PrP expression and replication by Schwann cells: implications in prion spreading

Potential Therapeutic Use of Stem Cells for Prion Diseases

Prion diseases are neurodegenerative disorders that are progressive, incurable, and deadly. The prion consists of PrPSc, the misfolded pathogenic isoform of the cellular prion protein (PrPC). PrPC is involved in a variety of physiological functions, including cellular proliferation, adhesion, differentiation, and neural development. Prion protein is expressed on the membrane surface of a variety of stem cells (SCs), where it plays an important role in the pluripotency and self-renewal matrix, as well as in SC differentiation. SCs have been found to multiply the pathogenic form of the prion protein, implying their potential as an in vitro model for prion diseases. Furthermore, due to their capability to self-renew, differentiate, immunomodulate, and regenerate tissue, SCs are prospective cell treatments in many neurodegenerative conditions, including prion diseases. Regenerative medicine has become a new revolution in disease treatment in recent years, particularly with the introduction of SC therapy. Here, we review the data demonstrating prion diseases' biology and molecular mechanism. SC biology, therapeutic potential, and its role in understanding prion disease mechanisms are highlighted. Moreover, we summarize preclinical studies that use SCs in prion diseases.

Genetically engineered cellular models of prion propagation

For over three decades, cultured cells have been a useful tool for dissecting the molecular details of prion replication and the identification of candidate therapeutics for prion disease. A major issue limiting the translatability of these studies has been the inability to reliably propagate disease-relevant, non-mouse strains of prions in cells relevant to prion pathogenesis. In recent years, fueled by advances in gene editing technology, it has become possible to propagate prions from hamsters, cervids, and sheep in immortalized cell lines originating from the central nervous system. In particular, the use of CRISPR-Cas9-mediated gene editing to generate versions of prion-permissive cell lines that lack endogenous PrP expression has provided a blank canvas upon which re-expression of PrP leads to species-matched susceptibility to prion infection. When coupled with the ability to propagate prions in cells or organoids derived from stem cells, these next-generation cellular models should provide an ideal paradigm for identifying small molecules and other biological therapeutics capable of interfering with prion replication in animal and human prion disorders. In this review, we summarize recent advances that have widened the spectrum of prion strains that can be propagated in cultured cells and cutting-edge tissue-based models.

Transport of Prions in the Peripheral Nervous System: Pathways, Cell Types, and Mechanisms

Prion diseases are transmissible protein misfolding disorders that occur in animals and humans where the endogenous prion protein, PrP^C, undergoes a conformational change into self-templating aggregates termed PrP^Sc. Formation of PrP^Sc in the central nervous system (CNS) leads to gliosis, spongiosis, and cellular dysfunction that ultimately results in the death of the host. The spread of prions from peripheral inoculation sites to CNS structures occurs through neuroanatomical networks. While it has been established that endogenous PrP^C is necessary for prion formation, and that the rate of prion spread is consistent with slow axonal transport, the mechanistic details of PrP^Sc transport remain elusive. Current research endeavors are primarily focused on the cellular mechanisms of prion transport associated with axons. This includes elucidating specific cell types involved, subcellular machinery, and potential cofactors present during this process.

Propagation and Dissemination Strategies of Transmissible Spongiform Encephalopathy Agents in Mammalian Cells

Transmissible spongiform encephalopathies or prion disorders are fatal infectious diseases that cause characteristic spongiform degeneration in the central nervous system. The causative agent, the so-called prion, is an unconventional infectious agent that propagates by converting the host-encoded cellular prion protein PrP into ordered protein aggregates with infectious properties. Prions are devoid of coding nucleic acid and thus rely on the host cell machinery for propagation. While it is now established that, in addition to PrP, other cellular factors or processes determine the susceptibility of cell lines to prion infection, exact factors and cellular processes remain broadly obscure. Still, cellular models have uncovered important aspects of prion propagation and revealed intercellular dissemination strategies shared with other intracellular pathogens. Here, we summarize what we learned about the processes of prion invasion, intracellular replication and subsequent dissemination from ex vivo cell models.

From Cell Culture to Organoids-Model Systems for Investigating Prion Strain Characteristics

Prion diseases are the hallmark protein folding neurodegenerative disease. Their transmissible nature has allowed for the development of many different cellular models of disease where prion propagation and sometimes pathology can be induced. This review examines the range of simple cell cultures to more complex neurospheres, organoid, and organotypic slice cultures that have been used to study prion disease pathogenesis and to test therapeutics. We highlight the advantages and disadvantages of each system, giving special consideration to the importance of strains when choosing a model and when interpreting results, as not all systems propagate all strains, and in some cases, the technique used, or treatment applied, can alter the very strain properties being studied.

An astrocyte cell line that differentially propagates murine prions

Prion diseases are fatal infectious neurodegenerative disorders in human and animals caused by misfolding of the cellular prion protein (PrP^C) into the pathological isoform PrP^Sc. Elucidating the molecular and cellular mechanisms underlying prion propagation may help to develop disease interventions. Cell culture systems for prion propagation have greatly advanced molecular insights into prion biology, but translation of in vitro to in vivo findings is often disappointing. A wider range of cell culture systems might help overcome these shortcomings. Here, we describe an immortalized mouse neuronal astrocyte cell line (C8D1A) that can be infected with murine prions. Both PrP^C protein and mRNA levels in astrocytes were comparable with those in neuronal and non-neuronal cell lines permitting persistent prion infection. We challenged astrocytes with three mouse-adapted prion strains (22L, RML, and ME7) and cultured them for six passages. Immunoblotting results revealed that the astrocytes propagated 22L prions well over all six passages, whereas ME7 prions did not replicate, and RML prions replicated only very weakly after five passages. Immunofluorescence analysis indicated similar results for PrP^Sc. Interestingly, when we used prion conversion activity as a readout in real-time quaking-induced conversion assays with RML-infected cell lysates, we observed a strong signal over all six passages, comparable with that for 22L-infected cells. These data indicate that the C8D1A cell line is permissive to prion infection. Moreover, the propagated prions differed in conversion and proteinase K–resistance levels in these astrocytes. We propose that the C8D1A cell line could be used to decipher prion strain biology.

