From the Department of Neurology, Hadassah University
Hospital, Jerusalem, 91120 Israel and the Departments of
§ Molecular Biology and ¶ Experimental Medicine and
Cancer Research, Hebrew University Medical School,
Jerusalem, 91120 Israel
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
PrPSc, an abnormal
conformational isoform of the normal prion protein, PrPC,
is the only known component of the prion, a proteinacious agent that
causes fatal neurodegenerative disorders in humans and other animals.
The hallmark properties of PrPSc are its insolubility in
nondenaturing detergents and its resistance to digestion by proteases.
Anions such as Congo red (CR) have been shown to reduce the
accumulation of PrPSc in a neuroblastoma cell line
permanently infected with prions as well as to delay disease onset in
rodents when administrated prophylactically. The mechanism by which
such anti-prion agents operate is unknown. We show here that in
vitro incubation with CR renders native PrPSc
resistant to denaturation by boiling SDS. This resulted from PrPSc conformation, since neither the properties of
PrPC nor those of predenatured PrPSc were
changed by the addition of CR. CR-PrPSc could only be
denatured by the addition of acidic 3 M guanidine thiocyanate. Since in vitro conversion experiments have
suggested that partial denaturation may be required for
PrPSc to serve as template in the PrPC PrPSc conversion, we propose that CR inhibits prion
propagation by overstabilizing the conformation of PrPSc
molecules.
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Prion diseases, such as Creutzfeldt-Jakob disease in humans or
scrapie and bovine spongiform encephalopathies in animals, are fatal
transmissible neurodegenerative diseases (1). The only component of the
prion is PrPSc, an abnormal conformer of the host encoded
PrPC. Both PrP isoforms share the same amino acid sequence,
but, as opposed to PrPC, PrPSc is relatively
resistant to digestion by proteases and insoluble in nondenaturing
detergents (2). Unlike PrPC, which is mainly -helical,
PrPSc contains a considerable amount of
-sheet
structures (3-5). Both isoforms are localized in cholesterol-rich
membrane microdomains called rafts or caveolaelike domains (6-8). The
cell biology of prion diseases as well as the metabolism of the prion
protein isoforms, have been investigated extensively in
scrapie-infected mouse neuroblastoma cells (ScN2a) (9, 10). These cells
are permanently infected with prions and produce both PrPSc
and PrPC. In addition, inoculation of ScN2a extracts into
mice causes prion disease (11). It has been postulated that prion
diseases propagate by the conversion of
-helical PrPC
molecules into
-sheet enriched PrPSc by a mechanism in
which PrPSc serves as a template (12-14). Such a pathway
may feature either the formation of PrPC-PrPSc
heterodimers alone or be also aided by mediator molecules. Experiments with transgenic mice favor the presence of such mediators and predict
some of them to be species-specific (15).
While there is currently no effective therapy for prion diseases, an impressive body of data indicates that polyanions in general, and sulfated sugars in particular, inhibit prion propagation both in animals and in cultured cells. An array of such reagents have been utilized in an effort to "cure" prion infection, either in ScN2a cells or in animal models (16-19). The criteria for the "cure" of ScN2a cells are reduction of cell infectivity titers and decrease of PrPSc levels produced by the cells. Several anti-prion agents, such as Congo red (CR),1 pentosan polysulfate, and others, fulfill these criteria (20). In rodents, coinoculation of prion samples with CR (21), amphotericin B (22), and lately iododoxorubicin (19) resulted in prolonged incubation time, a phenomenon that represents reduction in effective prion titers.
The mechanism by which these anti-prion agents operate has not been
elucidated, although it can be assumed that they must interfere with
one or more of the steps leading to the completion of the
PrPC PrPSc conversion. Diverse sulfated
polyanions seem to interfere with the subcellular metabolism of prions
in the host cell, for example by altering the recycling of PrP to the
interior of the cell (20). Interestingly, none of these reagents were
shown to inhibit PrPC production (23). Some of them, such
as CR, pentosan polysulfate, or low molecular weight heparin, were
shown to inhibit the binding of PrP to heparin (17, 24). Interestingly,
negatively charged molecules and, in particular, heparan sulfate are
found in plaques of every amyloid protein composition, and it has been
proposed that cellular heparan sulfate proteoglycans may be involved in the replication of prions (25). The fluorescent antracycline iododoxorubicin was also shown to decorate amyloid deposits in brain
sections of Creutzfeldt-Jakob disease patients (19). Thus, it is
possible that some of these reagents interact directly with the prion
proteins in the inoculum.
