Stimulation of plasminogen activation by recombinant cellular prion protein is conserved in the NH2-terminal fragment PrP23-110

Endoproteolysis of cellular prion protein by plasmin hinders propagation of prions

Many questions surround the underlying mechanism for the differential metabolic processing observed for the prion protein (PrP) in healthy and prion-infected mammals. Foremost, the physiological α-cleavage of PrP interrupts a region critical for both toxicity and conversion of cellular PrP (PrP ^C ) into its misfolded pathogenic isoform (PrP ^Sc ) by generating a glycosylphosphatidylinositol (GPI)-anchored C1 fragment. During prion diseases, alternative β-cleavage of PrP becomes prominent, producing a GPI-anchored C2 fragment with this particular region intact. It remains unexplored whether physical up-regulation of α-cleavage can inhibit disease progression. Furthermore, several pieces of evidence indicate that a disintegrin and metalloproteinase (ADAM) 10 and ADAM17 play a much smaller role in the α-cleavage of PrP ^C than originally believed, thus presenting the need to identify the primary protease(s) responsible. For this purpose, we characterized the ability of plasmin to perform PrP α-cleavage. Then, we conducted functional assays using protein misfolding cyclic amplification (PMCA) and prion-infected cell lines to clarify the role of plasmin-mediated α-cleavage during prion propagation. Here, we demonstrated an inhibitory role of plasmin for PrP ^Sc formation through PrP α-cleavage that increased C1 fragments resulting in reduced prion conversion compared with non-treated PMCA and cell cultures. The reduction of prion infectious titer in the bioassay of plasmin-treated PMCA material also supported the inhibitory role of plasmin on PrP ^Sc replication. Our results suggest that plasmin-mediated endoproteolytic cleavage of PrP may be an important event to prevent prion propagation.

Fibrinogen Mitigates Prion-Mediated Platelet Activation and Neuronal Cell Toxicity

Prion peptide (PrP) misfolds to infectious scrapie isoform, the β pleat-rich insoluble fibrils responsible for neurodegeneration and fatal conformational diseases in humans. The amino acid sequence 106–126 from prion proteins, PrP(106–126), is highly amyloidogenic and implicated in prion-induced pathologies. Here, we report a novel interaction between PrP(106–126) and the thrombogenic plasma protein fibrinogen that can lead to mitigation of prion-mediated pro-thrombotic responses in human platelets as well as significant decline in neuronal toxicity. Thus, prior exposure to fibrinogen-restrained PrP-induced rise in cytosolic calcium, calpain activation, and shedding of extracellular vesicles in platelets while it, too, averted cytotoxicity of neuronal cells triggered by prion peptide. Interestingly, PrP was found to accelerate fibrin-rich clot formation, which was resistant to plasmin-mediated fibrinolysis, consistent with enhanced thrombus stability provoked by PrP. We propose that PrP-fibrinogen interaction can be clinically exploited further for prevention and management of infectious prion related disorders. Small molecules or peptides mimicking PrP-binding sites on fibrinogen can potentially mitigate PrP-induced cellular toxicity while also preventing the negative impact of PrP on fibrin clot formation and lysis.

Metal Ions Bound to Prion Protein Affect its Interaction with Plasminogen Activation System

Prion diseases are a group of neurodegenerative diseases, which can progress rapidly. Previous data have demonstrated that prion protein (PrP) stimulates activation of plasminogen (Plg) by tissue plasminogen activator (tPA). In this study, using spectroscopic method, we aimed to determine whether PrP’s role in activating Plg is influenced by metal binding. We also investigated the region in PrP involved in binding to tPA and Plg, and whether PrP in fibrillar form behaves the same way as PrP unbound to any metal ion i.e., apo-PrP. We investigated the effect of recombinant mouse PrP (residues 23-231) refolded with nickel, manganese, copper, and a variant devoid of any metal ions, on tPA-catalyzed Plg activation. Using mutant PrP (H95A, H110A), we also investigated whether histidine residues outside the octarepeat region in PrP, which is known to bind tPA and Plg, are also involved in their binding. We demonstrated that apo-PrP is most effective at stimulating Plg. PrP refolded with nickle or manganese behave similar to apo-PrP, and PrP refolded with copper is least effective. The mutant form of PrP did not stimulate Plg activation to the same degree as apo-PrP indicating that the histidine residues outside the octarepeat region are also involved in binding to tPA and Plg. Similarly, the fibrillar form of PrP was ineffective at stimulating Plg activation. Our data suggest that upon loss of copper specifically, a structural rearrangement of PrP occurs that exposes binding sites to Plg and tPA, enhancing the stimulation of Plg activation.

