Expression of amino-terminally truncated PrP in the mouse leading to ataxia and specific cerebellar lesions

Determinants of the in vivo folding of the prion protein. A bipartite function of helix 1 in folding and aggregation

Misfolding of the mammalian prion protein (PrP) is implicated in the pathogenesis of prion diseases. We analyzed wild type PrP in comparison with different PrP mutants and identified determinants of the in vivo folding pathway of PrP. The complete N terminus of PrP including the putative transmembrane domain and the first beta-strand could be deleted without interfering with PrP maturation. Helix 1, however, turned out to be a major determinant of PrP folding. Disruption of helix 1 prevented attachment of the glycosylphosphatidylinositol (GPI) anchor and the formation of complex N-linked glycans; instead, a high mannose PrP glycoform was secreted into the cell culture supernatant. In the absence of a C-terminal membrane anchor, however, helix 1 induced the formation of unglycosylated and partially protease-resistant PrP aggregates. Moreover, we could show that the C-terminal GPI anchor signal sequence, independent of its role in GPI anchor attachment, mediates core glycosylation of nascent PrP. Interestingly, conversion of high mannose glycans to complex type glycans only occurred when PrP was membrane-anchored. Our study indicates a bipartite function of helix 1 in the maturation and aggregation of PrP and emphasizes a critical role of a membrane anchor in the formation of complex glycosylated PrP.

Abbreviated incubation times for human prions in mice expressing a chimeric mouse-human prion protein transgene

Transgenic (Tg) mouse lines that express chimeric mouse-human prion protein (PrP), designated MHu2M, are susceptible to prions from patients with sporadic Creutzfeldt-Jakob disease (sCJD). With the aim of decreasing the incubation time to fewer than 200 days, we constructed transgenes in which one or more of the nine human residues in MHu2M were changed to mouse. The construct with murine residues at positions 165 and 167 was expressed in Tg(MHu2M,M165V,E167Q) mice and resulted in shortening the incubation time to approximately 110 days for prions from sCJD patients. The construct with a murine residue at position 96 resulted in lengthening the incubation time to more than 280 days for sCJD prions. When murine residues 96, 165, and 167 were expressed, the abbreviated incubation times for sCJD prions were abolished. Variant CJD prions showed prolonged incubation times between 300 and 700 days in Tg(MHu2M) mice on first passage and incubation times of approximately 350 days in Tg(MHu2M,M165V,E167Q) mice. On second and third passages of variant CJD prions in Tg(MHu2M) mice, multiple strains of prions were detected based on incubation times and the sizes of the protease-resistant, deglycosylated PrP(Sc) fragments. Our discovery of a previously undescribed chimeric transgene with abbreviated incubation times for sCJD prions should facilitate studies on the prion species barrier and human prion diversity.

NMR structure of the human doppel protein

The NMR structure of the recombinant human doppel protein, hDpl(24-152), contains a flexibly disordered "tail" comprising residues 24-51, and a globular domain extending from residues 52 to 149 for which a detailed structure was obtained. The globular domain contains four alpha-helices comprising residues 72-80 (alpha1), 101-115 (alpha2(a)), 117-121 (alpha2(b)), and 127-141 (alpha3), and a short two-stranded anti-parallel beta-sheet comprising residues 58-60 (beta1) and 88-90 (beta2). The fold of the hDpl globular domain thus coincides nearly identically with the structure of the murine Dpl protein. There are close similarities with the human prion protein (hPrP) but, similar to the situation with the corresponding murine proteins, hDpl shows marked local differences when compared to hPrP: the beta-sheet is flipped by 180 degrees with respect to the molecular scaffold formed by the four helices, and the beta1-strand is shifted by two residues toward the C terminus. A large solvent-accessible hydrophobic cleft is formed on the protein surface between beta2 and alpha3, which has no counterpart in hPrP. The helix alpha2 of hPrP is replaced by two shorter helices, alpha2(a) and alpha2(b). The helix alpha3 is shortened by more than two turns when compared with alpha3 of hPrP, which is enforced by the positioning of the second disulfide bond in hDpl. The C-terminal peptide segment 144-149 folds back onto the loop connecting beta2 and alpha2. All but four of the 20 conserved residues in the globular domains of hPrP and hDpl appear to have a structural role in maintaining a PrP-type fold. The conservation of R76, E96, N110 and R134 in hDpl, corresponding to R148, E168, N183 and R208 in hPrP suggests that these amino acid residues might have essential roles in the so far unknown functions of PrP and Dpl in healthy organisms.

Three-dimensional structures of the prion protein and its doppel

This article discussed the implications of the structures of PrP and Dpl--with their unusual folds containing N-terminal flexible tails and C-terminal globular domains--to the physiologic functions of PrPC and Dpl, and investigations of a possible structural basis of familial human TSEs. Further relations between TSEs and the PrP structure would include the species barrier of TSEs (which seems to be associated with species-specific structural characteristics of PrPC [25,39,67]), and the conformational transition from PrPC to PrPSc using, for example, molecular dynamic simulations [68,69]. Due to the lack of knowledge on physiologic functions of PrPC, however, and the remaining uncertainty about the exact role of the PrP in TSE pathology, it appears that most or all of the physiologically relevant structure-function correlations of PrPC have yet to be identified.

