Is loss of function of the prion protein the cause of prion disorders?

Transmissible spongiform encephalopathies are fatal neurodegenerative diseases that involve misfolding of the prion protein. Recent studies have provided evidence that normal prion protein might have a physiological function in neuroprotective signaling, suggesting that loss of prion protein activity might contribute to the pathogenesis of prion disease. However, studies using knockout animals do not support the loss-of-function hypothesis and argue that prion neurodegeneration might be associated with a gain of a toxic activity by the misfolded prion protein. Thus, the mechanism of neurodegeneration in spongiform encephalopathies remains enigmatic.     

Prion protein interconversions and the transmissible spongiform encephalopathies.

Autocatalytic changes in the conformation and aggregation state of prion protein appear to be fundamental to transmissible spongiform encephalopathies or prion diseases. Here we review the considerable progress that has been made in describing the normal properties of prion protein and the changes that occur during these devastating neurodegenerative diseases.     

Spongiform encephalopathies and prions: An overview of pathology and disease mechanisms

Abstract The etiology of spongiform encephalopathies has been sharply contested for decades. At the heart of the issue is the question of disease origin: Are prion diseases representative of primary neurodegenerative genetic disorders, or are they bona fide infectious diseases? This article provides a brief outline of the progress made in the elucidation of prion disease mechanisms in the context of pathological support of the 'protein only' hypothesis. The answer to the above question appears to be that spongiform encephalopathies are uniquely both infectious and genetic neurodegenerative diseases.     

The role of PrP in health and disease

Transmissible spongiform encephalopathies (TSEs) such as scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle or Creutzfeldt-Jacob disease (CJD) and Gerstmann-Str?ussler-Scheinker syndrome (GSS) in humans, are caused by an infectious agent designated prion. The "protein only" hypothesis states that the prion consists partly or entirely of a conformational isoform of the normal host protein PrPc and that the abnormal conformer, when introduced into the organism, causes the conversion of PrPc into a likeness of itself. Since the proposal of the "protein only" hypothesis more than three decades ago, cloning of the PrP gene, studies on PrP knockout mice and on mice transgenic for mutant PrP genes allowed deep insights into prion biology. Reverse genetics on PrP knockout mice containing modified PrP transgenes was used to address a variety of problems: mapping PrP regions required for prion replication, studying PrP mutations affecting the species barrier, modeling familial forms of human prion disease, analysing the cell specificity of prion propagation and investigating the physiological role of PrP by structure-function studies. Many questions regarding the role of PrP in susceptibility to prions have been elucidated, however the physiological role of PrP and the pathological mechanisms of neurodegeneration in prion diseases are still elusive.     

Not on the menu

A common feature of neurodegenerative diseases is the accumulation of disease-specific, aggregated protein species in the nervous system. Transmissible spongiform encephalopathies are universally fatal neurodegenerative diseases involving the transconformation and aggregation of prion proteins. At the cellular level macroautophagy has been identified as an efficient pathway for the clearance of these toxic protein aggregates. Hence, recent research has focused on the pharmacological manipulation of autophagy as a potential treatment for neurodegenerative diseases. Independent of their effects on the estrogen receptor, tamoxifen and its metabolite 4-hydroxytamoxifen are well known inducers of autophagy. However, we recently reported that the ability of 4-hydroxytamoxifen to clear prion infection is independent of autophagy. In contrast, we provide a model whereby perturbation of cholesterol metabolism, and not autophagy, is the main mechanism whereby 4-hydroxytamoxifen is able to exert its anti-prion effects. Thus, while tamoxifen, a widely available pharmaceutical, may have applications in prion therapy, prions may also represent a special case and may require different pharmacological interventions than other proteinopathies.     

Copper and prion diseases

Transmissible spongiform encephalopathies are diseases of animals and humans that are also termed prion diseases. These diseases are linked together because a normal brain glycoprotein termed the prion protein is converted to a readily detectable protease-resistant isoform. There is now strong evidence to suggest that apart from this difference in resistance a major difference between the isoforms is that the normal prion protein binds copper and has an anti-oxidant function. Brains from Creutzfeldt-Jakob disease patients and brains from mice with experimental mouse scrapie have been shown to have changes in the levels of both copper and manganese. There is growing evidence that links prion diseases to disturbances of metal metabolism.     