Cellular models for discovering prion disease therapeutics: Progress and challenges

Prions, which cause fatal neurodegenerative disorders such as Creutzfeldt-Jakob disease, are misfolded and infectious protein aggregates. Currently, there are no treatments available to halt or even delay the progression of prion disease in the brain. The infectious nature of prions has resulted in animal paradigms that accurately recapitulate all aspects of prion disease, and these have proven to be instrumental for testing the efficacy of candidate therapeutics. Nonetheless, infection of cultured cells with prions provides a much more powerful system for identifying molecules capable of interfering with prion propagation. Certain lines of cultured cells can be chronically infected with various types of mouse prions, and these models have been used to unearth candidate anti-prion drugs that are at least partially efficacious when administered to prion-infected rodents. However, these studies have also revealed that not all types of prions are equal, and that drugs active against mouse prions are not necessarily effective against prions from other species. Despite some recent progress, the number of cellular models available for studying non-mouse prions remains limited. In particular, human prions have proven to be particularly challenging to propagate in cultured cells, which has severely hindered the discovery of drugs for Creutzfeldt-Jakob disease. In this review, we summarize the cellular models that are presently available for discovering and testing drugs capable of blocking the propagation of prions and highlight challenges that remain on the path towards developing therapies for prion disease.

The cellular and pathologic prion protein

The cellular prion protein, PrP^C, is a small, cell surface glycoprotein with a function that is currently somewhat ill defined. It is also the key molecule involved in the family of neurodegenerative disorders called transmissible spongiform encephalopathies, which are also known as prion diseases. The misfolding of PrP^C to a conformationally altered isoform, designated PrP^TSE, is the main molecular process involved in pathogenesis and appears to precede many other pathologic and clinical manifestations of disease, including neuronal loss, astrogliosis, and cognitive loss. PrP^TSE is also believed to be the major component of the infectious "prion," the agent responsible for disease transmission, and preparations of this protein can cause prion disease when inoculated into a naïve host. Thus, understanding the biochemical and biophysical properties of both PrP^C and PrP^TSE, and ultimately the mechanisms of their interconversion, is critical if we are to understand prion disease biology. Although entire books could be devoted to research pertaining to the protein, herein we briefly review the state of knowledge of prion biochemistry, including consideration of prion protein structure, function, misfolding, and dysfunction.

Cell biology of prion infection

The development of multiple cell culture models of prion infection over the last two decades has led to a significant increase in our understanding of how prions infect cells. In particular, new techniques to distinguish exogenous from endogenous prions have allowed us for the first time to look in depth at the earliest stages of prion infection through to the establishment of persistent infection. These studies have shown that prions can infect multiple cell types, both neuronal and nonneuronal. Once in contact with the cell, they are rapidly taken up via multiple endocytic pathways. After uptake, the initial replication of prions occurs almost immediately on the plasma membrane and within multiple endocytic compartments. Following this acute stage of prion replication, persistent prion infection may or may not be established. Establishment of a persistent prion infection in cells appears to depend upon the achievement of a delicate balance between the rate of prion replication and degradation, the rate of cell division, and the efficiency of prion spread from cell to cell. Overall, cell culture models have shown that prion infection of the cell is a complex and variable process which can involve multiple cellular pathways and compartments even within a single cell.

Flow Cytometric Detection of PrPSc in Neurons and Glial Cells from Prion-Infected Mouse Brains

In prion diseases, an abnormal isoform of prion protein (PrP^Sc) accumulates in neurons, astrocytes, and microglia in the brains of animals affected by prions. Detailed analyses of PrP^Sc-positive neurons and glial cells are required to clarify their pathophysiological roles in the disease. Here, we report a novel method for the detection of PrP^Sc in neurons and glial cells from the brains of prion-infected mice by flow cytometry using PrP^Sc-specific staining with monoclonal antibody (MAb) 132. The combination of PrP^Sc staining and immunolabeling of neural cell markers clearly distinguished neurons, astrocytes, and microglia that were positive for PrP^Sc from those that were PrP^Sc negative. The flow cytometric analysis of PrP^Sc revealed the appearance of PrP^Sc-positive neurons, astrocytes, and microglia at 60 days after intracerebral prion inoculation, suggesting the presence of PrP^Sc in the glial cells, as well as in neurons, from an early stage of infection. Moreover, the kinetic analysis of PrP^Sc revealed a continuous increase in the proportion of PrP^Sc-positive cells for all cell types with disease progression. Finally, we applied this method to isolate neurons, astrocytes, and microglia positive for PrP^Sc from a prion-infected mouse brain by florescence-activated cell sorting. The method described here enables comprehensive analyses specific to PrP^Sc-positive neurons, astrocytes, and microglia that will contribute to the understanding of the pathophysiological roles of neurons and glial cells in PrP^Sc-associated pathogenesis.IMPORTANCE Although formation of PrP^Sc in neurons is associated closely with neurodegeneration in prion diseases, the mechanism of neurodegeneration is not understood completely. On the other hand, recent studies proposed the important roles of glial cells in PrP^Sc-associated pathogenesis, such as the intracerebral spread of PrP^Sc and clearance of PrP^Sc from the brain. Despite the great need for detailed analyses of PrP^Sc-positive neurons and glial cells, methods available for cell type-specific analysis of PrP^Sc have been limited thus far to microscopic observations. Here, we have established a novel high-throughput method for flow cytometric detection of PrP^Sc in cells with more accurate quantitative performance. By applying this method, we succeeded in isolating PrP^Sc-positive cells from the prion-infected mouse brains via fluorescence-activated cell sorting. This allows us to perform further detailed analysis specific to PrP^Sc-positive neurons and glial cells for the clarification of pathological changes in neurons and pathophysiological roles of glial cells.

