Cryptic epitopes in N-terminally truncated prion protein are exposed in the full-length molecule: dependence of conformation on pH

The prion protein is required for normal responses to light stimuli by photoreceptors and bipolar cells

The prion protein, PrPC, is well known as an essential susceptibility factor for neurodegenerative prion diseases, yet its function in normal, healthy cells remains uncertain. A role in synaptic function has been proposed for PrPC, supported by its cell surface expression in neurons and glia. Here, in mouse retina, we localized PrPC to the junctions between photoreceptors and bipolar cells using synaptic proteins EAAT5, CtBP2, and PSD-95. PrPC localized most densely with bipolar cell dendrites synapsing with cone photoreceptors. In two coisogenic mouse strains, deletion of the gene encoding PrPC, Prnp, significantly altered the scotopic and/or photopic electroretinographic (ERG) responses of photoreceptors and bipolar cells. Cone-dominant pathways showed the most significant ERG changes. Retinal thickness, quantitated by high-resolution optical coherence tomography (OCT), and ribbon synapse morphology were not altered upon deletion of PrPC, suggesting that the ERG changes were driven by functional rather than structural alterations.

Efficacy of Wex-cide 128 disinfectant against multiple prion strains

Prion diseases are transmissible, fatal neurologic diseases that include Creutzfeldt-Jakob Disease (CJD) in humans, chronic wasting disease (CWD) in cervids, bovine spongiform encephalopathy (BSE) in cattle and scrapie in sheep. Prions are extremely difficult to inactivate and established methods to reduce prion infectivity are often dangerous, caustic, expensive, or impractical. Identifying viable and safe methods for treating prion contaminated materials is important for hospitals, research facilities, biologists, hunters, and meat-processors. For three decades, some prion researchers have used a phenolic product called Environ LpH (eLpH) to inactivate prions. ELpH has been discontinued, but a similar product, Wex-cide 128, containing the similar phenolic chemicals as eLpH is now available. In the current study, we directly compared the anti-prion efficacy of eLpH and Wex-cide 128 against prions from four different species (hamster 263K, cervid CWD, mouse 22L and human CJD). Decontamination was performed on either prion infected brain homogenates or prion contaminated steel wires and mouse bioassay was used to quantify the remaining prion infectivity. Our data show that both eLpH and Wex-cide 128 removed 4.0-5.5 logs of prion infectivity from 22L, CWD and 263K prion homogenates, but only about 1.25-1.50 logs of prion infectivity from human sporadic CJD. Wex-cide 128 is a viable substitute for inactivation of most prions from most species, but the resistance of CJD to phenolic inactivation is a concern and emphasizes the fact that inactivation methods should be confirmed for each target prion strain.

Deletion of Kif5c Does Not Alter Prion Disease Tempo or Spread in Mouse Brain

In prion diseases, the spread of infectious prions (PrPSc) is thought to occur within nerves and across synapses of the central nervous system (CNS). However, the mechanisms by which PrPSc moves within axons and across nerve synapses remain undetermined. Molecular motors, including kinesins and dyneins, transport many types of intracellular cargo. Kinesin-1C (KIF5C) has been shown to transport vesicles carrying the normal prion protein (PrPC) within axons, but whether KIF5C is involved in PrPSc axonal transport is unknown. The current study tested whether stereotactic inoculation in the striatum of KIF5C knock-out mice (Kif5c^−/−) with 0.5 µL volumes of mouse-adapted scrapie strains 22 L or ME7 would result in an altered rate of prion spreading and/or disease timing. Groups of mice injected with each strain were euthanized at either pre-clinical time points or following the development of prion disease. Immunohistochemistry for PrP was performed on brain sections and PrPSc distribution and tempo of spread were compared between mouse strains. In these experiments, no differences in PrPSc spread, distribution or survival times were observed between C57BL/6 and Kif5c^−/− mice.

Prion-induced photoreceptor degeneration begins with misfolded prion protein accumulation in cones at two distinct sites: cilia and ribbon synapses

Accumulation of misfolded host proteins is central to neuropathogenesis of numerous human brain diseases including prion and prion-like diseases. Neurons of retina are also affected by these diseases. Previously, our group and others found that prion-induced retinal damage to photoreceptor cells in mice and humans resembled pathology of human retinitis pigmentosa caused by mutations in retinal proteins. Here, using confocal, epifluorescent and electron microscopy we followed deposition of disease-associated prion protein (PrPSc) and its association with damage to critical retinal structures following intracerebral prion inoculation. The earliest time and place of retinal PrPSc deposition was 67 days post-inoculation (dpi) on the inner segment (IS) of cone photoreceptors. At 104 and 118 dpi, PrPSc was associated with the base of cilia and swollen cone inner segments, suggesting ciliopathy as a pathogenic mechanism. By 118 dpi, PrPSc was deposited in both rods and cones which showed rootlet damage in the IS, and photoreceptor cell death was indicated by thinning of the outer nuclear layer. In the outer plexiform layer (OPL) in uninfected mice, normal host PrP (PrPC) was mainly associated with cone bipolar cell processes, but in infected mice, at 118 dpi, PrPSc was detected on cone and rod bipolar cell dendrites extending into ribbon synapses. Loss of ribbon synapses in cone pedicles and rod spherules in the OPL was observed to precede destruction of most rods and cones over the next 2–3 weeks. However, bipolar cells and horizontal cells were less damaged, indicating high selectivity among neurons for injury by prions. PrPSc deposition in cone and rod inner segments and on the bipolar cell processes participating in ribbon synapses appear to be critical early events leading to damage and death of photoreceptors after prion infection. These mechanisms may also occur in human retinitis pigmentosa and prion-like diseases, such as AD.

Prion-associated cerebral amyloid angiopathy is not exacerbated by human phosphorylated tau aggregates in scrapie-infected mice expressing anchorless prion protein

Tau aggregates consisting of hyperphosphorylated tau fibrils are associated with many neurodegenerative diseases, including Alzheimer's disease, Pick's disease, frontotemporal dementia, and progressive supranuclear palsy. Tau may contribute to the pathogenesis of these diseases, collectively referred to as tauopathies. In human genetic prion diseases, tau aggregates are detected in association with amyloid plaques consisting of prion protein (PrP). However, the role of abnormal tau aggregates in PrP amyloid disease remains unclear. Previously we inoculated scrapie prions into transgenic mice expressing human tau, mouse tau, glycophosphatidylinositol (GPI) anchored PrP, and anchorless PrP. These mice developed both spongiform vacuolar pathology and PrP amyloid pathology, and human tau was detected near PrP amyloid plaques. However, the presence of human tau did not alter the disease tempo or prion-induced neuropathology. In the present study, we tested mice which more closely modeled familial human prion disease. These mice expressed human tau but lacked both mouse tau and GPI-anchored PrP. However, they did produce anchorless PrP, resulting in perivascular PrP amyloid plaques, i.e. cerebral amyloid angiopathy (CAA), without spongiform degeneration. Typical of PrP amyloid disease, the clinical course was very slow in this model. Nevertheless, the accumulation of aggregated, phosphorylated human tau and its association with PrP amyloid plaques failed to alter the timing or course of the clinical disease observed. These data suggest that human tau does not contribute to the pathogenesis of mouse PrP amyloid brain disease and raise the possibility that tau may also not be pathogenic in human PrP amyloid disease.

