Simone Hornemann

NMR characterization of the full-length recombinant murine prion protein, mPrP(23–231)

The recombinant murine prion protein, mPrP(23–231), was expressed in E. coli with uniform 15N‐labeling. NMR experiments showed that the previously determined globular three‐dimensional structure of the C‐terminal domain mPrP(121–231) is preserved in the intact protein, and that the N‐terminal polypeptide segment 23–120 is flexibly disordered. This structural information is based on nearly complete sequence‐specific assignments for the backbone amide nitrogens, amide protons and α‐protons of the polypeptide segment of residues 121–231 in mPrP(23–231). Coincidence of corresponding sequential and medium‐range nuclear Overhauser effects (NOE) showed that the helical secondary structures previously identified in mPrP(121–231) are also present in mPrP(23–231), and near‐identity of corresponding amide nitrogen and amide proton chemical shifts indicates that the three‐dimensional fold of mPrP(121–231) is also preserved in the intact protein. The linewidths in heteronuclear 1H–15N correlation spectra and 15N{1H}‐NOEs showed that the well structured residues 126–230 have correlation times of several nanoseconds, as is typical for small globular proteins, whereas correlation times shorter than 1 nanosecond were observed for all residues of mPrP(23–231) outside of this domain.

Prion (PrPSc)-specific epitope defined by a monoclonal antibody

Prions are infectious particles causing transmissible spongiform encephalopathies (TSEs). They consist, at least in part, of an isoform (PrPSc) of the ubiquitous cellular prion protein (PrPC). Conformational differences between PrPCand PrPScare evident from increased β-sheet content and protease resistance in PrPSc(refs 1,2,3). Here we describe a monoclonal antibody, 15B3, that can discriminate between the normal and disease-specific forms of PrP. Such an antibody has been long sought as it should be invaluable for characterizing the infectious particle as well as for diagnosis of TSEs such as bovine spongiform encephalopathy (BSE) or Creutzfeldt–Jakob disease (CJD) in humans. 15B3 specifically precipitates bovine, murine or human PrPSc, but not PrPC, suggesting that it recognizes an epitope common to prions from different species. Using immobilized synthetic peptides, we mapped three polypeptide segments in PrP as the 15B3 epitope. In the NMR structure of recombinant mouse PrP, segments 2 and 3 of the 15B3 epitope are near neighbours in space, and segment 1 is located in a different part of the molecule. We discuss models forthe PrPSc-specific epitope that ensure close spatial proximity of all three 15B3 segments, either by intermolecular contacts in oligomeric forms of the prion protein or by intramolecular rearrangement.

The adaptor ASC has extracellular and 'prionoid' activities that propagate inflammation.

Microbes or danger signals trigger inflammasome sensors, which induce polymerization of the adaptor ASC and the assembly of ASC specks. ASC specks recruit and activate caspase-1, which induces maturation of the cytokine interleukin 1β (IL-1β) and pyroptotic cell death. Here we found that after pyroptosis, ASC specks accumulated in the extracellular space, where they promoted further maturation of IL-1β. In addition, phagocytosis of ASC specks by macrophages induced lysosomal damage and nucleation of soluble ASC, as well as activation of IL-1β in recipient cells. ASC specks appeared in bodily fluids from inflamed tissues, and autoantibodies to ASC specks developed in patients and mice with autoimmune pathologies. Together these findings reveal extracellular functions of ASC specks and a previously unknown form of cell-to-cell communication.

Prion strain discrimination using luminescent conjugated polymers

The occurrence of multiple strains of prions may reflect conformational variability of PrPSc, a disease-associated, aggregated variant of the cellular prion protein, PrPC. Here we used luminescent conjugated polymers (LCPs), which emit conformation-dependent fluorescence spectra, for characterizing prion strains. LCP reactivity and emission spectra of brain sections discriminated among four immunohistochemically indistinguishable, serially mouse-passaged prion strains derived from sheep scrapie, chronic wasting disease (CWD), bovine spongiform encephalopathy (BSE), and mouse-adapted Rocky Mountain Laboratory scrapie prions. Furthermore, using LCPs we differentiated between field isolates of BSE and bovine amyloidotic spongiform encephalopathy, and identified noncongophilic deposits in prion-infected deer and sheep. We found that fibrils with distinct morphologies generated from chemically identical recombinant PrP yielded unique LCP spectra, suggesting that spectral characteristic differences resulted from distinct supramolecular PrP structures. LCPs may help to detect structural differences among discrete protein aggregates and to link protein conformational features with disease phenotypes.