Physiological Functions of the Cellular Prion Protein

The prion protein, PrP^C, is a small, cell-surface glycoprotein notable primarily for its critical role in pathogenesis of the neurodegenerative disorders known as prion diseases. A hallmark of prion diseases is the conversion of PrP^C into an abnormally folded isoform, which provides a template for further pathogenic conversion of PrP^C, allowing disease to spread from cell to cell and, in some circumstances, to transfer to a new host. In addition to the putative neurotoxicity caused by the misfolded form(s), loss of normal PrP^C function could be an integral part of the neurodegenerative processes and, consequently, significant research efforts have been directed toward determining the physiological functions of PrP^C. In this review, we first summarise important aspects of the biochemistry of PrP^C before moving on to address the current understanding of the various proposed functions of the protein, including details of the underlying molecular mechanisms potentially involved in these functions. Over years of study, PrP^C has been associated with a wide array of different cellular processes and many interacting partners have been suggested. However, recent studies have cast doubt on the previously well-established links between PrP^C and processes such as stress-protection, copper homeostasis and neuronal excitability. Instead, the functions best-supported by the current literature include regulation of myelin maintenance and of processes linked to cellular differentiation, including proliferation, adhesion, and control of cell morphology. Intriguing connections have also been made between PrP^C and the modulation of circadian rhythm, glucose homeostasis, immune function and cellular iron uptake, all of which warrant further investigation.

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.

Functional Role of the Disulfide Isomerase ERp57 in Axonal Regeneration

ERp57 (also known as grp58 and PDIA3) is a protein disulfide isomerase that catalyzes disulfide bonds formation of glycoproteins as part of the calnexin and calreticulin cycle. ERp57 is markedly upregulated in most common neurodegenerative diseases downstream of the endoplasmic reticulum (ER) stress response. Despite accumulating correlative evidence supporting a neuroprotective role of ERp57, the contribution of this foldase to the physiology of the nervous system remains unknown. Here we developed a transgenic mouse model that overexpresses ERp57 in the nervous system under the control of the prion promoter. We analyzed the susceptibility of ERp57 transgenic mice to undergo neurodegeneration. Unexpectedly, ERp57 overexpression did not affect dopaminergic neuron loss and striatal denervation after injection of a Parkinson’s disease-inducing neurotoxin. In sharp contrast, ERp57 transgenic animals presented enhanced locomotor recovery after mechanical injury to the sciatic nerve. These protective effects were associated with enhanced myelin removal, macrophage infiltration and axonal regeneration. Our results suggest that ERp57 specifically contributes to peripheral nerve regeneration, whereas its activity is dispensable for the survival of a specific neuronal population of the central nervous system. These results demonstrate for the first time a functional role of a component of the ER proteostasis network in peripheral nerve regeneration.

A bovine cell line that can be infected by natural sheep scrapie prions

Cell culture systems represent a crucial part in basic prion research; yet, cell lines that are susceptible to prions, especially to field isolated prions that were not adapted to rodents, are very rare. The purpose of this study was to identify and characterize a cell line that was susceptible to ruminant-derived prions and to establish a stable prion infection within it. Based on species and tissue of origin as well as PrP expression rate, we pre-selected a total of 33 cell lines that were then challenged with natural and with mouse propagated BSE or scrapie inocula. Here, we report the successful infection of a non-transgenic bovine cell line, a sub-line of the bovine kidney cell line MDBK, with natural sheep scrapie prions. This cell line retained the scrapie infection for more than 200 passages. Selective cloning resulted in cell populations with increased accumulation of PrP^res, although this treatment was not mandatory for retaining the infection. The infection remained stable, even under suboptimal culture conditions. The resulting infectivity of the cells was confirmed by mouse bioassay (Tgbov mice, Tgshp mice). We believe that PES cells used together with other prion permissive cell lines will prove a valuable tool for ongoing efforts to understand and defeat prions and prion diseases.

The potential of mesenchymal stem cell in prion research

Scrapie and bovine spongiform encephalopathy are fatal neurodegenerative diseases caused by the accumulation of a misfolded protein (PrP(res)), the pathological form of the cellular prion protein (PrP(C)). For the last decades, prion research has greatly progressed, but many questions need to be solved about prion replication mechanisms, cell toxicity, differences in genetic susceptibility, species barrier or the nature of prion strains. These studies can be developed in murine models of transmissible spongiform encephalopathies, although development of cell models for prion replication and sample titration could reduce economic and timing costs and also serve for basic research and treatment testing. Some murine cell lines can replicate scrapie strains previously adapted in mice and very few show the toxic effects of prion accumulation. Brain cell primary cultures can be more accurate models but are difficult to develop in naturally susceptible species like humans or domestic ruminants. Stem cells can be differentiated into neuron-like cells and be infected by prions. However, the use of embryo stem cells causes ethical problems in humans. Mesenchymal stem cells (MSCs) can be isolated from many adult tissues, including bone marrow, adipose tissue or even peripheral blood. These cells differentiate into neuronal cells, express PrP(C) and can be infected by prions in vitro. In addition, in the last years, these cells are being used to develop therapies for many diseases, including neurodegenerative diseases. We review here the use of cell models in prion research with a special interest in the potential use of MSCs.

Infectious prions accumulate to high levels in non proliferative C2C12 myotubes

Prion diseases are driven by the strain-specific, template-dependent transconformation of the normal cellular prion protein (PrP^C) into a disease specific isoform PrP^Sc. Cell culture models of prion infection generally use replicating cells resulting in lower levels of prion accumulation compared to animals. Using non-replicating cells allows the accumulation of higher levels of PrP^Sc and, thus, greater amounts of infectivity. Here, we infect non-proliferating muscle fiber myotube cultures prepared from differentiated myoblasts. We demonstrate that prion-infected myotubes generate substantial amounts of PrP^Sc and that the level of infectivity produced in these post-mitotic cells, 10^5.5 L.D.₅₀/mg of total protein, approaches that observed in vivo. Exposure of the myotubes to different mouse-adapted agents demonstrates strain-specific replication of infectious agents. Mouse-derived myotubes could not be infected with hamster prions suggesting that the species barrier effect is intact. We suggest that non-proliferating myotubes will be a valuable model system for generating infectious prions and for screening compounds for anti-prion activity.

This manuscript describes the generation of a new cell culture system to study the replication of infectious prions. While numerous cell lines exist that can replicate prions, these systems are usually based upon proliferating cells. As mammalian cell cultures double approximately every day, prions established in the culture must also, at least, double to be maintained. This is problematic, however, as prions replicate relatively slowly and cell replication may outpace prion replication. In fact, many cell culture systems do not replicate prions and those that do often do not replicate all strains of prions. Here we describe the use of differentiated non-proliferative muscle cells to replicate prions without the interfering effect of cell division. We observed that prions accumulate to very high levels in this muscle cell culture with infectivity approaching that observed in animals.

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.