Incomplete glycosylation during prion infection unmasks a prion protein epitope that facilitates prion detection and strain discrimination

The causative factors underlying conformational conversion of cellular prion protein (PrP^C) into its infectious counterpart (PrP^Sc) during prion infection remain undetermined, in part because of a lack of monoclonal antibodies (mAbs) that can distinguish these conformational isoforms. Here we show that the anti-PrP mAb PRC7 recognizes an epitope that is shielded from detection when glycans are attached to Asn-196. We observed that whereas PrP^C is predisposed to full glycosylation and is therefore refractory to PRC7 detection, prion infection leads to diminished PrP^Sc glycosylation at Asn-196, resulting in an unshielded PRC7 epitope that is amenable to mAb recognition upon renaturation. Detection of PRC7-reactive PrP^Sc in experimental and natural infections with various mouse-adapted scrapie strains and with prions causing deer and elk chronic wasting disease and transmissible mink encephalopathy uncovered that incomplete PrP^Sc glycosylation is a consistent feature of prion pathogenesis. We also show that interrogating the conformational properties of the PRC7 epitope affords a direct means of distinguishing different prion strains. Because the specificity of our approach for prion detection and strain discrimination relies on the extent to which N-linked glycosylation shields or unshields PrP epitopes from antibody recognition, it dispenses with the requirement for additional standard manipulations to distinguish PrP^Sc from PrP^C, including evaluation of protease resistance. Our findings not only highlight an innovative and facile strategy for prion detection and strain differentiation, but are also consistent with a mechanism of prion replication in which structural instability of incompletely glycosylated PrP contributes to the conformational conversion of PrP^C to PrP^Sc.

Antisense oligonucleotides extend survival of prion-infected mice

Prion disease is a fatal, incurable neurodegenerative disease of humans and other mammals caused by conversion of cellular prion protein (PrP^C) into a self-propagating neurotoxic conformer (prions; PrP^Sc). Strong genetic proofs of concept support lowering PrP expression as a therapeutic strategy. Antisense oligonucleotides (ASOs) can provide a practical route to lowering 1 target mRNA in the brain, but their development for prion disease has been hindered by 3 unresolved issues from prior work: uncertainty about mechanism of action, unclear potential for efficacy against established prion infection, and poor tolerability of drug delivery by osmotic pumps. Here, we test ASOs delivered by bolus intracerebroventricular injection to intracerebrally prion-infected WT mice. Prophylactic treatments given every 2–3 months extended survival times 61%–98%, and a single injection at 120 days after infection, near the onset of clinical signs, extended survival 55% (87 days). In contrast, a nontargeting control ASO was ineffective. Thus, PrP lowering is the mechanism of action of ASOs effective against prion disease in vivo, and infrequent — or even single — bolus injections of ASOs can slow prion neuropathogenesis and markedly extend survival, even when initiated near clinical signs. These findings should empower development of PrP-lowering therapy for prion disease.

ASO-mediated prion protein suppression delays disease and extends survival, even in mice with established prion infection.

Microglia are not required for prion-induced retinal photoreceptor degeneration

Degeneration of photoreceptors in the retina is a major cause of blindness in humans. Often retinal degeneration is due to inheritance of mutations in genes important in photoreceptor (PR) function, but can also be induced by other events including retinal trauma, microvascular disease, virus infection or prion infection. The onset of apoptosis and degeneration of PR neurons correlates with invasion of the PR cellular areas by microglia or monocytes, suggesting a causal role for these cells in pathogenesis of PR degenerative disease. To study the role of microglia in prion-induced retinal disease, we fed prion-infected mice a CSF-1 receptor blocking drug, PLX5622, to eliminate microglia in vivo, and the effects on retinal degeneration were analyzed over time. In mice not receiving drug, the main inflammatory cells invading the degenerating PR areas were microglia, not monocytes. Administration of PLX5622 was highly effective at ablating microglia in retina. However, lack of microglia during prion infection did not prevent degeneration of PR cells. Therefore, microglia were not required for the PR damage process during prion infection. Indeed, mice lacking microglia had slightly faster onset of PR damage. Similar results were seen in C57BL/10 mice and transgenic mice expressing GFP or RFP on microglia and monocytes, respectively. These results were supported by experiments using prion-infected Cx3cr1 knockout mice without PLX5622 treatment, where microglial expansion in retina was delayed, but PR degeneration was not. Contrary to predictions, microglia were not a causative factor in retinal damage by prion infection. Instead, newly generated PrPSc accumulated around the inner segment region of the PR cells and appeared to correlate with initiation of the pathogenic process in the absence of microglia.

Microglia Are Critical in Host Defense against Prion Disease

Microglial cells in the central nervous system play important roles in neurodevelopment and resistance to infection, yet microglia can become neurotoxic under some conditions. An early event during prion infection is the activation of microglia and astrocytes in the brain prior to damage or death of neurons. Previous prion disease studies using two different strategies to manipulate signaling through the microglial receptor CSF-1R reported contrary effects on survival from prion disease. However, in these studies, reductions of microglial numbers and function were variable, thus confounding interpretation of the results. In the present work, we used oral treatment with a potent inhibitor of CSF-1R, PLX5622, to eliminate 78 to 90% of microglia from cortex early during the course of prion infection. Oral drug treatment early after infection with the RML scrapie strain significantly accelerated vacuolation, astrogliosis, and deposition of disease-associated prion protein. Furthermore, drug-treated mice had advanced clinical disease requiring euthanasia 31 days earlier than untreated control mice. Similarly, PLX5622 treatment during the preclinical phase at 80 days postinfection with RML scrapie also accelerated disease and resulted in euthanasia of mice 33 days earlier than infected controls. PLX5622 also accelerated clinical disease after infection with scrapie strains ME7 and 22L. Thus, microglia are critical in host defense during prion disease. The early accumulation of PrPSc in the absence of microglia suggested that microglia may function by clearing PrPSc, resulting in longer survival.IMPORTANCE Microglia contribute to many aspects of health and disease. When activated, microglia can be beneficial by repairing damage in the central nervous system (CNS) or they can turn harmful by becoming neurotoxic. In prion and prionlike diseases, the involvement of microglia in disease is unclear. Previous studies suggest that microglia can either speed up or slow down disease. In this study, we infected mice with prions and depleted microglia from the brains of mice using PLX5622, an effective CSF-1R tyrosine kinase inhibitor. Microglia were markedly reduced in brains, and prion disease was accelerated, so that mice needed to be euthanized 20 to 33 days earlier than infected control mice due to advanced clinical disease. Similar results occurred when mice were treated with PLX5622 at 80 days after infection, which was just prior to the start of clinical signs. Thus, microglia are important for removing prions, and the disease is faster when microglia are depleted.

Phosphorylated human tau associates with mouse prion protein amyloid in scrapie-infected mice but does not increase progression of clinical disease

Tauopathies are a family of neurodegenerative diseases in which fibrils of human hyperphosphorylated tau (P-tau) are believed to cause neuropathology. In Alzheimer disease, P-tau associates with A-beta amyloid and contributes to disease pathogenesis. In familial human prion diseases and variant CJD, P-tau often co-associates with prion protein amyloid, and might also accelerate disease progression. To test this latter possibility, here we compared progression of amyloid prion disease in vivo after scrapie infection of mice with and without expression of human tau. The mice used expressed both anchorless prion protein (PrP) and membrane-anchored PrP, that generate disease associated amyloid and non-amyloid PrP (PrPSc) after scrapie infection. Human P-tau induced by scrapie infection was only rarely associated with non-amyloid PrPSc, but abundant human P-tau was detected at extracellular, perivascular and axonal deposits associated with amyloid PrPSc. This pathology was quite similar to that seen in familial prion diseases. However, association of human and mouse P-tau with amyloid PrPSc did not diminish survival time following prion infection in these mice. By analogy, human P-tau may not affect prion disease progression in humans. Alternatively, these results might be due to other factors, including rapidity of disease, blocking effects by mouse tau, or low toxicity of human P-tau in this model.

