Role of plasminogen in propagation of scrapie

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.

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.

In vitro screen of prion disease susceptibility genes using the scrapie cell assay

Prion diseases (transmissible spongiform encephalopathies) are fatal neurodegenerative diseases, including Creutzfeldt-Jakob disease in humans, scrapie in sheep and bovine spongiform encephalopathy in cattle. While genome-wide association studies in human and quantitative trait loci mapping in mice have provided evidence for multiple susceptibility genes, few of these have been confirmed functionally. Phenotyping mouse models is generally the method of choice. However, this is not a feasible option where many novel genes, without pre-existing models, would need to be tested. We have therefore developed and applied an in-vitro screen to triage and prioritize candidate modifier genes for more detailed future studies which is faster, far more cost effective and ethical relative to mouse bioassay models. An in vitro prion bioassay, the scrapie cell assay, uses a neuroblastoma-derived cell line (PK1) that is susceptible to RML prions and able to propagate prions at high levels. In this study, we have generated stable gene silencing and/or overexpressing PK1-derived cell lines to test whether perturbation of 14 candidate genes affects prion susceptibility. While no consistent differences were determined for seven genes, highly significant changes were detected for Zbtb38, Sorcs1, Stmn2, Hspa13, Fkbp9, Actr10 and Plg, suggesting that they play key roles in the fundamental processes of prion propagation or clearance. Many neurodegenerative diseases involve the accumulation of misfolded protein aggregates and 'prion-like' seeding and spread has been implicated in their pathogenesis. It is therefore expected that some of these prion-modifier genes may be of wider relevance in neurodegeneration.

Therapeutic activity of inhibition of the soluble epoxide hydrolase in a mouse model of scrapie

AIMS

The misfolding and the aggregation of specific proteins are key features of neurodegenerative diseases, specifically Transmissible Spongiform Encephalopathies (TSEs). In TSEs, neuronal loss and inflammation are associated with the accumulation of the misfolded isoform (PrP(sc)) of the cellular prion protein (PrP(c)). Therefore we tested the hypothesis that augmenting a natural anti-inflammatory pathway mediated by epoxygenated fatty acids (EpFAs) will delay lethality. EpFAs are highly potent but enzymatically labile molecules produced by the actions of a number of cytochrome P450 enzymes. Stabilization of these bioactive lipids by inhibiting their degradation mediated by the soluble epoxide hydrolase (sEH) results in potent anti-inflammatory effects in multiple disease models.

MAIN METHODS

Mice were infected with the mouse-adapted RML strain of scrapie by intracerebral or intraperitoneal routes. Animals received the sEH inhibitor, by oral route, administrated in drinking water or vehicle (PEG400). Infected mice were euthanized at a standard clinical end point. Histopathological, immunohistochemical and Western blot analyses of brain tissue confirmed the presence of pathology related to prion infection.

KEY FINDINGS

Oral administration of the sEHI did not affect the very short survival time of the intracerebral prion infection group. However, mice infected by intraperitoneal route and treated with t-AUCB survived significantly longer than the control group mice (p<0.001).

SIGNIFICANCE

These findings support the idea that inhibition of sEH or augmentation of the natural EpFA signaling in the brain offers a potential and different route to understand prion diseases and may become a therapeutic strategy for diseases involving neuroinflammation.

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.

Microglia and the urokinase plasminogen activator receptor/uPA system in innate brain inflammation

The urokinase plasminogen activator (uPA) receptor (uPAR) is a GPI-linked cell surface protein that facilitates focused plasmin proteolytic activity at the cell surface. uPAR has been detected in macrophages infiltrating the central nervous system (CNS) and soluble uPAR has been detected in the cerebrospinal fluid during a number of CNS pathologies. However, its expression by resident microglial cells in vivo remains uncertain. In this work, we aimed to elucidate the murine CNS expression of uPAR and uPA as well as that of tissue plasminogen activator and plasminogen activator inhibitor 1 (PAI-1) during insults generating distinct and well-characterized inflammatory responses; acute intracerebral lipopolysaccharide (LPS), acute kainate-induced neurodegeneration, and chronic neurodegeneration induced by prion disease inoculation. All three insults induced marked expression of uPAR at both mRNA and protein level compared to controls (naïve, saline, or control inoculum-injected). uPAR expression was microglial in all cases. Conversely, uPA transcription and activity was only markedly increased during chronic neurodegeneration. Dissociation of uPA and uPAR levels in acute challenges is suggestive of additional proteolysis-independent roles for uPAR. PAI-1 was most highly expressed upon LPS challenge, whereas tissue plasminogen activator mRNA was constitutively present and less responsive to all insults studied. These data are novel and suggest much wider involvement of the uPAR/uPA system in CNS function and pathology than previously supposed.