Cellular aspects of prion replication in vitro

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders in mammals that are caused by unconventional agents predominantly composed of aggregated misfolded prion protein (PrP). Prions self-propagate by recruitment of host-encoded PrP into highly ordered β-sheet rich aggregates. Prion strains differ in their clinical, pathological and biochemical characteristics and are likely to be the consequence of distinct abnormal prion protein conformers that stably replicate their alternate states in the host cell. Understanding prion cell biology is fundamental for identifying potential drug targets for disease intervention. The development of permissive cell culture models has greatly enhanced our knowledge on entry, propagation and dissemination of TSE agents. However, despite extensive research, the precise mechanism of prion infection and potential strain effects remain enigmatic. This review summarizes our current knowledge of the cell biology and propagation of prions derived from cell culture experiments. We discuss recent findings on the trafficking of cellular and pathologic PrP, the potential sites of abnormal prion protein synthesis and potential co-factors involved in prion entry and propagation.

Cobalamin and normal prions: a new horizon for cobalamin neurotrophism

It is known that cobalamin (Cbl) deficiency damages myelin by increasing tumor necrosis factor (TNF)-α and decreasing epidermal growth factor (EGF) levels in rat central nervous system (CNS), and affects the peripheral nervous system (PNS) morphologically and functionally. It is also known that some polyneuropathies not due to Cbl deficiency are connected with increased TNF-α levels, and that various cytokines (including TNF-α) and growth factors regulate the in vitro synthesis of normal prions (PrP(C)s). Given that there is extensive evidence that PrP(C)s play a key role in the maintenance of CNS and PNS myelin, we investigated whether the PrP(C) octapeptide repeat (OR) region is involved in the pathogenesis of rat Cbl-deficient (Cbl-D) polyneuropathy. After intracerebroventricularly administering antibodies (Abs) against the OR region (OR-Abs) to Cbl-D rats to prevent myelin damage and maximum nerve conduction velocity (MNCV) abnormalities, and PrP(C)s to otherwise normal rats to reproduce PNS Cbl-D-like lesions, we measured PrP(C) levels and MNCV of the sciatic and tibial nerves. PrP(C) and TNF-α levels were increased in sciatic and tibial nerves of Cbl-D and saline-treated rats, and the OR-Abs normalized the myelin ultrastructure, TNF-α levels, and MNCV values of the sciatic and tibial nerves of Cbl-D rats. The same peripheral nerves of the otherwise normal PrP(C)-treated rats showed typical Cbl-D myelin lesions, significantly increased TNF-α levels, and significantly decreased MNCV values. These findings demonstrate that Cbl deficiency induces excess PrP(C)s and thereby excess OR regions, which seem to be responsible for the PNS myelin damage, as has recently been found in the case of CNS myelin damage [66]. Furthermore, excess TNF-α is also involved in the pathogenesis of Cbl-D polyneuropathy. In conclusion, we have extended the list of prion diseases by adding one caused by excess PrP(C)s and the polyneuropathies related to excess TNF-α.

Immunohistochemical detection of disease-associated prion protein in the peripheral nervous system in experimental H-type bovine spongiform encephalopathy

H-type bovine spongiform encephalopathy (BSE) has been identified in aged cattle in Europe and North America. To determine the localization of disease-associated prion protein (PrP(Sc)) in the peripheral nerve tissues of cattle affected with H-type BSE, we employed highly sensitive immunohistochemical and immunofluorescence techniques with the tyramide signal amplification (TSA) system. PrP(Sc) deposition was detected in the inferior ganglia, sympathetic nerve trunk, vagus nerve, spinal nerves, cauda equina, and adrenal medulla, using this system. Notably, granular PrP(Sc) deposits were present mainly in the Schwann cells and fibroblast-like cells and occasionally along certain nerve fibers at the surface of the axons. In the adrenal gland, PrP(Sc) immunolabeling was observed within the sympathetic nerve fibers and nerve endings in the adrenal medulla. Although our results were limited to only 3 experimental cases, these results suggest that the TSA system, a highly sensitive immunohistochemical procedure, may help in elucidating the peripheral pathogenesis of H-type BSE.

Neuroimmune connections in ovine pharyngeal tonsil: potential site for prion neuroinvasion

Recent studies have established the involvement of nasal-associated lymphoid tissues, mainly the pharyngeal tonsil, in prion pathogenesis. However, the mechanisms of the associated neuroinvasion are still debated. To determine potential sites for prion neuroinvasion inside the ovine pharyngeal tonsil, the topography of heavy (200 kDa) and light (70 kDa) neurofilaments and of glial fibrillar acidic protein has been semi-quantitatively analysed inside the various compartments of the tonsil. The results show that the most innervated areas are the interfollicular area and the connective tissue located beneath the respiratory epithelium. The existence of rare synapses between follicular dendritic cells and nerve fibres inside the germinal centre indicates that this mechanism of neuroinvasion is possible but, since germinal centres of lymphoid follicles are poorly innervated, other routes of neuroinvasion are likely. The host PRNP genotype does not influence the pattern of innervation in these various tonsil compartments, unlike ageing during which an increase of nerve endings occurs in a zone of high trafficking cells beneath the respiratory epithelium. A minimal age-related increase of innervation inside the lymphoid follicles has also been observed. An increase in nerve fibre density around the lymphoid follicles, in an area rich in mobile cells such as macrophages and dendritic cells capable of capturing and conveying pathogen prion protein (PrPd), might ensure more efficient infectivity, not in the early phase but in the advanced phase of lymphoinvasion after the amplification of PrPd; alternatively, this area might even act as a direct site of entry during neuroinvasion.

Detection of disease-associated prion protein in the optic nerve and the adrenal gland of cattle with bovine spongiform encephalopathy by using highly sensitive immunolabeling procedures

A sensitive immunohistochemical procedure, the tyramide signal amplification (TSA) system, was applied to detect the localization of immunolabeled disease-associated prion protein (PrP(Sc)) in cattle affected with bovine spongiform encephalopathy (BSE). In this procedure, immunolabeling could be visualized in the optic nerve and the adrenal medulla. In the optic nerve, the dual immunofluorescent technique showed that the granular PrP(Sc) was occasionally detected in the astrocytes, microglia, and myelin sheath adjacent to the axon. Clustered PrP(Sc) was also scattered in association with microglial cells and astrocytes of the optic nerve. In the adrenal gland, PrP(Sc) immunolabeling was confined within the sympathetic nerve fibers and endings. The results suggest that (1) PrP(Sc) might centrifugally spread within and between glial cells and/or the non-axonal (also known as ad-axonal) region of nerve fibers, rather than the axonal and/or extracellular space pathway in the optic nerve, and (2) the sympathetic innervations might be important for the trafficking of BSE agent in the adrenal glands of cattle. This study also suggests that tyramide-based immunochemical analysis should be performed to detect immunolabeled PrP(Sc) in the extracerebral tissues of BSE-affected cattle.