Prion Strain Differences in Accumulation of PrPSc on Neurons and Glia Are Associated with Similar Expression Profiles of Neuroinflammatory Genes: Comparison of Three Prion Strains

Misfolding and aggregation of host proteins are important features of the pathogenesis of neurodegenerative diseases including Alzheimer’s disease, Parkinson’s disease, frontotemporal dementia and prion diseases. In all these diseases, the misfolded protein increases in amount by a mechanism involving seeded polymerization. In prion diseases, host prion protein is misfolded to form a pathogenic protease-resistant form, PrPSc, which accumulates in neurons, astroglia and microglia in the CNS. Here using dual-staining immunohistochemistry, we compared the cell specificity of PrPSc accumulation at early preclinical times post-infection using three mouse scrapie strains that differ in brain regional pathology. PrPSc from each strain had a different pattern of cell specificity. Strain 22L was mainly associated with astroglia, whereas strain ME7 was mainly associated with neurons and neuropil. In thalamus and cortex, strain RML was similar to 22L, but in substantia nigra, RML was similar to ME7. Expression of 90 genes involved in neuroinflammation was studied quantitatively using mRNA from thalamus at preclinical times. Surprisingly, despite the cellular differences in PrPSc accumulation, the pattern of upregulated genes was similar for all three strains, and the small differences observed correlated with variations in the early disease tempo. Gene upregulation correlated with activation of both astroglia and microglia detected in early disease prior to vacuolar pathology or clinical signs. Interestingly, the profile of upregulated genes in scrapie differed markedly from that seen in two acute viral CNS diseases (LaCrosse virus and BE polytropic Friend retrovirus) that had reactive gliosis at levels similar to our prion-infected mice.

Accumulation of aggregates of misfolded protein in brain is a common feature of the damage seen in several neurodegenerative diseases including prion disease, Alzheimer’s disease and Parkinson’s disease. In the present work three strains of prion disease differed in accumulation of the disease-associated prion protein (PrPSc) on neurons and astroglial cells. These patterns were first detectable in the thalamus at 40–60 days after inoculation. This coincided with initial detection of gliosis and PrPSc deposition, but was far in advance of clinical signs or spongiform pathology. In spite of the different patterns of cellular PrPSc deposition, these three strains had similar patterns of expression of a large number of genes known to be active during neuroinflammatory responses and gliosis. However, the gene upregulation in scrapie differed markedly from that seen in two neurovirulent viral diseases, which also had abundant glial responses similar to those observed with prion infection.

Early Generation of New PrPSc on Blood Vessels after Brain Microinjection of Scrapie in Mice

Aggregation of misfolded host proteins in the central nervous system is believed to be important in the pathogenic process in several neurodegenerative diseases of humans, including prion diseases, Alzheimer’s disease, and Parkinson’s disease. In these diseases, protein misfolding and aggregation appear to expand through a process of seeded polymerization. Prion diseases occur in both humans and animals and are experimentally transmissible orally or by injection, thus providing a controllable model of other neurodegenerative protein misfolding diseases. In rodents and ruminants, prion disease has a slow course, lasting months to years. Although prion infectivity has been detected in brain tissue at 3 to 4 weeks postinfection (p.i.), the details of early prion replication in the brain are not well understood. Here we studied the localization and quantitation of PrPSc generation in vivo starting at 30 min postmicroinjection of scrapie into the brain. In C57BL mice at 3 days p.i., generation of new PrPSc was detected by immunohistochemistry and immunoblot assays, and at 7 days p.i., new generation was confirmed by real-time quaking-induced conversion assay. The main site of new PrPSc generation was near the outer basement membrane of small and medium blood vessels. The finding and localization of replication at this site so early after injection have not been reported previously. This predominantly perivascular location suggested that structural components of the blood vessel basement membrane or perivascular astrocytes might act as cofactors in the initial generation of PrPSc. The location of PrPSc replication at the basement membrane also implies a role for the brain interstitial fluid drainage in the early infection process.

Neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, and prion diseases, of humans are characterized by misfolding and aggregation of certain proteins, resulting in the destruction of brain tissue. In these diseases, the damage process spreads progressively within the central nervous system, but only prion diseases are known to be transmissible between individuals. Here we used microinjection of infectious prion protein (PrPSc) into the mouse brain to model early events of iatrogenic prion transmission via surgical instruments or tissue grafts. At 3 and 7 days postinjection, we detected the generation of new PrPSc, mostly on the outer walls of blood vessels near the injection site. This location and very early replication were surprising and unique. Perivascular prion replication suggested the transport of injected PrPSc via brain interstitial fluid to the basement membranes of blood vessels, where interactions with possible cofactors made by astrocytes or endothelia might facilitate the earliest cycles of prion infection.

Molecular dynamics simulations capture the misfolding of the bovine prion protein at acidic pH

Bovine spongiform encephalopathy (BSE), or mad cow disease, is a fatal neurodegenerative disease that is transmissible to humans and that is currently incurable. BSE is caused by the prion protein (PrP), which adopts two conformers; PrPC is the native innocuous form, which is α-helix rich; and PrPSc is the β-sheet rich misfolded form, which is infectious and forms neurotoxic species. Acidic pH induces the conversion of PrPC to PrPSc. We have performed molecular dynamics simulations of bovine PrP at various pH regimes. An acidic pH environment induced conformational changes that were not observed in neutral pH simulations. Putative misfolded structures, with nonnative β-strands formed in the flexible N-terminal domain, were found in acidic pH simulations. Two distinct pathways were observed for the formation of nonnative β-strands: at low pH, hydrophobic contacts with M129 nucleated the nonnative β-strand; at mid-pH, polar contacts involving Q168 and D178 facilitated the formation of a hairpin at the flexible N-terminus. These mid- and low pH simulations capture the process of nonnative β-strand formation, thereby improving our understanding of how PrPC misfolds into the β-sheet rich PrPSc and how pH factors into the process.

Distinct patterns of spread of prion infection in brains of mice expressing anchorless or anchored forms of prion protein

Background

In humans and animals, prion protein (PrP) is usually expressed as a glycophosphatidylinositol (GPI)-anchored membrane protein, but anchorless PrP may be pathogenic in humans with certain familial prion diseases. Anchored PrP expressed on neurons mediates spread of prions along axons in the peripheral and central nervous systems. However, the mechanism of prion spread in individuals expressing anchorless PrP is poorly understood. Here we studied prion spread within brain of mice expressing anchorless or anchored PrP.

Results

To create a localized initial point of infection, we microinjected scrapie in a 0.5 microliter volume in the striatum. In this experiment, PrPres and gliosis were first detected in both types of mice at 40 days post-inoculation near the needle track. In mice with anchored PrP, PrPres appeared to spread via neurons to distant connected brain areas by the clinical endpoint at 150 days post-inoculation. This PrPres was rarely associated with blood vessels. In contrast, in mice with anchorless PrP, PrPres spread did not follow neuronal circuitry, but instead followed a novel slower pattern utilizing the drainage system of the brain interstitial fluid (ISF) including perivascular areas adjacent to blood vessels, subependymal areas and spaces between axons in white matter tracts.

Conclusions

In transgenic mice expressing anchorless PrP small amyloid-seeding PrPres aggregates appeared to be transported in the ISF, thus spreading development of cerebral amyloid angiopathy (CAA) throughout the brain. Spread of amyloid seeding by ISF may also occur in multiple human brain diseases involving CAA.

Unusual cerebral vascular prion protein amyloid distribution in scrapie-infected transgenic mice expressing anchorless prion protein

BACKGROUND

In some prion diseases, misfolded aggregated protease-resistant prion protein (PrPres) is found in brain as amyloid, which can cause cerebral amyloid angiopathy. Small diffusible precursors of PrPres amyloid might flow with brain interstitial fluid (ISF), possibly accounting for the perivascular and intravascular distribution of PrPres amyloid. We previously reported that PrPres amyloid in scrapie-infected transgenic mice appeared to delay clearance of microinjected brain ISF tracer molecules.

