Unhampered prion neuroinvasion despite impaired fast axonal transport in transgenic mice overexpressing four-repeat tau

Deletion of Kif5c Does Not Alter Prion Disease Tempo or Spread in Mouse Brain

In prion diseases, the spread of infectious prions (PrPSc) is thought to occur within nerves and across synapses of the central nervous system (CNS). However, the mechanisms by which PrPSc moves within axons and across nerve synapses remain undetermined. Molecular motors, including kinesins and dyneins, transport many types of intracellular cargo. Kinesin-1C (KIF5C) has been shown to transport vesicles carrying the normal prion protein (PrPC) within axons, but whether KIF5C is involved in PrPSc axonal transport is unknown. The current study tested whether stereotactic inoculation in the striatum of KIF5C knock-out mice (Kif5c^−/−) with 0.5 µL volumes of mouse-adapted scrapie strains 22 L or ME7 would result in an altered rate of prion spreading and/or disease timing. Groups of mice injected with each strain were euthanized at either pre-clinical time points or following the development of prion disease. Immunohistochemistry for PrP was performed on brain sections and PrPSc distribution and tempo of spread were compared between mouse strains. In these experiments, no differences in PrPSc spread, distribution or survival times were observed between C57BL/6 and Kif5c^−/− mice.

Prion-associated cerebral amyloid angiopathy is not exacerbated by human phosphorylated tau aggregates in scrapie-infected mice expressing anchorless prion protein

Tau aggregates consisting of hyperphosphorylated tau fibrils are associated with many neurodegenerative diseases, including Alzheimer's disease, Pick's disease, frontotemporal dementia, and progressive supranuclear palsy. Tau may contribute to the pathogenesis of these diseases, collectively referred to as tauopathies. In human genetic prion diseases, tau aggregates are detected in association with amyloid plaques consisting of prion protein (PrP). However, the role of abnormal tau aggregates in PrP amyloid disease remains unclear. Previously we inoculated scrapie prions into transgenic mice expressing human tau, mouse tau, glycophosphatidylinositol (GPI) anchored PrP, and anchorless PrP. These mice developed both spongiform vacuolar pathology and PrP amyloid pathology, and human tau was detected near PrP amyloid plaques. However, the presence of human tau did not alter the disease tempo or prion-induced neuropathology. In the present study, we tested mice which more closely modeled familial human prion disease. These mice expressed human tau but lacked both mouse tau and GPI-anchored PrP. However, they did produce anchorless PrP, resulting in perivascular PrP amyloid plaques, i.e. cerebral amyloid angiopathy (CAA), without spongiform degeneration. Typical of PrP amyloid disease, the clinical course was very slow in this model. Nevertheless, the accumulation of aggregated, phosphorylated human tau and its association with PrP amyloid plaques failed to alter the timing or course of the clinical disease observed. These data suggest that human tau does not contribute to the pathogenesis of mouse PrP amyloid brain disease and raise the possibility that tau may also not be pathogenic in human PrP amyloid disease.

Tau accumulation triggers STAT1-dependent memory deficits by suppressing NMDA receptor expression

Intracellular tau accumulation forming neurofibrillary tangles is hallmark pathology of Alzheimer's disease (AD), but how tau accumulation induces synapse impairment is elusive. By overexpressing human full-length wild-type tau (termed hTau) to mimic tau abnormality as seen in the brain of sporadic AD patients, we find that hTau accumulation activates JAK2 to phosphorylate STAT1 (signal transducer and activator of transcription 1) at Tyr701 leading to STAT1 dimerization, nuclear translocation, and its activation. STAT1 activation suppresses expression of N-methyl-D-aspartate receptors (NMDARs) through direct binding to the specific GAS element of GluN1, GluN2A, and GluN2B promoters, while knockdown of STAT1 by AAV-Cre in STAT1^flox/flox mice or expressing dominant negative Y701F-STAT1 efficiently rescues hTau-induced suppression of NMDAR expression with amelioration of synaptic functions and memory performance. These findings indicate that hTau accumulation impairs synaptic plasticity through JAK2/STAT1-induced suppression of NMDAR expression, revealing a novel mechanism for hTau-associated synapse and memory deficits.

Cellular mechanisms responsible for cell-to-cell spreading of prions

Prions are infectious agents that cause fatal neurodegenerative diseases. Current evidence indicates that they are essentially composed of an abnormally folded protein (PrPSc). These abnormal aggregated PrPSc species multiply in infected cells by recruiting and converting the host PrPC protein into new PrPSc. How prions move from cell to cell and progressively spread across the infected tissue is of crucial importance and may provide experimental opportunity to delay the progression of the disease. In infected cells, different mechanisms have been identified, including release of infectious extracellular vesicles and intercellular transfer of PrPSc-containing organelles through tunneling nanotubes. These findings should allow manipulation of the intracellular trafficking events targeting PrPSc in these particular subcellular compartments to experimentally address the relative contribution of these mechanisms to in vivo prion pathogenesis. In addition, such information may prompt further experimental strategies to decipher the causal roles of protein misfolding and aggregation in other human neurodegenerative diseases.

Phosphorylated human tau associates with mouse prion protein amyloid in scrapie-infected mice but does not increase progression of clinical disease

Tauopathies are a family of neurodegenerative diseases in which fibrils of human hyperphosphorylated tau (P-tau) are believed to cause neuropathology. In Alzheimer disease, P-tau associates with A-beta amyloid and contributes to disease pathogenesis. In familial human prion diseases and variant CJD, P-tau often co-associates with prion protein amyloid, and might also accelerate disease progression. To test this latter possibility, here we compared progression of amyloid prion disease in vivo after scrapie infection of mice with and without expression of human tau. The mice used expressed both anchorless prion protein (PrP) and membrane-anchored PrP, that generate disease associated amyloid and non-amyloid PrP (PrPSc) after scrapie infection. Human P-tau induced by scrapie infection was only rarely associated with non-amyloid PrPSc, but abundant human P-tau was detected at extracellular, perivascular and axonal deposits associated with amyloid PrPSc. This pathology was quite similar to that seen in familial prion diseases. However, association of human and mouse P-tau with amyloid PrPSc did not diminish survival time following prion infection in these mice. By analogy, human P-tau may not affect prion disease progression in humans. Alternatively, these results might be due to other factors, including rapidity of disease, blocking effects by mouse tau, or low toxicity of human P-tau in this model.

