Spongiform encephalopathies: insights from transgenic models

Prions: Protein Aggregation and Infectious Diseases

Transmissible spongiform encephalopathies (TSEs) are inevitably lethal neurodegenerative diseases that affect humans and a large variety of animals. The infectious agent responsible for TSEs is the prion, an abnormally folded and aggregated protein that propagates itself by imposing its conformation onto the cellular prion protein (PrPC) of the host. PrPCis necessary for prion replication and for prion-induced neurodegeneration, yet the proximal causes of neuronal injury and death are still poorly understood. Prion toxicity may arise from the interference with the normal function of PrPC, and therefore, understanding the physiological role of PrPCmay help to clarify the mechanism underlying prion diseases. Here we discuss the evolution of the prion concept and how prion-like mechanisms may apply to other protein aggregation diseases. We describe the clinical and the pathological features of the prion diseases in human and animals, the events occurring during neuroinvasion, and the possible scenarios underlying brain damage. Finally, we discuss potential antiprion therapies and current developments in the realm of prion diagnostics.

The spread of prions through the body in naturally acquired transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies are fatal neurodegenerative diseases that are caused by unconventional pathogens and affect the central nervous system of animals and humans. Several different forms of these diseases result from natural infection (i.e. exposure to transmissible spongiform encephalopathy agents or prions, present in the natural environment of the respective host). This holds true also for scrapie in sheep, bovine spongiform encephalopathy in cattle, chronic wasting disease in elk and deer, or variant Creutzfeldt-Jakob disease in humans, all of which are assumed to originate predominantly from peroral prion infection. This article intends to provide an overview of the current state of knowledge on the spread of scrapie, chronic wasting disease, bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease agents through the body in naturally affected hosts, and in model animals experimentally challenged via the alimentary tract. Special attention is given to the tissue components and spreading pathways involved in the key stages of prion routing through the body, such as intestinal uptake, neuroinvasion of nerves and the central nervous system, and centrifugal spread from the brain and spinal cord to peripheral sites (e.g. sensory ganglia or muscles). The elucidation of the pathways and mechanisms by which prions invade a host and spread through the organism can contribute to efficient infection control strategies and the improvement of transmissible spongiform encephalopathy diagnostics. It may also help to identify prophylactic or therapeutic approaches that would impede naturally acquired transmissible spongiform encephalopathy infections.

Scrapie strain transmission studies in ovine PrP transgenic mice reveal dissimilar susceptibility

The Tg(OvPrP4) mouse line, expressing the sheep prion protein, is a sensitive model crucial for the identification of the bovine spongiform encephalopathy agent possibly present in natural sheep spongiform encephalopathies. It was also previously demonstrated as susceptible to infection with natural scrapie isolates from sheep harbouring various genotypes. The performance of this new transgenic mouse line in scrapie strain characterization was further assessed by intracranial inoculation of five groups of Tg(OvPrP4) mice with brain homogenate of the wild type mouse-adapted scrapie strains, C506M3, 22A, 79A, 87V, or Chandler. The Tg(OvPrP4) mice were susceptible to the scrapie agent transmitted using mouse-adapted scrapie strains but not equivalently. Strains 87V and Chandler were most readily transmissible followed by 79A and C506M3. Strain 22A was the least transmissible. Clinical signs, survival data, spongiosis, and PrP(sc) distribution were also reported. These various data demonstrate the possibility of distinguishing between scrapie strains. Our findings are discussed with regard to agent strain and host factors and already demonstrate the dissimilar susceptibilities of Tg(OvPrP4) mice to the different murine strains studied, thus, reinforcing their potential use in strain typing studies.

