Therapy for prion diseases: Insights from the use of RNA interference

Accumulation Area of a Japanese PRNP P102L Variant Associated With Gerstmann-Sträussler-Scheinker Disease: The Ariake PRNP P102L Variant

BACKGROUND AND PURPOSE

The coast of Kyushu Island on Ariake Sea in Japan is known to be an accumulation area for patients with a proline-to-leucine substitution mutation at residue 102 (P102L) of the human prion protein gene (PRNP), which is associated with Gerstmann-Sträussler-Scheinker disease. We designated this geographical distribution as the "Ariake PRNP P102L variant." The purpose of this study was to characterize the clinical features of this variant.

METHODS

We enrolled patients with the PRNP P102L variant who were followed up at the Saga University Hospital from April 2002 to November 2019. The clinical information of patients were obtained from medical records, including clinical histories, brain magnetic resonance imaging (MRI), and electroencephalography (EEG). A brain autopsy was performed on one of the participants.

RESULTS

We enrolled 24 patients from 19 family lines, including 12 males. The mean age at symptom onset was 60.6 years (range, 41-77 years). The incidence rate of the Ariake PRNP P102L variant was 3.32/1,000,000 people per year in Saga city. The initial symptoms were ataxia (ataxic gait or dysarthria) in 19 patients (79.2%), cognitive impairment in 3 (12.5%), and leg paresthesia in 2 (8.3%). The median survival time from symptom onset among the 18 fatal cases was 63 months (range, 23-105 months). Brain MRI revealed no localized cerebellar atrophy, but sparse diffusion-weighted imaging abnormalities were detected in 16.7% of the patients. No periodic sharp-wave complexes were identified in EEG. Neuropathological investigations revealed uni- and multicentric prion protein (PrP) plaques in the cerebral cortex, putamen, thalamus, and cerebellum of one patient. Western blot analysis revealed 8-kDa proteinase-K-resistant PrP.

CONCLUSIONS

This is the first report of the accumulation area of a PRNP P102L variant on the coast of Ariake Sea. The Ariake PRNP P102L variant can be characterized by a relatively long disease duration with sparse abnormalities in brain MRI and EEG relative to previous reports. Detailed interviews to obtain information on the birthplace and the family history of related symptoms are important to diagnosing a PRNP P102L variant.

Genetic aspects of human prion diseases

Human prion diseases are rapidly progressive and fatal neurodegenerative conditions caused by a disease-causing isoform of the native prion protein. The prion protein gene (PRNP) encodes for the cellular prion protein, which is the biological substrate for prion disease transmission and neurotoxicity. Human prion diseases have three etiologies: sporadic, genetic, and acquired. PRNP polymorphisms and pathogenic variants play a large role in the frequency, age at onset, and clinicopathologic phenotype of prion diseases. Genetic prion diseases will be covered in detail and information necessary for clinical care, predictive genetic testing, and genetic counseling will be reviewed. Because the prion protein is necessary for transmission and neurotoxicity, many experimental treatments targeting its production are being investigated and hold potential promise as a disease modifying treatment for all forms of prion disease, including asymptomatic mutation carriers. This article will review genetic aspects of human prion disease and their influence on epidemiology, clinicopathologic phenotype, diagnostics, clinical management, and potential treatment approaches.

Tissue Engineering and Photodynamic Therapy: A New Frontier of Science for Clinical Application -An Up-To-Date Review

Tissue engineering (TE) connects principles of life sciences and engineering to develop biomaterials as alternatives to biological systems and substitutes that can improve and restore tissue function. The principle of TE is the incorporation of cells through a 3D matrix support (scaffold) or using scaffold-free organoid cultures to reproduce the 3D structure. In addition, 3D models developed can be used for different purposes, from studies mimicking healthy tissues and organs as well as to simulate and study different pathologies. Photodynamic therapy (PDT) is a non-invasive therapeutic modality when compared to conventional therapies. Therefore, PDT has great acceptance among patients and proves to be quite efficient due to its selectivity, versatility and therapeutic simplicity. The PDT mechanism consists of the use of three components: a molecule with higher molar extinction coefficient at UV-visible spectra denominated photosensitizer (PS), a monochromatic light source (LASER or LED) and molecular oxygen present in the microenvironment. The association of these components leads to a series of photoreactions and production of ultra-reactive singlet oxygen and reactive oxygen species (ROS). These species in contact with the pathogenic cell, leads to its target death based on necrotic and apoptosis ways. The initial objective of PDT is the production of high concentrations of ROS in order to provoke cellular damage by necrosis or apoptosis. However, recent studies have shown that by decreasing the energy density and consequently reducing the production of ROS, it enabled a specific cell response to photostimulation, tissues and/or organs. Thus, in the present review we highlight the main 3D models involved in TE and PS most used in PDT, as well as the applications, future perspectives and limitations that accompany the techniques aimed at clinical use.

