Human prion diseases (spongiform encephalopathies)

Mechanism of Unfolding of Human Prion Protein

Misfolding and aggregation of prion proteins are associated with several neurodegenerative diseases. Therefore, understanding the mechanism of the misfolding process is of enormous interest in the scientific community. It has been speculated and widely discussed that the native cellular prion protein (PrP^C) form needs to undergo substantial unfolding to a more stable PrP^C* state, which may further oligomerize into the toxic scrapie (PrP^Sc) form. Here, we have studied the mechanism of the unfolding of the human prion protein (huPrP) using a set of extensive well-tempered metadynamics simulations. Through multiple microsecond-long metadynamics simulations, we find several possible unfolding pathways. We show that each pathway leads to an unfolded state of lower free energy than the native state. Thus, our study may point to the signature of a PrP^C* form that corresponds to a global minimum on the conformational free-energy landscape. Moreover, we find that these global minima states do not involve an increased β-sheet content, as was assumed to be a signature of PrP^Sc formation in previous simulation studies. We have further analyzed the origin of metastability of the PrP^C form through free-energy surfaces of the chopped helical segments to show that the helices, particularly H2 and H3 of the prion protein, have the tendency to form either a random coil or a β-structure. Therefore, the secondary structural elements of the prion protein are only weakly stabilized by tertiary contacts and solvation forces so that relatively weak perturbations induced by temperature, pressure, pH, and so forth can lead to substantial unfolding with characteristics of intrinsically disordered proteins.

Sporadic Creutzfeldt-Jakob disease with unusual initial presentation as posterior reversible encephalopathy syndrome: a case report

BACKGROUND

Creutzfeldt - Jakob disease (CJD) is a rapidly progressive and fatal neurodegenerative prion disease. MRI findings are included in diagnostic criteria for probable CJD, giving a sensitivity and specificity more than 90%, but the atypical radiological presentations in the early stage of the disease could cause the diagnostic difficulties. CJD can be definitively diagnosed by histopathological confirmation, brain biopsy or at autopsy.

CASE PRESENTATION

We present a case of 53-year-old woman with a history of a rapidly progressive dementia with symptoms of visual impairment, increased extrapyramidal type muscle tonus, stereotypical movements and ataxic gait resulting in the patient's death after13 months. The clinical symptoms deteriorated progressively to myoclonus and akinetic mutism already on the 14th week. The series of diagnostic examinations were done to exclude the possible causes of dementia. Initial MRI evaluation as posterior reversible encephalopathy syndrome (PRES) on the 9th week after the onset of symptoms created us a diagnostic conundrum. Subsequent MRI findings of symmetrical lesions in the basal ganglia (nucleus caudatus, putamen) on the 13th week and EEG with periodic sharp wave complexes (PSWC) in frontal regions on the 18th week allowed us to diagnose the probable sCJD. The histopathological findings after brain biopsy on the 14th week demonstrated the presence of the abnormal prion protein deposits in the grey matter by immunohistochemistry with ICSM35, KG9 and 12 F10 antibodies and confirmed the diagnosis of sCJD.

CONCLUSIONS

In this article we focus our attention on a rare association between radiological PRES syndrome and early clinical stage of sCJD. Although concurrent manifestation of these conditions can be accidental, but the immunogenic or neuropeptide mechanisms could explain such radiological MRI findings. A thorough knowledge of differential diagnostic of PRES may be especially useful in earlier diagnosis of sCJD.

Small-molecule theranostic probes: a promising future in neurodegenerative diseases

Prion diseases are fatal neurodegenerative illnesses, which include Creutzfeldt-Jakob disease in humans and scrapie, chronic wasting disease, and bovine spongiform encephalopathy in animals. They are caused by unconventional infectious agents consisting primarily of misfolded, aggregated, β -sheet-rich isoforms, denoted prions, of the physiological cellular prion protein (PrP(C)). Many lines of evidence suggest that prions (PrP(Sc)) act both as a template for this conversion and as a neurotoxic agent causing neuronal dysfunction and cell death. As such, PrP(Sc) may be considered as both a neuropathological hallmark of the disease and a therapeutic target. Several diagnostic imaging probes have been developed to monitor cerebral amyloid lesions in patients with neurodegenerative disorders (such as Alzheimer's disease, Parkinson's disease, and prion disease). Examples of these probes are Congo red, thioflavin T, and their derivatives. We synthesized a series of styryl derivatives, denoted theranostics, and studied their therapeutic and/or diagnostic potentials. Here we review the salient traits of these small molecules that are able to detect and modulate aggregated forms of several proteins involved in protein misfolding diseases. We then highlight the importance of further studies for their practical implications in therapy and diagnostics.

