Current perspectives on bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease

Iron content of ferritin modulates its uptake by intestinal epithelium: implications for co-transport of prions

The spread of Chronic Wasting Disease (CWD) in the deer and elk population has caused serious public health concerns due to its potential to infect farm animals and humans. Like other prion disorders such a sporadic Creutzfeldt-Jakob-disease of humans and Mad Cow Disease of cattle, CWD is caused by PrP-scrapie (PrP^Sc), a β-sheet rich isoform of a normal cell surface glycoprotein, the prion protein (PrP^C). Since PrP^Sc is sufficient to cause infection and neurotoxicity if ingested by a susceptible host, it is important to understand the mechanism by which it crosses the stringent epithelial cell barrier of the small intestine. Possible mechanisms include co-transport with ferritin in ingested food and uptake by dendritic cells. Since ferritin is ubiquitously expressed and shares considerable homology among species, co-transport of PrP^Sc with ferritin can result in cross-species spread with deleterious consequences. We have used a combination of in vitro and in vivo models of intestinal epithelial cell barrier to understand the role of ferritin in mediating PrP^Sc uptake and transport. In this report, we demonstrate that PrP^Sc and ferritin from CWD affected deer and elk brains and scrapie from sheep resist degradation by digestive enzymes, and are transcytosed across a tight monolayer of human epithelial cells with significant efficiency. Likewise, ferritin from hamster brains is taken up by mouse intestinal epithelial cells in vivo, indicating that uptake of ferritin is not limited by species differences as described for prions. More importantly, the iron content of ferritin determines its efficiency of uptake and transport by Caco-2 cells and mouse models, providing insight into the mechanism(s) of ferritin and PrP^Sc uptake by intestinal epithelial cells.

The impact of social amplification and attenuation of risk and the public reaction to mad cow disease in Canada

Following the detection of bovine spongiform encephalopathy (BSE) in Canada, and subsequently in the United States, confidence in the safety of beef products remained high. Consumers actually increased their consumption of beef slightly after the news of an increased risk from mad cow disease, which has been interpreted as public support for beef farmers and confidence in government regulators. The Canadian public showed a markedly different reaction to the news of domestic BSE than the furious and panicked responses observed in the United Kingdom, Germany, and Japan. Using the social amplification of risk framework, we show that, while other countries displayed social amplification of risk, Canada experienced a social attenuation of risk. The attenuated reaction in Canada toward mad cow disease and increased human health risks from variant Creutzfeldt-Jakob disease (vCJD) was due to the social context at the time when BSE was discovered domestically. Mortality, morbidity, and psychosocial impacts resulting from other major events such as severe acute respiratory syndrome (SARS), West Nile virus (WNV), and the U.S.-Iraq war made the theoretical risks of BSE and vCJD a lower priority, reducing its concern as a risk issue.

Prophylactic effect of dietary seaweed Fucoidan against enteral prion infection

Dietary seaweed fucoidan delays the onset of disease of enterally infected mice with scrapie when given orally for 6 days after infection, but not when given before the infection. This effect was not modified at a tested fucoidan dose range and appeared to reach the maximum level at a concentration of 2.5% or less in feed. Daily uptake of fucoidan might be prophylactic against prion diseases caused by ingestion of prion-contaminated materials, although further evaluation of its pharmacology remains to be done.

Prions: Who Should Worry about Them?

Prion diseases, also called transmissible spongiform encephalopathies (TSEs), are a family of neurodegenerative disorders affecting both humans and animals. They are caused by the accumulation of an abnormal form of a protein known as prion that results in neuronal death and a characteristic spongiform appearance of the brain tissue. Human prion diseases can be sporadic, acquired or hereditary. Acquired prion diseases have been linked to entering contaminated food into the human food chain, failure to completely disinfect or sterilize contaminated surgical instruments, patients receiving tissues and organs from infected donors, recipients of blood and other biological contaminated products, and potentially to cross infection in dental procedures. At present, there is unfortunately no efficient therapy that can be administered to clinically infected patients with prion diseases. Moreover, there are no simple diagnostic tests that can be used to show the agent of transmissible spongiform encephalopathy during the preclinical phase of the disease. Therefore, to prevent the spread of this emerging infectious agent it is necessary to implement several health control strategies and maintain surveillance for subclinical infections.

Protease-resistant human prion protein and ferritin are cotransported across Caco-2 epithelial cells: implications for species barrier in prion uptake from the intestine

Foodborne transmission of bovine spongiform encephalopathy (BSE) to humans as variant Creutzfeldt-Jakob disease (CJD) has affected over 100 individuals, and probably millions of others have been exposed to BSE-contaminated food substances. Despite these obvious public health concerns, surprisingly little is known about the mechanism by which PrP-scrapie (PrP(Sc)), the most reliable surrogate marker of infection in BSE-contaminated food, crosses the human intestinal epithelial cell barrier. Here we show that digestive enzyme (DE) treatment of sporadic CJD brain homogenate generates a C-terminal fragment similar to the proteinase K-resistant PrP(Sc) core of 27-30 kDa implicated in prion disease transmission and pathogenesis. Notably, DE treatment results in a PrP(Sc)-protein complex that is avidly transcytosed in vesicular structures across an in vitro model of the human intestinal epithelial cell barrier, regardless of the amount of endogenous PrP(C) expression. Unexpectedly, PrP(Sc) is cotransported with ferritin, a prominent component of the DE-treated PrP(Sc)-protein complex. The transport of PrP(Sc)-ferritin is sensitive to low temperature, brefeldin-A, and nocodazole treatment and is inhibited by excess free ferritin, implicating a receptor- or transporter-mediated pathway. Because ferritin shares considerable homology across species, these data suggest that PrP(Sc)-associated proteins, in particular ferritin, may facilitate PrP(Sc) uptake in the intestine from distant species, leading to a carrier state in humans.