RESULTS

Here we studied distribution of PrPres amyloid on capillaries, arteries and veins to test whether vascular specificity of PrPres corresponded to distribution of ISF tracer molecules. To distinguish PrPres-positive arteries from veins and capillaries, scrapie-infected mouse brains were studied by immunodetection of alpha smooth muscle actin. ISF was studied using fluorescein-labeled ovalbumin microinjected into brain as a tracer. In infected preclinical or clinical mice, PrPres was found mostly on capillaries (73-78%). Lower levels were found on arteries (11-14%) and veins (11-13%). Compared to PrPres, ISF tracer was found at higher levels on capillaries (96-97%), and the remaining tracer was found at a skewed ratio of 4 to 1 on arteries and veins respectively.

CONCLUSIONS

PrPres association with blood vessels suggested that ISF flow might transport diffusible PrPres precursor molecules to perivascular sites. However, the different vascular specificity of PrPres and ISF tracer indicated that ISF flow did not alone control PrPres dissemination. Possibly blood vessel basement membrane (BM) components, such as glucosaminoglycans, might concentrate small PrPres aggregates and serve as scaffolds for PrP conversion on multiple vessel types.

Non-amyloid and amyloid prion protein deposits in prion-infected mice differ in blockage of interstitial brain fluid

AIMS

Prion diseases are characterized by brain deposits of misfolded aggregated protease-resistant prion protein (PrP), termed PrPres. In humans and animals, PrPres is found as either disorganized non-amyloid aggregates or organized amyloid fibrils. Both PrPres forms are found in extracellular spaces of the brain. Thus, both might block drainage of brain interstitial fluid (ISF). The present experiments studied whether ISF blockage occurred during amyloid and/or non-amyloid prion diseases.

METHODS

Various-sized fluorescein-labelled ISF tracers were stereotactically inoculated into the striatum of adult mice. At times from 5 min to 77 h, uninfected and scrapie-infected mice were compared. C57BL/10 mice expressing wild-type anchored PrP, which develop non-amyloid PrPres similar to humans with sporadic Creutzfeldt-Jakob disease, were compared with Tg44+/+ mice (transgenic mice secreting anchorless PrP) expressing anchorless PrP, which develop amyloid PrPres similar to certain human familial prion diseases.

RESULTS

In C57BL/10 mice, extensive non-amyloid PrPres aggregate deposition was not associated with abnormal clearance kinetics of tracers. In contrast, scrapie-infected Tg44+/+ mice showed blockage of tracer clearance and colocalization of tracer with perivascular PrPres amyloid.

CONCLUSIONS

As tracer localization and clearance was normal in infected C57BL/10 mice, ISF blockage was not an important pathogenic mechanism in this model. Therefore, ISF blockage is unlikely to be a problem in non-amyloid human prion diseases such as sporadic Creutzfeldt-Jakob disease. In contrast, partial ISF blockage appeared to be a possible pathogenic mechanism in Tg44+/+ mice. Thus this mechanism might also influence human amyloid prion diseases where expression of anchorless or mutated PrP results in perivascular amyloid PrPres deposition and cerebral amyloid angiopathy.

Early cytokine elevation, PrPres deposition, and gliosis in mouse scrapie: no effect on disease by deletion of cytokine genes IL-12p40 and IL-12p35

Neurodegenerative diseases are typically associated with an activation of glia and an increased level of cytokines. In our previous studies of prion disease, the cytokine response in the brains of clinically sick scrapie-infected mice was restricted to a small group of cytokines, of which IL-12p40, CCL2, and CXCL10 were present at the highest levels. The goal of our current research was to determine the relationship between cytokine responses, gliosis, and neuropathology during prion disease. Here, in time course studies of C57BL/10 mice intracerebrally inoculated with 22L scrapie, abnormal protease-resistant prion protein (PrPres), astrogliosis, and microgliosis were first detected at 40 days after intracerebral scrapie inoculation. In cytokine studies, IL-12p40 was first elevated by 60 days; CCL3, IL-1β, and CXCL1 were elevated by 80 days; and CCL2 and CCL5 were elevated by 115 days. IL-12p40 showed the most extensive increase throughout disease and was 30-fold above control levels at the terminal stage. Because of the early onset and dramatic elevation of IL-12p40 during scrapie, we investigated whether IL-12p40 contributed to the development of prion disease neuropathogenesis by using three different scrapie strains (22L, RML, 79A) to infect knockout mice in which the gene encoding IL-12p40 was deleted. We also studied knockout mice lacking IL-12p35, which combines with IL-12p40 to form active IL-12 heterodimers. In all instances, knockout mice did not differ from control mice in survival time, clinical tempo, or levels of spongiosis, gliosis, or PrPres in the brain. Thus, neither IL-12p40 nor IL-12p35 molecules were required for prion disease-associated neurodegeneration or neuroinflammation.

Environmental factors preceding aβ40 monomer to oligomers and the detection of oligomers in Alzheimer's disease patient serum

We present here environmental factors including pH shifts, temperature, and metal ions surrounding Aβ40 monomer to precede the oligomers. We also suggest a new idea to detect Aβ40 oligomers with anti-Aβ40 monoclonal antibody using enzyme-linked immunosorbent assay. This method involves the different sensitivity of the thermal shifts between Aβ40 monomer and the oligomers. The idea is useful for the diagnostics of Alzheimer's disease to detect Aβ40 oligomers in the serum from the patients.

Allosteric function and dysfunction of the prion protein

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases associated with progressive oligo- and multimerization of the prion protein (PrP(C)), its conformational conversion, aggregation and precipitation. We recently proposed that PrP(C) serves as a cell surface scaffold protein for a variety of signaling modules, the effects of which translate into wide-range functional consequences. Here we review evidence for allosteric functions of PrP(C), which constitute a common property of scaffold proteins. The available data suggest that allosteric effects among PrP(C) and its partners are involved in the assembly of multi-component signaling modules at the cell surface, impose upon both physiological and pathological conformational responses of PrP(C), and that allosteric dysfunction of PrP(C) has the potential to entail progressive signal corruption. These properties may be germane both to physiological roles of PrP(C), as well as to the pathogenesis of the TSEs and other degenerative/non-communicable diseases.

Role of cyclophilin A from brains of prion-infected mice in stimulation of cytokine release by microglia and astroglia in vitro

Prion diseases or transmissible spongiform encephalopathy diseases are typically characterized by deposition of abnormally folded partially protease-resistant host-derived prion protein (PrPres), which is associated with activated glia and increased release of cytokines. This neuroinflammatory response may play a role in transmissible spongiform encephalopathy pathogenesis. We previously reported that brain homogenates from prion-infected mice induced cytokine protein release in primary astroglial and microglial cell cultures. Here we measured cytokine release by cultured glial cells to determine what factors in infected brain contributed to activation of microglia and astroglia. In assays analyzing IL-12p40 and CCL2 (MCP-1), glial cells were not stimulated in vitro by either PrPres purified from infected mouse brains or prion protein amyloid fibrils produced in vitro. However, significant glial stimulation was induced by clarified scrapie brain homogenates lacking PrPres. This stimulation was greatly reduced both by antibody to cyclophilin A (CyPA), a known mediator of inflammation in peripheral tissues, and by cyclosporine A, a CyPA inhibitor. In biochemical studies, purified truncated CyPA fragments stimulated a pattern of cytokine release by microglia and astroglia similar to that induced by scrapie-infected brain homogenates, whereas purified full-length CyPA was a poor stimulator. This requirement for CyPA truncation was not reported in previous studies of stimulation of peripheral macrophages, endothelial cell cardiomyocytes, and vascular smooth muscle cells. Therefore, truncated CyPA detected in brain following prion infection may have an important role in the activation of brain-derived primary astroglia and microglia in prion disease and perhaps other neurodegenerative or neuroinflammatory diseases.