Variable tau accumulation in murine models with abnormal prion protein deposits

The conversion of cellular prion protein (PrP) into a misfolded isoform is central to the development of prion diseases. However, the heterogeneous phenotypes observed in prion disease may be linked with the presence of other misfolded proteins in the brain. While hyperphosphorylated tau (p.tau) is characteristic of Alzheimer's disease (AD), p.tau is also observed in human prion diseases. To explore this association in the absence of potential effects due to aging, drug treatment, agonal stage and postmortem delay we analyzed p.tau and PrP immunopositivity in mouse models. Analyses were performed on mice inoculated with prion agents, and mice with PrP amyloid in the absence of prion disease. We observed that p.tau was consistently present in animals with prion infectivity (models that transmit disease upon serial passage). In contrast, p.tau was very rarely observed or absent in mice with PrP amyloid plaques in the absence of prion replication. These data indicate that the formation of p.tau is not linked to deposition of misfolded PrP, but suggest that the interaction between replication of infectivity and host factors regulate the formation of p.tau and may contribute to the heterogeneous phenotype of prion diseases.

Mapping PrPScPropagation in Experimental and Natural Scrapie in Sheep with Different PrP Genotypes

Twenty-one orally inoculated and seven naturally infected sheep with scrapie were examined for PrPScin peripheral tissues and in the central nervous system (CNS), using immunohistochemistry. In the inoculated group, VRQ (valine at codon 136, arginine at codon 154 and glutamine at codon 171)/VRQ sheep generally had a greater accumulation of the pathologic form of prion protein (PrPSc) in peripheral tissues, as compared with VRQ/ARQ (alanine at codon 136, arginine at codon 154, and glutamine at codon 171) animals at corresponding time points after inoculation. PrPScwas not detected in the ileal Peyer's patch, the spleen, the superficial cervical lymph node, and peripheral nervous tissues of several inoculated VRQ/ARQ animals. All inoculated VRQ/VRQ sheep, but only one of eight inoculated VRQ/ARQ animals, were PrPSc-positive in the CNS. Thus, the propagation of PrPScseemed slower and more limited in VRQ/ARQ animals. Tissue and cellular localization of PrPScsuggested that PrPScwas disseminated through three different routes. PrPSc-positive cells in lymph node sinuses and in lymphatics indicated spreading by lymph. The sequential appearance of PrPScin the peripheral nervous system and the CNS, with satellite cells as early targets, suggested the periaxonal transportation of PrPScthrough supportive cells. Focal areas of vascular amyloid-like PrPScin the brain of five sheep, suggested the hematogenous dissemination of PrPSc. There was a poor correlation between the amount of PrPScin the CNS and clinical signs. One subclinically affected sheep showed widespread PrPScaccumulation in the CNS, whereas three sheep had early clinical signs without detectable PrPScin the CNS. A VV136(homozygous for valine at codon 136) sheep inoculated with ARQ/ARR (alanine at codon 136, arginine at codon 154, and arginine at codon 171) tissue succumbed to disease, demonstrating successful heterologous transmission. Less susceptible sheep receiving VRQ/VRQ or ARQ/ARR material were PrPSc-negative by immunohistochemistry, enzyme-linked immunosorbent assay, and western blot.

T-Tau and P-Tau in Brain and Blood from Natural and Experimental Prion Diseases

Synaptic abnormalities are prominent in prion disease pathogenesis and are responsible for functional deficits. The microtubule associated protein, Tau, binds to and stabilizes microtubules in axons ensuring axonal transport of synaptic components. Tau phosphorylation reduces its affinity for microtubules leading to their instability and resulting in disrupted axonal transport and synaptic dysfunction. We report on the levels of total Tau (T-Tau) and phosphorylated Tau (P-Tau), measured by highly sensitive laser-based immunoassays, in the central nervous system and biofluids from experimentally transmitted prion disease in mice and natural cases of sporadic Creutzfeldt-Jakob Disease (sCJD) in humans. We found that, in contrast to sCJD where only the levels of T-Tau in brain are increased, both T-Tau and P-Tau are increased in the brains of symptomatic mice experimentally infected with the ME7, 139A and 22L mouse-adapted scrapie strains. The increased levels of T-Tau in sCJD brain, compared to control samples, were also observed in patient plasma. In contrast, there was no detectable increase in T-Tau and P-Tau in plasma from symptomatic experimentally infected mice. Furthermore, our data suggests that in mice showing clinical signs of prion disease the levels and/or ratios of T-Tau and P-Tau are only a useful parameter for differentiating the mouse-adapted scrapie strains that differ in the extent of disease. We conclude that the neuropathogenesis associated with P-Tau and synaptic dysfunction is similar for at least two of the mouse-adapted scrapie strains tested but may differ between sporadic and experimentally transmitted prion diseases.

Axonal and transynaptic spread of prions

Natural transmission of prion diseases depends upon the spread of prions from the nervous system to excretory or secretory tissues, but the mechanism of prion transport in axons and into peripheral tissue is unresolved. Here, we examined the temporal and spatial movement of prions from the brain stem along cranial nerves into skeletal muscle as a model of axonal transport and transynaptic spread. The disease-specific isoform of the prion protein, PrP(Sc), was observed in nerve fibers of the tongue approximately 2 weeks prior to PrP(Sc) deposition in skeletal muscle. Initially, PrP(Sc) deposits had a small punctate pattern on the edge of muscle cells that colocalized with synaptophysin, a marker for the neuromuscular junction (NMJ), in >50% of the cells. At later time points PrP(Sc) was widely distributed in muscle cells, but <10% of prion-infected cells exhibited PrP(Sc) deposition at the NMJ, suggesting additional prion replication and dissemination within muscle cells. In contrast to the NMJ, PrP(Sc) was not associated with synaptophysin in nerve fibers but was found to colocalize with LAMP-1 and cathepsin D during early stages of axonal spread. We propose that PrP(Sc)-bound endosomes can lead to membrane recycling in which PrP(Sc) is directed to the synapse, where it either moves across the NMJ into the postsynaptic muscle cell or induces PrP(Sc) formation on muscle cells across the NMJ.

