondary consequence of degenerative brain pathology. The PrP-res species had lower apparent molecular masses than the normal, protease-sensitive PrP species and were not affected by moderate treatments with proteinase K. This suggested that the PrP-res species were partially proteolyzed by the neuroblastoma cells. Immunoblot analysis of PrP-res with a panel of monospecific anti-PrP peptide sera confirmed that the PrP-res species were quantitatively truncated at the N terminus. The metabolic labeling of PrP-res in serum-free medium did not prevent the proteolysis of PrP-res, showing that the protease(s) involved was cellular rather than serum-derived. The PrP-res truncation was inhibited in intact cells by leupeptin and NH4Cl. This provided evidence that a lysosomal protease(s) was involved, and therefore, that PrP-res was translocated to lysosomes. When considered with other studies, these results imply that the conversion of PrP to the protease-resistant state occurs in the plasma membrane or along an endocytic pathway before PrP-res is exposed to endosomal and lysosomal proteases.

In vitro expression and biosynthesis of prion protein

In addition to whatever function PrP may have normally, its involvement in scrapie-like neurodegenerative diseases has become clearer in recent years. In vitro studies have made important contributions to the understanding of normal PrP biosynthesis and turnover and how they can be influenced by scrapie infection. Cell-free transcription and translation experiments have indicated that PrP gene translation products are capable of assuming two different topologies, one spanning microsomal membranes and the other completely translocated into the microsomal lumen (Hay et al. 1987a, b). A novel stop transfer signal in the polypeptide is critical to the formation of the transmembrane topology (Yost et al. 1990). Expression of recombinant PrP genes has been accomplished in mouse (Caughey et al. 1988b), monkey (Scott et al. 1988), frog (Hay et al. 1987a), and insect (Scott et al. 1988) tissue culture cells. PrP products encoded by PrP cDNAs cloned from scrapie-infected brain tissues are not infectious and do not have the protease-resistance characteristic of the scrapie-associated form of PrP isolated from diseased tissue (Caughey et al. 1988b; Scott et al. 1988). Studies of PrP encoded by the endogenous gene of mouse neuroblastoma cells have identified the precursors (Caughey et al. 1989) and products (Race et al. 1988; Caughey et al. 1989) of normal PrP biosynthesis and shown that most of the PrP of normal cells is linked to the cell surface by phosphatidylinositol (Stahl et al. 1987; Caughey et al. 1989, 1990; Borchelt et al. 1990). In scrapie-infected clones, and additional pool of PrP is present which, unlike the normal PrP, aggregates (B. Caughey, unpublished observations) and is partially protease resistant (Butler et al. 1988; Caughey et al. 1990; Borchelt et al. 1990; Stahl et al. 1990). This scrapie-associated pool of PrP differs from the normal PrP in that it is primarily intracellular (Caughey et al. 1990; Borchelt et al. 1990; Taraboulos et al. 1990) and resistant to removal from cells by phospholipase or protease (Caughey et al. 1990; Borchelt et al. 1990; Stahl et al. 1990) treatments. Kinetic studies have shown that while PrP-sen is synthesized and degraded relatively rapidly (Caughey et al. Borchelt et al. 1990), PrP-res is synthesized slowly and has a very long half-life (Borchelt et al. 1990). Further studies with the scrapie-infected mouse neuroblastoma cells should lead toward the elucidation of the molecular details of the scrapie-associated modification of PrP and whether the modification is directly related to scrapie agent replication.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular biology and transgenetics of prion diseases

Considerable progress has been made deciphering the role of an abnormal isoform of the prion protein (PrP) in scrapie of animals and Gerstmann-Sträussler syndrome (GSS) of humans. Some transgenic (Tg) mouse (Mo) lines that carry and express a Syrian hamster (Ha) PrP gene developed scrapie 75 d after inoculation with Ha prions; non-Tg mice failed to show symptoms after greater than 500 d. Brains of these infected Tg(HaPrP) mice featured protease-resistant HaPrPSc, amyloid plaques characteristic for Ha scrapie, and 10(9) ID50 units of Ha-specific prions upon bioassay. Studies on Syrian, Armenian, and Chinese hamsters suggest that the domain of the PrP molecule between codons 100 and 120 controls both the length of the incubation time and the deposition of PrP in amyloid plaques. Ataxic GSS in families shows genetic linkage to a mutation in the PrP gene, leading to the substitution of Leu for Pro at codon 102. Discovery of a point mutation in the Prp gene from humans with GSS established that GSS is unique among human diseases--it is both genetic and infectious. These results have revised thinking about sporadic Creutzfeldt-Jakob disease, suggesting it may arise from a somatic mutation. These findings combined with those from many other studies assert that PrPSc is a component of the transmissible particle, and the PrP amino acid sequence controls the neuropathology and species specificity of prion infectivity. The precise mechanism of PrPSc formation remains to be established. Attempts to demonstrate a scrapie-specific nucleic acid within highly purified preparations of prions have been unrewarding to date. Whether transmissible prions are composed only of PrPSc molecules or do they also contain a second component such as small polynucleotide remains uncertain.

