The peripheral nervous system and the pathogenesis of prion diseases

Generation of novel neuroinvasive prions following intravenous challenge

Prions typically spread into the central nervous system (CNS), likely via peripheral nerves. Yet prion conformers differ in their capacity to penetrate the CNS; certain fibrillar prions replicate persistently in lymphoid tissues with no CNS entry, leading to chronic silent carriers. Subclinical carriers of variant Creutzfeldt-Jakob (vCJD) prions in the United Kingdom have been estimated at 1:2000, and vCJD prions have been transmitted through blood transfusion, however, the circulating prion conformers that neuroinvade remain unclear. Here we investigate how prion conformation impacts brain entry of transfused prions by challenging mice intravenously to subfibrillar and fibrillar strains. We show that most strains infiltrated the brain and caused terminal disease, however, the fibrillar prions showed reduced CNS entry in a strain-dependent manner. Strikingly, the highly fibrillar mCWD prion strain replicated in the spleen and emerged in the brain as a novel strain, indicating that a new neuroinvasive prion had been generated from a previously non-neuroinvasive strain. The new strain showed altered plaque morphology, brain regions targeted and biochemical properties and these properties were maintained upon intracerebral passage. Intracerebral passage of prion-infected spleen re-created the new strain. Splenic prions resembled the new strain biochemically and intracerebral passage of prion-infected spleen re-created the new strain, collectively suggesting splenic prion replication as a potential source. Taken together, these results indicate that intravenous exposure to prion-contaminated blood or blood products may generate novel neuroinvasive prion conformers and disease phenotypes, potentially arising from prion replication in non-neural tissues or from conformer selection.

Prion disease: experimental models and reality

The understanding of the pathogenesis and mechanisms of diseases requires a multidisciplinary approach, involving clinical observation, correlation to pathological processes, and modelling of disease mechanisms. It is an inherent challenge, and arguably impossible to generate model systems that can faithfully recapitulate all aspects of human disease. It is, therefore, important to be aware of the potentials and also the limitations of specific model systems. Model systems are usually designed to recapitulate only specific aspects of the disease, such as a pathological phenotype, a pathomechanism, or to test a hypothesis. Here, we evaluate and discuss model systems that were generated to understand clinical, pathological, genetic, biochemical, and epidemiological aspects of prion diseases. Whilst clinical research and studies on human tissue are an essential component of prion research, much of the understanding of the mechanisms governing transmission, replication, and toxicity comes from in vitro and in vivo studies. As with other neurodegenerative diseases caused by protein misfolding, the pathogenesis of prion disease is complex, full of conundra and contradictions. We will give here a historical overview of the use of models of prion disease, how they have evolved alongside the scientific questions, and how advancements in technologies have pushed the boundaries of our understanding of prion biology.

Genetic prion disease: no role for the immune system in disease pathogenesis?

Prion diseases, which can manifest by transmissible, sporadic or genetic etiologies, share several common features, such as a fatal neurodegenerative outcome and the aberrant accumulation of proteinase K (PK)-resistant PrP forms in the CNS. In infectious prion diseases, such as scrapie in mice, prions first replicate in immune organs, then invade the CNS via ascending peripheral tracts, finally causing death. Accelerated neuroinvasion and death occurs when activated prion-infected immune cells infiltrate into the CNS, as is the case for scrapie-infected mice induced for experimental autoimmune encephalomyelitis (EAE), a CNS inflammatory insult. To establish whether the immune system plays such a central role also in genetic prion diseases, we induced EAE in TgMHu2ME199K mice, a line mimicking for late onset genetic Creutzfeldt Jacob disease (gCJD), a human prion disease. We show here that EAE induction of TgMHu2ME199K mice neither accelerated nor aggravated prion disease manifestation. Concomitantly, we present evidence that PK-resistant PrP forms were absent from CNS immune infiltrates, and most surprisingly also from lymph nodes and spleens of TgMHu2ME199K mice at all ages and stages of disease. These results imply that the mechanism of genetic prion disease differs widely from that of the infectious presentation, and that the conversion of mutant PrPs into PK resistant forms occurs mostly/only in the CNS. If the absence of pathogenic PrP forms form immune organs is also true for gCJD patients, it may suggest their blood is devoid of prion infectivity.