Temporally controlled site-specific mutagenesis in the germ cell lineage of the mouse testis

We have obtained a PrP-Cre-ER(T) transgenic mouse line (28.8) that selectively expresses in testis the tamoxifen-inducible Cre-ER(T) recombinase under the control of a mouse Prion protein (PrP) promoter-containing genomic fragment. Cre-ER(T) is expressed in spermatogonia and spermatocytes, but not in Sertoli and Leydig cells. We also established reporter PrP-L-EGFP-L transgenic mice harboring a LoxP-flanked enhanced green fluorescent protein (EGFP) Cre reporter cassette under the control of the same PrP promoter-containing genomic fragment that exhibits prominent EGFP expression in brain and testis. Using the PrP-L-EGFP-L as well as other Cre-reporter mice, we demonstrate that tamoxifen administration efficiently and selectively induces Cre-mediated recombination in the germ cell lineage. The established PrP-Cre-ER(T) line should provide a valuable tool for studying functions of germ cell-expressed genes involved in spermatogenesis.

Prion protein as trans-interacting partner for neurons is involved in neurite outgrowth and neuronal survival

Many uncertainties remain regarding the physiological function of the prion protein PrP and the consequences of its conversion into the pathological scrapie isoform in prion diseases. Here, we show for the first time that different signal transduction pathways are involved in neurite outgrowth and neuronal survival elicited by PrP in cell culture of primary neurons. These pathways include the nonreceptor Src-related family member p59(Fyn), PI3 kinase/Akt, cAMP-dependent protein kinase A, and MAP kinase. Regulation of Bcl-2 and Bax expression also correlates with the survival effect elicited by PrP. The combined results, along with our observation that PrP carries the recognition molecule-related HNK-1 carbohydrate, argue strongly for a role of the molecule in neural recognition by interacting with yet unknown heterophilic neuronal receptors, as shown by comparison of neurite outgrowth from neurons of PrP-deficient and wild-type mice.

CNS pathogenesis of prion diseases

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative diseases, clinically characterised by cognitive decline, paralleled by severe damage to the central nervous system. Prion diseases have attracted a broad interest because of their unique mechanisms of replication and propagation; however, the underlying pathogenic mechanisms are still highly speculative. In this review, current knowledge about the pathogenesis of prion diseases in the CNS will be highlighted and the most revealing animal models will be discussed, with future perspectives to address immediate questions about the pathogenesis.

Prions and the immune system: a journey through gut, spleen, and nerves

For more than two decades it has been contended that prion infection does not elicit immune responses: transmissible spongiform encephalopathies do not go along with conspicuous inflammatory infiltrates, and antibodies to the prion protein are typically undetectable. Why is it, then, that prions accumulate in lymphoid organs, and that various states of immune deficiency prevent peripheral prion infection? This review revisits the current evidence of the involvement of the immune system in prion diseases, while attempting to trace the elaborate mechanisms by which peripherally administered prions invade the brain and ultimately cause damage. The investigation of these questions leads to unexpected detours, including the neurophysiology of lymphoid organs, and even the function of a prion protein homolog in male fertility.

Putative functions of PrP(C)

While the exact function of the cellular prion protein (PrP(C)) remains unknown, there are several leads due to increasing knowledge on the localisation and interaction of PrP(C) with other molecules. This chapter will concentrate on these aspects. Identified ligands of PrP(C) mainly belong to the categories of heat-shock proteins, membrane-bound receptors, or heparan sulphates. The possible synaptic role of PrP(C) has been exemplified by electrophysiological findings in PrP(o/o) mice and the studies of PrP(C) as a copper-binding molecule that could regulate the copper content of the synaptic cleft. The latter property of PrP(C) may also endow PrP(C) with the activity of a copper-dependent superoxide dismutase. Binding of PrP(C) to signalling molecules suggests a role as a transmitter of information from the extracellular milieu to the cell and a trigger for a molecular cascade. This agrees with new data on PrP(C) receptors and the role of PrP(C) in cell survival.

Physiological and pathological functions of the prion protein homologue Dpl

A misfolded version of the prion protein PrP(C), known as PrP(Sc), is the major component of scrapie infectivity, the pathological agent in transmissible spongiform encephalopathies. The Prnp gene that encodes the cellular PrP(C) protein was cloned almost 20 years ago, but remained without sequence or structural relatives for over a decade. Only recently a novel protein, named Doppel (Dpl), was identified, which shares significant biochemical and structural homology with PrP(C). When overexpressed, Dpl is neurotoxic and causes a neurological disease. Strikingly, Dpl neurotoxicity is counteracted and prevented by PrP(C). In contrast to its homologue PrP(C), Dpl is dispensable for prion disease progression and for the generation of PrP(Sc), but Dpl appears to have an essential function in male spermatogenesis. Although Dpl research is still in its infancy, the discovery of Dpl has already solved some enigmas of prion biology and an understanding of its physiological function is emerging.

PrP knock-out and PrP transgenic mice in prion research

Spongiform encephalopathies such as scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle or Creutzfeldt-Jacob disease (CJD) and Gerstmann-Sträussler-Scheinker syndrome (GSS) in humans is caused by a transmissible agent designated prion. The 'protein only' hypothesis proposes that the prion consists partly or entirely of a conformational isoform of the normal host protein PrP(C), designated PrP(*)(1) and that the abnormal conformer, when introduced into the organism, causes the conversion of PrP(C) into a likeness of itself. PrP(*) may be congruent with PrP(Sc), a protease-resistant, aggregated conformer of PrP that accumulates mainly in brain of almost all prion-infected organisms. PrP(C) consists of a flexible N-terminal half, comprising Cu(2+)-binding octapeptide repeats, and a globular domain consisting of three alpha-helices, one short antiparallel beta-sheet and a single disulphide bond. It is anchored at the outer cell-surface by a glycosyl phosphatidylinositol (GPI) tail and is present in almost all tissues, however, mainly in brain. Compelling linkage between the prion and PrP was established by biochemical and genetic data and led to the prediction that animals devoid of PrP should be resistant to experimental scrapie and fail to propagate infectivity. This prediction was indeed borne out, adding substantial support to the 'protein only' hypothesis. In addition, the availability of PrP knock-out mice provided an approach to carry out reverse genetics on PrP, both in regard to prion disease and to its physiological role.

