Evidence for an early inflammatory response in the central nervous system of mice with scrapie

Comparison of the neuropathological characteristics of bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (vCJD) in mice

Bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (vCJD) belong to a group of diseases called the transmissible spongiform encephalopathies (TSEs). Transmission studies in inbred mice (strain typing) provided overwhelming evidence that vCJD arose from BSE. In this study, we compare the patterns of neuropathology in a panel of three inbred mouse strains (RIII, C57BL and VM) and one cross (C57BL x VM) infected with either vCJD or BSE. For each mouse strain, patterns of abnormal prion protein (PrPres) deposition, astrocytosis and vacuolation were similar in the vCJD- and BSE-challenged mice. Prion protein (PrP)-positive plaques were prominent in the VM and C57BL x VM mice in addition to diffuse PrPres accumulation, whereas only diffuse PrPres labelling was observed in the RIII and C57BL mice. The hippocampus was targeted in all mouse strains, as was the cochlear nucleus in the medulla, both showing consistent severe vacuolation and heavy PrPres deposition. Although the targeting of PrPres was similar in the BSE- and vCJD-infected brains, the amount and intensity of PrPres observed in the brains treated with formic acid during fixation was reduced considerably. The distribution of astrocytosis was similar to the targeting of PrPres deposition in the brain, although some differences were observed in the hippocampi of mice challenged with vCJD. We conclude that there are no significant differences in the targeting of neuropathological changes observed in the BSE- and vCJD-infected mice, consistent with the previous evidence of a link between BSE and vCJD.

Synaptic changes characterize early behavioural signs in the ME7 model of murine prion disease

Prion diseases are fatal, chronic neurodegenerative diseases of mammals, characterized by amyloid deposition, astrogliosis, microglial activation, tissue vacuolation and neuronal loss. In the ME7 model of prion disease in the C57BL/6 J mouse, we have shown previously that these animals display behavioural changes that indicate the onset of neuronal dysfunction. The current study examines the neuropathological correlates of these early behavioural changes. After injection of ME7-infected homogenate into the dorsal hippocampus, we found statistically significant impairment of burrowing, nesting and glucose consumption, and increased open field activity at 13 weeks. At this time, microglia activation and PrPSc deposition was visible selectively throughout the limbic system, including the hippocampus, entorhinal cortex, medial and lateral septum, mamillary bodies, dorsal thalamus and, to a lesser degree, in regions of the brainstem. No increase in apoptosis or neuronal cell loss was detectable at this time, while in animals at 19 weeks postinjection there was 40% neuronal loss from CA1. There was a statistically significant reduction in synaptophysin staining in the stratum radiatum of the CA1 at 13 weeks indicating loss of presynaptic terminals. Damage to the dorsal hippocampus is known to disrupt burrowing and nesting behaviour. We have demonstrated a neuropathological correlate of an early behavioural deficit in prion disease and suggest that this should allow insights into the first steps of the neuropathogenesis of prion diseases.

T cells infiltrate the brain in murine and human transmissible spongiform encephalopathies

CD4 and CD8 T lymphocytes infiltrate the parenchyma of mouse brains several weeks after intracerebral, intraperitoneal, or oral inoculation with the Chandler strain of mouse scrapie, a pattern not seen with inoculation of prion protein knockout (PrP(-/-)) mice. Associated with this cellular infiltration are expression of MHC class I and II molecules and elevation in levels of the T-cell chemokines, especially macrophage inflammatory protein 1beta, IFN-gamma-inducible protein 10, and RANTES. T cells were also found in the central nervous system (CNS) in five of six patients with Creutzfeldt-Jakob disease. T cells harvested from brains and spleens of scrapie-infected mice were analyzed using a newly identified mouse PrP (mPrP) peptide bearing the canonical binding motifs to major histocompatibility complex (MHC) class I H-2(b) or H-2(d) molecules, appropriate MHC class I tetramers made to include these peptides, and CD4 and CD8 T cells stimulated with 15-mer overlapping peptides covering the whole mPrP. Minimal to modest K(b) tetramer binding of mPrP amino acids (aa) 2 to 9, aa 152 to 160, and aa 232 to 241 was observed, but such tetramer-binding lymphocytes as well as CD4 and CD8 lymphocytes incubated with the full repertoire of mPrP peptides failed to synthesize intracellular gamma interferon (IFN-gamma) or tumor necrosis factor alpha (TNF-alpha) cytokines and were unable to lyse PrP(-/-) embryo fibroblasts or macrophages coated with (51)Cr-labeled mPrP peptide. These results suggest that the expression of PrP(sc) in the CNS is associated with release of chemokines and, as shown previously, cytokines that attract and retain PrP-activated T cells and, quite likely, bystander activated T cells that have migrated from the periphery into the CNS. However, these CD4 and CD8 T cells are defective in such an effector function(s) as IFN-gamma and TNF-alpha expression or release or lytic activity.

CNS pathogenesis of prion diseases

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative diseases, clinically characterised by cognitive decline, paralleled by severe damage to the central nervous system. Prion diseases have attracted a broad interest because of their unique mechanisms of replication and propagation; however, the underlying pathogenic mechanisms are still highly speculative. In this review, current knowledge about the pathogenesis of prion diseases in the CNS will be highlighted and the most revealing animal models will be discussed, with future perspectives to address immediate questions about the pathogenesis.

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.

Autoimmune encephalomyelitis and uveitis induced by T cell immunity to self beta-synuclein

Beta-synuclein is a neuronal protein that accumulates in the plaques that characterize neurodegenerative diseases such as Parkinson's and Alzheimer's diseases. It has been proposed that immunization to peptides of plaque-forming proteins might be used therapeutically to help dissociate pathogenic plaques in the brain. We now report that immunization of Lewis rats with a peptide from beta-synuclein resulted in acute paralytic encephalomyelitis and uveitis. T cell lines and clones reactive to the peptide adoptively transferred the disease to naive rats. Immunoblotting revealed the presence of beta-synuclein in heavy myelin, indicating that the expression of beta-synuclein is not confined to neurons. These results add beta-synuclein to the roster of encephalitogenic self Ags, point out the potential danger of therapeutic autoimmunization to beta-synuclein, and alert us to the unsuspected possibility that autoimmunity to beta-synuclein might play an inflammatory role in the pathogenesis of neurodegeneration.