Strain specific resistance to murine scrapie associated with a naturally occurring human prion protein polymorphism at residue 171

Transmissible spongiform encephalopathies (TSE) or prion diseases are neurodegenerative disorders associated with conversion of normal host prion protein (PrP) to a misfolded, protease-resistant form (PrPres). Genetic variations of prion protein in humans and animals can alter susceptibility to both familial and infectious prion diseases. The N171S PrP polymorphism is found mainly in humans of African descent, but its low incidence has precluded study of its possible influence on prion disease. Similar to previous experiments of others, for laboratory studies we created a transgenic model expressing the mouse PrP homolog, PrP-170S, of human PrP-171S. Since PrP polymorphisms can vary in their effects on different TSE diseases, we tested these mice with four different strains of mouse-adapted scrapie. Whereas 22L and ME7 scrapie strains induced typical clinical disease, neuropathology and accumulation of PrPres in all transgenic mice at 99-128 average days post-inoculation, strains RML and 79A produced clinical disease and PrPres formation in only a small subset of mice at very late times. When mice expressing both PrP-170S and PrP-170N were inoculated with RML scrapie, dominant-negative inhibition of disease did not occur, possibly because interaction of strain RML with PrP-170S was minimal. Surprisingly, in vitro PrP conversion using protein misfolding cyclic amplification (PMCA), did not reproduce the in vivo findings, suggesting that the resistance noted in live mice might be due to factors or conditions not present in vitro. These findings suggest that in vivo conversion of PrP-170S by RML and 79A scrapie strains was slow and inefficient. PrP-170S mice may be an example of the conformational selection model where the structure of some prion strains does not favor interactions with PrP molecules expressing certain polymorphisms.

Transmissible spongiform encephalopathies (TSE) or prion diseases are infectious fatal neurological diseases that affect many mammals, including humans. In these diseases a misfolded form of host prion protein (PrP) leads to brain degeneration and death. The genetic code of PrP in individual animals or humans has minor variations, which in some cases are associated with altered susceptibility to disease. In humans a variation at residue 171 (N171S) has been found in people mainly of African descent. However, due to the low incidence of the variation and difficult accessibility of these individuals, studies of prion diseases in these populations have not been carried out. Therefore, to create a laboratory animal model to study the effect of this variation on prion diseases, we generated transgenic mice expressing the mouse version of the human PrP variation at residue 171. We then studied the susceptibility of these mice to 4 strains of mouse-adapted scrapie. In our experiments these transgenic mice were uniquely resistant to two scrapie strains, but showed high sensitivity to two others. This resistance appeared to be related to a slow or inefficient generation of the aggregated disease-associated form of PrP in these mice, and was not duplicated using in vitro assays. In summary, transgenic mice expressing this variant PrP provide an interesting model to study differences among prion strains and their interactions with PrP in vivo.

The prion protein binds thiamine

Although highly conserved throughout evolution, the exact biological function of the prion protein is still unclear. In an effort to identify the potential biological functions of the prion protein we conducted a small-molecule screening assay using the Syrian hamster prion protein [shPrP(90-232)]. The screen was performed using a library of 149 water-soluble metabolites that are known to pass through the blood-brain barrier. Using a combination of 1D NMR, fluorescence quenching and surface plasmon resonance we identified thiamine (vitamin B1) as a specific prion ligand with a binding constant of ~60 μM. Subsequent studies showed that this interaction is evolutionarily conserved, with similar binding constants being seen for mouse, hamster and human prions. Various protein construct lengths, both with and without the unstructured N-terminal region in the presence and absence of copper, were examined. This indicates that the N-terminus has no influence on the protein's ability to interact with thiamine. In addition to thiamine, the more biologically abundant forms of vitamin B1 (thiamine monophosphate and thiamine diphosphate) were also found to bind the prion protein with similar affinity. Heteronuclear NMR experiments were used to determine thiamine's interaction site, which is located between helix 1 and the preceding loop. These data, in conjunction with computer-aided docking and molecular dynamics, were used to model the thiamine-binding pharmacophore and a comparison with other thiamine binding proteins was performed to reveal the common features of interaction.

Detailed biophysical characterization of the acid-induced PrP(c) to PrP(β) conversion process

Prions are believed to spontaneously convert from a native, monomeric highly helical form (called PrP(c)) to a largely β-sheet-rich, multimeric and insoluble aggregate (called PrP(sc)). Because of its large size and insolubility, biophysical characterization of PrP(sc) has been difficult, and there are several contradictory or incomplete models of the PrP(sc) structure. A β-sheet-rich, soluble intermediate, called PrP(β), exhibits many of the same features as PrP(sc) and can be generated using a combination of low pH and/or mild denaturing conditions. Studies of the PrP(c) to PrP(β) conversion process and of PrP(β) folding intermediates may provide insights into the structure of PrP(sc). Using a truncated, recombinant version of Syrian hamster PrP(β) (shPrP(90-232)), we used NMR spectroscopy, in combination with other biophysical techniques (circular dichroism, dynamic light scattering, electron microscopy, fluorescence spectroscopy, mass spectrometry, and proteinase K digestion), to characterize the pH-driven PrP(c) to PrP(β) conversion process in detail. Our results show that below pH 2.8 the protein oligomerizes and conversion to the β-rich structure is initiated. At pH 1.7 and above, the oligomeric protein can recover its native monomeric state through dialysis to pH 5.2. However, when conversion is completed at pH 1.0, the large oligomer "locks down" irreversibly into a stable, β-rich form. At pH values above 3.0, the protein is amenable to NMR investigation. Chemical shift perturbations, NOE, amide line width, and T(2) measurements implicate the putative "amylome motif" region, "NNQNNF" as the region most involved in the initial helix-to-β conversion phase. We also found that acid-induced PrP(β) oligomers could be converted to fibrils without the use of chaotropic denaturants. The latter finding represents one of the first examples wherein physiologically accessible conditions (i.e., only low pH) were used to achieve PrP conversion and fibril formation.

Influence of pH on the human prion protein: insights into the early steps of misfolding

Transmissible spongiform encephalopathies, or prion diseases, are caused by misfolding and aggregation of the prion protein PrP. Conversion from the normal cellular form (PrP(C)) or recombinant PrP (recPrP) to a misfolded form is pH-sensitive, in that misfolding and aggregation occur more readily at lower pH. To gain more insight into the influence of pH on the dynamics of PrP and its potential to misfold, we performed extensive molecular-dynamics simulations of the recombinant PrP protein (residues 90-230) in water at three different pH regimes: neutral (or cytoplasmic) pH (∼7.4), middle (or endosomal) pH (∼5), and low pH (<4). We present five different simulations of 50 ns each for each pH regime, amounting to a total of 750 ns of simulation time. A detailed analysis and comparison with experiment validate the simulations and lead to new insights into the mechanism of pH-induced misfolding. The mobility of the globular domain increases with decreasing pH, through displacement of the first helix and instability of the hydrophobic core. At middle pH, conversion to a misfolded (PrP(Sc)-like) conformation is observed. The observed changes in conformation and stability are consistent with experimental data and thus provide a molecular basis for the initial steps in the misfolding process.