A cluster of familial Creutzfeldt-Jakob disease mutations recapitulate conserved residues in Doppel: a case of molecular mimicry?

Intrachromosomal deletions linking Dpl expression to the PrP promoter produce cerebellar degeneration that can be abrogated by the introduction of wild-type PrP transgenes. Since Dpl-like truncated forms of PrP are neuropathogenic in mice and likewise counterbalanced by expression of PrP(C) we asked whether naturally occurring mutant forms of human PrP have Dpl-like attributes. Five PRNP missense mutations causing familial Creutzfeldt-Jakob disease (F-CJD) map to a helical region found in both PrP(C) and Dpl and result in amino acids identical to conserved residues in Dpl. These F-CJD alleles may cause mutant PrP to become a weak mimetic of Dpl structure and/or function.

Grand ideas floating freely. Conference on the new prion biology: basic science, diagnosis and therapy

Conference on the new prion biology: basic science, diagnosis and therapy

Transgenesis applied to transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSE) are fatal neurodegenerative disorders present in various mammals. TSEs have been studies intensively, even more so following the BSE crisis and the subsequent threat of a human nvCJD epidemic. In the 'protein-only' hypothesis, the infectious agent, called prion, is assumed to be a misfolded host protein. Transgenesis has mainly been applied to study the role of this protein, its structure-function relationship with respect to its pathogenic properties and to assess the genetic origin of the well-recognised species barrier effect. This approach has somewhat supplemented the lack of in vitro models. This review will try to summarise the impressive work that has been done in this field. Although many questions remain unanswered, transgenic experiments have and will still improve our knowledge on this disease and might help us to develop critically needed therapeutic approaches.

Neurotoxicity and neurodegeneration when PrP accumulates in the cytosol

Changes in prion protein (PrP) folding are associated with fatal neurodegenerative disorders, but the neurotoxic species is unknown. Like other proteins that traffic through the endoplasmic reticulum, misfolded PrP is retrograde transported to the cytosol for degradation by proteasomes. Accumulation of even small amounts of cytosolic PrP was strongly neurotoxic in cultured cells and transgenic mice. Mice developed normally but acquired severe ataxia, with cerebellar degeneration and gliosis. This establishes a mechanism for converting wild-type PrP to a highly neurotoxic species that is distinct from the self-propagating PrP(Sc) isoform and suggests a potential common framework for seemingly diverse PrP neurodegenerative disorders.

Transmissible spongiform encephalopathies: a family of etiologically complex diseases--a review

The upsurge of 'mad cow disease' with its human implications has raised the problem of the etiological mechanisms and the similarities or differences underlying the family of transmissible spongiform encephalopathies. Structural properties of prions are reviewed in connection with their natural distribution and functions, factors of transmissibility and mechanisms of pathogenicity. Polymorphism is examined in relation to disease phenotype variants. The role of oxidative factors is emphasized, while raising complexity about the role of copper ions. Further investigation directions are suggested.

Endocytic intermediates involved with the intracellular trafficking of a fluorescent cellular prion protein

We have investigated the intracellular traffic of PrP(c), a glycosylphosphatidylinositol (GPI)-anchored protein implicated in spongiform encephalopathies. A fluorescent functional green fluorescent protein (GFP)-tagged version of PrP(c) is found at the cell surface and in intracellular compartments in SN56 cells. Confocal microscopy and organelle-specific markers suggest that the protein is found in both the Golgi and the recycling endosomal compartment. Perturbation of endocytosis with a dynamin I-K44A dominant-negative mutant altered the steady-state distribution of the GFP-PrP(c), leading to the accumulation of fluorescence in unfissioned endocytic intermediates. These pre-endocytic intermediates did not seem to accumulate GFP-GPI, a minimum GPI-anchored protein, suggesting that PrP(c) trafficking does not depend solely on the GPI anchor. We found that internalized GFP-PrP(c) accumulates in Rab5-positive endosomes and that a Rab5 mutant alters the steady-state distribution of GFP-PrP(c) but not that of GFP-GPI between the plasma membrane and early endosomes. Therefore, we conclude that PrP(c) internalizes via a dynamin-dependent endocytic pathway and that the protein is targeted to the recycling endosomal compartment via Rab5-positive early endosomes. These observations indicate that traffic of GFP-PrP(c) is not determined predominantly by the GPI anchor and that, different from other GPI-anchored proteins, PrP(c) is delivered to classic endosomes after internalization.

Mayhem of the multiple mechanisms: modelling neurodegeneration in prion disease

This review examines recent attempts to advance the understanding of the mechanism by which neurones die in prion disease. Prion diseases or transmissible spongiform encephalopathies are characterized by the conversion of a normal glycoprotein, the prion protein, to a protease-resistant form that is suggested to be both the infectious agent and the cause of the rapid neurodegeneration in the disease. Death of the patient results from this widespread neuronal loss. Thus understanding the mechanism by which the abnormal form of the prion protein causes neuronal death might lead to treatments that would prevent the life-threatening nature of these diseases.

Cellular prion protein transduces neuroprotective signals

To test for a role for the cellular prion protein (PrP(c)) in cell death, we used a PrP(c)-binding peptide. Retinal explants from neonatal rats or mice were kept in vitro for 24 h, and anisomycin (ANI) was used to induce apoptosis. The peptide activated both cAMP/protein kinase A (PKA) and Erk pathways, and partially prevented cell death induced by ANI in explants from wild-type rodents, but not from PrP(c)-null mice. Neuroprotection was abolished by treatment with phosphatidylinositol-specific phospholipase C, with human peptide 106-126, with certain antibodies to PrP(c) or with a PKA inhibitor, but not with a MEK/Erk inhibitor. In contrast, antibodies to PrP(c) that increased cAMP also induced neuroprotection. Thus, engagement of PrP(c) transduces neuroprotective signals through a cAMP/PKA-dependent pathway. PrP(c) may function as a trophic receptor, the activation of which leads to a neuroprotective state.