Heterogeneous expression of the triggering receptor expressed on myeloid cells-2 on adult murine microglia

Microglial activation is an early and common feature of almost all neuropathologies, including multiple sclerosis, Alzheimer's disease and mechanical injury. To better understand the relative contributions microglia make toward neurodegeneration and neuroprotection, we used TOGA(R) to identify molecules expressed by microglia and regulated by inflammatory signals. Triggering receptor expressed on myeloid cells-2 (TREM-2) was among the mRNAs identified as being expressed by unactivated microglia, but down-regulated by lipopolysaccharide/interferon gamma. In the healthy CNS, not all microglia expressed TREM-2. Microglial expression of TREM-2 varied not only between brain regions but also within each brain region. Brain regions with an incomplete blood-brain barrier had the lowest percentages of TREM-2- expressing microglia, whereas the lateral entorhinal and cingulate cortex had the highest percentages. A novel form of TREM-2b that lacked a transmembrane domain was detected, perhaps indicating a soluble form of the protein. Taken together, these data suggest that (1) subsets of microglia are specialized to respond to defined extracellular signals; and (2) regional variations in TREM-2 expression may contribute to the varying sensitivities of different brain regions to similar pathological signals.

Neuroinflammation in Alzheimer's disease and prion disease

Alzheimer's disease (AD) and prion disease are characterized neuropathologically by extracellular deposits of Abeta and PrP amyloid fibrils, respectively. In both disorders, these cerebral amyloid deposits are co-localized with a broad variety of inflammation-related proteins (complement factors, acute-phase protein, pro-inflammatory cytokines) and clusters of activated microglia. The present data suggest that the cerebral Abeta and PrP deposits are closely associated with a locally induced, non-immune-mediated chronic inflammatory response. Epidemiological studies indicate that polymorphisms of certain cytokines and acute-phase proteins, which are associated with Abeta plaques, are genetic risk factors for AD. Transgenic mice studies have established the role of amyloid associated acute-phase proteins in Alzheimer amyloid formation. In contrast to AD, there is a lack of evidence that cytokines and acute-phase proteins can influence disease progression in prion disease. Clinicopathological and neuroradiological studies have shown that activation of microglia is a relatively early pathogenetic event that precedes the process of neuropil destruction in AD patients. It has also been found that the onset of microglial activation coincided in mouse models of prion disease with the earliest changes in neuronal morphology, many weeks before neuronal loss and subsequent clinical signs of disease. In the present work, we review the similarities and differences between the involvement of inflammatory mechanisms in AD and prion disease. We also discuss the concept that the demonstration of a chronic inflammatory-like process relatively early in the pathological cascade of both diseases suggests potential therapeutic strategies to prevent or to retard these chronic neurodegenerative disorders.

Peripheral infection evokes exaggerated sickness behaviour in pre-clinical murine prion disease

Peripheral infections in mammals are characterised by local, systemic and CNS effects. The latter give rise to sickness behaviour. Pro-inflammatory cytokines such as interleukin-1beta (IL-1beta) are thought to be important mediators of this neuro-immune signalling (Cartmell et al., 1999). There is anecdotal evidence suggesting that peripheral infections in patients with Alzheimer's disease have more severe behavioural consequences than those in otherwise healthy elderly subjects, and it is well known that brain microglia are activated in the elderly and in Alzheimer's disease (McGeer et al., 1987). Using ME7-induced murine prion disease as a model of chronic neurodegeneration that displays chronic microglial activation, and the intra-peritoneal injection of bacterial lipopolysaccharide to mimic a peripheral infection, we have shown that the temperature and activity responses of animals with pre-clinical prion disease were exaggerated compared with controls, and that this was associated with a significant increase in brain levels of IL-1beta. We hypothesise that prior priming of microglia by the degenerative process, followed by further activation through signalling from the periphery, resulted in increased brain IL-1beta synthesis and the consequent acute sickness behavioural responses. These findings demonstrate an interaction between peripheral infection and pre-existing CNS inflammation and suggest that further stimulation of an already primed microglial population by a peripheral infection may drive disease progression in chronic inflammatory conditions such as Alzheimer's disease and prion disease.

Peripheral macrophage recruitment in cuprizone-induced CNS demyelination despite an intact blood-brain barrier

The contribution of peripheral macrophage was assessed in cuprizone intoxication, a model of demyelination and remyelination in which the blood-brain barrier remains intact. Flow cytometry of brain cells isolated from cuprizone-treated mice revealed an increase in the percentage of Mac-1(+)/CD45(hi) peripheral macrophage. To confirm these results in situ, C57BL/6 mice were lethally irradiated, transplanted with bone marrow from GFP-transgenic mice, and exposed to cuprizone. GFP(+) peripheral macrophages were seen in the CNS after 2 weeks of treatment, and infiltration continued through 6 weeks. While the peripheral macrophages were far outnumbered by the resident microglia, their recruitment across the blood-brain barrier alludes to a potentially important role.

Atypical inflammation in the central nervous system in prion disease

The inflammatory response in prion diseases is dominated by microglial activation. Contrary to their profile in vitro none of the pro-inflammatory cytokines interleukin-1beta, interleukin-6, or tumour necrosis factor-alpha are significantly upregulated in the ME7 model of prion disease. However, two major inflammatory mediators are elevated: transforming growth factor-beta1 and prostaglandin E2. This cytokine profile is the same as that reported for macrophages during phagocytosis of apoptotic cells and indeed transforming growth factor-beta1 and prostaglandin E2 are responsible for the downregulated phenotype of these macrophages. Transforming growth factor-beta1 may also have roles in extracellular matrix deposition and in amyloidogenesis and may play a direct role in disease pathogenesis. There is also now evidence to suggest that a peripheral infection, and its consequent systemic cytokine expression, may drive central nervous system cytokine expression and perhaps exacerbate disease.

