Killing of prion-damaged neurones by microglia

Microarray profiling emphasizes transcriptomic differences between hippocampal in vivo tissue and in vitro cultures

Primary hippocampal cell cultures are routinely used as an experimentally accessible model platform for the hippocampus and brain tissue in general. Containing multiple cell types including neurons, astrocytes and microglia in a state that can be readily analysed optically, biochemically and electrophysiologically, such cultures have been used in many in vitro studies. To what extent the in vivo environment is recapitulated in primary cultures is an on-going question. Here, we compare the transcriptomic profiles of primary hippocampal cell cultures and intact hippocampal tissue. In addition, by comparing profiles from wild type and the PrP 101LL transgenic model of prion disease, we also demonstrate that gene conservation is predominantly conserved across genetically altered lines.

Defining the Microglia Response during the Time Course of Chronic Neurodegeneration

Inflammation has been proposed as a major component of neurodegenerative diseases, although the precise role it plays has yet to be defined. We examined the role of key contributors to this inflammatory process, microglia, the major resident immune cell population of the brain, in a prion disease model of chronic neurodegeneration. Initially, we performed an extensive reanalysis of a large study of prion disease, where the transcriptome of mouse brains had been monitored throughout the time course of disease. Our analysis has provided a detailed classification of the disease-associated genes based on cell type of origin and gene function. This revealed that the genes upregulated during disease, regardless of the strain of mouse or prion protein, are expressed predominantly by activated microglia. In order to study the microglia contribution more specifically, we established a mouse model of prion disease in which the 79A murine prion strain was introduced by an intraperitoneal route into BALB/cJ(Fms-EGFP/-) mice, which express enhanced green fluorescent protein under the control of the c-fms operon. Samples were taken at time points during disease progression, and histological analysis of the brain and transcriptional analysis of isolated microglia was carried out. The analysis of isolated microglia revealed a disease-specific, highly proinflammatory signature in addition to an upregulation of genes associated with metabolism and respiratory stress. This study strongly supports the growing recognition of the importance of microglia within the prion disease process and identifies the nature of the response through gene expression analysis of isolated microglia.

IMPORTANCE

Inflammation has been proposed as a major component of neurodegenerative diseases. We have examined the role of key contributors to this inflammatory process, microglia, the major resident immune cell population of the brain, in a murine prion disease model of chronic neurodegeneration. Our study demonstrates that genes upregulated throughout the disease process are expressed predominantly by microglia. A disease-specific, highly proinflammatory signature was observed in addition to an upregulation of genes associated with metabolism and respiratory stress. This study strongly supports the growing recognition of the important contribution of microglia to a chronic neurodegenerative disease process.

Excitotoxicity through NMDA receptors mediates cerebellar granule neuron apoptosis induced by prion protein 90-231 fragment

Prion diseases recognize, as a unique molecular trait, the misfolding of CNS-enriched prion protein (PrP(C)) into an aberrant isoform (PrP(Sc)). In this work, we characterize the in vitro toxicity of amino-terminally truncated recombinant PrP fragment (amino acids 90-231, PrP90-231), on rat cerebellar granule neurons (CGN), focusing on glutamatergic receptor activation and Ca(2+) homeostasis impairment. This recombinant fragment assumes a toxic conformation (PrP90-231(TOX)) after controlled thermal denaturation (1 h at 53 °C) acquiring structural characteristics identified in PrP(Sc) (enrichment in β-structures, increased hydrophobicity, partial resistance to proteinase K, and aggregation in amyloid fibrils). By annexin-V binding assay, and evaluation of the percentage of fragmented and condensed nuclei, we show that treatment with PrP90-231(TOX), used in pre-fibrillar aggregation state, induces CGN apoptosis. This effect was associated with a delayed, but sustained elevation of [Ca(2+)]i. Both CGN apoptosis and [Ca(2+)]i increase were not observed using PrP90-231 in PrP(C)-like conformation. PrP90-231(TOX) effects were significantly reduced in the presence of ionotropic glutamate receptor antagonists. In particular, CGN apoptosis and [Ca(2+)]i increase were largely reduced, although not fully abolished, by pre-treatment with the NMDA antagonists APV and memantine, while the AMPA antagonist CNQX produced a lower, although still significant, effect. In conclusion, we report that CGN apoptosis induced by PrP90-231(TOX) correlates with a sustained elevation of [Ca(2+)]i mediated by the activation of NMDA and AMPA receptors.

