Bovine Spongiform Encephalopathy - A Review from the Perspective of Food Safety

Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative disease that belongs to transmissible spongiform encephalopathy (TSE). Since the first case was identified in the UK in 1986, BSE spread to other countries including Japan. Its incidence peaked in 1992 in the UK and from 2001 to 2006 in many other countries, but a feed ban aimed at eliminating the recycling of the BSE agent and other control measures aimed at preventing food and feed contamination with the agent were highly effective at reducing the spread of BSE. In 2004, two types of atypical BSE, H-type BSE (H-BSE) and L-type BSE (L-BSE), which differ from classical BSE (C-BSE), were found in France and Italy. Atypical BSE, which is assumed to occur spontaneously, has also been detected among cattle in other countries including Japan. The BSE agent including atypical BSE agent is a unique food-safety hazard with different chemical and biological properties from the microbial pathogens and toxic chemicals that contaminate food. In this review, we summarize the reported findings on the tissue distribution of BSE prions in infected cattle and other aspects of BSE, as well as the control measures against the disease employed in Japan. Topics that require further studies are discussed based on the summarized findings from the perspective of food safety.

Transmission of scrapie prions to primate after an extended silent incubation period

Classical bovine spongiform encephalopathy (c-BSE) is the only animal prion disease reputed to be zoonotic, causing variant Creutzfeldt-Jakob disease (vCJD) in humans and having guided protective measures for animal and human health against animal prion diseases. Recently, partial transmissions to humanized mice showed that the zoonotic potential of scrapie might be similar to c-BSE. We here report the direct transmission of a natural classical scrapie isolate to cynomolgus macaque, a highly relevant model for human prion diseases, after a 10-year silent incubation period, with features similar to those reported for human cases of sporadic CJD. Scrapie is thus actually transmissible to primates with incubation periods compatible with their life expectancy, although fourfold longer than BSE. Long-term experimental transmission studies are necessary to better assess the zoonotic potential of other prion diseases with high prevalence, notably Chronic Wasting Disease of deer and elk and atypical/Nor98 scrapie.

Foodborne-Transmitted Prions From the Brain of Cows With Bovine Spongiform Encephalopathy Ascend in Afferent Neurons to the Simian Central Nervous System and Spread to Tonsils and Spleen at a Late Stage of the Incubation Period

BACKGROUND

Protease-resistant prion protein (PrP(res)) accumulation in lymphoreticular tissues indicates prion infection. To date, tonsillectomy and appendectomy samples have been used in population prevalence surveys to detect clinically silent carriers of variant Creutzfeldt-Jakob disease (vCJD). However, the temporal sequence of prion spread in the human body is still not known. We therefore traced the temporal-spatial pattern of PrP(res) accumulation in the body of a simian vCJD model.

METHODS

Cynomolgus monkeys were fed brain of (eleven) cows with bovine spongiform encephalopathy, and some were euthanized before and some after onset of neurological signs. PrP(res) was detected in tissues by a paraffin-embedded tissue blot technique and a semiquantitative Western immunoblot assay.

RESULTS

Bovine spongiform encephalopathy (BSE)-associated prions were preferentially transported from the gut to the central nervous system (CNS) along sensory nerve fibers and initially entered the simian CNS at lumbar spinal cord levels. In asymptomatic animals, we found BSE in 50% and 12% of gut- and tonsil-derived samples, respectively.

CONCLUSIONS

Unlike in rodents and ruminants, foodborne BSE-associated prions entered the simian CNS via afferent neurons. From sites of initial CNS invasion, prions spread centrifugally to tonsils and spleen at an advanced stage of the incubation period, thus explaining why tonsil specimens were not reliable for detection of simian disease carriers before onset of clinical signs.

Foodborne transmission of bovine spongiform encephalopathy to non-human primates results in preclinical rapid-onset obesity

Obesity has become one of the largest public health challenges worldwide. Recently, certain bacterial and viral pathogens have been implicated in the pathogenesis of obesity. In the present study, we retrospectively analyzed clinical data, plasma samples and post-mortem tissue specimens derived from a risk assessment study in bovine spongiform encephalopathy (BSE)-infected female cynomolgus monkeys (Macaca fascicularis). The original study design aimed to determine minimal infectious doses after oral or intracerebral (i.c.) infection of macaques to assess the risk for humans. High-dose exposures resulted in 100% attack rates and a median incubation time of 4.7 years as described previously. Retrospective analyses of clinical data from high-dosed macaques revealed that foodborne BSE transmission caused rapid weight gain within 1.5 years post infection (β = 0.915; P<0.0001) which was not seen in age- and sex-matched control animals or i.c. infected animals. The rapid-onset obesity was not associated with impaired pancreatic islet function or glucose metabolism. In the early preclinical phase of oral transmission associated with body weight gain, prion accumulation was confined to the gastrointestinal tract. Intriguingly, immunohistochemical findings suggest that foodborne BSE transmission has a pathophysiological impact on gut endocrine cells which may explain rapid weight gain. To our knowledge, this is the first experimental model which clearly demonstrates that foodborne pathogens can induce obesity.