A multimolecular signaling complex including PrPC and LRP1 is strictly dependent on lipid rafts and is essential for the function of tissue plasminogen activator

Prion protein (PrP^C ) localizes stably in lipid rafts microdomains and is able to recruit downstream signal transduction pathways by the interaction with promiscuous partners. Other proteins have the ability to occasionally be recruited to these specialized membrane areas, within multimolecular complexes. Among these, we highlight the presence of the low-density lipoprotein receptor-related protein 1 (LRP1), which was found localized transiently in lipid rafts, suggesting a different function of this receptor that through lipid raft becomes able to activate a signal transduction pathway triggered by specific ligands, including Tissue plasminogen activator (tPA). Since it has been reported that PrP^C participates in the tPA-mediated plasminogen activation, in this study, we describe the role of lipid rafts in the recruitment and activation of downstream signal transduction pathways mediated by the interaction among tPA, PrP^C and LRP1 in human neuroblastoma SK-N-BE2 cell line. Co-immunoprecipitation analysis reveals a consistent association between PrP^C and GM1, as well as between LRP1 and GM1, indicating the existence of a glycosphingolipid-enriched multimolecular complex. In our cell model, knocking-down PrP^C by siRNA impairs ERK phosphorylation induced by tPA. Moreover the alteration of the lipidic milieu of lipid rafts, perturbing the physical/functional interaction between PrP^C and LRP1, inhibits this response. We show that LRP1 and PrP^C , following tPA stimulation, may function as a system associated with lipid rafts, involved in receptor-mediated neuritogenic pathway. We suggest this as a multimolecular signaling complex, whose activity depends strictly on the integrity of lipid raft and is involved in the neuritogenic signaling.

Cellular and Molecular Mechanisms of Prion Disease

Prion diseases are rapidly progressive, incurable neurodegenerative disorders caused by misfolded, aggregated proteins known as prions, which are uniquely infectious. Remarkably, these infectious proteins have been responsible for widespread disease epidemics, including kuru in humans, bovine spongiform encephalopathy in cattle, and chronic wasting disease in cervids, the latter of which has spread across North America and recently appeared in Norway and Finland. The hallmark histopathological features include widespread spongiform encephalopathy, neuronal loss, gliosis, and deposits of variably sized aggregated prion protein, ranging from small, soluble oligomers to long, thin, unbranched fibrils, depending on the disease. Here, we explore recent advances in prion disease research, from the function of the cellular prion protein to the dysfunction triggering neurotoxicity, as well as mechanisms underlying prion spread between cells. We also highlight key findings that have revealed new therapeutic targets and consider unanswered questions for future research.

Diverse functions of the prion protein - Does proteolytic processing hold the key?

Proteolytic processing of the cellular and disease-associated form of the prion protein leads to generation of bioactive soluble prion protein fragments and modifies the structure and function of its cell-bound form. The nature of proteases responsible for shedding, α-, β-, and γ-cleavage of the prion protein are only partially identified and their regulation is largely unknown. Here, we provide an overview of the increasingly multifaceted picture of prion protein proteolysis and shed light on physiological and pathological roles associated with these cleavages. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

Loss of Cellular Sialidases Does Not Affect the Sialylation Status of the Prion Protein but Increases the Amounts of Its Proteolytic Fragment C1

The central molecular event underlying prion diseases involves conformational change of the cellular form of the prion protein (PrP^C), which is a sialoglycoprotein, into the disease-associated, transmissible form denoted PrP^Sc. Recent studies revealed a correlation between the sialylation status of PrP^Sc and incubation time to disease and introduced a new hypothesis that progression of prion diseases could be controlled or reversed by altering the sialylation level of PrP^C. Of the four known mammalian sialidases, the enzymes that cleave off sialic acid residues, only NEU1, NEU3 and NEU4 are expressed in the brain. To test whether cellular sialidases control the steady-state sialylation level of PrP^C and to identify the putative sialidase responsible for desialylating PrP^C, we analyzed brain-derived PrP^C from knockout mice deficient in Neu1, Neu3, Neu4, or from Neu3/Neu4 double knockouts. Surprisingly, no differences in the sialylation of PrP^C or its proteolytic product C1 were noticed in any of the knockout mice tested as compared to the age-matched controls. However, significantly higher amounts of the C1 fragment relative to full-length PrP^C were detected in the brains of Neu1 knockout mice as compared to WT mice or to the other knockout mice. Additional experiments revealed that in neuroblastoma cell line the sialylation pattern of C1 could be changed by an inhibitor of sialylatransferases. In summary, this study suggests that targeting cellular sialidases is apparently not the correct strategy for altering the sialylation levels of PrP^C, whereas modulating the activity of sialylatransferases might offer a more promising approach. Our findings also suggest that catabolism of PrP^C involves its α-cleavage followed by desialylation of the resulting C1 fragments by NEU1 and consequent fast degradation of the desialylated products.