In this work, we investigated whether CR inhibition of prion
propagation may result from direct interaction of CR with
PrPSc. The activity of CR as an anti-prion agent is
particularly interesting, since this reagent is used as an amyloid dye
in many systems including prion diseases (26). In fact, its binding to
protein aggregates is considered diagnostic of an amyloidogenic process
(27). Our results show that such an interaction of CR with the prion
isoform exists and results in overstabilization of the
PrPSc molecules, as can be seen by their increased
resistance to denaturation. If so, CR inhibition of prion propagation
may result form the inability of CR-PrPSc to convert into
the partially denatured molecules required as template in the
PrPC PrPSc reaction.
![]() |
EXPERIMENTAL PROCEDURES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Chemicals and Tissue Culture Reagents--
Congo red (98%)
purchased from Aldrich was dissolved in distilled water and sterilized
before use. Reagents for cell culture were from Biological Industries
(Beit Haemek, Israel). G418 was from Calbiochem.
-L-Phosphatidylcholine was obtained from Avanti Biochemicals. Secondary antibodies were either from Promega (Madison, WI) or Jackson Immunoresearch (West Grove, PA). All other reagents were
from Sigma.
Antibodies-- Rabbit antiserum RO73 (10, 28) as well as monoclonal antibodies 3F4 (29, 30) and 13A5 (31) were used for immunoblotting. Antibodies were used at a dilution of 1:3000 or 1:1000, respectively (of the serum or the ascitic fluid).
Cell Culture-- ScN2a-c10 cells are scrapie-infected neuroblastoma cells that express the MHM2-PrP chimeric gene (30) driven by the commercial expression vector pCI-neo (Promega). The cells were grown at 37 °C in Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum, glutamine, penicillin/streptomycin, and 1 mg/ml G418.
ScN2a-c10 Cells Cultured with CR and Tunicamycin-- Identically seeded and confluent ScN2a-c10 cells were cultured in 10-cm plates (about 107 cells) and incubated for 16, 24, or 48 h in the presence of either 50 µg/ml CR, 1.5 µg/ml tunicamycin, or both. Cells were extracted in 1 ml of lysis buffer containing 10 mM Tris-HCl, pH 7.5, 100 mM NaCl, 1 mM EDTA, and 1% Nonidet P-40. The samples were centrifuged at 3000 rpm for 15 min at 4 °C. The supernatant was concentrated by methanol precipitation and digested with 40 µg/ml proteinase K (PK) for 20 min at 37 °C. 3 M acidic GndSCN was added to half of each sample before boiling in SDS sample buffer and immunoblotting with anti-PrP Ab 3F4.
Incubation of Syrian Hamster Brain Membranes with CR-- Normal and scrapie-infected Syrian hamster brains were kindly provided by Dr. S. B. Prusiner from the University of California, San Francisco. Membranes were prepared as follows. One brain (1 g) was homogenized in 10 ml of cold sucrose buffer (0.3 M sucrose, 10 mM Tris-HCl, pH 7.5) on ice. The homogenate was centrifuged at 2000 rpm for 15 min, and the supernatant was subsequently supplemented with 40 ml of sucrose buffer and centrifuged at 20,000 rpm for 30 min at 4 °C. The pellet was resuspended in with 40 ml of cold 10 mM Tris-HCl, pH 7.5, and centrifuged again at 20,000 rpm for 30 min. The final pellet was resuspended to a concentration of 5 mg/ml in STES buffer (10 mM Tris-HCl, pH 7.5, 100 mM NaCl, 1 mM EDTA, 2% sarkosyl).
Normal and scrapie-infected hamster brain membranes were incubated with increasing concentrations of CR for 1 h at room temperature. Scrapie samples were digested with 40 µg/ml PK for 20 min at 37 °C, and all samples were boiled in SDS sample buffer before immunoblotting with mAb 3F4. The percentage of acrylamide for SDS-PAGE (10% for the scrapie samples or 12% for the normal samples) was determined to avoid comigration of the PrP band with the dye.Incubation of Prion Rods with CR-- Syrian hamster prion rods (PrP-(27-30)) purified by the sucrose gradient method (32) were kindly provided by Dr. S. B. Prusiner. PrP-(27-30) was diluted to a final concentration of approximately 100 ng/ml in STES buffer and incubated with increasing concentrations of CR for 1 h at room temperature. Half of each sample was then incubated with 3 M guanidine thiocyanate, pH 2.5, for 10 min and concentrated by methanol precipitation prior to SDS-PAGE analysis and immunoblotting with mAb 3F4.