Transforming growth factor beta1, the dominant cytokine in murine prion disease: influence on inflammatory cytokine synthesis and alteration of vascular extracellular matrix

Previous studies from our laboratory have shown the ME7 model of murine scrapie to be accompanied by an atypical inflammatory response that is characterized by marked astroglial and microglial activation but also by the lack of significant expression of the pro-inflammatory cytokines interleukin (IL)-1beta and IL-6. The aim of this study was to determine whether, in the absence of IL-1beta and IL-6, tumour necrosis factor (TNF)-alpha may play an equivalent pro-inflammatory role, or if an anti-inflammatory cytokine profile dominates. We have used competitive polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA) to determine the levels of TNF-alpha, IL-10 and transforming growth factor (TGF)-beta1 in the ME7 model, using their expression in lipopolysaccharide (LPS)-induced acute inflammation as a positive control. Levels of mRNA were elevated for all three cytokines during acute inflammation, while TGF-beta1 mRNA alone was significantly elevated in ME7-injected brains. Similarly, by ELISA, we detected elevated IL-10, TNF-alpha and TGF-beta1 in LPS-injected animals but only significant elevation of TGF-beta1 in ME7-injected animals. An increase in laminin and collagen IV deposition around blood vessels was also observed and is consistent with up-regulation by active TGF-beta1. These findings suggest that TGF-beta1 may play a central role in maintenance of an atypical microglial phenotype and may also be involved in vascular and extracellular matrix changes.

What is the basis of transmissible spongiform encephalopathy induced neurodegeneration and can it be repaired?

Once an animal becomes infected with a prion disease, or transmissible spongiform encephalopathy (TSE), the progression of infection is relentless and inevitably fatal, although often with such prolonged incubation periods that an alternative cause of death can intervene. Infection has been compared to 'setting a clock' which then runs inexorably as the disease spreads, usually through the lymphoreticular system and then via peripheral nerves to the central nervous system (CNS), although the mechanism controlling the protracted progression is not known. Clinical disease develops as characteristic degenerative changes in the CNS progress, but the molecular basis for this pathology is not clear, particularly the relationship between the deposition of abnormal PrP and neuronal dysfunction. Recent research has identified several means of slowing (if not stopping) the clock when infection has not yet reached the CNS; although the potential for later stage therapies seems limited, neuroprotective strategies which have been shown to be effective in other neurodegenerative conditions may also ameliorate TSE induced CNS pathology. This review focuses on our current knowledge of the key events following infection of the CNS and the opportunities for intervention once the CNS has become infected.

Quantification of the pathological changes with laminar depth in the cortex in sporadic Creutzfeldt-Jakob disease

The laminar distribution of the vacuolation ('spongiform change'), surviving neurons, glial cell nuclei, and prion protein (PrP) deposits was studied in the frontal, parietal and temporal cortex in 11 cases of sporadic Creutzfeldt-Jakob disease (CJD). The distribution of the vacuolation was mainly bimodal with peaks of density in the upper and lower cortical laminae. The density of surviving neurons was greatest in the upper cortex while glial cell nuclei were distributed largely in the lower cortex. PrP deposits exhibited either a bimodal distribution or reached a maximum density in the lower cortex. The vertical density of the vacuoles was positively correlated with the surviving neurons in 12/44 of cortical areas studied, with glial cell nuclei in 16/44 areas and with PrP deposition in 15/28 areas. PrP deposits were positively correlated with glial cell nuclei in 12/31 areas. These results suggest that in sporadic CJD: (1) the lower cortical laminae are the most affected by the pathological changes; (2) the development of the vacuolation may precede that of the extracellular PrP deposits and the glial cell reaction; and (3) the pathological changes may develop initially in the lower cortical laminae and spread to affect the upper cortical laminae.

The transmissible spongiform encephalopathies: pathogenic mechanisms and strategies for therapeutic intervention

Primary neurodegenerative diseases tend to be intractable and largely affect the elderly. There is rarely the opportunity to identify individuals at risk and the appearance of clinical symptoms usually signifies the occurrence of irreversible neurological damage. This situation describes sporadic Creutzfeldt-Jakob disease which occurs world-wide, affecting one person per million per annum. The epidemic of bovine spongiform encephalopathy in the UK in the 1980s and the subsequent causal appearance of variant Creutzfeldt-Jakob disease in young UK residents in the 1990s has refocused attention on this whole group of diseases, known as the transmissible spongiform encephalopathies or prion diseases. The potentially lengthy incubation period of variant Creutzfeldt-Jakob disease, including perhaps an obligate peripheral phase, prior to neuroinvasion, marks variant Creutzfeldt-Jakob disease out as different from sporadic Creutzfeldt-Jakob disease. The formal possibility of detecting individuals infected with the bovine spongiform encephalopathy agent during this asymptomatic peripheral phase provides a strong incentive for the development of therapies for transmissible spongiform encephalopathies. This review focuses on recent advances in the understanding of the pathogenesis of these diseases, with particular reference to in vitro and animal model systems. Such systems have proved invaluable in the identification of potential therapeutic strategies that either specifically target the prion protein or more generally target peripheral pathogenesis. Furthermore, recent experiments in animal models suggest that even after neuroinvasion there may be pharmacological avenues to explore that might retard or even halt the degenerative process.

Modulation of proteinase-K resistant prion protein by prion peptide immunization

Prion diseases are caused by conformational alterations in the prion protein (PrP). The immune system has been assumed to be non-responsive to the self-prion protein, therefore, PrP autoimmunity has not been investigated. Here, we immunized various strains of mice with PrP peptides, some selected to fit the MHC class II-peptide binding motif. We found that specific PrP peptides elicited strong immune responses in NOD, C57BL/6 and A/J mice. To test the functional effect of this immunization, we examined the expression of proteinase-K-resistant PrP by a scrapie-infected tumor transplanted to immunized syngeneic A/J mice. PrP peptide vaccination did not affect the growth of the infected tumor transplant, but significantly reduced the level of protease-resistant PrP. Our results demonstrate that self-PrP peptides are immunogenic in mice and suggest that this immune response might affect PrP-scrapie levels in certain conditions.