Non-Oncologic Applications of Nanomedicine-Based Phototherapy

Phototherapy is widely applied to various human diseases. Nanomedicine-based phototherapy can be classified into photodynamic therapy (PDT) and photothermal therapy (PTT). Activated photosensitizer kills the target cells by generating radicals or reactive oxygen species in PDT while generating heat in PTT. Both PDT and PTT have been employed for treating various diseases, from preclinical to randomized controlled clinical trials. However, there are still hurdles to overcome before entering clinical practice. This review provides an overview of nanomedicine-based phototherapy, especially in non-oncologic diseases. Multiple clinical trials were undertaken to prove the therapeutic efficacy of PDT in dermatologic, ophthalmologic, cardiovascular, and dental diseases. Preclinical studies showed the feasibility of PDT in neurologic, gastrointestinal, respiratory, and musculoskeletal diseases. A few clinical studies of PTT were tried in atherosclerosis and dry eye syndrome. Although most studies have shown promising results, there have been limitations in specificity, targeting efficiency, and tissue penetration using phototherapy. Recently, nanomaterials have shown promising results to overcome these limitations. With advanced technology, nanomedicine-based phototherapy holds great potential for broader clinical practice.

Novel regulators of PrPC expression as potential therapeutic targets in prion diseases

INTRODUCTION

Prion diseases are rare and fatal neurodegenerative disorders. The key molecular event in these disorders is the misfolding of the physiological form of the cellular prion protein, PrP^C, leading to the accumulation of a pathological isoform, PrP^Sc, with unique features. Both isoforms share the same primary sequence, lacking detectable differences in posttranslational modification, a major hurdle for their biochemical or biophysical independent characterization. The mechanism underlying the conversion of PrP^C to PrP^Sc is not completely understood, so finding an effective therapy to cure prion disorders is extremely challenging.

AREAS COVERED

This review discusses the strategies for decreasing prion replication and throws a spotlight on the relevance of PrP^C in the prion accumulation process.

EXPERT OPINION

PrP^C is the key substrate for prion pathology; hence, the most promising therapeutic approach appears to be the targeting of PrP^C to block the production of the infectious isoform. The use of RNA interference and antisense oligonucleotide technologies may offer opportunities for treatment because of their success in clinical trials for other neurodegenerative diseases.

Elucidating Critical Proteinopathic Mechanisms and Potential Drug Targets in Neurodegeneration

Neurodegeneration entails progressive loss of neuronal structure as well as function leading to cognitive failure, apathy, anxiety, irregular body movements, mood swing and ageing. Proteomic dysregulation is considered the key factor for neurodegeneration. Mechanisms involving deregulated processing of proteins such as amyloid beta (Aβ) oligomerization; tau hyperphosphorylation, prion misfolding; α-synuclein accumulation/lewy body formation, chaperone deregulation, acetylcholine depletion, adenosine 2A (A2A) receptor hyperactivation, secretase deregulation, leucine-rich repeat kinase 2 (LRRK2) mutation and mitochondrial proteinopathies have deeper implications in neurodegenerative disorders. Better understanding of such pathological mechanisms is pivotal for exploring crucial drug targets. Herein, we provide a comprehensive outlook about the diverse proteomic irregularities in Alzheimer's, Parkinson's and Creutzfeldt Jakob disease (CJD). We explicate the role of key neuroproteomic drug targets notably Aβ, tau, alpha synuclein, prions, secretases, acetylcholinesterase (AchE), LRRK2, molecular chaperones, A2A receptors, muscarinic acetylcholine receptors (mAchR), N-methyl-D-aspartate receptor (NMDAR), glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) and mitochondrial/oxidative stress-related proteins for combating neurodegeneration and associated cognitive and motor impairment. Cross talk between amyloidopathy, synucleinopathy, tauopathy and several other proteinopathies pinpoints the need to develop safe therapeutics with ability to strike multiple targets in the aetiology of the neurodegenerative disorders. Therapeutics like microtubule stabilisers, chaperones, kinase inhibitors, anti-aggregation agents and antibodies could serve promising regimens for treating neurodegeneration. However, drugs should be target specific, safe and able to penetrate blood-brain barrier.