Stressing out the ER: a role of the unfolded protein response in prion-related disorders

Transmissible Spongiform Encephalopathies are fatal and infectious neurodegenerative diseases characterized by extensive neuronal apoptosis and the accumulation of an abnormally folded form of the cellular prion protein (PrP), denoted PrP(SC). Compelling evidence suggests the involvement of several signaling pathways in prion pathogenesis, including proteasome dysfunction, alterations in the protein maturation pathways and the unfolded protein response. Recent reports indicate that endoplasmic reticulum stress due to the PrP misfolding may be a critical factor mediating neuronal dysfunction in prion diseases. These findings have applications for developing novel strategies for treatment and early diagnosis of transmissible spongiform encephalopathies and other neurodegenerative diseases.     

Prions: disease propagation and disease therapy by conformational transmission

Transmissible spongiform encephalopathies - also known as prion-related diseases - are a group of fatal neurodegenerative disorders associated with the misfolding of prion protein. Several unprecedented scientific findings, which have directly confronted popular dogmas in biology, have put prion research in the spotlight. The experimental evidence supports an entirely novel disease mechanism, involving disease transmission by replication of protein conformation. Here, we describe exciting scientific findings that make the prion field attractively heretical, and we propose the transmission of protein conformation as a novel approach to producing drugs to combat a variety of diseases.     

Cell culture models of transmissible spongiform encephalopathies

Cell cultures represent versatile and useful experimental models of transmissible spongiform encephalopathies. These models include chronically prion infected cell lines, as well as cultures expressing variable amounts of wild-type, mutated or chimeric prion proteins. These cultures have been widely used to investigate the biology of both the normal and the pathological isoform of the prion protein. They have also contributed to the comprehension of the pathogenic processes occurring in transmissible spongiform encephalopathies and in the development of new therapeutic approaches of these diseases.     

Immunization with Recombinant Prion Protein Leads to Partial Protection in a Murine Model of TSEs through a Novel Mechanism

Transmissible spongiform encephalopathies are neurodegenerative diseases, which despite fervent research remain incurable. Immunization approaches have shown great potential at providing protection, however tolerance effects hamper active immunization protocols. In this study we evaluated the antigenic potential of various forms of recombinant murine prion protein and estimated their protective efficacy in a mouse model of prion diseases. One of the forms tested provided a significant elongation of survival interval. The elongation was mediated via an acute depletion of mature follicular dendritic cells, which are associated with propagation of the prion infectious agent in the periphery and in part to the development of humoral immunity against prion protein. This unprecedented result could offer new strategies for protection against transmissible encephalopathies as well as other diseases associated with follicular dendritic cells.     

Prion protein interconversions

The transmissible spongiform encephalopathies (TSEs), or prion diseases, remain mysterious neurodegenerative diseases that involve perturbations in prion protein (PrP) structure. This article summarizes our use of in vitro models to describe how PrP is converted to the disease-associated, protease-resistant form. These models reflect many important biological parameters of TSE diseases and have been used to identify inhibitors of the PrP conversion as lead compounds in the development of anti-TSE drugs.     

Investigating protein conformation-based inheritance and disease in yeast

Our work supports the hypothesis that a protein can serve as an element of genetic inheritance. This protein-only mechanism of inheritance is propagated in much the same way as hypothesized for the transmission of the protein-only infectious agent in the spongiform encephalopathies; hence these protein factors have been called yeast prions. Our work has focused on [PSI(+)], a dominant cytoplasmically inherited factor that alters translational fidelity.This change in translation is produced by a self-perpetuating change in the conformation of the translation-termination factor, Sup35. Most recently, we have determined that new elements of genetic inheritance can be created by deliberate genetic engineering, opening prospects for new methods of manipulating heredity. We have also uncovered evidence that other previously unknown elements of protein-based inheritance are encoded in the yeast genome. Finally, we have begun to use yeast as a model system for studying human protein folding diseases, such as Huntington's disease. Proteins responsible for some of these diseases have properties uncannily similar to those that produce protein-based mechanisms of inheritance.     

