Toward Therapy of Human Prion Diseases

Personalized brain MRI revealed distinct functional and anatomical disruptions in Creutzfeldt-Jakob disease and Alzheimer's disease

AIMS

Creutzfeldt-Jakob disease (CJD) is a lethal neurodegenerative disorder, which leads to a rapidly progressive dementia. This study aimed to examine the cortical alterations in CJD, changes in these brain characteristics over time, and the differences between CJD and Alzheimer's disease (AD) that show similar clinical manifestations.

METHODS

To obtain reliable, subject-specific functional measures, we acquired 24 min of resting-state fMRI data from each subject. We applied an individual-based approach to characterize the functional brain organization of 10 patients with CJD, 8 matched patients with AD, and 8 normal controls. We measured cortical atrophy as well as disruption in resting-state functional connectivity (rsFC) and then investigated longitudinal brain changes in a subset of CJD patients.

RESULTS

CJD was associated with widespread cortical thinning and weakened rsFC. Compared with AD, CJD showed distinct atrophy patterns and greater disruptions in rsFC. Moreover, the longitudinal data demonstrated that the progressive cortical thinning and disruption in rsFC mainly affected the association rather than the primary cortex in CJD.

CONCLUSIONS

CJD shows unique anatomical and functional disruptions in the cerebral cortex, distinct from AD. Rapid progression of CJD affects both the cortical thickness and rsFC in the association cortex.

Prediction of protein aggregation propensity employing SqFt-based logistic regression model

Here we present a novel machine-learning approach to predict protein aggregation propensity (PAP) which is a key factor in the formation of amyloid fibrils based on logistic regression (LR). Amyloid fibrils are associated with various neurodegenerative diseases (ND) such as Alzheimer's disease (AD) and Parkinson's disease (PD), which are caused by oxidative stress and impaired protein homeostasis. Accordingly, the paper uses a dataset of hexapeptides with known aggregation tendencies and eight physiochemical features to train and test the LR model. Also, it evaluates the performance of the LR model using F-measure and Matthews correlation coefficient (MCC) as metrics and compares it with other existing methods. Moreover, it investigates the effect of combining sequence and feature information in the prediction. In conclusion, the LR model with sequence and feature information achieves high F-measure (0.841) and MCC (0.6692), outperforming other methods and demonstrating its efficiency and reliability for PAP prediction. In addition, the overall performance of the concluded method was higher than the other known servers, for instance, Aggrescan, Metamyl, Foldamyloid, and PASTA 2.0. The LR model can be accessed at: https://github.com/KatherineEshari/Protein-aggregation-prediction.

Human prion disease: molecular pathogenesis, and possible therapeutic targets and strategies

INTRODUCTION

Human prion diseases are heterogeneous, and often rapidly progressive, transmissible neurodegenerative disorders associated with misfolded prion protein (PrP) aggregation and self-propagation. Despite their rarity, prion diseases comprise a broad spectrum of phenotypic variants determined at the molecular level by different conformers of misfolded PrP and host genotype variability. Moreover, they uniquely occur in idiopathic, genetically determined, and acquired forms with distinct etiologies.

AREA COVERED

This review provides an up-to-date overview of potential therapeutic targets in prion diseases and the main results obtained in cell and animal models and human trials. The open issues and challenges associated with developing effective therapies and informative clinical trials are also discussed.

EXPERT OPINION

Currently tested therapeutic strategies target the cellular PrP to prevent the formation of misfolded PrP or to favor its elimination. Among them, passive immunization and gene therapy with antisense oligonucleotides against prion protein mRNA are the most promising. However, the disease's rarity, heterogeneity, and rapid progression profoundly frustrate the successful undertaking of well-powered therapeutic trials and patient identification in the asymptomatic or early stage before the development of significant brain damage. Thus, the most promising therapeutic goal to date is preventing or delaying phenoconversion in carriers of pathogenic mutations by lowering prion protein expression.

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.

Recognition Mechanisms between a Nanobody and Disordered Epitopes of the Human Prion Protein: An Integrative Molecular Dynamics Study

Immunotherapy using antibodies to target the aggregation of flexible proteins holds promise for therapeutic interventions in neurodegenerative diseases caused by protein misfolding. Prions or PrP^Sc, the causal agents of transmissible spongiform encephalopathies (TSE), represent a model target for immunotherapies as TSE are prototypical protein misfolding diseases. The X-ray crystal structure of the wild-type (WT) human prion protein (HuPrP) bound to a camelid antibody fragment, denoted as Nanobody 484 (Nb484), has been previously solved. Nb484 was found to inhibit prion aggregation in vitro through a unique mechanism of structural stabilization of two disordered epitopes, that is, the palindromic motif (residues 113-120) and the β2-α2 loop region (residues 164-185). The study of the structural basis for antibody recognition of flexible proteins requires appropriate sampling techniques for the identification of conformational states occurring in disordered epitopes. To elucidate the Nb484-HuPrP recognition mechanisms, here we applied molecular dynamics (MD) simulations complemented with available NMR and X-ray crystallography data collected on the WT HuPrP to describe the conformational spaces occurring on HuPrP prior to Nb484 binding. We observe the experimentally determined binding competent conformations within the ensembles of pre-existing conformational states in solution before binding. We also described the Nb484 recognition mechanisms in two HuPrP carrying a polymorphism (E219K) and a TSE-causing mutation (V210I). Our hybrid approaches allow the identification of dynamic conformational landscapes existing on HuPrP and highly characterized by molecular disorder to identify physiologically relevant and druggable transitions.

