Full atomistic model of prion structure and conversion

Protein Fold Usages in Ribosomes: Another Glance to the Past

The analysis of protein fold usage, similar to codon usage, offers profound insights into the evolution of biological systems and the origins of modern proteomes. While previous studies have examined fold distribution in modern genomes, our study focuses on the comparative distribution and usage of protein folds in ribosomes across bacteria, archaea, and eukaryotes. We identify the prevalence of certain 'super-ribosome folds,' such as the OB fold in bacteria and the SH3 domain in archaea and eukaryotes. The observed protein fold distribution in the ribosomes announces the future power-law distribution where only a few folds are highly prevalent, and most are rare. Additionally, we highlight the presence of three copies of proto-Rossmann folds in ribosomes across all kingdoms, showing its ancient and fundamental role in ribosomal structure and function. Our study also explores early mechanisms of molecular convergence, where different protein folds bind equivalent ribosomal RNA structures in ribosomes across different kingdoms. This comparative analysis enhances our understanding of ribosomal evolution, particularly the distinct evolutionary paths of the large and small subunits, and underscores the complex interplay between RNA and protein components in the transition from the RNA world to modern cellular life. Transcending the concept of folds also makes it possible to group a large number of ribosomal proteins into five categories of urfolds or metafolds, which could attest to their ancestral character and common origins. This work also demonstrates that the gradual acquisition of extensions by simple but ordered folds constitutes an inexorable evolutionary mechanism. This observation supports the idea that simple but structured ribosomal proteins preceded the development of their disordered extensions.

Insight into the conserved structural dynamics of the C-terminus of mammal PrPC identifies structural core and possible structural role of pharmacological chaperones

Misfolding of the prion protein is central to prion disease aetiology. Although understanding the dynamics of the native fold helps to decipher the conformational conversion mechanism, a complete depiction of distal but coupled prion protein sites common across species is lacking. To fill this gap, we used normal mode analysis and network analysis to examine a collection of prion protein structures deposited on the protein data bank. Our study identified a core of conserved residues that sustains the connectivity across the C-terminus of the prion protein. We propose how a well-characterized pharmacological chaperone may stabilize the fold. Also, we provide insight into the effect on the native fold of initial misfolding pathways identified by others using kinetics studies.

The Smallest Infectious Substructure Encoding the Prion Strain Structural Determinant Revealed by Spontaneous Dissociation of Misfolded Prion Protein Assemblies

It is commonly accepted that the prion replicative propensity and strain structural determinant (SSD) are encoded in the fold of PrPSc amyloid fibril assemblies. By exploring the quaternary structure dynamicity of several prion strains, we revealed that all mammalian prion assemblies exhibit the generic property of spontaneously generating two sets of discreet infectious tetrameric and dimeric species differing significantly by their specific infectivity. By using perturbation approaches such as dilution and ionic strength variation, we demonstrated that these two oligomeric species were highly dynamic and evolved differently in the presence of chaotropic agents. In general, our observations of seven different prion strains from three distinct species highlight the high dynamicity of PrPSc assemblies as a common and intrinsic property of mammalian prions. The existence of such small infectious PrPSc species harboring the SSD indicates that the prion infectivity and the SSD are not restricted only to the amyloid fold but can also be encoded in other alternative quaternary structures. Such diversity in the quaternary structure of prion assemblies tends to indicate that the structure of PrPSc can be divided into two independent folding domains: a domain encoding the strain structural determinant and a second domain whose fold determines the type of quaternary structure that could adopt PrPSc assemblies.

Are fibrinaloid microclots a cause of autoimmunity in Long Covid and other post-infection diseases?

It is now well established that the blood-clotting protein fibrinogen can polymerise into an anomalous form of fibrin that is amyloid in character; the resultant clots and microclots entrap many other molecules, stain with fluorogenic amyloid stains, are rather resistant to fibrinolysis, can block up microcapillaries, are implicated in a variety of diseases including Long COVID, and have been referred to as fibrinaloids. A necessary corollary of this anomalous polymerisation is the generation of novel epitopes in proteins that would normally be seen as 'self', and otherwise immunologically silent. The precise conformation of the resulting fibrinaloid clots (that, as with prions and classical amyloid proteins, can adopt multiple, stable conformations) must depend on the existing small molecules and metal ions that the fibrinogen may (and is some cases is known to) have bound before polymerisation. Any such novel epitopes, however, are likely to lead to the generation of autoantibodies. A convergent phenomenology, including distinct conformations and seeding of the anomalous form for initiation and propagation, is emerging to link knowledge in prions, prionoids, amyloids and now fibrinaloids. We here summarise the evidence for the above reasoning, which has substantial implications for our understanding of the genesis of autoimmunity (and the possible prevention thereof) based on the primary process of fibrinaloid formation.

Role of sialylation of N-linked glycans in prion pathogenesis

Mammalian prion or PrPSc is a proteinaceous infectious agent that consists of a misfolded, self-replicating state of the prion protein or PrPC. PrPC and PrPSc are posttranslationally modified with N-linked glycans, which are sialylated at the terminal positions. More than 30 years have passed since the first characterization of the composition and structural diversity of N-linked glycans associated with the prion protein, yet the role of carbohydrate groups that constitute N-glycans and, in particular, their terminal sialic acid residues in prion disease pathogenesis remains poorly understood. A number of recent studies shed a light on the role of sialylation in the biology of prion diseases. This review article discusses several mechanisms by which terminal sialylation dictates the spread of PrPSc across brain regions and the outcomes of prion infection in an organism. In particular, relationships between the sialylation status of PrPSc and important strain-specific features including lymphotropism, neurotropism, and neuroinflammation are discussed. Moreover, emerging evidence pointing out the roles of sialic acid residues in prion replication, cross-species transmission, strain competition, and strain adaptation are reviewed. A hypothesis according to which selective, strain-specified recruitment of PrPC sialoglycoforms dictates unique strain-specific disease phenotypes is examined. Finally, the current article proposes that prion strains evolve as a result of a delicate balance between recruiting highly sialylated glycoforms to avoid an "eat-me" response by glia and limiting heavily sialylated glycoforms for enabling rapid prion replication.

Conversion of Helix 1 into a Loop in Prion Protein Misfolding

Cellular prion protein PrP^C consists of three α-helices, one β-sheet, and an unstructured N-terminal domain. Misfolding of this protein into the scrapie form (PrP^Sc) increases dramatically the β-sheet content. H1 is the most stable helix on PrP^C and contains an unusual number of hydrophilic amino acids. Its fate in PrP^Sc is not clear. We performed replica exchange molecular dynamics simulations on H1 alone, H1 together with an N-terminally flanking H1B1 loop and H1 in complex with other hydrophilic regions of the prion protein. In the presence of the H⁹⁹SQWNKPSKPKTNMK¹¹³ sequence, H1 is almost completely converted to a loop structure stabilized by a network of salt bridges. On the other hand, H1 retains its helical structure alone or together with the other sequences considered in this study. We carried out an additional simulation by restraining the distance between the two ends of H1, mimicking a possible geometric restriction by the rest of the protein. Even though the loop was the major conformation, a significant amount of helical structure was also observed. This suggests that the interaction with H⁹⁹SQWNKPSKPKTNMK¹¹³ is necessary for complete helix-to-loop conversion.