Microglia and prion disease

Gliosis is one of the hallmarks of the prion diseases. Prion diseases are fatal neurodegenerative conditions of low incidence made famous by both the hypothesis that a protein acts as the infectious agent without involvement of nucleic acid and the speculative idea that a disease of cattle, BSE, has spread to humans from the ingestion of prion-infected beef. Despite these unproved hypotheses, the aetiology of the prion diseases remains unsolved. The rapid degenerative course of the disease is preceded by a long incubation period with little or no symptoms. The rapid neurodegeneration in the disease follows from increased deposition of an abnormal isoform of a normal neuronal protein. Co-incident with the appearance of this abnormal protein is the activation of large numbers of microglia. Studies in cell culture with both the abnormal prion protein and a peptide-mimic suggest that neuronal degeneration occurs because of two concurrent effects. First, there is a reduction in neuronal resistance to toxic insults and, second, there is an increase in the production of toxic substances such as reactive oxygen species by microglia and a decrease in glutamate clearance by astrocytes. Microglia activated by the abnormal form of the prion protein also release cytokines, which stimulate changes in astrocytes such as proliferation. The implication of this is that microglia may play a major role in initiating the pathological changes in prion disease. This review discusses the role of microglia in these changes.

Burrowing into prion disease

Mice received intra-hippocampal injections of scrapie-infected brain homogenate. Open field activity increased from around week 12 post-injection. Concomitantly the tendency to displace food from a tube inside the home cage decreased. The food was generally dug out with the feet, rather than carried by mouth, so its displacement was called burrowing. Food restriction was unnecessary for this burrowing to occur. Only later, around 18 weeks, did more general motor impairments develop. As burrowing in scrapie-infected mice decreased when open field activity increased, and preceded later motor impairments, it was not due to motor dysfunction. Burrowing is a simple, sensitive, objective, ethological measure, sensitive to preclinical prion disease. Other potential applications are in transgenic and knockout mice, models of ageing and Alzheimer's disease, and pharmacology, particularly neuroleptics.

Early behavioural changes in scrapie-affected mice and the influence of dapsone

Behavioural testing can reveal effects in scrapie-infected mice long before overt clinical signs appear (Betmouni et al., 1999, Psychobiology, 27, 63-71). These effects may be partly attributable to an early, atypical inflammatory response in the brain (Betmouni et al., 1996, Neuroscience, 74, 1-5). The present study replicated and extended these findings, and examined the effect of chronic treatment with dapsone. This anti-inflammatory compound has been reported to delay disease onset in a rat model of Creutzfeldt-Jakob disease (Manuelidis et al., 1998, Lancet, 352, 456). Although the doses used in the present study were higher than those of Manuelidis et al. (1998), no attenuation of the disease was seen in either behavioural or subsequent histological tests. Burrowing, i.e. displacing food pellets from a tube in the home cage, decreased from around week 12 in scrapie-infected mice, as did consumption of palatable glucose solution. Concurrently, ambulation in an open field increased, as did rearing at around week 17. Spontaneous alternation was impaired around this time. Around 18 weeks, motor performance on an inverted screen, horizontal bar, rotating rod and static rods decreased. Nest construction was impaired at 20 weeks. Overt clinical signs (reduction in mobility, hunched posture, poor coat condition, bladder enlargement) only occurred after week 20, when the mice were prepared for histology. The ME7 scrapie-infected mice thus showed a characteristic complex of neurological and behavioural changes during the course of the disease that were not ameliorated by dapsone. These changes appeared well before clinical signs were prominent.

Mechanisms by which metals promote events connected to neurodegenerative diseases

Although the exact causative phenomenon responsible for the onset and progression of neurodegenerative disorders is at present unresolved, there are some clues as to the mechanisms underlying these chronic diseases. This review addresses mechanisms by which endogenous or environmental factors, through interaction with redox active metals, may initiate a common cascade of events terminating in neurodegeneration.

Prion glycoprotein: structure, dynamics, and roles for the sugars

The prion protein contains two N-linked glycosylation sites and a glycosylphosphatidylinositol (GPI) anchor. The large size of the N-linked sugars, together with their dynamic properties, enables them to shield two orthogonal faces of the protein almost completely. Thus, the sugars can protect large regions of the protein surface from proteases and from nonspecific protein-protein interactions. Immunoprecipitation of prion protein with calnexin suggests that in the ER the oligosaccharides may provide a route for protein folding via the calnexin pathway. Major questions relate to the relevance of the glycoform distribution (as defined by glycan site occupancy) to strain type and disease transmission. Glycan analysis has shown that prion protein contains at least 52 different sugars, that these consist of a subset of brain sugars, and that there is site specific glycan processing. PrP(Sc) from the brains of Syrian hamsters contains the same set of glycans as PrP(C), but a higher proportion of tri- and tetra-antennary sugars. This may be attributed to a decrease in the activity of GnTIII. The GPI anchor, which is modified with sialic acid, may allow the prion protein to be mobile in the lipid bilayer. Potentially, this provides a possible means for translocating the prions from one cell to another.

Increased expression of MIP-1 alpha and MIP-1 beta mRNAs in the brain correlates spatially and temporally with the spongiform neurodegeneration induced by a murine oncornavirus

The chimeric murine oncornavirus FrCas(E) causes a rapidly progressive paralytic disease associated with spongiform neurodegeneration throughout the neuroaxis. Neurovirulence is determined by the sequence of the viral envelope gene and by the capacity of the virus to infect microglia. The neurocytopathic effect of this virus appears to be indirect, since the cells which degenerate are not infected. In the present study we have examined the possible role of inflammatory responses in this disease and have used as a control the virus F43. F43 is an highly neuroinvasive but avirulent virus which differs from FrCas(E) only in 3' pol and env sequences. Like FrCas(E), F43 infects large numbers of microglial cells, but it does not induce spongiform neurodegeneration. RNAase protection assays were used to detect differential expression of genes encoding a variety of cytokines, chemokines, and inflammatory cell-specific markers. Tumor necrosis factor alpha (TNF-alpha) and TNF-beta mRNAs were upregulated in advanced stages of disease but not early, even in regions with prominent spongiosis. Surprisingly there was no evidence for upregulation of the cytokines interleukin-1 alpha (IL-1 alpha), IL-1 beta, and IL-6 or of the microglial marker F4/80 at any stage of this disease. In contrast, increased levels of the beta-chemokines MIP-1 alpha and -beta were seen early in the disease and were concentrated in regions of the brain rich in spongiosis, and the magnitude of responses was similar to that observed in the brains of mice injected with the glutamatergic neurotoxin ibotenic acid. MIP-1alpha and MIP-1beta mRNAs were also upregulated in F43-inoculated mice, but the responses were three- to fivefold lower and occurred later in the course of infection than was observed in FrCas(E)-inoculated mice. These results suggest that the robust increase in expression of MIP-1 alpha and MIP-1 beta in the brain represents a correlate of neurovirulence in this disease, whereas the TNF responses are likely secondary events.