IMPORTANCE

Prion diseases are transmissible and fatal neurodegenerative diseases in which prion dissemination to excretory or secretory tissues is necessary for natural disease transmission. Despite the importance of this pathway, the cellular mechanism of prion transport in axons and into peripheral tissue is unresolved. This study demonstrates anterograde spread of prions within nerve fibers prior to infection of peripheral synapses (i.e., neuromuscular junction) and infection of peripheral tissues (i.e., muscle cells). Within nerve fibers prions were associated with the endosomal-lysosomal pathway prior to entry into muscle cells. Since early prion spread is anterograde and endosome-lysosomal movement within axons is primarily retrograde, these findings suggest that endosome-bound prions may have an alternate fate that directs prions to the peripheral synapse.

Distinct patterns of spread of prion infection in brains of mice expressing anchorless or anchored forms of prion protein

Background

In humans and animals, prion protein (PrP) is usually expressed as a glycophosphatidylinositol (GPI)-anchored membrane protein, but anchorless PrP may be pathogenic in humans with certain familial prion diseases. Anchored PrP expressed on neurons mediates spread of prions along axons in the peripheral and central nervous systems. However, the mechanism of prion spread in individuals expressing anchorless PrP is poorly understood. Here we studied prion spread within brain of mice expressing anchorless or anchored PrP.

Results

To create a localized initial point of infection, we microinjected scrapie in a 0.5 microliter volume in the striatum. In this experiment, PrPres and gliosis were first detected in both types of mice at 40 days post-inoculation near the needle track. In mice with anchored PrP, PrPres appeared to spread via neurons to distant connected brain areas by the clinical endpoint at 150 days post-inoculation. This PrPres was rarely associated with blood vessels. In contrast, in mice with anchorless PrP, PrPres spread did not follow neuronal circuitry, but instead followed a novel slower pattern utilizing the drainage system of the brain interstitial fluid (ISF) including perivascular areas adjacent to blood vessels, subependymal areas and spaces between axons in white matter tracts.

Conclusions

In transgenic mice expressing anchorless PrP small amyloid-seeding PrPres aggregates appeared to be transported in the ISF, thus spreading development of cerebral amyloid angiopathy (CAA) throughout the brain. Spread of amyloid seeding by ISF may also occur in multiple human brain diseases involving CAA.

Tauopathy differentially affects cell adhesion molecules in mouse brain: early down-regulation of nectin-3 in stratum lacunosum moleculare

Cell adhesion molecules are important structural substrates, required for synaptic plasticity and synaptogenesis. CAMs differ widely in their expression throughout different brain regions and their specific structural and functional roles in the brain remain to be elucidated. Here, we investigated selected cell adhesion molecules for alterations in expression levels and neuronal localization in validated mouse models for Alzheimer's disease that mimic the age-related progression of amyloid accumulation and tauopathy. Among the cell adhesion molecules analyzed, Nectin-3 expression was affected most and specifically in all mouse models with tauopathy. In particular was Nectin-3 depleted from the specific region of the hippocampus, known as the stratum lacunosum and moleculare, in mice that express wild-type or mutant human protein Tau, either chronically or sub-acutely. Tauopathy progresses from the entorhinal cortex to the hippocampus by unknown mechanisms that could involve transport by the myelinated axons of the temporoammonic and perforant pathways. The decreased expression of Nectin-3 in the stratum lacunosum moleculare is an early marker of impaired transport, and eventual synaptic problems, caused by beginning tauopathy.

Filamentous white matter prion protein deposition is a distinctive feature of multiple inherited prion diseases

BACKGROUND

Sporadic, inherited and acquired prion diseases show distinct histological patterns of abnormal prion protein (PrP) deposits. Many of the inherited prion diseases show striking histological patterns, which often associate with specific mutations. Most reports have focused on the pattern of PrP deposition in the cortical or cerebellar grey matter.

RESULTS

We observed that the subcortical white matter in inherited prion diseases frequently contained filamentous depositions of abnormal PrP, and we have analysed by immunohistochemistry, immunofluorescence and electron microscopy 35 cases of inherited prion disease seen at the UK National Prion Clinic. We report here that filamentous PrP is abundantly deposited in myelinated fibres in inherited prion diseases, in particular in those with N-terminal mutations.

CONCLUSIONS

It is possible that the presence of filamentous PrP is related to the pathogenesis of inherited forms, which is different from those sporadic and acquired forms.

Targeted knock-down of cellular prion protein expression in myelinating Schwann cells does not alter mouse prion pathogenesis

In naturally acquired transmissible spongiform encephalopathies, the pathogenic agents or prions spread from the sites of initial peripheral uptake or replication to the brain where they cause progressive and fatal neurodegeneration. Routing via the peripheral nervous system is considered to be one of the main pathways to the central nervous system. Replication of prions in Schwann cells is viewed as a potentially important mechanism for efficient prion spread along nerves. Here we used a Cre-loxP mouse transgenetic approach to disrupt host-encoded prion protein (PrP(C)) specifically in myelinating Schwann cells. Despite the use of infection routes targeting highly myelinated nerves, there was no alteration in mouse prion pathogenesis, suggesting that conversion-dependent, centripetal spread of prions does not crucially rely on PrP(C) expressed by myelinating Schwann cells.

