Prion diseases: immunotargets and therapy

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.

A case of fatal familial insomnia: diagnostic and therapeutic approaches

Fatal Familial Insomnia (FFI) is an uncommon but fatal genetic condition that is characterized by severe progressive insomnia, dysautonomia, neuropsychiatric changes, and gait instability. Diagnostic workup includes genetic testing, EEG, MRI imaging of the brain, polysomnography, and CSF analysis. MRI brain imaging may be notable for areas of restricted diffusion in the thalamus. Therapeutic approaches are centered on symptom management, predominantly for insomnia. It is important for clinicians to consider FFI in patients presenting with progressive insomnia, cognitive deficits, and gait instability, and to direct patients and families toward genetic counseling and palliative care services.

Identification of molecular signatures associated with sleep disorder and Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) and sleep disorders are both neurodegenerative conditions characterized by impaired or absent sleep. However, potential common pathogenetic mechanisms of these diseases are not well characterized.

METHODS

Differentially expressed genes (DEGs) were identified using publicly available human gene expression profiles GSE5281 for AD and GSE40562 for sleep disorder. DEGs common to the two datasets were used for enrichment analysis, and we performed multi-scale embedded gene co-expression network analysis (MEGENA) for common DEGs. Fast gene set enrichment analysis (fGSEA) was used to obtain common pathways, while gene set variation analysis (GSVA) was applied to quantify those pathways. Subsequently, we extracted the common genes between module genes identified by MEGENA and genes of the common pathways, and we constructed protein-protein interaction (PPI) networks. The top 10 genes with the highest degree of connectivity were classified as hub genes. Common genes were used to perform Metascape enrichment analysis for functional enrichment. Furthermore, we quantified infiltrating immune cells in patients with AD or sleep disorder and in controls.

RESULTS

DEGs common to the two disorders were involved in the citrate cycle and the HIF-1 signaling pathway, and several common DEGs were related to signaling pathways regulating the pluripotency of stem cells, as well as 10 other pathways. Using MEGENA, we identified 29 modules and 1,498 module genes in GSE5281, and 55 modules and 1,791 module genes in GSE40562. Hub genes involved in AD and sleep disorder were ATP5A1, ATP5B, COX5A, GAPDH, NDUFA9, NDUFS3, NDUFV2, SOD1, UQCRC1, and UQCRC2. Plasmacytoid dendritic cells and T helper 17 cells had the most extensive infiltration in both AD and sleep disorder.

CONCLUSION

AD pathology and pathways of neurodegeneration participate in processes contributing in AD and sleep disorder. Hub genes may be worth exploring as potential candidates for targeted therapy of AD and sleep disorder.

A novel β-hairpin peptide derived from the ARC repressor selectively interacts with the major groove of B-DNA

Transcription factors (TFs) have a remarkable role in the homeostasis of the organisms and there is a growing interest in how they recognize and interact with specific DNA sequences. TFs recognize DNA using a variety of structural motifs. Among those, the ribbon-helix-helix (RHH) proteins, exemplified by the MetJ and ARC repressors, form dimers that insert antiparallel β-sheets into the major groove of DNA. A great chemical challenge consists of using the principles of DNA recognition by TFs to design minimized peptides that maintain the DNA affinity and specificity characteristics of the natural counterparts. In this context, a peptide mimic of an antiparallel β-sheet is very attractive since it can be obtained by a single peptide chain folding in a β-hairpin structure and can be as short as 14 amino acids or less. Herein, we designed eight linear and two cyclic dodeca-peptides endowed with β-hairpins. Their DNA binding properties have been investigated using fluorescence spectroscopy together with the conformational analysis through circular dichroism and solution NMR. We found that one of our peptides, peptide 6, is able to bind DNA, albeit without sequence selectivity. Notably, it shows a topological selectivity for the major groove of the DNA which is the interaction site of ARC and many other DNA-binding proteins. Moreover, we found that a type I' β-hairpin folding pattern is a favorite peptide structure for interaction with the B-DNA major groove. Peptide 6 is a valuable lead compound for the development of novel analogs with sequence selectivity.