PK Digestion of Denatured PrP-(27-30)-- Purified PrP-(27-30) was incubated in the presence or absence of 200 µg/ml CR for 1 h at room temperature. After the incubation, the samples were boiled in 1% SDS and then diluted 10-fold with STES buffer. Samples were digested with 20 µg/ml PK for 30 min at 37 °C and treated with or without 3 M acidic GndSCN as described above.
Dispersion of CR-treated PrP-(27-30) into Detergent-Lipid
Protein Complexes (DLPC)--
Purified PrP-(27-30) was resuspended in
Hepes buffer (100 mM NaCl, 10 mM Hepes sodium
salt, pH 7.4) to a final concentration of 100 µg/ml and incubated,
when applicable, with 200 µg/ml CR for 1 h at room temperature.
Prion rods were dispersed into DLPC as described by Gabizon et
al. (33). Shortly, the resuspended rods were mixed with 2% sodium
cholate and added to a glass tube containing dried
-L-phosphatidylcholine (10 mg). The sample was mixed,
sonicated, and centrifuged at 31,000 × g for 25 min.
Pellet and supernatant were separated, supplemented with 3 M acidic GndSCN, and methanol-precipitated. For some
experiments, the supernatant, which contained PrP-(27-30) in DLPC, was
incubated in the presence of 200 µg/ml CR for 1 h at room
temperature followed by the addition of 3 M acidic GndSCN
before methanol precipitation. All samples were boiled in SDS sample
buffer before immunoblotting with mAb 3F4.
Immunoprecipitation of PrP-(27-30) in DLPC--
150 ng/ml of
purified PrP-(27-30) in DLPC were incubated in the absence or presence
of 200 µg/ml CR for 1 h at room temperature and
immunoprecipitated with mAb 3F4 (diluted 1:250) for 18 h at 4 °C. Protein A-Sepharose was then added, and the mixture was incubated for an additional 30 min at room temperature. The beads were
rinsed five times in TNS (100 mM NaCl, 1% sarkosyl, 10 mM Tris-HCl, pH 7.5) and were separated from the
supernatant. Supernatant and beads were treated in the presence or
absence of 3 M acidic GndSCN followed by methanol
precipitation. All samples were boiled in sample buffer, which did not
contain -mercaptoethanol, and immunoblotted with antiserum RO73.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
CR Renders PrPSc Resistant to Denaturation by Boiling SDS-- To investigate whether direct interaction of PrPSc with CR may result in inhibition of prion propagation, we tested the biochemical properties of the PrP proteins after incubation of membranes from normal or scrapie-infected hamster brains with CR. The hallmark property of PrPSc is its protease resistance. While PrPC is highly sensitive to proteases, digestion of PrPSc with PK results in PrP-(27-30), the protease-resistant core of PrPSc (34). To test whether incubation of PrPSc with CR will render this protein sensitive to PK, normal and scrapie brain samples were incubated with increasing concentrations of CR. Scrapie-infected hamster samples presumably contain both PrP isoforms, as opposed to normal brain membranes that contained only PrPC (35). To differentiate the protease-sensitive PrPC from protease-resistant PrPSc, the scrapie-infected brain samples (CR-treated and controls) were digested with PK (see "Experimental Procedures") prior to PAGE analysis. All samples were subsequently boiled in SDS sample buffer and processed for immunoblotting. While the PrPC pattern on the gel carrying only normal brain samples was not affected by the preincubation with CR (Fig. 1A), the presence of PrP-(27-30) on the immunoblot loaded with samples from scrapie-infected brains was reduced in a dose-dependent manner (Fig. 1B). These results suggested the intriguing possibility that direct incubation of the scrapie isoform with CR in vitro is enough to render PrPSc sensitive to digestion by proteases.