Human neural stem cells enhance structural plasticity and axonal transport in the ischaemic brain

Stem cell transplantation promises new hope for the treatment of stroke although significant questions remain about how the grafted cells elicit their effects. One hypothesis is that transplanted stem cells enhance endogenous repair mechanisms activated after cerebral ischaemia. Recognizing that bilateral reorganization of surviving circuits is associated with recovery after stroke, we investigated the ability of transplanted human neural progenitor cells to enhance this structural plasticity. Our results show the first evidence that human neural progenitor cell treatment can significantly increase dendritic plasticity in both the ipsi- and contralesional cortex and this coincides with stem cell-induced functional recovery. Moreover, stem cell-grafted rats demonstrated increased corticocortical, corticostriatal, corticothalamic and corticospinal axonal rewiring from the contralesional side; with the transcallosal and corticospinal axonal sprouting correlating with functional recovery. Furthermore, we demonstrate that axonal transport, which is critical for both proper axonal function and axonal sprouting, is inhibited by stroke and that this is rescued by the stem cell treatment, thus identifying another novel potential mechanism of action of transplanted cells. Finally, we established in vitro co-culture assays in which these stem cells mimicked the effects observed in vivo. Through immunodepletion studies, we identified vascular endothelial growth factor, thrombospondins 1 and 2, and slit as mediators partially responsible for stem cell-induced effects on dendritic sprouting, axonal plasticity and axonal transport in vitro. Thus, we postulate that human neural progenitor cells aid recovery after stroke through secretion of factors that enhance brain repair and plasticity.

Cytoplasmic dynein in neurodegeneration

Cytoplasmic dynein 1 (later referred to as dynein) is the major molecular motor moving cargoes such as mitochondria, organelles and proteins towards the minus end of microtubules. Dynein is involved in multiple basic cellular functions, such as mitosis, autophagy and structure of endoplasmic reticulum and Golgi, but also in neuron specific functions in particular retrograde axonal transport. Dynein is regulated by a number of protein complexes, notably by dynactin. Several studies have supported indirectly the involvement of dynein in neurodegeneration associated with Alzheimer's disease, Parkinson's disease, Huntington's disease and motor neuron diseases. First, axonal transport disruption represents a common feature occurring in neurodegenerative diseases. Second, a number of dynein-dependent processes, including autophagy or clearance of aggregation-prone proteins, are found defective in most of these diseases. Third, a number of mutant genes in various neurodegenerative diseases are involved in the regulation of dynein transport. This includes notably mutations in the P150Glued subunit of dynactin that are found in Perry syndrome and motor neuron diseases. Interestingly, gene products that are mutant in Huntington's disease, Parkinson's disease, motor neuron disease or spino-cerebellar ataxia are also involved in the regulation of dynein motor activity or of cargo binding. Despite a constellation of indirect evidence, direct links between the motor itself and neurodegeneration are few, and this might be due to the requirement of fully active dynein for development. Here, we critically review the evidence of dynein involvement in different neurodegenerative diseases and discuss potential underlying mechanisms.

Crucial role for prion protein membrane anchoring in the neuroinvasion and neural spread of prion infection

In nature prion diseases are usually transmitted by extracerebral prion infection, but clinical disease results only after invasion of the central nervous system (CNS). Prion protein (PrP), a host-encoded glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein, is necessary for prion infection and disease. Here, we investigated the role of the anchoring of PrP on prion neuroinvasion by studying various inoculation routes in mice expressing either anchored or anchorless PrP. In control mice with anchored PrP, intracerebral or sciatic nerve inoculation resulted in rapid CNS neuroinvasion and clinical disease (154 to 156 days), and after tongue, ocular, intravenous, or intraperitoneal inoculation, CNS neuroinvasion was only slightly slower (193 to 231 days). In contrast, in anchorless PrP mice, these routes resulted in slow and infrequent CNS neuroinvasion. Only intracerebral inoculation caused brain PrPres, a protease-resistant isoform of PrP, and disease in both types of mice. Thus, anchored PrP was an essential component for the rapid neural spread and CNS neuroinvasion of prion infection.

Change in tau phosphorylation associated with neurodegeneration in the ME7 model of prion disease

Hyperphosphorylation of the microtubule-associated protein tau is a significant determinant in AD (Alzheimer's disease), where it is associated with disrupted axonal transport and probably causes synaptic dysfunction. Although less well studied, hyperphosphorylation has been observed in prion disease. We have investigated the expression of hyperphosphorylated tau in the hippocampus of mice infected with the ME7 prion agent. In ME7-infected animals, there is a selective loss of CA1 synapse, first discernable at 13 weeks of disease. There is a potential that dysfunctional axonal transport contributes to this synaptopathy. Thus investigating hyperphosphorylated tau that is dysfunctional in AD could illuminate whether and how they are significant in prion disease. We observed no differences in the levels of phosphorylated tau (using MC1, PHF-1 and CP13 antibodies) in detergent-soluble and detergent-insoluble fractions extracted from ME7- and NBH- (normal brain homogenate) treated animals across disease. In contrast, we observed an increase in phospho-tau staining for several epitopes using immunohistochemistry in ME7-infected hippocampal sections. Although the changes were not of the magnitude seen in AD tissue, clear differences for several phospho-tau species were seen in the CA1 and CA3 of ME7-treated animals (pSer(199-202)>pSer(214)>PHF-1 antibody). Temporally, these changes were restricted to animals at 20 weeks and none of the disease-related staining was associated with the axons or dendrites that hold CA1 synapses. These findings suggest that phosphorylation of tau at the epitopes examined does not underpin the early synaptic dysfunction. These data suggest that the changes in tau phosphorylation recorded here and observed by others relate to end-stage prion pathology when early dysfunctions have progressed to overt neuronal loss.