The cellular prion protein in multiple sclerosis: A potential target for neurotherapeutics?

Multiple sclerosis (MS) is a debilitating disease that affects millions. There is no known cure for the disease and neither is the cause of the disease known. Recent studies have indicated that it is a multi-factorial disease with several genes involved. Importantly, sunlight and vitamin D have been implicated in the progression of the disease. The pathogenesis of MS chiefly involves loss of oligodendrocytes, which in addition to being killed by inflammatory mediators in the CNS, also succumbs to loss of trophic support from astrocytes. Neurotrophins play an important role in myelination and the cellular prion protein (PrPC) is a key player in this process. Although the physiological roles of PrPC remain to be fully understood, increasing evidence suggests multiple roles for PrPC in regulation of cellular immunity and for its interaction with several neurotrophins that are necessary for homeostasis of the nervous system. This mini-review focuses on the findings establishing a crucial role for PrPC in the neuropathogenesis of MS, emphasizing its neuroprotective role. Since MS is a multi-factorial disease with unknown etiology and no cure, this review aims to highlight endogenous repair mechanisms mediated by PrPC that might contribute to functional recovery in MS patients.

Prion replication in the hematopoietic compartment is not required for neuroinvasion in scrapie mouse model

Fatal neurodegenerative prion diseases are caused by the transmissible PrP^Sc prion agent whose initial replication after peripheral inoculation takes place in follicular dendritic cells present in germinal centers of lymphoid organs. However, prion replication also occurs in lymphoid cells. To assess the role of the hematopoietic compartment in neuroinvasion and prion replication, we generated chimeric mice, on a uniform congenic C57/BL6J background, by bone marrow replacement with hematopoietic cells expressing different levels of PrP protein. Nine different types of chimeric mice were inoculated intraperitoneally either with the lymphotropic Rocky Mountain Laboratory (RML) strain or the non lymphotropic ME-7 scrapie strain, at different doses. Here, we clearly demonstrate that overexpression of PrP by the hematopoietic system, or the lack of PrP expression by the bone marrow derived cells, does not change the incubation time period of the disease, even when the mice are infected at limiting doses. We conclude that the hematopoietic compartment is more or less permissive to prion replication, both for RML and ME-7, but does not play a role in neuroinvasion.

Dramatic reduction of PrP C level and glycosylation in peripheral nerves following PrP knock-out from Schwann cells does not prevent transmissible spongiform encephalopathy neuroinvasion

Expression of the prion protein (PrP(C)) is a requirement for host susceptibility to the transmissible spongiform encephalopathies (TSEs) and thought to be necessary for the replication and transport of the infectious agent. The mechanism of TSE neuroinvasion is not fully understood, although the routing of infection has been mapped through the peripheral nervous system (PNS) and Schwann cells have been implicated as a potential conduit for transport of the TSE infectious agent. To address whether Schwann cells are a requirement for spread of the TSE agent from the site of infection to the CNS, PrP(C) expression was selectively removed from Schwann cells in vivo. This dramatically reduced total PrP(C) within peripheral nerves by 90%, resulting in the selective loss of glycosylated PrP(C) species. Despite this, 139A and ME7 mouse-passaged scrapie agent strains were efficiently replicated and transported to the CNS following oral and intraperitoneal exposure. Thus, the myelinating glial cells within the PNS do not appear to play a significant role in TSE neuroinvasion.

PrP expression, PrPSc accumulation and innervation of splenic compartments in sheep experimentally infected with scrapie

Background

In prion disease, the peripheral expression of PrP^C is necessary for the transfer of infectivity to the central nervous system. The spleen is involved in neuroinvasion and neural dissemination in prion diseases but the nature of this involvement is not known. The present study undertook the investigation of the spatial relationship between sites of PrP^Sc accumulation, localisation of nerve fibres and PrP^C expression in the tissue compartments of the spleen of scrapie-inoculated and control sheep.

Methodology/Principal Findings

Laser microdissection and quantitative PCR were used to determine PrP mRNA levels and results were compared with immunohistochemical protocols to distinguish PrP^C and PrP^Sc in tissue compartments of the spleen. In sheep experimentally infected with scrapie, the major sites of accumulation of PrP^Sc in the spleen, namely the lymphoid nodules and the marginal zone, expressed low levels of PrP mRNA. Double immunohistochemical labelling for PrP^Sc and the pan-nerve fibre marker, PGP, was used to evaluate the density of innervation of splenic tissue compartments and the intimacy of association between PrP^Sc and nerves. Some nerve fibres were observed to accompany blood vessels into the PrP^Sc-laden germinal centres. However, the close association between nerves and PrP^Sc was most apparent in the marginal zone. Other sites of close association were adjacent to the wall of the central artery of PALS and the outer rim of germinal centres.

Conclusions/Significance

The findings suggest that the degree of PrP^Sc accumulation does not depend on the expression level of PrP^C. Though several splenic compartments may contribute to neuroinvasion, the marginal zone may play a central role in being the compartment with most apparent association between nerves and PrP^Sc.

The role of the prion protein membrane anchor in prion infection

Normal cellular and abnormal disease-associated forms of prion protein (PrP) contain a C-terminal glycophosphatidyl-inositol (GPI) membrane anchor. The importance of the GPI membrane anchor in prion diseases is unclear but there are data to suggest that it both is and is not required for abnormal prion protein formation and prion infection. Utilizing an in vitro model of prion infection we have recently demonstrated that, while the GPI anchor is not essential for the formation of abnormal prion protein in a cell, it is necessary for the establishment of persistent prion infection. In combination with previously published data, our results suggest that GPI anchored PrP is important in the amplification and spread of prion infectivity from cell to cell.