Gerstmann-Sträussler-Scheinker syndrome misdiagnosed as conversion disorder

Gerstmann-Sträussler-Scheinker syndrome (GSS) is a rare cause of genetic prion disease. Overlapping neurological, cognitive and psychiatric symptoms make GSS difficult to diagnose based on clinical features alone. We present a 40-year-old man without relevant medical or family history who developed progressive neurocognitive and behavioural symptoms over 3 years. Initial extensive diagnostic workup of his variable motor symptoms was unrevealing and he was diagnosed with conversion disorder. This diagnosis persisted for over 2 years, despite progressive neurocognitive symptoms. He eventually developed dementia and severe neurological impairment. Repeat brain MRI revealed generalised cortical volume loss, establishing the diagnosis of a rapidly progressive neurodegenerative process. He ultimately died from aspiration pneumonia at age 43. Postmortem neuropathological examination showed widespread multicentric prion protein amyloid plaques characteristic of GSS. Ultimately, genetic testing of brain tissue revealed a heterozygous A117V variant in the PNRP gene, confirming the diagnosis.

In Vitro Prion Amplification Methodology for Inhibitor Screening

Prion (PrP^C) is an endogenous protein found mainly in the nervous system, and its misfolded isoform (PrP^Sc) is associated with a group of neurodegenerative disorders known as transmissible spongiform encephalopathies, or simply prion diseases. The PrP^Sc isoform shows an intriguing ability to self-perpetuate, acting as template for PrP^C misfolding and consequent aggregation. Aggregation in vitro and in vivo follows a fibrillation processes that is associated with neurodegeneration. Therefore, it is important to investigate and understand the molecular mechanisms involved in this process; such understanding also allows investigation of the action of possible candidate molecules to inhibit this process. Here, we highlight useful in vitro methodologies and analyses that were developed using PrP as a protein model but that, as other amyloid proteins also exhibit the same behavior, may be applied to understand other "prion-like" diseases such as Alzheimer's and Parkinson's disease.

Physiological role of Prion Protein in Copper homeostasis and angiogenic mechanisms of endothelial cells

The Prion Protein (PrP) is mostly known for its role in prion diseases, where its misfolding and aggregation can cause fatal neurodegenerative conditions such as the bovine spongiform encephalopathy and human Creutzfeldt–Jakob disease. Physiologically, PrP is involved in several processes including adhesion, proliferation, differentiation and angiogenesis, but the molecular mechanisms behind its role remain unclear. PrP, due to its well-described structure, is known to be able to regulate copper homeostasis; however, copper dyshomeostasis can lead to developmental defects. We investigated PrP-dependent regulation of copper homeostasis in human endothelial cells (HUVEC) using an RNA-interference protocol. PrP knockdown did not influence cell viability in silenced HUVEC (PrPKD) compared to control cells, but significantly increased PrPKD HUVEC cells sensitivity to cytotoxic copper concentrations. A reduction of PrPKD cells reductase activity and copper ions transport capacity was observed. Furthermore, PrPKD-derived spheroids exhibited altered morphogenesis and their derived cells showed a decreased vitality 24 and 48 hours after seeding. PrPKD spheroid-derived cells also showed disrupted tubulogenesis in terms of decreased coverage area, tubule length and total nodes number on matrigel, preserving unaltered VEGF receptors expression levels. Our results highlight PrP physiological role in cellular copper homeostasis and in the angiogenesis of endothelial cells.