Mammalian prion proteins: enigma, variation and vaccination

Although misfolding of the cellular prion protein PrP(C) into an alternative form, denoted PrP(Sc), is a key event in prion infections, the normal function of PrP(C) remains to be clearly defined. Many PrP(C)-binding proteins have been identified, but authentication of these interactions in functional assays is incomplete. Doppel (Dpl), a recently discovered PrP-like protein, might provide a new avenue by which to explore physiological and pathological functions of PrP. For example, overexpression of Dpl causes apoptotic cerebellar cell death that is abrogated by PrP(C), indicating that these two proteins can act in a common pathway. Despite our incomplete understanding of PrP(C), immunological targeting of this PrP(Sc) precursor has produced encouraging results, indicating a potential point of intervention against these fatal diseases.

Biology of the prion gene complex

The prion protein gene Prnp encodes PrPSc, the major structural component of prions, infectious pathogens causing a number of disorders including scrapie and bovine spongiform encephalopathy (BSE). Missense mutations in the human Prnp gene, PRNP, cause inherited prion diseases such as familial Creutzfeldt-Jakob Disease. In uninfected animals, Prnp encodes a GPI-anchored protein denoted PrPC, and in prion infections, PrPC is converted to PrPSc by templated refolding. Although Prnp is conserved in mammalian species, attempts to verify interactions of putative PrP-binding proteins by genetic means have proven frustrating in that two independent lines of Prnp gene ablated mice (Prnp0/0 mice: ZrchI and Npu) lacking PrPC remain healthy throughout development. This indicates that PrPC serves a function that is not apparent in a laboratory setting or that other molecules have overlapping functions. Shuttling or sequestration of synaptic Cu(II) via binding to N-terminal octapeptide residues and (or) signal transduction involving the fyn kinase are possibilities currently under consideration. A new point of entry into the issue of prion protein function has emerged from identification of a paralog, Prnd, with 25% coding sequence identity to Prnp. Prnd lies downstream of Prnp and encodes the Dpl protein. Like PrPC, Dpl is presented on the cell surface via a GPI anchor and has three alpha-helices: however, it lacks the conformationally plastic and octapeptide repeat domains present in its well-known relative. Interestingly, Dpl is overexpressed in two other lines of Prnp0/0 mice (Ngsk and Rcm0) via intergenic splicing events. These lines of Prnp0/0 mice exhibit ataxia and apoptosis of cerebellar cells, indicating that ectopic synthesis of Dpl protein is toxic to CNS neurons: this inference has now been confirmed by the construction of transgenic mice expressing Dpl under the direct control of the PrP promoter. Remarkably, Dpl-programmed ataxia is rescued by wt Prnp transgenes. The interaction between the Prnp and Prnd genes in mouse cerebellar neurons may have a physical correlate in competition between Dpl and PrPC within a common biochemical pathway that, when misregulated, leads to apoptosis.

Molecular genetics of transmissible spongiform encephalopathies: an introduction

Prnp knockout mice disrupted PrPC-related genes have played an essential role to elucidate the relationship between PrPC, a normal host gene product, and PrPSc, a protease-resistant, infectious PrP; Prnp knockout mice developed by Büeler et al. (1992) were completely protected against scrapie disease when challenged with mouse prions. Further, varying expression levels in PrPC were revisited along with a varying susceptibility of mouse prions, when mouse Prnp genes were introduced into Prnp% mice. How these murine models for human prion-related disease would contribute to the presently ongoing TSE research?

Similar and divergent features in mammalian and yeast prions

Mammalian transmissible spongiform encephalopathies are likely due to the propagation of an abnormal form of a constitutive protein instead of traditional genetic material (nucleic acids). Such infectious proteins, which are termed prions, exist in yeast. They are at the origin of a number of phenotypes that are inherited in a non-Mendelian manner. These prions are very useful to dissect the molecular events at the origin of this structure-based inheritance. The properties of mammalian and yeast prions are presented and compared. This review highlights a number of similarities and differences.

Small is not beautiful: antagonizing functions for the prion protein PrP(C) and its homologue Dpl

A conformational variant of the normal prion protein PrP(C) is believed to be identical to PrP(Sc), the agent that causes prion diseases. Recently, a novel protein, named Doppel (Dpl), was identified that shares significant biochemical and structural homology with PrP(C). In specific strains of PrP(C)-deficient mouse lines, Dpl is overexpressed and causes a neurological disease. Dpl neurotoxicity is counteracted and prevented by PrP(C), but the mechanism of antagonistic PrP(C)-Dpl interaction remains elusive. In contrast to its homologue PrP(C), initial studies suggest that Dpl is dispensable for prion disease progression and for the generation of PrP(Sc). Although we are only beginning to understand its function, the discovery of Dpl has already provided some answers to long-standing questions and is transforming our understanding of prion biology.