Functional, pathogenic, and pharmacological roles of protein folding intermediates

Protein expression and function in eukaryotic cells are tightly harmonized processes modulated by the combination of different layers of regulation, including transcription, processing, stability, and translation of messenger RNA, as well as assembly, maturation, sorting, recycling, and degradation of polypeptides. Integrating all these pathways and the protein quality control machinery, deputed to avoid the production and accumulation of aberrantly folded proteins, determines protein homeostasis. Over the last decade, the combined development of accurate time-resolved experimental techniques and efficient computer simulations has opened the possibility of investigating biological mechanisms at atomic resolution with physics-based models. A meaningful example is the reconstruction of protein folding pathways at atomic resolution, which has enabled the characterization of the folding kinetics of biologically relevant globular proteins consisting of a few hundred amino acids. Combining these innovative computational technologies with rigorous experimental approaches reveals the existence of non-native metastable states transiently appearing along the folding process of such proteins. Here, we review the primary evidence indicating that these protein folding intermediates could play roles in disparate biological processes, from the posttranslational regulation of protein expression to disease-relevant protein misfolding mechanisms. Finally, we discuss how the information encoded into protein folding pathways could be exploited to design an entirely new generation of pharmacological agents capable of promoting the selective degradation of protein targets.

Loss of small GTPase Rab7 activation in prion infection negatively affects a feedback loop regulating neuronal cholesterol metabolism

Prion diseases are fatal and infectious neurodegenerative diseases that occur in humans and animals. They are caused by the misfolding of the cellular prion protein PrP^c into the infectious isoform PrP^Sc. PrP^Sc accumulates mostly in endolysosomal vesicles of prion-infected cells, eventually causing neurodegeneration. In response to prion infection, elevated cholesterol levels and a reduction in membrane-attached small GTPase Rab7 have been observed in neuronal cells. Here, we investigated the molecular events causing an impaired Rab7 membrane attachment and the potential mechanistic link with elevated cholesterol levels in prion infection. We demonstrate that prion infection is associated with reduced levels of active Rab7 (Rab7.GTP) in persistently prion-infected neuronal cell lines, primary cerebellar granular neurons, and neurons in the brain of mice with terminal prion disease. In primary cerebellar granular neurons, levels of active Rab7 were increased during the very early stages of the prion infection prior to a significant decrease concomitant with PrP^Sc accumulation. The reduced activation of Rab7 in prion-infected neuronal cell lines is also associated with its reduced ubiquitination status, decreased interaction with its effector RILP, and altered lysosomal positioning. Consequently, the Rab7-mediated trafficking of low-density lipoprotein to lysosomes is delayed. This results in an impaired feedback regulation of cholesterol synthesis leading to an increase in cholesterol levels. Notably, transient overexpression of the constitutively active mutant of Rab7 rescues the delay in the low-density lipoprotein trafficking, hence reducing cholesterol levels and attenuating PrP^Sc propagation, demonstrating a mechanistic link between the loss of Rab7.GTP and elevated cholesterol levels.

Human prion diseases and the prion protein - what is the current state of knowledge?

Prion diseases and the prion protein are only partially understood so far in many aspects. This explains the continued research on this topic, calling for an overview on the current state of knowledge. The main objective of the present review article is to provide a comprehensive up-to-date presentation of all major features of human prion diseases bridging the gap between basic research and clinical aspects. Starting with the prion protein, current insights concerning its physiological functions and the process of pathological conversion will be highlighted. Diagnostic, molecular, and clinical aspects of all human prion diseases will be discussed, including information concerning rare diseases like prion-associated amyloidoses and Huntington disease-like 1, as well as the question about a potential human threat due to the transmission of prions from prion diseases of other species such as chronic wasting disease. Finally, recent attempts to develop future therapeutic strategies will be addressed.

Probing the origin of prion protein misfolding via reconstruction of ancestral proteins

Prion diseases are fatal neurodegenerative diseases caused by pathogenic misfolding of the prion protein, PrP. They are transmissible between hosts, and sometimes between different species, as with transmission of bovine spongiform encephalopathy to humans. Although PrP is found in a wide range of vertebrates, prion diseases are seen only in certain mammals, suggesting that infectious misfolding was a recent evolutionary development. To explore when PrP acquired the ability to misfold infectiously, we reconstructed the sequences of ancestral versions of PrP from the last common primate, primate-rodent, artiodactyl, placental, bird, and amniote. Recombinant ancestral PrPs were then tested for their ability to form β-sheet aggregates, either spontaneously or when seeded with infectious prion strains from human, cervid, or rodent species. The ability to aggregate developed after the oldest ancestor (last common amniote), and aggregation capabilities diverged along evolutionary pathways consistent with modern-day susceptibilities. Ancestral bird PrP could not be seeded with modern-day prions, just as modern-day birds are resistant to prion disease. Computational modeling of structures suggested that differences in helix 2 could account for the resistance of ancestral bird PrP to seeding. Interestingly, ancestral primate PrP could be converted by all prion seeds, including both human and cervid prions, raising the possibility that species descended from an ancestral primate have retained the susceptibility to conversion by cervid prions. More generally, the results suggest that susceptibility to prion disease emerged prior to ~100 million years ago, with placental mammals possibly being generally susceptible to disease.

Cryo-EM of prion strains from the same genotype of host identifies conformational determinants

Prion strains in a given type of mammalian host are distinguished by differences in clinical presentation, neuropathological lesions, survival time, and characteristics of the infecting prion protein (PrP) assemblies. Near-atomic structures of prions from two host species with different PrP sequences have been determined but comparisons of distinct prion strains of the same amino acid sequence are needed to identify purely conformational determinants of prion strain characteristics. Here we report a 3.2 Å resolution cryogenic electron microscopy-based structure of the 22L prion strain purified from the brains of mice engineered to express only PrP lacking glycophosphatidylinositol anchors [anchorless (a) 22L]. Comparison of this near-atomic structure to our recently determined structure of the aRML strain propagated in the same inbred mouse reveals that these two mouse prion strains have distinct conformational templates for growth via incorporation of PrP molecules of the same sequence. Both a22L and aRML are assembled as stacks of PrP molecules forming parallel in-register intermolecular β-sheets and intervening loops, with single monomers spanning the ordered fibril core. Each monomer shares an N-terminal steric zipper, three major arches, and an overall V-shape, but the details of these and other conformational features differ markedly. Thus, variations in shared conformational motifs within a parallel in-register β-stack fibril architecture provide a structural basis for prion strain differentiation within a single host genotype.

REMD Simulations of Full-Length Alpha-Synuclein Together with Ligands Reveal Binding Region and Effect on Amyloid Conversion

Alpha-synuclein is a key protein involved in the development and progression of Parkinson’s disease and other synucleinopathies. The intrinsically disordered nature of alpha-synuclein hinders the computational screening of new drug candidates for the treatment of these neurodegenerative diseases. In the present work, replica exchange molecular dynamics simulations of the full-length alpha-synuclein together with low-molecular ligands were utilized to predict the binding site and effect on the amyloid-like conversion of the protein. This approach enabled an accurate prediction of the binding sites for three tested compounds (fasudil, phthalocyanine tetrasulfonate, and spermine), giving good agreement with data from experiments by other groups. Lots of information about the binding and protein conformational ensemble enabled the suggestion of a putative effect of the ligands on the amyloid-like conversion of alpha-synuclein and the mechanism of anti- and pro-amyloid activity of the tested compounds. Therefore, this approach looks promising for testing new drug candidates for binding with alpha-synuclein or other intrinsically disordered proteins and at the same time the estimation of the effect on protein behavior, including amyloid-like aggregation.