Reduction of canine plasminogen leads to an expanded molecule which precipitates

Canine plasminogen is made up of seven domains. In each domain there are several cysteines that are linked by disulfide bonds. Reduction of a limited number of the cystines destabilizes the protein such that it precipitates. The bond or bonds that are broken provide about 14 kcal of stabilization energy. Circular dichroism and dynamic light scattering indicate that there is probably an intermediate that is formed prior to precipitation and that the intermediate is somewhat larger than the compact form of plasminogen.

Characterization of prion susceptibility in Neuro2a mouse neuroblastoma cell subclones

In this study, we established Neuro2a (N2a) neuroblastoma subclones and characterized their susceptibility to prion infection. The N2a cells were treated with brain homogenates from mice infected with mouse prion strain Chandler. Of 31 N2a subclones, 19 were susceptible to prion as those cells became positive for abnormal isoform of prion protein (PrP(Sc)) for up to 9 serial passages, and the remaining 12 subclones were classified as unsusceptible. The susceptible N2a subclones expressed cellular prion protein (PrP(C)) at levels similar to the parental N2a cells. In contrast, there was a variation in PrP(C) expression in unsusceptible N2a subclones. For example, subclone N2a-1 expressed PrP(C) at the same level as the parental N2a cells and prion-susceptible subclones, whereas subclone N2a-24 expressed much lower levels of PrP mRNA and PrP(C) than the parental N2a cells. There was no difference in the binding of PrP(Sc) to prion-susceptible and unsusceptible N2a subclones regardless of their PrP(C) expression level, suggesting that the binding of PrP(Sc) to cells is not a major determinant for prion susceptibility. Stable expression of PrP(C) did not confer susceptibility to prion in unsusceptible subclones. Furthermore, the existence of prion-unsusceptible N2a subclones that expressed PrP(C) at levels similar to prion-susceptible subclones, indicated that a host factor(s) other than PrP(C) and/or specific cellular microenvironments are required for the propagation of prion in N2a cells. The prion-susceptible and -unsusceptible N2a subclones established in this study should be useful for identifying the host factor(s) involved in the prion propagation.

The prion protein family: Diversity, rivalry, and dysfunction

The prion gene family currently consists of three members: Prnp which encodes PrP(C), the precursor to prion disease associated isoforms such as PrP(Sc); Prnd which encodes Doppel, a testis-specific protein involved in the male reproductive system; and Sprn which encodes the newest PrP-like protein, Shadoo, which is expressed in the CNS. Although the identification of numerous candidate binding partners for PrP(C) has hinted at possible cellular roles, molecular interpretations of PrP(C) activity remain obscure and no widely-accepted view as to PrP(C) function has emerged. Nonetheless, studies into the functional interrelationships of prion proteins have revealed an interesting phenomenon: Doppel is neurotoxic to cerebellar cells in a manner which can be blocked by either PrP(C) or Shadoo. Further examination of this paradigm may help to shed light on two prominent unanswered questions in prion biology: the functional role of PrP(C) and the neurotoxic pathways initiated by PrP(Sc) in prion disease.

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 degrees C and 37 degrees 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 degrees C to above 15-20 degrees 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.

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.

Human prions and plasma lipoproteins

Prions are composed solely of an alternatively folded isoform of the prion protein (PrP), designated PrP(Sc). The polyoxometalate phosphotungstic acid has been used to separate PrP(Sc) from its precursor PrP(C) by selective precipitation; notably, native PrP(Sc) has not been solubilized by using nondenaturing detergents. Because of the similarities between PrP(Sc) and lipoproteins with respect to hydrophobicity and formation of phosphotungstic acid complexes, we asked whether these molecules are bound to each other in blood. Here we report that prions from the brains of patients with sporadic Creutzfeldt-Jakob disease (CJD) bind to very low-density (VLDL) and low-density (LDL) lipoproteins but not to high-density lipoproteins (HDL) or other plasma components, as demonstrated both by affinity assay and electron microscopy. Immunoassays demonstrated that apolipoprotein B (apoB), which is the major protein component of VLDL and LDL, bound PrP(Sc) through a highly cooperative process. Approximately 50% of the PrP(Sc) bound to LDL particles was released after exposure to 4 M guanidine hydrochloride at 80 degrees C for 20 min. The apparent binding constants of native human (Hu) PrP(Sc) or denatured recombinant HuPrP(90-231) for apoB and LDL ranged from 28 to 212 pM. Whether detection of PrP(Sc) in VLDL and LDL particles can be adapted into an antemortem diagnostic test for prions in the blood of humans, livestock, and free-ranging cervids remains to be determined.

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.