Transmissible spongiform encephalopathies: the story of a pathogenic protein

Emerging and re-emerging infections at the turn of the millennium

After World War II, mankind believed that infectious diseases were on the way to being defeated. Unfortunately, they still are the second worldwide cause of death. Globalization changes promote the emergence of new infections and pandemics; international deliveries and travelling facilitate the dissemination of infectious agents; man-induced environmental changes create new opportunities for contacts between species, leading to infections in aberrant hosts, including man; global warming enables insects, a major vector of pathogens, to thrive in more countries. The main pandemics have been caused by viruses, such as HIV and novel subtypes of influenza viruses. In addition, prion proteins are a threat. The transmission of the Creutzfeld Jakob disease variant through blood transfusion and the recent discovery of prion protein in the spleen of a haemophilia patient are a matter of further concern. The end of the war against infectious diseases is not in sight. Mankind's battle with pathogens has lasted millennia and is destined to continue.

Squalestatin alters the intracellular trafficking of a neurotoxic prion peptide

Background

Neurotoxic peptides derived from the protease-resistant core of the prion protein are used to model the pathogenesis of prion diseases. The current study characterised the ingestion, internalization and intracellular trafficking of a neurotoxic peptide containing amino acids 105–132 of the murine prion protein (MoPrP105-132) in neuroblastoma cells and primary cortical neurons.

Results

Fluorescence microscopy and cell fractionation techniques showed that MoPrP105-132 co-localised with lipid raft markers (cholera toxin and caveolin-1) and trafficked intracellularly within lipid rafts. This trafficking followed a non-classical endosomal pathway delivering peptide to the Golgi and ER, avoiding classical endosomal trafficking via early endosomes to lysosomes. Fluorescence resonance energy transfer analysis demonstrated close interactions of MoPrP105-132 with cytoplasmic phospholipase A₂ (cPLA₂) and cyclo-oxygenase-1 (COX-1), enzymes implicated in the neurotoxicity of prions. Treatment with squalestatin reduced neuronal cholesterol levels and caused the redistribution of MoPrP105-132 out of lipid rafts. In squalestatin-treated cells, MoPrP105-132 was rerouted away from the Golgi/ER into degradative lysosomes. Squalestatin treatment also reduced the association between MoPrP105-132 and cPLA₂/COX-1.

Conclusion

As the observed shift in peptide trafficking was accompanied by increased cell survival these studies suggest that the neurotoxicity of this PrP peptide is dependent on trafficking to specific organelles where it activates specific signal transduction pathways.

Classification of sporadic Creutzfeldt-Jakob disease based on clinical and neuropathological characteristics

Creutzfeldt-Jakob disease (CJD) is a rare and fatal neurodegenerative disease of unknown cause. Patients are usually aged between 50 and 75 and typical clinical features include rapidly progressive dementia associated with myoclonus and a characteristic electroencephalographic pattern. Neuropathological examination reveals cortical spongiform change, hence the term 'spongiform encephalopathy'. Several statistical techniques were applied to classify patients with sporadic CJD (sCJD), based on clinical and neuropathological investigation. We focus on the classification of neuropathologically confirmed sCJD patients. In order to obtain a classification rule that correctly classifies this type of patients and at the same time controls the overall error rate, we apply several classification techniques, which in general, produce comparable results. The boosting method produces the best results and the variable 14-3-3 protein in cerebrospinal fluid plays the most important role in the prediction of neuropathologically confirmed sCJD.

Inducible proteopathies

Numerous degenerative diseases are characterized by the aberrant polymerization and accumulation of specific proteins. These proteopathies include neurological disorders such as Alzheimer's disease, Parkinson's disease, Huntington's disease and the prion diseases, in addition to diverse systemic disorders, particularly the amyloidoses. The prion diseases have been shown to be transmissible by an alternative conformation of the normal cellular prion protein. Other proteopathies have been thought to be non-transmissible, but there is growing evidence that some systemic and cerebral amyloidoses can be induced by exposure of susceptible hosts to cognate molecular templates. As we review here, the mechanistic similarities among these diseases provide unprecedented opportunities for elucidating the induction of protein misfolding and assembly in vivo, and for developing an integrated therapeutic approach to degenerative proteopathies.

Koch's postulates and infectious proteins

Koch's postulates were formulated in the late nineteenth century as guidelines for establishing that microbes cause specific diseases. Because the rules were developed for living agents--particularly bacteria--their applicability to inanimate pathogens such as viruses and infectious proteins has been problematic. The unorthodox mechanism by which prion diseases are transmitted, involving specific physicochemical characteristics of the protein as well as susceptibility traits of the host, has made these disorders refractory to analysis within the context of the original Koch's postulates. In addition, evidence is accumulating that other proteopathies, such as AA amyloidosis, apolipoprotein AII amyloidosis, and cerebral Abeta amyloidosis, can be induced in vulnerable recipients by cognate proteinaceous agents. In light of the salient differences in the mode of disease-transmission by microbes and proteins, we propose modifications of Koch's postulates that will specifically accommodate presumed infectious proteins.