Ultrasensitive detection of PrP(Sc) in the cerebrospinal fluid and blood of macaques infected with bovine spongiform encephalopathy prion

Prion diseases are characterized by the prominent accumulation of the misfolded form of a normal cellular protein (PrP(Sc)) in the central nervous system. The pathological features and biochemical properties of PrP(Sc) in macaque monkeys infected with the bovine spongiform encephalopathy (BSE) prion have been found to be similar to those of human subjects with variant Creutzfeldt-Jakob disease (vCJD). Non-human primate models are thus ideally suited for performing valid diagnostic tests and determining the efficacy of potential therapeutic agents. In the current study, we developed a highly efficient method for in vitro amplification of cynomolgus macaque BSE PrP(Sc). This method involves amplifying PrP(Sc) by protein misfolding cyclic amplification (PMCA) using mouse brain homogenate as a PrP(C) substrate in the presence of sulfated dextran compounds. This method is capable of amplifying very small amounts of PrP(Sc) contained in the cerebrospinal fluid (CSF) and white blood cells (WBCs), as well as in the peripheral tissues of macaques that have been intracerebrally inoculated with the BSE prion. After clinical signs of the disease appeared in three macaques, we detected PrP(Sc) in the CSF by serial PMCA, and the CSF levels of PrP(Sc) tended to increase with disease progression. In addition, PrP(Sc) was detectable in WBCs at the clinical phases of the disease in two of the three macaques. Thus, our highly sensitive, novel method may be useful for furthering the understanding of the tissue distribution of PrP(Sc) in non-human primate models of CJD.

Gene expression profiling of brains from bovine spongiform encephalopathy (BSE)-infected cynomolgus macaques

BACKGROUND

Prion diseases are fatal neurodegenerative disorders whose pathogenesis mechanisms are not fully understood. In this context, the analysis of gene expression alterations occurring in prion-infected animals represents a powerful tool that may contribute to unravel the molecular basis of prion diseases and therefore discover novel potential targets for diagnosis and therapeutics. Here we present the first large-scale transcriptional profiling of brains from BSE-infected cynomolgus macaques, which are an excellent model for human prion disorders.

RESULTS

The study was conducted using the GeneChip® Rhesus Macaque Genome Array and revealed 300 transcripts with expression changes greater than twofold. Among these, the bioinformatics analysis identified 86 genes with known functions, most of which are involved in cellular development, cell death and survival, lipid homeostasis, and acute phase response signaling. RT-qPCR was performed on selected gene transcripts in order to validate the differential expression in infected animals versus controls. The results obtained with the microarray technology were confirmed and a gene signature was identified. In brief, HBB and HBA2 were down-regulated in infected macaques, whereas TTR, APOC1 and SERPINA3 were up-regulated.

CONCLUSIONS

Some genes involved in oxygen or lipid transport and in innate immunity were found to be dysregulated in prion infected macaques. These genes are known to be involved in other neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Our results may facilitate the identification of potential disease biomarkers for many neurodegenerative diseases.

Foodborne transmission of bovine spongiform encephalopathy to nonhuman primates

Risk for human exposure to bovine spongiform encephalopathy (BSE)–inducing agent was estimated in a nonhuman primate model. To determine attack rates, incubation times, and molecular signatures, we orally exposed 18 macaques to 1 high dose of brain material from cattle with BSE. Several macaques were euthanized at regular intervals starting at 1 year postinoculation, and others were observed until clinical signs developed. Among those who received ≥5 g BSE-inducing agent, attack rates were 100% and prions could be detected in peripheral tissues from 1 year postinoculation onward. The overall median incubation time was 4.6 years (3.7–5.3). However, for 3 macaques orally exposed on multiple occasions, incubation periods were at least 7–10 years. Before clinical signs were noted, we detected a non-type 2B signature, indicating the existence of atypical prion protein during the incubation period. This finding could affect diagnosis of variant Creutzfeldt-Jakob disease in humans and might be relevant for retrospective studies of positive tonsillectomy or appendectomy specimens because time of infection is unknown.