Loss of cerebellar granule neurons is associated with punctate but not with large focal deposits of prion protein in Creutzfeldt-Jakob disease

Whether aggregates of prion protein (PrP) reflect neurotoxicity or are neuroprotective in prion diseases is unclear. To address this question, we performed a clinicopathologic study of cerebellar granular neurons in 100 patients affected with sporadic Creutzfeldt-Jakob disease (CJD). There was significant loss of these neurons in the subset of cases with Val/Val genotype at PRNP Codon 129 and Molecular Isotype 2 of abnormal PrP (sporadic CJD-VV2) (n=32) compared with both the other CJD subtypes and to controls. Pathological PrP deposits of the punctate-type (synaptic-type) in this subgroup correlated with neuronal loss and proliferation of astrocytes and microglia. By contrast, the numbers of large deposits (5- to 50-microm-diameter) and numbers of amyloid plaques did not correlate with neuronal loss. These findings are consistent with the view that large aggregates may protect neurons by sequestering neurotoxic PrP oligomers, whereas punctate deposits may indicate the location of neuronal death processes in CJD.

Small-ruminant lentivirus enhances PrPSc accumulation in cultured sheep microglial cells

Sheep scrapie is the prototypical transmissible spongiform encephalopathy (prion disease), which has a fundamental pathogenesis involving conversion of normal cellular prion protein (PrP(C) [C superscript stands for cellular]) to disease-associated prion protein (PrP(Sc) [Sc superscript stands for sheep scrapie]). Sheep microglial cell cultures, derived from a prnp 136VV/171QQ near-term fetal brain, were developed to study sheep scrapie in the natural host and to investigate potential cofactors in the prion conversion process. Two culture systems, a primary cell culture and a cell line transformed with the large T antigen of simian virus 40, were developed, and both were identified as microglial in origin as indicated by expression of several microglial phenotype markers. Following exposure to PrP(Sc), sheep microglial cells demonstrated relatively low levels (transformed cell line) to high levels (primary cell line) of PrP(Sc) accumulation over time. The accumulated PrP(Sc) demonstrated protease resistance, an inferred beta-sheet conformation (as determined by a commercial enzyme-linked immunosorbent assay), specific inhibition by anti-PrP antibodies, and was transmissible in a dose-dependent manner. Primary microglia coinfected with a small-ruminant lentivirus (caprine arthritis encephalitis virus-Cork strain) and PrP(Sc) demonstrated an approximately twofold increase in PrP(Sc) accumulation compared to that of primary microglia infected with PrP(Sc) alone. The results demonstrate the in vitro utility of PrP(Sc)-permissive sheep microglial cells in investigating the biology of natural prion diseases and show that small-ruminant lentiviruses enhance prion conversion in cultured sheep microglia.

Effect of intraventricular infusion of anti-prion protein monoclonal antibodies on disease progression in prion-infected mice

It is well known that anti-prion protein (PrP) monoclonal antibodies (mAbs) inhibit abnormal isoform PrP (PrPSc) formation in cell culture. Additionally, passive immunization of anti-PrP mAbs protects the animals from prion infection via peripheral challenge when mAbs are administered simultaneously or soon after prion inoculation. Thus, anti-PrP mAbs are candidates for the treatment of prion diseases. However, the effects of mAbs on disease progression in the middle and late stages of the disease remain unclear. This study carried out intraventricular infusion of mAbs into prion-infected mice before and after clinical onset to assess their ability to delay disease progression. A 4-week infusion of anti-PrP mAbs initiated at 120 days post-inoculation (p.i.), which is just after clinical onset, reduced PrPSc levels to 70-80 % of those found in mice treated with a negative-control mAb. Spongiform changes, microglial activation and astrogliosis in the hippocampus and thalamus appeared milder in mice treated with anti-PrP mAbs than in those treated with a negative-control mAb. Treatment with anti-PrP mAb prolonged the survival of mice infected with Chandler or Obihiro strain when infusion was initiated at 60 days p.i., at which point PrPSc is detectable in the brain. In contrast, infusion initiated after clinical onset prolonged the survival time by about 8 % only in mice infected with the Chandler strain. Although the effects on survival varied for different prion strains, the anti-PrP mAb could partly prevent disease progression, even after clinical onset, suggesting immunotherapy as a candidate for treatment of prion diseases.