Impaired axonal transport in motor neurons correlates with clinical prion disease

Prion diseases are fatal neurodegenerative disorders causing motor dysfunctions, dementia and neuropathological changes such as spongiosis, astroglyosis and neuronal loss. The chain of events leading to the clinical disease and the role of distinct brain areas are still poorly understood. The role of nervous system integrity and axonal properties in prion pathology are still elusive. There is no evidence of both the functional axonal impairments in vivo and their connection with prion disease. We studied the functional axonal impairments in motor neurons at the onset of clinical prion disease using the combination of tracing as a functional assay for axonal transport with immunohistochemistry experiments. Well-established and novel confocal and ultramicroscopy techniques were used to image and quantify labeled neurons. Despite profound differences in the incubation times, 30% to 45% of neurons in the red nucleus of different mouse lines showed axonal transport impairments at the disease onset bilaterally after intracerebral prion inoculation and unilaterally—after inoculation into the right sciatic nerve. Up to 94% of motor cortex neurons also demonstrated transport defects upon analysis by alternative imaging methods. Our data connect axonal transport impairments with disease symptoms for different prion strains and inoculation routes and establish further insight on the development of prion pathology in vivo. The alterations in localization of the proteins involved in the retrograde axonal transport allow us to propose a mechanism of transport disruption, which involves Rab7-mediated cargo attachment to the dynein-dynactin pathway. These findings suggest novel targets for therapeutic and diagnostic approaches in the early stages of prion disease.

For almost a century, prion disease symptoms, such as dementia and motor system defects, have been accompanied with neuropathological hallmarks in the central nervous system. In past decades, discrepancies between neuropathological changes and clinical symptoms showed that the processes triggering the disease remain elusive. We concentrated on the hypothesis that nervous system integrity and axonal properties may play an important role in the disease development. Since motor system defects are typical for prion disease, we investigated the centers of the motor system, red nucleus and hindlimb area of motor cortex. Although intracerebral prion infection led to a 30% to 45% bilateral reduction of labeled neurons in the red nucleus, infection into the right sciatic nerve—the major hindlimb nerve—led to unilateral reduction of labeled neurons in the red nucleus. Up to 94% reduction was found in the neurons of motor cortex hindlimb area. This reduction is probably caused by functional axonal impairments in motor neurons. Prion-induced alterations in protein distribution implicate a mechanism of transport disruption at cargo attachment to the retrograde axonal transport complex. Our work reveals a connection between axonal transport impairments and disease symptoms in vivo, providing further insight in the development of prion pathology and suggesting novel targets for therapeutic and diagnostic approaches.

Ultramicroscopy reveals axonal transport impairments in cortical motor neurons at prion disease

The functional imaging of neuronal circuits of the central nervous system is crucial for phenotype screenings or investigations of defects in neurodegenerative disorders. Current techniques yield either low penetration depth, yield poor resolution, or are restricted by the age of the animals. Here, we present a novel ultramicroscopy protocol for fluorescence imaging and three-dimensional reconstruction in the central nervous system of adult mice. In combination with tracing as a functional assay for axonal transport, retrogradely labeled descending motor neurons were visualized with >4 mm penetration depth. The analysis of the motor cortex shortly before the onset of clinical prion disease revealed that >80% neurons have functional impairments in axonal transport. Our study provides evidence that prion disease is associated with severe axonal transport defects in the cortical motor neurons and suggests a novel mechanism for prion-mediated neurodegeneration.

Molecular pathology of human prion diseases

Prion diseases are fatal neurodegenerative conditions in humans and animals. In this review, we summarize the molecular background of phenotypic variability, relation of prion protein (PrP) to other proteins associated with neurodegenerative diseases, and pathogenesis of neuronal vulnerability. PrP exists in different forms that may be present in both diseased and non-diseased brain, however, abundant disease-associated PrP together with tissue pathology characterizes prion diseases and associates with transmissibility. Prion diseases have different etiological background with distinct pathogenesis and phenotype. Mutations of the prion protein gene are associated with genetic forms. The codon 129 polymorphism in combination with the Western blot pattern of PrP after proteinase K digestion serves as a basis for molecular subtyping of sporadic Creutzfeldt-Jakob disease. Tissue damage may result from several parallel, interacting or subsequent pathways that involve cellular systems associated with synapses, protein processing, oxidative stress, autophagy, and apoptosis.

Oral Delivery of Pathogens from the Intestine to the Nervous System

Most therapeutic agents are delivered orally. Consequently, the major classes of therapeutically useful chemicals are partially lipophilic, small molecular weight compounds. They have reasonable permeability coefficient values across cell membranes, including those of intestinal epithelia and vascular endothelia. In contrast, large molecular weight biotechnology compounds have limited usefulness by non-injected routes as a consequence of their low membrane permeability and variable solubility. However, a wide range of infectious agents have developed strategies or have hijacked physiological routings in order to enter the host by the oral route. Efforts to address such issues have refreshed interest in mechanisms by which different types of payloads (including particulates and microorganisms) translocate across gut epithelia and then distribute to target tissues. Special attention is given to the potential role of the enteric nervous system and its plasticity.

Prion diseases: from protein to cell pathology

Prion diseases or transmissible spongiform encephalopathies are fatal neurodegenerative conditions in humans and animals that originate spontaneously, genetically or by infection. Conformational change of the normal (cellular) form of prion protein (PrP c) to a pathological, disease-associated form (PrP TSE) is considered central to pathogenesis and formation of the infectious agent or prion. Neuronal damage is central to clinical manifestation of prion diseases but poorly understood. In this review, we analyze the major pathogenetic pathways that lead to tissue pathology in different forms of disease. Neuropathogenesis of prion diseases evolves in complex ways on several front lines, most but not all of which exist also in other neurodegenerative as well as infectious diseases. Whereas intracellular accumulation of PrP forms might significantly impair cell function and lead to cytopathology, mere extracellular deposition of PrP TSE is questionable as a direct cytotoxic factor. Tissue damage may result from several parallel, interacting, or subsequent pathways. Future studies should clarify the trigger(s) and sequence of these processes and whether, and which, one is dominating or decisive.