Extraneural manifestations of prion infection in GPI-anchorless transgenic mice

Earlier studies indicated that transgenic (tg) mice engineered to express prion protein (PrP) lacking the glycophosphatidylinositol (GPI⁻/⁻) membrane anchor formed abnormal proteinase-resistant prion (PrPsc) amyloid deposits in their brains and hearts when infected with the RML strain of murine scrapie. In contrast, RML scrapie infection of normal mice with a GPI-anchored PrP did not deposit amyloid with PrPsc in the brain or the heart. Here we report that scrapie-infected GPI⁻/⁻ PrP tg mice also deposit PrP and transmissible infectious material in the gut, kidneys, and islets of Langerhans. Similar to previously reported amyloid deposits in the brain and heart, amyloid deposits were found in the gut; however, no amyloid deposited in the islets. By high-resolution electron microscopy, we show PrP is located primarily in α cells and also β cells. Islets contain abundant insulin and there is no abnormality in glucose metabolism in infected GPI⁻/⁻ PrP tg mice.

Molecular dynamics as an approach to study prion protein misfolding and the effect of pathogenic mutations

Computer simulation of protein dynamics offers unique high-resolution information that complements experiment. Using experimentally derived structures of the natively folded prion protein (PrP), physically realistic dynamics and conformational changes can be simulated, including the initial steps of misfolding. By introducing mutations in silico, the effect of pathogenic mutations on PrP conformation and dynamics can be assessed. Here, we briefly introduce molecular dynamics methods and review the application of molecular dynamics simulations to obtain insight into various aspects of the PrP, including the mechanism of misfolding, the response to changes in the environment, and the influence of disease-related mutations.

Susceptibilities of nonhuman primates to chronic wasting disease

A species barrier may protect humans from this disease.

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy, or prion disease, that affects deer, elk, and moose. Human susceptibility to CWD remains unproven despite likely exposure to CWD-infected cervids. We used 2 nonhuman primate species, cynomolgus macaques and squirrel monkeys, as human models for CWD susceptibility. CWD was inoculated into these 2 species by intracerebral and oral routes. After intracerebral inoculation of squirrel monkeys, 7 of 8 CWD isolates induced a clinical wasting syndrome within 33–53 months. The monkeys’ brains showed spongiform encephalopathy and protease-resistant prion protein (PrPres) diagnostic of prion disease. After oral exposure, 2 squirrel monkeys had PrPres in brain, spleen, and lymph nodes at 69 months postinfection. In contrast, cynomolgus macaques have not shown evidence of clinical disease as of 70 months postinfection. Thus, these 2 species differed in susceptibility to CWD. Because humans are evolutionarily closer to macaques than to squirrel monkeys, they may also be resistant to CWD.

Detection of prion infectivity in fat tissues of scrapie-infected mice

Distribution of prion infectivity in organs and tissues is important in understanding prion disease pathogenesis and designing strategies to prevent prion infection in animals and humans. Transmission of prion disease from cattle to humans resulted in banning human consumption of ruminant nervous system and certain other tissues. In the present study, we surveyed tissue distribution of prion infectivity in mice with prion disease. We show for the first time detection of infectivity in white and brown fat. Since high amounts of ruminant fat are consumed by humans and also incorporated into animal feed, fat-containing tissues may pose a previously unappreciated hazard for spread of prion infection.

Prion diseases, also known as transmissible spongiform encephalopathies, are infectious progressive fatal neurodegenerative diseases which affect humans as well as wild and domestic animals. Distribution of prion infectivity in organs and tissues is important in understanding prion disease pathogenesis and designing strategies to prevent prion infection in animals and humans. We show for the first time the presence of prion infectivity in white fat and brown fat tissues of mice with prion disease. Our results suggest that fat tissues of domestic or wild animals infected with prions may pose an unappreciated hazard for spread of infection to humans or domestic animals. The presence of prion infectivity in fat suggests that additional consideration may be required to eliminate from the food chain any fat from ruminants suspected of exposure to or infection with prions. Thus, this finding has implications for public health, food safety, and prion disease prevention strategies.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

Phosphorothioate oligonucleotides reduce PrP levels and prion infectivity in cultured cells

Prions are composed solely of the disease-causing prion protein (PrPSc) that is formed from the cellular isoform PrPC by a posttranslational process. Here we report that short phosphorothioate DNA (PS-DNA) oligonucleotides diminished the levels of both PrPC and PrPSc in prion-infected neuroblastoma (ScN2a) cells. The effect of PS-DNA on PrP levels was independent of the nucleotide sequence. The effective concentration (EC50) of PS-DNA required to achieve half-maximal diminution of PrPSc was approximately 70 nM, whereas the EC50 of PS-DNA for PrPC was more than 50-fold greater. This finding indicated that diminished levels of PrPSc after exposure to PS-DNA are unlikely to be due to decreased PrPC levels. Bioassays in transgenic mice demonstrated a substantial diminution in the prion infectivity after ScN2a cells were exposed to PS-DNAs. Whether PS-DNA will be useful in the treatment of prion disease in people or livestock remains to be established.

Phosphorothioate oligonucleotides reduce PrP levels and prion infectivity in cultured cells

Prions are composed solely of the disease-causing prion protein (PrPSc) that is formed from the cellular isoform PrPC by a posttranslational process. Here we report that short phosphorothioate DNA (PS-DNA) oligonucleotides diminished the levels of both PrPC and PrPSc in prion-infected neuroblastoma (ScN2a) cells. The effect of PS-DNA on PrP levels was independent of the nucleotide sequence. The effective concentration (EC50) of PS-DNA required to achieve half-maximal diminution of PrPSc was approximately 70 nM, whereas the EC50 of PS-DNA for PrPC was more than 50-fold greater. This finding indicated that diminished levels of PrPSc after exposure to PS-DNA are unlikely to be due to decreased PrPC levels. Bioassays in transgenic mice demonstrated a substantial diminution in the prion infectivity after ScN2a cells were exposed to PS-DNAs. Whether PS-DNA will be useful in the treatment of prion disease in people or livestock remains to be established.

Identification of pH-sensitive regions in the mouse prion by the cysteine-scanning spin-labeling ESR technique

We analyzed the pH-induced mobility changes in moPrP(C) alpha-helix and beta-sheets by cysteine-scanning site-directed spin labeling (SDSL) with ESR. Nine amino acid residues of alpha-helix1 (H1, codon 143-151), four amino acid residues of beta-sheet1 (S1, codon 127-130), and four amino acid residues of beta-sheet2 (S2, codon 160-163) were substituted for by cysteine residues. These recombinant mouse PrP(C) (moPrP(C)) mutants were reacted with a methane thiosulfonate sulfhydryl-specific spin labeling reagent (MTSSL). The 1/deltaH of the central (14N hyperfine) component (M(I) = 0) in the ESR spectrum of spin-labeled moPrP(C) was measured as a mobility parameter of nitroxide residues (R1). The mobilities of E145R1 and Y149R1 at pH 7.4, which was identified as a tertiary contact site by a previous NMR study of moPrP, were lower than those of D143R1, R147R1, and R150R1 reported on the helix surface. Thus, the mobility in the H1 region in the neutral solution was observed with the periodicity associated with a helical structure. On the other hand, the values in the S2 region, known to be located in the buried side, were lower than those in the S1 region located in the surface side. These results indicated that the mobility parameter of the nitroxide label was well correlated with the 3D structure of moPrP. Furthermore, the present study clearly demonstrated three pH-sensitive sites in moPrP, i.e., (1) the N-terminal tertiary contact site of H1, (2) the C-terminal end of H1, and (3) the S2 region. In particular, among these pH-sensitive sites, the N-terminal tertiary contact region of H1 was found to be the most pH-sensitive one and was easily converted to a flexible structure by a slight decrease of pH in the solution. These data provided molecular evidence to explain the cellular mechanism for conversion from PrP(C) to PrP(Sc) in acidic organelles such as the endosome.