Expression of doppel in the CNS of mice does not modulate transmissible spongiform encephalopathy disease

Late onset ataxia reported in three independently derived PrP null lines of mice has been attributed to the overexpression of the doppel protein in the CNS of these mice rather than to the loss of PrP. The central role of PrP in the transmissible spongiform encephalopathies (TSEs), the proximity of the gene which encodes doppel (Prnd) to the PrP gene (Prnp) and the structural similarity shared by PrP and doppel have led to the proposition that ataxia which develops during TSE disease could, in part, be due to doppel. In order to address this hypothesis, we have crossed our two inbred lines of PrP null mice, which either express (RCM) or do not express (NPU) the Prnd gene in the CNS, with mice expressing two Prnp(a[108F189V]) alleles of the PrP gene. We have found that the TSE infection does not influence the level of expression of Prnd in the CNS at the terminal stages of disease. Moreover, we have demonstrated that the level of expression of Prnd in the CNS has no influence on the incubation period, vacuolar pathology nor amount or distribution of PrP(Sc) deposition in the brains of the TSE-infected mice. Doppel has therefore no apparent influence on the outcome of TSE disease in transgenic mice, suggesting it is unlikely to be involved in the naturally occurring TSE diseases in other species.

PrP(c) expression influences the establishment of herpes simplex virus type 1 latency

PrP(c) is a glycophosphatidylinositol-linked cell-surface protein expressed principally by neural tissue. The normal function of this protein is unestablished, although a role in either transmembrane signaling, cell-cell adhesion, or copper metabolism has been proposed. In this study we have investigated the effect of the neurotropic virus herpes simplex virus type 1 (HSV-1) in strains of mice which express different levels of PrP(c). Viral gene expression under the control of the HSV-1 early promoter IE110, detected either by in situ hybridization for RNA transcripts or by beta-galactosidase (beta-Gal) activity from an inserted lacZ gene, showed that the magnitude of HSV replication was retarded in PrP-/- mice. This was reflected in the lower level of acute viral titers in tissues from these virus-inoculated mice. However, HSV-inoculated PrP-/- mice contained higher levels of latent virus in both peripheral and central nervous tissue than those seen in mice which express PrP(c). Our observations show that lack of PrP(c) expression favors the establishment of HSV latency whereas HSV replication proceeds more efficiently in neuronal tissue that expresses this protein. The data further suggest that PrP(c) may be involved in a metabolic pathway that culminates in apoptosis of neurons that have been infected by neurotropic viruses.

Sequence determination and expression of the ovine doppel-encoding gene in transgenic mice

The ovine Doppel-encoding gene transcription unit (TU) and proximal flanking regions were cloned from a bacterial artificial chromosome (BAC) and sequenced. The 4586 bp TU is composed of two exons and one intron. When compared with its human counterpart, beside the open reading frame and part of the 3' untranslated sequence, significant regions of homology were found within the intron and the proximal 5' flanking regions. Sequence analysis of the BAC clone revealed that the ovine Prnd gene is located around 52 kb downstream of the Prnp locus. Expression of the sheep and murine Prnd genes was observed in all tissues analyzed of transgenic mice bearing the BAC insert, with a noticeable highest expression level observed in the testis, with no associated noticeable phenotype. The present data suggest that, in contrast to ovine Prnp, ovine Prnd expression in transgenic mice has no obvious influence on conferred susceptibility to scrapie.

Post-natal knockout of prion protein alters hippocampal CA1 properties, but does not result in neurodegeneration

Prion protein (PrP) plays a crucial role in prion disease, but its physiological function remains unclear. Mice with gene deletions restricted to the coding region of PrP have only minor phenotypic deficits, but are resistant to prion disease. We generated double transgenic mice using the Cre-loxP system to examine the effects of PrP depletion on neuronal survival and function in adult brain. Cre-mediated ablation of PrP in neurons occurred after 9 weeks. We found that the mice remained healthy without evidence of neurodegeneration or other histopathological changes for up to 15 months post-knockout. However, on neurophysiological evaluation, they showed significant reduction of afterhyperpolarization potentials (AHPs) in hippocampal CA1 cells, suggesting a direct role for PrP in the modulation of neuronal excitability. These data provide new insights into PrP function. Furthermore, they show that acute depletion of PrP does not affect neuronal survival in this model, ruling out loss of PrP function as a pathogenic mechanism in prion disease and validating therapeutic approaches targeting PrP.

Involvement of caveolae and caveolae-like domains in signalling, cell survival and angiogenesis

Caveolae, the flask-shaped membrane invaginations abundant in endothelial cells, have acquired a prominent role in signal transduction. Evidence, that events occurring in caveolae participate in cell survival and angiogenesis, has been recently substantiated by the identification of two novel caveolar constituents: prostacyclin synthase (PGIS) and the cellular form of prion protein (PrP(c)). We have shown that PGIS, previously described as an endoplasmic reticulum component, is bound to caveolin-1 (cav-1) and localized in caveolae in human endothelial cells. By generating prostacyclin, PGIS is involved in angiogenesis. Previous observations regarding the localization of PrP(c) in caveolae-like membrane domains (CLDs) have been recently confirmed and extended. It has been demonstrated that PrP(c) is bound to cav-1 and, by recruiting Fyn kinase, can participate in signal transduction events connected to cell survival and differentiation. The new entries of PGIS and PrP(c) in caveolar components place caveolae and CLDs at the centre of a network, where cells decide whether to proliferate or differentiate and whether to survive or to suicide by apoptosis.

From stem cells to prion signalling

A strategy based upon the introduction of an adenovirus-SV40 plasmid into multipotential cells was designed to immortalize clones displaying properties of lineage stem cells. The murine 1C11 cell line behaves as a neuroepithelial progenitor. Upon appropriate induction, almost 100% of 1C11 precursor cells develop neurite extensions and convert into either serotonergic or noradrenergic neurons. The two mutually exclusive neuronal programs are autoregulated by serotonergic or adrenergic receptors. PrPc is constitutively expressed by 1C11 cells. Antibody-mediated cross-linking of PrPc promotes the dephosphorylation of the tyrosine kinase Fyn associated to a Fyn kinase activation. The coupling of PrPc to Fyn is dependent on caveolin-1. It is restricted to the fully differentiated serotonergic or noradrenergic cells and occurs mainly at neurites. Thus, PrPc may represent a signal transduction protein which may fine-tune neuronal functions. Since the 1C11 stem cell supports prion replication, it may provide a tool to investigate whether PrPSc accumulation interferes with PrPc signalling activity.