Conformation-Dependent Influences of Hydrophobic Amino Acids in Two In-Register Parallel β-Sheet Amyloids, an α-Synuclein Amyloid and a Local Structural Model of PrPSc

Prions are unconventional pathogens that encode the pathogenic information in conformations of the constituent abnormal isoform of prion protein (PrP^Sc), independently of the nucleotide genome. Therefore, conformational diversity of PrP^Sc underlies the existence of many prion strains and species barriers of prions, although the conformational information is extremely limited. Interestingly, differences between polymorphic or species-specific residues responsible for the species/strain barriers are often caused by conservative replacements between hydrophobic amino acids. This implies that subtle differences among hydrophobic amino acids are significant for PrP^Sc structures. Here we analyzed the influence of different hydrophobic residues on the structures of an in-register parallel β-sheet amyloid of α-synuclein (αSyn) using molecular dynamics (MD) simulation and applied the knowledge from the αSyn amyloid to modeling a local structure of human PrP^Sc encompassing residues 107-143. We found that mutations equivalent to polymorphisms that cause transmission barriers substantially affect the stabilities of the local structures; for example, the G127V mutation, which makes the host resistant to various human prion diseases, greatly destabilized the local structure of the model amyloid. Our study indicates that subtle differences among hydrophobic side chains can considerably affect the interaction network, including hydrogen bonds, and demonstrates specifically how and in what structures hydrophobic residues can exert unique effects on in-register parallel β-sheet amyloids.

Prion strains viewed through the lens of cryo-EM

Mammalian prions are lethal transmissible pathogens that cause fatal neurodegenerative diseases in humans and animals. They consist of fibrils of misfolded, host-encoded prion protein (PrP) which propagate through templated protein polymerisation. Prion strains produce distinct clinicopathological phenotypes in the same host and appear to be encoded by distinct misfolded PrP conformations and assembly states. Despite fundamental advances in our understanding of prion biology, key knowledge gaps remain. These include precise delineation of prion replication mechanisms, detailed explanation of the molecular basis of prion strains and inter-species transmission barriers, and the structural definition of neurotoxic PrP species. Central to addressing these questions is the determination of prion structure. While high-resolution definition of ex vivo prion fibrils once seemed unlikely, recent advances in cryo-electron microscopy (cryo-EM) and computational methods for 3D reconstruction of amyloids have now made this possible. Recently, near-atomic resolution structures of highly infectious, ex vivo prion fibrils from hamster 263K and mouse RML prion strains were reported. The fibrils have a comparable parallel in-register intermolecular β-sheet (PIRIBS) architecture that now provides a structural foundation for understanding prion strain diversity in mammals. Here, we review these new findings and discuss directions for future research.

Structural biology of ex vivo mammalian prions

The structures of prion protein (PrP)-based mammalian prions have long been elusive. However, cryo-EM has begun to reveal the near-atomic resolution structures of fully infectious ex vivo mammalian prion fibrils as well as relatively innocuous synthetic PrP amyloids. Comparisons of these various types of PrP fibrils are now providing initial clues to structural features that correlate with pathogenicity. As first indicated by electron paramagnetic resonance and solid-state NMR studies of synthetic amyloids, all sufficiently resolved PrP fibrils of any sort (n > 10) have parallel in-register intermolecular β-stack architectures. Cryo-EM has shown that infectious brain-derived prion fibrils of the rodent-adapted 263K and RML scrapie strains have much larger ordered cores than the synthetic fibrils. These bona fide prion strains share major structural motifs, but the conformational details and the overall shape of the fibril cross sections differ markedly. Such motif variations, as well as differences in sequence within the ordered polypeptide cores, likely contribute to strain-dependent templating. When present, N-linked glycans and glycophosphatidylinositol (GPI) anchors project outward from the fibril surface. For the mouse RML strain, these posttranslational modifications have little effect on the core structure. In the GPI-anchored prion structures, a linear array of GPI anchors along the twisting fibril axis appears likely to bind membranes in vivo, and as such, may account for pathognomonic membrane distortions seen in prion diseases. In this review, we focus on these infectious prion structures and their implications regarding prion replication mechanisms, strains, transmission barriers, and molecular pathogenesis.

Cryo-EM structure of anchorless RML prion reveals variations in shared motifs between distinct strains

Little is known about the structural basis of prion strains. Here we provide a high (3.0 Å) resolution cryo-electron microscopy-based structure of infectious brain-derived fibrils of the mouse anchorless RML scrapie strain which, like the recently determined hamster 263K strain, has a parallel in-register β-sheet-based core. Several structural motifs are shared between these ex vivo prion strains, including an amino-proximal steric zipper and three β-arches. However, detailed comparisons reveal variations in these shared structural topologies and other features. Unlike 263K and wildtype RML prions, the anchorless RML prions lack glycophosphatidylinositol anchors and are severely deficient in N-linked glycans. Nonetheless, the similarity of our anchorless RML structure to one reported for wildtype RML prion fibrils in an accompanying paper indicates that these post-translational modifications do not substantially alter the amyloid core conformation. This work demonstrates both common and divergent structural features of prion strains at the near-atomic level.

Cholesterol and its reciprocal association with prion infection

Prion diseases are incurable, infectious and fatal neurodegenerative diseases that affect both humans and animals. The pathogenesis of prion disease involves the misfolding of the cellular prion protein, PrP^C, to a disease-causing conformation, PrP^Sc, in the brain. The exact mechanism of conversion of PrP^C to PrP^Sc is not clear; however, there are numerous studies supporting that this process of misfolding requires the association of PrP^C with lipid raft domains of the plasma membrane. An increase in the cellular cholesterol content with prion infection has been observed in both in vivo and in vitro studies. As cholesterol is critical for the formation of lipid rafts, on the one hand, this increase may be related to, or aiding in, the process of prion conversion. On the other hand, increased cholesterol levels may affect neuronal viability. Here, we discuss current literature on the underlying mechanisms and potential consequences of elevated neuronal cholesterol in prion infection and advancements in prion disease therapeutics targeting brain cholesterol homeostasis.

Location of the cross-β structure in prion fibrils: A search by seeding and electron spin resonance spectroscopy

Prion diseases are transmissible fatal neurodegenerative disorders spreading between humans and other mammals. The pathogenic agent, prion, is a protease-resistant, β-sheet-rich protein aggregate, converted from a membrane protein called PrPC . PrPSc is the misfolded form of PrPC and undergoes self-propagation to form the infectious amyloids. Since the key hallmark of prion disease is amyloid formation, identifying and studying which segments are involved in the amyloid core can provide molecular details about prion diseases. It has been known that the prion protein could also form non-infectious fibrils in the presence of denaturants. In this study, we employed a combination of site-directed nitroxide spin-labeling, fibril seeding, and electron spin resonance (ESR) spectroscopy to identify the structure of the in vitro-prepared full-length mouse prion fibrils. It is shown that in the in vitro amyloidogenesis, the formation of the amyloid core is linked to an α-to-β structural transformation involving the segment 160-224, which contains strand 2, helix 2, and helix 3. This method is particularly suitable for examining the hetero-seeded amyloid fibril structure, as the unlabeled seeds are invisible by ESR spectroscopy. It can be applied to study the structures of different strains of infectious prions or other amyloid fibrils in the future.