Prion therapeutics: Lessons from the past

Prion diseases are a group of incurable zoonotic neurodegenerative diseases (NDDs) in humans and other animals caused by the prion proteins. The abnormal folding and aggregation of the soluble cellular prion proteins (PrPC) into scrapie isoform (PrPSc) in the Central nervous system (CNS) resulted in brain damage and other neurological symptoms. Different therapeutic approaches, including stalling PrPC to PrPSc conversion, increasing PrPSc removal, and PrPC stabilization, for which a spectrum of compounds, ranging from organic compounds to antibodies, have been explored. Additionally, a non-PrP targeted drug strategy using serpin inhibitors has been discussed. Despite numerous scaffolds being screened for anti-prion activity in vitro, only a few were effective in vivo and unfortunately, almost none of them proved effective in the clinical studies, most likely due to toxicity and lack of permeability. Recently, encouraging results from a prion-protein monoclonal antibody, PRN100, were presented in the first human trial on CJD patients, which gives a hope for better future for the discovery of other new molecules to treat prion diseases. In this comprehensive review, we have re-visited the history and discussed various classes of anti-prion agents, their structure, mode of action, and toxicity. Understanding pathogenesis would be vital for developing future treatments for prion diseases. Based on the outcomes of existing therapies, new anti-prion agents could be identified/synthesized/designed with reduced toxicity and increased bioavailability, which could probably be effective in treating prion diseases.

Cryo-EM structure of ex vivo fibrils associated with extreme AA amyloidosis prevalence in a cat shelter

AA amyloidosis is a systemic disease characterized by deposition of misfolded serum amyloid A protein (SAA) into cross-β amyloid in multiple organs in humans and animals. AA amyloidosis occurs at high SAA serum levels during chronic inflammation. Prion-like transmission was reported as possible cause of extreme AA amyloidosis prevalence in captive animals, e.g. 70% in cheetah and 57-73% in domestic short hair (DSH) cats kept in zoos and shelters, respectively. Herein, we present the 3.3 Å cryo-EM structure of AA amyloid extracted post-mortem from the kidney of a DSH cat with renal failure, deceased in a shelter with extreme disease prevalence. The structure reveals a cross-β architecture assembled from two 76-residue long proto-filaments. Despite >70% sequence homology to mouse and human SAA, the cat SAA variant adopts a distinct amyloid fold. Inclusion of an eight-residue insert unique to feline SAA contributes to increased amyloid stability. The presented feline AA amyloid structure is fully compatible with the 99% identical amino acid sequence of amyloid fragments of captive cheetah.

Aminoquinolones and Their Benzoquinone Dimer Hybrids as Modulators of Prion Protein Conversion

Prion Diseases or Transmissible Spongiform Encephalopathies are neurodegenerative conditions associated with a long incubation period and progressive clinical evolution, leading to death. Their pathogenesis is characterized by conformational changes of the cellular prion protein-PrP^C-in its infectious isoform-PrP^Sc-which can form polymeric aggregates that precipitate in brain tissues. Currently, there are no effective treatments for these diseases. The 2,5-diamino-1,4-benzoquinone structure is associated with an anti-prion profile and, considering the biodynamic properties associated with 4-quinolones, in this work, 6-amino-4-quinolones derivatives and their respective benzoquinone dimeric hybrids were synthesized and had their bioactive profile evaluated through their ability to prevent prion conversion. Two hybrids, namely, 2,5-dichloro-3,6-bis((3-carboxy-1-pentyl-4-quinolone-6-yl)amino)-1,4-benzoquinone (8e) and 2,5-dichloro-3,6-bis((1-benzyl-3-carboxy-4-quinolone-6-yl)amino)-1,4-benzoquinone (8f), stood out for their prion conversion inhibition ability, affecting the fibrillation process in both the kinetics-with a shortening of the lag phase-and thermodynamics and their ability to inhibit the formation of protein aggregates without significant cytotoxicity at ten micromolar.

Antibody binding increases the flexibility of the prion protein

Prion diseases are associated with the conversion of the cellular prion protein (PrP) into a pathogenic conformer (PrP^Sc). A proposed therapeutic approach to avoid the pathogenic transformation is to develop antibodies that bind to PrP and stabilize its structure. POM1 and POM6 are two monoclonal antibodies that bind the globular domain of PrP and have different biological responses, i.e., trigger neurotoxicity mimicking prion infections (POM1) or prevent neurotoxicity (POM6). The crystal structures of PrP in complex with the two antibodies show similar epitopes which seems inconsistent with the opposite phenotypes. Here, we investigate the influence of the POM1 and POM6 antibodies on the flexibility of the mouse PrP by molecular dynamics simulations. The simulations reveal that the POM6/PrP interface is less stable than the POM1/PrP interface, ascribable to localized polar mismatches at the interface, despite the former complex having a larger epitope than the latter. In the presence of any of the two antibodies, the flexibility of the globular domain increases everywhere except for the β₁-α₁ loop in the POM1/PrP complex which suggests the involvement of this loop in the pathological conversion. The secondary structure of PrP is preserved whereas the polar interactions involving residues Glu146, Arg156 and Arg208 are modified upon antibody binding.

Immunization with Genetically Modified Trypanosomes Provides Protection against Transmissible Spongiform Encephalopathies

Transmissible spongiform encephalopathies are incurable neurodegenerative diseases, associated with the conversion of the physiological prion protein to its disease-associated counterpart. Even though immunization against transmissible spongiform encephalopathies has shown great potential, immune tolerance effects impede the use of active immunization protocols for successful prophylaxis. In this study, we evaluate the use of trypanosomes as biological platforms for the presentation of a prion antigenic peptide to the host immune system. Using the engineered trypanosomes in an immunization protocol without the use of adjuvants led to the development of a humoral immune response against the prion protein in wild type mice, without the appearance of adverse reactions. The immune reaction elicited with this protocol displayed in vitro therapeutic potential and was further evaluated in a bioassay where immunized mice were partially protected in a representative murine model of prion diseases. Further studies are underway to better characterize the immune reaction and optimize the immunization protocol.