Assaying prions in cell culture: the standard scrapie cell assay (SSCA) and the scrapie cell assay in end point format (SCEPA)

Prions are usually quantified by bioassays based on intracerebral inoculation of animals, which are slow, imprecise, and costly. We have developed a cell-based prion assay that is based on the isolation of cell lines highly susceptible to certain strains (Rocky Mountain Laboratory and 22L) of mouse prions and a method for identifying individual, prion-infected cells and quantifying them. In the standard scrapie cell assay (SSCA), susceptible cells are exposed to prion-containing samples for 4 days, grown to confluence, passaged two or three times, and the proportion of rPrP(Sc)-containing cells is determined with automated counting equipment. The dose response is dynamic over 2 logs of prion concentrations. The SSCA has a standard error of +/-20-30%, is as sensitive as the mouse bioassay, 10 times faster, at least 2 orders of magnitude less expensive, and it is suitable for robotization. Assays performed in a more time-consuming end point titration format extend the sensitivity and show that infectivity titers measured in tissue culture and in the mouse are similar.

Evidence for prion protein expression in enteroglial cells of the myenteric plexus of mouse intestine

Transmissible spongiform encephalopathies (TSEs) are slowly progressive and fatal neurodegenerative diseases affecting man and animals. They are caused by pathological isoforms (PrP(Sc)) of the host-encoded cellular prion protein (PrP(C)). There are two crucial factors for the initiation of infection, namely host cells PrP(C) expression and sufficient sequence homology between the PrP(Sc) to which the animal is exposed and its own PrP(C). In acquired TSEs, the gastrointestinal tract (GIT) is the main prion entry site. Hence, it is of paramount importance to an understanding of the early pathogenesis of prion infections, to characterize the GIT cell types constitutively expressing PrP(C). Twenty-three mice were utilized, including wild-type (WT), Prnp knock-out (KO), and PrP(C)-overexpressing (tga20/tga20) animals, of 20-30 g in weight and of either sex. In all three groups of mice, PrP(C)-immunoreactivity (IR), along with glial fibrillary acidic protein (GFAP)-IR and synaptophysin (Syn)-IR were investigated by means of indirect immunofluorescence in wholemount preparations from several gut regions, from duodenum to rectum. In WT mice, PrP(C)-IR and GFAP-IR co-localization was observed in enteric glial cells (EGCs) from all intestinal segments. PrP(C)-overexpressing mice showed a stronger PrP(C)-IR in EGCs, whereas the same cells exhibited no PrP(C)-IR in Prnp-KO mice. Our findings clearly indicate that EGCs of the mouse intestine constitutively express PrP(C); thus they could be a potential target for infectious prions.

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.

Cell culture models to unravel prion protein function and aberrancies in prion diseases

From an early stage of prion research, tissue cultures that could support and propagate the scrapie agent were sought after. The earliest attempts were explants from brains of infected mice, and their growth and morphological characteristics were compared with those from uninfected mice. Using the explant technique, several investigators reported increased cell growth in cultures established from scrapie-sick brain compared with cultures from normal mice. These are odd findings in the light of the massive neuronal cell death known to occur in scrapie-infected brains; however, the cell types responsible for the increased cell growth in the scrapie-explants most probably were not neuronal. The first successful cell culture established in this way, in which the scrapie agent was serially and continuously passaged beyond the initial explant, was in the scrapie mouse brain culture, which is still used today. This chapter describes the generation and use of chronically prion-infected cell lines as cell culture models of prion diseases. These cell lines have been crucial for the current understanding of the cell biology of both the normal (PrP(C)) and the pathogenic isoform (PrP(Sc)) of the prion protein. They also have been useful in the development of antiprion drugs, prospectively used for therapy of prion diseases, and they offer an alternative approach for transmission/infectivity assays normally performed by mouse bioassay. Cell culture models also have been used to study prion-induced cytopathological changes, which could explain the typical spongiform neurodegeneration in prion diseases.

Cell models of prion infection

Due to recent renewal of interest and concerns in prion diseases, a number of cell systems permissive to prion multiplication have been generated in the last years. These include established cell lines, neuronal stem cells and primary neuronal cultures. While most of these models are permissive to experimental, mouse-adapted strains of prions, the propagation of natural field isolates from sheep scrapie and chronic wasting disease has been recently achieved. These models have improved our knowledge on the molecular and cellular events controlling the conversion of the PrP(C) protein into abnormal isoforms and on the cell-to-cell spreading of prions. Infected cultured cells will also facilitate investigations on the molecular basis of strain identity and on the mechanisms that lead to neurodegeneration. The ongoing development of new cell models with improved characteristics will certainly be useful for a number of unanswered critical issues in the prion field.

Molecular biology of prion protein and its first homologous protein

Conformational conversion of the normal cellular isoform of prion protein, PrP(C), a glycoprotein anchored to the cell membrane by a glycosylphosphatidylinositol moiety, into the abnormally folded, amyloidogenic prion protein, PrP(Sc), plays a pivotal role in the pathogenesis of prion diseases. It has been suggested that PrP(C) might be functionally disturbed by constitutive conversion to PrP(Sc) due to either the resulting depletion of PrP(C) or the dominant negative effects of PrP(Sc) on PrP(C) or both. Consistent with this, we and others showed that mice devoid of PrP(C) (PrP-/-) spontaneously developed abnormal phenotypes very similar to the neurological abnormalities of prion diseases, supporting the concept that functional loss of PrP(C) might at least be partly involved in the pathogenesis of the diseases. However, no neuronal cell death could be detected in PrP-/- mice, indicating that the functional loss of PrP(C) alone might not be enough to induce neuronal cell death, one of major pathological hallmarks of prion diseases. Interestingly, it was recently shown that the first identified PrP-like protein, termed PrPLP/Doppel (Dpl), is neurotoxic in the absence of PrP(C), causing Purkinje cell degeneration in the cerebellum of mice. Although it is not understood if PrP(Sc) could have a neurotoxic potential similar to PrPLP/Dpl, it is very interesting to speculate that accumulation of PrP(Sc) and the functional disturbance of PrP(C), both of which are caused by constitutive conversion, might be required for the neurodegeneration in prion diseases.

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.

