Prion disease: a tale of folds and strains

The Role of Glial Cells in Neurobiology and Prion Neuropathology

Prion diseases are rare and neurodegenerative diseases that are characterized by the misfolding and infectious spread of the prion protein in the brain, causing progressive and irreversible neuronal loss and associated clinical and behavioral manifestations in humans and animals, ultimately leading to death. The brain has a complex network of neurons and glial cells whose crosstalk is critical for function and homeostasis. Although it is established that prion infection of neurons is necessary for clinical disease to occur, debate remains in the field as to the role played by glial cells, namely astrocytes and microglia, and whether these cells are beneficial to the host or further accelerate disease. Here, we review the current literature assessing the complex morphologies of astrocytes and microglia, and the crosstalk between these two cell types, in the prion-infected brain.

Vaccination strategies

Currently all prion diseases are without effective treatment and are universally fatal. It is increasingly being recognized that the pathogenesis of many neurodegenerative diseases, such as Alzheimer disease (AD), includes "prion-like" properties. Hence, any effective therapeutic intervention for prion disease could have significant implications for other neurodegenerative diseases. Conversely, therapies that are effective in AD might also be therapeutically beneficial for prion disease. AD-like prion disease has no effective therapy. However, various vaccine and immunomodulatory approaches have shown great success in animal models of AD, with numerous ongoing clinical trials of these potential immunotherapies. More limited evidence suggests that immunotherapies may be effective in prion models and in naturally occurring prion disease. In particular, experimental data suggest that mucosal vaccination against prions can be effective for protection against orally acquired prion infection. Many prion diseases, including natural sheep scrapie, bovine spongiform encephalopathy, chronic wasting disease, and variant Creutzfeldt-Jakob disease, are thought to be acquired peripherally, mainly by oral exposure. Mucosal vaccination would be most applicable to this form of transmission. In this chapter we review various immunologically based strategies which are under development for prion infection.

PrPC Undergoes Basal to Apical Transcytosis in Polarized Epithelial MDCK Cells

The Prion Protein (PrP) is an ubiquitously expressed glycosylated membrane protein attached to the external leaflet of the plasma membrane via a glycosylphosphatidylinositol anchor (GPI). While the misfolded PrPSc scrapie isoform is the infectious agent of prion disease, the cellular isoform (PrPC) is an enigmatic protein with unclear function. Of interest, PrP localization in polarized MDCK cells is controversial and its mechanism of trafficking is not clear. Here we investigated PrP traffic in MDCK cells polarized on filters and in three-dimensional MDCK cysts, a more physiological model of polarized epithelia. We found that, unlike other GPI-anchored proteins (GPI-APs), PrP undergoes basolateral-to-apical transcytosis in fully polarized MDCK cells. Following this event full-length PrP and its cleavage fragments are segregated in different domains of the plasma membrane in polarized cells in both 2D and 3D cultures.

Structural studies on the mechanism of protein aggregation in age related neurodegenerative diseases

The progression of many neurodegenerative diseases is assumed to be caused by misfolding of specific characteristic diseases related proteins, resulting in aggregation and fibril formation of these proteins. Protein misfolding associated age related diseases, although different in disease manifestations, share striking similarities. In all cases, one disease protein aggregates and loses its function or additionally shows a toxic gain of function. However, the clear link between these individual amyloid-like protein aggregates and cellular toxicity is often still uncertain. The similar features of protein misfolding and aggregation in this group of proteins, all involved in age related neurodegenerative diseases, results in high interest in characterization of their structural properties. We review here recent findings on structural properties of some age related disease proteins, in the context of their biological importance in disease.

Disease Transmission by Misfolded Prion-Protein Isoforms, Prion-Like Amyloids, Functional Amyloids and the Central Dogma

In 1982, the term “prions” (proteinaceous infectious particles) was coined to specify a new principle of infection. A misfolded isoform of a cellular protein has been described as the causative agent of a fatal neurodegenerative disease. At the beginning of prion research scientists assumed that the infectious agent causing transmissible spongiform encephalopathy (TSE) was a virus, but some unconventional properties of these pathogens were difficult to bring in line with the prevailing viral model. The discovery that prions (obviously devoid of any coding nucleic acid) can store and transmit information similarly to DNA was initially even denoted as being “heretical” but is nowadays mainly accepted by the scientific community. This review describes, from a historical point of view, how the “protein-only hypothesis” expands the Central Dogma. Definition of both, the prion principle and the Central Dogma, have been essential steps to understand information storage and transfer within and among cells and organisms. Furthermore, the current understanding of the infectivity of prion-proteins after misfolding is summarized succinctly. Finally, prion-like amyloids and functional amyloids, as found in yeast and bacteria, will be discussed.