Genetically engineered cellular models of prion propagation

For over three decades, cultured cells have been a useful tool for dissecting the molecular details of prion replication and the identification of candidate therapeutics for prion disease. A major issue limiting the translatability of these studies has been the inability to reliably propagate disease-relevant, non-mouse strains of prions in cells relevant to prion pathogenesis. In recent years, fueled by advances in gene editing technology, it has become possible to propagate prions from hamsters, cervids, and sheep in immortalized cell lines originating from the central nervous system. In particular, the use of CRISPR-Cas9-mediated gene editing to generate versions of prion-permissive cell lines that lack endogenous PrP expression has provided a blank canvas upon which re-expression of PrP leads to species-matched susceptibility to prion infection. When coupled with the ability to propagate prions in cells or organoids derived from stem cells, these next-generation cellular models should provide an ideal paradigm for identifying small molecules and other biological therapeutics capable of interfering with prion replication in animal and human prion disorders. In this review, we summarize recent advances that have widened the spectrum of prion strains that can be propagated in cultured cells and cutting-edge tissue-based models.

Chronic Wasting Disease (CWD) in Cervids and the Consequences of a Mutable Protein Conformation

Chronic wasting disease (CWD) is a prion disease of cervids (deer, elk, moose, etc.). It spreads readily from CWD-contaminated environments and among wild cervids. As of 2022, North American CWD has been found in 29 states, four Canadian provinces and South Korea. The Scandinavian form of CWD originated independently. Prions propagate their pathology by inducing a natively expressed prion protein (PrPC) to adopt the prion conformation (PrPSc). PrPC and PrPSc differ solely in their conformation. Like other prion diseases, transmissible CWD prions can arise spontaneously. The CWD prions can respond to selection pressures resulting in the emergence of new strain phenotypes. Annually, 11.5 million Americans hunt and harvest nearly 6 million deer, indicating that CWD is a potential threat to an important American food source. No tested CWD strain has been shown to be zoonotic. However, this may not be true for emerging strains. Should a zoonotic CWD strain emerge, it could adversely impact the hunting economy and game meat consumers.

Extracellular vesicles with diagnostic and therapeutic potential for prion diseases

Prion diseases (PrD) or transmissible spongiform encephalopathies (TSE) are invariably fatal and pathogenic neurodegenerative disorders caused by the self-propagated misfolding of cellular prion protein (PrP^C) to the neurotoxic pathogenic form (PrP^TSE) via a yet undefined but profoundly complex mechanism. Despite several decades of research on PrD, the basic understanding of where and how PrP^C is transformed to the misfolded, aggregation-prone and pathogenic PrP^TSE remains elusive. The primary clinical hallmarks of PrD include vacuolation-associated spongiform changes and PrP^TSE accumulation in neural tissue together with astrogliosis. The difficulty in unravelling the disease mechanisms has been related to the rare occurrence and long incubation period (over decades) followed by a very short clinical phase (few months). Additional challenge in unravelling the disease is implicated to the unique nature of the agent, its complexity and strain diversity, resulting in the heterogeneity of the clinical manifestations and potentially diverse disease mechanisms. Recent advances in tissue isolation and processing techniques have identified novel means of intercellular communication through extracellular vesicles (EVs) that contribute to PrP^TSE transmission in PrD. This review will comprehensively discuss PrP^TSE transmission and neurotoxicity, focusing on the role of EVs in disease progression, biomarker discovery and potential therapeutic agents for the treatment of PrD.

Prion protein with a mutant N-terminal octarepeat region undergoes cobalamin-dependent assembly into high-molecular weight complexes

The cellular prion protein (PrP^C) has a C-terminal globular domain and a disordered N-terminal region encompassing five octarepeats (ORs). Encounters between Cu(II) ions and four OR sites produce interchangeable binding geometries; however, the significance of Cu(II) binding to ORs in different combinations is unclear. To understand the impact of specific binding geometries, OR variants were designed that interact with multiple or single Cu(II) ions in specific locked coordinations. Unexpectedly, we found that one mutant produced detergent-insoluble, protease-resistant species in cells in the absence of exposure to the infectious prion protein isoform, scrapie-associated prion protein (PrP^Sc). Formation of these assemblies, visible as puncta, was reversible and dependent upon medium formulation. Cobalamin (Cbl), a dietary cofactor containing a corrin ring that coordinates a Co³⁺ ion, was identified as a key medium component, and its effect was validated by reconstitution experiments. Although we failed to find evidence that Cbl interacts with Cu-binding OR regions, we instead noted interactions of Cbl with the PrP^C C-terminal domain. We found that some interactions occurred at a binding site of planar tetrapyrrole compounds on the isolated globular domain, but others did not, and N-terminal sequences additionally had a marked effect on their presence and position. Our studies define a conditional effect of Cbl wherein a mutant OR region can act in cis to destabilize a globular domain with a wild type sequence. The unexpected intersection between the properties of PrP^Sc's disordered region, Cbl, and conformational remodeling events may have implications for understanding sporadic prion disease that does not involve exposure to PrP^Sc.

Molecular simulations of proteins: From simplified physical interactions to complex biological phenomena

Molecular dynamics simulation is the most popular computational technique for investigating the structural and dynamical behaviour of proteins, in search of the molecular basis of their function. Far from being a completely settled field of research, simulations are still evolving to best capture the essential features of the atomic interactions that govern a protein's inner motions. Modern force fields are becoming increasingly accurate in providing a physical description adequate to this purpose, and allow us to model complex biological systems under fairly realistic conditions. Furthermore, the use of accelerated sampling techniques is improving our access to the observation of progressively larger molecular structures, longer time scales, and more hidden functional events. In this review, the basic principles of molecular dynamics simulations and a number of key applications in the area of protein science are summarized, and some of the most important results are discussed. Examples include the study of the structure, dynamics and binding properties of 'difficult' targets, such as intrinsically disordered proteins and membrane receptors, and the investigation of challenging phenomena like hydration-driven processes and protein aggregation. The findings described provide an overall picture of the current state of this research field, and indicate new perspectives on the road ahead to the upcoming future of molecular simulations.

Detecting Differences in Prion Protein Conformation by Quantifying Methionine Oxidation

A prion's pathogenic character is enciphered in its conformation, which also defines the chemical environments of its amino acids. Differences in chemical environments influence the reactivity of amino acid side chains, in a conformation-dependent manner. Chemical oxidation of susceptible methionines would identify those methionines on the surface of a prion, which would reveal conformation-dependent information. We identified a set of methionine-containing peptides derived from the tryptic, chymotryptic, or tryptic/chymotryptic digestion of recombinant prion protein and the Sc237 strain of hamster-adapted scrapie. We developed a multiple reaction monitoring-based method of quantifying the extent of the methionine oxidation in those peptides. This approach can be used to define a prion's conformation and to distinguish among prion strains, which is an important component of food safety.