Spatial pattern of prion protein deposits in patients with sporadic Creutzfeldt-Jakob disease

The spatial pattern of the prion protein (PrP) deposits was studied in the cerebral cortex and cerebellum in 10 patients with sporadic Creutzfeldt-Jakob disease (CJD). In all patients the PrP deposits were aggregated into clusters and, in 90% of cortical areas and in 50% of cerebellar sections, the clusters exhibited a regular periodicity parallel to the tissue boundary; a spatial pattern also exhibited by beta-amyloid (Abeta) deposits in Alzheimer's disease (AD). In the cerebral cortex, the incidence of regular clustering of the PrP deposits was similar in the upper and lower cortical laminae. The sizes of the PrP clusters in the upper and lower cortex were uncorrelated. No significant differences in mean cluster size of the PrP deposits were observed between brain regions. The size, location and distribution of the PrP deposit clusters suggest that PrP deposition occurs in relation to specific anatomical pathways and supports the hypothesis that prion pathology spreads through the brain via such pathways. In addition, the data suggest that there are similarities in the pathogenesis of extracellular protein deposits in prion disease and in AD.

Interleukin-1beta -induced changes in blood-brain barrier permeability, apparent diffusion coefficient, and cerebral blood volume in the rat brain: a magnetic resonance study

The cytokine interleukin-1beta (IL-1beta) is implicated in a broad spectrum of CNS pathologies, in which it is thought to exacerbate neuronal loss. Here, the effects of injecting recombinant rat IL-1beta into the striatum of 3-week-old rats were followed noninvasively from 2 to 123 hr using magnetic resonance imaging and spectroscopy. Four hours after injection of IL-1beta (1 ng in 1 microliter), cerebral blood volume was significantly increased, the blood-brain barrier (BBB) became permeable to intravenously administered contrast agent between 4.5 and 5 hr, and the apparent diffusion coefficient (ADC) of brain water fell by 6 hr (5.42 +/- 0. 35 x 10(-4) mm(2)/sec treated, 7.35 +/- 0.77 x 10(-)(4) mm(2)/sec control; p < 0.001). At 24 hr the BBB was again intact, but the ADC, although partially recovered, remained depressed at both 24 and 123 hr (p < 0.03). Depleting the animals of neutrophils before IL-1beta injection prevented the BBB permeability at all time points, but the ADC was still depressed at 6 hr (6.64 +/- 0.34 x 10(-4) mm(2)/sec treated, 7.49 +/- 0.38 x 10(-4) mm(2)/sec control; p < 0.005). No changes were seen in brain metabolites using proton spectroscopy at 6 hr after IL-1beta. Intraparenchymal injection of IL-1beta caused a neutrophil-dependent transient increase in BBB permeability. The presence of neutrophils within the brain parenchyma significantly contributed to the IL-1beta-induced changes in cerebral blood volume and the ADC of brain water. However, IL-1beta apparently had a direct effect on the resident cell populations, which persisted well after all recruited leukocytes had disappeared. Thus the action of IL-1beta alone can give rise to magnetic resonance imaging-visible changes that are normally attributed to alterations to cellular homeostasis.

Absence of detectable IL-1beta production in murine prion disease: a model of chronic neurodegeneration

Murine prion disease is accompanied by a modified inflammatory response characterized by early but prolonged microglial activation and T-lymphocyte recruitment. In this model, we look at the profile of cytokine production, particularly IL-1beta. Mice inoculated with prion-infected brain homogenate show typical signs of prion disease. We were unable to detect any IL-1beta using immunohistochemistry, with various fixation protocols, or ELISA between 8 and 24 wk post-inoculation. Also, there was no increase in mRNA for IL-1beta, IL-6, IFNgamma, and iNOS as measured by quantitative RT-PCR. Using the same procedures and examining tissues at the same time, IL-1beta immunostaining was detected in infiltrating inflammatory cells in mouse brains injected with LPS or in a delayed-type hypersensitivity response in the brain. Soluble IL-1beta was also increased, as measured by ELISA, and there was an increase in mRNA species for IL-1beta, IL-6, TNFalpha but not IFNgamma or iNOS in these brains. These data reveal that chronic neurodegeneration seen in prion disease does not induce production of a range of proinflammatory mediators despite showing marked microglial activation and raise the question as to whether IL-1beta would exacerbate the neurodegeneration as it does in acute neurodegeneration following head injury and stroke.

DNA immunization and central nervous system viral infection

This chapter discusses the virus infections of the central nervous system (CNS) and DNA vaccines. Mild central nervous system (CNS) symptoms, such as headache and drowsiness, can result from systemically elevated cytokine levels and therefore are common in many virus infections, even in the absence of the infection of the CNS. CNS infection is quite unusual and is initiated either as a result of the viremia or, more rarely, as a result of neural spread. The poliovirus infects the anterior horn motor neurons of the spinal cord, causing poliomyelitis, the disease for which the virus is named. DNA vaccination is a relatively new entrant in the vaccine sweepstakes, but is viewed with optimism, for a number of reasons. DNA vaccines encoding the nucleoprotein from lymphocytic choriomeningitis virus can confer protection against the normally lethal intracranial challenge. In rabies, in a mouse model, immunization with plasmids encoding the rabies glycoprotein conferred complete protection against subsequent viral challenge. Several virus-induced CNS diseases may be explained by their triggering of autoimmunity. Experimental autoimmune encephalomyelitis is a well-characterized CNS disease induced by the administration of certain CNS proteins.