Autophagy Intertwines with Different Diseases-Recent Strategies for Therapeutic Approaches

Autophagy is a regular and substantial “clear-out process” that occurs within the cell and that gets rid of debris that accumulates in membrane-enclosed vacuoles by using enzyme-rich lysosomes, which are filled with acids that degrade the contents of the vacuoles. This machinery is well-connected with many prevalent diseases, including cancer, HIV, and Parkinson’s disease. Considering that autophagy is well-known for its significant connections with a number of well-known fatal diseases, a thorough knowledge of the current findings in the field is essential in developing therapies to control the progression rate of diseases. Thus, this review summarizes the critical events comprising autophagy in the cellular system and the significance of its key molecules in manifesting this pathway in various diseases for down- or upregulation. We collectively reviewed the role of autophagy in various diseases, mainly neurodegenerative diseases, cancer, inflammatory diseases, and renal disorders. Here, some collective reports on autophagy showed that this process might serve as a dual performer: either protector or contributor to certain diseases. The aim of this review is to help researchers to understand the role of autophagy-regulating genes encoding functional open reading frames (ORFs) and its connection with diseases, which will eventually drive better understanding of both the progression and suppression of different diseases at various stages. This review also focuses on certain novel therapeutic strategies which have been published in the recent years based on targeting autophagy key proteins and its interconnecting signaling cascades.

Neurofilament light chain and tau concentrations are markedly increased in the serum of patients with sporadic Creutzfeldt-Jakob disease, and tau correlates with rate of disease progression

OBJECTIVES

A blood-based biomarker of neuronal damage in sporadic Creutzfeldt-Jakob disease (sCJD) will be extremely valuable for both clinical practice and research aiming to develop effective therapies.

METHODS

We used an ultrasensitive immunoassay to measure two candidate biomarkers, tau and neurofilament light (NfL), in serum from patients with sCJD and healthy controls. We tested longitudinal sample sets from six patients to investigate changes over time, and examined correlations with rate of disease progression and associations with known phenotype modifiers.

RESULTS

Serum concentrations of both tau and NfL were increased in patients with sCJD. NfL distinguished patients from controls with 100% sensitivity and 100% specificity. Tau did so with 91% sensitivity and 83% specificity. Both tau and NfL appeared to increase over time in individual patients, particularly in those with several samples tested late in their disease. Tau, but not NfL, was positively correlated with rate of disease progression, and was particularly increased in patients homozygous for methionine at codon 129 of PRNP.

CONCLUSIONS

These findings independently replicate other recent studies using similar methods and offer novel insights. They show clear promise for these blood-based biomarkers in prion disease. Future work should aim to fully establish their potential roles for monitoring disease progression and response to therapies.

Treatment with a non-toxic, self-replicating anti-prion delays or prevents prion disease in vivo

Transmissible spongiform encephalopathies (TSEs) are fatal neurological disorders caused by prions, which are composed of a misfolded protein (PrP^Sc) that self-propagates in the brain of infected individuals by converting the normal prion protein (PrP^C) into the pathological isoform. Here, we report a novel experimental strategy for preventing prion disease based on producing a self-replicating, but innocuous PrP^Sc-like form, termed anti-prion, which can compete with the replication of pathogenic prions. Our results show that a prophylactic inoculation of prion-infected animals with an anti-prion delays the onset of the disease and in some animals completely prevents the development of clinical symptoms and brain damage. The data indicate that a single injection of the anti-prion eliminated ~99% of the infectivity associated to pathogenic prions. Furthermore, this treatment caused significant changes in the profile of regional PrP^Sc deposition in the brains of animals that were treated, but still succumbed to the disease. Our findings provide new insights for a mechanistic understanding of prion replication and support the concept that prion replication can be separated from toxicity, providing a novel target for therapeutic intervention.