Variant Creutzfeldt–Jakob disease: between lymphoid organs and brain

Prion diseases are often caused by peripheral uptake of the infectious agent. To reach their ultimate target, the central nervous system (CNS), prions enter their host, replicate in lymphoid organs and spread via peripheral nerves. Once the agent has reached the CNS disease progression is rapid, resulting in neurodegeneration and death. many of these mechanisms have been uncovered using genetically modified mice. A recently published study demonstrated the presence of pathological prion protein in sympathetic ganglia of patients suffering from variant Creutzfeldt-Jakob disease, suggesting that these mechanisms might apply to humans.

Dimethyl sulfoxide delays PrP sc accumulation and disease symptoms in prion-infected hamsters

PrP(Sc), an aberrantly folded protein, is the only identified component of the prion, an agent causing fatal neurodegenerative diseases such as scrapie and bovine spongiform encephalopathy. Dimethyl sulfoxide (DMSO) has been shown to reduce the accumulation of PrP(Sc) in scrapie-infected (ScN2a) cells, and to inhibit its aggregation in vitro. In humans, DMSO was used successfully in the treatment of various peripheral amyloidotic diseases. Here we show that administration of DMSO to scrapie-infected hamsters significantly prolonged disease incubation time, as well as delayed the accumulation of PrP(Sc) in Syrian hamster brains. Interestingly, administration of DMSO to scrapie sick hamsters resulted in increased clearance of protease-resistant PrP in their urine. We conclude that although DMSO by itself may not be sufficient to cure prion diseases, it may be considered as a component in a 'cocktail' drug approach for these disorders. Also, urine PrP testing should be considered for the assessment of treatment efficacy.

Unexpected high testis-specific transcriptional activity of the cyclin T1 promoter in transgenic mice

The ubiquitously expressed cyclin T1 gene encodes for a protein involved in human immunodeficiency virus type 1 (HIV-1) transcription activation. The goat gene was recently shown to share an expression pattern similar to that of its endogenous counterpart when incorporated into mice using a BAC insert. To assess if its promoter could target ubiquitous expression of the bovine Prnp in transgenic mice, two constructs carrying either 1 or 30 kb of cyclin T1 5'-flanking sequences were built and microinjected. Both constructs resulted in the unexpected high male germ cell-specific expression of the prion protein. These data re-question the suspected location of the cyclin T1 gene regulatory elements.

The new variant of the Creutzfeldt-Jakob disease accounts for no relative increase of the Creutzfeldt-Jakob disease mortality rate in the United Kingdom; this fits ill with the new variant being the consequence of consumption of food infected with the agent of Bovine Spongiform Encephalopathy

BACKGROUND

A new variant of Creutzfeldt-Jakob disease was described in the United Kingdom. It is often claimed that it is caused by consumption of food infected with the agent of bovine spongiform encephalopathy. However, this remains open to question because the number of cases of the variant is, at the present time, less than would be expected from a major food-borne source.

DISCUSSION

The EUROCJD cooperative study presents currently available epidemiological data of Creutzfeldt-Jakob disease and its new variant, for nine European countries plus Australia and Canada. Unexpectedly, for the United Kingdom where all but a few cases of the new variant have been reported, these cases have to be included in the incidence curve of the sporadic forms of the disease in order to obtain the best fit with the median curve from all the countries. This variant could be merely a rare clinical phenotype within the sporadic disease. The published clinical and experimental data which suggest that it is linked with bovine spongiform encephalopathy, lead us to propose that this link could be a common etiological origin other than consumption of bovine infected food. In any case, public health recommendations hold and further investigation is required.

SUMMARY

The lack of a relative increase of the Creutzfeldt-Jakob-disease mortality rate in the United Kingdom, does not fit well with the new variant being the consequence of consumption of food infected with the agent of bovine spongiform encephalopthy.

Efficient and specific down-regulation of prion protein expression by RNAi

Prion diseases are fatal neurodegenerative disorders associated with an abnormal isoform of the PrPc host-encoded protein. Invalidation of the Prnp gene, that encodes PrPc, led to transgenic mice that are viable, apparently healthy, and resistant to challenge by the infectious agent. These results indicated that a down-regulation of the Prnp gene expression is a potential therapeutic approach. In the present report, we demonstrate that RNAi targeted towards the Prnp mRNA can efficiently and highly specifically reduce the level of PrPc in transfected cells. It, thus, indicates that RNAi is an attractive therapeutic approach to fight against prion diseases.

Preventing accidental transmission of human transmissible spongifom encephalopathies

The mechanism by which humans became infected with the BSE agent is discussed, and the matter of potential person-to person transmission of TSEs through contagion or medical procedures is considered. There is some discussion regarding the current evidence relating to whether or not the blood of humans infected with TSEs is infectious. Considerable emphasis is placed on the fact that TSE agents are known to be relatively resistant to decontamination by procedures that are effective with conventional micro-organisms, including (under worst-case conditions) the autoclaving procedures used to sterilise surgical instruments. Methods for providing additional re-assurance with regard to the safety of instruments are described. Safety in the pathology laboratory is discussed extensively because TSE agents are not inactivated by the usual processes used to fix tissues, and such laboratories will receive fixed tissues that are still highly infectious as far as TSE agents are concerned.