Increased brain synthesis of prostaglandin E2 and F2-isoprostane in human and experimental transmissible spongiform encephalopathies

The levels of 2 arachidonic acid metabolites formed either by enzymatic activity of cyclooxygenase, i.e. prostaglandin E2 (PGE2), or by free radical-catalyzed peroxidation, i.e. F2-isoprostane 8-epi-prostaglandin F2alpha (8-epi-PGF2alpha), were measured in the CSF of subjects with sporadic and familial Creutzfeldt-Jakob disease (CJD) and in brain homogenates of scrapie-infected mice. The CSF levels of both metabolites were increased in sporadic CJD (n = 52) and familial CJD (n = 10) patients when compared with a group of patients with noninflammatory disorders. Similarly, PGE2 and 8-epi-PGF2alpha levels were higher in brain homogenates obtained from C57BL/6J mice infected with the ME7 scrapie strain than in brain homogenates from control animals. As PGE2 is 1 of the most abundant prostaglandins released during inflammation and 8-epi-PGF2alpha is a quantitative marker of lipid peroxidation, our results provide in vivo biochemical evidence for the occurrence of inflammation and oxidative stress in human and experimental transmissible spongiform encephalopathies (TSEs), a concept so far based mainly on histopathological and in vitro evidence. Interestingly, in sporadic CJD patients, high CSF levels of PGE2, but not 8-epi-PGF2alpha, correlated with short survival time, suggesting that the inflammatory response correlates with the clinical duration of disease.

The spatial pattern of the vacuolation in patients with sporadic Creutzfeldt-Jakob disease

1600 microm in diameter and, in the majority of tissue sections, the vacuole clusters were distributed with regular periodicity parallel to the tissue boundary. The size of the vacuole clusters was positively correlated with patient age in the lower laminae of the occipital cortex and the inferior temporal gyrus (ITG) and negatively correlated with age in the hippocampus. In addition, the size of the vacuole clusters was positively correlated with disease duration in the upper laminae of the ITG. The size and distribution of the vacuole clusters suggests that the vacuolation in CJD reflects the degeneration of specific brain pathways and supports the hypothesis that prion pathology may spread through the brain along well defined anatomical pathways.

Nuclear DNA fragmentation and immune reactivity in bovine spongiform encephalopathy

To investigate whether apoptosis contributes to neuronal degeneration in bovine spongiform encephalopathy (BSE), morphological changes consistent with apoptosis were sought and in-situ end labelling (ISEL) was applied, in a series of 20 BSE cases and 10 age-matched normal control cattle. Apoptotic changes were not found in neurons but were occasionally seen in glial cells. Relatively few ISEL-positive neurons were found, but many labelled nuclei were seen in glial cells in certain areas. None of the labelled cells showed morphological features of apoptosis. ISEL(+)cells occurred in areas of spongiform change and other areas of grey matter lacking spongiform change. Some association was found between degree of cellular DNA fragmentation and accumulation of abnormal prion protein (PrP(Sc)). Interestingly, small or moderate numbers of T lymphocytes, not present in the normal central nervous system (CNS), were detected in the CNS parenchyma in most BSE cases. There was a pronounced astrogliosis, but markers of macrophage or microglial activation were only slightly increased. The results indicate that nuclear DNA vulnerability is enhanced in certain neuroanatomical areas in BSE, but evidence that apoptosis plays a role in neuronal loss in BSE was very limited. 1999 Harcourt Publishers Ltd.

Non-nuclear histone H1 is upregulated in neurones and astrocytes in prion and Alzheimer's diseases but not in acute neurodegeneration

A non-nuclear isoform of histone H1 is constitutively expressed in neurones. This protein is the major lipopolysaccharide (LPS)-binding protein in the brain. Since the major systemic LPS-binding protein is released in the liver and is an acute phase reactant, we were interested to learn whether this novel CNS histone showed altered expression following neuronal injury. We have therefore examined the changes in the expression of this molecule in acute neuronal injury and in two neurodegenerative pathologies, murine scrapie and Alzheimer's disease. No upregulation or change in H1 staining was observed in acute neurodegeneration induced by the intrastriatal injection of the glutamate antagonist N-methyl d-aspartic acid. In contrast, Western blotting indicated that histone H1 is upregulated in the brains of mice with clinical signs of scrapie. Immunohistochemistry revealed that in the regions of pathology there was increased staining for histone H1 in the neurones and the surrounding neuropil. Cells with an astrocytic appearance were also seen to stain positively for H1 but only in the regions of pathology. Immunofluorescent double staining for glial fibrillary acid protein (GFAP) and histone H1 confirmed that these cells were indeed astrocytes. Alzheimer's disease brain also showed an increase in the neuronal and astrocytic staining but only in regions of pathology. The function of histone in the CNS is unknown but the data presented here demonstrate an upregulation in areas of neuronal degeneration, which indicates that it may be involved in disease pathogenesis.

Prion protein peptide neurotoxicity can be mediated by astrocytes

A peptide based on amino acids 106-126 of the sequence of human prion protein (PrP106-126) is neurotoxic in culture. A role for astrocytes mediating PrP106-126 toxicity was investigated. The toxicity of PrP106-126 to cerebellar cell cultures was reduced by aminoadipate, a gliotoxin. Normally, PrP106-126 is not toxic to cultures containing neurones deficient in the cellular isoform of prion protein (PrPc). However, PrP106-126 was toxic to cerebellar cells derived from Prnp(0/0) mice (deficient in PrPc expression) when those cerebellar cells were cocultured with astrocytes. This toxicity was found to occur only in the presence of PrPc-positive astrocytes and to be mediated by glutamate. Furthermore, PrPc-positive astrocytes were shown to protect Prnp(0/0) cerebellar cells from glutamate toxicity. This effect could be inhibited by PrP106-126. PrP106-126 did not enhance the toxicity of glutamate to neurones directly. When cerebellar cells were cocultured with astrocytes, the neurones became dependent on astrocytes for protection from glutamate toxicity and expressed an increased sensitivity to glutamate. In such a system, the protective effects of astrocytes against glutamate toxicity to neurones were inhibited by PrP106-126, resulting in a greater reduction in neuronal survival than would have been caused by PrP106-126 when astrocytes were not present. This new model provides a possible mechanism by which the gliosis in prion disease may accelerate the neurodegeneration seen in the later stages of the disease.