Recombinant full‐length murine prion protein, mPrP(23–231): purification and spectroscopic characterization

The cellular prion protein of the mouse, mPrPC, consists of 208 amino acids (residues 23–231). It contains a carboxy‐terminal domain, mPrP(121–231), which represents an autonomous folding unit with three α‐helices and a two‐stranded antiparallel β‐sheet. We expressed the complete amino acid sequence of the prion protein, mPrP(23–231), in the cytoplasm of Escherichia coli. mPrP(23–231) was solubilized from inclusion bodies by 8 M urea, oxidatively refolded and purified to homogeneity by conventional chromatographic techniques. Comparison of near‐UV circular dichroism, fluorescence and one‐dimensional 1H‐NMR spectra of mPrP(23–231) and mPrP(121–231) shows that the amino‐terminal segment 23–120, which includes the five characteristic octapeptide repeats, does not contribute measurably to the manifestation of three‐dimensional structure as detected by these techniques, indicating that the residues 121–231 might be the only polypeptide segment of PrPC with a defined three‐dimensional structure.

De novo generation of a transmissible spongiform encephalopathy by mouse transgenesis.

Most transmissible spongiform encephalopathies arise either spontaneously or by infection. Mutations of PRNP, which encodes the prion protein, PrP, segregate with phenotypically similar diseases. Here we report that moderate overexpression in transgenic mice of mPrP(170N,174T), a mouse PrP with two point mutations that subtly affect the structure of its globular domain, causes a fully penetrant lethal spongiform encephalopathy with cerebral PrP plaques. This genetic disease was reproduced with 100% attack rate by intracerebral inoculation of brain homogenate to tga20 mice overexpressing WT PrP, and from the latter to WT mice, but not to PrP-deficient mice. Upon successive transmissions, the incubation periods decreased and PrP became more protease-resistant, indicating the presence of a strain barrier that was gradually overcome by repeated passaging. This shows that expression of a subtly altered prion protein, with known 3D structure, efficiently generates a prion disease.

NMR structure of the bovine prion protein isolated from healthy calf brains.

NMR structures of recombinant prion proteins from various species expressed in Escherichia coli have been solved during the past years, but the fundamental question of the relevancy of these data relative to the naturally occurring forms of the prion protein has not been directly addressed. Here, we present a comparison of the cellular form of the bovine prion protein isolated and purified from healthy calf brains without use of detergents, so that it contains the two carbohydrate moieties and the part of the GPI anchor that is maintained after enzymatic cleavage of the glycerolipid moiety, with the recombinant bovine prion protein expressed in E. coli. We show by circular dichroism and 1H‐NMR spectroscopy that the three‐dimensional structure and the thermal stability of the natural glycoprotein and the recombinant polypeptide are essentially identical. This result indicates possible functional roles of the glycosylation of prion proteins in healthy organisms, and provides a platform and validation for future work on the structural biology of prion proteins, which will have to rely primarily on the use of recombinant polypeptides.