Doppel-induced cerebellar degeneration in transgenic mice

Doppel (Dpl) is a paralog of the mammalian prion protein (PrP); it is abundant in testes but expressed at low levels in the adult central nervous system. In two Prnp-deficient (Prnp(0/0)) mouse lines (Ngsk and Rcm0), Dpl overexpression correlated with ataxia and death of cerebellar neurons. To determine whether Dpl overexpression, rather than the dysregulation of genes neighboring the Prn gene complex, was responsible for the ataxic syndrome, we placed the mouse Dpl coding sequence under the control of the Prnp promoter and produced transgenic (Tg) mice on the Prnp(0/0)-ZrchI background (hereafter referred to as ZrchI). ZrchI mice exhibit neither Dpl overexpression nor cerebellar degeneration. In contrast, Tg(Dpl)ZrchI mice showed cerebellar granule and Purkinje cell loss; the age of onset of ataxia was inversely proportional to the levels of Dpl protein. Crosses of Tg mice overexpressing wild-type PrP with two lines of Tg(Dpl)ZrchI mice resulted in a phenotypic rescue of the ataxic syndrome, while Dpl overexpression was unchanged. Restoration of PrP expression also rendered the Tg(Dpl) mice susceptible to prion infection, with incubation times indistinguishable from non-Tg controls. Whereas the rescue of Dpl-induced neurotoxicity by coexpression of PrP argues for an interaction between the PrP and Dpl proteins in vivo, the unaltered incubation times in Tg mice overexpressing Dpl in the central nervous system suggest that Dpl is unlikely to be involved in prion formation.

Toxicity of novel C-terminal prion protein fragments and peptides harbouring disease-related C-terminal mutations

Mice expressing a C-terminal fragment of the prion protein instead of wild-type prion protein die from massive neuronal degeneration within weeks of birth. The C-terminal region of PrPc (PrP121-231) expressed in these mice has an intrinsic neurotoxicity to cultured neurones. Unlike PrPSc, which is not neurotoxic to neurones lacking PrPc expression, PrP121-231 was more neurotoxic to PrPc-deficient cells. Human mutations E200K and F198S were found to enhance toxicity of PrP121-231 to PrP-knockout neurones and E200K enhanced toxicity to wild-type neurones. The normal metabolic cleavage point of PrPc is approximately amino-acid residue 113. A fragment of PrPc corresponding to the whole C-terminus of PrPc (PrP113-231), which is eight amino acids longer than PrP121-231, lacked any toxicity. This suggests the first eight amino residues of PrP113-121 suppress toxicity of the toxic domain in PrP121-231. Addition to cultures of a peptide (PrP112-125) corresponding to this region, in parallel with PrP121-231, suppressed the toxicity of PrP121-231. These results suggest that the prion protein contains two domains that are toxic on their own but which neutralize each other's toxicity in the intact protein. Point mutations in the inherited forms of disease might have their effects by diminishing this inhibition.

Temporally controlled targeted somatic mutagenesis in the mouse brain

To develop spatio-temporally controlled somatic mutagenesis in the adult mouse nervous system, we established transgenic mice expressing the tamoxifen-inducible Cre-ERT recombinase under the control of the mouse prion protein (PrP) promoter. Cre-ERT was expressed in most regions of the brain and in the retina of one transgenic line, whereas its expression was mostly restricted to the hippocampus and the cerebellum in another line. As tamoxifen efficiently induced Cre-mediated recombination in the various neuronal cell types expressing Cre-ERT in the brain of adult mice, the PrP-Cre-ERT lines should be valuable tools to study the functions of genes involved in neurodegenerative diseases or regeneration, and in complex processes such as behaviour, learning and memory. Some limitations of presently available reporter lines for Cre-mediated recombination in adult mouse CNS are discussed.

Chaperonin-mediated de novo generation of prion protein aggregates

The infectious prion protein, PrP(Sc), a predominantly beta-sheet aggregate, is derived from PrP(C), the largely alpha-helical cellular isoform of PrP. Conformational conversion of PrP(C) into PrP(Sc) has been suggested to involve a chaperone-like factor. Here we report that the bacterial chaperonin GroEL, a close homolog of eukaryotic Hsp60, can catalyze the aggregation of chemically denatured and of folded, recombinant PrP in a model reaction in vitro. Aggregates form upon ATP-dependent release of PrP from chaperonin and have certain properties of PrP(Sc), including a high beta-sheet content, the ability to bind the dye Congo red, detergent-insolubility and increased protease-resistance. A conserved sequence segment of PrP (residues 90-121), critical for PrP(Sc) generation in vivo, is also required for chaperonin-mediated aggregate formation in vitro. Initial binding of refolded, alpha-helical PrP to chaperonin is mediated by the unstructured N-terminal segment of PrP (residues 23-121) and is followed by a rearrangement of the globular PrP core-domain. These results show that chaperonins of the Hsp60 class can, in principle, mediate PrP aggregation de novo, i.e. independently of a pre-existent PrP(Sc) template.