Mechanisms of prion-induced toxicity

Prion diseases are devastating neurodegenerative diseases caused by the structural conversion of the normally benign prion protein (PrP^C) to an infectious, disease-associated, conformer, PrP^Sc. After decades of intense research, much is known about the self-templated prion conversion process, a phenomenon which is now understood to be operative in other more common neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. In this review, we provide the current state of knowledge concerning a relatively poorly understood aspect of prion diseases: mechanisms of neurotoxicity. We provide an overview of proposed functions of PrP^C and its interactions with other extracellular proteins in the central nervous system, in vivo and in vitro models used to delineate signaling events downstream of prion propagation, the application of omics technologies, and the emerging appreciation of the role played by non-neuronal cell types in pathogenesis.

Carnosic Acid and Carnosol Display Antioxidant and Anti-Prion Properties in In Vitro and Cell-Free Models of Prion Diseases

Prion diseases are transmissible encephalopathies associated with the conversion of the physiological form of the prion protein (PrP^C) to the disease-associated (PrP^Sc). Despite intense research, no therapeutic or prophylactic agent is available. The catechol-type diterpene Carnosic acid (CA) and its metabolite Carnosol (CS) from Rosmarinus officinalis have well-documented anti-oxidative and neuroprotective effects. Since oxidative stress plays an important role in the pathogenesis of prion diseases, we investigated the potential beneficial role of CA and CS in a cellular model of prion diseases (N2a22L cells) and in a cell-free prion amplification assay (RT-QuIC). The antioxidant effects of the compounds were confirmed when N2a22L were incubated with CA or CS. Furthermore, CA and CS reduced the accumulation of the disease-associated form of PrP, detected by Western Blotting, in N2a22L cells. This effect was validated in RT-QuIC assays, indicating that it is not associated with the antioxidant effects of CA and CS. Importantly, cell-free assays revealed that these natural products not only prevent the formation of PrP aggregates but can also disrupt already formed aggregates. Our results indicate that CA and CS have pleiotropic effects against prion diseases and could evolve into useful prophylactic and/or therapeutic agents against prion and other neurodegenerative diseases.

Therapeutic strategies for identifying small molecules against prion diseases

Prion diseases are fatal neurodegenerative disorders, for which there are no effective therapeutic and diagnostic agents. The main pathological hallmark has been identified as conformational changes of the cellular isoform prion protein (PrP^C) to a misfolded isoform of the prion protein (PrP^Sc). Targeting PrP^C and its conversion to PrP^Sc is still the central dogma in prion drug discovery, particularly in in silico and in vitro screening endeavors, leading to the identification of many small molecules with therapeutic potential. Nonetheless, multiple pathological targets are critically involved in the intricate pathogenesis of prion diseases. In this context, multi-target-directed ligands (MTDLs) emerge as valuable therapeutic approach for their potential to effectively counteract the complex etiopathogenesis by simultaneously modulating multiple targets. In addition, diagnosis occurs late in the disease process, and consequently a successful therapeutic intervention cannot be provided. In this respect, small molecule theranostics, which combine imaging and therapeutic properties, showed tremendous potential to cure and diagnose in vivo prion diseases. Herein, we review the major advances in prion drug discovery, from anti-prion small molecules identified by means of in silico and in vitro screening approaches to two rational strategies, namely MTDLs and theranostics, that have led to the identification of novel compounds with an expanded anti-prion profile.

Zn(II) binding causes interdomain changes in the structure and flexibility of the human prion protein

The cellular prion protein (PrPC) is a mainly α-helical 208-residue protein located in the pre- and postsynaptic membranes. For unknown reasons, PrPC can undergo a structural transition into a toxic, β-sheet rich scrapie isoform (PrPSc) that is responsible for transmissible spongiform encephalopathies (TSEs). Metal ions seem to play an important role in the structural conversion. PrPC binds Zn(II) ions and may be involved in metal ion transport and zinc homeostasis. Here, we use multiple biophysical techniques including optical and NMR spectroscopy, molecular dynamics simulations, and small angle X-ray scattering to characterize interactions between human PrPC and Zn(II) ions. Binding of a single Zn(II) ion to the PrPC N-terminal domain via four His residues from the octarepeat region induces a structural transition in the C-terminal α-helices 2 and 3, promotes interaction between the N-terminal and C-terminal domains, reduces the folded protein size, and modifies the internal structural dynamics. As our results suggest that PrPC can bind Zn(II) under physiological conditions, these effects could be important for the physiological function of PrPC.

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.

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.

Therapeutic Assay with the Non-toxic C-Terminal Fragment of Tetanus Toxin (TTC) in Transgenic Murine Models of Prion Disease

The non-toxic C-terminal fragment of the tetanus toxin (TTC) has been described as a neuroprotective molecule since it binds to Trk receptors and activates Trk-dependent signaling, activating neuronal survival pathways and inhibiting apoptosis. Previous in vivo studies have demonstrated the ability of this molecule to increase mice survival, inhibit apoptosis and regulate autophagy in murine models of neurodegenerative diseases such as amyotrophic lateral sclerosis and spinal muscular atrophy. Prion diseases are fatal neurodegenerative disorders in which the main pathogenic event is the conversion of the cellular prion protein (PrP^C) into an abnormal and misfolded isoform known as PrP^Sc. These diseases share different pathological features with other neurodegenerative diseases, such as amyotrophic lateral sclerosis, Parkinson's disease or Alzheimer's disease. Hitherto, there are no effective therapies to treat prion diseases. Here, we present a pilot study to test the therapeutic potential of TTC to treat prion diseases. C57BL6 wild-type mice and the transgenic mice Tg338, which overexpress PrP^C, were intracerebrally inoculated with scrapie prions and then subjected to a treatment consisting of repeated intramuscular injections of TTC. Our results indicate that TTC displays neuroprotective effects in the murine models of prion disease reducing apoptosis, regulating autophagy and therefore increasing neuronal survival, although TTC did not increase survival time in these models.