Evaluation of the zoonotic potential of transmissible mink encephalopathy

Successful transmission of Transmissible Mink Encephalopathy (TME) to cattle supports the bovine hypothesis for the still controversial origin of TME outbreaks. Human and primate susceptibility to classical Bovine Spongiform Encephalopathy (c-BSE) and the transmissibility of L-type BSE to macaques indicate a low cattle-to-primate species barrier. We therefore evaluated the zoonotic potential of cattle-adapted TME. In less than two years, this strain induced in cynomolgus macaques a neurological disease similar to L-BSE but distinct from c-BSE. TME derived from another donor species (raccoon) induced a similar disease with even shorter incubation periods. L-BSE and cattle-adapted TME were also transmissible to transgenic mice expressing human prion protein (PrP). Secondary transmissions to transgenic mice expressing bovine PrP maintained the features of the three tested bovine strains (cattle TME, c-BSE and L-BSE) regardless of intermediate host. Thus, TME is the third animal prion strain transmissible to both macaques and humanized transgenic mice, suggesting zoonotic potentials that should be considered in the risk analysis of animal prion diseases for human health. Moreover, the similarities between TME and L-BSE are highly suggestive of a link between these strains, and therefore the possible presence of L-BSE for many decades prior to its identification in USA and Europe.

Spontaneous Obesity-Linked Type 2 Diabetes in the Absence of Islet Amyloid in a Cynomolgus Monkey Infected With Bovine Spongiform Encephalopathy

A 9-year-old cynomolgus monkey ( Macaca fascicularis) infected orally with bovine spongiform encephalopathy (BSE) was presented for necropsy following euthanasia 4 years post infection (p.i.). This macaque R984 was exposed to a BSE dose that causes a simian form of variant Creutzfeldt-Jakob disease (vCJD) within 5 years p.i. in other macaques. All orally BSE-infected macaques developed a significant weight gain within the first 2 years p.i. compared with non-BSE-infected age- and sex-matched control animals, suggesting increased risk of type 2 diabetes (T2D). In contrast, macaque R984 developed rapid weight loss, hyperglycemia, and glucosuria and had to be euthanatized 4 years p.i. before clinical signs of vCJD. Pancreas histopathological evaluation revealed severe islet degeneration but, remarkably, no islet amyloid deposits were present. Immunostaining of pancreas sections for insulin and glucagon confirmed the loss of endocrine cells. In addition, prions were present in the adenohypophysis but not in other areas of the brain, indicating centripetal prion spread from the gut during the preclinical phase of BSE infection. Plasma glucose and insulin concentrations of macaque R984 became abnormal with age and resembled T2D. This unusual case of spontaneous T2D in the absence of islet amyloid deposits could have been due to early prion spread from the periphery to the endocrine system or could have occurred spontaneously.

14-3-3 proteins as potential therapeutic targets

The 14-3-3 family of phosphoserine/phosphothreonine-binding proteins dynamically regulates the activity of client proteins in various signaling pathways that control diverse physiological and pathological processes. In response to environmental cues, 14-3-3 proteins orchestrate the highly regulated flow of signals through complex networks of molecular interactions to achieve well-controlled physiological outputs, such as cell proliferation or differentiation. Accumulating evidence now supports the concept that either an abnormal state of 14-3-3 protein expression, or dysregulation of 14-3-3/client protein interactions, contributes to the development of a large number of human diseases. In particular, clinical investigations in the field of oncology have demonstrated a correlation between upregulated 14-3-3 levels and poor survival of cancer patients. These studies highlight the rapid emergence of 14-3-3 proteins as a novel class of molecular target for potential therapeutic intervention. The current status of 14-3-3 modulator discovery is discussed.

Comparison of mRNA expression levels of selected genes in the brain stem of cattle naturally infected with classical and atypical BSE