α-Cleavage of cellular prion protein

The cellular prion protein (PrP^C) is subjected to various processing under physiological and pathological conditions, of which the α-cleavage within the central hydrophobic domain not only disrupts a region critical for both PrP toxicity and PrP^C to PrP^Sc conversion but also produces the N1 fragment that is neuroprotective and the C1 fragment that enhances the pro-apoptotic effect of staurosporine in one report and inhibits prion in another. The proteases responsible for the α-cleavage of PrP^C are controversial. The effect of ADAM10, ADAM17, and ADAM9 on N1 secretion clearly indicates their involvement in the α-cleavage of PrP^C, but there has been no report of direct PrP^C α-cleavage activity with any of the three ADAMs in a purified protein form. We demonstrated that, in muscle cells, ADAM8 is the primary protease for the α-cleavage of PrP^C, but another unidentified protease(s) must also play a minor role. We also found that PrP^C regulates ADAM8 expression, suggesting that a close examination on the relationships between PrP^C and its processing enzymes may reveal novel roles and underlying mechanisms for PrP^C in non-prion diseases such as asthma and cancer.

Plasminogen: A cellular protein cofactor for PrPSc propagation

The biochemical essence of prion replication is the molecular multiplication of the disease-associated misfolded isoform of prion protein (PrP), termed PrPSc, in a nucleic acid-free manner. PrP(Sc) is generated by the protein misfolding process facilitated by conformational conversion of the host-encoded cellular PrP to PrP(Sc). Evidence suggests that an auxiliary factor may play a role in PrP(Sc) propagation. We and others previously discovered that plasminogen interacts with PrP, while its functional role for PrPSc propagation remained undetermined. In our recent in vitro PrP conversion study, we showed that plasminogen substantially stimulates PrP(Sc) propagation in a concentration-dependent manner by accelerating the rate of PrP(Sc) generation, while depletion of plasminogen, destabilization of its structure, and interference with the PrP-plasminogen interaction hinder PrP(Sc) propagation. Further investigation in cell culture models confirmed an increase of PrP(Sc) formation by plasminogen. Although molecular basis of the observed activity for plasminogen remain to be addressed, our results demonstrate that plasminogen is the first cellular protein auxiliary factor proven to stimulate PrP(Sc) propagation.

Plasminogen stimulates propagation of protease-resistant prion protein in vitro

To clarify the role of plasminogen as a cofactor for prion propagation, we conducted functional assays using a cell-free prion protein (PrP) conversion assay termed protein misfolding cyclic amplification (PMCA) and prion-infected cell lines. Here, we report that plasminogen stimulates propagation of the protease-resistant scrapie PrP (PrP(Sc)). Compared to control PMCA conducted without plasminogen, addition of plasminogen in PMCA using wild-type brain material significantly increased PrP conversion, with an EC(50) = ∼56 nM. PrP conversion in PMCA was substantially less efficient with plasminogen-deficient brain material than with wild-type material. The activity stimulating PrP conversion was specific for plasminogen and conserved in its kringle domains. Such activity was abrogated by modification of plasminogen structure and interference of PrP-plasminogen interaction. Kinetic analysis of PrP(Sc) generation demonstrated that the presence of plasminogen in PMCA enhanced the PrP(Sc) production rate to ∼0.97 U/μl/h and reduced turnover time to ∼1 h compared to those (∼0.4 U/μl/h and ∼2.5 h) obtained without supplementation. Furthermore, as observed in PMCA, plasminogen and kringles promoted PrP(Sc) propagation in ScN2a and Elk 21(+) cells. Our results demonstrate that plasminogen functions in stimulating conversion processes and represents the first cellular protein cofactor that enhances the hypothetical mechanism of prion propagation.