Human cerebral organoids as a therapeutic drug screening model for Creutzfeldt-Jakob disease

Creutzfeldt-Jakob Disease (CJD) is a fatal, currently incurable, neurodegenerative disease. The search for candidate treatments would be greatly facilitated by the availability of human cell-based models of prion disease. Recently, an induced pluripotent stem cell derived human cerebral organoid model was shown to take up and propagate human CJD prions. This model offers new opportunities to screen drug candidates for the treatment of human prion diseases in an entirely human genetic background. Here we provide the first evidence that human cerebral organoids can be a viable model for CJD drug screening by using an established anti-prion compound, pentosan polysulfate (PPS). PPS delayed prion propagation in a prophylactic-like treatment paradigm and also alleviated propagation when applied following establishment of infection in a therapeutic-like treatment paradigm. This study demonstrates the utility of cerebral organoids as the first human 3D cell culture system for screening therapeutic drug candidates for human prion diseases.

Involvement of Cellular Prion Protein in Invasion and Metastasis of Lung Cancer by Inducing Treg Cell Development

The cellular prion protein (PrP^C) is a cell surface glycoprotein expressed in many cell types that plays an important role in normal cellular processes. However, an increase in PrP^C expression has been associated with a variety of human cancers, where it may be involved in resistance to the proliferation and metastasis of cancer cells. PrP-deficient (Prnp^0/0) and PrP-overexpressing (Tga20) mice were studied to evaluate the role of PrP^C in the invasion and metastasis of cancer. Tga20 mice, with increased PrP^C, died more quickly from lung cancer than did the Prnp^0/0 mice, and this effect was associated with increased transforming growth factor-beta (TGF-β) and programmed death ligand-1 (PD-L1), which are important for the development and function of regulatory T (Treg) cells. The number of FoxP3⁺CD25⁺ Treg cells was increased in Tga20 mice compared to Prnp^0/0 mice, but there was no significant difference in either natural killer or cytotoxic T cell numbers. In addition, mice infected with the ME7 scrapie strain had decreased numbers of Treg cells and decreased expression of TGF-β and PD-L1. These results suggest that PrP^C plays an important role in invasion and metastasis of cancer cells by inducing Treg cells through upregulation of TGF-β and PD-L1 expression.

A patient with sporadic Creutzfeldt-Jakob disease: challenges of rare diseases in the COVID-19 era

We describe a patient who presented to hospital during the coronavirus disease 2019 (COVID-19) pandemic with sporadic Creutzfeldt–Jakob disease (sCJD). The case demonstrates the typical clinical, radiological and laboratory features of this condition. It also highlights some of the challenges associated with diagnosis and care of patients with rare diseases such as sCJD, and how these have been intensified by COVID-19.

Recent developments in antibody therapeutics against prion disease

Preclinical evidence indicates that prion diseases can respond favorably to passive immunotherapy. However, certain antibodies to the cellular prion protein PrPC can be toxic. Comprehensive studies of structure-function relationships have revealed that the flexible amino-terminal tail of PrPC is instrumental for mediating prion toxicity. In a first-in-human study, an anti-prion antibody has been recently administered to patients diagnosed with sporadic Creutzfeldt-Jakob's disease, the most prevalent human prion disease. Moreover, large-scale serosurveys have mapped the prevalence of naturally occurring human anti-prion autoantibodies in health and disease. Here, we provide a perspective on the limitations and opportunities of therapeutic anti-prion antibodies.

Improved Anti-Prion Nucleic Acid Aptamers by Incorporation of Chemical Modifications

Nucleic acid aptamers are innovative and promising candidates to block the hallmark event in the prion diseases, that is the conversion of prion protein (PrP) into an abnormal form; however, they need chemical modifications for effective therapeutic activity. This communication reports on the development and biophysical characterization of a small library of chemically modified G-quadruplex-forming aptamers targeting the cellular PrP and the evaluation of their anti-prion activity. The results show the possibility of enhancing anti-prion aptamer properties through straightforward modifications.

Multimodal small-molecule screening for human prion protein binders

Prion disease is a rapidly progressive neurodegenerative disorder caused by misfolding and aggregation of the prion protein (PrP), and there are currently no therapeutic options. PrP ligands could theoretically antagonize prion formation by protecting the native protein from misfolding or by targeting it for degradation, but no validated small-molecule binders have been discovered to date. We deployed a variety of screening methods in an effort to discover binders of PrP, including 19F-observed and saturation transfer difference (STD) NMR spectroscopy, differential scanning fluorimetry (DSF), DNA-encoded library selection, and in silico screening. A single benzimidazole compound was confirmed in concentration-response, but affinity was very weak (Kd > 1 mm), and it could not be advanced further. The exceptionally low hit rate observed here suggests that PrP is a difficult target for small-molecule binders. Whereas orthogonal binder discovery methods could yield high-affinity compounds, non-small-molecule modalities may offer independent paths forward against prion disease.

Guijarro L, García-Honduvilla N, Álvarez-Mon M, Buján J, García-Gallego S. Dendrimers and Dendritic Materials: From Laboratory to Medical Practice in Infectious Diseases

Infectious diseases are one of the main global public health risks, predominantly caused by viruses, bacteria, fungi, and parasites. The control of infections is founded on three main pillars: prevention, treatment, and diagnosis. However, the appearance of microbial resistance has challenged traditional strategies and demands new approaches. Dendrimers are a type of polymeric nanoparticles whose nanometric size, multivalency, biocompatibility, and structural perfection offer boundless possibilities in multiple biomedical applications. This review provides the reader a general overview about the uses of dendrimers and dendritic materials in the treatment, prevention, and diagnosis of highly prevalent infectious diseases, and their advantages compared to traditional approaches. Examples of dendrimers as antimicrobial agents per se, as nanocarriers of antimicrobial drugs, as well as their uses in gene transfection, in vaccines or as contrast agents in imaging assays are presented. Despite the need to address some challenges in order to be used in the clinic, dendritic materials appear as an innovative tool with a brilliant future ahead in the clinical management of infectious diseases and many other health issues.