Is the pathogen of prion disease a microbial protein?

Though considerable circumstantial evidence suggests that the pathogen of prion disease is proteinaceous, it has not yet been conclusively identified. Epidemiological observations indicate that a microbial vector is responsible for the transmission of natural prion disease in sheep and goats and that the real causative agent may correspond to a structural protein of that microorganism. The microbial protein should resemble prion protein (PrP) and may replicate itself in the host by using mammalian DNA. A similar phenomenon was already described with a protein antigen of the ameba Naegleria gruberi. The various serotypes of the microbial protein may account for the existence of scrapie strains. It is proposed that many microbial proteins may be capable of replicating themselves in mammalian cells eliciting and sustaining thereby degenerative and/or autoimmune reactions subsequent to infections with microorganisms.

Inflammatory response and retinal ganglion cell degeneration following intraocular injection of ME7

Scrapie is a prion disease which occurs naturally in sheep and which can be transmitted experimentally to rodents. After intracerebral injection of ME7 into mouse, an atypical inflammatory response, characterized by T-lymphocytes and activated microglia is present early in the course of the disease. In the present work, we have investigated the relationship between this inflammatory response, astrocytosis and neuronal loss along the visual pathway after intraocular injection (intraocular) of ME7 in C57BL/6J mice. We have demonstrated that microglia activation and T-lymphocyte recruitment accompanies the spread of prion pathology along the visual pathway and in the early stages of the disease is restricted to the subcortical visual pathway. Inflammation was also present in non-visual areas in association with PrPsc deposition at late stages of the disease, possibily indicating that diffusion of the scrapie agent also contributes to the spread of the disease. After intraocular injection of the prion agent, the disease is believed to be transported into the brain via axons of retinal ganglion cells (RGCs). Despite the high levels of infectivity reported to be present in the retina early in the disease after intraocular injection of ME7, retinal pathology has not been extensively investigated. We have studied the RGCs response in whole mount retinas after intraocular injection of ME7. We have shown that RGCs degenerate after intraocular injection of ME7 whereas amacrine cells, retinal interneurones, are more resistant. Our results suggest that two distinct population of neurones, exposed in vivo at the same time to the same agent scrapie strain, show different susceptibility to the toxic effects of PrPsc.

Identification of microglial signal transduction pathways mediating a neurotoxic response to amyloidogenic fragments of beta-amyloid and prion proteins

Microglial interaction with amyloid fibrils in the brains of Alzheimer's and prion disease patients results in the inflammatory activation of these cells. We observed that primary microglial cultures and the THP-1 monocytic cell line are stimulated by fibrillar beta-amyloid and prion peptides to activate identical tyrosine kinase-dependent inflammatory signal transduction cascades. The tyrosine kinases Lyn and Syk are activated by the fibrillar peptides and initiate a signaling cascade resulting in a transient release of intracellular calcium that results in the activation of classical PKC and the recently described calcium-sensitive tyrosine kinase PYK2. Activation of the MAP kinases ERK1 and ERK2 follows as a subsequent downstream signaling event. We demonstrate that PYK2 is positioned downstream of Lyn, Syk, and PKC. PKC is a necessary intermediate required for ERK activation. Importantly, the signaling response elicited by beta-amyloid and prion fibrils leads to the production of neurotoxic products. We have demonstrated in a tissue culture model that conditioned media from beta-amyloid- and prion-stimulated microglia or from THP-1 monocytes are neurotoxic to mouse cortical neurons. This toxicity can be ameliorated by treating THP-1 cells with specific enzyme inhibitors that target various components of the signal transduction pathway linked to the inflammatory responses.

The acute inflammatory response in CNS following injection of prion brain homogenate or normal brain homogenate

The neuropathological hallmarks of end-stage prion disease are vacuolation, neuronal loss, astrocytosis and deposition of PrPSc amyloid. We have also shown that there is an inflammatory response in the brains of scrapie-affected mice from 8 weeks post-injection. In this study we have investigated the acute CNS response to the intracerebral injection of scrapie-affected brain homogenate. The ME7 strain of scrapie (Neuropathogenesis Unit, Edinburgh) was used, and control mice were injected with brain homogenate derived from normal C57BL/6 J mice. One microlitre of 10% w/v ME7 (n = 33) and normal brain homogenate (n = 28) was injected stereotaxically into the right dorsal hippocampus. Cryostat sections of brains taken at 1, 2, 5, 7, 14 and 28 days post-injection were examined histologically for neuronal loss, and immunocytochemically to study the inflammatory response. This study shows that ME7 is not acutely neurotoxic in vivo. There is also no difference (ANOVA) in the inflammatory response, which peaked between 2 and 5 days and resolved by 4 weeks after intracerebral injection of either ME7 or normal brain homogenate. The well circumscribed inflammatory response seen previously at 8 weeks is therefore a consequence of a disease process rather than a surgical artefact. This disease process may be related to a localized accumulation of PrPSc sufficient to stimulate an inflammatory response which in turn may contribute to neuronal loss. The role of the inflammatory response in chronic neurodegeneration can be usefully studied using this mouse model of prion disease, and this will undoubtedly shed light on the pathogenic mechanisms underlying other chronic neurodegenerative diseases.

Prion protein-overexpressing cells show altered response to a neurotoxic prion protein peptide

A peptide fragment of the prion protein, PrP106-126 is toxic to neuronal cells in culture. This toxicity is dependent on neuronal expression of the prion protein (PrPc) and also the presence of microglia. The role of expression of the PrPc in neurotoxicity of this peptide was investigated using mice that overexpress the prion protein. Cells derived from two different strains of PrPc-overexpressing mice were used (Tg20 and Tg35). PrP106-126 was more toxic to Tg35 cerebellar cells than wild-type or Tg20 cells. This increased toxicity required the presence of microglia. Analysis of microglia derived from wild-type and PrPc-overexpressing cells showed that Tg35 microglia were more easily activated than wild-type microglia, were more easily stimulated to proliferate by astrocytes, and had a higher level of PrPc expression. This may explain the increased PrP106-126 toxicity to Tg35 PrPc-overexpressing cerebellar cells. These results suggest that the toxicity of PrP106-126 may depend on the level of expression of PrPc by microglia as well as by neurones.