Canine Plasminogen: Spectral Responses to Changes in 6-Aminohexanoate and Temperature

We studied the near UV absorption spectrum of canine plasminogen. There are 19 tryptophans, 19 phenylalanines and 34 tyrosines in the protein. 4th derivative spectra optimized for either tryptophan or tyrosine give a measure of the polarity of the environments of these two aromatic amino acids. Plasminogen at temperatures between 0°C and 37°C exists as a mixture of four conformations: closed-relaxed, open-relaxed, closed-compact, and open-compact. The closed to open transition is driven by addition of ligand to a site on the protein. The relaxed to compact transition is driven by increasing temperature from 0°C to above 15-20°C.

When the conformation of plasminogen is mainly closed-relaxed, the 4th derivative spectra suggest that the average tryptophan environment is similar to a solution of 20% methanol at the same temperature. Under the same conditions, 4th derivative spectra suggest that the average tyrosine environment is similar to water. These apparent polarities change as the plasminogen is forced to assume the other conformations. We try to rationalize the information based on the known portions of the plasminogen structure.

Current and future molecular diagnostics for prion diseases

It is now widely held that the infectious agents underlying the transmissible spongiform encephalopathies are prions, which are primarily composed of a misfolded, protease-resistant isoform of the host prion protein. Untreatable prion disorders include some human diseases, such as Creutzfeldt-Jakob disease, and diseases of economically important animals, such as bovine spongiform encephalopathy (cattle) and chronic wasting disease (deer and elk). Detection and diagnosis of prion disease (and presymptomatic incubation) is contingent upon developing novel assays, which exploit properties uniquely possessed by this misfolded protein complex, rather than targeting an agent-specific nucleic acid. This review highlights some of the conventional and disruptive technologies developed to respond to this challenge.

Unaltered prion protein cleavage in plasminogen-deficient mice

In normal brains and cultured cells, cellular prion protein (PrP) is partially found as N-terminally truncated fragments, designated C1 and C2. The cleavage of recombinant PrP to a fragment corresponding to C1 can be mediated by the protease plasmin (Pln) in vitro, suggesting that plasmin might be responsible for the generation of the C1 fragment in vivo as well. The cleavage pattern of PrP found in both brain lysates and other tissues of plasminogen knock-out mice, however, is unaltered. The presence of C1 fragment in homogenates from plasminogen-deficient mice in a comparable ratio with full-length PrP as can be found in wild-type animals indicates that other proteases in addition to plasmin are responsible for PrP cleavage in vivo.

Screening a library of potential prion therapeutics against cellular prion proteins and insights into their mode of biological activities by surface plasmon resonance

The conversion of cellular prion protein (PrP(C)) to the protease resistant isoform (PrP(Sc)) is considered essential for the progression of transmissible spongiform encephalopathies (TSEs). A potential therapeutic strategy for preventing the accumulation of PrP(Sc) is to stabilize PrP(C) through the direct binding of a small molecule to make conversion less energetically favourable. Using surface plasmon resonance (SPR)-based technology we have developed a procedure, based on direct binding, for the screening of small molecules against PrP(C) immobilized on a sensor chip. In this paper we report some problems associated with the immobilization of PrP(C) onto the sensor surface for conducting drug screening and how these problems were overcome. We demonstrated that the conformational change of PrP(C) on the chip surface leads to increased exposure of the C-terminal which was observed by the increase in quinacrine binding over time, and loss of heparin binding to the N-terminal. In addition, we also report the results of the successful screening of a library of 47 compounds of known activity in cell line or cell free conversion studies for direct binding to three forms of PrP(C) (huPrP(C), t-huPrP(C) and moPrP(C)). These results show the usefulness of this technique for the identification of PrP(C) binding ligands and to gain some insight as to their potential mode of action.

Proteolytic processing of the ovine prion protein in cell cultures

The cellular compartment and purpose of the proteolytic processing of the prion protein (PrP) are still under debate. We have studied ovine PrP constructs expressed in four cell lines; murine neuroblastoma cells (N2a), human neuroblastoma cells (SH-SY5Y), dog kidney epithelial cells (MDCK), and human furin-deficient colon cancer cells (LoVo). Cleavage of PrP in LoVo cells indicates that the processing is furin independent. Neither is it reduced by some inhibitors of lysosomal proteinases, proteasomes or zinc-metalloproteinases, but incubation with bafilomycin A1, an inhibitor of vacuolar H+/ATPases, increases the amount of uncleaved PrP in the apical medium of MDCK cells. Mutations affecting the putative cleavage site near amino acid 113 reveal that the cleavage is independent of primary structure at this site. Absence of glycosylphosphatidylinositol anchor and glycan modifications does not influence the proteolytic processing of PrP. Our data indicate that PrP is cleaved during transit to the cell membrane.