Prion propagation in a nerve conduit model containing segments devoid of axons

Prions, the putative causative agents of transmissible spongiform encephalopathies, are neurotropic pathogens that spread to the central nervous system via synaptically linked neural conduits upon peripheral infection. Axons and their transport processes have been suggested as mediators of nerve-associated prion dissemination. However, the exact cellular components and molecular mechanisms underlying neural spread are unknown. This study used an established hamster scrapie model to pursue a novel experimental approach using nerve conduits containing segments devoid of neurites generated by incomplete nerve regeneration following Wallerian degeneration to probe the necessity of axons for the neural propagation of prions. For this purpose, animals were subjected to unilateral sciatic neurectomy 4 weeks before footpad inoculation with scrapie agent. The results showed that the regional nerve is the prime conduit for cerebral neuroinvasion and revealed, as evidenced by the accumulation of pathological prion protein PrP TSE, that prions can proceed along segments of peripheral neural projections without detectable axonal structures.

Myelin proteolipid protein (PLP) as a marker antigen of central nervous system contaminations for routine food control

Spreading transmissible spongiform encephalopathies (TSE) have been widely attributed to transmission by ingestion of mammalian central nervous system (CNS) tissue. Reliable exclusion of this epidemiological important route of transmission relies on an effective surveillance of food contamination. Here, myelin proteolipid protein (PLP) is identified as a specific and largely heat-resistant marker for detection of food contaminations by CNS tissue. PLP is a component of oligodendritic glial sheaths of neuronal processes that is specifically expressed in the CNS. A highly selective polyclonal antibody was developed directed against an epitope present in the full-length PLP protein, but absent from the developmentally regulated splice variant DM-20. In combination with a hydrophobic extraction of PLP from tissue samples, the antibody reliably detected PLP from spinal cord, cerebellum, and cortex of different mammalian species. Consistent with earlier reports on PLP expression, no cross-reactivity was observed with peripheral nerve or extraneural tissue, except for a very faint signal obtained with heart. When applied to an artificial CNS contamination present in sausages, the antibody reliably detected a low concentration (1%) of the contaminant. Application of heat, as used during conventional sausage manufacturing, led to a predominant alteration of arginine residues in the PLP protein and a partial loss of immunoreactivity. In contrast, a stretch of hydrophilic amino acids(112-122) proved to be heat-resistant, preserving the immunogenicity of this PLP epitope during heating. Taken together, the excellent CNS specificity of PLP immunodetection and the presence of a heat-resistant epitope have permitted the development of a highly sensitive immunoassay for CNS contamination in routine food control.

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.

Study on interaction between microtubule associated protein tau and prion protein

Microtubule-associated protein tau is considered to play roles in many neurodegenerative diseases including some transmissible spongiform encephalopathies. To address the possible molecular linkage of prion protein (PrP) and tau, a GST-fusion segment of human tau covering the three-repeat region and various PrP segments was used in the tests of GST pull-down and immunoprecipitation. We found tau protein interacted with various style prion proteins such as native prion protein (PrPc) or protease-resistant isoform (PrPSc). Co-localization signals of tau and PrP were found in the CHO cell tranfected with both PrP and tau gene. The domain of interaction with tau was located at N-terminal of PrP (residues 23 to 91). The evidence of molecular interactions between PrP and tau protein highlights a potential role of tau in the biological function of PrP and the pathogenesis of TSEs.

Prions and peripheral nerves: A deadly rendezvous

The infectious particle causing transmissible spongiform encephalopathy (TSE), a fatal neurodegenerative disease of humans and animals, has been termed prion. Its major component is an aggregated variant of the cellular prion protein, PrP(C). The main target of prion pathology is the central nervous system (CNS), yet most prion diseases are initiated or accompanied by prion replication at extracerebral locations, including secondary lymphoid organs, muscle and, in some instances, blood. How do prions travel from the periphery into the CNS? Is this an active or a passive process and does neuronal prion transport explain the long incubation times in prion diseases? Alternatively, if prion infectivity arises spontaneously in the CNS, as believed from sporadic Creutzfeldt-Jakob patients, how do prions manage to travel from the CNS into the periphery (e.g., spleen, muscle) of the infected host? The mechanisms of neuronal prion transport from the periphery into the CNS or vice versa are heavily investigated and debated but poorly understood. Although research in the past has accumulated knowledge on prion progression from the periphery to the brain, we are far from understanding the molecular mechanisms of neuronal prion transport.

Propagation of scrapie in peripheral nerves after footpad infection in normal and neurotoxin exposed hamsters

As is known from various animal models, the spread of agents causing transmissible spongiform encephalopathies (TSE) after peripheral infection affects peripheral nerves before reaching the central nervous system (CNS) and leading to a fatal end of the disease. The lack of therapeutic approaches for TSE is partially due to the limited amount of information available on the involvement of host biological compartments and processes in the propagation of the infectious agent. The in vivo model presented here can provide information on the spread of the scrapie agent via the peripheral nerves of hamsters under normal and altered axonal conditions. Syrian hamsters were unilaterally footpad (f.p.) infected with scrapie. The results of the spatiotemporal ultrasensitive immunoblot-detection of scrapie-associated prion protein (PrP(Sc)) in serial nerve segments of both distal sciatic nerves could be interpreted as a centripetal and subsequent centrifugal neural spread of PrP(Sc) for this route of infection. In order to determine whether this propagation is dependent on main components in the axonal cytoskeleton (e.g. neurofilaments, also relevant for the component ;a' of slow axonal transport mechanisms), hamsters were treated -in an additional experiment- with the neurotoxin beta,beta-iminodiproprionitrile (IDPN) around the beginning of the scrapie infection. A comparison of the Western blot signals of PrP(Sc) in the ipsilateral and in the subsequently affected contralateral sciatic nerve segments with the results revealed from IDPN-untreated animals at preclinical and clinical stages of the TSE disease, indicated similar amounts of PrP(Sc). Furthermore, the mean survival time was unchanged in both groups. This in vivo model, therefore, suggests that the propagation of PrP(Sc) along peripheral nerves is not dependent on an intact neurofilament component of the axonal cytoskeleton. Additionally, the model indicates that the spread of PrP(Sc) is not mediated by the slow component ;a' of the axonal transport mechanism.