Neuroimmune connections in jejunal and ileal Peyer’s patches at various bovine ages: potential sites for prion neuroinvasion

During preclinical stages of cattle orally infected with bovine spongiform encephalopathy (BSE), the responsible agent is confined to ileal Peyer's patches (IPP), namely in nerve fibers and in lymph follicles, before reaching the peripheral and central nervous systems. No infectivity has been reported in other bovine lymphoid organs, including jejunal Peyer's patches (JPP). To determine the potential sites for prion neuroinvasion in IPP, we analyzed the mucosal innervation and the interface between nerve fibers and follicular dendritic cells (FDC), two dramatic influences on neuroinvasion. Bovine IPP were studied at three ages, viz., newborn calves, calves less than 12 months old, and bovines older than 24 months, and the parameters obtained were compared with those of JPP. No differences in innervation patterns between IPP and JPP were found. The major difference observed was that, in calves of less than 12 months, IPP were the major mucosal-associated lymphoid organ that possessed a large number of follicles with extended FDC networks. Using a panel of antibodies, we showed that PP in 24-month-old bovines were highly innervated at various strategic sites assumed to be involved in the invasion and replication of the BSE pathogen: the suprafollicular dome, T cell area, and germinal centers. In PP in calves of less than 12 months old, no nerve fibers positive for the neurofilament markers NF-L (70 kDa) and NF-H (200 kDa) were observed in contact with FDC. Thus, in view of the proportion of these protein subunits present in neurofilaments, the innervation of the germinal centers can be said to be an age-dependent dynamic process. This variation in innervation might influence the path of neuroinvasion and, thus, the susceptibility of bovines to the BSE agent.

Neural stem cell model for prion propagation

The study of prion transmission and targeting is a major scientific issue with important consequences for public health. Only a few cell culture systems that are able to convert the cellular isoform of the prion protein into the pathologic scrapie isoform of the prion protein (PrP(Sc)) have been described. We hypothesized that central nervous system neural stem cells (NSCs) could be the basis of a new cell culture model permissive to prion infection. Here, we report that monolayers of differentiated fetal NSCs and adult multipotent progenitor cells isolated from mice were able to propagate prions. We also demonstrated the large influence of neural cell fate on the production of PrP(Sc), allowing the molecular study of prion neuronal targeting in relation with strain differences. This new stem cell-based model, which is applicable to different species and to transgenic mice, will allow thoughtful investigations of the molecular basis of prion diseases, and will open new avenues for diagnostic and therapeutic research.

Prion diseases: current understanding of epidemiology and pathogenesis, and therapeutic advances

The bovine spongiform encephalopathy (BSE) epidemic, along with the related threat to human health posed by the transmission of the BSE agent to humans, has highlighted the importance of prion diseases. These fatal neurodegenerative diseases are characterised by spongiform changes in the CNS, and comprise a wide spectrum of clinicopathological entities in humans and animals, such as Creutzfeldt-Jakob disease (CJD) and its emerging new variant (vCJD) in humans, and BSE and scrapie in animals. This article reviews the geographical distribution and the temporal trends of CJD and vCJD; the major events in the pathogenesis of prion diseases; the risk factors for sporadic CJD and vCJD; and the possible strategies for treating them. Worldwide statistics indicate that sporadic CJD has a stable incidence of one case per million people per year; in contrast, the incidence of vCJD appears to have increased exponentially from its characterisation in 1994 to a peak in 2000. As of December 2005, 183 definite or probable cases of vCJD had been reported worldwide. The crucial event in the pathogenesis of prion diseases is the conversion of the normally occurring cellular prion protein (PrP(c)) into a pathogenic form, called protease-resistant PrP (PrP(res)) or scrapie PrP (PrP(sc)). Pathogenetic studies in rodent models have shown that PrP(sc) is found in the enteric nervous system and in the gut-associated lymphoid tissue following oral scrapie ingestion. The role of the lymphoreticular system in the pathogenesis of TSE seems to be related to the strains of agents and the host genotype. Therapeutic approaches to vCJD are mainly based on the inhibition or prevention of the pathological change that creates PrP(sc). Derivatives of acridine (such as mepacrine [quinacrine]) and the phenothiazine psychotropics have been proposed as possible therapies because of their activity in cellular models; however, neither class was able to affect the protease resistance of preexisting PrP fibrils. More encouragingly, in animal models of prion disease, tetracyclines were found to reduce prion infectivity by direct inactivation of PrP(sc). While these findings are promising, the suitability of these compounds for clinical use is still limited by their low efficacy once symptoms are apparent. Treatments based on the vaccination approach have also produced positive results, but further investigations are necessary to establish their clinical application.

Mouse-adapted scrapie infection of SN56 cells: greater efficiency with microsome-associated versus purified PrP-res

The process by which transmissible spongiform encephalopathy agents, or prions, infect cells is unknown. We employed a new differentiable cell line (SN56) susceptible to infection with three mouse-adapted scrapie strains to gain insight into the cellular infection process. The effect of disease-associated PrP (PrP-res) association with microsomal membranes on infection efficiency was examined by comparing sustained PrP-res production in cells treated with either scrapie brain microsomes or purified, detergent-extracted PrP-res. When normalized for quantity of input PrP-res, scrapie brain microsomes induced dramatically enhanced persistent PrP-res formation compared to purified PrP-res. Infected SN56 cells released low levels of PrP-res into the culture supernatant, which also efficiently initiated infection in recipient cells. Interestingly, microsomes labeled with a fluorescent marker were internalized by SN56 cells in small vesicles, which were subsequently found in neuritic processes. When bound to culture wells to reduce internalization during the infection process, scrapie microsomes induced less long-term PrP-res production than suspended microsomes. Long-term differentiation of infected SN56 cells was accompanied by a decrease in PrP-res formation. Our observations provide evidence that infection of cells is aided by the association of PrP-res with membranes and/or other microsomal constituents.