Tissue plasminogen activator in brain tissues infected with transmissible spongiform encephalopathies

Prion propagation involves conversion of host PrP(C) to a disease-related isoform, PrP(Sc), which accumulates during disease and is the principal component of the transmissible agent. Proteolysis seems to play an important role in PrP metabolism. Plasminogen, a serine protease precursor, has been shown to interact with PrP(Sc). Plasminogen can be proteolytically activated by tissue plasminogen activator (tPA). Recent reports imply a crosstalk between tPA-mediated plasmin activation and PrP. In our study, both tPA activity and tPA gene expression were found elevated in TSE-infected brains as compared to their normal counterparts. Furthermore, it was proved that PrP(Sc), in contrast to PrP(C), could not be degraded by plasmin. In addition, it was observed that TSE symptoms and subsequent death of plasminogen-deficient and tPA-deficient scrapie challenged mice preceded that of wild-type controls. Our data imply that enhanced tPA activity observed in prion infected brains may reflect a neuro-protective response.

Plasminogen activities and concentrations in patients with sporadic Creutzfeldt-Jakob disease

Human plasminogen has been shown to interact with the abnormal disease-specific prion protein. Till now, no data are available for patients with Creutzfeldt-Jakob disease (CJD). Therefore, we compared plasminogen concentrations and plasminogen activities in patients with sporadic CJD and controls with other dementia, which were collected in the framework of the German CJD Surveillance study. Patients with CJD had significantly higher plasminogen concentrations than patients with other forms of dementia and plasminogen specific activities were lower in CJD patients. The reasons for these abnormalities are not clear at the moment. The results may reflect a disease-specific prion protein and plasminogen interaction in patients with CJD. Other possible explanations are plasminogen polymorphisms and genotypes with distinct plasminogen activity levels in CJD than in controls, which should be a subject for further studies.

Diagnosing prion diseases: needs, challenges and hopes

Prion diseases are among the most intriguing infectious diseases and are associated with unconventional proteinaceous infectious agents known as prions. Prions seem to lack nucleic acid and propagate by transmission of protein misfolding. The nature of prions and their unique mode of transmission present challenges for early diagnosis of prion diseases. In this article, state-of-the-art prion diagnostic techniques, together with the new strategies that are being used to develop sensitive, early and non-invasive diagnoses for these diseases are reviewed.

Prion protein stimulates tissue-type plasminogen activator-mediated plasmin generation via a lysine-binding site on kringle 2

Recombinant human prion-protein (PrP23-231) stimulates plasminogen activation by tissue-type plasminogen activator (t-PA). The stimulatory activity is conserved in the N-terminal fragment (PrP23-110). It has further been shown by others that PrP(c) binds to kringle-domains of plasminogen. We compared the stimulatory activity of recombinant PrP23-231 and PrP23-110 on plasminogen activation catalyzed by t-PA, urokinase (u-PA), streptokinase and Desmodus salivary plasminogen activator (DSPAalpha1). As these plasminogen activators are distinct, with respect to their kringle domains we studied their binding to immobilized PrP23-110. Plasminogen activation was measured in a chromogenic assay in vitro and binding studies were carried out using surface plasmon resonance technology. We found that recombinant full-length prion protein, PrP23-231, and PrP23-110 specifically stimulate t-PA mediated plasminogen activation. Two hundred nanomoles per liter of PrP23-110 stimulated 1.8 nmol L(-1) t-PA 48-fold, 180 nmol L(-1) DSPA(alpha1) 2.5-fold, 1.8 nmol L(-1) u-PA 1.1-fold, and 1.8 nmol L(-1) streptokinase 1.8-fold. Our data show no specific binding for streptokinase. In contrast all plasminogen activators carrying a kringle domain bound to PrP23-110. We further studied the effect of lysine on binding to PrP23-110 and on plasminogen activation by DSPA(alpha1) or t-PA. Lysine decreased both the binding of t-PA to PrP23-110 and the stimulation of plasmin generation by t-PA. Both binding and plasminogen activation of DSPA(alpha1) were not influenced by the presence of lysine. All plasminogen activators tested bearing kringle domains bind to PrP23-110. Binding to PrP23-110 is not sufficient for stimulation of plasmin generation. Thus the lysine-binding site of kringle 2 that is unique to t-PA appears to mediate the specific stimulation of plasminogen activation by the cellular prion protein.