Contributions of the Prion Protein Sequence, Strain, and Environment to the Species Barrier

Amyloid propagation requires high levels of sequence specificity so that only molecules with very high sequence identity can form cross-β-sheet structures of sufficient stringency for incorporation into the amyloid fibril. This sequence specificity presents a barrier to the transmission of prions between two species with divergent sequences, termed a species barrier. Here we study the relative effects of protein sequence, seed conformation, and environment on the species barrier strength and specificity for the yeast prion protein Sup35p from three closely related species of the Saccharomyces sensu stricto group; namely, Saccharomyces cerevisiae, Saccharomyces bayanus, and Saccharomyces paradoxus. Through in vivo plasmid shuffle experiments, we show that the major characteristics of the transmission barrier and conformational fidelity are determined by the protein sequence rather than by the cellular environment. In vitro data confirm that the kinetics and structural preferences of aggregation of the S. paradoxus and S. bayanus proteins are influenced by anions in accordance with their positions in the Hofmeister series, as observed previously for S. cerevisiae. However, the specificity of the species barrier is primarily affected by the sequence and the type of anion present during the formation of the initial seed, whereas anions present during the seeded aggregation process typically influence kinetics rather than the specificity of prion conversion. Therefore, our work shows that the protein sequence and the conformation variant (strain) of the prion seed are the primary determinants of cross-species prion specificity both in vivo and in vitro.

Phosphatidylethanolamine Metabolism in Health and Disease

Phosphatidylethanolamine (PE) is the second most abundant glycerophospholipid in eukaryotic cells. The existence of four only partially redundant biochemical pathways that produce PE, highlights the importance of this essential phospholipid. The CDP-ethanolamine and phosphatidylserine decarboxylase pathways occur in different subcellular compartments and are the main sources of PE in cells. Mammalian development fails upon ablation of either pathway. Once made, PE has diverse cellular functions that include serving as a precursor for phosphatidylcholine and a substrate for important posttranslational modifications, influencing membrane topology, and promoting cell and organelle membrane fusion, oxidative phosphorylation, mitochondrial biogenesis, and autophagy. The importance of PE metabolism in mammalian health has recently emerged following its association with Alzheimer's disease, Parkinson's disease, nonalcoholic liver disease, and the virulence of certain pathogenic organisms.

Prion infection in cells is abolished by a mutated manganese transporter but shows no relation to zinc

The cellular prion protein has been identified as a metalloprotein that binds copper. There have been some suggestions that prion protein also influences zinc and manganese homeostasis. In this study we used a series of cell lines to study the levels of zinc and manganese under different conditions. We overexpressed either the prion protein or known transporters for zinc and manganese to determine relations between the prion protein and both manganese and zinc homeostasis. Our observations supported neither a link between the prion protein and zinc metabolism nor any effect of altered zinc levels on prion protein expression or cellular infection with prions. In contrast we found that a gain of function mutant of a manganese transporter caused reduction of manganese levels in prion infected cells, loss of observable PrP(Sc) in cells and resistance to prion infection. These studies strengthen the link between manganese and prion disease.

Cyclin-dependent kinase 5 phosphorylation of familial prion protein mutants exacerbates conversion into amyloid structure

Familial prion protein (PrP) mutants undergo conversion from soluble and protease-sensitive to insoluble and partially protease-resistant proteins. Cyclin-dependent kinase 5 (Cdk5) phosphorylation of wild type PrP (pPrP) at serine 43 induces a conversion of PrP into aggregates and fibrils. Here, we investigated whether familial PrP mutants are predisposed to Cdk5 phosphorylation and whether phosphorylation of familial PrP mutants increases conversion. PrP mutants representing three major familial PrP diseases and different PrP structural domains were studied. We developed a novel in vitro kinase reaction coupled with Thioflavin T binding to amyloid structure assay to monitor phosphorylation-dependent amyloid conversion. Although non-phosphorylated full-length wild type or PrP mutants did not convert into amyloid, Cdk5 phosphorylation rapidly converted these into Thioflavin T-positive structures following first order kinetics. Dephosphorylation partially reversed conversion. Phosphorylation-dependent conversion of PrP from α-helical structures into β-sheet structures was confirmed by circular dichroism. Relative to wild type pPrP, most PrP mutants showed increased rate constants of conversion. In contrast, non-phosphorylated truncated PrP Y145X (where X represents a stop codon) and Q160X mutants converted spontaneously into Thioflavin T-positive fibrils after a lag phase of over 20 h, indicating nucleation-dependent polymerization. Phosphorylation reduced the lag phase by over 50% and thus accelerated the formation of the nucleating event. Consistently, phosphorylated Y145X and phosphorylated Q160X exacerbated conversion in a homologous seeding reaction, whereas WT pPrP could not seed WT PrP. These results demonstrate an influence of both the N terminus and the C terminus of PrP on conversion. We conclude that post-translational modifications of the flexible N terminus of PrP can cause or exacerbate PrP mutant conversion.