Biochemical Principles in Prion-Based Inheritance

Prions are proteins that can stably fold into alternative structures that frequently alter their activities. They can self-template their alternate structures and are inherited across cell divisions and generations. While they have been studied for more than four decades, their enigmatic nature has limited their discovery. In the last decade, we have learned just how widespread they are in nature, the many beneficial phenotypes that they confer, while also learning more about their structures and modes of inheritance. Here, we provide a brief review of the biochemical principles of prion proteins, including their sequences, characteristics and structures, and what is known about how they self-template, citing examples from multiple organisms. Prion-based inheritance is the most understudied segment of epigenetics. Here, we lay a biochemical foundation and share a framework for how to define these molecules, as new examples are unearthed throughout nature.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

The unique ability to adapt and thrive in inhospitable, stressful tumor microenvironments (TME) also renders cancer cells resistant to traditional chemotherapeutic treatments and/or novel pharmaceuticals. Cancer cells exhibit extensive metabolic alterations involving hypoxia, accelerated glycolysis, oxidative stress, and increased extracellular ATP that may activate ancient, conserved prion adaptive response strategies that exacerbate multidrug resistance (MDR) by exploiting cellular stress to increase cancer metastatic potential and stemness, balance proliferation and differentiation, and amplify resistance to apoptosis. The regulation of prions in MDR is further complicated by important, putative physiological functions of ligand-binding and signal transduction. Melatonin is capable of both enhancing physiological functions and inhibiting oncogenic properties of prion proteins. Through regulation of phase separation of the prion N-terminal domain which targets and interacts with lipid rafts, melatonin may prevent conformational changes that can result in aggregation and/or conversion to pathological, infectious isoforms. As a cancer therapy adjuvant, melatonin could modulate TME oxidative stress levels and hypoxia, reverse pH gradient changes, reduce lipid peroxidation, and protect lipid raft compositions to suppress prion-mediated, non-Mendelian, heritable, but often reversible epigenetic adaptations that facilitate cancer heterogeneity, stemness, metastasis, and drug resistance. This review examines some of the mechanisms that may balance physiological and pathological effects of prions and prion-like proteins achieved through the synergistic use of melatonin to ameliorate MDR, which remains a challenge in cancer treatment.

Body-first Parkinson's disease and variant Creutzfeldt-Jakob disease - similar or different?

In several neurodegenerative disorders, proteins that typically exhibit an α-helical structure misfold into an amyloid conformation rich in β-sheet content. Through a self-templating mechanism, these amyloids are able to induce additional protein misfolding, facilitating their propagation throughout the central nervous system. This disease mechanism was originally identified for the prion protein (PrP), which misfolds into PrP^Sc in a number of disorders, including variant Creutzfeldt-Jakob disease (vCJD) and bovine spongiform encephalopathy (BSE). More recently, the prion mechanism of disease was expanded to include other proteins that rely on this self-templating mechanism to cause progressive degeneration, including α-synuclein misfolding in Parkinson's disease (PD). Several studies now suggest that PD patients can be subcategorized based on where in the body misfolded α-synuclein originates, either the brain or the gut, similar to patients developing sporadic CJD or vCJD. In this review, we discuss the human and animal model data indicating that α-synuclein and PrP^Sc misfolding originates in the gut in body-first PD and vCJD, and summarize the data identifying the role of the autonomic nervous system in the gut-brain axis of both diseases.

The role of microglia in prion diseases and possible therapeutic targets: a literature review

Creutzfeldt-Jakob disease (CJD) is a rare and fatal condition that leads to progressive neurodegeneration due to gliosis, vacuolation of central nervous system tissue, and loss of neurons. Microglia play a crucial role in maintaining Central Nervous System (CNS) homoeostasis, both in health and disease, through phagocytosis and cytokine production. In the context of CJD, the immunomodulatory function of microglia turns it into a cell of particular interest. Microglia would be activated by infectious prion proteins, initially acquiring a phagocytic and anti-inflammatory profile (M2), and producing cytokines such as IL-4, IL-10, and TGF-β. Therefore, microglia are seen as a key target for the development of new treatment approaches, with many emerging strategies to guide it towards a beneficial role upon neuroinflammation, by manipulating its metabolic pathways. In such a setting, many cellular targets in microglia that can be involved in phenotype modulation, such as membrane receptors, have been identified and pointed out as possible targets for further experiments and therapeutic approaches. In this article, we review the major findings about the role of microglia in CJD, including its relationship to some risk factors associated with the development of the disease. Furthermore, considering its central role in neural immunity, we explore microglial connection with other elements of the immune system and cell signalling, such as inflammasomes, the complement and purinergic systems, and the latest finding strategies to guide these cells from harmful to beneficial roles.

Virus Infection, Genetic Mutations, and Prion Infection in Prion Protein Conversion

Conformational conversion of the cellular isoform of prion protein, PrP^C, into the abnormally folded, amyloidogenic isoform, PrP^Sc, is an underlying pathogenic mechanism in prion diseases. The diseases manifest as sporadic, hereditary, and acquired disorders. Etiological mechanisms driving the conversion of PrP^C into PrP^Sc are unknown in sporadic prion diseases, while prion infection and specific mutations in the PrP gene are known to cause the conversion of PrP^C into PrP^Sc in acquired and hereditary prion diseases, respectively. We recently reported that a neurotropic strain of influenza A virus (IAV) induced the conversion of PrP^C into PrP^Sc as well as formation of infectious prions in mouse neuroblastoma cells after infection, suggesting the causative role of the neuronal infection of IAV in sporadic prion diseases. Here, we discuss the conversion mechanism of PrP^C into PrP^Sc in different types of prion diseases, by presenting our findings of the IAV infection-induced conversion of PrP^C into PrP^Sc and by reviewing the so far reported transgenic animal models of hereditary prion diseases and the reverse genetic studies, which have revealed the structure-function relationship for PrP^C to convert into PrP^Sc after prion infection.

High-resolution structure and strain comparison of infectious mammalian prions

Within the extensive range of self-propagating pathologic protein aggregates of mammals, prions are the most clearly infectious (e.g., ∼10⁹ lethal doses per milligram). The structures of such lethal assemblies of PrP molecules have been poorly understood. Here we report a near-atomic core structure of a brain-derived, fully infectious prion (263K strain). Cryo-electron microscopy showed amyloid fibrils assembled with parallel in-register intermolecular β sheets. Each monomer provides one rung of the ordered fibril core, with N-linked glycans and glycolipid anchors projecting outward. Thus, single monomers form the templating surface for incoming monomers at fibril ends, where prion growth occurs. Comparison to another prion strain (aRML) revealed major differences in fibril morphology but, like 263K, an asymmetric fibril cross-section without paired protofilaments. These findings provide structural insights into prion propagation, strains, species barriers, and membrane pathogenesis. This structure also helps frame considerations of factors influencing the relative transmissibility of other pathologic amyloids.

Epitope-specific anti-PrP antibody toxicity: a comparative in-silico study of human and mouse prion proteins

Despite having therapeutic potential, anti-PrP antibodies caused a major controversy due to their neurotoxic effects. For instance, treating mice with ICSM antibodies delayed prion disease onset, but both were found to be either toxic or innocuous to neurons by researchers following cross-linking PrP^C. In order to elucidate and understand the reasons that led to these contradictory outcomes, we conducted a comprehensive in silico study to assess the antibody-specific toxicity. Since most therapeutic anti-PrP antibodies were generated against human truncated recombinant PrP^91-231 or full-length mouse PrP^23-231, we reasoned that host specificity (human vs murine) of PrP^C might influence the nature of the specific epitopes recognized by these antibodies at the structural level possibly explaining the 'toxicity' discrepancies reported previously. Initially, molecular dynamics simulation and pro-motif analysis of full-length human (hu)PrP and mouse (mo)PrP 3D structure displayed conspicuous structural differences between huPrP and moPrP. We identified 10 huPrP and 6 moPrP linear B-cell epitopes from the prion protein 3D structure where 5 out of 10 huPrP and 3 out of 6 moPrP B-cell epitopes were predicted to be potentially toxic in immunoinformatics approaches. Herein, we demonstrate that some of the predicted potentially 'toxic' epitopes identified by the in silico analysis were similar to the epitopes recognized by the toxic antibodies such as ICSM18 (146-159), POM1 (138-147), D18 (133-157), ICSM35 (91-110), D13 (95-103) and POM3 (95-100). This in silico study reveals the role of host specificity of PrP^C in epitope-specific anti-PrP antibody toxicity.