Detection of New Quantitative Trait Loci for Susceptibility to Transmissible Spongiform Encephalopathies in Mice

Susceptibility to scrapie is largely controlled by the PRNP gene in mice and in several other species. However, individuals with identical scrapie susceptibility Prnp alleles may have very different incubation periods, suggesting the influence of other environmental and genetic factors. To detect loci influencing susceptibility to TSE, two mouse lines carrying the same PRNP genotype (C57BL and RIII) were crossed to produce an F2 population inoculated intracerebrally with a mouse-adapted scrapie strain. Linkage was studied between 72 markers and the age of death of F2 animals. Six QTL were detected, two at a genome-wide significant level (chromosomes 5 and 7) and four at a genome-wide suggestive level (chromosomes 4, 6, 8, and 17). Our results confirmed the existence of some QTL that were detected previously (chromosomes 4, 6, 7, and 8) while others were found only in the present study (chromosomes 5 and 17). Furthermore, it seems that some QTL (chromosomes 4 and 8) are involved in resistance to scrapie as well as to BSE.

Prions and the immune system: a journey through gut, spleen, and nerves

For more than two decades it has been contended that prion infection does not elicit immune responses: transmissible spongiform encephalopathies do not go along with conspicuous inflammatory infiltrates, and antibodies to the prion protein are typically undetectable. Why is it, then, that prions accumulate in lymphoid organs, and that various states of immune deficiency prevent peripheral prion infection? This review revisits the current evidence of the involvement of the immune system in prion diseases, while attempting to trace the elaborate mechanisms by which peripherally administered prions invade the brain and ultimately cause damage. The investigation of these questions leads to unexpected detours, including the neurophysiology of lymphoid organs, and even the function of a prion protein homolog in male fertility.

Immune system and peripheral nerves in propagation of prions to CNS

Prions are not only unique in the way they replicate. Also the sequence of events triggered by peripheral prion infection, generically termed 'peripheral pathogenesis', sets prions aside from all other known pathogens. Whereas most bacteria, parasites, and viruses trigger innate and adaptive immune responses, the mammalian immune system appears to be remarkably oblivious to prions. Transmissible spongiform encephalopathies (TSEs) do not go along with inflammatory infiltrates, and antibodies to the prion protein are not typically raised during the course of the disease. On the other hand, there is conspicuous involvement of lymphoid organs, which accumulate sizeable concentrations of the infectious agent early during disease. Moreover, various states of immune deficiency can abolish peripheral pathogenesis and prevent 'take' of infection when prions are administered to peripheral sites. Here, we critically re-visit the current evidence for an involvement of the immune system in prion diseases, and will attempt to trace the elaborate mechanisms by which prions, upon entry into the body from peripheral sites, reach the brain.

Transgenesis applied to transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSE) are fatal neurodegenerative disorders present in various mammals. TSEs have been studies intensively, even more so following the BSE crisis and the subsequent threat of a human nvCJD epidemic. In the 'protein-only' hypothesis, the infectious agent, called prion, is assumed to be a misfolded host protein. Transgenesis has mainly been applied to study the role of this protein, its structure-function relationship with respect to its pathogenic properties and to assess the genetic origin of the well-recognised species barrier effect. This approach has somewhat supplemented the lack of in vitro models. This review will try to summarise the impressive work that has been done in this field. Although many questions remain unanswered, transgenic experiments have and will still improve our knowledge on this disease and might help us to develop critically needed therapeutic approaches.

Dementia, the fate of brain? Neuropathological point of view

The prevalence of dementia dramatically increases during ageing, and this puts a serious strain on the optimism brought by the continuous increase in life expectancy observed in most industrialised countries. Diseases that produce dementia are numerous, and the cognitive deficit results from lesions of various regions and from different mechanisms. This modulates the possible prediction, prevention and cure of dementia. Emphasis is put on the necessity of, and prerequisite for, efficient research in the field of dementia. Three paradigmatic dementing disorders are reviewed. Subacute spongiform encephalopathies (prion diseases) constitute a biological enigma and a public health concern. In Alzheimer's disease and vascular or mixed dementia, the clinical diagnosis is still imperfect, and this hinders research. Distinguishing and accurately identifying the various types of dementia is essential for understanding their mechanism and for developing efficient therapeutic strategies, preventive and curative. For such objectives, the study of human brain tissue will remain mandatory until non-invasive markers and additional models are available. Ethical reasons banish the use of cerebral biopsy and favour the promotion of autopsy.