The POM Monoclonals: A Comprehensive Set of Antibodies to Non-Overlapping Prion Protein Epitopes

PrPSc, a misfolded and aggregated form of the cellular prion protein PrPC, is the only defined constituent of the transmissible agent causing prion diseases. Expression of PrPC in the host organism is necessary for prion replication and for prion neurotoxicity. Understanding prion diseases necessitates detailed structural insights into PrPC and PrPSc. Towards this goal, we have developed a comprehensive collection of monoclonal antibodies denoted POM1 to POM19 and directed against many different epitopes of mouse PrPC. Three epitopes are located within the N-terminal octarepeat region, one is situated within the central unstructured region, and four epitopes are discontinuous within the globular C-proximal domain of PrPC. Some of these antibodies recognize epitopes that are resilient to protease digestion in PrPSc. Other antibodies immunoprecipitate PrPC, but not PrPSc. A third group was found to immunoprecipitate both PrP isoforms. Some of the latter antibodies could be blocked with epitope-mimicking peptides, and incubation with an excess of these peptides allowed for immunochromatography of PrPC and PrPSc. Amino-proximal antibodies were found to react with repetitive PrPC epitopes, thereby vastly increasing their avidity. We have also created functional single-chain miniantibodies from selected POMs, which retained the binding characteristics despite their low molecular mass. The POM collection, thus, represents a unique set of reagents allowing for studies with a variety of techniques, including western blotting, ELISA, immunoprecipitation, conformation-dependent immunoassays, and plasmon surface plasmon resonance-based assays.

The toxicity of antiprion antibodies is mediated by the flexible tail of the prion protein

Prion infections cause lethal neurodegeneration. This process requires the cellular prion protein (PrPC; ref. 1), which contains a globular domain hinged to a long amino-proximal flexible tail. Here we describe rapid neurotoxicity in mice and cerebellar organotypic cultured slices exposed to ligands targeting the α1 and α3 helices of the PrPC globular domain. Ligands included seven distinct monoclonal antibodies, monovalent Fab1 fragments and recombinant single-chain variable fragment miniantibodies. Similar to prion infections, the toxicity of globular domain ligands required neuronal PrPC, was exacerbated by PrPC overexpression, was associated with calpain activation and was antagonized by calpain inhibitors. Neurodegeneration was accompanied by a burst of reactive oxygen species, and was suppressed by antioxidants. Furthermore, genetic ablation of the superoxide-producing enzyme NOX2 (also known as CYBB) protected mice from globular domain ligand toxicity. We also found that neurotoxicity was prevented by deletions of the octapeptide repeats within the flexible tail. These deletions did not appreciably compromise globular domain antibody binding, suggesting that the flexible tail is required to transmit toxic signals that originate from the globular domain and trigger oxidative stress and calpain activation. Supporting this view, various octapeptide ligands were not only innocuous to both cerebellar organotypic cultured slices and mice, but also prevented the toxicity of globular domain ligands while not interfering with their binding. We conclude that PrPC consists of two functionally distinct modules, with the globular domain and the flexible tail exerting regulatory and executive functions, respectively. Octapeptide ligands also prolonged the life of mice expressing the toxic PrPC mutant, PrP(Δ94–134), indicating that the flexible tail mediates toxicity in two distinct PrPC-related conditions. Flexible tail-mediated toxicity may conceivably play a role in further prion pathologies, such as familial Creutzfeldt-Jakob disease in humans bearing supernumerary octapeptides.

A molecular switch controls interspecies prion disease transmission in mice.

Transmissible spongiform encephalopathies are lethal neurodegenerative disorders that present with aggregated forms of the cellular prion protein (PrPC), which are known as PrPSc. Prions from different species vary considerably in their transmissibility to xenogeneic hosts. The variable transmission barriers depend on sequence differences between incoming PrPSc and host PrPC and additionally, on strain-dependent conformational properties of PrPSc. The beta2-alpha2 loop region within PrPC varies substantially between species, with its structure being influenced by the residue types in the 2 amino acid sequence positions 170 (most commonly S or N) and 174 (N or T). In this study, we inoculated prions from 5 different species into transgenic mice expressing either disordered-loop or rigid-loop PrPC variants. Similar beta2-alpha2 loop structures correlated with efficient transmission, whereas dissimilar loops correlated with strong transmission barriers. We then classified literature data on cross-species transmission according to the 170S/N polymorphism. Transmission barriers were generally low between species with the same amino acid residue in position 170 and high between those with different residues. These findings point to a triggering role of the local beta2-alpha2 loop structure for prion transmissibility between different species.