Squalestatin alters the intracellular trafficking of a neurotoxic prion peptide

Background

Neurotoxic peptides derived from the protease-resistant core of the prion protein are used to model the pathogenesis of prion diseases. The current study characterised the ingestion, internalization and intracellular trafficking of a neurotoxic peptide containing amino acids 105–132 of the murine prion protein (MoPrP105-132) in neuroblastoma cells and primary cortical neurons.

Results

Fluorescence microscopy and cell fractionation techniques showed that MoPrP105-132 co-localised with lipid raft markers (cholera toxin and caveolin-1) and trafficked intracellularly within lipid rafts. This trafficking followed a non-classical endosomal pathway delivering peptide to the Golgi and ER, avoiding classical endosomal trafficking via early endosomes to lysosomes. Fluorescence resonance energy transfer analysis demonstrated close interactions of MoPrP105-132 with cytoplasmic phospholipase A₂ (cPLA₂) and cyclo-oxygenase-1 (COX-1), enzymes implicated in the neurotoxicity of prions. Treatment with squalestatin reduced neuronal cholesterol levels and caused the redistribution of MoPrP105-132 out of lipid rafts. In squalestatin-treated cells, MoPrP105-132 was rerouted away from the Golgi/ER into degradative lysosomes. Squalestatin treatment also reduced the association between MoPrP105-132 and cPLA₂/COX-1.

Conclusion

As the observed shift in peptide trafficking was accompanied by increased cell survival these studies suggest that the neurotoxicity of this PrP peptide is dependent on trafficking to specific organelles where it activates specific signal transduction pathways.

Polyanion induced fibril growth enables the development of a reproducible assay in solution for the screening of fibril interfering compounds, and the investigation of the prion nucleation site

The misfolded conformer of the prion protein (PrP) that aggregates into fibrils is believed to be the pathogenic agent in transmissible spongiform encephalopathies. In order to find fibril interfering compounds a screening assay in solution would be the preferred format to approximate more closely to physical conditions and enable the performance of kinetic studies. However, such an assay is hampered by the high irreproducibility because of the stochastic nature of the fibril formation process. According to published fibril models, the fibrillar core may be composed of stacked parallel beta-strands. In these models positive charge repulsion may reduce the chance of favorable stacking and cause the irreproducibility in the fibril formation. This study shows that the charge compensation by polyanions induced a very strong fibril growth which made it possible to develop a highly reproducible fibril interference assay. The stimulating effect of the polyanions depended on the presence of the basic residues Lys(106), Lys(110) and His(111). The assay was validated by comparison of the 50% fibril inhibition levels of peptide huPrP106-126 by six tetracyclic compounds. With this new assay, the fibrillogenic core (GAAAAGAVVG) of peptide huPrP106-126 was determined and for the first time it was possible to test the inhibition potentials of peptide analogues. Also it was found that variants of peptide huPrP106-126 with proline substitutions at positions Ala(115), Ala(120), or Val(122) inhibited the fibril formation of huPrP106-126.

Prion protein expression differences in microglia and astroglia influence scrapie-induced neurodegeneration in the retina and brain of transgenic mice

Activated microglia and astroglia are known to be involved in a variety of neurodegenerative diseases, including prion diseases. In the present experiments, we studied activation of astroglia and microglia after intraocular scrapie infection in transgenic mice expressing prion protein (PrP) in multiple cell types (tg7 mice) or in neurons only (tgNSE mice). In this model, scrapie infection and protease-resistant PrP deposition occurs in the retinas of both strains of mice, but retinal degeneration is observed only in tg7 mice. Our results showed that the retinas of tg7 and tgNSE mice both had astroglial activation with increased chemokine expression during the course of infection. However, only tg7 retinas exhibited strong microglial activation compared to tgNSE retinas, which showed little microglial activation by biochemical or morphological criteria. Therefore, microglial PrP expression might be required for scrapie-induced retinal microglial activation and damage. Furthermore, microglial activation preceded retinal neurodegeneration in tg7 mice, suggesting that activated microglia might contribute to the degenerative process, rather than being a response to the damage. Surprisingly, brain differed from retina in that an altered profile of microglial activation markers was upregulated, and the profiles in the two mouse strains were indistinguishable. Microglial activation in the brain was associated with severe brain vacuolation and neurodegeneration, leading to death. Thus, retinal and brain microglia appeared to differ in their requirements for activation, suggesting that different activation pathways occur in the two tissues.