Identification of interaction domains of the prion protein with its 37-kDa/67-kDa laminin receptor

Cell-binding and internalization studies on neuronal and non-neuronal cells have demonstrated that the 37-kDa/67-kDa laminin receptor (LRP/LR) acts as the receptor for the cellular prion protein (PrP). Here we identify direct and heparan sulfate proteoglycan (HSPG)-dependent interaction sites mediating the binding of the cellular PrP to its receptor, which we demonstrated in vitro on recombinant proteins. Mapping analyses in the yeast two-hybrid system and cell-binding assays identified PrPLRPbd1 [amino acids (aa) 144-179] as a direct and PrPLRPbd2 (aa 53-93) as an indirect HSPG-dependent laminin receptor precursor (LRP)-binding site on PrP. The yeast two-hybrid system localized the direct PrP-binding domain on LRP between aa 161 and 179. Expression of an LRP mutant lacking the direct PrP-binding domain in wild-type and mutant HSPG-deficient Chinese hamster ovary cells by the Semliki Forest virus system demonstrates a second HSPG-dependent PrP-binding site on LRP. Considering the absence of LRP homodimerization and the direct and indirect LRP-PrP interaction sites, we propose a comprehensive model for the LRP-PrP-HSPG complex.

Prion diseases: what is the neurotoxic molecule?

A great deal of effort has been devoted during the past 20 years to defining the chemical nature of prions, the infectious agents responsible for transmissible spongiform encephalopathies. In contrast, much less attention has been paid to elucidating how prions actually damage the central nervous system. Although it is commonly assumed that PrP(Sc), the protein constituent of infectious prions, is the primary culprit, increasing evidence indicates that this may not be the case. Several alternative molecular forms of PrP are reasonable candidates for the neurotoxic species in prion diseases, although it is still too early to tell whether these or other ones will turn out to be the true instigating factors. The cellular pathways activated by neurotoxic forms of PrP that ultimately result in neuronal death are also being investigated, and several possible mechanisms have been uncovered, including the operation of quality control processes in the endoplasmic reticulum. Elucidating the distinction between the infectious and neurotoxic forms of PrP has important implications for designing therapy of prion diseases, as well as for understanding pathogenic mechanisms operative in other neurodegenerative disorders and the role of prion-like states in biology.

Internalization of mammalian fluorescent cellular prion protein and N-terminal deletion mutants in living cells

The cellular prion protein (PrP(c)) is a glycosylphosphatidylinositol (GPI)-anchored plasma membrane protein whose conformational altered forms (PrP(sc)) are known to cause neurodegenerative diseases in mammals. In order to investigate the intracellular traffic of mammalian PrP(c) in living cells, we have generated a green fluorescent protein (GFP) tagged version of PrP(c). The recombinant protein was properly anchored at the cell surface and its distribution pattern was similar to that of the endogenous PrP(c), with labeling at the plasma membrane and in an intracellular perinuclear compartment. Comparison of the steady-state distribution of GFP-PrP(c) and two N-terminal deletion mutants (Delta32-121 and Delta32-134), that cause neurological symptoms when expressed in PrP knockout mice, was carried out. The mutant proteins accumulated in the plasma membrane at the expense of decreased labeling in the perinuclear region when compared with GFP-PrP(c). In addition, GFP-PrP(c), but not the two mutants, internalized from the plasma membrane in response to Cu2+ treatment and accumulated at a perinuclear region in SN56 cells. Our data suggest that GFP-PrP(c) can be used to follow constitutive and induced PrP(c) traffic in living cells.

Prion diseases of humans and animals: their causes and molecular basis

Prion diseases are transmissible neurodegenerative conditions that include Creutzfeldt-Jakob disease (CJD) in humans and bovine spongiform encephalopathy (BSE) and scrapie in animals. Prions appear to be composed principally or entirely of abnormal isoforms of a host-encoded glycoprotein, prion protein. Prion propagation involves recruitment of host cellular prion protein, composed primarily of alpha-helical structure, into a disease specific isoform rich in beta-sheet structure. The existence of multiple prion strains has been difficult to explain in terms of a protein-only infections agent, but recent studies suggest that strain specific phenotypes can be encoded by different prion protein conformations and glycosylation patterns. The ability of a protein to encode phenotypic information has important biological implications. The appearance of a novel human prion disease, variant CJD, and the clear experimental evidence that it is caused by exposure to BSE has highlighted the need to understand the molecular basis of prion propagation, pathogenesis, and the barriers limiting intermammalian transmission. It is unclear if a large epidemic of variant CJD will occur in the years ahead.

The prion gene complex encoding PrP(C) and Doppel: insights from mutational analysis

The prion protein gene, Prnp, encodes PrP(Sc), the major structural component of prions, infectious pathogens causing a number of disorders including scrapie and bovine spongiform encephalopathy (or BSE). Missense mutations in the human Prnp gene cause inherited prion diseases such as familial Creutzfeldt-Jakob disease. In uninfected animals Prnp encodes a glycophosphatidylinositol (GPI)-anchored protein denoted PrP(C) and in prion infections PrP(C) is converted to PrP(Sc) by templated refolding. Though Prnp is conserved in mammalian species, attempts to verify interactions of putative PrP binding proteins by genetic means have proven frustrating and the ZrchI and Npu lines of Prnp gene-ablated mice (Prnp(0/0) mice) lacking PrP(C) remain healthy throughout development. This indicates that PrP(C) serves a function that is not apparent in a laboratory setting or that other molecules have overlapping functions. Current possibilities involve shuttling or sequestration of synaptic Cu(II) via binding to N-terminal octapeptide residues and/or signal transduction involving the fyn kinase. A new point of entry into the issue of prion protein function has emerged from identification of a paralogue, Prnd, with 24% coding sequence identity to Prnp. Prnd lies downstream of Prnp and encodes the doppel (Dpl) protein. Like PrP(C), Dpl is presented on the cell surface via a GPI anchor and has three alpha-helices: however, it lacks the conformationally plastic and octapeptide repeat domains present in its well-known relative. Interestingly, Dpl is overexpressed in the Ngsk and Rcm0 lines of Prnp(0/0) mice via intergenic splicing events. These lines of Prnp(0/0) mice exhibit ataxia and apoptosis of cerebellar cells, indicating that ectopic synthesis of Dpl protein is toxic to central nervous system neurons: this inference has now been confirmed by the construction of transgenic mice expressing Dpl under the direct control of the PrP promoter. Remarkably, Dpl-programmed ataxia is rescued by wild-type Prnp transgenes. The interaction between the Prnp and Prnd genes in mouse cerebellar neurons may have a physical correlate in competition between Dpl and PrP(C) within a common biochemical pathway that when mis-regulated leads to apoptosis.