BSE infectivity in jejunum, ileum and ileocaecal junction of incubating cattle

To establish bovine spongiform encephalopathy (BSE) public health protection measures it is important to precisely define the cattle tissues considered as specified risk materials (SRM). To date, in pre-clinical BSE infected cattle, no evidence of the BSE agent had been found in the gut outside of the ileal Peyer's Patches. This study was undertaken to determine when and where the pathological prion protein (PrP^Sc) and/or BSE infectivity can be found in the small intestine of cattle 4 to 6 months of age, orally challenged with BSE. Samples of the jejunum, the ileum and the ileocaecal junction from 46 BSE infected cattle, culled from 1 up to 44 months post infection (mpi) were examined by immunohistochemistry. Samples from cattle 8 mpi to 20 mpi were additionally studied by PTA Western blot, rapid tests, and by mouse (TgbovXV) bioassay. In doing so nearly all of the cattle, from 4 up to 44 mpi, had detectable amounts of PrP^Sc and/or infectivity in the distal ileum. In the distal ileum clear time-dependent variations were visible concerning the amount of PrP^Sc, the tissue structures affected, and the cells involved. BSE infectivity was found not only in the ileum and ileocaecal junction but also in the jejunum. The systematic approach of this study provides new data for qualitative and quantitative risk assessments and allows defining bovine SRM more precisely.

Rapid diagnosis of human prion disease using streptomycin with tonsil and brain tissues

The use of streptomycin in the pathological prion protein (PrP(sc)) detection procedures represents a new and attractive way for diagnostic purpose. With this agent, western blot readily detected PrP(sc) in 263K scrapie hamster and C57Bl/6 wild-type mice challenged with C506M3 scrapie strain. Our aim was to evaluate this new diagnosis procedure in the field of human transmissible spongiform encephalopathies (TSEs). First, we had confirmed the ability of streptomycin to precipitate PrP(res) from human brain of Creutzfeldt-Jakob disease (CJD) patient. Second, we compared the detection of PrP(res) with streptomycin against three other protocols using other precipitations. Then we assessed PrP(res) detection with streptomycin in 98 brain tissue samples from various aetiologies of human TSEs and 52 brain samples from other dementia. Finally, we applied this protocol for tonsils examination of five patients suspected of variant CJD (v-CJD). Sensitivity and specificity obtained with the streptomycin protocol were both 100% on brain tissue. For tonsil tissues, PrP(res) was clearly identified in the two post-mortem confirmed v-CJD cases, whereas no characteristic three-band pattern was seen in the three confirmed non-v-CJD samples. In this study, streptomycin demonstrated its efficiency to detect PrP(res) both in the central nervous system and in the lymphoid tissue without practical difficulty and with rapid preparation. Because of its ability to act as a good agent for PrP(sc) examination in different tissues, recovery of PrP(sc) in biological fluids using streptomycin should open further perspectives of applications in CJD diagnostics. Streptomycin effects in vivo might thus also be questioned.

Relevance of the regional lymph node in scrapie pathogenesis after peripheral infection of hamsters

BACKGROUND

The exact role of the lymphoreticular system in the spread of peripheral prion infections to the central nervous system still needs further elucidation. Against this background, the influence of the regional lymph node (Ln. popliteus) on the pathogenesis of scrapie was monitored in a hamster model of prion infection via the footpad.

METHODS

Surgical lymphadenectomy was carried out at different time points after infection, or prior to inoculation, in order to elucidate the impact of the lymph node on lethal neuroinvasion.

RESULTS

The Ln. popliteus did not show an influence on pathogenesis when a high dose of infectivity was administered. However, it was found to modulate the interval of time until the development of terminal scrapie in a subset of animals lymphadenectomized after low-dose infection. In additon, lymphadenectomy performed four weeks before inoculation prevented cerebral PrP(TSE) deposition and development of disease during the period of observation (314 days) in the majority of hamsters challenged with a very low dose of scrapie agent.

CONCLUSION

Our findings suggest the regional lymph node as a potentially facilitating or even essential factor for invasion of the brain after peripheral challenge with low doses of infectious scrapie agent. The invasive in vivo approach pursued in this study may be applied also to other animal species for further elucidating the involvement of lymphoid tissue in the pathogenesis of experimental and natural TSEs.