Sporadic Creutzfeldt-Jakob disease--a review

The objective is to study a patient with sporadic Creutzfeldt-Jakob disease (CJD). The patient, a 70-year-old woman with a history spanning over 1 month, with acute onset, progressive abnormal behavior, and cognitive decline with generalized asymmetrical myoclonic jerking, startle phenomenon, and cortical blindness, was referred to the hospital. On observation of clinical symptoms, metabolic and hematological investigations, MRI (magnetic resonance imaging), and EEG (electroencephalogram) were done. The clinical symptoms, MRI, and diagnostic EEG were suggestive of sporadic CJD. Other metabolic encephalopathies were ruled out. With sodium valproate and clonezepam, her myoclonic jerks improved slightly. As CJD is an incurable disease, no definitive treatment could be given.

Computational studies on prion proteins: effect of Ala(117)-->Val mutation

Molecular dynamics calculations demonstrated the conformational change in the prion protein due to Ala(117)-->Val mutation, which is related to Gerstmann-Sträussler-Sheinker disease, one of the familial prion diseases. Three kinds of model structures of human and mouse prion proteins were examined: (model 1) nuclear magnetic resonance structures of human prion protein HuPrP (125-228) and mouse prion protein MoPrP (124-224), each having a globular domain consisting of three alpha-helices and an antiparallel beta-sheet; (model 2) extra peptides including Ala(117) (109-124 in HuPrP and 109-123 in MoPrP) plus the nuclear magnetic resonance structures of model 1; and (model 3) extra peptides including Val(117) (109-124 in HuPrP and 109-123 in MoPrP) plus the nuclear magnetic resonance structures of model 1. The results of molecular dynamics calculations indicated that the globular domains of models 1 and 2 were stable and that the extra peptide in model 2 tended to form a new alpha-helix. On the other hand, the globular domain of model 3 was unstable, and the beta-sheet region increased especially in HuPrP.

Prion diseases. An overview

Prion disease is the new designation of a group of spongiform encephalopathies, all invariably fatal, which show similar clinical and neuropathological changes. They comprise a range of distinct diseases in both animals and man, and spontaneous, hereditary and transmissible forms are recognized. Until the sudden occurrence in the mid-1980s of an epizootic of a formerly unknown disease, popularly named 'mad cow disease', in cattle in the UK, very little attention had been paid to these rather obscure diseases. Concurrently it was asserted that the disease-causing agent appeared to be a ubiquitous mammalian brain constituent, and the disease mechanism a conformational change of its structure. These events have not only led to a new understanding of these extraordinary diseases, but have also provided insight into both neurodegeneration and disease mechanisms at the molecular level. Moreover, in 1997 the prion concept earned its originator the second Nobel price for medicine within this scientific field. In this introduction and overview of prion diseases, historical and philosophical perspectives are presented along with descriptions of the diseases in both animals and man. Epidemiology, genetics and transmissibility are also covered.

Copper and prion disease

The prion protein is a cell surface glyco-protein expressed by neurones. Its function has remained elusive until it was recently shown to be a copper binding protein. There is now strong evidence that the prion protein has a role in normal brain copper metabolism. Prion protein expression alters copper uptake into cells and enhances copper incorporation into superoxide dismutase. Furthermore the prion protein itself can act as a superoxide dismutase. One aspect of prion disease is the conversion of functional prion protein into an aggregated amyloid. This conversion may alter the function of the prion protein or abolish it. These results suggest that prion disease may involve disturbance to brain copper homeostasis.

Determination of 14-3-3 protein levels in cerebrospinal fluid from Creutzfeldt-Jakob patients by a highly sensitive capture assay

The level of 14-3-3gamma protein was determined in the cerebrospinal fluid (CSF) from patients with Creutzfeldt-Jakob disease (CJD) and non-CJD patients applying a new and fast microplate assay (14-3-3 protein capture assay), based on the binding to a peptide comprising a phosphorylated recognition motif of 14-3-3 protein. The levels of the gamma-isoform of 14-3-3 protein in CSF samples from CJD patients (n=41) were significantly higher than those observed in patients with non-CJD dementias (n=36) suggesting that this capture assay is a reliable method in the diagnosis of CJD. Since this assay allows the direct measurement of 14-3-3 protein in the CSF without prior concentration it is an easy and simple alternative to the conventionally applied immunoblotting procedures.