When dementia progresses quickly: a practical approach to the diagnosis and management of rapidly progressive dementia

Making a diagnosis of rapidly progressive dementia requires practical adaptation of the skills used to assess patients with chronic causes of cognitive impairment. An expedited assessment, commensurate with the accelerated pace of the disease, is required to identify the cause of symptoms amidst a myriad of possibilities. Features upon history, physical examination and cognitive assessment that support specific diagnoses are reviewed, and a stratified approach to testing is presented. The use of readily-accessible investigations is prioritized, acknowledging the implications and applications of novel diagnostic tests. The coordinated use of clinical and laboratory measures are promoted as a means of facilitating rapid evaluation, with the ultimate goal of identifying patients with potentially reversible causes of rapidly progressive dementia.

Alpha-synuclein biology in Lewy body diseases

α-Synuclein is an abundantly expressed neuronal protein that is at the center of focus in understanding a group of neurodegenerative disorders called α-synucleinopathies, which are characterized by the presence of aggregated α-synuclein intracellularly. Primary α-synucleinopathies include Parkinson’s disease (PD), dementia with Lewy bodies and multiple system atrophy, with α-synuclein also found secondarily in a number of other diseases, including Alzheimer’s disease. Understanding how α-synuclein aggregates form in these different disorders is important for the understanding of its pathogenesis in Lewy body diseases. PD is the most prevalent of the α-synucleinopathies and much of the initial research on α-synuclein Lewy body pathology was based on PD but is also relevant to Lewy bodies in other diseases (dementia with Lewy bodies and Alzheimer’s disease). Polymorphism and mutation studies of SNCA, the gene that encodes α-synuclein, provide much evidence for a causal link between α-synuclein and PD. Among the primary α-synucleinopathies, multiple system atrophy is unique in that α-synuclein deposition occurs in oligodendrocytes rather than neurons. It is unclear whether α-synuclein originates from oligodendrocytes or whether it is transmitted somehow from neurons. α-Synuclein exists as a natively unfolded monomer in the cytosol, but in the presence of lipid membranes it is thought to undergo a conformational change to a folded α-helical secondary structure that is prone to forming dimers and oligomers. Posttranslational modification of α-synuclein, such as phosphorylation, ubiquitination and nitration, has been widely implicated in α-synuclein aggregation process and neurotoxicity. Recent studies using animal and cell models, as well as autopsy studies of patients with neuron transplants, provided compelling evidence for prion-like propagation of α-synuclein. This observation has implications for therapeutic strategies, and much recent effort is focused on developing antibodies that target extracellular α-synuclein.

Targeting prion propagation using peptide constructs with signal sequence motifs

Synthetic peptides with sequences derived from the cellular prion protein (PrP(C)) unprocessed N-terminus are able to counteract the propagation of proteinase K resistant prions (PrP(Res), indicating the presence of the prion isoform of the prion protein) in cell cultures (Löfgren et al., 2008). The anti-prion peptides have characteristics like cell penetrating peptides (CPPs) and consist of the prion protein hydrophobic signal sequence followed by a polycationic motif (residues KKRPKP), in mouse PrP(C) corresponding to residues 1-28. Here we analyze the sequence elements required for the anti-prion effect of KKRPKP-conjugates. Neuronal GT1-1 cells were infected with either prion strain RML or 22L. Variable peptide constructs originating from the mPrP1-28 sequence were analyzed for anti-prion effects, measured as disappearance of proteinase K resistant prions (PrP(Res)) in the infected cell cultures. We find that even a 5 amino acid N-terminal shortening of the signal peptide abolishes the anti-prion effect. We show that the signal peptide from PrP(C) can be replaced with the signal peptide from the Neural cell adhesion molecule-1; NCAM11-19, with a retained capacity to reduce PrP(Res) levels. The anti-prion effect is lost if the polycationic N-terminal PrP(C)-motif is conjugated to any conventional CPP, such as TAT48-60, transportan-10 or penetratin. We propose a mechanism by which a signal peptide from a secretory or cell surface protein acts to promote the transport of a prion-binding polycationic PrP(C)-motif to a subcellular location where prion conversion occurs (most likely the Endosome Recycling Compartment), thereby targeting prion propagation.