Neurotheranostics as personalized medicines

The discipline of neurotheranostics was forged to improve diagnostic and therapeutic clinical outcomes for neurological disorders. Research was facilitated, in largest measure, by the creation of pharmacologically effective multimodal pharmaceutical formulations. Deployment of neurotheranostic agents could revolutionize staging and improve nervous system disease therapeutic outcomes. However, obstacles in formulation design, drug loading and payload delivery still remain. These will certainly be aided by multidisciplinary basic research and clinical teams with pharmacology, nanotechnology, neuroscience and pharmaceutic expertise. When successful the end results will provide "optimal" therapeutic delivery platforms. The current report reviews an extensive body of knowledge of the natural history, epidemiology, pathogenesis and therapeutics of neurologic disease with an eye on how, when and under what circumstances neurotheranostics will soon be used as personalized medicines for a broad range of neurodegenerative, neuroinflammatory and neuroinfectious diseases.

Dual MicroRNA to Cellular Prion Protein Inhibits Propagation of Pathogenic Prion Protein in Cultured Cells

Prion diseases are fatal transmissible neurodegenerative disorders affecting humans and various mammals. In spite of intensive efforts, there is no effective cure or treatment for prion diseases. Cellular forms of prion protein (PrP^C) is essential for propagation of abnormal isoforms of prion protein (PrP^Sc) and pathogenesis. The effect of an artificial dual microRNA (DmiR) on PrP^C suppression and resultant inhibition of prion replication was determined using prion-infectible cell cultures: differentiated C2C12 culture and primary mixed neuronal and glial cells culture (MNGC). Processing of DmiR by prion-susceptible myotubes, but not by reserve cells, in differentiated C2C12 culture slowed prion replication, implying an importance of cell type-specific PrP^C targeting. In MNGC, reduction of PrP^C with DmiR was effective for suppressing prion replication. MNGC lentivirally transduced with non-targeting control miRNAs (scrambled) reduced prion replication at a level similar to that with a synthetic analogue of viral RNA, poly I:C. The results suggest that a synergistic combination of the immunostimulatory RNA duplexes (miRNA) and PrP^C silencing with DmiR might augment a therapeutic potential of RNA interference.

Prions: Beyond a Single Protein

Since the term protein was first coined in 1838 and protein was discovered to be the essential component of fibrin and albumin, all cellular proteins were presumed to play beneficial roles in plants and mammals. However, in 1967, Griffith proposed that proteins could be infectious pathogens and postulated their involvement in scrapie, a universally fatal transmissible spongiform encephalopathy in goats and sheep. Nevertheless, this novel hypothesis had not been evidenced until 1982, when Prusiner and coworkers purified infectious particles from scrapie-infected hamster brains and demonstrated that they consisted of a specific protein that he called a "prion." Unprecedentedly, the infectious prion pathogen is actually derived from its endogenous cellular form in the central nervous system. Unlike other infectious agents, such as bacteria, viruses, and fungi, prions do not contain genetic materials such as DNA or RNA. The unique traits and genetic information of prions are believed to be encoded within the conformational structure and posttranslational modifications of the proteins. Remarkably, prion-like behavior has been recently observed in other cellular proteins-not only in pathogenic roles but also serving physiological functions. The significance of these fascinating developments in prion biology is far beyond the scope of a single cellular protein and its related disease.

Systemic delivery of siRNA down regulates brain prion protein and ameliorates neuropathology in prion disorder

One of the main challenges for neurodegenerative disorders that are principally incurable is the development of new therapeutic strategies, which raises important medical, scientific and societal issues. Creutzfeldt-Jakob diseases are rare neurodegenerative fatal disorders which today remain incurable. The objective of this study was to evaluate the efficacy of the down-regulation of the prion protein (PrP) expression using siRNA delivered by, a water-in-oil microemulsion, as a therapeutic candidate in a preclinical study. After 12 days rectal mucosa administration of Aonys/PrP-siRNA in mice, we observed a decrease of about 28% of the brain PrP^C level. The effect of Aonys/PrP-siRNA was then evaluated on prion infected mice. Several mice presented a delay in the incubation and survival time compared to the control groups and a significant impact was observed on astrocyte reaction and neuronal survival in the PrP-siRNA treated groups. These results suggest that a new therapeutic scheme based an innovative delivery system of PrP-siRNA can be envisioned in prion disorders.