Novel approaches in diagnosis and therapy of Creutzfeldt-Jakob disease

The scrapie prion protein, PrP(Sc), as well as its peptide fragment, PrP106-126, are toxic on neuronal cells, resulting in cell death by an apoptotic, rather than necrotic mechanism. The apoptotic process of neuronal cells induced by prion protein supports diagnosis and offers potential targets for therapeutic intervention of the prion diseases. Among the cerebrospinal fluid (CSF) proteins, which may serve as markers of neuronal cell death associated with prion diseases, the 14-3-3 protein(s) turned out to be the most promising one. A new sensitive assay allows the detection of even small changes in the normally low levels of these proteins. In vitro, the toxic effects displayed by PrP(Sc) and its peptide fragment can be blocked by antagonists of N-methyl-D-aspartate (NMDA) receptor channels, like Memantine. Also Flupirtine, a non-opiod analgesic drug, which is already in clinical use, was found to display in vitro a strong cytoprotective effect on neurons treated with PrP(Sc) or PrP106-126. This drug acts like a NMDA receptor antagonists, but does not bind to the receptor. Clinical trials on prion diseases with Flupirtine are in progress. Flupirtine was found to enhance the intracellular levels of the antiapoptotic protein Bcl-2 and the antioxidative agent glutathione (GSH). Due to its favourable pharmacokinetic profile, Flupirtine is considered to be a promising drug to prevent neuronal death in Creutzfeldt-Jakob disease (CJD) and other neurodegenerative disorders occurring with age, e.g. Alzheimer's disease.

Transmissible spongiform encephalopathies in humans

Creutzfeldt-Jakob disease (CJD), the first transmissible spongiform encephalopathy (TSE) to be described in humans, occurs in a sporadic, familial, or iatrogenic form. Other TSEs in humans, shown to be associated with specific prion protein gene mutations, have been reported in different parts of the world. These TSEs compose a heterogeneous group of familial diseases that traditionally have been classified as familial CJD, Gerstmann-Sträussler-Scheinker syndrome, or fatal familial insomnia. In 1996, a newly recognized variant form of CJD among young patients (median age, 28 years) with unusual clinical features and a unique neuropathologic profile was reported in the United Kingdom. In the absence of known CJD risk factors or prion protein gene abnormalities, the UK government concluded that the clustering of these cases may represent transmission to humans of the agent causing bovine spongiform encephalopathy. Additional epidemiologic and recent laboratory data strongly support the UK government's conclusion.

Prion protein expression and superoxide dismutase activity

The function of the prion protein (PrPc) remains uncertain. It has been suggested that prion protein expression may aid cellular resistance to oxidative stress by influencing the activity of Cu/Zn superoxide dismutase (Cu,Zn SOD). The activity of Cu,Zn SOD was investigated in mice with different levels of PrPc expression. Increasing levels of PrPc expression were linked to increased levels of Cu,Zn SOD activity. Western-blot and Northern-blot analysis indicated that mice either lacking or overexpressing PrPc had levels of Cu,Zn SOD mRNA equivalent to those expressed in wild-type mice. Mice overexpressing the prion protein had lower levels of resistance to oxidative stress but higher expression levels of glutathione peroxidase, probably due to increased levels of hydrogen peroxide produced by increased Cu,Zn SOD activity. When cells were metabolically labelled with radioactive copper, increased radioactivity was immunoprecipitated with Cu,Zn SOD from mice with higher levels of PrPc. In addition, diethyldithiocarbamate, a copper chelator that inactivates Cu,Zn SOD by capturing copper from the molecule, is more able to inactivate Cu,Zn SOD expressed in animals with higher levels of PrPc. As recent studies have suggested that PrPc may regulate some aspect of copper metabolism, it is suggested that PrPc expression may regulate Cu,Zn SOD activity by influencing copper incorporation into the molecule.

Mossy fibre reorganization in the hippocampus of prion protein null mice

Mice lacking prion protein (PrP-null) are resistant to transmissible spongiform encephalopathies. However, the normal functions of this highly conserved protein remain controversial. This study examines whether PrP-null mice develop normal neuronal pathways, specifically the mossy fibre pathway, within the hippocampus. Timm stained hippocampal sections from the PrP-null group had more granules than the controls in: the granule cell layer, the inner molecular layer of the dentate gyrus, and the infrapyramidal region of CA3. This resembles the mossy fibre collateral and terminal sprouting seen in certain epilepsies. The abnormal connectivity might be predicted to promote epileptiform activity, but extracellular electrophysiological recordings from the granule cell layer revealed a reduced excitability in the PrP-null group, both with and without blockade of GABA(A) receptor-mediated inhibition. We propose that reorganization of neuronal circuity is a feature of PrP-null mice.

Cerebrospinal fluid concentration of neuron-specific enolase in diagnosis of Creutzfeldt-Jakob disease

Neuron-specific enolase (NSE) is among the biochemical markers in cerebrospinal fluid reported to be useful in the differential diagnosis of Creutzfeldt-Jakob disease from other dementing illnesses. In a group of 58 patients with definite and probable Creutzfeldt-Jakob disease, NSE concentrations (median 94.0, interquartile range 256 ng/mL) were significantly higher (p < 0.001) than in 26 control patients (9.5, 15.5 ng/mL). At a cut-off of 35 ng/mL an optimum sensitivity of 80% with a specificity of 92% for the diagnosis of Creutzfeldt-Jakob disease by NSE in cerebrospinal fluid was obtained.