Transgenic mouse models for Alzheimer's disease: the role of GSK-3β in combined amyloid and tau-pathology

Describing and understanding the pathological processes which devastate the brain of Alzheimer's disease (AD) patients remains a major target for experimental biology. We approached this problem by generating different types of single and double transgenic mice that develop pathological hallmarks of AD. In APP-V717 mice, the progression from intracellular amyloid to diffuse and senile plaques with vascular deposits, is preceded by early defects in cognition and LTP. In Tau-P301L mice, the morbid tauopathy with intracellular filaments, cause mortality before age 1 year. Ageing APP-V717IxTau-P301L double tg mice (14-17 months) have combined AD-like pathology in hippocampus and cortex consisting of amyloid plaques and neurofibrillary tangles. Remarkably, while Tau-P301L mice die before age 1 year, the APP-V717IxTau-P301L double tg mice survive much longer, which correlates with alleviation of tauopathy in hindbrain, despite aggravation in forebrain. This hypothesis is corroborated in Tau-P301LxGSK-3B double transgenic mice, which have also an extended lifespan relative to Tau-P301L mice, that correlates with reduction of brainstem tauopathy. At the same time, Tau-P301LxGSK-3B mice have dramatic forebrain tauopathy, with "tangles in almost all neurons", although without hyper-phosphorylation of Tau. The data corroborate the hypothesis that GSK-3B is the missing link between the amyloid and tau-pathology, and position GSK-3B as prominent player in the pathogenesis in AD.

Anterograde axonal transport of the exogenous cellular isoform of prion protein in the chick visual system

The cellular isoform of endogenous, newly synthesized prion protein (PrPc) can be transported by axons in the anterograde direction. To determine whether a mechanism exists for secreted PrPc to be internalized and then axonally transported, we analyzed internalization and anterograde axonal transport of radiolabeled recombinant PrPc after its intraocular injection in chick embryos. Internalization and axonal transport of exogenous PrPc to the midbrain by retinal ganglion cells (RGCs) is efficient, saturable and likely receptor-mediated. Ultrastructural quantitative localization of radiolabeled PrPc within RGC soma showed significant labeling of vesicular/endosomal compartments and much less labeling present over the Golgi apparatus and lysosomes, which indicates slow degradation of exogenous PrPc in this system. These data show that a mechanism exists to internalize a secreted form of PrPc and then to axonally transport such PrPc in an anterograde direction. This may provide an additional, novel mechanism for prion protein to spread among neurons.

Subcellular localization of disease-associated prion protein in the human brain

Disease-associated prion protein (PrP(TSE)) deposits in distinct immunostaining patterns in the brain in Creutzfeldt-Jakob disease, including synaptic, extracellular, and cell-associated localizations. After having developed an appropriate pretreatment protocol to enhance immunostaining for PrP(TSE) without damaging epitopes of other antigens, we systematically evaluated co-localization patterns of distinct PrP(TSE) immunodeposits by confocal laser microscopy, including optical serial sectioning. As shown by quantification, the most prominent co-localization of PrP(TSE) is with synaptophysin, but PrP(TSE) may also co-deposit with connexin-32, a gap junction-related protein. Furthermore, neuronal cell bodies, dendrites, axons, astrocytes, and microglia harbor granular PrP(TSE) deposits. Highly aggregated deposits are focally ubiquitinated. We conclude that PrP(TSE) is not exclusively associated with chemical but also with electric synapses, axonal transport may be a relevant route of PrP(TSE) spread in the brain, and activated microglia and astrocytes may play a role in PrP(TSE) processing, degradation, or removal.

Pathogenesis of prion diseases

Prion diseases are rare neurological disorders that may be of genetic or infectious origin, but most frequently occur sporadically in humans. Their outcome is invariably fatal. As the responsible pathogen, prions have been implicated. Prions are considered to be infectious particles that represent mainly, if not solely, an abnormal, protease-resistant isoform of a cellular protein, the prion protein or PrP(C). As in other neurodegenerative diseases, aggregates of misfolded protein conformers are deposited in the CNS of affected individuals. Pathogenesis of prion diseases comprises mainly two equally important, albeit essentially distinct, topics: first, the mode, spread, and amplification of infectivity in acquired disease, designated as peripheral pathogenesis. In this field, significant advances have implicated an essential role of lymphoid tissues for peripheral prion replication, before a likely neural spread to the CNS. The second is the central pathogenesis, dealing, in addition to spread and replication of prions within the CNS, with the mechanisms of nerve cell damage and death. Although important roles for microglial neurotoxicity, oxidative stress, and complement activation have been identified, we are far from complete understanding, and therapeutic applications in prion diseases still need to be developed.

Changed conformation of mutant Tau-P301L underlies the moribund tauopathy, absent in progressive, nonlethal axonopathy of Tau-4R/2N transgenic mice

Protein tau-3R/4R isoform ratio and phosphorylation regulates binding to microtubules and, when disturbed by aging or mutations, results in diverse tauopathies and in neurodegeneration. The underlying mechanisms were studied here in three transgenic mouse strains with identical genetic background, all expressing the tau-4R/2N isoform driven specifically in neurons by the thy1 gene promoter. Two strains, expressing human tau-4R/2N or mutant tau-4R/2N-P301L at similar, moderate levels, developed very different phenotypes. Tau-4R/2N mice became motor-impaired already around age 6-8 weeks, accompanied by axonopathy (dilatations, spheroids), but no tau aggregates, and surviving normally. In contrast, tau-P301L mice developed neurofibrillary tangles from age 6 months, without axonal dilatations and, despite only minor motor problems, all succumbing before the age of 13 months. The third strain, obtained by tau knock-out/knock-in (tau-KOKI), expressed normal levels of wild-type human tau-4R/2N replacing all mouse tau isoforms. Tau-KOKI mice survived normally with minor motor problems late in life and without any obvious pathology. Biochemically, a fraction of neuronal tau in aging tau-P301L mice was hyperphosphorylated concomitant with conformational changes and aggregation, but overall, tau-4R/2N was actually more phosphorylated than tau-P301L. Significantly, tau with changed conformation and with hyperphosphorylation colocalized in the same neurons in aging tau-P301L mice. Taken together, we conclude that excessive binding of tau-4R/2N as opposed to reduced binding of tau-P301L to microtubules is responsible for the development of axonopathy and tauopathy, respectively, in tau-4R/2N and tau-P301L mice and that the conformational change of tau-P301L is a major determinant in triggering the tauopathy.