Green fluorescent protein as a reporter of prion protein folding

Background

The amino terminal half of the cellular prion protein PrP^c is implicated in both the binding of copper ions and the conformational changes that lead to disease but has no defined structure. However, as some structure is likely to exist we have investigated the use of an established protein refolding technology, fusion to green fluorescence protein (GFP), as a method to examine the refolding of the amino terminal domain of mouse prion protein.

Results

Fusion proteins of PrP^c and GFP were expressed at high level in E.coli and could be purified to near homogeneity as insoluble inclusion bodies. Following denaturation, proteins were diluted into a refolding buffer whereupon GFP fluorescence recovered with time. Using several truncations of PrP^c the rate of refolding was shown to depend on the prion sequence expressed. In a variation of the format, direct observation in E.coli, mutations introduced randomly in the PrP^c protein sequence that affected folding could be selected directly by recovery of GFP fluorescence.

Conclusion

Use of GFP as a measure of refolding of PrP^c fusion proteins in vitro and in vivo proved informative. Refolding in vitro suggested a local structure within the amino terminal domain while direct selection via fluorescence showed that as little as one amino acid change could significantly alter folding. These assay formats, not previously used to study PrP folding, may be generally useful for investigating PrP^c structure and PrP^c-ligand interaction.

Modification of blood cell PrP epitope exposure during prion disease

PrPC [normal cellular PrP (prion-related protein)] is a glycosylphosphatidylinositol-linked cell-surface glycoprotein that is expressed primarily by cells of the central and peripheral nervous system and the lymphoreticular system. During prion disease, PrPC undergoes structural modification to PrPSc (abnormal disease-specific conformation of PrP). The appearance of prion infectivity and PrPSc within different peripheral lymphoid tissue sites during natural scrapie infection in sheep is suggestive of haematogenic dissemination. For this to occur, blood cells may harbour or carry disease-associated PrP and in doing so present altered conformations of PrP on their cell-surface. In the present study, we show that changes in PrP epitope expression, or accessibility, can be detected on peripheral blood mononuclear cells during the course of experimental scrapie in susceptible sheep. Peripheral blood mononuclear cells isolated from VRQ homozygous lambs inoculated orally with scrapie were probed with either N- or C-terminal-specific anti-PrP monoclonal antibodies and analysed by flow cytometry. During the progression of scrapie, significant alterations were seen in the exposure of particular cell-surface PrP epitopes. These modifications included increased accessibility to N-terminal regions of the PrP molecule, to the region between beta-strand-2 and residue 171, and to the C-terminal region of helix-3. Increased accessibility in the globular C-terminal domain of PrP occurred in the vicinity of tyrosine dimers, which are believed to have increased solvent exposure in disease-associated PrP. We suggest that the alterations in anti-PrP monoclonal antibody recognition of cell-surface PrP on blood cells from scrapie-infected sheep are indicative of structural changes within this molecule that may be relevant to prion disease.

Local environmental effects on the structure of the prion protein

Prion diseases are neurodegenerative diseases causally linked to the partial unfolding and subsequent misfolding and aggregation of the prion protein (PrP). While most proteins fold into a single low energy state, PrP can fold into two distinct isoforms. In its innocuous state, denoted as PrPC, the protein has predominantly alpha-helical secondary structure, however, PrPC can misfold into an isoform rich in extended structure capable of forming toxic and infectious aggregates. While prion disease is believed to be a protein-only disease, one not requiring any non-protein elements for propagation, the different environments the protein finds itself in vivo likely influence its ability to misfold and aggregate. In this review we will examine various molecules, covalent modifications and environments PrP faces in vivo and the effect they have on PrP's local environment and, potentially, conformation. Included in this discussion are: (1) pH, (2) carbohydrates, (3) lipid membranes, (4) metal ions, and (5) small molecules.

An improved Western blot assay to assess the clearance of prion protein from plasma-derived therapeutic proteins

Specific detection of the pathogenic prion protein, PrP(Sc), is essential for determining the prion clearance capacity of purification processes for therapeutic proteins. Use of a previously described indirect (two-antibody) Western blot assay sometimes resulted in the appearance of non-specific protein bands that interfered with the detection of small amounts of PrP(Sc)-specific signal, limiting the amount of clearance that could be determined for steps so affected. It is shown that these non-specific signals are due to the interaction between immunoglobulin fragments in the sample and the secondary antibody used in the assay. To circumvent this problem, a direct Western blot assay using a prion-specific primary antibody conjugated to the reporter enzyme alkaline phosphatase was developed. Application of the direct Western blot assay resulted in a significant reduction of non-specific signal while retaining the detection sensitivity for PrP(Sc)-specific signal. Therefore, the direct Western blot assay format is an improved tool for determining prion clearance capacity, particularly for immunoglobulin-rich samples.

Probing the origins, diagnosis and treatment of amyloid diseases using antibodies

The deposition of proteins in the form of amyloid fibrils is the characteristic feature of more than 20 medical conditions affecting the central nervous system or a variety of peripheral tissues. These disorders, which include Alzheimer's disease, the prion diseases and type II diabetes, are of enormous importance in the context of present-day human health and welfare. Extensive research is therefore being carried out to define the molecular details of the mechanism of the pathological conversion of amyloidogenic proteins from their soluble forms into fibrillar structures. This review focuses on recent studies that demonstrate the power of using antibodies or antibody fragments to probe the process of fibril formation, and discusses the emerging potential of these species as diagnostic and therapeutic agents.

Propagation of a protease-resistant form of prion protein in long-term cultured human glioblastoma cell line T98G

Human prion diseases, such as Creutzfeldt–Jakob disease (CJD), a lethal, neurodegenerative condition, occur in sporadic, genetic and transmitted forms. CJD is associated with the conversion of normal cellular prion protein (PrPC) into a protease-resistant isoform (PrPres). The mechanism of the conversion has not been studied in human cell cultures, due to the lack of a model system. In this study, such a system has been developed by culturing cell lines. Human glioblastoma cell line T98G had no coding-region mutations of the prion protein gene, which was of the 129 M/V genotype, and expressed endogenous PrPC constitutively. T98G cells produced a form of proteinase K (PK)-resistant prion protein fragment following long-term culture and high passage number; its deglycosylated form was approximately 18 kDa. The PK-treated PrPres was detected by immunoblotting with the mAb 6H4, which recognizes residues 144–152, and a polyclonal anti-C-terminal antibody, but not by the mAb 3F4, which recognizes residues 109–112, or the anti-N-terminal mAb HUC2-13. These results suggest that PrPC was converted into a proteinase-resistant form of PrPres in T98G cells.

Accumulation of prion protein in muscle fibers of experimental chloroquine myopathy: in vivo model for deposition of prion protein in non-neuronal tissues

Prion protein (PrP) is known to accumulate in some non-neuronal tissues under conditions unrelated to prion diseases. The biochemical and biological nature of such accumulated PrP molecules, however, has not been fully evaluated. In this study, we established experimental myopathy in hamsters by long-term administration of chloroquine, and we examined the nature of the PrP molecules that accumulated. PrP accumulation was immunohistochemically demonstrated in autophagic vacuoles in degenerated muscle fibers, and this was accompanied by the accumulation of other molecules related to the neuropathogenesis of prion diseases such as clathrin, cathepsin B, heparan sulfate, and apolipoprotein J. Accumulated PrP molecules were partially insoluble in detergent solution and were slightly less sensitive to proteinase K digestion than normal cellular PrP. Muscle homogenates containing these PrP molecules did not cause disease in inoculated hamsters. The findings indicate that the PrP molecules that accumulated in muscle fibers have distinct biochemical and biological properties. Therefore, experimental chloroquine myopathy is a novel and useful model to investigate the mechanism of deposition of PrP in non-neuronal tissues and might provide new insights in the pathogenesis of prion diseases.