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

Prion diseases are diseases of protein conformation. Structure-dependent antibodies have been sought to probe conformations of the prion protein (PrP) resulting from environmental changes, such as differences in pH. Despite the absence of such antibodies for full-length PrP, a recombinant Fab (D13) and a Fab derived from mAb 3F4 showed pH-dependent reactivity toward epitopes within the N-terminus of N-terminally truncated PrP(90-231). Refolding and maintaining this protein at pH > or =5.2 before immobilization on an ELISA plate inhibited reactivity relative to protein exposed to pH < or =4.7. The reactivity was not affected by pH changes after immobilization, showing retention of conformation after binding to the plate surface, although guanidine hydrochloride at 1.5-2 M was able to expose the cryptic epitopes after immobilization at pH > or =5.2. The alpha-helical CD spectrum of PrP(90-231) refolded at pH 5.5 was reduced somewhat by these pH changes, with a minor shift toward beta-sheet at pH 4 and then toward coil at pH 2. No covalent changes were caused by the pH differences. This pH dependence suggests titration of an acidic region that might inhibit the N-terminal epitopes. A similar pH dependence for a monoclonal antibody reactive to the central region identified an acidic region incorporating Glu152 as a significant participant.

Electron paramagnetic resonance evidence for binding of Cu(2+) to the C-terminal domain of the murine prion protein

Transmissible spongiform encephalopathies in mammals are believed to be caused by scrapie form of prion protein (PrP(Sc)), an abnormal, oligomeric isoform of the monomeric cellular prion protein (PrP(C)). One of the proposed functions of PrP(C) in vivo is a Cu(II) binding activity. Previous studies revealed that Cu(2+) binds to the unstructured N-terminal PrP(C) segment (residues 23-120) through conserved histidine residues. Here we analyzed the Cu(II) binding properties of full-length murine PrP(C) (mPrP), of its isolated C-terminal domain mPrP(121-231) and of the N-terminal fragment mPrP(58-91) in the range of pH 3-8 with electron paramagnetic resonance spectroscopy. We find that the C-terminal domain, both in its isolated form and in the context of the full-length protein, is capable of interacting with Cu(2+). Three Cu(II) coordination types are observed for the C-terminal domain. The N-terminal segment mPrP(58-91) binds Cu(2+) only at pH values above 5.0, whereas both mPrP(121-231) and mPrP(23-231) already show identical Cu(II) coordination in the pH range 3-5. As the Cu(2+)-binding N-terminal segment 58-91 is not required for prion propagation, our results open the possibility that Cu(2+) ions bound to the C-terminal domain are involved in the replication of prions, and provide the basis for further analytical studies on the specificity of Cu(II) binding by PrP.

Insights into the physiological function of cellular prion protein

Prions have been extensively studied since they represent a new class of infectious agents in which a protein, PrPsc (prion scrapie), appears to be the sole component of the infectious particle. They are responsible for transmissible spongiform encephalopathies, which affect both humans and animals. The mechanism of disease propagation is well understood and involves the interaction of PrPsc with its cellular isoform (PrPc) and subsequently abnormal structural conversion of the latter. PrPc is a glycoprotein anchored on the cell surface by a glycosylphosphatidylinositol moiety and expressed in most cell types but mainly in neurons. Prion diseases have been associated with the accumulation of the abnormally folded protein and its neurotoxic effects; however, it is not known if PrPc loss of function is an important component. New efforts are addressing this question and trying to characterize the physiological function of PrPc. At least four different mouse strains in which the PrP gene was ablated were generated and the results regarding their phenotype are controversial. Localization of PrPc on the cell membrane makes it a potential candidate for a ligand uptake, cell adhesion and recognition molecule or a membrane signaling molecule. Recent data have shown a potential role for PrPc in the metabolism of copper and moreover that this metal stimulates PrPc endocytosis. Our group has recently demonstrated that PrPc is a high affinity laminin ligand and that this interaction mediates neuronal cell adhesion and neurite extension and maintenance. Moreover, PrPc-caveolin-1 dependent coupling seems to trigger the tyrosine kinase Fyn activation. These data provide the first evidence for PrPc involvement in signal transduction.

Normal neurogenesis and scrapie pathogenesis in neural grafts lacking the prion protein homologue Doppel

The agent that causes prion diseases is thought to be identical to PrPSc, a conformer of the normal prion protein PrPC. Recently a novel protein, termed Doppel (Dpl), was identified that shares significant biochemical and structural homology with PrPC. To investigate the function of Dpl in neurogenesis and in prion pathology, we generated embryonic stem (ES) cells harbouring a homozygous disruption of the Prnd gene that encodes Dpl. After in vitro differentiation and grafting into adult brains of PrPC-deficient Prnp0/0 mice, Dpl-deficient ES cell-derived grafts contained all neural lineages analyzed, including neurons and astrocytes. When Prnd-deficient neural tissue was inoculated with scrapie prions, typical features of prion pathology including spongiosis, gliosis and PrPSc accumulation, were observed. Therefore, Dpl is unlikely to exert a cell-autonomous function during neural differentiation and, in contrast to its homologue PrPC, is dispensable for prion disease progression and for generation of PrPSc.