Structural and electronic analysis of the octarepeat region of prion protein with four Cu2+ by polarizable MD and QM/MM simulations

Prions have been linked to neurodegenerative diseases that affect various species of mammals including humans. The prion protein, located mainly in neurons, is believed to play the role of metal ion transporter. High levels of copper ions have been related to structural changes. A 32-residue region of the N-terminal domain, known as octarepeat, can bind up to four copper ions. Different coordination modes have been observed and are strongly dependent on Cu2+ concentration. Many theoretical studies carried out so far have focused on studying the coordination modes of a single copper ion. In this work we investigate the octarepeat region coordinated with four copper ions. Molecular dynamics (MD) and hybrid quantum mechanics/molecular mechanics (QM/MM) simulations using the polarizable AMOEBA force field have been carried out. The polarizable MD simulations starting from a fully extended conformation indicate that the tetra-Cu2+/octarepeat complex forms a globular structure. The globular form is stabilized by interactions between Cu2+ and tryptophan residues resulting in some coordination sites observed to be in close proximity, in agreement with experimental results. Subsequent QM/MM simulations on several snapshots suggests the system is in a high-spin quintet state, with all Cu2+ bearing one single electron, and all unpaired electrons are ferromagnetically coupled. NMR simulations on selected structures provides insights on the chemical shifts of the first shell ligands around the metals with respect to inter-metal distances.

Genetic prion disease-related mutation E196K displays a novel amyloid fibril structure revealed by cryo-EM

Prion diseases are caused by the conformational conversion of prion protein (PrP). Forty-two different mutations were identified in human PrP, leading to genetic prion diseases with distinct clinical syndromes. Here, we report the cryo–electron microscopy structure of an amyloid fibril formed by full-length human PrP with E196K mutation, a genetic Creutzfeldt-Jakob disease–related mutation. This mutation disrupts key interactions in the wild-type PrP fibril, forming an amyloid fibril with a conformation distinct from the wild-type PrP fibril and hamster brain–derived prion fibril. The E196K fibril consists of two protofibrils. Each subunit forms five β strands stabilized by a disulfide bond and an unusual hydrophilic cavity stabilized by a salt bridge. Four pairs of amino acids from opposing subunits form four salt bridges to stabilize the zigzag interface of the two protofibrils. Our results provide structural evidences of the diverse prion strains and highlight the importance of familial mutations in inducing different strains.

Unraveling the Histidine Tautomerism Effect on the Initial Stages of Prion Misfolding: New Insights from a Computational Perspective

The aggregation and structural conversion of normal prion peptide (PrP^C) into the pathogenic scrapie form (PrP^Sc), which can act as a seed to enhance prion amyloid fiber formation, is believed to be a crucial event in prionopathies. Previous research suggests that the prion monomer may play an important role in oligomer generation during disease pathogenesis. In the present study, extensive replica-exchange molecular dynamics (REMD) simulations were conducted to explore the conformational characteristics of the huPrP (125-160) monomer under the histidine tautomerism effect. Investigating the structural characteristics and fibrilization process is challenging because two histidine tautomers [N_ε2-H (ε) and N_δ1-H (δ)] can occur in the open neutral state. Molecular dynamics (MD) simulation outcomes have shown that the toxic εδ and δδ isomer (containing several and broader local minima) had the highest α-helix structures, with contents of 21.11% and 21.01%, respectively, and may have a strong influence on the organizational behavior of a monomeric prion. The amino acids aspartate 20 (D20)-asparagine 29 (N29) and isoleucine 15 (I15)-histidine 16 (H16), D20-arginine 27 (R27) as well as N29 formed α-helix with the highest probabilities in the δδ and εδ isomer, accordingly. On the basis of our findings, we propose the histidine tautomerization hypothesis as a new prion accumulation mechanism, which may exist to induce the formation of prion accumulates. Overall, our tautomerism hypothesis constitutes a promising perspective for enhancing understanding of prion disease pathobiology and may help in the design of a good inhibitor.

Prion protein and prion disease at a glance

Prion diseases are neurodegenerative disorders caused by conformational conversion of the cellular prion protein (PrPC) into scrapie prion protein (PrPSc). As the main component of prion, PrPSc acts as an infectious template that recruits and converts normal cellular PrPC into its pathogenic, misfolded isoform. Intriguingly, the phenomenon of prionoid, or prion-like, spread has also been observed in many other disease-associated proteins, such as amyloid β (Aβ), tau and α-synuclein. This Cell Science at a Glance and the accompanying poster highlight recently described physiological roles of prion protein and the advanced understanding of pathogenesis of prion disease they have afforded. Importantly, prion protein may also be involved in the pathogenesis of other neurodegenerative disorders such as Alzheimer's and Parkinson's disease. Therapeutic studies of prion disease have also exploited novel strategies to combat these devastating diseases. Future studies on prion protein and prion disease will deepen our understanding of the pathogenesis of a broad spectrum of neurodegenerative conditions.

Multiscale simulations of large complexes in conjunction with cryo-EM analysis

The cellular environment is highly crowded with most proteins and RNA/DNA forming homomeric and heteromeric complexes. Essential questions regarding how these complexes switch between functional, rest, and abnormal states with regulators or modifications remain challenging and complicated. Here, we review the recent progress integrating cryoelectron microscopy and multiscale molecular modeling to understand the dynamics and function-related mechanism in protein-RNA/DNA complexes, protein-protein complexes/assemblies, and membrane protein complexes. One future direction of multiscale simulations will be to interpret the large complex multibody regulation in assembly-induced function enhancement in conjunction with advanced atomic resolution structural-biology techniques and specialized computing architectures.

New and distinct chronic wasting disease strains associated with cervid polymorphism at codon 116 of the Prnp gene

Chronic wasting disease (CWD) is a prion disease affecting cervids. Polymorphisms in the prion protein gene can result in extended survival of CWD-infected animals. However, the impact of polymorphisms on cellular prion protein (PrP^C) and prion properties is less understood. Previously, we characterized the effects of a polymorphism at codon 116 (A>G) of the white-tailed deer (WTD) prion protein and determined that it destabilizes PrP^C structure. Comparing CWD isolates from WTD expressing homozygous wild-type (116AA) or heterozygous (116AG) PrP, we found that 116AG-prions were conformationally less stable, more sensitive to proteases, with lower seeding activity in cell-free conversion and reduced infectivity. Here, we aimed to understand CWD strain emergence and adaptation. We show that the WTD-116AG isolate contains two different prion strains, distinguished by their host range, biochemical properties, and pathogenesis from WTD-116AA prions (Wisc-1). Serial passages of WTD-116AG prions in tg(CerPrP)1536^+/+ mice overexpressing wild-type deer-PrP^C revealed two populations of mice with short and long incubation periods, respectively, and remarkably prolonged clinical phase upon inoculation with WTD-116AG prions. Inoculation of serially diluted brain homogenates confirmed the presence of two strains in the 116AG isolate with distinct pathology in the brain. Interestingly, deglycosylation revealed proteinase K-resistant fragments with different electrophoretic mobility in both tg(CerPrP)1536^+/+ mice and Syrian golden hamsters infected with WTD-116AG. Infection of tg60 mice expressing deer S96-PrP with 116AG, but not Wisc-1 prions induced clinical disease. On the contrary, bank voles resisted 116AG prions, but not Wisc-1 infection. Our data indicate that two strains co-existed in the WTD-116AG isolate, expanding the variety of CWD prion strains. We argue that the 116AG isolate does not contain Wisc-1 prions, indicating that the presence of 116G-PrP^C diverted 116A-PrP^C from adopting a Wisc-1 structure. This can have important implications for their possible distinct capacities to cross species barriers into both cervids and non-cervids.