微小RNA在朊病毒病等神经退行性疾病中的研究进展

朊病毒病是一类具有传染性、不可逆且致命的神经退行性疾病,其致病机制为体内正常编码的细胞型朊蛋白(cellular prion protein,PrP~C)构象发生变化,形成了具有感染性的异常痒病型朊病毒(scrapie prion protein,PrP~(Sc)),但具体机制不清楚,目前为止尚无有效治疗方法。微小RNA(microRNA,miRNA)可在转录水平调控细胞蛋白表达,对神经系统发育及功能起重要作用。近年来,对一些特定miRNA在朊病毒病中相应调控机制、自发免疫、炎症信号转导及靶基因预测方面的研究可为治疗朊病毒病提供新的角度。本文就miRNA在朊病毒病发生中的相关研究进展进行综述,并详细探讨其中研究较为深入的miRNA。     

Interactions between prion protein isoforms: the kiss of death?

Direct interactions between the normal and aberrant forms of prion protein appear to be crucial in the transmission and pathogenesis of transmissible spongiform encephalopathies (TSEs) or prion diseases. Recent studies of such interactions in vitro have provided mechanistic insight into how TSE-associated prion protein might promote its own propagation in a manner that is specific enough to account, at least in part, for TSE strains and species barriers.     

Metallic prions

Prion diseases, also referred to as transmissible spongiform encephalopathies, are characterized by the deposition of an abnormal isoform of the prion protein in the brain. However, this aggregated, fibrillar, amyloid protein, termed PrPSc, is an altered conformer of a normal brain glycoprotein, PrPc. Understanding the nature of the normal cellular isoform of the prion protein is considered essential to understanding the conversion process that generates PrPSc. To this end much work has focused on elucidation of the normal function and activity of PrPc. Substantial evidence supports the notion that PrPc is a copper-binding protein. In conversion to the abnormal isoform, this Cu-binding activity is lost. Instead, there are some suggestions that the protein might bind other metals such as Mn or Zn. PrPc functions currently under investigation include the possibility that the protein is involved in signal transduction, cell adhesion, Cu transport and resistance to oxidative stress. Of these possibilities, only a role in Cu transport and its action as an antioxidant take into consideration PrPc's Cu-binding capacity. There are also more published data supporting these two functions. There is strong evidence that during the course of prion disease, there is a loss of function of the prion protein. This manifests as a change in metal balance in the brain and other organs and substantial oxidative damage throughout the brain. Thus prions and metals have become tightly linked in the quest to understand the nature of transmissible spongiform encephalopathies.     

Human and animal spongiform encephalopathies are the result of chronic autoimmune attack in the CNS: a novel medical theory supported by overwhelming experimental evidence

Spongiform encephalopathies, also called "prion diseases", are fatal degenerative diseases of the central nervous system which can occur in animals (such as the "mad cow disease" in cattle) and also in humans. This paper presents a novel medical theory concerning the pathogenic mechanisms for various human and animal spongiform encephalopathies. It is hypothesized that various forms of prion diseases are essentially autoimmune diseases, resulting from chronic autoimmune attack of the central nervous system. A key step in the pathogenic process leading towards the development of spongiform encephalopathies involves the production of specific autoimmune antibodies against the disease-causing prion protein (PrPsc) and possibly other immunogenic macromolecules present in the brain. As precisely explained in this paper, the autoimmune antibodies produced against PrPsc are responsible for the conversion of the normal cellular prion protein (PrPc) to PrPsc, for the accumulation of PrPsc in the brain and other peripheral tissues, and also for the initiation of an antibody-mediated chronic autoimmune attack of the central nervous system neurons, which would contribute to the development of characteristic pathological changes and clinical symptoms associated with spongiform encephalopathies. The validity and correctness of the proposed theory is supported by an overwhelming body of experimental observations that are scattered in the biomedical literature. In addition, the theory also offers practical new strategies for early diagnosis, treatment, and prevention of various human and animal prion diseases.     