Cofactor molecules: Essential partners for infectious prions

The protein-only hypothesis predicts that infectious mammalian prions are composed solely of PrP^Sc, a misfolded conformer of the normal prion protein, PrP^C. However, to date, all wild type protein-only PrP^Sc preparations lack significant levels of prion infectivity. Using a systemic biochemical approach, our laboratory isolated and identified two different endogenous cofactor molecules, RNA (Deleault et al., 2003 [50]; Deleault et al., 2007 [59]) and phosphatidylethanolamine (Deleault et al., 2012 [61]; Deleault et al., 2012 [18]), which facilitate the formation of prions with high levels of specific infectivity, leading us to propose to the alternative hypothesis that cofactor molecules are required to form wild type infectious prions (Deleault et al., 2007 [59]; Deleault et al., 2012 [18]; Geoghegan et al., 2007 [57]). In addition, we found that purified cofactor molecules restrict the strain properties of chemically defined infectious prions (Deleault et al., 2012 [18]), suggesting a "cofactor selection" model in which natural variation in the distribution of strain-specific cofactor molecules in different parts of the brain may be responsible for strain-dependent patterns of neurotropism (Deleault et al., 2012 [18]; Geoghegan et al., 2007 [57]).

Prion infection, transmission, and cytopathology modeled in a low-biohazard human cell line

Expanding the toolbox of prion research to a low-biohazard, scalable human cell model.

Transmission of prion infectivity to susceptible murine cell lines has simplified prion titration assays and has greatly reduced the need for animal experimentation. However, murine cell models suffer from technical and biological constraints. Human cell lines might be more useful, but they are much more biohazardous and are often poorly infectible. Here, we describe the human clonal cell line hovS, which lacks the human PRNP gene and expresses instead the ovine PRNP VRQ allele. HovS cells were highly susceptible to the PG127 strain of sheep-derived murine prions, reaching up to 90% infected cells in any given culture and were maintained in a continuous infected state for at least 14 passages. Infected hovS cells produced proteinase K–resistant prion protein (PrP^Sc), pelletable PrP aggregates, and bona fide infectious prions capable of infecting further generations of naïve hovS cells and mice expressing the VRQ allelic variant of ovine PrP^C. Infection in hovS led to prominent cytopathic vacuolation akin to the spongiform changes observed in individuals suffering from prion diseases. In addition to expanding the toolbox for prion research to human experimental genetics, the hovS cell line provides a human-derived system that does not require human prions. Hence, the manipulation of scrapie-infected hovS cells may present fewer biosafety hazards than that of genuine human prions.

Genome-wide transcriptomics identifies an early preclinical signature of prion infection

The clinical course of prion diseases is accurately predictable despite long latency periods, suggesting that prion pathogenesis is driven by precisely timed molecular events. We constructed a searchable genome-wide atlas of mRNA abundance and splicing alterations during the course of disease in prion-inoculated mice. Prion infection induced PrP-dependent transient changes in mRNA abundance and processing already at eight weeks post inoculation, well ahead of any neuropathological and clinical signs. In contrast, microglia-enriched genes displayed an increase simultaneous with the appearance of clinical signs, whereas neuronal-enriched transcripts remained unchanged until the very terminal stage of disease. This suggests that glial pathophysiology, rather than neuronal demise, could be the final driver of disease. The administration of young plasma attenuated the occurrence of early mRNA abundance alterations and delayed signs in the terminal phase of the disease. The early onset of prion-induced molecular changes might thus point to novel biomarkers and potential interventional targets.

Combining autophagy stimulators and cellulose ethers for therapy against prion disease

Prion diseases are fatal transmissible neurodegenerative disorders that affect animals and humans. Prions are proteinaceous infectious particles consisting of a misfolded isoform of the cellular prion protein PrP^C, termed PrP^Sc. PrP^Sc accumulates in infected neurons due to partial resistance to proteolytic digestion. Using compounds that interfere with the production of PrP^Sc or enhance its degradation cure prion infection in vitro, but most drugs failed when used to treat prion-infected rodents. In order to synergize the effect of anti-prion drugs, we combined drugs interfering with the generation of PrP^Sc with compounds inducing PrP^Sc degradation. Here, we tested autophagy stimulators (rapamycin or AR12) and cellulose ether compounds (TC-5RW or 60SH-50) either as single or combination treatment of mice infected with RML prions. Single drug treatments significantly extended the survival compared to the untreated group. As anticipated, also all the combination therapy groups showed extended survival compared to the untreated group, but no combination treatment showed superior effects to 60SH-50 or TC-5RW treatment alone. Unexpectedly, we later found that combining autophagy stimulator and cellulose ether treatment in cultured neuronal cells mitigated the pro-autophagic activity of AR12 and rapamycin, which can in part explain the in vivo results. Overall, we show that it is critical to exclude antagonizing drug effects when attempting combination therapy. In addition, we identified AR-12 as a pro-autophagic drug that significantly extends survival of prion-infected mice, has no adverse side effects on the animals used in this study, and can be useful in future studies.

Immunotherapy against Prion Disease

The term "prion disease" encompasses a group of neurodegenerative diseases affecting both humans and animals. Currently, there is no effective therapy and all forms of prion disease are invariably fatal. Because of (a) the outbreak of bovine spongiform encephalopathy in cattle and variant Creutzfeldt-Jakob disease in humans; (b) the heated debate about the prion hypothesis; and (c) the availability of a natural prion disease in rodents, the understanding of the pathogenic process in prion disease is much more advanced compared to that of other neurodegenerative disorders, which inspired many attempts to develop therapeutic strategies against these fatal diseases. In this review, we focus on immunotherapy against prion disease. We explain our rationale for immunotherapy as a plausible therapeutic choice, review previous trials using either active or passive immunization, and discuss potential strategies for overcoming the hurdles in developing a successful immunotherapy. We propose that immunotherapy is a plausible and practical therapeutic strategy and advocate more studies in this area to develop effective measures to control and treat these devastating disorders.