Chronic wasting disease belongs to the family of prion diseases. It is considered the most contagious prion disease and the only one that affects free ranging wildlife. The disease range is expanding in North America and Northern Europe. This work describes the emergence and characterization of new chronic wasting disease strains related to a polymorphism in the prion protein gene. It supports the concept that strains are a dynamic mixture of substrains that can influence and interfere with each other. Because transmission barriers are governed by the compatibility of a particular prion strain with the new host’s prion protein, it is critical to understand the emergence and variety of chronic wasting disease strains circulating in wild animals and their ability to infect new host species including humans.

Structures of Pathological and Functional Amyloids and Prions, a Solid-State NMR Perspective

Infectious proteins or prions are a remarkable class of pathogens, where pathogenicity and infectious state correspond to conformational transition of a protein fold. The conformational change translates into the formation by the protein of insoluble amyloid aggregates, associated in humans with various neurodegenerative disorders and systemic protein-deposition diseases. The prion principle, however, is not limited to pathogenicity. While pathological amyloids (and prions) emerge from protein misfolding, a class of functional amyloids has been defined, consisting of amyloid-forming domains under natural selection and with diverse biological roles. Although of great importance, prion amyloid structures remain challenging for conventional structural biology techniques. Solid-state nuclear magnetic resonance (SSNMR) has been preferentially used to investigate these insoluble, morphologically heterogeneous aggregates with poor crystallinity. SSNMR methods have yielded a wealth of knowledge regarding the fundamentals of prion biology and have helped to solve the structures of several prion and prion-like fibrils. Here, we will review pathological and functional amyloid structures and will discuss some of the obtained structural models. We will finish the review with a perspective on integrative approaches combining solid-state NMR, electron paramagnetic resonance and cryo-electron microscopy, which can complement and extend our toolkit to structurally explore various facets of prion biology.

From System Modeling to System Analysis: The Impact of Resolution Level and Resolution Distribution in the Computer-Aided Investigation of Biomolecules

The ever increasing computer power, together with the improved accuracy of atomistic force fields, enables researchers to investigate biological systems at the molecular level with remarkable detail. However, the relevant length and time scales of many processes of interest are still hardly within reach even for state-of-the-art hardware, thus leaving important questions often unanswered. The computer-aided investigation of many biological physics problems thus largely benefits from the usage of coarse-grained models, that is, simplified representations of a molecule at a level of resolution that is lower than atomistic. A plethora of coarse-grained models have been developed, which differ most notably in their granularity; this latter aspect determines one of the crucial open issues in the field, i.e. the identification of an optimal degree of coarsening, which enables the greatest simplification at the expenses of the smallest information loss. In this review, we present the problem of coarse-grained modeling in biophysics from the viewpoint of system representation and information content. In particular, we discuss two distinct yet complementary aspects of protein modeling: on the one hand, the relationship between the resolution of a model and its capacity of accurately reproducing the properties of interest; on the other hand, the possibility of employing a lower resolution description of a detailed model to extract simple, useful, and intelligible information from the latter.

The ultrastructure of infectious L-type bovine spongiform encephalopathy prions constrains molecular models

Bovine spongiform encephalopathy (BSE) is a prion disease of cattle that is caused by the misfolding of the cellular prion protein (PrPC) into an infectious conformation (PrPSc). PrPC is a predominantly α-helical membrane protein that misfolds into a β-sheet rich, infectious state, which has a high propensity to self-assemble into amyloid fibrils. Three strains of BSE prions can cause prion disease in cattle, including classical BSE (C-type) and two atypical strains, named L-type and H-type BSE. To date, there is no detailed information available about the structure of any of the infectious BSE prion strains. In this study, we purified L-type BSE prions from transgenic mouse brains and investigated their biochemical and ultrastructural characteristics using electron microscopy, image processing, and immunogold labeling techniques. By using phosphotungstate anions (PTA) to precipitate PrPSc combined with sucrose gradient centrifugation, a high yield of proteinase K-resistant BSE amyloid fibrils was obtained. A morphological examination using electron microscopy, two-dimensional class averages, and three-dimensional reconstructions revealed two structural classes of L-type BSE amyloid fibrils; fibrils that consisted of two protofilaments with a central gap and an average width of 22.5 nm and one-protofilament fibrils that were 10.6 nm wide. The one-protofilament fibrils were found to be more abundant compared to the thicker two-protofilament fibrils. Both fibrillar assemblies were successfully decorated with monoclonal antibodies against N- and C-terminal epitopes of PrP using immunogold-labeling techniques, confirming the presence of polypeptides that span residues 100-110 to 227-237. The fact that the one-protofilament fibrils contain both N- and C-terminal PrP epitopes constrains molecular models for the structure of the infectious conformer in favour of a compact four-rung β-solenoid fold.

Asymmetric-flow field-flow fractionation of prions reveals a strain-specific continuum of quaternary structures with protease resistance developing at a hydrodynamic radius of 15 nm

Prion diseases are transmissible neurodegenerative disorders that affect mammals, including humans. The central molecular event is the conversion of cellular prion glycoprotein, PrPC, into a plethora of assemblies, PrPSc, associated with disease. Distinct phenotypes of disease led to the concept of prion strains, which are associated with distinct PrPSc structures. However, the degree to which intra- and inter-strain PrPSc heterogeneity contributes to disease pathogenesis remains unclear. Addressing this question requires the precise isolation and characterization of all PrPSc subpopulations from the prion-infected brains. Until now, this has been challenging. We used asymmetric-flow field-flow fractionation (AF4) to isolate all PrPSc subpopulations from brains of hamsters infected with three prion strains: Hyper (HY) and 263K, which produce almost identical phenotypes, and Drowsy (DY), a strain with a distinct presentation. In-line dynamic and multi-angle light scattering (DLS/MALS) data provided accurate measurements of particle sizes and estimation of the shape and number of PrPSc particles. We found that each strain had a continuum of PrPSc assemblies, with strong correlation between PrPSc quaternary structure and phenotype. HY and 263K were enriched with large, protease-resistant PrPSc aggregates, whereas DY consisted primarily of smaller, more protease-sensitive aggregates. For all strains, a transition from protease-sensitive to protease-resistant PrPSc took place at a hydrodynamic radius (Rh) of 15 nm and was accompanied by a change in glycosylation and seeding activity. Our results show that the combination of AF4 with in-line MALS/DLS is a powerful tool for analyzing PrPSc subpopulations and demonstrate that while PrPSc quaternary structure is a major contributor to PrPSc structural heterogeneity, a fundamental change, likely in secondary/tertiary structure, prevents PrPSc particles from maintaining proteinase K resistance below an Rh of 15 nm, regardless of strain. This results in two biochemically distinctive subpopulations, the proportion, seeding activity, and stability of which correlate with prion strain phenotype.