Current concepts in human prion protein (Prp) misfolding, Prnp gene polymorphisms and their contribution to Creutzfeldt-Jakob Disease (CJD)

Transmissible spongiform encephalopathies are a group of neural degenerative diseases that may be infectious, sporadic, or hereditary and are associated with an abnormally folded prion protein. Unfortunately at the current time it is not at all clear what the normal structure of the prion protein actually is or how it is toxic to cells. Extensive research on prion diseases has led to a dramatic increase in understanding of the pathogenesis of prion disorders, which will hopefully lead to the development of effective treatments. The inability to detect the disease in blood using current technology has made screening difficult. While fortunately there has been a decline in the number of clinical cases of transmissible variant CJD, evidence indicates that very long incubation periods can occur in humans so there may be a long slow, gradual epidemic. In particular, clinical cases in genotypes other than those homozygous for methionine at codon 129 of PRNP have not yet occurred, but such cases might be expected to have longer incubation periods and show differences in pathology to those seen to date. Transgenic animal studies have shown that a large proportion of infected animals develop sub-clinical disease. Moreover, results from a large prevalence study in humans show that several cases test positive but do not develop clinical disease. It is possible therefore that further cases of secondary transmission could occur by iatrogenic spread, which could result in vCJD persisting in the UK at low levels for many years, highlighting the importance of continued vigilance.     

Disease-associated F198S mutation increases the propensity of the recombinant prion protein for conformational conversion to scrapie-like form

The critical step in the pathogenesis of transmissible spongiform encephalopathies (prion diseases) is the conversion of a cellular prion protein (PrP(c)) into a protease-resistant, beta-sheet rich form (PrP(Sc)). Although the disease transmission normally requires direct interaction between exogenous PrP(Sc) and endogenous PrP(C), the pathogenic process in hereditary prion diseases appears to develop spontaneously (i.e. not requiring infection with exogenous PrP(Sc)). To gain insight into the molecular basis of hereditary spongiform encephalopathies, we have characterized the biophysical properties of the recombinant human prion protein variant containing the mutation (Phe(198) --> Ser) associated with familial Gerstmann-Straussler-Scheinker disease. Compared with the wild-type protein, the F198S variant shows a dramatically increased propensity to self-associate into beta-sheet-rich oligomers. In a guanidine HCl-containing buffer, the transition of the F198S variant from a normal alpha-helical conformation into an oligomeric beta-sheet structure is about 50 times faster than that of the wild-type protein. Importantly, in contrast to the wild-type PrP, the mutant protein undergoes a spontaneous conversion to oligomeric beta-sheet structure even in the absence of guanidine HCl or any other denaturants. In addition to beta-sheet structure, the oligomeric form of the protein is characterized by partial resistance to proteinase K digestion, affinity for amyloid-specific dye, thioflavine T, and fibrillar morphology. The increased propensity of the F198S variant to undergo a conversion to a PrP(Sc)-like form correlates with a markedly decreased thermodynamic stability of the native alpha-helical conformer of the mutant protein. This correlation supports the notion that partially unfolded intermediates may be involved in conformational conversion of the prion protein.     

朊蛋白相关蛋白Shadoo与朊病毒病

朊病毒病,即传染性海绵状脑病（transmissible spongiform encephalopathies, TSEs）,是一类传染性、致死性神经退行性疾病。在朊病毒病的病理过程中,细胞正常朊蛋白PrP。转化为异常构象的PrP是至关重要的,但是PrP‘的正常生理功能仍不清楚。国外学者利用比较基因组学发现了-个新的朊蛋白相关蛋白-shadoo（Sho）。Sho与PrP。在氨基酸序列和细胞定位的相似性及主要在脑组织表达,使它成为-个非常值得研究的PrP相关蛋白。对Sho可能存在的与PrP。重叠的功能甚至直接相互作用的研究工作,将对今后揭示PrPc正常生理功能以及揭示Pfion病发病机制具有重要现实意义。