Molecular biology and genetics of neurodegenerative diseases caused by prions

POSCAbilities: The Application of the Prion Organotypic Slice Culture Assay to Neurodegenerative Disease Research

Prion diseases are fatal, transmissible neurodegenerative disorders whose pathogenesis is driven by the misfolding, self-templating and cell-to-cell spread of the prion protein. Other neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis and Huntington’s disease, share some of these prion-like features, with different aggregation-prone proteins. Consequently, researchers have begun to apply prion-specific techniques, like the prion organotypic slice culture assay (POSCA), to these disorders. In this review we explore the ways in which the prion phenomenon has been used in organotypic cultures to study neurodegenerative diseases from the perspective of protein aggregation and spreading, strain propagation, the role of glia in pathogenesis, and efficacy of drug treatments. We also present an overview of the advantages and disadvantages of this culture system compared to in vivo and in vitro models and provide suggestions for new directions.

Familial Creutzfeldt-Jakob disease. Codon 200 prion disease in Libyan Jews

Creutzfeldt-Jakob disease (CJD) is the most prevalent of the human prion diseases, a group of fatal neurodegenerative disorders afflicting both humans and animals. The unique characteristic of these diseases, whether sporadic, dominantly inherited, or acquired by transmission, is the accumulation in the brain of an abnormal isoform (PrPSc) of the cellular prion protein (PrPc). Progress has been made in understanding inherited prion diseases by genetically linking clusters of familial CJD (fCJD) to mutations of the PrP gene (PRNP). One of the largest clusters of fCJD exists among Jews of Libyan origin. The clinical and pathologic manifestations of CJD in this community resemble those seen with sporadic CJD (sCJD), but the incidence is about 100 times higher than in the general population. Initially, this high incidence was attributed to infection via consumption of sheep brains or eyeballs, but a mutation at codon 200 in PRNP resulting in the substitution of lysine (K) for glutamate (E), designated E200K, was identified in this population. The onset of fCJD (E200K) is age dependent and shows nearly complete penetrance by age 85 years. fCJD in Libyan Jews is invariably associated with accumulation of the pathologic isoform PrPSc in the central nervous system. Using mutation-specific antibodies, it was shown that most PrPSc in the brain of these patients originated from the mutant protein. Some studies suggest that mutant PrP may accumulate in brain and other organs due to an impaired degradation, and its accumulation has been postulated to promote conversion into PrPSc. fCJD (E200K) has been transmitted to primates and transgenic mice, highlighting the need to address ethical and public health issues surrounding the possibility of human to human transmission.

Separating the environmental and genetic factors that may be causes of bovine spongiform encephalopathy

The initial cause of the bovine spongiform encephalopathy (BSE) epidemic is generally accepted to have been the feeding of infected animal protein to cattle. The proportion of animals affected in any year in a particular herd has generally been low. This suggests either considerable variation in the extent of challenge of the individual animals or variation in their susceptibility to challenge or both. There is known to be genetic variation in susceptibility in other spongiform encephalopathies, such as scrapie in sheep. However, earlier indications that there may be associations between the incidence of BSE in cattle and polymorphisms and mutations in the PrP gene have not been confirmed (Hunter et al. 1994). Here, we attempt to model the likely extent of challenge of the individual animals in five Holstein Friesian pedigree herds and also the distribution of incubation times to the date of clinical onset. By studying the incidence of the disease in related animals we first found that single locus genetic models fitted the data much better than a non-genetic model. This was the first statistical evidence found of genetic variation in susceptibility to BSE. A check on the model in which individual animals were randomly allocated to 'parents' showed that the result was due to the lack of allowance in the non-genetic model for those animals insufficiently challenged or, for non-genetic reasons, resistant to their level of challenge. Thus there is still no evidence, molecular or statistical, for genetic variation in susceptibility. The importance of checking the attribution of genetic effects in complex models by the random allocation of progeny to parents is clear.

PrP-deficient mice are resistant to scrapie

Prusiner proposed that the infectious agent of scrapie, the prion, is PrPSc, a modified form of the normal host protein PrPC. Prn-p0/0 mice devoid of PrPC showed normal development and behavior. When inoculated with mouse scrapie prions they remained free of scrapie symptoms for at least 16 months while wild type controls all died within 6 months. Propagation of infectivity in the PrP null mice, if any, was less than 10(-5) that in wild type animals. Surprisingly, heterozygous Prn-p0/+ mice also showed enhanced resistance to scrapie. After introduction of Syrian hamster PrP transgenes, Prn-p0/0 mice became highly susceptible to hamster but not to mouse prions. These experiments show that PrPC, possibly at close to normal levels, is required for the usual susceptibility to scrapie and that lack of homology between incoming prions and the host's PrP genes retards disease.

Susceptibility to scrapie in mice is dependent on PrPC

Mice devoid of functional PrP genes (Prn-p(o/o) mice) showed normal development and behaviour. When inoculated with mouse scrapie prions they remained free of scrapie symptoms for at least 18 months whereas wild-type controls all died within 6 months. No propagation of infectivity could be detected in the PrP null mice. Surprisingly, heterozygous Prn-p(o/+) mice also showed enhanced resistance to scrapie. After introduction of Syrian hamster PrP transgenes, Prn-p(o/o) mice became highly susceptible to hamster but not to mouse prions. These experiments show that PrPC, possibly at close to normal levels, is required for the usual susceptibility to scrapie and that lack of homology between incoming prions and the host's PrP genes retards disease.