Since 2004 cases of atypical bovine spongiform encephalopathy (BSE) in older cattle are recorded on the basis of aberrant glycoprofiles of prion protein resistant to proteolysis (PrP(res)). The nature of those types of PrP(res) is still not fully understood but the epidemiological data indicate that their occurrence is rare. Hitherto, most BSE cases were studied on the basis of the features of pathological form of prion protein (PrP(Sc)) or lesions observed in the gray matter of the brain. Here we propose the gene expression profiling as a method to characterize and distinguish BSE types. Thus, the aim of the study was to compare the activity of some genes which are known to play a role in the pathogenesis of transmissible spongiform encephalopathies (TSEs). Significant differences in the expression level of the selected genes in the brain stem were observed for 7 out of 11 genes tested when the results for BSE affected and healthy control animals were compared. Significant up-regulation of caspase 3, Bax and 14-3-3 protein encoding genes was apparent in the obex of all BSE affected cattle regardless of the prion type. Significant and unique to BSE H-type up-regulation was detected in prion and SOD1 genes, while BSE C-type was characterized by higher Bcl-2 and Fyn gene expression levels in respect to other BSE types and control animals. Different gene expression profiles of bovine brains infected with classical and atypical BSE indicate possible different pathogenesis or origin of the disease.

Reduction of PrP(C) in human cerebrospinal fluid after spinal cord injury

It has been estimated that cerebrospinal fluid (CSF) contains approximately 80 proteins that significantly increase or decrease in response to various clinical conditions. Here we have evaluated the CSF protein PrP(C) (cellular prion protein) for possible increases or decreases following spinal cord injury. The physiological function of PrP(C) is not yet completely understood; however, recent findings suggest that PrP(C) may have neuroprotective properties. Our results show that CSF PrP(C) is decreased in spinal cord injured patients 12 h following injury and is absent at 7 days. Given that normal PrP(C) has been proposed to be neuroprotective we speculate that the decrease in CSF PrP(C) levels may influence neuronal cell survival following spinal cord injury.

Inducible expression of chimpanzee prion protein (PrP) in murine PrP knock-out cells

In transmissible spongiform encephalopathy (TSE) pathogenesis the cellular prion protein (PrP(C)) is converted into its pathogenic PrP(Sc) isoform. Prion protein gene (Prnp) deficient mice (PrP(0/0)) are resistant to PrP(Sc) infection, but following reconstitution of Prnp they regain their susceptibility to infection. Therefore, it is challenging to simulate this natural situation in a cell culture model. We have previously reported the inducible stable expression of a human PrP(C) in murine 3T3 cells. In this study, we used murine PrP(0/0) cells stably expressing exemplarily the chimpanzee Prnp under the control of inducible tetracycline (Tet) system. The Prnp was integrated using a lentiviral vector. Its expression in the engineered PrP(0/0)Chimp1/Tet-Off cell line was analyzed by Western blot (Wb) and fluorescence activated cell sorting (FACS) analyses. PrP(C) was partially purified by using immobilized metal affinity chromatography (IMAC). Compared to all the other cell systems which possess an endogenous PrP(C) expression, here described cell line contains only an overexpressing species specific PrP(C) expression which is tightly regulated and can be turned-off at any time without showing any endogenous host PrP(C) expression. Consequently, a contamination of the isolated PrP(C) is impossible. This cell line potentially offers a new tool for simulation of mice bioassays widely used in TSE infection studies.

Atypical BSE (BASE) transmitted from asymptomatic aging cattle to a primate

Background

Human variant Creutzfeldt-Jakob Disease (vCJD) results from foodborne transmission of prions from slaughtered cattle with classical Bovine Spongiform Encephalopathy (cBSE). Atypical forms of BSE, which remain mostly asymptomatic in aging cattle, were recently identified at slaughterhouses throughout Europe and North America, raising a question about human susceptibility to these new prion strains.

Methodology/Principal Findings

Brain homogenates from cattle with classical BSE and atypical (BASE) infections were inoculated intracerebrally into cynomolgus monkeys (Macacca fascicularis), a non-human primate model previously demonstrated to be susceptible to the original strain of cBSE. The resulting diseases were compared in terms of clinical signs, histology and biochemistry of the abnormal prion protein (PrPres). The single monkey infected with BASE had a shorter survival, and a different clinical evolution, histopathology, and prion protein (PrPres) pattern than was observed for either classical BSE or vCJD-inoculated animals. Also, the biochemical signature of PrPres in the BASE-inoculated animal was found to have a higher proteinase K sensitivity of the octa-repeat region. We found the same biochemical signature in three of four human patients with sporadic CJD and an MM type 2 PrP genotype who lived in the same country as the infected bovine.

Conclusion/Significance

Our results point to a possibly higher degree of pathogenicity of BASE than classical BSE in primates and also raise a question about a possible link to one uncommon subset of cases of apparently sporadic CJD. Thus, despite the waning epidemic of classical BSE, the occurrence of atypical strains should temper the urge to relax measures currently in place to protect public health from accidental contamination by BSE-contaminated products.