Scrapie protein degradation by cysteine proteases in CD11c+ dendritic cells and GT1-1 neuronal cells

Dendritic cells (DC) of the CD11c(+) myeloid phenotype have been implicated in the spread of scrapie in the host. Previously, we have shown that CD11c(+) DC can cause a rapid degradation of proteinase K-resistant prion proteins (PrP(Sc)) in vitro, indicating a possible role of these cells in the clearance of PrP(Sc). To determine the mechanisms of PrP(Sc) degradation, CD11c(+) DC that had been exposed to PrP(Sc) derived from a neuronal cell line (GT1-1) infected with scrapie (ScGT1-1) were treated with a battery of protease inhibitors. Following treatment with the cysteine protease inhibitors (2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane (E-64c), its ethyl ester (E-64d), and leupeptin, the degradation of PrP(Sc) was inhibited, while inhibitors of serine and aspartic and metalloproteases (aprotinin, pepstatin, and phosphoramidon) had no effect. An endogenous degradation of PrP(Sc) in ScGT1-1 cells was revealed by inhibiting the expression of cellular PrP (PrP(C)) by RNA interference, and this degradation could also be inhibited by the cysteine protease inhibitors. Our data show that PrP(Sc) is proteolytically cleaved preferentially by cysteine proteases in both CD11c(+) DC and ScGT1-1 cells and that the degradation of PrP(Sc) by proteases is different from that of PrP(C). Interference by protease inhibitors with DC-induced processing of PrP(Sc) has the potential to modify prion spread, clearance, and immunization in a host.

Phosphorylation of the amino terminus of maize sucrose synthase in relation to membrane association and enzyme activity

Sucrose synthase (SUS) is phosphorylated on a major, amino-terminal site located at Ser-15 (S15) in the maize (Zea mays) SUS1 protein. Site- and phospho-specific antibodies against a phosphorylated S15 (pS15) peptide allowed direct analysis of S15 phosphorylation in relation to membrane association. Immunoblots of the maize leaf elongation zone, divided into 4-cm segments, demonstrated that the abundance of soluble (s-SUS) and membrane (m-SUS) SUS protein showed distinct positional profiles. The content of m-SUS was maximal in the 4- to 8-cm segment where it represented 9% of total SUS and occurred as a peripheral membrane protein. In contrast, s-SUS was highest in the 12- to 16-cm segment. Relative to s-SUS, m-SUS was hypophosphorylated at S15 in the basal 4 cm but hyperphosphorylated in apical segments. Differing capabilities of the anti-pS15 and anti-S15 peptide antibodies to immunoprecipitate SUS suggested that phosphorylation of S15, or exposure of unphosphorylated SUS to slightly acidic pH, altered the structure of the amino terminus. These structural changes were generally coincident with the increased sucrose cleavage activity that occurs at pH values below 7.5. In vitro S15 phosphorylation of the S170A SUS protein by a maize calcium-dependent protein kinase (CDPK) significantly increased sucrose cleavage activity at low pH. Collectively, the results suggest that (1) SUS membrane binding is controlled in vivo; (2) relative pS15 content of m-SUS depends on the developmental state of the organ; and (3) phosphorylation of S15 affects amino-terminal conformation in a way that may stimulate the catalytic activity of SUS and influence membrane association.

Amino terminal interaction in the prion protein identified using fusion to green fluorescent protein

In contrast to the well-characterized carboxyl domain, the amino terminal half of the mature cellular prion protein has no defined structure. Here, following fusion of mouse prion protein fragments to green fluorescence protein as a reporter of protein stability, we report extreme variability in fluorescence level that is dependent on the prion fragment expressed. In particular, exposure of the extreme amino terminus in the context of a truncated prion protein molecule led to rapid degradation, whereas the loss of only six amino terminal residues rescued high level fluorescence. Study of the precise endpoints and residue identity associated with high fluorescence suggested a domain within the amino terminal half of the molecule defined by a long-range intramolecular interaction between 23KKRPKP28 and 143DWED146 and dependent upon the anti-parallel beta-sheet ending at residue 169 and normally associated with the structurally defined carboxyl terminal domain. This previously unreported interaction may be significant for understanding prion bioactivity and for structural studies aimed at the complete prion structure.

Detection of bovine spongiform encephalopathy, ovine scrapie prion-related protein (PrPSc) and normal PrPc by monoclonal antibodies raised to copper-refolded prion protein

Prion-related protein (PrP) is a glycosylphosphatidylinositol-linked cell-surface protein expressed by a wide variety of cells, including those of the nervous system and the immune system. Several functions of normal cellular PrP (PrPc) have been proposed that may be associated with the capacity of this protein to bind copper. In the present study, we describe the generation of a panel of monoclonal antibodies raised to copper-refolded PrP, which may be used to analyse the normal and disease-associated forms of this protein. The anti-PrP monoclonal antibodies were reactive by Western blot and ELISA with recombinant murine PrPc refolded in the presence or absence of either copper or manganese, and with the disease-susceptible allelic form V136R154Q171 ('VRQ'; where single-letter amino-acid notation has been used) and disease-resistant allelic form A136R154R171 ('ARR') of recombinant ovine PrPc. FACS analysis of lymphoid cells using these monoclonal antibodies showed that wild-type non-activated mouse lymphocytes expressed little, if any, PrPc. These monoclonal antibodies were shown to react with the unglycosylated and monoglycosylated forms of PrPSc (abnormal disease-specific conformation of PrP) in prion-infected tissue samples from all of the different species tested by Western blot. In addition, this analysis allowed one to make a distinction between bovine spongiform encephalopathy ('BSE') and scrapie PrPSc) isolates from experimentally infected sheep on the basis of their different electrophoretic mobilities.

[Study on the detection of prion protein in food products by a competitive enzyme-linked immunosorbent assay]

We developed a competitive enzyme-linked immunosorbent assay (ELISA) to detect prion protein contained in materials derived from cattle, aiming at establishing a method to detect abnormal prion protein (PrPSc) in food products. Rabbit polyclonal antibodies were raised against bovine prion peptides. Using these antibodies, we have established a competitive ELISA that is capable of detecting recombinant bovine prion protein (rBoPrP) in the range of 12 to 1,200 ng and we used it to determine prion protein contents in bovine cerebral cortex. This assay system was evaluated by spiking food products with various amounts of rBoPrP. The determination gave 2-fold higher values in minced meat homogenates and lower values in large intestine homogenates than the values expected from the spiked amounts. This assay provides a simple determination method of spiked rBoPrP, and therefore is expected to be useful for investigating sample pretreatment methods.

Antiaggregating antibody raised against human PrP 106-126 recognizes pathological and normal isoforms of the whole prion protein

Antibodies to the prion protein (PrP) have been critical to the neuropathological and biochemical characterization of PrP-related degenerative diseases in humans and animals. Although PrP is highly conserved evolutionarily, there is some sequence divergence among species; as a consequence, anti-PrP antibodies have a wide spectrum of reactivity when challenged with PrP from diverse species. We have produced an antibody [monoclonal antibody (mAb) 2-40] raised against a synthetic peptide corresponding to residues (106-126 of human PrP and have characterized it by epitope mapping, Western immunoblot analysis, and immunohistochemistry. The antibody recognizes not only human PrP isoforms but also pathological PrP from all species tested (i.e., sheep, hamsters, and mice). Together with the fact that it recognizes the whole PrP in both cellular and scrapie isoforms, mAb 2-40 may be helpful in studying conformational changes of the PrP, as well as establishing a possible connection between human and animal diseases.