Neuroinflammation: An overview of neurodegenerative and metabolic diseases and of biotechnological studies

Neuroinflammation is an important factor contributing to cognitive impairment and neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic lateral sclerosis (ALS), ischemic injury, and multiple sclerosis (MS). These diseases are characterized by inexorable progressive injury of neuron cells, and loss of motor or cognitive functions. Microglia, which are the resident macrophages in the brain, play an important role in both physiological and pathological conditions. In this review, we provide an updated discussion on the role of ROS and metabolic disease in the pathological mechanisms of activation of the microglial cells and release of cytotoxins, leading to the neurodegenerative process. In addition, we also discuss in vivo models, such as zebrafish and Caenorhabditis elegans, and provide new insights into therapeutics bioinspired by neuropeptides from venomous animals, supporting high throughput drug screening in the near future, searching for a complementary approach to elucidating crucial mechanisms associated with neurodegenerative disorders.

Autoantibodies against the prion protein in individuals with PRNP mutations

OBJECTIVE

To determine whether naturally occurring autoantibodies against the prion protein are present in individuals with genetic prion disease mutations and controls, and if so, whether they are protective against prion disease.

METHODS

In this case-control study, we collected 124 blood samples from individuals with a variety of pathogenic PRNP mutations and 78 control individuals with a positive family history of genetic prion disease but lacking disease-associated PRNP mutations. Antibody reactivity was measured using an indirect ELISA for the detection of human immunoglobulin G_1-4 antibodies against wild-type human prion protein. Multivariate linear regression models were constructed to analyze differences in autoantibody reactivity between (1) PRNP mutation carriers vs controls and (2) asymptomatic vs symptomatic PRNP mutation carriers. Robustness of results was examined in matched cohorts.

RESULTS

We found that antibody reactivity was present in a subset of both PRNP mutation carriers and controls. Autoantibody levels were not influenced by PRNP mutation status or clinical manifestation of prion disease. Post hoc analyses showed anti-PrP^C autoantibody titers to be independent of personal history of autoimmune disease and other immunologic disorders, as well as PRNP codon 129 polymorphism.

CONCLUSIONS

Pathogenic PRNP variants do not notably stimulate antibody-mediated anti-PrP^C immunity. Anti-PrP^C immunoglobulin G autoantibodies are not associated with the onset of prion disease. The presence of anti-PrP^C autoantibodies in the general population without any disease-specific association suggests that relatively high titers of naturally occurring antibodies are well-tolerated.

CLINICALTRIALSGOV IDENTIFIER

NCT02837705.

Protein aggregation in bacteria

Protein aggregation occurs as a consequence of perturbations in protein homeostasis that can be triggered by environmental and cellular stresses. The accumulation of protein aggregates has been associated with aging and other pathologies in eukaryotes, and in bacteria with changes in growth rate, stress resistance and virulence. Numerous past studies, mostly performed in Escherichia coli, have led to a detailed understanding of the functions of the bacterial protein quality control machinery in preventing and reversing protein aggregation. However, more recent research points toward unexpected diversity in how phylogenetically different bacteria utilize components of this machinery to cope with protein aggregation. Furthermore, how persistent protein aggregates localize and are passed on to progeny during cell division and how their presence impacts reproduction and the fitness of bacterial populations remains a controversial field of research. Finally, although protein aggregation is generally seen as a symptom of stress, recent work suggests that aggregation of specific proteins under certain conditions can regulate gene expression and cellular resource allocation. This review discusses recent advances in understanding the consequences of protein aggregation and how this process is dealt with in bacteria, with focus on highlighting the differences and similarities observed between phylogenetically different groups of bacteria.

Metformin reduces prion infection in neuronal cells by enhancing autophagy

Prion diseases are fatal infectious neurodegenerative disorders in human and animals that are caused by misfolding of the cellular prion protein (PrP^C) into the infectious isoform PrP^Sc. No effective treatment is available for prion diseases. Metformin is a first-line medication for treatment of type 2 diabetes which is known to activate AMPK and induce autophagy through the inhibition of mammalian target of rapamycin (mTOR1) signaling. Metformin was reported to be beneficial in various protein misfolding and neurodegenerative diseases like Alzheimer's and Huntington's diseases. In this study we investigated the anti-prion effect of metformin in persistently prion-infected neuronal cells. Our data showed that metformin significantly decreased the PrP^Sc load in the treated cells, as shown by less PK resistant PrP in Western blots and reduced prion conversion activity in Real-Time Quaking-Induced Conversion (RT-QuIC) assay in both 22L-ScN2a and RML-ScCAD5 cells. Additionally, metformin induced autophagy as shown by higher levels of LC3-II in treated cells compared with control cells. On the other hand, our mouse bioassay showed that oral metformin at a dose of 2 mg/ml in drinking water had no effect on the survival of prion-infected mice. In conclusion, our findings describe the anti-prion effect of metformin in two persistently prion-infected neuronal cell lines. This effect can be explained at least partially by the autophagy inducing activity of metformin. This study sheds light on metformin as an anti-prion candidate for the combination therapy of prion diseases.

Structural evidence for the critical role of the prion protein hydrophobic region in forming an infectious prion

Prion or PrPSc is the proteinaceous infectious agent causing prion diseases in various mammalian species. Despite decades of research, the structural basis for PrPSc formation and prion infectivity remains elusive. To understand the role of the hydrophobic region in forming infectious prion at the molecular level, we report X-ray crystal structures of mouse (Mo) prion protein (PrP) (residues 89-230) in complex with a nanobody (Nb484). Using the recombinant prion propagation system, we show that the binding of Nb484 to the hydrophobic region of MoPrP efficiently inhibits the propagation of proteinase K resistant PrPSc and prion infectivity. In addition, when added to cultured mouse brain slices in high concentrations, Nb484 exhibits no neurotoxicity, which is drastically different from other neurotoxic anti-PrP antibodies, suggesting that the Nb484 can be a potential therapeutic agent against prion disease. In summary, our data provides the first structure-function evidence supporting a crucial role of the hydrophobic region of PrP in forming an infectious prion.