Microglial cell line established from prion protein-overexpressing mice is susceptible to various murine prion strains

Several lines of evidence suggest that microglia have important roles in the pathogenesis of prion diseases. Here, we establish a novel microglial cell line (MG20) from neonatal tga20 mice that overexpress murine prion protein. After exposure to Chandler scrapie, we observed the replication and accumulation of disease-associated forms of the prion protein in MG20 cells up to the 15th passage. Furthermore, MG20 cells were susceptible to ME7, Obihiro scrapie, and bovine spongiform encephalopathy agents. Thus, MG20 cell lines persistently infected with various murine prion strains provide a useful model for the study of the pathogenesis of prion diseases.

Interferon-gamma increases neuronal death in response to amyloid-beta1-42

Background

Alzheimer's disease is a neurodegenerative disorder characterized by a progressive cognitive impairment, the consequence of neuronal dysfunction and ultimately the death of neurons. The amyloid hypothesis proposes that neuronal damage results from the accumulation of insoluble, hydrophobic, fibrillar peptides such as amyloid-β_1-42. These peptides activate enzymes resulting in a cascade of second messengers including prostaglandins and platelet-activating factor. Apoptosis of neurons is thought to follow as a consequence of the uncontrolled release of second messengers. Biochemical, histopathological and genetic studies suggest that pro-inflammatory cytokines play a role in neurodegeneration during Alzheimer's disease. In the current study we examined the effects of interferon (IFN)-γ, tumour necrosis factor (TNF)α, interleukin (IL)-1β and IL-6 on neurons.

Methods

Primary murine cortical or cerebellar neurons, or human SH-SY5Y neuroblastoma cells, were grown in vitro. Neurons were treated with cytokines prior to incubation with different neuronal insults. Cell survival, caspase-3 activity (a measure of apoptosis) and prostaglandin production were measured. Immunoblots were used to determine the effects of cytokines on the levels of cytoplasmic phospholipase A₂ or phospholipase C γ-1.

Results

While none of the cytokines tested were directly neurotoxic, pre-treatment with IFN-γ sensitised neurons to the toxic effects of amyloid-β_1-42 or HuPrP82-146 (a neurotoxic peptide found in prion diseases). The effects of IFN-γ were seen on cortical and cerebellar neurons, and on SH-SY5Y neuroblastoma cells. However, pre-treatment with IFN-γ did not affect the sensitivity to neurons treated with staurosporine or hydrogen peroxide. Pre-treatment with IFN-γ increased the levels of cytoplasmic phospholipase A₂ in SH-SY5Y cells and increased prostaglandin E₂ production in response to amyloid-β_1-42.

Conclusion

Treatment of neuronal cells with IFN-γ increased neuronal death in response to amyloid-β_1-42 or HuPrP82-146. IFN-γ increased the levels of cytoplasmic phospholipase A₂ in cultured neuronal cells and increased expression of cytoplasmic phospholipase A₂ was associated with increased production of prostaglandin E₂ in response to amyloid-β_1-42 or HuPrP82-146. Such observations suggest that IFN-γ produced within the brain may increase neuronal loss in Alzheimer's disease.

Prostaglandin D2 mediates neuronal damage by amyloid-β or prions which activates microglial cells