Cellular prion protein interaction with vitronectin supports axonal growth and is compensated by integrins

The physiological functions of the cellular prion protein, PrP(C), as a cell surface pleiotropic receptor are under debate. We report that PrP(C) interacts with vitronectin but not with fibronectin or collagen. The binding sites mediating this PrP(C)-vitronectin interaction were mapped to residues 105-119 of PrP(C) and the residues 307-320 of vitronectin. The two proteins were co-localized in embryonic dorsal root ganglia from wild-type mice. Vitronectin addition to cultured dorsal root ganglia induced axonal growth, which could be mimicked by vitronectin peptide 307-320 and abrogated by anti-PrP(C) antibodies. Full-length vitronectin, but not the vitronectin peptide 307-320, induced axonal growth of dorsal root neurons from two strains of PrP(C)-null mice. Functional assays demonstrated that relative to wild-type cells, PrP(C)-null dorsal root neurons were more responsive to the Arg-Gly-Asp peptide (an integrin-binding site), and exhibited greater alphavbeta3 activity. Our findings indicate that PrP(C) plays an important role in axonal growth, and this function may be rescued in PrP(C)-knockout animals by integrin compensatory mechanisms.

Prions spread via the autonomic nervous system from the gut to the central nervous system in cattle incubating bovine spongiform encephalopathy

To elucidate the still-unknown pathogenesis of bovine spongiform encephalopathy (BSE), an oral BSE challenge and sequential kill study was carried out on 56 calves. Relevant tissues belonging to the peripheral and central nervous system, as well as to the lymphoreticular tract, from necropsied animals were analysed by highly sensitive immunohistochemistry and immunoblotting techniques to reveal the presence of BSE-associated pathological prion protein (PrPSc) depositions. Our results demonstrate two routes involving the autonomic nervous system through which BSE prions spread by anterograde pathways from the gastrointestinal tract (GIT) to the central nervous system (CNS): (i) via the coeliac and mesenteric ganglion complex, splanchnic nerves and the lumbal/caudal thoracic spinal cord (representing the sympathetic GIT innervation); and (ii) via the Nervus vagus (parasympathetic GIT innervation). The dorsal root ganglia seem to be subsequently affected, so it is likely that BSE prion invasion of the non-autonomic peripheral nervous system (e.g. sciatic nerve) is a secondary retrograde event following prion replication in the CNS. Moreover, BSE-associated PrPSc was already detected in the brainstem of an animal 24 months post-infection, which is 8 months earlier than reported previously. These findings are important for the understanding of BSE pathogenesis and for the development of new diagnostic strategies for this infectious disease.

An immunoassay for the pathological form of the prion protein based on denaturation and time resolved fluorometry

Concern about the possible secondary spread of variant Creutzfeldt-Jakob disease (vCJD) through blood transfusion and blood products has increased the need for a sensitive and rapid test for the identification of PrP(Sc) in specimens collected non-invasively from living persons. Furthermore, an accurate estimate of the prevalence of pre-clinical vCJD in the British population would be possible if there were such a test that could be applied to specimens available readily (e.g. blood and urine). As a first step towards that goal, we have developed a simple and sensitive test for the detection of PrP(Sc) in peripheral tissues and brain of vCJD patients, based on the differential extraction of PrP(Sc) with guanidine hydrochloride. The prion protein (PrP) isoforms are extracted sequentially from homogenized tissue by applying two different concentrations of this chaotropic agent. Each extraction yields a fraction of the PrP isoforms with different solubilities in guanidine hydrochloride. Quantitation of the two fractions (relatively insoluble or relatively soluble) using time resolved fluorescence (DELFIA) as a reporter system allows differentiation between PrP(Sc) infected and non-infected tissues. The assay has a detection limit of 10 pg PrP, is robust and could be automated.

Uptake and neuritic transport of scrapie prion protein coincident with infection of neuronal cells

Invasion of the nervous system and neuronal spread of infection are critical, but poorly understood, steps in the pathogenesis of transmissible spongiform encephalopathies or prion diseases. To characterize pathways for the uptake and intraneuronal trafficking of infectious, protease-resistant prion protein (PrP-res), fluorescent-labeled PrP-res was used to infect a neuronally derived murine cell line (SN56) and adult hamster cortical neurons in primary culture. Concurrent with the establishment of persistent scrapie infection, SN56 cells internalized PrP-res aggregates into vesicles positive for markers for late endosomes and/or lysosomes but not synaptic, early endocytic, or raft-derived vesicles. Internalized PrP-res was then transported along neurites to points of contact with other cells. Similar trafficking was observed with dextran, Alzheimer's Abeta1-42 fibrils and noninfectious recombinant PrP fibrils, suggesting that PrP-res is internalized by a relatively nonspecific pinocytosis or transcytosis mechanism. Hamster cortical neurons were also capable of internalizing and disseminating exogenous PrP-res. Similar trafficking of exogenous PrP-res by cortical neurons cultured from the brains of PrP knock-out mice showed that uptake and neuritic transport did not require the presence of endogenous cellular PrP. These experiments visualize and characterize the initial steps associated with prion infection and transport within neuronal cells.