Role of prion protein glycosylation in replication of human prions by protein misfolding cyclic amplification

Prion diseases are transmissible neurological disorders associated with the presence of abnormal, disease-related prion protein (PrP^D). The detection of PrP^D in the brain is the only definitive diagnostic evidence of prion disease and its identification in body fluids and peripheral tissues are valuable for pre-mortem diagnosis. Protein misfolding cyclic amplification (PMCA) is a technique able to detect minute amount of PrP^D and is based on the conversion of normal or cellular PrP (PrP^C) to newly formed PrP^D, sustained by a self-templating mechanism. Several animal prions have been efficiently amplified by PMCA, but limited results have been obtained with human prions with the exception of variant-Creutzfeldt-Jakob-disease (vCJD). Since the total or partial absence of glycans on PrP^C has been shown to affect PMCA efficiency in animal prion studies, we attempted to enhance the amplification of four major sporadic-CJD (sCJD) subtypes (MM1, MM2, VV1, and VV2) and vCJD by single round PMCA using partially or totally unglycosylated PrP^C as substrates. The amplification efficiency of all tested sCJD subtypes underwent a strong increase, inversely correlated to the degree of PrP^C glycosylation and directly related to the matching of the PrP polymorphic 129 M/V genotype between seed and substrate. This effect was particularly significant in sCJDMM2 and sCJDVV2 allowing the detection of PK-resistant PrP^D (resPrP^D) in sCJDMM2 and sCJDVV2 brains at dilutions of 6 × 10⁷ and 3 × 10⁶. vCJD, at variance with the tested sCJD subtypes, showed the best amplification with partially deglycosylated PrP^C substrate reaching a resPrP^D detectability at up to 3 × 10¹⁶ brain dilution. The differential effect of substrate PrP^C glycosylations suggests subtype-dependent PrP^C-PrP^D interactions, strongly affected by the PrP^C glycans. The enhanced PMCA prion amplification efficiency achieved with unglycosylated PrP^C substrates may allow for the developing of a sensitive, non-invasive, diagnostic test for the different CJD subtypes based on body fluids or easily-accessible-peripheral tissues.

Prions, prionoids and protein misfolding disorders

Prion diseases are progressive, incurable and fatal neurodegenerative conditions. The term 'prion' was first nominated to express the revolutionary concept that a protein could be infectious. We now know that prions consist of PrP^Sc, the pathological aggregated form of the cellular prion protein PrP^C. Over the years, the term has been semantically broadened to describe aggregates irrespective of their infectivity, and the prion concept is now being applied, perhaps overenthusiastically, to all neurodegenerative diseases that involve protein aggregation. Indeed, recent studies suggest that prion diseases (PrDs) and protein misfolding disorders (PMDs) share some common disease mechanisms, which could have implications for potential treatments. Nevertheless, the transmissibility of bona fide prions is unique, and PrDs should be considered as distinct from other PMDs.

Identification of anti-prion drugs and targets using toxicity-based assays

Prion diseases are untreatable and invariably fatal, making the discovery of effective therapeutic interventions a priority. Most candidate molecules have been discovered based on their ability to reduce the levels of PrP^Sc, the infectious form of the prion protein, in cultured neuroblastoma cells. We have employed an alternative assay, based on an abnormal cellular phenotype associated with a mutant prion protein, to discover a novel class of anti-prion compounds, the phenethyl piperidines. Using an assay that monitors the acute toxic effects of PrP^Sc on the synapses of cultured hippocampal neurons, we have identified p38 MAPK as a druggable pharmacological target that is already being pursued for the treatment of other human diseases. Organotypic brain slices, which can propagate prions and mimic several neuropathological features of the disease, have also been used to test inhibitory compounds. An effective anti-prion regimen will involve synergistic combination of drugs acting at multiple steps of the pathogenic process, resulting not only in reduction in prion levels but also suppression of neurotoxic signaling.

Prionoid Proteins in the Pathogenesis of Neurodegenerative Diseases

There is a growing body of evidence that prionoid protein behaviors are a core element of neurodegenerative diseases (NDs) that afflict humans. Common elements in pathogenesis, pathological effects and protein-level behaviors exist between Alzheimer's Disease (AD), Parkinson's Disease (PD), Huntington's Disease (HD) and Amyotrophic Lateral Sclerosis (ALS). These extend beyond the affected neurons to glial cells and processes. This results in a complicated system of disease progression, which often takes advantage of protective processes to promote the propagation of pathological protein aggregates. This review article provides a current snapshot of knowledge on these proteins and their intrinsic role in the pathogenesis and disease progression seen across NDs.

The Potential Use of Plant Natural Products and Plant Extracts with Antioxidant Properties for the Prevention/Treatment of Neurodegenerative Diseases: In Vitro, In Vivo and Clinical Trials

Neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease and Huntington's disease, present a major health issue and financial burden for health care systems around the world. The impact of these diseases will further increase over the next decades due to increasing life expectancies. No cure is currently available for the treatment of these conditions; only drugs, which merely alleviate the symptoms. Oxidative stress has long been associated with neurodegeneration, whether as a cause or as part of the downstream results caused by other factors. Thus, the use of antioxidants to counter cellular oxidative stress within the nervous system has been suggested as a potential treatment option for neurological disorders. Over the last decade, significant research has focused on the potential use of natural antioxidants to target oxidative stress. However, clinical trial results have lacked success for the treatment of patients with neurological disorders. The knowledge that natural extracts show other positive molecular activities in addition to antioxidant activity, however, has led to further research of natural extracts for their potential use as prevention or treatment/management of neurodegenerative diseases. This review will cover several in vitro and in vivo research studies, as well as clinical trials, and highlight the potential of natural antioxidants.