Microglial cells killed neurons damaged following incubation with sub-lethal concentrations of peptides derived from either the human prion protein (HuPrP82-146) or amyloid-beta1-42 (a peptide found in Alzheimer's disease). HuPrP82-146 or amyloid-beta1-42 induced phenotypic changes in neurons that caused them to bind a CD14-IgG chimera. In co-cultures microglial cells produced interleukin (IL)-6 in response to HuPrP82-146 or amyloid-beta1-42 damaged neurons. The binding of the CD14-IgG chimera to HuPrP82-146 or amyloid-beta1-42 damaged neurons was reduced by pre-treatment with cyclo-oxygenase (COX)-1 inhibitors and in co-cultures, COX-1 inhibitors significantly increased neuronal survival. Studies with individual prostaglandins demonstrated that the addition of prostaglandin D2, or prostaglandin E2, but not other prostaglandins (F2alpha, H2, I2 or 15-dJ2), mimicked the effects of amyloid-beta1-42 on neurons. Thus, prostaglandin D2 or E2 damaged neurons bound the CD14-IgG chimera, and in co-cultures prostaglandin D2 damaged neurons activated microglial cells. These effects were mediated via the DP prostanoid receptor; DP receptor agonists BW245C or SQ27986 induced neuronal damage, while the DP receptor antagonist BWA868C was neuroprotective in co-cultures. These results indicate that prostaglandin D2, produced following activation of COX-1 by sub-lethal concentrations of HuPrP82-146 or amyloid-beta1-42, causes phenotypic changes in neurons that activates microglial cells and leads to neuronal loss.

Stressing out the ER: a role of the unfolded protein response in prion-related disorders

Transmissible Spongiform Encephalopathies are fatal and infectious neurodegenerative diseases characterized by extensive neuronal apoptosis and the accumulation of an abnormally folded form of the cellular prion protein (PrP), denoted PrP(SC). Compelling evidence suggests the involvement of several signaling pathways in prion pathogenesis, including proteasome dysfunction, alterations in the protein maturation pathways and the unfolded protein response. Recent reports indicate that endoplasmic reticulum stress due to the PrP misfolding may be a critical factor mediating neuronal dysfunction in prion diseases. These findings have applications for developing novel strategies for treatment and early diagnosis of transmissible spongiform encephalopathies and other neurodegenerative diseases.

Microglial cells kill prion-damaged neurons in vitro by a CD14 dependent process

Microglial cells killed primary cortical neurons exposed to the prion (PrP)-derived peptide HuPrP106-126. The survival of HuPrP106-126-damaged neurons was increased by pre-treating microglial cells with anti-CD14 antibodies, while microglial cells from CD14 knockout mice failed to kill HuPrP106-126-damaged neurons. In addition, HuPrP106-126-damaged neurons selectively bound a CD14-IgG chimera. The killing of HuPrP106-126-damaged neurons by human monocytes was epitope specific; it was reduced by pre-treating monocytes with some anti-CD14 monoclonal antibodies (mabs) (60bca, 3C10 or MAB3832), but not others (26ic or MAB3831). None of the mabs affected the survival of HuPrP106-126-damaged neurons in the absence of monocytes.

Sporadic Creutzfeldt-Jakob disease: the extent of microglia activation is dependent on the biochemical type of PrPSc

In prion-related encephalopathies, microglial activation occurs early and is dependent on accumulation of disease-specific forms of the prion protein (PrPSc) and may play a role in nerve cell death. Previously, we found that different types of PrPSc (i.e. type 1 and type 2) coexisted in approximately 25% of patients with sporadic Creutzfeldt-Jakob disease (CJD); and a close relationship was detected between PrPSc type, the pattern of PrP immunoreactivity, and extent of spongiform degeneration. To investigate whether microglial reaction is related to the biochemical type and deposition pattern of PrPSc, we carried out a neuropathologic and biochemical study on 26 patients with sporadic CJD, including all possible genotypes at codon 129 of the prion protein gene. By quantitative analysis, we demonstrated that strong microglial activation was associated with type 1 PrPSc and diffuse PrP immunoreactivity, whereas type 2 PrPSc and focal PrP deposits were accompanied by mild microglia reaction. These findings support the view that the phenotypic heterogeneity of sporadic CJD is largely determined by the physicochemical properties of distinct PrPSc conformers.

Activation of human microglia by fibrillar prion protein-related peptides is enhanced by amyloid-associated factors SAP and C1q

Complement activation products C1q and C3d, serum amyloid P component (SAP) and activated glial cells accumulate in amyloid deposits of conformationally changed prion protein (PrP(Sc)) in Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease and scrapie-infected mouse brain. Biological properties, including the potential to activate microglia, relate to prion (PrP) peptide fibrillogenic abilities. We investigated if SAP and C1q influence the fibrillogenic properties of human and mouse PrP peptide and concomitantly their stimulatory effects on human microglia in vitro. PrP-peptide induced microglial IL-6 and TNF-alpha release significantly increased in the presence of SAP and C1q. Also, SAP and C1q enhanced PrP-peptide fibril formation as revealed by electron microscopy and thioflavin S-based quantitative assays. This suggests that SAP and C1q contribute to fibrillar state-dependent cellular effects of PrP. Combined, ultrastructural and thioflavin assays, together with microglial cytokine release measurements, provide a test system to screen potential, fibrillarity impeding therapeutics for prion disease.