|
|
|
CR Does Not Interfere with the Dispersion of PrP-(27-30) into DLPC-- In addition to protease resistance, another hallmark property of the scrapie prion protein is its insolubility in nondenaturing detergents (Fig. 4A). The insolubility of PrPSc has been utilized in the protocol developed for its purification as PrP-(27-30) rods (37). Interestingly, as opposed to its behavior in many detergents, the insoluble rods could be dispersed into DLPC (33). When PrP-(27-30) in prion rods was sonicated in the presence of phosphatidylcholine and 2% cholate and subsequently centrifuged at high speed, most of the PrP remained in the supernatant, as opposed to the samples without lipids, which are pelleted at these conditions (Fig. 4B). Although it is difficult to conclude whether PrP-(27-30) is completely solubilized to monomers by this procedure, it is obviously more dispersed than in rods and, as a result of this, more infectious (38). To investigate whether CR competes with lipids for binding sites on PrPSc, we tested whether CR-PrP-(27-30) could also be incorporated into DLPC. As can be seen in Fig. 4B, when CR-treated PrP-(27-30) was dispersed into DLPC by the same protocol used for control PrP, most of the CR-treated protein remained in the supernatant. As in the previous experiments, this could be visualized only after the addition of acidic GndSCN to the samples prior to their analysis by PAGE (Fig. 4B). Moreover, PrP-(27-30) in DLPC was still susceptible to CR, since acidic GdnSCN was required to visualized on the immunoblot PrP-(27-30) to which CR was applied after the dispersion into DLPC (Fig. 4C). This result suggests that dispersion into DLPC did not hide the CR binding site of PrP-(27-30).
|
|
CR Inhibited New PrPSc Synthesis and Old PrPSc Degradation-- The best cell culture model developed to date for prion infection is the ScN2a system. These scrapie-infected mouse neuroblastoma cells produce PrPSc as well as infectivity (11, 40). When ScN2a cells were cultured in the presence of CR, no new PrPSc was synthesized and subsequently accumulated (41). We tested the fate of PrPSc in these cells to resolve whether in previous experiments some of the "old" scrapie protein may not have been accounted for because of the insufficient denaturation that results from the addition of CR. ScN2a-c10 cells, a ScN2a clone expressing a chimeric mouse/hamster PrP (47), were incubated with and without CR and in the presence or absence of tunicamycin, a glycosylation inhibitor, for 18, 30, or 48 h. Cell lysates were then digested with 20 µg/ml PK for 30 min at 37 °C. In the presence of tunicamycin, new PrP is synthesized as an unglycosylated protease-resistant protein (42), while fully glycosylated PrPSc, higher in Mr, present prior to the addition of tunicamycin is degraded very slowly. Such an experiment is shown in Fig. 6. In the absence of CR, fully glycosylated "old" PrP (upper band of the three that comprise the PrP immunoblot profile) was partially degraded after 48 h in tunicamycin, while the lower band signal (representing unglycosylated PrP) was considerably enhanced as compared with the signal obtained before the addition of tunicamycin. When tunicamycin was added to the cells incubated in the presence of CR, no new protease-resistant PrP was synthesized, as seen by the fact that there was no increase in the signal of the low molecular weight band. In effect, no difference at all could be appreciated in the banding pattern of PK-resistant PrP in the presence or absence of tunicamycin. These results suggest that no new PrPSc was made by ScN2a-c10 cells in the presence of CR, although the concentration of the old PrPSc remained high. As with the previous experiments performed on membranes or purified PrP-(27-30), the total concentration of PrPSc in the CR-treated cells could be visualized only when acidic GndSCN was added to the denaturing sample buffer.
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Prions appear to propagate by refolding mature PrPC, a
membrane glycoprotein of unknown function, into an isoform of aberrant conformation denominated PrPSc. This refolded protein is
the only known component of the prion. Thus, the two isoforms of PrP
serve, at the same time, as substrate and template in the conversion
process that results in the accumulation of more prions. During the
conversion, PrPSc somehow confers to PrPC its
own and distinct structural conformation, which is extremely stable and
comprises large -sheet segments (5). Although the very nature of the
conversion process is unknown, in vitro studies have
suggested that partial denaturation of PrPSc may be a
crucial requirement for this molecule to act as a template in the
further conformational conversion of PrPC (43). In
addition, theoretical considerations also suggested the involvement of
a partially denatured molten globule in the formation of
PrPSc (44). The requirement of the chaperon HSP104 for the
propagation of the prion like
of yeast also suggested that template
"breathing" may feature in the propagation of conformers (45).
Therefore, it is possible that reagents that stabilize
PrPSc so that it cannot be partially denatured will inhibit
prion propagation.
Our results show that CR can act directly on PrPSc, even in vitro, and that its effect can be evaluated in a simple assay. Since reagents such as CR inhibit further propagation of prions, they must interfere with one or more steps in the conversion process. While other anti-prion agents may disrupt the PrPSc conformation and render this protein protease-sensitive, as expected from an anti-prion agent, CR seems to reinforce PrPSc structure and, as a result of this, inhibit its denaturation. In view of this, we suggest that CR may hinder prion propagation and PrPSc formation by stabilizing the template PrPSc so that the partial denaturation needed for the conversion of additional PrPC molecules is prevented.