The search for scrapie agent nucleic acid

Despite decades of research, the identity of the scrapie agent has remained elusive. Recent studies have discovered much about the influence of the host genome upon scrapie infection, yet relatively little is known about the causative agent itself. The predominant hypothesis in the scrapie field (the prion hypothesis) argues that the disease is the result of an infectious protein and that nucleic acid is not required for infection. Biological studies of the scrapie agent, however, suggest that a nucleic acid may be involved in the disease. Sensitive molecular biology techniques have yet to identify this putative nucleic acid.

Normal and scrapie-associated forms of prion protein differ in their sensitivities to phospholipase and proteases in intact neuroblastoma cells

Previous studies have indicated that scrapie infection results in the accumulation of a proteinase K-resistant form of an endogenous brain protein generally referred to as prion protein (PrP). The molecular nature of the scrapie-associated modification of PrP accounting for proteinase K resistance is not known. As an approach to understanding the cellular events associated with the PrP modification in brain tissue, we sought to identify proteinase K-resistant PrP (PrP-res) in scrapie-infected neuroblastoma cells in vitro and to compare properties of PrP-res with those of its normal proteinase K-sensitive homolog, PrP-sen. PrP-res was detected by immunoblot in scrapie-infected but not uninfected neuroblastoma clones. Densitometry of immunoblots indicated that there was two- to threefold more PrP-res than PrP-sen in one infected clone. Metabolic labeling and membrane immunofluorescence experiments indicated that PrP-sen was located on the cell surface and could be removed from intact cells by phosphatidylinositol-specific phospholipase C and proteases. In contrast, PrP-res was not removed after reaction with these enzymes. Thus, either the scrapie-associated PrP-res was not on the cell surface or it was there in a form that is resistant to these hydrolytic enzymes. Attempts to detect intracellular PrP-res by immunofluorescent staining of fixed and permeabilized cells revealed that PrP was present in discrete perinuclear Golgi-like structures. However, the staining pattern was similar in both scrapie-infected and uninfected clones, and thus the intracellular staining may have represented only PrP-sen. Analysis of scrapie infectivity in cells treated with extracellular phospholipase, proteinase K, and trypsin indicated that, like PrP-res, the scrapie agent was not removed from the infected cells by any of these enzymes.

Prion protein biosynthesis in scrapie-infected and uninfected neuroblastoma cells

Numerous studies have indicated that a modified proteinase K-resistant form of an endogenous brain protein, prion protein (PrP), is associated with scrapie infection in animals. This scrapie-associated PrP modification appears to occur posttranslationally in brain, but its molecular nature is not known. To learn about the normal PrP biosynthesis and whether it is altered by scrapie infection in vitro, we did metabolic labeling experiments with uninfected and scrapie-infected mouse neuroblastoma tissue culture cells. Pulse-chase labeling experiments indicated that, in both cell types, two major PrP precursors of 28 and 33 kilodaltons (kDa) were processed to mature 30- and 35- to 41-kDa forms. Endoglycosidase H, tunicamycin, and phospholipase treatments revealed that the 28- and 33-kDa precursors resulted from the addition of high-mannose glycans to a 25-kDa polypeptide containing a phosphatidylinositol moiety and that maturation of the precursors involved the conversion of the high-mannose glycans to hybrid or complex glycans. Treatments of the live cells with trypsin and phosphatidylinositol-specific phospholipase C indicated that the mature PrP species were expressed solely on the cell surface, where they were anchored by covalent linkage to phosphatidylinositol. Once on the cell surface, the major PrP forms had half-lives of 3 to 6 h. No differences in PrP biosynthesis were observed between the scrapie-infected versus uninfected neuroblastoma cells.