Pathogenesis of prion diseases

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

Manipulation of PrPres production in scrapie-infected neuroblastoma cells

In the present study the accumulation of protease resistant prion protein (PrPres) in scrapie-infected neuroblastoma cells (ScN2a cells) was shown to be dependent on culture conditions. The highest levels of PrPres were found in slow growing cells. Further increases in PrPres accumulation were observed in ScN2a cells treated with retinoic acid, a compound that is associated with neuronal differentiation. The effects of retinoic acid were dose-dependent with a maximal effect at 200 ng/ml. A similar increase in PrPres was observed in another prion-infected cell line, scrapie-mouse brain (SMB) cells, treated with retinoic acid while retinoic acid increased the amount of PrPC in non-infected cells. Other drugs reported to cause neuronal differentiation, such as phorbol esters, did not increase the PrPres content of ScN2a cells. The survival of retinoic acid-treated ScN2a cells co-cultured with microglia was significantly reduced when compared to untreated ScN2a cells and an inverse correlation was demonstrated between the PrPres content of cells and their survival when co-cultured with microglia. The production of interleukin-6 by microglia cultured with retinoic acid-treated ScN2a cells was significantly higher than that of microglia cultured with untreated ScN2a cells.

Microglia kill amyloid-β1-42 damaged neurons by a CD14-dependent process

Activated microglia are closely associated with neuronal damage in Alzheimer's disease. In the present study, neurons exposed to low concentrations of amyloid-beta1-42, a toxic fragment of the amyloid-beta protein, were killed by microglia in a process that required cell-cell contact. Pre-treating microglia with polyclonal antibodies to the CD14 protein, or treating neurons exposed to amyloid-beta1-42 with a CD14-IgG chimera, prevented the killing of amyloid-beta1-42 damaged neurons by microglia. Moreover, microglia from CD14 null mice failed to kill amyloid-beta1-42 damaged neurons. Increased neuronal survival was accompanied by a significant reduction in the production of interleukin-6 indicative of reduced microglial activation. These results indicate an important role for CD14 in the recognition and subsequent killing of amyloid-beta damaged neurons by microglia.

Identification of up-regulated genes by array analysis in scrapie-infected mouse brains

The major neuropathological features of the transmissible spongiform encephalopathies (TSEs) are well documented, however, the underlying molecular events are poorly defined. We have applied cDNA expression arrays and quantitative RT-PCR to the study of gene expression in the brain, and more specifically in the hippocampus, of the well-characterized ME7/CV mouse model of scrapie. The number of genes showing consistent, scrapie-associated changes in expression was limited, and was primarily restricted to glial-associated genes. Increased expression of genes encoding glial fibrillary acidic protein, vimentin, complement component 1q (alpha and beta polypeptides), cathepsin D, clusterin and cystatin C was evident in the hippocampus from 170 days after inoculation (dpi), with expression increasing thereafter to terminal disease (225-235 dpi). Elevation of gene expression preceded clinical disease by approximately 30 days, and coincided with a 20-day period in the ME7/CV model during which 50% of the CA1 hippocampal neurones are lost. Increased expression of cystatin C, an inhibitor of lysosomal cysteine proteases, is a novel finding in the context of TSE neuropathology and was confirmed by Western analysis and immunocytochemistry.