From high-throughput cell culture screening to mouse model: identification of new inhibitor classes against prion disease

Transmissible spongiform encephalopathies (TSE) or prion diseases belong to a category of fatal and so far untreatable neurodegenerative conditions. All prion diseases are characterized by both degeneration in the central nervous system (CNS) in humans and animals and the deposition and accumulation of Proteinase K-resistant prion protein (PrP(res)). Until now, no pharmaceutical product has been available to cure these diseases or to alleviate their associated symptoms. Here, a cell-culture screening system is described that allows for the large-scale analysis of the PrP(res) inhibitory potential of a library of compounds and the identification of structural motifs leading potent compounds able to cause PrP(res) clearance at the cellular level. Based on different scrapie-infected cell lines, 10,000 substances were tested, out of which 530 potential inhibitors were identified. After re-screening and validation using a series of dilutions, 14 compounds were identified as the most effective. These 14 compounds were then used for therapeutic studies in a mouse bioassay to test and verify their in vivo potency. Two compounds exhibited therapeutic potential in the mouse model by significantly extending the survival time of intracerebrally infected mice, when treated 90 days after infection with scrapie.

Establishment and characterization of Prnp knockdown neuroblastoma cells using dual microRNA-mediated RNA interference

Prion diseases are fatal transmissible neurodegenerative disorders. In the pathogenesis of the disease, the cellular prion protein (PrPC) is required for replication of abnormal prion (PrPSc), which results in accumulation of PrPSc. Although there have been extensive studies using Prnp knockout systems, the normal function of PrPC remains ambiguous. Compared with conventional germline knockout technologies and transient naked siRNA-dependent knockdown systems, newly constructed durable chained-miRNA could provide a cell culture model that is closer to the disease status and easier to achieve with no detrimental sequelae. The selective silencing of a target gene by RNA interference (RNAi) is a powerful approach to investigate the unknown function of genes in vitro and in vivo. To reduce PrPC expression, a novel dual targeting-microRNA (miRdual) was constructed. The miRdual, which targets N- and C- termini of Prnp simultaneously, more effectively suppressed PrPC expression compared with conventional single site targeting. Furthermore, to investigate the cellular change following PrPC depletion, gene expression analysis of PrPC interacting and/or associating genes and several assays including proliferation, viability and apoptosis were performed. The transcripts 670460F02Rik and Plk3, Ppp2r2b and Csnk2a1 increase in abundance and are reported to be involved in cell proliferation and mitochondrial-mediated apoptosis. Dual-targeting RNAi with miRdual against Prnp will be useful for analyzing the physiological function of PrPC in neuronal cell lines and may provide a potential therapeutic intervention for prion diseases in the future.

Alkylating antitumor drug mechlorethamine conceals a structured PrP domain and inhibits in vitro prion amplification

Prion diseases are a group of incurable transmissible neurodegenerative disorders. The key molecular event in the pathogenesis of prion diseases is the conversion of the cellular prion protein (PrP(C)) into its pathological isoform (PrP(Sc)), accompanied by a conformational transition of α-helix into β-sheet structure involving the structured α-helix 1 domain from residues 144-154 of the protein (PrP144-154). Blocking the accessibility of PrP144-152 with anti-PrP antibody 6H4 was found to prevent PrP conversion and even to cure prion infection in cell models ( Enari et al. 2001 ). Previously, Yuan et al. (2005 ) demonstrated that the reduction and alkylation of PrP induced concealment of the 6H4 epitope. This study examined the ability of mechlorethamine (MCT), an alkylating antitumor drug, to conceal the 6H4 epitope and block PrP conversion in the presence of a reducing reagent. Mechlorethamine treatment significantly decreased in vitro amplification of PrP(Sc) in the highly efficient protein misfolding cyclic amplification system. Our findings suggest that MCT may serve as a potential therapeutic agent for prion diseases.

Dysfunction and recovery of synapses in prion disease: implications for neurodegeneration

Synaptic dysfunction is a key early process in many neurodegenerative diseases, but how this ultimately leads to neuronal loss is not clear. In health, there is ongoing remodelling of synapses and spines in the adult brain: their elimination and formation are continual physiological processes fundamental to learning and memory. But in neurodegenerative disease, including prion disease, lost synapses are not replaced, and their loss is followed by neuronal death. These two processes are separately regulated, with mechanistic, spatial and temporal segregation of the respective death routines of synapses and cell bodies. Mice with prion disease can be cured at the stage of early synaptic dysfunction, when they have reversible impairments at neurophysiological, behavioural and morphological levels. Critically, reversing synaptic dysfunction at this stage of disease rescues neurons, preventing its otherwise inevitable progression to synapse loss and cell death. These findings call for a deeper analysis of the mechanisms underlying neurotoxicity at the synapse, and have important implications for the therapy of prion and other neurodegenerative disorders.