The effect of CR on PrPSc is not a function of the PrP sequence or its glycosylation profile but rather of its conformation. We conclude this from the fact that neither PrPC (Fig. 1) nor predenatured PrPSc (Fig. 3) changes its basic properties when incubated in the presence of CR. It has been shown in the past that CR does not inhibit the synthesis of PrPC (24).
CR association abolished the affinity of PrP-(27-30) to its antibodies. This affinity, very poor in purified PrP rods, was significantly enhanced only after PrP-(27-30) dispersion into DLPC. In addition, the comparison of the lines in Fig. 6 with and without guanidine denaturation suggests that inhibition of synthesis of new PrPSc by CR occurs even in the presence of the PrPSc molecules that were present in the cells before the addition of CR.
Our results are consistent with yet another plausible conclusion. Since
in the presence of CR, PrPSc seems to be "invisible" to
either antibodies or metabolic cell components, it may also be
invisible to components of the PrPC PrPSc
conversion site. Whether this effect is due to direct inhibition of
binding by CR or is also a result of the increased stability conferred
to PrPSc by CR remains to be established.
Scrapie incubation time may be determined not only by the concentration of PrPSc in the inoculum, but also by the number of PrPSc molecules available for binding to conversion sites. Indeed, PrP-(27-30) in DLPC, a more dispersed form of PrP, has been shown to be more infectious, by a factor of 10 or 100, than the same sample in the more aggregated form (33, 46). Conversely, it is possible that highly aggregated as well as metabolically stable PrP will be less infectious, since it cannot interact efficiently with the crucial cell components needed for the conversion of PrPC. This is probably why it has been very difficult to correlate PrPSc concentrations with infectivity titer (47). As stated above, ScN2a cells did not produce new PrPSc in the presence of CR, although acidic guanidinium revealed the presence of large concentrations of very stable PrPSc.
The results presented here suggest an interesting feature of transmissible amyloid diseases. While it is necessary for the aberrant isoform of the amyloidogenic protein to be stable enough to metabolic degradation, so that it can be present long enough in the cell to transform many normal molecules, an overstabilized aberrant conformation will inhibit the partial denaturation required for template formation. Interestingly, the amyloid aggregates from a nontransmissible brain neurodegenerative disorder, Alzheimer's disease, cannot be denatured by SDS boiling but by much harsher conditions such as 6 M urea (48).
Whether the anti-prion effect of CR is caused by stabilizing the scrapie conformation only or also by blocking the binding of PrPSc molecules to conversion sites, the fact that direct interaction of CR with PrPSc inhibits new PrPSc synthesis reinforces the notion that PrPSc is probably the crucial if not the only prion component.
What molecules in the cell, in addition to PrPC, participate in the conversion reaction? Several molecules have been shown to either bind to PrP or be altered in scrapie-infected cells or brains (49-52). Experiments with transgenic mice have postulated the existence of a species-specific factor, denominated Protein X, which is yet to be identified (15). The presence of glycosaminoglycan molecules such as heparan sulfate in every amyloid was proposed as evidence of their possible role in the scrapification of PrP (25). However, the fact that PrPC, although at a low efficiency, can be converted in vitro to a PrPSc-like molecule (43), suggests that such molecules may be important mostly for the kinetics of the reaction or for the pathogenesis of the prion disease. As proposed above, it is the very nature of PrPSc conformation that probably hinders the finding of such molecules.
Elucidating the diverse mechanisms by which the known anti-prion agents operate may help in understanding the biology of prion diseases and may also be a crucial step for the preparation of reagents that can be used in in vivo experiments and clinical trials. Some of them will eventually also be useful for the treatment of other amyloid disorders.
![]() |
FOOTNOTES |
---|
* This work was supported by grants from the National Institutes of Health and the Israeli Academy of Science.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
To whom correspondence should be addressed: Dept. of
Neurology, Hadassah University Hospital, Jerusalem 91120, Israel. Fax: 972-2-6437782; E-mail: gabizonr{at}hadassah.org.il.
1 The abbreviations used are: CR, Congo red; PK, proteinase K; Ab, antibody; mAb, monoclonal antibody; PAGE, polyacrylamide gel electrophoresis; GndSCN, guanidine thiocyanate; DLPC, detergent-lipid protein complexes.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|