Squalestatin Cures Prion-infected Neurons and Protects Against Prion Neurotoxicity

A key feature of prion diseases is the conversion of the normal, cellular prion protein (PrP(C)) into beta-sheet-rich disease-related isoforms (PrP(Sc)), the deposition of which is thought to lead to neurodegeneration. In the present study, the squalene synthase inhibitor squalestatin reduced the cholesterol content of cells and prevented the accumulation of PrP(Sc) in three prion-infected cell lines (ScN2a, SMB, and ScGT1 cells). ScN2a cells treated with squalestatin were also protected against microglia-mediated killing. Treatment of neurons with squalestatin resulted in a redistribution of PrP(C) away from Triton X-100 insoluble lipid rafts. These effects of squalestatin were dose-dependent, were evident at nanomolar concentrations, and were partially reversed by cholesterol. In addition, uninfected neurons treated with squalestatin became resistant to the otherwise toxic effect of PrP peptides, a synthetic miniprion (sPrP106) or partially purified prion preparations. The protective effect of squalestatin, which was reversed by the addition of water-soluble cholesterol, correlated with a reduction in prostaglandin E(2) production that is associated with neuronal injury in prion disease. These studies indicate a pivotal role for cholesterol-sensitive processes in controlling PrP(Sc) formation, and in the activation of signaling pathways associated with PrP-induced neuronal death.

The role of platelet activating factor in prion and amyloid-β neurotoxicity

In the prion diseases, neurodegeneration is preceded by the accumulation of the disease-associated isoform of the prion protein (PrP). In the present study, neurones treated with three different phospholipase A2 inhibitors were resistant to the toxic effects of PrP peptides or a synthetic miniprion (sPrP106). Phospholipase A2 inhibitors also protected neurones against a toxic peptide found in Alzheimer's disease (amyloid-beta1-42). Further studies showed that neurones pre-treated with platelet activating factor (PAF) antagonists were equally resistant to PrP peptides or amyloid-beta1-42. Moreover, both phospholipase A2 inhibitors and PAF antagonists reduced the activation of caspase-3, a marker of apoptosis, and the production of prostaglandin E2 that is closely associated with neuronal death in prion or Alzheimer's diseases.

Astrocytic regulation of NMDA receptor subunit composition modulates the toxicity of prion peptide PrP106–126

Prion diseases are neurodegenerative conditions. The main pathological alterations common to these diseases include the loss of neurones, gliosis and the deposition of an abnormal isoform of the prion protein in aggregates in the nervous tissue. Prevention of the devastating effects of prion disease requires prevention of neuronal death. Therefore, understanding the mechanism by which this occurs is essential. Cell culture studies using the synthetic peptide PrP106-126 have been central to developing a model of this mechanism. Using a coculture system, we have shown that PrP106-126 caused neuronal death mediated by glutamate. This neuronal death resulted from modification of the expression of NMDA receptor subtypes stimulated by the exposure of neurones to the combination of astrocytic factors, elevated Cu and PrP106-126. The results of these experiments suggest neuronal death in prion disease might be reduced by the use of NMDA receptor antagonists such as MK801 or inhibitors of the arachidonic acid metabolism pathway.

Neurones treated with cyclo-oxygenase-1 inhibitors are resistant to amyloid-β1-42

Epidemiological studies have shown that the risk of developing Alzheimer's disease is reduced by the chronic use of classical non-steroidal anti-inflammatory drugs (NSAIDs), drugs that inhibit the cyclo-oxygenase (COX) enzymes that convert arachidonic acid to prostaglandins. In the present study, human SH-SY5Y neuroblastoma cells or murine primary cortical neurones treated with NSAIDs were protected against the otherwise toxic effects of amyloid-beta1-42. COX-1 selective inhibitors provided greater protection than did COX-2 selective inhibitors or lipoxygenase inhibitors, suggesting that activation of COX-1 is required for amyloid-beta1-42-induced neurotoxicity. Although the production of neuronal prostaglandin E2 in response to amyloid-beta1-42 was reduced by the presence of COX-1 inhibitors, no neurotoxic effects of prostaglandin E2, or any other prostaglandin, were observed.

Evaluation of quinacrine treatment for prion diseases

Based on in vitro observations in scrapie-infected neuroblastoma cells, quinacrine has recently been proposed as a treatment for Creutzfeldt-Jakob disease (CJD), including a new variant CJD which is linked to contamination of food by the bovine spongiform encephalopathy (BSE) agent. The present study investigated possible mechanisms of action of quinacrine on prions. The ability of quinacrine to interact with and to reduce the protease resistance of PrP peptide aggregates and PrPres of human and animal origin were analyzed, together with its ability to inhibit the in vitro conversion of the normal prion protein (PrPc) to the abnormal form (PrPres). Furthermore, the efficiencies of quinacrine and chlorpromazine, another tricyclic compound, were examined in different in vitro models and in an experimental murine model of BSE. Quinacrine efficiently hampered de novo generation of fibrillogenic prion protein and PrPres accumulation in ScN2a cells. However, it was unable to affect the protease resistance of preexisting PrP fibrils and PrPres from brain homogenates, and a "curing" effect was obtained in ScGT1 cells only after lengthy treatment. In vivo, no detectable effect was observed in the animal model used, consistent with other recent studies and preliminary observations in humans. Despite its ability to cross the blood-brain barrier, the use of quinacrine for the treatment of CJD is questionable, at least as a monotherapy. The multistep experimental approach employed here could be used to test new therapeutic regimes before their use in human trials.