Proteomics analysis of amyloid and nonamyloid prion disease phenotypes reveals both common and divergent mechanisms of neuropathogenesis

Prion diseases are a heterogeneous group of neurodegenerative disorders affecting various mammals including humans. Prion diseases are characterized by a misfolding of the host-encoded prion protein (PrP^C) into a pathological isoform termed PrP^Sc. In wild-type mice, PrP^C is attached to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor and PrP^Sc typically accumulates in diffuse nonamyloid deposits with gray matter spongiosis. By contrast, when mice lacking the GPI anchor are infected with the same prion inoculum, PrP^Sc accumulates in dense perivascular amyloid plaques with little or no gray matter spongiosis. In order to evaluate whether different host biochemical pathways were implicated in these two phenotypically distinct prion disease models, we utilized a proteomics approach. In both models, infected mice displayed evidence of a neuroinflammatory response and complement activation. Proteins involved in cell death and calcium homeostasis were also identified in both phenotypes. However, mitochondrial pathways of apoptosis were implicated only in the nonamyloid form, whereas metal binding and synaptic vesicle transport were more disrupted in the amyloid phenotype. Thus, following infection with a single prion strain, PrP^C anchoring to the plasma membrane correlated not only with the type of PrP^Sc deposition but also with unique biochemical pathways associated with pathogenesis.

STRUCTURAL INSIGHT INTO THE POLYMORPHISM OF NNQNTF PROTOFIBRIL: IMPORTANCE OF INTERFACIAL WATER, POLAR AND AROMATIC RESIDUES

Polymorphism is widely observed in amyloid fibrils associated with many neurodegenerative diseases. Recent experimental study reported that fibrils formed by the segment NNQNTF of elk prion protein exhibited facial polymorphism with the two β-sheets either in back-to-back (BB) or in face-to-face (FF) packing arrangement. In the BB packing, the side chains of N2, N4 and F6 are interdigitated to form steric zipper, while in the FF packing, the side chains of N1, Q3 and T5 form the interdigitated interface. In this study, we investigate the water-mediated assembly of two preformed β-sheets and the physical interactions that stabilize the two different fibrils using all-atom molecular dynamics (MD) simulations. Multiple MD simulations have been carried out by starting from FF or BB packing of two β-sheets according to the facial polymorphism revealed by X-ray microcrystallography. For both packing patterns, we observe that the assembly of β-sheets is mediated by water molecules in the interface between β-sheets, leading to a long-lived protofibrils with wet interface prior to the formation of dry amyloid fibrils. Detailed structural analysis shows that besides the side chain steric zipper interactions, intra-sheet hydrogen bonding and aromatic stacking interactions play an important role on the stabilization of the protofibril with BB packing, while the intra-sheet and inter-sheet hydrogen bonding interactions are crucial for the formation of BB protofibril. These findings provide insights into the mechanism that lead to the facial polymorphism of NNQNTF fibrils.

PrP(Sc)-specific antibodies do not induce prion disease or misfolding of PrP(C) in highly susceptible Tga20 mice

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders caused by misfolding of a cellular protein PrP(C) into an infectious conformation PrP(Sc). Previously our group demonstrated induction of PrP(Sc)-specific antibodies with a SN6b vaccine that targets regions of the protein that are exposed upon misfolding. There are concerns that these antibodies could function as templates to promote misfolding and cause disease. To evaluate the consequences of prolonged exposure to PrP(Sc)-specific antibodies in a prion sensitized animal, tga20 mice were vaccinated with the SN6b vaccine. No clinical signs of disease were detected up to 255 d post-vaccination, and postmortem assay of brains and spleens revealed no proteinase-K resistant PrP. These results suggest that vaccinating against TSEs with the SN6b antigen is safe from the standpoint of prion disease induction.

Molecular mechanisms of disease-causing missense mutations

Genetic variations resulting in a change of amino acid sequence can have a dramatic effect on stability, hydrogen bond network, conformational dynamics, activity and many other physiologically important properties of proteins. The substitutions of only one residue in a protein sequence, so-called missense mutations, can be related to many pathological conditions and may influence susceptibility to disease and drug treatment. The plausible effects of missense mutations range from affecting the macromolecular stability to perturbing macromolecular interactions and cellular localization. Here we review the individual cases and genome-wide studies that illustrate the association between missense mutations and diseases. In addition, we emphasize that the molecular mechanisms of effects of mutations should be revealed in order to understand the disease origin. Finally, we report the current state-of-the-art methodologies that predict the effects of mutations on protein stability, the hydrogen bond network, pH dependence, conformational dynamics and protein function.

Strain phenomenon in protein aggregation: Interplay between sequence and conformation

Studies of yeast and mammalian prions introduced the idea that the protein aggregates can exist in multiple stable conformations that can be propagated by seeding. These conformational states (aka strains) were shown to have distinct physical (secondary structure, stability) and biological (cytotoxicity, infectivity) properties. For mammalian prions they were also tied to differences in disease pathology and incubation time. It was later shown that this phenomenon is not limited to prion proteins, and distinct conformational states of amyloid fibrils and oligomers derived from a variety of proteins can be propagated both in vitro and in vivo. Moreover, in some cases these conformations were preserved even when propagated into a protein with a different sequence. There is now an increasing body of evidence that strain phenomenon is a generic feature of protein aggregation, and characteristic features of amyloid strains can be transmitted between unrelated sequences.