Harnessing the Physiological Functions of Cellular Prion Protein in the Kidneys: Applications for Treating Renal Diseases

A cellular prion protein (PrP^C) is a ubiquitous cell surface glycoprotein, and its physiological functions have been receiving increased attention. Endogenous PrP^C is present in various kidney tissues and undergoes glomerular filtration. In prion diseases, abnormal prion proteins are found to accumulate in renal tissues and filtered into urine. Urinary prion protein could serve as a diagnostic biomarker. PrP^C plays a role in cellular signaling pathways, reno-protective effects, and kidney iron uptake. PrP^C signaling affects mitochondrial function via the ERK pathway and is affected by the regulatory influence of microRNAs, small molecules, and signaling proteins. Targeting PrP^C in acute and chronic kidney disease could help improve iron homeostasis, ameliorate damage from ischemia/reperfusion injury, and enhance the efficacy of mesenchymal stem/stromal cell or extracellular vesicle-based therapeutic strategies. PrP^C may also be under the influence of BMP/Smad signaling and affect the progression of TGF-β-related renal fibrosis. PrP^C conveys TNF-α resistance in some renal cancers, and therefore, the coadministration of anti-PrP^C antibodies improves chemotherapy. PrP^C can be used to design antibody-drug conjugates, aptamer-drug conjugates, and customized tissue inhibitors of metalloproteinases to suppress cancer. With preclinical studies demonstrating promising results, further research on PrP^C in the kidney may lead to innovative PrP^C-based therapeutic strategies for renal disease.

Tunneling nanotubes: A novel pharmacological target for neurodegenerative diseases?

Diversiform ways of intercellular communication are vital links in maintaining homeostasis and disseminating physiological states. Among intercellular bridges, tunneling nanotubes (TNTs) discovered in 2004 were recognized as potential pharmacology targets related to the pathogenesis of common or infrequent neurodegenerative disorders. The neurotoxic aggregates in neurodegenerative diseases including scrapie prion protein (PrPSc), mutant tau protein, amyloid-beta (Aβ) protein, alpha-synuclein (α-syn) as well as mutant Huntington (mHTT) protein could promote TNT formation via certain physiological mechanisms, in turn, mediating the intercellular transmission of neurotoxicity. In this review, we described in detail the skeleton, the formation, the physicochemical properties, and the functions of TNTs, while paying particular attention to the key role of TNTs in the transport of pathological proteins during neurodegeneration.

Time of Detection of Prions in the Brain by Nanoscale Liquid Chromatography Coupled to Tandem Mass Spectrometry Is Comparable to Animal Bioassay

Prions cause transmissible and inevitably fatal neurological diseases in agriculturally important animals, including bovine spongiform encephalopathy in domestic cattle, scrapie in sheep and goats, and chronic wasting disease in cervids. Because animals are largely asymptomatic throughout the course of the disease, early detection of prion disease is important. Hamsters were peripherally (ip) inoculated with hamster-adapted (Sc237) prions. By week 13 of a 14-week disease course, clinical signs appeared. A multiple-reaction-monitoring-based method was used to quantitate the amount of proteinase-K-digested prions (PrP 27-30) and the extent of methionine 213 oxidation present in the brains of infected hamsters. Detectable amounts of PrP 27-30 were present in all animals after 4 weeks. The extent of methionine 213 oxidation decreased over time. When we compared our quantitation results to those from other researchers using bioassay, we observed that consistent detection of PrP 27-30 by mass spectrometry occurs at a time when prions are reliably detected by bioassay.

Multisite interactions of prions with membranes and native nanodiscs

Although prions are known as protein-only infectious particles, they exhibit lipid specificities, cofactor dependencies and membrane-dependent activities. Such membrane interactions play key roles in how prions are processed, presented and regulated, and hence have significant functional consequences. The expansive literature related to prion protein interactions with lipids and native nanodiscs is discussed, and provides a unique opportunity to re-evaluate the molecular composition and mechanisms of its infectious and cellular states. A family of crystal and solution structures of prions are analyzed here for the first time using the membrane optimal docking area (MODA) program, revealling the presence of structured binding elements that could mediate specific lipid recognition. A set of motifs centerred around W99, L125, Y169 and Y226 are consistently predicted as being membrane interactive and form an exposed surface which includes α helical, β strand and loop elements involving the prion protein (PrP) structural domain, while the scrapie form is radically different and doubles the size of the membrane interactive site into an extensible surface. These motifs are highly conserved throughout mammalian evolution, suggesting that prions have long been intrinsically attached to membranes at central and N- and C-terminal points, providing several opportunities for stable and specific bilayer interactions as well as multiple complexed orientations. Resistance or susceptibility to prion disease correlates with increased or decreased membrane binding propensity by mutant forms, respectively, indicating a protective role by lipids. The various prion states found in vivo are increasingly resolvable using native nanodiscs formed by styrene maleic acid (SMA) and stilbene maleic acid (STMA) copolymers rather than classical detergents, allowing the endogenous states to be tackled. These copolymers spontaneously fragment intact membranes into water-soluble discs holding a section of native bilayer, and can accommodate prion multimers and mini-fibrils. Such nanodiscs have also proven useful for understanding how β amyloid and α synuclein proteins contribute to Alzheimer's and Parkinson's diseases, providing further biomedical applications. Structural and functional insights of such proteins in styrene maleic acid lipid particles (SMALPs) can be resolved at high resolution by methods including cryo-electron microscopy (cEM), motivating continued progress in polymer design to resolve biological and pathological mechanisms.

From Posttranslational Modifications to Disease Phenotype: A Substrate Selection Hypothesis in Neurodegenerative Diseases

A number of neurodegenerative diseases including prion diseases, tauopathies and synucleinopathies exhibit multiple clinical phenotypes. A diversity of clinical phenotypes has been attributed to the ability of amyloidogenic proteins associated with a particular disease to acquire multiple, conformationally distinct, self-replicating states referred to as strains. Structural diversity of strains formed by tau, α-synuclein or prion proteins has been well documented. However, the question how different strains formed by the same protein elicit different clinical phenotypes remains poorly understood. The current article reviews emerging evidence suggesting that posttranslational modifications are important players in defining strain-specific structures and disease phenotypes. This article put forward a new hypothesis referred to as substrate selection hypothesis, according to which individual strains selectively recruit protein isoforms with a subset of posttranslational modifications that fit into strain-specific structures. Moreover, it is proposed that as a result of selective recruitment, strain-specific patterns of posttranslational modifications are formed, giving rise to unique disease phenotypes. Future studies should define whether cell-, region- and age-specific differences in metabolism of posttranslational modifications play a causative role in dictating strain identity and structural diversity of strains of sporadic origin.

Pharmacological inactivation of the prion protein by targeting a folding intermediate

Recent computational advancements in the simulation of biochemical processes allow investigating the mechanisms involved in protein regulation with realistic physics-based models, at an atomistic level of resolution. These techniques allowed us to design a drug discovery approach, named Pharmacological Protein Inactivation by Folding Intermediate Targeting (PPI-FIT), based on the rationale of negatively regulating protein levels by targeting folding intermediates. Here, PPI-FIT was tested for the first time on the cellular prion protein (PrP), a cell surface glycoprotein playing a key role in fatal and transmissible neurodegenerative pathologies known as prion diseases. We predicted the all-atom structure of an intermediate appearing along the folding pathway of PrP and identified four different small molecule ligands for this conformer, all capable of selectively lowering the load of the protein by promoting its degradation. Our data support the notion that the level of target proteins could be modulated by acting on their folding pathways, implying a previously unappreciated role for folding intermediates in the biological regulation of protein expression.

Spagnolli, Massignan, Astolfi et al. design a new drug discovery approach, termed Pharmacological Protein Inactivation by Folding Intermediate Targeting, in which folding intermediates of disease-causing proteins are targeted. They test it on the cellular prion protein, identifying ligands stabilizing a folding intermediate and consequently promoting its degradation by the cellular quality control machinery.