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.

Cultured peripheral neuroglial cells are highly permissive to sheep prion infection

Transmissible spongiform encephalopathies arise as a consequence of infection of the central nervous system (CNS) by prions. Spreading of the infectious agent through the peripheral nervous system (PNS) may represent a crucial step toward CNS neuroinvasion, but the modalities of this process have yet to be clarified. Here we provide further evidence that PNS glial cells are likely targets for infection by prions. Glial cell clones originating from dorsal root ganglia of transgenic mice expressing ovine PrP (tgOv) and simian virus 40 T antigen were found to be readily infectible by sheep scrapie agent. This led us to establish two stable cell lines that exhibited features of Schwann cells. These cells were shown to sustain an efficient and stable replication of sheep prion based on the high level of accumulation of abnormal PrP and infectivity in exposed cultures. We also provide evidence for abnormal PrP deposition in peripheral neuroglial cells from scrapie-infected tgOv mice and sheep. These findings have potential implications in terms of designing new cell systems permissive to prions and of peripheral pathobiology of prion infections.

Enhanced detection and retrograde axonal transport of PrPc in peripheral nerve

Neuroinvasion of the CNS during orally acquired transmissible spongiform encephalopathies (TSEs) may involve the transport of the infectious agent from the periphery to the CNS via the peripheral nerves. If this occurs within axons, the mechanism of axonal transport may be fundamental to the process. In studies of peripheral nerve we observed that the cellular prion protein (PrPc) is highly resistant to detergent extraction. The implication of this is an underestimation of the abundance of PrPc in peripheral nerve. We have developed nerve extraction conditions that enhance the quantification of the protein in nerve 16-fold. Application of these conditions to evaluate the accumulation of PrPc distal to a cut nerve now reveals that PrPc is retrogradely transported from the axon ending. These results provide a potential cellular mechanism for TSE infectivity to gain entry to the CNS from the periphery.

Cytosolic prion protein in neurons

Localizing the cellular prion protein (PrPC) in the brain is necessary for understanding the pathogenesis of prion diseases. However, the precise ultrastructural localization of PrPC still remains enigmatic. We performed the first quantitative study of the ultrastructural localization of PrPC in the mouse hippocampus using high-resolution cryoimmunogold electron microscopy. PrPC follows the standard biosynthetic trafficking pathway with a preferential localization in late endosomal compartments and on the plasma membrane of neurons and neuronal processes. PrPC is found with the same frequency within the synaptic specialization and perisynaptically, but is almost completely excluded from synaptic vesicles. Unexpectedly, PrP is also found in the cytosol in subpopulations of neurons in the hippocampus, neocortex, and thalamus but not the cerebellum. Cytosolic PrP may have altered susceptibility to aggregation, suggesting that these neurons might play a significant role in the pathogenesis of prion diseases, in particular those mammals harboring mutant PrP genes.

Connecting the dots: trafficking of neurotrophins, lectins and diverse pathogens by binding to the neurotrophin receptor p75NTR

The common receptor for neurotrophins, p75, has important roles in internalization and trafficking of neurotrophins along axons. Recent studies show that an astonishing array of proteins, including lectins, pathogens and neurotoxins, bind the p75 receptor, suggesting that they can hijack and utilize this receptor for trafficking between neuronal populations within the nervous system. Such pathogens include the neurologically important rabies viruses, prion proteins, beta-amyloid and possibly tetanus toxin. These proteins may hijack existing transport machineries designed to traffick neurotrophins, thus allowing the infiltration and distribution of pathogens and toxins among vulnerable neuronal populations with devastating effects, as seen in rabies, prion encephalopathies, Alzheimer's disease and tetanic muscle spasm. The discovery of an entry and transport machinery that is potentially shared between pathogens and neurotrophins sheds light ono trafficking systems in the nervous system and may assist the design of novel therapeutic avenues that prevent or slow the progression of diverse chronic and acute neurological disorders.

Axonal transport, tau protein, and neurodegeneration in Alzheimer's disease

The molecular causes and the genetic and environmental modifying factors of the sporadic form of Alzheimer's disease (AD) remain elusive. Extrapolating from the known mutations that cause the rare familial forms and from the typical post-mortem pathological lesions in all AD patients--e.g., amyloid plaques and neurofibrillary tangles (NFTs)-the evident molecular candidates are amyloid precursor protein (APP), presenilin, and tau protein. To include ApoE4 as the only certain genetic modifier known leaves us to face the challenge of implementing these very different molecules into an evident pathological partnership. In more than one respect, the proposition of disturbed axonal transport appears attractive with more details becoming available on APP processing and microtubular transport and also of the pathology in the model systems--e.g., transgenic mice expressing APP or protein tau. Conversely, the resistance of APP-transgenic mice with full-blown amyloid pathology to also develop tau-related neurofibrillar pathology is a major challenge for this hypothesis. From the most relevant data discussed here, we conclude that the postulate of disturbed axonal transport as the primary event in AD is difficult to defend. On the other hand, failing axonal transport appears to be of major importance in the later stages in AD, by further compromising tau protein, APP metabolism, and synaptic functioning. Protein tau may thus be the central "executer" in the chain of events leading from amyloid neurotoxicity to tau hyperphosphorylation, microtubular destabilization, disturbed axonal transport, and synaptic failure to neurodegeneration. In order to identify normal physiological processes and novel pathological targets, definition is needed--in molecular detail--of the complex mechanisms involved.

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.