Immunization of cervidized transgenic mice with multimeric deer prion protein induces self-antibodies that antagonize chronic wasting disease infectivity in vitro

Chronic wasting disease (CWD) is the most contagious prion disease. It is expanding rapidly in North America, was found recently in Europe, and the potential for transmission to humans cannot be excluded yet. We hypothesized that it is possible to prevent peripheral CWD infection and CWD prion shedding by inducing auto-antibodies against the cellular prion protein (PrP^C) by active vaccination. Our objective is to overcome self-tolerance against PrP by using a multimeric recombinant PrP (recPrP) as an immunogen. We expressed in E. coli, purified and refolded four immunogens: cervid and murine recPrP in monomeric and dimeric form. Testing immunogenicity in sera of the vaccinated transgenic mice expressing cervid PrP revealed that all four immunogens effectively overcame self-tolerance against the prion protein as shown by high antibody titers. Confocal microscopy analysis revealed effective binding of post-immune sera to surface-located PrP^C in both murine and cervid PrP expressing cultured cells. Remarkably, the post-immune auto-antibodies effectively inhibited CWD-induced prion conversion in RT-QuIC assay when incubated with either PrP substrate or CWD seed. Furthermore, they mitigated prion propagation in CWD-infected cervid-PrP expressing RK13 cells. Together, multimeric recombinant cervid PrP effectively overcomes self-tolerance to PrP and induces auto-antibodies that interfere with CWD conversion in vitro.

Unraveling Prion Protein Interactions with Aptamers and Other PrP-Binding Nucleic Acids

Transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative disorders that affect humans and other mammals. The etiologic agents common to these diseases are misfolded conformations of the prion protein (PrP). The molecular mechanisms that trigger the structural conversion of the normal cellular PrP (PrP^C) into the pathogenic conformer (PrP^Sc) are still poorly understood. It is proposed that a molecular cofactor would act as a catalyst, lowering the activation energy of the conversion process, therefore favoring the transition of PrP^C to PrP^Sc. Several in vitro studies have described physical interactions between PrP and different classes of molecules, which might play a role in either PrP physiology or pathology. Among these molecules, nucleic acids (NAs) are highlighted as potential PrP molecular partners. In this context, the SELEX (Systematic Evolution of Ligands by Exponential Enrichment) methodology has proven extremely valuable to investigate PrP–NA interactions, due to its ability to select small nucleic acids, also termed aptamers, that bind PrP with high affinity and specificity. Aptamers are single-stranded DNA or RNA oligonucleotides that can be folded into a wide range of structures (from harpins to G-quadruplexes). They are selected from a nucleic acid pool containing a large number (10¹⁴–10¹⁶) of random sequences of the same size (~20–100 bases). Aptamers stand out because of their potential ability to bind with different affinities to distinct conformations of the same protein target. Therefore, the identification of high-affinity and selective PrP ligands may aid the development of new therapies and diagnostic tools for TSEs. This review will focus on the selection of aptamers targeted against either full-length or truncated forms of PrP, discussing the implications that result from interactions of PrP with NAs, and their potential advances in the studies of prions. We will also provide a critical evaluation, assuming the advantages and drawbacks of the SELEX (Systematic Evolution of Ligands by Exponential Enrichment) technique in the general field of amyloidogenic proteins.

Pharmacological Modulation of Functional Phenotypes of Microglia in Neurodegenerative Diseases

Microglia are the resident innate immune cells of the central nervous system that mediate brain homeostasis maintenance. Microglia-mediated neuroinflammation is a hallmark shared by various neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis. Numerous studies have shown microglial activation phenotypes to be heterogeneous; however, these microglial phenotypes can largely be categorized as being either M1 or M2 type. Although the specific classification of M1 and M2 functionally polarized microglia remains a topic for debate, the use of functional modulators of microglial phenotypes as potential therapeutic approaches for the treatment of neurodegenerative diseases has garnered considerable attention. This review discusses M1 and M2 microglial phenotypes and their relevance in neurodegenerative disease models, as described in recent literature. The modulation of microglial polarization toward the M2 phenotype may lead to development of future therapeutic and preventive strategies for neuroinflammatory and neurodegenerative diseases. Thus, we focus on recent studies of microglial polarization modulators, with a particular emphasis on the small-molecule compounds and their intracellular target proteins.