Prion protein expression in bovine podocytes and extraglomerular mesangial cells

The cellular form of the prion protein (PrP(c)) is thought to be a substrate for an abnormal isoform of the prion protein (PrP(sc)). One emerging hypothesis is that the proposed conversion phenomenon takes place at the site at which the infectious agent meets PrP(c). PrP(c) is abundant in the central nervous system, but little is known about the cell-type-specific distribution of PrP(c) in non-neuronal tissues of cattle. We have studied whether PrP(c), a protein found predominantly in neurons, also exists in bovine podocytes, since neurons and podocytes share a large number of similarities. We have therefore examined the expression of PrP(c) by immunohistochemistry, reverse transcription/polymerase chain reaction and enzyme-linked immunosorbent analysis. Immunostained serial sections and specific antibodies against PrP(c) have revealed that PrP(c) is selectively localized in podocytes and is particularly strongly expressed in extraglomerular mesangial cells but not in endothelial or intraglomerular mesangial cells. The selective expression of PrP(c) in podocytes is of special importance, as it suggests that these cells represent possible targets for peripheral infection with prions and demonstrates that PrP(c) can be added to the list of neuronal factors expressed in mammalian podocytes.

Interfaces between dendritic cells, other immune cells, and nerve fibres in mouse Peyer's patches: potential sites for neuroinvasion in prion diseases

In this study, we examined where immune cells and nerve fibres are located in mouse Peyer's patches, with a view to identifying potential sites for neuroinvasion by prions. Special attention was paid to dendritic cells, viewed as candidate transporters of infectious prion. Double immunofluorescence labellings with anti-CD11c antibody and marker for other immune cells (B cells, T cells, follicular dendritic cells) were carried out and analysed by confocal microscopy on Peyer's patch cryosections. To reveal the extensive ganglionated networks of the myenteric and submucosal plexi and the sparse meshworks of nerve strands, we used antibodies directed against different neurofilament subunits or against glial fibrillary acidic protein. In the suprafollicular dome, dendritic cells connect, via their cytoplasmic extensions, enterocytes with M cells of the follicle-associated epithelium. They are also close to B and T cells. Nerve fibres are detected in the suprafollicular dome, notably in contact with dendritic cells. Similar connections between dendritic cells, T cells, and nerve fibres are seen in the interfollicular region. Germinal centres are not innervated; inside them dendritic cells establish contacts with follicular dendritic cells and with B cells. After immunolabelling of normal prion protein, dendritic cells of the suprafollicular dome are intensely positive labelled.

Axonal transport of the cellular prion protein is increased during axon regeneration

The cellular prion protein, PrPc, is a glycosylphosphatidylinositol-anchored cell surface glycoprotein and a protease-resistant conformer of the protein may be the infectious agent in transmissible spongiform encephalopathies. PrPc is localized on growing axons in vitro and along fibre bundles that contain elongating axons in developing and adult brain. To determine whether the growth state of axons influenced the expression and axonal transport of PrPc, we examined changes in the protein following post-traumatic regeneration in the hamster sciatic nerve. Our results show (1) that PrPc in nerve is significantly increased during nerve regeneration; (2) that this increase involves an increase in axonally transported PrPc; and (3) that the PrPc preferentially targeted for the newly formed portions of the regenerating axons consists of higher molecular weight glycoforms. These results raise the possibility that PrPc may play a role in the growth of axons in vivo, perhaps as an adhesion molecule interacting with the extracellular environment through specialized glycosylation.

BACE overexpression alters the subcellular processing of APP and inhibits Abeta deposition in vivo

Introducing mutations within the amyloid precursor protein (APP) that affect β- and γ-secretase cleavages results in amyloid plaque formation in vivo. However, the relationship between β-amyloid deposition and the subcellular site of Aβ production is unknown. To determine the effect of increasing β-secretase (BACE) activity on Aβ deposition, we generated transgenic mice overexpressing human BACE. Although modest overexpression enhanced amyloid deposition, high BACE overexpression inhibited amyloid formation despite increased β-cleavage of APP. However, high BACE expression shifted the subcellular location of APP cleavage to the neuronal perikarya early in the secretory pathway. These results suggest that the production, clearance, and aggregation of Aβ peptides are highly dependent on the specific neuronal subcellular domain wherein Aβ is generated and highlight the importance of perikaryal versus axonal APP proteolysis in the development of Aβ amyloid pathology in Alzheimer's disease.

Intrinsic resistance of oligodendrocytes to prion infection

Within the CNS, the normal form of cellular prion protein (PrP(C)) is expressed on neurons, oligodendrocytes, and astrocytes. The contribution of these cell types to prion replication and pathogenesis is unclear. To assess the role of oligodendrocytes, we expressed PrP(C) under the control of the myelin basic protein (MBP) promoter in mice lacking endogenous PrP(C). PrP(C) was detected in oligodendrocytes and Schwann cells but not in neurons and astrocytes. MBP-PrP mice never developed scrapie after intracerebral, intraperitoneal, or intraocular challenge with scrapie prions. Transgenic brains did not contain protease-resistant prion protein and did not transmit scrapie when inoculated into PrP(C)-overexpressing indicator mice. To investigate whether prion spread within the CNS depends on oligodendrocytic PrP(C), we implanted PrP(C)-overexpressing neuroectodermal grafts into MBP-PrP brains. After intraocular prion inoculation, none of the grafts showed spongiform encephalopathy or prion infectivity. Hence oligodendrocytes do not support cell-autonomous prion replication, establishment of subclinical disease, and neural spread of prions. Prion resistance sets oligodendrocytes aside from both neurons and astrocytes.

Propagation of a protease-resistant form of prion protein in long-term cultured human glioblastoma cell line T98G

Human prion diseases, such as Creutzfeldt–Jakob disease (CJD), a lethal, neurodegenerative condition, occur in sporadic, genetic and transmitted forms. CJD is associated with the conversion of normal cellular prion protein (PrPC) into a protease-resistant isoform (PrPres). The mechanism of the conversion has not been studied in human cell cultures, due to the lack of a model system. In this study, such a system has been developed by culturing cell lines. Human glioblastoma cell line T98G had no coding-region mutations of the prion protein gene, which was of the 129 M/V genotype, and expressed endogenous PrPC constitutively. T98G cells produced a form of proteinase K (PK)-resistant prion protein fragment following long-term culture and high passage number; its deglycosylated form was approximately 18 kDa. The PK-treated PrPres was detected by immunoblotting with the mAb 6H4, which recognizes residues 144–152, and a polyclonal anti-C-terminal antibody, but not by the mAb 3F4, which recognizes residues 109–112, or the anti-N-terminal mAb HUC2-13. These results suggest that PrPC was converted into a proteinase-resistant form of PrPres in T98G cells.