Lymphocyte activation gene 3 (Lag3) expression is increased in prion infections but does not modify disease progression

Prion diseases, Alzheimer’s disease and Parkinson’s disease (PD) are fatal degenerative disorders that share common neuropathological and biochemical features, including the aggregation of pathological protein conformers. Lymphocyte activation gene 3 (Lag3, also known as CD223) is a member of the immunoglobulin superfamily of receptors expressed on peripheral immune cells, microglia and neurons, which serves as a receptor for α-synuclein aggregates in PD. Here we examined the possible role of Lag3 in the pathogenesis of prion diseases. Through quantitative real-time PCR and RNA-sequencing, we found that the expression levels of Lag3 were relatively low in the adult mouse brains, yet its expression was increased after prion infection. However, we failed finding significant differences regarding the incubation time, PrP^Sc load, neurodegeneration, astrocyte and microglia reactions and inflammatory gene expression between the Lag3 knockout mice and wild-type littermate controls after prion infection. We conclude that loss of Lag3 has no significant influence on prion disease pathogenesis. Considering that Lag3 is an immune checkpoint receptor, our results suggest that immune checkpoint inhibition (an increasingly prevalent therapeutic modality against many types of cancer) might not exert positive or negative effects on the progression of prion diseases.

Application of the fragment molecular orbital method to discover novel natural products for prion disease

Conformational conversion of the normal cellular isoform of the prion protein PrPC into an infectious isoform PrPSc causes pathogenesis in prion diseases. To date, numerous antiprion compounds have been developed to block this conversion and to detect the molecular mechanisms of prion inhibition using several computational studies. Thus far, no suitable drug has been identified for clinical use. For these reasons, more accurate and predictive approaches to identify novel compounds with antiprion effects are required. Here, we have applied an in silico approach that integrates our previously described pharmacophore model and fragment molecular orbital (FMO) calculations, enabling the ab initio calculation of protein-ligand complexes. The FMO-based virtual screening suggested that two natural products with antiprion activity exhibited good binding interactions, with hotspot residues within the PrPC binding site, and effectively reduced PrPSc levels in a standard scrapie cell assay. Overall, the outcome of this study will be used as a promising strategy to discover antiprion compounds. Furthermore, the SAR-by-FMO approach can provide extremely powerful tools in quickly establishing virtual SAR to prioritise compounds for synthesis in further studies.

Prion infectivity is encoded exclusively within the structure of proteinase K-resistant fragments of synthetically generated recombinant PrPSc

Transmissible spongiform encephalopathies, also known as prion diseases, are a group of fatal neurodegenerative disorders affecting both humans and animals. The central pathogenic event in prion disease is the misfolding of normal prion protein (PrP^C) into the pathogenic conformer, PrP^Sc, which self-replicates by converting PrP^C to more of itself. The biochemical hallmark of PrP^Sc is its C-terminal resistance to proteinase K (PK) digestion, which has been historically used to define PrP^Sc and is still the most widely used characteristic for prion detection. We used PK-resistance as a biochemical measure for the generation of recombinant prion from bacterially expressed recombinant PrP. However, the existence of both PK- resistant and -sensitive PrP^Sc forms in animal and human prion disease led to the question of whether the in vitro-generated recombinant prion infectivity is due to the PK-resistant or -sensitive recombinant PrP forms. In this study, we compared undigested and PK-digested recombinant prions for their infectivity using both the classical rodent bioassay and the cell-based prion infectivity assay. Similar levels of infectivity were detected in PK-digested and -undigested samples by both assays. A time course study of recombinant prion propagation showed that the increased capability to seed the conversion of endogenous PrP in cultured cells coincided with an increase of the PK-resistant form of recombinant PrP. Moreover, prion infectivity diminished when recombinant prion was subjected to an extremely harsh PK digestion. These results demonstrated that the infectivity of recombinant prion is encoded within the structure of the PK-resistant PrP fragments. This characteristic of recombinant prion, that a simple PK digestion is able to eliminate all PK-sensitive (non-infectious) PrP species, makes possible a more homogenous material that will be ideal for dissecting the molecular basis of prion infectivity.

Prolonged follicular helper T cell responses in ME7 scrapie-infected mice

We previously reported that mice intracerebrally inoculated with the mouse-adapted scrapie strain ME7 have markedly diminished T zones in the spleen due to the decreased expression of CCL19 and CCL21. In addition, follicular dendritic cell networks in germinal centers were larger in ME7-infected spleens compared to uninfected spleens. As an extension of that study, we set out to determine how ME7 infection affects spleen structure and follicular helper T (Tfh) cell responses in mice. For this study, mice were intraperitoneally inoculated with brain homogenate of the ME7 inoculum and spleens were analyzed 50, 130, and 200 days after inoculation and compared with those from uninfected mice. The result showed that ME7- infected mice had increased Tfh cell responses which were maintained until end-stage prion disease. Although CD4 T cells decreased in white pulps, they increased in germinal centers, and expressed higher levels of the Tfh-related genes, such as Bcl6, Il21, Cxcr5, Icos, and Pdcd1. In addition, ME7-infected spleens had increased numbers of CD4 memory T cells. These data indicate that although ME7 infection led to impaired splenic white pulp structure, CD4 memory T cells were increased and Tfh cell responses were required and prolonged to provide help for the replication and accumulation of pathogenic prion protein in germinal centers.

A role for astroglia in prion diseases

In this issue of JEM, Krejciova et al. (https://doi.org/10.1084/jem.20161547) report that astrocytes derived from human iPSCs can replicate human CJD prions. These observations provide a new, potentially very valuable model for studying human prions in cellula and for identifying antiprion compounds that might serve as clinical candidates. Furthermore, they add to the evidence that astrocytes may not be just innocent bystanders in prion diseases.