Microglial expression of the prion protein

The prion protein (PrPc) is a normal cellular protein expressed by neurones and astrocytes. An altered isoform, PrPSc is thought to transmit spongiform encephalopathies. Here we show that microglia also express PrPc. Sensitivity of microglia to activation is enhanced by increased expression of PrPc. Bacterial endotoxin increases superoxide production and inhibits profileration of microglia more effectively when microglia express PrPc. PrPc expression is therefore important for the normal function of microglia.

Vaccination with an attenuated Creutzfeldt-Jakob disease strain prevents expression of a virulent agent

Although slow and persistent viruses often escape host defenses infection may be prevented by live vaccines. To determine whether an attenuated "slow" strain of the Creutzfeldt-Jakob disease agent (SY) could block expression of a virulent "fast" strain (FU), outbred CD-1 mice were inoculated intracerebrally with low infectious doses of SY and challenged 80 days later with higher doses of FU. For comparison, the same SY and FU samples were inoculated in two parallel control groups. All 18 superinfected mice showed incubation times identical to those inoculated with only the SY strain, yielding clinical disease >110 days later than predicted for the FU strain. Neurological signs, such as scratching and an extended clinical phase, were also characteristic for SY but not FU infection. Moreover, the widespread cortical pathology of FU was not detectable in superinfected mice. Western blot analyses further showed no strain-specific differences in prion protein (PrP) band profiles for all experimental groups, although there was approximately 10-fold more protease-resistant PrP (PrP-res) in FU brains during terminal disease. In contrast, infectivity assays revealed an approximately 10,000-fold difference between SY and FU at terminal stages, indicating that PrP-res content does not correlate with infectivity. In summary, an attenuated strain of the Creutzfeldt-Jakob disease agent evokes substantial interference against a virulent agent. Because superinfected mice had little PrP-res just before the onset of clinical disease and retained abundant cellular PrP, cellular PrP was not the factor limiting FU replication. The mechanisms underlying SY interference are not understood but could be based on host recognition of foreign molecular features shared by this class of invasive agents involving antibody production, and possibly involve defective viral particles produced by attenuated variants.

Effects of oxidative stress on prion protein expression in PC12 cells

PC12 cells are known to express the prion protein, a normal cell surface glycoprotein. This protein is upregulated in PC12 cells differentiated with nerve growth factor. A neurotoxic prion protein peptide, PrP106-126, is not toxic to PC12 cells alone. PrP106-126 is toxic to PC12 cells co-cultured with microglia and more so to NGF-differentiated PC12 cells. PC12 cells selected for resistance to either copper toxicity or oxidative stress have higher levels of PrP(C) expression. Both PC12 variants are more sensitive to the toxicity of PrP106-126. This suggests that PC12 sensitivity to PrP106-126 toxicity is related to prion protein expression and not to a state of high differentiation induced by NGF. Variants of PC12 cells that are more resistant to copper toxicity have higher levels of anti-oxidant enzymes, superoxide dismutase and glutathione peroxidase. Our results suggest that cells expressing higher levels of PrP(C) have higher resistance to oxidative stress or copper toxicity but are more sensitive to PrP106-126 toxicity. Prion protein expression may be involved in both the metabolism of copper and resistance to oxidative stress. Increased cellular resistance to copper toxicity may be partly related to increased activity of anti-oxidant enzymes.

Increase of intracellular free Ca2+ in microglia activated by prion protein fragment

A synthetic peptide consisting of amino acid residues 106 to 126 of the human prion protein (PrPc) that forms fibrils in vitro is toxic to cultured neurons. We have previously shown that the neurotoxic effect of this peptide is related to microglia activation (Brown et al., 1996a). For closer insight into this process of activation, we investigated the effect of the peptide on the intracellular free Ca2+ concentration ([Ca2+]i) in cultured microglia using Fura-2. Cultured microglia from wild-type as well as from PrPc gene-ablated mice (Prn-p0/0) responded to exposure to PrP106-126 with an increase in intracellular free calcium within 30 min. We observed two types of responses. Both in wild-type and Prn-p0/0 mice about half of the tested cells presented a small and often transient calcium increase after peptide application which was found to be independent of the extracellular calcium concentration. However, a further 33% of wild-type cells showed a strong and often permanent calcium increase depending on the extracellular calcium concentration, which was only rarely observed in Prn-p0/0 cells. To determine whether the response depended on the activation state of the microglia, we also examined LPS-treated activated microglia. The character of the calcium response remained unchanged, but significantly fewer cells responded. Our findings demonstrate the earliest reaction of microglia to a PrP fragment known to date.

The blood-brain barrier and the inflammatory response

The environment of the brain is controlled by a sophisticated endothelial barrier that prevents the free entry of solutes from the blood. It is commonly assumed that this blood-brain barrier (BBB) also prevents the entry of leukocytes into the central nervous system. However, recent evidence in animal models shows that this is not the case, and leukocytes can cross an intact BBB during health and disease. Indeed, in many neurological diseases, including Alzheimer's disease, prion diseases and AIDS-related dementia, leukocytes enter the brain parenchyma without concomitant BBB breakdown. Current research is concentrating on factors that control the integrity of the BBB and the mechanisms that leukocytes use to enter the brain.

Evolution of a strain of CJD that induces BSE-like plaques

Bovine spongiform encephalopathy (BSE) has become a public health issue because a recently evolved BSE agent has infected people, yielding an unusual form of Creutzfeld-Jakob disease (CJD). A new CJD agent that provokes similar amyloid plaques and cerebellar pathology was serially propagated. First-passage rats showed obvious clinical signs and activated microglia but had negligible PrP-res (the more protease-resistant form of host PrP) or cerebellar lesions. Microglia and astrocytes may participate in strain selection because the agent evolved, stabilized, and reproducibly provoked BSE-like disease in subsequent passages. Early vacuolar change involving activated microglia and astrocytes preceded significant PrP-res accumulation by more than 50 days. These studies reveal several inflammatory host reactions to an exogenous agent.