Activation by prion peptide PrP106-126 induces a NF-kappaB-driven proinflammatory response in human monocyte-derived dendritic cells

Specific prion peptides have been shown to mimic the pathologic isoform of the prion protein (PrP) and to induce a neurotoxic effect in vitro and in vivo. As monocytic cells are thought to play a role in the transmission and pathogenesis of prion disease, the use of these peptides in regulating monocytic cell function is under intense investigation. In the current study, we characterize the ability of prion peptide PrP(106-126) to activate specific signaling pathways in human monocyte-derived dendritic cells (DCs). Electrophoretic mobility shift assays establish the activation of transcription factor nuclear factor-kappaB within 15 min of exposure, with as little as 25 micro M peptide. This signaling cascade results in the up-regulation of inflammatory cytokines interleukin (IL)-1beta, IL-6, and tumor necrosis factor alpha (TNF-alpha) at the mRNA and protein levels. Phenotypic activation of DCs exposed to PrP(106-126) is partly a result of an autocrine TNF-alpha response and results in an increased ability of these cells to induce lymphocyte proliferation. The effects of PrP(106-126) on DCs were elicited through a receptor complex distinct from that used by human monocytes, demonstrating the ability of this peptide to interact with a multiplicity of receptors on various cell types. Together, these data suggest an involvement of DCs in prion disease pathogenesis.

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.

Cyclo-oxygenase inhibitors protect against prion-induced neurotoxicity in vitro

The mechanisms of neuronal loss during the course of the prion diseases are not fully understood. In this study, neurones treated with certain non-steroidal anti-inflammatory drugs (NSAIDs) were protected against the otherwise toxic effects of a peptide derived from the prion protein, or extracts containing infectious prions (PrP ). These NSAIDs inhibit the cyclo-oxygenase (cox) enzymes that metabolise arachidonic acid to prostaglandins (PG). Conversely, drugs that inhibited the metabolism of arachidonic acid to leucotrienes enhanced neurotoxicity. Studies with selective inhibitors highlighted the importance of the cox-1 isoform in prion-induced neurotoxicity. The cox-1 inhibitors also inhibited neuronal PGE production and protected both neuroblastoma cells and primary cortical neurones against prions. They also reduced microglia-mediated killing of prion-treated neurones.

Temporal and spatial relationship between the death of PrP-damaged neurones and microglial activation

Previous studies have demonstrated a role for microglia in the neuronal loss that occurs in the transmissible spongiform encephalopathies or prion diseases. In the present studies, the processes that lead to the death of neurones treated with synthetic peptides derived from the prion protein (PrP) were fully activated within 1 h, although neuronal cell death was not seen until 24 h later. Similarly, neurones exposed to PrP peptides for only 1 h activated microglia and a temporal relationship between the production of interleukin-6, an indicator of microglial activation, and microglial killing of PrP-treated neurones was also demonstrated. Activation of microglia and microglia-mediated killing of PrP-treated neurones or scrapie-infected neuroblastoma cells were maximal only when microglia were in direct contact with neurones.

Mayhem of the multiple mechanisms: modelling neurodegeneration in prion disease

This review examines recent attempts to advance the understanding of the mechanism by which neurones die in prion disease. Prion diseases or transmissible spongiform encephalopathies are characterized by the conversion of a normal glycoprotein, the prion protein, to a protease-resistant form that is suggested to be both the infectious agent and the cause of the rapid neurodegeneration in the disease. Death of the patient results from this widespread neuronal loss. Thus understanding the mechanism by which the abnormal form of the prion protein causes neuronal death might lead to treatments that would prevent the life-threatening nature of these diseases.

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.